FTIR spectroscopic study of the zeolitic adsorption of hydrogen cyanide on acidic sites

The uptake of hydrogen cyanide by protonic forms of zeolites: Y, steam-treated Y, X, mordenite, L, beta and ZSM-5, has been studied using Fourier-transform infrared (FTIR) spectroscopic measurements of the —CN stretch vibration to characterize the binding of hydrogen cyanide by the various Brønsted acid sites of each zeolite. Diminished pressure and thermal desorption of zeolitically bound hydrogen cyanide, monitored by reduction in FTIR spectral band intensities, have been used to distinguish the strength of binding of hydrogen cyanide by the various acid sites. A relationship exists between the type of Brønsted acid site, characterized by the —CN stretch wavenumber of bound hydrogen cyanide, and the zeolite framework structural features in terms of the presence of sodalite units and the salient channel structures. A consideration of the acid sites of greatest strength for each zeolite indicates that the decreasing order of strength of hydrogen bonding of hydrogen cyanide is ZSM-5 > mordenite > L ≈ Y, X beta. The IR spectral features of hydrogen cyanide bound by the Brønsted acid sites on the protonic form of zeolite Y steamed at various temperatures have been interpreted in terms of acidic sites on extra-framework material and those associated with the zeolite framework. The extra-framework component has been interpreted to be a highly acidic, hydrated alumina-like material of much higher acidic strength than that of the framework acidic sites and whose formation is critically dependent on the temperature of steaming, and an aluminosilicate phase formed throughout the range of steaming temperatures. While there is no correlation between the strength of the various zeolitic Brønsted acid sites and the IR spectral wavenumber of hydrogen cyanide bound to such sites, the wavenumber serves to characterize the sites and makes possible their identification in various zeolites and the elucidation of their relationship to zeolitic structural features.