Promoting Effects of Some Metal Additives on the Methanol

The most active catalyst doped with the Ca additive yielded 720 g kg-cat-1 h-1 of methanol at 593 K and 5.1 MPa, which was about 50% of the space−ti...
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Energy & Fuels 2003, 17, 829-835

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Promoting Effects of Some Metal Additives on the Methanol Synthesis Activity of Sulfided Pd/SiO2 Catalyst from Syngas Containing H2S Naoto Koizumi, Kazuhito Murai, Seiko Tamayama, Toshihiko Ozaki, and Muneyoshi Yamada* Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan Received October 15, 2002

A sulfided Pd/SiO2 catalyst was doped with various kinds of metal additives (M: Li, K, Cs, Mg, Ca, Sr, Ba, Sc, Y, La, Nd, Mn, Zn, or Al) and used for methanol synthesis from CO hydrogenation. The addition of Ca, Y, La, or Nd significantly improved the activity for methanol synthesis, and among these additives, Ca was the most effective additive. Besides, the methanol synthesis activity of the sulfided Pd/SiO2 doped with the Ca additive changed, depending on the preparation method of the precursor. The calcination of the precursor after impregnating with the Pd-containing solution was helpful for improving the methanol synthesis activity. The most active catalyst doped with the Ca additive yielded 720 g kg-cat-1 h-1 of methanol at 593 K and 5.1 MPa, which was about 50% of the space-time yield of methanol that is obtained with a commercial Cu/Zn/Al catalyst at 593 K and 5.1 MPa from a syngas containing CO2. Besides, even in the presence of H2S, the sulfided Pd/SiO2 catalyst doped with the Ca additive preserved 35% of the initial activity. The undoped catalyst showed a much lower methanol synthesis activity than the doped catalyst in the presence of H2S as well. Thus, even in the presence of a small amount of H2S in syngas, the Ca additive shows the promoting effect on increasing the methanol synthesis activity. In contrast with the sulfided catalysts, in the presence of H2S, the methanol synthesis activity of the Cu/Zn/Al catalyst decreased linearly with time on-stream and eventually dropped to zero.

Introduction Methanol is currently synthesized from a syngas containing CO2 using a Cu/Zn-type catalyst. This type of catalyst is usually operated at around 523 K and 5-10 MPa and yields above 1000 g kg-cat-1 h-1 of methanol. However, due to thermodynamic equilibrium, a single-pass CO conversion is restricted below 60% (523 K and 5.0 MPa), which requires a gas-recycle system. On the other hand, reduced supported Pd catalysts have been claimed for many years to be effective for the methanol synthesis.1 Recent studies by Matsumura et al.2 showed that a reduced Pd/CeO2 catalyst yields 300 g kg-cat-1 h-1 of methanol from the syngas at 443 K and 3.0 MPa. Because the Cu/Zn catalyst shows the comparable space-time yield (STY) of methanol at 503 K and 3.0 MPa from the syngas containing CO2, the reduced Pd/CeO2 catalyst shows higher activity than the Cu/Zn catalyst at the lower reaction temperature. To search for a new type of methanol synthesis catalysts, the present authors investigated CO hydrogenation activities of various metal sulfides and found * Author to whom correspondence should be addressed. Tel: +81 (22) 217 7214. Fax: +81 (22) 217 7293. E-mail: yamada@ erec.che.tohoku.ac.jp. (1) Lee, G. V. D.; Ponec, V. Catal. Rev.sSci. Eng. 1987, 29 (2&3), 183. (2) Matsumura, Y.; Shen, W.-J.; Ichihashi, Y.; Okumura, M. J. Catal. 2001, 197, 267.

that both Rh17S153 and Pd16S74 yield methanol from the syngas. Rh17S15 yields 800 g kg-cat-1 h-1 of methanol at 593 K and 5.1 MPa, while Pd16S7 yields 80 g kg-cat-1 h-1 of methanol at 613 K and 5.1 MPa. It was also found that these sulfides showed stable activities even in the presence of H2S in the syngas (100 ppm in concentration). On the contrary, the conventional Cu/Zn/Al catalyst is reported to be poisoned with a very small amount of H2S in the feed (1.6 ppm in concentration).5 Although most of methanol is used as chemical feedstocks at present, methanol is also suitable for transportation fuels because it is easily converted into dimethyl ether, which is one of most promising substituted diesel fuels. To synthesize methanol as the transportation fuels, it is an important issue to utilize small-scale and dispersed carbon resources such as remote gas field, biomass, and waste materials for raw materials. On one hand, the Cu/Zn/Al catalyst is easily deactivated without a complete desulfurization of the syngas because the syngas produced by noncatalytic partial oxidation of biomass and waste plastics contains H2S several hundred ppm in concentration.6,7 On the other hand, Rh17S15 and Pd16S7 can be used to synthe(3) Yamada, M.; Koizumi, N.; Miyazawa, A.; Furukawa, T. Catal. Lett. 2002, 78, 195. (4) Koizumi, N.; Miyazawa, A.; Furukawa, T.; Yamada, M. Chem. Lett. 2001, p 1282. (5) Wood, B. J.; Isakon, W. E.; Wise, H. Ind. Eng. Chem. Res. Dev. 1980, 2, 33.

