CO2-Induced Miscibility of Fluorous and Organic Solvents for

is sufficiently close (in temperature) to miscibility to use this method, then the low-temperature solubility of the fluorous compound in the organic ...
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Ind. Eng. Chem. Res. 2004, 43, 4827-4832

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CO2-Induced Miscibility of Fluorous and Organic Solvents for Recycling Homogeneous Catalysts Kevin N. West, Jason P. Hallett, Rebecca S. Jones, David Bush, Charles L. Liotta, and Charles A. Eckert* Schools of Chemical & Biomolecular Engineering and Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0100

Fluorous biphasic chemistry enables the recovery of homogeneous catalysts but presents engineering challenges because of issues concerning phase contacting and solvent loss. The addition of gaseous CO2 to fluorous-organic biphasic systems results in a single homogeneous liquid phase at temperatures well below the upper critical solution temperature of the binary liquid mixture. This phenomenon is due to the high solubility of CO2 in both organic liquids and fluorocarbons, and it facilitates reactions and can also reduce fluorous solvent losses. We demonstrate two homogeneously catalyzed reactions, a hydrogenation and an epoxidation, which result in an enhancement in the turnover frequency of 70% and 50%, respectively, for the CO2merged phase relative to the fluorous biphasic system. This creates new opportunities for the use of fluorous-sequestered catalysts. Scheme 1. CO2 as a Miscibility Switcha

Introduction Fluorous biphasic chemistry1-5 is an interesting alternative solvent concept for reactions and separations. In this system, a homogeneous catalyst is modified with fluorinated ligands, imparting preferential solubility in the fluorous phase of a biphasic system. The mutual immiscibility of fluorous and organic solvents6-12 provides an opportunity for facile separations of reaction components and reuse of homogeneous catalysts that must be recycled for reasons of toxicity and/or cost. However, virtually all of the systems reported were studied on a very small scale (a few cubic centimeters) and are not amenable to scale-up for industrial application. First, mass-transfer limitations in biphasic systems may limit the overall reaction rate, and the vigorous agitation of a small separatory funnel does not scale well. In systems containing nonpolar solvents, such as toluene or cyclohexane, it is often possible to increase the temperature to induce miscibility.1 However, for more polar or thermally labile substrates, this is not a viable option because the consulate point is often more than 100 °C.13,14 Thus, any polar reactants must be diluted into a nonpolar solvent, introducing a volatile organic solvent into the process. Further, if the system is sufficiently close (in temperature) to miscibility to use this method, then the low-temperature solubility of the fluorous compound in the organic is appreciable and results in expensive and environmentally undesirable losses upon separation. A more feasible alternative is needed. In this work, CO2 is used to induce miscibility of fluorous-organic mixtures, even with more polar compounds such as methanol. When the reaction is complete, depressurizing the system releases the CO2 and induces a phase split. This provides a homogeneous reaction medium where the reactants are in intimate contact with the catalyst while maintaining the facile separation of a biphasic system. A cartoon schematic of * To whom correspondence should be addressed. Tel.: (404) 894-7070. Fax: (404) 894-9085. E-mail: [email protected].

a The first picture is a biphasic system under ambient conditions, with the reactants and catalyst in separate phases. CO2 is then added, homogenizing the liquids for reaction. Depressurization splits the liquids apart once more, segregating the catalyst from the products for easy separation and recycle.

the proposed process is provided in Scheme 1 and an example of the homogenization in Figure 1, where the two liquids are toluene and a commercial fluorocarbon solvent FC-75 (primarily a fluorinated butyltetrahydrofuran), and the color results from a fluorinated cobalt catalyst predominantly soluble in the fluorous phase prior to merging. It is important to note that there is a vapor phase present in each of these systems at the pressures investigated. Therefore, “biphasic” and “monophasic” refer only to the number of liquid phases present. The physical properties of fluorous solvents make them excellent candidates for alternative solvents. They are immiscible with water and are typically immiscible with organic compounds under ambient conditions, affording a facile separation process. Typical fluorous solvents have densities of around 1.8 g/cm3 and are clear, colorless, and odorless. Many gases have a high

