Sr2+–SAPO-34 Prepared via Coupled Partial Detemplation and Solid State Ion Exchange: Effect on Textural Properties and Carbon Dioxide Adsorption

Sr2+–SAPO-34 materials were prepared via solid-state ion exchange (SSIE) to improve their CO2 adsorptive properties, particularly at low partial pressure, and study the effect of the ion exchange treatments on the structural and textural properties of the materials. In the past, these materials have been prepared with traditional liquid-state ion exchange (LSIE) methods yielding a strontium(II) content of about one cation per unit cell, well below the theoretical maximum and probably due to aqueous phase equilibrium constraints. Characterization of the SSIE materials included coupled thermal gravimetric analyses/Fourier transform infrared spectroscopy (TGA/FT-IR), X-ray diffraction (XRD), energy-dispersive analysis by X-rays (EDAX), surface area, and pure component CO2 equilibrium adsorption. Coupled TGA/FT-IR studies were used for the selection of the SSIE temperature for both NH4+–SAPO-34 and as-synthesized Na+–SAPO-34 starting materials. In general, the results indicated that temperatures well above the Tammann point are necessary to achieve acceptable strontium(II) loadings via SSIE while minimizing the loss of effective surface area due to pore clogging with unexchanged SrCl2. Furthermore, in situ partial detemplation (PD) of the as-synthesized material during SSIE avoided the formation of excess proton (acid) sites and allowed further loading of strontium(II) onto sites suitable for interaction with CO2. In order to increase the strontium(II) loading per unit cell, a combined PD/SSIE/LSIE strategy was used to remove some of the remaining tenacious sodium(I) cations remaining after SSIE. This approach resulted in materials with a loading of nearly two strontium cations per unit cell and, as a result, improved the overall CO2 adsorption performance of the materials in a remarkable fashion.