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Energy & Fuels 2003, 17, 810-816
Low-Pressure Hydroformylation of Middle Olefins over Co and Rh Supported on Active Carbon Catalysts Baitao Li,† Xiaohong Li,‡ Kenji Asami,‡ and Kaoru Fujimoto*,‡ Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Faculty of Environmental Engineering, The University of Kitakyushu, 1-1 Hibikino, Fukuoka 808-0135, Japan Received October 15, 2002
The hydroformylation of olefin has been investigated on Co and Rh catalysts supported on active carbon at mild reaction conditions (403 K, 3.0 MPa) in the slurry phase reactor. Of the catalysts, Co/activated carbon catalysts showed excellent catalytic performance only in the alcoholic solvent, while Rh/activated carbon catalyst exhibited good activity in the nonpolar solvent. Rh/activated carbon catalyst showed stable activity for 14 h in the continuous feeding system, giving the total turnover number of 500. The effects of solvents were discussed in detail. The details of reaction conditions that affected the performance of the catalysts have been optimized. The higher catalytic activity was found at the lower temperature, about 403 K. About 20% of metal was dissolved from the support, a determination based on inductively coupled plasma (ICP) atomic emission spectrometry. The dissolved part was thought to deposit again on the support and work as active species as well.
Introduction Due to their high efficiencies and low emissions, oxygenates are potentially useful as good engine and vehicle fuels. Considerable efforts have been concentrated on the syntheses of oxygenates, including longchain alcohols, aldehydes, and acetals.1-3 Synthesis gas, or syngas (CO and H2) obtained from coal, natural gas, and biomass can be converted into paraffins and olefins via Fischer-Tropsch process, mainly aiming at the production of diesel fuel. Our research group is devoted to the conversion of oxygenates from syngas (CO and H2) and olefin, where the latter is the main product in the Fischer-Tropsch reaction. Hydroformylation, the synthesis of aldehyde and alcohol from olefins and synthesis gas, is a well-known homogeneous cobalt-catalyzed reaction.4-6 To keep the carbonyl intermediate (active species) stable, the high operation pressure of syngas (about 10-20 MPa) must be applied to the system,4 but this will result in the extra economic investment for the high-pressure reactor and other systems. Moreover, the drawbacks of homogeneous catalysis, such as the recovery of metal and the separation problem of catalyst and products, retard its practical application into wider fields.7-10 Thus, the †
The University of Tokyo. The University of Kitakyushu. * Corresponding author. (1) Qiu, X.; Tsubaki, N.; Fujimoto, K. Catal. Commun. 2001, 2, 75. (2) Qiu, X.; Tsubaki, N.; Fujimoto, K. Fuel 2002, 81, 1625. (3) Qiu, X.; Tsubaki, N.; Fujimoto, K. J. Chem. Eng. Jpn. 2001, 34, 1366. (4) Lenarda, M.; Storaro, L.; Ganzerla, R. J. Mol. Catal. 1996, 111, 203. (5) Parshall, G. W. Homogeneous Catalyst; Wiley-Interscience: New York, 1980; p 85. (6) Ertl, G.; Knozinger, H.; Weitkamp, J. Handbook of Heterogeneous Catalyst; VCH: Berlin, 1997; Vol. 5, p 2232. ‡
development of a practical heterogeneous catalyst would be desirable. Most of the studies have been concerned with supported rhodium and cobalt catalysts,11-14 which have been tested in high-pressure autoclaves under liquid-phase reaction conditions. Kainulainen7 et al. applied the Co/SiO2 catalyst to the liquid-phase hydroformylation of 1-hexene at 423 K and 7.5 MPa in the typical autoclave reactor and studied the effect of the different cobalt-containing precursors. They also reported that Co/SiO2 was very active and stable in the gas-phase hydroformylation of ethene in flow conditions at 546 K and 0.5 MPa.15 Our interest is concentrated on the development of the low-pressure process where solid catalysts are utilized.16 The process is of great importance when R-olefins with carbon numbers of C5-C10 are considered to be directly used as raw materials at the completion stage of the Fischer-Tropsch reaction (2.0-3.0 MPa). This paper reports the results of hydroformylation which was operated in several solvents over Rh and Co supported on active carbon catalysts. The reason for (7) Kainulainen, T. A.; Niemela, M. K.; Krause, A. O. I. J. Mol. Catal. 