Incorporation of metal-organic frameworks (MOFs) in polymeric membranes for gas separation applications

The concept of mixed-matrix membranes (MMM) has been highly attractive for gas separation research since the 1970s, when it was discovered that the addition of zeolite 5A in a polymer results in an increase in the gas diffusion time-lag compared to the neat polymer. MMMs are, in essence, composite materials consisting of filler particles homogeneously dispersed in a polymeric matrix. They are designed for the purpose of exploiting the desirable properties of their counterparts. Over the last few decades, numerous types of fillers ranging from carbon molecular sieves, carbon nanotubes, ordered mesoporous silica and zeolites have been used to prepare highly productive MMMs. Non-porous fillers have been utilised to manipulate polymer chain packing and modify the free volume of the polymer, while porous fillers achieved molecular sieving to separate gases according to their size and shape. Fillers that possess well-defined pore sizes, such as zeolites, showed great potential but their distribution in the polymer matrix and adhesion therein has proven to be problematic. Distribution problems are largely caused by the necessity of post-synthesis calcination for removing leftover templates, which also causes the particles to form strong aggregates. Breaking down these aggregates is possible by strong mixing methods such as ultrasonication, which makes the membrane fabrication method more complicated and inefficient in terms of time and energy. On the other hand, the intrinsic lack of affinity between the inorganic zeolite and the organic polymer phase causes the formation of non-selective defects across the membrane cross-section, resulting in significant losses in selectivity. The solutions suggested for promoting adhesion, such as compatibilisers, silylation etc. have also proven to be material-intensive. An alternative filler material, metal organic frameworks (MOF) have stirred up excitement in MMM research, as well as many of their fields of application. MOFs are a new class of hybrid materials that consist of metal ions bound together with organic linkers that form a porous framework. In terms of gas separation, MOFs are very attractive materials owing to their tailorable chemistry, tunable composition, well-defined pore size, pore flexibility, and breathing effects. They essentially consist of metal ions bridged with organic linkers that form a porous framework. Contrary to inorganic fillers such as zeolites, the organic linkers in MOFs offer better adhesion to the polymer, providing an advantage in preventing membrane defects. Unfortunately, MOFs are not completely free of problems of aggregation and aggregate detachment. Recent research has shown that breaking down aggregates by ultrasonication triggers drastic distortion in the morphology and particle size distribution of MOF particles. This work aimed at preventing the formation of aggregates to prepare membranes with high loading, so that the selective behaviour of MOFs can be exploited to the fullest. As the first step of this research, a novel method was devised to use non-dried MOFs to prepare MMMs. Attempts at synthesising MOFs inside a polymer solution did not yield the desired MOF loading. Moreover, the unreacted MOF precursors plasticised the membrane, resulting in gel-like forms. Higher MOF loading was achieved by using separately synthesised MOFs without drying. A comparison with MMMs comprising dried MOFs showed that, MMMs with non-dried MOFs did nost suffer from MOF aggregation or MOF-polymer detachment, and showed better gas separation performance. This trend was consistent for MMMs with ZIF-8, ZIF-7 and NH2-MIL-53(Al), proving that this principle is generic. These MOF-loaded membranes were further subjected to high thermal treatment conditions to achieve very high mixed-gas selectivities. Two well-known MOFs for gas separation, ZIF-8 and ZIF-7 were used. The controlled thermal treatment resulted in a synergy of MOF amorphisation and cross-linking. Amorphisation refers to the disruption of long-range ordering of MOF building units, followed by densification and loss of crystallinity. The Zn-N bonds in ZIF-8 is broken during amorphisation, creating unsaturated Zn2+ sites, which in turn can act as an additional cross-linker for the polymer chains. Combined with the heat-induced polymer-polymer cross-linking, polymer-MOF cross-linking creates a strong interwoven structure that increases the CO2/CH4 selectivity, and stabilises the MMMs against CO2-induced plasticisation. Mixed-gas separation analyses have revealed the outstanding separation performance of these membranes, which surpassed the Robeson upper-bound of 1991, and reached that of 2008.This work is the first reported concept of this synergy between MOF amorphisation and cross-linking in membranes. Finally, the same method was applied to the incorporation of NH2-MIL-53(Al), a MOF that shows breathing behaviour, in MMMs. Following the thermal treatment as described in the previous part, even higher gas separation performances were achieved. XRD analysis revealed that the polymer chains penetrated the MOF pores, partially blocking the pore entrances. Owing to this partial blockage, the MOF pores were not completely sealed upon transition to the narrow-pore form of the MOF. Moreover, these MOF particles are thought to provide a cross-linked network at the mouth of the pore, which resulted in improved selectivities. The polymer chains acting like foundation piles provided a network that was able to maintain its separation properties at pressures as high as 40 bar.