Enhancing hydrophobicity and catalytic activity of nano-Sn-Beta for alcohol ring opening of epoxides through post-synthetic treatment with fluoride

Abstract

Elucidation of synthesis-structure–reactivity relationships can help to enhance the catalytic performance of zeolites for a broad range of reactions. Considerable focus has been on examining structure–reactivity behavior of the catalytic site, but it has become increasingly evident that molecular interactions beyond the catalytic site in the outer sphere impact catalytic activity. The key challenge is to create synthetic methods that can create well-defined catalytic materials with control over catalytic sites and the catalytic reaction environment. After synthesizing nano-Sn-Beta using a hydrothermal method involving hydroxide ions that produce a hydrophilic material, these materials are treated with tetraethylammonium fluoride (TEAF) using mild conditions to produce a hydrophobic framework while maintaining the small particle size. The fluoride treated material is determined to be more active in the epoxide ring opening reaction than the as-synthesized nano-Sn-Beta (90% increase in TOF0), indicating that a hydrophobic framework is beneficial for the alcohol ring opening of epoxides. This is supported by site quantification experiments and an analysis of residual fluorine, which is found to have limited effect on activity. When accounting for the fraction of active sites, it is demonstrated for the methanol ring opening of epichlorohydrin that all hydrophobic materials are not diffusion limited under the reaction conditions and have the same turnover frequencies, which are higher than the turnover frequency of hydrophilic catalysts. Overall, this work reports a technique to improve the hydrophobicity of zeolite beta and demonstrates the importance of a hydrophobic reaction environment for the alcohol ring opening of epoxides.