Nanoporous Organically Pillared Layered Silicates for CO ₂ Capture

Carbon dioxide (CO2) is a major contributor to today’s climate change. Capturing this greenhouse gas from point sources with high CO2 concentrations is therefore a highly interesting method for counteracting this problem. This work investigates the use of microporous organically pillared layered silicates (MOPSs) for selective physisorptive capture of CO2 from flue gas streams, addressing the challenge of separating CO2 from mixtures containing substantial water vapor (H2O) and nitrogen (N2). MOPSs allow tuning selectivity in an orthogonal way by adjusting the pore height and the pore width of the slit-shaped (ultra)micropores independently of each other in a highly modular manner. We have previously shown that a MOPS intercalated with diammonium butane (MOPS-7) exhibits perfect selectivity for CO2 over N2 by creating a slit too small for N2 to enter the pores. Here, we systematically vary the pore width by varying the chain length of the pillar molecules (diammonium n-alkanes). With longer alkyl chains, this pillar moiety contributes more prominently to the pore surface, and concomitantly, the nature of the micropores is rendered more hydrophobic. Under nonequilibrium conditions, the materials can adsorb substantial amounts of CO2 even in competition with significantly smaller H2O molecules at conditions mimicking a typical coal-fired power plant flue gas mixture (CO2/H2O/N2 ≈ 1:1:8 at 40 °C). CO2 breakthrough capacities up to 0.77 mmol/g are reported under these humid conditions. The CO2/H2O selectivity was found to be 1 for all studied pillars, indicating no influence of pore width and hydrophobicity. The materials could be regenerated at the measurement temperature of 40 °C without loss of CO2/H2O selectivity. These findings confirm MOPSs as highly modular, energy-efficient sorbents for industrial CO2 capture capable of maintaining robust performance even in the presence of water vapor.