Bimetallic Fe-Mn loaded H-ZSM-5 zeolites for excellent VOCs catalytic oxidation at low-temperatures: Synergistic effects and catalytic mechanisms

The development of catalysts with high adsorption and low-temperature catalytic characteristics for the decomposition of volatile organic compounds (VOCs) is of great importance for air purification. Herein, H-ZSM-5 (proton form Zeolite Socony Mobil-5) zeolite -supported Fe-Mn elements were synthesized via a two-step procedure (hydrothermal and precipitation methods). These zeolite-supported bimetal Fe-Mn catalysts exhibited a mesoporous structure, uniform distribution of Fe-Mn elements, excellent catalytic-oxidation capacity for toluene at low-temperatures, and high stability. Due to interactive synergies between the bimetallic Fe and Mn, the Fe2-Mn1/H-ZSM-5 (ratio of Fe and Mn was 2:1 with a total amount of 15 % to H-ZSM-5) exhibited 100 % toluene-conversion at 200 °C and T90 = 195 °C (temperature of toluene-conversion at 90 %) with GHSV = 20,000 mL/(g∙h). Its distinguishing catalytic-oxidation abilities at low temperatures were due to abundant oxygen-vacancies, adequate acidic-sites and various active oxygen-species, which improved the adsorption of toluene and expedited its degradation. The Fe2-Mn1/H-ZSM-5 catalyst also demonstrated a good resistance to humidity and renewable catalytic properties over five cyclic tests. The in-situ DRIFTS and LC-MS results revealed that toluene was decomposed to benzyl-alcohol, benzaldehyde, benzoate-acid, formates or/and acetate step by step, and finally CO2 and H2O. The mechanism of toluene oxidation over Fe2-Mn1/H-ZSM-5 catalysts was proposed and elucidated via the Mars–van-Krevelen (MVK) model. This study comprehensively explored the adsorption and decomposition kinetics of VOCs over bimetal decorated H-ZSM-5, which was useful toward the design of highly efficient catalysts for the removal of VOCs.