Preparation of Functional Microporous Organic Polymers and Their Applications for Heterogeneous Catalysis and Dye Adsorption

Abstract

Microporous organic polymers (MOPs) have many remarkable properties such as high surface area, low density, microporosity, and physicochemical stability. Various functional groups can be introduced to MOPs by the selection of reactants and post-modification, which facilitates practical uses of MOPs in various fields. However, most MOPs are obtained by insoluble powders because of their highly crosslinked structures, which causes poor processability of MOPs. In this study, various functional MOPs having shape-controlled structures were prepared and used for heterogeneous catalysis and dye adsorption. Firstly, a MOP sponge was prepared using a homogenized electrospun nanofiber as the reinforcement. The Sonogashira-Hagihara coupling reaction of 2,5-dibromoaniline and 1,3,5-triethynylbenzene in a dispersion of homogenized electrospun nanofibers (PVASi) produced the compressible MOP composite with a core-shell structure (PVASi@TEDB-NH2). The polymer was uniformly grown on the surface of the nanofibers because TEDB-NH2 had primary amino groups that could form hydrogen bonds with the hydroxyl groups on the surface of PVASi. PVASi@TEDB-NH2 showed an average density and a BET surface area of 30.4 mg cm-3 and 447 m2g-1, respectively. The composite sponge was used for the removal of an organic dye dissolved in water. When PVASi@TEDB-NH2 was manually compressed and released in an aqueous methylene blue (MB) solution, the dye adsorption occurred rapidly. Secondly, a microporous catalytic membrane based on a hypercrosslinked polymer (HCP) was prepared. A HCP-based nanofibrous membrane was synthesized via Friedel-Crafts reaction of 1,1-bi-2-naphthol in the presence of an aminated polyacrylonitrile (APAN) nanofibrous membrane as a substrate. The HCP was uniformly grown on the surface of the APAN nanofiber, which conferred a hierarchical porosity to the membrane. The HCP-based nanofibrous membrane showed a good mechanical strength and microporosity with a Brunaure-Emmett-Teller (BET) surface area of 375 m2 g-1. The HCP-based nanofibrous catalytic membrane (APAN-HCP-Pd) was prepared via the in-situ growth of palladium nanoparticles inside the membrane. The application of APAN-HCP-Pd as a catalytic membrane was investigated for the reduction of 4-nitrophenol. Thirdly, a compressible monolithic catalyst based on a MOP nanotube sponge was prepared. The monolithic MOP sponge was synthesized via Sonogashira-Hagihara coupling reaction between 1,4-diiodotetrafluorobenzene and 1,3,5-triethynylbenzene in a co-solvent of toluene and TEA (2:1, v/v) without stirring. The MOP sponge had an intriguing microstructure, where tubular polymer fibers having a diameter of hundreds of nanometers were entangled. It showed hierarchical porosity with a Brunauer-Emmett-Teller (BET) surface area of 512 m2 g-1. The MOP sponge was functionalized with sulfur groups by the thiol-yne reaction. The functionalized MOP sponge exhibited a higher BET surface area than the MOP sponge by 13 % due to the increase in the total pore and micropore volumes. A MOP sponge-Ag heterogeneous catalyst (S-MOPS-Ag) was prepared by in-situ growth of silver nanoparticles inside the sulfur-functionalized MOP sponge by the reduction of Ag+ ions. The catalytic activity of S-MOPS-Ag was investigated for the reduction reaction of 4-nitrophenol in an aqueous condition. When S-MOPS-Ag was compressed and released during the reaction, the rate of the reaction was considerably increased. S-MOPS-Ag was easily removed from the reaction mixture owing to its monolithic character and was reused after washing and drying. Lastly, compressible polyimide composite having metal binding sites was prepared by in situ polymerization inside a melamine sponge. 1,3,5-Tris(4-aminophenyl)benzene and pyromellitic dianhydride were used as a triamine and a dianhydride monomer, respectively, to construct the microporous framework, and 5-amino-1,10-phenanthroline was used as a functional monomer. The microporous polyimide containing phenanthroline groups (MPI-Phen) was obtained as insoluble powders. However, when the polymerization was carried out in the melamine sponge, MPI-Phen formed a coating layer on the sponge skeletons. The melamine sponge/microporous polyimide composite (MS/MPI-Phen) had an open cellular structure with a hierarchical porosity composed of macropores between the sponge skeletons and meso- and micropores of the MPI-Phen coating. It showed the higher compressive strength than the melamine sponge, indicating the reinforcement by the microporous polymer. The BET surface areas of MPI-Phen and MS/MPI-Phen were 723 m2g-1 and 524 m2g-1, respectively. Pd(II) ions were coordinated with the phenanthroline groups of MS/MPI-Phen for heterogeneous catalysis (MS/MPI-Phen-Pd). The catalytic activity of MS/MPI-Phen-Pd was evaluated for the Suzuki coupling reaction between bromobenzene and phenylboronic acid.