Introduction

Thenard, a French chemist, discovered hydrogen peroxide (H2O2) in 1818 (Chalmers 1996). The industrial manufacturing of H2O2 was introduced by reacting barium peroxide (BaO2) with sulfuric acid, hydrochloric acid, and phosphoric acid in late 1885 (Van 1968). The demand for H2O2 is increasing constantly because of its favorable oxidation properties. It is an essential raw material for industrial green chemicals and environmentally friendly oxidizing agents (Jose et al. 2006; Yao et al., 2016a; Gema et al. 2016; Han et al., 2015a; Tan et al. 2012; Melada et al. 2006; Garcia-Serna et al., 2014). H2O2 is being used in various grades in the food storage and cosmetic and electronic industries. In the recent COVID-19 era, demand of healthcare products is increasing as H2O2 has high antiseptic properties which directly boost its market value. H2O2 is used as an antiseptic, which is commonly applied on the skin cuts and bruises to avoid infection. Furthermore, in light of the recent COVID-19 pandemic, demand for H2O2 has grown significantly worldwide due to its high consumption in the formulation of disinfectant products. H2O2 is the main ingredients in hand sanitizers whose demand has increased simultaneously all over the world.

The H2O2 has been used in the textile, food, mining, pulp, and paper industries and for the wastewater treatment. It is also used in the manufacture of printed circuit boards for semiconductors and also has good demand in aquaculture market (Jose et al. 2006; Rosaria et al. 2016). The average plant capacity of H2O2 production was 20–40 kT per year with a world capacity of 1.5 million tons up to mid of 1990 (Ranganathan and Sieber, 2018). The plant capacity increases to 300 kT per year with a world capacity of 100% H2O2 which is around 5.5 million tons in 2015 (Ciriminna et al. 2016; Ranganathan and Sieber, 2018). The global producers of H2O2 involves Solvay (30%), Evonik (20%), and Arkema (13%) (Garcia-Serna et al. 2014).

Production of hydrogen peroxide

Autoxidation (AO) of anthraquinone (AQ), oxidation of primary and secondary alcohols, and electrochemical process are the commercial processes for the production of H2O2 (Foller and Bombard, 1995). The direct introduction of hydrogen (H2) and oxygen (O2) over hydrogenation catalyst (Chen et al. 2013) semiconductor oxide photocatalytic reactions (Gabriele et al. 2003) and the incorporation of metal complexes in CO/O2/H2O mixture (Zudin et al. 1979) are the new synthesis processes for the production of H2O2. The different routes of the production of H2O2 are depicted in Fig. 1. However, the abovementioned processes carry the high risk of explosion and also suffer from low concentration of H2O2. Due to larger production scale and high concentration of H2O2, the AQ process overcomes the flaws of high energy consumption and larger production cost and surpasses all commercial processes with mild reaction temperatures and lower production costs. The conventional AQ process involved the sequential hydrogenation of an AQ derivative, mainly 2-ethylanthraquinone (EAQ), and the oxidation of 2-ethylanthrahydroquinone (EAQH2). The world production of H2O2 is dominated by the AQ process in which EAQ is dissolved in a mixture of polar and nonpolar solvents. The resulting working solution is hydrogenated to EAQH2 in the presence of hydrogenation catalysts. EAQH2 is then converted to H2O2 and EAQ after oxidation. A simplified production route of this AQ process is shown in Fig. 2. H2O2 is miscible with water in all proportions; hence, it can be recovered in extract phase by multiple washing of organic phase and enhances the concentration up to 50% by distillation process (Liu et al. 2005; Cheng et al. 2008). The overall world share of H2O2 production by the AQ process is about 98% (Guo et al. 2019).

Fig. 1
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A typical production route of hydrogen peroxide

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Fig. 2
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A simplified production route by the AQ process

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The hydrogenation of EAQ in the presence of a catalyst is the key step in the production of H2O2 via the AQ process. I. G. Farbenindustries (1939) investigated the first commercial process in Germany for hydrogenation of EAQ in which EAQH2 was formed in a slurry reactor over a Pd/Al2O3 catalyst (Goor et al. 1989; Kirchner 1979). The working solution was a combination of two organic solvents, namely benzene and a mixture of secondary alcohols (C7–C11). The concentration of the H2O2 solution obtained from the AQ process was about 0.6–1.8 wt%. It is important to enhance the efficiency of the AQ process by improving the purity of the resulting H2O2 solution (Ren et al. 2011). Tang et al. (2014) describe the importance of catalyst support to enhance the diffusion rate as well as the dispersion of active metals. The spherical alumina has been used widely in the fixed bed reactor as catalyst support due to its excellent physical and chemical properties. Li et al. (2012) invented the Pd supported on whisker-modified spherical alumina, resulting in better catalytic activity than the convention pelleted catalyst.

This study is a review-based finding of different researchers, predominating on the hydrogenation of EAQ using Pd-supported catalysts. The focus is given on the progress in the research and development of the catalytic hydrogenation of EAQ for the production of H2O2 by the AQ process. The study discusses primarily the development in the field of new hydrogenation catalysts mainly oxide-supported catalysts for the AQ process. Furthermore, the preparation of Pd catalysts supported on Al2O3, SiO2, and other novel catalyst systems was performed with an attempt to possibly elaborate the significant improvement in the literature. Thus, the present paper describes the advances in the preparation of Pd catalysts on various oxide supports and also other advanced catalysts with structured supports and summarized the general outlook of the developments in the associated area.

Role of oxidizing agent

An oxidizing agent (often referred to as an oxidizer or an oxidant) is a chemical species that tends to oxidize other substances, i.e., causing an increase in the oxidation state of the substance by making it lose electrons. Common examples of oxidizing agents include halogens (such as chlorine and fluorine), O2, and H2O2.

