Mixed Alcohol Synthesis from Syngas on Sulfided K−Mo-Based

The mixed alcohol synthesis from syngas is an important reaction in C1 chemistry. Increased attention has been paid to it, because it offers a possibl...
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Ind. Eng. Chem. Res. 1998, 37, 1736-1743

Mixed Alcohol Synthesis from Syngas on Sulfided K-Mo-Based Catalysts: Influence of Support Acidity Guo-zhu Bian,† Li Fan,*,‡ Yi-lu Fu,† and Kaoru Fujimoto‡ Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China, and Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan

The mixed alcohols were synthesized at different reaction temperatures, different GHSVs, and different pressures over K-MoO3 supported on different supports such as γ-Al2O3 and SiO2. The ammonia adsorption over the catalysts was studied using temperature program desorption (TPD). The ethanol decomposition was studied over these catalysts, which has shown that the decomposition activity was higher for the catalyst calcined at 500 °C than for the catalyst calcined at 800 °C. However, the activity for ethanol decomposition was low over a K-Mo-unsupported sample. The activity and selectivity of the catalysts increased in the order of K-MoO3/γ-Al2O3 < K-MoO3/SiO2 < unsupported K-Mo. If the oxidized supported catalysts were recalcined at 800 °C, the yields of CO2 and hydrocarbons were suppressed and the yields of mixed alcohols were remarkably enhanced. However, the unsupported K-Mo has not shown any effect. Introduction The mixed alcohol synthesis from syngas is an important reaction in C1 chemistry. Increased attention has been paid to it, because it offers a possible way for production of a clean fuel from coal, natural gas, or some hydrocarbon waste via gasification. The mixed alcohols can also be used as an additive stock to increase the octane number of the traditional fuel. Molybdenumbased catalysts promoted by alkali compounds have shown high activities for mixed alcohol synthesis (Storm, 1995; Murchison et al., 1988), in which the sulfided catalyst systems have been given a special interest because of their sulfur-resistant property. γ-Al2O3-supported, Mo-based catalysts have been widely used in many industrial hydrotreating processes due to their high activity for hydrodesulfurization, hydrodenitrogenation, and methane synthesis from hydrogenation. These facts have found out a close correlation with the well-dispersed Mo species on the γ-Al2O3 support. The γ-Al2O3-supported K-Mo catalysts has been reported to have a lower selectivity for mixed alcohols in the reaction of CO hydrogenation, compared to SiO2 and MgO-supported or unsupported K-Mo catalysts (Murchison et al., 1988; Tatsumi et al., 1987). Sulfided K-MoO3/Al2O3 catalysts have been studied in our group for mixed alcohol synthesis (Jiang et al., 1994; Bian et al., 1996). The results obtained demonstrate that K-Mo species formed on the oxidized catalyst are the precursors of the active sites for mixed alcohol synthesis. The high dispersion of the molybdenum species on the support favored the rise in the activity for hydrocarbons synthesis; however, the aggregation of the molybdenum species favored the rise in the activity for mixed alcohol synthesis (Jiang et al., 1994). * To whom correspondence should be addressed. Fax/ Voice: 81-3-5689-0476. E-mail: [email protected]. † University of Science and Technology of China. ‡ The University of Tokyo.

The calcination process of the oxidized K-MoO3/γAl2O3 catalyst is important for the interaction of K and Mo components and the support and also the dispersion control of the K-Mo species. The oxidized Mo/γ-Al2O3 catalysts are usually prepared by calcination at about 500 °C, after which Mo species were dispersed on the support very well. The influence of recalcination of the oxidized K-MoO3/γ-Al2O3 sample at high temperature up to 900 °C has been investigated. The results indicate that the sample recalcined at about 800 °C has a low surface area and the aggregated K-Mo species. The activity for hydrocarbon synthesis decreased, but the activity for mixed alcohol synthesis increased sharply. The rise in the activity for mixed alcohol synthesis after the recalcination of oxidized K-MoO3/γ-Al2O3 catalyst is at a temperature of about 800 °C and also directly results in aggregation of K-Mo species or a decrease in the BET surface area. It was reported that the acidic site on the γ-Al2O3 support was the active site for alcohol dehydration (Seiyama et al., 1991). The interaction between the potassium component and the support was considered to suppress most of the acidic sites. But some acidic sites may still remain on the K-MoO3/γ-Al2O3 catalyst and reduce the selectivity to mixed alcohol synthesis. The effects of the recalcination at 800 °C may be attributed to a decrease in the acidity. Therefore, it is interesting to search for new Mo-based catalysts with high activity and selectivity for the mixed alcohol synthesis; it is also necessary to elucidate the intrinsic reasons of the influence of the support and recalcination process in the mixed alcohol synthesis. The K-Mo supported on γ-Al2O3, SiO2, and unsupported oxidized samples were prepared by calcination at 500 °C, and parts of the samples were recalcined at 800 °C. These oxidized samples were sulfided for mixed alcohol synthesis. The activity for ethanol decomposition was measured. The influence of reaction conditions on 800 °C recalcined K-MoO3/Al2O3 sample, in the synthesis of mixed alcohols, was investigated. The effect of supports and the mechanism for mixed alcohol synthesis are discussed as well.

