Thermal Detemplation of Na-SAPO-34: Effect on Sr²⁺ Ion Exchange and CO₂ Adsorption

Abstract

Silicoaluminophosphate Na-SAPO-34 was studied with regard to the decomposition of tetraethylammonium (TEA+) and its effect on the ion exchange of Sr2+ and subsequent CO2 adsorption. TEA+ is the template used during the synthesis of the material. X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma mass spectrometry (ICP-MS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), and pure component gas adsorption were used to characterize the materials' structural and textural properties. Coupled thermal gravimetric analyses (TGA)/Fourier transform infrared (FT-IR) spectroscopy results indicate that the decomposition of the TEA+ in Na-SAPO-34 follows a three-step mechanism and is faster in air when compared to tests performed in helium. The transient spectroscopy and DRIFT methods also indicate that a detemplation temperature of about 430 °C gives formation to ammonium cations and possibly to other organic small fractions. Ion exchange of the resulting SAPO-34 materials with Sr2+ followed by calcination at 500 °C resulted in materials with good textural properties for CO2 adsorption, including surface area. Compared to materials detemplated straightforwardly at 500 °C, the two-step method treatment resulted in an increase of ca. 70% in CO2 uptake at low partial pressure because of an increase in the amount of Sr2+ cations in the SAPO-34 framework. Traditional one-step detemplation methods produce protons that occupy cation sites that are forbidden or inaccessible to divalent species during the ion exchange, which is the principal limitation to increase the concentration of particular adsorption sites in zeolitic adsorbents.