Metal organic frameworks (MOF) is a relatively new class of porous material that has attracted great interest in the last two decades. This is because it offers numerous advantages such as high surface area and tailorability. Recent developments have also shown the possibility to render them with a stimuli-responsive property. Such MOFs will exhibit different behaviors depending upon the condition of the external stimulants. This study then focuses on developing MOFs that are responsive towards light as the external stimulant. The application of the materials were then studied both as adsorbents for low-energy post-combustion CO2 capture and porous fillers in mixed matrix membranes (MMMs) for various applications.
Two new generation-2 light-responsive MOFs with azobenzene compound protruding into the pores were then successfully synthesized. The MOFs were built based on the structure of dabco MOF-1 (DMOF-1) and UiO-66. Compared with PCN-123 as their predecessor, both MOFs are relatively more stable and thus did not require special handling for storing. The application of the MOFs were then investigated regarding their applicability for low-energy post-combustion CO2 capture thanks to their satisfactory CO2/N2 selectivity and the possibility to utilize UV light as the sustainable source for adsorbent regeneration because of the presence of azobenzene functionality. Both MOFs then showed a highly efficient CO2 dynamic photoswitching where they could instantaneously release the adsorbed CO2 upon UV light irradiation.
To extend the study, one generation-3 light responsive MOF and another class of porous materials called covalent organic polymers (COP) were also successfully synthesized. Differing from the generation-2 light-responsive MOFs, both materials are built with azobenzene as part of the main framework so their pores are not obstructed with azobenzene functionality. Despite this difference, both porous materials also managed to exhibit a highly efficient CO2 dynamic photoswitching and thus also a suitable candidate to be applied for low-energy post-combustion CO2 capture adsorbent.
As has been stated, apart from just studying the porous materials as adsorbent, the applicability of these porous materials were also further explored when they were incorporated as a porous filler in mixed matrix membranes (MMMs). This approach is chosen thanks to its fabrication simplicity yet could offer satisfactory performance enhancement. This study has then shown the efficacy of having light-responsive MOFs and COP as porous fillers in MMMs.
The resulting MMMs had better CO2/N2 separation compared with their pristine polymers. Moreover, the light-responsive property of the porous materials could also be translated to change the CO2 permeability of the MMMs resulting in a responsive membrane. Lastly, they could also contribute to slow the membrane aging. This approach could then be used as a strategy to fabricate a multifunctional membranes that could be simply fabricated yet applicable in various fields.