Abstract
Compared to standard time and solvent consuming procedures, mechanically-assisted processes offer numerous environmentally-friendly advantages for nano-catalytically active materials design. Mechanochemistry displays high reproducibility, simplicity, cleanliness and versatility, avoiding, in most cases, the use of any solvent. Moreover, mechanically-assisted procedures are normally faster and cheaper as compared to conventional processes. Due to these outstanding characteristics, mechanochemistry has evolved as an exceptional technique for the synthesis of novel and advanced catalysts designed for a large range of applications. The literature reports numerous works showing that mechanosynthetic procedures offer more promising paths than traditional solvent-based techniques. This review aims to disclose the latest advances in the mechanochemical assisted synthesis of catalytically active materials, focusing on nanocatalysts designed for biomass conversion and on bio-based catalysts.
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Introduction
Mechanochemistry timeline
As formalized by IUPAC, a mechanical-assisted reaction is “a reaction caused by the mechanical energy” [1]. In fact, mechanical actions, such as compression, stress, or friction, usually provides the energy to activate a process. According to Takacs, the most ancient document concerning a mechanical-assisted process is described in a book in 315 B.C.. The document, titled “On Stones”, was written by Theophrastus, one of Aristotle’s students. The philosopher-scientist described the reduction of cinnabar (HgS) to mercury (Hg0) using a copper vessel and a copper pestle filled with some vinegar (containing acetic acid) [2, 3]. After that first experiment reported by Theophrastus, no mechanical-based protocols were reported for the following 2000 years. Only in 1820 mechanochemistry appeared again, when Faraday carried out mechanical-assisted trials, reducing AgCl to elemental Ag using zinc, copper, tin or iron in a pestle. Faraday noticed that mechanical-assisted reaction could give different products compared to the ones obtained by normal thermal heating. Specifically, he proved that the mechanical-assisted processes favoured the decomposition of Ag and Hg halides to their elements by chemical reaction, rather than by melting or sublimation [4]. A few years later, Wilhelm Ostwald (1853–1932) definided “mechano-chemistry” as one of four chemistry disciplines (together with photo-, electro- and thermo-chemistry). In 1894, Gerard Heinicke formalized mechanochemistry as “the discipline relative to physical-chemical modifications of the solid produced by the action of mechanical factors” [5]. Figure 1 gives a summary of the milestones of mechanochemistry evolution.
Milestones of mechanochemistry
Mechanochemistry theory
The principal feature of mechanical-assisted process is the achievement of chemical changes by the only action of grinding (or milling), without needing to dissolve reagents (therefore without using any solvent). Grinding is a broad term that describes the effect of mechanical forces on a compound that allow a solid breaking into small parts. By grinding, the improved potential energy together with friction and shear contributions, generate surface and shape defects in the reactants. These defects can considerably change the reactivity of chemicals, giving the final product, as described in Fig. 2.
Mechanochemical reactions: from reactants to products
Mechanochemistry equipment
Mechanical-assisted processes could be carried out using different equipment based on manual methods (mortar and pestle) or non-manual methods, such as mixer mills [6]. Mortars and pestles were largely studied in the past especially because they are the cheapest tools for conducting mechanical-assisted protocols. Unfortunately, many environmental factors can influence the aforementioned instruments. In addition, these manual methods do not allow properly controlling the protocol parameters such as frequency or grinding force. Consequently, nowadays the use of this type of equipment can be considered obsolete. Recently, more advanced non-manual instrumentations have been developed in order to achieve highly reproducibility of the mechanochemical synthesis. These tools include, among the others, mixer and planetary mills, and offer the possibility to accomplish solvent-free processes through well-defined reaction parameters, such as milling force and grinding speed.
Mechanochemical-related literature
The study of mechanical-assisted processes has increased considerably, especially in the last 5 years [7], as showed in Fig. 3.
