Catalysis Today, Vol.345, 165-174, 2020
Identification of the most active sites for tetrahydropyranylation in zeolites: MFI as a test case
Tetrahydropyranylation (THP) has been extensively studied experimentally with both Bronsted and Lewis acid-based catalysts, such as zeolites, considered. However, our atomic-level understanding of the underlying mechanisms of different types of catalytic sites remains limited, particularly regarding zeolites. Thus, we combined an experimental catalytic study with density functional theory (DFT) calculations to identify the active sites and corresponding reaction mechanism in MFI zeolite. Both experimental and computational data clearly show that Bronsted acid sites are more reactive than Lewis acid sites. Furthermore, calculated reaction barriers for Bronsted acid site catalysis (5 kcal mol(-1)) are much lower than those for the diffusion of bulky products in microporous channels (approximately 20 kcal mol(-1)). In full agreement with theoretical calculations, the results of Madon-Boudart test clearly show that the effect of diffusion on the H-MFI catalysts performance cannot be neglected even at low temperature. Therefore, the diffusion becomes the rate-determining step. Overall, our findings suggest that designing zeolites with improved THP catalysis performance requires focusing on acid zeolites with Bronsted acid sites of average strength with facilitated diffusion, e.g., due to auxiliary mesoporous system.