Benzene in hydrogen zeolites. Infrared spectroscopic and catalytic investigation of variously exchanged hydrogen Y systems

Abstract

Infrared spectra of sorbed benzene on several hydrogen Y zeolites, where the extent of proton exchange varied from 0 to 100%, are reported together with catalytic data for the ethylation of benzene over the same samples. Shifts of the zeolite hydroxyl band at 3640 cm^–1 to lower frequencies with increasing proton content are considered. A study of the out-of-plane C—H vibrations indicates an abrupt change in the strength of the benzene–surface bond at ca. 35 protons per unit cell; below this level of exchange the benzene is more tightly bound. The activity of the catalysts in the ethylation of benzene with ethanol is discussed; tightly held benzene at low levels of proton exchange is likely to reduce observed rates through hindered diffusion whereas for proton contents > 35 per unit cell the absence of hindered diffusion allows an exponential-type increase in alkylation rate, due to increased acidity, to be fully manifested. The paper shows how zeolitic cations can limit catalysis by localizing reactant or product molecules which in turn render the catalysis diffusion controlled.