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Experimental Section 1. Catalyst Preparation. The oxidized K-MoO3/ Al2O3 and K-MoO3/SiO2 catalysts were prepared by first impregnating γ-Al2O3 and SiO2 supports with an aqueous KCl solution, followed by drying and calcination in air at 300 °C for 1 h. BET surface areas of γ-Al2O3 and SiO2 supports were 270 and 400 m2/g, respectively. The obtained KCl/Al2O3 and KCl/SiO2 samples were then impregnated with a (NH4)6Mo7O24‚4H2O solution, followed by drying and calcination in air at 500 °C for 12 h. Half of each sample was recalcined at 800 °C in air. The contents of the Mo component in the K-MoO3/ Al2O3 and K-MoO3/SiO2 samples, calculated as a weight ratio of MoO3/Al2O3 or MoO3/SiO2, were 0.24. The content of the K component, as an atomic ratio of K/Mo, was 0.8. The oxidized unsupported K-Mo catalyst was prepared by mixing the calculated amount of KCl and (NH4)6Mo7O24‚4H2O. The mixture was ground for more than 0.5 h and then calcined in air at 500 °C for 12 h. Half of the sample was recalcined in air at 800 °C for 12 h. The atomic ratio of K/Mo in the K-Mo catalyst was also 0.8. The oxidized samples were sulfided by calcination in a flow of H2S/H2 (1:3 mol ratio, 40 mL/min) at 400 °C for 3 h. 2. Activity Measurement. The activity of sulfided catalysts was measured by using a fixed-bed stainless reactor (9 mm i.d.) with an on-line gas chromatograph. The feed gas was composed of 31% CO, 66% H2, and 3% Ar. The liquid products were collected in a trap filled with distilled water and kept at 0 °C. The contents of CO and CO2 in the exit gas were analyzed by a thermal conductivity detector (TCD) with a 2-m active carbon column in. Ar was used as an internal standard for the analysis. The hydrocarbons in the exit gas and the composition of the liquid products were detected by a flame ionization detector (FID) with a 2-m Porapak Q column in. A temperature program was used to separate C1OH-C6OH. The reaction of sulfided Mo-based catalyst needs a long time to reach steady state. In the first period of the reaction, the yields of hydrocarbons were high and the yields of mixed alcohols were low. With increasing reaction period, the yields of hydrocarbons decreased and the yields of mixed alcohols increased progressively. The data shown in this paper were obtained after the reaction was performed for more than 20 h. 3. Ammonia Saturation and Temperature-Programmed Desorption (TPD). The surface acidity of the γ-Al2O3 support and the oxidized catalysts was investigated with ammonia saturation and TPD. For each experiment, a 0.1-g sample was packed in a quartz microreactor tube between plugs of glass wool, which was placed in a tube furnace and in a flow of He (30 mL/min). The sample was dehydrated by ramping to 500 °C for 0.5 h. The cell was evacuated to 10-2 Torr for 0.5 h and finally cooled to 100 °C. The isothermal curve for ammonia adsorption was measured at 100 °C, with ammonia pressure increased up to about 100 Torr, after which the cell was evacuated to 10-2 Torr for 5min, to make weakly adsorbed ammonia desorbed. Then the second isothermal curve of ammonia adsorption up to weak acid sites was measured in the same procedure. Finally the ammonia in the cell was evacuated, and TPD was measured with temperature increased up to 500 °C at a rate of 10 °C/min. The desorption species from

Figure 1. Yields of CO2, hydrocarbons, and mixed alcohols from syngas over sulfided K-Mo/Al2O3 catalyst whose oxidized precursor calcined at different temperatures.