Consumption of H2O2 is increasing day by day in the current forecast period and gaining importance as an environmentally friendly and efficient disinfecting agent because of the increased environmental awareness globally and more stringent legislative requirements. It can be added to the coloring liquid for efficient bleaching operation in the textile industry (Asghar et al. 2015; Karmakar 1999). It has been mixed with the reaction mixture to accelerate the oxidation reaction in an organosynthesis process for the production of fine and bulk chemicals (Sato et al. 1998). It is a promising substitute as an energy carrier used in the production of fuel cells because of its high density, safe handling, and easy storage (Yamazaki et al. 2008; Shaegh et al. 2012; Yamada et al. 2013; Yang et al. 2013). It can be united with a light source, catalyst, and ozone to remove various gaseous and aqueous pollutants from the effluent treatment plant. H2O2 is generally used in the wastewater treatment plant for wet peroxide oxidation, Fenton and photo-Fenton oxidations, sono-Fenton, electro-Fenton, and photocatalytic degradation reactions (Vieira et al. 2022; Perathoner and Centi, 2005; Rokhina and Virkutyte, 2010; Debellefontaine et al. 1996; Domingues et al. 2017; Gomes et al. 2010; Ribeiro et al. 2013, 2016, 2015). In situ generation of H2O2 or may be combining it with the photocatalytic water treatment process enhances the degradation of pollutants due to its promoting effect (Cui et al. 2012; Yao et al. 2016b; Kumar et al. 2017; Zhou et al. 2018; Deng et al. 2017).

In municipal wastewater treatment, H2O2 is used to remove hydrogen sulfide (H2S) from the sewer pipes. It is mostly significantly used in an environmental application for the treatment of various types of industrial wastes. The H2O2 treatment can easily remove the cyanide, thiocyanate, nitrite, chloride, hypochlorite, and organic matter (Chidambara Raj and Quen, 2005). It can also be used in an advanced oxidation process involving more complex reactions as a source of hydroxyl radicals (Chidambara Raj and Quen, 2005; Vieira et al. 2020a, 2020b). H2O2 is used in the bulk sludge biological treatment as a source of oxygen and to prevent denitrification in the settling tanks. H2O2 can also be combined in the injection as a source of oxygen for microorganisms and nutrients to inject into the ground for in situ bioremediation of contaminated soils. H2O2 can be employed in the manufacturing of many organic and inorganic chemicals either directly or followed by conversion into a peroxycarboxylic acid using oxidation, epoxidation, hydroxylation, and oxohalogenation reactions, and for the initiation of emulsion polymerization reactions to produce chemicals like hydrazine, cyanogen, cyanogen chloride, bromine, iodic acid, high-purity ferric sulfate, perborates, percarbonates, benzyl peroxide, and lauryl peroxide (Centi et al. 2009; Bremner et al. 2000; Niwa et al. 2002; Jiajian et al. 2003; Ligtenbarg et al. 2003; Immanuel et al. 2003).

H2O2 can be used in a variety of applications in the mining and metal processing industry (Jennings et al. 2000). The major applications of H2O2 in the mining operations consist of extraction, separation, and purification of uranium, chromium, copper, cobalt, tungsten, molybdenum, gold recovery, and other precious metals. H2O2 can also be used in the metal finishing process for etching, stainless steel pickling, and non-ferrous metal finishing. High-purity H2O2 is used in the electronic industry for cleaning germanium and silicon semiconductor wafers and etching printed circuit boards. In chemical and pharmaceutical syntheses, hydrocarbons, nitrogen oxides, and sulfur dioxide in H2S coming from the exhaust gases can be removed by H2O2 (Jose et al. 2006).

Hydrogenation of 2-ethylanthraquinone

The hydrogenation of EAQ has been studied extensively using various kinds of catalyst. However, the modified Pd-based catalysts are the significant catalytic system. Therefore, the improvements in the existing structure of the catalysts are the important point of consideration for the process development (Liu et al. 2005; Cheng et al. 2008; Li et al. 2012; Isaka et al. 2016; Freakley et al. 2016; Hong et al. 2015; Kosydar et al. 2011; Drelinkiewicz et al., 2005, 2006a, b). In the conventional AQ process, Rn-Ni catalysts have been used in the hydrogenation of EAQ. Rn-Ni catalysts have been replaced by Pd-supported catalysts in the industry due to its higher catalytic activity and selectivity of active quinone observed in the hydrogenation reactions (Liu et al. 2001; Hou et al. 2004). The supported catalysts provide better catalytic activity due to a more effective surface area available with smaller catalyst particles and the higher active metal component dispersion. They are thermally stable and can undergo high temperature and carry higher specific surface area (Hermans and Geus, 1979). The Pd metal on various supports has been used in the hydrogenation of EAQ in both batch and continuous processes.

The supported catalysts are gaining recognition in the field of catalysis due to their superior catalytic performance. Zhang and co-workers (2017) proposed the hydrogenation of EAQ in presence of the bimetallic Pd-Ru catalyst on γ-alumina. Yuan et al. (2017) investigated the effects of synergistic forces due to the addition of second metal such as Co, Ag, and Cu to the conventional catalyst Pd/Al2O3 in the hydrogenation of EAQ. The performance of the supported catalysts has been enhanced due to the size, crystal facet, and electronic structure of Pd particles. Li and Luo enhanced the supports by alternate modification using various methods of preparations to increase the dispersion of metal for higher catalyst activity and selectivity (Tang et al. 2014; Li et al. 2012; Feng et al. 2010; Liu et al. 2002).

In the previous studies, the catalytic hydrogenation of EAQ has been done using the monometallic (Ni, Pd, Ag, Cu) and the bimetallic (Pd-Co, Pd-Ag, Pd-Cu) catalysts. The Ni-B catalysts have been used in the hydrogenation of EAQ, and the results show the influence of the support structure on the catalytic activity and selectivity (Liu et al. 2001, 2002). Ni-Al and Ni-Cr-B-supported catalysts have also been used in the hydrogenation of EAQ. Han et al. (2015a) focused on the modification of supports with the facet-controlled Pd nanocrystals in the hydrogenation of EAQ. A specific method to enhance the catalytic performance of Pd nanocrystals is to place specific highly active facets instead of single-crystal nanoparticles (Jin et al. 2012). Drelinkiewicz and Hasik (2001) and Kosydar et al. (2010) studied extensively to lead the high-performance catalyst for hydrogenation reaction. It was found that the modification in the existing Al2O3 or SiO2 support to enhance the adsorption properties may retard the rate of reaction in the “deep hydrogenation” stage due to the addition of Na2SiO3, polyaniline, or alkali metals such as Li, Na, K, and Cs (Kosydar et al. 2010, 2011; Drelinkiewicz and Hasik 2001; Drelinkiewicz et al., 2004a, b, 2005, 2007; Drelinkiewicz and Waksmundzka-Gora, 2006a, b). It was also investigated that the dispersion of Pd metal enhances with a decrease in the diffusion resistance due to the modification in the pore structure of the support. Han et al. (2015a) have focused on the modification of supports with preparing the high-performance catalysts by enhancing the catalytic chemistry of the Pd-based catalysts.

