(715b) Photoresponsive Azo-Uio-66 Mixed Matrix Membranes for CO2/N2 Separation

Novel responsive molecular separation materials are interesting from a variety of perspectives. For example, materials that can be easily switched between adsorption mode and desorption mode could find new applications in sensors, environmental remediation and controlled release (including as in vivo controlled drug release materials). In another sense, materials that have existing applications with conventional release mechanisms, such as gas sorbents which rely on temperature or pressure swing, could be used in quite radically new applications if they could be operated with new stimuli that is faster acting, cheaper, or more energy efficient.

Our research group has worked with international collaborators for several years to develop photo-responsive gas sorbent materials, which can be rapidly and repeatedly switched between adsorption and desorption mode by illumination with light (usually UV light, but other wavelengths are possible). We showed in small-scale laboratory tests that it was possible to produce a range of different materials that exploit different strategies to impart photo-responsive nature to metal organic framework adsorbents, including through the use of guest molecules, photo-responsive groups pendant to ligand, and directly photo-responsive ligands.

In this most recent work, we have successful produced a highly photoresponsive sorbent based on the very stable UiO-66 MOF topology, through the synthesis and systematic incorporation of a modified ligand with azobenzene functional group (which adsorbs strongly in the UV region, leading to a cis-trans photoisomerization and subsequent transformation of the localized gas sorption environment). We denote this material Azo(X)-UiO-66 where X represents the proportion of the ligands which have been substituted with the modified Azobenzene-functionalised ligand. The sorbents were comprehensively characterized for textural properties using nitrogen sorption at 77K, composition (including quantifying ligand substitution) using 1H-NMR spectroscopy, crystallinity and structure using PXRD, chemical properties using FTIR and Raman spectroscopy, and thermal stability using TGA. The static and dynamic CO₂ sorption was evaluated in a custom-designed adsorption cell which allowed for in-situ illumination with UV light, while carefully controlling sample temperature to ensure local heating effects were minimized.

As shown in the Figure, samples with Azo content ranging from 16.7% (A), through to 33.3% (B), 66.7% (C) and 100% (D) all demonstrated some degree of dynamic photo-switching capability. The optimum was obtained for 66.7% Azo-content, with the samples at higher loadings typically displaying reduced overall CO2 sorption capacity, likely due to the steric hindrance imparted by the relatively bulky pendant azobenzene groups. For all samples, the isoteric heat of adsorption was relatively constant, although slightly higher for the 100% Azo-content sample.

The performance of mixed matrix membranes using Matrimid and PIM-1 as the continuous phase and Azo(X)-UiO-66 as the dispersed phase, was evaluated for CO2/N2 separations. PIM-1 membranes had CO₂ permeability around 7500 Barrer (similar to other reports in the literature), and incorporating Azo(100)- and Azo(66.7)-UiO-66 increased it to around 11 000 Barrer. Azo(33.3)- and Azo(16.7)-UiO-66 increased the permeability further to around 13 000 Barrer, indicating that all filler particles introduced additional free volume, but the samples with higher CO₂ sorption capacity (16.7 and 33.3) led to a higher mixed-matrix membrane permeability, consistent with standard models for gas transport in mixed-matrix membranes. Both single gas studies and mixed-gas experiments were used to evaluate the selectivity of the mixed-matrix membranes, revealing that the mixed-ligand filler particles could increase selectivity for CO₂ over N₂ from around 14 for pristine PIM-1 to 19 for mixed matrix membranes with Azo(100)-UiO-66.

The photo-response properties of mixed-matrix membranes containing photo-responsive filler particles is more subtle than the powder properties, but at a first approximation follows the trend that would be expected assuming mixed-properties response. Our initial results show that it is possible to prepare mixed-matrix membranes that demonstrate photo-responsive behavior that can be dynamically switched with application of external light, however the effect is modest (while still being statistically significant). This presentation will highlight the latest developments in our research in this area, including the development of a new test station with robust capabilities to evaluation photo-responsive membranes (not just mixed-matrix membranes), and new collaborations with materials scientists to test a wide range of novel membrane materials. We also present a collaboration with data scientists to incorporate exceptional levels of data storage, analysis and open data sharing through the adoption of the Chemotion electronic lab notebook and data repository.