Zeolite Structure and Reactivity by Combined Quantum-Chemical−Classical Calculations

Abstract

Proton-energy differences, ammonia adsorption, and D/H-exchange barriers for methane at selected isolated Brønsted sites in zeolites FAU, MFI, BEA, ERI, and CHA are studied by combined quantum-chemical−classical (QM/MM) calculations in an attempt to understand the factors that determine the reactivity at these Brønsted sites. The barrier of the D/H-exchange reaction for methane was found to correlate well with the calculated ammonia chemisorption energy, but even better with the O−Al−O angle of the free zeolite Brønsted site the reaction is taking place on, provided the Si−O−Al−O−Si moiety over which the reaction takes place is more or less collinear. The barrier is considerably higher if this collinearity is weaker, which may be explained by the necessity of costly backbone distortions to accommodate the geometrical requirements of the transition state. This is confirmed by similarly strong correlations with the O−Al−O angle change going from the free acid site to zeolite−ammonium ion bidentate structures, which may be thought of as a measure of the backbone distortion. A new measurement of the D/H-exchange barrier in BEA is also reported. It was found to be 88 ± 18 kJ/mol, lower than the experimental barriers in both FAU and MFI.