Carbonylation of Dimethyl Ether to Methyl Acetate over SSZ-13

Abstract

The small-pore zeolite with the chabazite framework topology, SSZ-13, is found to be an active catalyst in the carbonylation of dimethyl ether to methyl acetate (MA). The production of MA over SSZ-13 after 24 h on stream at 165 °C and 1 bar approaches that obtained from mordenite and is significantly higher than from ferrierite at comparable Si/Al of ca. 10. To understand the origin of the activity, SSZ-13 materials are synthesized with variable Si/Al, characterized via several techniques including multinuclear magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and evaluated for their carbonylation activity. While MA production rates increase with decreasing Si/Al, the correlation is nonlinear due to the effect of Si/Al on the acid site distribution within different confining environments, and the associated impact of the latter on the rate-determining transition state barrier. Enhanced MA production rates trend with acid sites located at the eight-membered ring (8MR) that are increasingly populated as framework Al content increases. Density functional theory analyses of transition state energies as a function of active site location support the experimental findings, where the lowest apparent barriers are associated with the methoxy groups that orient within the plane of the 8MR window. This is due to an optimal charge stabilization of the cationic transition states with the negatively charged oxygens within the 8MR window. The effects of catalyst chemical composition, separate from framework topology, are also investigated using SAPO-34 (the silicoaluminophosphate analog of SSZ-13). Analyses of the 1 H MAS NMR signals and carbonylation activity suggest that the higher acid site strength of SSZ-13 compared to that of the SAPO material is required for effective Brønsted acid catalysis of Koch-type carbonylation pathways.

Analysis and control of acid sites in zeolites

“…The fact that this relation is not linear is owing to the highest methylacetate production rates on methoxy groups residing in the 8MRs. The lowest carbonylation rates are attained for the methoxy groups in the CHA cage above the 6MR, as predicted by the DFT calculations [218].…”

“…Recently, Lusardi et al prepared a series of SSZ-13 materials with varied aluminium content, and hence the materials presented framework Si/Al ratio ranging from 9 to 35 [218]. As the Al amount increased, both the number of Brønsted acid sites and extra-framework Al sites increased too.…”

Formation of HCO+ ion by protonation of carbon monoxide in zeolites: the origin of catalytic activity in methanol carbonylation

“…In summary, we show unprecedented formation of super-electrophilic HCO + ions in zeolite H-MOR under mild conditions. We suggest it is the catalytically active intermediate for methanol carbonylation (and not CH3 + ions as suggested in all previous studies on solid catalysts [8][9][10][11]). HCO + can be observed with 13 C NMR studies presumably due to favorable energetics (confinement) in 8-membered-ring side-pockets of H-MOR.…”

Initial Carbon−Carbon Bond Formation during the Early Stages of Methane Dehydroaromatization

“…This same observation was also observed for unreacted methane (Figure b and b), as its multiple adsorbed forms were also identified and hence, further advocates for the local heterogeneity within the material. The existence of surface‐formate species, i.e., the 13 C correlations observed at 170–173 ppm correlating with a 1 H‐signal at ≈8 ppm (Figure d), is indicative of the occurrence of CO insertion, which is known to generate hydrocarbon pool species during the zeolite‐catalyzed MTH process (see below) . However, it should also be mentioned that these species can only be formed in the very early stages of the reaction making use of O atoms present in the zeolite and metal oxide Mo precursor (cf.…”