Core-shell SAPO-34@ZSM-5 composite via in situ solid-solid transformation of pre-coating MCM-41 shell and its application in methanol-to-olefins

Abstract

Hierarchically core-shell-structured SAPO-34@ZSM-5 composite zeolite was successfully achieved by an in situ solid-solid transformation of a pre-coating MCM-41 shell layer with the help of ZSM-5 seeds under a traditional hydrothermal crystallization route. As a result, a uniform nano-sized polycrystalline ZSM-5 shell closely grows on the core SAPO-34 microsphere, thus forming a desirable core-shell structured zeolite composite. Besides, the SAPO-34 core and the polycrystalline ZSM-5 shell are intimately connected through mesosilica substrates as a “bridge” which guarantees the integrity of the core-shell structure very well, even under harsh ultrasonic treatment at a high frequency. More crucially, this innovative method effectively overcomes the chemical and structural incompatibility between the core crystals and the shell zeolite during synthesis, and simultaneously avoids the collapse and dissolution of SAPO-34 crystals mainly derived from the shielding effect of MCM-41 shell layer under a highly alkaline gel precursor yielding ZSM-5. Furthermore, the methanol-to-olefins (MTO) was selected as a probe reaction so as to investigate the catalytic performance of the core-shell SAPO-34@ZSM-5. It is found that besides obtaining high light olefins yield (12.3% more than that on the reference catalyst), the catalytic lifetime over core-shell SAPO-34@ZSM-5 catalyst is greatly improved, about 3 times as long as that over the pristine SAPO-34 and 2.6 times as long as that on the corresponding physically-blended SAPO-34+ZSM-5. The superior catalytic performances on the core-shell SAPO-34@ZSM-5 can be attributed to the synergetic effects between SAPO-34 (core) and ZSM-5 (shell) including different framework structures, moderate acidic properties, and well-defined hierarchical architectures.