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γ-Al2O3 catalysts for mixed alcohols synthesis from syngas: Effects of Mo loadings
Applied Catalysis A: General 170 (1998) 255±268
High temperature calcined K±MoO3/g-Al2O3 catalysts for mixed alcohols synthesis from syngas: Effects of Mo loadings Guo-zhu Biana, Li Fanb,*, Yi-lu Fua, Kaoru Fujimotob a
Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan
b
Received 15 October 1997; received in revised form 14 January 1998; accepted 26 January 1998
Abstract One series of oxidized K±MoO3/g-Al2O3 samples with different Mo loadings (MoO3/Al2O3 (wt ratio)0.05±0.45) was prepared by impregnating K and Mo compounds and successive calcination in air at 8008C. The oxidized samples were sul®ded and then utilized for mixed alcohols synthesis from syngas. The structural information from laser Raman spectroscopy (LRS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ammonia saturation, temperature programmed desorption (TPD) and ethanol decomposition were studied to elucidate the reaction properties. The results indicated that with Mo loading increased from MoO3/Al2O30.05 to 0.25, the total yields of mixed alcohols and hydrocarbons decreased, but the selectivity to mixed alcohols was enhanced sharply from 3% to 50%. With Mo loading increased from MoO3/Al2O30.25 to 0.45, the CO conversion was enhanced, but the selectivity to mixed alcohols leveled off. On these catalysts, Fischer±Tropsch (FT) synthesis to linear alcohols and the condensation reaction of low alcohols to form branched i-C4OH occurred at the same time. With increased Mo loading, activity of the alcohols condensation became high. Structural studies demonstrated that on oxidized samples with increased Mo loading the same K±Mo±O species was formed, but the dispersion of these K±Mo species decreased. The catalyst's acidity decreased remarkably with Mo loading up to MoO3/Al2O30.25, and stayed unchanged as Mo loading was further increased to MoO3/Al2O30.45. With increased Mo loading, the activity for ethanol dehydration changed parallel to the acidity. Results of the activity experiments for mixed alcohols' synthesis and the structural measurements indicated that the dispersion state of Mo species and the content of unreduced Mo species in¯uenced the total CO conversion, and that the acidity of the catalyst controlled the selectivity to mixed alcohols. # 1998 Elsevier Science B.V. All rights reserved. Keywords: K±MoO3/Al2O3 catalyst; Mo loading; Synthesis of mixed alcohols; Structure and acidity
1. Introduction Mo-based catalysts have been widely applied in many hydrotreating reactions due to their high activ*Corresponding author. Tel.: 0081 35689 0469; e-mail: [email protected] 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(98)00053-2
ities for the hydrodesulfurization, hydrodenitrogenation reaction and methane synthesis from CO hydrogenation. Doped with alkali compounds, molybdenum-based catalysts showed relatively high activities for mixed alcohols synthesis [1±3]. g-Al2O3 has been regarded as a superior support for Mo-based catalysts because Mo species could be well
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dispersed on it and therefore possessed high speci®c reaction activity. But g-Al2O3 supported Mo-based catalysts were reported to show quite low selectivity to mixed alcohols from syngas compared to active carbon, SiO2 or MgO supported and unsupported catalysts [4±6]. For supported Mo-based catalysts, it has been demonstrated that K±Mo interaction species which was formed on the oxidized sample were the precursors of the active sites for mixed alcohols synthesis; poor dispersion of the molybdenum species could suppress the yield of hydrocarbons and enhance the selectivity to mixed alcohols [7,8]. The fact that g-Al2O3-supported catalysts possessed low selectivity to mixed alcohols may be related to good dispersion of the Mo species on the support, but its mechanism is not clear until now. Calcination process of the oxidized precursors is important for the dispersion control of the molybdenum species. We have studied the in¯uence of recalcination at high temperature of an oxidized K±MoO3/ Al2O3 catalyst on the property for mixed alcohols' synthesis. It was found that while the oxidized sample was recalcined at 8008C, both the yields of mixed alcohols and the selectivity to mixed alcohols were sharply enhanced [9]. Structural studies indicated that after recalcination at high temperature of 8008C, the BET surface area of the catalyst samples decreased, and K±Mo interaction species were aggregated and became dif®cult to be reduced and sul®ded [9]. For further characterization of the sul®ded K± MoO3/Al2O3 catalyst for mixed alcohol synthesis, one series of samples with different Mo loading was investigated. All the oxidized samples were prepared through recalcination at 8008C. The property for mixed alcohols synthesis and the structure characterization are reported. The effect of the support and the reaction mechanism are discussed as well. 2. Experimental 2.1. Catalyst preparation The oxidized K±MoO3/Al2O3 catalysts were prepared by ®rst impregnating g-Al2O3 support with the aqueous solutions of KCl compound, followed by drying and calcination at 3008C for 1 h. The oxidized KCl/Al2O3 samples were impregnated with
Table 1 Mo content and BET surface area of the oxidized K±MoO3/gAl2O3 catalyst sample (K/Mo molar ratio0.8) Sample
MoO3/Al2O3 (wt%)
BET surface area (m2/g)
g-Al2O3 KCl±MoO3/Al2O3 KCl±MoO3/Al2O3 KCl±MoO3/Al2O3 KCl±MoO3/Al2O3 KCl±MoO3/Al2O3
Ð 5 15 25 35 45
270.0 91.7 27.8 21.4 10.7 3.7
(NH4)6Mo7O244H2O solutions, followed by drying and calcination in air at 5008C for 12 h. Finally, the obtained oxidized samples were recalcined in air at 8008C for 12 h. The contents of K and Mo components and the BET surface areas of the oxidized K±MoO3/ Al2O3 samples are presented in Table 1. With increased Mo loading, the BET surface area decreased linearly. The oxidized samples were sul®ded in a ¯ow of H2S/H2 (1:3 mol ratio, 40 ml/min) for 3 h at 4008C. The samples were kept in H2 atmosphere until the synthesis experiment was performed to prevent reoxidation by air. 2.2. Activity for mixed alcohols synthesis Activity of the sul®ded catalysts for mixed alcohols synthesis was measured by using a ®xed-bed reactor with an on-line gas chromatograph. The inner diameter of the reactor was 9 mm. 1 ml sul®ded sample was used for each experiment. The feed gas was composed of CO 31%, H2 66% and argon 3%. The liquid products were collected in an on-line trap, which was ®lled with distilled water and kept at 08C. The contents of CO and CO2 in the ef¯uent gas were detected by TCD with 2 m active carbon column. Argon was used as internal standard for quantitative calculation. The contents of hydrocarbons in the ef¯uent gas and the compositions of the liquid products were detected by FID with 2 m Porapak Q column. An aqueous solution of mixed alcohols made of linear C1±C6 alcohols, with a total content of 1.0% (wt ratio) alcohol in the solution, was used as reference for the analysis of mixed alcohols in the product solution.
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The reaction needed a long time to reach a steady state. The data shown in this paper were obtained after the reaction was performed for more than 20 h. 2.3. X-ray diffraction (XRD) XRD patterns were obtained using a D/MAX-gA rotatory target diffractometer with Cu K radiation. Each sample was ground into ®ne powders and packed in a thin sample holder for the measurement. 2.4. Laser Raman spectroscopy (LRS) Raman spectra of the oxidized and sul®ded K± MoO3/Al2O3 samples were run on a Spex-1403 spectrometer with the 488.0 nm line from a Spectra-Physics-2020 argon laser. The intensity of the laser beam was 150 mW for the oxidized sample and 200 mW for the sul®ded samples. The spectrum slit width was 3.5 cm and the spectrometer resolution was 2 cmÿ1. The samples were pressed into thin pellets for the measurements. 2.5. X-ray photoelectron spectroscopy (XPS) Spectra of the sul®ded K±MoO3/Al2O3 samples were recorded on an ESCA LAB MK-II spectrometer using an Al K X-ray source (1486.6 eV). The sample chamber was evacuated to around 410ÿ7 Pa. The Al2p peak (74.5 eV) from the sample was used as internal standard for binding energy calibration. The composition of different components on the surface of the sul®ded samples were calculated from the peak areas with Wagner's sensitivity factors. 2.6. Ammonia saturation and temperature programmed desorption (TPD) The surface acidity of the g-Al2O3 support and the oxidized samples was investigated with ammonia saturation and TPD. For each experiment, 0.1 g of the sample was packed in quartz cell. The sample was degassed at 5008C for 0.5 h. The cell was evacuated to 10ÿ2 Torr for 0.5 h and ®nally cooled to 1008C. The isothermal curve of the ammonia adsorption was measured at 1008C with ammonia pressure up to about 100 Torr. Then the cell was evacuated to 10ÿ2 Torr for 5 min; thus the weakly adsorbed ammonia desorbed.
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Then a second isothermal curve of ammonia adsorption up to weak acid sites was obtained. Finally, the ammonia in the cell was evacuated and TPD was conducted with temperature raised up to 5008C at a rate of 108C/min. The desorption species from the catalyst surface were carried to mass spectrometer in a ¯ow of He and analyzed continuously at an interval of 6 s. The acid amount of the catalyst sample was calculated according to the two thermal curves of ammonia adsorption. The acid strength was evaluated by the TPD trace of NH3. 2.7. Ethanol decomposition The properties of both the oxidized and sul®ded K± MoO3/Al2O3 samples for ethanol decomposition were tested in a U-tube reactor. 100 mg of the sample was used for each experiment. All the samples were ®rst activated by being pretreated at 3008C for 2 h in ¯owing argon (20 ml/min). The reactant stream consisted of 20 ml/min of argon bubbled through ethanol liquid at room temperature. The reactivity tests were conducted at atmospheric pressure between temperatures of 1508C and 3008C. Only the data of the reaction which occurred at 3008C were reported in this paper. The ethanol±argon stream was allowed to ¯ow through the catalyst bed for a period of 30 min before being injected into a gas chromatography for analysis. The gas chromatography had a 2 m Porapak Q column maintained at 1508C for the separation of the reaction products and FID for the detection. 3. Results 3.1. Activity for mixed alcohols synthesis Figs. 1 and 2 present the activity results over sul®ded K±Mo/Al2O3 catalysts with MoO3/Al2O3 weight ratio increased from 0.05 to 0.45 (K/Mo atomic ratio kept at 0.8) for mixed alcohols synthesis. For the sample of MoO3/Al2O30.05, the yields of hydrocarbons were high and those of mixed alcohols were remarkably low, the total CO conversion was 18% and the selectivity to mixed alcohols was only 3.0%. With Mo loading increased from MoO3/Al2O30.05 to 0.25, the yields of hydrocarbons decreased and the yields of mixed alcohols increased gradually. Over the
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Fig. 1. Effect of Mo loading of K±MoO3/g-Al2O3 catalyst on CO conversion and the yields of CO2 and hydrocarbons from syngas. Reaction conditions: 3208C, 5.0 MPa, 6000 hÿ1, and H2/CO2.0.
