Adsorption of carbon dioxide and methane on alkali metal exchanged silicoaluminophosphate zeolite

Abstract

Aluminosilicate zeolites remain the most widely used microporous materials for important industrial gas separation applications. Aluminophosphate molecular sieves (AlPO-n) are the first microporous materials synthesized without silica. Substitution of silica in the AlPO-n framework result in the formation of silicoaluminophosphate (SAPO-n) molecular sieve. In this study, Na-SAPO-34 zeolite was synthesized and ion exchanged with alkali metal cations (K+ and Li+) using a two-step ion exchange procedure, with each step lasting for 24 hours. The prepared materials were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), inductive coupled plasma optical emission spectroscopy (ICP-OES), and Fourier transform infrared spectroscopy (FTIR). Thermal analysis was carried out using thermographymetric analyzer (TGA) and differential scanning calorimetry (DSC). The textural properties (surface area, pore size and pore volume) were calculated from N2 adsorption isotherm at 77 K. Equilibrium adsorption isotherms and adsorption capacity were measured volumetrically at 273, 298 and 323 K respectively for CO2 and at 298 K for CH4 at pressure of 101.3 KPa. The corresponding isosteric heats of adsorption of CO2 was calculated using the Clausius-Claperon equation. The Langmuir, Freundlich and Sips models were applied to correlate the adsorption isotherm data. The equilibrium performance of the prepared materials for CO2 separation from CO2/ CH4 mixtures operating at room temperature were calculated based on working capacity, regenerability and selectivity. The results showed that although all the prepared materials show changes in structural properties, the structure did not collapse after ion exchange. All the prepared materials showed high preference for CO2 over CH4. At 273 K, the 1st stage K+ ion exchange material had the highest CO2 adsorption capacity (2.90 mmol/g), whereas at 323 K, the 1st stage Li+ material had the highest capacity (1.90 mmol/g). However, at 298 K, the adsorption capacity did not improve. The 1st stage K+ material had highest adsorption capacity for CH4 (0.54 mmol/g) at 298 K. Sips model provides the best fit with experimental data. The 1st stage K+ ion exchange material had the highest heat of adsorption reflecting stronger interaction between the extra framework cation and CO2. Based on the adsorbent performance evaluation, all the prepared materials showed potential for CO2 removal from CO2/ CH4 mixtures at room temperature with the exception of the 2nd stage K+ ion exchange material. The lithium ion exchange series had higher working capacity and regenerability than the potassium ion exchanged series at 298 K. This study showed that adsorption capacity of CO2 and CH4 depends not only on temperature but also on the properties of the exchangeable cation such as size, charge, location, and distribution within the channels and cavities of the prepared materials.