The Performance of C2 Oxygenates Synthesis from Syngas over Rh

in the catalysts for syngas conversion to produce C2 oxygenated compounds such as ethanol, acetic acid, and acetaldehyde.5 Extensive studies have been...
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VOLUME 17, NUMBER 6

NOVEMBER/DECEMBER 2003

© Copyright 2003 American Chemical Society

Articles The Performance of C2 Oxygenates Synthesis from Syngas over Rh-Mn-Li-Fe/SiO2 Catalysts with Various Rh Loadings Hongmei Yin,† Yunjie Ding,*,† Hongyuan Luo,† Li Yan,† Tao Wang,† and Liwu Lin‡ Natural Gas Utilization and Applied Catalysis Laboratory and State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China Received January 9, 2003. Revised Manuscript Received June 23, 2003

The promotion effect of additives on Rh-Mn-Li-Fe/SiO2 catalyst was investigated by means of an in-situ FTIR technique. IR results suggested that the additives Fe, Mn, and Li might be in close contact with Rh, and form new active sites, which promoted CO activation over Rh surface and favored the formation of C2-oxygenated intermediates. The catalysts with rhodium loading of 1.0-5.0 wt % were tested by CO hydrogenation to find the optimized rhodium loading. The results showed that the high-rhodium-loading catalysts exhibited high activity and selectivity toward C2 oxygenates at low temperature, but the selectivity was very sensitive to reaction temperature, low-rhodium-loading catalysts showed low activity, while the selectivity was not so sensitive to reaction temperature, therefore high space time yield (STY) of C2 oxygenates can be obtained by increasing the reaction temperature. Among the catalysts investigated, the 1.5 wt % rhodium catalyst exhibited the highest selectivity and relative higher rhodium efficiency for the synthesis of C2 oxygenates.

Introduction Ethanol can be used pure or as an additive to gasoline as fuels for automobiles, which can enhance the gasoline octane number and improve the gasoline burning efficiency.1-4 Rhodium is the most versatile noble metal * Author to whom correspondence should be addressed. Fax: 86411-4379143. E-mail: [email protected]. † Natural Gas Utilization and Applied Catalysis Laboratory. ‡ State Key Laboratory of Catalysis. (1) von der Decken, C. B.; Fedders, H.; Ho¨hlein, B. Proceedings of the 6th International Symposium on Alcohol Fuels Technology, Ottawa, Canada, May, 1984; pp 21-25. (2) Ponec, V. Catal. Today 1982, 12, 227-254. (3) Ichikawa, M. CHEMTECH 1982, 12, 674-680. (4) Xu, X. D.; Doesburg, E. B. M.; Scholten, J. J. F. Catal. Today 1987, 2, 125-170.

in the catalysts for syngas conversion to produce C2 oxygenated compounds such as ethanol, acetic acid, and acetaldehyde.5 Extensive studies have been reported to synthesize these C2 oxygenates on rhodium-based catalysts to replace limited petroleum and grain resources.1-7 However an un-promoted rhodium catalyst produces mainly methane and the activity of CO conversion is poor, so various promoters have widely been employed to improve the performance of the Rh-based catalysts (5) Arakawa, H.; Fukushima, T.; Ichikawa, M. Chem. Lett. 1985, 163, 881-884. (6) Luo, H. Y.; Zhang, W.; Zhou, H. W.; Huang, S. Y.; Lin, P. Z.; Ding, Y. J.; Lin, L. W. Appl. Catal. A 2001, 214, 161-166. (7) Ichikawa, M.; Fukuoka, A.; Kimura, T. In Proceedings of the 9th International Congress on Catalysis, Ottawa, Calgary, 30 November, 1987; Philips, M. J., Ternan, M., Eds.; 1988; Vol, 2, pp 569-576.

