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15 September 2022

Molybdenum, Vanadium, and Tungsten-Based Catalysts for Sustainable (ep)Oxidation

and
1
LCC-CNRS, Université de Toulouse, CNRS, UPS, CEDEX 4, F-31077 Toulouse, France
2
Department of Chemistry, Institut Universitaire de Technologie Paul Sabatier, University of Toulouse, Av. G. Pompidou, BP20258, CEDEX, F-81104 Castres, France
3
Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia
*
Author to whom correspondence should be addressed.
Molecules2022, 27(18), 6011;https://doi.org/10.3390/molecules27186011
This article belongs to the Special Issue Sustainable Chemistry in France
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Abstract

This article gives an overview of the research activity of the LAC2 team at LCC developed at Castres in the field of sustainable chemistry with an emphasis on the collaboration with a research team from the University of Zagreb, Faculty of Science, Croatia. The work is situated within the context of sustainable chemistry for the development of catalytic processes. Those processes imply molecular complexes containing oxido-molybdenum, -vanadium, -tungsten or simple polyoxometalates (POMs) as catalysts for organic solvent-free epoxidation. The studies considered first the influence of the nature of complexes (and related ligands) on the reactivity (assessing mechanisms through DFT calculations) with model substrates. From those model processes, the work has been enlarged to the valorization of biomass resources. A part concerns the activity on vanadium chemistry and the final part concerns the use of POMs as catalysts, from molecular to grafted catalysts, (ep)oxidizing substrates from fossil and biomass resources.

1. Introduction

Among the several challenges that the chemical industry has to face in the future, the most visible is to diminish/erase its negative image, soiled for years by several unfortunate contaminations issues, hazards being mainly bad handling procedures and storage mistakes (ammonium nitrate explosions, chemical leaks from tankers, burning of chemicals) [1]. For a century, the market of organic molecules has mainly been based on relatively cheap and available fossil resources. The “cheap” version of those raw sources is diminishing and industries have to face some geopolitical issues, increasing their prices. Maintaining the production of such organic molecules, with a relative low price/cost, obliges academic/industrial chemists to consider the development of new sources. New fossil sources can still be discovered for several years (ca. 50–100 according to Association for the Study of Peak Oil-ASPO) but with low accessibility [2,3]. The use of fossil sources being one cause of global warming, the quest toward renewable and sustainable sources seems more than urgent. In the current time, there is high interest in the carbon footprints of all processes/products for public policies but also for energy- and matter-saving processes. This was among the reasons that academic and industrials scientists began to think about solutions for a “better chemistry”. Those ideas have been gathered in 12 principles in the beginning of 1990 called “Green Chemistry” [4]. This concept points out the urgent need for new processes that are more sustainable and less energy demanding. Presented solutions recommend using cleaner and safer processes to replace the actual chemical processes where hazards might more frequently occur [5]. All of those ideas are a straight line to solutions anticipating fossil depletion.
To save energy in a chemical process, one point is to diminish the activation energy of a chemical reaction. For this, catalytic processes, among the 12 principles pointed out by Anastas and Warner, are one relevant solution [6]. In addition to reactions in which the catalyst diminishes the activation energy barrier, making the chemical reaction faster and preferably more selective, the process should become cleaner by replacing, diminishing, or eliminating organic solvents (sometimes toxic and often from fossil sources) and finding new, renewable sources [7]. Several research groups developed new answers with one or more of those solutions. In the presented research, we have followed Green Chemistry principles, i.e., namely catalysis, catalysts recovery through grafting, organic solvent-free processes, and biomass valorization. Those objectives are realized in the LCC research group in close collaboration with international research groups, especially with the one from Croatia. All that is presented herein corresponds to the work developed by Castres for several years, within the frame of the LCC research group devoted to catalysis. Within the numerous possible simple chemical transformations, the work developed herein focused on oxidation reactions, i.e., olefin epoxidation and alcohol oxidations.
Oxidation processes are at the origin of the formation of numerous molecules present in nature. From a fundamental point of view, studying those processes helps to understand the formation of those compounds. From an applicative point of view, chemists tend to mimic faster natural oxidation processes in order to obtain in abundant quantities (and preferably with a decent price) molecules present in nature (but often in too small a quantity considering commercial application) [8] or to create new molecules for several other purposes (often pharmaceutical). The advantage of oxidation protocols is to be under air, in agreement with some principles of Green Chemistry (simple process). The impact of such reactions is huge since oxidation represents a big part of industrial chemical transformation. The pharmaceutical industry [9,10,11], polymer industry [12,13], as well as the flavor and fragrance industries [14,15] need simple building blocks, and starting reagents have to be easily accessible. For example, the most known efficient synthetic processes to perform olefin epoxidation or alcohol oxidation use non-green conditions. The use of toxic inorganic oxidants in a stoichiometric amount, strong acids and/or organic solvents represent the non-green area that has to be replaced in light of the previously cited Green Chemistry principles and safety regulations [16,17].
Some existing processes are found to be cleaner, including metal complexes and/or metal oxides as catalysts, with the use of a cleaner oxidant as H2O2 [18], TBHP [19], or O2 [20]. Among efficient metals, we focused on transition metals with low toxicity, i.e., Mo, W, and V. Most of those metal-catalyzed processes used organic solvents and, in the case of epoxidation especially, dichloroethane (DCE) has been found to the most efficient. The replacement of DCE and extension to any organic solvent is an interesting challenge that has been discussed in the context of the industrial sector [21,22,23].
“No solvent is the best solvent” is the motto of the catalyzed processes presented herein. For this, we have separated the work into several aspects, that dealing with molybdenum, tungsten, and vanadium coordination complexes containing mainly two types of tridentate ligands, including some mechanistic studies as well as valorization of biomass. The second aspect will focus on the use of commercial polyoxometalates as oxidation catalysts under organic solvent-free protocols, using organic salts and grafted salts, for simple model studies and on applied processes toward the synthesis of useful species or the use of biomass substrates.