10.1021/ef020242f CCC: $25.00 © 2003 American Chemical Society Published on Web 06/05/2003

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size methanol without the desulfurization unit from the syngas produced from these carbon resources. Before our findings, only MoS2-based catalysts were known as a sulfur-tolerant catalyst. Without additives, MoS2 shows only a weak activity for the hydrocarbon synthesis,8 while alcohols can be obtained by the addition of alkaline metal carbonates.8,9 STYs of methanol with the Rh17S15 and Pd16S7 catalysts are higher than those with the alkaline metal-promoted MoS2 catalysts. To increase the methanol synthesis activity per Pd atoms over Pd16S7, we investigated effects of using various metal oxides as supports.10 Because basic metal oxides11 and lanthanide oxides2,11 are effective supports for the methanol synthesis over the reduced Pd catalysts, we investigated effects of these oxides and found that the formation rates of methanol normalized to the total amount of Pd atoms over sulfided Pd/MxOy (M ) Mg, Si, Ca, La, and Nd) are 10-30 times higher than that over the unsupported Pd16S7, similar to the reduced Pd catalysts.11 SiO2 was the most effective support among them. On the other hand, methane was selectively formed over the sulfided Pd/CeO2. The sulfided Pd/SiO2 catalyst yielded 100 g kg-cat-1 h-1 of methanol at 613 K and 5.1 MPa, which is 1.1 times higher than that obtained with Pd16S7. However, the STY of methanol with the sulfided Pd/SiO2 catalyst is still much lower than that with the Cu/Zn/Al catalyst. Because the methanol synthesis activity of the reduced Pd/SiO2 catalyst is significantly improved by the addition of some metal oxides,12-14 in the present study the sulfided Pd/SiO2 catalyst doped with various metal additives was investigated for methanol synthesis from the syngas to develop a highly active methanol synthesis catalyst with the sulfur tolerance. Effects of preparation methods of the precursor were also examined. The STY of methanol obtained with the prepared sulfide was compared with those with Rh17S15 and the commercial Cu/Zn/Al catalyst in the absence and presence of H2S. Experimental Section Preparation of the Precursor Material. A SiO2-supported Pd oxide precursor (Pdc/SiO2) was prepared by impregnating SiO2 powder (Fuji Silysia, 560 m2 g-1) with an aqueous solution containing Pd(NH3)4(NO3)2 (Aldrich, 99.99%) followed by drying and calcination in air at 723 K for 2 h. The subscript “c” indicates that the sample was calcined after the drying step. The weight ratio of Pd (as metal) to SiO2 was kept at 0.045. The Pdc/SiO2 precursor was then impregnated with an aqueous solution containing a metal nitrate (M: Li, K, Cs, Mg, Ca, Sr, Ba, Sc, Y, La, Nd, Mn, Zn, or Al). When effects of various Ca salts were examined, acetate and chloride solutions of Ca were used as well. Unless otherwise stated, the Ca nitrate was used (6) Berg, M.; Koningen, J.; Sjo¨stro¨m, K.; Waldheim, L. Development in Thermochemical Biomass Conversion; Blackie: London, 1997; p 1117. (7) Jahnek, F. C. PETROTECH 1999, 22, 11. (8) Santiesteban, J. G.; Bogdan, C. E.; Herman, R. G.; Klier, K. Proc. 9th Int. Congr. Catal. 1988, 2, 561. (9) Quarderer, G. J.; Cochran, G. A. EP-0-0119609, assigned to Dow Chemical Company. (10) Koizumi, N.; Murai, K.; Takasaki, S.; Yamada, M. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 2001, 46 (2), 437. (11) Ichikawa, M.; Shikakura, K. Shokubai 1979, 21, 253. (12) Kikuzono, Y.; Kagami, S.; Naito, S.; Onishi, T.; Tamaru, K. Chem. Lett. 1981, p 1249. (13) Gotti, A.; Prins, R. J. Catal. 1998, 175, 302. (14) Gusovius, A. F.; Walting, T. C.; Prins, R. Appl. Catal. A.; General 1999, 188, 187.

