Facilitated transport membranes by incorporating different divalent metal ions as CO2 carriers
Facilitated transport membranes by utilizing π complexation reactions between metal ions and penetrants have been actively explored, however, the different facilitated transport abilities of metal ions remains to be disclosed. In this study, poly(N-vinylimidazole) coated carbon nanotube particles (PVI@CNT) were prepared via precipitation polymerization of N-vinylimidazole monomers on a CNT surface. The PVI@CNT particles were then loaded with four kinds of divalent metal ions, Cu2+, Fe2+, Ca2+ and Mg2+, and incorporated into polyimide (PI) to prepare M2+–PVI@CNT hybrid membranes. The structure of M2+–PVI@CNT particles and PI–M2+–PVI@CNT membranes was analyzed by different characterization tools. Taking CO2/CH4 as the model separation system, the hybrid membranes containing Cu2+ and Fe2+ at filler content of 7 wt% showed the maximum increase of CO2 permeability of 89% and 87% compared with those of a pristine PI membrane. Meanwhile, the selectivity of these membranes shows little increase. However, membranes containing Ca2+ and Mg2+ show only little enhancement in the separation properties. Such results can be interpreted based on the π complexation mechanism, transition metal ions Cu2+ and Fe2+ possess a strong CO2 facilitated transport ability whereas main-group metal ions Ca2+ and Mg2+ possess a weak facilitated transport ability. Finally, a correlation of the electronegativity of metal ions with their CO2 facilitated transport abilities was explored.