the catalyst surface were carried out to a mass spectrometer in a flow of He and analyzed continuously at intervals of 6 s until the cell temperature increased up to 500 °C. The acid amount of the catalyst sample was calculated with the two isothermal curves for ammonia adsorption. The acidic strength was evaluated by the TPD of ammonia. 4. Ethanol Decomposition. The properties of the γ-Al2O3 support, the K-MoO3/Al2O3 samples, and the 500 °C calcined K-Mo sample for ethanol decomposition were tested in a U-tube reactor. A total of 100 mg of sample was used for each experiment. All the samples were first activated by pretreating them at 300 °C for 2 h in flowing Ar (20 mL/min). The reactant stream consisted of 20 mL/min of argon bubbled through liquid ethanol at room temperature. The reactivity tests were conducted between 150 and 300 °C. Only the data of the reaction occurring at 300 °C were reported in this paper. The ethanol/argon stream was allowed to flow through the catalyst bed for a period of 30 min before being injected into a gas chromatograph for analysis. The gas chromatograph had a 2-m Porapak Q column maintained at 150 °C for the separation of the reaction products and a flame ionization detector for the detection. Results 1. Influence of Recalcination at High Temperature on the Properties for Mixed Alcohol Synthesis. 1.1. K-MoO3/Al2O3 Catalyst. Figure 1 presents the activity for mixed alcohol synthesis on the two sulfided K-MoO3/Al2O3 catalysts. The reaction conditions were P ) 5.0 MPa, T ) 320 °C, and GHSV ) 6000 h-1. The results indicated that, for 500 °C calcined K-MoO3/Al2O3 catalyst, the yields of hydrocarbons were high and the yields of mixed alcohols were very low. CO conversion to total mixed alcohols and hydrocarbons was 16%, but the yield of mixed alcohols was only 0.54% and the selectivity to mixed alcohols (CO2 free) was 3.4%. Methane was predominant in the hydrocarbons, and methanol was predominant in the mixed alcohols. For 800 °C recalcined catalyst, CO conversion to mixed alcohols and hydrocarbons decreased to 7.4%, but in contrast the yield of mixed alcohols increased to 3.7% and the selectivity to mixed alcohols increased sharply to 51%. The recalcination process favored the synthesis of mixed alcohols. For 800 °C recalcined catalyst, the

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Figure 2. Yields of CO2, hydrocarbons, and mixed alcohols from syngas over sulfided K-Mo/SiO2 catalyst whose oxidized precursor calcined at different temperatures. Figure 4. Ammonia TPD profiles of γ-Al2O3 support and oxidized K-MoO3/γ-Al2O3 catalysts. Table 1. Ammonia Saturation Measurements Reflecting Surface Acid Amount

Figure 3. Yields of CO2, hydrocarbons, and mixed alcohols from syngas over sulfided unsupported K-Mo catalyst whose oxidized precursors calcined at different temperatures.

content of methane in hydrocarbons increased and that of methanol in mixed alcohols decreased. 1.2. K-MoO3/SiO2 Catalyst. The activity results of the two sulfided K-MoO3/SiO2 catalysts, one calcined at 500 °C and the other recalcined at 800 °C, are presented in Figure 2. For 500 °C calcined K-MoO3/ SiO2 catalyst, CO conversion to mixed alcohols and hydrocarbons was 6.9%, the yields of mixed alcohols were 2.0%, and the selectivity to mixed alcohols was 29%. For 800 °C recalcined K-MoO3/SiO2 catalyst, CO conversion to mixed alcohols and hydrocarbons was 3.2%, the yields of mixed alcohols were 1.6% and the selectivity to mixed alcohols was 50%. The C molar ratio of C1OH/C2+OH did not show a dramatic change for the two SiO2 supported samples. It was found that the activity for CO2 and hydrocarbon synthesis was high on 500 °C calcined K-MoO3/SiO2 catalyst and decreased obviously on 800 °C recalcined catalyst, which was similar to the γ-Al2O3-supported catalyst system. However, the activity for mixed alcohol synthesis was found to be significantly high over 500 °C calcined K-MoO3/ SiO2 catalyst and was not increased on the sample recalcined at 800 °C, which was different from γ-Al2O3supported catalyst system. 1.3. K-Mo Catalyst. The activity results over the two K-Mo catalyst samples were shown in Figure 3. This figure indicates that, on 500 °C calcined K-Mo catalyst, the yield of hydrocarbons was 4.4%, which was obviously low, but the yield of mixed alcohols was about 4.1%, which was reversely higher than those on 500 °C calcined K-MoO3/Al2O3 and K-MoO3/SiO2 catalysts. The selectivity to mixed alcohols was 47%. For 800 °C

sample

strong NH3 adsorption (µmol/g)

total NH3 adsorption (µmol/g)

γ-Al2O3 500 °C, K-Mo/Al2O3 800 °C, K-Mo/Al2O3 500 °C, K-Mo/SiO2 800 °C, K-Mo/SiO2 500 °C, K-Mo

