Photocatalytic degradation and selective oxidation of typical gas phase molecules with TS-1 and HZSM-5 zeolites

Abstract

In this study, we selected zeolite molecular sieves (TS-1 and HZSM-5) with highly dispersed transition metal (Ti or Fe) in their frameworks and TiO2 P25 as photocatalysts and typical gas as probe molecules. The structure and crystallization, specific surface area, pore size distribution, light absorption and active radical species were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller surface area (BET), diffuse reflectance spectroscopy (DRS) and electron spin resonance spectroscopy (ESR). The influences of the intrinsic structure, adsorption property and the kind of active sites of the catalytic materials on their photocatalytic behaviors of different molecules were investigated. The emphases were placed on the photocatalytic conversion of hydrophobic halohydrocarbons and hydrophilic lower alcohol over zeolite molecular sieves. The photocatalytic reactions were performed in a tubular vessel microreactor operating in a continuous-flow mode under UV irradiation. Analysis of the reactor effluent was conducted by a gas chromatograph (HP6890). The concentrations of gas molecules and carbon dioxide were determined by using the flame ionization detector (FID) and thermal conductivity detector (TCD), respectively. Under UV irradiation, the excited state between the isolated and highly dispersed Fe3+/Ti4+ and bridging oxygen atom can activate the molecular oxygen into superoxide radical via the surface electron transfer, which can participate in the subsequent selective oxidation reactions. Due to zeolites with excellent adsorption relative to their unique MFI pore structure, active sites environment and the separation mechanism of photogenerated holes and electrons, these resulted that TS-1 and HZSM-5 zeolites with regular pore structure of molecular dimensions showed good degradation rate of chloroform (about 80%) for 16 h, obviously higher than that of TiO2. Also, they exhibited excellent selectivity for photocatalytic oxidation of methanol and isopropanol into formaldehyde and acetone, respectively. This is different from TiO2 as a typical representative of conventional semiconductor photocatalysts, in which the hydroxyl radicals with strong oxidation were produced as main active species and can mineralize the organic compounds into inorganic small molecules completely. This study provides ideas for the photocatalytic transformation of small organic molecules over molecular sieves with highly dispersed transition metal. © 2013 Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences.