Effect of Si : Al ratio and type of binder on the catalytic properties of HZSM-5 catalysts

Different HZSM-5 catalysts have been prepared by mixing and shaping dealuminated HZSM-5 zeolite samples with aerosil and boehmite as binder components. Catalytic investigations, adsorption measurements, temperature-programmed desorption studies and infrared spectroscopy with probe molecules were carried out to characterize the catalyst properties. The catalysts were tested in the coupled methanol–hydrocarbon cracking (CMHC) reaction with n-butane as hydrocarbon feed. In general, the catalytic data show inhibition of the deactivation resulting in lower coking, increased alkene yields and longer working periods with increasing Si : Al ratio independent of the nature of the binder. Comparing the data obtained from the two series with different types of binder we concluded that boehmite has not only a binder function but also a promoting effect on the catalyst properties. Thus, the catalyst activity was maintained at a more stable level owing to higher cracking activity and reduced rate of dealumination. Temperature-programmed desorption and IR spectroscopy were applied to determine the Brønsted and Lewis acidity of fresh and used samples. It was established that the addition of boehmite causes a partial neutralization of Brønsted sites. Moreover, the acid sites of the admixed boehmite influence considerably the acidic and catalytic properties of the catalyst.