Probing the Morphology and Mobility of Amines in Porous Silica CO2 Sorbents by 1H T 1-T 2 Relaxation Correlation NMR

Molecular and oligomeric amines supported in porous oxidesupportsare a promising class of CO2 sorbent materials studiedfor CO2 removal from diverse streams such as flue gas andambient air. Among the various amines investigated, low molecularweight, hyperbranched poly(ethyleneimine) (PEI), and tetraethylenepentamine(TEPA) are among the most extensively studied. While macroscopic structure-performancerelationships relating the support structure, amine loading, and otherfactors affecting CO2 sorption capacities and kineticshave been developed, structural and dynamic information about theorganic amine phase in the porous support is less plentiful. The structureand mobility of amines impregnated in the pores of porous supportsdirectly impact gas sorption, as the accessibility of amine sitesin the pores directly relates to amine distribution in the pores andoverall pore filling as well as the dynamics of the amine chains.Here, we prepare a family of mesoporous silica SBA-15 materials containingvarying loadings of oligomeric (PEI) and molecular (TEPA) amines. H-1 T (1)-T (2) relaxation correlation solid-state NMR experiments areused to characterize the structural and dynamic properties of theconfined amines. Both TEPA and PEI are shown to form multiple differentdomains in the pores, each with distinguishable dynamic properties.TEPA and PEI form more rigid layers around the silica support wallsat lower organic loading fractions, characterized by lower mobilities,followed by the formation of more mobile domains less engaged in porewall interactions at higher loadings. TEPA shows faster mobilitiesthan PEI because of its lower molecular weight. TEPA also appearsto more easily transfer between domains within the pores, leadingto generally faster CO2 uptake rates with higher sorptioncapacities, while PEI located closer to the pore walls remained muchless mobile and is thus less engaged in CO2 capture.