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 supercriticalcarbon dioxide (SCCO2) was studied in a batch reactor. The experiments were carried out attemperatures of 66-108 °C and at pressures of 1350-2700 psig. The pseudo-first-order rateconstant was calculated for each run employing an empirical kinetic expression from theliterature. It has been observed that at constant temperature the observed rate constantincreased with pressure. The activation energy of the reaction in SCCO2 was determined at1650 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 atdifferent temperatures and pressures, and it was observed that, at the constant temperature of88 °C, the product selectivity increases from 2.7 to 4.3 as the pressure doubles from 1350 to2700 psig.

Introduction

Synthesis of many specialty chemicals involves theuse of selective homogeneous catalysts, and the reac-tions are usually carried out in an organic solvent. Thesolvents used are coming under close scrutiny becauseof environmental regulatory restrictions due to theirtoxicity. There is a great push in industry today toreplace these solvents with environmentally benignsolvents, such as water-based solvents. However, mostof the catalytic materials used in homogeneous catalysisare not soluble in aqueous media. Furthermore, evenif a water-soluble catalyst is synthesized, the organicreactants and products may not be soluble in water,hence resulting in solubility and/or mass-transfer-limited reaction rates in water-based solvents.A new approach is to use supercritical fluids (SCFs),

specifically supercritical carbon dioxide (SCCO2), as thereaction medium for homogeneous catalysis. AlthoughSCFs have seldom been explored for this purpose, theyhave properties that could make them nearly the idealmedium for conducting homogeneous catalytic pro-cesses. Specifically, SCCO2 is inert to most reactions,nontoxic, cheap, readily available, and environmentallyacceptable. In addition, SCCO2 is also nonflammable;thus, its use does not introduce a safety hazard duringoperation. SCCO2 has already been proven useful as asolvent for extractions and separations (McHugh andKrukonis, 1994), and there are a number of commercial-ized applications.The research on the use of supercritical fluids as the

reaction medium in place of more conventional solventshas been receiving increasing attention. The mainreactions where SCFs (most of the time SCCO2) are usedare Diels-Alder reactions, enzymatic catalysis, orga-nometallic reactions, polymerization, hydroformylation,hydrogenation, oxidation, and co-oligomerization reac-tions. Recently, Kaupp (1994) and Savage et al. (1995)reviewed the published studies on reactions in super-critical fluids. They noted that many reactions, suchas radical brominations and polymerizations, hydro-formylations, CO2 hydrogenations, catalytic additions/cycloadditions on CO2, and enzymatic reactions, havebeen shown to proceed as good or even better in SCCO2

than in conventional solvents. Savage et al. (1995)present an excellent summary of the literature onfundamental studies on simple elementary reactions inSCFs. The major advantage of using SCFs as thereaction medium is the possibility to adjust the reactionrate constant by the system pressure (Johnston andHaynes, 1987; Paulaitis and Alexandre, 1987) becauseof the very large negative partial molar volumes insupercritical systems (Erkey and Akgerman, 1990).Further, gases are completely miscible with supercriti-cal fluids; therefore, gas-phase reactants’ concentrationsin the supercritical media would be much higher thanachievable in normal liquid solvents. In addition, dueto high diffusivities in supercritical fluids combined withlow viscosities, mass-transfer rates in supercriticalmedia are expected to be higher than those in liquidsolvents.The objective of our study was to assess the potential

benefits of using supercritical carbon dioxide as thesolvent in homogeneous catalysis. We were specificallyinterested in (1) the replacement of organic solventstraditionally used in homogeneous catalysis with envi-ronmentally benign supercritical fluids and (2) thecontrol of reaction rate and product selectivity byadjustment of solvent density (varying the pressure and/or temperature). We used propylene hydroformylationas the model reaction.Hydroformylation is a type of CO insertion reaction,

typically represented by the overall reaction:

