1744
Ind. Eng. Chem. Res. 1998, 37, 1744-1747
Combination of CO2 Reforming and Partial Oxidation of Methane over NiO/MgO Solid Solution Catalysts Eli Ruckenstein* and Yun Hang Hu Department of Chemical Engineering, State University of New York at Buffalo, Amherst, New York 14260
To overcome the explosions that can occur during the partial oxidation of methane, the latter exothermic reaction is coupled with an endothermic one, namely, the CO2 reforming of CH4. These combined reactions have been carried out over NiO/MgO solid solution, NiO/Al2O3, and NiO/SiO2 catalysts. About 90% conversion of CH4 and about 98% selectivities to CO and H2 were obtained at 790 °C and a GHSV(gas hourly space velocity) of 90 000 cm3 g-1 h-1 (O2/CO2/ CH4 ) 14.5/26.9/58.6), over a reduced NiO/MgO solid solution catalyst. Almost no change in activity and selectivity occurred during 50 h of reaction. Compared to the reduced NiO/MgO, the reduced NiO/SiO2 and NiO/Al2O3 catalysts provided lower activities and stabilities. The effects of the reaction temperature, space velocity, and feed gas composition for a ratio CH4/ (CO2 + 2O2) ) 1 were investigated. The increase in O2 in the feed gas resulted in a higher conversion of CH4, but the apparent conversion of CO2 passed through a maximum. The CH4 conversion decreased with increasing space velocity, while during the partial oxidation, because of the hot spots, it would have increased. This means that the coupling can, indeed, control the thermal behavior of the reactor. 1. Introduction
Table 1. BET Surface Area and Carbon Deposition
The synthesis gas (CO/H2) is an important feed for the methanol and the Fischer-Tropsch syntheses. Although the dominant commercial method employed to produce synthesis gas is the steam reforming of methane (CH4 + H2O f CO + 3H2, ∆H ) 225.4 kJ/ mol), this process has poor selectivity for CO and a too high H2/CO product ratio for the methanol and the Fischer-Tropsch syntheses. In recent years, the research to produce the synthesis gas from methane was centered on two other processes: the catalytic partial oxidation of methane (CH4 + 1/2O2 f CO + 2H2, ∆H ) -38 kJ/mol) (Ashcroft et al., 1990; Dissanayake et al., 1991; Hickman et al., 1993; Hu and Ruckenstein, 1996a) and the CO2 reforming of methane (CH4 + CO2 f 2CO + 2H2, ∆H ) 247 kJ/mol) (Ashcroft et al., 1991; Yamazaki et al., 1992; Rostrup-Nielsen et al.; Ruckenstein and Hu, 1995, 1997; Hu and Ruckenstein, 1996b). Both processes have high activity and selectivity (Dissanayake et al., 1991; Ashcroft et al., 1990, 1991; Ruckenstein and Hu, 1995). The catalytic partial oxidation of methane to CO is mildly exothermic. However, even a low conversion to CO2 generates a large amount of heat which coupled with a high space velocity leads to hot spots (Dissanayake et al., 1993). Since it is difficult to remove the reaction heat from the reactor, particularly from a large-scale operation, the process becomes hazardous and/or difficult to control. Because the CO2 reforming of methane is an endothermic process, its coupling with the catalytic partial oxidation of methane can overcome the overheating hazard. In addition, by combining the two reactions, one can control the ratio H2/CO and thus the selectivity for various Fischer-Tropsch synthesis products. This coupling was carried out over Ir/Al2O3 (Ashcroft et al., 1991). However, the catalyst is very expensive. Recently, we found that the reduced NiO/MgO catalyst has high activity * To whom correspondence should be addressed.
catalyst
BET surface area (m2/g)
NiO/MgO NiO/Al2O3 NiO/SiO2
50 63 362
deposited carbon (g/g of catalyst)a from CH4 from CO b 0.08 0.17
b b b
a Deposited carbon was obtained during the decomposition of pure CH4 or CO at 790 °C for 1 h. b Negligible.
