Strengthening the Connection between Science, Society and Environment to Develop Future French and European Bioeconomies: Cutting-Edge Research of VAALBIO Team at UCCS.

The development of the future French and European bioeconomies will involve developing new green chemical processes in which catalytic transformations are key. The VAALBIO team (valorization of alkanes and biomass) of the UCCS laboratory (Unité de Catalyse et Chimie du Solide) are working on various catalytic processes, either developing new catalysts and/or designing the whole catalytic processes. Our research is focused on both the fundamental and applied aspects of the processes. Through this review paper, we demonstrate the main topics developed by our team focusing mostly on oxygen- and hydrogen-related processes as well as on green hydrogen production and hybrid catalysis. The social impacts of the bioeconomy are also discussed applying the concept of the institutional compass.

How Catalysts and Experimental Conditions Determine the Selective Hydroconversion of Furfural and 5-Hydroxymethylfurfural.

Furfural and 5-hydroxymethylfurfural stand out as bridges connecting biomass raw materials to the biorefinery industry. Their reductive transformations by hydroconversion are key routes toward a wide variety of chemicals and biofuels, and heterogeneous catalysis plays a central role in these reactions. The catalyst efficiency highly depends on the nature of metals, supports, and additives, on the catalyst preparation procedure, and obviously on reaction conditions to which catalyst and reactants are exposed: solvent, pressure, and temperature. The present review focuses on the roles played by the catalyst at the molecular level in the hydroconversion of furfural and 5-hydroxymethylfurfural in the gas or liquid phases, including catalytic hydrogen transfer routes and electro/photoreduction, into oxygenates or hydrocarbons (e.g., furfuryl alcohol, 2,5-bis(hydroxymethyl)furan, cyclopentanone, 1,5-pentanediol, 2-methylfuran, 2,5-dimethylfuran, furan, furfuryl ethers, etc.). The mechanism of adsorption of the reactant and the mechanism of the reaction of hydroconversion are correlated to the specificities of each active metal, both noble (Pt, Pd, Ru, Au, Rh, and Ir) and non-noble (Ni, Cu, Co, Mo, and Fe), with an emphasis on the role of the support and of additives on catalytic performances (conversion, yield, and stability). The reusability of catalytic systems (deactivation mechanism, protection, and regeneration methods) is also discussed.

Upgrading furfural to bio-fuels using supported molybdenum carbides: study of the support effect.

Materials exhibiting different textural and surface properties (SiO2, TiO2, ZrO2 and ZSM-5) were investigated as supports for Mo carbides in the upgrading of furfural (FF) in liquid phase to produce 2-methylfuran (2MF). The state of the catalysts after carburization, passivation, and reactivation under a hydrogen atmosphere was investigated by XAS analysis. The effect of the supports was observed in the first step of the reaction, i.e., the hydrogenation of FF to furfuryl acid and related to Lewis acidic and basic sites. The nature of the supports was also relevant to the final state of the Mo carbides after carburization, passivation, and reactivation. The comparison of the materials showed that Mo2C/SiO2 was the least decarburized catalyst after reactivation, and the most active in converting furfural, while the Mo2C/TiO2 system presented smaller carbide particles after carburization and more disorganized particles after reactivation. Mo carbide supported on SiO2 and on TiO2 were found to be suitable catalysts for producing a mixture containing 2-methylfuran and C10 compounds with potential application as biofuel.

Supported Gold Catalysts for Base-Free Furfural Oxidation: The State of the Art and Machine-Learning-Enabled Optimization.

Supported gold nanoparticles have proven to be highly effective catalysts for the base-free oxidation of furfural, a compound derived from biomass. Their small size enables a high surface-area-to-volume ratio, providing abundant active sites for the reaction to take place. These gold nanoparticles serve as catalysts by providing surfaces for furfural molecules to adsorb onto and facilitating electron transfer between the substrate and the oxidizing agent. The role of the support in this reaction has been widely studied, and gold-support interactions have been found to be beneficial. However, the exact mechanism of furfural oxidation under base-free conditions remains an active area of research and is not yet fully understood. In this review, we delve into the essential factors that influence the selectivity of furfural oxidation. We present an optimization process that highlights the significant role of machine learning in identifying the best catalyst for this reaction. The principal objective of this study is to provide a comprehensive review of research conducted over the past five years concerning the catalytic oxidation of furfural under base-free conditions. By conducting tree decision making on experimental data from recent articles, a total of 93 gold-based catalysts are compared. The relative variable importance chart analysis reveals that the support preparation method and the pH of the solution are the most crucial factors determining the yield of furoic acid in this oxidation process.