Pd/γ-Al2O3 catalyst was synthesized by Chen et al. (2015) by plant-mediated strategy for hydrogenation of EAQ with the adsorption-reduction method. Pd-Au/Al2O3 catalyst with special structural and electronic properties shows better catalytic performance in the hydrogenation reaction of EAQ (Han et al., 2015b). A combination of impregnation (IM) and deposition-precipitation (DP) methods was incorporated for the preparation of the bimetallic Pd-Au/Al2O3 catalyst with higher performance for the hydrogenation reaction of EAQ. The mass transfer–controlled hydrogenation of EAQ in the presence of the Pd catalyst was carried out by Santacesaria et al. (1994a, b), and they observed it as a very fast reaction. The hydrogenation of EAQ was confirmed to be zero- and first-order kinetic with respect to hydrogen and EAQ concentrations, respectively (Shang et al. 2012; Santacesaria et al. 1999, 1988; Berglin and Schoon, 1981). The glass beads with an eggshell structure have been used to support Pd nanoparticles and found to be a new catalyst in the hydrogenation of EAQ. Na2SiO3 and MgSiO3 contents in glass beads resulted in its alkalinity, and hence, the glass beads with porous eggshell structure are well suited to combine with the Pd nanoparticles (Shen et al. 2011; Sun et al. 2008a, 2008b).

Catalytic performance and reaction mechanism

The supported catalysts for EAQ hydrogenation can be classified into three categories: (1) Al2O3-supported catalysts, comprised of noble metal Pd and bimetallic Pd (Pd-Ru, Pd-Au, and Pd-Ag) catalysts supported on Al2O3; (2) SiO2-supported catalysts; and (3) catalysts with advanced structure such as Al2O3-SiO2, ZrO2-γ-Al2O3, glass beads, functionalized SBA-15, and honeycomb cordierite monolith catalysts.

Al2O3-supported Pd catalysts

An extensive study on the investigations of hydrogenation catalysts with experimental runs and theoretical discussions was attempted in the past (Han et al. 2015a; Tang et al. 2014; Li et al. 2012; Drelinkiewicz et al. 2005; Shi et al. 2016). Although the various metal-based catalysts were used in the production of H2O2 by different methods, the Pd-supported catalysts provide a better hydrogenation effect among all catalysts. The various Pd catalysts supported on Al2O3 are presented in Table 1. Ulmann (1994) mentioned the H2O2 is produced by mixing 30% EAQ, 70% 2-ethyl-tetrahydroanthraquinone (THEAQ), and organic solvents followed by hydrogenation and oxidation as depicted in Fig. 3. The kinetic of hydrogenation of EAQ can be affected by internal diffusion mass transfer limitation because of a very fast reaction occurring over the Pd catalyst in a semi-batch reactor. Hence, they studied the kinetic for hydrogenation reaction measuring in terms of hydrogen consumption and rotating speed. It can be observed that the kinetics follow a zero-order kinetic for hydrogen and a first-order kinetic for THEAQ. Thus, the kinetics confirm that the THEAQ hydrogenation follows the Rideal-Eley mechanism. The hydrogenation reaction over the Pd catalyst has great affinity; hence, dissociative adsorption of hydrogen on the Pd catalyst is very high (Santacesaria et al. 1988).

Table 1 Hydrogenation of EAQ using various Pd-supported catalysts
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Fig. 3
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Reaction scheme for cyclic reduction and oxidation of EAQ and THEAQ

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The necessity to add a fresh catalyst in the industrial reactor to enhance productivity is a relevant consequence due to catalyst poisoning. Here, two types of catalyst poisoning were observed: reversible poisoning with water molecules adsorbed on Pd catalytic sites and irreversible poisoning with aromatic rings partially hydrogenated and precipitated with EAQ molecules on the catalyst surface. The ring hydrogenation reactions are given in Fig. 4. 2-Ethyloctahydro-9,10-anthrahydroquinone (H8EAQH2) cannot be converted into H2O2. Hence, the other intermediate reactions have to occur to give anthrones and dianthrones (Ulmann 1994). A modified reaction is given in Fig. 5. Santacesaria et al. (1994b) mentioned that the EAQ inhibited the formation of H8EAQH2. The ring hydrogenation reaction of the aromatic rings of EAQ molecules in the presence of the Pd catalyst follows a dual-site mechanism at lower temperature and pressure as shown in Fig. 6 (Santacesaria et al. 1999, 1995).

Fig. 4
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Reaction scheme for ring hydrogenation

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Fig. 5
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A simplified reaction scheme for hydrogenation of EAQH2

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Fig. 6
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Schematic representation of the dual-site mechanism

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Chen et al. (2015) prepared the Pd/γ-Al2O3 catalysts by the adsorption-reduction method and used in the hydrogenation of EAQ. CP extract which was used for the adsorption-reduction method can also be treated as a reductive and protective agent in the hydrogenation of EAQ. They performed the several experiments to find the effect of various parameters like reaction temperature, pressure, hydrogenation time, Pd loading, the volume of working solution, the flow rate of H2, and the support modified by pretreatment on the yield of H2O2 and active EAQ contents. It was found that the highest yield of H2O2 could be achieved at optimized values of the working solution of 10 mL, the hydrogenation time of 1.5 h, and the flow rate of H2 of 40 mL min−1 operated at 40 °C. The sufficient work had been carried out on the pretreatment with different alkali modifiers (Na2SiO3) on the γ-Al2O3-supported catalysts which show better catalytic activity (Drelinkiewicz et al. 2004a, 2004b). It was found that the different pretreated catalyst supports (HCl, NaOH, Na2CO3, and Na2SiO3) provide the different values of H2O2 yield with the enhancement of about 5–10%. However, Na2SiO3-pretreated supports provide the highest value of yield of H2O2 (96.4%). The pretreated catalyst supports can also reduce the formation of the degradation products.