Fig. 2. Effect of Mo loading of K±MoO3/g-Al2O3 catalyst on the yields of mixed alcohols from syngas. Reaction conditions: 3208C, 5.0 MPa, 6000 hÿ1, and H2/CO2.0.
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Table 2 Influence of Mo loading on the distribution of the reaction products Sample (MoO3/Al2O3)
C1 H=C 2H
C1 OH=C 2 OH
2-C3OH/1-C3OH
i-C4OH/n-C4OH
0.05 0.15 0.25 0.35 0.45
0.66 0.87 2.01 2.44 2.75
0.75 0.74 0.62 0.48 0.46
0.03 0.03 0.05 0.09 0.06
0.56 0.73 1.27 1.38 1.33
sample of MoO3/Al2O30.25, the total CO conversion decreased to about 11% but the selectivity to mixed alcohols was enhanced abruptly to 50%. With Mo loading increased further into MoO3/Al2O30.45, the yields of hydrocarbons increased parallel to those of mixed alcohols, and the selectivity to mixed alcohols was kept at about 50%. With increased Mo loading, the content of methane in the hydrocarbons increased but the content of the methanol in the mixed alcohols decreased. Table 2 shows the changes of the distribution of mixed alcohols in the product. For MoO3/Al2O30.05 sample, C1 H=C 2 H was 0.66 and C1 OH=C2 OH was 0.75. While MoO3/Al2O3 increased to 0.45, C1 H=C 2 H rose
to 2.75 and C1 OH=C 2 OH decreased to 0.46. The ratio of 2-C3OH/1-C3OH was less than 0.1, so 1-C3OH predominated in C3-alcohols. But in C4-alcohols, the content of i-C4OH was high. More interestingly, with Mo loading increased, the content of i-C4OH in C4alcohols increased obviously. Molar ratio of i-C4OH/ n-C4OH was only 0.56 for the sample of MoO3/ Al2O30.05, but it went up to 1.33 for the sample of MoO3/Al2O30.45. 3.2. LRS results Fig. 3 presents the Raman spectra for the oxidized K±MoO3/Al2O3 samples. Raman spectra could deter-
Fig. 3. Raman spectra of the oxidized K±MoO3/g-Al2O3 samples with different Mo loading.
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mine Mo species on the surface of the catalyst within a depth of about 2 nm. Strong bands at 941, 896, 360 and 219 cmÿ1 were the characteristics of octahedral molybdenum species [10±12]. Bands at 932, 352 and 220 cmÿ1 were assigned to some tetrahedral Mo species containing potassium component [6]. Small bands at 1002 and 378 cmÿ1 demonstrated the formation of a little Al2(MoO4)3 [6], a species resulting from the interaction of the molybdenum with the support. Raman spectra of the different Mo loading samples were similar, so the same species existed on the surface of these oxidized samples. With increased Mo loading, Raman bands became stronger, this was due to more Mo species on the catalyst surface. Fig. 4 presents the Raman spectra of the sul®ded K±MoO3/Al2O3 samples with different Mo loadings. Two bands at 378 and 406 cmÿ1 were pronounced for the sample of MoO3/Al2O30.05. These two bands were the characteristics of MoS2 crystalline indicating that MoS2-like species were formed on the surface of the catalyst. For the sample of MoO3/ Al2O30.25, the Raman spectrum was the same as that for the sample of MoO3/Al2O30.05 but the bands due to MoS2 crystallites became a little larger.
This suggested that a similar MoS2 species was formed on this catalyst surface. Besides MoS2 crystallites, no other Mo species were observed by Raman spectra for these two samples. With Mo loading increased to MoO3/Al2O30.45, the band at 378 cmÿ1 became stronger than that at 406 cmÿ1, which was different from that for the crystalline of MoS2. So, the band at 378 cmÿ1 may contain the contribution from other Mo species; this was supported by the appearance of a strong band at 940 cmÿ1, which indicated that part of the oxidized Mo species still existed after sul®dation. The band at 406 cmÿ1 was obviously weaker than that for low Mo loading sample, which demonstrated that the MoS2 crystallite formed on the sample of MoO3/ Al2O30.45 became poorer. 3.3. XRD results XRD patterns for the oxidized samples of MoO3/ Al2O30.05 and 0.15 were identical to that for amorphous g-Al2O3 support. For the sample of MoO3/ Al2O30.25, only much weak diffraction patterns, besides those for the support, were observed. Mo
Fig. 4. Raman spectra of the sulfided K±MoO3/g-Al2O3 samples with different Mo loading.
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Fig. 5. XRD patterns of the oxidized and the sulfided K±MoO3/g-Al2O3 samples with a Mo loading of MoO3/Al2O30.45 (wt/wt).
species on these oxidized samples with low Mo loading were possibly in amorphous state and the crystallites were too small to give XRD signals. For the sample of MoO3/Al2O30.45, new diffraction patterns were clearly observed and the intensities increased as shown in Fig. 5, which demonstrated that the crystallites grew larger with increased Mo loading. To distinguish the XRD patterns of K±Mo±O species from that of the possible species formed by the interaction of the Al2O3 support with K and Mo components, one unsupported oxidized K±Mo sample was prepared by mixing KCl and ammonia heptamolybdate compounds (K/Mo0.8), followed by calcination at 5008C and then recalcination at 8008C. This unsupported sample was detected by XRD, and the same diffraction patterns as that of the supported sample of MoO3/Al2O30.45 (except for those assigned to g-Al2O3 support) were observed. So for g-Al2O3 supported sample, all of the XRD patterns were surely assigned to complex K±Mo±O species. They included K0.85Mo6O17, K2Mo4O13, K2Mo7O22, K2Mo3O10, K2MoO4 and K6Mo2O9 phases. These phases were formed by K±Mo interaction during calcination at 8008C [9].
For the sul®ded samples of MoO3/Al2O30.05 and 0.15, only XRD patterns which contributed to g-Al2O3 support were observed. With Mo loading increased to MoO3/Al2O30.25, small diffraction patterns due to MoS2 compound appeared, and those weak patterns assigned to K±Mo±O species disappeared at the same time. For the sample of MoO3/Al2O30.45, new diffraction patterns different from those for oxidized sample were observed; these were assigned to MoS2 species, K±Mo±O±S species such as K2MoO2S2, K2MoOS3, and K±Mo±S species such as K2MoS4, respectively, which indicated that the aggregated K±Mo±O species on this high loading sample were only partly sul®ded and reduced during the sul®dation. 3.4. XPS results Sul®ded samples with different Mo loading values were analyzed by XPS. The binding energies of the detected components indicated that Al, K and O components existed with valences of 3, 1 and 2ÿ, respectively. Mo and S components existed as a mixture of complex species with valences of 4, 5 and 6, and valences of 2ÿ, 0 and 6, respectively
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Fig. 6. XPS spectra of the Mo3d region of the sulfided K±MoO3/g-Al2O3 catalysts with (a) MoO3/Al2O30.05, (b) MoO3/Al2O30.25, and (c) MoO3/Al2O30.45.
