γ−Al2O3 catalyst for syngas obtainment by simultaneous catalytic reaction of alkanes with carbon dioxide and oxygen

Studies in Surface Science and Catalysis J.J. Spivey,E. lglesia and T.H. Fleisch(Editors) | 2001 ElsevierScience B.V. All rights reserved.

51

LiLaNiO/y-Al203 catalyst for syngas obtainment by simultaneous catalytic reaction of alkanes with carbon dioxide and oxygen Shenglin Liu a*, Longya Xu b, Sujuan Xie a, Qingxia Wang a and Guoxing Xiong b a. Laboratory of Natural Gas Utilization and Applied Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, P. R. China b. State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, CAS, P.R.China The performance of a LiLaNiO/7-A1203 catalyst for the simultaneous catalytic reaction of alkanes with CO2 and 02 to syngas was investigated in a fixed-bed flow microreactor using XRD and TG techniques. The catalyst exhibits high activity for the reactions of CHa-C2H6CO2-O2 and CH4-C2H6-C3Hs-C4Hlo-CO2-O2 to syngas. The life tests for the two reactions show that the catalyst is stable and has resistance to carbon deposition at high temperatures. At these high temperatures, the CH4-CaH6-CO2-O2 mixture could be directly converted to syngas over the LiLaNiO/7-A1203, and the presence of C2H6 does not lead to catalyst deactivation by carbon deposition. On the other hand, the CHn-C2H6-C3Hs-CaHlo-CO2-O2 mixtures could not be directly converted to syngas due to a gas phase reaction which yielded coke on the reactor wall but not on the catalyst. Instead, syngas could be produced by following two steps: first, the C3H8 and CaHlo in the mixture gases were converted to CH4, C2H6, and CO2 below 873K, then the CHa-C2H6-CO2-O2 mixture was converted into syngas over the LiLaNiO/7-AI203 catalyst at high temperatures (>873K). 1. INTRODUCTION There are abundant supplies of gases containing CH4, C2H6,C3H8and C4HI0,etc. from FCC (Fluidized Catalytic Cracking) tail gas, refinery gas, etc. Normally, these gas mixtures are combusted to carbon dioxide, since the complete separation of CH4, C2H6, C3H8 and CaHIo is not economical. If syngas could be produced from these mixtures over supported nickel catalysts with high selectivity and conversion, then syngas can be obtained directly from FCC tail gas, refinery gas, etc. This will lead to a better utilization of the light fractions of the FCC tail gas and refineries, etc. Both partial oxidation of alkanes (POA) and CO2 reforming of alkanes are effective routes for syngas production. The POA reaction is an exothermic process, and the heat produced is strongly dependent on the selectivity, which makes the process very difficult to control and to operate safely. Meanwhile, CO2 reforming of alkanes is a highly * Corresponding author e-mail: [email protected]

52 endothermic process which often leads to rapid carbon deposition, particularly on nickel catalysts. Coupling the endothermic CO2 reforming with the exothermic POA reaction has the following advantages: (i) the coupled process can be made mildly endothermic, thermoneutral or mildly exothermic by manipulating the process conditions; (ii). the energy efficiency of the process can be optimized, and hot spots can be avoided, which eliminates runaway reaction conditions makes the process safe to operate; (iii). the process can be made environmentally beneficial. Combined partial oxidation of methane and CO2 reforming with methane has been reported by Vernon et al [ 1]. They reported that transition metals supported on inert oxides were active catalysts for combined partial oxidation and CO2 reforming. They varied the CH4/CO2/O2 ratio in order to achieve a thermoneutral reaction over a 1%Ir/A1203 catalyst. Ross et al [2] reported that hot spots in the catalyst bed were reduced significantly by combining the partial oxidation reaction with the CO2 reforming reaction. Inui et al [3] carded out the reaction using a feed containing CH4, C2H6, C3H8, CO2, O2 and N2 over the Ni-CeEO3-Pt-Rh catalyst, and found that an extraordinarily high space-time yield of syngas could be obtained using high flow rates at around 400 ~ Long et al [4] reported that the nickel supported catalysts showed were active for making syngas from a feed including a low molecular alkane, 02, CO2 and H20 in a fluidized bed of catalyst. Recent studies conducted in this laboratory. [5,6,7,8] showed that high reaction rates and resistance to carbon deposition, combined with excellent stability could be achieved over the LiLaNiO/7-A1203 catalyst for CH4-O2, C2H6-O2,and CH4-C2H6-O2reactions to syngas. In the present work, a fixed-bed flow microreactor, and XRD and TG techniques, were used to study the reactions of a series of (CH4, C2H6, C3Hs and CaHlo) with 02 and CO2 produces to syngas over a LiLaNiO/7-A1203 catalyst. The obvious difference between the higher alkanes and methane is that C2H6, C3Hs and C4H~o decompose rapidly at high temperatures, leading to catalyst deactivation by carbon deposition. Hence, the key to the simultaneous catalytic reactions is to optimize the supported nickel catalysts to reaction activity and selectivity, and to resist carbon deposition. 2. EXPERIMENTAL