Lactic Acid Conversion to Acrylic Acid Over Fluoride-Substituted Hydroxyapatites.

One of the most interesting intermediates for the chemical industry is acrylic acid, which can be derived from lactic acid by catalytic dehydration in the gas phase. The realization of this reaction is complex due to a strong thermal activation leading to the formation of undesired by-products (acetaldehyde, propanoic acid…) as well as polymerization. We studied this reaction over hydroxyapatites modified by substitution of the hydroxyl groups by fluoride. This notably enabled increasing the selectivity to acrylic acid while reducing the formation of the undesired acetaldehyde. Introduction of fluoride induced a modification of the phosphate ( PO 4 3 - ) groups. In the presence of water, fluoride prevented the formation of hydrogenophosphate species ( HPO 4 2 - ), which are well-known acid sites responsible for the formation of acetaldehyde by decarboxylation/decarbonylation. Further, we evidenced an important impact of fluoride substitution on crystallinity, specific surface area and on the surface Ca/P ratio. This latter is known to be a key parameter to control the acidity and the basicity of the hydroxyapatites. Using FT-IR spectroscopy with propyne as a probe molecule, we could show that lactic acid was concertedly adsorbed on basic and acid sites, which might be at the origin of the observed superior performances.

Glycerol to Glyceraldehyde Oxidation Reaction Over Pt-Based Catalysts Under Base-Free Conditions.

Glycerol valorization through partial oxidation is a good way of obtaining many different molecules with high added value such as glyceric acid, tartronic acid, dihydroxyacetone, etc. Among the potential products, glyceraldehyde is an interesting chemical compound for its various applications in different domains such as organic chemistry, medical, and cosmetic industries. In the present paper, we studied the effect of different supports on the glycerol oxidation reaction in a batch reactor applying base-free conditions. The tested catalysts were Pt-based materials deposited on various supports (i.e., SiO2, TiO2, ZSM-5, γ-Al2O3), which were synthesized using a deposition method followed by a chemical reduction. The catalysts were extensively characterized (BET, ICP, XRD, TEM, XPS), highlighting differences in terms of specific surface areas, textural properties, and Pt nanoparticles sizes. We evidenced a direct relation between glycerol conversion and glyceraldehyde selectivity (i.e., an increase in glycerol conversion leads to a decrease in glyceraldehyde selectivity). The Pt/γ-Al2O3 catalysts exhibited the highest activity, but their selectivity to glyceraldehyde significantly decreased with time on stream. Pt/SiO2 presented the highest selectivity to glyceraldehyde owing to a slower reaction rate, which allows envisioning technical opportunities to continuously extract the formed glyceraldehyde from the mixture.

Au-based bimetallic catalysts: how the synergy between two metals affects their catalytic activity.

Supported bimetallic nanoparticles are particularly attractive catalysts due to increased activity and stability compared to their monometallic counterparts. In this work, gold-based catalysts have been studied as catalysts for the selective base-free oxidation of glucose. TiO2-supported Au-Pd and Au-Cu series prepared by the sol-immobilization and precipitation-reduction methods, respectively, showed a significant synergistic effect, particularly when the theoretical weight ratio of the two metals was close to 1 : 1 (with an actual experimental bulk Au/Pd molar ratio of ca. 0.8 and ca. 0.4 for Au/Cu) in both cases. XPS analysis showed that the presence of Au δ+, Pd2+ and CuOH species played an important role in the base-free glucose oxidation.

Extending Catalyst Life in Glycerol-to-Acrolein Conversion Using Non-thermal Plasma.

Booming biodiesel production worldwide demands valorization of its byproduct of glycerol. Acrolein, an important intermediate chemical, can be produced by gas-phase glycerol dehydration catalyzed by solid acids. Because catalysts that lead to high acrolein selectivity usually deactivate rapidly due to the formation of coke that blocks the active sites on their surface, one major challenge of this method is how to extend the service life of the catalyst. Silica-supported silicotungstic acid (HSiW-Si) is a good example of such a catalyst that shows good activity in glycerol dehydration to acrolein initially, but deactivates quickly. In this study, HSiW-Si was selected to probe the potential of using non-thermal plasma with oxygen-containing gas as the discharge gas (NTP-O2) to solve the catalyst deactivation problem. NTP-O2 was found to be effective in coke removal and catalyst regeneration at low temperatures without damaging the Keggin structure of the HSiW-Si catalyst.

