Study of the dynamics of NH3 adsorption in ZSM-5 zeolites and the acidity of the sorption sites using the frequency-response technique

The frequency response (FR) method has been applied to a study of the dynamics of NH3 adsorption in various ZSM-5 zeolites. Rate spectra have been recorded with samples differing in crystallite size, Si-to-Al ratio, and cationic composition. The effect of temperature and ammonia pressure has been examined. Independent of sorbent composition and measurement conditions, either macropore diffusion or soption was found to be the rate determining transport step. Diffusion control could be removed, leaving pure sorption rate spectra which have been used to characterize acid-base interactions. Under the quasi-equilibrium conditions of the measurements practically all of the sites examined are covered by the sorbate permitting rapid diffusional transport in the zeolitic micropores. The total and the relative amount of the different kinds of sites involved in the sorption processes have been shown to depend not only on sample characteristics but also on NH3 pressure and temperature. Both the frequency and the intensity of the FR resonances vary if the sorption equilibrium conditions are varied, making the direct interpretation of the spectra difficult. Generally a slow and a fast process could be distinguished assignable to sorption on Bronsted and Lewis acid sites, respectively. The time constants of the parallel sorption processes were determined and correlated to the strength of the acid sites. Spectra determined for the H-forms are characteristic for the processes of (i) NH3 + H+ half arrow right over half arrow left NH4+ and (ii) NH4+ + NH3 half arrow right over half arrow left NH3-H+-NH3 equilibria. Under conditions where protonic acid sites are saturated with the sorbate, only process ii is detected with significant intensity. The FR method was found to be less sensitive in detecting the differences in the acidity of the weaker Lewis acid sites, such as Na+ and NH4+ ions and positively charged extraframework Al species, but could be applied for characterizing the process of ammine complex formation over Cu-ZSM-5.