Energetics of ammonia sorption in alkali metal exchanged analogues of linde type X zeolites

The parent zeolite NaX was prepared from hydrogel system 3.6 Na2O:3 SiO2:Al2O3:144 H2O at 368 K for 8 h under static condition. Postsynthesis modification was carried out using the conventional ion-exchange technique to obtain different ion-exchanged forms. The parent as well as exchanged samples were characterized by chemical analysis, IR, SEM, powder XRD, and low temperature nitrogen sorption. The ammonia sorption isotherms in parent NaX (Si/Al = 1.15) and its modified forms with a nearly equal degree of exchange by K+, Rb+, and Cs' cations have been measured in the temperature range of 303-453 K up to 500 Torr. The equilibrium sorption uptake at 50 Torr and especially at temperatures above 363 K was found to follow the sequence NaX > NaK(53)X > NaRb(53)X > NaCs(58)X. The analysis of the ammonia sorption data in terms of different isotherm equations revealed satisfactory representation by Langmuir, Dubinin, BET, and Sips equations and statistical models of Langmuir and Volmer. However, the Freundlich equation failed to represent ammonium sorption data at higher pressures. All of the parent and exchanged samples were compared in terms of chemical potential of ammonia sorption. Isosteric heat of ammonia sorption data revealed the higher heterogeneity of sorbent surface of Cs+-exchanged samples than those of other analogues. The variations in the charge density of the extraframework cations were found to influence the ammonia sorption energetics via alteration in Lewis acid-base character.