Ind. Eng. Chem. Res. 1997, 36, 4581-4585
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Hydroformylation of Propylene in Supercritical Carbon Dioxide Yang Guo and Aydin Akgerman* Chemical Engineering Department, Texas A&M University, College Station, Texas 77843-3122
Homogeneously catalyzed propylene hydroformylation using Co2(CO)8 precatalyst in supercritical carbon dioxide (SCCO2) was studied in a batch reactor. The experiments were carried out at temperatures of 66-108 °C and at pressures of 1350-2700 psig. The pseudo-first-order rate constant was calculated for each run employing an empirical kinetic expression from the literature. It has been observed that at constant temperature the observed rate constant increased with pressure. The activation energy of the reaction in SCCO2 was determined at 1650 and 2400 psig, and the value 23.3 ( 1.4 kcal/mol was comparable to values of 27-35 kcal/ mol obtained in conventional organic solvents. The product selectivity was determined at different temperatures and pressures, and it was observed that, at the constant temperature of 88 °C, the product selectivity increases from 2.7 to 4.3 as the pressure doubles from 1350 to 2700 psig. Introduction Synthesis of many specialty chemicals involves the use of selective homogeneous catalysts, and the reactions are usually carried out in an organic solvent. The solvents used are coming under close scrutiny because of environmental regulatory restrictions due to their toxicity. There is a great push in industry today to replace these solvents with environmentally benign solvents, such as water-based solvents. However, most of the catalytic materials used in homogeneous catalysis are not soluble in aqueous media. Furthermore, even if a water-soluble catalyst is synthesized, the organic reactants and products may not be soluble in water, hence resulting in solubility and/or mass-transferlimited reaction rates in water-based solvents. A new approach is to use supercritical fluids (SCFs), specifically supercritical carbon dioxide (SCCO2), as the reaction medium for homogeneous catalysis. Although SCFs have seldom been explored for this purpose, they have properties that could make them nearly the ideal medium for conducting homogeneous catalytic processes. Specifically, SCCO2 is inert to most reactions, nontoxic, cheap, readily available, and environmentally acceptable. In addition, SCCO2 is also nonflammable; thus, its use does not introduce a safety hazard during operation. SCCO2 has already been proven useful as a solvent for extractions and separations (McHugh and Krukonis, 1994), and there are a number of commercialized applications. The research on the use of supercritical fluids as the reaction medium in place of more conventional solvents has been receiving increasing attention. The main reactions where SCFs (most of the time SCCO2) are used are Diels-Alder reactions, enzymatic catalysis, organometallic reactions, polymerization, hydroformylation, hydrogenation, oxidation, and co-oligomerization reactions. Recently, Kaupp (1994) and Savage et al. (1995) reviewed the published studies on reactions in supercritical fluids. They noted that many reactions, such as radical brominations and polymerizations, hydroformylations, CO2 hydrogenations, catalytic additions/ cycloadditions on CO2, and enzymatic reactions, have been shown to proceed as good or even better in SCCO2 * To whom all correspondence should be addressed. Telephone and Fax: 409-845-3375. Email: akg9742@ chennov2.tamu.edu. S0888-5885(97)00137-1 CCC: $14.00
than in conventional solvents. Savage et al. (1995) present an excellent summary of the literature on fundamental studies on simple elementary reactions in SCFs. The major advantage of using SCFs as the reaction medium is the possibility to adjust the reaction rate constant by the system pressure (Johnston and Haynes, 1987; Paulaitis and Alexandre, 1987) because of the very large negative partial molar volumes in supercritical systems (Erkey and Akgerman, 1990). Further, gases are completely miscible with supercritical fluids; therefore, gas-phase reactants’ concentrations in the supercritical media would be much higher than achievable in normal liquid solvents. In addition, due to high diffusivities in supercritical fluids combined with low viscosities, mass-transfer rates in supercritical media are expected to be higher than those in liquid solvents. The objective of our study was to assess the potential benefits of using supercritical carbon dioxide as the solvent in homogeneous catalysis. We were specifically interested in (1) the replacement of organic solvents traditionally used in homogeneous catalysis with environmentally benign supercritical fluids and (2) the control of reaction rate and product selectivity by adjustment of solvent density (varying the pressure and/ or temperature). We used propylene hydroformylation as the model reaction. Hydroformylation is a type of CO insertion reaction, typically represented by the overall reaction:
