J. Comb. Chem. 2009, 11, 169–174
169
Catalyst Screening for Oxidative Reforming of Methane in Direct Route using High Pressure HTS Reactor with Syngas Detection System by Colorimetric Reaction and Gas Chromatograph Kohji Omata, Hidetomo Ishii, Junpei Horiguchi, Seishiro Kobayashi, Yuichiro Yamazaki, and Muneyoshi Yamada* Department of Applied Chemistry, Graduate School of Engineering, Tohoku UniVersity, Aoba 6-6-07, Aramaki, Aoba-ku, Sendai 980-8579, Japan ReceiVed September 30, 2008 A high-throughput screening (HTS) reactor for high-pressure oxidative reforming of methane in a direct reaction route was developed. With a combination of catalyst preparation by a split-and-pool method and HTS, Ni-K/R-Al2O3 catalyst was found to show high activity under 1 MPa at 650 °C with high selectivity even when O2 conversion is less than 100%. The HTS reactor required a new simple syngas detector operable under high pressure because the number of parallel reactor is limited when equipped with the conventional detection system. The complexity of the pressure reducing unit is the main reason of the limitation. Reduction of metal oxide accompanied with the color change was applied to the detection system. Copper oxide was supported on the filter disk made of alumina, and the filter was placed underneath the catalyst bed. After the methane was oxidatively reformed under 1 MPa at 650 °C, color change of spots from dark brown to light brown was observed just under the catalyst which produced hydrogen. Color change of the disk can be used to detect hydrogen formation from the reforming catalyst under pressure. Introduction Combinatorial chemistry attracts much attention in research field of solid catalyst development. The technology consists of high-speed library synthesis, high-throughput screening (HTS), and informatics.1 Among them, the most demanding is HTS because the environment of the catalyst in a laboratory should be similar to those in an industrial process. When the discrepancy between the laboratory application and the industry application is not negligible, ultra-HTS, such as thermographic observation of a thin film catalyst,2,3 Q-mass detection of products at surface,4,5 and a reactor system using 384 well microplate,6 are valid. However, the industrial catalysts are often tailor-made to give the best performance under constrained conditions. To estimate the in situ catalytic performance under nearindustrial conditions, that is, pressurized conditions, other types of HTS with smaller library scales were also reported.7-9 In the present study, new catalysts have been screened for high-pressure oxidative reforming of methane to produce synthesis gas (syngas). For reforming catalysts, it was reported that both oxidation state of the catalyst and deactivation by coke formation strongly depend on reaction pressure.10 In such a case, an HTS system operable under pressure and at high temperature is required. We also reported a methanol detection system, where methanol synthesized from syngas in a high-pressure reactor was transferred through capillary tube into a water trap and was analyzed by reagent oxidation with color change.11 Such a * To whom correspondence should be addressed. E-mail: yamada@ erec.che.tohoku.ac.jp.
colorimetric reaction was proven to be useful for simultaneous detection, and other detection systems were reported for pH,12 NOx,13 acetic acid,14 aniline,15 and olefin.16 Hydrogen was also detected by a colorimetric reaction of tungsten oxide reduction by spillover hydrogen17 or of molybdenum oxide reduction in hydrogen detector-tube.18 Because such a color change of metal oxide seemed applicable under high pressure and high temperature, an in situ colorimetric reaction for hydrogen detection was investigated in the present study. Methane is the main component of natural gas, and steam reforming of methane is currently used industrially on a large scale to produce hydrogen and syngas. Whereas the steam reforming process is currently the most prevalent method, the heat requirement is huge, and the heat is supplied by many external burners heating the tubular reactors by combustion of natural gas. An attractive alternative process for syngas production is the oxidative reforming of methane.19 If the reaction proceeds as is presented by eq. 1, the reaction is mildly exothermic, and therefore the reforming process would be more economical from viewpoint of heat supply. In addition, the H2/CO ratio produced in stoichiometry is around 2, and this ratio is ideal for downstream processes, such as methanol synthesis and Fischer-Tropsch reaction with cobalt catalysts. Thus the oxidative reforming reaction is suitable for gas to liquid (GTL) processes. 1 (1) CH4 + O2 f CO + 2H2 2 To take full advantage of oxidative reforming, the reaction should proceed in a direct route. Usually the oxidative
10.1021/cc800156g CCC: $40.75 2009 American Chemical Society Published on Web 12/05/2008
170 Journal of Combinatorial Chemistry, 2009 Vol. 11, No. 1
Omata et al.
reforming proceeds in an indirect route on Ni catalyst,20 where methane is first combusted by eq 2 and then reformed by eqs 3 and 4. While total reaction is same, the indirect route reaction spoils the advantages of oxidative reforming reaction. Only few Ni catalysts were reported as a direct route catalyst, such as Ni/Ca-Al-O21 and Ni-Ca hydroxyapatite22 in steady state reaction and Ni/Al2O323 in pulse reaction. CH4 + 2O2 f CO2 + 2H2O
(2)
CH4 + H2O f CO + 3H2
(3)
CH4 + CO2 f 2CO + 2H2
(4)
Activity tests were conducted using a diluted gas and a diluted catalyst bed under the criteria developed by Verykios24 to suppress the hot-spot formation and to decrease the influence of diffusion. High hydrogen selectivity at O2 conversion