Koizumi et al. as the Ca source. After the impregnation, the samples were only dried in the same manner as the Pdc/SiO2 precursor. An M/Pd atomic ratio was varied from 0.1 to 2.0. The precursor thus prepared is denoted as M/Pdc/SiO2 in this paper. To investigate effects of preparation methods of the precursor on the performance of the sulfided catalyst, the precursors doped with the Ca additive were prepared in different ways. A Cac/Pdc/SiO2 precursor was prepared by calcining the Ca/ Pdc/SiO2 after the drying step after the impregnation with the Ca nitrate solution. We also prepared Pd/Ca/SiO2, Pdc/Ca/SiO2, and Pd/Cac/SiO2 precursors by changing the impregnation sequence. The subscript “c” indicates that the sample was calcined after the drying step after the impregnation with the aqueous solution containing Pd or Ca salt. For example, the Pdc/Ca/SiO2 precursor was prepared by impregnating the SiO2 powder with the Ca nitrate solution, followed by the drying step. Thereafter, this sample was impregnated with the Pd(NH3)4(NO3)2 solution followed by the drying and calcination steps. Catalyst Pretreatment and Activity Measurement. Catalyst pretreatment and activity measurements on the sulfided catalysts were performed using a conventional fixedbed reactor. For each test, 0.2 g of the precursor was charged in a reactor and then sulfided in a stream of 5% H2S/H2 at 673 K and 0.1 MPa. The cumulative amount of H2S fed during the pretreatment was 160 mol-H2S mol-Pd-1. After the sulfidation, the H2S/H2 stream was replaced by a stream of a syngas composed of CO 33%, H2 62%, and Ar 5% at room temperature. Ar was used as an internal standard. Typical reaction temperature and pressure were 613 K and 5.1 MPa, respectively. An effluent gas was analyzed by an on-line gas chromatograph with TCD and FID (Shimadzu, GC-14BPTF). The concentrations of sulfur compounds in the methanol condensed in an ice trap were measured with an atomic emission detector (Agilent, G2350A). When effects of small amounts of H2S in the syngas were investigated, the syngas was mixed with 0.1% H2S/H2 and fed into the reactor. The concentrations of H2S in the syngas at the inlet and the outlet of the reactor were measured with a sulfur chemiluminescence detector (Sievers, model 355). When the reduced Pd catalysts and a commercial Cu/Zn/Al catalyst supplied by ICI Corp. were tested, to avoid a possible contamination with sulfur compounds, an apparatus that had not been tainted with sulfur compounds was used. The Pd-containing precursor was reduced in a stream of H2 at 673 K and 0.1 MPa before the test. The Cu/Zn/Al catalyst was reduced in the same feed as that employed for the reaction (CO 33%/H2 62%/Ar 5%, or CO 30%/ H2 60%/CO2 5%/Ar 5%). CO Uptake Measurements. To determine the amounts of surface Pd sites on the sulfided M/Pd/SiO2 catalysts, CO uptakes of the sulfided M/Pdc/SiO2 after use for the CO hydrogenation were measured by an in-situ pulse adsorption technique. An amount of 0.2 g of the precursor was charged in a stainless steel reactor and sulfided in the stream of H2S/ H2 and then subjected to the CO hydrogenation reaction at 613 K and 5.1 MPa. After the reaction, the temperature was reduced to room temperature. Thereafter, the pressure was reduced to 0.2 MPa and then the syngas stream was replaced by a helium flow. Then, the temperature was raised to 613 K and then held for 30 min. The CO adsorption was performed by injecting 3.3 mL of 10% CO/He into the catalyst bed at 308 K. This procedure was repeated until the amount of CO in the effluent gas became unchanged.

Results and Discussion Activities for CO Hydrogenation over the Sulfided M/Pdc/SiO2 Catalysts. Sulfided M/Pdc/SiO2 (M: Li, K, Cs, Mg, Ca, Sr, Ba, Sc, Y, La, Nd, Mn, Zn, or Al) with M/Pd atomic ratios of 0.5 were subjected to CO

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Figure 1. Formation rates of products obtained with the sulfided Pdc/SiO2 and sulfided M/Pdc/SiO2 with the M/Pd atomic ratio of 0.5. Reactions were performed at 613 K, 5.1 MPa, and 20 m3 (STP) kg-cat-1 h-1.