olefin + CO + H2 f aldehyde Hydroformylation is of tremendous industrial importance: butyraldehyde and several higher aliphatic aldehydes and alcohols (detergents) are prepared industrially all over the world by this reaction using either cobalt- or rhodium-based homogeneous catalysts. The reaction has been thoroughly studied (Cornils, 1980; Wender and Pino, 1977). Hydroformylation is done in various solvents ranging from the reactants themselves to solvents like benzene and halogenated alkanes. Recently, Rathke et al. (1991, 1992) reported on cobaltcatalyzed hydroformylation of propylene in SCCO2. They have studied the reaction by means of highpressure NMR and reported that the reaction proceeded cleanly in SCCO2 with somewhat of an improved yield of linear to branched butyraldehyde. The concentration of catalyst they used is 0.017 M (5.8 g/L). They carried © 1997 American Chemical Society
4582 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 1. Schematic diagram of the experimental assembly.
out the reaction at 80 °C and SCCO2 density of 0.5 g/mL (i.e., P ) 2400 psi). Their focus was more on the determination of the catalytic intermediates, and the reaction rate studies were not well-defined since they had no means of mixing in the NMR cell. We have extended their studies in a well-defined reactor and studied the reaction at different conditions up to 100% propylene conversion. The experimental results showed that (1) at a constant temperature the observed rate constant increased with an increase of the system pressure; (2) the activation energy of the reaction in SCCO2 was comparable to those values in conventional organic solvents; and (3) the product selectivity (linear to branched aldehyde ratio), which normally depends on the catalyst ligands as well as the type of organic solvents, can be affected with the change of the density of the reaction mixture. Experimental Section Materials and Apparatus. Propylene (Scott Specialty Gases, CP grade, 99.0%), carbon monoxide (Airco, 99.5%), carbon dioxide (Airco, 99.5%), and hydrogen (Airco, 99.9%) were used as received. The catalyst used in this study was Co2(CO)8, octacarbonyldicobalt (Johnson Matthey, 95%, with 5% n-hexane as stabilizer), which in reality is the precatalyst. The catalyst was stored in a jar in an argon box. A certain amount of the catalyst was weighed and sealed under vacuum in an ampule (Wheaton, 1 mL Gold Band) before it was placed into the reactor. A schematic diagram of the experimental setup is shown in Figure 1. The main part of the system consisted of a 300 mL vessel with a Magnodrive stirrer (Autoclave Engineers). The actual volume of the reactor vessel was 282 mL. The pressure was measured with a Heise 12401 gauge. The temperature was monitored with a thermocouple (Omega 115KC) that was placed in a thermowell inside the reactor. The reactor vessel was heated by a constant-temperature bath employing an Isotemp Immersion Circulator (Fisher Scientific 730)
and heating fluid (water for temperatures below 90 °C and CALFLO HTF for temperatures above). Gas samples from the reactor were trapped in a 75 mL stainless steel sample cylinder (Swagelok). The whole setup was connected with 1/8 in. 316 SS tubing and associated valves and fittings (Autoclave Engineers). Procedure. The reactor was cleaned before the start of each experiment. Since the catalyst is sensitive to air, it was weighed in an argon gas box and placed into ampules, and the ampules were sealed under vacuum. The catalyst was placed in the reactor in the ampule, and the vessel was then sealed. Helium gas was used to flush air from the whole system. Propylene was then charged to the vessel from the cylinder with a dip-tube. The amount of propylene charged to the reactor was determined and adjusted by monitoring the pressure in the reactor. Predetermined amounts of H2 and CO were then charged into the reactor from the gas cylinders via the pressure control. The concentrations of gas reactants are very low; e.g., at 2400 psig and 88 °C, the concentrations of propylene, H2, and CO are 0.8%, 4%, and 4% (mole), respectively. The reactor was then heated to the desired temperature (66-108 °C). When the temperature attained was about 5 °C less than the desired temperature, liquid CO2 was pumped through an ice bath to the reactor, employing an LDC Analytical MiniPump (Milton Roy) to bring the system close to the desired pressure. The final pressure adjustment was made by charging additional CO2 when the desired temperature was reached. The reactor stirrer was then switched on which also breaks the ampule and introduces the catalyst into the reaction mixture which is already at the desired temperature and pressure. The stirrer speed was 500 rpm for each run, which was sufficient to enable perfect mixing. The procedure of getting the reactor system to the desired conditions takes about 1-2 h. During this period there was no reaction, since the catalyst was inside the ampule and isolated from the reactants (also verified by blank runs using no catalyst). The reaction starts at the time when the stirrer was switched on and broke the ampule.