Total publications (yearly) in the mechanochemistry field. Data taken from SciFinder Web
Historically, most of the publication has focused on the mechanical-assisted synthesis of different intrinsically insoluble inorganic material. In the last years, an increasing number of publications were related to the application of mechanical-assisted protocols in organic chemistry. Finally, the newest works re-designed the concept of mechanochemistry, exploiting mechanochemical-assisted techniques for the preparation of novel nanocatalytic materials [8].
Scope of the review
Notably, most studied methods for the synthesis of nanocatalysts include sol-gel methods, impregnation, precipitation, hydrothermal procedures and microwave-assisted techniques [9,10,11]. These protocols have successfully led to the synthesis of advanced catalysts, but they also show numerous drawbacks. In details, these methods are normally time and solvent-consuming, expensive and need aggressive reaction conditions. In this context, mechanical-assisted processes have been proposed as alternative paths for the industrially applicable synthesis of nanocatalysts. In fact, thanks to easiness, versatility and solvent-free conditions, mechanochemistry can compete with standard synthetic methods, avoiding multi-step routes, traditional heating and any addition of toxic reagents. A wide range of nanocatalysts have been synthetized using mechanical-assisted methods. Most studied applications of mechanical-synthesized catalysts include energy and environmental uses or applications in organic synthesis [12,13,14,15,16,17]. Among all these applications, an interesting sub-class is represented by mechanochemical-synthesized nanocatalyst used for biomass valorization. This review aims to illustrate key examples of these types of novel materials. In particular, a first section is dedicated to the mechanochemically assisted synthesis of nanocatalysts used for biomass conversion reactions. The subsequent part discloses mechanochemical-assisted protocols for the preparation of bio-based catalytically active materials. Figure 4 schematizes the scope of the review.
Scheme of the different fields of applications of mechanochemically-synthesized nanocatalysts described in the review
Mechanochemically-assisted protocols for nanocatalysts preparation
Mechanochemically prepared nanocatalysts for biomass conversion
The design of nanocatalysts for biomass conversion has become a very hot topic since the society have started the ambitious transition to a bio-based circular economy [18].
In fact, especially in the last decades, biomass has emerged as an alternative and renewable feedstock which can be converted into valuable materials and chemicals using different protocols that include greener and environmentally friendly paths respect to traditional routes [19, 20]. A key factor for the valorization of biomass through the different processes, is the utilization of an efficient catalyst. The most important characteristic that a catalyst used for biomass valorization must have is the stability under the conditions in which the biomass is normally treated. These include highly stability to moderate-high pressure/temperature and to the presence of water. These characteristics can be effectively found in nanocatalysts prepared with a suitable mechanochemical synthesis.
For examples, highly active nanocatalysts prepared by mechanosynthesis have been employed to produce vanillin. Vanillin is a broadly used aromatic compound in the food industry and in cosmetic formulations and consequently its synthesis is of considerable economic interest. However, the traditional synthetic procedures of this compound requires petro-derivatives and non-environmental friendly protocols. Therefore, more green and alternative methodologies have been studied. Specifically, isoeugenol and vanillyl alcohol can be employed as bio-based starting chemical. In fact, both isoeugenol and vanillyl alcohol are producted from lignin, one of the most abundant biowaste.
In a recent publication, an effective process to obtain vanillin from raw materials derived from lignin (isoeugenol) has been studied [21]. Fast kinetics and high selectivity were achieved using transition metal-based catalysts. The metals were supported on reduced graphene using simple, clean and fast mechanical-assisted protocols. In details, the supporting material of reduced graphene oxide was mixed with the iron salt (FeCl2 ∙ 4H2O) in order to support 1% weight of metal and subsequently subjected to grinding under mild conditions (350 rpm, 10 min) in a ball mill. The preparation of the 1% wt. cobalt catalyst was carried out via the same mechanical-assisted protocol, using Co(NO3)2 ∙ 6H2O as metal precursor. The iron or cobalt-based nanocatalyst were tested in the reaction of isoeugenol oxidation, achieving good conversion and remarkable selectivity using hydrogen peroxide as oxidant. The mechanical-assisted procedure was proved to be a valid alternative synthesis for Fe and Co catalysts, showing outstanding activity for biomass conversion, as schematized in Fig. 5.