3. Vanadium Species

As for molybdenum, vanadium is an element used in oxidation processes. In addition to being present in nature in some enzymatic processes, vanadium can activate smoother and cleaner oxidants, such as TBHP or H2O2 and the most convenient oxidant, O2. Several vanadium-containing compounds have been shown to be active in catalysis, for example the species used by Mimoun [58], Rehder [59], Maurya [60], or Hartung [61]. The mechanisms are numerous according to the nature of the ligands.

3.1. SAP

It was interesting to take advantage of the H2L ligands (H2SAE, H2SAMP, and H2SAP) presented in the molybdenum section and to study the activity of equivalent vanadium species. Through reaction with [VO(acac)2] as a vanadium precursor and subsequent oxidation, the dinuclear complexes [(L)VO]2O complexes have been isolated (Figure 19) and the structure of the compound (L = SAE, SAMP) was determined through X-ray crystallography. The catalytic activity of 1 mol% complex vs. substrate was tested with [VO(SAP)]2O as a catalyst, with both TBHP or H2O2 as oxidant, and for the first time under organic solvent-free conditions. The epoxidation of cyclo-octene gave a 94% conversion (and 83% selectivity towards the epoxide) after 5.5 h with TBHP and no reaction with H2O2 [62].
Figure 19. Schematic representation of [VO(L)]2O complexes (L = SAP, SAE, SAMP).