2.1. Preparation of catalyst and test of reaction performance Catalysts were investigated in a fixed-bed flow microreactor under atmospheric pressure. Reaction performance was tested using a microreactor with an internal diameter of 8 mm, using 1 mL of catalyst with an average particle size of 0.37-0.25mm. An EU-2 type thermocouple was placed in the quartz reactor, which was located in the middle of the catalyst bed to control the temperature in the electric furnace. This temperature was taken as the reaction temperature. The analysis methods for the reaction products and preparation of the LiLaNiO/7-A1203 catalyst have been described earlier [5].

2.2. Characterization of catalyst X-ray diffraction (XRD) characterization of the catalysts was performed with a Riguku D/Max-RB X-ray diffractometer using a copper target at 40KV x 100mA and a scanning speed of 8 degree/min. TG tests were recorded and treated by a Perkin-Elmer 3600 work-

53 station at a programmed temperature rate of 10 K/min in air, with the flow rate 25 mL/min. 3. RESULTS AND DISCUSSION 3.1. Reaction performance of CH4-C2H6-CO2-O2 to syngas over the LiLaNiO/7-AI203 The reactions of CH4-CO2-O2 and C2H6-CO2-O2 mixtures to form syngas over the LiLaNiO/7-A1203 were carried out. The results indicate that the catalyst possesses high activity for these two reactions. On this basis, the reaction of CH4-C2H6-CO2-O2to syngas was performed next. The effect of reaction temperature on the performance of the LiLaNiO/7A1203 is shown in Fig.1. When the reaction temperature is subsequently increased from 973 to 1123K, the CO2 and CH4 conversions increase, while the H2 selectivity does not change significantly, and the H2/CO ratio of the product remains 1.1/1. Ethane and oxygen are converted almost completely (not shown). Below 1053K and at a CH4/CO2/C2H6/O2/Ar ratio of 0.5/0.19/0.31/0.48/1.5, the influence of space velocity on the reaction over the LiLaNiO/7A1203 is studied. The results in Fig.2 indicate that the influence of space velocity is not appreciable. This means that the catalyst has sufficientactive centers for the reaction over this wide range of space velocities. ~

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Effect of temperature on the performance of the LiLaNiO/7-A1203 (CHa/CO2/CzH6/O2/Ar = 0.42/0.43/0.15/0.28/1.1; SV = 3.6xl 03h-l) Fig.2. (Right) Performance of the LiLaNiO/y-AleO3 at different space velocities (CH4/CO2/C2H6/O2/Ar = 0.5/0.19/0.31/0.48/1.5; T = 1053K) At a constant space velocity of 1.9 x 10 3 h-I (based on the combined flow rate of CHa+CO2+C2H6) and a temperature of 1053K, the flow rate of 02 was changed to obtain different O2/C2H6 ratios. As shown in Table 1, as the O2/C2H6 ratio changes from 1.35 to 2.19, the CO2 conversion and the H2 selectivity decrease, while the CH4 conversion increases. The effect of diluting the reaction gases on reaction performance is investigated by addition of Ar, at an Ar/O2 ratio of 4/1. As shown in Table 1, at the same O2/C2H6 ratio, the CH4 conversion, H2 selectivity, and H2/CO ratio without dilution of Ar are lower than those with dilution, and the CO2 conversion of the former is higher than that of the latter. Since the methane reforming with carbon dioxide is a slow process compared to that of methane partial oxidation, adding Ar reduces the contact time of CO2 and CH4 with the catalyst, which decreases the CO2 conversion. For methane partial oxidation, increasing the space velocity increases the rate of

~4

the direct partial oxidation (DPO) scheme[5], so that the H2 selectivity increases. This work i,, under way. These results indicate that the addition of dilute Ar in the feed affects the reactior performance.

