Biofuel production, hydrogen production and water remediation by photocatalysis, biocatalysis and electrocatalysis

Abstract

The energy crisis and environmental pollution have recently fostered research on efficient methods such as environmental catalysis to produce biofuel and to clean water. Environmental catalysis refers to green catalysts used to breakdown pollutants or produce chemicals without generating undesirable by-products. For example, catalysts derived from waste or inexpensive materials are promising for the circular economy. Here we review environmental photocatalysis, biocatalysis, and electrocatalysis, with focus on catalyst synthesis, structure, and applications. Common catalysts include biomass-derived materials, metal–organic frameworks, non-noble metals nanoparticles, nanocomposites and enzymes. Structure characterization is done by Brunauer–Emmett–Teller isotherm, thermogravimetry, X-ray diffraction and photoelectron spectroscopy. We found that water pollutants can be degraded with an efficiency ranging from 71.7 to 100%, notably by heterogeneous Fenton catalysis. Photocatalysis produced dihydrogen (H₂) with generation rate higher than 100 μmol h⁻¹. Dihydrogen yields ranged from 27 to 88% by methane cracking. Biodiesel production reached 48.6 to 99%.

Introduction

Industrialization and urbanization have resulted in a significant increase in the consumption of finite fossil fuel reserves, leading to a cascade of environmental issues that pose a considerable threat to human health, environmental preservation, and energy demand. Recent years have seen a growing emphasis on utilizing environmentally friendly, efficient, and cost-effective technology to eliminate pollutants, produce clean energy, and synthesize valuable compounds. Among many methods, catalysis plays a major role in reducing production costs and boosting reaction efficiency, which will significantly advance the development of human society. Sustainability, on the other hand, is the development that meets the needs of the current generation without compromising the ability of future generations to meet their own needs. Environmental catalysis involves developing catalysts to degrade undesirable substances or enable alternative catalytic synthesis of essential molecules without generating environmentally unacceptable by-products, and their role in achieving sustainability is crucial. Environmental catalysts have made remarkable strides in pollutant elimination as well as energy and material production over the past decades. Novel environmental catalysts derived from waste or cost-effective materials reduce waste and promote a circular economy while improving catalytic performance.

Homogeneous catalysts have high catalytic performance and well-defined structures, but their stability and recyclability are low. Due to the significant potential for separation, reusability, and stability, heterogeneous catalysts are subsequently considered efficient for environmental applications. Although much progress has been achieved in heterogeneous catalysts, producing catalysts with high catalytic selectivity, high atom utilization efficiency, low cost, easy preparation, and well-defined active sites remains a significant challenge. For instance, designing catalytic systems such as single-atom catalysts enhance atom utilization efficiency to reduce catalytic costs and inherit the benefits of heterogeneous and homogeneous catalysts for several environmental applications (Xu et al. 2022). Another example is biochar; due to its porous and functional structure along with superior performance, it has been widely employed in agriculture, composting, gas storage, animal feed, energy storage, construction, and environmental remediation, with the potential to eliminate environmental contaminants, improve soil fertility, and reduce greenhouse gas emissions (Osman et al. 2022a). Adsorption is the most efficient method for decontaminating the environment; however, unsatisfied adsorption capacity, sluggish equilibrium rate, unstable adsorption conditions, and others continue to be obstacles to biochar adsorption applications (Song et al. 2022a).

Herein, we review the advances in photocatalysis, biocatalysis, and electrocatalysis as environmental catalysts and identify their promising and challenging issues. The main commonly used environmental catalysts discussed are biomass- and carbon-based, metal–organic frameworks, nanocomposites, non-noble metal nanoparticles, and enzymes. The advances and challenges in preparation methods for environmental catalysis are also highlighted. In addition, linking the chemical structure of environmental catalysis with their catalytic performance is exhaustively analyzed and discussed. In addition, the applications of environmental catalysis in three major fields, namely recent progress in water and soil remediation, biomass valorization, product upgrading, and biofuel and hydrogen production, are summarized and evaluated, as shown in Fig. 1.

Fig. 1

Applications of environmental catalysis, ranging from sustainable applications and technologies to sustainable catalysis approaches. Using catalysts, such as biomass- and carbon-based catalysts, metal–organic frameworks, nanocomposites, non-noble metal nanoparticles, and enzymes for environmental catalysis is a promising area of research. Notably, while some nano-based materials are sustainable, others are not and may be toxic or environmentally damaging. Applying environmental catalysis in three main areas: water and soil remediation, biomass valorization, product upgrading, and biofuel and hydrogen production can provide valuable insights for achieving global environmental protection

Environmental catalysis

Photocatalysis

Photocatalysis has emerged as the most promising solution for the world’s current energy crisis, environmental pollution, and global warming (Byrne et al. 2018). Since 1972, when Fujishima and Honda discovered the photoelectrochemical properties of titanium oxide (Fujishima and Honda 1972; Byrne et al. 2018; Yap and Lim 2011, 2012), significant effort has been made to develop photocatalysts for efficient photocatalytic processes. Other semiconducting materials, including zinc oxide and cadmium sulfide, have also been proposed as potential photocatalysts due to their similar band gap to titanium oxide. In contrast, their chemical stability and environmental toxicity limit their application in environmental remediation. As a result, titanium oxide remains one of the most researched semiconductor photocatalysts in academia and industry, and its photocatalytic properties have been commercialized in air-purifying (nitrogen oxide and volatile organic compounds conversion) and self-cleaning window film systems (Cha et al. 2019; Lyulyukin et al. 2018). Although their practical application in water and wastewater treatment is still limited, titanium oxide-based photocatalysts have been widely used in persistent pollution-burdened environments, implying that titanium oxide systems may be a viable solution for the removal of widespread emerging pollutants from aquatic environments.

A photocatalytic reaction, in general, consists of three steps. First, photocatalysis is initiated by bombarding a photocatalyst with ultraviolet light photons. Second, suppose the photon energy is greater than the band gap. In that case, these photons cause the generation of electrons (e⁻) on the surface of the photocatalyst to become ‘excited’ in the valence band (VB), causing them to move to the conduction band (CB). Simultaneously, a positive hole (h⁺_VB) is formed on the valence band. Electrons and holes are excited and migrate to the surface of photocatalysts, where they react with adsorbed electron acceptors and donors, respectively (Fig. 2).

Fig. 2

Photocatalytic redox reaction for the degradation of pollutants. Photocatalysis is initiated by bombarding a photocatalyst with ultraviolet light. When the photon energy is greater than the band gap energy, electrons can be generated on the surface of the photocatalyst, where becoming excited in the valence band, which moves to the conduction band. Simultaneously a positive hole is formed on the valence band. Generated electrons and holes further developed superoxide anions and hydroxyl radicals used to remove pollutants. H₂O, O₂, OH, H⁺, and H₂ refer to water, oxygen, hydroxyl, hydrogen ion, and hydrogen, respectively

In the preceding decades, bare titanium oxide nanomaterials have attracted considerable interest due to their potential applications in environmental pollution removal and photocatalytic hydrogen production (Osman et al. 2020a). Nevertheless, due to its relatively large band gap energy (3.2 eV and 3.0 V for anatase and rutile phases, respectively) (Fig. 3), it can only absorb approximately 6% of the solar energy that reaches the earth at any given time. Thus, significant effort has been devoted to enhancing titanium oxide's absorption properties in the visible spectrum and developing new photocatalytic materials that can capture a broad range, from ultraviolet to visible light and even the near-infrared region. This strategy will lead to the efficient use of solar energy as a clean, abundant, and renewable energy source. The photocatalysts' surface modification, alteration, and structure design can be optimized to increase and broaden light absorption.

Fig. 3

Band gaps of non-metal oxide and metal oxide photocatalysts at a pH of 7. Other non-metals and metals have various band gap energies than titanium dioxide. Solar energy can be more effectively adapted to prepare visible-light-driven photocatalysts using their properties, which require narrow band gap energy. Additionally, doping with other non-metal dopants can narrow the band gap energy between the valence and conduction bands. The unit used herein is eV (electron volt), which is a unit of energy commonly used in photocatalysis research to express the energy of photons or electron excitations. It measures the amount of energy gained or lost by an electron when it moves between energy levels in a material. H₂O, O₂, OH, and H₂ refer to water, oxygen, hydroxyl, and hydrogen

Doping titanium oxide with non-metal dopants (such as carbon, nitrogen, sulfur, and fluorine) or metal dopants (such as iron, silver, chromium, and manganese) can narrow the band gap between the valence and conduction bands, thereby enhancing photocatalytic properties (Akpan and Hameed 2011; Elbanna et al. 2016; Luo et al. 2019). There has been a growing interest in the development of novel photocatalysts, such as zinc oxide (Lee et al. 2016; Ani et al. 2018), zinc sulfide (Antoniadou et al. 2011; Hojamberdiev et al. 2020; Zhang et al. 2011), zirconium dioxide (Basahel et al. 2015; Tian et al. 2019; Pirzada et al. 2015), perovskites (Wei et al. 2021a; Bresolin et al. 2020), molybdenum disulfide (Yuan et al. 2019; Li et al. 2018a; Chang et al. 2014), tungsten trioxide (Wang et al. 2012a; Yu et al. 2017; Dutta et al. 2021), cadmium sulfide (Cheng et al. 2018a; Jing and Guo 2006; Zhu et al. 2019), and iron oxide (Hitam and Jalil 2020; Shi et al. 2012; Palanisamy et al. 2013) due to their potential to improve photocatalytic performance and efficiency.

In recent years, lightweight and abundant elements such as carbon, phosphorus, binary carbon nitride, hexagonal boron nitride, and boron carbide have given rise to a new class of metal-free materials opens up new photochemical possibilities. In photocatalytic fields, graphene has been extensively studied as an efficient electron acceptor capable of enhancing charge transfer and decreasing electron–hole pair recombination. Polymer graphite carbon nitride (g-C₃N₄), an analog of graphene, has become a hot material in photocatalysis due to its unique electronic band structure, low cost, and ease of preparation (Cao et al. 2015). The visible light can excite the graphite carbon nitride with a narrow bandgap of 2.7 eV, indicating its potential use in photocatalytic degradation, photocatalytic sterilization, hydrogen generation, and carbon dioxide reduction (Li et al. 2016a; Zheng et al. 2012).

However, a narrower band gap restricts the reduction and oxidation of photogenerated electrons/holes. In addition, the rapid recombination of photogenerated electron–hole pairs in semiconductors with a narrow bandgap diminishes their photocatalytic activity. Thus, significant effort is being put into developing an efficient photocatalyst that uses doping, coupling with other nanomaterials, precipitation with metal particles, crystal growth designs, and heterojunctions. Compared to single-phase semiconductor photocatalysts, hybrids of two or more semiconductor systems, such as heterojunction, appear to be one of the most promising methods for optimizing solar light utilization efficiency. In addition, the design of heterojunctions prevents photogenerated electron–hole pair recombination and permits rapid charge transport. Based on their adjacent band structures, heterojunctions can be classified as conventional type-I and type-II heterojunctions, Z-scheme heterojunctions, p-n heterojunctions, and homojunction band alignments (Wang et al. 2012a; Wang et al. 2012b; Su et al. 2011; Liu et al. 2011). Recent advances in heterojunction-based photocatalysts indicate a promising strategy for boosting photocatalytic activity in environmental pollution degradation, hydrogen production, and carbon dioxide reduction (De Wolf et al. 2012). While significant progress has been made in photocatalysis over the past few decades, its practical application is currently limited due to insufficient activity, poor stability, and high cost. There is still a great deal of work to improve photocatalyst systems.

Electrocatalysis

In recent years, there has been a growing interest in electrochemical processes for treating polluted waters. The ability of electrochemical systems to operate at ambient temperature and pressure, as well as their robust performance and adaptability to changes in the influent composition and flow rate, are a few advantages over alternative methods (Chen 2004). In addition, they typically do not require additional chemicals and produce no waste. Besides, electrochemical processes can be easily integrated with other technologies, such as electrochemical advanced oxidation processes (e.g., anodic oxidation, electro-Fenton, and electrocoagulation) (Sirés et al. 2014). Furthermore, the coupling efficiency with biological technologies (e.g., aerobic, anaerobic, membrane bioreactors, and microbial fuel cells) is also evaluated (Mousset et al. 2021).

In the 1920s, Bowden and Rideal developed electrocatalysis to measure hydrogen evolution reactions (Wu and Hu 2021; Popovski 2004; Boudjemaa 2020). Since then, electrochemistry has been extensively studied for energy conversion devices (such as batteries, fuel cells, and solar cells), electroanalytical sensors, organic synthesis, corrosion science, and wastewater treatment applications (Carlesi Jara et al. 2007; Zhang et al. 2019a; Comninellis 1994). Using palladium dioxide anodes, the degradation of phenolic compounds was studied, creating new opportunities for electrochemical applications in wastewater treatment (Wu and Hu 2021; Nilsson et al. 1973). The ability of electrocatalysis to remove various organic and inorganic contaminants, including dyes, phenols, pesticides, herbicides, and antibiotics, was then investigated (Comninellis 1994; Quiroz et al. 2014; Ansari and Nematollahi 2020; Xing et al. 2018).

Electrocatalysis is responsible for initiating or accelerating redox reactions in the presence of electrodes by supplying an external potential, which an electric field can provide. As depicted in Fig. 4, micropollutant degradation in an electrocatalysis system is primarily accomplished by direct and indirect oxidation processes. Electrocatalytic degradation processes consider several variables, including electrode materials, electrolytes, water matrices' physical and chemical properties (e.g., pH, coexisting ions, and ionic strength), and operating conditions. An electrode conducts electricity, initiates reactions, accelerates electron transfer, and selectively promotes electrochemical reactions. Moreover, catalysts are essential components of the electrodes used in the electrocatalytic degradation of environmental contaminants.

Fig. 4

Electrocatalytic system's direct and indirect oxidation mechanisms. The anode surface and the target contaminants undergo direct oxidation, whereas the anions or OH⁻ generated on the anode surface interact with the contaminants via indirect oxidation. e⁻, OH⁻, H⁺, Fe⁺², and H₂O₂ refer to electrons, hydroxide ions, ferrous ions, hydrogen ions, and hydrogen peroxide, respectively

The choice of electrode material affects the efficiency of electrochemical treatment and the possibility of byproduct formation. Electrocatalysts are categorized as either metal oxides such as ruthenium dioxide, iridium dioxide, lead dioxide, or tin dioxide, or carbon, e.g., boron-doped diamonds. Metal-oxide electrodes are categorized as either active anodes, such as iridium dioxide and ruthenium dioxide, or inactive anodes, e.g., ruthenium dioxide, lead dioxide, and tin dioxide. Titanium/lead dioxide is an excellent electrode, for instance, due to its high oxygen evolution potential, excellent electrical conductivity, chemical stability, and low cost. However, the widespread use of titanium/lead dioxide electrodes is limited due to their distinct disadvantages, including relatively high interface resistance, toxic lead ion leaching, film instability, and low current efficiency. To overcome this disadvantage, either a three-dimensional ordered microporous lead dioxide film based on a porous titanium substrate (Liu et al. 2017) or an aluminum-doped lead dioxide electrode via doping was synthesized (Chen et al. 2015). Tin dioxide-based electrodes are commonly used, but their wide band gap (3.6 eV) restricts their applications. Due to its high oxygen evolution potential, stability, and excellent electrocatalytic properties, antimony is the most used dopant. A tin dioxide-antimony/titanium electrode was synthesized using the sol–gel method for the electrochemical oxidation of antibiotics (e.g., ciprofloxacin) (Wang et al. 2016a). Recently, a titanium/carbon nanotube/tin dioxide-antimony-erbium electrode was prepared as the anode, with the carbon nanotube exhibiting a high specific surface area and high oxygen evolution potential in increased hydroxyl radical production at the anode (Lei et al. 2020).

