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The CO2−amine chemistry in gas−solid processes was investigated under both humid and dry conditions using aminopropyl-grafted pore-expanded MCM-41 silica (MONO-PE-MCM-41). To draw accurate conclusions, a set of conditions had to be met including (i) the use of an adsorbent with open pore structure and readily accessible adsorption sites, e.g. MONO-PE-MCM-41 with a mean pore size of 7.2 nm; (ii) the CO2 concentration in the feed should be high enough to achieve saturation via chemisorption, but low enough to avoid any additional physisorption, e.g., 5% CO2 in N2; (iii) the use of a reliable method for the accurate measurement of CO2/N ratio. Under such conditions, the obtained CO2/N ratios were reminiscent of those obtained in the CO2 scrubbing process using ethanolamine solutions. Under dry conditions, the CO2/N ratio was close to 0.5, consistent with the formation of carbamate. Streams with relative humidity (RH) of 27, 61, and 74% were studied as well. As RH in the feed increased, CO2/N ratio increased from 0.57 to 0.88, in line with the gradual formation of bicarbonate. As for the determination of CO2/N ratio under dry conditions, both thermogravimetry (TG) and mass spectrometry (MS) were suitable, whereas in the presence of moisture, TG was found to drastically underestimate the CO2 uptake. The seemingly disparate CO2/N ratios reported in the literature for various propylamine-bearing adsorbents were rationalized on the basis of the adsorbent pore structure and/or the experimental conditions used.

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Adsorption of CO2 on triamine-grafted pore-expanded mesoporous silica, TRI-PE-MCM-41, was investigated from very low pressure to 1 bar at four temperatures (298, 308, 318, 328 K) using gravimetric measurements. TRI-PE-MCM-41 exhibited one of the highest equilibrium capacities compared to other typical CO2 adsorbents such as zeolites, activated carbons, and metal−organic frameworks (MOFs). In contrast, under the same pressure and temperature conditions, TRI-PE-MCM-41 exhibited very small uptakes of N2, CH4, H2, and O2. Column-breakthrough measurements of CO2 in mixtures with other species showed exceedingly high selectivity of CO2 over N2, CH4, H2, and O2 even at very low CO2 concentrations, indicating that TRI-PE-MCM-41 is suitable adsorbent for gas purification applications. Moreover, water vapor was found to have a beneficial effect on CO2 adsorption capacity even at very low CO2 partial pressure, e.g. 400 ppm, without adverse effect on CO2 selectivity.