Table 1 Performance of the LiLaNiO/),-A1203 at different 02/C2H6 ratios* CH4/CO2/C2H6 = 0.5/0.19/0.31" SV CH4-CO2-C2H6-" 1.9 x 10 3 h-1 ;T = 1053K 02/C2H6 ratio CH4 Conv (%) CO2 Conv (%) H2 Sele(%) H2/CO ratio 1.35 76.2 (82.0) 98.3 (96.9) 94.7 (99.9) 1.4 (1.5) 1.58 82.2 (86.7) 98.0 (96.6) 88.9 (93.7) 1.3 (1.4) 1.84 93.0 (97.6) 88.9 (83.2) 83.8 (91.2) 1.3 (1.4) 2.19 95.8 (99.5) 84.0 ( 6 8 . 5 ) 71.8 (83.7) 1.1 (1.4) *' 1. C2H6 and 02 conversions are all 100% 2. The value in parentheses is for a CH4/CO2/C2H6 ratio of 0.5/0.19/0.31, Ar/O2 ratio of 4/1, (CH4+COE+CEH6) space velocity of 1.9 x 10 3 h-! , and a temperature of 1053K. The stability of the LiLaNiO/),-AI203 was studied at 1053K, a CH4/CO2/C2H6/O2 ratio o 0.42/0.43/0.15/x (where x represents the oxygen content of the mixture gases), a O2/Ar rati~ of 1/4, and a (CH4+CO2+C2H6) space velocity, of 1.8x103 h ~ (Table 2). During the 50h lift test, it is interesting that CH4 and C2H6 are almost completely converted. Meanwhile, the C( and H2 selectivities, and H2/CO ratio remain ~100%,-~ 85% and-~1.1, respectively. Th, values approach to those of the thermodynamic equilibrium. The result of TG shows th, carbon deposition of the used catalyst is only 0.22 wt%. These results indicate that th, LiLaNiO/),-A1203 is quite stable and resists carbon deposition during the high temperatur, reaction. A longer life test is under way. Table 2 Reaction performance of the LiLaNiO/~/-AI203 as a function of time * CH4/COE/CEH6/O2 = 0.42/0.43/0.15/x; Ar/O2 = 4/1; SVcH4.CO2.C2H6-" 1.8 x 10 3 h "1",T = 1053I~ x CO2 Cony ( % ) cO Sele(%) H2 Sele (%) H2/CO ratio Time (h) 0.2 0.45 54.6 100 77.7 1.1 2 0.37 71.9 100 83.6 1.1 7 0.34 77.4 100 85.5 1.1 14 0.32 75.2 100 88.6 1.1 27 0.36 67.9 100 85.8 1.1 29 0.37 69.4 100 84.3 1.1 39 0.35 70.3 100 86.3 1.1 43 0.31 79.2 100 87.8 1.1 49 0.33 74.2 100 87.9 1.1 50 0.34 71.7 100 87.2 1.1 *" CH4, C2H6 and 02 conversions are all 100% 3.2. Reaction performance of alkane-CO2-O2 to syngas over the LiLaNiO/~/-AI203

Because C3H8 and C4HIo are present in FCC tail gas, refinery gas, etc., the simultaneou catalytic reaction of these alkanes (containing CH4, C2H6, C3H8 and CaHI0) with carbo dioxide and oxygen to syngas over the LiLaNiO/7-A1203 catalyst was investigated. The alkan

55 concentrations were: 47.2%(vol)CH4, 40.0%(vol)C2H6, 9.6%(vol)C3H8 and 3.2%(vol)C4H10. Tests to measure the effect of temperature show that the conversions of CH4 and CO2 increase with increasing temperature, while Hz/CO ratio is constant (- 0.90), at an 02/alkane ratio of 0.36, CO2/alkane ratio of 0.65 and alkane space velocity of 1.7x103 h l. In addition, C2H6, C3H8, and C4H8 conversions, and CO selectivity all remain-100% between 923K and 1073K. The CH4 conversion increases, while CO2 conversion and H2 selectivity decrease with the increasing of 02/alkane ratio (from 0.19 to 0.61) at a temperature of1073K, CO2/alkane ratio of 0.65 and alkane space velocity of 1.7xl 03 h l. The LiLaNiO/7-A1203 catalyst is active over a wide range of space velocity at these conditions. The life test experiment of the catalyst was performed using a microreactor with an internal diameter of 8mm and the volume of catalyst of 1 mL, while the stability of the LiLaNiO/7A1203 catalyst was studied at 1073K, under O2/alkane ratio of 0.36, CO2/alkane ratio of 0.65 and alkane space velocity of 1.7xl 03 h~ (Fig.3). During the 40-h life test, CH4 conversion and H2/CO ratio remain -~94%, - 0.9, respectively, CO2 conversion and H2 selectivity are >75%, >80% respectively. In addition, C2H6, C3H8, and C4H8 conversions, and CO selectivity all remain-100%. These results indicate that the LiLaNiO/7-A1203 is quite stable during the high temperature reaction.

