Enhanced H2/CO2 separation of ZIF-8/NPBI mixed matrix membranes by forming cross-linking networks and crystal defects via acid doping

[Display omitted] •Complex cross-linking networks and tiny defects can be formed by doping MMMs with H3PO4.•Adjusting the doping level of H3PO4 can adjust the cross-linking degree of MMMs and the defect degree of ZIF-8.•The 10% ZIF-8/NPBI-9 h MMM with the optimum interaction between H3PO4 and polymer/ZIF-8 showing the best H2/CO2 separation performance. Fabricating a membrane with high H2/CO2 separation performance at high temperatures is highly desired. Herein, a cross-linked zeolitic imidazolate framework 8/poly[2,2′-(1,4-naphthalene)-5,5′-bibenzimidazoles] (ZIF-8/NPBI) mixed matrix membranes (MMMs) with tiny defects in ZIF-8 is fabricated via a phosphoric acid doping approach. The cross-linked NPBI chains and partially etched ZIF-8 by the acid result in significantly improved H2 permeability and H2/CO2 mixed gas selectivity. At 150 °C, compared to pure PBI membrane, the H2 permeability of the acid-treated 10% ZIF-8/NPBI membrane is enhanced from 39.97 Barrer to 98.36 Barrer, and the H2/CO2 mixed gas selectivity increases from 4.6 to 8.9, eventually exceeding the Robeson's upper bound predicted for 150 °C. When tested with the gas mixtures of 50% H2/50% CO2 continuously, the resulting membrane exhibits a stable H2 permeability of around 74 Barrer and H2/CO2 mixed gas selectivity of ≈8.69 at 150 °C for 24 h. This simple approach of improving H2 permeability and H2/CO2 mixed gas selectivity through crosslinking and etching provides a new approach for designing MMMs for H2/CO2 separation at high temperatures.