Hydroformylation is of tremendous industrial impor-tance: butyraldehyde and several higher aliphatic al-dehydes and alcohols (detergents) are prepared indus-trially all over the world by this reaction using eithercobalt- or rhodium-based homogeneous catalysts. Thereaction has been thoroughly studied (Cornils, 1980;Wender and Pino, 1977). Hydroformylation is done invarious solvents ranging from the reactants themselvesto solvents like benzene and halogenated alkanes.Recently, Rathke et al. (1991, 1992) reported on cobalt-catalyzed hydroformylation of propylene in SCCO2.They have studied the reaction by means of high-pressure NMR and reported that the reaction proceededcleanly in SCCO2 with somewhat of an improved yieldof linear to branched butyraldehyde. The concentrationof catalyst they used is 0.017 M (5.8 g/L). They carried

* To whom all correspondence should be addressed.Telephone and Fax: 409-845-3375. Email: akg9742@chennov2.tamu.edu.

olefin + CO + H2 f aldehyde

4581Ind. Eng. Chem. Res. 1997, 36, 4581-4585

S0888-5885(97)00137-1 CCC: $14.00 © 1997 American Chemical Society

out the reaction at 80 °C and SCCO2 density of 0.5 g/mL(i.e., P ) 2400 psi). Their focus was more on thedetermination of the catalytic intermediates, and thereaction rate studies were not well-defined since theyhad no means of mixing in the NMR cell.We have extended their studies in a well-defined

reactor and studied the reaction at different conditionsup to 100% propylene conversion. The experimentalresults showed that (1) at a constant temperature theobserved rate constant increased with an increase of thesystem pressure; (2) the activation energy of the reactionin SCCO2 was comparable to those values in conven-tional organic solvents; and (3) the product selectivity(linear to branched aldehyde ratio), which normallydepends on the catalyst ligands as well as the type oforganic solvents, can be affected with the change of thedensity of the reaction mixture.

Experimental Section

Materials and Apparatus. Propylene (Scott Spe-cialty 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 usedin this study was Co2(CO)8, octacarbonyldicobalt (JohnsonMatthey, 95%, with 5% n-hexane as stabilizer), whichin reality is the precatalyst. The catalyst was storedin a jar in an argon box. A certain amount of thecatalyst was weighed and sealed under vacuum in anampule (Wheaton, 1 mL Gold Band) before it was placedinto the reactor.A schematic diagram of the experimental setup is

shown in Figure 1. The main part of the systemconsisted of a 300 mL vessel with a Magnodrive stirrer(Autoclave Engineers). The actual volume of the reactorvessel was 282 mL. The pressure was measured witha Heise 12401 gauge. The temperature was monitoredwith a thermocouple (Omega 115KC) that was placedin a thermowell inside the reactor. The reactor vesselwas heated by a constant-temperature bath employingan Isotemp Immersion Circulator (Fisher Scientific 730)

and heating fluid (water for temperatures below 90 °Cand CALFLO HTF for temperatures above). Gassamples from the reactor were trapped in a 75 mLstainless steel sample cylinder (Swagelok). The wholesetup was connected with 1/8 in. 316 SS tubing andassociated valves and fittings (Autoclave Engineers).Procedure. The reactor was cleaned before the start

