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

Although spectroscopic investigation of surface chemisorbed CO2 species has been the focus most studies, identifying different domains weakly interacting (physisorbed) 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 first time a thorough solid-state analysis physisorbed prior and after amine-functionalization silica surfaces; combining 13C anisotropy (CSA) longitudinal relaxation times (T1). These methods were used quantitatively distinguish otherwise overlapping signals, which contributed an empirical model speciation physi- fractions. The measured T1 values confirm presence molecular dynamics on microsecond, millisecond, second scales, strongly supporting existence up three with proportions about 15%, 70%, respectively. Our approach takes advantage from using adsorbed 13C-labeled probe quantitative cross-polarization magic-angle spinning study both species, showing that 45% versus 55% formed overall amine-modified hybrid silicas. A total six distinct environments identified discriminated, coined here “gas, liquid, solid-like” species. complex nature absence revealed, shedding light what fractions affected upon pore functionalization. This work extends current knowledge sorption mechanisms providing new clues toward sorbent optimization.