Density-functional theory characterization of acid sites in chabazite

Abstract

The nature of the acid sites in zeolites and the factors contributing to enhanced catalytic activity have been the subject of much study in the literature. In particular, the issue of whether all of the acid sites in a particular zeolite are homogeneous or heterogeneous in acid strength requires the development of a systematic way to quantify acidity. To address this, we performed a detailed density-functional theory (DFT) investigation of the reactivity of the acid sites in the zeolite chabazite. We calculated energies of adsorption of bases, deprotonation energies, and vibrational frequencies on a periodic chabazite (SSZ-13) model with various loadings of acid sites per unit cell, and with various structural framework defects. We found that the four acidic oxygens at the aluminum T site all have roughly the same proton affinity, and the deprotonation energy is not correlated to the O H bond length or vibrational stretch frequency. Furthermore, we found that the adsorption energy of various bases at each acid site oxygen is roughly the same and correlated only to the gas-phase proton affinity of the base; it does not vary significantly with acid-site concentration or framework defects near the acid site. Given the range of local chemical structure that we investigated, our results suggest that the strength of the acid sites in chabazite is not influenced significantly by chemical or structural variations in the framework near the acid site.