Kinetics of the Homogeneous Catalytic Hydroformylation of 1

The hydroformylation of 1-octene using HRh(CO)[P(p-CF3C6H4)3]3 (1) in scCO2 was investigated at 50 °C and 273 atm. A kinetic rate expression was deve...
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetics of the Homogeneous Catalytic Hydroformylation of 1-Octene in Supercritical Carbon Dioxide with HRh(CO)[P(p-CF3C6H4)3]3 Daniel R. Palo and Can Erkey* Environmental Engineering Program, Department of Chemical Engineering, 191 Auditorium Road, University of Connecticut, Storrs, Connecticut 06269-3222

The first kinetic study of the rhodium-catalyzed homogeneous hydroformylation of olefins in supercritical carbon dioxide (scCO2) is reported. The hydroformylation of 1-octene using HRh(CO)[P(p-CF3C6H4)3]3 (1) in scCO2 was investigated at 50 °C and 273 atm. A kinetic rate expression was developed to represent the experimental data, having the form r1-octene ) kA0.48C0.84D0.50/(1 + KBB2.2), where A ) [H2], B ) [CO], C ) [1], and D ) [1-octene]. The observed kinetic behavior differs significantly from behavior of conventional systems employing the nonfluorinated analogue of 1, HRh(CO)(PPh3)3, in organic solvents. Most notable are the ∼0.5 order H2 dependence of the reaction rate, the lack of substrate inhibition, and the absence of a critical catalyst concentration. The altered kinetic behavior relative to conventional systems may be due to scCO2 solvent effects, the modified phosphine ligand, or the high concentrations of reactant gases in the system. This study also illustrates the usefulness of scCO2 as a mechanistic tool in homogeneous catalysis, especially for reactions involving gases such as H2 and CO. Introduction Hydroformylation is the oldest and most widely used homogeneous catalytic reaction involving olefins.1 This reaction involves the formation of branched or linear aldehydes by the addition of H2 and CO to a double bond according to the reaction

Once formed, aldehydes can be further reacted to produce a wide range of compounds such as alcohols (C4 and higher), diols, carboxylic acids, acroleins, acetals, ethers, and amines. Linear alcohols are by far the most important products and find extensive use in the PVC and detergent industries.2 While a number of transition metals show activity for the hydroformylation of olefins, only cobalt and rhodium are utilized commercially. Rhodium is far more active than cobalt, but is used much less frequently.1 Several different industrial processes have been developed that utilize cobalt- and rhodium-based complexes as homogeneous catalysts, mainly the species HCo(CO)4, HCo(CO)3PBu3, and HRh(CO)(PR3)3, where R is usually a phenyl group. The major difficulty with hydroformylation, as with any homogeneous catalytic process, is the separation and recovery of the catalyst from the reactant/ * To whom correspondence should be addressed: E-mail: [email protected]. Fax: (860) 486-2959.

product mixture. For lower olefins such as propylene and butene, Ruhrchemie/Rhone-Poulenc developed an elegant two-phase system. In this process, the gaseous reactants are contacted with an aqueous phase containing the catalyst (a water soluble analogue of HRh(CO)(PPh3)3 (2) with meta-sulfonated phenyl groups). The resulting liquid products are highly hydrophobic and are easily separated from the aqueous phase.3,4 However, the limitation of this two-phase process is that higher olefins will not dissolve in sufficient quantities in aqueous solutions for industrial application. Therefore, innovative catalyst recovery/recycle schemes are required to utilize rhodium-based catalysts in the hydroformylation of higher olefins. Supercritical carbon dioxide (scCO2), with its tunable solvent properties,5 has the potential to solve this problem by providing efficient operation, separation, and recycle by pressure-tuning under mild conditions. However, before an industrial hydroformylation process using scCO2 can be realized, much fundamental research needs to be performed on these systems, and the present study is one step toward such a goal. The area of reactions in supercritical fluids has been under increased investigation in recent years, as indicated by the number of review articles that have been published.6-10 Specifically, homogeneous catalysis in scCO2 has been the focus of many recent studies, including hydrogenation,11-15 carbon-carbon bond formation,16,17 and olefin metathesis.18 Several groups have also investigated homogeneous hydroformylation in scCO2 in the past few years. Rathke et al. focused on the kinetics and thermodynamics of reactions involving