Overview of the preparation and application of the 1% wt. Fe (or Co) /graphene oxide catalysts
A mechanochemical-assisted protocol was also designed to obtain a magnetic material Fe2O3-based using mesoporous silica (Al-SBA-15) as supporting material [22]. The mechanical-assisted preparation of Fe2O3 particles on Al-SBA-15 was achieved by milling together propionic acid, Fe(NO3)3·9H2O and Al-SBA-15 at 350 rpm for 10 min. The following step of the protocol was a calcination one at 300 °C for half an hour. The synthesized materials were tested in the synthesis of vanillin through the oxidation of vanillyl alcohol, showing great selectivity and conversion, as displayed in Fig. 6. Interesting, the material was demonstrated to possess a great stability in the aforementioned oxidation, since the activity did not decrease also after 10 cycles of reuse.
a Kinetic analysis of the oxidation of vanillyl alcohol for 20 min at 50 °C and b catalytic behavior at different temperature for 120 min. Reprinted with permission from Ref. [22] Copyright (2019) Elsevier B.V
Another captivating biomass-derived chemical is benzyl alcohol, which can be oxidized to benzyl aldehyde. This last compound is a highly demanded product as it is extensively employed as ingredient in the food or in the pharmaceutical industry or as a fragrance for cosmetic formulations, or even as an intermediate in many chemicals synthesis.
Recently, graphitic carbon nitride (g-C3N4) doped with zinc oxide or iron oxide were prepared using a one-step mechanical-assisted protocols [23]. The oxide incorporation on graphitic carbon nitride was obtained by a simple mechanical-assisted step in a planetary ball mill for 10 min at 350 rpm. Zinc oxide and Fe2NO3 were used as precursors. Lastly, the material was calcined at 300 °C for 3 h. The prepared materials were used as catalysts for the selective benzyl alcohol photo-oxidation to benzaldehyde. Both prepared composite materials showed an improvement of selectivity (70%) and conversion (20%) with respect to pure g-C3N4 used as reference, as displayed in Fig. 7.
Activity and selectivity in the photo-oxidation of benzyl alcohol for the graphitic carbons nitride enriched with zinc or iron. Reprinted with permission from Ref. [23] Copyright (2019) Elsevier B.V
The selective oxidation of benzyl alcohol could be also achieved using cobalt oxide nanoparticles supported on mesoporous silica (SBA-15). The catalyst was prepared through a mechanochemical-assisted protocol. Briefly, the appropriated amount of cobalt precursor was milled with 2 g of SBA-15 metallosillicate at 350 rpm for 10 min. This first step was followed by a calcination at 400 °C for 4 h. The so-prepared material was tested in the selective oxidation of benzyl alcohol, allowing conversions up to 40%. Moreover, the efficiency of the cobalt-based catalysts was proved in the alkylation of toluene with benzyl chloride, achieving complete conversion to alkylated derivatives in a very short time [24].
Similarly, novel copper-containing aluminosilicate materials were synthetized using a mechanical-assisted protocols. Two types of aluminosilicate catalyst with and without zinc were tested. A typical preparation includes the milling of 1 g of Al-SBA-15 or AlZn-SBA-15 and the correct quantity of a copper precursor (CuCl2 ·2 H2O) in order to reach 2 wt%. The reactants were milled together in a ball mill (Retsch 100) at 350 rpm for 10 min [25]. Lastly, the materials were calcined in air at 400 °C for 4 h, as showed in Fig. 8.
