New element organic frameworks via Suzuki coupling with high adsorption capacity for hydrophobic molecules† Marcus Rose, a Nicole Klein, a Winfried B€ ohlmann, b Bertram B€ ohringer, c Sven Fichtner d and Stefan Kaskel * a Received 15th February 2010, Accepted 27th May 2010 DOI: 10.1039/c003130e We present new highly microporous element organic frameworks synthesized by the Pd catalyzed Suzuki coupling reaction. They show specific surface areas of up to 1380 m 2 g 1 with a strong hydrophobic character. Thus, they are interesting for the adsorption of non-polar substances. By variation of the organic linkers, the modular concept of the materials in analogy to the metal–organic frameworks is demonstrated. The polymeric materials have thermal stability up to 573 K and show no decomposition in aqueous environment, allowing excellent handling and processing. They are accessible by a basic synthetic approach, and by their chemical and thermal stabilities they may provide adequate properties for applications in many fields, especially in adsorptive separation processes and storage of non-polar gases. Introduction The search for novel, highly porous adsorbents as alternatives to established materials such as zeolites and activated carbons has led to the discovery of several new classes of predominantly microporous materials. A bottom-up approach for the produc- tion of such materials uses a modular concept to build a frame- work out of rigid, mainly organic molecules. In the case of metal– organic frameworks (MOFs), 1–3 the most widely explored class of that kind of materials, the polyfunctional organic linkers are coordinated to metal ions or clusters, the so called connectors. Depending on the potential application, some MOFs show disadvantages such as low stability against water 4 and toxicity through metals like chromium. 5 In recent years quite a number of novel classes of metal-free materials have been developed. Most of them consist of poly- meric networks. One of the established approaches of the clas- sical polymeric chemistry to produce microporous polymers is the crosslinking of linear polymers in solution or swollen pre- crosslinked polymers to produce a hypercrosslinked polymer (HCP) with voids between the inflexible polymeric chains. For example materials consisting of hypercrosslinked polystyrene and polyaniline were reported. 6,7 In contrast to the postsynthetic crosslinking, HCPs can also be synthesized by hypercrosslinking of polyfunctional monomers from solution. 6,8 Another group of polymers, polyamide and -imide (PA and PI) frameworks are built up from a rigid tetrafunctional spirobifluorene monomer. 9 The PI frameworks show specific surface areas of up to 1000 m 2 g 1 while the PA frameworks have a pore system not accessible for nitrogen molecules but for hydrogen molecules. A similar approach of polymerization of larger molecules to obtain a porous structure by their rigidity and therefore non-effective packing with remaining voids between the chains was used to obtain the polymers of intrinsic microporosity (PIMs). 10 Their advantage is the solubility of the porous network. They maintain their porosity after removal of the solvent, rendering them as interesting candidates for the production of porous membranes. 11 A disadvantage of the so far mentioned porous polymers is the disordered structure of the frameworks, resulting in a broad pore size distribution. For a high specific surface area and size specific molecular sieving, ordered frameworks with monomodal pore size are needed. Therefore, the modular concept used in the MOFs was applied in the synthesis of metal-free organic frameworks, for instance the covalent organic frameworks (COFs) using the formation of boroxine rings by Yaghi et al. 12–14 and the covalent triazine frameworks (CTFs) from the group of Thomas et al. synthesized by cyclotrimerization of polyfunc- tional cyanides to triazine rings. 15 Other efforts to obtain metal- free framework structures like the element organic frameworks (EOFs) 16,17 or polymers derived via palladium catalyzed coupling reactions 18,19 resulted in amorphous structures with reasonable porosity. The use of tetrafunctional linker molecules connected by bifunctional groups via Sonogashira coupling leads to high specific surface areas of around 1200 m 2 g 1 . 20 It was shown that the variation of the length of the linker units is applicable similar to MOFs, but due to the amorphous structures an inverse effect was observed, meaning that with increasing size of the linker the specific surface area and pore volume decrease. In the following we present a novel strategy for the synthesis of EOFs composed of tetrahedral nodes and bifunctional linkers. However, in order to avoid defects at the tetrahedral node caused by side reactions in the original EOF synthesis scheme, we have developed a new scheme using Suzuki coupling in the middle of the bifunctional linker resulting in porous polymers with high a Department of Inorganic Chemistry, Dresden University of Technology, Mommsenstraße 6, D-01062 Dresden, Germany. E-mail: stefan.kaskel@ chemie.tu-dresden.de; Fax: +49 351 46337287; Tel: +49 351 46334885 b Faculty of Physics and Earth Science, University of Leipzig, Linn estraße 5, D-04103 Leipzig, Germany c Bl € ucher GmbH, Mettmanner Straße 25, D-40699 Erkrath, Germany d Adsor-Tech GmbH, Von-Bl € ucher-Straße 2, D-14727 Premnitz, Germany † Electronic supplementary information (ESI) available: Optimized synthesis parameters (S1), DTA/TG analysis (S2), EDX analysis (S3), elemental analysis (S4), SEM micrographs (S5), FtIR spectra (S6), low pressure nitrogen adsorption isotherms (S7), and NLDFT pore size distributions (S8). See DOI: 10.1039/c003130e 3918 | Soft Matter , 2010, 6, 3918–3923 This journal is ª The Royal Society of Chemistry 2010 PAPER www.rsc.org/softmatter | Soft Matter Downloaded by UNIVERSITATSBIBLIOTHEK LEIPZIG on 04 August 2010 Published on 17 June 2010 on http://pubs.rsc.org | doi:10.1039/C003130E View Online