Unraveling High Chemoselectivity in Oxidative Dehydration of Glycerol into Acrylic Acid over a Bifunctional Metal–Organic Framework–Polyoxometalate Composite Catalyst

The selective conversion of glycerol into acrylic acid (AA) using polyoxometalate catalysts poses a challenge due to low conversion and selectivity, as the acrolein intermediate is not effectively stabilized on the catalyst surface. In this study, we introduce a novel approach to develop a bifunctional active catalyst that incorporates dehydration and oxidation active sites while also confining these sites within a porous material to enhance product selectivity. A unique synthetic method was devised to create such a bifunctional catalyst (MIL88B-800PMA) comprising a metal–organic framework (MIL88B) and polyoxometalate (PMA). These catalysts demonstrated 100% conversion with 96.4% selectivity toward AA under mild reaction conditions. The complete encapsulation of PMA can account for the high catalytic activity within the pores of the MOF, facilitating the chemical bond formation and structural modulation between PMA and the MOF, as confirmed by detailed X-ray absorption spectroscopy (XAS) analysis. The presence of oxidative species such as Mo6+ and Fe2+, along with oxygen vacancies, promotes the highest catalytic conversion and selectivity. A comprehensive kinetic study, including in situ ATR-IR analysis, illustrates the formation and stabilization of the specific intermediate acrolein, guiding the selective formation of AA. This finding is corroborated by theoretical quantum chemistry calculations, offering a quantitative understanding of the surface interaction between glycerol and the Brønsted acid sites present on the MIL88B and PMA composite catalysts. Additionally, the activation energy of the synthesized catalysts is evaluated, aiding in the comprehension of the underlying reasons for their high catalytic activity.