A durable Cu-beta zeolite via simple yet controlled pre-removal of framework Al for hydrocarbon trapping

Abstract

The development of robust, high-performance hydrocarbon (HC) trapping materials that can withstand harsh hydrothermal treatment (HT) at temperatures >= 800 degrees C remains a great challenge. In this study, to the best of our knowledge, we, for the first time, delicately removed framework Al from the BEA zeolite support (Si/Al ratio of ca. 18.1) using acids of varying strength and concentration prior to Cu impregnation. Use of moderately dealuminated BEA zeolites (Si/Al ratio: ca. 25-35) for Cu impregnation allowed for forming a sufficient amount of the active Cu species (both Cu+ ions inside for preferred HC adsorption and tiny CuO particles outside for effective HC oxidation). More importantly, the Cu-impregnated ones preserved these active Cu species as well as the original zeolite structure after HT. These features led to both excellent cold-start test (a simplified trap test for simulated HC emissions) activity and hydrothermal stability as desired for HC traps. In particular, the Cuimpregnated BEA zeolite with optimal pre-dealumination (treated with a 0.1 M oxalic acid solution) showed unprecedentedly marked HC removal ability (11.3 % of total HC removal efficiency vs. 0 % for the parent BEA zeolite-based counterpart) even after very harsh HT at 850 degrees C, bringing the HC traps closer to real-world applications.