10.1021/ef0300085 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/19/2003

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in the past decades.8-15 The Rh-based catalysts promoted with oxide promoters, such as oxides of Fe, Mn, Zr, V, and La, are highly active and selective for the formation of C2 oxygenates. Luo et al.16 introduced Mn, Li into the Rh/SiO2, and the activity and selectivity toward C2 oxygenates were greatly improved. So far, the Rh-Mn-Li/SiO2 catalyst is one of the most favorable catalyst systems for the effective synthesis of C2 oxygenates.17 Recently, an iron-promoted Rh-Mn-Li/SiO2 catalyst, which showed superior activity in C2 oxygenates synthesis reaction, was developed in our group.18 As well-known, the scarcity and the price of rhodium for the manufacture of the catalyst is one of the crucial factors for the commercialization of C2 oxygenates synthesis. It has been reported that the activity of the Rh/SiO2 catalyst is remarkably influenced by rhodium dispersion.15 A lot of attention has been paid to the Rh loading in a wide range of 1∼5 wt % in the literature.7,8,16,19 Unfortunately, little optimized rhodiumloading information about the additive-promoted rhodium-based catalyst for the formation of C2 oxygenates appeared. In this paper, Rh-Mn-Li-Fe/SiO2 catalysts with Rh loading in the range of 1.0-5.0 wt % were prepared and tested by CO hydrogenation in attempt to find out the factors which influence the improvement of the Rh efficiency for C2 oxygenates. Experimental Section Material and Catalyst Preparation. Rhodium chloride was purchased from Johnson Matthey Precious Metal Company. Catalysts were prepared by impregnating SiO2 (20-40 mesh, BET surface area 260 m2/g) with an aqueous mixture solution of RhCl3. xH2O, Mn(NO3)2, LiNO3, and Fe(NO3)3; the water used per gram of catalyst was 5 mL. The catalysts were dried for ca. 6 days at room temperature and then dried at 393 K for 4 h. Rhodium loading of all the catalysts varied from 1.0 to 5.0 wt %. The weight ratio of Rh/Mn/Li/Fe always was 1:1:0.075:0.05. H2 Uptake. H2 uptake experiments were conducted to determine the Rh metal dispersion of the catalysts. It was performed on an America Micromeritics Autochem 2910. A 200-mg catalyst sample was placed in a quartz reactor and was reduced by flowing H2 at 623 K, then purged with Ar at the same temperature for 0.5 h, and finally cooled to 323 K. Uptakes of H2 were measured by injecting 10% H2-Ar into the Ar carrier gas. (8) Van den Berg, F. G. A.; Glezer, J. H. E.; Sachtler, W. M. H. J. Catal. 1985, 93, 340-352. (9) Stevenson, S. A.; Lisitsyn, A. S.; Knozinger, H. J. Phys. Chem. 1990, 94, 1576-1581. (10) Orita, H.; Naito, S.; Tamaru, K. J. Chem. Soc., Chem. Commun. 1984, 150-151. (11) Lisitsyn, A. S.; Stevenson, S. A.; Knozinger, H. J. Mol. Catal. A 1990, 63, 201-211. (12) Trevin˜o, H.; Sachtler, W. M. H. Catal. Lett. 1994, 27, 251-258. (13) Underwood, R. P.; Bell, A. T. Appl. Catal. A 1986, 21, 157168. (14) Kip, B. J.; Smeets, P. A. T.; Van Wolput, J. H. M. C.; Zandbergen, H. W.; Van Grondelle J.; Prins, R. Appl. Catal. A 1987, 33, 157-180. (15) Arakawa, H.; Takeuchi, K.; Matsuzaki, T.; Sugi, Y. Chem. Lett. 1984, 153, 1607-1610. (16) Luo, H.-Y.; Lin, P.-Z.; Xie, S.-B.; Zhou, H.-W.; Xu, C.-H.; Huang, S.-Y.; Lin, L.-W.; Liang, D.-B.; Yin, P.-L.; Xin, Q. J. Mol. Catal. A 1997, 122, 115-123. (17) Lin, P.-Z.; Liang, D.-B.; Luo, H.-Y.; Xu, C.-H.; Zhou, H.-W.; Huang, S.-Y.; Lin, L.-W. Appl. Catal. A 1995, 131, 207-214. (18) Yin, H. M.; Ding, Y. J.; Luo, H. Y.; Zhu, H. J.; Xiong, J. M.; Lin, L. W. Appl. Catal. A 2003, 243, 155-164. (19) Luo, H. Y.; Bastein, A. G. T. M.; Mulder, A. A. J. P.; Ponec, V. Appl. Catal. A 1988, 38, 241-253.

