Hydrophobic, Nitrogen-Free Organic Molecule-Protected Alkali Desilication for Hierarchical Zeolites with High Crystallinity and Porosity

Desilication by alkali etching is generally used to prepare hierarchical zeolites to enhance their mass transfer properties; however, for high Si/Al ratio zeolites, the alkali etching would greatly reduce the framework crystallinity or even give rise to amorphous product. Though organic amines have been used as additives during alkali etching to protect the microporous framework, the problems of toxicity, volatility, high cost, and nitrogen-containing wastes induced by organic amines would hinder their scalable application. In this study, we provide a different viewpoint on the protective mechanism of zeolite micropores during alkali etching, and the attention is focused on the framework polarity, which has not been studied before. We develop a hydrophobic nitrogen-free organic molecule (such as acetophenone)-protected alkali etching method to prepare hierarchical zeolites with high crystallinity and high porosity, and the prepared hierarchical Y zeolites showed enhanced performance in the catalytic cracking of bulky molecules. This method is based on the principle that the hydrophobic micropores of high-silica zeolites could adsorb hydrophobic organic molecules from aqueous solution, which would fill in the zeolite micropores and prevent excessive OH^- damage to Si-O-Si bonds. Thus, our method is facile for scalable production without the tedious pretreatment of vacuum dehydration, organic amine impregnation, and drying before alkali etching. Furthermore, concerning the characteristics of a nitrogen-free process and low volatility, using acetophenone as a protective agent would reduce the cost and avoid nitrogen-containing wastes in practical applications.