Highly Permeable Mixed Matrix Membranes of Thermally Rearranged Polymers and Porous Polymer Networks for Gas Separations

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.