[13±15]. Fig. 6 shows the Mo3d spectra of the sul®ded K±MoO3/Al2O3 catalysts. For the sample of MoO3/ Al2O30.05, about 70% of total Mo components was reduced to Mo4 species. For the sample of MoO3/ Al2O30.25, above 80% of total Mo components was reduced to Mo4 species. So, the percentage of unreduced Mo species decreased obviously. While Mo loading further increased up to MoO3/Al2O30.45, only 60% of total Mo component was reduced to Mo4 species. More of the Mo species was unreduced on this sample, which was in accordance with the results from LRS and XRD measurements. XPS results demonstrated that the content of Cl component on the catalyst surface was low, which suggested that Cl component was removed during the catalyst preparation [7]. The relative compositions of different components on the surface of the sul®ded samples were calculated according to XPS data. The results expressed as
atomic ratios of the different elements to total Mo component are shown in Table 3. These indicated that on the surface of these catalysts, S/Mo atomic ratios were higher than 2, the S/Mo molar ratio in MoS2. So the sulfur component was obviously rich on the catalyst surface. But S2ÿ/Mo atomic ratios were obviously lower than 2.0 for the samples of MoO3/ Al2O30.05 and 0.45, which indicated that sul®dation process was not as adequate for these two samples as for the sample of MoO3/Al2O30.25. K/Mo atomic ratio on the surface of the sample of MoO3/ Al2O30.05 was near to the bulk molar ratio, but the K component became rich on samples with higher Mo loading. For the sample of MoO3/Al2O30.05, Al/Mo atomic ratio was 19 and O/Mo atomic ratio was 28. But on the sample of MoO3/Al2O30.45, Al/Mo atomic ratio decreased to 2.8 and the O/Mo atomic ratio decreased to 6.6, respectively. With increased Mo loading, Al/Mo and O/Mo atomic ratios decreased
Table 3 Atomic concentrations of the sulfided K±MoO3/Al2O3 catalysts obtained from XPS Sample (MoO3/Al2O3)
Total Mo
O
Al
K
S
Mo4
Mo5Mo6
S2ÿ
0.05 0.25 0.45
1.0 1.0 1.0
28.5 7.5 6.6
19.3 4.2 2.8
0.9 1.1 1.6
2.4 2.8 3.9
0.7 0.8 0.6
0.3 0.2 0.4
1.3 1.9 1.4
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gradually; this was caused by the covering of an increased amount of Mo species. 3.5. Ammonia saturation and TPD g-Al2O3 was a support with strong acidity, which would be changed after loading with K and Mo compounds. It was reported that the acidity of the catalyst was strongly correlated to the dehydration activity of alcohol [16]. Investigating the acidity of the K±MoO3/Al2O3 catalysts may be essential to understand the changes of the reaction tendency in CO hydrogenation. Ammonia saturation data and TPD spectra for the g-Al2O3 support and some oxidized K±MoO3/Al2O3 samples with different Mo loading are presented in Table 4 and Fig. 7. Strong ammonia adsorption shown in Table 4 referred to the adsorbed ammonia which could not be desorbed at 1008C. The results in Table 4 indicated that the acid amount of sites interacting with ammonia was high for the g-Al2O3 support, and decreased markedly upon the addition of K and Mo components. For the sample of MoO3/Al2O30.05, the acid amount were about 1/4 of that for the support. For the samples of MoO3/Al2O30.25 and 0.45, the acid amount further decreased to about 1/25 of that for
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Table 4 Ammonia saturation measurements reflecting surface acid amounts Sample (MoO3/Al2O3)
Strong NH3 adsorption (mmol/g)
Total NH3 adsorption (mmol/g)
0.00 0.05 0.25 0.45
494 131 21 18
912 401 92 66
the g-Al2O3 support. These results could be related to the fact that the K and Mo species interacted with acid sites on the support to suppress the acidity of the support. With increased Mo loading, more acid sites were suppressed by the K and Mo compounds. The TPD patterns in Fig. 7 showed that for g-Al2O3 support, the desorption peak of adsorbed ammonia was at 1808C and the desorption amount stayed high up to about 4008C. But for the sample of MoO3/ Al2O30.05, the desorption peak shifted to 1608C and the desorption amount decreased to a much lower level at the temperature of 2608C. For the sample of MoO3/Al2O30.25 and 0.45, the desorption peak shifted to about 1508C and the peak height became very low. These results indicated that K±Mo species
Fig. 7. Ammonia TPD profiles of g-Al2O3 support and the oxidized K±MoO3/g-Al2O3 samples with different Mo loading.
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Fig. 8. Activity for ethanol decomposition over the oxidized K±MoO3/g-Al2O3 samples with different Mo loading.
®rst interacted with strong acid sites and then with weak acid sites. 3.6. Ethanol decomposition To illustrate the in¯uence of a sample's acidity on its property for mixed alcohols synthesis, g-Al2O3 support and some oxidized K±MoO3/g-Al2O3 samples were chosen to be tested for ethanol decomposition. Fig. 8 presents the test results. The main products of the reaction were ethene and acetaldehyde. The ethene was formed from ethanol dehydration and the acetaldehyde from ethanol dehydrogenation. It was reported that the acidic sites on g-Al2O3 support were the active sites for ethanol dehydration and the basic sites were the active sites for ethanol dehydrogenation [17]. In the catalytic process of mixed alcohols synthesis from syngas, no acetaldehyde was detected, which demonstrated that alcohol dehydrogenation did not occur under the reaction conditions. Our experimental results indicated that the activity for both the ethanol dehydration and the ethanol dehydrogenation was high over the g-Al2O3 support and decreased over K±Mo based catalysts. For the sample of MoO3/Al2O30.05, the STY of ethene was about 50% of the STY for the g-Al2O3 support.
For the sample of MoO3/Al2O30.25, the STY of ethene decreased further to about 10%. But for the sample of MoO3/Al2O30.45, the activity for ethanol dehydration was near that for the sample of MoO3/ Al2O30.25. With increased Mo loading, the activity for ethanol dehydration showed a parallel change to sample's acidity. The experimental results also showed that for these oxidized K±MoO3/Al2O3 samples, activity for ethanol dehydrogenation changed in a similar tendency to the activity for ethanol dehydration. The test results of ethanol decomposition over the sul®ded samples are shown in Fig. 9. It is found that the activity for ethanol dehydration over the sul®ded sample was near to that over the oxidized sample with the same Mo loading, which indicated that the sample's acidity did not show obvious changes during the sul®dation. But the activity for ethanol dehydrogenation over the sample of MoO3/Al2O30.05 enhanced sharply after sul®dation, which became even higher than that over the g-Al2O3 support. This result demonstrated that the existence of some potassium component on the g-Al2O3 surface gave a rise in the basic sites after sul®dation. But for higher Mo loading samples, the activity for ethanol dehydrogenation was unchanged during sul®dation.
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Fig. 9. Activity for ethanol decomposition over the sulfided K±MoO3/g-Al2O3 samples with different Mo loading.
4. Discussion 4.1. Distribution of the mixed alcohols in the product Mixed alcohols in the product were mainly composed of linear C1±C4OH and a small amount of i-C4OH. The content of a linear alcohol in the total alcohols decreased if its carbon number increased. The content of i-C4OH was near to that of n-C4OH. Compared to the linear 1-C3OH, the amount of 2C3OH was quite low. The distribution of the mixed alcohols in the products here was similar to that on the unsupported MoS2-based catalyst and Co-based catalyst [18,19], but remarkably different from that over Cu±Zn based catalyst, where branched alcohols were prevailing [20]. The reaction for linear mixed alcohols synthesis over Co-based alcohols has been proved to occur by the mechanism of carbene polymerization scheme of Fischer±Tropsch (FT) process, since the product distribution obeyed the Anderson±Schulz±Flory distribution [18]. It is suggested that CO insertion into the adsorbed alkyl-containing species produced acyl intermediates [21,22]. The reaction over Cu±Zn based
catalyst which resulted in an abundance of branched alcohols was proposed to proceed by a condensation mechanism, where a higher alcohol was 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 [23,24]. MoS2-based catalyst has been reported to be FT type catalyst and its product was mainly composed of linear alcohols. The composition of the linear mixed alcohols was similar to that of Co-based catalyst [25]. The existence of a small amount of the branched alcohol (i-C4OH) in the product indicated that over MoS2based catalyst, to some extent, condensation reaction of the mixed alcohols happened [4]. As exhibited in Table 2, the molar ratio of i-C4OH/n-C4OH was small for the sample with low Mo loading and increased with enhanced Mo loading. This demonstrated that the reaction over the catalyst with low Mo loading was mainly an FT process to produce linear alcohols. The activity of the alcohol condensation was promoted with increased Mo loading. It should be noticed that the decrease of the content of C1OH in the mixed alcohols with increased Mo loading may be caused by the alcohols condensation reaction where C1OH was
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consumed. The reason for increased Mo loading which changed the condensation activity was not clear until now; in literature it has been suggested that the catalyst acidity in¯uenced the condensation activity to produce branched hydrocarbons over ZrO2-based catalyst [26]. 4.2. Dispersion state and activity Structural studies by LRS and XRD demonstrated that for oxidized samples, the same K±Mo±O species was formed. For sul®ded catalysts, MoS2 crystallites were formed as the main phase. One of the observed differences about these sul®ded catalysts was that the dispersion decreased with increased Mo loading. Activity results demonstrated that the selectivity to mixed alcohols increased from 3% for the sample of MoO3/Al2O30.05 to 50% for the sample of MoO3/ Al2O30.25. The aggregation of Mo species with increased Mo loading was closely related to the sharp increase of the mixed alcohols selectivity over these samples. However, while Mo loading increased from MoO3/Al2O30.25 to 0.45, the selectivity to mixed alcohols leveled off, even if Mo species aggregated further. Over Mo-based catalysts, it has been reported that potassium loading, support property and calcination temperature of the oxidized precursor showed marked effect on the activity for mixed alcohols synthesis [7± 9,27]. These results suggested that a signi®cant dispersion of the active phase was necessary to obtain high activity, but while the dispersion decreased, the selectivity to mixed alcohols increased. A good correlation between the selectivity to mixed alcohols and the dispersion state was observed. But the results in this work demonstrated that the aggregation of the K and Mo species should not be the immediate cause for the increase of the selectivity to mixed alcohols. 4.3. Reducibility and activity XPS, XRD and LRS results indicated that a part of the K±Mo species was not sul®ded and reduced during the sul®dation. It was reported that the interactions of Mo species with support, with potassium species and the dispersion state controlled the reducibility and the sul®dation of the Mo species [7±9,28]. For our cata-