10.1021/ie0308745 CCC: $27.50 © 2004 American Chemical Society Published on Web 07/09/2004

4828 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004

Figure 1. CO2 used to homogenize an organic (toluene, clear liquid) and a fluorous (FC-75, colored liquid) phase. The fluorous phase is colored because of a dissolved cobalt catalyst. Note the slight coloration of the organic phase (middle panel), indicating extensive mutual solubility just prior to miscibility.

solvents,5

solubility in these making them particularly attractive media for oxidations,15-19 hydrogenations,20-22 and hydroformylations.1,3,23 They are inert even at high temperatures, nonflammable, not ozone depleting,24 and nontoxic. However, they do persist in the environment and have high global warming potentials,25,26 making recovery of the fluorous solvent a critical concern in this research. The phase behavior of CO2 and fluorous moieties has been investigated previously. Ponytails similar to those used to make compounds soluble in a fluorous solvent also impart solubility in CO2. Perfluorinated species are very “CO2-philic” for several reasons, but the simplest is that they both have very low cohesive energy densities, substantially lower than hydrocarbons. The synthesis and CO2 solubility of many of these compounds, with fluorous ponytails or fluorous chelating agents, have been reported in the literature, for both supercritical CO227-33 and CO2-expanded organic solvents.34,35 The binary phase behavior of CO2 and perfluorohexane has been measured at 40 and 80 °C36 as part of a work examining the formation of gels from CO2-swollen semifluorinated alkanes. The data demonstrated that fluorination of an alkane can enhance the solubility of CO2 at a given pressure by comparing the phase behavior of CO2-hexane with that of CO2-perfluorohexane. However, no ternary systems were reported. The phase behavior in this work examines the ability of CO2 to make the fluorous and organic phases merge. CO2 is used as a cosolvent to homogenize the organic reactant phase with the catalyst-containing fluorous solvent phase. The study is an effort to understand where this phenomenon can be useful for making biphasic reactions homogeneous and removing masstransfer limitations, thus increasing the reaction rate without the addition of an organic diluent. Additionally, the hydrogenation of allyl alcohol and the epoxidation of cyclohexene are used as model reactions to demonstrate the effectiveness of using CO2 as a cosolvent for reactions in fluorous-organic media. Experimental Methods Materials. All organic chemicals used in the experiments were high-performance liquid chromatography grade, obtained from Sigma-Aldrich Chemical Co. and not further purified. Supercritical fluid chromatography grade carbon dioxide (Matheson Gas Co.) was purified to remove solid particulates and water. Ultrahigh-purity

Figure 2. Schematic of the apparatus used in all phase behavior measurements.

(UHP)-grade hydrogen and UHP/zero-grade oxygen were obtained from AirGas and used as received. FC40 (95% perfluorotributylamine) and FC-75 (90% perfluoro-2-butyltetrahydrofuran) were obtained from 3M. Perfluorohexane (99%) was obtained from Sigma-Aldrich. All liquids were degassed by three freeze-pumpthaw cycles prior to use. Apparatus. (i) Phase Behavior Apparatus. A schematic of the phase behavior apparatus is shown in Figure 2. The viewable high-pressure cell used was a 60 mL Jerguson gauge mounted on a rotor arm to facilitate mixing. Graphite seals were used in all experiments. All degassed organic and fluorous liquids were added from evacuated 100 mL Whitey pressure vessels using a gastight SGE Luer-lock syringe and 316 stainless steel HiP valves. CO2 was supplied by a highpressure syringe pump (Isco, Inc., model 500D) through stainless steel tubing (3.18 mm o.d. and 1.52 mm i.d.) to HiP valves at one end of the boiler gauge. An HiP hand syringe pump was used to increase or decrease the volume in the system through the same valve. The air bath was constructed from 1/2-in.-thick sheets of 20 in. × 24 in. polycarbonate. The air-bath temperature was monitored using an Omega K-type thermocouple accurate to (0.1 °C and controlled using an Omega CN76000 proportional-integral-derivative (PID) controller. The pressure was measured using a Druck DPI 260 gauge with a PDCR 910 transducer having a range of 0-20 MPa and calibrated to an accuracy of (0.01 MPa. (ii) Reactor Apparatus. Figure 3 is a schematic of the reaction apparatus. The reactor was a stainless steel batch reactor (Parr Instrument Co., model 4561) with a maximum working pressure of 208 bar and a maximum working temperature of 350 °C. Stirring was provided by a magnetic drive (Parr Instrument Co., model A1120HC) equipped with a paddle-type impeller. The seal between the stirrer and the reactor was made with a silver gasket, and the seal between the reactor body and the reactor head was made with a flat Teflon gasket held in a confined recess in the reactor head. The internal volume of the reactor was determined to be approximately 305 mL by filling the reactor with a known mass of pure CO2 and measuring the pressure at several temperatures. A PID temperature controller and tachometer (Parr Instrument Co., model 4842) were used to control the