2-Amyl anthraquinone (AAQ) was hydrogenated in the presence of the Pd catalyst prepared by the polyol synthesis method. The effect of the size of the catalysts has also been studied in terms of the selectivity of the catalyst. The production of H2O2 by the AQ process needs to be focused on the selectivity of the catalyst. The consumption of active quinone in the hydrogenation/oxidation process increases with the generation of degradation products, leading to the decrease in catalyst activity. The bigger size particles easily activate the C=O bond, though AAQ has a unique configuration with reaction centers to activate the carbonyl group or aromatic ring. Also, the larger size of the Pd catalyst promotes the rate of deactivation which may be attributed to numerous side reactions (Li et al., 2016a, b).

Similarly, Yuan et al. (2016) have used one-pot evaporation-induced self-assembly (EISA), and hydrothermal (HT) methods for the synthesis of a Pd/Al2O3 catalyst by using the typical synthesis process of impregnation. The one-pot synthesis method provides the higher specific surface area, precise mesopores with the confined distribution of the pores, and the Pd particles effectively encapsulated over the inner part of the mesopores of Al2O3. Most probably, Pd and Pd black catalysts with and without supports have been used after the Rn-Ni catalyst by most of the manufacturers (Edvinsson Albers et al., 2001). The size and dispersion of the Pd particles are the essential variables to decide the activity of the supported catalysts. The dispersion of the Pd particles depends upon some of the favorable conditions such as the higher surface area of the support, uniform pore structure, and smaller pore size distribution. The elemental characteristics of the supports affect the dispersion of the Pd particles, which depends on the catalyst precursors (Tang et al. 2014), preparation methods (Li et al. 2012, 2014; Hong et al. 2015), and chemical compositions (Feng et al. 2010) of the catalysts.

Drelinkiewicz and Waksmundzka-Gora (2006a, b) performed the hydrogenation of EAQ with various catalyst supports such as Pd/C, Pd/SiO2, and Pd/Al2O3. It was found that the catalyst with alumina support provides the higher catalyst activity. The selectivity towards active quinone and the generation of undesirable products get affected by the acidity of the support, whereas the acidity of the Al2O3-supported catalyst decreases the formation of undesirable products and enhances the selectivity of hydrogenation reaction (Yuan et al. 2016). The alumina-supported metal oxide composites have been prepared by HT or EISA synthesis processes for the numerous applications in the catalysis (Xu et al. 2012). Yuan et al. (2016) systematically studied the hydrogenation of EAQ using four types of catalysts. The higher catalyst activity was observed in the hydrogenation reaction in case of smaller Pd particles size (Feng et al. 2010). However, the surface morphologies and the electronic state of Pd particles affect the catalyst activity more than the size of particles. Thus, the increase in the catalytic activity may be attributed to the greater electron density of Pd particles at the interface.

The adsorbed EAQ structure on perfect Pd and the low coordinated Pd were evaluated by density functional theory (DFT). Figure 7 describes the most stable structure of EAQ adsorption. It has two benzene rings and carbonyl oxygen atoms of EAQ located at bridge sites, along with the elongation of the carbonyl bond (0.03 Å) on the perfect Pd(111) surface. The distance between the carbonyl oxygen atom and the nearest Pd atom (Oc-PdOn) is longer than the distance between the carbonyl carbon atom and the nearest Pd atom (Cc-PdCn). Hence, it can be confirmed the formation of an acyl intermediate (η1(C)-acyl) on the Pd(100) surface. The EAQ adsorption energy was found to be increased over low-coordinate Pd atoms (53.7 kcal/mol and 60.0 kcal/mol). The elongation of the carbonyl bond from 0.05 to 0.06 Å might be the reason to decrease the distance between Oc-PdOn and Cc-PdCn. Also, Fig. 7c confirms the formation of a di-sigma complex (η2(C–O)-aldehyde), when the distance between Oc-PdOn and Cc-PdCn almost vanishes. Hence, the hydrogenation of EAQ using low-coordinated Pd atoms allows the possible adsorption of the EAQ structure.

Fig. 7
figure 7

DFT-optimized structures of EAQ (down left), adsorbed EAQ on perfect (a) and defective (b) and c Pd(111) facets. Red, gray, white, and blue spheres represent O, C, H, and Pd atoms, respectively (light blue stands for the top layer) (Yang et al. 2012) (reprinted with permission)

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In the past, many researchers focused on the improvement of the catalytic activity due to the synergetic effect (Alonso et al. 2012) of the incorporation of second metal to the Pd-based catalyst (Au (Yang et al. 2012; Kittisakmontree et al. 2013), Ag (Zhang et al. 2000), and Pt (Kim et al. 2014; Xu et al. 2012)). Zhang et al. (2017) observed the promotion of hydrogenation reaction due to the introduction of the second metal with the Pd catalyst. The addition of Ru on Pd improved the hydrogenation efficiency of the Pd catalyst with higher selectivity toward active quinone. It also increases the desorption of hydrogen atoms as compared to single-Pd-based catalysts. They confirmed that the promotion effect of bimetallic Pd-Ru catalysts improves the catalytic activity of Pd-based catalysts. It was found that both the catalyst activity and selectivity have been improving with the incorporation of Ru to the Pd catalyst in the hydrogenation of EAQ.

The superior performance of Pd-Ru catalysts can be attributed to the adsorption of H2 and the activation of C=O bond in the hydrogenation of EAQ. The dissociative adsorption of the H2 molecules provides the hydride ions to the surface of the Pd and Ru catalyst which enhances the reaction activity. The proposed mechanism for adsorption of H2 and activation of the C=O bond of the aromatic ring of EAQ on the surface of the catalyst may be due to the lone pair of electrons of the oxygen atoms of Pd2+ and Ru4+ acting as an electrophilic site and hence improves the performance of Pd-Ru catalysts (Fig. 8).

