Sorption of ammonia in cation-exchanged omega zeolite and gallium analogue

Aluminum and gallium isomorphs of omega zeolites in Na+, H+, and La3+ catio nic forms were characterized by equilibrium sorption uptakes of different p robe molecules such as water, cyclohexane, n-hexane, and benzene at 298 K a t P/P-0 = 0.8 (2 h). Specific surface areas (BET) and void volumes were obt ained by low-temperature (78 K) nitrogen sorption. Ammonia sorption isother ms in these zeolites have been measured in the temperature range of 323-523 K up to 400 Torr. The amount of ammonia retained irreversibly (on account of chemisorption) during desorption measurements was found to be correlated with the acidic character of the zeolite. The analysis of these isotherm d ata in terms of different isotherm models revealed satisfactory representat ion of Dubinin, Langmuir, Freundlich, BET, and Sips equations in the low-pr essure region. Sorption models of Langumir and Volmer with and without sorb ate-sorbate interaction show limited applicability of these models. All the zeolites were compared in terms of chemical affinity of the sorption. Isos teric heat computed from linear isosters revealed the surface heterogeneity . Qst values were found to decrease with the coverage.