Thermal, Oxidative and CO2 Induced Degradation of Primary Amines Used for CO2 Capture: Effect of Alkyl Linker on Stability

The stability of primary amine grafted mesoporous SBA-15 silica adsorbents with alkyl linker length varying from methyl to propyl (SBA-Methyl, SBA-Ethyl, SBA-Propyl) is investigated with respect to thermal, oxidative, and CO2 induced degradation to assess how differences in stability may result from different chain lengths. Methyl-based materials are found to have no thermal stability, with a severe reduction in amine content of up to 80% upon heating to 135 °C, and were therefore unable to be assessed for oxidative or CO2 specific instabilities. SBA-Ethyl and SBA-Propyl adsorbents are both thermally and oxidatively stable but are prone to urea induced deactivation in the presence of high-temperature, dry CO2 for prolonged periods. A greater extent of CO2 capacity deactivation is observed for SBA-Propyl materials, which is attributed to the larger amine efficiency of the material as compared to that of SBA-Ethyl adsorbents, as this would translate to a greater percentage of amines interacting with CO2 that could then be susceptible to deactivation upon heating. DFT calculations are used to assess CO2 induced urea formation pathways, leading to an energetically favored pathway for CO2 induced degradation, as well as to compare differences in stability between the varied alkyl chain lengths. We show that an isocyanate intermediate provides the lowest-energy route to urea formation and also that amine or silanol assisted deactivation lowers the energy barrier of deactivation, thus supporting the observation that high efficiency SBA-Propyl adsorbents are prone to a higher degree of deactivation under dry, high-temperature CO2 exposure conditions.