of each experiment. Since the catalyst is sensitive toair, it was weighed in an argon gas box and placed intoampules, 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 usedto flush air from the whole system. Propylene was thencharged to the vessel from the cylinder with a dip-tube.The amount of propylene charged to the reactor wasdetermined and adjusted by monitoring the pressure inthe reactor. Predetermined amounts of H2 and CO werethen charged into the reactor from the gas cylinders viathe pressure control. The concentrations of gas reac-tants are very low; e.g., at 2400 psig and 88 °C, theconcentrations of propylene, H2, and CO are 0.8%, 4%,and 4% (mole), respectively. The reactor was thenheated to the desired temperature (66-108 °C). Whenthe temperature attained was about 5 °C less than thedesired temperature, liquid CO2 was pumped throughan ice bath to the reactor, employing an LDC AnalyticalMiniPump (Milton Roy) to bring the system close to thedesired pressure. The final pressure adjustment wasmade by charging additional CO2 when the desiredtemperature was reached. The reactor stirrer was thenswitched on which also breaks the ampule and intro-duces the catalyst into the reaction mixture which isalready at the desired temperature and pressure. Thestirrer speed was 500 rpm for each run, which wassufficient to enable perfect mixing. The procedure ofgetting the reactor system to the desired conditionstakes about 1-2 h. During this period there was noreaction, since the catalyst was inside the ampule andisolated from the reactants (also verified by blank runsusing no catalyst). The reaction starts at the time whenthe stirrer was switched on and broke the ampule.

Figure 1. Schematic diagram of the experimental assembly.

4582 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997

Samples were taken by a double-valve sampling system.The samples were trapped in the tubing line (about 0.2mL) by opening and then shutting off valve V5. Thesamples were then allowed to expand through V6 intoa sampling cylinder (SCF phase to gas phase). Gas-phase samples were then injected to a Carle GCequipped with a FID and a TCD, to quantify theamounts of propylene, CO, H2, and CO2. The reactionwas stopped by switching off the stirrer and tempera-ture bath circulator and removing the temperature bath.For most of the runs, the reactions were stopped aftercomplete conversion of propylene (all limiting reactantconsumed). For reactions at low temperatures, e.g., 78°C, the reaction was stopped after 12 h; since thereaction is very slow at this condition, propylene conver-sion was not complete. While the temperature of thereactor decreased to room temperature (which tookabout 1 h), the pressure of the system was slowlydecreased. The vessel was then charged with about 40mL of n-hexane (Sigma, GC grade) and was cleaned.The washings were collected, and liquid samples wereanalyzed by a Varian 3000 GC equipped with a FID toquantify the linear and branched aldehyde products.

Results and Discussion

Reaction Rates. Figure 2 shows the propyleneconversion as a function of reaction time at the constanttemperature of 88 °C and different pressures of 1350,1650, 2100, 2400, and 2700 psig. The amount ofcatalyst in each of these runs was 1 g/batch. As can beseen from the figure, at a given reaction time, conversionincreases with increasing pressure up to 2100 psig andthen levels off.A proposed empirical equation for the hydroformyla-

tion reaction rate is given by (Cornils, 1980):

where Cp is the concentration of propylene, Wcat is theamount of catalyst, PH2 and PCO are the partial pres-sures of hydrogen and carbon monoxide, respectively,and k is the rate constant. We assumed this rateexpression to be valid for synthesis in SCCO2 as well.The data presented in Figure 2 were obtained bycharging the reactor with the same amount of reactants(propylene, hydrogen, and carbon monoxide) and the

same amount of catalyst for each run. Hence, the initialconcentrations are the same in each run. In addition,the ratio of CO:H2 is 1 in each run. The reaction rategiven above then reduces to a pseudo-first-order expres-sion:

Figure 3 shows the first-order fitting of the data givenin Figure 2. The fits indicate that there is a trend interms of the residuals, but for qualitative evaluation ofthe results they can be considered satisfactory.Pressure and Temperature Effects. The pressure

effect of the observed pseudo-first-order rate constantis shown in Figure 4. The rate constant more thandoubles as the pressure increases from 1350 to 2700 psigat 88 °C.The experiments were also carried out at constant

pressure (1650 and 2400 psig) but at different temper-atures. Propylene conversions at 2400 psig and at fivedifferent temperatures from 66 to 108 °C are presentedin Figure 5. The Arrhenius dependency of the rateconstant at two pressures is given in Figure 6. Althoughthere is a distinct difference between the values of the

Figure 2. Propylene conversion as a function of time at differentpressures; T ) 88 °C and Wcat ) 1.0 g/batch.

r ) kCpWcat

PH2

PCO(1)

Figure 3. First-order fitting of the experimental data at differentpressures and constant temperature, 88 °C; Wcat ) 1.0 g/batch; xis propylene conversion.

