Energy & Fuels 2000, 14, 899-903
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Oxidative Dehydrogenation of Ethane over Alkali Metal Chloride Modified Silica Catalysts Shaobin Wang,* K. Murata, T. Hayakawa, S. Hamakawa, and K. Suzuki Department of Surface Chemistry, National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan Received December 3, 1999
The catalytic oxidative dehydrogenation of ethane into ethylene was investigated over a series of alkali metal chloride supported on silica catalysts. It is found that alkali metal chloride modified silica catalysts show much higher activity in the selective oxidation of ethane than silica support itself, and their catalytic activity depends on the nature of alkali metal and its loading. LiCl/ SiO2 exhibits the highest ethane conversion and ethylene yield, giving 99% ethane conversion and 80% ethylene selectivity at 600 °C. However, it shows rapid deactivation. NaCl/SiO2 shows high conversion and a longer stability. On the contrary, KCl/SiO2 displays the lowest ethane conversion but a high deactivation rate. Characterization of the catalysts reveals that the activity variation can be related to the redox and acid-base properties of the catalysts. The deactivation of catalytic activity can be attributed to the loss of chlorine and chemical phase transformation.
Introduction In the past decade much attention has been paid to the oxidative dehydrogenation of ethane (ODE) to ethylene due to the potential interest in utilization of ethane, an abundant component in natural gas, and its advantages over the thermal cracking process. It has been shown that alkali metal oxides supported on transitional metal oxides or rare earth oxides are active catalysts for this reaction.1 The most successful one of these candidates is probably the Li/MgO catalyst. Recently, it has been reported that addition of chlorine ions in catalysts or feeding chlorine-containing compounds into the reactor showed promoting effects on conversion and selectivity to ethylene in the reaction.2-5 It is suggested that the presence of Cl- ions on the catalysts can suppress the poisoning of active sites causing by CO2 produced in the reaction. In addition, chlorine radicals are thought to favor the homogeneous decomposition of ethyl radicals to ethylene at high temperatures.1 Therefore, it is deduced that modification of catalysts directly using metal halides will probably have a good effect on catalyst performance in the ODE. Wang et al.4 reported that they obtained 75% ethane conversion and 77% ethylene selectivity over a Li+MgO-Cl- catalyst at 620 °C after 50 h on stream. Several researches have revealed that LiCl supported on ZnO6 or NiO7 also exhibited high activities for the * Corresponding author, present address: Department of Chemical Engineering, 230 Ross Hall, Auburn University, AL 36849. (1) Cavani F.; Trifiro, F. Catal. Today. 1995, 24, 307-313. (2) Burch, R.; Crabb, E. M.; Squire, G. D.; Tsang, S. C. Catal. Lett. 1989, 2, 249. (3) Ahmed S.; Moffat, J. B. J. Catal. 1990, 121, 408. (4) Wang, D.; Rosynek, M. P.; Lunsdord, J. H. J. Catal. 1995, 151, 155-67. (5) Sugiyama, S.; Sogabe, K.; Miyamoto, T.; Hayashi, H.; Moffat, J. B. Catal. Lett. 1996, 42, 127-133.
oxidative dehydrogenation of ethane and methane oxidative coupling to C2 compounds. Other investigations have demonstrated the promoting effect of alkaline earth metal halides on some rare earth oxides for this reaction.8-10 All above investigations employed basic supports; however, it seems likely that strongly basic catalysts are not always required and in some cases may be a drawback. We have recently found that LiCl supported on sulfated zirconia catalysts showed much better activity and selectivity to ethylene in the oxidative dehydrogenations of ethane and propane.11-13 Sulfation or tungstation of zirconia increases the acidity of catalysts, helps to balance the acidity/basicity of alkali metal doped catalysts, and thus is believed to have some beneficial effects on catalytic activity. Silica is a commonly used acidic support. It has been found to exhibit unexpected oxidized properties, which can produce active oxygen species owing to surface defects and can exhibit activity in some oxidation reactions.14 However, few researchers have employed it as a support or catalyst in the oxidative dehydrogenation of ethane. Erdohelyi et al.15,16 have studied the (6) Wang, D.; Rosynek, M. P.; Lunsford, J. H. Chem. Eng. Technol. 1995, 18, 118-124. (7) Otsuka, K.; Hatano, M.; Komotsu, T. Catal. Today. 1989, 4, 409419. (8) Zhou, X. P.; Chao, Z. S.; Luo, J. Z.; Wan, H. L.; Tsai, K. R. Appl. Catal. A. 1995, 133, 263-268. (9) Au, C. T.; Chen, K. D.; Dai, H. X.; Liu, Y. W.; Luo, J. Z.; Ng, C. F. J. Catal. 1998, 179, 300-308. (10) Luo, J. Z.; Wan, H. L. Appl. Catal. A. 1997, 158, 137-144. (11) Wang, S.; Murata, K.; Hayakawa, T., Hamakawa, S.; Suzuki, K. Chem. Commun. 1999, 103-104. (12) Wang, S.; Murata, K.; Hayakawa, T.; Hamakawa, S.; Suzuki, K. Catal. Lett. 1999, 59, 173-178. (13) Wang, S.; Murata, K.; Hayakawa, T.; Hamakawa, S.; Suzuki, K. Chem. Lett. 1999, 25-26. (14) Cavani, F.; Trifiro, F. Catal. Today. 1999, 51, 561-580. (15) Erdohely, A.; Solymosi, F. J. Catal. 1991, 129, 497-510. (16) Erdohely, A.; Mate, F.; Solymosi, F. J. Catal. 1992, 135, 563575.
