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

authors	Vieira, R; Marin-Montesinos, I; Pereira, J; Fonseca, R; Ilkaeva, M; Sardo, M; Mafra, L
nationality	International
journal	JOURNAL OF PHYSICAL CHEMISTRY C
keywords	SOLID-STATE NMR; MESOPOROUS SILICA; METHANE ADSORPTION; CARBON-DIOXIDE; C-13 NMR; CAPTURE; SPECTROSCOPY; RELAXATION
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 C-13 NMR chemical shift anisotropy (CSA) and longitudinal relaxation times (T-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 T-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 C-13-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 CO, 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.
publisher	AMER CHEMICAL SOC
issn	1932-7447
isbn	1932-7455
year published	2021
volume	125
issue	27
beginning page	14797
ending page	14806
digital object identifier (doi)	10.1021/acs.jpcc.1c02871
web of science category	10
subject category	Chemistry, Physical; Nanoscience & Nanotechnology; Materials Science, Multidisciplinary
unique article identifier	WOS:000674871100032