Physical aging in glassy mixed matrix membranes; tuning particle interaction for mechanically robust nanocomposite films

Despite the exceptional separation performance of modern glassy mixed matrix membranes, these materials are not being utilized to improve the performance of existing membrane technologies. Nano-sized additives can greatly enhance separation performance, and have recently been used to overcome age-related performance loss of high performance MMMs. However nano-additives also compromise the structural integrity of films and little is known on how physical aging affects their mechanical properties over time. A solution for both physical aging and mechanical instability is required before these high performance materials can be utilised in industrial membrane applications. Here, we examine physical aging in mixed matrix membranes through mechanical properties and gas permeation experiments using three glassy polymers, Matrimid® 5218, poly-1-trimethylsilyl-1-propyne (PTMSP), and a Polymer of Intrinsic Microporosity (PIM-1); and a range of nano-scale additives previously shown to enhance gas separation performance. We find polymer-additive interactions strongly influence local physical aging and play a key role in determining the overall material properties of glassy nanocomposite films. Strong interface interactions can slow physical aging, and may not correlate to reinforced or age-stable films. Whereas traditionally ‘incompatible’ nanocomposites exhibit mechanical properties that can improve over time and even outperform their native polymers. Tuning polymer-additive interactions is vital to achieving the physical aging, mechanical stability, and permselectivity requirements of advanced mixed matrix membrane technologies and reducing the enormous global energy cost of separation processes

Matrimid-JUC-62 and Matrimid-PCN-250 mixed matrix membranes displaying light-responsive gas separation and beneficial ageing characteristics for CO2/N2 separation

The performance of two generation-3 light-responsive metal-organic framework (MOF), namely JUC-62 and PCN-250, was investigated in a mixed matrix membrane (MMM) form. Both of them were incorporated inside the matrimid as the polymer matrix. Using our custom-designed membrane testing cell, it was observed that the MMMs showed up to 9% difference in CO2 permeability between its pristine and UV-irradiated condition. This shows that the light-responsive ability of the light-responsive MOFs could still be maintained. Thus, this finding is applicable in designing a smart material. Apart from that, the MMMs also has the potential to be applied for post-combustion carbon capture. At loadings up to 15 wt%, both CO2 permeability and CO2/N2 ideal selectivity could be significantly improved and surpassed the value exhibited by most of the MOF-matrimid MMM. Lastly the long term performance of the MMM was also evaluated and it was observed that both MMM could maintain their performance up to 1 month with only a slight decrease in CO2 permeability observed for 10 wt% PCN-250-matrimid. This study then opens up the possibility to fabricate a novel anti-aging multifunctional membrane material that is applicable as a smart material and also in post combustion carbon capture applications

Missing Linker Defects in a Homochiral Metal-Organic Framework: Tuning the Chiral Separation Capacity

Efficient chiral separation remains a very challenging task due to the identical physical and chemical properties of the enantiomers of a molecule. Enantiomers only behave differently from each other in the presence of other chiral species. Homochiral metal–organic frameworks (MOFs) have received much attention for their promising enantioseparation properties. However, there are still challenges to overcome in this field such as high enantiomeric separation. Structural defects play an important role in the properties of MOFs and can significantly change the pore architecture. In this work, we introduced missing linker defects into a homochiral metal–organic framework [Zn2(bdc)(l-lac)(dmf)] (ZnBLD; bdc = 1,4-benzenedicarboxylic acid, l-lac = l-lactic acid, dmf = N,N′-dimethylformamide) and observed an increase in enantiomeric excess for 1-phenylethanol of 35% with the defective frameworks. We adjusted the concentration of monocarboxylic acid ligand l-lactic acid by varying the ratio of Zn2+ to ligand from 0.5 to 0.85 mmol. Additionally, a defective framework was synthesized with propanoic acid as modulator. In order to elucidate the correlation between defects and enantiomeric excess, five characterization techniques (FTIR, TGA, 1H NMR, ICP, and PXRD) were employed. Full width at half-maximum analysis (fwhm) was performed on the powder X-ray diffraction traces and showed that the higher concentration of monocarboxylic acid MOFs were isostructural but suffered from increased fwhm values

Upcycling a plastic cup: one-pot synthesis of lactate containing metal organic frameworks from polylactic acid

Waste PLA can be upcycled to metal organic frameworks of potential high value in a one-pot synthesis scheme, where PLA depolymerisation occurs in situ. Three homochiral lactate based frameworks were successfully synthesised and characterised from PLA as a feed source, including ZnBLD. The chiral separation ability of ZnBLD was maintained

Investigation of mass transport processes in a microstructured membrane reactor for the direct synthesis of hydrogen peroxide