10.1021/ef990247l CCC: $19.00 © 2000 American Chemical Society Published on Web 06/10/2000
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partial oxidation of ethane using N2O as an oxidant over silica-supported alkali metal molybdate and vanadate catalysts. Hong and Moffat17,18 investigated the oxidative dehydrogenation of ethane on silica-supported metal-oxygen cluster compounds. To our knowledge, no research has been reported on the behavior of metal chloride based silica catalysts in this reaction. In this work, several alkali metal chloride supported on silica catalysts were prepared and tested in the oxidative dehydrogenation of ethane. Their physicochemical properties and catalytic behaviors were characterized and investigated.
Wang et al. Table 1. Physicochemical Properties of the Alkali Metal Chloride/Silica Catalysts SBET (m2/g)
Cl (%)
catalyst
fresh
reacted
fresh
reacted
3.5 wt % LiCl/SiO2 3.5 wt % NaCl/SiO2 3.5 wt % KCl/SiO2
0.5 187.7 205.0
0.05 10.3 15.1
6.7 2.8 2.5
1.7 2.3 1.8
Experimental Section Catalyst Preparation and Characterization. A commercial catalyst support, SiO2 (SBET ) 338 m2/g, 99.99%), obtained from Wako Chemicals, was employed. The catalysts were prepared by a wetness impregnation method. An alkali metal chloride salt (LiCl, NaCl, and KCl, 99.99% from Wako Chemicals) was dissolved in distilled water to make a solution at an appropriate concentration and then was mixed with the silica. All chemicals were used as received without further treatment. After evaporation and drying at 105 °C overnight, the samples were calcined at 700 °C for 3 h. The BET surface areas of the catalysts were measured by nitrogen adsorption at -196 °C on Micromeritics volumetric adsorption equipment provided by Shimadzu. Before adsorption, those samples were degassed at 200 °C for 2 h. The chemical structure of the support and catalysts was determined by X-ray diffraction (XRD) measurements. The XRD patterns were obtained on a Philips PW 1800 X-ray diffractometer at 40 kV, and 40 mA. The radiation source was Cu KR with a Ni filter. The surface chemical compositions of the catalysts were also determined by X-ray photoelectron spectroscopy (XPS). Those measurements were carried out on a PHI 5500 ESCA system (Perkin-Elmer) with Mg KR as a radiation source. Data were acquired at 14 kV and 30 mA under the vacuum of 9.4 × 10-10 Torr. The temperatureprogrammed reduction (TPR) experiments were conducted in a fixed-bed reactor. Samples (0.5 g) were loaded in the fixedbed reactor and underwent heat treatment under Ar gas flow of 30 mL/min from ambient temperature to 700 °C at a heating rate of 5 °C/min. After being maintained at 700 °C for 30 min, the temperature was reduced to the ambient temperature under the same gas flow. And then a 10% H2/Ar gas flow at a rate of 30 mL/min was introduced into the reactor and a heating program was started to raise the temperature to 700 °C at a heating rate of 3 °C/min. The concentrations of H2 in the products were determined by a GC-8A equipped with a TCD. Catalytic Testing. The selective oxidative dehydrogenation of ethane was performed at atmospheric pressure in a fixedbed vertical-flow reactor constructed from a high-purity alumina tube (i.d. ) 6 mm) packed with 1 g of catalysts and 2 g of quartz sand and mounted inside a tube furnace. The reactant stream consisting of 10% ethane, 10% oxygen, and 80% nitrogen was introduced into the reactor at a flow rate of 60 mL/min. The reaction temperature ranged between 500 and 650 °C. The samples were taken after 5 min on stream at each temperature and the products were analyzed by two gas chromatographs (Shimadzu, GC-8A) equipped with a Porapak Q column using a FID for hydrocarbons and a 5A molecular sieve column for CO, CO2, CH4, O2, N2, and H2 using a TCD, respectively. Before catalyst testing, a reactor tube loaded with only quartz sand was evaluated for the oxidative dehydrogenation of ethane under various conditions. It was found that (17) Hong, S. S.; Moffat, J. B. Appl. Catal. A. 1994, 109, 117-34. (18) Hong, S. S.; Moffat, J. B. Catal. Lett. 1996, 40, 1-7.