Figure 4. Pseudo-first-order rate constant k′ as a function ofpressure at T ) 88 °C and Wcat ) 1.0 g/batch.

r ) k′Cp k′ ) kWcat

PH2

PCO(2)

Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4583

rate constant at the two pressures at each temperature,the uncertainties in the k′ values do overlap a little andhence 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 27-35kcal/mol measured in organic solvents (Wender andPino, 1977).The only products identified were n-butyraldehyde

and isobutyraldehyde. The reaction selectivity wasdefined as the linear aldehyde (desired product) tobranched aldehyde (byproduct) ratio and was measuredat the termination of each run. Table 1 shows theproduct selectivities at different temperatures and dif-ferent pressures. When the pressure is constant (2400psig), the product selectivity decreases with an increaseof the temperature; however, the reaction is not com-plete at the low temperature of 78 °C (Figure 5).

Similar behavior with temperature was observed inorganic solvents as well. At the constant temperatureof 88 °C, the product selectivity increases from 2.7 to4.3 as the pressure doubles from 1350 to 2700 psig. Itshould be noted that in this case 100% conversion wasachieved at all conditions and the selectivity is mea-sured after 100% conversion. This is the most signifi-cant observation of this study.It has been observed that the rate constant k′ and the

product selectivity are a function of pressure at constanttemperature. As mentioned above, the same amountof reactants (propylene, hydrogen, and carbon monoxide)and the same amount of catalyst were used in aconstant-volume reactor with identical initial concentra-tions in each run. Therefore, it was believed that theobserved effect is due to the pressure (the amount ofcarbon dioxide in the reactor). The increase in thereaction rate constant can also be explained in termsof the catalyst solubility in the reaction mixture sincek′ ∝ Wcat. Although the amount of catalyst used in eachrun is less than that corresponding to the reportedsolubility in SCCO2 (Rathke et al., 1991), the solubilityof the catalyst in the reaction mixture (involving pro-pylene, 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 catalystmay be soluble and driving the reaction, whereas at highpressures all of it is becoming soluble. However, thisdoes not explain the selectivity increase. There areseveral difficulties in the measurement of solubility ofthe catalyst in SCCO2. Co2(CO)8 is not a stable com-pound and will decompose in air at 51 °C, which is lowerthan the temperatures of all the experiments. Second,in hydroformylation, Co2(CO)8 is not the catalyst but isthe catalyst precursor and the real catalyst is HCo(CO)4(Cornils, 1980), which is formed by

If the proposed kinetics is correct (eq 1), the rateconstant should be proportional to the amount of the

Figure 5. Propylene conversion at different temperatures; P )2400 psig and Wcat ) 1.0 g/batch.

Figure 6. Arrhenius plot of the rate constants at P ) 1600 and2400 psig; T° is the average temperature, Wcat ) 1.0 g/batch.

Table 1. Reaction Selectivity at Different Pressures andTemperatures, with 1.0 g of Catalyst/Batch (L/B: Linearvs Branched Butyraldehyde)

pressure(psig)

temperature(°C) L/B

temperature(°C)

pressure(psig) L/B

2400 78 4.2 88 1350 2.72400 88 4.1 88 1650 3.02400 98 3.1 88 2100 4.22400 108 2.7 88 2400 4.1

88 2700 4.3

Figure 7. Change in the pseudo-first-order rate constant k′ withthe 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 vsBranched Butyraldehyde)

pressure (psig) L/B pressure (psig) L/B

1350 2.7 2400 3.92000 3.4

Co2(CO)8 + H2 / 2HCo(CO)4 (3)