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Figure 2. Propylene conversion as a function of time at different pressures; T ) 88 °C and Wcat ) 1.0 g/batch.
Samples were taken by a double-valve sampling system. The samples were trapped in the tubing line (about 0.2 mL) by opening and then shutting off valve V5. The samples were then allowed to expand through V6 into a sampling cylinder (SCF phase to gas phase). Gasphase samples were then injected to a Carle GC equipped with a FID and a TCD, to quantify the amounts of propylene, CO, H2, and CO2. The reaction was stopped by switching off the stirrer and temperature bath circulator and removing the temperature bath. For most of the runs, the reactions were stopped after complete conversion of propylene (all limiting reactant consumed). For reactions at low temperatures, e.g., 78 °C, the reaction was stopped after 12 h; since the reaction is very slow at this condition, propylene conversion was not complete. While the temperature of the reactor decreased to room temperature (which took about 1 h), the pressure of the system was slowly decreased. The vessel was then charged with about 40 mL of n-hexane (Sigma, GC grade) and was cleaned. The washings were collected, and liquid samples were analyzed by a Varian 3000 GC equipped with a FID to quantify the linear and branched aldehyde products. Results and Discussion Reaction Rates. Figure 2 shows the propylene conversion as a function of reaction time at the constant temperature of 88 °C and different pressures of 1350, 1650, 2100, 2400, and 2700 psig. The amount of catalyst in each of these runs was 1 g/batch. As can be seen from the figure, at a given reaction time, conversion increases with increasing pressure up to 2100 psig and then levels off. A proposed empirical equation for the hydroformylation reaction rate is given by (Cornils, 1980):
PH2 r ) kCpWcat PCO
(1)
where Cp is the concentration of propylene, Wcat is the amount of catalyst, PH2 and PCO are the partial pressures of hydrogen and carbon monoxide, respectively, and k is the rate constant. We assumed this rate expression to be valid for synthesis in SCCO2 as well. The data presented in Figure 2 were obtained by charging the reactor with the same amount of reactants (propylene, hydrogen, and carbon monoxide) and the
Figure 3. First-order fitting of the experimental data at different pressures and constant temperature, 88 °C; Wcat ) 1.0 g/batch; x is propylene conversion.
Figure 4. Pseudo-first-order rate constant k′ as a function of pressure at T ) 88 °C and Wcat ) 1.0 g/batch.
same amount of catalyst for each run. Hence, the initial concentrations are the same in each run. In addition, the ratio of CO:H2 is 1 in each run. The reaction rate given above then reduces to a pseudo-first-order expression:
r ) k′Cp
k′ ) kWcat
PH2 PCO
(2)
Figure 3 shows the first-order fitting of the data given in Figure 2. The fits indicate that there is a trend in terms of the residuals, but for qualitative evaluation of the results they can be considered satisfactory. Pressure and Temperature Effects. The pressure effect of the observed pseudo-first-order rate constant is shown in Figure 4. The rate constant more than doubles as the pressure increases from 1350 to 2700 psig at 88 °C. The experiments were also carried out at constant pressure (1650 and 2400 psig) but at different temperatures. Propylene conversions at 2400 psig and at five different temperatures from 66 to 108 °C are presented in Figure 5. The Arrhenius dependency of the rate constant at two pressures is given in Figure 6. Although there is a distinct difference between the values of the
4584 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 5. Propylene conversion at different temperatures; P ) 2400 psig and Wcat ) 1.0 g/batch.
