Near-Infrared Spectroscopic Identification of Ethylene Adsorption States on Ag + -Exchanged ZSM-5 Zeolites

Near-infrared (NIR) spectroscopy was applied to investigate the adsorption behavior of ethylene on ZSM-5 zeolites containing Na+, Ag+, and H+ sites. Ethylene adsorbed on Na+ and Ag+ sites could be distinguished by its characteristic NIR absorption band. Further, ethylene adsorbed on Brønsted acid sites was identified by the formation of protonated ethyl species. Temperature-dependent measurements also revealed distinct desorption behaviors associated with different cation sites. The applicability of NIR spectroscopy was further demonstrated by in situ monitoring of ethanol dehydration over Ag+0.5/H+0.5/ZSM-5. Upon ethanol adsorption, ethoxy species were first formed on Brønsted acid sites. At reaction temperatures above 200 °C, ethylene was generated and subsequently adsorbed on Ag+ sites, while water was simultaneously produced. In contrast, the characteristic C–H stretching bands of ethylene were not detected in the mid-IR region under the same reaction conditions. This may lead to the misleading conclusion that ethylene produced during ethanol dehydration is not adsorbed on the catalyst surface. These results demonstrate that NIR spectroscopy provides complementary information to conventional FT-IR measurements for investigating adsorption phenomena and catalytic reactions.