1997, 122, 39. (8) Chuang, S. S. C.; Srinuvas, G.; Mukherjee, A. J. Catal. 1993, 139, 490. (9) Takahashi, N.; Tobise, T.; Mogi, I.; Sasaki, M.; Mijin, A.; Fujimoto, T.; Ichikawa, M. Bull. Chem. Soc. Jpn. 1992, 65, 2565. (10) Naito, S.; Tanimoto, M. J. Chem. Soc., Chem. Commun. 1989, 1403. (11) Davis, M. E.; Rode, E.; Taylor, D.; Hanson, B. E. J. Catal. 1984, 86, 67. (12) Ichikawa, M.; Rao, L. F.; Kimura, T.; Fukuoka, A. J. Mol. Catal. 1990, 62, 15. (13) Allum, K. G.; Hancock, R. D.; Howell, I. V.; Lester, T. E.; Mckenzie, S.; Pitkethly, R. C.; Robinson, P. F. J. Catal. 1976, 43, 331. (14) Hanaoka, T.; Arakawa, H.; Matsuzaki, T.; Sugi, Y.; Kanno, K.; Abe, Y. Catal. Today. 2000, 58, 271. (15) Kainulainen, T. A.; Niemela, M. K.; Krause, A. O. I. Catal. Lett. 1998, 53, 97. (16) Li, B.; Li, X.; Asami, K.; Fujimoto, K. Chem. Lett. 2002, 8, 836.
10.1021/ef0202440 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/16/2003
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Figure 1. Flow diagram of hydroformylation reaction in a slurry phase reactor.
cobalt catalyst was chosen due not only to its low cost but also to its high activity. Active carbon has been shown to exhibit beneficial characteristics as the strong support for active metal as well as the excellent hydroformylation activity and oxygenate selectivity.17-20 Experimental Section Preparation of Catalyst. Active carbon (20-40 mesh) was obtained from Kanto Chemical Co. with a surface area of 1071.7 m2/g and an average pore volume of 0.43 cm3/g. Cobalt or rhodium supported on active carbon catalyst was prepared by the conventional impregnation method. The support was degassed at 413 K for 2 h and impregnated with aqueous solution of Co(NO3)2‚6H2O or Rh(NH3)2(NO2)2 (Tanaka Noble Metal). Ru-promoted cobalt catalyst was prepared by the coimpregnation with aqueous solution of Co(NO3)2‚6H2O and Ru(NO3)3. The catalyst precursor was dried in a rotary evaporator at 333 K and heat treated in nitrogen flow at 673 K for 4 h. The catalyst was reduced under hydrogen flow at the same temperature for 6 h and passivated at room temperature with 1% O2 in N2 before use. Co2(CO)8 (Kanto Chemical Co.) was used as a homogeneous catalyst for reference. Reaction Apparatus and Procedure. Catalytic reactions were carried out in a semibatch slurry-phase reactor with an inner volume of 85 mL, where olefin, solvent, and catalyst remained in the reactor while synthesis gas passed through the reactor (Figure 1). The amount of catalyst was 0.2 g. Hexene-1 (120 mmol) was used as the model olefin. The moar ratio of solvent to olefin was 2:1. The flow rate of syngas (CO: H2:Ar ) 47.8:48.2:4.0) in the reaction was set at 80 mL/min (NTP). A dry ice reflux condenser was set down stream of the reactor to prevent the loss of solvent. A dry ice methanol trap was set between the reactor exit and the pressure regulator to collect the solvent and products. After the reaction was completed, the reactor was cooled to room temperature and depressurized. All products were identified with a GC-MS (Shimadzu GCMS QP 5050). The gaseous products were analyzed by an on-line GC equipped with an active charcoal column. The liquid products collected in the dry ice trap and the products (17) Robert, J. D.; Joseph, A. R.; Mark, E. D. J. Catal. 1986, 98, 477. (18) Omata, K.; Fujimoto, K.; Shikada, T.; Tominaga, H. Ind. Eng. Chem. Res. 1998, 27, 2211. (19) Kainulainen, T. A.; Niemela, M. K.; Krause, A. O. I. J. Mol. Catal. 1999, 140, 173. (20) Tsubaki, N.; Sun, S. L.; Fujimoto, K. J. Catal. 2001, 199, 236.