Figure 1. XRD patterns of the support and alkali metal chloride promoted silica catalysts. the homogeneous contribution was negligible because ethane conversion was less than 2% at 650 °C.
Results and Discussion Catalyst Characterization. The surface areas of the catalysts prepared are given in Table 1. It is seen that addition of alkali metal chloride greatly reduces the surface areas of the catalysts. The reduction degree depends on the nature of alkali metal. KCl/SiO2 shows the least reduction in SBET, while LiCl/SiO2 has the lowest SBET, less than 1 m2/g. The reduction in surface area was caused by the phase transformation that occurred on those catalysts. Figure 1 shows the XRD patterns of the support and alkali metal chloride/silica catalysts. As shown, the silica support presents an amorphous phase. However, it gradually transforms into crystallites when it was modified by alkali metal chloride, and the extent of crystallization varies with the nature of the alkali metal. For NaCl/SiO2 and KCl/ SiO2, apart from the amorphous SiO2, the crystallites of NaCl and KCl are present in the corresponding catalysts. However, a complete phase transformation from the amorphous phase to crystallites occurs on LiCl/ SiO2, which can account for the greatest extent of decrease in surface areas. Quartz, Li2SiO3, and LiCl are found as the major phases in LiCl/SiO2 catalysts. The phase transformation indicates that alkali metals have a different interaction with the support silica. The surface concentration of chlorine on catalysts was determined by XPS and values are presented also in Table 1. Due to the crystallization, most of LiCl is present on the surface of catalyst while NaCl and KCl can penetrate into the pores of the support, resulting in the highest surface concentration of Cl on LiCl/SiO2.
Oxidative Dehydrogenation of Ethane
Figure 2. TPR profiles of alkali metal chloride/SiO2 catalysts.
The surface concentration of chlorine follows the order LiCl/SiO2 > NaCl/SiO2 > KCl/SiO2. The TPR profiles of the support and alkali metal chloride/silica catalysts are presented in Figure 2. These catalysts exhibit quite different TPR profiles. There is no significant reduction peak in SiO2 TPR spectra. Several reduction peaks were observed in the TPR profile of LiCl/SiO2. In the temperature range of 200580 °C, there are three reduction peaks at ca. 310, 415, and 505 °C, respectively. At high temperatures a sharp reduction peak also appears at 700 °C. For NaCl/SiO2, a weak reduction peak and a strong one can be observed with the maximum hydrogen consumption at 310 and 620 °C, respectively. However, only one peak appears in the TPR profile of KCl/SiO2, which is centered at 610 °C. These results indicate that addition of alkali metal chloride produces the oxygen defects in catalysts and that LiCl/SiO2 possesses the higher redox potential. Catalytic Performance. Table 2 presents the catalytic activity and product distribution over the support SiO2 and alkali metal chloride modified silica catalysts at the alkali metal loading of 3.5 wt % in the oxidative dehydrogenation of ethane at various temperatures. It is seen that SiO2 shows a moderate activity and high ethylene selectivity. The ethane and O2 conversions increase with the increasing temperature while ethylene selectivity first decreases and then increases as the ethane conversion increases, suggesting that SiO2 favors the homogeneous reaction at high temperatures. At 650 °C, 20% ethane conversion with 72% ethylene selectivity can be obtained. When SiO2 is modified by an alkali metal chloride, the ethane and oxygen conversions over those catalysts are all remarkably improved. Like the behavior of SiO2, ethane and oxygen conversions increase with the increasing temperature. However, ethylene selectivity displays a different pattern. For NaCl/ SiO2 and KCl/SiO2, ethylene selectivity increases as the increasing ethane conversion. This behavior is a typical characteristic of halides-containing catalysts, due to the fact that the formed ethyl radical at high-temperature desorbes and forms ethylene in the gas phase via reaction with O2.1 For LiCl/SiO2, ethylene selectivity first increases with the increasing ethane conversion