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Figure 1. The schematic diagram of the apparatus for CO hydrogenation. In-Situ FT-IR Studies. In-situ FT-IR experiments were conducted under 120 mL/min gas flow, H2/CO ) 2, 323-593 K, and 0.1-3.0 MPa. The IR spectra were recorded by a BRUKER EQUINOX 55 single-beam Fourier Transform infrared spectrometer. All spectra were recorded with 16 scans with a resolution of 4 cm-1. The sample wafer (11 mg, φ13 mm) was mounted in a high-temperature/high-pressure cell (SPECAC INC P/N 5850) fitted with ZnSe widows and with automatic temperature controller; the cell is a water-cooled, stainless steel chamber. The sample was first reduced in flowing H2 at 623 K for 1 h. After reduction, the catalyst was cooled to 323 K. An FT-IR spectrum (denoted as spectrum A) of the catalyst in H2 flow at a given reaction temperature was recorded. Subsequently, flowing H2/CO was introduced into this system, the temperature and pressure of the reactants were increased to realize the desirable reaction conditions and maintained for 30 min, relieving the pressure and purging the chamber with high-purity nitrogen for 30 min, spectrum B was recorded. Subtracting spectrum A from spectrum B gave a final IR spectrum. CO Hydrogenation. The experiments were carried out in an apparatus as shown in Figure 1. The catalyst was in-situ reduced in a flow of H2 before the test. The temperature was raised at 2 K/min from room temperature up to 623 K, and then held at constant for 1 h. The H2 flow rate was 4 L/h at atmospheric pressure. Then the catalyst was shifted into syngas (H2/CO ) 2) after cooling, and reacted under specified temperature, pressure, and space velocity for 4 h. The charge of catalyst was 0.4 g (∼0.8 mL) for each run. The testing apparatus consisted of a small fixed bed tubular reactor with an external heating system, which was made of 316 L stainless steel with 300 mm length, 4.6 mm i.d. The effluent passed through a condenser filled with 150 mL of cold de-ionic water. The oxygenated compounds from the product were captured by complete dissolution into the water in the condenser. The aqueous solution containing oxygenates obtained was off-line analyzed by Varian CP-3800 gas chromatography with an FFAP column, using an FID detector and 1-pentanol as an internal standard. The tail gas was on-line analyzed by Varian CP-3800 GC with a Porapak QS column and TCD detector.

Results and Discussion H2 Uptake. Data of H2 uptake of the catalysts with different Rh loading are shown in Table 1. When the Rh loading increased from 1.0 to 5.0 wt %, the H/Rh molar ratio decreased from 0.51 to 0.26, indicating that the Rh particles grew up with Rh loading. FTIR Studies. After reaction at 593 K and 3.0 MPa over Rh/SiO2 (see Figure 2a), only bands for the linearCO (2051 cm-1), the gem-dicarbonyl Rh(CO)2 (2093, 2022 cm-1) were observed.20,21 For the spectra after reaction at 593 K and 0.1 MPa over Rh-Mn-Li-Fe/ SiO2 (Figure 2b), bands for linear-CO (2038 cm-1), bridge-CO (1840 cm-1), and gem-dicarbonyl CO (2093