and selectivity, as well as excellent stability (Ruckenstein and Hu, 1995; Hu and Ruckenstein, 1996b). In those cases, the support (MgO), with which NiO forms a solid solution, inhibited the carbon deposition. In this paper, we investigate the coupling between the catalytic partial oxidation and the CO2 reforming of methane, at high space velocities, over the NiO/MgO (containing 13.6 wt % Ni) solid solution catalyst, composition for which the above catalyst provided the optimum activity and selectivity in the CO2 reforming of CH4 (Hu and Ruckenstein, 1996b). For comparison purposes we also investigate the NiO/Al2O3 and NiO/SiO2 catalysts. 2. Experiment 1.1. Catalyst. NiO/Al2O3, NiO/SiO2, and NiO/MgO (containing 13.6 wt % Ni) catalysts were prepared by impregnating the support with an aqueous solution of nickel nitrate. The obtained paste was dried at room temperature in air, then decomposed, and calcined at 800 °C in air for 1.5 h. The solid solution of NiO/MgO thus formed was identified by X-ray powder diffraction (XRD) using a Nicolet X-ray diffraction instrument, equipped with a Cu KR source, at 40 kV and 20 mA. The surface areas of the catalysts, listed in Table 1, were determined via nitrogen adsorption, using a Micromeritics ASAP2000 instrument, after the sample was degassed at 200 °C for 3 h in high vacuum. 1.2. Catalytic Reaction. The catalytic reaction was carried out at 790, 700, and 650 °C, in a flow system,
S0888-5885(97)00788-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/13/1998
Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1745
using a vertical quartz tube (2 mm inside diameter) as the reactor; the entrance pressure was 1.3 atm and the exit one 1 atm. The catalyst powder (weight: 0.02 g) was held on quartz wool. Mixtures of various compositions of CO2/O2/CH4, in which the ratio CH4/(CO2 + 2O2) ) 1, were employed in the investigation. Because there is a low activity over the unreduced NiO/MgO catalyst both for CO2 reforming (Hu and Ruckenstein, 1996b) and for the partial oxidation of methane (Hu and Ruckenstein, 1998), the catalyst was first reduced in H2 (20 mL/min) by temperature-programmed heating from room temperature to 790 °C, at a rate of 20 °C/min. The temperature of the catalyst was measured with a thermocouple located in the catalyst bed. Experiments carried out in a reactor free of catalyst indicated a conversion lower than 2%. The analysis of the reactants/ products mixtures was performed with an in situ gas chromatograph equipped with both Porapak Q and 5A molecular sieve columns. 1.3. Decomposition of CH4 and CO. The catalytic decomposition of CH4 or CO was investigated under atmospheric pressure, at 790 °C, in a flow system, using a vertical quartz tube (4 mm inside diameter) as the reactor. The catalyst powder (weight: 0.1 g) was held on quartz wool, and pure CH4 or CO (40 mL/min) was employed. The catalyst was first reduced in H2 (20 mL/ min) by temperature-programmed heating from room temperature to 790 °C, at a rate of 20 °C/min. The amount of carbon deposited on the catalyst was determined after 1 h of decomposition at 790 °C, and the results are listed in Table 1. 3. Results and Discussion The coupling of CO2 reforming and partial oxidation of methane was carried out over NiO/MgO solid solution, NiO/SiO2, and NiO/Al2O3 catalysts at a high space velocity (GHSV ) 90 000 cm3 g-1 h-1) and at 790 °C, using a feed gas of CH4/CO2/O2 with the composition 58.6/26.9/14.5. Figure 1a shows that the CH4 conversion has an initial value of 88% and remains almost unchanged during 50 h of reaction over NiO/MgO; it has an initial value of 84% but decreases to 70% after about 50 h over NiO/SiO2 and an initial value of 80% and decreases to 75% after 50 h over NiO/Al2O3. Figure 1b shows that the CO2 conversion has an initial value of 95% but remains constant during 50 h of reaction over NiO/MgO; it has an initial value of 93% and decreases to 82% after 50 h over NiO/SiO2 and an initial value of 85% and decreases to 75% after 50 h over NiO/Al2O3. These results indicate that the reduced NiO/MgO catalyst has a high stability. The conversions of O2 are 100% over all catalysts. Parts a and b of Figures 2 reveal that the selectivity for CO is greater than 95% over NiO/MgO and NiO/SiO2 and equal to 90% over NiO/Al2O3 and that the selectivity for H2 is greater than 95% in all cases. The ratio H2/CO is equal to 1.3 over all three catalysts (Figure 2c). Table 1 shows that the surface area of NiO/MgO is smaller than those of NiO/Al2O3 and NiO/SiO2. This indicates that the activity per square meter is much higher over the reduced NiO/MgO than over the reduced NiO/Al2O3, which, in turn, is higher than that over the reduced NiO/SiO2. NiO and MgO are face-centered-cubic oxides with very close lattice parameters (4.1946 and 4.2112 Å for NiO and MgO, respectively) and bond distances (2.10 and 2.11 Å for NiO and MgO, respectively). For this reason, MgO and NiO can form solid solutions. The formation
Figure 1. (a) CH4 conversion and (b) CO2 conversion over reduced catalysts against reaction time. Reaction conditions: T ) 790 °C, CH4/CO2/O2 ) 58.6/26.9/14.5, GHSV ) 90 000 cm3 g-1 h-1 (2, Ni/ MgO; 9, Ni/Al2O3; [, Ni/SiO2).