lysts, XPS measurements indicated that the percentage of unreduced Mo species decreased with Mo loading increased from MoO3/Al2O30.05 to 0.25, which should be attributed to the fact that the interaction between Mo species and g-Al2O3 support was strong on the low Mo-loaded sample and became weak on the high Mo-loaded sample. With Mo loading increased to MoO3/Al2O30.45, percentage of unreduced Mo species on the catalyst surface was enhanced signi®cantly, which should be due to the sintering of the Mo species. For the samples with Mo loading increased from MoO3/Al2O30.25 to 0.45, the selectivity to mixed alcohols was kept unchanged under the same reaction conditions; this demonstrated that the content of unreduced Mo species on the surface hardly showed any relationship to the reaction selectivity. With Mo loading increased from MoO3/Al2O30.05 to 0.25, the selectivity to mixed alcohols increased sharply, but the content of unreduced Mo species on the surface decreased obviously. These results were different from the reported works which declared that the existence of some unreduced Mo species favored mixed alcohols formation [29,30]. For the catalysts with Mo loading increased from MoO3/Al2O30.25 to 0.45, the unchanged selectivity to mixed alcohols suggested that the surface circumstances on these catalysts were similar for the synthesis of mixed alcohols. The enhanced total CO conversion could be attributed to the increase of the amount of Mo species on the catalyst surface. Comparisons of the total CO conversion and the contents of the different Mo species between the two samples of MoO3/Al2O30.25 and 0.45 were made. The results were shown in Table 5. For these two samples, the ratio of total CO conversions should be near the ratio of the active species, so the active species was closely correlated with the reduced Mo4 species. Table 5 Comparison of the CO conversion and different Mo species between the samples of MoO3/Al2O30.25 and 0.45 Item
Ratio
Act0.45/Act0.25 (mol CO converted) (Total Mo)0.45/(Total Mo)0.25
1.58 2.04
4 Mo4 0:45 =Mo0:25
1.53
(Mo5Mo6)0.45/(Mo5Mo6)0.25
4.08
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4.4. Acidity and activity With Mo loading increased up to MoO3/ Al2O30.25, the acid amount connected to ammonia adsorption decreased sharply. The acid amount for the sample of MoO3/Al2O30.25 also became very low, compared to that of g-Al2O3 support. But the acid amount for the sample of MoO3/Al2O30.45 was near that for the sample of MoO3/Al2O30.25. According to literature, Mo species on g-Al2O3 support surface would interact primarily with nonacidic hydroxyl groups and the acid amount was still signi®cant [16,31,32]. On potassium promoted g-Al2O3 and TiO2 samples, potassium interacted with acidic sites and suppressed the acidity of these samples effectively [16,33]. For our catalysts, the decrease of acidity should be attributed to the interaction between the potassium component and the support. With increased Mo loading, the content of K component enhanced as well (K/Mo0.8) and the acidity of the catalyst decreased further. On the sample of MoO3/ Al2O30.25, potassium species interacted with most of the acidic sites and the acidity became rather weak. With Mo loading increased further, the acidity of the sample was almost unchanged. It was shown that while the sample possessed a strong acidity, the selectivity to mixed alcohols was low; and while the sample showed a weak acidity, the selectivity to mixed alcohols became high. The acidic sites of the catalyst sample may play an important role in the reaction of mixed alcohols synthesis. According to literature [21,22,25], acyl-contained intermediate was formed on the catalyst during the reaction, which could be hydrogenated to form mixed alcohols or hydrogenated and dehydrated immediately to form hydrocarbons. Acidic sites on the support were the active sites for alcohols' dehydration [16,34,35]. On the sample of MoO3/ Al2O30.05, the strong acidic sites which had not been covered by K±Mo species may be the principal reason for its high hydrocarbon selectivity. With Mo loading increased to MoO3/Al2O30.25, most of the acidic sites was covered, so the selectivity to mixed alcohols rose sharply. The results from ethanol decomposition strongly supported this opinion. It was reported that the selectivity to mixed alcohols could be correlated to the in¯uence of structural
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changes such as dispersion state, content of unreduced Mo species, the re-oxidation of Mo species and utilizing different potassium salts [7±9,36,37]. In fact, most of these in¯uences could be related to the acidity changes of the catalysts. We have reported that the increase of potassium loading over K±MoO3/Al2O3 catalyst resulted in the enhancement of the selectivity to mixed alcohols. It was known that the existence of a potassium component would make the acidity of the catalyst weaken. So the activity results were in agreement with the consideration that the acidity controlled the selectivity of the reaction. The increase of potassium loading over the K±MoO3/Al2O3 catalysts resulted in the aggregation of Mo species and the latter became dif®cult to be reduced to Mo4 species [7]. The aggregation of Mo species and the enhancement of unreduced Mo species may be the surface factors in increasing the selectivity to mixed alcohols. Avila et al. reported [8] that for K±Mo based catalyst on different supports such as ZnCr2O4, ZrO2, Cr2O3 and ZnO, the selectivity to mixed alcohols was rather different and increased in the order: ZrO2
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5. Conclusions The activity results of a series of sul®ded K±MoO3/ Al2O3 catalysts indicated that with Mo loading increased from MoO3/Al2O30.05 to 0.25, the total CO conversion to mixed alcohols and hydrocarbons decreased, but the selectivity to mixed alcohols enhanced sharply from 3% to 50%. With Mo loading increased from MoO3/Al2O30.25 to 0.45, the CO conversion was enhanced but the selectivity to mixed alcohols leveled off. Over these catalysts, FT synthesis to linear alcohols and the condensation reaction of a part of low alcohols to form i-C4OH occurred at the same time. With increased Mo loading, activity of the alcohols condensation became higher. Structural information from LRS, XRD, XPS, ammonia adsorption, TPD measurements and ethanol decomposition demonstrated that on the oxidized samples with Mo loading increased, same K±Mo±O species was formed but the dispersion decreased. After sul®dation the content of unreduced Mo species was high for the samples of MoO3/Al2O30.05 and 0.45, and was low for the sample of MoO3/Al2O30.25. Catalyst acidity and its activity for ethanol decomposition of the K±MoO3/Al2O3 decreased sharply with Mo loading increased up to MoO3/Al2O30.25, and leveled off with Mo loading increased further up to MoO3/Al2O30.45. Activity results and structural studies indicated that the dispersion state of the active species and the content of unreduced Mo species in¯uenced the total CO conversion but the acidity of the catalyst controlled the selectivity to mixed alcohols. References [1] G.J. Quarderer, K.A. Cochran, Eur. Patent, 119 609, 1984. [2] R.R. Stevens, Eur. Patent, 172 431, 1986. [3] Y. Xie, B.M. Naasz, G.A. Somorjai, Appl. Catal. 27 (1986) 233. [4] C.B. Murchison, M.M. Conway, R.R. Stevens, G.J. Quarderer, in: M.J. Phillips, M. Ternan (Eds.), Proceedings of the Ninth International Congress on Catalysis, vol. 2, Calgary, 1988, Chemical Institute of Canada, Ottawa, 1988, p. 626. [5] T. Tatsumi, A. Muramatsu, H. Tominaga, Appl. Catal. 34 (1987) 77. [6] J. Zhang, Y. Wang, L. Chang, Appl. Catal. A 126 (1995) 205.
[7] M. Jiang, G.Z. Bian, Y.L. Fu, J. Catal. 146 (1994) 144. [8] Y. Avila, C. Kappenstein, S. Pronier, J. Barrault, Appl. Catal. A 132 (1995) 97. [9] Y. Fu, K. Fujimoto, P. Lin, K. Omata, Y. Yu, Appl. Catal. A 126 (1995) 273. [10] C.P. Cheng, G.L. Schrader, J. Catal. 60 (1979) 276. [11] L. Wang, W.K. Hall, J. Catal. 66 (1980) 251. [12] D.S. Kim, K. Segawa, T. Soeya, I.E. Wachs, J. Catal. 136 (1992) 539. [13] A. Mull, R. Joster, W. Jaegermann, Inorg. Chim. Acta 41 (1980) 259. [14] P.A. Spevack, S. McIntyre, Appl. Catal. 64 (1990) 191. [15] P.A. McIntyre, T.C. Chan, P.A. Spevack, Appl. Catal. 63 (1990) 391. [16] E.E. Lowenthal, S. Schwarz, H.C. Foley, J. Catal. 156 (1995) 96. [17] T. Seiyama, M.M. Huang, Metal Oxide and their Catalytic Effects, The University of Science and Technology of China Publishing, 1991, p. 116. [18] K. Fujimoto, T. Oba, Appl. Catal. 13 (1985) 289. [19] P. Courty, P. Chaumette, Energy Prog. 7 (1987) 23. [20] C.E. Hofstadt, M. Schneider, O. Bock, K. Kochloefl, Stud. Surf. Sci. Catal. 16 (1983) 709. [21] A. Kiennemann, S. Boujana, C. Diagne, P. Chaumette, Stud. Surf. Sci. Catal. 75 (1993) 1479. [22] A. Kiennemann, S. Boujana, C. Diagne, P. Courty, P. Chaumette, Stud. Surf. Sci. Catal. 61 (1991) 243. [23] S.C. Tseng, N.B. Jackson, J.G. Ekerdt, J. Catal. 109 (1988) 284. [24] T.J. Mazanec, J. Catal. 98 (1986) 115. [25] J.G. Santiesteban, C.E. Bogdan, R.G. Herman, K. Klier, in: M.J. Phillips, M. Ternan (Eds.), Proceedings of the Ninth International Congress on Catalysis, vol. 2, Calgary, 1988, Chemical Institute of Canada, Ottawa, 1988, p. 561. [26] N.B. Jackson, J.G. Ekerdt, J. Catal. 124 (1990) 46. [27] K. Klier, R.G. Herman, J.G. Nunan, K.J. Smith, C.E. Bogdan, C.W. Young, J.G. Santiesteban, in: D.M. Bibby, C.D. Chang, R.F. Howe, S.Y. Yurchak (Eds.), 2-Methane Conversion, Elsevier, Amsterdam, 1988, p. 109. [28] Y. Okamoto, H. Tomioka, Y. Katoh, T. Imanaka, S. Teranishi, J. Phys. Chem. 84 (1980) 1833. [29] A. Muramatsu, T. Tatsumi, H. Tominaga, J. Phys. Chem. 96 (1992) 1334. [30] J.M. Driessen, E.K. Poels, J.P. Hindermann, V. Ponec, J. Catal. 82 (1983) 26. [31] A. Brenner, R.L. Burwell, J. Catal. 52 (1978) 353. [32] R.L. Burwell, A. Brenner, J. Mol. Catal. 1 (1975) 77. [33] C. Martin, I. Martin, C. Moral, V. Rives, J. Catal. 146 (1994) 415. [34] F.M. Bautista, B. Delmon, Appl. Catal. 130 (1995) 47. [35] S. Srinivasan, C.R. Narayanan, A. Biaglow, R. Gorte, A.K. Datye, Appl. Catal. 132 (1995) 271. [36] H.C. Woo, I.S. Nam, J.S. Lee, J.S. Chung, K.H. Lee, Y.G. Kim, J. Catal. 138 (1992) 525. [37] J.S. Lee, S. Kim, Y.G. Kim, Top. Catal. 2 (1995) 127.