Table 1. Characteristics of the Catalysts
catalyst 0.1 wt % Rh/activated carbon 0.5 wt % Rh/activated carbon 1.0 wt % Rh/activated carbon 3.0 wt % Rh/activated carbon 10 wt % Co/activated carbon 10 wt % Co/activated carbon + 0.5 wt % Ru/activated carbon
uptake H2 dispersion (µmol/g) (%) 0.36 1.48 2.37 6.21 49.51 71.84
10.10 8.31 6.67 5.82 7.95 11.57
surface area (m2/g) 1145 1123 1102 1056 972 960
remaining in the reactor were combined and analyzed by a gas chromatograph with a DB-1 capillary column of the J&W Scientific Co. Catalyst Characterization. Chemisorption experiments were carried out by using the ASAP2010 chemisorption system (Shimadzu Co.). Before adsorption of H2, the passivated catalyst was treated under H2 at 673 K for 4 h. Then H2 chemisorption was conducted at 373 K. The total gas uptake was determined by extrapolation of the straight-line portion of the adsorption isotherm to zero pressure. The properties of the catalysts are presented in Table 1. To estimate the amounts of metal elution of catalyst during the reaction, the cobalt and rhodium contents of catalyst before and after the reaction were determined by inductively coupled plasma (ICP) atomic emission spectrometer (Perkin-Elmer Co.). TEM photos were also taken to evaluate the particle size of cobalt catalyst. Transmission electronic miscroscopy (TEM) was measured by JEOL TEM-2010. It showed that the average particle grew from 6 to 10 nm for fresh and used catalysts, respectively.
Results and Discussion Catalytic Performance of Active Carbon Supported Catalysts. Figure 2 shows the catalytic performances of the 1 wt % Rh/carbon catalyst for 1-hexene hydroformylation. It is clear from the figure that the CO conversion, which was measured from the effluent gas composition out of the reactor, showed the catalytic activity rose quickly as the reaction time increased to about 2 h. Then the CO conversion rate decreased slowly with the reaction time. On the other hand, the yields of the products, which were calculated from in situ liquid analysis in the reactor, showed that the isomerized products (2-hexene and 3-hexene) were produced quickly and, then, that
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Table 2. Hydroformylation of 1-Hexene in Various Solvents over Co/Activated Carbona solvent
1-hexene converted (%)
C7-aldehyde
C7-olefin
benzene n-heptane n-octane n-octanec THF toluene methanol ethanol 1-propanol 2-propanol n-octaned methanold
0.3 0.1 0.2 0.2 0.1 0.2 49.2 72.9 74.1 80.0 84.4 80.6
0.1 0 0 0 0 0 8.8 28.9 26.8 34.3 67.8 27.4
0.1 0 0 0 0 0 0.4 3.8 3.7 6.8 0 1.6
product yield (%) acetal 0 0 0 0 0 0 24.8 31.8 36.6 31.1 0 38.5
ester
isomer
n:ib
0 0 0 0 0 0 0.3 0.8 1.1 0 0 1.3
0.1 0.1 0.2 0.2 0.1 0.1 14.9 7.6 5.9 7.8 16.6 11.8
0.3 0.4 0.4 0.5 1.0 0.4
a 3.0 MPa and 403 K. b Ratio of normal C -aldehyde to iso-C -aldehyde. c 10 wt %Co/active carbon without passivation. 7 7 (0.06 g) was used as catalyst.