In addition, carbon-based electrodes such as boron-doped diamond, graphite, and carbon nanotubes are commonly used due to their low resistance and high chemical stability. Still, there is a high cost associated with boron-doped diamond electrodes. Boron-doped diamond is the most frequently used carbon-based electrode due to its high oxygen evolution potential, high corrosion resistance, wide electrochemical potential window, excellent electrochemical stability, and low background current, which produces more reactive radicals to achieve a higher mineralization rate of antibiotics (such as sulfamethoxazole and trimethoprim) than other carbon-based electrodes (de Amorim et al. 2013). However, the reported catalysts for electrochemical have been evaluated for environmental remediation applications; limited efforts have been made for the toxic assessment of pollutants that are not efficiently mineralized via electrochemical and electrode materials, which should be considered in future research.

Biocatalysis

Biocatalysis is an integral component of the 'green chemistry' concept pioneered in the 1990s, and its impact on sustainability is now established beyond dispute, as shown in Fig. 5 (Alcalde et al. 2006). Compared to conventional physical and chemical methods, which have several significant drawbacks, such as insufficient purification, low efficiency, high costs, the production of hazardous byproducts, and application to a narrow concentration range for mineralizing organic compounds from wastewater, this method has several significant advantages (Bilal et al. 2019a; Wong et al. 2019). To overcome these obstacles, environmental engineers and biotechnologists are developing an innovative bioremediation technique that is efficient, cost-effective, and environmentally safe. Enzymes provide new options for treating effluent streams containing organic contaminants resistant to treatment (Bilal et al. 2019a). Enzymes are preferred environmental candidates over whole organisms because isolated enzymes are more specific and easier to handle, and their activity can be tailored to the reaction conditions more precisely (Sutherland et al. 2004; Pieper et al. 2004). Numerous biocatalysts, including hydrolases, oxidoreductases, laccases, and peroxidases, are actively involved in biological treatment (Kadri et al. 2017).

Fig. 5

Biocatalytic evaluation and improvement strategies. Biocatalysis involves the application of microbes and biocatalysts (or enzymes). Many biocatalysts have been engineered by rational and directed evolution, contributing to the catalytic activity, enantioselectivity, and stability that are essential for biocatalytic applications. This evolution allows using of biocatalysts for the biosynthesis of value-added pharmaceuticals and fine chemicals. Investigation of the modification of biocatalysts in a short time and recovery of water-soluble enzymes by enzyme immobilization or assistance with solid support attachment greatly enhances the catalytic features of enzymes

In addition, it was determined that other microorganisms, such as fungi, algae, and bacteria, could use a catabolic process to degrade pollutants. Recent interest has centered on microbial lipolytic enzymes because of their ability to catalyze biotransformation reactions involving compounds with ester bonds (e.g., converting waste into biofuel or other value-added products such as fatty acid esters, mono- and diacylglycerols, and others), as listed in Table 1 (Kumar et al. 2020a, b). Since enzyme catalysts are typically water-soluble and difficult to recover from aqueous solutions, using modified and immobilized enzymes is one of the emerging strategies for treating target wastes and lipids today. Enzymes were immobilized using various techniques (Bilal et al. 2017a, b, c, 2018; Zhang et al. 2015) to reuse expensive biocatalysts. It was found that enzyme attachment on solid carriers, either through physical adsorption or covalent bonds, is one of the most practical techniques (Bilal et al. 2018). Therefore, lipolytic enzymes can reduce the massive amount of lipid waste environmentally friendly and address energy security concerns.

Table 1 Waste as a source of lipolytic enzymes and high-value products produced using microbial lipases (Kumar et al. 2020a). Several microbes produces high-economic-value products using lipases as biocatalysts

Furthermore, various soil-derived enzymes, such as lipases, dehydrogenases, ureases, and catalases, have been proposed as bioindicators for pollution assessments (Margesin et al. 2000). Lipase@ZIF-8 nanoparticle-based biosensor was used to directly detect methyl parathions and organophosphorus pesticides with higher sensitivity and lower cost than laboratory-based methods (Herranz et al. 2018). However, lipase-based biosensors for bioremediation are not yet suitable for commercial use, and additional research is necessary for implementation.

Common environmental catalysts

Fields associated with the environment and energy have adopted the fostered approaches that use benign solvents and mild reaction conditions and produce limited amounts of waste using cost-effective techniques (Chen et al. 2017a). The utilization of environmental catalyst materials contributes to numerous aspects of the United Nations’ seventeen Sustainability Development Goals, particularly Goal 12 (enabling responsible production) and Goal 13 (climate action). Short preparation time, ambient temperature, less hazardous solvents, solvent-free conditions, simple methods, low energy consumption, and fewer by-products are generally considered to be among the most essential characteristics of a green synthesis route (Guesh et al. 2017). Remarkably, environmental catalysts materials have been utilized in various applications, such as electrocatalysis, organic synthesis, biocatalysis, desulfurization, water and soil remediation, biomass valorization, biofuel, and hydrogen production, among others. To reduce environmental pollution and achieve sustainability objectives, numerous materials have been engineered to serve as environmental catalysts instead of traditional catalysts. Metal–organic frameworks, biomass- and carbon-based materials, non-noble metal nanoparticles, nanocomposites, and enzymes are frequently used as environmental catalysts among these materials. A diagrammatic illustration of commonly used environmental catalysts is shown in Fig. 6.

Fig. 6

Common environmental catalysts. Metal–organic frameworks are synthesized via metal-linker coordination, forming a uniform porous structure. Carbon-based catalysts are widely used as environmental catalysts in environmental applications. Non-noble metal nanoparticles are considered environmental catalysts under green conditions. Nanocomposites have exhibited remarkable performance in many environmental applications, and enzymes are biological catalysts consisting of protein and active metal sites

Biomass-derived carbon materials

To scale up the technology, scientists must prioritize several essential criteria, such as production cost, efficiency, durability, and environmental impact. Recently, biomass-derived carbon-based materials have been heavily utilized in many applications, including energy storage and conversion, sensing, catalysis, and environmental applications (Park et al. 2022). This was attributed to the outstanding properties of carbonaceous materials in terms of good mechanical stability, unique electronic properties of carbon atoms, electrical conductivity, and structural tenability (Xiao et al. 2022). Notably, researchers have introduced many fabrication strategies to scale carbonaceous materials, such as arc discharge, laser ablation, chemical vapor deposition, pyrolysis, and electrochemical methods, depending on non-renewable petroleum carbon sources such as methane, ethylene, and other petrochemicals (Wang et al. 2021a). However, these strategies unveiled some negative aspects, such as high energy consumption, emission of a large amount of greenhouse gas carbon dioxide, and environmental pollution. For instance, the production of activated carbon consumes large amounts of energy (97 MJ/Kg) with an emission of 6.6 kg/Kg of carbon dioxide. Similarly, graphene production's energy demand was 1100–1640 MJ/Kg with a high carbon dioxide emission (80 kg/Kg) (Boyjoo et al. 2021). On this basis, researchers have attempted to produce novel next generations of carbon materials with desirable characteristics such as affordable cost, facile and sustainable fabrication strategies, low energy consumption, and eco-friendly merit. From an economical and sustainable point of view, researchers have considered biomass an excellent alternative, renewable organic feedstock to crude oil and natural gas (Osman et al. 2022b; Farghali et al. 2022). This is ascribed to the abundance of enormous biomass resources, including plant and animal-derived materials.

The worldwide annual disposal of food waste was found to be equivalent to a third of food production (1300 million tons) (Matharu et al. 2016); also, agricultural practices generate 570 million tons of waste annually (Morrison and Golden 2015). For instance, plant-derived biomass comprises three components: (1) inedible portions (lignin, cellulose, and hemicellulose); (2) edible portions (starch, free sugars, protein, and vegetable oils (triglycerides); and (3) essential oils and other secondary metabolites of high value (Sherwood 2020). Accordingly, the exclusion of unnecessary wastes and the rational upcycling of those wastes guarantee viable, high-quality products, minimize the environmental problems related to unwise waste disposal, i.e., waste management, and achieve the circular economy aspects (Sherwood 2020; Clark et al. 2016; Peng et al. 2023).

Recently, biomass-derived carbon materials were established as the most sustainable and scalable strategy due to the enormous annual production of biomass from agriculture and forestry (104.9 billion tons/year) (Field et al. 1998). Interestingly, there are numerous macrostructures with tunable porosity in biomass-derived carbon materials, including spherical (Xia et al. 2012), reticular (Ubeyitogullari and Ciftci 2016), fibrous (Hu et al. 2016), ribbon-like (Ai et al. 2018), bubble-like (Xie et al. 2018), and plate-like materials (Ling et al. 2015), expanding their applications. Besides, the remarkable characteristics of such materials outperform the noble metals in many aspects, including (1) cost-effectiveness, i.e., carbon price is 0.03$ per gram while gold price is 60$ per gram, silver price is 25$ per gram, and platinum price is 34$ per gram; (2) ease of fabrication due to the abundance of raw materials; (3) tailorable structure due to the presence of heteroatoms, for example nitrogen, oxygen, and sulfur, that facilitate the functionalization and enhance the catalytic activity (Li et al. 2020; Wu and Zhang 2020). However, the synthesis strategy played a crucial role in carbon materials' porosity and surface chemistry. Therefore, a challenging issue is fabricating a uniform porous structure with a well-defined shape of sustainable biomass-derived carbon materials.

The world demand for energy has become a quintessential priority for better and sustainable life for humanity. The massive emissions of carbon dioxide into the atmosphere, which amount to about 30 billion tons annually, have severely threatened the environment with issues like global warming and air pollution as a result of the overuse of fossil fuels as a major source of energy (about 80%) to support the industrial revolution and satisfy population growth (Seh et al. 2017; Zhang et al. 2020; Panwar et al. 2011). According to the international energy agency, atmospheric carbon dioxide concentration increased from 280 to 480 ppm in 2019 (Chu et al. 2017; Swann et al. 2016). As a result, this dangerous environmental crisis has drawn considerable public concern. Therefore, many researchers have devoted their efforts to creating innovative technologies and strategies and generating an alternative, green, and renewable energy source.

The exploitation of hydrogen gas as a clean energy source has become a promising approach owing to its non-toxic combustion by-product and the higher caloric value (142 MJ/kg) comparable to fossil and hydrocarbon fuels (Muradov and Veziroğlu 2008; Schlapbach and Züttel 2011). Therefore, great efforts have been exerted to foster an alternative strategy instead of the conventional methods like steam reforming fossil fuel and water electrolysis, which consume a large amount of energy and money (Karayilan et al. 2019; Hosseini and Wahid 2016). Subsequently, photocatalytic water splitting has gained significant interest as a sustainable and green strategy to produce hydrogen (Gopinath and Nalajala 2021). Especially, photocatalytic water splitting has demonstrated an excellent efficiency of solar energy conversion into hydrogen, reaching 5–10%, indicating its proficiency and possible application on a large scale (Arunachalam et al. 2021; Wang et al. 2019a).

Three-dimensional biomass-derived carbon materials revealed an excellent performance in different catalytic applications due to their large specific surface area, high porosity, good conductivity, and propitious thermal and chemical stability, as shown in Table 2. In this perspective, a sustainable and cost-effective three-dimensional carbon aerogel support from seaweed biomass (carrageenan) was prepared to boost the efficiency and photostability of cadmium sulfide photocatalyst (CdS@CAs) (Quan et al. 2018). Carrageenan provided a non-toxic sulfur source, i.e., sulfated galactons, that extended across the double helix structure of carrageenan and was integrated by cadmium cations, resulting in photocatalysts with interconnected macropores and mesopores that accelerated the mass transfer of reactants. The results exhibited an excellent hydrogen evolution rate of 113.5 µmol h⁻¹ with a photocurrent density of 100 µA cm⁻². This finding may be attributed to the good electrical conductivity of the photocatalyst that accelerated the photogenerated charge separation and the photocorrosion resistance of cadmium sulfide. A certain carbon coating thickness for cadmium sulfide inhibited the self-oxidation of cadmium-sulfur bonds by the photogenerated holes. Therefore, cadmium sulfide photocatalyst achieved excellent photostability after 4000 s under light illumination.

Table 2 Carbon-based and biochar-based catalysts: carbon source, preparation conditions, surface area, catalytic applications, and efficiency. Biomass-derived carbon materials with large specific surface areas served as catalytic species hosts, enhancing stability and inhibiting agglomeration

Further, a sustainable strategy for the catalytic hydrolysis of ammonia borane (ammonia, borane trihydridoboron, ammonia borane) was proposed using magnetically recoverable ruthenium and cobalt (Ru and Co) bimetallic nanoparticles supported on costless cotton-derived carbon fibers (Ru@Co/CCF) (Yang et al. 2018a). Transmission electron microscope and elemental mapping images revealed the uniform dispersion of ruthenium and cobalt bimetallic nanoparticles with an average diameter of 5–15 nm onto helical-shaped carbon fiber support. This was related to the large number of –OH functional groups on the surface of the carbon fibers that uniformly adsorbed Co²⁺. Interestingly, ruthenium and cobalt on cotton-derived carbon fibers showed a satisfactory turnover frequency (139.59 mol_H2 mol_Ru⁻¹ min⁻¹) at 30 °C. Due to the cobalt nanoparticles' ferromagnetic properties, ruthenium and cobalt on cotton-derived carbon fibers were involved in several catalytic cycles with good catalytic activity and facile recovery by an external magnet. The chemical storage of hydrogen could be a promising strategy rather than the traditional storage methods such as compression, liquefication, and adsorption due to its high volumetric and gravimetric hydrogen content (i.e., ammonia. borane trihydridoboron has a molecular weight of 30 g mol⁻¹ with a hydrogen content of 19.6 wt%) (Sadhasivam et al. 2017).

Metal–organic frameworks

Metal–organic frameworks are a dominant category of catalysis owing to their unparalleled characteristics that are not wholly exploited, including flexibility, superb reusability, immense surface area, unique physicochemical properties, and intrinsic structures. In addition, metal–organic frameworks are synthesized via metal-linker coordination bonds in three directions, forming a uniform porous structure (Vasanthakumar et al. 2020). Solvothermal is generally the most common method to fabricate metal–organic frameworks; however, it involves dissolving metals and ligands in organic solvents such as dimethyl formamide (Omer et al. 2021; Abd El-Monaem et al. 2022). Notably, dimethyl formamide is a quite detrimental solvent that decomposes to dimethylamine at high temperatures, causing environmental and handling problems (Chen et al. 2017a). Hence, for applying the concept of sustainability and avoiding environmental issues, the preparation of metal–organic frameworks via solvent-free approaches has become a research hotspot. The alternative green synthesis methods of metal–organic frameworks are sorted into two categories according to solvent amount and energy consumption; the first one includes aging (Cliffe et al. 2012; Mottillo et al. 2013), mechanochemical (Užarević et al. 2016; Do and Friščić 2017), and thermochemical (Lanchas et al. 2012). In comparison, the second category contains microwave-assisted (Liang et al. 2013; Klinowski et al. 2011), sonochemical (Song et al. 2016; Yuan et al. 2016), and electrochemical (Liang et al. 2013; Klinowski et al. 2011). Overall, to fabricate metal–organic frameworks in a sustainable and green approach, Julien et al. (2017) summarized the main aspects that should be considered (Fig. 7); (1) utilizing biocompatible building blocks, (2) eschewing bulk solvents, (3) utilizing supercritical liquids or water as a solvent, and (4) rationalizing the energy inputs.

Fig. 7

Environmental criteria for metal–organic framework-based catalysts. Green metal–organic can be obtained using green solvents and less toxic metal salts. The linker should be eco-friendly and produces biodegradable products with less hazardous by-products. The process should be energy efficient, and the metal–organic framework-based catalysts should have good recyclability and be applied in green applications

Indeed, metal–organic frameworks' heterogeneous character and porous nature render them outstanding catalysts compared to other catalysts, such as aluminosilicates, zeolites, and others. (Table 3). In addition, the synergistic effect between the Brønsted acid (linker) and Lewis acid (metal node) greatly promotes the strength of catalytic reactions (Jiang et al. 2014). More importantly, the functionality of metal–organic frameworks endows them one more merit than conventional catalysis since it facilitates the modification of metal–organic frameworks with active species, enhancing their catalytic performance (Chen et al. 2017b). The introduction of active species to metal–organic frameworks typically takes place via three strategies: post-synthetic approach, encapsulation of active species into metal–organic frameworks, and functionalization via the unsaturated metal sites or the organic ligand (Dhakshinamoorthy and Garcia 2014; Liu et al. 2014a).

