Methane Oxidation DOI: 10.1002/anie.200902009 Solid Catalysts for the Selective Low-Temperature Oxidation of Methane to Methanol** Regina Palkovits, Markus Antonietti, Pierre Kuhn, Arne Thomas, and Ferdi Schüth* The development of catalyst systems for the direct low- temperature oxidation of methane to methanol has been one of the major challenges in catalysis over the last decades. [1–8] The high binding energy of the CH 3 H bond (435 kJ mol 1 ) together with the ease of overoxidation to form CO 2 require not only a highly active but also a highly selective catalyst system to tackle this reaction. [9] In the past, various inves- tigations addressed this challenge. [10–16] However, the catalysts mostly suffered from irreversible reduction and bulk metal formation, together with consequently poor selectivity. [5, 6, 13] Some palladium, gold, and mercury complexes with superior stability initially appeared to be promising but still suffer from turnover frequencies (TOFs) below 1 h 1 . In the field of heterogeneous catalysis, nearly all reported investigations involve temperatures far above 250 8C over basic oxides, [7, 17] transition-metal oxides, [13] and iron complexes encapsulated in zeolites. [16] All these catalysts showed poor selectivity owing to overoxidation, and maximum methanol yields were around 5 %. [7, 18] Promising progress in molecular catalysis, however, has recently been made by Periana et al., who demonstrated the selective low-temperature oxidation of methane at temper- atures around 200 8C over platinum bipyrimidine complexes in concentrated sulfuric acid. [19–22] Methane conversions above 90 % at 81 % selectivity to methylbisulfate were reached. However, despite these promising results, commer- cial application seems to be hampered by difficult separation and recycling of the molecular catalyst. We report herein on the development of solid catalysts for the direct low-temperature oxidation of methane to methanol reaching high activity at high selectivity and stability over several recycling steps, which could provide a breakthrough for this reaction. The development is based on the recent discovery of a new class of high-performance polymer frameworks that are formed by the trimerization of aromatic nitriles in molten ZnCl 2 . [23, 24] The materials are thermally stable up to 400 8C and resist strongly oxidizing conditions, which made them appear promising as a solid matrix for methane oxidation along the lines of Perianas work for liquid-phase conditions. Utilizing 2,6-dicyanopyridine as monomer, a covalent triazine-based framework (CTF) with numerous bipyridyl structure units is accessible, which should allow coordination of platinum and resemble the coordina- tion sites for platinum coordination in the molecular Periana catalyst (Scheme 1). The CFT material was characterized with physicochem- ical techniques. Nitrogen sorption analysis of CTF reveals a type I isotherm corresponding to a microporous material with a specific surface area of 1061 m 2 g 1 , a pore volume of 0.934 cm 3 g 1 , and an average micropore diameter of 1.4 nm as determined by nonlocal DFT analysis. Pore volume and specific surface area are somewhat higher than reported in the initial publications on this material by Kuhn et al. [23, 24] Although CTF materials based on 1,4-dicyanobenzene exhibit some regularity, X-ray diffraction measurements of the material based on 2,6-dicyanopyridine indicate a pre- dominantly amorphous structure, and the material has at most short-range ordering. In line with this finding, TEM micro- graphs support the amorphous nature of the CTF, with pores in the micropore range and neither long-range nor short- range order. For modification with platinum, two different routes were chosen, either an in situ pathway by simply combining CTF and the platinum precursor in the reaction mixture for the methane oxidation reaction (K 2 [PtCl 4 ]-CTF), or by pre- coordination of platinum (Pt-CTF) in a separate step. The platinum-modified material was tested in the direct methane oxidation in concentrated sulfuric acid according to the conditions described by Periana et al. [19] In principle, utilization of sulfuric acid and sulfur trioxide as oxidants, as schematically described in Equations (a)–(d), would allow design of a continuous process. All process steps, including methane oxidation to methyl bisulfate (a), hydrolysis to form free methanol (b), and reoxidation of SO 2 (c) could be integrated in such a system. A solid catalyst, with its advantages of easy separation and recyclability, would facilitate the implementation of such processes to allow efficient conversion of natural gas on-site. CH 4 þ H 2 SO 4 þ SO 3 ! CH 3 OSO 3 H þ H 2 O þ SO 2 ðaÞ CH 3 OSO 3 H þ H 2 O ! CH 3 OH þ H 2 SO 4 ðbÞ SO 2 þ 1=2O 2 ! SO 3 ðcÞ SCH 4 þ 1=2O 2 ! CH 3 OH ðdÞ [*] Dr. R. Palkovits, Prof. Dr. F. Schüth Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1, 45470 Mülheim (Germany) Fax: (+ 49) 208-306-2995 E-mail: schueth@mpi-muelheim.mpg.de Prof.Dr. M. Antonietti, Dr. P. Kuhn, Dr. A. Thomas Max-Planck-Institut für Kolloid- und Grenzflächenforschung Am Mühlenberg 1, 14476 Potsdam-Golm (Germany) [**] This work was supported by the Project House “ENERCHEM” of the Max Planck Society. We thank B. Spliethoff (MPI für Kohlenfor- schung) for TEM measurements, S. Palm for SEM measurements, and Dr. C. Weidenthaler for XRD and XPS measurements and for helpful discussions. Angewandte Chemie 6909 Angew. Chem. Int. Ed. 2009, 48, 6909 –6912  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim