Renewable production of acrylic acid and its derivative: New insights into the aldol condensation route over the vanadium phosphorus oxides

Vanadium phosphorus oxides (VPOs) fabricated by employing poly ethylene glycol (PEG) additive were used as catalysts for efficient conversion of acetic acid (methyl acetate) and formaldehyde to acrylic acid (methyl acrylate). The highest formation rate (19.8 μmol g_cat^-1 min ^-1) of desired products (acrylic acid + methyl acrylate) was accomplished over a VPO catalyst comprising mainly vanadyl pyrophosphate ((VO)₂P₂O₇) and vanadyl phosphate in δ form (δ-VOPO₄). This catalyst is nearly three times more active than the analogue reported in literature. The VPO catalyst activated in 1.5% butane-air is superior to that activated in air or nitrogen. Different from the PEG-derived VPO catalysts for n-butane oxidation to maleic anhydride, a better VPO catalyst for the current reaction requires a higher fraction of δ-VOPO₄ entity and contains the medium strong acid sites of high density. Through systematic catalyst characterizations and evaluations, an unambiguous correlation between catalyst structure/constitution and performance was established.