Aluminum Location and Acid Strength in an Aluminum-Rich Beta Zeolite Catalyst: A Combined Density Functional Theory and Solid-State NMR Study

摘要

Density functional theory was performed to investigate the specific Al distribution and the origination of Bronsted acid strength in the Al-rich Beta zeolite catalyst. The most preferable sites for Al atoms of Al-rich and Si-rich Beta zeolites represented by 1Al and 2Al atoms are compared by electrostatic potential analysis and substitution energies. IT1 and T9 sites are the most favorable locations for 1Al distribution, while 5MR1-T92, 5MR2-T15 and 6MR1-T66 sites are inclined to be occupied by Al atoms for 2Al distribution. Al atoms in 5MR1-T17 sites would be dealuminated more easily to become the extra-framework Al species when Al-rich Na-Beta is ion-exchanged to H-Beta. As for NNNN sequences in Al-rich Beta, 6MR1-T66 sites are the most easily substituted by Al atoms, facing different channels and showing the properties of isolated Al site. Proton affinities, NH3 adsorption energies and H-1 chemical shifts of [D-5]pyridine adsorbed in Beta zeolites were used to analyze the Bronsted acidity. Si-rich Beta has stronger Bronsted acid strength than the Al-rich counterpart. This agrees with the experimental results from H-1 MAS NMR with [D-5]pyridine as probe molecule. The Bronsted acid strength of Al-rich and Si-rich H-Beta zeolites was correlated to the Al location at the specific T-site on the zeolitic framework.