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

Abstract

The wealth of site-selective structural information on CO speciation, obtained by spectroscopic techniques, is often hampered by the lack of easy-to-control synthetic routes. Herein, an alternative experimental protocol that relies on the high sensitivity of C chemical shift anisotropy (CSA) tensors to proton transfer, is presented to unambiguously distinguish between ionic/charged and neutral CO species, formed upon adsorption of CO in amine-modified porous materials. Control of the surface amine spacing was achieved through the use of amine protecting groups during functionalisation prior to CO adsorption. This approach enabled the formation of either "isolated" or "paired" carbamate/carbamic acid species, providing a first experimental NMR proof towards the identification of both aggregation states. Computer modelling of surface CO -amine adducts assisted the solid-state NMR assignments and validated various hydrogen-bond arrangements occurring upon formation of isolated/aggregated carbamic acid and alkylammonium carbamate ion species. This work extends the understanding of chemisorbed CO structures formed at pore surfaces and reveals structural insight about the protonation source responsible for the proton-transfer mechanism in such aggregates.

Elucidating CO2 Chemisorption in Diamine-Appended Metal–Organic Frameworks

“…Indeed, recent work on amine-functionalized silicas has shown that the use of the anisotropy of the 13 C chemical shift (rather than simply isotropic chemical shifts) can enable identification of neutral versus charged chemisorption products. 52 Overall, these studies demonstrate that 1 H, 13 C, and 15 N NMR experiments, combined with DFT calculations, provide far more reliable structural assignments than 1D NMR experiments and calculations on a single nucleus. Figure 8) with key 1 H - 13 C HETCOR correlations indicated by blue arrows and labels A-E. Gas dosing times were >13 hours in all cases, and care was taken to avoid air exposure between sample activation and gas dosing, as water vapor has a large effect on the chemisorption mechanism (see below).…”

A Unified Approach to CO2–Amine Reaction Mechanisms

“…As for solid-supported amines, because of the limited mobility of amine groups, it is surmised that water catalytic assistance becomes prominent for a low amine surface density, leading to carbamic acid or hydronium carbamate (CO 2 /N = 1), often stabilized by hydrogen bonding. 15 , 19 , 24 , 25 Notice that hydroxyl groups may play the same assisting role as water vapor. 29 This is consistent with experimental evidence as the enhancement of amine efficiency under humid conditions was found to be most pronounced for low amine surface coverage.…”

“… 26 Furthermore, the nature of the chemical species formed and the amine efficiency, that is, the CO 2 /N ratio, were found to depend on the surface density of amine groups. 16 , 19 , 24 , 27 Although the formation of alkylammonium carbamate is often dominating under dry conditions, Cendak et al 19 found that at low surface density, supported amines give rise mainly to carbamic acid, whereas at high density, the formation of alkylammonium carbamate prevails. Carbamic acid was also demonstrated to occur in the absence of moisture, on primary amine-bearing metal–organic frameworks, 21 or other hydrophobic media.…”

Unravelling the Structure of Chemisorbed CO2 Species in Mesoporous Aminosilicas: A Critical Survey

“…Traditionally, Fourier-transform infrared (FTIR) spectroscopy has been used as the leading tool in species identification. More recently, nuclear magnetic resonance (NMR) has emerged as a powerful alternative 39 , able to discriminate not only between different species 25, 27-28, 30, 34-35, 40-46 , but also different conformations of the same species 32,47 . These studies use mainly 13 C NMR, in order to detect CO2-amine adducts.…”

“…The silica surface model was intentionally faded out to better emphasize the propylamine chains. Resonance C is associated with model 1 from Figure 1, resonance B with 3, resonance B' with 4, resonance A with 5 and resonance A' withMany authors, including us, have tried to investigate how amine loading impacts the nature of CO2 species formed in silica-based materials29,47,[62][63] ; however, computational studies modelling CO2 structures in conditions of high-and low-amine loadings are extremely scarce42 . Whether CO2 species are isolated or establishing hydrogen bonds with neighbouring amines, with or without the involvement of hydrogen bonded silanol groups, is still very debatable and very difficult to verify through experimental evidence.…”