3.2. ONO, ONS, and Mechanism

Based on the pyridoxal moiety used for the Mo complexes mentioned above, different families of vanadium complexes were synthesized with an ONS coordination sphere and containing one vanadium atom with general formulas [VO2(LH)] and an ONO coordination sphere around the vanadium creating neutral [V2O3L2] or charged [V2O3(HL)]Cl2 molecules containing two vanadium atoms (Figure 20) [63]. Those species have been tested as catalysts for the epoxidation of cyclo-octene under organic solvent-free conditions (Table 10). With TBHP in water as oxidant. Results were moderate but it was interesting to try to elucidate the mechanism through DFT. Thus, from the complexes of general formulas [VO2(LH)] (X = C-OH), calculations showed that the most energetically favorable pathway went through the formation of hydroxido-alkylperoxido [VO(OH)(OOMe)(HL)] (Figure 21) [63].
Figure 20. Schematic representation of vanadium complexes containing pyridoxal fragments being monomeric when containing hydrazide moieties and dimeric with thiosemicarbazones.
Table 10. Results of CO epoxidation under organic solvent-free conditions with vanadium complexes from Figure 20.
Figure 21. Schematic catalytic cycle proposed after DFT calculations with the [VO2(LH)] (X = C-OH) complex. The ligand was schemed with the green symbol.

4. Keggin-Type Polyoxometalates as (ep)Oxidation Catalyst

Keggin Polyoxometalates (POMs) is the second class of catalysts studied using the sustainable methods presented herein. Known for a very long time for fundamental research but also for their applications in biology and in catalysis, in both homogeneous and heterogeneous conditions, POMs have the advantage of being an extremely stable species, very simple to be synthesized (mostly in solution methods but recently using solvent-free methods using mechanochemical activation). Those species can very easily activate smooth oxidants (H2O2, TBHP, O2, UHP) for several oxidation reactions. Among the active species relative to POMs; the classical Venturello–Ishii catalyst, a peroxo-oxomolybdenum complex, has been deeply studied and Keggin species were more explored for the sulfoxidation reaction.

4.1. Pyridinium Salts

Very simple [PMo12O40]3− and [PW12O40]3− Keggin type heteropolyanions have been tested for catalysts as organic salts using tetrabutylammonium (TBA), butyl- (BP) or cetyl-pyridinium (CP) as cation, in order to use the catalyst in the organic medium, i.e., the substrate itself [64]. The reactions were done without organic solvent and oxidants (H2O2 or TBHP) in aqueous solution (Figure 22). The results (Table 11) exhibited activity depending on all parameters, i.e., PMo12 vs. PW12, nature of cation and nature of oxidant. With 0.1% POM vs. cyclo-octene, at 80 °C, TBHP gave a better selectivity toward epoxide without formation of the cyclo-octanediol. This latter compound was observed when H2O2 was used as oxidant, this certainly explained the low selectivity. The catalysts could be recycled and separated easily from the reaction mixture very easily at room temperature in the case of TBA and BP salts, the CP giving an emulsion that was hard to separate (although efficient). The reaction certainly takes place in the organic phase, and it was found that homogenous and heterogeneous reactions coexist (the solubility of the POMs being strongly cation dependent), explaining the difference within the selectivity. The alkyl pyridinium being the most soluble, a different test with a low catalyst charge was performed, exhibiting activity until 2 or 5 ppm POM vs. substrate ratio but a lower selectivity. Thus, it was supposed that a heterogeneous process gave better selectivity within the reaction media. In addition, it was also shown that [PMo12O40]3− species were more efficient with TBHP and [PW12O40]3− with H2O2.
Figure 22. Epoxidation of cyclooctene catalyzed by organic salts of polyoxometalates Q3[PM12O40] (M = Mo or W, Q = Bu4N (TBA), Pyr-C4H9 (BP), Pyr-(CH2)15CH3 (CP).
Table 11. CO epoxidation under organic solvent free conditions with organic salts of POMs from Figure 22. TBHP/cyclooctene = 1.5, 24 h, 80 °C.

4.2. Supported Catalysts

Within the aim of a sustainable and recyclable process, it was interesting to further study the catalytic activity of the simple Keggin polyanions once ionically grafted on solid support. Two types of supports have rightly been studied: functionalized organic polymer and silica nanoparticles.