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cobalt-carbonyl catalysts, finding improved n:iso ratios under supercritical conditions, and determining several thermodynamic parameters for the system using highpressure spectroscopic techniques.19-21 Guo and Akgerman investigated the same system and found both rate and n:iso ratio to be dependent upon pressure.22,23 Bach and Cole-Hamilton studied rhodium trialkylphosphine complexes and found similar rates as in toluene, with slightly improved selectivity.24 Several recent studies, however, have focused on the use of modified triphenylphosphine ligands to increase the solubility of rhodium species in scCO2. Leitner and co-workers synthesized perfluoroalkyl-substituted arylphosphines and aryl phosphites.25,26 These ligands allowed for increased solubility of rhodium species in scCO2 under hydroformylation conditions, illustrating a general method for the use of arylphosphorus compounds in homogeneous catalysis in scCO2. Our group recently reported the synthesis and use of RhCl(CO)[P(p-CF3C6H4)3]2 and HRh(CO)[P(p-CF3C6H4)3]3 (1) as hydroformylation catalysts in scCO2.27,28 These catalysts exhibited solubilities in the reaction mixture of at least 5.5 × 10-3 mol dm-3 (T ) 343 K, P ) 273 atm) and 7.6 × 10-3 mol dm-3 (T ) 323 K, P ) 273 atm), respectively, indicating that small perfluoroalkyl substituents have a dramatic effect on the scCO2 solubilities of rhodium/phosphine complexes. The behavior of 1 in the hydroformylation of various unsaturated compounds was also investigated, yielding similar behavior to that found for the conventional catalyst in organic media.29 Concerning 2, Wilkinson and co-workers performed much detailed study, setting forth most of the mechanistic information that is referenced elsewhere, and proposing a suitable catalytic cycle.30-35 More recently, thorough investigations of 2 were performed by Chaudhari and co-workers, who developed kinetic rate expressions using several different substrates and calculated the rate parameters and activation energy for each system.36-44 A comprehensive review article on 2 has also been published.45 The present study was conducted to determine the kinetic behavior of 1 in scCO2. A kinetic rate expression has been developed to describe the behavior of this catalyst in scCO2, and comparison is made to the kinetic behavior of the conventional, organic solvent based system employing 2. Experimental Section Materials. Tris[p-(trifluoromethyl)phenyl]phosphine was purchased from Strem Chemical Co. and used as received. The catalyst 1 was synthesized according to a procedure described elsewhere.28 Glass ampules were filled with 1 and then sealed under vacuum for use in the experiments. 1-Octene was obtained from Acros Chemicals, and was distilled from sodium metal under nitrogen immediately before each experiment. Hydrogen, carbon monoxide, and carbon dioxide (all 99.999% purity, grade 5.0) were obtained from Northeast Airgas, and were further deoxygenated before use. Nitrogen (99.999% purity, grade 5.0), also obtained from Northeast Airgas, was used as received. Experimental Setup. All kinetic experiments were conducted batchwise in a high-pressure, custom-manufactured, 51.5-cm3 stainless steel reactor equipped with a high-pressure sampling system, as shown in Figure 1. The reactor was fitted with a sapphire window (Sapphire Engineering, Inc.), poly-ether-ether-ketone

Figure 1. Experimental setup for hydroformylation in supercritical carbon dioxide (see the text for a description).

