Exploring the effect of polyacrylamide/graphene oxide composite hydrogel on the synthesis and application of ZSM-5 molecular sieves

Abstract

Herein, we demonstrate the synthesis of coffin-shaped ZSM-5 molecular sieves in polyacrylamide (PAM)/graphene oxide (GO) composite hydrogel using the hydrothermal method. The as-synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS), and Thermogravimetry analyses (TGA). The effect of PAM and GO addition on the morphology and structure of ZSM-5 was investigated. The results indicated that the introduction of PAM and GO promoted the growth of the ZSM-5 (101) plane. Moreover, samples synthesized in the PAM/GO hydrogel exhibited a longer c-axis and a more pronounced coffin-shaped morphology compared to samples synthesized solely in PAM hydrogel. The Si/Al ratio decreased with the addition of GO, possibly due to interface effects leading to the formation of ZSM-5 precursors with a higher Al content. TG, FT-IR, and XPS characterization revealed hydrogen bonding interactions between PAM and GO, and the addition of GO likely restricted the movement of polymers, improving cross-linking between monomers and forming a more uniform network structure. The sample G-15 (P-5) exhibited higher mesopore volume and average pore diameter, contributing to enhanced catalytic performance. However, with increasing GO content, the average pore diameter decreased, potentially due to functional groups on the surfaces of graphene oxide and polyacrylamide forming a more uniform and structurally stable three-dimensional network structure. This allowed for controlled modulation of the molecular sieve morphology and pore size. The introduction of PAM/GO hydrogel resulted in a moderate amount of weak acid and an increased amount of strong acid, both shifting to lower temperatures. The ZSM-5 molecular sieves synthesized in the PAM/GO composite hydrogel demonstrated outstanding MTO catalytic performance, exhibiting high selectivity for light olefins and an extended catalytic lifespan. This work is expected to provide new ideas and methods for developing efficient ZSM-5 catalysts.