hydrogenation at 613 K, 5.1 MPa, and 20 m3 (STP) kgcat-1 h-1. Figure 1 shows the formation rates of the products normalized to the total amount of Pd atoms. In this figure, formation rates of products over the sulfided Pdc/SiO2 catalyst are also shown as a reference. The M/Pdc/SiO2 yield methanol, methane, dimethyl ether (DME), and C2+ hydrocarbons. Rates of CO conversion, the sum of the formation rates of all the products, over the sulfided M/Pdc/SiO2 (except M ) K, Cs, and Zn) are 1.2-4.7 times higher than that over the sulfided Pdc/SiO2. Methanol is obtained as a main product over the sulfided M/Pdc/SiO2 catalysts (M ) Mg, Ca, Sr, Ba, Sc, Y, La, Nd, or Mn). Thus, the formation rates of methanol over these catalysts are higher than that over the sulfided Pdc/SiO2. Especially, the addition of the Ca, Y, La, or Nd shows the significant promoting effect. On the other hand, sulfided M/Pdc/SiO2 (M ) Li and Al) yield methane as main products, although the rates of CO conversion are higher than that over the sulfided Pdc/SiO2. Dependency of the Activity for Methanol Synthesis on the M/Pd Ratio. Sulfided M/Pdc/SiO2 with various M/Pd atomic ratios were subjected to the CO hydrogenation at 613 and 573 K. The formation rates of methanol as a function of the M/Pd atomic ratio are shown in Figure 2, parts A and B. At 613 K, the formation rate of methanol over the sulfided Pdc/SiO2 is ca. 2.5 C-mmol mol-Pd-1 s-1. The addition of a small amount of the Ca additive (Ca/Pd atomic ratio ) 0.1) significantly improves the formation rate of methanol. It then increases more slowly with increasing the Ca/ Pd ratio and shows a maximum at the ratio between 0.5 and 1.0. Dependencies of the formation rates of methanol over the sulfided M/Pdc/SiO2 (M ) Y, Mg, or Mn) show trends similar to those of the sulfided Ca/ Pdc/SiO2. Especially, the addition of Y shows comparable promoting effects with the Ca additive in the M/Pd atomic ratios of 0.1-1.0. On the contrary, the addition of Cs shows the negative effects on the formation rate of methanol, irrespective of the Cs/Pd ratio. As shown in the later (Figure 7), the STY of methanol obtained with the sulfided Ca/Pdc/SiO2 at 613 K is near the equilibrium value. To avoid the thermodynamic

Figure 2. Dependencies of the formation rate of methanol over the sulfided (closed symbols) and reduced (open symbols) M/Pdc/SiO2 on the M/Pd atomic ratio. Reactions were performed at 613 K (A) and 573 K (B). The reaction pressure and gas-hourly space velocity are the same as those indicated in Figure 1.

equilibrium, the CO hydrogenation was also performed at 573 K. The formation rate of methanol over the sulfided Ca/Pdc/SiO2 catalyst at 573 K is comparable with that at 613 K because of the thermodynamic equilibrium at the higher temperature (Figure 2, parts A and B). On the other hand, the maximum formation rates of methanol with the other catalysts are lower at the lower reaction temperature. The differences in the maximum formation rate of methanol over the sulfided Ca/Pdc/SiO2 and sulfided Y/Pdc/SiO2 are evident at 573 K. Thus, the Ca additive shows a stronger promoting effect than the Y additive. Comparison with the Reduced Pd Catalysts. As concerns the role of the metal additives on the reduced Pd catalysts, Gotti and Prins13 reported the effects of various metal additives (M ) Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Mn, Zn, and La) on the methanol synthesis activity of the reduced Pd/SiO2 catalyst. They found that

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Figure 3. Relationship between the formation rates of methanol over the sulfided M/Pdc/SiO2 (Pdc/SiO2) and their CO uptakes. “None” indicates the sulfided Pdc/SiO2 while “M” indicates the sulfided M/Pdc/SiO2 with the M/Pd atomic ratio of 0.5. The reaction was performed at the same conditions as indicated in Figure 1.

