Insights into CO2 Adsorption Motifs in Functionalized Mesoporous Silica Using Solid-State NMR

Abstract

The immobilization of amine active sites onto porous supports, such as polyethylenimine (PEI) impregnated mesoporous silica, is well explored for carbon dioxide adsorption. However, a qualitative investigation of the molecular-level interactions among PEI, water molecules, and CO2 within the pore walls of mesoporous silica remains challenging. Probing such interactions requires spectroscopic methods that can access the local environment of the PEI/silica composite. A systematic investigation is needed to elucidate the atomistic picture of physically impregnated PEI within silica pores, particularly in relation to CO2 adsorption processes. Based on existing literature and feasible scenarios, we formed three distinct cases for the physical impregnation of PEI into the pores of silica. We synthesized a model system comprising porous silica impregnated with PEI at various weight percentages. We performed solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy experiments on a series of PEI-impregnated porous silica materials. The observed peak shifts and line shape changes in the 1H and 13C MAS NMR spectra, together with the complementary spectroscopic data, indicate that PEI forms a conformal layer along the silica pore walls at lower loadings and agglomerates within the pores at higher loadings. Moreover, PEI agglomeration becomes pronounced at PEI loadings above 40 wt %. A custom-built CO2 adsorption setup was used to perform CO2 adsorption on PEI/silica composites packed within solid-state NMR rotors. Complementary quantum mechanical calculations support the experimental observations and indicate the formation of carbamic acid species, characterized by a 13C resonance at 164 ppm. These molecular-level insights into the interactions within the PEI/silica CO2 system provide a framework for the design of high-performance amine-functionalized adsorbents for carbon capture applications.