Figure 7. Change in the pseudo-first-order rate constant k′ with the amount of catalyst at T ) 88 °C and P ) 1350 psig. Table 2. Reaction Selectivity at Different Pressures, T ) 88 °C with 0.5 g of Catalyst/Batch (L/B: Linear vs Branched Butyraldehyde)
Figure 6. Arrhenius plot of the rate constants at P ) 1600 and 2400 psig; T° is the average temperature, Wcat ) 1.0 g/batch. Table 1. Reaction Selectivity at Different Pressures and Temperatures, with 1.0 g of Catalyst/Batch (L/B: Linear vs Branched Butyraldehyde) pressure (psig)
temperature (°C)
L/B
2400 2400 2400 2400
78 88 98 108
4.2 4.1 3.1 2.7
temperature (°C)
pressure (psig)
L/B
88 88 88 88 88
1350 1650 2100 2400 2700
2.7 3.0 4.2 4.1 4.3
rate constant at the two pressures at each temperature, the uncertainties in the k′ values do overlap a little and hence a single line is fitted to the data at both pressures. The activation energy obtained is 23.3 ( 1.4 kcal/mol, which is comparable to the reported values of 27-35 kcal/mol measured in organic solvents (Wender and Pino, 1977). The only products identified were n-butyraldehyde and isobutyraldehyde. The reaction selectivity was defined as the linear aldehyde (desired product) to branched aldehyde (byproduct) ratio and was measured at the termination of each run. Table 1 shows the product selectivities at different temperatures and different pressures. When the pressure is constant (2400 psig), the product selectivity decreases with an increase of the temperature; however, the reaction is not complete at the low temperature of 78 °C (Figure 5).
pressure (psig)
L/B
pressure (psig)
L/B
1350 2000
2.7 3.4
2400
3.9
Similar behavior with temperature was observed in organic solvents as well. At the constant temperature of 88 °C, the product selectivity increases from 2.7 to 4.3 as the pressure doubles from 1350 to 2700 psig. It should be noted that in this case 100% conversion was achieved at all conditions and the selectivity is measured after 100% conversion. This is the most significant observation of this study. It has been observed that the rate constant k′ and the product selectivity are a function of pressure at constant temperature. As mentioned above, the same amount of reactants (propylene, hydrogen, and carbon monoxide) and the same amount of catalyst were used in a constant-volume reactor with identical initial concentrations in each run. Therefore, it was believed that the observed effect is due to the pressure (the amount of carbon dioxide in the reactor). The increase in the reaction rate constant can also be explained in terms of the catalyst solubility in the reaction mixture since k′ ∝ Wcat. Although the amount of catalyst used in each run is less than that corresponding to the reported solubility in SCCO2 (Rathke et al., 1991), the solubility of the catalyst in the reaction mixture (involving propylene, hydrogen, carbon monoxide and carbon dioxide) is not known but is expected to increase with pressure. Hence, at low pressures, only a portion of the catalyst may be soluble and driving the reaction, whereas at high pressures all of it is becoming soluble. However, this does not explain the selectivity increase. There are several difficulties in the measurement of solubility of the catalyst in SCCO2. Co2(CO)8 is not a stable compound and will decompose in air at 51 °C, which is lower than the temperatures of all the experiments. Second, in hydroformylation, Co2(CO)8 is not the catalyst but is the catalyst precursor and the real catalyst is HCo(CO)4 (Cornils, 1980), which is formed by
Co2(CO)8 + H2 / 2HCo(CO)4
(3)
If the proposed kinetics is correct (eq 1), the rate constant should be proportional to the amount of the
