CO2 Adsorption and Oxidative Degradation of Silica-Supported Branched and Linear Aminosilanes

An array of aminosilanes is synthesized and grafted onto a mesoporous silica support to evaluate the effect of aminosilane structure on CO2 adsorption and oxidative stability. Both linear (NH2(CH2)(n)NH(CH2)(3)Si(OEt)(3), L-ethyl for n = 2, L-propyl for n = 3) and branched ((NH2(CH2)(n))(2)N(CH)(3)Si(OEt)(3), B-ethyl for n = 2, B-propyl for n = 3) aminosilanes containing multiple amine moieties (primary and secondary or primary and tertiary amines) with ethyl or propyl spacers between the amine moieties are prepared. The CO2 uptakes of the adsorbents are measured using a 400 ppm of CO2 stream, representing the concentration of CO2 in ambient air. The adsorbents containing branched silanes show higher CO2 uptakes than the adsorbents with linear silanes per silane molecule due to the presence of more effective amine moieties (primary amines > secondary amines) for CO2 sorption. The thermal and oxidative stabilities of the adsorbents are evaluated with multiple cycles of CO2 adsorption and desorption using flowing, inert helium or oxidizing air. While all the prepared adsorbents show good thermal stability, the L-propyl adsorbent shows the highest oxidative stability, followed by a stability trend of B-propyl > B-ethyl > L-ethyl materials. The nature of the oxidized species on the adsorbents is monitored by in situ IR spectroscopy, where amide and imine species are observed. Depending on the oxidative stability of the adsorbents, the order of the normalized H/(C+N) ratios for the oxidized adsorbents varied, showing good agreement with the order of the normalized CO2 capacities of the oxidized adsorbents.