494 71 21 41 12 15

912 412 92 122 22 24

recalcined K-Mo catalyst, the yield of hydrocarbons was 4.0% and the yield of mixed alcohols was 3.6%. The activity for hydrocarbon synthesis and for mixed alcohol synthesis was parallelly decreased, and the selectivity to mixed alcohols was kept unchanged. The effect of the recalcination process on the catalytic properties of mixed alcohol synthesis was quite different for γ-Al2O3 and SiO2-supported and unsupported K-Mo samples. 2. Influence of the Recalcination Process on the Acidity of the Oxidized Samples. The acidity of γ-Al2O3 and SiO2 supports has been correlated with their catalytic properties for alcohol dehydration; the acidity of these γ-Al2O3 and SiO2-supported K-Mo catalysts may be the essential factor for determining the reaction tendency of CO hydrogenation to form alcohols or hydrocarbons. 2.1. γ-Al2O3-Supported Samples. Ammonia saturation data and TPD spectra for the γ-Al2O3 support and two oxidized K-MoO3/Al2O3 samples, one calcined at 500 °C and another recalcined at 800 °C, were presented in Table 1 and Figure 4. In Table 1 the strong ammonia adsorption referred to adsorbed ammonia which could not be desorbed at 100 °C. The results in Table 1 showed that the acid amount of the sites interacting with ammonia was high for the γ-Al2O3 support and decreased markedly upon the addition of K and Mo components. For the 500 °C calcined sample, the acid amount was about 1/7 of the γ-Al2O3 support. For the 800 °C recalcined sample, the acid amount decreased to about 1/25 of that for the support. These results could be related to the fact that K and Mo species on the support interacted with acid sites to suppress the acidic sites on the support. By recalcination at 800 °C, the interaction between the active species and the γ-Al2O3 support was strengthened, which suppressed more acid sites on the support.

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Figure 6. Activity for ethanol decomposition on oxidized catalysts.

Figure 5. Ammonia TPD profiles of oxidized K-MoO3/SiΟ2 catalysts and K-Mo catalyst.

The ammonia TPD traces showed that, for the γ-Al2O3 support, the desorption peak was at 180 °C and the desorption amount was kept high up to 400 °C. For 500 °C calcined catalyst, the desorption peak was at 160 °C and the desorption amount became much low while the temperature increased to 250 °C. For the 800 °C recalcined sample the desorption peak shifted to 150 °C and the peak height was lowest. The TPD traces indicated that the acidity of the γ-Al2O3 support was strong. K and Mo species first connected with strong acid sites and then connected with weak acid sites. 2.2. SiO2-Supported and Unsupported Samples. Ammonia saturation data for the two K-MoO3/SiO2 samples and the 500 °C calcined K-Mo sample were shown in Table 1. Ammonia TPD traces for these samples were presented in Figure 5. The acid amount for the 500 °C calcined K-MoO3/SiO2 sample was obviously lower than that for the 500 °C calcined K-MoO3/γ-Al2O3 sample but higher than that for the 800 °C recalcined K-MoO3/γ-Al2O3 sample. After recalcination at 800 °C, the acid amount decreased to a quite low value and no obvious desorption peak was observed in the ammonia TPD trace. For the 500 °C calcined K-Mo sample, the acidity was very weak and no desorption peak was observed in the ammonia TPD trace. 3. Ethanol Decomposition. To illustrate the effect of the acidity on the property for alcohol decomposition, the γ-Al2O3 support, two K-MoO3/γ-Al2O3 samples, and the 500 °C calcined K-Mo sample were chosen to be tested for ethanol decomposition. Figure 6 presents the test results. The main products of the reaction were ethene and acetaldehyde; they were formed from ethanol dehydration and ethanol dehydrogenation, respectively. It is clearly indicated that the activity for ethanol decomposition was high over the γ-Al2O3 support and decreased over the K-MoO3/Al2O3 and K-Mo samples. The space-time-yield (STY) of ethene over the 500 °C calcined K-MoO3/Al2O3 sample was about 50% of the STY over the γ-Al2O3 support. The STY of ethene over the 800 °C recalcined K-MoO3/Al2O3 sample was only about 10% of that over the support. The recalcination process reduced the activity of the K-MoO3/Al2O3 sample for alcohol dehydration to a very low degree. For the unsupported K-Mo sample, the activity for ethanol

Figure 7. Activity for ethanol decomposition on sulfided catalysts.

dehydration was near that for the 800 °C recalcined K-MoO3/Al2O3 sample. The experimental results also showed that, for these oxidized samples, the activity for ethanol dehydrogenation changed in a way similar to the activity for ethanol dehydration. It was reported that the acidic sites on the γ-Al2O3 support were the active sites for ethanol dehydration and the basic sites were the active sites for ethanol dehydrogenation (Seiyama et al., 1991). The changes in the activity for ethanol dehydration were similar to the changes in the acidity for these catalyst samples. These results suggested that the acidity of the catalyst played an important role in the reaction of mixed alcohol synthesis. In the catalytic process of mixed alcohol synthesis from syngas, no acetaldehyde was detected, which demonstrated that alcohol dehydrogenation did not occur under the reaction conditions. The test results for ethanol decomposition over the sulfided samples are shown in Figure 7. On each sulfided sample, the activity for ethanol dehydration showed little difference from that on the corresponding oxidized sample, which indicated that the acidity of these oxidized samples did not show obvious changes during the sulfidation. However, for the 500 °C calcined K-MoO3/Al2O3 sample, the activity for ethanol dehydrogenation enhanced sharply after sulfidation, which became even higher than that on the γ-Al2O3 support. This result demonstrated that the existence of some potassium component on the γ-Al2O3 surface gave a rise in the amount of basic sites during the sulfidation. For the 800 °C recalcined K-MoO3/Al2O3 and unsupported K-Mo samples, the activity for ethanol dehydrogenation was unchanged during the sulfidation. The different changes in the activity for ethanol dehydrogenation during the sulfidation may be correlated with the interactions between the support and the K and Mo species.