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4829

Figure 3. Schematic of the reactor.

temperature of the reactor to (1 °C and the stirring speed to (5 rpm. The temperature inside the reactor was monitored with a type J thermocouple, and heat was provided by a high-temperature heating mantle housed in an aluminum shell. A digital pressure transducer (Heise, model 901B) was used to monitor constantly the reactor pressure, and a safety rupture disk, made of Inconel, was used to ensure that the pressure in the reactor remained below the reactor pressure rating. The Parr reactor was modified by replacing all of the standard valves and fittings with low-volume HiP valves and fittings to decrease the reactor dead volume. CO2 was supplied by a high-pressure syringe pump (Isco, Inc., model 500D) through stainless steel tubing (3.18 mm o.d. and 1.52 mm i.d.) to one inlet port on the reactor head. H2 gas was added to the reactor through stainless steel tubing (3.18 mm o.d. and 1.52 mm i.d.) to a second inlet port. All liquids were added to the vessel immediately prior to sealing. (iii) Fluorous G4-PAMAM Dendrimer Synthesis. This synthesis is taken from the literature.22 A total of 0.0001 mol of G4-PAMAM dendrimer was dissolved in 20 mL of water in a 125 mL Erlenmeyer flask. A total of 0.001 mol of K2PdCl4 (10:1 Pd-dendrimer) was then added to the mixture. HCl (0.1 M) was added and the solution pH monitored until a value of approximately 2 was achieved. Under these conditions, the terminal amine groups of the PAMAM dendrimer are protonated, and the Pd2+ ions are solvated through specific interactions with the interior amine groups only because of protonation of the 64 terminal amines. Addition of 0.006 mol of aqueous NaBH4 (5 M excess) reduced Pd2+ to Pd0 nanoparticles in the center of the dendrimer. Aqueous KOH (0.1 M) was next added to neutralize the acid and bring the solution pH to approximately 7. According to Chechik and Crooks,22 the size of the nanoparticles encapsulated within the dendrimer is easily controlled by the molar ratio of the metal salt to the dendrimer. A 10:1 loading creates Pd0 particles an average of 10 molecules (approximately 4.5 nm) in diameter. This was confirmed by scanning electron microscopy. A total of 0.096 mol of poly(hexafluoropropylene oxide-codifluoromethylene oxide)-carboxylic acid was dissolved in the aqueous solution to complex with the 64 terminal amines (plus 50% excess), creating fluorous quaternary ammonium salts. The mixture was allowed to stir for 30 min. The (brown) contents of the flask was then transferred to a 125 mL separatory funnel. Next, approximately 5