d
Co2(CO)8
1-hexene) was constant over 14 h and the total turnover number was 500. The main reactions occurred in this system are shown as follows:
CH2dCH-(CH2)3CH3 h CH3CHdCH (CH2)2CH3 h CH3CH2CHdCHCH2CH3 (1) CH2dCH-(CH2)3CH3 + CO + H2 f CHO(CH2)5CH3 + CH3CHO(CH2)4CH3 (2)
Figure 2. The catalytic performance of 1 wt % Rh/activated carbon (403 K, 3.0 MPa, and n-octane as solvent.).
Equation 1 presents the isomerization reaction, during which the isomers of 1-hexene were formed. Equation 2 shows the hydroformylation of 1-hexene, where naldehyde and i-aldehyde were produced. Catalytic Performance over Co/Active Carbon Catalyst. Reaction Products. In the blank test without any solvents or catalyst, no 1-hexene conversion was observed at 403 K and 3.0 MPa over 10 wt % Co/ activated carbon. The comparative effects of solvents on the hydroformylation of 1-hexene are shown in Table 2. In the solvents such as benzene, hydrocarbon, toluene and THF, no oxygenates were formed and only a small amount of 1-hexene was converted to 2-hexene (isomer), even when the catalyst was not passivated. It is worth noting that the enhanced 1-hexene conversion was observed over Co/activated carbon in alcohol solvents, suggesting that alcohols dramatically promoted the hydroformalytion reaction under conditions of 3.0 MPa and 403 K. The catalytic activity in the presence of 1-propanol, 2-propanol, and ethanol were higher than that of methanol. The formation of acetal is shown as follows:
RCHO + 2R′OH f RCH(OR′)2 (acetal) + H2O (3) Figure 3. The performance of 1 wt % Rh/activated carbon in liquid and syngas continuously feed system (cetane as solvent; 3.0 MPa; 1 wt % Rh/activated carbon, 0.5 g; syngas (flow rate) ) 80 cm3/min; and 1-hexene (flow rate) ) 0.7 cm3/min).
the hydroformylated products increased slowly with the reaction time, consuming 1-hexene and other hexenes. In 6 h of reaction time, the yield of hydroformylated products reached about 60%. Therefore, the gradual decrease in the CO conversion rate should be attributed to the decrease in the reactants (hexenes) in the reactor. As shown in Figure 3, if 1-hexene and syngas were fed continuously, the reaction rate (CO conversion and
where R and R′ stand for the hydrocarbyl in the aldehyde and alcohol, respectively. RCHO is the aldehyde produced in the hydroformylation process, and R′OH is the alcohol solvent used in the reaction. However, in case of the Rh/activated carbon catalyst, the alcohol solvent gave worse results than those in n-octane or the nonsolvent system. Over the homogeneous catalyst, Co2(CO)8 showed high activities both in n-octane and methanol solvents. The reaction rate was high, but the life was short. After the reaction was completed, a black precipitate was detected. XRD failed to characterize the composition because of the black precipitate’s small quantity. It was
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Table 3. Effect of Ru-Added Catalyst on the Hydroformylation of 1-Hexene (403 K and 3.0 MPa) product yield (%)
catalyst
solvents
1-hexene converted (%)
oxygen
C7-aldehyde
C7-olefin
acetal
ester
isomer
hexane
0.5 wt % Ru/activated carbon 0.5 wt % Ru/activated carbon 10 wt %Co + 0.5 wt % Ru/activated carbon 10 wt %Co + 0.5 wt % Ru/activated carbon 10 wt %Co + 0.5 wt % Ru/activated carbon
none methanol none methanol n-octane
28.2 56.4 11.6 78.8 15.8
0.3 1.3 3.3 51.5 6.3
0.3 0.2 3.3 17.3 6.3
0 0 0 0.6 0
0 1.1 0 33.0 0
0 0 0 0.6 0
26.0 53.3 6.5 25.6 7.7
1.9 1.8 1.8 1.7 1.8
Figure 4. CO conversion versus time on stream over cobalt catalysts (403 K, 3.0 MPa, and methanol as solvent).
