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

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.