Interaction Mechanism Study of IL@MOF Based Mixed Matrix Membranes for CO2 Separation Processes

Mixed Matrix Membranes (MMMs) are an attractive alternative as a carbon dioxide mitigation technology. The combination of the properties of polymers and porous metal-organic frameworks (MOFs) with incorporated ionic liquids (ILs) provides a potential enhancement of the membrane performance in CO₂ removal processes (Kinik et al. 2017, Vinoba et al. 2017). However, the good affinity of the materials involved in the synthesis of the membrane is one of the downsides to evaluate in the selection of the constituents. Hence, MOFs have the advantage of their organic nature, making them highly suitable for their incorporation in polymers, as their other multiple characteristics: high surface areas, porous volume, gas adsorption capacities, tunable pore sizes, good thermal and chemical stabilities. However, the rigidity and brittleness of MOFs suppose a disadvantage for their utilization, making the integration of ILs highly appealing. Though, there is still an important lack of knowledge about the chemical structure of the resulting MMM and the nature of the separation involved. Therefore, the main objective of this work is the understanding of the separation performance of synthesized MMMs, composed by a polymeric material and a metal-organic framework supporting ionic liquid (IL@MOF) through the insight into their chemical structure, interaction between the materials involved and separation mechanism.

For this purpose and due to the innumerous possible combinations, four different polymers: Matrimid@5218, PEBAX, polysulfone and polyether sulfone have been considered. The materials selected for the disperse phase in the polymeric matrices include two types of MOFs (ZIF-8 and UiO-66) and two low viscosity ILs and high CO₂ affinity: 1-ethyl-3-methyimizadolium dicyanamide and 1-ethyl-3-methylimidazolium tricyanomethanide ([EMIM][DCA] and [EMIM][TCM], respectively).

An exhaustive characterization of the pristine materials and the synthesized MMMs has been carried out through the following techniques: energy-dispersive x-ray spectroscopy (EDS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), mechanical properties and gas permeation experiments with pure gases N₂, CH₄ and CO₂.

The membranes prepared were found to be dense, thermally stable, with the dispersive filler homogenously distributed along the membrane surface. Besides, the presence of IL made the membranes more flexible than when only a MOF was incorporated in the polymer. Through the results of FT-IR, it was possible to analyse and understand the composition of the membranes in terms of chemical structures and compare them to the pristine materials. Gas permeation results showed an improvement of the membrane performance with higher loading of the filler). The CO₂ permeability increased, while N₂ and CH₄ permeability suffered an important decrease, improving at both cases the selectivity of the membranes.

As future work, different loadings of IL@MOFs will be studied to correlate the content of filler with the transport properties. In addition, to obtain a further understanding of the interactions established and separation mechanism(s) at a molecular level, molecular dynamics simulations will be performed, complementing the experimental results. References

Kinik F.T., Uzun A. and Keskin S., (2017) Ionic Liquid/Metal-Organic Framework Composites: From Synthesis to Applications, ChemSusChem, 10, 2842-2863.

Robeson L.M., (2008) The Upper Bound Revisited, J. Memb. Sci., 320, 1-2.

Vinoba M., Bhagiyalakshmi M., Akqaheem Y., Alomair A.A., Peréz A. and Rana M.S., (2017) Recent Progress of Fillers in Mixed Matrix Membranes for CO₂ separation: A Review, Sep. Purif. Technol., 188, 431-450.