Dimethoxymethane carbonylation and disproportionation over extra-large pore zeolite ZEO-1: Reaction network and mechanism
Methyl methoxyacetate (MMAc) and methyl formate (MF) can be produced directly by heterogeneous zeolite-catalyzed carbonylation and disproportionation of dimethoxymethane (DMM), with near 100% selectivity for each process. Despite continuous research efforts, the insight into the reaction mechanism and kinetics theory are still in their nascent stage. In this study, ZEO-1 material, a zeolite with up to now the largest cages comprising 16×16-MRs, 16×12-MRs, and 12×12-MRs, was explored for DMM carbonylation and disproportionation reactions. The rate of MMAc formation based on accessible Brönsted acid sites is 2.5 times higher for ZEO-1 (Si/Al = 21) relative to the previously investigated FAU (Si/Al = 15), indicating the positive effect of spatial separation of active sites in ZEO-1 on catalytic activity. A higher MF formation rate is also observed over ZEO-1 with lower activation energy (79.94 vs. 95.19 kJ/mol) compared with FAU (Si/Al = 30). Two types of active sites are proposed within ZEO-1 zeolite: Site 1 located in large cages formed by 16×16-MRs and 16×12-MRs, which is active predominantly for MMAc formation, and Site 2 located in smaller cages for methyl formate/dimethyl ether formation. Kinetics investigation of DMM carbonylation over ZEO-1 exhibit a first-order dependence on CO partial pressure and a slightly inverse-order dependence on DMM partial pressure. The DMM disproportionation is nearly first-order dependence on DMM partial pressure, while it reveals a strongly inverse dependence with increasing CO partial pressure. Furthermore, ZEO-1 exhibits good catalytic stability, and almost no deactivation is observed during the more than 70 h test with high carbonylation selectivity of above 89%, due to the well-enhanced diffusion property demonstrated by intelligent-gravimetric analysis.
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