the promoting factors of the metal additives, the ratio of the formation rates of methanol over the reduced M/Pd/SiO2 catalyst to that over the reduced Pd/SiO2 catalyst, follow a volcano curve when plotted as a function of the electronegativity of the additive cations with a maximum at the Ca additive. Thus, they concluded that the metal additives work as cocatalysts; that is, formate species are formed on the surface of the additive oxide from CO, while metallic Pd sites create dissociated hydrogen atoms. The effective additives such as the Mg and Ca additives are considered to stabilize the formate species moderately. To investigate the role of the metal additives on the methanol synthesis over the sulfided catalysts, the effects of the metal additives were compared between the sulfided and the reduced. In Figure 2B, formation rates of methanol over the reduced Ca/Pdc/SiO2 catalysts with various Ca/Pd atomic ratio values are also shown. Although the formation rates of methanol over the reduced Ca/Pdc/SiO2 catalysts are about 2 times higher than that over the sulfided Ca/Pdc/SiO2, its dependency on the Ca/Pd ratio is similar to that on the sulfided catalyst. That is, the formation rate of methanol over the reduced catalyst shows a plateau at the Ca/Pd ratio above 0.5. It is worthwhile to note here that the equilibrium STY of methanol at the reaction conditions is much higher than the STYs of methanol obtained with the reduced catalysts. Figure 4 compares the promoting factors of the additives on the sulfided and the reduced catalysts. The promoting factors of the Mg, Ca, and Y additives on the sulfided and the reduced catalysts are comparable with each other. From these results it is suggested that the metal additives work as cocatalysts in the sulfided catalysts as well. Effects of the Metal Additives on the CO Uptakes. To investigate the role of the metal additive in more detail, changes in the number of coordinatively unsaturated sites (CUS) of the sulfided M/Pdc/SiO2 were

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Figure 4. Rate enhancements of the methanol formation by the various metal additives on the sulfided and reduced M/Pdc/ SiO2 with the M/Pd atomic ratios of 0.5. Reactions were performed at 613 and 573 K. The reaction pressure and gashourly space velocity are the same as those indicated in Figure 1.

investigated by CO adsorption measurements. The sulfided Ca/Pdc/SiO2 with various Ca/Pd ratios were subjected to CO hydrogenation at 613 K, 5.1 MPa, and 20 m3 (STP) kg-cat-1 h-1, and then the CO uptakes of the used catalysts were measured by the in-situ pulse adsorption technique. The results showed that the CO uptakes of the sulfided Ca/Pdc/SiO2 are constant (0.04 mol mol-Pd-1) in the Ca/Pd ratio from 0 to 0.5. A slightly lower value was obtained for the catalyst with a Ca/Pd ratio of 1.0. Since no adsorption occurred on the sulfided Ca/SiO2 catalyst, these results clearly indicate that the addition of Ca does not change the number of the CUS on the supported Pd sulfides. Then, effects of various metal additives were also investigated. In Figure 3, the formation rates of methanol over the sulfided M/Pdc/SiO2 (M/Pd ratio ) 0.5) are plotted as a function of their CO uptakes. As shown in the figure, CO uptakes of the sulfided M/Pdc/SiO2 (except M ) Mn) were in a range from 0.03 to 0.05 mol mol-Pd-1. For the reduced supported Pd catalysts, the dispersion of the metallic Pd species are hardly affected by the addition of the alkaline metal oxides,13,15 lanthanide oxides,13,16 and basic metal oxides such as Ca and Mg oxides.13 Our results indicate that, similar to the reduced supported Pd catalysts, the additions of the metals have little influence on changing the number of the CUS on the supported Pd sulfides. On the other hand, the formation rates of methanol sharply increase by the addition of Ca, Y, La, and/or Nd. Therefore, the formation rate of methanol normalized to the CUS on the supported Pd sulfides was significantly improved by the additions of these metals. These results do not conflict with the above-mentioned notion that the metal additives work as cocatalysts in the sulfided catalysts. Effects of the Preparation Methods of the Catalyst Precursor. To prepare highly active Pd sulfide (15) Rieck, J. S.; Bell, A. T. J. Catal. 1986, 100, 305. (16) Rieck, J. S.; Bell, A. T. J. Catal. 1986, 99, 278.

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Figure 5. Effects of the preparation methods of the precursor for the sulfided Pd catalysts doped with the Ca additive (Ca/ Pd atomic ratio ) 0.5) on their formation rates of methanol (left side bar) and product selectivities (right side bar). Reactions were performed at the same conditions as indicated in Figure 1.