4584 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997

catalyst. Therefore, we conducted experiments withdifferent amounts of the catalyst at 1350 psig (thelowest pressure where the expected solubility would beminimum) and 88 °C. The amount of catalyst used wasset at 0.15, 0.25, 0.50, and 0.75 g for the runs. If theproposed kinetics is correct and if the catalyst iscompletely dissolved at these reaction conditions, theobserved rate constant should be a linear function ofthe amount of the catalyst. The result of the pseudo-first-order rate constant vs the amount of the catalystis shown in Figure 7. Considering the data uncertainty,up to a catalyst concentration of 0.5 g/batch the rela-tionship may be considered linear, but it deviates fromlinearity beyond that point. These results indicate thateither the applied kinetics (eq 2) might be incorrect orthere is a solubility limitation at low pressures. At thisstage this issue is still not resolved. We have conductedexperiments using 0.5 g of catalyst/batch at 88 °C anddifferent pressures and observed that the pseudo-first-order rate constant did not vary appreciably withpressure. On the other hand, the product selectivitystill increases with an increase of the pressure as shownin Table 2.

Conclusions

This is one of the very first detailed studies of ahomogeneously catalyzed reaction in a supercriticalfluid at conversions up to 100%. A proposed kineticsfrom the literature was used to analyze the experimen-tal data. The determined activation energy for thereaction in SCCO2 is comparable to those measured inconventional organic solvents. We observed that thepseudo-first-order rate constant at a constant temper-ature is a function of pressure. The reaction selectivityis also affected with pressure and temperature. Al-though we do not have all the explanations yet, we areintrigued by the observations. A different, and poten-tially beneficial, phenomenon is taking place.

Acknowledgment

We thank Professor John P. Fackler, Jr., and Ms.Tiffany Grant of the Chemistry Department of TexasA&M University for the preparation of ampules ofcatalyst and very helpful discussions. This project has

been funded by Grants 104TAM0441 and 026TAM2441,in part with Federal Funds as part of the program ofthe Gulf Coast Hazardous Substance Research Centerwhich is supported under cooperative agreement R815197with the United States Environmental Protection Agencyand in part with funds from the State of Texas as partof the program of the Texas Hazardous Waste ResearchCenter. The contents do not necessarily reflect the viewsand policies of the U.S. EPA or the State of Texas nordoes the mention of trade names or commercial productconstitute endorsement or recommendation for use.

Literature CitedCornils, B. Hydroformylation, Oxo Synthesis, Roelen Reaction. InNew Synthesis with Carbon Monoxide; Falbe, J., Ed.;Springer-Verlag: Berlin, 1980; pp 16 and 17.

Erkey, C.; Akgerman, A. Chromatography Theory: Application toSupercritical Extraction. AIChE J. 1990, 36, 1715.

Johnston, K. P.; Haynes, C. Extreme Solvent Effects on ReactionRate 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 aDiels-Alder Reaction in Supercritical Carbon Dioxide. PureAppl. Chem. 1987, 59 (1), 61.

Rathke, J. W.; Klingler, R. J.; Krause, T. R. Propylene Hydro-formylation in Supercritical Carbon Dioxide. Organometallics1991, 10, 1350.

Rathke, J. W.; Klingler, R. J.; Krause, T. R. Thermodynamics ofthe Hydrogenation of Dicobalt Octacarbonyl in SupercriticalCarbon Dioxide. Organometallics 1992, 11, 585.

Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E.E. Reaction at Supercritical conditions: Applications andFundamentals. AIChE J. 1995, 41 (7), 1723.

Wender, I.; Pino, P. Organic Syntheses via Metal Carbonyls; JohnWiley & Sons: New York, 1977; Vol. 2, pp 44-126.

Received for review February 10, 1997Revised manuscript received May 16, 1997

Accepted June 3, 1997X

IE9701377

X Abstract published in Advance ACS Abstracts, October 1,1997.

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