O-rings (Valco Instruments, Inc.), T-type thermocouple assembly (5-Omega Engineering, DP41-TC-MDSS), pressure transducer (6-Omega Engineering, PX01K15KGV), vent line (7), and rupture disk assembly (9Autoclave Engineers). For a typical hydroformylation experiment, the reactor (3) was charged with a catalyst ampule, a magnetic stir bar, and freshly distilled 1-octene under nitrogen in a glovebox. The reactor was sealed, placed on a magnetic stir plate (4), and then heated to reaction temperature by a circulating heater (12-Haake FJ) via an external heating coil. Once the reaction temperature was reached, the reactor was charged to the desired pressure with H2 and CO from gas cylinders (1a-b), and then pressurized with CO2 from a syringe pump (2-ISCO, 260D) to the reaction pressure of 273 atm. The ampule shattered upon pressurization of the reactor with CO, and the elapsed time from the introduction of CO until a homogeneous mixture was obtained was less than 5 min. The presence of a single fluid phase was confirmed visually, with the catalyst being completely dissolved in the reaction mixture. Internal reactor temperature was maintained at 50 ( 0.5 °C during each experiment. Kinetic data were obtained by sampling through the high-pressure sample loop (10) by filling with the supercritical fluid mixture, depressurizing into a sample vial (11), and flushing the loop with solvent from a reservoir (8). Concentrations of 1-octene, nonanal, and 2-methyloctanal were determined using a gas chromatograph (HP 6890, with 30-m HP-5 capillary column) which was calibrated using solutions of known concentration prepared gravimetrically. Caution: When working with high-pressure equipment, appropriate safety devices should be used, including but not limited to pressure relief mechanisms and explosion barriers. Results and Discussion Several major differences exist between the conventional organic solvent based system and the scCO2 system, and these are shown in Table 1. First of all, the concentrations of H2 and CO employed in the scCO2 experiments can be up to 2 orders of magnitude greater than those used in ethanol or toluene, due to very high solubilities of gases in scCO2. Second, the total pressure in the scCO2 system is significantly higher than for the conventional system, where the total pressure is determined by the partial pressures of H2 and CO (and any volatile substrates such as ethylene). Finally, depending on the temperature and total pressure in the scCO2

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Table 1. Comparison of Kinetic Rate Expressions for 1 in scCO2 and 2 in Organic Solventsa rate expression kA0.48C0.84D0.50b 1 + KBB2.2

103A (mol dm-3) 103B (mol dm-3) 103C (mol dm-3) D (mol dm-3)

solvent Ptot (atm)

0.63-2.5

0.04-0.96

1-octene

scCO2

21.4-62.1

6.2-153

0.33-1.2

0.20-1.56

1-hexene

ethanol

12-34

30-50

kABCD d (1 + KBB)2(1 + KDD)

8.9-30.8

8.4-121

1.0-8.0

0.18-2.2

1-dodecene toluene

29-54

50-70

kA1.5BD e (1 + KBB)2(1 + KDD)2

2.7-44.7

14.0-149

0.5-4.0

0.045-1.45

ethylene

24-48

60-100

toluene

273

T (°C)

130-2200

kABCD c (1 + KBB)2.5(1 + KDD)2.1

130-2600

substrate

50

a A ) [H ], B ) [CO], C ) [catalyst], D ) [substrate]. b Present study. c From ref 44; C is a corrected catalyst concentration. d From ref 2 37. e From ref 36; the rate showed linear dependence on catalyst concentration, but the rate expression was developed only for a catalyst concentration of 1.0 × 10-3 mol dm-3.

system, fluid densities (F(scCO2) ) 0.2-0.9 g cm-3) may be significantly different from those of organic systems (F = 0.7 g cm-3), and this may contribute to unique kinetic behavior in scCO2. Solubility and Phase Behavior. All kinetic experiments were conducted at 273 atm and 50 °C (F(scCO2) ) 0.86 g cm-3). In each experiment in this investigation, the reactor contents consisted of a single fluid phase in which complete dissolution of the catalyst was observed. Detailed thermodynamic investigations on the phase behavior of this multicomponent system were not performed. The solubility of 1 in scCO2 has not been precisely measured. However, in the course of conducting kinetic experiments, it was established that the solubility of 1 in the hydroformylation reaction mixture is at least 7.6 × 10-3 mol dm-3, which is the highest yet reported solubility in scCO2 for a catalyst of this type. All kinetic experiments were conducted at catalyst concentrations well below this value. This roughly 1000-fold solubility increase over the conventional catalyst was achieved using relatively small trifluoromethyl groups as modifiers. Further investigations into how the amounts and locations of perfluorinated substituents affect catalyst properties are currently being conducted. Reproducibility. The reproducibility of our experimental technique for the hydroformylation of 1-octene was determined using three separate experiments run under identical conditions. The agreement among separate experiments was excellent, with an average experimental error of ∼8%. Development of the Rate Expression. A kinetic rate expression for the hydroformylation of 1-octene in scCO2 using 1 was developed for the concentration ranges listed in the first row of Table 1 over a 1-octene conversion range of 0.0-0.9. The concentrations of H2 and CO at each data point were calculated from their initial concentrations using the reaction stoichiometry. The 1-octene concentration versus time data for each experiment were fitted to a polynomial, which was then differentiated to determine the slope (experimental reaction rate in mol dm-3 min-1) at each data point. These rate versus concentration data were then used to determine the kinetic rate expression by fitting the experimental reaction rate to an expression involving the concentrations of the four components. A sum-ofsquares minimization routine (Marquardt method) was utilized to determine the optimal rate expression. The expression which best represented the experimental

