Assessing CO2 Capture in Porous Sorbents via Solid-State NMR-Assisted Adsorption Techniques

Abstract

Adsorption isotherms obtained through volumetric measurements are widely used to estimate the gas adsorption performance of porous materials. Nonetheless, there is always ambiguity regarding the contributions of chemi- and physisorption processes to the overall retained gas volume. In this work, we propose, for the first time, the use of solid-state NMR (ssNMR) to generate isotherms of CO₂ adsorbed onto an amine-modified silica sorbent. This method enables the separation of six individual isotherms for chemi- and physisorbed CO₂ components, a feat only possible using the discrimination power of NMR spectroscopy. The adsorption mechanism for each adsorbed species was ascertained by tracking their adsorption profiles at various pressures. The proposed method was validated against conventional volumetric adsorption measurements. The isotherm curves obtained by the proposed ssNMR-assisted approach enable advanced analysis of the sorbents, revealing the potential of variable-pressure NMR experiments in gas adsorption applications.

Figure 1

Schematics of the methodology used to perform qualitative and quantitative characterization of different chemi- (A, B, and C) and physisorbed (D, E, and F) CO₂ species formed at different CO₂ partial pressures. Adsorbent materials are packed in a zirconia NMR rotor and placed in a sorption apparatus comprising a custom-made high-vacuum gas line equipped with a turbomolecular pumping station, a borosilicate glass cell enclosing the NMR rotor, a mantle heater with temperature controller, and a thermocouple. Additionally, the setup includes connections to introduce gases and a manometer. This apparatus allows drying samples by a degassing/heating procedure and introducing desired gases at precise partial pressures (up to 770 Torr). After the sample is prepared by equilibrating with gas at a certain partial pressure, the gas-adsorbed species are analyzed and quantified by ssNMR. The quantification of the chemisorbed components is performed by multiple cross-polarization NMR spectroscopy, while the physisorbed fraction is quantified by means of T₁ measurements. This methodology allowed us to plot adsorption isotherms for each one of the six adsorbed CO₂ species (cf. Figures 4 and 5).

Figure 2

CO₂ adsorption isotherms (recorded at 298 K) for the APTES-modified SBA-15 obtained by the manometric (red) and ssNMR (blue) techniques (vertical lines depict the error bars).

Figure 3

Curve fitting of the isotherm recorded with the ssNMR-assisted (a) and manometric (b) methods, using the Langmuir, Freundlich, and Sips models (circles and squares represent the experimental adsorption data, and continuous lines depict the corresponding fitted adsorption models). The parameters obtained from the various adsorption models used to fit the experimental curves are presented in Table 1.

Figure 4

(a) ssNMR isotherms of the chemisorbed CO₂ components (A, B, and C) fitted with the Freundlich model. (b–d) Selected ¹³C MAS NMR spectra corresponding to the isotherm data points recorded at 0.01, 0.39, and 1.00 atm, respectively.

Figure 5

(a) ssNMR isotherm of the three physisorbed CO₂ components (D, E, and F) fitted using the Langmuir (E and F) and the Freundlich (D) models. (b, c) NMR saturation recovery curves of the corresponding isotherm data points recorded at 0.13 and 1.00 atm.