RELATIONSHIP BETWEEN SURFACE-ACIDITY AND ACTIVITY OF CATALYSTS IN THETRANSFORMATION OF METHANOL INTO HYDROCARBONS

The effect of Si/Al ratio of HZSM5 zeolite-based catalysts on surface acidity and on selectivity in the transformation of methanol into hydr ocarbons has been studied. The acidic strength distribution (measured by combining the differential scanning calorimetry and the chromatogra phic analysis) and the total acidity (measured by calorimetry and temp erature programmed desorption), using ammonia and tert-butylamine, hav e been studied as acidity measurements. The nature of the acidic sites has been determined by FTIR analysis of both the surface OH groups an d the interaction of Bronsted and Lewis sites with pyridine. The incre ase in light olefin selectivity and the decrease in methanol conversio n with increasing Si/Al ratio are a consequence of the kinetic scheme of the reaction, which has steps in series, and can be explained by th e decrease in total acidity. The same acidic sites are responsible for the formation of light olefins and for the transformation of these ol efins into higher hydrocarbons.