Fig. 8
figure 8

The proposed mechanism of promoting effect of a second metal in bimetallic Pd-Ru/γ-Al2O3 samples and the preferential modes of 2-ethylanthraquinone activation on their surface (Jin et al. 2012). Reprinted with permission

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Han et al. (2015a) prepared single-crystal Pd nanocrystals over the immobilized γ-Al2O3 catalyst with exposed (100) and (111) facets which were used in the hydrogenation of EAQ. Both Pd(100) and Pd(111) facets were simultaneously more active toward the hydrogenation of C=O and the activation of the aromatic ring in EAQ. Hence, single-Pd nanocrystals were prepared by one-step synthesis and used in the hydrogenation of EAQ (Tan et al. 2012). It can handle the Pd(100) facet exposure and enhance the stability of Pd metal components. The principal mechanism of hydrogenation reaction of EAQ involved two important steps: the activation of the C=O bond of the aromatic ring of EAQ and adsorption of H2 molecules. The previous studies reveal that the low-coordinate Pd(111) facet is less active in the adsorption and activation of H2 molecules than the perfect Pd(100) facet. However, there are no studies related to the activation of EAQ by Pd catalysts. Figure 9 describes the modified adsorbed structures of EAQ over the Pd(100) and the Pd(111) facets. The stability of the Pd catalyst was found to be improved due to an increase in the interactions between the support and the nanocrystals formed by using the one-step synthesis method.

Fig. 9
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Optimized structures of EAQ adsorbed on the a Pd (100) and b Pd (111) surfaces (Tan et al. 2012). Reprinted with permission

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The incorporation of Au into the Pd/Al2O3 catalyst with the combination of IM and DP methods can enhance the catalytic activity and selectivity for the hydrogenation of EAQ (Han et al. 2015a). Au remained inactive for the EAQ hydrogenation which was confirmed by the negligible yield of 1.11 g L−1 for the Au/Al2O3 catalyst. It indicates that the Au atom was neutral, but after loading with the Pd atom, it could provide a significant amount of charge transfer to the Al2O3 surface, leading to enhancement of the oxidation of Pd. When Au was being added to the Pd/Al2O3 catalyst, the Pd/Au/Al2O3 (14.27 g L−1) and Au/Pd/Al2O3 (12.79 g L−1) catalysts have exhibited higher hydrogenation efficiency than Pd/Al2O3 (12.04 g L−1). Li et al. (2012) developed highly dissipated Pd/Al2O3 catalysts with large surface area and the flowerlike shape design whisker alumina supports. The alumina crystallites can be activated on top of the alumina surface after the evolution of OH− ions by decomposition of urea. Then, activated Al3+ mixed with an aqueous solution of NH3 and CO2 to form ammonium aluminum carbonate hydroxide NH4[Al(OOH)HCO3] nanocrystals. The higher hydrogenation efficiency and selectivity of EAQ hydrogenation is reported with whisker-modified alumina. The formation of degradation products affects the hydrogenation efficiency as the reaction time proceeds (Biasi et al. 2010).

Hong et al. (2015) have involved a low potential-anodizing method to prepare anodic alumina oxide (AAO) supports. They prepared θ-ring-shaped Pd/AAO@Al catalysts by impregnating the Pd catalyst over AAO supports. The controlled preparation method involved different anodizing potentials over a reaction time to adjust the pore size and depth of the prepared AAO supports. In this case, the diffusion is the main driving force for the reaction to occur. The external and internal mass transfers for the hydrogenation reaction of EAQ by using various heterogeneous catalysts, mainly supported Pd catalysts, usually depend upon the shape and pore structure of the support. The performance of the Pd/AAO@Al catalysts in the hydrogenation of EAQ has been enhanced due to the confined effect of cylindrical-shaped pores arranged in a highly ordered array in the AAO support with narrower diffusion distance.

The hydrogenation of AQ and THAQ can be estimated using DFT calculations. The cluster models (Christmann, 1988; Baykara et al. 1986; Rochefort et al. 1990; Roques et al. 2001, 2007; Bertin et al. 2005) and slab models (Dong et al. 1996; Kamachi et al. 2013) were used to investigate the behavior of the Pd(111) facet (Kamachi et al. 2015). The Pd atoms can adsorb atomic H2 to form H2 atoms with the two neighboring 3-fold hollow fcc positions. The H2 dissociation is facile, and the adsorbed AQ remains at the bridge sites on the Pd(111) surface (Kamachi et al. 2015). Thus, anthrahydroquinone (AHQ) and tetrahydroanthrahydroquinone (THAHQ) could be formed with the combination of the two carbonyl O2 atoms of AQ and THAQ with the surface H2 atoms. The Pd catalyst with various supports (C, SiO2, and Al2O3) has been used in the hydrogenation of AQ and THAQ to form AHQ and THAHQ (Santacesaria et al. 1994a, 1994b; Albers et al. 2001; Fajt et al., 2008). The hydrogenation of THAQ is reported as more productive than that of AQ to convert into THAHQ and the degradation of AQ and THAQ to form degradation products such as tetrahydroanthrone (THAN), anthrone (AN), tetrahydro-oxanthrone (THOAN), and oxanthrone (OAN) which were slow (Santacesaria et al. 1988). Hence, demand is growing for process intensification for an industrial AQ process where the side reactions could be suppressed.

SiO2-supported catalysts

Silica supports have been used in the field of catalysis due to its excellent properties. Highly dispersed catalysts can be prepared using silica as supporting material. It has enormous properties like high surface area and high pore volume, and the mesoporous channels make them comfortably used for impregnation of metal catalyst for better adsorption and dispersion of metal salt and trap active metal particles that can resist deactivation during the reaction.

Hydrogenation of EAQ by using the Pd/SiO2 catalyst has been carried out with almost 100% selectivity in a slurry reactor. Drelinkiewicz (1992) investigated the effect of catalyst dosages on the rate of hydrogenation, the yield of 2-ethyltetrahydro-9,10-anthrahydroquinone (H4EAQH2), and the formation of the degradation products. It indicates that the formation of EAQH2 is dependent on the intermediate product H4EAQH2. It was found that the average value of degree of hydrogenation for the generation of degradation products can be evaluated from the individual degree of hydrogenation (U). The value of U changes during reaction, depending on the amount of catalyst. Hence, the formation of degradation products or undesirable products due to side reaction depends on the catalyst concentration.

