Energy & Fuels 2005, 19, 49-53
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Combined Carbon Dioxide Reforming and Partial Oxidation of Methane to Syngas over Ni-La2O3/SiO2 Catalysts in a Fluidized-Bed Reactor Liuye Mo, Xiaoming Zheng,* Qiangshan Jing, Hui Lou, and Jinhua Fei Institute of Catalysis, Zhejiang University (Xixi Campus), Hangzhou, 310028, People’s Republic of China Received June 22, 2004. Revised Manuscript Received October 21, 2004
Catalysts of Ni/SiO2 and Ni-La/SiO2 were prepared using the incipient-wetness impregnation method. The prepared catalysts were used to produce syngas from a combined reaction of CO2 reforming and partial oxidation of methane in a fluidized-bed reactor. Ni/SiO2 exhibited excellent resistance to carbon deposition and stable catalytic performance upon promotion with La2O3. The promoter (La2O3) increased the dispersion of nickel crystallites and prevented their sintering during the reaction. Consequently, the adsorption abilities of oxygen and CO2 were enhanced, and the rate of carbon elimination was therefore increased. Temperature-programmed reduction and X-ray diffraction characterization revealed that the promotion effect of La2O3 originated from its strong interaction with nickel.
1. Introduction Syngas (CO/H2) is an important feed for the FischerTropsch reaction, methanol synthesis, and other chemical processes. Recently, research on syngas production from methane (CH4) has been focused mainly on two processes, i.e., catalytic partial oxidation of CH4 (CH4 + 1/2O2 f CO + 2H2, ∆H ) -38 kJ/mol)1-3 and CO2 reforming of CH4 (CH4 + CO2 f 2CO + 2H2, ∆H ) 247 kJ/mol).4-9 (∆H is the enthalpy of reaction.) The partial oxidation of CH4 is an exothermic reaction and has a tendency to cause hot spots in catalyst beds. Therefore, this process becomes hazardous and/or difficult to control, particularly in a large-scale operation. The overheating hazard of catalytic partial oxidation of CH4 can be lessened by coupling it with the endothermic CO2 reforming reaction. Negligible hot spots have been attained in fixed-bed reactor10-14 in laboratory scale. * Author to whom correspondence should be addressed. Fax: +86571-88273283. E-mail address:
[email protected]. (1) Choudhary, V. R.; Rajput, A. M.; Prabhakar, B.; Mamman, A. S. Fuel 1998, 77, 1803. (2) Choudhary, V. R.; Rajput, A. M.; Prabhakar, B.; Mamman, A. S. J. Catal. 1993, 139, 326. (3) Ashcroft, A. T.; Cheetham, A. K.; Foord, J. S.; Green, M. L. H.; Grey, C. P.; Murrell, A. J.; Vernon, P. D. F. Nature 1990, 344, 319. (4) Ashcroft, A. T.; Cheetham, A. K.; Green, M. L. H.; Vernon, P. D. F. Nature 1991, 352, 225. (5) Wurzel, T.; Malcus, S.; Mleczko, L. Chem. Eng. Sci. 2000, 55, 3955. (6) Huang, C. J.; Fei, J. H.; Wang, D. J.; Zheng, X. M. Chin. Chem. Lett. 2000, 2, 181. (7) Huang, C. J.; Zheng, X. M.; Mo, L. Y.; Fei, J. H. Chin. Chem. Lett. 2001, 3, 249. (8) Wang, H. Y.; Ruckenstein, E. Appl. Catal., A 2001, 209, 207. (9) Huang, C. J.; Zheng, X. M.; Mo, L. Y.; Fei, J. H. Chin. J. Chem. 2001, 4, 340. (10) Vernon, P. D. F.; Green, M. L. H.; Cheetham, A. K.; Ashcroft, A. T. Catal. Today 1992, 13, 417. (11) Ruckenstein, E.; Hu, Y. H. Ind. Eng. Chem. Res. 1998, 37, 1744. (12) Choudhary, V. R.; Rajput, A. M.; Prabhakar, B. Catal. Lett. 1995, 32, 391. (13) Choudhary, V. R.; Mamman, A. S. Appl. Energy 2000, 66, 167.