catalysts, the present work tried to optimize the preparation method of the catalyst precursor containing the Ca additive. First, effects of the calcination after the drying step were investigated. Figure 5 shows the formation rates of methanol and the product selectivity over the sulfided catalysts containing the Ca additive prepared in different ways. These catalysts were subjected to the CO hydrogenation reaction at 613 K, 5.1 MPa, and 20 m3 (STP) kg-cat-1 h-1. Formation rate of methanol and product selectivity over the sulfided Pdc/ SiO2 are also shown in this figure. All the sulfided catalysts containing the Ca additive show higher formation rates of methanol than the sulfided Pdc/SiO2. However, the formation rates over the sulfided catalysts containing the Ca additive change, depending on the preparation method, i.e., the addition or omission of the calcination step(s). Because the formation rates over these catalysts increase in the following order: sulfided Pd/Ca/SiO2 < sulfided Pd/Cac/SiO2, sulfided Cac/Pdc/ SiO2 < sulfided Pdc/Ca/SiO2, sulfided Ca/Pdc/SiO2, it is suggested that the calcination after impregnation with the Pd(NH3)4(NO3)2 solution is helpful for improving the catalytic activity for methanol synthesis, which may decompose the Pd(NH3)4(NO3)2 species to form fine PdO particles on the SiO2 surface. On the other hand, the calcination after the impregnation of Pdc/SiO2 with the Ca nitrate solution shows a negative effect. It is also noteworthy that the methanol selectivity of the sulfided catalysts containing the Ca additive changes, depending on the preparation methods as well. Especially, the sulfided Pdc/Ca/SiO2 shows the highest selectivity for methanol among them and makes no CO2. Since the methanol selectivity of the sulfided Ca/Pdc/SiO2 depended on its CO conversion when the space velocity was varied, we examined the dependencies of the product selectivities of the sulfided catalysts prepared in the different ways on their CO conversion. It was found that the methanol selectivity tends to decrease with increasing the CO conversion. Besides, the catalysts that showed higher CO conversion produced CO2. These dependencies were similar to that observed in the

Figure 6. Formation rates of products obtained with the sulfided Ca/Pdc/SiO2 prepared from the different Ca salts (Ca/ Pd atomic ratio ) 0.5). Reaction conditions are the same as those indicated in Figure 1.

methanol synthesis with the sulfided Ca/Pdc/SiO2 at various space velocities. Thus, the variation in the product selectivities of the sulfided catalysts prepared in the different ways is mainly attributed to the variation in the CO conversions. Although the formation rates of methanol over the sulfided Ca/Pdc/SiO2 and the sulfided Pdc/Ca/SiO2 are comparable with each other, the former catalyst shows a higher CO conversion than the latter one. In addition, effects of using various Ca salts (chloride, acetate, and nitrate) on the CO hydrogenation activity of sulfided Ca/Pdc/SiO2 catalysts were examined. Formation rates of the products over various sulfided Ca/ Pdc/SiO2 catalysts are shown in Figure 6. These catalysts were subjected to the reactions at 613 K, 5.1 MPa, and 20 m3 (STP) kg-cat-1 h-1. Formation rates over the sulfided Pdc/SiO2 catalysts are also shown in this figure. Rates of CO conversion over the sulfided Ca/Pdc/SiO2 catalysts are 3-4.5 times higher than that over the sulfided Pdc/SiO2 catalyst. From the viewpoint of increasing the rate of CO conversion, Ca nitrate is the most effective additive. However, the formation rates of methanol over the sulfided Ca/Pdc/SiO2 catalysts are not so sensitive to the different Ca salts. The sulfided Pdc/SiO2 doped with Ca nitrate shows a slightly higher formation rate of methanol. Promoting Effects of the Ca Additive in the Presence of H2S in Syngas. Then, we examined an effect of the Ca additive in the presence of H2S. For this purpose, the most active catalyst, i.e., the sulfided Ca/ Pdc/SiO2 with the Ca/Pd ratio of 0.5 (the Ca nitrate was used as the Ca source), and the sulfided Pdc/SiO2 were subjected to the reactions in the presence of H2S. Before examinations, suitable reaction temperatures, where the maximum STY of methanol is obtained, were searched in the absence of H2S. Figure 7 shows the STYs of methanol obtained with the sulfided Ca/Pdc/SiO2 and sulfided Pdc/SiO2 as a function of the reaction temper-

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Figure 8. Effect of H2S on the STY of methanol over the sulfided (Ca/)Pdc/SiO2 and Cu/Zn/Al catalyst. Reaction conditions are 613 K, 5.1 MPa, and 20 (30) m3 (STP) kg-cat-1 h-1 for the sulfided (Ca/)Pdc/SiO2 and 523 K, 5.1 MPa, and 6.0 m3 (STP) kg-cat-1 h-1 for the Cu/Zn/Al catalyst.

Figure 7. Dependencies of the STY of methanol over the sulfided (Ca/)Pdc/SiO2 and a commercial Cu/Zn/Al catalyst on the reaction temperature. A 33% CO/62% H2/5% Ar feed was used for the sulfided catalysts while a 30% CO/60% H2/5% CO2/ 5% Ar feed was used for the Cu/Zn/Al catalyst. Reaction conditions are 5.1 MPa and 20 (30) m3 (STP) kg-cat-1 h-1 for the sulfided (Ca/)Pdc/SiO2, and 5.1 MPa and 6.0 (or 30) m3 (STP) kg-cat-1 h-1 for the Cu/Zn/Al catalyst.

