Insights into the Reaction Network and Mechanism of Green Aerobic Oxidative Esterification of Methacrolein over Different Heterogeneous Catalysts

The oxidative esterification of methacrolein (MAL) is an important way to prepare high-valued methyl methacrylate (MMA), but this process is ultra-complex due to the high reactivity of both C=O and C=C bonds in MAL molecule. In order to further improve MMA selectivity, the reaction network and relevant mechanisms have been proposed and profoundly investigated in this paper. Five kinds of fundamental reactions are involved in this process, including (a) the acetal reaction; (b) the aerobic oxidation of hemiacetal; (c) the alkoxylation of C=C double bond; (d) the Diels-Alder reaction; and (e) the hydrogenation reaction of unsaturated double bond. Among them, the Diels-Alder reaction of MAL is non-catalyzed, and the Br?nsted acid sites or the Lewis acid sites favor promoting acetal reaction of MAL with methanol, while the alkoxylation of C=C bond with methanol is enhanced under alkaline condition. In particular, by employing the Pd-based catalysts, hydrogenation products are formed in alkaline methanol solution, hence with lower than those obtained by the Au-based catalysts. Notably, it is necessary to match the hemiacetal fromation and aerobic oxidation of hemiacetal, which is relevant with the amount and strength of acid and redox sites. Consequently, this work can provide a good guidance for the further design of both catalysts and processes in future.