Figure 2. Comparison of calculated and experimental reaction rates.

data was

r1-octene )

kAaCcDd 1 + KBBb

where A ) [H2], B ) [CO], C ) [1], and D ) [1-octene], the rate is in units of mol dm-3 min-1, and concentrations are expressed in mol dm-3. The optimized rate parameters were determined to be k ) 6.2 ( 1.2 dm2.46 min-1 mol-0.82, KB ) 0.69 ( 0.16 dm6.6 mol-2.2, a ) 0.48 ( 0.04, b ) 2.2 ( 0.3, c ) 0.84 ( 0.03, d ) 0.50 ( 0.05, which yielded a minimized sum of squares value of 3.0 × 10-5 and an average absolute error of 15.5%. Figure 2 shows the correlation between the calculated and experimental reaction rates. Once the above rate expression was established, it was inserted into the design equation for a batch reactor, which was then numerically integrated (fourthorder Runge-Kutta method). Figures 3-6 show the predicted values (solid curves) along with the experimental data, illustrating the degree of fit for the calculated rate expression. The rate expression developed from this study differs significantly from those previously derived for 2 in organic solvents, as described below for each component. These results are interpreted using the generally ac-

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Figure 3. Hydroformylation of 1-octene using 1: effect of catalyst concentration (T ) 50 °C, P ) 273 atm, [1-octene]0 ) 0.96 mol dm-3, [H2]0 ) [CO]0 ) 1.1 mol dm-3).

Figure 4. Hydroformylation of 1-octene using 1: effect of initial 1-octene concentration (T ) 50 °C, P ) 273 atm, [H2]0 ) [CO]0 ) 1.1 mol dm-3, [1] ) 0.63 × 10-3 mol dm-3).

Figure 5. Hydroformylation of 1-octene using 1: effect of initial hydrogen concentration (T ) 50 °C, P ) 273 atm, [1-octene]0 ) 0.96 mol dm-3, [CO]0 ) 1.1 mol dm-3, [1] ) 0.63 × 10-3 mol dm-3).

cepted catalytic cycle for hydroformylation of olefins with 2, as shown in Figure 7. Effect of Catalyst Concentration. The observed 0.84 order rate dependence on [1] is not far from the first-order behavior expected based on previous studies. The studies conducted by Brown and Wilkinson35 and Chaudhari37,42-44 for 2 showed first-order rate dependence on catalyst concentration in benzene, toluene, or ethanol for several different unsaturated substrates.

Figure 6. Hydroformylation of 1-octene using 1: effect of initial carbon monoxide concentration (T ) 50 °C, P ) 273 atm, [1-octene]0 ) 0.96 mol dm-3, [H2]0 ) 1.0 mol dm-3, [1] ) 0.63 × 10-3 mol dm-3).