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Figure 8. CO conversion and yields of CO2 and hydrocarbons from syngas over sulfided K-Mo/Al2O3 catalyst. Reaction conditions: P ) 5.0 MPa, GHSV ) 6000 h-1, and T ) 310-340 °C.

Figure 9. Yields of mixed alcohols from syngas over sulfided K-Mo/Al2O3 catalyst. Reaction conditions: P ) 5.0 MPa, GHSV ) 6000 h-1, and T ) 310-340 °C.

Figure 10. CO conversion and yields of CO2 and hydrocarbons from syngas over sulfided K-Mo/Al2O3 catalyst. Reaction conditions: P ) 5.0-8.0 MPa, GHSV ) 6000 h-1, and T ) 320 °C.

Figure 11. Yields of mixed alcohols from syngas over sulfided K-Mo/Al2O3 catalyst. Reaction conditions: P ) 5.0-8.0 MPa, GHSV ) 6000 h-1, and T ) 320 °C.

Table 2. Influence of Reaction Conditions on the Distribution of Reaction Products reaction conditions

C1OH/ C2+OH

2-C3OH/ 1-C3OH

i-C4OH/ n-C4OH

5.0 MPa, 320 °C, 6000 h-1 5.0 MPa, 340 °C, 6000 h-1 8.0 MPa, 320 °C, 6000 h-1 8.0 MPa, 320 °C, 18 000 h-1

0.60 0.52 0.58 0.69

0.05 0.05 0.04 0.03

1.27 1.36 1.30 0.79

4. Mixed Alcohol Synthesis over 800 °C Recalcined K-Mo/Al2O3 Catalyst. 4.1. Effect of Reaction Temperature. Figures 8 and 9 present the activity results over 800 °C recalcined K-MoO3/Al2O3 catalyst with P ) 5.0 MPa, GHSV ) 6000 h-1, and reaction temperature 310-340 °C. In Figure 8 it is indicated that the yields of CO2 and hydrocarbons increased with increased reaction temperature. Figure 9 indicates that the yields of the mixed alcohols reached a maximum while the reaction temperature was 320 °C. When the reaction temperature was raised above 320 °C, the yields of mixed alcohols decreased progressively. When the reaction temperature was increased from 310 to 340 °C, the selectivity to mixed alcohols decreased from 60.2% to 32.7%. It was also shown in Table 2 that the content of C2+OH in mixed alcohols increased and the C mole ratio of C1OH/C2+OH decreased from 0.60 to 0.52 when the reaction temperature was increased from 320 to 340 °C. In the meantime, the ratio of i-C4OH/n-C4OH increased. The amount of branched 2-C3OH was quite low compared to that of the linear 1-C3OH. 4.2. Effect of Total Pressure. Activity results over 800 °C recalcined K-MoO3/γ-Al2O3 catalyst with T ) 320 °C, GHSV ) 6000 h-1, and total pressure 5.0-8.0 MPa are shown in Figures 10 and 11. With increased total pressure, the yields of CO2, hydrocarbons, and

Figure 12. CO conversion and yields of CO2 and hydrocarbons from syngas over sulfided K-Mo/Al2O3 catalyst. Reaction conditions: P ) 8.0 MPa, GHSV ) 3000-18 000 h-1 and T ) 320 °C.

mixed alcohols all increased linearly. The selectivity to mixed alcohols remained at 50% when the total pressure was increased from 5.0 to 8.0 MPa. The contents of C2+OH in the mixed alcohols showed a weak increase and the C mole ratio of C1OH/C2+OH decreased from 0.6 to 0.58 when the total pressure was increased from 5.0-8.0 MPa (Table 2). The ratio of i-C4OH/n-C4OH showed no significant increase. 4.3. Effect of GHSV. Figures 12 and 13 present the activity results over 800 °C recalcined K-MoO3/Al2O3 catalyst with P ) 8.0 MPa, T ) 320 °C, and GHSV 3000-18 000 h-1. With increased GHSV, the yields of CO2 and hydrocarbons decreased, but the yields of mixed alcohols reached a maximum when GHSV was 6000 h-1. The selectivity to mixed alcohols increased gradually when GHSV was enhanced. The enhancement of GHSV favored the synthesis of mixed alcohols. As shown in Table 2, the content of C2+OH in mixed

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Figure 13. Yields of mixed alcohols from syngas over sulfided K-Mo/Al2O3 catalyst. Reaction conditions: P ) 8.0 MPa, GHSV ) 3000-18 000 h-1, and T ) 320 °C. Table 3. Space-Time-Yield of Mixed Alcohols over Sulfided K-MoO3/Al2O3 Catalyst with P ) 8.0 MPa, T ) 320 °C, and GHSV ) 3000-18 000 h-1 GHSV (h-1)

