Distribution of Brønsted acid sites on beta zeolite H-BEA: A periodic density functional theory calculation

Abstract

We performed a density functional theory calculation of a beta zeolite (H-BEA) crystal unit cell containing two Al atoms and two protons. Two protons were put in a zeolite framework in order to compensate the excess charge caused by the substitution of Si (4+) atoms by Al (3+) atoms. These protons act as Brønsted acids. Thus, our unit cell model contains two Brønsted acids distributed among possible T sites of zeolite framework. The overall structure was optimized in a periodic boundary condition. In the most stable structure, we found that these two Brønsted acid sites were mutually spatially very close. In addition, we found in this stable structure that two protons were shared by a hydrogen bond between these Brønsted acid sites. This hydrogen bond was revealed to be a cause of the stability of the most stable positioning of the two Brønsted acid sites.