thought to be the complex of Cox(CO)y or Co metal (X and Y are uncertain.). Role of Alcohol Solvents. The characteristic feature of the heterogeneous catalyst was that its high activity can be generated under low pressure only in the alcohol solvent. The cobalt catalyst is easily oxidized, for example, by O2 or H2O, but difficult to reduce. The reduction of Con+ species by synthesis gas under the reaction conditions is hard to achieve but is promoted by alcohol. It is clearly shown in Table 2 that the promotive effect of alcohol is in the following order: i-C3H7OH > n-C3H7OH > C2H5OH > CH3OH, which is the order of the generation of reactive hydrogen. Ratio of n:i in the Products. The ratio of normal aldehyde to branched aldehyde is low, about 0.3-0.5, compared with the corresponding homogeneous cobalt catalyst (n:i is about 1.0, where n:i is the ratio of normal C7-aldehyde to iso-C7-aldehyde). The quite low n:i ratios should be attributed to the solvent effect of alcohol. The n:i ratio over Co2(CO)8-catalyzed reaction was about 0.4 in methanol, while it was about 1.0 in octane solvent. Both the branched and straight aldehydes and acetals are acceptable as clean fuels. Lower pressures, such as 2.0 MPa, were not applicable to this system. It should be noted that 3.0 MPa is the minimum pressure under our reaction conditions. Induction Period. The CO conversion over 10 wt % Co/activated carbon as a function of reaction time is presented in Figure 4. For the passivated Co/activated carbon, an induction period exists at the beginning of the reaction, lasting for about 2 h. No induction period was observed for the Co/activated carbon without passivation catalyst and for the Ru-promoted catalyst. We can assume that the induction period partly involves a reduction process in the presence of methanol solvent and synthesis gas, which provided more active metallic sites available for the hydroformylation.
The performance of the passivated catalyst with 0.5 wt % of added Ru promoter was improved significantly, with about 50% yield of oxygenates, a yield 17% higher than that for 10 wt % Co/activated carbon. The short induction period for the Ru-added catalyst in Figure 4 should be attributed to the spillover effect which promotes the reduction of oxidized cobalt, as was shown for Ru-Co/SiO2 catalyst.20 As shown in Table 3, without cobalt, even in the methanol solvent, 0.5 wt % Ru/activated carbon catalyst showed a high yield of isomer with 1-hexene conversion of about 57% while the oxygenates yield was only less than 2%. The results suggest that the active component of this catalyst system be cobalt. In addition, the hydroformylation activity of the Ru-added catalyst was also promoted by methanol solvent. Its hydroformylation activity was very low either in nonsolvent or in the octane solvent system. Decrease in the CO Conversion. As shown in Figure 4, CO conversion sharply decreased after reaching the maximum point. This decrease in CO conversion was thought due to the decrease of the reactants or to the deactivation of the catalyst. After a 6 h reaction time, over passivated Co/activated carbon, about 50% of 1-hexene was converted to products, which should result in the decrease in the rate of hydroformylation of 1-hexene. The variation in CO conversion with time on stream after the 1-hexene addition (36 mmol) was from 8.8% to 24.8% (Figure 5). This is the clear indication that the decrease in CO conversion was not because of the deactivation of the catalyst but because of the decrease in the reactant. Effect of Reaction Conditions. Effect of Temperature and Pressure. Figure 6 shows the results of 1-hexene hydroformylation over 10 wt % Co/activated carbon catalyst as a function of temperature. It should be noticed that the conversion of 1-hexene increased dramatically in the interval of 383-403 K and declined at higher temperatures. As expected, the isomeric product compositions of 1-hexene increased at elevated temperature and the yield of target oxygenates (aldehyde and acetal) decreased at temperatures higher than 403 K. A lower temperature is of benefit for hydroformylation, while higher temperature accelerates the isomerization of 1-hexene to 2- and 3-hexene. The influence of pressure of CO/H2 (1:1) on the hydroformylation of 1-hexene was also investigated at 403 K for 10 wt % Co/activated carbon catalyst. As shown in Table 4, high pressure resulted in the high yield of target products. It was found that the reaction hardly took place at 2.0 MPa. In addition, the induction period becomes shorter under high pressure, which implied that the formation of active cobalt species was easy and quick to form under high pressure. Catalytic Performance over Rh/Activated Carbon Catalyst. Effect of Solvent and Rh Loading Con-
