Pore engineering of MOFs through in-situ polymerization of dopamine into the cages to boost gas selective screening of mixed-matrix membranes

Reducing the effective pore size of MOFs is necessary to improve gas screening of mixed matrix membrane (MMMs). However, continuous and precise regulation on pore size of MOFs is still challenging. In this work, dopamine (DA) monomers are proposed to polymerize in situ into pore cages of UIO-66, achieving the tuning of effective pore size for CO2 screening. Meanwhile, the abundant hydroxyl and amino groups in polydopamine (PDA) offer high affinity with CO2 to improve the selective permeation of CO2. Furthermore, additional PDA existing outside UIO-66 at longer polymerization time can intensify the interface compatibility to promote CO2/N2 selectivity. Through increasing PDA content in the cage by prolonging polymerization time, the effective pore size and total pore volume of UIO-66 are decreased by 13.2% and 59.2%, respectively, compared with pure UIO-66. Correspondingly, PDA@UIO-66/Pebax MMM presents greatly improved CO2 separation performances, especially the CO2/N2 selectivity, compared with UIO-66/Pebax MMM. Specifically, both the permeability and selectivity of CO2 increase with the PDA content into the cages increasing, which is attributed to the synergy of reduced pore size and abundant CO2-philic groups in PDA. The PDA10.3@UIO-66/Pebax MMM shows the best CO2/N2 selectivity of 70.55 and CO2 permeability of 94.56 Barrer, 29.2% and 27.9% higher than those of UIO-66/Pebax MMM, respectively. The proposed PDA@UIO-66 based MMMs exhibit superior CO2 separation performance, especially high CO2/N2 selectivity compared with most reported UIO-66 based MMMs. Together with the good long-term stability, the proposed regulating strategy on effective pore engineering of MOFs is promising and competitive in CO2 screening of MOFs based MMM.