Synthesis of hierarchical beta zeolite by using a bifunctional cationic polymer and the improved catalytic performance

Abstract

Hierarchical beta zeolites have been hydrothermally synthesized by using commercial cationic polymer PDADMA as both microporogen and mesoporogen. The influence of various synthetic parameters on the products was systematically investigated. Products with narrow SiO₂/Al₂O₃ ratios were obtained under static crystallization conditions. By employing rotational conditions and adding seeds in the initial gel, higher SiO₂/Al₂O₃ ratios over a wider range of 25 to 50 could be successfully achieved, which showed an obvious improvement. The ¹³C NMR, TG, XRF, N₂ sorption as well as molecular mechanics simulation results indicated that PDADMA was incorporated in the final product without decomposition, acting as a SDA for the formation of beta zeolite and mesoporogen simultaneously. Further studies on the crystallization process revealed that hierarchical structures templated by PDADMA had been formed in the early solid. The amorphous Si–Al species around the micropore channels gradually evolved to the beta structure with the assistance of PDADMA, whereas the mesopores formed in the initial period remained less changed. A solid-mediated mechanism is thus proposed for the synthesis. Characterization results showed that the obtained products had sphere-like morphology composed of 10–20 nm crystalline domains, high mesopore volumes, and large external surface areas. More importantly, the hierarchical beta zeolites exhibited greatly enhanced catalytic activity and stability in the cracking reaction of triisopropylbenzene.