Table 3 Synthesis conditions and applications of metal–organic frameworks (MOF)-based, non-noble metal-based, and nanocomposite-based catalysts

Despite the merits mentioned above of metal–organic framework-based catalysts, they suffer a critical shortcoming: the poor stability of their structure, particularly in organic solvents and water, limiting their applications in water treatment (Tan et al. 2015). The robustness study of metal–organic frameworks in water, acidic and basic media, and water vapor found that the weak coordination bond between metal and linker is the leading cause of oxidative leaching of metal nodes. Thus, the metal-linker coordination bonds' strength has controlled the metal–organic frameworks' stability (Rashid et al. 2020). Nevertheless, few metal–organic frameworks possess high stability in water, especially MIL-family, such as MIL-101 (chromium, iron, aluminum), MIL-100 (chromium, iron), and MIL-53 (chromium, aluminum), as well as zirconium-based metal–organic frameworks (Schoenecker et al. 2012; Lan et al. 2016). Furthermore, porous coordination network-222 and porous coordination network-224 showed relatively high stability in both acidic and basic conditions, while the linker protonation of aluminum- metal–organic frameworks endows excellent stability in the acidic medium (Feng et al. 2013; Feng et al. 2012). The stability of zeolitic imidazolate frameworks was examined in boiling water, sodium hydroxide, and benzene at different temperatures for seven days. The results revealed the superb structural stability of ZIF-8 in boiling benzene and water for seven days and only 24 h in sodium hydroxide solution (Panda et al. 2019). Several studies have focused on enhancing the metal–organic framework's stability, suggesting that the linker's basicity is the key parameter in the robustness of the metal–organic framework since it controls the bond strength between metal and linker (Ali et al. 2021). Owing to these unparalleled advantages, metal–organic frameworks are intensively utilized in diversified applications as follows:

Pascanu et al. (2013) fabricated palladium-loaded MIL-101(Cr)-NH₂ composite (Pd@ MIL-101(Cr)-NH₂) with different loading ratios of Pd (4, 8, 12, and 16 wt%). It was observed a diminution in the specific surface area (S_BET) of MIL-101(Cr)-NH₂ from 2869 to 1321 m²/g and pore volume from 1.403 to 0.625 m³/g with the increase in the Pd loading ratios from 4 to 16 wt%, respectively. This finding may be attributed to the particle aggregation of the extra amount of Pd. Furthermore, the catalytic activity of the as-fabricated Pd@ MIL-101(Cr)-NH₂ composite was examined in the Suzuki–Miyaura cross-coupling reaction. It was found that the optimal Pb-loading ratio was 8 wt% since aryl and heteroaryl bromides and chlorides were coupled in eco-friendly solvents (H₂O and ethanol) within a quite short reaction time and at an ambient temperature of 80 °C. More importantly, the recyclability study depicted the promising catalytic activity of 8 wt% for Pd@MIL-101Cr-NH₂ even after the 10th cycle.

In that context, Nuri et al. (2020) investigated the catalytic activity of Pd-supported amino magnetic MIL-101(Cr) composite (Pd@Fe₃O₄-NH₂@MIL-101(Cr)-NH₂) toward Mizoroki–Heck Cross-Coupling reaction of iodobenzene and methyl acrylate. They recorded a decline in the saturation magnetization (Ms) of iron oxide (Fe₃O₄) after blinding with Pd and MIL-101(Cr). However, the Ms of Pd@Fe₃O₄-NH₂@MIL-101(Cr)-NH₂ was still sufficient for the separation by an external magnet instead of the conventional techniques that consumes a long time. Such perfect and easy separation is the prominent role of iron oxide since it does not possess catalytic activity toward Heck cross-coupling reaction. The reusability study displayed the ability of Pd@Fe₃O₄-NH₂@MIL-101(Cr)-NH₂ to reuse for seven cycles with no significant decrease in its activity. Furthermore, it was suggested that the reduction of the catalytic activity of the as-fabricated magnetic composite is most likely due to the Pd leaching. Notably, Pd@Fe₃O₄-NH₂@MIL-101(Cr)-NH₂ revealed a high catalytic activity and stability toward cross-coupling iodobenzene and methyl acrylate since the turnover frequency reached 2438 h⁻¹ within 30 min at 120 °C.

In one investigation, Alamgholiloo et al. (2021) inspected the catalytic activity of magnetic ZIF-67 composite (Fe₃O₄/ZIF-67) toward the degradation of ciprofloxacin through peroxymonosulfate activation. The rhombic dodecahedron ZIF-67 was greenly synthesized in a benign solvent (methanol). However, ZIF-67 was successfully synthesized via a green approach in an eco-friendly solvent, and the yield was low (about 57%). Consequently, the green strategies for metal–organic framework preparation need further developments to be applicable in industrial applications.

Notably, Fe₃O₄/ZIF-67 composite showed high efficiency in degrading ciprofloxacin since the degradation rate reached 98% during 48 h using 0.4 g/L Fe₃O₄/ZIF-67 and 0.5 g/L peroxymonosulfate. Furthermore, the pure Fe₃O₄ and ZIF-67 revealed a lower ciprofloxacin degradation efficiency compared to the Fe₃O₄/ZIF-67 composite, reflecting the synergistic effect between the pristine components. To determine the predominant radical species in the degradation of ciprofloxacin, the scavenger effect was studied in the presence of isopropyl alcohol, benzoquinone, and ethanol. The result clarified that both SO₄·⁻ and ·OH are the controlled radicals. Besides, Electron spin resonance confirmed the same result since only the characteristic signals of SO₄·⁻ and ·OH appeared in the Fe₃O₄/ZIF-67/peroxymonosulfate system. These findings indicated that the degradation mechanism of ciprofloxacin most probably occurs via the radical pathway.

In another investigation, Chandra et al. (2019) adopted the green synthesis approach to fabricate another ZIF MOF to photodegrade methylene blue from an aqueous solution. The core–shell tin dioxide (SnO₂)-ZIF-8 composite was fabricated with various ratios of tin dioxide, revealing a decrease in the degradation efficiency of methylene blue with the increase in the ratios of tin dioxide due to particle aggregation. It was reported that the photocatalytic degradation mechanism of methylene blue using tin dioxide-ZIF-8 composite occurred as follows: the movement of the generated e⁻ and h⁺ in the conduction and valance bands of tin dioxide to ZIF-8, reducing the recombination of e⁻ and h⁺ pair. Then, this e⁻ and h⁺ pair could form ·OH via the oxygenation of H₂O. Thus, ·OH is the responsible radical for the photocatalytic degradation of methylene blue, agreeing with several previous studies (Chandra and Nath 2017; Jing et al. 2014; Kim et al. 2016).

Further, Gong et al. (2019) successfully prepared a magnetic core–shell Fe₃O₄@GO@MIL-100(Fe) microsphere catalyst using water as a solvent instead of dimethyl formamide in a green and sustainable route. The scanning electron microscope and transmission electron microscope images of the synthesized materials demonstrated that Fe₃O₄ particles have rough surface spheres with particle sizes ranging from 300 to 350 nm. However, a crinkled surface texture was obtained after being wrapped with graphene oxide (GO) with a 4.5 nm thickness of the GO layer. Upon coating with MIL-100(Fe), the MOF layer was 61 nm thick, confirming the formation of magnetic core–shell Fe3O4@GO@MIL-100(Fe) microspheres. The fabricated catalyst was used for 2,4-dichlorophenol (2,4-DCP) degradation via the photo-Fenton process with almost 100% efficiency in 40 min. The reusability tests revealed that the magnetic core–shell Fe₃O₄@GO@MIL-100(Fe) catalyst has good stability and recycling ability.

Non-noble metals nanoparticles

Undoubtedly, metal nanoparticles have received immense interest in the last few years in the catalysis field owing to their unique physical and chemical properties (Song et al. 2015). Interestingly, it was deduced that the metal catalysts in a nano-size possess advanced catalytic performances compared to their bulk equivalents since the size and shape play significant roles in the chemical activity of the catalyst (Table 3). The catalyst shape impacts its activity and selectivity toward the catalytic reactions, while the catalyst size controls its specific surface area (Mandić et al. 2017). Nonetheless, the fabrication of shape-controlled metal nanoparticles is still a big challenge and a complicated process. For this purpose, several strategies have been developed to synthesize stable metal nanoparticles with a defined shape, such as the addition of inorganic capping agents, organic ligands, colloids, polymers, or the fabrication of core–shell materials (Campelo et al. 2009).

Among the metals nanoparticles, noble metals have revealed exceptional catalytic performance in various potential applications (Wang and Astruc 2017). However, they could not be classified as sustainable catalysts due to their high price, rare abundance on earth, and detrimental impacts on the biological system. Contrariwise, the availability, premium activity, and selectivity of non-noble metals have acquired considerable fame among the diverse as more favorable catalyst types (Ilies et al. 2020). Therefore, extensive studies have been implemented focusing on preparing non-noble metal nanoparticle-based catalysts under green conditions, considering using nontoxic solvents and consuming low energy. In addition, the utilization of bio-reducing agents such as algal, sugars, glutathione, and mainly plant extract (Fig. 8) instead of the toxic reducing agents such as sodium borohydride or lithium borohydride. For instance, Lohrasbi et al. (2019) fabricated an iron nanocatalyst using Plantago as a bio-reducing agent. The transmission electron microscope image revealed that the particles are uniform spherical with a size range between 4.6 and 30.6 nm.

Fig. 8

Synthesis of non-noble metal nanoparticles from plant materials, such as leaves, seeds, flowers, stems, roots, or fruits. The plant extract contains a variety of bioactive agents, such as polyphenols and flavonoids. These bioactive agents can reduce the metal ions to produce metallic nanoparticles. The capping agents in the plant extract act as capping agents for the metallic nanoparticles. These non-noble metal nanoparticles are suitable more preferably-type catalysts. M⁰ refers to metal nanoparticles

Furthermore, Din et al. (2018) utilized the wild plant Calotropis gigantea as a bio-reducing and stabilizing agent to fabricate nickel nanocatalysts. The various characterization tools inferred the successful fabrication of nickel in a nano-size of 20–40 nm. Thus, such bio-reducing agents could fabricate non-noble metal nanoparticles with controlled particle size and shape in some cases (Murphy 2008).

Unfortunately, there are some flaws in non-noble metal nanoparticles as catalysts, including their rapid deactivation, poor recyclability, and difficult separation. Several previous studies have reported that using supported nanoparticles is the most effective method among the other approaches to overcome these demerits of non-noble metals (Wang et al. 2021b). However, supported metal nanoparticles provoke researchers' anxiety about whether the quite active and dispersed supported metal nanoparticles are harmful to human tissues or not. On the other hand, no cytotoxicity or cellular oxidative stress was inferred for the unsupported metals, although they were retained in human tissues (Campelo et al. 2009). In this perspective, Musa et al. (2017) fabricated copper nanoparticle-supported nanocrystalline cellulose (Cu@NCC) for the catalytic reduction of methylene blue in the presence of sodium borohydride (NaBH₄) as a reducing agent. The following equations represent the proposed preparation mechanism of copper nanoparticle-supported nanocrystalline cellulose;

$${\text{Cu}}^{{{2} + }} + {\text{ NCC }} \to \, \left[ {\left( {{\text{NCC}}} \right)/{\text{Cu}}} \right]^{{2}}$$

(1)

$$\left[ {\left( {{\text{NCC}}} \right)/{\text{Cu}}} \right]^{{{2} + }} + {\text{ 2NaOH }} \to \, \left[ {\left( {{\text{NCC}}} \right)/{\text{Cu}}\left( {{\text{OH}}} \right)_{{2}} } \right]$$

(2)

$$\left[ {\left( {{\text{NCC}}} \right)/{\text{Cu}}\left( {{\text{OH}}} \right)_{{2}} } \right]^{{{2} + }} + {\text{ N}}_{{2}} {\text{H}}_{{4}} \to \, \left[ {{\text{NCC}}/{\text{Cu}}} \right]^{{}} + {\text{ N}}_{{2}} \uparrow \, + {\text{ 2H}}_{{2}} {\text{O }} + {\text{ H}}_{{2}}$$

(3)

To infer the successful preparation of copper nanoparticle-supported nanocrystalline cellulose, the elemental composition was examined by X-ray fluorescence, clarifying the presence of Cu in nanocrystalline cellulose (NCC). Moreover, the thermogravimetric analysis elucidated an improvement in the thermal behavior of NCC after blinding with Cu nanoparticles, proving the interaction between NCC and Cu nanoparticles. At the same time, a noticeable decline in the surface area of copper nanoparticle-supported nanocrystalline cellulose was recorded compared to that of NCC, most likely due to the dispersion of Cu nanoparticles onto the NCC surface and its pores (Ghosh et al. 2015). Results confirmed the auspicious catalytic activity of copper nanoparticle-supported nanocrystalline cellulose toward reducing methylene blue since the reaction completely terminated within merely 12 min. In comparison, the unsupported Cu nanoparticles reduced methylene blue within 44 min. This finding revealed that the supported nanoparticles possessed higher catalytic activity than the unsupported ones. Contrariwise, Kamal et al. (2019) pointed out that the unsupported carboxymethyl cellulose-stabilized cobalt nanoparticles (CMC-Co) contained higher catalytic activity toward reducing methylene blue than the supported carboxymethyl cellulose-stabilized cobalt bacterial-cellulose nanofibers (CMC-Co-bacterial cellulose nanofibers). This behavior may be explained by the availability of the whole surface area of cobalt nanoparticles of carboxymethyl cellulose-stabilized cobalt nanoparticles to adsorb the methylene blue molecules. In contrast, in carboxymethyl cellulose-stabilized cobalt-bacterial cellulose nanofibers, the cobalt nanoparticles are partly available.

In another attempt, Wang et al. (2017) adopted an innovative technique to prepare an electrocatalyst for the oxygen reduction reaction. The platinum-coated cobalt nanoparticles (Co@Pt) were fabricated via in situ seed growth approach. Such a preparation method enables self-nucleation and growth by adjusting the Pt deposition rate to be slower than its diffusion rate on the surface (Park et al. 2016). Besides the generated CO during the Co growth from cobalt carbonyl facilities, the coating of Pt on the Co. Transmission electron microscope image inferred the core–shell-like structure of platinum-coated cobalt with an average diameter of 10 nm, whereas the diameter of the platinum shell was 1 nm. It was found that the catalytic activity of platinum-coated cobalt for oxygen reduction reaction was enhanced 10 times compared to pure platinum.

Furthermore, platinum-coated cobalt exhibited a high stability potential cycling since the non-noble Co nanoparticles were conserved from leaching out by the noble platinum nanoparticles shell. This result was consistent with Park et al. (2016), which enhanced the durability of nickel–platinum (Ni–Pt) electrocatalyst for oxygen reduction reaction by coating it with a thin layer of platinum. It was deduced that Ni–Pt@Pt protected Ni nanoparticles from leaching; however, 11% of Ni was leaching from Ni–Pt. In addition, the activity of Ni–Pt dwindles by about 75% after 10,000 cycles of the oxygen reduction reaction, while the activity of Ni–Pt@Pt decreased by only 25%.

Nanocomposites

Using any pure substance as a catalyst has advantages and disadvantages, so combining two or more to form composites is a superior avenue in modern technology. Such an approach enables the exploitation of the best merits of each substance by overcoming some of its demerits (Ates et al. 2017). Composites are classified into three groups based on their size; micro-composites, macro-composites, and nanocomposites (Paul and Robeson 2008). The extremely high surface area, strong interaction between the matrix, and high stability of nanocomposites have revealed remarkable activity (Yu et al. 2006; Xu et al. 2015a). Notably, it was evinced that the combination of two active components dramatically enhances their catalytic activity (Lin et al. 2019a).

