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Ind. Eng. Chem. Res. 1999, 38, 1740-1741
CORRESPONDENCE Comments on “Catalyst Temperature Oscillations during Partial Oxidation of Methane” J. Gunther Cohn P.O. Box 240 WOB, West Orange, New Jersey 07052
Sir: Hu and Ruckenstein1 discovered thermal oscillations during partial oxidation of methane to CO and H2 over nickel catalyst when the catalyst was prepared from NiO supported on SiO2, but there were no oscillations when catalysts were prepared from NiO supported on Al2O3 or MgO. During oscillations the hot spot moved from the top of the bed down and up alternatingly. The explanation for the different behavior of the catalysts was that the interaction between NiO and SiO2 is weak but strong between NiO and Al2O3 or MgO. Hence, with SiO2 as the support, an oxidation-reduction cycle is established for NiO. For a full assessment of this interpretation, more details would be required such as the surface area and particle size of NiO, the surface area, particle size, porosity, and pore size distribution of the supports, the temperature of the feed entering the catalyst bed, and temperatures inside the bed. However, some basic arguments can be made. (1) To catalyze partial oxidation, NiO has to be reduced to Ni0. With NiO on Al2O3 and SiO2, this was accomplished by passing a feed (CH4/O2, 2/1) through the catalyst bed, raising the temperature from room temperature to 800 °C. To obtain an effective catalyst with MgO as the support required reduction with H2 at 800 °C. There is no rationale presented for a mechanism by which active Ni0 would become reoxidized and deactivated while producing CO and H2 and subsequently is rereduced and activated by contact with the feed. (2) The premise for the proposed mechanism of the oscillations was that interaction between NiO and SiO2 is weak but strong between NiO and Al2O3 and MgO. No documentary proof for this difference was given. Interactions between NiO and the supports were possible, as the last step of catalyst preparation was heating at 800 °C for 1.5 h. Strong interaction of metal oxides with oxide supports will cause incomplete reduction to the metal, e.g., of base metals with oxides of alumina, silica, or zeolites,2 and it is not obvious why NiO should be an exception. Smirnov and Serikov3 found indeed that mixtures of NiO and SiO2 formed nickel silicates at temperatures from 800 to 900 °C. (3) The investigation of Choudhary et al.4 of catalytic partial oxidation of methane to syngas (CO and H2) also showed that strong interaction occurs with the support. The catalyst was NiO/MgO on ceramic supports of low surface area but fair porosity to provide the mechanical strength required for practical application. One of the supports consisted of 4.1% Al2O3 and 95% SiO2. The catalyst, NiO/MgO, was prepared in two
Table 1. Test Results
catalyst
ratio Ni/Mg
surface CH4 selectivity area conversion ratio 2 (m /g) (%) CO H2 H2/CO
Data from Choudhary et al. NiO/MgO 1.2 93.5 with no support NiO/MgO 3.0 0.88 56.7 together on the support NiO on 1.2 3.3 91.9 the support precoated with MgO/(Mg silicate) NiO/SiO2
92.9 96.9
2.07
86.9 73.6
1.69
95.0
2.00
Data from Hu and Ruckenstein1 0.16 62 57 (Ni/Si)
71
2.49
ways: One by impregnation of the support with a solution containing nickel nitrate and magnesium nitrate, drying, and calcining in air at 900 °C for 4 h and the other by first impregnating the support with a solution of magnesium nitrate, drying, and calcining at 900 °C for 4 h and thereafter repeating the same procedure with a nickel nitrate solution. By this procedure a protective layer of magnesium silicate was formed on the support, preventing interaction of NiO with the support. The test procedure employed by Choudhary et al.4 was similar to that of Hu and Ruckenstein1 in regards to feed composition and process conditions. NiO became essentially reduced to Ni0 during a short period by autocatalytic reaction with methane. Test results obtained at 800 °C by Choudhary et al.4 and by Hu and Ruckenstein1 are shown in Table 1. No oscillations occurred in the tests of Choudhary et al.4 The beneficial effect of precoating the support is due to elimination or at least drastic reduction of the formation of catalytically inactive nickel silicate phases. The same was also observed when NiO was directly supported on silica containing catalyst carriers. The evidence suggests that the oscillations were not due to weak interaction between NiO and SiO2. Perhaps the oscillations can be explained by a coking/decoking mechanism. (4) Nickel is a very active catalyst for coke laydown which poses problems at the inlet section of the catalyst for partial oxidation and also in steam reforming. To minimize or prevent coking, Anilin and Soda Fabrik5 proposed the installation for partial oxidation of a first stage with platinum catalyst, which is less active for carbon formation. Recently, Besenbacher et al.6 could
10.1021/ie9808312 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/17/1999
Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1741
avoid coking in steam reforming of butane by alloying gold into the surface of the nickel catalyst. Furthermore, carbon laydown on nickel catalysts used in steam reforming is also dependent on the acidity of the support, which should also apply to partial oxidation. Trimm7 reports that the strongest contributor to coke formation is the group of SiO2/Al2O3, SiO2/MgO, SiO2, and weakest CaO and MgO. Coke laydown on Ni0/ SiO2 and reactivation by carbon burnoff by the oxygen in the feed appears possible because the molar ratio of NiO/SiO2 is 0.23 and the density ratio of NiO/SiO2 is 3.4. This should leave sufficient area of free SiO2 to affect coke laydown on Ni0/nickel silicate particles. This mechanism could be tested by alkalizing the SiO2 support, which should eliminate the coke formation. Conditions in the work of Choudhary et al.4 were different. The support area alone was 0.22 m2/g and thus could be completely covered by the protective layer of magnesium silicate, leaving enough free MgO to cover the magnesium silicate layer. The result was an area of 3.3 m2/g and a finished catalyst without inactivating coke laydown.
Literature Cited (1) Hu, Y. H.; Ruckenstein, E. Catalyst Temperature Oscillations during Partial Oxidation of Methane. Ind. Eng. Chem. Res. 1998, 37, 2333. (2) Farrauto, R. J.; Bartholomew, C. H. Fundamentals of Industrial Catalysis; Blackie Academic & Professional: Glasgow, U.K. 1997; p 44. (3) Smirnov, V. I.; Serikov, A. P. The Temperature Conditions for the Formation of Copper and Nickel Silicates. Khim. Ref. Zh. 1940, No. 9, 75; Chem. Abstr. 1943, 37, 841. (4) Choudhary, V. R.; Uphade, B. S.; Mamman, A. S. Oxydative Conversion of Methane to Syngas over Nickel Supported on Commerical Low Surface Area Porous Catalyst Carriers Precoated with Alkaline and Rare Earth Oxides. J. Catal. 1997, 172, 281. (5) Anilin, B.; Soda Fabrik, A. G. Anonym. Chem. Eng. 1966, 3, 24. (6) Besenbacher, F.; Chorkendorff, I.; Clausen, B. S.; Hammer, B.; Molenbroek, A. M.; Nørskov, J. K.; Stensgaard, I. Design of a Surface Alloy Catalyst for Steam Reforming. Science 1998, 279, 1913. (7) Trimm, D. L. Design of Industrial Catalysts; Elsevier Scientific Publishing Co.: New York, 1980; p 170.
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