Functionalized porous materials for adsorption and chemical fixation of CO2 into industrially relevant cyclic carbonates

Abstract

This thesis presents a small-scale demonstration of the CO2 capture and utilization approach using functionalized porous adsorbents such as metal organic framework (MOF), various amine-functionalized mesoporous silica and dendritic fibrous nanosilica (DFNS). Due to the functionalization, these materials possess Lewis base and Lewis acid or hydrogen bond donor sites. These sites facilitate the adsorption of CO2 on the material surfaces and catalyze its fixation into a value-added product, cyclic carbonate, in the presence of epoxide and co-catalyst tetrabutylammonium bromide (TBAB) under solvent-free conditions. The porous nature of the material provides a large surface area for the adsorption, while amine functionalities help boost the selective CO2 adsorption due to acid-base interaction between acidic CO2 and basic amine nitrogen. At the same time, Lewis acid centers (metal ions in the case of MOF) or H-bond donors (carbamide groups in functionalized silica and DFNS) activate the epoxide ring for cycloaddition with the adsorbed CO2. The detailed CO2 adsorption and catalytic CO2 fixation studies indicate that the reported materials are selective CO2 adsorbents and excellent catalysts for the cycloaddition of CO2 and epoxide into cyclic carbonates with broad substrate scope. Moreover, due to their heterogeneous character, these catalysts can be easily recycled without any considerable loss in their catalytic performances. The main features of this thesis are the inexpensive synthetic methodology involving benign precursors, high selective CO2 adsorption with moderate enthalpy helping easier regeneration, and efficient chemical transformation of CO2 into a value-added product. Overall, this work emphasizes a sustainable approach to mitigate the rising CO2 level.