High temperature calcined K±MoO3/g-Al2O3 catalysts for mixed alcohols synthesis from syngas: Effects of Mo loadings Guo-zhu Biana, Li Fanb,*, Yi-lu Fua, Kaoru Fujimotob a
Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan
b
Received 15 October 1997; received in revised form 14 January 1998; accepted 26 January 1998
Abstract One series of oxidized K±MoO3/g-Al2O3 samples with different Mo loadings (MoO3/Al2O3 (wt ratio)0.05±0.45) was prepared by impregnating K and Mo compounds and successive calcination in air at 8008C. The oxidized samples were sul®ded and then utilized for mixed alcohols synthesis from syngas. The structural information from laser Raman spectroscopy (LRS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ammonia saturation, temperature programmed desorption (TPD) and ethanol decomposition were studied to elucidate the reaction properties. The results indicated that with Mo loading increased from MoO3/Al2O30.05 to 0.25, the total yields of mixed alcohols and hydrocarbons decreased, but the selectivity to mixed alcohols was enhanced sharply from 3% to 50%. With Mo loading increased from MoO3/Al2O30.25 to 0.45, the CO conversion was enhanced, but the selectivity to mixed alcohols leveled off. On these catalysts, Fischer±Tropsch (FT) synthesis to linear alcohols and the condensation reaction of low alcohols to form branched i-C4OH occurred at the same time. With increased Mo loading, activity of the alcohols condensation became high. Structural studies demonstrated that on oxidized samples with increased Mo loading the same K±Mo±O species was formed, but the dispersion of these K±Mo species decreased. The catalyst's acidity decreased remarkably with Mo loading up to MoO3/Al2O30.25, and stayed unchanged as Mo loading was further increased to MoO3/Al2O30.45. With increased Mo loading, the activity for ethanol dehydration changed parallel to the acidity. Results of the activity experiments for mixed alcohols' synthesis and the structural measurements indicated that the dispersion state of Mo species and the content of unreduced Mo species in¯uenced the total CO conversion, and that the acidity of the catalyst controlled the selectivity to mixed alcohols. # 1998 Elsevier Science B.V. All rights reserved. Keywords: K±MoO3/Al2O3 catalyst; Mo loading; Synthesis of mixed alcohols; Structure and acidity
1. Introduction Mo-based catalysts have been widely applied in many hydrotreating reactions due to their high activ*Corresponding author. Tel.: 0081 35689 0469; e-mail: [email protected] 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(98)00053-2
ities for the hydrodesulfurization, hydrodenitrogenation reaction and methane synthesis from CO hydrogenation. Doped with alkali compounds, molybdenum-based catalysts showed relatively high activities for mixed alcohols synthesis [1±3]. g-Al2O3 has been regarded as a superior support for Mo-based catalysts because Mo species could be well
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dispersed on it and therefore possessed high speci®c reaction activity. But g-Al2O3 supported Mo-based catalysts were reported to show quite low selectivity to mixed alcohols from syngas compared to active carbon, SiO2 or MgO supported and unsupported catalysts [4±6]. For supported Mo-based catalysts, it has been demonstrated that K±Mo interaction species which was formed on the oxidized sample were the precursors of the active sites for mixed alcohols synthesis; poor dispersion of the molybdenum species could suppress the yield of hydrocarbons and enhance the selectivity to mixed alcohols [7,8]. The fact that g-Al2O3-supported catalysts possessed low selectivity to mixed alcohols may be related to good dispersion of the Mo species on the support, but its mechanism is not clear until now. Calcination process of the oxidized precursors is important for the dispersion control of the molybdenum species. We have studied the in¯uence of recalcination at high temperature of an oxidized K±MoO3/ Al2O3 catalyst on the property for mixed alcohols' synthesis. It was found that while the oxidized sample was recalcined at 8008C, both the yields of mixed alcohols and the selectivity to mixed alcohols were sharply enhanced [9]. Structural studies indicated that after recalcination at high temperature of 8008C, the BET surface area of the catalyst samples decreased, and K±Mo interaction species were aggregated and became dif®cult to be reduced and sul®ded [9]. For further characterization of the sul®ded K± MoO3/Al2O3 catalyst for mixed alcohol synthesis, one series of samples with different Mo loading was investigated. All the oxidized samples were prepared through recalcination at 8008C. The property for mixed alcohols synthesis and the structure characterization are reported. The effect of the support and the reaction mechanism are discussed as well. 2. Experimental 2.1. Catalyst preparation The oxidized K±MoO3/Al2O3 catalysts were prepared by ®rst impregnating g-Al2O3 support with the aqueous solutions of KCl compound, followed by drying and calcination at 3008C for 1 h. The oxidized KCl/Al2O3 samples were impregnated with
Table 1 Mo content and BET surface area of the oxidized K±MoO3/gAl2O3 catalyst sample (K/Mo molar ratio0.8) Sample
MoO3/Al2O3 (wt%)
BET surface area (m2/g)
g-Al2O3 KCl±MoO3/Al2O3 KCl±MoO3/Al2O3 KCl±MoO3/Al2O3 KCl±MoO3/Al2O3 KCl±MoO3/Al2O3
Ð 5 15 25 35 45
270.0 91.7 27.8 21.4 10.7 3.7
(NH4)6Mo7O244H2O solutions, followed by drying and calcination in air at 5008C for 12 h. Finally, the obtained oxidized samples were recalcined in air at 8008C for 12 h. The contents of K and Mo components and the BET surface areas of the oxidized K±MoO3/ Al2O3 samples are presented in Table 1. With increased Mo loading, the BET surface area decreased linearly. The oxidized samples were sul®ded in a ¯ow of H2S/H2 (1:3 mol ratio, 40 ml/min) for 3 h at 4008C. The samples were kept in H2 atmosphere until the synthesis experiment was performed to prevent reoxidation by air. 2.2. Activity for mixed alcohols synthesis Activity of the sul®ded catalysts for mixed alcohols synthesis was measured by using a ®xed-bed reactor with an on-line gas chromatograph. The inner diameter of the reactor was 9 mm. 1 ml sul®ded sample was used for each experiment. The feed gas was composed of CO 31%, H2 66% and argon 3%. The liquid products were collected in an on-line trap, which was ®lled with distilled water and kept at 08C. The contents of CO and CO2 in the ef¯uent gas were detected by TCD with 2 m active carbon column. Argon was used as internal standard for quantitative calculation. The contents of hydrocarbons in the ef¯uent gas and the compositions of the liquid products were detected by FID with 2 m Porapak Q column. An aqueous solution of mixed alcohols made of linear C1±C6 alcohols, with a total content of 1.0% (wt ratio) alcohol in the solution, was used as reference for the analysis of mixed alcohols in the product solution.
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The reaction needed a long time to reach a steady state. The data shown in this paper were obtained after the reaction was performed for more than 20 h. 2.3. X-ray diffraction (XRD) XRD patterns were obtained using a D/MAX-gA rotatory target diffractometer with Cu K radiation. Each sample was ground into ®ne powders and packed in a thin sample holder for the measurement. 2.4. Laser Raman spectroscopy (LRS) Raman spectra of the oxidized and sul®ded K± MoO3/Al2O3 samples were run on a Spex-1403 spectrometer with the 488.0 nm line from a Spectra-Physics-2020 argon laser. The intensity of the laser beam was 150 mW for the oxidized sample and 200 mW for the sul®ded samples. The spectrum slit width was 3.5 cm and the spectrometer resolution was 2 cmÿ1. The samples were pressed into thin pellets for the measurements. 2.5. X-ray photoelectron spectroscopy (XPS) Spectra of the sul®ded K±MoO3/Al2O3 samples were recorded on an ESCA LAB MK-II spectrometer using an Al K X-ray source (1486.6 eV). The sample chamber was evacuated to around 410ÿ7 Pa. The Al2p peak (74.5 eV) from the sample was used as internal standard for binding energy calibration. The composition of different components on the surface of the sul®ded samples were calculated from the peak areas with Wagner's sensitivity factors. 2.6. Ammonia saturation and temperature programmed desorption (TPD) The surface acidity of the g-Al2O3 support and the oxidized samples was investigated with ammonia saturation and TPD. For each experiment, 0.1 g of the sample was packed in quartz cell. The sample was degassed at 5008C for 0.5 h. The cell was evacuated to 10ÿ2 Torr for 0.5 h and ®nally cooled to 1008C. The isothermal curve of the ammonia adsorption was measured at 1008C with ammonia pressure up to about 100 Torr. Then the cell was evacuated to 10ÿ2 Torr for 5 min; thus the weakly adsorbed ammonia desorbed.
257
Then a second isothermal curve of ammonia adsorption up to weak acid sites was obtained. Finally, the ammonia in the cell was evacuated and TPD was conducted with temperature raised up to 5008C at a rate of 108C/min. The desorption species from the catalyst surface were carried to mass spectrometer in a ¯ow of He and analyzed continuously at an interval of 6 s. The acid amount of the catalyst sample was calculated according to the two thermal curves of ammonia adsorption. The acid strength was evaluated by the TPD trace of NH3. 2.7. Ethanol decomposition The properties of both the oxidized and sul®ded K± MoO3/Al2O3 samples for ethanol decomposition were tested in a U-tube reactor. 100 mg of the sample was used for each experiment. All the samples were ®rst activated by being pretreated at 3008C for 2 h in ¯owing argon (20 ml/min). The reactant stream consisted of 20 ml/min of argon bubbled through ethanol liquid at room temperature. The reactivity tests were conducted at atmospheric pressure between temperatures of 1508C and 3008C. Only the data of the reaction which occurred at 3008C were reported in this paper. The ethanol±argon stream was allowed to ¯ow through the catalyst bed for a period of 30 min before being injected into a gas chromatography for analysis. The gas chromatography had a 2 m Porapak Q column maintained at 1508C for the separation of the reaction products and FID for the detection. 3. Results 3.1. Activity for mixed alcohols synthesis Figs. 1 and 2 present the activity results over sul®ded K±Mo/Al2O3 catalysts with MoO3/Al2O3 weight ratio increased from 0.05 to 0.45 (K/Mo atomic ratio kept at 0.8) for mixed alcohols synthesis. For the sample of MoO3/Al2O30.05, the yields of hydrocarbons were high and those of mixed alcohols were remarkably low, the total CO conversion was 18% and the selectivity to mixed alcohols was only 3.0%. With Mo loading increased from MoO3/Al2O30.05 to 0.25, the yields of hydrocarbons decreased and the yields of mixed alcohols increased gradually. Over the
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Fig. 1. Effect of Mo loading of K±MoO3/g-Al2O3 catalyst on CO conversion and the yields of CO2 and hydrocarbons from syngas. Reaction conditions: 3208C, 5.0 MPa, 6000 hÿ1, and H2/CO2.0.
