Luís Mafra, Tomaž Čendak, Sarah Schneider, Paul V. Wiper, João Pires, José R.B. Gomes, Moisés L. Pinto (2018). Amine functionalized porous silica for CO2/CH4 separation by adsorption: Which amine and why. Chemical Engineering Journal, 336, 612-621. DOI: 10.1016/j.cej.2017.12.061.
The separation of CO2/CH4 gas mixtures on mesoporous SBA-15 functionalized with primary, secondary and tertiary amines, and a diamine (primary and secondary), was studied by gas adsorption, solid state nuclear magnetic resonance (ssNMR) and electronic structure density functional theory (DFT) techniques to evaluate the possible application of amine functionalized mesoporous silicas in the purification of natural or biogas sources. High pressure adsorption, up to 10 bar, showed a strong selective interaction of CO2 with primary amine, secondary amine and diamine that give separation selectivities up to 25, 3300 and 15000, respectively, due to the chemisorption of CO2. Notably, not all amines are saturated at pressures below 40 kPa, as demonstrated by the 13C ssNMR spectra with variable 13CO2pressure. Furthermore, ssNMR and DFT results indicated that physisorbed CO2 interacts with the formed chemisorbed species, providing a possible explanation for the high selectivity observed even beyond the saturation of amines. Reaction Gibbs energies of CO2with the amines estimated by DFT computational methods are in excellent agreement with observed differences in the experimental Henry’s constants, selectivities and phase diagrams. Preliminary assessment of the materials for possible application on cyclic separation by pressure modulation indicated that although secondary amine and diamine originate very high selectivities, they are not very suitable for applications because of the lower working capacities (below 1.3 mmol g−1). The material functionalized with primary amine presents a better compromise between the high selectivity for separation and a good working capacity (about 2 mmol g−1).
Keywords: Methane upgrading; Gas separation; Amine-functionalized silicas; Solid-state NMR; DFT computer simulations; Adsorption