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Table 4. Effect of Pressure on Hydroformylation of 1-Hexene over 10 wt % Co/Activated Carbona pressure (MPa)
1-hexene converted (%)
oxygen
C7-aldehyde
2.0 3.0 4.0 5.0
0.4 49.2 76.3 84.9
0 34.3 66.9 81.0
0 8.8 23.4 28.3
a
product yield (%) C7-olefin acetal 0 0.4 0.5 0.9
0 24.8 42.3 50.7
ester
isomer
n:ib
0 0.3 0.7 1.1
0.4 14.9 9.4 3.9
0 0.3 0.4 0.4
Reaction conditions: 403 K and methanol as solvent. b Ratio of normal C7-aldehyde to iso-C7-aldehyde. Table 5. Effects of Solvents on the Hydroformylation of 1-Hexenea
a
solvent
1-hexene converted (%)
C7-aldehyde
C7-olefin
none methanol H2O benzene n-octane
93.6 88.2 91.4 94.2 93.8
38.5 9.3 19.4 46.7 56.8
0 0.2 0 0 0
product yield/% acetal ester 0 15.2 0 0 0
0 0.1 0 0 0
isomer
hexane
52.8 61.9 70.7 45.3 34.1
2.3 1.5 1.3 2.2 2.9
Reaction conditions: 1 wt %Rh/activated carbon (0.2 g), 3.0 MPa, and 403 K. Table 6. Effects of Rh Loading on the 1-Hexene Hydroformylationa Rh (wt %)
1-hexene converted (%)
0.1 0.5 1.0 3.0
88.1 94.5 93.8 92.1
yield (%) C7-aldehyde isomer 25.4 41.2 56.8 58.6
60.8 50.9 34.1 30.3
hexane
n:ib
1.9 2.4 2.9 3.2
0.6 0.6 0.6 0.6
a Reaction conditions: n-octane as solvent, 3.0 MPa, and 403 K. b Ratio of normal C7-aldehyde to iso-C7-aldehyde.
Figure 5. Hydroformylation of 1-hexene over 10 wt % Co/ activated carbon after liquid input (403 K, 3.0 MPa, and methanol as solvent).
Figure 6. Effect of temperature on hydroformylation of 1-hexene over 10 wt % Co/activated carbon (3.0 MPa, methanol as solvent, and 6 h).
tent. Table 5 presents the catalytic performance of 1-hexene hydroformylation over 1 wt % Rh/activated carbon in various solvents, including the reaction result without any solvent for reference. At 3.0 MPa and 403 K, the yield of hexane is less than 3%, indicating that the hydrogenation of 1-hexene was almost suppressed. The main products were C7-aldehyde and isomers of 1-hexene. Acetal was also detected as product when methanol was used as solvent. Without any solvent, 1
wt % Rh/activated carbon showed catalytic activity with 1-hexene conversion of about 95% and C7-aldehyde yield of about 40%. The yield of oxygenates was about 20% in H2O and about 25% (including C7-aldehyde, C7alcohol, acetal, and ester) in methanol solvent. The results indicate that the existence of H2O and methanol in the reaction system suppresses the 1-hexene hydroformylation reaction. The higher catalytic activity was observed in the n-octane and benzene solvents. Relative higher activity was obtained with n-octane as solvent, with the yield of oxygenates 10% higher than that in benzene. Therefore, the results show that n-octane and benzene might facilitate the formation of active sites effective in the hydroformylation, while the solvents, such as H2O and methanol, might yield a negative effect. The reverse effect of alcohol solvent on cobalt catalyst (promotive) and rhodium catalyst (suppressive) should be explained as follows. The passivated Co catalyst was hard to reduce under the reaction conditions, and the reduction was improved by the alcohol solvent. On the other hand, over the Rh catalyst, since Rh can be easily reduced by H2 under reaction condition, the use of an alcohol promoter was not necessary. The protonic solvent (alcohol or water) will be dissociatively adsorbed (oxidative addition). This effect will reduce the electron density of Rh and then retard the CO adsorption because of the insufficient contribution of back-donation of electron from Rh to CO. The results on the Rh/activated carbon catalysts are summarized in Table 6. The CO conversion and yield of C7-aldehyde greatly increased with an increase with the Rh loading up to 1 wt %. It is noteworthy that, with increasing rhodium loading (from 0.1 wt % to 1.0 wt %), the yield of C7-aldehyde was improved from 25% to 57%, while the yield of isomer decreased from 61% to 34%.
