Journal of Catalysis, Vol.370, 424-433, 2019
Promotion of protolytic pentane conversion on H-MFI zeolite by proximity of extra-framework aluminum oxide and Bronsted acid sites
The activity of strong Brensted acid sites in H-MFI zeolites for pentane cracking and dehydrogenation is enhanced by the presence of extra-framework aluminum in spatial proximity. The steric proximity of these two sites is deduced from the perturbation of OH groups on extra-framework aluminum oxide by pyridine or pentane adsorbed on Bronsted acid sites manifested in the IR spectra. The turnover frequencies of overall cracking and dehydrogenation on such sites are about 50 and 80 times higher than on isolated strong Brensted acid sites. While pentane does not adsorb stronger, the site pair stabilizes cracking and dehydrogenation transition states mainly via increasing the activation entropy. This is interpreted as a later transition state for the cracking and dehydrogenation. The results suggest that controlled steaming can be used to enhance catalytic activity of zeolite Brensted acid sites. (C) 2019 Elsevier Inc. All rights reserved.