ature at 5.1 MPa and 30 m3 (STP) kg-cat-1 h-1. The STY of methanol with the sulfided Ca/Pdc/SiO2 shows a maximum at 593 K, where this catalyst yields 720 g kgcat-1 h-1 of methanol. No deactivation was observed under these conditions for more than 60 h. In the case of the sulfided Pdc/SiO2, the reactions were performed at 20 m3 (STP) kg-cat-1 h-1. The sulfided Pdc/SiO2 shows the highest STY at 613 K, where this catalyst yields 100 g kg-cat-1 h-1 of methanol. After the STY of methanol with each catalyst reached the steady state under the sulfur-free conditions, the syngas mixed with 0.1% H2S/H2 was fed to the reactor. STYs of methanol with the sulfided Pdc/SiO2 and sulfided Ca/Pdc/SiO2 as a function of amounts of H2S fed during the reactions are shown in Figure 8. The amounts of H2S were normalized to the total amount of Pd atoms. Soon after the H2S-containing syngas (H2S concentration: 120 ppm) was fed into the reactor, STY of methanol with the sulfided Ca/Pdc/SiO2 decreases to 35% of the initial value. Thereafter, this catalyst preserves a constant STY, i.e., 250 g kg-cat-1 h-1 (5.7 C-mmol mol-Pd-1 s-1), even when the amount of H2S reaches 6.0 mol-H2S mol-Pd-1. The STY of methanol with the sulfided Pdc/SiO2 shows a similar trend toward the sulfided Ca/Pdc/SiO2. This catalyst also preserves a constant STY, i.e., 40 g kg-cat-1 h-1 (0.9 C-mmol molPd-1 s-1). Thus, even in the presence of H2S, the formation rate of methanol over the sulfided Ca/Pdc/SiO2 is ca. 6 times higher than that over the sulfided Pdc/ SiO2. In other words, the Ca additive shows the promoting effect on increasing the activity for methanol synthesis in the presence of H2S as well. Comparison with the Rh Sulfide Catalyst and the Cu/Zn/Al Catalyst. In this paragraph, the STY of

methanol with the sulfided Ca/Pdc/SiO2 was compared with that with Rh17S15 and commercial Cu/Zn/Al catalyst in the absence and presence of H2S. In the previous paper,3 the authors reported that Rh17S15 yields 800 g kg-cat-1 h-1 of methanol at 593 K, 5.1 MPa, and 30 m3 (STP) kg-cat-1 h-1 in the absence of H2S. Because the sulfided Ca/Pdc/SiO2 yields 720 g kg-cat-1 h-1 of methanol at the same reaction conditions, Rh17S15 shows a slightly higher STY than the sulfided Ca/Pdc/SiO2. However, it is noted that the precious metal content of the sulfided Ca/Pdc/SiO2 is much lower than that of Rh17S15. Therefore, the sulfided Ca/Pdc/SiO2 is more suitable than the Rh17S15 from economic points of view. Being normalized by the total amount of Pd or Rh atoms, the formation rate of methanol over the sulfided Ca/Pdc/SiO2 is ca. 18 times higher than that over Rh17S15. Then, methanol synthesis activity of the sulfided Ca/ Pdc/SiO2 catalyst was compared with that of a commercial Cu/Zn/Al catalyst. It is well-known that the methanol synthesis activity of the Cu/Zn/Al catalyst is significantly enhanced by the presence of a small amount of CO2 in the syngas.17 Figure 7 also shows the STY of methanol with the Cu/Zn/Al catalyst from the syngas containing CO2 as a function of the reaction temperature. We first examined the activity of the Cu/ Zn/Al catalyst at the conventional reaction conditions, i.e., 513 K, 5.1 MPa, and 6.0 m3 (STP) kg-cat-1 h-1. At these conditions, the Cu/Zn/Al catalyst yields 1200 g kgcat-1 h-1 of methanol. This STY value is comparable with that reported by Chinchen et al.18 Then, methanol synthesis was performed with the Cu/Zn/Al catalyst at the same reaction temperature and space velocity as employed for the sulfided Ca/Pdc/SiO2 (593 K and 30 m3 (STP) kg-cat-1 h-1). At the reaction conditions, the Cu/Zn/Al catalyst yields 1500 g kg-cat-1 h-1 of methanol that is near the equilibrium value. The STY of methanol with the sulfided Ca/Pdc/SiO2 is 50% of that with the Cu/Zn/Al catalyst. (17) Zhang, Y.; Sun, Q.; Deng, J.; Wu, D.; Chen, S. Appl. Catal. A.; General 1997, 158, 105. (18) Chinchen, G. C.; Waugh, K. C.; Whan, D. A. Appl. Catal. 1986, 25, 101.