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and then decreases at high temperature (over 600 °C). This is probably due to the secondary oxidative reaction of ethylene with oxygen to form carbon oxides. In addition, ethylene selectivity over the LiCl/SiO2 shows a higher value at all temperatures, but the other two catalysts exhibit lower ethylene selectivity than SiO2. A 99% ethane conversion with 80% ethylene selectivity can be achieved over the LiCl/SiO2 at 600 °C while 30% ethane conversion and 50% ethylene selectivity can only be obtained over the NaCl/SiO2 and KCl/SiO2 catalysts. From Table 2, one can find that the ethane conversion and ethylene yield follow the order of LiCl/SiO2 > NaCl/ SiO2 > KCl/SiO2 > SiO2 at the same temperature. In previous investigations, Wang et al.4 obtained 75% ethane conversion and 77% ethylene over a chloridemodified Li/MgO catalyst at 620 °C. Au et al.9 reported that C2H6 conversion of 70.6% with C2H4 selectivity of 80.2% could be attained over a 50 mol % BaBr2/Ho2O3 catalyst at 640 °C. We have found that LiCl supported on sulfated zirconia could produce 70% selectivity to ethylene at 98% ethane conversion, giving 68% ethylene yield at 650 °C.11 Thus, it is seen that LiCl/SiO2 prepared in this investigation shows a better catalytic conversion than that of those catalysts reported so far. It is believed that the catalyst activity in the oxidative dehydrogenation of ethane has a close relationship with the redox and acid/base properties of catalysts, in particular for transitional metal oxides on which the reaction proceeds via a redox mechanism. For alkali metal doped catalysts, a heterogeneous-homogeneous reaction scheme dominates the reaction mechanism. Addition of an alkali metal can increase the surface defects and oxygen activation. A more oxidized surface usually results in more active catalysts, because the activation of oxygen requires the surface oxygen defects. TPR results have demonstrated that LiCl/SiO2 shows a high redox potential and can be easily reduced at lower temperatures while KCl/SiO2 only exhibits reduction at much high temperatures. From Table 2 and Figure 2, it is found that the activity of the catalysts has the same order as the reducibility. Lunsford et al.4,19 studied the catalytic activity of Li+MgO-Cl- catalysts in the ODE and found that the activity was influenced by the surface acid-base property. If the surface basicity of a catalyst was very large, strong CO2 adsorption would occur on the catalyst surface, resulting in low catalytic activity. The addition of chlorine may inhibit the centers responsible for total oxidation. Chlorine radicals can also contribute measurably to the activation of ethane and ethylene formation, thus increasing both conversion and selectivity. For alkali metal chloride/silica catalysts in this investigation, it is deduced that the lower activity of KCl/SiO2 and NaCl/SiO2 could be partly due to the higher basicity induced by the doped K+ and Na+ ions. The XPS measurements show the lower concentration of chlorine on the surface of NaCl/SiO2 and KCl/SiO2 catalysts. The effect of LiCl loading on catalyst activity was investigated. Figure 3 presents the catalytic behavior over a series of LiCl/SiO2 catalysts at 600 °C. It is shown that addition of LiCl on SiO2 greatly influences the catalytic performance by increasing the ethane conversion and ethylene selectivity. However, the conversion (19) Conway, S. J.; Lunsford, J. H. J. Catal. 1991, 131, 513-522.
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Table 2. Performances of Alkali Metal Chloride Doped on Silica Catalysts for the ODH of Ethane at Various Temperatures catalyst
SiO2
LiCl/SiO2
NaCl/SiO2
KCl/SiO2
temp (°C) 550 570 600 620 650 500 550 570 600 550 570 600 620 650 550 570 600 620 650
conv (%) C2H6 O2
C2H4
CH4
1.5 4.0 9.5 13.1 20.0 8.0 43.8 75.8 99.1 6.0 12.1 32.4 56.2 88.2 6.7 13.0 30.5 48.5 70.2
97.3 69.4 60.9 66.0 71.6 69.2 87.8 88.8 79.4 35.3 40.4 50.0 57.2 68.9 31.2 37.0 48.9 54.5 75.1
2.7 2.4 3.1 1.3 1.8 2.0 1.5 1.6 1.7 0.8 1.2 0.8 1.3 1.8 1.3 1.8 1.2 1.7 2.1
2.7 7.6 11.3 16.3 24.6 11.8 33.4 52.1 86.4 19.0 29.3 54.7 76.4 92.5 16.3 25.3 48.5 67.0 85.2
selectivity (%) CO CO2 0 14.9 25.0 23.8 19.1 15.3 7.2 7.2 16.0 37.5 35.9 32.9 29.3 20.6 60.1 53.6 43.4 36.6 12.9
0 13.2 11.0 8.9 7.3 13.4 2.3 1.2 2.6 26.4 22.4 15.9 11.5 7.1 7.4 7.6 6.2 6.6 8.7
C2H5OH
yield (%) C2H4
0 0 0 0 0.2 0 0.6 0.6 0.1 0 0 0.2 0.7 1.1 0 0 0.2 0.4 0.8
1.5 2.8 5.8 8.6 14.3 5.5 38.4 67.2 78.6 2.1 4.9 16.2 32.2 60.7 2.1 4.8 14.9 26.4 52.7
Figure 3. Effect of LiCl loading on catalytic activity of LiCl/ SiO2 catalysts.