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Table 7. Effect of Olefins on the Hydroformylation Reaction yield (%) C7-olefin acetal
catalyst
olefin
olefin converted (%)
oxygen
C7-aldehyde
10 wt % Co/activated carbon 0.5 wt % Rh/activated carbonb 10 wt % Co/activated carbona 0.5 wt % Rh/activated carbonb 10 wt % Co/activated carbona 0.5 wt % Rh/activated carbonb
1-hexene
49.2
34.3
8.8
1-hexene
94.5
41.2
41.2
0
2-hexene
48.4
42.1
11.9
0.5
2-hexene
58.9
42.5
42.5
0
1-octene
61.5
29.9
17.6
1.2
1-octene
93.3
40.8
40.8
0
a
0.4
24.8 0 29.4 0 10.9 0
ester
isomer
0.3
14.9
0
0
50.9
2.4
0.3
paraffin
6.3
0
0
13.4
3.0
0.2
26.3
5.3
0
47.1
5.4
Methanol as solvent. b n-Octane as solvent, solvent:olefin ) 2:1 (molar ratio), 3.0 MPa, and 403 K.
Table 8. Metal Content of Catalyst before and after Reaction (403 K)
catalyst
reaction pressure (MPa)
metal content of fresh catalyst (wt %)
metal content of used catalyst (wt %)
Co/activated carbona Co/activated carbona Co/activated carbona Rh/activated carbonb
3.0 4.0 5.0 3.0
10 10 10 1.0
8.1 7.2 5.1 0.8
a
Methanol as solvent. b n-Octane as solvent.
The increase in the activity with Rh content should be attributed to the increase in the metal site as shown in Table 1. Reactivity of Different Olefins. As shown in Table 7, 2-hexene and 1-octene were used as reactants for the hydroformylation over Co/activated carbon and Rh/ activated carbon catalysts. It was found that these two catalysts were also very active and selective for the hydroformylation of 2-hexene and 1-octene. It indicates that the heterogeneous process is feasible in the hydroformylation of long-chain olefins. It is important to note that the conversion to oxygenates is almost similar for 1-hexene and 2-hexene on cobalt and rhodium catalysts. With a small amount of 2-hexene isomerizing to 1-hexene and 3-hexene, the yield of oxygenates was very high in the products. For aldehydes, the selectivity toward different isomers decreased in the following order: 2-methylhexanal > n-heptanal > 2-ethylpentanal. The high activity and n:i ratio of 2-hexene hydroformylation here might suggest that the transformation between the adsorbed hexene isomersis very quick over the surface of the solid catalysts. Active Species of Catalyst. For the heterogeneous catalyst, some soluble species, such as cobalt carbonyls or rhodium carbonyls, could be formed and eluted from the support. In this work, the metal contents of catalyst were evaluated before and after the reaction by ICP (Table 8). It was found that even under mild reaction conditions (3.0 MPa and 403 K), the supported metal was dissolved, and the content of the dissolved metal changed considerably with the reaction pressure. About 20% of cobalt (or rhodium) was leached from the catalyst under 3.0 MPa, but about 50% under 5.0 MPa. It is difficult to distinguish the activity of homogeneous and heterogeneous catalyst in one reaction system. Thus, to clarify the different behavior of the homogeneous cata-
Figure 7. Catalytic performance of homogeneous and heterogeneous catalyst (Co2(CO)8, 0.06 g; 10 wt % Co/activated carbon, 0.2 g; and methanol as solvent).