However, when vinyl acetate or 1-hexene was the substrate, Chaudhari noted a critical catalyst concentration (C*) of 0.2-0.4 × 10-3 mol dm-3, below which no reaction was observed. Beyond C*, the dependence on catalyst concentration was first order.43,44 The inactivity at low catalyst concentrations was attributed to a high substrate/catalyst ratio, leading to catalytically inactive dimer formation. This dimer formation probably involves the species IV and VIII of Figure 7. Initial reaction rate as a function of catalyst concentration for the scCO2 system is plotted in Figure 8. The data lie on a curve that passes through the origin, indicating that there is no critical catalyst concentration in the scCO2 system using 1 as catalyst when 1-octene is the substrate. Catalyst concentration also had a direct effect on the n:iso ratio of the aldehyde products. Selectivity increased from 3.0 to 3.3 to 3.4 to 3.9 as [1] increased from (0.63 to 1.27 to 2.54 to 7.61) × 10-3 mol dm-3. Such selectivity dependence on catalyst concentration is typical of rhodium/phosphine systems and is related to the degree of phosphine dissociation from 1 in solution (i.e., the formation of species II and VI in Figure 7). Effect of 1-Octene Concentration. The reaction shows a 0.5 order rate dependence on [1-octene] over the concentration range studied. Authors of previous studies have noted distinct substrate inhibition, which has been attributed to inactive dimeric rhodium species that form at high substrate/catalyst ratios.36,37,42-44 The nature of these inactive species has not been confirmed. For the conventional system, the substrate/catalyst ratio at which inhibition took over could be as low as 150. However, this ratio generally had values between 400 and 1000, depending on temperature and substrate. In the present study, substrate inhibition was absent for the substrate and catalyst concentrations studied, where the ratio varied from 600 to 1500. This is a significant change in kinetic behavior. It may be that in scCO2 the dimeric rhodium species do not form, or that they only form at substrate/catalyst ratios above 1500. [1-Octene]0 does not significantly affect the final n:iso ratio. However, it was observed that selectivity decreased slightly over the course of the reaction for each experiment in this study. Effect of Hydrogen Concentration. The rate of reaction is ∼0.5 order in [H2], a rate dependence not previously observed in organic solvents. This is a

Figure 7. Catalytic rate cycle for the hydroformylation of olefins using HRh(CO)L3 (adapted from ref 45).

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r1-octene )

Figure 8. Initial reaction rate as a function of catalyst concentration in the hydroformylation of 1-octene using 1 in scCO2 (T ) 50 °C, P ) 273 atm, [1-octene]0 ) 0.96 mol dm-3, [H2]0 ) [CO]0 ) 1.1 mol dm-3).

significant departure from the conventional system, where the rate is at least first order in [H2], depending on the substrate. Such first-order dependency is commonly interpreted to mean that the oxidative addition of H2 (conversion of IX to X in Figure 7) is the rate determining step.36,37,43,44 Since first-order behavior is not observed here, that may imply a change in the ratedetermining step. While this shift in behavior is most likely due to the high H2 concentrations employed, it may also be the result of scCO2 solvent effects. This result illustrates the usefulness of scCO2 not only as an alternative solvent, but also as a mechanistic tool for investigating kinetic behavior over a much wider range of conditions than normally employed. No correlation between n:iso ratio and [H2]0 was observed. Effect of Carbon Monoxide Concentration. Increasing [CO] had no positive effect on the reaction rate at all, which is easily explained by the high value of [CO]0 in the reactor. The conventional system exhibits positive dependence on [CO] only at very low concentrations [(0-17) × 10-3 mol dm-3] and shows drastic rate inhibition at higher concentrations, due to the formation of “dead-end” species XI and XII (see Figure 7). These species are coordinatively saturated, so oxidative addition of H2 cannot take place, preventing aldehyde formation. Since the scCO2 system is operated at [CO] values of at least 0.13 mol dm-3, the purely negative rate dependence on [CO] is not surprising. Increases in [CO] also have a detrimental effect on the n:iso ratio. The selectivity of the reaction decreased from 3.0 to 2.7 to 2.3 as [CO]0 increased from 1.1 to 1.6 to 2.2 mol dm-3. Mechanistically, increases in [CO] enhance the formation of the active species VI over against the active species II (see Figure 7). Since the CO ligand is much less sterically demanding than the phosphine, selectivity decreases as the [VI]/[II] ratio increases. Conclusion The kinetic behavior of 1 in the hydroformylation of 1-octene in scCO2 was investigated at 50 °C and 273 atm over a range of concentrations of H2, CO, 1, and 1-octene. Through a nonlinear least-squares fitting routine, the following rate expression (expressed in mol dm-3 min-1) was determined to be the best fit