of a solid solution, which was revealed by XRD (Hu and Ruckenstein, 1996c), can explain why the NiO/MgO catalyst has a high stability and activity in the above coupled reactions. With the electronegativity of Mg (1.293) being lower than that of Ni (1.8) (Naray-Szabo, 1969), the binding of oxygen is stronger to Mg than to Ni. Because of the coordination of each O atom to both Mg and Ni, it is more difficult to reduce NiO in the solid solution, and thus only a small amount of NiO is reduced to Ni0. Indeed, our previous TPR experiments confirmed that NiO/MgO is more difficult to reduce with H2 than NiO (Hu and Ruckenstein, 1996b). Consequently, during the prereduction with H2, a small amount of NiO in the NiO/MgO catalyst is reduced, resulting in the formation of dispersed Ni over the surface layer of the catalyst (Hu and Ruckenstein, 1997). These Ni0 atoms constitute the active sites, because without prereduction the activity of the catalyst is low. Since the activity of the reduced catalyst remains high and unchanged for 55 h (Figure 1), it is clear that Ni0 is maintained during reaction, even though the feed gas contains some oxygen. The strong interactions between the highly dispersed Ni and MgO inhibit the sintering of Ni. As a result, the reduced NiO/MgO has a high activity and, because the clustering of Ni, which is necessary for coke formation, is prevented, the Ni/MgO catalyst inhibits carbon deposition. The latter can occur by CH4 decomposition and CO disproportionation. The experiments which we performed regarding the CH4 decomposition and the CO disproportionation at 790 °C for 1 h revealed that there is no carbon deposition on the reduced NiO/MgO catalyst. In contrast, carbon did deposit during the CH4 decomposition over the reduced NiO/SiO2 and NiO/Al2O3 catalysts, with the amount deposited over the former being twice as large as that over the latter. However, negligible carbon deposition occurred during the CO disproportionation over all the catalysts employed. This happens for thermodynamic reasons. At 790 °C, the methane decomposition is
1746 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998
Figure 3. CH4 conversion (a), CO2 conversion (b), CO selectivity (c), H2 selectivity (d), and H2/CO ratio (e) over reduced NiO/MgO catalysts against space velocity. Reaction conditions: T ) 790 °C, CH4/CO2/O2 ) 58.6/26.9/14.5.
Figure 2. (a) CO selectivity, (b) H2 selectivity, and (c) H2/CO ratio over reduced catalysts against reaction time. Reaction conditions: T ) 790 °C, CH4/CO2/O2 ) 58.6/26.9/14.5, GHSV ) 90 000 cm3 g-1 h-1 (2, Ni/MgO; 9, Ni/Al2O3; [, Ni/SiO2).
favored compared to CO disproportionation, and at relatively low temperatures the disproportionation reaction is favored (Claridge et al., 1993). Of course, during the combined reaction, the amount of carbon deposited should be smaller than that during the decomposition of CH4, because the oxidants O2 and CO2 are also present. Consequently, the reduced NiO/MgO catalyst has a high stability because the sintering of Ni and the carbon deposition are inhibited. In contrast, over the catalysts NiO/Al2O3 and NiO/SiO2 both sintering and carbon deposition occur. The effect of various conditions on the coupling between the CO2 reforming and the partial oxidation of methane was also investigated. Figure 3 indicates that, as the space velocity of the feed increases, the conversions of CH4 and CO2 decrease. In contrast, during the partial oxidation of CH4 (Matsumura and Moffat, 1994), the conversion increases with increasing space velocity. This most likely occurs because of the formation of hot spots. This means that the coupling can, indeed, control the partial oxidation. Figure 3 also shows that, with increasing space velocity, the selectivity to H2 does not change, while the selectivity to CO and the ratio H2/CO decrease slightly. Figure 4 reveals that, as the ratio O2/CO2 increases, the CH4 conversion increases, but the apparent CO2 conversion defined as
Figure 4. CH4 conversion (a), CO2 conversion (b), CO selectivity (c), H2 selectivity (d), and H2/CO ratio (e) over reduced NiO/MgO catalyst against O2/CO2 ratio in the feed. Reaction conditions: T ) 790 °C, CH4/(CO2 + 2O2) ) 1, GHSV ) 135 000 cm3 g-1 h-1.
CO2exit/CO2feed gas passes through a maximum. At low O2/CO2 ratios, the conversion of CO2 increases with increasing O2 concentration probably because O2 promotes the CH4 activation (Hu and Ruckenstein, 1995) and the activated CH4 can react with CO2. At large O2/ CO2 ratios, the concentration of CO2 in the feed gas is small, but additional CO2 is formed via the reaction between CH4 and O2. Even a small amount of CO2 formed during reaction will strongly affect the apparent CO2 conversion. Figure 4 also reveals that, as the ratio O2/CO2 increases, there are small changes in the selectivities to CO and H2, whereas the ratio of H2/CO increases. The latter ratio increases because the contribution of the partial oxidation reaction increases with increasing O2/CO2 ratio. As expected, Figure 5 shows that the decrease of the reaction temperature decreases the conversions of CH4 and CO2 and the selectivities to CO and H2. 4. Conclusions The partial oxidation was coupled with the CO2 reforming of methane and the main conclusions are as follows: 1. The reduced NiO/MgO2 has higher activity and stability than the reduced NiO/SiO2 and NiO/Al2O3.
Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1747
Figure 5. CH4 conversion (a), CO2 conversion (b), CO selectivity (c), H2 selectivity (d), and H2/CO ratio (e) over reduced NiO/MgO catalysts against reaction temperature. Reaction conditions: CH4/ CO2/O2 ) 58.6/26.9/14.5, GHSV ) 135 000 cm3 g-1 h-1.
2. As the ratio O2/CO2 increases, the conversion of CH4 increases while the apparent conversion of CO2 passes through a maximum. 3. The coupling with the CO2 reforming can control the thermal behavior of the partial oxidation. Literature Cited Ashcroft, A. T.; Cheetham, A. K.; Foord, J. S.; Green, M. L. H.; Grey, C. P.; Murrell, A. J.; Vernon, P. D. F. Selective oxidation of methane to synthesis gas using transition metal catalysts. Nature 1990, 344, 319. Ashcroft, A. T.; Cheetham, A. K.; Green, M. L. H.; Vernon, P. D. F. Partial oxidation of methane to synthesis gas using carbon dioxide. Nature 1991, 352, 225. Claridge, J. B.; Green, M. L. U.; Tsang, S. C.; York, A. P. E.; Ashcroft, A. T.; Battle, P. D. A study of carbon deposition on catalysts during the partial oxidation of methane to synthesis gas. Catal. Lett. 1993, 22, 299.
Dissanayake, D.; Rosynek, M. P.; Kharas, K. C. C.; Lunsford, J. H. Partial oxidation of methane to carbon monoxide and hydrogen over a Ni/Al2O3 catalyst. J. Catal. 1991, 132, 117. Dissanayake, D.; Rosynek, M. P.; Lunsford, J. H. Are the equilibrium concentrations of CO and H2 exceeded during the oxidation of CH4 over a Ni/Yb2O3 catalyst? J. Phys. Chem. 1993, 97, 3644. Hickman, D. A.; Schmidt, L. D. Syngas formation by direct catalytic oxidation of methane. Science 1993, 259, 343. Hu, Y. H.; Ruckenstein, E. Pulse-MS study of the partial oxidation of methane over Ni/La2O3 catalyst. Catal. Lett. 1995, 34, 41. Hu, Y. H.; Ruckenstein, E. Transient kinetic studies of partial oxidation of CH4. J. Catal. 1996a, 158, 260. Hu, Y. H.; Ruckenstein, E. An optimum NiO content in the CO2 reforming of CH4 with NiO/MgO solid solution. Catal. Lett. 1996b, 36, 145. Hu, Y. H.; Ruckenstein, E. Temperature-programmed desorption of CO adsorbed on NiO/MgO. J. Catal. 1996c, 163, 306. Hu, Y. H.; Ruckenstein, E. The characterization of a highly effective NiO/MgO solid solution catalyst in the CO2 reforming of CH4. Catal. Lett. 1997, 43, 71. Hu, Y. H.; Ruckenstein, E. Catalyst temperature oscillations during the partial oxidation of methane. Ind. Eng. Chem. Res. 1998, submitted for publication. Matsumura, Y.; Moffat, J. B. Oxidative methane conversion to carbon monoxide and hydrogen at low reactor wall temperatures over ruthenium supported on silica. Catal. Lett. 1994, 24, 59. Naray-Szabo, I. Inorganic Crystal Chemistry; Alkademiai Kiado; Budapest, 1969; p 237. Rostrup-Nielsen, J. R.; Hansen, J.-H. B. CO2-reforming of methane over transition metals. J. Catal. 1993, 144, 38. Ruckenstein, E.; Hu, Y. H. Carbon dioxide reforming of methane over nickel/alkaline earth metal oxide catalysts. Appl. Catal. A 1995, 133, 149. Ruckenstein, E.; Hu, Y. H. The efect of precursor and preparation conditions of MgO on the CO2 reforming of CH4 over NiO/MgO catalysts. Appl. Catal. A 1997, 154, 185. Yamazaki, O.; Nozaki, T.; Omata, K.; Fujimoto, K. Reduction of carbon dioxide by methane with Ni-on-MgO-CaO containing catalysts. Chem. Lett. 1992, 10, 1953.
Received for review November 10, 1997 Revised manuscript received January 23, 1998 Accepted January 29, 1998 IE9707883
