Facile synthesis of hierarchical NaX zeolite from natural kaolinite for efficient Knoevenagel condensation

Abstract

Zeolite catalysts have found extensive applications in the synthesis of various fine chemicals. However, the micropores of zeolites impose diffusion limitations on bulky molecules, greatly reducing the catalytic efficiency. Herein, we explore an economic and environmentally friendly method for synthesizing hierarchical NaX zeolite that exhibits improved catalytic performance in the Knoevenagel condensation reaction for producing the useful fine chemical 2-cyano-3-phenylacrylate. The synthesis was achieved via a low-temperature activation of kaolinite and subsequent in-situ transformation strategy without any template or seed. Systematic characterizations reveal that the synthesized NaX zeolite has both inter-crystalline and intra-crystalline mesopores, smaller crystal size, and larger external specific surface area compared to commercial NaX zeolite. Detailed mechanism investigations show that the inter-crystalline mesopores are generated by stacking smaller crystals formed from in-situ crystallization of the depolymerized kaolinite, and the intra-crystalline mesopores are inherited from the pores in the depolymerized kaolinite. This synthesis strategy provides an energy-saving and effective way to construct hierarchical zeolites, which may gain wide applications in fine chemical manufacturing.