A novel method of shortening the pathway between surface reactive oxygen species and organic receptors under surface lattice metal atom substitution over HZSM-5 for intensifying catalytic oxidation CH2Cl2

Abstract

Efficient catalytic oxidation of chlorine-containing volatile organic compounds (CVOCs) is a major research focus, but challenges include chlorine resistance, high HCl generation rates and maintaining high conversion rates. To overcome these technical bottlenecks, a series of transition metal (Cu, Cr, Fe, Co and Mn)-doped ZSM-5 molecular sieves were successfully synthesized with lattice Si⁴⁺ ions substituted by a one-step hydrothermal process. CH₂Cl₂ (DCM) served as a probe molecule to evaluate catalytic oxidation performance, while advanced physicochemical characterization elucidated the catalytic reaction, chlorine resistance and HCl generation mechanisms over the doped HZSM-5. The results show that in situ targeted substitution of transition metal atoms over HZSM-5 on the lattice Si⁴⁺ sites can induce the emergence of the twinning between the surface lattice single metal atoms and surface reactive oxygen nearby. Compared with supported HZSM-5, metal-doped HZSM-5 has greater catalytic performance and better resistance to chlorine poisoning and Cl deposition, due to the shortened pathway of surface reactive oxygen migration between surface reactive oxygen species and organic receptors, which can play an important role in catalytic oxidation and facilitate deep oxidation and inhibit Cl deposition. Mn- and Cr-doped HZSM-5 catalysts possess a high content of surface active oxygen, relatively high transfer ability, relatively high low-temperature reducibility and more appropriate intensities and quantities of surface Brønsted acids and Lewis acids, which results in a higher rate of DCM conversion, accompanied by relatively high CO₂ generation rate and HCl generation rate, as well as a strong ability to inhibit Cl deposition. Furthermore, the different synergistic reaction routes of DCM oxidation over metal-doped HZSM-5 are also proposed. We believe that this work can provide new insights into the design of catalysts for purifying CVOCs.