High performance and commercially attractive mixed-matrix membranes were developed for H₂/CO₂ separation via a scalable hollow fiber spinning process. Thin (~300 nm) and defect-free selective layers were successfully created with a uniform distribution of the nanosized (~60 nm) zeolitic-imidazole framework (ZIF-8) filler within the polymer (polybenzimidazole, PBI) matrix. These membranes were able to operate at high temperature (150 °C) and pressure (up to 30 bar) process conditions required in treatment of pre-combustion and syngas process gas streams. Compared with neat PBI hollow fibers, filler incorporation into the polymer matrix leads to a strong increase in H₂ permeance from 65 GPU to 107 GPU at 150 °C and 7 bar, while the ideal H₂/CO₂ selectivity remained constant at 18. For mixed gas permeation, there is competition between H₂ and CO₂ transport inside ZIF-8 structure. Adsorption of CO₂ in the nanocavities of the filler suppresses the transport of the faster permeating H₂ and consequently decreases the H₂ permeance with total feed pressure down to values equal to the pure PBI hollow fibers for the end pressure of 30 bar. Therefore, the improvement of fiber performance for gas separation with filler addition is compromised at high operating feed pressures, which emphasizes the importance of membrane evaluation under relevant process conditions.

Mixed-matrix membranes comprising NH₂-MIL-53(Al) and Matrimid or 6FDA-DAM have been investigated. The metal organic framework (MOF) loading has been varied between 5 and 20 wt%, while NH₂-MIL-53(Al) with three different morphologies, nanoparticles, nanorods, and microneedles has been dispersed in Matrimid. The synthesized membranes have been tested in the separation of CO₂ from CH₄ in an equimolar mixture. At 3 bar and 298 K for 8 wt% MOF loading, incorporation of NH₂-MIL-53(Al) nanoparticles leads to the largest improvement compared to nanorods and microneedles. The incorporation of the best performing filler, i.e., NH₂-MIL-53(Al) nanoparticles, into the highly permeable 6FDA-DAM has a larger effect, and the CO₂ permeability increases up to 85% with slightly lower selectivities for 20 wt% MOF loading. Specifically, these membranes have a permeability of 660 Barrer with a CO₂/CH₄ separation factor of 28, leading to a performance very close to the Robeson limit of 2008. Furthermore, a new non-destructive technique based on Raman spectroscopy mapping is introduced to assess the homogeneity of the filler dispersion in the polymer matrix. The MOF contribution can be calculated by modeling the spectra. The determined homogeneity of the MOF filler distribution in the polymer is confirmed by focused ion beam scanning electron microscopy analysis.

The preparation and performance of mixed matrix membranes (MMMs) based on polybenzimidazole (PBI) and ZIF-8 nanoparticles of different average sizes (50, 70 and 150 nm) as filler are reported. MMMs containing 10 and 20 wt% of ZIF-8 were tested for H₂/CO₂ separation (pre-combustion CO₂ capture) at 150 °C and feed pressures from 3 to 6 bar. The addition of ZIF-8 resulted in a clear improvement in membrane performance. Embedding 20 wt% of ZIF-8 resulted in a H₂ permeability increase of six times and the H₂/CO₂ selectivity increased nearly by 55% compared to the bare PBI polymer membrane. Both permeability and selectivity improved as the filler size increased, due to the lower degree of agglomeration of the largest particles, that may be less active owing to their smaller external surface area. MMMs synthesized using dry 150 nm ZIF-8 filler showed a better performance than those containing wet filler. Apart from agglomeration concerns favoring wet filler handling as evidenced by infrared characterization, the MMM preparation with wet filler is simpler than with dry filler. Finally, the reproducibility of the membranes was confirmed by a European interlaboratory Round Robin test involving three different institutions.