"Hidden" CO 2 in Amine-Modified Porous Silicas Enables Full Quantitative NMR Identification of Physi- and Chemisorbed CO 2 Species.

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.
Competing Interests: The authors declare no competing financial interest.
(© 2021 American Chemical Society.)