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 non-noble 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.