4.2.1. Grafted POMs on Merrifield Resins [65]

The protocol consisted in the functionalization of a commercial Merrifield resin by quaternization of alkyl imidazoles by the chloromethyl pending functions present on the polymer. From those functionalizations, [PM12O40]3− (M= Mo, W) were ionically grafted on those polymers. In order to compare the activity when grafted or as a free molecule, molecular analogs with same type of imidazolium countercations have been also synthesized. (Figure 23) The species were stable and could be characterized through several methods. It was possible to load more Mo Keggin than W Keggin on the Merrifield resins with a range of 55.6–66.7 μmol/g polymer for PMo12O40 and 12.2–18.9 μmol/g polymer for PW12O40.
Figure 23. molecular and grafted catalysts on functionalized Merrifield resin.
The organic and the grafted salts of POMs have been tested for the epoxidation of cyclohexene, an interesting precursor for the synthesis of adipic acid (AA) (Figure 24). With four equivalents of oxidant starting from cyclohexene (Table 12), AA yields from 46–61% were obtained with molecular catalysts and 33–51% with grafted catalysts. The interesting fact lies in the low POM content in general (0.025% POM vs. substrate with molecular catalysts and within 0.001–0.007% range for the grafted catalysts). The study considered each step of the postulated mechanism.
Figure 24. Epoxidation of cyclohexene leading to adipic acid.
Table 12. Results of AA formation according to substrate using molecular of grafted POM-based catalysts from Figure 23.

4.2.2. Grafted POMs on Functionalized Silica

The ionic grafting of POMs being a convenient recovery method, we continued in this area by using an inorganic support, i.e., ca. 76 nm diameter sized non-mesoporous silica nanoparticles functionalized at their surface by aminopropyltriethoxysilane [66]. The strategy implied a limited number of synthetic steps, ionically grafting the POM at the surface of the functionalized bead through protonation of the pending NH2 functions. (Figure 25) Using this synthetic strategy and several characterization methods, the objects contain 0.12–0.14 mmol POM/g of sample, i.e., 2–10 times more than for the Merrifield resin. Those objects were used for the epoxidation of cyclooctene (CO) (Table 13), cyclohexene (CH) (Table 14), and limonene (Lim) (Table 15) and for the oxidation of cyclohexanol (CYol) (Table 16). For CO, the conversion was a bit slower for the grafted catalysts and the selectivity was better for the H3PMo12O40 catalytic objects. The same trend between the metals was observed with CH, the grafted W-catalyst being more active than the heteropolyacids precursors and the reaction giving other products than CHD (maybe AA). With Mo, the CHD was more visible showing that the reaction was slower. With Lim, the reaction was very fast and mainly led to the formation of LimDs, as well as a few quantities of carveol (Col) and carvone (Cone).
Figure 25. Schematic representation of POMs grafted on functionalized silica.
Table 13. CO epoxidation into COE with heteropolyacids and corresponding anionic species from Figure 25.
Table 14. CH epoxidation with heteropolyacids and corresponding anionic species from Figure 25. Analyzed species are in Figure 26.
Table 15. Lim epoxidation with heteropolyacids and corresponding anionic species from Figure 24. Analyzed species can be found in Figure 16 and Figure 18.
Table 16. CYol oxidation toward CYone with heteropolyacids and corresponding anionic species grafted on silica schemed in Figure 22. Studied products are those from Figure 27.
CyOH gave interesting information, exhibiting better activity for the heteropolyacids, certainly due to the inner acidity (compared to the grafted ones).
Those objects were even reused and showed recyclability after the third run.

5. Conclusions

It has been shown here all the different directions taken in Castres, France toward sustainable processes. Ligand engineering (with some mechanistic DFT explanations) for the coordination complexes and catalysts grafting were the strategies employed to use a very low quantity of catalysts for different (ep)oxidation processes. All has been progressively oriented recently toward the valorization of biomass substrates, in order to situate this research in the context of circular economy. The advances in this research are still in progress.

Author Contributions

Conceptualization, D.A.; methodology, D.A. and J.P.; writing—original draft preparation, D.A.; writing—review and editing, D.A. and J.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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