mL of perfluorohexane was added to the funnel, which was shaken vigorously for 5 min. The phases were allowed to separate for 1 h, and the characteristic brown palladium color was observed only in the perfluorohexane phase. (iv) Fluorous Cobalt Carboxylate Synthesis. This synthesis is also taken from the literature,37 but the perfluoropolyether acid is substituted for the straightchain acid. A total of 0.0050 mol of poly(hexafluoropropylene oxide-co-difluoromethylene oxide)-carboxylic acid was dissolved in 50 mL of water in a 125 mL Erlenmeyer flask. A total of 0.0050 mol of triethylamine was then added to this mixture, which was allowed to stir for 30 min. Next, 0.0025 mol of cobalt perchlorate was added to the flask. The solution was stirred for an additional 1 h. After this time, a second liquid phase was observed at the bottom of the flask. The contents of the flask was then transferred to a 125 mL separatory funnel. Approximately 15 mL of perfluorohexane was added to the funnel, which was then shaken vigorously for 5 min. The phases were allowed to separate for 1 h, and the characteristic purple cobalt color was only observed in the perfluorohexane phase. The lower fluorous phase was drained from the funnel and placed under vacuum overnight to evaporate the solvent. The cobalt salt was recovered as a highly viscous blue liquid. The cobalt carboxylate salt was confirmed by elemental analysis (QTI). Experimental Procedure. For the ternary phase behavior measurements, equal volumes of the organic and fluorous compounds were added into the empty, evacuated cell inside an air bath at 25 °C. The cell was rotated end-over-end vigorously as CO2 was added slowly from the syringe pump until the phases merged. Once thermal equilibrium had been achieved, the hand syringe pump was used to increase or decrease the pressure in the cell in order to pinpoint accurately the pressure at which the fluorous and organic phases merged. The miscibility pressure was determined to within (0.02 MPa. The pressure was cycled through the miscibility point three times and an average of the readings taken. All reactions were run in a 300 mL stirred Parr autoclave (see Figure 3). The dendrimer-encapsulated Pd0 catalyst used for the hydrogenations was synthesized according to the literature22 and dissolved in 45 mL of FC-40 (95% perfluorotributylamine), and then the resulting solution was transferred into the reactor. A total of 5 mL of allyl alcohol was added to the vessel, which was then sealed. H2 pressure of 1.45 MPa was added and a stoichiometric excess of 100% calculated using the ideal gas law and the vapor headspace of the cell. Next, CO2 pressure was introduced until the total pressure was 7.00 MPa. Merging of the phases with hydrogen present was verified using a windowed version of the autoclave. Depressurization of the reactor split the homogeneous phase, with the brown color characteristic of palladium observed in the fluorous phase only. The substrate-to-catalyst ratio was 5000:1. In the biphasic system, loading conditions and stirring were identical with those of the CO2-expanded system but without carbon dioxide. The vessel was held at 25 °C and stirred at approximately 700 rpm for 7.5 min to dissolve the gases. Stirring was then held at a constant rate of 250 rpm for the remainder of the reaction, usually 20 min. The cell was then depressurized, cooled, and emptied. Dur-

4830 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 Table 1. Miscibility Pressures Measured at 25 °C for Three Fluorous Solvents: Perfluorohexane, FC-75, and FC-40a

Figure 4. Miscibility pressures measured at 25 °C. Equal volumes of each organic liquid with perfluorohexane were used, and the pressure of CO2 required to merge the liquid phases was recorded.

ing depressurization, the vapor exhaust was bubbled through acetone, to recover organics, and added to the recovered organic phase. The organic phase was diluted in acetone and analyzed by mass spectroscopy (Agilent model 6890 gas chromatograph-mass spectrometer). The epoxidation catalyst was synthesized according to a literature procedure37 but with a perfluoropolyether acid substituted for the straight-chain acid. The catalyst was dissolved in 50 mL of FC-75, and the resulting solution was transferred into the reaction vessel, along with cyclohexene and a 3-fold excess of pivaldehyde, used as a co-oxidant. A 100% excess O2 pressure (0.7 MPa) was added, and CO2 pressure was introduced until the total pressure was 6.2 MPa. Merging of the phases with oxygen present was verified by using a windowed version of the Parr instrument. The substrate-tocatalyst ratio was 20:1. The reactions were run at 25 °C for 3-5 h.

solvent

C6F14

FC-75

FC-40

ethyl acetate tetrahydrofuran chloroform acetone cyclohexane propionic acid acetic acid toluene decane acetonitrile dimethylformamide nitromethane ethanol methanol decalin

1.65 1.92 1.93 2.15 2.64 2.74 2.76 3.23 3.61 4.00 4.41 4.42 4.44 4.59 5.39

1.78 1.92 nm 2.37 2.69 nm nm 3.35 nm 4.02 nm nm nm 4.74 5.77

2.57 2.58 nm 3.04 3.40 nm nm 3.42 4.45 nm nm nm nm nm nm

a

Not measured.