However, heat manipulation remains as a big problem for fixed-bed reactors upon large-scale operation. This problem may be overcome, using a fluidized-bed reactor. A good result was reported on the combined reaction over a Ni/MgO catalyst, using a fluidized-bed reactor.15 Notably, the MgO support has poor mechanical strength and is not suitable for fluidized-bed reactors. Previous reports from our laboratory have shown that Pt/CoAl2O4/Al2O3 and Ni/Al2O3 catalysts were active for the combined reaction in fixed-bed and fluidized-bed reactors, respectively.14,16 Nevertheless, high mechanical strength is necessary for the supports of industrial catalysts, especially those used in fluidized-bed reactors. Commercial microspherical silica (SiO2) has good mechanical strength and is an excellent support for a fluidized-bed reactor. Herein, we report syngas production from the combined reaction in a fluidized-bed reactor, using Ni/SiO2 catalysts that are promoted by La2O3. 2. Experimental Section 2.1. Catalyst Preparation. Ni/SiO2 and Ni-La2O3/SiO2 catalysts were prepared by the incipient-wetness impregnation method. Nitrate salts were used as precursors, and commercial microspherical silica (from Nanjing Tianyi Inorganic Chemical Factory; Brunauer-Emmett-Teller (BET) surface area (SBET) of 370 m2/g, particle diameter of 0.28-0.45 mm) was used as support. After impregnation, the samples were dried overnight at 120 °C and subsequently calcined in air at 700 °C for 4 h. The prepared Ni-La2O3/SiO2 and Ni/SiO2 catalysts were designated as NixLaSi and NiSi, respectively. The x symbol (14) Mo, L. Y.; Zheng, X. M.; Huang, C. J.; Fei, J. H. Catal. Lett. 2002, 3-4, 165. (15) Matsuo, Y.; Yoshinaga, Y.; Sekine, Y.; Tomishige, K.; Fujimoto, K. Catal. Today 2000, 63, 439. (16) Mo, L. Y.; Zheng, X. M.; Chen, Y. H.; Fei, J. H. React. Kinet. Catal. Lett. 2003, 78, 233.
10.1021/ef0498521 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/07/2004
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in the terminology of NixLaSi catalysts represents the lanthanum loading (x wt %) over Ni-La2O3/SiO2 catalysts. The loading of nickel on NiSi and NixLaSi catalysts was always 4 wt %. 2.2. Catalytic Reaction. The catalytic reaction was performed in a fluidized-bed reactor that was comprised of a quartz tube (inner diameter (ID) of 20 mm, height (H) of 750 mm) under atmospheric pressure at 700 °C. Prior to reaction, 2 mL of catalyst was reduced at 700 °C for 60 min under a flow of hydrogen at atmospheric pressure. A reactant gas stream that consisted of methane (CH4), carbon dioxide (CO2), and oxygen (O2), with a molar ratio of 1/0.4/0.3 (unless otherwise stated), was used with a gas hourly space velocity (GHSV) ) 9000 h-1. The feed gas was controlled by a mass flow controller. The effluent gas was analyzed with an online gas chromatograph (Shimadzu, model GC-8A) that was equipped with a packed column (TDX-01) and a thermal conductivity detector, after it was cooled in an ice trap. Under our reaction conditions, the oxygen in the feed gas was completely consumed in all cases. The conversion and the selectivity were calculated as given in the literature:16
X(CH4) (%) ) X(CO2) (%) ) S(H2) (%) ) S(CO) (%) )
FCH4,in - FCH4,out
× 100
FCH4,in FCO2,in - FCO2,out FCO2,in
Figure 1. Effect of the lanthanum loading (x) on the conversion of CH4 and stability over the NixLaSi catalysts. Reaction conditions: temperature (T), 700 °C; FCH4/FCO2/FO2 ) 1/0.4/0.3; and gas hourly space velocity (GHSV), 9000 h-1.