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Figure 2. FT-IR spectra of adsorbed species on Rh-based catalysts after various reactions. (a) Rh/SiO2, (b) and (c) RhMn-Li-Fe/SiO2. Table 1. Uptake of H2 on Various Catalyst Samples with Different Rh Loading Rh loading (%) H/Rh molar ratio

1.0 0.51

1.5 0.41

2.0 0.35

3.0 0.31

5.0 0.26

cm-1) were detected, another gem-dicarbonyl CO band around 2020 cm-1 may overlap with linear-CO and was indiscernible. The downshift of linear-CO from 2051 to 2038 cm-1 over Rh-Mn-Li-Fe/SiO2 compared to the spectra of Rh/SiO2 could be attributed to the promotion effect of additives toward the CO activation on the Rh surface. When the syngas pressure was elevated from 0.1 to 3.0 MPa (Figure 2c), the Rh(CO)2 bands at 2093 cm-1 disappeared, which may be due to the fact that under high-pressure reaction condition, more Rh1+ species was reduced to Rh0. Meanwhile, the bands at 1716, 1566, 1442, and 1343 cm-1 became distinct and sharp. Ichikawa22-24 assigned the bands at 1579 and 1444 cm-1 to bidentated acetate species, the bands at 1745, 1444, and 1382 cm-1 to monodentated acetate species, and proposed the acetate species be oxygenated intermediates. Trevin˜o25 postulated that oxygenated precursors (a surface acetate species), CxHyOz, were formed at MnO sites in close proximity to the Rh-MnO interface. The role of Rh was the formation and delivery of CHx groups and H atoms. Bands at 1716, 1566, 1442, and 1343 cm-1 could not be observed over un-promoted Rh/SiO2, indicating that the addition of Mn, Li, and Fe favored the C2 oxygenates intermediates formation. Extensive work has been done to investigate the unexpected effect of promoters on the activity and (20) Yang, A. C.; Garland, C. W. J. Phys. Chem. 1957, 61, 15041512. (21) Sachtler, W. M. H.; Ichikawa, M. J. Phys. Chem. 1986, 90, 4752-4758. (22) Fukushima, T.; Arakawa, H.; Ichikawa, M. J. Phys. Chem. 1985, 89, 4440-4443. (23) Fukushima, T.; Arakawa, H.; Ichikawa, M. J. Chem. Soc. Chem. Commun. 1985, 729-731. (24) Arakawa, H.; Fukushima, T.; Ichikawa, M. Appl. Spectrosc. 1986, 40, 884-886. (25) Trevin˜o, H.; Hyeon, T.; Sachtler, W. M. H. J. Catal. 1997, 170, 236-243.

Figure 3. Effect of temperature on the performance of CO hydrogenation Rh: 2.0 wt %, H2/CO: 2 (molar ratio), GHSV: 12000 h-1. (4) Selectivity toward MeOH; (b) CO conversion; (3) selectivity toward AcOH; (9) selectivity toward AcH; (O) selectivity toward EtOH; (2) selectivity toward C1-C4 hydrocarbon; (0) selectivity toward C2 oxygenates; (1) the STY of C2 oxygenates.

