Cooperativity of adjacent Bronsted acid sites in MFI zeolite channel leads to enhanced polarization and cracking of alkanes

The spatial proximity between Bronsted acid sites (BAS) in HZSM-5 zeolites is demonstrated by H-1 double quantum (DO) MAS NMR measurements. This proximity results in more pronounced polarization of adsorbed acetone and alkanes in zeolites, evident from C-13 MAS NMR spectra. The adjacent BAS with synergistic interactions on alkane reactants (propane, n-butane, and n-pentane) cooperatively catalyze alkane cracking at higher turnover rates than on isolated BAS. Apparent activation energies are similar on HZSM-5 catalysts with different concentrations of isolated and adjacent BAS, while apparent activation entropies become less negative at higher BAS concentrations. Kinetic experiments in conjunction with adsorption measurement and DFT calculations prove that cracking rates at these Al-site pairs are mainly due to more positive intrinsic activation entropies, suggesting that the protonation transition state occurs later along the reaction coordinate on adjacent BAS. Adjacent Bronsted acid sites favor cracking over dehydrogenation and favor central cracking over terminal cracking. (C) 2017 Elsevier Inc. All rights reserved.