× 100
FH2,out 2(FCH4,in - FCH4,out)
× 100
FCO,out (FCH4,in - FCH4,out) + (FCO2,in - FCO2,out)
× 100
Fi ) CiFtotal where X, S, and F are the conversion, selectivity, and gas flow rate, respectively; Ftotal is the total feed gas flow rate or reaction effluent gas rate; and Ci is the molar fraction of component i in the feed gas or reaction effluent gas, which is detected using gas chromatography (GC). 2.3. Catalyst Characterization. X-ray diffraction (XRD) analysis was performed with an automated powder X-ray diffractometer (Rigaku, model D/max-B). A copper anode (Cu KR, 45 kV, 40 mA) was used as the X-ray source. Temperatureprogrammed reduction/desorption (TPR/TPD) experiments were performed on the AMI-200 characterization system (Zeton Altamira, USA). Before the TPR experiment was performed, the catalyst was pretreated at 300 °C for 30 min under an argon flow (99.999%). After the catalyst was cooled to 30 °C, it was heated to 700 °C, at a rate of 20 °C/min in a 10% H2/argon stream (30 mL/min). Prior to the O2-TPD or CO2TPD experiment, the sample was pretreated at 300 °C for 30 min under a helium atmosphere. After the sample was cooled to 30 °C, a gas flow of 5% O2/helium or CO2 was introduced to the sample for 30 min. The sample then was purged with helium at 30 °C for 30 min. TPD experiments were performed using a ramp rate of 20 °C/min from 30 °C to a predetermined temperature under the helium stream. The carbon deposition on the catalysts that were used was measured using a thermogravimetry (TG) technique on a Perkin-Elmer model PE TAC7/DX instrument. Before the TG analysis was conducted, the sample was pretreated with N2 (99.999% pure) at 300 °C for 30 min. After the sample was cooled to 50 °C, O2 was introduced to it. The temperature was increased to 800 °C, at a ramp rate of 20 °C/min.
3. Results and Discussion 3.1. Catalytic Performance. 3.1.1. Effect of Lanthanum Loading (x) on Catalytic Activities of the
Figure 2. Effect of the reaction temperature on the catalytic activity of the Ni5LaSi catalyst. Reaction conditions: FCH4/FCO2/ FO2 ) 1/0.4/0.3 and GHSV ) 9000 h-1.
NixLaSi Catalysts. The catalytic activities of NixLaSi catalysts are plotted in Figure 1. The initial activity of the NixLaSi catalysts did not vary with the lanthanum loading (CH4 conversion of ∼74%). However, the NiSi catalyst deactivated rapidly and the conversion of CH4 decreased to 43% in 6 h of reaction on stream. The stability of the Ni1LaSi catalyst was greatly enhanced and the conversion of CH4 decreased from 74% to 68% after 6 h of reaction on stream. The catalytic stability of NixLaSi catalysts was remarkably promoted when x g 5, and the catalysts did not exhibit any deactivation during 6 h of reaction on stream. 3.1.2. Effect of Reaction Temperature on the Catalytic Activity of the Ni5LaSi Catalyst. Figure 2 shows the effect of reaction temperature on the catalytic activity of the Ni5LaSi catalyst. The conversion of CH4 and CO2 and the selectivity of H2 and CO increased as the reaction temperature increased. The conversion of CH4 and CO2 was 39% and ∼0% at 550 °C, respectively, but increased to 96% and 92%, respectively, at 850 °C. As the reaction temperature increased from 550 °C to 850 °C, the selectivity of CO and H2 increased from 84% to 94% and from 75% to ∼100%, respectively. The fact that the conversion of CO2 was ∼0% at 550 °C demonstrated that high-temperature conditions are required for reforming by CO2. The CH4 combustion reaction was the major reaction that occurred at low temperatures. 3.1.3. Effect of Feed Composition on Catalytic Activity of the Ni5LaSi Catalyst. CO2 and O2 are used as oxidative reagents in the combined reaction to activate
CO2 Reforming and Oxidation of Methane to Syngas
Figure 3. Effect of the oxidant ratio of FO2/FCO2 on catalytic activity of the Ni5LaSi catalyst. Reaction conditions: T ) 700 °C, FCH4 ) FCO2 + 2FO2, and GHSV ) 9000 h-1.
Figure 4. Effect of the oxidant ratio of FO2/FCO2 on the ratio of nH2/nCO. Reaction conditions: T ) 700 °C, FCH4 ) FCO2 + 2FO2, and GHSV ) 9000 h-1.
