Structure of Chemisorbed CO₂ 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 with C-labeled CO, "under controlled partial pressures", have been performed in this work, revealing unprecedented structural details about the formation of CO adducts from its reaction with various amine-functionalized SBA-15 containing amines having distinct steric hindrances (e.g., primary, secondary) and similar loadings. Three chemisorbed CO species were identified by NMR from distinct carbonyl environments resonating at δ ≈ 153, 160, and 164 ppm. The newly reported chemisorbed CO species at δ ≈ 153 ppm was found to be extremely moisture dependent. A comprehensive H-based SSNMR study [1DH and 2D H-X heteronuclear correlation (HETCOR, X =C, Si) experiments] was performed on samples subjected to different treatments. It was found that all chemisorbed CO species are involved in hydrogen bonds (HBs) with either surface silanols or neighboring alkylamines. H chemical shifts up to 11.8 ppm revealed that certain chemisorbed CO species are engaged in very strong HBs. We effectively demonstrate that NMR may help in discriminating among free and hydrogen-bonded functional groups. C{N} 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 the H/C assignments and validated various HB arrangements that may occur upon formation of carbamic acid. This work extends the understanding of the chemisorbed CO structures that are formed upon bonding of CO with surface amines and readily released from the surface by pressure swing.

Detecting Proton Transfer in CO2 Species Chemisorbed on Amine‐Modified Mesoporous Silicas by Using 13C NMR Chemical Shift Anisotropy and Smart Control of Amine Surface Density

“…Resonance B lies somewhere between these two regions as ar esult of its ÀCOOH group being neither free nor deprotonated, but strongly hydrogen bonded, as shown in our model ( Figure 2) and evidenced experimentally through the correlation of B carbon atoms with strongly hydrogen-bonded protons( 1 H d iso % 11.8 ppm). [11] Parameter A of our computational models shows as imilar trend, that is, it decreasesf rom 242 (A)t o2 04 (B)a nd 178 ppm (C). We emphasise that the model for A matches well with the experimental 13 CC SA tensor elements and is also properly positioned in the protonated (carbamic acid) region (A > 190 ppm;T able1 and Figure 4).…”

“…Indeed, ac areful comparison of the textural parameters presented in Ta ble S1 in the Supporting Information demonstrate that samples AP-TES@SBA-15-LL and APTES@SBA-15-ISO are almost identical to that of the parent SBA-15, and indicate that thesep arameters are not significantly affected at low amine contents.T he grafting density,w hich is estimated from the amine loading and surfacea rea, can be very informative of the existence of hydrogen bonds between neighbouring amines, and can influence the nature of the CO 2 chemisorbed species. [11] For APTES@SBA-15-ISO,t he density is significantly below the threshold value (0.96 molecules nm À2 )c alculated for the existence of hydrogen bondsb etween neighbouring amines. [11] In the case of AP- Figure 1.…”

“…[11] For APTES@SBA-15-ISO,t he density is significantly below the threshold value (0.96 molecules nm À2 )c alculated for the existence of hydrogen bondsb etween neighbouring amines. [11] In the case of AP- Figure 1. Schematic representation of the distinct synthetic routes used to manipulate alkylamine spacing and yield the amine-functionalised materials investigatedherein.…”

“…Theoretical models in which amine-amine distances were incremented also showedathreshold after which the formation of amine pairs was no longerf avourable. [11] Therefore, controlling amine spacing could provide an interesting approacht of avour either the formation of isolated or clustered amines and evaluateh ow this influences chemi-sorbedC O 2 speciation. To exploit this idea, we prepared distinct amine-modified mesoporous silicas (SBA-15) through two synthetic routes:o ne to yield materials with high/low amine loadings by using ac onventionalg rafting protocol, and a second to produce materials with enlarged amine-amine spacing (favouring isolated surface amines)b yi ncorporating a bulky spacer into the amine group of propylalkylamine ligands.…”

Evaluation of porous clay heterostructures modified with amine species as adsorbent for the CO2 capture

“…It has been established by 13 C NMR that the carbamic acid is the only structure involved in the CO 2 capture in the absence de H 2 O as well the -OH group of the carbamic acid are not involved in the adsorption process [51].…”