and selectivity depend on LiCl content. Ethane conversion increases with increasing Li loading and reaches the highest level at a Li loading of 3.5 wt % and then shows a decreasing trend afterward. Ethylene selectivity is also enhanced when LiCl is added at low content and shows a slightly lower value at 3.5 wt % Li loading, but it will increase again at higher Li loading. In terms of ethane conversion and ethylene yield, a maximum can be achieved at 3.5 wt % Li on LiCl/SiO2 catalysts. As discussed above, the ethane conversion and ethylene selectivity are strongly influenced by the acid/base property of the catalyst. A large amount of LiCl on the catalysts will increase the basicity and suppress the contact of active surface species with reactants because adsorbed CO2 will cover the active sites and LiCl itself is not the active site for the reaction. The catalytic performance of the alkali metal chloride modified catalysts as a function of time at different temperatures is shown in Figure 4. One can see that these catalysts show deactivation at an extended reaction time with the varying deactivation rates. The ethylene selectivity increases as the conversion de-
Figure 4. Catalytic behavior as a function of time over alkali metal chloride/silica catalysts: (a) LiCl/SiO2 at 600 °C, (b) NaCl/SiO2 at 650 °C, (c) KCl/SiO2 at 650 °C.
creases. Ethylene yield over LiCl/SiO2 and NaCl/SiO2 first shows an increasing trend at the initial stage, but thereafter, a decreasing trend can be observed. The conversion decreases at the fastest rate over LiCl/SiO2 and reaches, at 5% ethane conversion, an almost inactive degree, after 15 h, whereas for NaCl/SiO2, ethane conversion can be maintained at a high level after the initial stage of deactivation. After 20 h of testing, ethane conversion and ethylene yield decrease to 61% and 51%
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in the reaction. Quartz and Na2SiO3 diffraction peaks can be found in XRD profiles of NaCl/SiO2. Some quartz crystallites are also formed on KCl/SiO2. It has been reported that chlorine-promoted catalysts would deactivate during the reaction due to the leaching of chlorine.6 The chlorine concentration of the reacted catalysts is shown in Table 1. It is seen that the chlorine concentration on all reacted catalysts decreases, suggesting the loss of chlorine during the catalyst testing. LiCl/SiO2 exhibits the greatest reduction in chlorine concentration. This is probably the second reason for its fastest deactivation. Conclusions
Figure 5. XRD patterns of alkali metal chloride/silica catalysts after reaction.
respectively. For KCl/SiO2, ethane conversion and ethylene yield show a medium level of deactivation. After 5 h, the deactivation rate becomes much slower. The ethane conversion and ethylene yield decrease from 70% and 52% to 29% and 25%, respectively in 20 h. The reacted catalysts were also characterized by XRD and XPS techniques. Figure 5 presents the XRD patterns of three reacted alkali metal chloride/silica catalysts. It is seen that the XRD patterns of the reacted catalysts display patterns somewhat different from those of the fresh catalysts, which indicates that a phase transformation occurred on all three catalysts during the reaction. More crystallites were formed on LiCl/SiO2
Silica shows a moderate catalytic activity in the oxidative dehydrogenation of ethane. Addition of alkali metal chloride on silica changes the crystalline structure and redox and acid-base properties of the catalysts, resulting in a significant improvement in the catalytic conversion. However, those alkali metal chloride modified catalysts still exhibit varying catalytic behavior, depending on the nature of the metal chloride. The alkali metal and chloride ions play a different role in the reaction. It is found that the catalytic activity follows the order LiCl/SiO2 > NaCl/SiO2 > KCl/SiO2 > SiO2. A 3.5 wt % loading of alkali metal is the optimal amount at which the highest ethane conversion and ethylene yield can be attained. For all three catalysts, LiCl/SiO2 exhibits the highest ethane conversion and ethylene yield of 79% at 600 °C, but it deactivates rapidly due to leaching of chlorine and chemical phase transformation. NaCl/SiO2 can exhibit a high activity and a longer time of stable performance. EF990247L
