A microporous Al trimesate-based metal-organic framework (MOF), denoted MIL-96-(Al), was selected as a porous hybrid filler for the processing of mixed matrix membranes (MMMs) for CO₂/N₂ postcombustion separation. First, the structural model of MIL-96-(Al) initially reported was revisited using a combination of synchrotron-based single-crystal X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, and density functional theory (DFT) calculations. In a second step, pure MIL-96-(Al) crystals differing by their size and aspect ratio, including anisotropic hexagonal platelets and nanoparticles of about 70 nm in diameter, were prepared. Then, a combination of in situ IR spectroscopy, single-gas, and CO₂/N₂ coadsorption experiments, calorimetry, and molecular simulations revealed that MIL-96-(Al) nanoparticles show a relatively high CO₂ affinity over N₂ owing to strong interactions between CO₂ molecules and several adsorption sites such as Al³⁺ Lewis centers, coordinated water, and hydroxyl groups. Finally, the high compatibility between MIL-96-(Al) nanoparticles and the 6FDA-DAM polymer allowed the processing of homogeneous and defect-free MMMs with a high MOF loading (up to 25 wt %) that outperform pure polymer membranes for CO₂/N₂ separation.

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.

Adsorption of CO₂ on MIL-53(Al) and NH₂-MIL-53(Al) has been studied by Fourier transform infrared (FTIR) spectroscopy at different temperatures and equilibrium pressures. For better interpretation of the spectra ¹³CO₂ was also utilized. It is established that with both samples at low coverages CO₂ forms O-bonded complexes with the structural OH groups (OH⋯O¹²CO). These species are characterized by μ₃(¹²CO₂) at 2337-2338 cm^-1 and two μ₂(¹²CO₂) modes around 662 and 650 cm^-1. Simultaneously, the μ(OH) modes of the hydroxyl groups are red-shifted, while the δ(OH) modes are blue-shifted. At higher coverages (OH⋯O¹²CO)₂ dimeric species are formed and this leads to a decrease of the μ₃(CO₂) frequency by 2-4 cm^-1. This change is due to vibrational interaction as proven by the observation that the frequency remains practically unaffected for (OH⋯O¹²CO) (OH⋯O¹³CO) dimeric species. Interaction between dimers leads to additional slight decrease of the value of μ₃(CO₂). In parallel with the CO₂ adsorption a partial transformation of the material from large-pore to narrow-pore form occurs. Far before CO₂ interacts with all hydroxyl groups, polymeric CO₂ species are produced within the MIL-53(Al) sample. They are characterized by a split μ₃(CO₂) mode with a pronounced component at 2340 cm^-1. The formation of these species involves some of the dimers and is accompanied by a reopening of the MIL-53 structure. Analysis of the shift of the OH modes led to the conclusion that the polymeric moiety interacts strongly with one OH group and more weakly with several other hydroxyls. No polymeric species were observed with the NH₂-MIL-53(Al) sample which is associated with the more stable narrow-pore structure of this material. However, evidence of interaction between CO₂ and the hydroxyls H-bonded to amino groups was found.

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.