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Project

Development of MOF-filled polymeric membranes for gas separation.

State of the art: Currently, membranes with gas separation properties are of fundamental importance to accelerate progress toward an energy-efficient economy and reduce greenhouse gas emission. Polymeric membranes with easy fabrication and low cost have been explored for gas separation. Many efforts to improve the performance of polymeric membranes to broaden the different membrane applications have been reported, including polymer crosslinking to enhance the plasticization resistance. HT screening methods proved in this to be a powerful tool allowing rapid screening of a large number of parameters in membrane development and membrane process optimisation. However, there is a ubiquitous limitation of polymer-based membranes: a trade-off between permeability and selectivity. To overcome this obstacle, a popular approach is to introduce inorganic fillers, e.g. metal-organic frameworks (MOFs), into the polymeric matrix to fabricate organic/inorganic hybrid membranes. MOFs, consisting of metal ions and organic ligands, show superior compatibility with polymers over traditional inorganic fillers, and hence allow preparing high quality hybrid membranes. MOFs with tailored porosity, size- and shape-selective properties, demonstrating superior selective adsorption efficiency, have been widely studied in gas separation and gas storage. Recently, many attractive new types of MOFs, flexible MOFs, with expansion and contraction or ‘‘breathing” properties show interesting adsorption behaviours and separation ability to gas pairs through both their well-defined window aperture dimensions and the dynamic molecular gateways. The distribution and long term stability of MOFs into a polymeric matrix is of vital importance. The biggest challenge is the poor adhesion between fillers and polymer, leading to the formation of defects at their interface. A great number of efforts have been taken to improve their compatibility and interface morphology, including surface functionalization, adding interface agents, etc. Nevertheless, several unavoidable problems, particularly MOF’s cavity or pore blockage, limit the practical applications. Recently, a thermal treatment inducing crosslinking of the polymer together with aporphisation of the filler to enhance interfacial filler/polymer compatibility has been reported by Prof. Vankelecom’s group. Resulting membranes showed a record high CO2/CH4 selectivity of 164 (above the Robeson plot) as well as more resistance to plasticisation owing to the densely packed network structure after cross-linking of the polymer itself and the covalent bond between the ZIF-8 particles and the polymer. However, the resulting ZIF-8/PI Matrimid® membranes were not porous enough to allow sufficient CO2 permeation, mainly due to substantial collapse of the MOF structure. A higher porosity of the fillers after the thermal treatment is thus still needed. Project rationale: To avoid the drastic drop in permeability of the MOF-filled membranes upon thermal treatment, a much more porous core of the fillers will be designed to ensure high fluxes, while the shell of the structure will remain capable of amorphising to create highly selective channels and keep the excellent interaction with the surrounding polymer phase. The proposal thus aims to explore the incorporation of core/shell MOF structures in polymer matrices to increase the fluxes of highly selective membranes for gas separation. Other MOFs than in the original paper or other types of porous inorganic materials, e.g. silica or zeolites, can be used as core materials for a ZIF-8 shell, thus obtaining porous@ZIF-8 hybrid materials with a highly porous core structure and a very selective skin. Thermal oxidative cross-linking (XL) of porous@ZIF-8 with the polymer matrix, e.g. polyimide Matrimid®, will further enhance adhesion between fillers and polymer matrix in MOF filled PI-XL membranes. The resulting hybrid membranes combine the highly porous structure of the core aterial with the highly selective nature of the amorphised MOF shell, while the polymer/filler interaction will be enhanced. In addition, the mechanical properties of the hybrid membranes would be further increased owing to the integrity of the core porous. These are expected to provide high stability, tunable porosity, and both high permeability and separation selectivity. Two types of MOF-based core/shell structures will be taken into account: 1) Concerning CO2/CH4 separation, we will develop selective MOFs, including MOF-74-Mg, UiO-66-NH2, and HKUST-1, which already demonstrated their high stability to further thermal treating. MOF-74-Mg with 1D hexagonal tunnel and UiO-66-NH2 with restricted pore size and rigid cages demonstrated high selectivity to CO2 of CO2/CH4 gas pair owing to open Mg sites and –NH2 functional groups respectively. On the other hand, HKUST-1 is one of the best performing materials with respect to methane storage, which can be used as filler slowing permeability rate of CH4. In addition, post-synthesis modification strategies can be explored to prepare more stable membranes. Recently, fine-tuning of the hydrophobicity of the membrane surface via incorporation of amine functionalities demonstrated improved stability. Unsaturated Zn2+, originated from the building blocks of the amorphous ZIF-8 shell, can coordinate with hydrophobicity amine groups to overcome the vulnerable of Zn2+ open sites to water molecules to enhance stability of hybrid membranes under harsh conditions. Another approach to improve the stability of the obtained MOF-filled amorphous ZIF-8/PI Matrimid® membranes is to eliminate Zn2+ by a weak acid etching method, to obtain hybrid membranes with high separation in humid environment. 2) Concerning flexible MOFs, we will focus on MIL-53 and the IRMOF and ZIF series. In this case, flexible MOFs as fillers are expected to adjust different gas permeability triggered by external stimuli, such as temperature, pressure or light, which significantly extend the functionality of the fillers and results in smart membranes. Project description: The nano-sized MOFs will be prepared in gram–scale using literature procedures. Furthermore, novel MOF@ZIF-8 core shell hybrid materials will be prepared by using in-situ preparative hydro/solvothermal methods, including internal extended growth in-situ seeding and secondary crystal growth. The MOF@ZIF-8 hybrid materials will be characterized by XRD, ICP-AES, 1H-NMR, TEM, SEM, N2-adsorption and TG. Amorphised ZIF-8 in crosslinked PI membranes will be obtained by in-situ, thermal oxidation cross-linking methods. The MOF@ZIF-8 will be embedded in the polymer, subsequently treated at high temperature, up to 350℃. The composite polymers will be characterized by ATR-FTIR, Raman spectroscopy and TGA. SEM and TEM will allow analyzing the morphology of the final membranes and BET will be used to establish the porosity of the fillers. Resulting membranes will then be studied for gas separation, including CO2/CH4 and multi gas mixtures including water vapour by high-throughput gas separation system (HTGS) . We expect to develop innovative separation membranes with enhanced performance. The preliminary data will be used to optimize the structure of the hybrids, by a careful optimization of their molecular constituents: the porous core (zeolites with different pore structure, counterions, selectivity and stability (e.g. zeolite A, X, Y, silicalite,…), silica with different porosities and polarities, MOFs with different thermal behavior and porosity), the MOF-shell, the polymer selection (e.g. fluorinated PI), the thermal treatment procedure (time, T, atmosphere), separation procedures (pressure, temperature, gas composition, addition of minor components).

Date:21 Aug 2019 →  21 Aug 2023
Keywords:polymeric membranes, Metal-Organic Frameworks, gas separation
Disciplines:Polymer composites, Membrane technologies
Project type:PhD project