Investigating new porous organic materials and their application in porous liquids

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The field of porous materials research is rapidly expanding, with continuing interest in established classes such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), sitting alongside increasing focus on porous organic cages (POCs) and porous liquids.1 These materials are currently in the limelight due to their wide array of applications. This includes gas storage and separation due to their specific pore size allowing entrapment of gases, or the prevention of larger gas molecules to pass through, acting as a molecular sieve.2 Research has also moved into catalysis, as these materials can produce very large surface area to volume ratios, allowing for a high dispersion of catalytic sites, and can also provide a sterically-tailored active site which can allow for enantioselective reactions.3 However, larger frameworks such as MOFs and COFs have their drawbacks, including the inability to be dissolved in organic solvents. Porous organic cages are a relatively new class of porous material, and, unlike extended frameworks, are comprised of discrete molecular units containing a permanent cavity that can assemble in the solid state to form interconnected pore networks. The discrete nature of the cages means they are solution processable and makes them significantly easier to dissolve in organic solvents.The concept of a novel class of porous material, referred to as 'porous liquids', was proposed by James in 2007, and described these materials as liquids with permanent intrinsic porosity.4 Three different types of porous liquids were proposed: Type 1 - neat liquids consisting of molecules with an internal cavity, and rigidity to prevent the collapse and loss of porosity; Type 2 - empty hosts dissolved in a sterically hindered solvent (again, rigid discrete molecules are required to prevent the collapse of the pore, such as that of POCs); and Type 3 - solid microporous frameworks dispersed within a solution to produce a fluid porous material. In all cases, one of the major factors to consider is the structure, and whether they will interact with an intermolecular pore. An example of this includes using long chain alkanes which can pass through the 'window' of a neighbouring molecule and occupy the empty cavity, resulting in the net loss of porosity. This project aims to explore the synthesis of new porous organic cages and porous liquids, building on the work by Cooper et al. who reported the first Type 2 porous liquids in 2015.5 To produce these type 2 porous liquids, the use of highly soluble porous organic cages dissolved in a variety of bulky solvents were investigated. This project also aims to make improvements to gas sorption, as many of the current systems lose porosity as a porous liquid compared to those in the solid-state. This project also aims to explore the possibility of developing molecular cages that have the ability to change under external stimuli such as heat or light, and how this can then be applied to porous liquids.ReferencesP. A. Wright, Microporous Framework Solids, Royal Society of Chemistry, 2007.S. Kitagawa, R. Kitaura, and S. Noro, Angew. Chem. Int. Ed., 2004, 43, 2334-2375.J. Cejka, G. Centi, J, Perez-Pariente, and W. J.Roth, Catal. Today., 2012, 179, 2-15.N. O'Reilly, N. Giri, and S. L. James, Chem. Eur. J., 13, 2007, 3020-3025.N. Giri, M. G. Del Pópolo, G. Melaugh, R. L. Greenaway, K. Rätzke, T. Koschine, L. Pison, M. F. Costa Gomes, A. I. Cooper and S. L. James, Nature, 527, 2015, 216-220.