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 hav e been recorded with samples differing in crystallite size, Si-to-Al r atio, and cationic composition. The effect of temperature and ammonia pressure has been examined. Independent of sorbent composition and mea surement conditions, either macropore diffusion or soption was found t o be the rate determining transport step. Diffusion control could be r emoved, leaving pure sorption rate spectra which have been used to cha racterize acid-base interactions. Under the quasi-equilibrium conditio ns of the measurements practically all of the sites examined are cover ed by the sorbate permitting rapid diffusional transport in the zeolit ic micropores. The total and the relative amount of the different kind s of sites involved in the sorption processes have been shown to depen d not only on sample characteristics but also on NH3 pressure and temp erature. Both the frequency and the intensity of the FR resonances var y 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 sor ption 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 NH 4+ and (ii) NH4+ + NH3 half arrow right over half arrow left NH3-H+-NH 3 equilibria. Under conditions where protonic acid sites are saturated with the sorbate, only process ii is detected with significant intens ity. The FR method was found to be less sensitive in detecting the dif ferences in the acidity of the weaker Lewis acid sites, such as Na+ an d NH4+ ions and positively charged extraframework Al species, but coul d be applied for characterizing the process of ammine complex formatio n over Cu-ZSM-5.