STY (g/kg of catalyst/h)

GHSV (h-1)

STY (g/kg of catalyst/h)

3000 6000 9000

62.6 165 227

12 000 15 000 18 000

271 322 375

Table 4. BET Surface Areas of the Oxidized Catalysts (m2/g) sample

500 °C calcined

800 °C recalcined

K-Mo K-Mo/Al2O3 K-Mo/SiO2

4.1 118.0 38.0

0.4 22.2 0.4

alcohols decreased and the C mole ratio of C1OH/C2+OH increased from 0.58 to 0.69 when GHSV was increased from 6000 to 18 000 h-1. The ratio of i-C4OH/n-C4OH decreased obviously. Table 3 shows that with increased GHSV the STY of mixed alcohols increased. The STY reached 375 g/kg of catalyst/h under the conditions of P ) 8.0 MPa, T ) 320 °C, and GHSV ) 10 800 h-1. Discussion 1. Recalcination Process and the Activity Sites. The activity results indicate that, for 500 °C calcined K-Mo catalysts, the selectivity to mixed alcohols was quite different. Support type showed a significant influence on the catalytic property, which was in agreement with the results reported by Avila et al. (1995). However, for 800 °C recalcined catalysts, the selectivity to mixed alcohols was similar for all tested supports. The characteristics of the support showed a weak influence on the reaction tendency to form alcohols or to form hydrocarbons. The recalcination at high temperature of these γ-Al2O3-supported, SiO2-supported, and unsupported K-Mo catalysts showed a different influence on their properties for CO hydrogenation. Table 4 shows the changes in BET surface area by recalcination at 800 °C. The data indicated that, for all catalysts, the recalcination process decreased their BET surface area. The recalcination process decreased the CO conversion to hydrocarbons and mixed alcohols, which can be attributed to the decrease in the surface area (Lee et al., 1995). The high selectivity to mixed alcohols over 500 °C calcined K-Mo catalyst demonstrated that the precursor of the active sites for mixed alcohol synthesis could be formed by K-Mo interaction during the calcination

at 500 °C. On 500 °C calcined K-MoO3/γ-Al2O3 catalyst, the selectivity to mixed alcohols was obviously low. It has been reported that the amount of K-Mo interaction species on support played an important role for the enhancement of the selectivity to mixed alcohols (Jiang et al., 1994; Avila et al., 1995), but the low selectivity to mixed alcohols on this Al2O3-supported catalyst did not mean a small amount of the K-Mo interaction species formed on the support. Because the contents of K-Mo components on the samples were high enough to cover the support for the above three layers, K and Mo components could interact sufficiently to form K-Mo species. Structural studies also demonstrated the formation of a large amount of K-Mo interaction species on the catalyst surface (Jiang et al., 1994). The low selectivity to mixed alcohols on 500 °C calcined K-MoO3/Al2O3 catalyst should be due to the characteristics of the support. The acid sites on the γ-Al2O3 surface were demonstrated to be the active sites for alcohol dehydration. The interaction between the potassium component and the support was promised to suppress these active sites, but Ammonia adsorption and TPD results demonstrated that the acidity for the 500 °C calcined K-MoO3/γ-Al2O3 sample was still stronger, compared to 500 °C calcined unsupported catalyst. The results from the test in ethanol decomposition indicated clearly that the activity for alcohols decomposition on the 500 °C calcined K-MoO3/γ-Al2O3 sample was much higher than that on the 500 °C calcined K-Mo sample. So, part of the alcohols formed during the reaction were decomposed immediately at the support. Avila reported that, on the Mo/ZrO2 catalyst, the primary product was methanol and the support ZrO2 participated in the transformation of methanol into hydrocarbons (Avila et al., 1995). Xie et al. (1986) reported that a secondary reaction of methanol to produce methane, H2O, and CO2 took place on MoS2based catalyst. Santiesteban et al. (1988) also reported that, on alkali/MoS2 catalyst, methane can be formed by the decomposition of methanol. These results demonstrated that the ability of the catalyst for alcohol decomposition determined the reaction direction. The 500 °C calcined K-MoO/SiO2 sample showed a relatively higher selectivity to mixed alcohol synthesis than 500 °C calcined K-MoO3/γ-Al2O3 catalyst, which may be attributed to the fact that 500 °C calcined K-MoO/SiO2 catalyst showed a relatively weak acidity. So, its activity for alcohol decomposition was lower than that of the γ-Al2O3-supported sample. Tatsumi et al. (1987) reported that SiO2 was a better support than γ-Al2O3 for mixed alcohol synthesis. Avila et al. (1995) reported that high selectivity to mixed alcohols could be obtained when ZnO was used as the support instead of ZrO2, although the CO conversion was higher with latter as the support. These results also can be explained by acidity change of the samples. The 800 °C recalcined K-MoO3/γ-Al2O3 sample showed an obvious higher activity for mixed alcohol synthesis than the 500 °C calcined K-MoO3/γ-Al2O3 sample, which could correspond to the disappearance of active sites for alcohol decomposition, although a decreased BET surface area resulted in a small total CO conversion to mixed alcohols and hydrocarbons. For unsupported K-Mo catalyst, the selectivity to mixed alcohols was unchanged after recalcination at 800 °C, which