The selectivity towards active quinone in the hydrogenation of EAQ can be determined by the concentration of H2 on the surface of the catalyst and the type of reaction regime used. Mainly, two types of reaction regimes can be used in the hydrogenation reaction such as kinetically controlled and diffusion-controlled regimes. The “hydrogen poor” mode has observed when the rate of reaction was associated with the rate of H2 transport. However, the “hydrogen-rich” mode existed when the rate of the chemical reaction was interpreted by the overall reaction rate. Ni catalyst has been dealt with the hydrogenation of AQ under high pressure of H2 with the quantitative formation of AN. Hydrogenation of AQ on Pd/Al2O3 and Pd/SiO2-Al2O3 could also form H4EAQ and other unidentified products. Drelinkiewicz (1992, 1995) investigated in the deep hydrogenation of AQ that the temperature, the catalyst dose, and the grain size can manage the internal diffusion, or external mass transport processes and the chemical reaction. The different reaction regimes can have different impacts on the hydrogenation reaction and, more specifically, the yield of H4EAQ. The yield of H4EAQ increases when the external mass transport is the driving force for the reaction kinetics involved (Binder et al. 2009). Drelinkiewicz et al. (2004a, b, 2005, 2007) studied extensively the SiO2 and Al2O3-supported eggshell catalysts with various types of supports and Pd concentration profile in the hydrogenation of EAQ. The precipitation technique was used to precipitate palladium hydroxide onto SiO2 and Al2O3 supports to prepared catalysts which were then impregnated with various alkaline (NaHCO3, NaH2PO4, Na2SiO3) solutions. The SiO2-supported catalyst provides poor deactivation than the Al2O3-supported catalysts, whereas the SiO2-Al2O3-mixed supported catalyst with Na2SiO3 impregnation gives the better catalyst activity (Drelinkiewicz et al. 2004a, 2004b, 2006a,b).

The hydrogenation of EAQ has also been found to be affected consequently by the humidity. The humidity plays a crucial role in the hydrogenation reaction, the dissociation of EAQ to EAQH2, and also the conversion of EAQH2. The utilization of EAQH2 and desirable and undesirable products varies depending upon the alkalinity of Pd/SiO2 catalysts. The humidity dominates the process of EAQH2 transformation with less-alkalinity Pd catalysts.

Advanced catalytic systems

The mass transfer of EAQ in the hydrogenation of EAQ using the Pd catalyst is a fast reaction and rate-controlling step (Santacesaria et al. 1988). On this prediction, Feng and co-investigator (2010) performed the hydrogenation of EAQ in a slurry reactor using the newly synthesized catalyst Pd/SiO2-Al2O3, which can attain 10.3 g L−1 of the yield of H2O2. Shen et al. (2011) achieved 11.2 g L−1 of the yield of H2O2 with a Pd/eggshell glass catalyst. The particle size can be controlled by varying the reflux timing and the calcination temperature (Yuan et al. 2016). As catalytic properties of catalyst depend upon the surface structure and particle size, the focus on the design of catalysts is desirable for well-defined highly dispersed particles. The correlation between the particle size and the reaction activity or selectivity has been introduced by various authors (Binder et al. 2009; Zhou et al. 2006).

Eggshell structure catalysts used in the hydrogenation of EAQ decrease the diffusion distance to enhance the reaction rate. Carbon or Al2O3 supports have been used for the commercial Pd catalysts in the conventional processes. The acidity of the support material has imparted a specific role in the hydrogenation reaction. Shen et al. (2011) have used eggshell-structured glass beads to support the Pd catalyst. They have used subcritical water treatment as the pretreatment of support and the ion exchange method for loading Pd metal. The hydrogenation reaction was performed in a fixed-bed reactor packed with a Pd-supported eggshell catalyst. The metal ions contained in glass beads exchange with the metal ions with higher electronegativity. Sixty percent of conversion of EAQ was achieved in less than 2 s at a liquid flow rate of 0.6 mL min−1, gas-to-liquid ratio of 30, and Pd loading of 0.34 wt%. The space-time yield (STY) and hydrogenation efficiency were found to be 3800 \({\mathrm{g}}_{{\mathrm{H}}_2{\mathrm{O}}_2}\ {{\mathrm{g}}_{\mathrm{Pd}}}^{-1}\ {\mathrm{h}}^{-1}\) and 11.2 \({\mathrm{g}}_{{\mathrm{H}}_2{\mathrm{O}}_2}\ {\mathrm{L}}^{-1}\), respectively. A similar study has been done by Yao et al. (2016a) for the hydrogenation of EAQ by using the eggshell structure of the Pd catalyst in a fixed-bed reactor. In situ prepared catalyst has a diameter of 100 μm, shell thickness of 3–4 μm, and small mono-dispersed Pd nanoparticles ranging from 4 to 5 nm. The hydrogenation efficiency of 6.6 g L−1 and the yield of H2O2 of 35.5% were obtained at an optimum Pd content of 0.32% and a residence time of less than 4 s. Thiele modulus and effective internal diffusion factors were used to know the effect of internal diffusion resistance. It was found that it weakens the internal diffusion resistance for fast reactions. The STY for hydrogenation reaction was found to be 567.5 \({\mathrm{g}}_{{\mathrm{H}}_2{\mathrm{O}}_2}\ {{\mathrm{g}}_{\mathrm{Pd}}}^{-1}\ {\mathrm{h}}^{-1}\) at the liquid flow rate of 0.2 mL min−1 and the gas-to-liquid ratio of 30.

