An environment-friendly and acid-degradable polymer templated synthesis of single-crystal hierarchical zeolites

Abstract

Hierarchical zeolites show great promise in green chemical and petrochemical processes, but their syntheses usually involve the use of nitrogen-/halogen-containing templates whose removal from the as-synthesized zeolites via calcination inevitably leads to serious environmental pollution, high energy consumption and sometimes destruction of zeolite frameworks. We report an eco-friendly and acid-degradable polymer denoted as PK3 as a multifunctional template to synthesize single-crystal hierarchical zeolites. It is shown that by strongly interacting with zeolite aluminosilicate gel precursors, PK3 acts as a structure-directing agent for the nucleation and crystallization of the amorphous gel into protozeolite crystals, and via its adsorption on the surfaces of protozeolite crystals, PK3 greatly inhibits their growth and thereby results in the formation of zeolitic nanocrystals. Furthermore, these thermodynamically unstable nanocrystals encapsulating PK3 spontaneously aggregate, then align in an oriented manner, and at last ripen, leading to the formation of single-crystal zeolite particles with interconnected mesopores. Importantly, the removal of PK3 from the as-synthesized zeolites can be achieved by a simple acid treatment. Thanks to these unique characteristics of PK3, a nitrogen-/halogen-free and combustion-free route is successfully achieved for synthesizing hierarchical MFI, MOR and GME zeolites. Moreover, the synthesized MFI zeolite exhibits superior catalytic activity in cracking low-density polyethylene.