selectivity of Rh/SiO2 catalyst. Sachtler and Ichikawa21 suggested that oxophilic ions Mn, Zr, Ti, and Nb, when located at the Rh surface, enhanced CO dissociation possibly through direct interaction with the oxygen atom of titled adsorbed CO. We have proposed in previous work18 on the basis of the results of CO-TPD and COTPSR that promoters Fe, Mn, and Li might be in close contact with Rh, and formed new active sites which were responsible for the high yield of C2 oxygenates. Therefore, it seemed that the new active sites promoted CO activation over Rh surface and favored the formation of C2-oxygenated intermediates. Effect of Process Conditions on the Reactivity of Rh-Mn-Li-Fe/SiO2 Catalysts. Reaction Temperature. Figure 3 gives the effect of temperature on the reactivity over 1 wt % Rh-Mn-Li-Fe/SiO2 catalyst. The results showed that CO conversion and the space time yield (STY) of C2 oxygenates increased with the temperature. The selectivity toward ethanol (EtOH) increased, while acetaldehyde (AcH) selectivity decreased, and acetic acid (AcOH) selectivity did not obviously change when the reaction temperature increased. It is well-known that the formation of hydrocarbon is thermodynamically more favorable than that of oxygenates. Reaction Pressure. The influence of reaction pressure on the catalyst reactivity is shown in Figure 4. Increasing the pressure was favorable for the synthesis of C2 oxygenates. Consequently, the STY of C2 oxygenates increased remarkably with the pressure. When the pressure increased from 2.0 to 5.0 MPa, the CO conversion increased from 2.74 to 5.38%, the STY of C2 oxygenates increased from 138.8 to 383.4 g/kg-cat.h, and the selectivity toward C2 oxygenates increased from 50.5 to 66.5%. The reasonable interpretation of this observation was that the formation of C2 oxygenates and hydrocarbon is a volume-contraction reaction. The above FT-IR results showed that high pressure favored the formation of the acetate species on the surface of RhMn-Li-Fe/SiO2, therefore, favored the selectivity to C2 oxygenates.

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Figure 4. Influence of pressure on the reactivity of CO hydrogenation Rh: 1.0 wt %, T: 578 K, H2/CO: 2, GHSV: 12000 h-1. (4) Selectivity toward MeOH; (b) CO conversion; (3) selectivity toward AcOH; (9) selectivity toward AcH; (O) selectivity toward EtOH; (2) selectivity toward C1-C4 hydrocarbon; (0) selectivity toward C2 oxygenates; (1) the STY of C2 oxygenates.

The work of Xu and Sachtler26 showed that the CO conversion on Rh/NaY was lower by more than an order of magnitude while the selectivity for acetic acid is two orders higher at 1 MPa than at 0.1 MPa. The authors proposed that Rh carbonyl clusters formed under high CO pressure and were instrumental for the formation of oxygenates. They rationalized that CO dissociation and the hydrogenation of surface carbon atoms toward CHx took place over metal atoms, while CO insertion was favored on metal carbonyl clusters; under high pressure of CO, the metal ensembles required for CO dissociation became scarce, while the metal carbonyl clusters favoring CO insertion became abundant, thus CO conversion deteriorated but the selectivity to acetic acid was improved. It sounds reasonable because metal carbonyl clusters in zeolite cavities form preferentially at high CO pressure27,28 for the zeolite cages are especially favorable for the formation of metal carbonyl clusters. However, over conventional silica-supported Rh catalysts, no formation of Rh6(CO)16 was detected even under high pressure of syngas in the literature,22-24,29 and no Rh6(CO)16 (2080, 1800 cm-1) and Rh4(CO)12 (2076, 2046, 1881-1870 cm-1) bands30 were observed in our IR study. In addition, both the CO conversion and the selectivity toward C2 oxygenates was enhanced over Rh-Mn-Li-Fe/SiO2 when the syngas pressure was increased, which was different from the work of Xu.26 Kinetic investigation over SiO2-supported Rhbased catalysts by Burch31 suggested that high syngas pressure kinetically favored the formation of C2 oxygenates, and which could rationalize our work. (26) Xu, B. Q.; Sachtler, W. M. H. J. Catal. 1998, 180, 194-206. (27) Sachtler, W. M. H.; Huang, Y.-Y. Appl. Catal. A 2000, 191, 3544. (28) Stakheev, A. Y.; Shapiro, E. S.; Jaeger. N. I.; Schulz-Ekloff, G. Catal. Lett. 1995, 34, 293-303. (29) Demri, D.; Hindermann, J. P.; Diagne, C.; Kiennemann. J. Chem. Soc., Faraday. Trans. 1994, 90, 501-506. (30) Theolier, A.; Smith, A. K.; Leconte, M.; Basset. J. M.; Zanderighi, G. M.; Psaro, R.; Ugo, R. J. Organomet. Chem. 1980, 191, 415424. (31) Burch, R.; Petch, M. I. Appl. Catal. A 1992, 88, 77-99.