The monolithic catalysts have been used extensively in various catalytic reactions. The basic characteristic of the ideal monolithic catalyst consists of enough durability, high specific surface area, and catalytically active phases. The structure of the porous layer plays an important part in the enlargement of the surface area of the catalyst and affects the active phase required for catalysis. The catalytically active phase in the monolithic catalyst improves the active sites needed for chemical reactions due to its high distribution capacity. Zhang et al. (2008) compared a novel Pd/Al2O3/cordierite monolithic catalyst with the Pd catalyst having Al2O3 coating. It was found that the STY of H2O2 over a monolithic catalyst is better than that of commercial catalysts with only Al2O3 coating. In the hydrogenation of EAQ using the monolithic catalyst, the diffusion distance is very short which improves the diffusion of the active metal. Li et al. (2015) have performed the 1000-h-long stability runs and determined that the monolithic catalyst with SiO2 coating (PSC) provides better catalytic activity than the Al2O3 coating (PAC). The regular structure and weak acidity of the PSC monolithic catalyst made them durable in such a long test. The yield of H2O2 increases up to 5.5 g L−1 with the incorporation of MgO to the SiO2 wash coat (Li et al. 2016a, b). It was also investigated that the mass transfer distance was very short and the mass transfer resistance also becomes less in the case of monolithic catalyst, leading to the improved hydrogenation efficiency. Thus, the application of a monolithic catalyst in the hydrogenation of EAQ has an important contribution.

Shi et al. (2016) investigated the Pd/oxide/cordierite monolithic catalysts prepared by the impregnation method. The Al2O3 layer with about 6 μm of the modest thickness in the prepared catalyst was used in the flowing tube reactor for hydrogenation of EAQ. The highest conversion of EAQ was found to be 99.1%, and the hydrogenation efficiency was found to be 10.0 g L−1. It resulted to the successful Pd penetration and the wider pore size, which promotes the distribution of Pd and receptiveness of the active sites by the reactants. The catalyst activity generally depends upon the type and thickness of the oxide layer. The catalyst with γ-Al2O3 as a layer gives higher reactivity than SiO2 and SiO2-Al2O3 as layers.

The metal-organic frameworks (MOFs) and modified zeolites have been introduced for the hydrogenation of EAQ. The ordered structure, higher surface area, and constant distribution of pore size of the zeolites (e.g., MCM-41 and SBA-15) make them better to support Pd nanoparticles and reduce the diffusion resistance. It enhances the surface for systematic dispersion of metal in the three-phase catalytic reactor (Beck et al. 1992; Kresge et al. 1992; Tanev and Pinnavaia, 1996). Guo et al. (2017) determined the pore diameter of zeolites (MCM-22 and Beta) and the effect on the EAQ hydrogenation. It was found that the monolithic catalyst (0.8% Pd/MCM-41/COR) demonstrated the highest yield of H2O2 (7.54 g L−1) and selectivity (85.3%) in the production of H2O2 by AQ process. The efficiency of 0.8% Pd/MCM-41/COR monolithic catalyst (1573 \({\mathrm{g}}_{{\mathrm{H}}_2{\mathrm{O}}_2}\ {{\mathrm{g}}_{\mathrm{Pd}}}^{-1}\ {\mathrm{h}}^{-1}\)) was much higher than that of a commercial pellet catalyst (500 \({\mathrm{g}}_{{\mathrm{H}}_2{\mathrm{O}}_2}\ {{\mathrm{g}}_{\mathrm{Pd}}}^{-1}\ {\mathrm{h}}^{-1}\Big)\). The monolithic catalysts possess higher mass transfer coefficients (5–20 times) than the commercial catalyst. Guo et al. (2016) have presented the hydrogenation of EAQ using monolith catalysts (Pd/SiO2/COR) with low-pressure drop which has a great impact on the mass transfer performance than the commercial pellet catalyst. It was found that the maximum STY obtained was 793.2 \({\mathrm{g}}_{{\mathrm{H}}_2{\mathrm{O}}_2}\ {{\mathrm{g}}_{\mathrm{Pd}}}^{-1}\ {\mathrm{h}}^{-1}\) with 1.5% Pd/SiO2/COR catalyst at 80 °C and atmospheric pressure.

Cai et al. (2019) prepared the nanorod by adding EAQ to aluminum isopropoxide (AIP) and isopropyl alcohol (IPA). The characteristics of the support, structure of the catalyst, metal distribution, and the interactivity of the metal and the support are necessary factors to enhance the catalytic activity of hydrogenation reaction (Augustyn et al. 2010). The modification of the support is generally used to enhance catalytic activity. The results reveal that the addition of a certain fixed amount of EAQ (4 wt%) in the hydrogenation step helps in the modification of the textural properties of the supports. The hydrogenation efficiency is reported to improve from 7.49 to 10.28 g L−1, which is 37.3% higher than the catalyst prepared without incorporation of EAQ. The formation mechanism of the Pd/γ-Al2O3 catalyst is shown in Fig. 10. In this, EAQ was added to AIP and IPA and followed by the vigorous blending. Hydrolysis of AIP converts it into γ-AlOOH during the sol-gel process. In the aging process, the EAQ molecules were adsorbed on the crystals formed. The recognition effects of the addition of the structure-directing agent (EAQ) before the formation of Al(OH)3 and separation after calcination (550oC) towards γ-AlOOH (Li et al. 2013) generate a definite pore structure over the commercially used γ-Al2O3 supports.

Fig. 10
figure 10

Schematic representation of the EAQ-directed development mechanism of the Pd/γ-Al2O3 catalyst (Li et al. 2018). Reprinted with permission

Full size image

Nano-hybrid catalysts (bimetallic and trimetallic) have been used for various hydrogenation reactions (Chen et al. 2015; Hong et al. 2017; Carvalho et al. 2004; Surisetty et al. 2010; Hungria et al., 2006; Toshima et al. 2007; Adams et al. 2007; Liew et al. 2019). The novel nano-hybrid trimetallic catalyst support Zr-Al-La (ZAL) was synthesized by co-precipitation and impregnated with Pd by an incipient wetness impregnation method (Ingle et al., 2020a, b, 2022). The hydrogenation reaction was performed in a high-pressure autoclave reactor with Pd supported on ZAL catalyst. The co-precipitation by the direct contact of oxide supports gained much attention due to its accessibility and economy. By this method of the preparation of trimetallic oxide support which precipitates with small size, narrow size distribution and high crystalline structure may be obtained. Pd/ZAL catalyst improves the catalytic activity (9.15 g L−1) and provides the high selectivity (100%) simultaneously, which has ascendancy over existing processes. The hydrogenation of 2-EAQ provides the conversion as high as 93% with only 0.5 g of the catalyst with a hydrogenation efficiency of 9.15 g L−1 at operating conditions of 75 °C and 0.3 MPa.