Nonetheless, the rapid growth of nanocomposites during the preparation results in hazardous environmental issues due to their complex decomposition (Ates et al. 2020). One of the most feasible solutions to this problem is utilizing templates to control the size and shape of nanocomposites and protect their surfaces, thereby preventing particle aggregation (Yadav et al. 2019). Diversified commercial temples have been utilized as functionalizing agents to ameliorate nanocomposites' morphology, size, and properties (Table 3). Still, they have revealed a fatal environmental risk due to their difficulty degrading or removing. Hence, synthesizing nanocomposites using bio-renewable resources like starch, chitin, vegetable oils, lignin, natural rubber, and cellulose has drawn vast concern to fabricating catalysts with no harsh synthetic approach (Borah et al. 2017). Such nanocomposite-based sustainable catalysts possess outstanding features, including easy preparation, excellent biocompatibility, low density, good biodegradability, low cost, flexibility, and suitability for modifications (Fig. 9) (Ulu et al. 2018). However, these fabricated nanocomposites from bio-renewable resources still suffer drawbacks limiting their industrial applicability, such as low thermal, chemical, and mechanical stability (Ates et al. 2020).

Fig. 9

Characteristics of the nanocomposite-based sustainable catalysts fabricated from renewable resources. Bio-renewable resources such as lignin, starch, and chitin are widely used in nanocomposite-based sustainable catalysts preparation. Nanocomposite-based catalysts should be flexible and suitable for modifications. Biodegradability is the primary condition for nanocomposites to be considered sustainable catalysts. Low density, biocompatibility, and ease of preparation are among the main criteria of nanocomposite-based sustainable catalysts

Nanocomposites derived from bioresources have exhibited remarkable performance in diverse potential applications. For instance, Wang et al. (2019b) fabricated chelated copper onto polydopamine-coated sand composite (Cu-PDA@Sand) for the catalytic degradation of the anionic Congo red and cationic methylene blue as well as reduction of 4-nitrophenol. It was found that 4-nitrophenol was wholly reduced to 4-aminophenol within 13 min. Furthermore, the color of Congo red vanished after only 6 min, while the color of methylene blue dye disappeared after 9 min, suggesting the reduction of methylene blue (Kim et al. 2016; Subair et al. 2016). Moreover, to examine the applicability of copper onto polydopamine-coated sand composite in industrial applications, the long-term catalytic activity was studied after 30 days. It recorded a slight diminution in the apparent reaction rate of 4-nitrophenol, Congo red, and methylene blue (about 6%), inferring the stability of the copper onto polydopamine-coated sand composite. Interestingly, the reusability test showed that the catalytic activity of copper onto polydopamine-coated sand composite toward the reduction of 4-nitrophenol did not decrease yet, even after 20 cycles. These findings confirmed the superb catalytic activity, stability, and reusability of copper onto polydopamine-coated sand composite.

In another study, Hajipour and Malek (2021) inspected the catalytic performance of Co/N-heterocyclic carbene-supported magnetic chitosan composite (Co/NHC@mCs) toward Suzuki and Sonogashira reactions of aryl chlorides. Several key parameters were studied to determine the optimal condition for both reactions, including solvent, base, catalyst dose, and reaction temperature. It was recorded that Suzuki cross-coupling reaction of 1-chloro-4-nitrobenzene with (4-methoxyphenyl) boronic acid achieved a high yield (90%) in polyethylene glycol as a reaction medium and at 70 °C within 8 h using 3 mol% of Co/NHC@mCs. On the other hand, the best yield (84%) in Sonogashira cross-coupling between 1-chloro-4-nitrobenzene and 1-ethynyl-4- methoxybenzene was obtained in polyethylene glycol at 100 °C, using 6 mol% of Co/NHC@mCs. Notably, both reactions proceeded without adding Co/NHC@mCs, and no product was obtained in these cases. Importantly, the recyclability test pointed out the propitious recyclability of Co/NHC@mCs since its catalytic activity dwindled to only 7% after the seven cycles. Furthermore, the used Co/NHC@mCs were characterized by transmission electron microscope, and scanning electron microscope displayed no change in the morphology compared to the fresh Co/NHC@mCs. Additionally, the Co-leaching amount that inductively coupled plasma mass spectrometry recorded was less than 2%. At the same time, X-ray diffraction revealed no change in the peak intensity after using Co/NHC@mCs for 7 cycles. These results suggested the excellent efficiency, stability, and reusability of Co/NHC@mCs. In addition to the sustainability of the earth-abundant Co-based catalyst instead of the high-cost Pt-based catalysts.

In this perspective, Mohammadinezhad and Akhlaghinia (2017) fabricated a costless and efficient nanocatalyst (Fe₃O₄@Boehmite-NH₂-CoII) for both Hecks–Mizoroki and Suzuki–Miyaura reactions. The transmission electron microscope image showed the core–shell structure of Fe₃O₄@Boehmite-NH₂-CoII, where Fe₃O₄ is the core and Boehmite is the shell. Furthermore, the particle size of Fe₃O₄@Boehmite-NH₂-CoII was found to be in the range of 13–54 nm. Field emission scanning electron microscope revealed an excellent distribution of Fe₃O₄@Boehmite-NH₂-CoII with an average particle size of 20–30 nm. To determine the optimum conditions of the Suzuki–Miyaura coupling reaction between 4-iodobenzene and phenylboronic acid, the effect of solvents was studied in the presence of dimethyl sulfoxide, n-hexane, polyethylene glycol, n-hexane, toluene, tetrahydrofuran, n-hexane, ethanol, acetonitrile, and water. It was found that water could be utilized as a benign, costless, efficient solvent for the reaction. Furthermore, the optimal dose of Fe₃O₄@Boehmite-NH₂-CoII was 0.33 mol%, while the further increase in the catalyst dose (22 mol%) did not change the rate or the yield of the reaction. However, the lower catalyst dose (0.22 mol%) significantly diminished the reaction yield. Moreover, to prove the synergistic effect between the components of the catalyst, the reaction has proceeded in the presence of pristine CoCl₂·6H₂O, Fe₃O₄, magnetic Fe₃O₄@Boehmite, Fe₃O₄@Boehmite-Pr-Cl, and Fe₃O₄ @Boehmite-NH₂ as catalysts, revealing incomplete coupling between 4-iodobenzene and phenylboronic acid with conversion yields of 40, 20, 20, 5, 5 and 5% after 24 h, respectively.

In another study, Basaveni et al. (2019) fabricated nickel (Ni) nanoparticle-based catalyst encapsulated in a supporting and stabilizing matrix hyperbranched poly-aromatic polymer anched poly-aromatic polymer for the catalytic transfer hydrogenation of alkenes and nitroarenes. The hyperbranched poly-aromatic polymer matrix acted as a control matrix for the nickel nanoparticles' particle size. The nickel nanoparticles' size was controlled by encapsulation into the cavities of hyperbranched poly-aromatic polymer that prevent their aggregations. The ni-hyperbranched poly-aromatic polymer had high activity and good air stability. The catalytic transfer hydrogenation was carried out under base-free conditions without the need for high pressure, highly acidic conditions, or a strongly flammable hydrogen source. The Ni-hyperbranched poly-aromatic polymer catalytic system showed good tolerance toward alkyne, hydroxyl, or halogen substituents.

Enzymes

Enzyme biocatalysis has drawn vast attention in industrial applications, such as producing antibiotic precursors and synthesizing valuable chemical products such as acrylamide, detergents, and others (Ottone et al. 2021; Madhavan et al. 2021). Enzymes are biological catalysts that constitute a complex protein chain connected to active metal sites offering unique selectivity toward the substrate molecules and good catalytic activity (Yi et al. 2021; Nazor et al. 2021). Besides, enzymes have demonstrated excellent catalytic activity in various chemical conversions from simple hydrolysis to reaction syntheses, outperforming other traditional chemical processes (Bell et al. 2021). Therefore, enzymes have been involved in many applications, as listed in Table 4. Thanks to their intrinsic properties, high selectivity and turnover number, cost-effectiveness, sustainability, and eco-friendly advantage (Zhang et al. 2021a). Owing to the biodegradability and sustainability of enzymes, they have been extensively utilized in environmental applications such as the bioremediation of water and soil pollution (Singh et al. 2021; Sellami et al. 2022). Additionally, enzymes have exhibited an auspicious catalytic performance in the biorefinery of waste polysaccharides and agro-industrial residues into valuable products such as bioplastics, biofuels, and sweeteners (Shiva et al. 2022; Mirpoor et al. 2021; Baptista et al. 2021) as well as the revalorization of lignocellulose or cellulose wastes into costless biofuels (Fig. 10) (Mathew et al. 2021; Li et al. 2021). Notably, the high reaction kinetics, selectivity, cost-effectiveness, recyclability, and non-toxicity of enzymes render them promising candidates in wastewater remediation.

Table 4 Sustainable enzyme-based catalysts and their immobilization support, applications, and recyclability

Fig. 10

Applications of enzymatic biocatalysis for sustainability. Enzymes are utilized in the bioremediation of industrial wastewater and biofuel production from lignin residues and waste oil. Many enzymes are used in the biorefinery field. Enzymes have shown excellent activity in chemical conversion processes

Due to the extensive production of oils, greases, and fats over the past few years, estimated as 20–3.8 million metric tons of edible vegetable oils in 2018 (Luo et al. 2020), large amounts of waste oil containing organic and inorganic loads have been disposed into the aquatic environment, producing filamentous microorganism blooms onto the water’s surface (Feng et al. 2021). Activated sludge was involved in mitigating such aquatic problems, but the floating oil effluent limited performance, causing poor sedimentation and reduced sludge biomass (Cammarota and Freire 2006). Lipases exhibited efficient biodegradation of triglycerides (i.e., vegetable oils, fats, greases) rather than traditional activated sludge. For example, Meng and co-workers investigated the anaerobic degradation of three different types of triglycerides (animal fat, vegetable oil, and floatable grease) involving lipase derived from three different sources, namely Aspergillus (lipase I), Candida (lipase II), and Porcine pancreatic (lipase III) (Meng et al. 2017). Lipase I and II showed an efficient biodegradation rate under appropriate hydrolysis conditions (24 h, 1000–1500 µL of lipase volume, and 40–50 °C), producing byproducts of long chains of fatty acids.

In contrast, lipase III showed a modest biodegradation performance. These findings confirmed the influence of the enzyme’s source on their activity. In another attempt, Theerachat et al. (2017) exhibited excellent degradation efficiency (93%) of palm oil mill effluent with a high oil concentration (7762 mg L⁻¹) using lipase derived from candida rugosa-cultured yeast cells. However, these enzymatic treatments were considered primary processes and needed to be accompanied by another treatment process, such as activated sludge or anaerobic fermentation, to attain complete treatment of the fatty acids and glycerol byproducts.

Despite the harsh removal of many micropollutants due to their resistance to natural biodegradation, toxicity, and presence in nanogram or microgram concentrations in the aquatic medium, enzymes demonstrated proficient catalytic degradation performance against a diverse range of micropollutants such as pharmaceuticals, pesticides, phenolic compounds, and organic dyes. However, their vulnerable stability under extreme pH and temperature conditions and their poor reusability due to the difficult separation of enzymes from reaction media remain challenging. Accordingly, directed evolution and genetic engineering techniques (i.e., tuning the sequential structure of genes) have been conducted to boost enzymes' stability and catalytic performance (Saravanan et al. 2021; Palomo 2021). Recently, researchers found that immobilization of enzymes into solid support such as organics (natural or synthetic polymers), inorganics (silica, zeolites, graphene, titania, alumina), or organic–inorganic hybrid materials improves the catalytic performance of enzymes, the biocompatibility as well as the stability and the reusability considering the economic point of view (Ashkan et al. 2021; Zahirinejad et al. 2021). Notably, the immobilization strategy and the type of enzyme carrier represent crucial factors that directly affect the stability of the enzymes due to the possible undesired interactions with the solid support and the sluggish mass transfer of the target molecules retarding the catalytic interaction with enzymes (Nunes et al. 2021). In this context, Mahmoodi and Saffar-Dastgerdi (2020) developed a novel biocatalyst of laccase covalently immobilized onto zeolite (NZ)-graphene oxide (GO) for the effective removal of Direct red 23 organic pollutants. Covalent immobilization of laccase through salinization and crosslinking of the surface support via (3-aminopropyl) trimethoxy and glutaraldehyde, respectively, provided super stability and higher loading of the enzyme. Laccase covalently immobilized onto zeolite (NZ)-graphene oxide (GO) exhibited higher degradation efficiency by increasing the graphene oxide content from 3 to 7% due to the accelerated electron transfer from the enzyme to the support. Besides, graphene oxide content the loading amount of enzyme (350 mg/g) compared to bare zeolite support (180 mg/g) (i.e., 1.7 times higher than zeolite). This was attributed to the increased surface area of the NZ-GO nanocomposite support concerning bare zeolite, enabling a greater loading amount of enzyme.

Interestingly, Laccase covalently immobilized onto zeolite (NZ)-graphene oxide (GO) demonstrated remarkable retained efficiency of 95% over 5 cycles, verifying the outstanding stability of the immobilized laccase. Additionally, the immobilized enzyme revealed its superior storage stability than the free enzyme, achieving higher catalytic activity of 83% after 8 days of incubation comparable to the latter (60%). It is worth noting that covalent immobilization passively affects the catalytic activity of enzymes due to the conformational distortion and change in the chemical structure of enzymes after covalent attachment.

In this stream, Ahmed et al. (2018) exhibited an innovative strategy to immobilize cellulase for sustainable cellulose biorefinery into valuable products. Considering the advantage of enzyme immobilization via physical adsorption, retaining the structured entity, and catalytic activity of enzymes, cellulase was successfully immobilized onto UiO-66-NH₂ (cellulase@UiO-66-NH₂) under mild conditions. Brunauer–Emmett–Teller (BET) measurements declared cellulase@UiO-66-NH₂ possessed a lower specific surface area (269.3 m² g⁻¹) than pristine UiO-66-NH₂ (593 m² g⁻¹), attributed to the pore-clogging by the loaded enzymes, confirming the successful immobilization approach. In fact, UiO-66-NH₂ offered large numbers of -NH₂ adsorptive sites compared with UiO-66 and achieved a greater cellulase loading capacity (350 mg/g) than UiO-66 (102 mg/g). Cellulase@UiO-66-NH₂ accomplished outstanding hydrolysis of cellulose (85%) rather than free cellulase (60%) under conditions of 80 °C and pH = 3–6 in 30 min due to the extra stability of the immobilized cellulase. Besides, cellulase@UiO-66-NH₂ demonstrated 72% conversion efficiency after 10 cycles, outperforming covalently loaded cellulase onto Fe₃O₄@ UiO-66-NH₂ via precipitation and glutaraldehyde crosslinking that attained 70% after only 5 cycles. These findings established that physical adsorption is a decent strategy for improving the catalytic performance of immobilized enzymes.

Synthesis of environmental catalysts

The methods to prepare environmental catalysts are essential determinants of their catalytic behavior. Integrating green chemistry principles during large-scale syntheses, such as moderating energy input, organic solvent issues, and problematic wastes, is a significant challenge (Rodríguez-Padrón et al. 2019a). Jahangiri et al. (2014) defined a triangular concept to be tailored in catalyst design to achieve optimal performance, which included: (1) chemical-physical properties (i.e., surface area, porosity, dimension, acidity, composition, density), (2) catalytic properties (i.e., activity, selectivity, stability), and (3) mechanical properties (i.e., strength, attrition).

