Frequency-response characterization of acid sites in zeolite catalysts using NH3 as the probe molecule

Abstract

The frequency-response (FR) technique has been shown to be a novel, useful method for studying the dynamics of ammonia adsorption–desorption processes and the rates obtained can be used to characterize the acidic sites present, for example, in zeolite catalysts. The FR method has been proved to be capable of recognizing mass transfer effects and distinguishing the different strengths and concentrations of acidic sites using ammonia sorbate pressures of ≈1–10 mbar and temperatures close to those applied under real catalytic reaction conditions. The method, therefore, has advantages over techniques which require high vacuum or very low probe molecule pressures to obtain the relevant information. Combination of the FR method and other methods is necessary, however, to reach a complete characterization of all of the zeolitic acid sites present in a sample.