Methanol Synthesis Activity of Sulfided Pd/SiO2 Catalyst

Energy & Fuels, Vol. 17, No. 4, 2003 835

In the presence of the small amount of H2S in the syngas (100 ppm in concentration), the STY of methanol obtained with the sulfided Ca/Pdc/SiO2 decreases to 35% of that attained under the sulfur-free conditions. On the other hand, H2S (100 ppm in concentration) in the syngas has little influence on the STY of methanol with the Rh17S15.3 Such a difference may be attributed to the difference in the gas-solid equilibrium between the sulfide phase and the gas-phase H2S. In the case of the Cu/Zn/Al catalyst, an effect of H2S in the syngas on the STY of methanol is shown in Figure 8. By introducing the syngas containing 100-300 ppm H2S, the STY over the Cu/Zn/Al catalyst decreases monotonically with increasing the amount of H2S, and then dropped to zero at 0.4 mol-H2S mol-Cu-1. Similarly, the STY over the Cu/Zn/Al catalyst decreased monotonically when the syngas without CO2 was used as a feed.3 Advantage of the Methanol Synthesis with the Sulfided Ca/Pd/SiO2 Catalyst. Finally, the authors mention advantages of the methanol synthesis with the sulfided Ca/Pd/SiO2 catalyst. As mentioned earlier, it is an important issue to utilize the small-scale and dispersed carbon resources such as remote gas fields, biomass, and waste materials for raw materials when the methanol is used as the transportation fuels. To utilize these carbon resources, it is necessary to omit and/or simplify some huge and complex units in a series of the processes. The syngas produced from the biomass and waste plastics, however, contains significant amounts of H2S. Thus, the conventional Cu/Zn-type catalyst is easily deactivated unless H2S is almost completely removed from the syngas before the methanol synthesis, as indicated in Figure 8. In other words, a huge and complex desulfurization unit is necessary for this type of catalyst. On the other hand, the sulfided Ca/Pd/SiO2 catalyst shows high and stable activities for the methanol synthesis even in the presence of H2S 100 ppm in concentration. Thus, by using the sulfided Ca/Pd/SiO2 catalyst, the methanol synthesis can be performed without the desulfurization unit when the syngas is produced from the biomass and waste plastics. On the other hand, one problem of the methanol synthesis from the syngas containing H2S is that the methanol is contaminated with the sulfur compounds. When the methanol synthesis was performed with the sulfided Ca/Pdc/SiO2 in the presence of H2S, approximately 40 massppm-S of sulfur compounds were detected with an atomic emission detector. Such sulfur contamination is deleterious, especially when the metha-

nol is used as the chemical feedstocks. However, we think that the contamination of methanol with the sulfur compounds is not a severe disadvantage for the usage of the methanol as the transportation fuels. That is, the sulfur content of the methanol obtained with the sulfided Ca/Pdc/SiO2 is much lower than the maximum sulfur content of the commercial diesel fuel at present (500 massppm-S in Japan). Although the maximum sulfur content of the commercial diesel fuel is reduced to 50 massppm-S from 2005 to meet severe environmental regulations, the sulfur content of the methanol with the sulfided Ca/Pdc/SiO2 is still lower. Thus, the methanol synthesis with the sulfided Ca/Pd/SiO2 catalyst is quite advantageous for the production of the clean transportation fuels from the small-scale and dispersed carbon resources because the methanol synthesis can be performed without the huge and complex desulfurization unit. Conclusion Our results showed that the Ca nitrate additive is the most effective for improving the activity for the methanol synthesis from the syngas over the sulfided Pd/SiO2. In the absence of H2S, the sulfided Pd/SiO2 promoted with the Ca nitrate additive yielded 720 g kg-cat-1 h-1 of methanol at 593 K and 5.1 MPa, which is 60% of the STY over the commercial Cu/Zn/Al catalyst from the syngas containing CO2. It was also found that the Ca nitrate additive shows the promoting effect, even in the presence of H2S. In the presence of H2S, the promoted catalyst yielded 250 g kg-cat-1 h-1 of methanol while the unpromoted catalyst yielded 40 g kg-cat-1 h-1 of methanol. In contrast with the sulfided catalysts, the STY over the Cu/Zn/Al catalyst decreased linearly with increasing time on-stream and eventually dropped to zero in the presence of H2S. Therefore, the developed catalyst is effective for the synthesis of methanol without a desulfurization unit from the syngas produced by the noncatalytic partial oxidation of biomass and waste materials such as plastics. Acknowledgment. This work was supported by Research for the Future Program of Japan Society for the Promotion of Science under the Project “Synthesis of Ecological High Quality Transportation Fuels” (JSPS-RFTF98P01001). EF020242F