Surface-Protection-Induced Controllable Restructuring of Pores and Acid Sites of the Nano-ZSM-5 Catalyst and Its Influence on the Catalytic Conversion of Methanol to Hydrocarbons.

Abstract

Creating mesopores for the nano-ZSM-5 catalyst could further promote the diffusion of molecules in its micropores and improve the catalytic activity and stability. Inorganic alkali treatment of ZSM-5 usually removes internal silica for the existence of an aluminum distribution gradient and leads to a hollow structure. Herein, surface TPA+ adsorption-induced protective desilication and recrystallization successively occurred during hydrothermal treatment, and controllable mesopore fabrication was achieved. The evolution of mesopores and acid sites was characterized by N2 physisorption, XRD, XRF, TEM, NH3-TPD, Py-IR, 27Al MAS NMR, 29Si MAS NMR, and TG techniques. It was found that the TPAOH concentration influenced the formation of internal cavity and mesopores in the shell. Introducing TPABr into TPAOH solution increased the surface protection because of the increased TPA+ adsorption, and coated hollow ZSM-5 was obtained. The acidity was restructured during the above mesopore fabrication. High-concentration TPAOH solution promoted the insertion of destructive Al into the skeleton structure to form strong acid sites, and the catalytic lifetime was recovered and even obviously prolonged. This reflected the key role of strong acid sites on the catalytic performance. Applying hollow nano-ZSM-5 with a mesoporous shell and strong acidity increased the lifetime by 50% and the conversion capacity for liquid hydrocarbon by 20% compared to the parent sample.