OH-enriched organo-montmorillonites for potential applications in carbon dioxide separation and concentration

Abstract

Carbon dioxide (CO2) was reversibly trapped using low cost hybrid matrices obtained through a mere incorporation of organic molecules bearing terminal hydroxyl groups in Na-montmorillonite. For this purpose various organic compounds ranging from bulky Boltorn dendrimers to light molecules like ethylene–glycol were used. At low loadings, the adsorption capacity increased almost proportionally with the number of OH groups. The latter acted as the main adsorption sites. Dendrimer contents exceeding 3wt.% were found to enhance the hydrophobic character of the organic moiety, which aggregates into clusters and reduces the number of accessible sites. Unlike amine-based adsorbents, such matrices displayed weak interaction with CO2, but sufficient adsorption sites to satisfactorily retain CO2. The latter was contacted with adsorbent powder (0.01–0.1mm particle size) at room temperature and normal pressure in the presence of nitrogen. Consecutive measurements through thermal programmed desorption gave CO2 retention capacity (CRC) values in the range 2–14μmolg−1, expressed in terms of CO2 amount desorbed between 20°C and 200°C under a nitrogen throughput of 1–15mL/min. The CRC value was found to be strongly depending on the structure of the incorporated organic molecule and the nitrogen flow rate during impregnation and saturation by CO2 and purge. Increasing the N2 flow rate was found to reduce the CRC value, due to CO2 removal by forced convection. The major part of CO2 can be released at temperature not exceeding 40–70°C or even at room temperature upon exposure to strong nitrogen stream or in CO2-free media. This provides clear evidence of a truly reversible retention of CO2, and opens new prospects for ideal clay-based respiratory membranes bearing other chemical groups, capable to release the adsorbed gas through mere convection by a carrier gas stream, without heating.