Thermodynamic adsorptive properties of exchanged chabazite-like aluminophosphate SAPO-34

The carbon dioxide adsorption behavior of synthesized chabazite-like M-SAPO-34 and modified with alkali metals (M = Li, Na, K, Cs), processed using volumetric method at different temperatures, was explored. The study allowed to see that the adsorption capacity increases according to the sequence: Na + > Li + > K + > Cs +, and all adsorption isothermal are type I according to Brunauer classification. Thermodynamic studies have been treated by the determination of the adsorption affinity, entropy and isosteric heat of these systems CO2-M-SAPO-34 with temperature variation between 273 and 298 K. The adsorption affinity shows the same trend for all the M-SAPO-34 at low coverage at 273 K. However, at high coverage, the isosteric heats of exchanged (H, Na, Li)-SAPO-34 present a homogeneous profile indicating dominance of adsorbate-adsorbate interaction. The differentials entropies were found to increase slightly with the coverage. Furthermore, the entropy of K-SAPO-34 at 273, exhibits values less than other exchanged M-SAPO-34. This Confirms pore blockage effect in the case of K-SAPO-34 and leads to reduction of free volume, while the Li-SAPO-34 shows the highest values of entropies with large free volume due to small size of Li+ cation.