Mixed matrix membranes (MMMs) have been obtained by blending an aromatic ortho-hydroxypolyimide (PIOH) or an ortho-acetylpolyimide (PIOAc) with different loading amounts (15 and 30 wt %) of a microporous polymer network (PPN), which was obtained from the reaction of triptycene and isatin. The excellent thermal resistance of the PPN (above 500 °C) allowed it to be used as a filler to successfully prepare thermally rearranged polybenzoxazole (TR-PBO)-MMMs obtained from the thermal treatment of the above MMMs. Moreover, PPN showed relatively good compatibility with the polyimide matrix, which improved the TR-PBO formation. The gas separation performances of these MMMs before and after the thermal process were studied for five representative gases (He, O₂, N₂, CO₂, and CH₄). For the MMMs derived from ortho-functional polyimides, the gas permeability considerably increased for all of the gases, whereas the selectivity for gas pairs, such as CO₂/N₂ and CO₂/CH₄, remained similar. Thus, the selectivity-permeability performance of PIOH- and PIOAc-MMMs containing 30 wt % of PPN (PIOH30 and PIOAc30) surpassed the 1991 Robeson limit for the CO₂/CH₄ gas pair. In the case of TR-PBO-MMMs (TROH and TROAc-MMMs), the thermal rearrangement process led to an increase in the gas permeability, showing values much better than those observed for the TR-PBO material employed as a MMM matrix. The selectivity values were a little bit lower than the pristine TR-PBO membranes. The CO₂ permeability of TROAc30 was 1036 barrer with a CO₂/CH₄ selectivity of 28, surpassing the 2008 Robeson limit.

Mixed matrix membranes (MMMs), derived from three aromatic polyimides (PIs), and an affordable porous organic polymer (POP) having basic bipyridine moieties were prepared. Matrimid and two fluorinated polyimides, which were derived from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride and 2,2′-bis(4-aminophenyl)hexafluoropropane (6F6F) or 2,4,6-trimethyl-m-phenylenediamine (6FTMPD), were employed as polymer matrixes. The used POP was a highly microporous material (surface area of 805 m² g⁻¹) with excellent thermal and chemical stability. The MMMs showed good compatibility between the PIs and POP, high thermal stabilities and glass transition temperatures superior to those of the neat PI membranes, and good mechanical properties. The addition of POP to the matrix led to an increase in the gas diffusivity and, thus, in permeability, which was associated with an increase in the fractional free volume of MMMs. The increase in permeability was higher for the less permeable matrix. For example, at 30 wt.% of POP, the permeability to CO₂ and CH₄ of the MMMs increased by 4- and 7-fold for Matrimid and 3- and 4-fold for 6FTMPD. The highest CH₄ permeability led to a decrease in CO₂/CH₄ selectivity. The CO₂/N₂ separation performance was interesting, as the selectivity remained practically constant. Finally, the POP showed no molecular sieving effect towards the C₂H₄/C₂H₆ and C₃H₆/C₃H₈ gas pairs, but the permeability increased by about 4-fold and the selectivity was close to that of the matrix. In addition, because the POP can form metal ion bipyridine complexes, modified POP-based MMMs could be employed for olefin/paraffin separations.