Vapor-enhanced CO2 adsorption mechanism of composite PEI@ZIF-8 modified by polyethyleneimine for CO2/N2 separation

Novel PEI impregnated ZIF-8 (PEI@ZIF-8) composites, with characteristic of water vapor enhanced adsorption property, were synthesized for CO2/N-2 separation, and then characterized by N-2 adsorption, PXRD, IR and TG. CO2 and N-2 isotherms were measured by static adsorption, and the breakthrough curves of CO2/N-2 mixture (v(co2)/v(N2) of 1/1) through the fixed bed of the composites were determined in presence of water vapor. Results showed that CO2 working capacity and CO2/N-2 selectivity of as-synthesized PEI@ZIF-8 at 338 K were up to 1.61 mmol/g and 62, being 6.2 and 27 times of those of ZIF-8 at 303 K, respectively. PEI@ZIF-8 is thermally stable until 450 K. More importantly, it was found that the presence of water vapor in feed stream further enhanced the CO2 capacity and CO2/N-2 selectivity of PEI@ZIF-8, which separately reached 1.99 mmol/g and 89.3 at RH of 55% and 338 K, having increases of 23.6% and 44% in comparison with those under dry condition. The enhancement could be attributed to the synergy action mechanism of water steam and PEI. Calculation by density functional theory revealed that the presence of water vapor resulted in the formation of bicarbonate, and thus promoted more CO2 molecules to be adsorbed on PEI@ZIF-8. This water vapor enhanced action mechanism can be applied to design rationally more efficient adsorbents for CO2 capture under realistic situation where water vapor is always present. (C) 2015 Elsevier B.V. All rights reserved.