Furthermore, these materials' electronic and optical properties can be crucial in catalyst optimization (Rodríguez-Padrón et al. 2019a). Each catalyst's synthesis may involve a series of complex processes, some of which may be unknown (Schwarz et al. 1995). A minor alteration in the preparation details can result in a significant change in the final catalyst properties. Typically, the trial-and-error method was determined to be a viable solution. The conventional procedures for environmental catalyst preparation are precipitation (Geus and Van Dillen 2008; Wang et al. 2021c; Munnik et al. 2015), impregnation (via deposition, grafting, ion exchange, and others) (Munnik et al. 2015; Hafdi et al. 2021; Baeza et al. 2016), precipitation-impregnation (Ayati et al. 2014; Orooji et al. 2021), sol–gel (Mahy et al. 2021; Esposito 2019), chemical deposition (Tuna et al. 2022), hydrothermal (Mamaghani et al. 2019; Ayati et al. 2015), microwave-assisted (Ahmad and Hossain 2022), along with other methods (Zhang 2020; Yin et al. 2022). Because of its ease of use and low cost, precipitation is the most commonly used in preparing environmental catalysts (Rajput et al. 2022), where anions and cations combine in solution and form insoluble solids or supersaturated forms. Reagent addition, precipitate formation by pH, temperature and/or concentration adjustment, flocculation, sedimentation, and solid–liquid separation are all steps involved (Rajput et al. 2022; Wang et al. 2005), as shown in Fig. 11. It typically suffered from the separation of the product after precipitation and large volumes of salt-containing solutions (Hutchings and Védrine 2004). It has been extensively used to synthesize single-component, supported, and mixed catalysts (Perego and Villa 1997). For example, Chetri et al. (2014) successfully used this method to fabricate core–shell TiO₂/SnO₂ and SnO₂/TiO₂ nanocomposites with high potential for dye degradation under visible light irradiation. Magdalane et al. (2019) prepared the tin (IV) oxide/titanium dioxide nanostructure via a low-temperature precipitation method using tin chloride and titanium isopropoxide as main precursors and starch as a template.

Fig. 11

Parameters affecting the catalyst characteristics using the precipitation method. All these parameters must be optimized to obtain the catalyst-specific features. They can impact the nucleation and/or crystal growth of structures. So, the size and porosity of catalysts can be tuned by the variation of these parameters. Amongst, the solution composition and precipitating agent play the central role. For instance, the type of precipitation agent, such as sodium carbonate, sodium bicarbonate, sodium hydroxide, and ammonium carbonate_, significantly impacts the final catalyst's characteristics

Coprecipitation is one of the most practical approaches for fabricating bi-metallic catalysts (Munnik et al. 2015; Yao et al. 2018). The coprecipitation technique and deposition–precipitation for the fabrication of supported bimetallic catalysts are illustrated in Fig. 12. In this approach, the nucleation and growth of combined active metal and support are obtained in a single step in the solutions containing both active metal and support salts (Benhiti et al. 2020; Chen et al. 2016).

Fig. 12

Synthesis of a supported bimetallic catalyst via a coprecipitation and b deposition precipitation. The support precursor is added to precipitated metal precursors in the coprecipitation approach. In contrast, the support material is added to the pH-adjusted metal precursor and urea solution in the deposition precipitation technique. After drying, the second metal is deposited by deposition precipitation with urea

Impregnation is another method for improving the dispersion of an active phase on either inert or active support that already possesses desired porous texture and mechanical toughness (Hutchings and Védrine 2004). It involves three steps (1) contacting the support with the solution containing a precursor, (2) drying, and (3) activating by calcination, reduction, or other suitable treatment (Hutchings and Védrine 2004; Perego and Villa 1997). The impregnation method can be divided into equivalent impregnation, using a particular carrier quality, and excessive impregnation, employing more than the amount of impregnation liquid metal salt solution volume. The impregnation method produces a catalyst with large particle size, easy recovery, and reagent-saving advantages. Alumina is commonly used support (Kim et al. 2022; Lerici et al. 2022), and silica (Xu et al. 2016; Gai et al. 2022), titania (Huang et al. 2022; Lincho et al. 2021; Martín-Hernández et al. 2012), carbon nanotubes (Afifeh et al. 2019; Li et al. 2022), metal–organic frameworks (Beni et al. 2020), cerium(III) oxide (Liu et al. 2015; Wang et al. 2016b), are some other employed supports in the synthesis of environmental catalysts via the impregnation way.

Actual ion exchange reactions between the precursor ions and those of the support surface are required for effective interaction in the impregnation process. By adding other competitive adsorption and exchange ions, the exchange process can be delayed to improve dispersion on the support. In preparing supported Pt° particles, for example, Cl⁻ or NH⁴⁺ ions are commonly added to the precursor solutions to improve particle dispersion (Hutchings and Védrine 2004). The literature (Schwarz et al. 1995; Hutchings and Védrine 2004; Quiton et al. 2021) examines the impact of several effective parameters in the impregnation approach.

The mechanochemistry synthesis of catalysts for environmental applications has also gained the scientific community's interest as a promising strategy (He et al. 2020a; Muñoz-Batista et al. 2018). This alternative solvent-free route has the advantages of high versatility, simplicity, and reproducibility (Rodríguez et al. 2007). It relies on directly absorbing mechanical energy by reagents, usually solids, during milling or grinding (Głowniak et al. 2021). Several mechanochemical methodologies, including top-down and bottom-up, have been developed based on milling equipment to transform precursors into the desired structures.

The commonly laboratory-level mechanochemistry synthesis are planetary ball mills, shaker mills, and attritor mills (Yin et al. 2022). Each mill possesses its advantages and disadvantages, which are extensively compared in the review articles (Yin et al. 2022; Espro and Rodríguez-Padrón 2021). Various mechanochemical synthesis techniques have recently progressed, such as solid-state grinding and liquid-assisted grinding (Xu et al. 2015b).

The mechanochemical routes have been employed to fabricate various environmental catalysts (Ralphs et al. 2013; Szczęśniak et al. 2020). High efforts have been made in the mechanochemical approach to narrow titanium oxide and zinc oxide bandgap and improve its photocatalytic behavior through its anionic and cationic doping through a ball mill (Pillai et al. 2020; Wu et al. 2019a; Reddy et al. 2019), or it's incorporated with various semiconductors, such as iron(III) oxide/titanium oxide (Subramonian et al. 2017), hexagonal boron nitride/titanium oxide (Fu et al. 2013), vanadium(V) oxide/titanium oxide (Mondal et al. 2020), and silver phosphate/zinc oxide (Liu et al. 2013a) hybrid composite. This considerable enhancement could be due to the decrease in particle size and therefore increase in surface area and enhancement of charge separation associated with the two components.

Similarly, by high-energy ball milling (8 days) and wet milling (1 h) to exfoliate graphite carbon nitride, higher photocatalytic efficiencies were achieved compared to that of pristine bulk (Ma et al. 2021). The different graphite carbon nitride heterojunctions could form under ball millings, such as ultrathin layer sheets (Wei et al. 2021b), sandwich-like three-dimensional structures (Ni et al. 2021), and multi-layer core–shell structures (Zhou et al. 2015). Also, the synthesis or decoration of carbonaceous material [e.g., graphite oxide (Ahmad et al. 2018), biochar (Yu et al. 2021a; Fawzy et al. 2021), and carbon nanotubes (Panahian and Arsalani 2017)] using high-energy ball milling provides a rapid and solventless process to intensify their environmental catalytic activity. Ball milling or liquid-assisted grinding was remarkably used as a promising alternative for metal–organic framework synthesis and modifications (Głowniak et al. 2021; Chen et al. 2019a; Stolar and Užarević 2020). Yin et al. (2022) well-reviewed the mechanochemical synthesis of various catalysts and reagents for water decontamination.

The conventional sol–gel process based on forming oxo bridges by hydrolysis and polycondensation of molecular precursors is a widely spread technique employed to synthesize inorganic and organic–inorganic hybrids catalytic materials (Debecker et al. 2013; Ciriminna et al. 2011). It has the inherent advantages of versatility, controlling the composition, structure, and morphology of the final materials, and high product homogeneity and purity, allowing the direct production of materials cast upon substrates (Agrafiotis et al. 2002). Due to the easy availability of molecular silicon sources with moderate reactivities, the most extensive research on the sol–gel synthesis of environmental catalysts has been conducted on silicate systems (e.g., tetraalkoxysilanes) (Kajihara 2013). The polycondensation and hydrolysis of tetraalkoxysilanes resulted in silica gels, which can be converted into silica glasses. It involves the partial hydrolysis of metallic alkoxides, condensation to create M–O–M bonds, and formation of sol by dealcoholation or dehydration, and finally, the formation of a cross-linked gel during aging or drying.

The most critical parameters in this process are the type and number of the alkyl group, water-to-alkoxide ratio, oxidation state and size of cation, pH, time, and temperature. Occasionally, chelating agents, such as glycol, acetylacetone, and acetyl acetoacetate, are exploited to prevent agglomeration and control crystallite morphology (Kazemi et al. 2020; Kim et al. 2020; Debecker 2018). The slow rate of hydrolysis and condensation resulted in sols. In contrast, precipitates form at slow hydrolysis and fast condensation, polymeric gels are synthesized at rapid hydrolysis and slow condensation (Hutchings and Védrine 2004), and colloidal gels are formed at fast hydrolysis and condensation processes. Figure 13 shows the differences between colloidal and polymeric sol–gel methods.

Fig. 13

Comparison between colloidal and polymeric sol–gel methods. The sol–gel technique creates metal oxides, mainly silicon (Si) and titanium (Ti). During the procedure, monomers are transformed into a colloidal solution (sol), which serves as the precursor for an integrated network (or gel) of discrete particles or network polymers. In colloidal sol–gel, the hydrolysis of the metal precursor is fast and uncontrolled via the charge generated by the catalyst. On the other hand, in the polymeric route, the partial hydrolysis of the metal precursor is provoked by slow and controlled hydrolysis. The catalyst facilitates the hydrolysis reaction in this approach

There are many studies carried out on the sol–gel synthesized dopants titanium oxide, such as Rh-doped titanium oxide-anatase nanoparticulate (Borlaf et al. 2012), Cerium-doped titanium oxide nanoparticulate (Xu et al. 2009), and some other metal cations (iron, silver, copper, zinc, chromium, aluminum, manganese or cobalt) doped titanium dioxide, to decrease the band gap of titanium dioxide and enhance its photocatalytic activity in degradation of different organic molecules (Mahy et al. 2018). For example, Ping et al. (2002) showed that the Eu³⁺ and Fe³⁺ doping of titanium dioxide nanoparticles improves the photoinduced charge separation in semiconductors and the interfacial charge transfer process at the semiconductor/solution interface. The sol–gel-assisted microwave or hydrothermal treatment has also been employed to accelerate the synthesis and prepare the model catalysts with ordered mesoporosity (Debecker et al. 2013; Falk et al. 2017; Falk et al. 2018). Additionally, the binary and ternary oxide catalysts have been prepared by the sol–gel process, while by controlling the relative reactivities of the two alkoxides, the homogeneity of the mixed oxide gels can be engineered (Padmanabhan et al. 2022; Ramakrishnan et al. 2021; Mahdi et al. 2022; Shubha et al. 2022). As shown in Table 5, a large variety of catalytic materials were prepared by the sol–gel approach.

Table 5 Catalysts and photocatalysts prepared by the sol–gel method, adapted from Debecker and Mutin (2012)

Hydrothermal synthesis is another method induced by temperature under aging in the presence of the mother liquor, used to modify precipitates, gels, or flocculates, and in particular, the preparation of mixed oxides zeolites, and other molecular sieves (Mamaghani et al. 2019; Yang et al. 2013). The concentration, pH, temperature, pressure, and time are the most crucial variable in given catalysts (Wang et al. 2021d, 2022). It is also carried out during other preparation procedures, such as precipitation, drying, washing, and extrusion (Ayati et al. 2015). The molecular sieve materials were well synthesized hydrothermally. It involves a classical precipitation method at a given pH under atmospheric pressure. For example, Khan et al. (2020) hydrothermally prepared the MFI TS-1 molecular sieve nanosheets using a tailored diquaternary ammonium surfactant as the structure-directing agent. Introducing Ni²⁺ cations at its ion-exchange sites remarkably enhanced its aerobic alcohol photo-oxidation. In another study, Zhou et al. (2022) synthesized the controllable MnSAPO-18 molecular sieves at the optimum crystallization time of 2 h for ammonia-selective catalytic reduction. This massive effort in hydrothermally synthesized materials resulted in numerous new structures with a large variety of monodisperse pore sizes, mono or tridimensional networks, and acidic or redox properties, which has opened tremendous hope for new catalysts.

While the synthesis and growth of nanostructures are highly sensitive to the reaction conditions, microwave and ultrasound irradiation's efficient and controlled energy can be a great promise (Gawande et al. 2014). Numerous studies have focused on the microwave-assisted method as a powerful approach (Kokel et al. 2017) in smart nanoarchitecture materials synthesis, particularly environmental catalysts (Kumar et al. 2020c). Compared to conventional heating, microwave heating operates in deep “inside-out,” high temperature and selective heating mode (Polshettiwar et al. 2009; Baig and Varma 2012) via the chemical polarity in the target compound (Kunal and Toops 2020). So, it is considered a fast, high-yield, high-purity product, highly reproducible, easily optimized, and more efficient synthesis route, which requires lower energy than the conventional methods.

A thorough review of the increasing growth of this research field and its performance is provided in the literature (Gawande et al. 2014; Kokel et al. 2017; State et al. 2019). Nanostructured photocatalysts, including oxides, sulfides, and nanocomposites, were fabricated using microwave-assisted methods (Singh et al. 2019). For instance, ceric oxide (CeO₂), Ce_xSm_1−xO₂ (Polychronopoulou et al. 2017), and copper oxide (CuO)-promoted CeO₂–M_xO_y (M = Zr, La, Pr, and Sm) (Reddy et al. 2012) were fabricated by microwave-assisted method and exploited for carbon monoxide oxidation. The tin dioxide quantum dots (Liu et al. 2013b), titanium dioxide- reduced graphene oxide (TiO₂-rGO) photocatalyst, synthesized through the reduction of graphene oxide in titanium dioxide slurry (Yang et al. 2011), ZnO–Y₃Al₅O₁₂: Ce³⁺, UiO-66-NH₂ (Solís et al. 2022), and dichalcogenide (NiTe₂) (Hussain et al. 2022) some of the environmental photocatalysts were prepared under microwave conditions. (Yang et al. 2014) utilized the microwave-assisted hydrothermal route to prepare permeable titanium dioxide microspheres as an efficient photocatalyst to degrade a binary of chromium (Cr(VI)) and methyl orange dye.

The uniform distribution of different metal oxides is a significant challenge in preparing improved mixed metal oxides catalysts. For example, due to the differences in salt solubility, preparing homogeneous mutual-dispersed catalysts is difficult through coprecipitation (Chen et al. 2017c). The double hydrolysis reaction is a simple, efficient, and environmentally friendly metathesis reaction involving the ionic compounds swapping their ionic partners (Bai et al. 2005). A low-cost and speedy preparation method was sought for thermally stable alumina-based two-component and multi-component materials with high specific surface area and loading (Bai et al. 2005; Bai et al. 2009; Do et al. 2022). (Ullah et al. 2016) exploited the double hydrolysis method to synthesize Ni/ZnO-Al₂O₃, which exhibited improved reactive adsorption desulfurization behavior. In another study, Do et al. (2022) used this approach to prepare highly active Ru-free Ni/Al₂O₃ for ammonia (NH₃) decomposition. Recently, (Dong et al. 2022) reported the loading of Nb₂O₅ nanoparticles on the surface of graphite carbon nitride using in-situ hydrolysis-calcination, which showed exemplary behavior in the rate of photocatalytic NH₃ decomposition. The hydrolysis-driven method (Ling et al. 2021) was also reported in the highly efficient synthesis of homogeneous distributed Mn-Fe binary oxides, which showed a more exposed surface area and lower crystallinity.

The flame aerosol processes, including liquid-fed and vapor-fed, are the most well-known technologies for the large low-cost industrial-scale production of nanocatalysts (Sheng et al. 2018; Li et al. 2016b). As an important competitive advantage compared to traditional wet chemistry synthesis methods, this approach promotes the ability to form both active species and support in a single step (see Fig. 14) (Liu et al. 2021b). It gives further flexibility to tailor the catalyst structure with desirable performance (Zhao et al. 2019a; Schimmoeller et al. 2010; Schimmoeller et al. 2011). Its most notable challenges are the requirement of volatile metal precursors and achieving homogeneous distributions in multicomponent catalysts. Figure 15 shows a dependency on catalyst characteristics and flame synthesis parameters.