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could be attributed to the fact that the recalcination process hardly changed the catalyst acidity. 2. Influence of Reaction Conditions and the Distribution of Mixed Alcohols. Mixed alcohols in the product were mainly composed of linear C1OH-C4OH and a small amount of i-C4OH. The content of each linear alcohol in the product decreased gradually with increasing carbon number; the content of i-C4OH was near that of n-C4OH. The amount of branched 2-C3OH was quite low compared to the linear 1-C3OH. The distribution of mixed alcohols was the same as that of MoS2-based catalyst in the literature and similar to that on Co-based catalyst (Fujimoto and Oba, 1985) but far away from that over Cu-Zn-based catalyst for branched alcohol synthesis (Hofstadt et al., 1983). The reaction for linear mixed alcohol synthesis over Co-based alcohols has been proven to occur by a mechanism of the carbide polymerization scheme of the Fischer-Tropsch reaction since the distribution obeyed the Anderson-Schulz-Flory distribution (Fujimoto and Oba, 1985), which suggested CO insertion into adsorbed alkyls to produce acyl intermediates (Kiennemann et al., 1991). These acyl intermediates can be hydrogenated to produce alcohols or can be hydrogenated and dehydrated to produce hydrocarbons (Santiesteban et al., 1988; Chaumette et al., 1995). The reaction such as that over Cu-Zn-based catalyst, which resulted in an abundance of branched alcohols, was proposed to occur via a condensation mechanism which indicated that higher alcohol formed from two lower alcohols. For example, one i-C4OH molecule was produced by the condensation of one C1OH molecule and one linear C3OH molecule (Tseng et al., 1988; Mazanec, 1986). MoS2based catalyst has been reported to be a F-T catalyst because the product was mainly composed of linear alcohols, and the composition of the linear alcohols was similar to that on a Co-based catalyst (Santiesteban et al., 1988). However, the existence of a small amount of branched alcohols in the product indicated that, on a MoS2-based catalyst, the condensation reaction of mixed alcohols took place as well (Murchison et al., 1988). The increase of the total pressure favored both the synthesis of mixed alcohols and the synthesis of hydrocarbons, which was due to the fact that both reactions decreased the volume of the reaction gas. Usually the rise in the reaction temperature can activate more adsorbed H2 and CO molecules, which should increase the reaction activity, but higher reaction temperature makes the acyl intermediates dehydrate easily. This is why the yields of hydrocarbons increased progressively when the reaction temperature was increased from 310 to 340 °C, but the yields of mixed alcohols reached a maximum at 320 °C. By increasing GHSV, the yields of hydrocarbons decreased abruptly but the yields of mixed alcohols decreased slowly and the selectivity to mixed alcohols increased. These results could also be attributed to the fact that the dehydration reaction of mixed alcohols was suppressed in high GHSV. The molar ratio of i-C4OH/n-C4OH was enhanced with raised reaction temperature and decreased GHSV, which suggested the increase in the activity for alcohol condensation. In the meantime, the content of methanol in the mixed alcohols decreased obviously. Similar results were reported on the MoS2-based catalyst (Murchison et al., 1988) and reduced K-Mo/SiO2 catalysts (Muramatru et al., 1987; Tatsumi et al., 1988). The