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Figure 5. Effect of space velocity on performance of CO hydrogenation Rh: 1.0 wt %, T: 578 K, H2/CO: 2. (4) Selectivity toward MeOH; (b) CO conversion; (3) selectivity toward AcOH; (9) selectivity toward AcH; (O) selectivity toward EtOH; (2) selectivity toward C1-C4 hydrocarbon; (0) selectivity toward C2 oxygenates; (1) the STY of C2 oxygenates.

Space Velocity. The effect of space velocity on catalyst reactivity was studied. As depicted in Figure 5, increase of the STY of C2 oxygenates and changes in selectivity toward C2 oxygenates were observed when space velocity was enhanced. It was found that the STY of C2 oxygenates increased sharply at lower feed speed, but the increasing rate became slower when the space velocity exceeded 15000 h-1. Similar tendencies were also observed in the selectivity toward AcH and AcOH, and the changes in the selectivity toward EtOH and methanol (MeOH) were not obvious, while the CO conversion and the selectivity of hydrocarbon decreased with the increase of space velocity. The improvement of the STY and selectivity toward C2 oxygenates might imply that higher linear velocity of syngas favored the C2 oxygenate species desorbing from the catalyst surface, and avoided being further hydrogenated. Ratio of H2/CO. The influence of H2/CO molar ratio on the catalytic activity is illustrated in Figure 6. CO conversion and the selectivity toward EtOH increased while selectivity toward AcH and AcOH decreased when the H2/CO ratio enlarged. The increase of ethanol selectivity with the ratio of H2/CO was resulted from that high partial pressure of hydrogen led to the enhancement of hydrogenation capability of acetaldehyde and acetic acid. It was worth noting that the STY and the selectivity toward C2 oxygenates reached a maximum at the H2/CO molar ratio of ca. 2. The Influence of Rh Loading on the Performance of the Catalysts. Effect of Rh Loading on the Reactivity of Catalysts. The effect of Rh loading on the performance of CO hydrogenation was investigated (Figure 7), The CO conversion increased from 1.55 to 15.94% and the STY of C2 oxygenates from 81.3 to 695.0 g/kg-cat‚h when the Rh loading went up from 1 to 5 wt %. The rate of C2 oxygenate formation increased rapidly and then went up slowly when the Rh loading exceeded 3.0 wt %, while the selectivity toward C2 oxygenates increased at first, attained a maximum, and then decreased when Rh loading increased continuously. It is worth noting that the maximum of selectivity toward

C2 Oxygenates Synthesis from Syngas

Figure 6. Influence of H2/CO molar ratio on the performance of CO hydrogenation Rh: 1.0 wt %, T: 588 K, GHSV: 12000 h-1 (4) selectivity toward MeOH; (b) CO conversion; (3) selectivity toward AcOH; (9) selectivity toward AcH; (O) selectivity toward EtOH; (2) selectivity toward C1-C4 hydrocarbon; (0) selectivity toward C2 oxygenates; (1) the STY of C2 oxygenates.

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Figure 8. Relationship between temperature and the STY of C2 oxygenates on various catalysts with different Rh loadings (2) 1%; (4) 1.5%; (9) 2%; (0) 3%; (b) 5%.

Figure 9. Relationship between temperature and CO conversion of various catalysts with different Rh loadings (2)1%; (4) 1.5%; (9) 2%; (0) 3%; (b) 5%. Figure 7. Effect of Rh loading on product distribution, CO conversion, and the STY of C2 oxygenates, T: 563 K, H2/CO: 2.0, GHSV: 12000 h-1. (4) Selectivity toward MeOH; (b) CO conversion; (3) selectivity toward AcOH; (9) selectivity toward AcH; (O) selectivity toward EtOH; (2) selectivity toward C1C4 hydrocarbon; (0) selectivity toward C2 oxygenates; (1) the STY of C2 oxygenates.