CH4 decomposition. In this study, the following feed relation between the CH4 and oxidant was generally maintained for a C/O atomic molar ratio of 1:
FCH4 ) FCO2 + 2FO2 However, the ratio of oxidants CO2 and O2 in the feed was varied, to control the ratio of H2 and CO in the reaction product. The effect of the oxidant ratio of FO2/ FCO2 was investigated, while maintaining a gas hourly space velocity (GHSV) of 9000 h-1. Figure 3 presents the effect of the ratio of oxidants on catalytic activity of the Ni5LaSi catalyst. The conversion of CH4 increased as the FO2/FCO2 ratio increased, which indicated that CH4 is easier to be activated by oxygen than CO2. However, the conversion of CO2 evidently decreased as the FO2/FCO2 ratio increased. The selectivity of H2 decreased as the chance of H2 being oxidized increased with O2 content in feed gas; however, the selectivity of CO was almost unchanged. Figure 4 shows the influence of feed composition on the molar ratio of syngas (nH2/nCO). The figure shows that the nH2/nCO ratio increases monotonically from 0.97 to 1.91 as the FO2/FCO2 ratio increases from 0 to 2.0. The combined reaction can provide a nH2/nCO ratio between 1 and 2, corresponding to the stoichiometric values of the CO2 reforming and partial oxidation reaction, respectively. Therefore, one of the most valuable advantages of combined partial oxidation and CO2 reforming of CH4 is that the ratio of syngas (nH2/nCO) can be
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Figure 5. Effect of the lanthanum loading (x) on the particle size of nickel over the NixLaSi catalysts.
Figure 6. Hydrogen-based temperature-programmed reduction (H2-TPR) profiles of the NixLaSi catalysts: NiSi (profile a), Ni1LaSi (profile b), Ni5LaSi (profile c), and Ni8LaSi (profile d).
controlled by varying the feed gas composition, based on the needs of the post-processing. 3.2. Catalyst Characterization. 3.2.1. XRD Study. Figure 5 gives the effect of the lanthanum loading (x) on the particle size of nickel over NixLaSi catalysts reduced at 700 °C for 1 h. The particle size of nickel was determined by XRD and calculated using the Scherrer equation. The lanthanum loading showed a great effect on the particle size of nickel over NixLaSi catalysts. The particle size of nickel over the NiSi catalyst was 42 nm; however, it decreased drastically, to 13 nm, when 5 wt % of lanthanum (x ) 5) was added and remained ∼12 nm when the lanthanum loading increased further. On XRD patterns (not shown here), no significant diffraction peak of La2O3 appeared from the NixLaSi catalysts, even at values up to x ) 15. Evidently, the La2O3 oxide was well-dispersed on the SiO2 support. The XRD results suggest that the highly dispersed La2O3 promotes a high dispersion to nickel crystallites that are dispersed over NixLaSi catalysts. 3.2.2. Temperature-Programmed Reduction (TPR) Results. Figure 6 exhibits the reduction behaviors of the NixLaSi and NiSi catalysts. For the NiSi catalyst, two peaks were observed: a small shoulder peak at ∼420 °C and a major peak at ∼475 °C. These two peaks can be attributed to the reductions of small particles of NiO and large particles of NiO, respectively. The two peaks also appeared over the NixLaSi catalysts. As the lanthanum loading increased, the area of the peak at ∼420
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Figure 7. Oxygen-based temperature-programmed desorption (O2-TPD) patterns of the NixLaSi catalysts: NiSi (pattern a), Ni1LaSi (pattern b), Ni5LaSi (pattern c), and Ni8LaSi (pattern d).
Figure 8. Carbon dioxide-based temperature-programmed desorption (CO2-TPD) patterns of the NixLaSi catalysts: NiSi (pattern a), Ni1LaSi (pattern b), Ni5LaSi (pattern c), and Ni8LaSi (pattern d).