increase in the content of i-C4OH was accompanied by the decrease in the content of methanol. This result supported the theory that i-C4OH was formed from alcohol condensation. Conclusion Activity results demonstrated that, on 500 °C calcined K-MoO3/γ-Al2O3 catalyst, the yields of CO2 and hydrocarbons were high and the yields of mixed alcohols were very low. If the oxidized catalyst was recalcined at 800 °C, the yields of CO2 and hydrocarbons were suppressed and the yield of mixed alcohols was remarkably enhanced. On 500 °C calcined K-MoO3/SiO2 catalyst and 500 °C calcined unsupported K-Mo catalyst, the yields of CO2 and hydrocarbons were lower and the yields of mixed alcohols were obviously higher than those on 500 °C calcined K-MoO3/Al2O3 catalyst. After recalcination at 800 °C, all the yields of CO2 and hydrocarbons and the yields of mixed alcohols decreased; the selectivity to mixed alcohols increased for SiO2-supported catalyst but did not change for K-Mo catalyst. The 500 °C calcined K-MoO3/Al2O3 and K-MoO3/ SiO2 catalysts showed obvious low selectivity to mixed alcohols in CO hydrogenation, which was due to the existence of the remaining acid sites on the support. These acid sites were responsible for alcohol decomposition. The 800 °C recalcined catalysts showed high selectivity to mixed alcohols, which was attributed to the fact that the strong interaction between the potassium component and the support suppressed most acid sites on the support. The effects of reaction temperature, total pressure, and GHSV were investigated for sulfided K-MoO3/ Al2O3 catalyst, whose oxidized precursor was recalcined at 800 °C. The optimum reaction temperature was 320 °C while P ) 5.0 MPa and GHSV ) 6000 h-1. Higher reaction pressure such as 8.0 MPa showed a parallel increase in alcohols yield and hydrocarbons yield. Higher GHSV favored mixed alcohol synthesis. High reaction temperature and low GHSV has promoted the reaction of alcohol condensation. Literature Cited Avila, Y.; Kappenstein, C.; Pronier, S.; Barrault, J. Alcohol Synthesis from Syngas over Supported Molybdenum Catalysts. Appl. Catal. A 1995, 132, 97. Chaumette, P.; Courty, P.; Kiennemann, A.; Ernst, B. Higher Alcohol and Paraffin Synthesis on Cobalt Based Catalysts: Comparison of Mechanistic Aspects. Top. Catal. 1995, 2, 117. Fujimoto, K.; Oba, T. Synthesis of C1-C7 Alcohols from Synthesis Gas with Supported Cobalt Catalysts. Appl. Catal. 1985, 13, 289. Hofstadt, C. E.; Schneider, M.; Bock, O.; Kochloefl, K. Effect of Preparation Methods and Promoters on Activity and Selectivity of Cu-Zn-Al-K Catalysts in Aliphatic Alcohols Synthesis from CO and H2. Stud. Surf. Sci. Catal. 1983, 16, 709. Jiang, M.; Bian, G. Z.; Fu, Y. L. Effect of the K-Mo Interaction in K-MoO3/γ-Al2O3 Catalysts on the Properties for Alcohol Synthesis from Syngas. J. Catal. 1994, 146, 144. Kiennemann, A.; Boujana, S.; Diagne, C.; Courty, P.; Chaumette, P. Use of Probe Molecules to Predict the Performances of Alcohols Synthesis Catalysts. Nat. Gas Convers. 1991, 243250. Lee, J.; Kim, S.; Kim, Y. G. Electronic and Geometric Effects of Alkali Promoters in CO Hydrogenation over K/Mo2C Catalysts. Top. Catal. 1995, 2, 127.

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1743 Mazanec, T. J. On the Mechanism of Higher Alcohol Formation over Metal Oxide Catalysts. J. Catal. 1986, 98, 115. Muramatru, A.; Tatsumi, T.; Tominaga, H. Mixed Alcohol Synthesis from CO-H2 by Use of KCl-promoted Mo/SiO2 Catalysts. Bull. Chem. Soc. Jpn. 1987, 60, 3157. Murchison, C. B.; Conway, M. M.; Stevens, R. R.; Quarderer, G. J. Mixed Alcohols from Syngas over Mo Catalysts. Proc. 9th Int. Congr. Catal. 1988, 2, 626. Santiesteban, J. G.; Bogdan, C. E.; Herman, R. G.; Klier, K. Mechanism of C1-C4 Alcohol Synthesis over Alkali/MoS2 and Alkali/Co/MoS2 Catalysts. Proc. 9th Int. Congr. Catal. 1988, 2, 561. Seiyama, T.; Huang, M. M. Metal Oxide and Their Catalytic Effects; The University of Science and Technology of China Publishing: Anhui, China, 1991; p 116. Storm, D. A. The Production of Higher Alcohols from Syngas using Potassium Promoted Co/Mo/Al2O3 and Rh/Co/Mo/Al2O3. Top. Catal. 1995, 2, 91.

Tatsumi, T.; Muramatsu, A.; Tominaga, H. Supported Molybdenum Catalysts for Alcohol Synthesis from CO-H2. Appl. Catal. 1987, 34, 77. Tatsumi, T.; Muramatsu, A.; Fukunaga, T.; Tominaga, H. NickelPromoted Molybdenum Catalysts for Synthesis of Mixed Alcohols. Proc. 9th Int. Congr. Catal. 1988, 2, 618. Tseng, S. C.; Jackson, N. B.; Ekerdt, J. G. Isosynthesis Reactions of CO/H2 over Zirconium Dioxide. J. Catal. 1988, 109, 284. Xie, Y. C.; Naasz, B. M.; Somojai, G. A. Alcohol Synthesis from CO and H2 over Molybdenum Sulfide. The Effect of Pressure and Promotion by Potassium Carbonate. Appl. Catal. 1986, 27, 233.

Received for review November 10, 1997 Revised manuscript received February 23, 1998 Accepted February 23, 1998 IE970792E