Polymer-metal organic frameworks (MOFs) mixed matrix membranes for gas separation applications

The performance of polymeric membranes is often limited by a trade-off between membrane permeability and selectivity, the so-called Robeson upper bound. Additionally, in high pressure CO2 capture applications, excessive swelling of the polymer membrane often leads to plasticization resulting in decreased separation performances. The main goal of this work was to develop high-performance polymeric Mixed Matrix Membranes (MMMs) with Metal Organic Frameworks (MOFs) as the inorganic additive for low and high-pressure gas separation applications. Approaches to understand and overcome the rather complex behavior of plasticization in MOF-MMMs and the aspect of poor MOF-polymer interfacial compatibility were developed in this work. The respective increase in performance of the MMMs is very much dependent on the MOF crystal structure and its interactions with CO2. Although moderate improvements were shown for the MOF-MMMs over native polyimide membranes at low pressures, the benefits of MOF incorporation (i.e. high CO2 permeability and CO2/CH4 selectivity) became more significant at higher pressures. All MOF-MMMs showed a comparable delay in plasticization pressure irrespective of the MOF structure. As a consequence, membrane selectivity of the MMMs remained sufficiently high, also at higher pressures. This in contrast to the selectivity of the native polymer polyimide, which selectivity significantly decreased at higher pressures. To enhance the integration of the MOF particles in the polymer matrix, a novel route for the preparation of defect free MMMs via a particle fusion approach was developed. This approach improved the MOF-polymer interaction and eliminated MOF incompatibility, agglomeration and particle distribution problems, even at high loadings of MOF. Additionally, surface modification of the matrix polymer polyimide with 1-(3-aminopropyl)-imidazole further improved the interactions between polymer and MOF and led to an excellent ZIF-8/polyimide interfacial compatibility. With this method, we showed it was possible to successfully prepare MMMs with MOF loadings as high as 30 wt.% without any non-selective defects. Upon increasing the ZIF-8 loading, MMMs showed significantly better performance in the separation of CO2/CH4 mixtures as compared to both the native polymer and the MMM prepared by simple polymer blending and solution casting. The CO2 permeability increased up to 200 % combined with a 65 % increase in CO2/CH4 selectivity, compared to native polyimide. Gas sorption studies further confirmed that selective (CO2/CH4) gas transport is mainly governed by the increase in diffusivity selectivity, which in all cases increased more than the solubility selectivity.