Coke deposition study on Cs-Zr/SiO2 catalysts in aldol condensation of methyl propionate with formaldehyde

The Cs-Zr/SiO 2 exhibits excellent catalytic performance on the aldol condensation of methyl propionate with formaldehyde to methyl methacrylate (MMA), however, the catalyst deactivation caused by coking still remains challenging. In this work, the coke deposition induced deactivation behavior of Cs-Zr/SiO 2 samples prepared by ion-exchange and impregnation methods were systematically investigated using NMR, Raman spectroscopy, physical N 2 adsorption–desorption, HR-TEM and TG analysis. The results show that the catalyst prepared by the ion-exchange strategy mainly forms removable amorphous carbon of aliphatic species and polycyclic aromatic hydrocarbons, whereas the impregnated sample tends to evolve from similar sources into carbonaceous compounds with higher graphitization degree and structural stability. Further analysis of the deactivation mechanism indicates that different amounts and types of coke deposited on the catalyst surface induce pore blockage, ultimately leading to catalyst deactivation. MMA is verified as the primary source of coke formation, with proposal of detailed pathway, through comparative experiments using different feed streams. Furthermore, a kinetic model for carbon deposition on the catalyst prepared by ion-exchange method was established by modification of classical Voorhies equation with feed component, and the apparent activation energy for coke formation is determined as 79.4 kJ/mol.