Modulating hierarchical ZSM-5 crystals from etching-assisted growth with substituted phenols

Abstract

The rational modulation of zeolite microstructures tailoring crystal morphology and hierarchy, is an effective way to promote their mass transport and catalytic performance. However, practical realization remains challenging. We recently demonstrated a novel etching-assisted crystallization process that allows fine control over zeolite architecture by eliminating intermediate defective nanocrystals with active proton-bearing organic molecules such as phenol. Here, we investigated in detail the phenolic etching effect on the crystallization of ZSM-5 zeolites using a combination of techniques including diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) and 2D 27Al multiple quantum magic angle spinning nuclear magnetic resonance (MQ MAS NMR) and systematically elucidated the structure modulation with a homologous series of phenolic compounds. Time-series characterizations confirmed the initial seed formation, intermediate dissolution, and final perfected crystallization stages during ZSM-5 synthesis in the presence of phenolic compounds. Since deprotonation to phenolate anion in-situ is required during alkaline zeolite synthesis, the electron-withdrawing (EW) groups on the phenol para-position lead to bigger ZSM-5 crystals with more mesopores, while the phenolic molecules with para- electron-donating (ED) groups exhibit less significant effect. Under the same initial pH, the phenolic compounds show unparalleled compatibility with zeolite crystallization by stabilizing the system pH values, whereas inorganic acids invariably fail the zeolite crystallization. Given the remarkable kinetic control over zeolite growth through the simultaneous but opposite nucleophilic etching process, our observation provides a versatile way to manipulate zeolite crystallization with high tunability.