Synthesis of COF-5 using microwave irradiation and conventional solvothermal routes Lyndsey K. Ritchie, Abbie Trewin, Aida Reguera-Galan, Tom Hasell, Andrew I. Cooper * Department of Chemistry and Centre for Materials Discovery, University of Liverpool, Crown Street, Liverpool L69 3BX, United Kingdom article info Article history: Received 16 November 2009 Received in revised form 16 February 2010 Accepted 17 February 2010 Available online 20 February 2010 Keywords: Microporous materials Covalent organic networks abstract A covalent organic framework, COF-5, was synthesized by microwave heating and by conventional solvo- thermal synthesis. The highest apparent Brunauer–Emmett–Teller surface areas were obtained for mate- rials produced by a combination of microwave heating and microwave extraction. The total surface area for the samples could be sub-divided into external and internal surface areas, arising from the macro- scopic morphology and the molecular crystal structure, respectively. Variations in external surface area could be attributed to the different sample morphologies resulting from the various synthesis and puri- fication methods. The experimental internal surface areas correlated well with the simulated Connolly and solvent-accessible surface area (2034 m 2 g À1 and 1737 m 2 g À1 ). Pore filling simulations can rational- ize the stepped gas uptake behavior and identify a physisorbed monolayer followed by filling of the pore interior. This study suggests that microwave irradiation is a convenient alternative route for the synthesis of COF materials, and underlines the importance of considering sample morphology in evaluating surface area data. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Microporous materials such as crystalline metal organic frame- works (MOFs) [1] and amorphous microporous organic polymers [2] have been of great interest recently. Covalent organic frame- works [3] (COFs) were first reported in 2005 by Yaghi, who syn- thesised crystalline porous aromatic framework materials linked by boroxine or boronate-ester groups. Following on from this ini- tial report [3], there have been various crystalline boroxine or bor- onate-ester COFs published in the literature [4,5], with apparent Brunauer–Emmett–Teller (BET) surfaces areas (S BET ) of up to 4210 m 2 g À1 in some cases (COF-103) [5]. This is interesting, for example, in terms of the potential of these materials for gas phys- isorption [6]. COF-1 [3], which was the first to be reported, was synthesized by solvothermal self-condensation in sealed Pyrex tubes of 1,4-benzenediboronic acid (BDBA). The resulting product was reported to be a staggered P6 3 /mmc hexagonal layered mate- rial prior to guest removal linked by planar boroxine rings (S BET = 711 m 2 g À1 ). COF-5 was formed via the condensation reac- tion of hexahydroxy triphenylene (HHTP) and BDBA and was shown to be mesoporous (S BET = 1590 m 2 g À1 ; average pore si- ze = 2.7 nm) and exhibited an eclipsed boron nitride structure [3]. The solvent selected was shown to be important, as was con- trol over reaction rate to ensure that the thermodynamic, ordered crystalline products were obtained. A sealed reaction vessel (e.g., a sealed Pyrex tube) ensured reversible reaction conditions were maintained as the condensate was contained. Other COFs, includ- ing COF 18 Å have been synthesized solvothermally in sealed tubes (reaction times typically 72 h) under a variety of conditions [4,5]. It has been shown that in the case of small molecule organic chemistry microwave reactors can be utilized to accelerate reac- tion times, produce cleaner products, and give higher yields in many cases [7]. Advantages which are less widely reported include the potential for continual online monitoring, simultaneous control of reaction temperature and pressure, and (recently) visual moni- toring of the phase behavior in reactions. The synthesis of organic polymers by microwave heating [8] has been reported but less attention has been given to microporous materials. IRMOF materi- als using microwaves [9] have been synthesized by Ni and Masel and also Bae et al. used microwave heating to produce MOFs with very high CO 2 /CH 4 gas separation selectivities [10]. We showed that [11] both COF-5 and COF-102 can be synthesized using micro- wave irradiation. In particular, we showed that products compara- ble with the materials described by Yaghi and coworkers could be obtained with a reaction time of just 20 min – that is, more than two hundred times faster than the reported solvothermal reaction time of 72 h [3,12]. The synthesis of COF-5 was also shown to be successful when carried out in an open vessel using microwave heating. The success of this open vessel approach demonstrates that an overpressure is not necessarily required in these syntheses [4]. This does not however disprove the need for H 2 O to facilitate 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.02.010 * Corresponding author. Tel.: +44 0151 7943548; fax: +44 0151 7942304. E-mail addresses: aicooper@liverpool.ac.uk, abbiet@liv.ac.uk (A.I. Cooper). Microporous and Mesoporous Materials 132 (2010) 132–136 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso