Effect of Ni Loading and Reaction Conditions on Partial Oxidation of Methane to Syngas

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Journal of Natural Gas Chemistry 12(2003)205 209 Effect of Ni Loading and Reaction Conditions on Partial Oxidation of Methane to Syngas Haitao Wang, Zhenhua Li, Shuxun Tian School of Chemical

in this paper over Ni/Al 2O 3 catalysts under atmospheric pressure.

1 Journal of Natural Gas Chemistry 12(2003) Effect of Ni Loading and Reaction Conditions on Partial Oxidation of Methane to Syngas Haitao Wang, Zhenhua Li, Shuxun Tian School of Chemical Engineering and Technology, Tianjin University, Tianjin , China [Manuscript received June 3, 2003; revised August 8, 2003] Abstract: The partial oxidation of methane to synthesis gas is studied in this paper over Ni/Al 2O 3 catalysts under atmospheric pressure. The effects of Ni loading on the activity and stability of catalysts with 5 mm α-al 2O 3 and θ-al 2O 3 pellets as supports were measured in a continuous fixed bed reactor. It is found that the optimum Ni loading is 10%. And the effect of reaction conditions on partial oxidation of methane is also studied. The methane conversion and CO selectivity increase with the increase of the reaction temperature and the space velocity on 10%Ni/α-Al 2O 3 catalysts. The best CH 4/O 2 mole ratio is 2 for CO selectivity, and the optimum space velocity is h 1. Key words: methane, syngas, nickel, partial oxidation, alumina 1. Introduction 2. Experimental 2.1. Catalyst preparation Natural gas is abundant in western China. It is a pressing subject of how to utilize this natural resource. A promising way is to convert methane to syngas then to methanol or hydrocarbons. The syngas can be produced by the partial oxidation of methane (POM) at a mole ratio of H 2 /CO of 2/1, which can be directly used as feed for the methanol synthesis or the Fischer- Tropsch reaction. POM process has been extensively studied in recent years [1 4]. Ni catalysts have attracted much attention for the high activity and low cost [5 6]. Most studies reported that the support with the small size particles of mesh is usually used, which needs to be shaped to fit for the industrial process. The scale-up effect may exist in this process. In this paper, 5 mm α-al 2 O 3 and θ-al 2 O 3 pellets were adopted as supports and the effects of Ni loading on POM process is studied. The optimum reaction conditions to the POM is also studied on 10%Ni/α-Al 2 O 3 catalysts. The catalysts were prepared by wetness impregnation of the supports with Ni (NO 3 ) 2 6H 2 O solutions. After stirred and heated at 80 to evaporate the excess water, the catalysts were dried at 120 for 2 h, calcined at 400 for 4 h and 800 for 8 h in air. Before the reaction was performed, the catalysts were reduced by hydrogen at 500 for 2 h The catalytic activity evaluation The activity of the catalyst was measured in a continues fixed bed reactor under atmospheric pressure. The N 2, O 2 and CH 4 from the cylinders were measured through a rotermeter and mixed thoroughly before entering the reactor. The catalyst charge is 70 mg, and the catalyst bed height is 5 mm. To measure and control the temperature of the catalyst bed, an Al-708 TA Artificial Intelligential Controller produced by Yu Guang Electrical Technology Institute Corresponding author. Tel: (022) ; zhenhua@tju.edu.cn. This work was supported by National Science Foundation Committee of China (Grant number ).

2 206 Haitao Wang et al./ Journal of Natural Gas Chemistry Vol. 12 No was used, the accuracy of which is ±0.5. The discharge gas was condensed by a cold trap, and the non-condensed gas was analyzed before ventilation. The feed and tail gases were analyzed by SQ-206 GC provided by Beijing Analyzed Instrument Manufacturer. The carrier gas was Ar with a flow rate of 30 ml/min. The column temperature and detection temperature are 50. The amounts of H 2, O 2, N 2, CH 4 and CO were analysed by the 5A molecular sieves column (φ 3 m 2 m), and the amounts of CO 2 were analyzed by the TDX-01 column (φ 3 m 1 m). The liquid sample collected was proved to be pure water. Since CH 4, CO and H 2 are explosive gases, the reaction conditions were selected to beyond the explosion limit. The space velocity was at the range of h 1, the temperature was of , the CH 4 /O 2 mole ratio was of 1 8, and the N 2 /O 2 mole ratio was Measurement of specific surface area The CHEMBET-3000 Instrument was used to measure the specific surface area of the catalysts at the temperature of liquid nitrogen. The specific surface area was calculated by the BET equation. 3. Results and discussion 3.1. Effects of Ni loading on the catalytic activity and stability The performances of 5%Ni/α-Al 2 O 3 and 10%Ni /α-al 2 O 3 catalysts are shown in Figure 1. 5%Ni/α- Al 2 O 3 catalyst shows lower CH 4 conversion, though the two catalysts exhibit similar CO selectivity especially at high reaction temperature. This could be due to its lower Ni content that induced fewer Ni 0 active sites for the POM reaction. The 20%Ni/α-Al 2 O 3 catalyst deactivates very quickly because the serious carbon deposition result in that no useful data could be got on the Ni supported α-al 2 O 3 catalyst. The bulk NiO particles are also formed on 20%Ni/α-Al 2 O 3 catalyst, which was considered as the main reason of carbon deposition [7]. Figure 1. Catalytic activity vs reaction temperature on the α-al 2 O 3 supported catalyst (1) 5%Ni, (2) 10%Ni The performances of 10%Ni/θ-Al 2 O 3 and 20%Ni/θ-Al 2 O 3 catalysts are shown in Figure 2. It is found that 5%Ni/θ-Al 2 O 3 catalyst was oxidized and lost its activity immediately after the experiment starts, whereas 10% catalyst shows equivalent CH 4 conversion and CO selectivity with 20%Ni/θ-Al 2 O 3 catalyst. It is considered that higher specific surface area of θ-al 2 O 3 to that of α-al 2 O 3 makes the dispersion of Ni 0 species increased and oxidized easier [8,9], which results in the deactivation.

3 Journal of Natural Gas Chemistry Vol. 12 No Figure 2. Catalytic activity vs reaction temperature on the θ-al 2 O 3 supported catalyst (1) 10%Ni, (2) 20%Ni 3.2. The activity of 10%Ni/α-Al 2 O 3, 10%Ni/θ- Al 2 O 3, 10%Ni/γ-Al 2 O 3 The specific surface area of the supports and catalysts are listed in Table 1. The data shows that, the specific surface area of θ-al 2 O 3 and γ-al 2 O 3 supports decreased after they are supported with 10%Ni. Their specific surface area further decreased after two catalysts are reduced at 800 and reacted at 650 for 4 h. However, the specific surface area of 10%Ni/α- Al 2 O 3 has a little increase compared with that of α- Al 2 O 3 support, which indicates that the θ-al 2 O 3 and γ-al 2 O 3 supports are not so stable as α-al 2 O 3 during catalyst preparation and reaction process. The catalytic activity results shows that the partial oxidation of methane does not occur on the reduced 10%Ni/θ-Al 2 O 3 and 10%Ni/γ-Al 2 O 3 catalysts at the reaction temperature range of Only 10%Ni/α-Al 2 O 3 catalyst has significant activity to the reaction. Thus the effect of reaction conditions on the catalytic activity of 10%Ni/α-Al 2 O 3 catalyst was further studied Effect of reaction temperature on catalytic activity for POM process Table 1. Catalyst Specific surface area of the supports and 10%Ni catalysts Specific surface area (m 2 /g) 10%Ni/α-Al 2 O %Ni/β-Al 2 O %Ni/γ-Al 2 O support, 2 the catalyst before reduction, 3 the catalyst reacted at 650 for 4 h after reduction at 800 Figure 3. The catalytic activity of 10%Ni/α-Al 2 O 3 catalyst vs reaction temperature at overall flow rate of 260 ml/min and n(o 2 ):n(ch 4 ):n(n 2 )=1:2:4

4 208 Haitao Wang et al./ Journal of Natural Gas Chemistry Vol. 12 No The experiment result shows that no syngas is detected on the 10%Ni/α-Al 2 O 3 until the temperature reaches 500. Figure 3 shows the detailed profile of the catalytic activity and selectivity of 10%Ni/α- Al 2 O 3 catalysts as a function of the reaction temperature. As shown in Figure 3, the methane conversion, CO and H 2 selectivity increase with the reaction temperature. Similar results are obtained on other catalysts. Therefore, it has proved that higher reaction temperature is more favorable for the POM process Effect of CH 4 /O 2 mole ratio on activity for POM process CH 4 /O 2 mole ratio is an important factor affecting the POM process. In order to investigate the effect of CH 4 /O 2 mole ratio, four experiments with different flow rates were devised, in which the flow rate of CH 4, O 2 and N 2 is listed in Table 2. Figure 4 show the effects of CH 4 /O 2 mole ratio on methane conversion and CO selectivity over 10%Ni/α-Al 2 O 3 catalyst. As the CH 4 /O 2 mole ratio range of 1 8, the optimum CH 4 /O 2 mole ratio is 2 for the methane conversion and CO selectivity. It is analyzed that higher O 2 concentration would be favorable for methane conversion when the CH 4 /O 2 mole ratio is lower than 2; but with the O 2 concentration further increase, the syngas (CO and H 2 ) will be further oxidized to CO 2 and H 2 O in that decrease the syngas selectivity. Table 2. The flow rate of CH 4, O 2 and N 2 in the four different experiments Experiment Flow rate (ml/min) Flow rate ratio No. CH 4 O 2 N 2 of CH 4 to O Figure 4. The catalytic activity vs mole ratio of CH 4 to O 2 on the 10%Ni/α-Al 2 O 3 catalyst at different reaction temperatures 3.5. Effect of space velocity on activity for POM reaction It is reported that high space velocity is necessary for keeping the reaction at the partial oxidation stage. Otherwise, methane will be further oxidized to CO 2 and H 2 O. Therefore space velocity is a key factor in the POM process. Figure 5 illustrate the effects of the space velocity on methane conversion and CO selectivity at different reaction temperatures. It can be seen that methane conversion and CO selectivity increase with the space velocity increased to h 1 and then have a little decrease. The optimum space velocity is h 1. The similar results have been reported by Bi Xian-jun and other researchers [10,11], and they agree that CH 4 and CO

5 Journal of Natural Gas Chemistry Vol. 12 No are adsorbed at the same active center and higher space velocity is favorable for the desorption of CO, which will stimulate the adsorption of methane on the active sites and promote the formation of syngas. Figure 5. Effect of space velocity on catalytic activity over the 10%Ni/α-Al 2 O 3 at different reaction temperatures 4. Conclusions (1) The optimum nickel loading on α-al 2 O 3 and θ-al 2 O 3 is 10%. (2) CH 4 conversion and CO selectivity increase with the increase of reaction temperature on 10%Ni/α-Al 2 O 3 catalyst. Both the CH 4 conversion and CO selectivity will be higher than 90% at reaction temperature of 850. (3) Higher space velocity is favorable for POM process and the optimum space velocity is h 1. The optimum CH 4 /O 2 mole ratio for CO selectivity and methane conversion is 2. References [1] Ashcroft A T, Cheetham A K, Foord J S et al. Nature (London), 1990, 344(6264): 319 [2] Hickman D A, S chmidt L D. J Catal, 1992, 138(1): 267 [3] Torniainen P M, Chu X, Schmidt L D. J Catal, 1994, 146(1): 1 [4] Lu Y, Deng C, Ding X J et al. Cuihua Xuebao (Chin J Catal), 1996, 17(1): 28 [5] Vermeiren W J M, Blomsma E, Jacobs P A. Catal Today, 1992, 13: 427 [6] Bi X J, Hong P J, Dai Sh Sh. Fenzi Cuihua (J Mol Catal), 1998, 12(5): 342 [7] Zhu T, Flytzani-Stephanopoulos M. Appl Catal A, 2001, 208: 403 [8] Kuijpers E G M, Jansen J W, Van Dillen A J et al. J catal, 1981, 72: 75 [9] Rostrup-Nielsen J R. In: Anderson J R ed. Catalysis: Science and Technology, Vol 5. Berlin: Springer- Verlag, [10] Cao L X, Chen Y X, Li W Zh. Fenzi Cuihua (J Mol Catal), 1994, 8(5): 375 [11] Yan Q G, Wu T H, Li J T. Yingyong Cuihua (Chin J Appl Chem), 1999, 16(4): 20

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