Regulating the pore engineering of MOFs by the confined dissolving of PSA template to improve CO2 capture

Abstract

Pore size adjustment has been considered to be a successful strategy for enhancing the sieving of MOF-based mixed matrix membranes (MMMs). However, the increase in CO2/N2 selectivity usually comes at the expense of permeability. Herein, the regulating on the pore size of polystyrene-modified hollow NH2-MIL-101 (PHNM) is achieved by controlling the confined dissolving of PSA polymer chain. During the etching of PSA template, the dissolved PSA polymer chains diffused outward through the pores and intergranular space of NH2-MIL-101 under the concentration difference. As a consequence, the pores of NH2-MIL-101 were unavoidably constrained by the partly dissolved PSA segments, achieving the regulation on the pore size. Meanwhile, there are also some PSA segments adhering to the surface of NH2-MIL-101 to form a PSA coating layer. In this unique PHNM, the reduced pore size endows it with better CO2/N2 selectivity than NH2-MIL-101. Meanwhile, the high interface compatibility provided by surface-modified PSA segments and the enhanced CO2 affinity of PHNM resulting from amino groups also synergistically improve the CO2/N2 selectivity. Additionally, the hollow core offers gas molecules a low-resistance transfer channel, improving the permeability of CO2. The hollow size of PHNM was adjusted from 33 to 57 nm by controlling the etching time of PSA, and the pore size of PHNM was regulated from 0.58 to 0.83 nm by controlling the confined dissolving process of PSA. Thus, the PHNM-based MMMs exhibit high CO2 permeability as well as high CO2/N2 selectivity. Compared with NH2-MIL-10/Pebax MMM, the MMM with PHNM-2 loading of 10 wt% shows 30% and 21% enhancements in the CO2 permeability (226.3 Barrer) and CO2/N2 selectivity (71.3), respectively, far exceeding the Robeson upper bound (2008). These results reveal that the PHNM based MMMs express a great potential in CO2 separation.