Elucidation of Active Sites and Mechanistic Pathways of a Heteropolyacid/Cu-Metal–Organic Framework Catalyst for Selective Oxidation of 5-Hydroxymethylfurfural via Ex Situ X-ray Absorption Spectroscopy and In Situ Attenuated Total Reflection-Infrared Studies

Abstract

Chemoselective oxidation of 5-hydroxymethylfurfural (HMF) over nonnoble metals to produce a bioplastic monomer, 2,5-furandicarboxylic acid (FDCA), under alkaline-free conditions is challenging and worthy of investigation. HMF oxidation into FDCA involves the concurrent oxidation of primary alcohol and an aldehyde functional group into carboxylic groups, which therefore demand a bifunctional catalyst containing dual active sites and chemoselective oxidation of HMF. The present work demonstrated the formation of new selective active sites in a composite porous material (Cu-BTC_PMA) that consists of Cu-BTC (metal−organic framework (MOF)) and polyoxometalate (POM). The porous framework provides (Cu-BTC_PMA) the desired chemoselectivity, while a selective Cu metal center in Cu-BTC (MOF) and Cu−O−Mo sites functions as active sites for the concurrent oxidation of HMF into FDCA. This catalyst exhibited a HMF conversion of 89% and an FDCA selectivity of 92.3% under base-free and mild reaction conditions. In detail, X-ray absorption spectroscopy analysis demonstrated the chemical bond tuning, as well as electronic structural modulations of MOF and POM at the molecular level, which directs the formation of new synergistic interfacial active sites and charge transfer states. This phenomenon causes the generation of the unique redox environment of copper and the multiple oxidation states along with the oxygen vacancy in the Cu-BTC_PMA catalyst, which most likely behaves as active sites for base-free oxidation. A kinetics study of this reaction was followed using in situ attenuated total reflection-infrared spectroscopy, demonstrating the stabilization of the specific intermediates that lead to the formation of FDCA. Moreover, we made comparative density functional theory and quantum theory of atoms in molecules investigations on the surface interaction between the reactant (HMF) and two catalyst models of Cu-BTC and Cu-BTC_PMA to interpret quantitatively the higher catalytic activity of the Cu-BTC_PMA catalyst. The kinetics study also evaluates the rate-determining step and activation energy for the multistep oxidation reactions.

Immobilizing nanozymes on 3D-printed metal substrates for enhanced peroxidase-like activity and trace-level glucose detection

“…In continuation of our recent computational studies on the catalytic activities exhibited by various metal active sites in porous metal organic framework and polyoxometalate composite materials (Cu-BTC_PMA) and thermally degraded MIL88B, 40,41 in this study, we investigated the enzymatic peroxidase properties of degraded MIL88B via quantum chemistry approaches. 42,43 Recently, we examined the thermal transformation of MIL88B MOF and its catalytic role in accelerating the conversion of CO 2 into acetic acid, in conjunction with molecular modelling.…”

Ce‐Modified MgFe−LDH Supported Au Particles: An Efficient Catalyst for Base‐Free Selective Oxidation of 5‐Hydroxymethylfurfural to 2,5‐Furandicarboxylic Acid

“…[7,80] In a recent investigation, a composite catalyst comprising a Cu-containing metal-organic framework (CuÀ MOF) and a polyoxometalate (CuÀ BTC_PMA) was examined for the base-free selective oxidation of HMF to FDCA (entry 9). [81] Even though a high oxygen pressure (20 bar) was used during this reaction, the excellent catalytic performance along with the unique catalytic structure that eliminates the need for noble metals, presents an intriguing avenue for future developments in environmentally friendly catalysts for HMF oxidation.…”

High Performance Polyoxometalate-Stabilizing Pt Nanocatalysts for Quinoline Hydrogenation with Water-Mediated Dynamic Hydrogen

“…Polyoxometalates (POMs) are a type of metal oxide cluster (typically W 6+ , Mo 6+ , Nb 5+ and V 5+ ) with unique structures and properties, including acidity, basicity, redox and photochemical properties. − POMs have the well-defined structures and distinct sizes; therefore, they represent ideal systems for understanding the role of solid state metal oxides in catalysis. , Especially, lacunary POMs as inorganic multidentate ligands can stabilize effectively Ru single-atom, atomically precise Ag nanoclusters, and Pt, Au nanoparticles, etc., which act as catalysts exhibiting great application potential in catalytic hydrogenation, oxidation and carbon dioxide conversion. − Notably, it has been demonstrated that oxygen vacancies could form as one of the important defects in metal oxide clusters, which played a crucial role in the catalytic reactions, as revealed by both theoretical calculations and experimental observations. − In this scenario, oxygen vacancies in POMs can facilitate oxygen activation and boost the oxidation of various intermediate in oxidation of 5-hydroxymethylfurfural . On the other aspect, the oxygen vacancies in reduced polyoxovanadate-alkoxide can mediate the catalytic deoxygenation of styrene oxide to styrene .…”