First-principles molecular modeling of structure-property relationships and reactivity in the zeolite chabazite

Abstract

Zeolites are crystalline, porous aluminosilicates; while a pure silicate structure is charge-neutral, the substitution of A1³⁺ for Si⁴⁺ creates in the framework a negative charge, which can be compensated by a proton that acts as a strong, acid-donating Bronsted site. Zeolites are widely used in industry, most commonly for catalysis and separations. Unfortunately, they have not yet been able to replace all homogeneous catalysts in industrial processes due to the difficulties in reactant and product diffusion to and from the zeolite surface in the absence of a solvent. However, it is believed that if we had a thorough understanding of how solid acids, especially zeolites, catalyze reactions, then we would be able to design heterogeneous catalysts to overcome these difficulties. 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, a detailed density functional theory (DFT) investigation of the reactivity of the acid sites in the zeolite chabazite was performed. Energies of adsorption of bases, deprotonation energies, and vibrational frequencies were calculated on a periodic chabazite (SSZ-13) model with various loadings of acid sites per unit cell, and with various structural framework defects. The four acidic oxygens at the aluminum T-site were found to 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, the adsorption energy of various bases at

(cont.) each acid site oxygen was found to be 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, these 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. A comprehensive methodology was also developed and implemented for studying the mechanism for the coupling reaction of two methanol molecules to form ethanol and water in the zeolite chabazite. This test reaction models an initial carbon-carbon bond formation, which is thought to be the rate limiting step in the industrial methanol-to-gasoline and methanol-to-olefins processes. Transition path sampling and constrained molecular dynamics, within the Car-Parrinello approach, were used to study this reaction. A new mechanism was found for the carbon-carbon bond formation, which proceeds at 400⁰C via stable intermediates of water, methane, and protonated formaldehyde. The carbon-carbon bond forms directly and concurrently with a proton transfer from methane to water. This mechanism does not involve the formation of dimethyl ether or surface methoxy groups at the acid site, as previously postulated. Also, the free energy barriers for the reaction in chabazite were compared to the free energy barriers ...