Frequency-response NH3 sorption study of acidic sites in H-ZSM-5 catalysts

Abstract

The frequency-response (FR) technique has been used to study the kinetics of ammonia adsorption–desorption processes and to characterise the acidic sites in two different, well characterized H-ZSM-5 catalysts. The FR method has been shown to be capable of distinguishing the different strengths and concentrations of acid sites using sorbate pressures close to those applied under real reaction conditions. The method, therefore, has advantages over techniques which require high vacuum or low probe molecule pressures to obtain relevant information. The FR observations were in agreement with the results of other examinations. Although in the range of this study the direct interaction between ammonia and the acidic sites can be detected most probably only above 623 K, the rate spectra of hydrogen-bonded ammonia sorption on ammonium ions seems to be characteristic for acidic catalysts. Adspecies detected by the FR technique must also play a role in any dynamic system of practical interest. Combination of the technique with other methods is suggested to obtain a more complete description of zeolitic acid sites.