title: Structure of Chemisorbed CO(2) Species in Amine-Functionalized Mesoporous Silicas Studied by Solid-State NMR and Computer Modeling.
abstract: Two-dimensional (2D) solid-state nuclear magnetic resonance (SSNMR) experiments on samples loaded with13C-labeled CO2, "under controlled partial pressures", have been performed in this work, revealing unprecedented structural details about the formation of CO2adducts from its reaction with various amine-functionalized SBA-15 containing amines having distinct steric hindrances (e.g., primary, secondary) and similar loadings. Three chemisorbed CO2species were identified by NMR from distinct carbonyl environments resonating at δC≈ 153, 160, and 164 ppm. The newly reported chemisorbed CO2species at δC≈ 153 ppm was found to be extremely moisture dependent. A comprehensive1H-based SSNMR study [1D1H and 2D1H-X heteronuclear correlation (HETCOR, X =13C,29Si) experiments] was performed on samples subjected to different treatments. It was found that all chemisorbed CO2species are involved in hydrogen bonds (HBs) with either surface silanols or neighboring alkylamines.1H chemical shifts up to 11.8 ppm revealed that certain chemisorbed CO2species are engaged in very strong HBs. We effectively demonstrate that NMR may help in discriminating among free and hydrogen-bonded functional groups.13C{14N} dipolar-recoupling NMR showed that the formation of carbonate or bicarbonate is excluded. Density functional theory calculations on models of alkylamines grafted into the silica surface assisted the1H/13C assignments and validated various HB arrangements that may occur upon formation of carbamic acid. This work extends the understanding of the chemisorbed CO2structures that are formed upon bonding of CO2with surface amines and readily released from the surface by pressure swing.
authors.affiliation: CICECO - Aveiro Institute of Materials, Department of Chemistry, University of Aveiro , Campus Universitário de Santiago, 3810-193 Aveiro, Portugal.
authors.forename: Luís
authors.initials: L
authors.lastname: Mafra
authors.affiliation: CICECO - Aveiro Institute of Materials, Department of Chemistry, University of Aveiro , Campus Universitário de Santiago, 3810-193 Aveiro, Portugal.
authors.forename: Tomaž
authors.initials: T
authors.lastname: Čendak
authors.affiliation: CICECO - Aveiro Institute of Materials, Department of Chemistry, University of Aveiro , Campus Universitário de Santiago, 3810-193 Aveiro, Portugal.
authors.forename: Sarah
authors.initials: S
authors.lastname: Schneider
authors.affiliation: CICECO - Aveiro Institute of Materials, Department of Chemistry, University of Aveiro , Campus Universitário de Santiago, 3810-193 Aveiro, Portugal.
authors.forename: Paul V
authors.initials: PV
authors.lastname: Wiper
authors.affiliation: Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa , 1749-016 Lisboa, Portugal.
authors.forename: João
authors.identifier: 0000-0002-8374-558X
authors.initials: J
authors.lastname: Pires
authors.affiliation: CICECO - Aveiro Institute of Materials, Department of Chemistry, University of Aveiro , Campus Universitário de Santiago, 3810-193 Aveiro, Portugal.
authors.forename: José R B
authors.identifier: 0000-0001-5993-1385
authors.initials: JR
authors.lastname: Gomes
authors.affiliation: CERENA, Departamento de Engenharia Quı́mica, Instituto Superior Técnico, Universidade de Lisboa , Av. Rovisco Pais, no. 1, 1049-001 Lisboa, Portugal.
authors.forename: Moisés L
authors.identifier: 0000-0003-3061-9632
authors.initials: ML
authors.lastname: Pinto