Unraveling the Coking and Deactivation Behavior of Al–Cs/SiO2 Catalyst in the Aldol Condensation of Methyl Propionate with Formaldehyde

The loaded acid–base bifunctional catalysts exhibit good catalytic performance in the synthesis of methyl methacrylate via aldol condensation of methyl propionate with formaldehyde, but they still suffer from deactivation caused by coke deposition. In this paper, coke deposition as well as consequential deactivation behavior of Cs/SiO2 catalyst loaded with various amounts of Al was investigated under different reactant compositions by using nuclear magnetic resonance, thermogravimetric, Brunauer–Emmett–Teller, and Raman analysis. As a result, coke on the catalyst was characterized by amorphous coke deposition, which was mainly composed of polycyclic aromatic hydrocarbons and aliphatic substances. Al added to the 10Cs/SiO2 catalyst significantly reduced the amount of coke deposition and inhibited the C/H ratio of coke. Appropriate amounts of Al and reactant compositions could ultimately increase the stability of the catalyst. Moreover, the kinetics of coke deposition observed on the Al–Cs/SiO2 catalyst was studied. It was found that the Voorhies equation modified using the variables of reaction temperature and reactants composition could accurately forecast coke accumulation on the 3.5Al-10Cs/SiO2 catalyst under specific conditions.