Facile synthesis of ultrafine nanosized ZSM-5 zeolite using a hydroxyl radical initiator for enhanced catalytic performance in the MTG reaction

Abstract

ZSM-5 zeolite is extensively used as a catalyst in the methanol-to-gasoline (MTG) reaction, but many secondary reactions occur because of its large grain size and strong acidity. Thus, it is vital to explore new methods for the preparation of nanosized ZSM-5 zeolite with intercrystalline mesopores to facilitate the diffusion of substances while simultaneously decreasing the acidity. Here, we present a novel procedure for the synthesis of ultrafine nanosized ZSM-5 zeolites (Z5-Sx) at a low temperature (90 °C) that includes the addition of a hydroxyl radical initiator to the initial gel. For comparison, a series of ZSM-5 zeolites (Z5-Ty) were synthesized by crystallization at 170 °C using the traditional hydrothermal method. The influence of hydroxyl radicals on the characteristic properties of the Z5-Sx zeolites was investigated by XRD, SEM, HRTEM, N2 adsorption, and 29Si and 27Al MAS NMR spectroscopy as well as NH3-TPD and Py-IR measurements. The results revealed that the Z5-Sx series samples had smaller crystal sizes and higher mesoporosity than the control samples. In particular, a high content of framework Al atoms was observed at the channel intersections of the Z5-Sx series samples, resulting in a weaker acid strength in comparison with the Z5-Ty samples. This distribution of Al atoms resulted in the excellent catalytic performance of the synthesized Z5-Sx catalysts towards the MTG reaction, and the highest gasoline yield of 65.5% with a methanol conversion of 97.2% was obtained in the reaction with the Z5-S3 catalyst. Markedly, the Z5-S3 sample showed significantly improved catalytic stability and coke tolerance than the control samples.