Fig. 14

Catalyst synthesis procedures of the flame aerosol process (a) versus typical conventional wet impregnation methods (b). The use of flame aerosol processing provides additional freedom for customizing catalyst properties by adjusting a process parameter. The flame aerosol process approach allows active species to nucleate from individual atoms. Wet chemical techniques sometimes need several posttreatment procedures, such as solvent washing and calcination, which may modify the composition and structure of the catalyst

Fig. 15

Effective process property parameters for catalyst design through flame aerosol processing. The flame temperature and residence time are the main factor affecting particle formation. However, they are individually dependent on some other factors. Therefore, the actual relationship is much more complex. Their interpretation is essential to prepare nanocatalysts with desirable characteristics and behavior

The liquid-fed flame aerosol is the most famous industrial continuous method for large-scale production of titanium dioxide, photocatalysts and fumed silica (Liu et al. 2021b; Teoh et al. 2010; Kho et al. 2010), and the vapor-fed flame aerosol was widely applied to synthesize high surface area oxides, such as iron(III) oxide, aluminum oxide (Pratsinis 1998), the aluminum oxide-supported (Jensen et al. 2003) and titanium dioxide-based (Almquist and Biswas 2002) catalysts. The liquid-fed flame aerosol can select a more comprehensive range of precursors (Sheng et al. 2018). It includes the sub-classes of flame spray pyrolysis and flame-assisted spray pyrolysis (Campagnoli et al. 2005), depending on the precursor solution combustion enthalpy, which is well-reviewed by Liu et al. (2021b). Most kinds of homogeneous and highly crystalline nanocatalysts in terms of composition have been prepared by flame spray pyrolysis by far (Sheng et al. 2018), particularly a wide spectrum of metal oxide and noble metal nanoparticles or composites, which are in growing in the catalytic processes. Some environmental catalysts prepared via flame aerosol processing are listed in Table 6. Some of the most recently synthesized photocatalysts and electrocatalysts prepared by flame spray pyrolysis are also presented in Table 7.

Table 6 Environmental catalysts synthesized by flame spray pyrolysis

Table 7 Some recently used photocatalysts and electrocatalysts prepared by flame spray pyrolysis

Analysis of catalyst structure

It is well established that the activity and the selection of a particular catalytic application of the catalyst strongly depend on its structure and intrinsic properties. On this basis, the structural characterization of a catalyst is a central topic to be considered. Moreover, different characterization techniques should be involved to assess the successful fabrication of heterogeneous catalysts and explore their inherent properties. Thus, the following section will discuss some intrinsic properties of heterogeneous catalysts such as surface area, porosity, crystallinity, thermal stability, and electronic properties based on several characterization tools, including Brunauer–Emmett–Teller analysis, powder X-ray diffraction, thermogravimetric analysis, electron microscopy, and X-ray photoelectron spectroscopy as follows:

Brunauer–Emmett–Teller isotherm analysis

The catalytic activity of heterogeneous catalysts is controlled by two intrinsic properties: (1) the high surface area of the catalyst that offers large numbers of active sites, and so boosting the catalytic performance; and (2) the sufficient pore size of the catalyst that facilitates the diffusion of reactants to the internal active sites. Brunauer–Emmett–Teller isotherm measurement is the most commonly used tool to assess heterogeneous catalysts' specific surface area, pore volume, and pore size distribution. It measures the amount of nitrogen gas molecules adsorbed on the surface of porous material at liquid nitrogen temperature, so the sample capacity to adsorb certain amounts of nitrogen gas molecules represents the specific surface area. For instance, surface analysis shows a type I adsorption–desorption isotherm model with microporous nature of citrate-capped Fe₃O₄@UiO-66-NH₂ nanocomposite (MU-2) (Prakash Tripathy et al. 2022). Remarkably, MU-2 exhibited higher specific surface area and pore sizes (572.13 m²/g, 1.65 Å, 6.72 Å) than pristine UiO-66-NH₂ (510.38 m²/g, 1.58 A°, 6.23 A°), confirming the in situ growth of the UiO-66-NH₂ particles onto the magnetic nanoparticles without clogging the metal–organic frameworks’ pores. Accordingly, MU-2 demonstrated higher photocatalytic hydrogen evolution (417 µmole h⁻¹) rate than UiO-66-NH₂ (115 µmole h⁻¹).

Thermogravimetric analysis

Thermogravimetric analysis is the most commonly used tool to inspect the thermal stability of heterogeneous catalysts. The thermogravimetric analysis explores the weight loss (%) over different temperatures. The weight loss profile for a material is usually donated in the first derivative form, and the peak position refers to a specific component decomposition temperature (Zhang et al. 2019b). Notably, the thermal assessment environment (i.e., air, oxygen, nitrogen) plays a crucial role in the thermal behavior of the catalyst.

Powder X-ray diffraction

It is noteworthy that the crystal phase directly influences heterogeneous catalysts' adsorption and catalytic performance. Besides, the catalytic process possibly changes the structural integrity and crystallinity of the catalyst. Powder X-ray diffraction provides a powerful route to recognize the crystallographic structure, purity, and crystal phase of the prepared catalysts, as well as the crystal phase stability and integrity after the catalytic process. In addition, powder X-ray diffraction can primarily measure the crystalline particle size using Scherrer’s equation (D = Kλ/β cosθ, where D represents the mean size of crystallite domains, K represents a dimensionless shape factor, λ represents X-ray wavelength, β represents the broadening of the diffraction peak at its half maximum, and θ is the Bragg angle). Song et al. (2022b) studied the effect of different crystalline phases on the photocatalytic performance of Ni-MOF. Based on density functional theory (DFT) calculations, Ni-MOF(H₂O) with rhombic crystalline phase attained more negative adsorption energy for carbon dioxide molecules (− 0.85 eV) than monoclinic Ni-MOF (− 0.62 eV).

Moreover, the Ni-C bond in Ni-MOF(H₂O) (1.87 Å) was shorter than Ni-MOF (1.95 Å). These findings verified the better carbon dioxide adsorption performance of Ni-MOF(H₂O) than Ni-MOF and the decisive role of the crystal phase differences. Accordingly, Ni-MOF(H₂O) accomplished 1.8 times higher photocatalytic conversion of carbon dioxide into carbon monoxide (34 µmole after 6 h) than Ni-MOF.

Morphological characterization

Scanning electron microscopy has been commonly used to visualize the nanocatalyst's crystalline morphology and ascertain the catalyst's structural stability beyond the catalytic reaction. Wu et al. (2019b) used scanning electron microscopy to explore the morphology of Ni nanoparticles entrapped in microporous graphene-like carbon (Ni@MGC). Scanning electron microscope images depicted that the MGC support showed a crystalline morphology similar to NaY zeolite. Notably, Ni@MGC revealed structural defects after the catalytic carbon dioxide methanation process, which was attributed to the reaction of the carbon material with the by-product water at high temperatures. It should be noted that the electron beam scanning the surface of the specimen may cause damage, producing a distorted image. Field emission scanning electron microscope overcomes this issue, engaging a highly focused electron beam that raster the surface of the specimen, forming a spot (0.4–5 nm) and generating a high-quality three-dimensional image for the sample.

Transmission electron microscopy involves a highly accelerated electron beam that passes through a thin specimen or fine powder, providing information about the internal structure and morphology, size of tiny particles, dispersity of the metal nanoparticles onto a solid support, and the atomic arrangement.

X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy is a non-destructive technique that studies the electronic state, composition, and content of the elements of the surface atoms. X-ray photoelectron spectroscopy can not only identify the elements but also can recognize the other elements bonded to these elements. Based on the photoelectric effect, X-ray photoelectron spectroscopy measures the kinetic energy and number of emitted core electrons from the surface atoms, giving a compositional analysis of the surface elements (i.e., element content and oxidation state). Furthermore, X-ray photoelectron spectroscopy can elucidate the catalytic mechanisms and stability of the catalyst by tracking the change in the oxidation states of each element after the catalytic reaction. For example, Wang et al. studied the mechanism of the catalytic peroxymonosulfate activation using mesoporous carbon framework-supported Cu–Fe oxides (Wang et al. 2022b). The intensity of the two peaks corresponding to Cu²⁺ and Cu⁺ at 934.4 eV (26.2–12.4%) and 933 eV (36.4–47.5%) of Cu 2p_3/2 spectrum changed after the catalytic reaction. But, the Fe 2p_3/2 spectrum survey showed a slight change in the peak’s intensity, revealing the remarkable activity of Cu instead of Fe within the mesoporous carbon support.

Moreover, the O is spectrum survey assured the catalytic contribution of the surface –OH groups, exhibiting observable changes in the terminal and bridging -OH groups at 532.9 eV and 531.2 eV, respectively. It is worth noting that X-ray photoelectron spectroscopy is a surface-sensitive technique that can only recognize the elemental composition of the atoms at only 0–10 nm depths. Nevertheless, X-ray photoelectron spectroscopy can identify the content and chemical composition of the deeper atomic layer using the ion-beam etching method. Such an approach has paved the way to study the electronic properties of several types of samples, such as thin films, graphene, and aligned carbon nanotubes at different depths.

Applications

Water remediation

In light of recent demographic developments, the global population is projected to increase by 29.9%, or 9.8 billion people, by 2025 (Bielecki et al. 2020). Similarly, numerous studies indicate that 30% of the world's population lacks access to safe drinking water (Wan and Wang 2021a, a, b, c, b). This is attributable to improperly discharging contaminated effluents into aquatic environments (Liu et al. 2021c). Organic and inorganic pollutants and dangerous biological infections are the primary contributors to water contamination (Lim et al. 2011). Heavy metals, dyes, and emerging organic contaminants (i.e., antibiotics, detergents, veterinary, pesticides, personal care, insecticides, oil spills, and food additives industries) are a diverse category of highly toxic water pollutants that have attracted significant attention from the legislative and scientific communities due to their widespread presence in the environment (Elgarahy et al. 2021a; Hüesker and Lepenies 2022; Cheng et al. 2020; Singh et al. 2020). Researchers have studied a multitude of operational methods, such as filtration (Zhang et al. 2021b), ion exchange (Liu et al. 2021d), coagulation (Cui et al. 2020), and chemical precipitation (Kalaitzidou et al. 2020) for wastewater treatment.

Recent years have seen the widespread application of environmental catalysis in various commercial and industrial sectors to protect the environment and enhance the quality of our lives. It can effectively mitigate water pollution, oxidize organic particles, reduce nitrogen oxide emissions from power plants, and regulate volatile organic compound emissions. Researchers have extensively researched water and wastewater remediation (Ali et al. 2021; Ihsanullah 2020). Among various scenarios, catalytic oxidation or reduction strategies (i.e., photocatalysis and advanced oxidation processes) have substantially progressed in treating wastewater containing pollutants (Kumar et al. 2020d; Russo 2021).

Advanced oxidation pathways (i.e., catalytic wet oxidation, electrochemical oxidation, sonochemical oxidation, photochemical oxidation, and ozone oxidation) can generate active transient species such as hydroxyl (HO·), chlorine (Cl·), alkoxyl (R·), hydroperoxyl (HO₂·), superoxide (O₂·⁻), and sulfate (SO₄·⁻), based on the employed catalyst or the oxidant type, and breakdown the hazardous pollutants into safe and biodegradable intermediate molecules (i.e., acetone, acetic, and oxalic acids) (Antonopoulou and Konstantinou 2019). Carbon dioxide, water, and inorganic ions are oxidation end products with no sludge formation. The process involving a catalyst may be suppressed whenever the catalyst is removed from the reaction or by the action of inhibitors (i.e., alcohols, phenols, and sugars, among others). The catalysis process can generally be classified as either homogenous or heterogeneous. During homogenous catalysis, the catalyst is dispersed in an aqueous solution or gas mixture with the reactants (i.e., a single phase of catalyst and reactants). In contrast, heterogeneous catalysis occurs when there are multiple phases (i.e., the catalyst is solid, and the reactants are liquid) (Antonopoulou et al. 2021). Advanced oxidation processes have evolved into promising wastewater remediation technologies in this context.

Fenton catalysis

Because of its broad applicability, the Fenton catalysis process is regarded as one of the most promising advanced oxidation technologies for cleaning aquatic systems. These reactions generate transient species, primarily through decomposing the oxidant H₂O₂ into hydroxyl radicals (·OH), which further attack and oxidize the target pollutants. Its distinguishing characteristics are its operational capabilities under ambient conditions, the nonselective nature of ·OH radicals, and the conversion of contaminants into harmless byproducts of carbon dioxide and H₂O. Combining these procedures with others, such as ultraviolet or sonic irradiation, which produce more OH· and facilitate the regeneration of the Fe²⁺ catalyst from Fe³⁺, can improve their efficacy (Bokare and Choi 2014; Ochando-Pulido et al. 2017). For instance, the mechanism of OH^• formation via Fenton catalysis can be classified into homogenous and heterogeneous processes (Brink et al. 2017; Vorontsov 2019). In turn, the homogeneous process is subdivided into conventional and modified (i.e., Photo-Fenton, Electro-Fenton, Sono-Fenton, and combinations/hybrid Fenton pathways) processes (Zhang et al. 2019c; Nidheesh et al. 2018; Miklos et al. 2018), whereas the heterogeneous process utilizes zero-valent state metal catalysts, synthesized iron supporting catalysts, iron minerals and waste-derived catalysts, and nanomaterials (Thomas et al. 2021). The entire homogenous Fenton catalysis is highly complex, involving oxidation and coagulation processes as presented in Eqs. (4–17) (Babuponnusami and Muthukumar 2014; Garcia-Segura et al. 2016).

$${\text{Fe}}^{{{2}+}} + {\text{H}}_{{2}} {\text{O}}_{{2}} \to {\text{Fe}}\left( {{\text{OH}}} \right)_{{2}}^{{{2} + }} \to {\text{Fe}}^{{{3} + }} +^{ \bullet } {\text{O H}} + {\text{OH}}^{ - } \quad k \, = { 4}0 - {8}0\,{\text{L}} \cdot {\text{mol}}^{{ - { 1}}} \cdot {\text{s}}^{{ - { 1}}}$$

(4)

$${\text{Fe}}^{{{3} + }} + {\text{ H}}_{{2}} {\text{O}}_{{2}} \to {\text{Fe}}^{{{2} + }} +^{ \bullet }{\text{O}}_{{2}} {\text{H }} + {\text{ H}}^{ + } \quad k = {1}0^{{ - { 3}}} - {1}0^{{ - { 2}}} \,{\text{L}} \cdot {\text{mol}}^{{ - { 1}}} \cdot {\text{s}}^{{ - { 1}}}$$

(5)

$${\text{R }} +^{ \bullet}{\text{O H}} \to {\text{ R}}^{ \bullet } + {\text{ H}}_{{2}} {\text{O}}\quad k = {1}0^{{7}} {-}{1}0^{{{1}0}} \,{\text{L}} \cdot {\text{mol}}^{{ - { 1}}} \cdot {\text{s}}^{{ - { 1}}}$$

(6)

$${\text{R }} +^{ \bullet}{\text{O H}} \to {\text{ R}}^{ \bullet } {\text{O H}}\quad k = {1}0^{{7}} - {1}0^{{{1}0}} \,{\text{L}} \cdot {\text{mol}}^{{ - { 1}}} \cdot {\text{s}}^{{ - { 1}}}$$

(7)

$${\text{R}}^{ \bullet } + {\text{ Fe}}^{{{3} + }} \to {\text{R}}^{ + } + {\text{ Fe}}^{{{2} + }}$$

(8)

$${\text{R}}^{ + } + {\text{ OH}}^{ - } \to {\text{R}}{-}{\text{OH}}$$

(9)

$${\text{Fe}}^{{{2} + }} +^{ \bullet } {\text{O H}} \to {\text{Fe}}^{{{3} + }} + {\text{ OH}}^{ - } \quad k = \left( {{2}.{5 } - { 5}.0} \right) \, \times { 1}0^{{8}} \,{\text{L}} \cdot {\text{mol}}^{{ - { 1}}} \cdot {\text{s}}^{{ - { 1}}}$$