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Time (hour) Fig.3 Reaction performance of the LiLaNiO/7-A1203 as a function of time* (O2/alkane = 0.36/1; CO2/alkane = 0.65/1; SV alkane =1.7x103 h'l; T=1073K) *: The alkane contains 47.2%CH4, 40.0%C2H6, 9.6%C3H8 and 3.2%C4H10. After the 40h life test, the reactor was cooled to room temperature in N2 gas flow, then the catalyst was characterized by XRD and TG. The XRD measurements indicate that the support for the fresh and used catalysts are composed of y-A1203. This demonstrates that the catalyst has a stable crystal phase during the high temperature reaction. The TG results show that the carbon deposition on the used catalyst is only 0.1 wt%. This shows the LiLaNiO/y-A1203 resists carbon deposition during the high temperature reaction. However, after the 40-h life test, there is some carbon on the reactor wall, resulting from the

56 thermal decomposition of propane and butane. The coke will plug the reactor and cause the reaction rate to decrease over time. In other words, over the LiLaNiO/7-A1203 catalyst, the mixture gases of CH4-C2H6-C3Hs-CaH~o-CO2-O2 may not be directly converted to syngas due to coke formation on the reactor wall at high temperatures, but may be converted to syngas by two steps: first, below 873K there is little gas phase reaction, and the C3H8 and C4HI0react to form CH4, C2H6and CO2 [9,10]. Next, the CH4+C2H6+CO2+O2 gas mixture is converted to syngas over the catalyst at high temperatures (>873K). This is being studied now. For the CH4-C2H6-CO2-O2reaction, no gas phase reaction is observed at these high temperatures, thus the CH4-C2H6-CO2-O2 gas mixture can be converted directly to syngas over the LiLaNiO/7A1203 at high temperatures. Schmidt et al. [ 11 ] investigated partial oxidation of alkanes over noble metal coated monolith. They reported that the presence of C2H6 in the natural gas would not lead to catalyst deactivation by carbon deposition. C3H8 oxidation over Rh has led to carbon deposition, but only in severely fuel rich regimes. The amount of C3H8 in natural gas should not be sufficient for coking to be a concern. 4. CONCLUSION At high temperatures(>873K), the CH4-C2H6-CO2-O2gas mixture can be directly converted to syngas over the LiLaNiO/7-A1203 without any detrimental coke formation on the wall. While the CH4-C2H6-C3Hs-C4Hlo-CO2-O2 gas mixture may not be directly converted to syngas due to a gas phase reaction resulting in coke formation on the reactor wall (but not on the catalyst), this mixture may be converted to syngas in two steps. REFERENCES [ 1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [ 11 ]

P.D.F.Vemon, M.L.H.Green, et al., Catal. Today, 13 (1992) 417. A.M. O'Connor, J. R. H. Ross, Catal. Today, 46 (1998) 203. T. Inui, Sekiyu Gakkai Shi, 40 (1997) 243. Long, et al., Low hydrogen syngas using CO2 and a nickel catalyst, US Patent No.5985178 (1999). Q. Miao, G. X. Xiong, S. S. Sheng, et al., React. Kinet. Catal. Lett. 66(1999)273. S.L. Liu, G. X. Xiong, S. S. Sheng, et al., Stud. Surf. Sci. Catal., 119 (1998) 747. S.L. Liu, G. X. Xiong, W. S. Yang, et al., Catal. Lett., 63 (1999)167. S.L. Liu, G. X. Xiong, S. S. Sheng, et al., Appl. Catal. A., 202 (2000) 141. S.L. Liu, G. X. Xiong, L.Y. Xu, et al., Appl. Catal. A., (In press). S.L. Liu, G. X. Xiong, L.Y. Xu, et al., Chin. Chem. Lett., (In press). M. Huff, P. M. Tomiainen, L. D. Schmidt, Catal. Today, 21 (1994) 113.