Organically modified activated bentonites for the reversible capture of CO2

Abstract Effective adsorbents for the reversible capture of CO2 at ambient temperature were prepared by acid activation and chemical grafting of 3-amino-propyltriethoxysilane (γ-APTS) or 3-diethanolamino-propyltriethoxysilane (3-diEtOH-APTS). Full characterization of the resulting organo-AAB materials through X-ray diffraction, nitrogen adsorption-desorption isotherms, thermogravimetric analysis, Fourier transform infrared spectroscopy, differential scanning calorimetry and 29Si solid-state nuclear magnetic resonance (NMR) revealed thermally stable porous structures with higher affinity towards CO2. Thermal programmed desorption analyses (TPD) gave appreciable CO2 retention capacity (CRC) with easy release of ca. 90% of the absorbed CO2 at temperature not exceeding 80 °C. Total regeneration of adsorbents was achieved at less than 160 °C, without affecting the stability of the grafted groups. Acid activation produced an increase of the number of silanol groups and of the silylation capacity, but unavoidably a marked CRC decay. Further grafting of amino and diethanol-amino groups was found to revive the affinity towards CO2. The CRC increase with increasing amount of impregnating CO2 and decreasing carrier gas throughput allows predicting even much higher CO2 uptakes than those obtained in the present work through predominant non-stoichiometric physical condensation around the amino groups. Such low-cost hybrid materials appear as promising alternative to others prepared by physical incorporation of amine, and the results obtained herein allow envisaging effective reversible capture of CO2 without amine loss upon thermal stresses.