Seed-assisted Template Control of ZSM-5 Structure for Mitigating Propylene Emissions over Cu/ZSM-5 Catalysts During Cold Start

Abstract

Cu/zeolite catalysts have attracted considerable attention for cold-start applications; however, their efficiency in propylene adsorption remains limited. To address this challenge, Cu/ZSM-5 catalysts with enhanced metal-support interactions were successfully constructed via a seed-assisted, template-mediated synthesis method, leading to improved propylene capture and oxidation efficiency. The catalysts were systematically characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), N₂ adsorption-desorption, scanning electron microscope (SEM), transmission electron microscope (TEM), NH₃-TPD, X-ray photoelectron spectroscope (XPS), and in situ infrared spectroscopy to investigate the influence of acid site density and metal-support interactions on propylene adsorption behavior. The results demonstrated that Cu@ZSM-5 catalysts synthesized with different structure-directing agents exhibited distinct acidity profiles and varying strengths of metal-support interaction. Among them, the Cu@ZSM-5-130 sample prepared using the TPAOH template showed a high specific surface area (474 m²/g) and a more optimal acidity balance. It was found that an excessive density of strong acid sites can be detrimental to propylene adsorption and oxidation. The intimately associated Cu species within the zeolite matrix facilitated selective adsorption of hydrocarbons, particularly propylene. In situ infrared spectroscopy further revealed a temperature-dependent oxidation pathway of propylene over the Cu@ZSM-5-TPAOH-130 catalyst, starting with epoxide formation at low temperatures, progressing to partial oxidation products (e.g., acrolein) at intermediate temperatures, and ultimately yielding complete oxidation products, such as CO₂ and H₂O at higher temperatures.

Keywords

ZSM-5 zeolite / Catalytic oxidation of propylene / Reaction mechanism / Acidity

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Jilin University, The Editorial Department of Chemical Research in Chinese Universities and Springer-Verlag GmbH