“Hidden” CO₂ in Amine-Modified Porous Silicas Enables Full Quantitative NMR Identification of Physi- and Chemisorbed CO₂ Species

High Resolution Image Download MS PowerPoint Slide Although spectroscopic investigation of surface chemisorbed CO 2 species has been the focus of most studies, identifying different domains of weakly interacting (physisorbed) CO 2 molecules in confined spaces is less trivial as they are often indistinguishable resorting to (isotropic) NMR chemical shift or vibrational band analyses. Herein, we undertake for the first time a thorough solid-state NMR analysis of CO 2 species physisorbed prior to and after amine-functionalization of silica surfaces; combining 13 C NMR chemical shift anisotropy (CSA) and longitudinal relaxation times ( T 1 ). These methods were used to quantitatively distinguish otherwise overlapping physisorbed CO 2 signals, which contributed to an empirical model of CO 2 speciation for the physi- and chemisorbed fractions. The quantitatively measured T 1 values confirm the presence of CO 2 molecular dynamics on the microsecond, millisecond, and second time scales, strongly supporting the existence of up to three physisorbed CO 2 species with proportions of about 15%, 15%, and 70%, respectively. Our approach takes advantage from using adsorbed 13 C-labeled CO 2 as probe molecules and quantitative cross-polarization magic-angle spinning to study both physi- and chemisorbed CO 2 species, showing that 45% of chemisorbed CO 2 versus 55% of physisorbed CO 2 is formed from the overall confined CO 2 in amine-modified hybrid silicas. A total of six distinct CO 2 environments were identified from which three physisorbed CO 2 were discriminated, coined here as “gas, liquid, and solid-like” CO 2 species. The complex nature of physisorbed CO 2 in the presence and absence of chemisorbed CO 2 species is revealed, shedding light on what fractions of weakly interacting CO 2 are affected upon pore functionalization. This work extends the current knowledge on CO 2 sorption mechanisms providing new clues toward CO 2 sorbent optimization.