(10)

$${\text{Fe}}^{{{2} + }} +^{ \bullet } {\text{O}}_{{2}} {\text{H}} \to {\text{Fe}}^{{{3} + }} + {\text{ HO}}^{ - }_{{2}} \quad k = \left( {0.{7 } - { 1}.{5}} \right) \times {1}0^{{6}} \,{\text{L}} \cdot {\text{mol}}^{{ - { 1}}} \cdot {\text{s}}^{{ - { 1}}}$$

(11)

$${\text{Fe}}^{{{3} + }} +^{ \bullet } {\text{O}}_{{2}} {\text{H}} \to {\text{Fe}}^{{{2} + }} + {\text{ O}}_{{2}} + {\text{ H}}^{ + } \quad k = \left( {0.{3 } - { 2}.{1}} \right) \times {1}0^{{6}} \,{\text{L}} \cdot {\text{mol}}^{{ - { 1}}} \cdot {\text{s}}^{{ - { 1}}}$$

(12)

$$^{ \bullet } {\text{O H}} +^{ \bullet } {\text{O}}_{{2}} {\text{H}} \to {\text{H}}_{{2}} {\text{O }} + {\text{ O}}_{{2}} \quad k = {1}.{4} \times {1}0^{{{1}0}} \,{\text{L}} \cdot {\text{mol}}^{{ - { 1}}} \cdot {\text{s}}^{{ - { 1}}}$$

(13)

$${2}^{ \bullet } {\text{O}}_{{2}} {\text{H}} \to {\text{H}}_{{2}} {\text{O}}_{{2}} + {\text{ O}}_{{2}} \quad k = \left( {0.{1 } - { 9}.{7}} \right) \times {1}0^{{7}} \,{\text{L}} \cdot {\text{mol}}^{{ - { 1}}} \cdot {\text{s}}^{{ - { 1}}}$$

(14)

$${2}^{ \bullet } {\text{OH}} \to {\text{H}}_{{2}} {\text{O}}_{{2}} \quad k = \left( {{5}.0 \, - { 8}.0} \right) \times {1}0^{{9}} \,{\text{L}} \cdot {\text{mol}}^{{ - { 1}}} \cdot {\text{s}}^{{ - { 1}}}$$

(15)

$$^{ \bullet } {\text{OH }} + {\text{ H}}_{{2}} {\text{O}}_{{2}} \to^{ \bullet } {\text{O}}_{{2}} {\text{H }} + {\text{ H}}_{{2}} {\text{O}}\quad k = \left( {{1}.{7 } - { 4}.{5}} \right) \, \times { 1}0^{{7}} \,{\text{L}} \cdot {\text{mol}}^{{ - { 1}}} \cdot {\text{s}}^{{ - { 1}}}$$

(16)

$$^{ \bullet } {\text{O}}_{{2}} {\text{H }} + {\text{ H}}_{{2}} {\text{O}}_{{2}} \to^{ \bullet } {\text{O H}} + {\text{H}}_{{2}} {\text{O}} + {\text{O}}_{{2}} \quad k = {3}.0\,{\text{L}} \cdot {\text{mol}}^{{ - { 1}}} \cdot {\text{s}}^{{ - { 1}}}$$

(17)

Numerous variables (i.e., catalyst concentration, surface area, solution pH, oxidant, and ionic strength) significantly impact the efficacy of Fenton reactions. In addition to the differences in operational parameters and wastewater characteristics, the need for optimization studies before their use in wastewater treatment is emphasized by the differences in operating parameters (Guo et al. 2018). Table 8 displays the wastewater remediation applications of homogenous (i.e., conventional and modified) Fenton catalysis processes.

Table 8 Applications of homogeneous (i.e., conventional and modified), Fenton catalysis process for wastewater remediation. BOD5, COD, BOD, AOX, and TOC refer to five-day biochemical oxygen demand, chemical oxygen demand, biological oxygen demand, adsorbable organic halides, and total organic carbon, respectively. H₂O₂ refers to hydrogen peroxide

The heterogeneous process operates on the same principles as the homogeneous process; however, the generation of ·OH is catalyzed on the surface of heterogeneous catalysts (X), as depicted in Eqs. (18–19). Both Fenton catalysis and reactant adsorption occur concurrently on the binding sites on the substrate's surface. After completing the process, the produced molecules are released (desorbed) from the binding sites, allowing new reactant molecules to bind to the vacant adsorptive sites (Queirós et al. 2015; Sreeja and Sosamony 2016).

$${\text{X}} - {\text{Fe}}^{{{2} + }} + {\text{H}}_{{2}} {\text{O}}_{{2}} \to {\text{X}} - {\text{Fe}}^{{{3} + }} +^{ \bullet}{\text{O H}} + {\text{OH}}^{ - }$$

(18)

$${\text{Fe}}^{{{3} + }} + {\text{ H}}_{{2}} {\text{O}}_{{2}} \to {\text{X}} - {\text{Fe}}^{{{2} + }} +^{ \bullet}{\text{O}}_{{2}} {\text{H}} + {\text{H}}^{ + }$$

(19)

The as-used catalyst should possess several desirable physicochemical properties, including cost-effectiveness, high reactivity for ·OH generation, a large surface area, a porous structure, physical and chemical stability, resistance to attrition and poisoning phenomena, and reusability. Table 9 summarizes the wastewater reclamation applications of the heterogeneous Fenton catalysis method. In addition, the heterogeneous Fenton process has several advantages over the widespread process, such as operational capacity over a broad pH range, a reduction in the formation of iron sludge and the associated cost issues, easy handling, and efficient recovery of as-used catalysts.

Table 9 The applications of heterogeneous Fenton catalysis process for wastewater remediation. Several wastewater organics contents and other parameters can be reduced using catalysts

Photocatalysis

Photocatalysis is a photo-promoted chemical reaction that occurs on the surface of the semiconductor substrate, i.e., photocatalyst, to produce reactive species, which are primarily forwarded to various applications, i.e., water pollutants degradation, microbes’ disinfection, and hydrocarbon fuels production. The induced electrons (e⁻) and holes (h⁺) resulting from the photo-excitation state migrate to the surface of the catalyst, undergo a series of reactions, and produce non-selective oxidizing active radicals, which react with various water pollutants, i.e., heavy metals, organic dyes, pesticides, herbicides, phenols, antibiotics, and others, via various redox reactions (Ohtani 2013). One way to operate photocatalysis is to utilize solar energy to produce chemical energy from the collected solar energy, transfer the simulated electron through photocatalysis, and eliminate organic pollutants (Huang et al. 2021). Several variables influence the efficacy of the degradation process, including solution pH, system temperature, initial pollutant concentration, irradiation intensity, catalyst type, catalyst dose, radiant flux, and oxygen concentration (Anwer et al. 2019; Adeel et al. 2021; Nisar et al. 2022). Equations (20–26) represent the overall photocatalysis process (Antonopoulou and Konstantinou 2019; Wang and Zhuan 2020).

$${\text{Photocatalyst}} + hv \to {\text{Photocatalyst}} + \left( {e^{ - }_{CB} + \, h^{ + }_{VB} } \right)$$

(20)

$${\text{Photocatalyst}}\left( {h^{ + }_{VB} } \right) + {\text{H}}_{{2}} {\text{O}} \to {\text{Photocatalyst}} + {\text{H}}^{ + } + {\text{ HO}}^{ \bullet }$$

(21)

$${\text{Photocatalyst}}\left( {h^{ + }_{VB} } \right) + {\text{OH}}^{ - } \to {\text{Photocatalyst}} + {\text{HO}}^{ \bullet }$$

(22)

$${\text{Photocatalyst}}\left( {e^{ - }_{CB} } \right) + {\text{O}}_{{2}} \to {\text{Photocatalyst}} + {\text{O}}_{{2}}^{ \bullet - }$$

(23)

$${\text{O}}_{{2}}^{ \bullet - } + {\text{H}}^{ + } \to {\text{HO}}^{ \bullet }_{{2}}$$

(24)

$${\text{2 HO}}^{ \bullet }_{{2}} \to {\text{O}}_{{2}} + {\text{ H}}_{{2}} {\text{O}}_{{2}}$$

(25)

$${\text{H}}_{{2}} {\text{O}}_{{2}} + {\text{O}}_{{2}}^{ \bullet - } \to {\text{HO}}^{ \bullet } + {\text{ OH}}^{ - } + {\text{O}}_{{2}}$$

(26)

First-generation, second-generation, and third-generation photocatalysts are distinguished from one another. Examples of first-generation photocatalysts include metal oxides, sulfides, phosphates, and nitride compounds. TiO_2, as previously mentioned, is the most common and well-known single-component (first-generation) type. The primary drawback of the first-generation kind is the large band gap (i.e., > 3 eV) and the rapid recombination of photoinduced (e⁻) and (h⁺), which reduces the catalytic efficiency (Li et al. 2015; Intarasuwan et al. 2017). Several second-generation photocatalyst modifications, such as heterojunction construction, co-catalyst loading, quantum dot sensitization, and metal oxide doping with metals/nonmetals, have been implemented to address the issues mentioned above (Luo et al. 2021). BiVO₄/Ag₃VO₄, WO₃/NiWO, graphite carbon nitride/Ag₃VO₄, and Bi₂O₂CO₃/Bi₂S are typical examples of second-generation photocatalysts used for photocatalytic degradation of methyl red, methylene blue, malachite green, and rhodamine B, with the activity of 78%, 92%, 97%, and 98%, respectively (Yan et al. 2016; Mohamed et al. 2014; Wang et al. 2014; Wang et al. 2013). However, the disadvantages of second-generation photocatalysts include difficulties in separation and recovery of as-used photocatalysts, high operational costs (i.e., high energy and pressure requirements), and the possibility of catalyst leaching into the aqueous medium, and a decrease in the catalytic activity of reusable photocatalysts (Munshi et al. 2017). Immobilizing photocatalysts on solid substrates has been proposed to overcome these obstacles with photocatalysts of the third generation. Metallic nanocrystals, carbon nitrides, and boron nitrides are some of their varieties. The photocatalysis applications for the degradation of various water pollutants are presented in Table 10.

Table 10 Applications of photocatalysis for the degradation of water pollutants

Electrocatalysis

Electrocatalysis is a chemical-free electrochemical process in which an electrode at the electrode–electrolyte interface serves as both an electron donor or acceptor and a catalyst (i.e., accelerates the rate of charge transfer). Similar to the photocatalytic process, where the cathodes and anodes represent the reduction and oxidation reaction sites, respectively. The only variation is when reduction and oxidation reactions occur separately on separate electrodes. Electrocatalysts are substrates that can reduce the activation energy of a reaction by modulating the binding energies without altering the thermodynamics of the reaction (Qian et al. 2021). The homogenous electrocatalysts are not commercially applicable but are of scientific interest. Generally, high-quality catalysts should be cost-effective, chemically and physically stable, and possess excellent porosity for efficient mass transfer. Enzymes and inorganic coordination complexes are homogenous electrocatalyst examples, whereas nanomaterials, metal–organic frameworks, and carbon-based materials are heterogenous electrocatalyst examples (Khan et al. 2018b).

Biomass valorization and product upgrading

Sustainability is the development that meets the current generation's needs without compromising the ability of future generations to meet their own needs. The role of environmental catalysis in achieving sustainability is crucial. The valorization of biomass is one of the most significant issues addressed by environmental catalysis (Rodríguez-Padrón et al. 2019b). Biomass is one of the most abundant and renewable sources of energy. The majority of biomass, about 60–80% is composed of carbohydrates, which can be divided into structural polysaccharides such as chitin, cellulose, and hemicellulose, and storage carbohydrates, i.e., starch, sucrose, and inulin (Sheldon 2016). Lignocellulosic biomass, the fibrous substance that makes up plant cell walls, is composed of lignin, hemicellulose, and cellulose of (10–25%), (15–30%), and (40–60%), respectively (Wang et al. 2017b; Osman et al. 2020b). Due to its accessibility, biomass feedstock has become one of the most valuable sources for obtaining alternative fuels and commodity chemical compounds, including alcohols, alkanes, bio-hydrocarbons, glycerol, succinic acid, lactic acid, olefins, levulinic acid, vanillin, muconic acid, furfural, sorbitol, and others (Bayu et al. 2019; Elgarahy et al. 2021b). Since then, biomass valorization has been regarded as an intriguing subject. The effective upcycling of waste biomass (such as corn stover, rice husks, and wheat straw) into valuable biofuels, biomaterials, and commodity chemicals can be accomplished via so-called chemo-catalytic and bio-catalytic strategies, depending on the biomass type, availability, desired end products, and process economics. The pretreatment (fractionation process) is an essential step for modifying the lignocellulosic structure into the primary constituents, thereby generating highly reactive lignocellulosic materials for subsequent catalytic upgrading to valuable biofuels and chemicals (Jing et al. 2019).

Chemical pretreatments, e.g., acid, alkaline, organosolv, ionic liquid, co-solvent, and deep eutectic solvents, physical pretreatments, e.g., steam explosion, ammonia fiber explosion, liquid hot water, and supercritical fluid, and biological pretreatment stages can be distinguished, i.e., enzymatic, and whole microbes (Bhatia et al. 2020; Chuetor et al. 2021). By utilizing thermochemical treatments, e.g., pyrolysis and gasification, and biological treatment, e.g., hydrolysis, fermentation, and anaerobic digestion, lignocellulosic feedstock valorization by fractionation, depolymerization, and upgrading can be accomplished (Akor et al. 2021). Each product of pyrolysis and gasification, such as bio-oil and syngas, can be further processed to generate liquid fuels or leading chemicals. Syngas can be used as a fermentation feedstock to produce acids, diols, and other compounds (Widayatno et al. 2016; Kurnia et al. 2017).

The lignocellulosic feedstock can also be hydrolyzed into lignin, cellulose, hemicellulose, and residual proteins. Their further hydrolysis may yield the monosaccharides pentose (C5) and hexose (C6), which constitute their primary structural components. Several platform lignin-derived commodities can be obtained via aqueous phase reforming, i.e., alkanes, hydrolysis, i.e., hydroxymethylfurfural, and separate hydrolysis and the fermentation/simultaneous saccharification and fermentation by diverting (processing) the intermediates resultant monosaccharides, i.e., acids, alcohols, and diols (Ishola et al. 2013). The use of catalysts, i.e., homogeneous, heterogeneous, and enzymatic, has a significant effect on optimizing process parameters and enhancing the distribution and quality of the products. Typically, an efficient catalyst should be supported by three essential characteristics: activity, stability, and selectivity. For instance, various types of catalysts have been adopted, including alkali or alkaline metals, activated carbon, fly ash, dolomite, zeolites, i.e., natural, hierarchical, two-dimensional, and three-dimensional, and others (Yu et al. 2021b; Duan et al. 2021; Islam 2020; Mardiana et al. 2022; Trinh and Chang 2021; Gao et al. 2011; El-Nahas et al. 2020).

Biofuels, e.g., bio-ethanol, biodiesel, jet fuel, bio-butanol, and biogas, as well as petroleum hydrocarbons, e.g., ethylene, propylene, butenes, butadiene, ethanol, propanol, butanols, furfural, gasoline, kerosene, aromatics (BTX), isoprene, isobutene, butadiene, and farnesene, can be produced through depolymerization, isomerization, epimerization, dehydration and hydration, retro-aldol, and reduction or oxidation reactions, operated by diverse chemo-catalytic and bio-catalytic pathways (Yang et al. 2016b; Delidovich and Palkovits 2016; Bayu et al. 2018). Moreover, the resultant monosaccharide sugar units produced from the depolymerization of biomass content can be converted into countless value-added chemicals such as hydroxymethylfurfural, levulinic acid, 2,5-Furandicarboxylic acid, γ-valerolactone (GVL), and others (Mariscal et al. 2016).