Microstructured membrane reactors present a promising approach to master the productivity and safety challenges during the direct synthesis of hydrogen peroxide. However, various mass transport processes occur in this complex system. In order to gain a deeper understanding of these processes, the saturation and desaturation behaviour of the liquid reaction medium with the gaseous reactants is investigated experimentally to examine possible cross-contamination. Moreover, the employed PDMS membrane’s permeances to hydrogen and oxygen are researched at different pressures, by using a variable-pressure/constant-volume setup for the behaviour at ambient pressure and a constant-pressure/variable-volume setup for the behaviour at elevated pressures. A mathematical model in MATLAB is applied to simulate the results. It is shown that a certain desaturation of the gasses through the membrane occurs, and the results are underlined by the modelled ones using a solution-diffusion model in MATLAB. Thus a constant flushing of the gas channels of the reactor is required for safety reasons. Moreover, the measured permeance values indicate that the species transport is mainly limited by the diffusion in the liquid phase and not the membrane resistance

Metal-organig framework MIL-68(In)-NH2 on the membrane test bench for dye removal and carbon capture

The metal-organic framework (MOF) MIL-68(In)-NH2 was tested for dye removal from wastewater and carbon capture gas separation. MIL-68(In)-NH2 was synthesized as a neat, supported MOF thin film membrane and as spherical particles using pyridine as a modulator to shape the morphology. The neat MIL-68(In)-NH2 membranes were employed for dye removal in cross-flow geometry, demonstrating strong molecular sieving. MIL-68(In)-NH2 particles were used for electrospinning of poylethersulfone mixed-matrix membranes, applied in dead-end filtration with unprecedented adsorption values. Additionally, the neat MOF membranes were used for H2/CO2 and CO2/CH4 separation

Dynamic photo-switching in light-responsive JUC-62 for CO2 capture

In this paper, we demonstrate the highly efcient photo-switching ability of a Cu-azobenzene tetracarboxylate MOF (JUC-62) for low-energy CO2 capture. Under UV light irradiation, both at 273 and 298K, JUC-62 showed 51% and 34% lower CO2 uptake, respectively, than when UV light was of. Its dynamic CO2 uptake also matched well with its static condition. Storing it at ambient condition was also found not to destroy its framework structure and its dynamic photoswitching property could still be maintained

High ion-exchange capacity semi-homogeneous cation exchange membranes prepared via a novel polymerization and sulfonation approach in porous polypropylene

Semi-homogeneous cation exchange membranes with superior ion exchange capacity (IEC) were synthesized via a novel polymerization and sulfonation approach in porous polypropylene support. The IEC of membranes could reach up to 3 mmol/g, due to high mass ratio of functional polymer to membrane support. Especially, theoretical IEC threshold value agreed well with experimental threshold value, indicating that IEC could be specifically designed without carrying out extensive experiments. Also, sulfonate groups were distributed both on membrane surface and across the membranes, which corresponded well with high IEC of the synthesized membranes. Besides, the semi-finished membrane showed hydrophobic property due to formation of polystyrene. In contrast, the final membranes demonstrated super hydrophilic property, indicating the adequate sulfonation of polystyrene. Furthermore, when sulfonation reaction time increased, the conductivity of membranes also showed a tendency to increase, revealing the positive relationship between conductivity and IEC. Finally, the final membranes showed sufficient thermal stability for electrodialysis applications such as water desalination

A new and highly robust light-responsive Azo-UiO-66 for highly selective and low energy post-combustion CO₂ capture and its application in a mixed matrix membrane for CO₂/N₂ separation

A new and robust generation-2 light-responsive MOF with UiO-66 topology applicable for post combustion CO2 capture has been successfully synthesized and is described in this article. Azo-UiO-66 shows a satisfactory performance for CO2/N2 separation as observed through high CO2/N2 selectivity. Furthermore, due to the presence of azobenzene groups, Azo-UiO-66 also exhibits a very efficient CO2 photoswitching uptake, a characteristic that has never been observed in any generation-2 light-responsive MOF. Combined together with its robust character, this makes Azo-UiO-66 a promising candidate for highly selective and low energy CO2 capture applications. To further apply this material, Azo-UiO-66 was incorporated in Matrimid to form mixed matrix membranes (MMM). Composites with up to 20 wt% of Azo-UiO-66 were fabricated and tested. The resulting MMM showed increased performance in terms of CO2 permeability and CO2/N2 selectivity compared with the similar MOF-based MMM composites. This then shows another promising application of Azo-UiO-66 as a filler to enhance polymeric membrane performance for CO2 separation