Direct cracking of bulky molecules such as heavy oil and biomass to produce value-added compounds (e.g., light olefins) requires catalysts with sufficiently large pore sizes and strong acidity. Conventional zeolite-based catalysts are not suitable for these reactions, suffering from the limitation of small pore sizes. We developed a completely new approach to synthesize a series of hierarchically structured mesoporous zeolites with various pore structures to tangle this issue, effectively cracking bulky raw reactants to produce light olefins with high yields.
Our synthetic strategy features the use of non-surfactant polymers as dual-functional templates for the fabrication of hierarchical zeolites. With this strategy, we are able to prepare hierarchical zeolites through one-pot synthesis by functionalizing the polymer template with different structure-directing groups. In addition to creating mesopores in zeolite crystals, our strategy has advantages in many other aspects. First, the minimal intermolecular interactions of non-surfactant polymers impose little interference on the crystallization of zeolites, favoring the formation of three-dimensionally continuous zeolite frameworks with a long-range order. Second, the mutual interpenetration of the polymer and the zeolite networks renders disordered but highly interconnected mesopores in zeolite crystals. These two factors allow for the synthesis of single-crystalline, mesoporous zeolites of varied compositions and framework types. A representative example, meso-ZSM-5, has been carefully characterized. It has a unique branched fibrous structure, and far outperforms bulk ZSM-5 as a catalyst in two model reactions: conversion of methanol to aromatics and catalytic cracking of canola oil. Third, extra functional groups in the polymer template can be utilized to incorporate desired functionalities into hierarchical zeolites. Last and most importantly, polymer-based templates permit heterogeneous nucleation and growth of mesoporous zeolites on existing surfaces, forming a continuous zeolitic layer. In a proof-of-concept experiment, we synthesized unprecedented core-shell-structured hierarchical zeolites by coating mesoporous zeolites on the surfaces of bulk zeolites.
We are also interested in fundamental scientific questions, e.g., the influence of mesoporosity in zeolites on their catalytic performance for different reactions, and the deactivation mechanisms of zeolite-based catalysts.