°C increased and the position of the peak at 475 °C shifted to higher temperature. The TPR results of the NixLaSi catalyst are consistent with the XRD results of NixLaSi catalysts: La2O3 promotes the dispersion of nickel over NixLaSi catalysts. The promotion effect of La2O3 on the dispersion of nickel may be attributed to a strong interaction between La2O3 and NiO. 3.2.3. Oxygen-Based Temperature-Programmed Desorption (O2-TPD) Results. Figure 7 provides O2-TPD patterns of the NixLaSi catalysts. The peaks of O2 desorption appeared at ∼120 °C. The peak area of O2 desorption peak increased as the loading of La2O3 increased. These results indicated that the oxide La2O3 enhanced the adsorption ability of O2, which may be beneficial in removing carbon deposition on the catalysts. 3.2.4. Carbon Dioxide-Based Temperature-Programmed Desorption (CO2-TPD) Results. Figure 8 presents the effect of lanthanum loading on the CO2 adsorption property of NixLaSi catalysts. The peak of CO2 desorption appeared at ∼100 °C for all the catalysts. Similar to O2-TPD, the amount of desorption of CO2 increased as the lanthanum loading increased. It is well-known that CO2 preferentially adsorbed on La2O3 to form La2O2CO3.17,18 (17) Squire, C. D.; Luc, H.; Puxley, D. C. Appl. Catal., A 1994, 108, 261. (18) Becker, S.; Baerns, M. J. Catal. 1991, 128, 512.
Mo et al.
Figure 9. Effect of the lanthanum loading (x) on the amount of carbon deposition of the NixLaSi catalysts, using the CO2 reforming of methane as the probe reaction. Reaction conditions: T ) 700 °C; GHSV ) 9000 h-1; and reaction time, 2 h.
3.3. Anti-carbon Deposition Property of the NixLaSi Catalysts. To investigate the anti-carbon deposition property of NixLaSi catalysts, CO2 reforming of the CH4 reaction was chosen as a probe reaction. Figure 9 shows that the carbon deposition of NiSi was 13.13%; however, the carbon deposition was drastically decreased to 5.23% as the value of x was increased to 5. The lanthanum loading showed no significant effect on carbon deposition for x > 5. From Figure 8, it can be observed that the amount of adsorption for CO2 increased as the addition of La2O3 increased. It was reported that the surface acidity of the support was an important factor that affected its resistance to coke deposition.19 Increasing the basicity of the catalyst can improve its CO2 adsorption ability, which results in not only an increase in the chance of reaction between carbon deposition and CO2 adsorption, but also inhibition of the Boudouard reaction (2CO H CO2 + C).20 According to the aforementioned results, the anti-carbon deposition property of NixLaSi catalysts should be enhanced with increased lanthanum loading (x); however, the amount of carbon deposited on the NixLaSi catalysts showed no evident decrease as the value of x increased to >5. Therefore, in addition to the reason that the basicity of the catalyst support can be correlated to the anti-carbon deposition property of the catalysts, the particle size of the active metal may be the other important relative factor. Figure 5 has indicated that the particle size of nickel in the NixLaSi catalyst decreased at the initial addition of La2O3 but leveled off as the value of x increased to >5. A similar profile of carbon deposition is obtained in Figure 9. Evidently, the anti-carbon ability of nickel catalysts is dependent on the particle size of the nickel. A similar conclusion has been proposed in the literature by Ji et al.21 Under the high temperatures applied for the combined reaction, the La2O2CO3 species at the interfacial area may become mobile, to some extent, and have a tendency to attack the carbon species deposited on the nickel metal at the sites that are close to the La2O3 or LaOx species, producing two CO molecules.22 (19) Yamazaki, O.; Nozaki, T.; Omata, K.; Fujimoto, K. Chem. Lett. 1992, 1953. (20) Kim, G. J.; Cho, D. S.; Kim, K. H.; Kim, J. H. Catal. Lett. 1994, 28, 41. (21) Ji, M.; Zhou, M. Z.; Bi, Y. L.; Zheng, K. J. J. Mol. Catal. 1997, 6, 1 (in Chin.).