Fig. 2. Effect of Mo loading of K±MoO3/g-Al2O3 catalyst on the yields of mixed alcohols from syngas. Reaction conditions: 3208C, 5.0 MPa, 6000 hÿ1, and H2/CO2.0.
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259
Table 2 Influence of Mo loading on the distribution of the reaction products Sample (MoO3/Al2O3)
C1 H=C 2H
C1 OH=C 2 OH
2-C3OH/1-C3OH
i-C4OH/n-C4OH
0.05 0.15 0.25 0.35 0.45
0.66 0.87 2.01 2.44 2.75
0.75 0.74 0.62 0.48 0.46
0.03 0.03 0.05 0.09 0.06
0.56 0.73 1.27 1.38 1.33
sample of MoO3/Al2O30.25, the total CO conversion decreased to about 11% but the selectivity to mixed alcohols was enhanced abruptly to 50%. With Mo loading increased further into MoO3/Al2O30.45, the yields of hydrocarbons increased parallel to those of mixed alcohols, and the selectivity to mixed alcohols was kept at about 50%. With increased Mo loading, the content of methane in the hydrocarbons increased but the content of the methanol in the mixed alcohols decreased. Table 2 shows the changes of the distribution of mixed alcohols in the product. For MoO3/Al2O30.05 sample, C1 H=C 2 H was 0.66 and C1 OH=C2 OH was 0.75. While MoO3/Al2O3 increased to 0.45, C1 H=C 2 H rose
to 2.75 and C1 OH=C 2 OH decreased to 0.46. The ratio of 2-C3OH/1-C3OH was less than 0.1, so 1-C3OH predominated in C3-alcohols. But in C4-alcohols, the content of i-C4OH was high. More interestingly, with Mo loading increased, the content of i-C4OH in C4alcohols increased obviously. Molar ratio of i-C4OH/ n-C4OH was only 0.56 for the sample of MoO3/ Al2O30.05, but it went up to 1.33 for the sample of MoO3/Al2O30.45. 3.2. LRS results Fig. 3 presents the Raman spectra for the oxidized K±MoO3/Al2O3 samples. Raman spectra could deter-
Fig. 3. Raman spectra of the oxidized K±MoO3/g-Al2O3 samples with different Mo loading.
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mine Mo species on the surface of the catalyst within a depth of about 2 nm. Strong bands at 941, 896, 360 and 219 cmÿ1 were the characteristics of octahedral molybdenum species [10±12]. Bands at 932, 352 and 220 cmÿ1 were assigned to some tetrahedral Mo species containing potassium component [6]. Small bands at 1002 and 378 cmÿ1 demonstrated the formation of a little Al2(MoO4)3 [6], a species resulting from the interaction of the molybdenum with the support. Raman spectra of the different Mo loading samples were similar, so the same species existed on the surface of these oxidized samples. With increased Mo loading, Raman bands became stronger, this was due to more Mo species on the catalyst surface. Fig. 4 presents the Raman spectra of the sul®ded K±MoO3/Al2O3 samples with different Mo loadings. Two bands at 378 and 406 cmÿ1 were pronounced for the sample of MoO3/Al2O30.05. These two bands were the characteristics of MoS2 crystalline indicating that MoS2-like species were formed on the surface of the catalyst. For the sample of MoO3/ Al2O30.25, the Raman spectrum was the same as that for the sample of MoO3/Al2O30.05 but the bands due to MoS2 crystallites became a little larger.
This suggested that a similar MoS2 species was formed on this catalyst surface. Besides MoS2 crystallites, no other Mo species were observed by Raman spectra for these two samples. With Mo loading increased to MoO3/Al2O30.45, the band at 378 cmÿ1 became stronger than that at 406 cmÿ1, which was different from that for the crystalline of MoS2. So, the band at 378 cmÿ1 may contain the contribution from other Mo species; this was supported by the appearance of a strong band at 940 cmÿ1, which indicated that part of the oxidized Mo species still existed after sul®dation. The band at 406 cmÿ1 was obviously weaker than that for low Mo loading sample, which demonstrated that the MoS2 crystallite formed on the sample of MoO3/ Al2O30.45 became poorer. 3.3. XRD results XRD patterns for the oxidized samples of MoO3/ Al2O30.05 and 0.15 were identical to that for amorphous g-Al2O3 support. For the sample of MoO3/ Al2O30.25, only much weak diffraction patterns, besides those for the support, were observed. Mo
Fig. 4. Raman spectra of the sulfided K±MoO3/g-Al2O3 samples with different Mo loading.
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261
Fig. 5. XRD patterns of the oxidized and the sulfided K±MoO3/g-Al2O3 samples with a Mo loading of MoO3/Al2O30.45 (wt/wt).
species on these oxidized samples with low Mo loading were possibly in amorphous state and the crystallites were too small to give XRD signals. For the sample of MoO3/Al2O30.45, new diffraction patterns were clearly observed and the intensities increased as shown in Fig. 5, which demonstrated that the crystallites grew larger with increased Mo loading. To distinguish the XRD patterns of K±Mo±O species from that of the possible species formed by the interaction of the Al2O3 support with K and Mo components, one unsupported oxidized K±Mo sample was prepared by mixing KCl and ammonia heptamolybdate compounds (K/Mo0.8), followed by calcination at 5008C and then recalcination at 8008C. This unsupported sample was detected by XRD, and the same diffraction patterns as that of the supported sample of MoO3/Al2O30.45 (except for those assigned to g-Al2O3 support) were observed. So for g-Al2O3 supported sample, all of the XRD patterns were surely assigned to complex K±Mo±O species. They included K0.85Mo6O17, K2Mo4O13, K2Mo7O22, K2Mo3O10, K2MoO4 and K6Mo2O9 phases. These phases were formed by K±Mo interaction during calcination at 8008C [9].
For the sul®ded samples of MoO3/Al2O30.05 and 0.15, only XRD patterns which contributed to g-Al2O3 support were observed. With Mo loading increased to MoO3/Al2O30.25, small diffraction patterns due to MoS2 compound appeared, and those weak patterns assigned to K±Mo±O species disappeared at the same time. For the sample of MoO3/Al2O30.45, new diffraction patterns different from those for oxidized sample were observed; these were assigned to MoS2 species, K±Mo±O±S species such as K2MoO2S2, K2MoOS3, and K±Mo±S species such as K2MoS4, respectively, which indicated that the aggregated K±Mo±O species on this high loading sample were only partly sul®ded and reduced during the sul®dation. 3.4. XPS results Sul®ded samples with different Mo loading values were analyzed by XPS. The binding energies of the detected components indicated that Al, K and O components existed with valences of 3, 1 and 2ÿ, respectively. Mo and S components existed as a mixture of complex species with valences of 4, 5 and 6, and valences of 2ÿ, 0 and 6, respectively
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Fig. 6. XPS spectra of the Mo3d region of the sulfided K±MoO3/g-Al2O3 catalysts with (a) MoO3/Al2O30.05, (b) MoO3/Al2O30.25, and (c) MoO3/Al2O30.45.
