Reversible interconversion behavior of Ag species in AgHZSM-5: XRD, 1H MAS NMR, TPR, TPHE, and catalytic studies

Abstract

1H MAS NMR, XRD, TPR, TPHE (temperature-programmed hydrogen evolution) techniques, and catalytic characterization by ethanol conversion were employed to investigate the reducibility and reversible interconversion behavior of Ag species in Ag-modified HZSM-5 (3 wt.%). The samples were prepared by ion-exchange of ZSM-5 with AgNO3 (aq). It was found that charge-balancing Ag cations in AgHZSM-5(Si/Al ∼11) and AgHZSM-5(Si/Al ∼28) can be readily reduced to “cationic Ag clusters” at ∼220 °C, while a higher temperature is required for the Ag species in AgHZSM-5(28) to be reduced to “metallic Ag clusters”, as compared with that for AgHZSM-5(11). Both cationic and metallic Ag clusters formed can be reversibly conversed into charge-balancing Ag cations by the reaction with neighboring Brønsted acid sites (reversible interconversion). The metallic Ag clusters were found to be more resistant to the reversible interconversion than the cationic Ag clusters. Under hydrogen stream, the reduced Ag species could be preserved and “silver hydride” species were observed, together with the active Brønsted acid sites. This phenomenon is suggested to play a decisive role on the catalytic activity of AgHZSM-5 towards BTX formation in the presence/absence of hydrogen.