lyst and heterogeneous catalyst, Co2(CO)8 and Co/ activated carbon (without passivation) were used at 3.0 MPa and 403 K, respectively, as shown in Figure 7. It is clear that the activity of the homogeneous catalyst was very high but decreased very quickly. Compared with the homogeneous catalyst, the activity of the heterogeneous catalyst was lower and longer. The contribution of the dissolved part can be evaluated from the data in Figure 7 as follows:
activity(total) ) activity(homogeneous) + activity (heterogeneous) ) activity(Co2(CO)8) + activity (Co/activatedcarbon) 27 ) (0.2)(52) + (0.8)[activity(Co/activatedcarbon)] (data at the summit point) Therefore, activity (Co/activatedcarbon) ) 21 From this rough evaluation, it is estimated that the contribution of homogeneous and heterogeneous part are 38% and 62%, respectively, at 3.0 MPa and 403 K, indicating the intrinsic activity for hydroformylation performance mainly comes from the heterogeneous part. The synergetic effect of heterogeneous and homogeneous catalyst was evaluated in the continuous liquid (1hexene) and gas feeding reactor by using 1 wt % Rh/
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activated carbon catalyst and high-boiling solvent (cetane), as shown in Figure 2. With the continuous liquid feeding, the constant CO conversion and hexene conversion were obtained over a reaction time of 14 h. In addition, the turnover number (TON) of the heterogeneous catalysts (Rh) reached about 500 after a reaction time of14 h. The stable catalytic activity gave us the convincing evidence that the solid catalyst was stable and that no serious metal loss occurred from the reaction system. The present system can be illustrated as follows: deposition
HRh(CO)x y\ z dissolution Rh(CO)x/activated carbon h Rh/activated carbon It means that some part of supported metal is dissolved as the metal-carbonyl complex (it also works as catalyst), which may occasionally decompose to metal or metal complex. Then this metal or metal complex deposit on the active carbon carrier and perform hydroformylation reaction effectively. During the reaction, the equilibrium of deposition and dissolution process exists. Consequently, both heterogeneous and homogeneous catalysts improve the hydroformylation process. Conclusions At 3.0 MPa and 403 K, Co/ showed the catalytic activity for the hydroformylation of olefins only in the
Li et al.
presence of alcohols, giving acetals and aldehydes. Induction period (1-2 h) was observed for the passivated catalyst. The use of lower alcohols is suggested to promote the reduction of cobalt mainly. The addition of small amount of Ru to the cobalt catalyst remarkably shortened the induction period, which should be attributed to the hydrogen spillover effect. For the Rh/activated carbon catalyst, on the other hand, water or methanol solvent had a negative effect on the hydroformylaition of 1-hexene. The activity of Rh/ activated carbon increased with the increasing of Rh up to 1 wt %. Some part of the metal was dissolved from the support, while the metal or metal complex in the solution deposit again on the support during the reaction. The equilibrium of deposition and dissolution exists. Both the supported metal and dissolved metal complex showed catalytic activity, while the supported metal showed lower activity than that of the dissolved. Continuous feed of olefin and syngas over the catalyst (Rh/activated carbon) showed a stable reaction rate over 14 h, giving the turnover number of 500. Acknowledgment. The authors wish to express their gratitude to Research for the Future Program of Japan Society for the Promotion of Science (JSPS) under the Project “Synthesis of Ecological High Quality Transportation Fuels” (Grant JSPS-RFTF98P01001). EF0202440