[13±15]. Fig. 6 shows the Mo3d spectra of the sul®ded K±MoO3/Al2O3 catalysts. For the sample of MoO3/ Al2O30.05, about 70% of total Mo components was reduced to Mo4 species. For the sample of MoO3/ Al2O30.25, above 80% of total Mo components was reduced to Mo4 species. So, the percentage of unreduced Mo species decreased obviously. While Mo loading further increased up to MoO3/Al2O30.45, only 60% of total Mo component was reduced to Mo4 species. More of the Mo species was unreduced on this sample, which was in accordance with the results from LRS and XRD measurements. XPS results demonstrated that the content of Cl component on the catalyst surface was low, which suggested that Cl component was removed during the catalyst preparation [7]. The relative compositions of different components on the surface of the sul®ded samples were calculated according to XPS data. The results expressed as
atomic ratios of the different elements to total Mo component are shown in Table 3. These indicated that on the surface of these catalysts, S/Mo atomic ratios were higher than 2, the S/Mo molar ratio in MoS2. So the sulfur component was obviously rich on the catalyst surface. But S2ÿ/Mo atomic ratios were obviously lower than 2.0 for the samples of MoO3/ Al2O30.05 and 0.45, which indicated that sul®dation process was not as adequate for these two samples as for the sample of MoO3/Al2O30.25. K/Mo atomic ratio on the surface of the sample of MoO3/ Al2O30.05 was near to the bulk molar ratio, but the K component became rich on samples with higher Mo loading. For the sample of MoO3/Al2O30.05, Al/Mo atomic ratio was 19 and O/Mo atomic ratio was 28. But on the sample of MoO3/Al2O30.45, Al/Mo atomic ratio decreased to 2.8 and the O/Mo atomic ratio decreased to 6.6, respectively. With increased Mo loading, Al/Mo and O/Mo atomic ratios decreased
Table 3 Atomic concentrations of the sulfided K±MoO3/Al2O3 catalysts obtained from XPS Sample (MoO3/Al2O3)
Total Mo
O
Al
K
S
Mo4
Mo5Mo6
S2ÿ
0.05 0.25 0.45
1.0 1.0 1.0
28.5 7.5 6.6
19.3 4.2 2.8
0.9 1.1 1.6
2.4 2.8 3.9
0.7 0.8 0.6
0.3 0.2 0.4
1.3 1.9 1.4
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gradually; this was caused by the covering of an increased amount of Mo species. 3.5. Ammonia saturation and TPD g-Al2O3 was a support with strong acidity, which would be changed after loading with K and Mo compounds. It was reported that the acidity of the catalyst was strongly correlated to the dehydration activity of alcohol [16]. Investigating the acidity of the K±MoO3/Al2O3 catalysts may be essential to understand the changes of the reaction tendency in CO hydrogenation. Ammonia saturation data and TPD spectra for the g-Al2O3 support and some oxidized K±MoO3/Al2O3 samples with different Mo loading are presented in Table 4 and Fig. 7. Strong ammonia adsorption shown in Table 4 referred to the adsorbed ammonia which could not be desorbed at 1008C. The results in Table 4 indicated that the acid amount of sites interacting with ammonia was high for the g-Al2O3 support, and decreased markedly upon the addition of K and Mo components. For the sample of MoO3/Al2O30.05, the acid amount were about 1/4 of that for the support. For the samples of MoO3/Al2O30.25 and 0.45, the acid amount further decreased to about 1/25 of that for
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Table 4 Ammonia saturation measurements reflecting surface acid amounts Sample (MoO3/Al2O3)
Strong NH3 adsorption (mmol/g)
Total NH3 adsorption (mmol/g)
0.00 0.05 0.25 0.45
494 131 21 18
912 401 92 66
the g-Al2O3 support. These results could be related to the fact that the K and Mo species interacted with acid sites on the support to suppress the acidity of the support. With increased Mo loading, more acid sites were suppressed by the K and Mo compounds. The TPD patterns in Fig. 7 showed that for g-Al2O3 support, the desorption peak of adsorbed ammonia was at 1808C and the desorption amount stayed high up to about 4008C. But for the sample of MoO3/ Al2O30.05, the desorption peak shifted to 1608C and the desorption amount decreased to a much lower level at the temperature of 2608C. For the sample of MoO3/Al2O30.25 and 0.45, the desorption peak shifted to about 1508C and the peak height became very low. These results indicated that K±Mo species
Fig. 7. Ammonia TPD profiles of g-Al2O3 support and the oxidized K±MoO3/g-Al2O3 samples with different Mo loading.
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Fig. 8. Activity for ethanol decomposition over the oxidized K±MoO3/g-Al2O3 samples with different Mo loading.
®rst interacted with strong acid sites and then with weak acid sites. 3.6. Ethanol decomposition To illustrate the in¯uence of a sample's acidity on its property for mixed alcohols synthesis, g-Al2O3 support and some oxidized K±MoO3/g-Al2O3 samples were chosen to be tested for ethanol decomposition. Fig. 8 presents the test results. The main products of the reaction were ethene and acetaldehyde. The ethene was formed from ethanol dehydration and the acetaldehyde from ethanol dehydrogenation. It was reported that the acidic sites on g-Al2O3 support were the active sites for ethanol dehydration and the basic sites were the active sites for ethanol dehydrogenation [17]. In the catalytic process of mixed alcohols synthesis from syngas, no acetaldehyde was detected, which demonstrated that alcohol dehydrogenation did not occur under the reaction conditions. Our experimental results indicated that the activity for both the ethanol dehydration and the ethanol dehydrogenation was high over the g-Al2O3 support and decreased over K±Mo based catalysts. For the sample of MoO3/Al2O30.05, the STY of ethene was about 50% of the STY for the g-Al2O3 support.
For the sample of MoO3/Al2O30.25, the STY of ethene decreased further to about 10%. But for the sample of MoO3/Al2O30.45, the activity for ethanol dehydration was near that for the sample of MoO3/ Al2O30.25. With increased Mo loading, the activity for ethanol dehydration showed a parallel change to sample's acidity. The experimental results also showed that for these oxidized K±MoO3/Al2O3 samples, activity for ethanol dehydrogenation changed in a similar tendency to the activity for ethanol dehydration. The test results of ethanol decomposition over the sul®ded samples are shown in Fig. 9. It is found that the activity for ethanol dehydration over the sul®ded sample was near to that over the oxidized sample with the same Mo loading, which indicated that the sample's acidity did not show obvious changes during the sul®dation. But the activity for ethanol dehydrogenation over the sample of MoO3/Al2O30.05 enhanced sharply after sul®dation, which became even higher than that over the g-Al2O3 support. This result demonstrated that the existence of some potassium component on the g-Al2O3 surface gave a rise in the basic sites after sul®dation. But for higher Mo loading samples, the activity for ethanol dehydrogenation was unchanged during sul®dation.
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Fig. 9. Activity for ethanol decomposition over the sulfided K±MoO3/g-Al2O3 samples with different Mo loading.
4. Discussion 4.1. Distribution of the mixed alcohols in the product Mixed alcohols in the product were mainly composed of linear C1±C4OH and a small amount of i-C4OH. The content of a linear alcohol in the total alcohols decreased if its carbon number increased. The content of i-C4OH was near to that of n-C4OH. Compared to the linear 1-C3OH, the amount of 2C3OH was quite low. The distribution of the mixed alcohols in the products here was similar to that on the unsupported MoS2-based catalyst and Co-based catalyst [18,19], but remarkably different from that over Cu±Zn based catalyst, where branched alcohols were prevailing [20]. The reaction for linear mixed alcohols synthesis over Co-based alcohols has been proved to occur by the mechanism of carbene polymerization scheme of Fischer±Tropsch (FT) process, since the product distribution obeyed the Anderson±Schulz±Flory distribution [18]. It is suggested that CO insertion into the adsorbed alkyl-containing species produced acyl intermediates [21,22]. The reaction over Cu±Zn based
catalyst which resulted in an abundance of branched alcohols was proposed to proceed by a condensation mechanism, where a higher alcohol was 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 [23,24]. MoS2-based catalyst has been reported to be FT type catalyst and its product was mainly composed of linear alcohols. The composition of the linear mixed alcohols was similar to that of Co-based catalyst [25]. The existence of a small amount of the branched alcohol (i-C4OH) in the product indicated that over MoS2based catalyst, to some extent, condensation reaction of the mixed alcohols happened [4]. As exhibited in Table 2, the molar ratio of i-C4OH/n-C4OH was small for the sample with low Mo loading and increased with enhanced Mo loading. This demonstrated that the reaction over the catalyst with low Mo loading was mainly an FT process to produce linear alcohols. The activity of the alcohol condensation was promoted with increased Mo loading. It should be noticed that the decrease of the content of C1OH in the mixed alcohols with increased Mo loading may be caused by the alcohols condensation reaction where C1OH was
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consumed. The reason for increased Mo loading which changed the condensation activity was not clear until now; in literature it has been suggested that the catalyst acidity in¯uenced the condensation activity to produce branched hydrocarbons over ZrO2-based catalyst [26]. 4.2. Dispersion state and activity Structural studies by LRS and XRD demonstrated that for oxidized samples, the same K±Mo±O species was formed. For sul®ded catalysts, MoS2 crystallites were formed as the main phase. One of the observed differences about these sul®ded catalysts was that the dispersion decreased with increased Mo loading. Activity results demonstrated that the selectivity to mixed alcohols increased from 3% for the sample of MoO3/Al2O30.05 to 50% for the sample of MoO3/ Al2O30.25. The aggregation of Mo species with increased Mo loading was closely related to the sharp increase of the mixed alcohols selectivity over these samples. However, while Mo loading increased from MoO3/Al2O30.25 to 0.45, the selectivity to mixed alcohols leveled off, even if Mo species aggregated further. Over Mo-based catalysts, it has been reported that potassium loading, support property and calcination temperature of the oxidized precursor showed marked effect on the activity for mixed alcohols synthesis [7± 9,27]. These results suggested that a signi®cant dispersion of the active phase was necessary to obtain high activity, but while the dispersion decreased, the selectivity to mixed alcohols increased. A good correlation between the selectivity to mixed alcohols and the dispersion state was observed. But the results in this work demonstrated that the aggregation of the K and Mo species should not be the immediate cause for the increase of the selectivity to mixed alcohols. 4.3. Reducibility and activity XPS, XRD and LRS results indicated that a part of the K±Mo species was not sul®ded and reduced during the sul®dation. It was reported that the interactions of Mo species with support, with potassium species and the dispersion state controlled the reducibility and the sul®dation of the Mo species [7±9,28]. For our cata-
