Understanding the Active Sites of Ag/Zeolites and Deactivation Mechanism of Ethylene Catalytic Oxidation at Room Temperature

Abstract

Microporous Ag/zeolite (ZSM-5, Beta, Y, and Mordenite) catalysts were found to be potentially good catalytic materials for ethylene oxidation at room temperature. These catalysts were evaluated under both dry and humid atmospheres, achieving 100% conversion of 100 ppm of ethylene mineralized into CO2 at 25 °C. Moreover, the zeolite framework type and relative humidity had a significant effect on catalytic stability. Pyridine Fourier transform infrared spectra (FTIR) and solid-state hydrogen-1 (1H) magic angle spinning nuclear magnetic resonance (MAS NMR) studies revealed that Brønsted acid sites were the active sites in Ag/zeolites; the deactivated Ag/zeolites had no available Brønsted acid sites. When the number of Brønsted acid sites of Ag/ZSM-5-humid and Ag/Beta-humid were reduced, the catalytic activities of Ag/ZSM-5-humid and Ag/Beta-humid decreased. Temperature-programmed oxidation mass spectrometry (TPO-MS) and water adsorption/desorption results indicated that H2O adsorbed on Brønsted acid sites led to the disappearance of available Brønsted acid sites and thus resulted in Ag/zeolite catalyst deactivation. An understanding of the active sites and deactivation mechanism in ethylene oxidation on zeolite catalysts is helpful in the synthesis of a better ethylene oxidation catalyst and development of an effective technology of eliminating trace ethylene.