Pictorial representation of the synthetic procedure of CuAl-SBA and CuAlZn-SBA
The catalysts were tested in the microwave-assisted valorization of glucose to the value added product 5-methylfurfuryl alcohol (5-MFA). Firstly, glucose was dehydrated with formic acid and subsequently hydrogenated to 5-MFA. This unprecedented mechanical-assisted protocol could pave the way for upcoming studies for the preparation of a wide range of products with high added value from sugars.
Table 1, briefly summarizes the works described above for biomass conversion using mechanochemically synthesized materials.
Mechanochemically prepared bio-based materials for energy conversion/storage and photodegradation
A recent innovative approach in mechanochemistry is the utilization of biological/natural compounds as sacrificial templates or as bio-conjugates in the synthesis of nanocatalysts. In the last years, the interest in the use of diverse biomass sources as sacrificial template has been growing. In fact, the use of natural template sources is extremely attractive for the preparation of nanocatalysts in ecological friendly ways, avoiding the utilization of toxic or expensive classical templates [26]. These bio-templates materials include starch [27,28,29,30], cellulose [31], chitosan [32,33,34,35], lignin [36, 37] and alginate [38, 39]. Compared to classical templates, these materials are also often employed in milder reaction conditions [40].
For example, the synthesis of porous zinc oxide nano-materials was carried on employing zinc nitrate with various polysaccharides including a biomass-derived agar extracted from Gracilia gracilis, as sacrificial template [41]. An easy mechanical-assisted step was efficiently carried out. The milling step was followed by calcination at 600 °C in order to remove the template. The prepared materials were tested for phenol degradation displaying an encouraging photocatalytic activity. Due to its simplicity, large applicability and reproducibility, the proposed mechanical-assisted process has a remarkable potential and could be employed to obtain alternative nanocatalysts from different metal oxides.
Schneidermann et al. have prepared nitrogen-doped carbon using a mixture of lignin with a mechanical-assisted one-pot process [36]. The synthesis was carried out using a sustainable, available, cheap and largely diffuse precursor. The nitrogen-doped carbons were synthetized milling together, in a zirconia vessel for 30 min, the product of the carbonization of a mixture of lignin (wasted from pulp industry) as carbon precursors, urea as nitrogen source and potassium carbonate as activation agent. The obtained materials were sequentially carbonized at 800 °C. Remarkably, carbons showed excellent performance as supercapacitor. The mechanical-assisted protocol was demonstrated to be an environmentally friendly alternative route to obtain nitrogen-doped materials from sustainable precursor. The protocol is schematized in Fig. 9.
Pictorial representation of the synthetic procedure of N-doped porous carbon. Reprinted with permission from Ref. [36] Copyright (2017) Wiley-VCH
More recently, the aforementioned researchers have carried out a mechanical-assisted synthesis of N-doped carbons using renewable biomass waste. In particular they used sawdust, an agricultural by-product, as sacrificial template [37]. Sawdust was used as carbon precursor, melamine and/or urea as a nitrogen precursor, and K2CO3 as an activation agent. In a typical mechanochemical procedures, the three precursors were milled for 30 min. The mechanical-assisted step was followed by a carbonization of the obtained polymer at 800 °C. The nitrogen-doped carbon materials showed a good performance as cathode for lithium–sulfur batteries. The adopted approach is schematically presented in Fig. 10.
Mechanical step and carbonization of a mixture of sawdust, urea and/or melamine and K2CO3 to prepare N-doped carbons as electrode for lithium-sulfur batteries. Reprinted with permission from Ref. Reprinted with permission from Ref. [37] Copyright (2019) Wiley-VCH
Usually, the preparations of nitrogen-doped carbons involve multiple process steps, which are time-, energy- and solvent-consuming and they often employ expensive chemicals. Furthermore, many traditional routes produce large amounts of wastes, especially solvents, which are potentially harmful to the environment or even toxic to humans [42, 43]. The two syntheses of nitrogen-doped carbon materials described above are based on economic and non-toxic feedstock and follow environmentally friendly synthetic paths. Synthetic methods and applications of mechanically obtained nanocatalysts using biomass-template materials are summarized in Table 2.