Catalytic Dehydration of Glycerol to Acrolein in a Two-Zone Fluidized Bed Reactor.

The gas-phase catalytic dehydration of glycerol to acrolein was carried out in a Two-Zone Fluidized-Bed Reactor (TZFBR) using a 20 wt. % phosphotungstic acid (H3PW12O40) catalyst supported on CARIACT-Q10 commercial silica. In the first step, a hydrodynamic study of the reactor was performed. A quality of fluidization of more than 80% was obtained. In the second step, the mechanical stability of the catalyst was studied. It was found that only the external layer of active phase is eliminated under the conditions of operation whereas the global composition of the catalyst was not significantly affected after 44 h of fluidization. Finally, in a third step, the influence of the main operating parameters on the overall catalytic performances (glycerol/oxygen molar ratio and relative volumes of the reaction and regeneration zones) was investigated, showing notably the importance of the O2/glycerol ratio, resulting in an inverse trend between conversion and selectivity. Increasing O2/glycerol ratio led to higher conversion (lower coke deposit as shown by TGA analysis), but to the detriment of the selectivity to acrolein, supposedly due to the presence of O2 in the reaction zone causing the degradation of glycerol and acrolein.

Amorphous oxide as a novel efficient catalyst for direct selective oxidation of methanol to dimethoxymethane.

We report for the first time the use of an amorphous oxide catalyst for the selective oxidation of methanol in the gas phase, leading at 553 K to the production of dimethoxymethane with a selectivity as high as 90% at high methanol conversion (68%).

Plasmon-Induced Electrocatalysis with Multi-Component Nanostructures.

Noble metal nanostructures are exceptional light absorbing systems, in which electron⁻hole pairs can be formed and used as "hot" charge carriers for catalytic applications. The main goal of the emerging field of plasmon-induced catalysis is to design a novel way of finely tuning the activity and selectivity of heterogeneous catalysts. The designed strategies for the preparation of plasmonic nanomaterials for catalytic systems are highly crucial to achieve improvement in the performance of targeted catalytic reactions and processes. While there is a growing number of composite materials for photochemical processes-mediated by hot charge carriers, the reports on plasmon-enhanced electrochemical catalysis and their investigated reactions are still scarce. This review provides a brief overview of the current understanding of the charge flow within plasmon-enhanced electrochemically active nanostructures and their synthetic methods. It is intended to shed light on the recent progress achieved in the synthesis of multi-component nanostructures, in particular for the plasmon-mediated electrocatalysis of major fuel-forming and fuel cell reactions.

Catalytic Conversion of Alcohols into Carboxylic Acid Salts in Water: Scope, Recycling, and Mechanistic Insights.

The catalytic conversion of alcohols into carboxylic acid salts in water was performed in the presence of ruthenium complexes supported by aliphatic PNP pincer ligands preformed or formed in situ. High activity toward a wide substrate scope was achieved with turnover number values of up to 4000. The air-stable catalytic system can be recycled by using toluene as a catalyst-immobilizing phase; the activity is maintained after five consecutive runs. Finally, mechanistic studies allowed some fundamental aspects related to water activation to be unveiled and to the mechanism postulated.

Novel La3Fe(MoO4)6 phase: magnetic properties and ethanol reactivity.

Single crystals of a new oxide, La3Fe(MoO4)6, were grown from fluxes of oxide precursors, and a polycrystalline sample was also prepared by a standard solid state reaction. La3Fe(MoO4)6 crystallizes in the orthorhombic space group Pbca with unit cell parameters a = 19.3164(11), b = 10.4143(5) and c = 22.0594(12) Å. This crystal structure exhibits a singular architectural type built on infinite chains of Fe(MoO4)4, each of them being surrounded by two isolated MoO4 tetrahedra and three isolated La(3+) cations. Fe(3+) ions in La3Fe(MoO4)6 are antiferromagnetically ordered below TN = 6.6 K in chains and between chains, as refined from neutron diffraction data. Further the redox stability of this compound - pure powder - was checked using temperature-programmed X-ray diffraction under a controlled atmosphere; under air, we observed a reversible phase transition above 523 K. The same phenomenon was observed under a reductive atmosphere, followed by a destruction of the as-formed phase above 923 K owing to iron III to II reduction. Reactivity of ethanol was then evaluated to get insights into the redox properties of the material under working conditions. After 4 hours of reaction at 648 K, the ethanol conversion was 97% with a selectivity to acetaldehyde of ∼60%, the other products being formaldehyde (∼10%) and CO2 (∼30%), underlining a better acetaldehyde selectivity than that of the La-free conventional Fe2(MoO4)3 catalytic formulation.