CO2 Reforming and Oxidation of Methane to Syngas
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the nickel metal was examined using XRD, which revealed that the particle sizes of the nickel in the freshly reduced and used catalysts were 46 and 89 nm, respectively. Sintering of the nickel metal particle over the NiSi catalyst had occurred. However, the particle size of the used Ni5LaSi catalyst reacted after 6 h was maintained at 13 nm. This result demonstrated that La2O3 had an important role in preventing the nickel metal over the NixLaSi catalysts from sintering, which may be attributed to the strong interaction between the nickel and La2O3. From the aforementioned analysis, the cause of deactivation of the NiSi catalyst is the sintering of the nickel metal particles during reaction on stream. Figure 10. Effect of the gas composition on carbon deposition over the Ni5LaSi catalysts. Reaction conditions: T ) 700 °C; FCH4 ) FCO2 + 2FO2; GHSV ) 9000 h-1; and reaction time, 2 h.
The interfacial area between the nickel metal and the La2O3 was dependent on the particle size of the nickel. The smaller the particle size of the nickel metal was, the larger the interfacial area between the La2O3 and the nickel became. Therefore, the CO2 that was adsorbed on La2O3 easily reacted with the carbon that was deposited on the nickel metal to form CO. The enhancement of the anti-carbon deposition ability over the NixLaSi catalysts contributed to the enhancement of the CO2 adsorption and the high dispersion of nickel, because of the addition of La2O3. 3.4. Effect of Oxidant Ratio on Carbon Deposition over the Ni5LaSi Catalyst. Figure 10 presents the amount of carbon deposition over the Ni5LaSi catalyst after 2 h of reaction under different oxidant ratios. The amount of carbon deposition drastically decreased from 5.23% to 0.71% when the FO2/FCO2 ratio was increased from 0 to 0.125. At FO2/FCO2 ) 0.750, negligible carbon deposition was observed. The drastic decrease of carbon deposition was attributed to an effective oxidation of deposited carbon by O2 in the feed gas. It is an additional advantage of the combined reaction, on comparison to the CO2 reforming reaction, in addition to the thermal control that was mentioned in the Introduction. 3.5. Reasons for Deactivation of the NiSi Catalyst. Figure 1 illustrates that the NiSi catalyst is rapidly deactivated after 6 h of reaction on stream. A TG technique has been used to detect the amount of carbon deposition when the NiSi catalyst was deactivated; no carbon deposition was observed. There are no significant differences in the pore volume and the pore diameter both of the fresh and used catalyst, through the texture measurement of the NiSi catalyst. The particle size of (22) Zhang, Z. L.; Verykios, X. E.; MacDonald, S. M.; Affrossman, S. J. Phys. Chem. 1996, 100, 744.
4. Conclusions The combined partial oxidation and CO2 reforming of methane (CH4) to produce syngas was investigated over the NixLaSi catalysts in a fluidized bed reactor (where x represents the lanthanum loading). The lanthanum loading (x) was proven to have a great effect on the stability of catalysts. The stability of NixLaSi catalysts was remarkably promoted as the value of x increased to g5. By changing the feed composition, according to the needs of the post-processing, the molar ratio of syngas (nH2/nCO) in the product can be controlled between 1.0 and 2.0. The carbon deposition over the Ni5LaSi catalyst can be suppressed significantly with the introduction of O2 to the feed gas, which is one of the advantages for the combined reaction. The adsorption ability of O2 and CO2 for the NixLaSi catalysts studied by carbon dioixde- and oxygen-based temperature-programmed desorption (CO2-TPD and O2TPD, respectively) was enhanced, because of the addition of La2O3. The adsorption ability of O2 and CO2 and the particle size of the nickel metal were the most important factors that affected the anti-carbon deposition ability of the NixLaSi catalysts. Sintering of the nickel particles over the NiSi catalyst was the main cause of the rapid deactivation of the NiSi catalyst during reaction on stream. X-ray diffraction (XRD) and temperature-programmed reduction (TPR) results showed that the high dispersion of La2O3 promoted the dispersion of nickel and prevented the nickel metal particles from sintering during reaction. The promotion is derived from the strong interaction between nickel and La2O3 over NixLaSi catalysts. Acknowledgment. We thank the financial support of National Natural Science Foundation of the People’s Republic of China (under Grant No. 20343002). We also thank Professor Chuin-Tih Yeh (National Tsinghua University, Taiwan) for beneficial discussion and revision of this paper. EF0498521