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catalyst. Therefore, we conducted experiments with different amounts of the catalyst at 1350 psig (the lowest pressure where the expected solubility would be minimum) and 88 °C. The amount of catalyst used was set at 0.15, 0.25, 0.50, and 0.75 g for the runs. If the proposed kinetics is correct and if the catalyst is completely dissolved at these reaction conditions, the observed rate constant should be a linear function of the amount of the catalyst. The result of the pseudofirst-order rate constant vs the amount of the catalyst is shown in Figure 7. Considering the data uncertainty, up to a catalyst concentration of 0.5 g/batch the relationship may be considered linear, but it deviates from linearity beyond that point. These results indicate that either the applied kinetics (eq 2) might be incorrect or there is a solubility limitation at low pressures. At this stage this issue is still not resolved. We have conducted experiments using 0.5 g of catalyst/batch at 88 °C and different pressures and observed that the pseudo-firstorder rate constant did not vary appreciably with pressure. On the other hand, the product selectivity still increases with an increase of the pressure as shown in Table 2. Conclusions This is one of the very first detailed studies of a homogeneously catalyzed reaction in a supercritical fluid at conversions up to 100%. A proposed kinetics from the literature was used to analyze the experimental data. The determined activation energy for the reaction in SCCO2 is comparable to those measured in conventional organic solvents. We observed that the pseudo-first-order rate constant at a constant temperature is a function of pressure. The reaction selectivity is also affected with pressure and temperature. Although we do not have all the explanations yet, we are intrigued by the observations. A different, and potentially beneficial, phenomenon is taking place. Acknowledgment We thank Professor John P. Fackler, Jr., and Ms. Tiffany Grant of the Chemistry Department of Texas A&M University for the preparation of ampules of catalyst and very helpful discussions. This project has
been funded by Grants 104TAM0441 and 026TAM2441, in part with Federal Funds as part of the program of the Gulf Coast Hazardous Substance Research Center which is supported under cooperative agreement R815197 with the United States Environmental Protection Agency and in part with funds from the State of Texas as part of the program of the Texas Hazardous Waste Research Center. The contents do not necessarily reflect the views and policies of the U.S. EPA or the State of Texas nor does the mention of trade names or commercial product constitute endorsement or recommendation for use. Literature Cited Cornils, B. Hydroformylation, Oxo Synthesis, Roelen Reaction. In New Synthesis with Carbon Monoxide; Falbe, J., Ed.; Springer-Verlag: Berlin, 1980; pp 16 and 17. Erkey, C.; Akgerman, A. Chromatography Theory: Application to Supercritical Extraction. AIChE J. 1990, 36, 1715. Johnston, K. P.; Haynes, C. Extreme Solvent Effects on Reaction Rate Constants at Supercritical Fluid Conditions. AIChE J. 1987, 33, 2017. Kaupp, G. Reactions in Supercritical Carbon Dioxide. Angew. Chem., Int. Ed. Engl. 1994, 33 (14), 1452. McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice, 2nd ed.; Butterworths: Boston, 1994. Paulaitis, M. E.; Alexandre, G. C. Reactions in Supercritical Fluids. A Case Study of the Thermodynamics Solvent Effects on a Diels-Alder Reaction in Supercritical Carbon Dioxide. Pure Appl. Chem. 1987, 59 (1), 61. Rathke, J. W.; Klingler, R. J.; Krause, T. R. Propylene Hydroformylation in Supercritical Carbon Dioxide. Organometallics 1991, 10, 1350. Rathke, J. W.; Klingler, R. J.; Krause, T. R. Thermodynamics of the Hydrogenation of Dicobalt Octacarbonyl in Supercritical Carbon Dioxide. Organometallics 1992, 11, 585. Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reaction at Supercritical conditions: Applications and Fundamentals. AIChE J. 1995, 41 (7), 1723. Wender, I.; Pino, P. Organic Syntheses via Metal Carbonyls; John Wiley & Sons: New York, 1977; Vol. 2, pp 44-126.
Received for review February 10, 1997 Revised manuscript received May 16, 1997 Accepted June 3, 1997X IE9701377
X Abstract published in Advance ACS Abstracts, October 1, 1997.