Regeneration of silica-supported silicotungstic acid as a catalyst for the dehydration of glycerol.

The dehydration reaction of glycerol to acrolein is catalyzed by acid catalysts. These catalysts tend to suffer from the formation of carbonaceous species on their surface (coking), which leads to substantial degradation of their performances (deactivation). To regenerate the as-deactivated catalysts, various techniques have been proposed so far, such as the co-feeding of oxygen, continuous regeneration by using a moving catalytic bed, or alternating between reaction and regeneration. Herein, we study the regeneration of supported heteropolyacid catalysts. We show that the support has a strong impact on the thermal stability of the active phase. In particular, zirconia has been found to stabilize silicotungstic acid, thus enabling the nondestructive regeneration of the catalyst. Furthermore, the addition of steam to the regeneration feed has a positive impact by hindering the degradation reaction by equilibrium displacement. The catalysts are further used in a periodic reaction/regeneration process, whereby the possibility of maintaining long-term catalytic performances is evidenced.

Quasi-homogeneous oxidation of glycerol by unsupported gold nanoparticles in the liquid phase.

A quasi-homogeneous solution of gold nanoparticles prepared by the Turkevich method was used as an unconventional catalyst in the oxidation of glycerol (GLY) in the liquid phase. The highest obtained conversion was 100 % after 3 h of reaction at 100 °C under an oxygen atmosphere (5 bar). The main products were glyceric, glycolic, formic, tartronic, and oxalic acid with selectivities of 28, 36, 25, 9, and 2 %, respectively. Traces of hydroxypyruvic and acetic acid were also detected (combined selectivities below 1 %). To elucidate the reaction mechanism and specify the role of gold nanoparticles in the oxidation process, a series of experiments under various reaction conditions were carried out. The effect of reaction temperature, oxygen pressure, gold concentration, and GLY/base molar ratio was investigated. All catalytic results were systematically compared to the corresponding noncatalytic base-induced transformations (blank tests). Such an approach allowed us to separate and clarify the respective driving parameters for the transformation of GLY (presence of a base and activity of the gold catalyst). The reaction mechanism comprised a series of oxidation and C-C cleavage reactions, whereas additional oxidation-reduction reactions (of the Cannizzaro type) could also occur in the presence of the base.

Transesterification of diethyl oxalate with phenol over sol-gel MoO(3)/TiO(2) catalysts.

The transesterification of diethyl oxalate (DEO) with phenol to form diphenyl oxalate (DPO) has been carried out in the liquid phase over very efficient MoO(3)/TiO(2) solid-acid sol-gel catalysts. A selectivity of 100 % with a remarkable maximum yield of 88 % were obtained, which opens the route to downstream phosgene-free processes for the synthesis of polycarbonates. Interpretation of the results of various acidity measurements (NH(3) and pyridine desorption, methanol oxidation as a probe reaction) allowed us to identify the catalytic sites as Lewis acid sites.

Towards the sustainable production of acrolein by glycerol dehydration.

The massive increase in biodiesel production by transesterification of vegatable oils goes hand-in-hand with the availability of a large volume of glycerol, which must be valorized. Glycerol dehydration to acrolein over acid catalysts is one of the most promising ways of valorization, because this compound is an important chemical intermediate used in, for example, the DL-methionine synthesis. In this Minireview, we give a detailed critical view of the state-of-the-art of this dehydration reaction. The processes developed in both the liquid and the gas phases are detailed and the best catalytic results obtained so far are reported as a benchmark for future developments. The advances on the understanding of the reaction mechanism are also discussed and we further focus particularly on the main obstacles for an immediate industrial application of this technology, namely catalyst coking and crude glycerol direct-use issues.