Characterization of a Mixture of CO₂ Adsorption Products in Hyperbranched Aminosilica Adsorbents by ¹³C Solid-State NMR

Abstract

Hyperbranched amine polymers (HAS) grown from the mesoporous silica SBA-15 (hereafter "SBA-15-HAS") exhibit large capacities for CO2 adsorption. We have used static in situ and magic-angle spinning (MAS) ex situ (13)C nuclear magnetic resonance (NMR) to examine the adsorption of CO2 by SBA-15-HAS. (13)C NMR distinguishes the signal of gas-phase (13)CO2 from that of the chemisorbed species. HAS polymers possess primary, secondary, and tertiary amines, leading to multiple chemisorption reaction outcomes, including carbamate (RnNCOO(-)), carbamic acid (RnNCOOH), and bicarbonate (HCO3(-)) moieties. Carbamates and bicarbonate fall within a small (13)C chemical shift range (162-166 ppm), and a mixture was observed including carbamic acid and carbamate, the former disappearing upon evacuation of the sample. By examining the (13)C-(14)N dipolar coupling through low-field (B0 = 3 T) (13)C{(1)H} cross-polarization MAS NMR, carbamate is confirmed through splitting of the (13)C resonance. A third species that is either bicarbonate or a second carbamate is evident from bimodal T2 decay times of the ∼163 ppm peak, indicating the presence of two species comprising that single resonance. The mixture of products suggests that (1) the presence of amines and water leads to bicarbonate being present and/or (2) the multiple types of amine sites in HAS permit formation of chemically distinct carbamates.

Elucidating CO2 Chemisorption in Diamine-Appended Metal–Organic Frameworks

“…This chemical shift is similar to those previously assigned to carbamic acids in the literature. 35,40,41,[44][45][46] The weak resonance at ~164 ppm likely arises from a minor amount of ammonium carbamate (see Figure S5). Importantly, a two dimensional 1 H → 13 C HETCOR experiment (short contact time) revealed two major correlations at 1 H chemical shifts of 12.2 and 5.7 ppm (Figure 4b), assigned to hydrogens of the carbamic acid (NHCOOH and NHCOOH, respectively).…”

Elucidation of Surface Species through in Situ FTIR Spectroscopy of Carbon Dioxide Adsorption on Amine‐Grafted SBA‐15

“…Furthermore, APS‐SBA15 has the highest fraction of residual chemisorbed species after desorption; this suggests that it has a higher ratio of carbamate ions to carbamic acid than MAPS‐SBA15. Carbamic acid is a less stable product during desorption …”

“…observed the formation of carbamic acid and ammonium carbamate ion pairs on amine‐functionalized porous clays, and Moore et al. observed the formation of a third surface species on hyperbranched amine polymers functionalized on SBA‐15 . It was concluded that the third resonance could be a different carbamate or bicarbonate species.…”

CO2adsorption on different organo-modified SBA-15 silicas: a multidisciplinary study on the effects of basic surface groups

“…Moreover, an additional resonance at 164 ppm was clearly visible in all the samples and was attributed to the presence of carbamate groups 54 . Moore et.al has studied CO 2 adsorption on hyperbranched amine polymers grown from the mesoporous silica SBA--15 which exhibited mixture of chemisorption products, including carbamate, carbamic acid and bicarbonate moieties [55] . The resonance at 164 ppm lies near the chemical shift values expected for typical carbamic acid (160 ppm), bicarbonate (163 ppm) and carbamate (164 ppm) species.…”