Besides the aforementioned application of biomass as carbon precursors, our group has extended the mechanochemistry field to synthetize bioconjugates-based materials. In the literature, different paths have been explored to functionalize biological molecules on magnetic nanoparticle surfaces. However, almost all reported protocols need the use of solvents. In order to obtain bio-modified magnetically recoverable nanocatalyst in an easier and less toxic way, mechanical-assisted synthesis was employed reducing reaction time and avoiding solvent consumption [44]. For example, a bio-modified nanomaterial was prepared using horse hemoglobin (Hb) and cobalt oxide magnetic nanoparticles (Co3O4 MNPs) through a solvent-free mechanical-assisted step [45]. Firstly, dopamine (DA) hydrochloride was solubilized in water and added to pre-synthesized Co3O4 magnetic nanoparticles. The mixture was milled in a planetary mill (200 rpm, 10 min) obtaining DA–Co3O4. This first step was followed by another ball milling-assisted step: using the same milling parameters, a dispersion of horse hemoglobin (Hb) in NaH2PO4 buffer was milled together with DA–Co3O4, obtaining Hb–DA–Co3O4. This novel nanocomposite was used as catalyst in durable supercapacitor. The dry mechanical-assisted preparation of the bio-modified catalytic material was demonstrated to be an easy, green and effective unconventional route.
Basing on the mechanical-assisted approach described above, another bioconjugate was synthetized using a redox-active protein and Fe2O3 nanoparticles. For the synthesis, dopamine (DA) previously coated with Fe2O3 particles (DA-Fe2O3) was functionalized with hemoglobin, using two successive mechanical-assisted steps [46]. The so-prepared materials were employed as catalysts to polymerize ortho-, meta- and para-substituted phenylenediamines. The products achieved through the polymerization showed outstanding fluorescence behavior and could be used in optoelectronic devices.
The mechanically functionalization of Fe2O3 nanoparticles with laccase has been also recently reported [47]. The procedure involved two mechanical-assisted steps and allowed the exploitation of a biomass, orange peel waste as sacrificial template and also of an enzyme for the synthesis of a bioconjugates. Firstly, Fe2O3 nanoparticles supported over carbon were prepared using iron nitrate and orange peel waste as carbon source using a mechanochemical-based approach. Sequentially, a mechanical-assisted step was performed milling iron oxide magnetic nanoparticles, dopamine hydrochloride (DA-HCl) and commercial laccase from Trametes Versicolor (LAC) for 10 min at 200 rpm. Finally, the materials were dried in the oven at 100 °C for 24 h, and consecutively heated up to 300 °C for 30 min. The bioconjugate catalysts were used in the direct electrochemically reduction of oxygen, showing good performances. Figure 11 represents an overview of the mechanical-assisted synthesis of bioconjugate-based materials and their application in the electroreduction of oxygen.
Overview of the preparation and application of LAC-DA-Fe2O3
The synthetic methods and applications of the mechanochemically obtained bioconjugates materials are summarized in Table 3.
Mechanochemically prepared bio-based catalysts for biomass conversion
Other novel and captivating examples of mechanical-synthetic protocols describe the preparation of bio-template nanocatalysts used for the biomass conversion, as schematized in Fig. 12. This paragraph combines the two aspects previously presented: the mechanochemical-assisted synthesis of nanocatalysts for biomass conversion and the mechanical preparation of bio-based materials.
