Mesoporous Organosilicas for CO2 Capture and Utilization: Reaction Insight and Material Development

Abstract

As mankind attempts to halt climate change and global warming, large-scale carbon dioxide (CO2) capture, utilization and storage (CCUS) technologies are viewed as an indispensable approach to curb CO2 emission. This thesis focused on better understanding CO2-amine interactions during adsorption, while developing in parallel covalently immobilized polyethylenimine (PEI) adsorbents for CO2 adsorption. In addition, catalyst reusability issues reported in the synthesis of cyclic carbonates (CCs) from CO2 and epoxides using metal-free supported immobilized quaternary ammonium salts are addressed, while developing new organosilicas for the synthesis of CCs. The reaction between CO2 and amine was investigated at the gas-solid interface in an attempt to provide a unified CO2-amine interaction both in adsorption and absorption. A combination of density functional theory calculations and experimental data (FTIR and 13C NMR) showed that the formation of the zwitterion intermediate often reported in the literature is highly unlikely, instead a six-atom centered zwitterion mechanism involving the “assisting” effect of water, amine or other functional groups was found to be more feasible due to its lower activation energy. Moreover, evidence was provided to suggest that under humid conditions, bicarbonate and carbonate are formed from the reaction between water and CO2, and not the widely reported carbamate hydrolysis. With a goal of minimizing the leaching of amines on PEI-impregnated adsorbents, PEI was covalently immobilized on mesoporous aluminosilica using 3-glycidoxypropyltrimethoxysilane or 3-triethoxysilylpropyl isocyanate as linkers. The resultant materials were found to be more resistant to leaching (in ethanol) and degradation (air at 100 oC) compared to their impregnated counterparts. Further enhancement in oxidation stability was achieved by covalently grafting epoxide-functionalized PEI onto mesoporous aluminosilica. CO2 uptake over amine-containing adsorbents is widely reported to be enhanced in the presence of moisture. However, the same cannot be said for other adsorbents, such as, carbonaceous and zeolite-based materials, and most MOFs. In a soon to be submitted review manuscript, a comprehensive analysis on the role of water on CO2 uptake (equilibrium and kinetics), material structure and regeneration over a wide range of adsorbents is presented. As for CO2-epoxides fixation to cyclic carbonates, a quaternary ammonium salt supported on SBA-15 was used to investigate the observed literature trend between product yield and substrate type with catalyst reuse. Under mild reaction conditions (1.0 MPa CO2, 100 oC and 4 h), 1,2-butylene carbonate was obtained in high yields (> 95%) over 5 cycles as the substrate is easy to activate and the product can be completely removed from the catalyst surface due to its low boiling point. Nonetheless, using styrene oxide led to decrease in yield over reuse cycles, mainly because styrene carbonate crystals were trapped on the catalysts surface (13C MAS NMR and TGA data), thereby blocking access to active sites. By extensively washing all spent catalysts in acetone and using chromatographic grade SiO2 as support material, styrene carbonate was obtained in very good yield (> 93%) over five cycles. Finally, novel quaternary ammonium iodide-based organosilicas, grouped into disordered, ordered and periodic mesoporous organosilicas, were prepared and tested for the cycloaddition of CO2 to epoxide to yield cyclic carbonates. Under mild reaction conditions (0.5 MPa CO2, 50 oC and 10 – 15 h) catalysts with the ordered mesoporous organosilicas structure were found to be more active owing to their larger surface area and pore volume, enhancing the accessibility of active sites by epoxides.