lysts, XPS measurements indicated that the percentage of unreduced Mo species decreased with Mo loading increased from MoO3/Al2O30.05 to 0.25, which should be attributed to the fact that the interaction between Mo species and g-Al2O3 support was strong on the low Mo-loaded sample and became weak on the high Mo-loaded sample. With Mo loading increased to MoO3/Al2O30.45, percentage of unreduced Mo species on the catalyst surface was enhanced signi®cantly, which should be due to the sintering of the Mo species. For the samples with Mo loading increased from MoO3/Al2O30.25 to 0.45, the selectivity to mixed alcohols was kept unchanged under the same reaction conditions; this demonstrated that the content of unreduced Mo species on the surface hardly showed any relationship to the reaction selectivity. With Mo loading increased from MoO3/Al2O30.05 to 0.25, the selectivity to mixed alcohols increased sharply, but the content of unreduced Mo species on the surface decreased obviously. These results were different from the reported works which declared that the existence of some unreduced Mo species favored mixed alcohols formation [29,30]. For the catalysts with Mo loading increased from MoO3/Al2O30.25 to 0.45, the unchanged selectivity to mixed alcohols suggested that the surface circumstances on these catalysts were similar for the synthesis of mixed alcohols. The enhanced total CO conversion could be attributed to the increase of the amount of Mo species on the catalyst surface. Comparisons of the total CO conversion and the contents of the different Mo species between the two samples of MoO3/Al2O30.25 and 0.45 were made. The results were shown in Table 5. For these two samples, the ratio of total CO conversions should be near the ratio of the active species, so the active species was closely correlated with the reduced Mo4 species. Table 5 Comparison of the CO conversion and different Mo species between the samples of MoO3/Al2O30.25 and 0.45 Item
Ratio
Act0.45/Act0.25 (mol CO converted) (Total Mo)0.45/(Total Mo)0.25
1.58 2.04
4 Mo4 0:45 =Mo0:25
1.53
(Mo5Mo6)0.45/(Mo5Mo6)0.25
4.08
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4.4. Acidity and activity With Mo loading increased up to MoO3/ Al2O30.25, the acid amount connected to ammonia adsorption decreased sharply. The acid amount for the sample of MoO3/Al2O30.25 also became very low, compared to that of g-Al2O3 support. But the acid amount for the sample of MoO3/Al2O30.45 was near that for the sample of MoO3/Al2O30.25. According to literature, Mo species on g-Al2O3 support surface would interact primarily with nonacidic hydroxyl groups and the acid amount was still signi®cant [16,31,32]. On potassium promoted g-Al2O3 and TiO2 samples, potassium interacted with acidic sites and suppressed the acidity of these samples effectively [16,33]. For our catalysts, the decrease of acidity should be attributed to the interaction between the potassium component and the support. With increased Mo loading, the content of K component enhanced as well (K/Mo0.8) and the acidity of the catalyst decreased further. On the sample of MoO3/ Al2O30.25, potassium species interacted with most of the acidic sites and the acidity became rather weak. With Mo loading increased further, the acidity of the sample was almost unchanged. It was shown that while the sample possessed a strong acidity, the selectivity to mixed alcohols was low; and while the sample showed a weak acidity, the selectivity to mixed alcohols became high. The acidic sites of the catalyst sample may play an important role in the reaction of mixed alcohols synthesis. According to literature [21,22,25], acyl-contained intermediate was formed on the catalyst during the reaction, which could be hydrogenated to form mixed alcohols or hydrogenated and dehydrated immediately to form hydrocarbons. Acidic sites on the support were the active sites for alcohols' dehydration [16,34,35]. On the sample of MoO3/ Al2O30.05, the strong acidic sites which had not been covered by K±Mo species may be the principal reason for its high hydrocarbon selectivity. With Mo loading increased to MoO3/Al2O30.25, most of the acidic sites was covered, so the selectivity to mixed alcohols rose sharply. The results from ethanol decomposition strongly supported this opinion. It was reported that the selectivity to mixed alcohols could be correlated to the in¯uence of structural
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changes such as dispersion state, content of unreduced Mo species, the re-oxidation of Mo species and utilizing different potassium salts [7±9,36,37]. In fact, most of these in¯uences could be related to the acidity changes of the catalysts. We have reported that the increase of potassium loading over K±MoO3/Al2O3 catalyst resulted in the enhancement of the selectivity to mixed alcohols. It was known that the existence of a potassium component would make the acidity of the catalyst weaken. So the activity results were in agreement with the consideration that the acidity controlled the selectivity of the reaction. The increase of potassium loading over the K±MoO3/Al2O3 catalysts resulted in the aggregation of Mo species and the latter became dif®cult to be reduced to Mo4 species [7]. The aggregation of Mo species and the enhancement of unreduced Mo species may be the surface factors in increasing the selectivity to mixed alcohols. Avila et al. reported [8] that for K±Mo based catalyst on different supports such as ZnCr2O4, ZrO2, Cr2O3 and ZnO, the selectivity to mixed alcohols was rather different and increased in the order: ZrO2
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5. Conclusions The activity results of a series of sul®ded K±MoO3/ Al2O3 catalysts indicated that with Mo loading increased from MoO3/Al2O30.05 to 0.25, the total CO conversion to mixed alcohols and hydrocarbons decreased, but the selectivity to mixed alcohols enhanced sharply from 3% to 50%. With Mo loading increased from MoO3/Al2O30.25 to 0.45, the CO conversion was enhanced but the selectivity to mixed alcohols leveled off. Over these catalysts, FT synthesis to linear alcohols and the condensation reaction of a part of low alcohols to form i-C4OH occurred at the same time. With increased Mo loading, activity of the alcohols condensation became higher. Structural information from LRS, XRD, XPS, ammonia adsorption, TPD measurements and ethanol decomposition demonstrated that on the oxidized samples with Mo loading increased, same K±Mo±O species was formed but the dispersion decreased. After sul®dation the content of unreduced Mo species was high for the samples of MoO3/Al2O30.05 and 0.45, and was low for the sample of MoO3/Al2O30.25. Catalyst acidity and its activity for ethanol decomposition of the K±MoO3/Al2O3 decreased sharply with Mo loading increased up to MoO3/Al2O30.25, and leveled off with Mo loading increased further up to MoO3/Al2O30.45. Activity results and structural studies indicated that the dispersion state of the active species and the content of unreduced Mo species in¯uenced the total CO conversion but the acidity of the catalyst controlled the selectivity to mixed alcohols. References [1] G.J. Quarderer, K.A. Cochran, Eur. Patent, 119 609, 1984. [2] R.R. Stevens, Eur. Patent, 172 431, 1986. [3] Y. Xie, B.M. Naasz, G.A. Somorjai, Appl. Catal. 27 (1986) 233. [4] C.B. Murchison, M.M. Conway, R.R. Stevens, G.J. Quarderer, in: M.J. Phillips, M. Ternan (Eds.), Proceedings of the Ninth International Congress on Catalysis, vol. 2, Calgary, 1988, Chemical Institute of Canada, Ottawa, 1988, p. 626. [5] T. Tatsumi, A. Muramatsu, H. Tominaga, Appl. Catal. 34 (1987) 77. [6] J. Zhang, Y. Wang, L. Chang, Appl. Catal. A 126 (1995) 205.
[7] M. Jiang, G.Z. Bian, Y.L. Fu, J. Catal. 146 (1994) 144. [8] Y. Avila, C. Kappenstein, S. Pronier, J. Barrault, Appl. Catal. A 132 (1995) 97. [9] Y. Fu, K. Fujimoto, P. Lin, K. Omata, Y. Yu, Appl. Catal. A 126 (1995) 273. [10] C.P. Cheng, G.L. Schrader, J. Catal. 60 (1979) 276. [11] L. Wang, W.K. Hall, J. Catal. 66 (1980) 251. [12] D.S. Kim, K. Segawa, T. Soeya, I.E. Wachs, J. Catal. 136 (1992) 539. [13] A. Mull, R. Joster, W. Jaegermann, Inorg. Chim. Acta 41 (1980) 259. [14] P.A. Spevack, S. McIntyre, Appl. Catal. 64 (1990) 191. [15] P.A. McIntyre, T.C. Chan, P.A. Spevack, Appl. Catal. 63 (1990) 391. [16] E.E. Lowenthal, S. Schwarz, H.C. Foley, J. Catal. 156 (1995) 96. [17] T. Seiyama, M.M. Huang, Metal Oxide and their Catalytic Effects, The University of Science and Technology of China Publishing, 1991, p. 116. [18] K. Fujimoto, T. Oba, Appl. Catal. 13 (1985) 289. [19] P. Courty, P. Chaumette, Energy Prog. 7 (1987) 23. [20] C.E. Hofstadt, M. Schneider, O. Bock, K. Kochloefl, Stud. Surf. Sci. Catal. 16 (1983) 709. [21] A. Kiennemann, S. Boujana, C. Diagne, P. Chaumette, Stud. Surf. Sci. Catal. 75 (1993) 1479. [22] A. Kiennemann, S. Boujana, C. Diagne, P. Courty, P. Chaumette, Stud. Surf. Sci. Catal. 61 (1991) 243. [23] S.C. Tseng, N.B. Jackson, J.G. Ekerdt, J. Catal. 109 (1988) 284. [24] T.J. Mazanec, J. Catal. 98 (1986) 115. [25] J.G. Santiesteban, C.E. Bogdan, R.G. Herman, K. Klier, in: M.J. Phillips, M. Ternan (Eds.), Proceedings of the Ninth International Congress on Catalysis, vol. 2, Calgary, 1988, Chemical Institute of Canada, Ottawa, 1988, p. 561. [26] N.B. Jackson, J.G. Ekerdt, J. Catal. 124 (1990) 46. [27] K. Klier, R.G. Herman, J.G. Nunan, K.J. Smith, C.E. Bogdan, C.W. Young, J.G. Santiesteban, in: D.M. Bibby, C.D. Chang, R.F. Howe, S.Y. Yurchak (Eds.), 2-Methane Conversion, Elsevier, Amsterdam, 1988, p. 109. [28] Y. Okamoto, H. Tomioka, Y. Katoh, T. Imanaka, S. Teranishi, J. Phys. Chem. 84 (1980) 1833. [29] A. Muramatsu, T. Tatsumi, H. Tominaga, J. Phys. Chem. 96 (1992) 1334. [30] J.M. Driessen, E.K. Poels, J.P. Hindermann, V. Ponec, J. Catal. 82 (1983) 26. [31] A. Brenner, R.L. Burwell, J. Catal. 52 (1978) 353. [32] R.L. Burwell, A. Brenner, J. Mol. Catal. 1 (1975) 77. [33] C. Martin, I. Martin, C. Moral, V. Rives, J. Catal. 146 (1994) 415. [34] F.M. Bautista, B. Delmon, Appl. Catal. 130 (1995) 47. [35] S. Srinivasan, C.R. Narayanan, A. Biaglow, R. Gorte, A.K. Datye, Appl. Catal. 132 (1995) 271. [36] H.C. Woo, I.S. Nam, J.S. Lee, J.S. Chung, K.H. Lee, Y.G. Kim, J. Catal. 138 (1992) 525. [37] J.S. Lee, S. Kim, Y.G. Kim, Top. Catal. 2 (1995) 127.