Lignin is considered one of the most promising renewable resources for producing abundant bio-products such as aerogels, bioplastics, carbon-derived materials, dye dispersants, guaiacol, and vanillin. In addition, the lignin content can serve as a suitable carbon precursor for producing various carbonaceous nanomaterials, including carbon nanotubes, fullerene, graphene, and graphene quantum dots. Due to their unique properties, these materials have gained great attention and are employed in various industrial sectors of energy storage materials, i.e., supercapacitors, photocatalysts, and lithium-ion batteries. Alternatively, lignin-based compounds, such as sodium lignosulfonate and hydroxypropyl sulfated alkaline lignin, can be used as dyes dispersing substrates due to their high adsorption dispersive efficiency, potent thermal stability, and durability (Liu et al. 2019). Economically, biodegradable lignin-based bioplastics are viewed as a viable alternative to non-biodegradable conventional plastics, i.e., petroleum-based polymers, due to the presence of multiple reactive moieties, i.e., carboxyl and hydroxyl, reasonable glass transmission temperatures, and their thermoplastic properties. Aerogels are a distinct class of advanced solid-state materials with exceptional physicochemical properties, such as an open mesoporous framework, a large surface area, enormous pore volumes, and a very low density. They can be used in numerous industries, such as adsorbents, acoustics, electromagnetic interference shielding, and others (Cho et al. 2018).

It is believed that biomass-derived cellulose is the most sustainable material capable of producing inexpensive bio-derived ionic liquids, i.e., cellulose-tetrabutylammonium bromide and cationic cellulosic poly ionic liquid. They have proven to be effective carbon dioxide absorbers with enhanced recyclability. In addition, they have exceptional loading capacities for various water pollutants, superior selectivity, accelerated loading rates, and excellent recyclability. In addition, biocompatible cellulose-based composites can be utilized as catalysts, supercapacitors, drug carriers, antibacterial, and batteries, among other applications (Suhas et al. 2016). Due to their low cost, large surface area, potential for functionalization, and fluorescence, carbon dots derived from cellulose could be utilized in several scientific fields, including biosensors, bioimaging, photocatalysis, solar cells, and optoelectronic devices. Cellulose-based aerogels are primarily used in thermal insulators, electrode materials, and wastewater treatment applications due to their hydrophilicity, biodegradability, low thermal conductivity, sonic velocity, and refractive index (Long et al. 2018).

Compared to lignocellulosic and cellulosic bioproducts, hemicellulose-derived bioproducts are the most limited. Due to their cost-effectiveness, biocompatibility, and renewability, they are ideally suited for certain potential applications, such as the production of pharmaceutical carriers. The hemicellulose-based films are excellent packaging materials, wound dressings, and drug capsules. In addition, hemicellulose-derived hydrogels served as pharmaceutically viable candidates for drug delivery systems due to their tolerance to various pH, solvent composition, temperature, and ionic strength variables (Sun et al. 2013). In addition, the biosorbents derived from hemicellulose exhibit significant adsorption capacities for various water pollutants (i.e., dyes, heavy metals, phenol, and others) (Xiang et al. 2022).

Biofuel and hydrogen production

Biofuel production

Biomass is viewed as a promising resource for producing biofuels, one of the best alternatives to fossil fuels for sustainable energy production. Attempting to generate energy from biomass components (such as lignin, cellulose, and hemicellulose, among others) simultaneously reduces waste production and satisfies our future energy demand. Biomass can be converted into valuable biofuels such as bioethanol, biodiesel, and biohydrogen via multiple bio-refinery and biotransformation scenarios. Biomass is a renewable, sustainable, and cost-effective resource (Cai et al. 2017; Li et al. 2018b; Al-Mawali et al. 2021; Al-Muhtaseb et al. 2021). Due to the inexhaustible supply of feedstock, biofuels can be utilized in various contexts. Depending on feedstock type, biofuels can be divided into the first, second, third, and fourth generations (Osman et al. 2021a). The first generation is produced directly from edible biomass (starch), whereas the second generation uses non-edible biomass, i.e., crop wastes. Marine-derived raw materials, i.e., algal biomass, can be incorporated into the third generation of biofuel production. Lastly, the fourth-generation harnesses bioengineered (genetically modified) microorganisms to produce biofuels (Osman et al. 2021a).

To produce biofuels, thermochemical, i.e., combustion, hydrothermal liquefaction, torrefaction, pyrolysis, and gasification, biochemical, i.e., anaerobic digestion and microbial fermentation, and chemical, i.e., transesterification, conversion techniques are widely utilized (Osman et al. 2021b; Najeeb et al. 2021). Using either homogeneous or heterogeneous catalysts throughout the process, the catalytic conversion of biomass is a rapid and effective expanding technique for biofuel production. Homogeneous catalysts, e.g., acid and base catalysts, enzyme-based biocatalysts, e.g., lipase-based and acyl acceptor catalysts, and heterogeneous catalysts are among the various types of catalysts, i.e., solid acid catalysts, solid base catalysts, and ion exchange resin catalysts. The heterogeneous solid acid catalysts are divided into metal-based and carbon-based catalysts. In contrast, the heterogeneous solid base catalysts are divided into oxide-based, boron-based, carbon-based, and waste-derived catalysts (Vasić et al. 2020). They have been studied to convert biomass into biofuels efficiently. Fundamentally, there are challenges associated with using homogeneous catalysts, such as the energy-intensive separation process. However, they yield better yields in a shorter amount of time.

In contrast, heterogeneous catalysts are inexpensive and environmentally benign materials with simple product separation, but they pose a challenge for rational nanoscale catalyst design (Najeeb et al. 2021; Bohlouli and Mahdavian 2021). Typically, homogeneous catalysts are used to convert biomass into biofuels. The primary disadvantages of these homogeneous catalysts are soap production under alkaline catalytic conditions and the caustic nature of the acidic catalyst. Typically, the reaction rate of biocatalysts is slow, but they have high selectivity for product synthesis and minimal environmental impact. Heterogeneous catalysis is chosen over homogeneous and biocatalyst catalysis because it is more effective in selectivity and reactivity. In the presence of heterogeneous catalysts, thermochemical processes may improve product selectivity and increase energy efficiency by decreasing activation energy.

The catalytic pyrolysis process has been identified as an efficient and effective method for producing biochar, liquid oil, and syngas from biomass in an oxygen-free environment using supporting catalysts. Approximately 40% of the produced biofuels (crude bio-oil) contain oxygenated compounds. In addition to its physicochemical properties, the bio-oil production yield depends heavily on the feedstock source and the various operational parameters, i.e., heating temperature, heating rate, inert gas type, inert gas flow rate, and others. Numerous studies have applied solid acid catalysts such as silica (Tan et al. 2018), zeolites (Paysepar et al. 2018), Nickel-supported catalysts (Santamaria et al. 2019), and silicoaluminophosphate (SAPO) catalysts (Chen et al. 2018) during the conversion of biomass into hydrocarbons (i.e., pyrolysis) with varying selectivity. The gasification of biomass is a subject of significant scientific interest at present. Producing gaseous fuel by burning biomass in a gasification medium, i.e., air, oxygen, and steam, at high temperatures, i.e., 500–1500 ℃, and pressures, i.e., 30–40 bars, maximizes the released gas.

Several studies have been performed on biomass gasification (Zhu and Zhuang 2012). Calcined dolomites or their related minerals, i.e., magnesite, calcite, and limestone, are frequently employed for hot gas cleaning (Gil et al. 1999). Some common catalysts (such as Ni- and Re-based catalysts) are also used to limit the coke production from tar (Kıpçak and Akgün 2018; Jin et al. 2018). Moreover, the introduced noble metal Rh catalyst admirably decreased the tar content in the product (Zhang et al. 2018b). Various types of catalysts such as native/modified zeolites, metal oxides, metal phosphates, metal–organic frameworks, and siliceous supporting materials are used to decompose biomass feedstock at moderate running (hydrothermal conditions) temperatures into phenolic compounds and sugars, i.e., mono- or oligosaccharides, which are then catalyzed to produce C₅–C₆ sugars (Li et al. 2016c; Huang and Fu 2013).

In the transesterification process, the triglycerides in vegetable oils, animal fats, or waste cooking oil feedstocks typically react with alcohols, i.e., methanol, ethanol, butanol, and hexanol, via a series of chemical reactions in the presence of an acid or base catalyst to form fatty acid methyl esters (FAME biodiesel) and glycerol (reaction byproduct). Diverse critical parameters, including catalyst loading, catalyst porosity, catalyst surface area, catalyst particle size, hydrophilic or hydrophobic characteristics of catalyst, calcination temperature, leaching phenomenon, reaction time, working temperatures, alcoholic factor, blending and stirring, and alcohol: oil molar ratios, have a significant impact on the biodiesel production yield and quality (Kumar and Ali 2013; Dhamodaran et al. 2017). The catalyst in the reaction provides a greater number of active sites for the catalytic reaction process, resulting in increased catalytic activity and a higher biodiesel conversion yield. However, adequate quantities of as-used catalysts must be optimized to prevent an uncontrollable increase in the viscosity of the alcohol-oil mixture. To purify its stream, produced crude diesel must be neutralized and passed through various cleaning-up scenarios (such as dry-washing, membrane extraction, water washing, and washing with adsorbent or water) (Bertram et al. 2009). Viscosity kinetics and dynamics, density, acid or neutralization number, cetane number, cloud point, and pour point should be thoroughly investigated (Agarwal and Das 2001). Table 11 displays a variety of distinct biodiesel production catalysts currently in use.

Table 11 Catalysts for biodiesel production

Hydrogen production

Due to limited fossil fuel suppliers, rising energy demand, and the urgent need to protect diverse forms of life on our planet to mitigate the effects of climate change, recent efforts have been concentrated on exploiting various renewable resources, e.g., solar and wind. Converting renewable natural resources into valuable, transportable fuels is an attractive concept (Avcıoğlu et al. 2019). From this point of view, hydrogen appears to be one of the promising fuels due to its zero emissions and subsequent reacquisition of chemical energy during combustion (Eljack and Kazi 2021). Hydrogen production from various feedstock such as non–renewables, i.e., natural gas, or renewables, i.e., biomass, organic solid wastes, and water, can be carried out through a variety of methods, including thermochemical (i.e., pyrolysis, gasification, catalytic reforming, partial oxidation, and chemical looping), biological (i.e., direct/indirect biophotolysis, dark/photo fermentation, enzymatic, and electrochemical (i.e., water electrolysis). The downstream reformate gas can be purified to produce hydrogen with a reduced carbon monoxide content, making it suitable for further applications (Carneiro and Gomes 2019; Morosuk and Tsatsaronis 2019).

In general, the decomposition of water through water electrolysis, whether powered by electricity or solar energy, can be divided into oxygen evolution reaction and hydrogen evolution reaction. Anodes and cathodes undergo oxidation–reduction reactions that result in the release of oxygen and hydrogen gases, respectively. The released hydrogen can be used solely as fuel or combined with oxygen to produce oxyhydrogen gas, which is then used for welding and other industrial applications (Wang et al. 2021e). Slow reaction kinetics of oxygen evolution reaction and hydrogen evolution reaction attributable to high overpotentials, a metric for kinetic energy barriers, is one of the most significant operational obstacles to water splitting (Suen et al. 2017). Catalysis plays a vital role in the energy cycle described previously during the water electrolysis process.

There are currently three significant electrolysis scenarios: proton exchange membrane (acidic condition), high-temperature solid oxide water electrolysis (high temperature), and alkaline electrolysis (alkaline conditions) (Wang et al. 2021e). Therefore, developing novel, highly effective, and stable electrocatalysts is suitable for different process parameters (i.e., working media, temperature, and others) and contain the proper stimulator species to accelerate reaction kinetics by enhancing their physicochemical properties is a significant challenge. The hydrogen evolution reaction electrocatalysts are separated into noble-metal-based (palladium, platinum, rhodium, ruthenium, iridium) and non-noble metal-based electrocatalysts. Many critical parameters for activity (i.e., exchange current density, overpotential, and Tafel slope), stability (i.e., current/potential over time), and efficiency are used to evaluate the performance of the employed catalyst (i.e., faradaic efficacy and turnover frequency) (Wang et al. 2021e). Table 12 shows different as-used catalysts for hydrogen evolution reactions through water electrolysis.

Table 12 Catalysts for hydrogen evolution reaction through the water electrolysis process

Both catalytic reforming and catalytic cracking are used to convert hydrocarbons into valuable products. Using mild temperatures and pressures in the presence of catalysts, catalytic cracking is the process by which larger hydrocarbon compounds are broken down into smaller (cracked) hydrocarbon molecules. While catalytic reforming (i.e., steam reforming, autothermal reforming, aqueous phase reforming, partial oxidation, and dry reforming) is the process by which hydrocarbons are reconfigured to form different valuable (reformate) products with the aid of a catalyst, reforming is the rearrangement of hydrocarbons into different products (Naikoo et al. 2021). Among various hydrocarbons, methane_, the primary component of natural gas and biogas, can be an abundant source of hydrogen production. Compared to steam methane reforming (63.3 kJ/mol hydrogen₎, methane cracking with a lower energy requirement (37.8 kJ/mol hydrogen) provides a more sustainable hydrogen production approach because it does not produce carbon monoxide or carbon dioxide emissions. However, catalysts with a low tendency for carbon formation, long-term stability, and high catalytic and reforming efficiencies are strongly suggested. Various catalyst types have been developed for catalytic and reforming processes, including metal (i.e., noble and transition)-based catalysts and carbon (i.e., activated carbon and carbon black). Tables 13 and 14 detail various catalysts used in catalytic cracking and reforming.

Table 13 Catalysts for methane cracking

Table 14 Performance of diverse catalysts during oxygenates reforming processes for hydrogen production

Conclusion

Recent advancements in photocatalysis, biocatalysis and electrocatalysis have been evaluated, and the challenges faced in environmental catalysis have been identified. Research is currently focused on developing efficient photocatalysts through doping, coupling with other nanomaterials, precipitation with metal particles, crystal growth designs, and heterojunctions. Heterojunction-based photocatalysts promise to improve photocatalytic activity for environmental pollution degradation, hydrogen production, and carbon dioxide reduction. However, the practical application of photocatalysis is currently limited due to insufficient activity, poor stability, and high cost. Electrocatalysts have been evaluated for environmental remediation applications, but limited efforts have been made to assess the toxicity of pollutants that are not efficiently mineralized. Developing novel, highly effective, and stable electrocatalysts suitable for various process parameters and containing the proper stimulator species to accelerate reaction kinetics by enhancing their physicochemical properties is a significant challenge.

In biocatalysis, enzymes found in soil, such as lipases, dehydrogenases, ureases, and catalases, have been used as bioindicators for evaluating pollution. However, lipase-based biosensors for bioremediation are not yet suitable for commercial use and require further research. Protein modeling and computational design have been utilized to improve known enzymatic functions and design new catalytic enzymes. New protein engineering tools, such as computational protein design, next-generation sequencing, and machine learning, will also aid in developing new enzymatic activities.

Materials such as biomass-derived carbon materials, metal–organic frameworks, non-noble metals, and nanocomposites have also been discussed as catalysts. The preparation and catalytic applications of these materials, as well as their impact on the circular economy, have been evaluated. The preparation of metal–organic frameworks via solvent-free methods is currently a popular research topic. Enzymes have also been discussed as sustainable catalysts with various supports and catalytic applications. The relationship between the chemical structure of sustainable catalysts and their end-application and catalytic performance has been critically evaluated.

The production of shape-controlled metal nanoparticles remains challenging. Several methods have been developed to synthesize stable metal nanoparticles with a defined shape, including the addition of inorganic capping agents, organic ligands, colloids, polymers, and core–shell materials. Using functionalizing agents improves nanocomposites' morphology, size, and properties, but their inability to degrade or be eliminated poses a significant environmental threat. Consequently, there is a growing interest in synthesizing nanocomposites from renewable resources such as starch, chitin, vegetable oils, lignin, natural rubber, and cellulose. The sensitivity of enzymes to extreme pH and temperature conditions and their poor reusability continues to be obstacles in enzyme-based catalysts. Techniques of directed evolution and genetic engineering have been employed to improve enzymes' stability and catalytic performance.

Integrating green chemistry principles into large-scale synthesis is a significant challenge, including moderating energy input, addressing organic solvent issues, and managing problematic wastes. As an alternative, solvent-free routes such as mechanochemistry synthesis offer high versatility, simplicity, and reproducibility. Additionally, microwave and ultrasound irradiation can provide highly efficient and precisely controlled energy for the synthesis and growth of nanostructures. However, preparing enhanced mixed metal oxide catalysts with a uniform distribution of various metal oxides remains challenging.