Contribution of Fe and Protonic Sites in Calcined and Steamed ZSM-5 Zeolites to Oxidation of Benzene with N2O to Phenol and Selective Catalytic Reduction of NO with Propane to Nitrogen

Abstract

NH4-(Fe)ZSM-5 zeolites with concentrations of Fe ranging from 30 to 2000 ppm, introduced into zeolites during their synthesis, either calcined in dry oxygen at 720 K or steamed at 870 K in oxygen stream with 30% water vapor, were investigated in benzene oxidation with N2O to phenol and selective catalytic reduction of NOx to nitrogen. The concentration of Brönsted and Lewis sites was determined by quantitative analysis of IR spectra of adsorbed d3-acetonitrile. The changes in Fe ion coordination were monitored by ESR spectra of Fe(III) ions. The Fe ions have been shown to control the activity of zeolites in the above reactions, while the Al-related acid sites play only a minor role. The steamed zeolites compared to calcined ones exhibited substantially higher activity in oxidation of benzene with N2O to phenol, while the reduction of NOx to nitrogen was considerably suppressed. The steaming resulted in a dramatic decrease in concentration of Brönsted sites, low concentration of Lewis sites and relocalization of Fe ions into cationic sites exhibiting distorted tetrahedral coordination (ESR signals at g = 6.0 and 5.6). It has been concluded that specific Fe sites are required for benzene oxidation to phenol without participation of Brönsted sites, while with the complex SCR-NOx reaction both Fe cationic sites and protonic sites are involved in the catalytic process. Fe sites contribute to oxidation of NO to NO2 and protonic sites control reduction of NO2 to molecular nitrogen.