Cs, V, Cu Keggin-type catalysts partially oxidize 2-methyl-1,3-propanediol to methacrylic acid

Abstract

Heteropoly acids are among the best catalysts to produce carboxylic acids from light hydrocarbons. The Keggin structure maintains metals at a high oxidation particularly in the presence of cesium, vanadium, and copper. They effectively catalyze the partial oxidation of 2-methyl-1,3-propanediol (2MPDO)—a co-product in the hydroformylation of allyl alcohol to 1–4 butanediol. Converting 2MPDO to methacrylic acid would improve the economics of this route to 1–4 butanediol. Here, we establish the relationship between heteropoly acid performance and calcination temperature at 300 °C and 350 °C. The molybdenum oxidation state was higher after the high temperature treatment but it reduced to a greater extent during reaction (from Mo6+ to Mo5+). The higher extent of reduction confirms the higher O2− → Mo6+ charge-transfer in the Keggin anion. Maximum MAA and MAC selectivities were 61% and 28%, respectively, at 250 °C after 480 min over cycling oxidation–reduction and 2MPDO/O2/Ar = 13/10/77 as the best reaction condition and feed composition. Though, the route of direct dehydration of 2MPDO (C–OH bond cleavage) followed by oxidation to MAC is the most direct path to the products detected, we propose a mechanism that passes through the homolytic dissociation of the tertiary C–H bond,considering that bonding energies energies are very similar.