6.2A0.48C0.84D0.50 1 + 0.69B2.2

yielding an average absolute error of 15.5%. The developed kinetic rate expression differs significantly from those previously obtained for 2 in organic media. The most significant observations are the ∼0.5 order rate dependence on [H2], the lack of substrate inhibition, and the absence of a critical catalyst concentration. This may be due to several factors, including scCO2 solvent effects, the modified phosphine ligands, and the increased H2 and CO concentrations relative to conventional systems. These results illustrate the usefulness of scCO2 as a mechanistic tool for reactions that involve reactant gases such as H2 and CO, where much higher concentrations can be investigated. Further studies, including in situ spectroscopy, should shed more light on the observed kinetic behavior. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the ACS for partial support of this research (ACS-PRF #32299-AC1). Additional support was provided by the State of Connecticut Critical Technologies Fund. We would also like to thank Dr. Michael Cutlip for his assistance during the preparation of the manuscript. Literature Cited (1) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis; 2nd ed.; Wiley: New York, 1992. (2) Applied Homogeneous Catalysis with Organometallic Compounds; Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim, Germany, 1996; Vol. 1. (3) Kuntz, E. G. Homogeneous Catalysis in Water. CHEMTECH 1987, 17, 570. (4) Cornils, B.; Kuntz, E. Introducing TPPTS and Related Ligands for Industrial Biphasic Processes. J. Organomet. Chem. 1995, 502, 177. (5) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice, 2nd ed.; Butterworth: Boston, 1994. (6) Jessop, P. G.; Ikariya, T.; Noyori, R. Homogeneous Catalysis in Supercritical Fluids. Chem. Rev. 1999, 99, 475. (7) Morgenstern, D. A.; LeLacheur, R. M.; Morita, D. K.; Borkowsky, S. L.; Feng, S.; Brown, G. H.; Luan, L.; Gross, M. F.; Burk, M. J.; Tumas, W. In Green Chemistry: Designing Chemistry for the Environment; Anastas, P. T., Williamson, T. C., Eds.; American Chemical Society: Washington, D.C., 1996. (8) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at Supercritical Conditions: Applications and Fundamentals. AIChE J. 1995, 41, 1723. (9) Kaupp, G. Reactions in Supercritical Carbon Dioxide. Angew. Chem., Int. Ed. Engl. 1994, 33, 1452; Angew. Chem. 1994, 106, 1519. (10) Subramaniam, B.; McHugh, M. A. Reactions in Supercritical Fluids-A Review. Ind. Eng. Chem. Process Des. Dev. 1986, 25, 1. (11) Jessop, G. P.; Hsiao, Y.; Ikariya, T.; Noyori, R. Homogeneous Catalysis in Supercritical Fluids: Hydrogenation of Supercritical Carbon Dioxide to Formic Acid, Alkyl Formates, and Formamides. J. Am. Chem. Soc. 1996, 118, 344. (12) Jessop, P. G.; Ikariya, T.; Noyori, R. Selectivity for Hydrogenation or Hydroformylation of Olefins by Hydridopentacarbonylmanganese(I) in Supercritical Carbon Dioxide. Organometallics 1995, 14, 1510. (13) Jessop, G. P.; Ikariya, T.; Noyori, R. Homogeneous Catalysis in Supercritical Fluids. Science 1995, 269, 1065. (14) Burk, M. J.; Feng, S.; Gross, M. F.; Tumas, W. Asymmetric Catalytic Hydrogenation Reactions in Supercritical Carbon Dioxide. J. Am. Chem. Soc. 1995, 117, 8278. (15) Jessop, G. P.; Ikariya, T.; Noyori, R. Homogeneous Catalytic Hydrogenation of Supercritical Carbon Dioxide. Nature 1994, 368, 231.

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Received for review May 13, 1999 Accepted July 13, 1999 IE990336L