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

Abstract

Although spectroscopic investigation of surface chemisorbed CO₂ species has been the focus of most studies, identifying different domains of weakly interacting (physisorbed) CO₂ 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 CO₂ species physisorbed prior to and after amine-functionalization of silica surfaces; combining ¹³C NMR chemical shift anisotropy (CSA) and longitudinal relaxation times (<i>T</i> ₁). These methods were used to quantitatively distinguish otherwise overlapping physisorbed CO₂ signals, which contributed to an empirical model of CO₂ speciation for the physi- and chemisorbed fractions. The quantitatively measured <i>T</i> ₁ values confirm the presence of CO₂ molecular dynamics on the microsecond, millisecond, and second time scales, strongly supporting the existence of up to three physisorbed CO₂ species with proportions of about 15%, 15%, and 70%, respectively. Our approach takes advantage from using adsorbed ¹³C-labeled CO₂ as probe molecules and quantitative cross-polarization magic-angle spinning to study both physi- and chemisorbed CO₂ species, showing that 45% of chemisorbed CO₂ versus 55% of physisorbed CO₂ is formed from the overall confined CO₂ in amine-modified hybrid silicas. A total of six distinct CO₂ environments were identified from which three physisorbed CO₂ were discriminated, coined here as "gas, liquid, and solid-like" CO₂ species. The complex nature of physisorbed CO₂ in the presence and absence of chemisorbed CO₂ species is revealed, shedding light on what fractions of weakly interacting CO₂ are affected upon pore functionalization. This work extends the current knowledge on CO₂ sorption mechanisms providing new clues toward CO₂ sorbent optimization.