The size of Pd particles plays a vital role in controlling the activity of the catalyst, and the smaller dimensions of Pd particles benefit the hydrogenation of EAQ (Feng et al. 2010). The Pd supported on hollow ceramic microsphere (HCM) exhibits high catalytic activity, selectivity, and stability in the liquid-phase hydrogenation of EAQ (Ingle et al. 2020, 2021). Catalytic activity was found to be 9.42 g L−1 with 100% selectivity over the Pd/HCM catalyst. The calculated mass transfer resistance of the reaction over the Pd/Zr-Al-La catalyst was found to be moderate with a diffusion coefficient of the reactant (H2) of 0.0133 × 10−6 m2 s−1 at 75 °C. It was also verified and confirmed with the Thiele modulus (calculated as 0.0289) indicating no mass transfer resistance. The effectiveness factor (ηs) was found to be 1.0, indicating the negligible mass transfer resistance in the hydrogenation reaction using the Pd/HCM catalyst

Ma et al. (2019) grafted silica support (SBA-15) with alkyltriethoxysilane to support commercial Pd catalysts. The silica supports can be functionalized by grafting method with alkyltriethoxysilane and deposition of Pd by the incipient wetness impregnation method. Both the grafting sequence and the grafting amount were varied over each hydrogenation cycle. It was reportedly found that the grafting agent improves the catalytic activity by increasing the hydrophobicity of the silica support. The effect of grafting amount in the hydrogenation of EAQ over Pd/SBA-15 catalysts provides better performance. The highest grafting amount of catalysts gives the enhancement in the activity and the selectivity of the hydrogenation reaction (98.3%).

Conclusions

The hydrogenation of EAQ into EAQH2 for the production of H2O2 gains more interest to meet the environmental demands of H2O2 as a green oxidant. The commercial production of H2O2 by the AO process proposed in Germany in the late 1930s has been used for large-scale production. The homogeneous catalysts (Ni, Cu, and Pd) have been used by many researchers in the hydrogenation of EAQ for the commercial production of H2O2 from the AO process, and it provided the satisfactory activity and selectivity. Moreover, the separation and regeneration of the catalysts were still complicated. Intensification and investigation of reaction mechanisms of the AO process are the utmost important issues for the technical development of liquid-phase hydrogenation of EAQ in the presence of the Pd catalysts. Thus, the development of heterogeneous catalyst is relatively secondarily recognized in the field of catalysis. The development of the heterogeneous catalysts in the hydrogenation reactions is looked forward to increase the production of H2O2, as the heterogeneous catalysts were not used for commercial production of H2O2. Nonetheless, the H2O2 process needs to fortify further in wholesomeness. Therefore, the heterogeneous catalysts are designed to have better stability, easy separation, safe handling, regeneration of the catalyst, and the reactor designing, which optimized the costs of commercial production of H2O2.

The alumina-supported catalysts can be significantly applied for the production of H2O2 by the AQ process and in the direct synthesis of H2O2. The Al2O3-supported catalyst provides an enhancement in the efficiency of H2O2 synthesis in the AQ process. Al2O3-supported catalysts are reported to provide more yield of H2O2 than the conventional Ni- and Pd-based catalysts. These catalysts can show high activity than the conventional Pd catalysts. Bimetallic catalysts (Pd-Co, Pd-Ag, Pd-Cu, Pd-Au, Pd-Ru, Pd-La), amorphous alloys, and facet-controlled Pd nanocrystal catalysts gave better hydrogenation efficiency than the commercial pellet catalysts. The novel metal catalysts generally provide much higher activity in contrast with the oxide-supported catalyst at lower reaction temperatures. Furthermore, Pd-based catalysts with stronger interactions between promoter and the major component, definite structure, better metal dispersion, and superfine particle exhibits higher selectivity, stability, and catalyst activity in the hydrogenation of EAQ. Disregarding with process development in the structure of the catalyst, one must be focused on developing reactors designed to intensify the activity and selectivity of EAQ hydrogenation. In recent years, the fluidized-bed reactor has become the substitute for a fixed-bed reactor. The fluidized-bed reactor has been a trending technology with important findings used in the hydrogenation reaction of EAQ for the production of H2O2. Sinopec has developed hydrogenation reaction by using bimetallic supported catalysts in the fluidized-bed and slurry stirring tank reactor for the production of H2O2 (Li et al. 2018).

Cordierite monolith catalysts exhibit much higher selectivity towards active quinones (> 99%) and can be obtained over Pd/γ-Al2O3/cordierite monolithic catalysts at the reaction temperature of 60 °C and atmospheric pressure, however at a lower yield of H2O2 (10 g L−1). Furthermore, the nanocrystals of Pd metal with oxide support provide better catalytic activity. Thus, the high solidity of the abovementioned catalyst in the hydrogenation of EAQ forged them better than metal-based catalysts. The maximum H2O2 yield was reported up to 15 g L−1 and was achieved over the Pd/Au/Al2O3 catalyst with 99.56% selectivity towards active quinones. Commercially, H2O2 can be produced from sequential auto-oxidation of EAQ with Pd-based catalysts, the technique which is often employed for useful production of H2O2. Nonetheless, numerous significantly active metal catalysts were employed for EAQ hydrogenation. The eminently promising hydrogenation catalyst with almost 100% selectivity was Pd/SiO2. However, a substantial amount of investigation in the field of hydrogenation catalysts for EAQ hydrogenation has been carried out with analysis and theoretical interpretation, and the mechanism behind hydrogenation reaction is still invariance. The advancement in recent findings should have assuredly accentuated basic acknowledgment regarding the characteristics of active sites and the active component mutual actions of a catalyst booster including support mechanisms of reaction, which play a significant part in the highly effective catalyst designed for the hydrogenation of EAQ.