Mechanochemically synthesized bio-based catalysts for biomass conversion
Recently, a humins valorization through a mechanical-assisted preparation of humin-based iron oxide nanocatalysts was reported [48]. Humins are a class of biowaste organic compounds derived from the catalytic conversion of biomass in acid conditions. However, they are generally an undesirable feedstock for chemical purposes. In the mechanical-assisted process FeNO3·9H2O and FeCl2·4H2O were used as iron precursors and these chemicals were milled with 4 g of humins in a planetary ball mill for 45 min at 350 rpm. Materials were subsequently dried at 100 °C for 12 h in the oven and finally subjected to a calcination for 4 h at 400 °C. The so-prepared catalysts were tested in a reaction for biomass valorization. The catalysts displayed a significant activity in the production of vanillin from isoeugenol, obtaining conversion > 87%. For the first time, humins were employed as sacrificial template for the mechanochemical preparation of catalysts for biomass conversion to obtain high added value products like vanillin.
In a more recent work, mechano-chemically prepared polysaccharides-based niobium nanomaterials were tested in the same isoeugenol oxidation reaction [49]. The mechanical-assisted preparation of the novel nanocatalysts was performed milling, at 350 rpm for 30 min, a niobium precursor and polysaccharides, derived from natural source and employed as sacrificial templates. Sequentially, the materials were ovendried at 100 °C for 24 h and calcined at 600 °C for 3 h. In this study, niobium-based biotemplate composites were prepared using a green and facile mechanical-assisted process, milling a niobium precursor and different polysugars. The so-prepared materials allowed isoeugenol conversion up to 60% with selectivity to vanillin up to 60%, as Fig. 13 schematically showed.
a Isoeugenol conversion (%) and b vanillin selectivity (%) of mechanochemically prepared polysaccharide-based Nb catalysts. Reproduced from Ref. [49]
The synthetic methods and applications in the conversion of biomass of the mechanochemically obtained bio-based materials are summarized in Table 4.
Conclusions and perspectives
Selected literature examples have been used to highlight the potential and broad perspectives of the mechanical-assisted preparation of advanced catalytically active nanomaterials. Particular emphasis was given to stability and activity enhancement in view of their utilization in biomass conversion and to the mechanical-assisted synthesis of bio-based nanocatalyst. In many cases, mechanically prepared nanocatalysts exhibited comparable or improved catalytic activities respect to the activities observed in nanocatalysts synthesized by traditional methods. The described examples clearly highlighted the extraordinary characteristics of mechanochemistry. These features include greater efficiency in terms of time, costs, sustainability and reproducibility as well as the possibility to discovery new products unreproducible with traditional techniques. In addition, the solventless quality of mechanochemistry implies greener reaction conditions, low E-factor and high atom efficiency.
The chemical industry has already started the transition to sustainable technologies, including some application of mechanochemistry. Remarkably, also various patent have been successfully published [50]. One extraordinary step will be the application of biomass into mechanochemistry for the massive production of bio-based materials, in order to full fill the concept of a green economy free of petroleum based chemicals.
Abbreviations
- 5-MFA:
-
5-methylfurfuryl alcohol
- Co3O4 MNPs:
-
Cobalt oxide magnetic nanoparticles
- DA:
-
Dopamine
- DA-HCl:
-
Dopamine hydrochloride
- g-C3N4 :
-
Graphitic carbon nitride
- Hb:
-
Horse hemoglobin
- LAC:
-
Laccase
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Acknowledgments
The authors gratefully acknowledge MINECO for funding under project CTQ2016-78289-P, co-financed with FEDER Funds including a contract for Camilla Cova in the framework of such project. The publication has been prepared with support from RUDN University Program 5-100.
Funding
This work was conducted in the framework of MINECO project CTQ2016–78289-P. The funding body (MINECO, Spain) provided support for the design of the study, analysis and interpretation of data and in writing the manuscript.
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RL conceived of the study and participated in its design and coordination as well as revised/finalized the manuscript for submission. CC made the most substantial contributions to the draft writing of the manuscript. Both authors read and approved the final manuscript.
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Cova, C.M., Luque, R. Advances in mechanochemical processes for biomass valorization. BMC Chem Eng 1, 16 (2019). https://doi.org/10.1186/s42480-019-0015-7
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DOI: https://doi.org/10.1186/s42480-019-0015-7