C2 oxygenates appeared at Rh loading 1.5 wt % in which Rh dispersion was about 0.41. Arakawa et al.15 have investigated the effect of Rh dispersion on product selectivity for the un-promoted Rh/SiO2 catalysts. Their work showed that the selectivity toward C2 oxygenates reached a maximum at around rhodium dispersion of 0.40 corresponding to Rh loading of 4.7%, which was in good agreement in Rh dispersion with the result of this study. The selectivity toward ethanol decreased when the Rh loading increased, and that toward acetic acid did not remarkably change; while the selectivity of acetaldehyde increased at low Rh loading, further attained a maximum at the Rh loading of 2.0 wt %, and finally decreased with the increase of Rh loading. It seemed that acetaldehyde and ethanol were derived from different intermediates.

Effect of Temperature on the Performance of Various Rh Loadings Catalysts. Figure 8 presents the effect of temperature on the STY of C2 oxygenates over Rh-based catalysts with various metal loadings. It was apparent that the STY of C2 oxygenates increased with the temperature for all catalysts, and the increasing rate became slower after the temperature reached a certain value over the catalyst with Rh loading more than 3.0 wt %. The STY of C2 oxygenates of the catalysts with low Rh loading could be improved and reached a value similar to that of the catalysts with high Rh loading by increasing the reaction temperature, even though the latter showed high STY of C2 oxygenates at low temperature. It is worth noting that the STY of C2 oxygenates of 5.0 wt % Rh-based catalyst was 562 g/kg-cat.h and the selectivity of C2 oxygenates was only ca. 48% based on carbon number at 557 K, the same STY could be attained and the selectivity toward C2 oxygenates was 57.3% for 1.5 wt % Rh-based catalyst at 593 K. Figure 9 exhibits the relationship between the temperature and the CO conversion of the catalysts with different Rh loadings. For all catalysts, the CO conversion increased with the temperature, it was found that

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the improvement of Rh efficiency of C2 oxygenates in a certain range of temperature. It was worth noting that the highest Rh efficiency moved toward the low-Rhloading region with an increase of reaction temperature due to deterioration of the selectivity toward C2 oxygenates for high-Rh-loading catalysts at high temperature. Conclusion

Figure 10. Influence of Rh loading on the efficiency of Rh metal. (2) Rh efficiency for the reaction at 563 K; (4) Rh efficiency for the reaction at 573 K; (b) Rh efficiency for the reaction at 583 K.

the reaction temperature was more sensitive for highRh-loading catalysts. The Rhodium Efficiency of Catalysts with Different Rh Loadings. The effect of the Rh loading on the Rh efficiency was shown in Figure 10. The Rh efficiency (gC2 oxy/g-Rh.h) increased with Rh loadings and attained a maximum value at all testing temperatures and decreased thereafter. Increasing the reaction temperature favored the enhancement of catalyst activity and

The addition of Mn, Li, and Fe, which might be in close contact with Rh and form new active sites, favored the acetate intermediate species formation, and consequently, enhanced the STY and selectivity to C2 oxygenates. The Rh-Mn-Li-Fe/SiO2 catalysts with high Rh loading exhibited high activity and selectivity at low temperature, but the selectivity was very sensitive to reaction temperature; low-rhodium-loading catalysts showed low activity, and the selectivity toward C2 oxygenates was not so sensitive to reaction temperature, thus high STY of C2 oxygenates could be obtained by increasing the reaction temperature. The catalyst with Rh dispersion 0.41, which corresponded to 1.5 wt % Rh loading, exhibited the highest selectivity toward C2 oxygenates and relatively higher rhodium efficiency among all Rh-based catalysts studied in this work. Acknowledgment. This work was supported financially by the Chinese Science and Technology Ministry (Grant No. G1999022404) EF0300085