Reaction mechanism of toluene tert-butylation with tert-Butyl alcohol over H-β and H-MOR zeolite: A QM/MM study

Abstract

The tert-butylation mechanisms of toluene with tert-butyl alcohol catalyzed by H-β and H-MOR were systematically investigated using the own-N-layered integrated molecular orbit+molecular mechanics (ONIOM) method in conjunction with density functional theory (DFT) calculations. Based on structural parameter analysis and relative free energy results, the stabilities of the co-adsorbed complex and transition state configurations along the tert-butylation paths were determined, and the catalytic performance of the zeolites was comparatively studied. In 116T ONIOM2 models, H-MOR shows better catalytic activities for toluene tert-butylation than H-β due to a lower free energy barrier. With the temperature increasing from 298 to 453 K, the barriers of toluene tert-butylation decreased for the both catalysts, implying that the temperature played an important role in improving the toluene conversion. The H-β and H-MOR zeolites were both more selective to the desired 4-tert-butyltoluene through a more stable C4-tert-butylation transition state structure as compared to the 3-tert-butyltoluene. Computational relative reaction rate constants for toluene tert-butylation reaction indicated that 4-tert-butyltoluene was the kinetically preferred product. H-β was found to be more para-selective than H-MOR.