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 Fo urier-transform infrared (FTIR) spectroscopic measurements of the -C=N stretch vibration to characterize the binding of hydrogen cyanide by the various Bronsted acid sites of each zeolite. Diminished pressure a nd thermal desorption of zeolitically bound hydrogen cyanide, monitore d by reduction in FTIR spectral band intensities, have been used to di stinguish the strength of binding of hydrogen cyanide by the various a cid sites. A relationship exists between the type of Bronsted acid sit e, characterized by the -C=N stretch wavenumber of bound hydrogen cyan ide, and the zeolite framework structural features in terms of the pre sence of sodalite units and the salient channel structures. A consider ation of the acid sites of greatest strength for each zeolite indicate s that the decreasing order of strength of hydrogen bonding of hydroge n cyanide is ZSM-5 > mordenite > L almost-equal-to Y, X much greater t han beta. The IR spectral features of hydrogen cyanide bound by the Br onsted 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. Th e extra-framework component has been interpreted to be a highly acidic , hydrated alumina-like material of much higher acidic strength than t hat of the framework acidic sites and whose formation is critically de pendent on the temperature of steaming, and an aluminosilicate phase f ormed throughout the range of steaming temperatures. While there is no correlation between the strength of the various zeolitic Bronsted aci d sites and the IR spectral wavenumber of hydrogen cyanide bound to su ch sites, the wavenumber serves to characterize the sites and makes po ssible their identification in various zeolites and the elucidation of their relationship to zeolitic structural features.