Supported MoOx and WOx Solid Acids for Biomass Valorization: Interplay of Coordination Chemistry, Acidity, and Catalysis

Review on synthesis of lactic acid and lactates from biomass derived carbohydrates via chemocatalysis routes

The utilization of renewable lignocellulosic biomass resources is a promising solution to deal with the deficit of fossil resources and the associated environmental concerns. Among diverse biomass-derived products, lactic acid (LA) stands out as one of the most successful commodities and also a platform to connect raw biomass feedstocks with value-added chemicals and degradable polymers. Herein, we critically review the recent advances in the design and development of base, acid, and multifunctional catalytic systems for the conversion of different carbohydrates to LA and alkyl lactates via chemical routes. In addition to critically evaluating the advantages and disadvantages of different catalytic systems, we provide deep insights into the reaction mechanisms, including the reaction pathways of different feedstocks, the catalytic roles of different kinds of active sites, and the structure-activity relationship. We conclude with our perspective on the key challenges and future opportunities.

Termination-acidity tailoring of molybdenum carbides for alkaline hydrogen evolution reaction

Transition-metal carbides have been advocated as the promising alternatives to noble-metal platinum-based catalysts in electrocatalytic hydrogen evolution reaction over half a century. However, the effectiveness of transition-metal carbides catalyzing hydrogen evolution in high-pH electrolyte is severely compromised due to the lowered proton activity and intractable alkaline-leaching issue of transition-metal centers. Herein, on the basis of validation of molybdenum-carbide model-catalyst system by taking advantage of surface science techniques, Mo2C micro-size spheres terminated by Al3+ doped MoO2 layer exhibit a notable performance of alkaline hydrogen evolution with a near-zero onset-potential, a low overpotential (40 mV) at a typical current density of 10 mA/cm2, and a small Tafel slope (45 mV/dec), as well as a long-term stability for continuous hydrogen production over 200 h. Advanced morphology and spectroscopy characterizations demonstrate that the local -Al-OH-Mo- structures within Al-MoO2 terminations serve as strong Brønsted acid sites that accelerate the deprotonation kinetics in alkaline HER process. Our work paves an interesting termination-acidity-tailoring strategy to explore cost-effective catalysts towards water electrolysis and beyond.

Synthesis and catalytic application of nanostructured metal oxides and phosphates

The design and development of new high-performance catalysts is one of the most important and challenging issues to achieve sustainable chemical and energy production. This Feature Article describes the synthesis of nanostructured metal oxides and phosphates mainly based on earth-abundant metals and their thermocatalytic application to selective oxidation and acid-base reactions. A simple and versatile methodology for the control of nanostructures based on crystalline complex oxides and phosphates with diverse structures and compositions is proposed as another approach to catalyst design. Herein, two unique and verstile methods for the synthesis of metal oxide and phosphate nanostructures are introduced; an amino acid-aided method for metal oxides and phosphates and a precursor crystallization method for porous manganese oxides. Nanomaterials based on perovskite oxides, manganese oxides, and metal phosphates can function as effective heterogeneous catalysts for selective aerobic oxidation, biomass conversion, direct methane conversion, one-pot synthesis, acid-base reactions, and water electrolysis. Furthermore, the structure-activity relationship is clarified based on experimental and computational approaches, and the influence of oxygen vacancy formation, concerted activation of molecules, and the redox/acid-base properties of the outermost surface are discussed. The proposed methodology for nanostructure control would be useful not only for the design and understanding of the complexity of metal oxide catalysts, but also for the development of innovative catalysts.

Syngas Production by Chemical Looping Dry Reforming of Methane over Ni-modified MoO3/ZrO2

We investigated supported-MoO3 materials effective for the chemical looping dry reforming of methane (CL-DRM) to decrease the reaction temperature. Ni-modified molybdenum zirconia (Ni/MoO3/ZrO2) showed CL-DRM activity under isothermal reaction conditions of 650 °C, which was 100-200 °C lower than the previously reported oxide-based materials. Ni/MoO3/ZrO2 activity strongly depends on the MoO3 loading amount. The optimal loading amount was 9.0 wt.% (Ni/MoO3(9.0)/ZrO2), wherein two-dimensional polymolybdate species were dominantly formed. Increasing the loading amount to more than 12.0 wt.% resulted in a loss of activity owing to the formation of bulk Zr(MoO4)2 and/or MoO3. In situ Mo K-edge XANES studies revealed that the surface polymolybdate species serve as oxygen storage sites. The Mo6+ species were reduced to Mo4+ species by CH4 to produce CO and H2. The reduced Mo species reoxidized by CO2 with the concomitant formation of CO. The developed Ni/MoO3(9.0)/ZrO2 was applied to the long-term CL-DRM under high concentration conditions (20 % CH4 and 20 % CO2) at 650 °C, with two pathways possible for converting CH4 and CO2 to CO and H2 via the redox reaction of the Mo species and coke formation.

Selective Hydrogenation of Azobenzene to Hydrazobenzene via Proton-Coupled Electron Transfer from a Polyoxotungstate Cluster

In this report, we describe proton-coupled electron transfer (PCET) reactivity at the surface of the Keggin-type polyoxotungstate cluster [nBu4N]3[PWVI12O40] (PW12) in acetonitrile. Bond dissociation free energies (BDFEs) of the O-H groups generated upon reduction of PW12 in the presence of acid are determined through the construction of a potential-pKa diagram. The surface O-H bonds are found to be weak (BDFE(O-H)avg < 48 kcal mol-1), comparable to the BDFE of H2. This is consistent with the observed formation of H2 upon addition of a suitably strong organic acid, H2NPh2+ (pKa MeCN = 5.98), to the reduced form of the cluster. The one-electron reduced form of PW12 is isolated and used in conjunction with acid to realize the stoichiometric semihydrogenation of azobenzene via PCET from the surface of the reduced cluster.

Multifunctional WO3-ZrO2-Supported Platinum Catalyst for Remarkably Efficient Hydrogenolysis of Esters to Alkanes

The hydrogenolysis of esters to alkanes is a key protocol for the synthesis of high-quality hydrocarbon fuels from renewable plant oils or fats. However, performing this process under mild energy-efficient conditions is challenging. Herein, we report a robust tungsten- and zirconium-oxide-supported platinum catalyst (Pt/WO₃-ZrO₂) for the hydrogenolysis of esters to alkanes at low temperatures (as low as 70 °C) and under ambient pressure (1 atm) of H₂. For example, tristearin undergoes a complete conversion at 130 °C with more than 95% selectivity for the corresponding alkanes without carbon loss. In addition, the heterogeneous nature of the catalyst system reported herein permits multiple reuse of the catalyst without any significant loss of its high activity and selectivity. Mechanistic studies suggest that the multifunctional nature (acid and redox properties) of the WO₃-ZrO₂ support plays an important role in the high activity of the catalyst.

Basicity of Isostructural Porous Ionic Crystals Composed of Nb/Ta-Substituted Keggin-Type Polyoxotungstates

Three isostructural porous ionic crystals (PICs) based on Keggin-type POMs with different compositions but equal negative charge ([BW12O40]5– (BW12), [SiW11NbO40]5– (SiW11Nb), and [SiW11TaO40]5– (SiW11Ta)) are synthesized. Experimental and theoretical characterizations...