Scheme 2.a (a) Hydrogenation of Allyl Alcohol with (b) a Fluorous-Soluble Dendrimer-Encapsulated Pd0 Nanoparticle

Results and Discussion Phase Behavior. Figure 4 shows the CO2 pressure required to make a variety of organic solvents miscible with perfluorohexane. It appears that CO2 solubility in the organic phase is the biggest driving force for miscibility because the compounds in which CO2 is most soluble (ethyl acetate, tetrahydrofuran, and acetone) have low miscibility pressures while the compounds in which CO2 is less soluble (dimethylformamide, ethanol, and decalin) possess higher miscibility pressures. This is consistent with the view of CO2 as a cosolvent used to create a homogeneous, CO2-rich phase. In general, highly polar or self-associating solvents (in which CO2 is less soluble) require higher CO2 pressures (more dilution in CO2) for miscibility to be achieved. It is noteworthy that there are several solvents that do not follow this trend. These anomalies can probably be explained by the other binary interaction of interest, the perfluorohexane-organic interaction. For example, CO2 is highly soluble in both acetonitrile and nitromethane and less soluble in cyclohexane. However, the miscibility pressure of cyclohexane (2.64 MPa) is much lower than that of either acetonitrile (4.00) or nitromethane (4.42). This could be explained by the (qualitative) observation that perfluorocarbons are generally more soluble in nonpolar solvents than in polar solvents. Also, propionic acid and acetic acid possess low miscibility pressures; however, it is known that both of these compounds readily form dimers in solution, which may explain this

a The dendrimer was a generation 4 poly(amidoamine) modified with (c) fluorous carboxylates.

result. A full list of the miscibility pressures for each organic solvent with three fluorous solvents (perfluorohexane, FC-75, and FC-40) at 25 °C is provided in Table 1. It is clear that CO2 is an effective cosolvent for inducing miscibility between a fluorous catalyst phase and an organic reactant phase. The pressures required for this process are much milder (2-5 MPa) than those for supercritical fluid reactants (generally 10-20 MPa). Demonstration reactions were used to test the effect of this homogenization on the reaction rates. Hydrogenation Reaction. The hydrogenation of allyl alcohol to form n-propanol (Scheme 2) was used to test the hypothesis that a homogeneous reaction phase, with miscibility induced by dissolution of CO2, would offer enhancements in the reaction rate relative to the corresponding biphasic reaction. A fluorous-soluble dendrimer catalyst22 was used in these experiments. To make the comparison between biphasic and monophasic rates effectively, we ran each reaction in the same apparatus under similar conditions (i.e., reactant and catalyst amounts, stirring speed, temperature, and H2 partial pressure). Rates were determined from 20 min batch experiments and are expressed as turnover frequencies (TOFs), defined as the number of moles of

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4831 Table 2. Effect of Phase Merging on TOF for the Hydrogenation of Allyl Alcohol at 25 °Ca biphasic monophasic a

PCO2 (MPa)

TOF (h-1)

0 5.55

1093 ( 62 1858 ( 57

The fluorous solvent was FC-40. No organic solvent was used.

Scheme 3. (a) Epoxidation of Cyclohexene and (b) Fluorous Cobalt Carboxylate

While merging of the immiscible phases did lead to significant gains in the reaction rate, questions about the environmental impact of fluorous solvents cannot be ignored. Fluorous chemicals, though nontoxic, are known to be environmentally persistent.24,25 This, as well as the expense, constitutes another reason to use a fluorous-organic pair with very low mutual miscibility at ambient temperature and to use CO2 instead of temperature to homogenize the system. As a further attempt at eliminating the fluorous solvent, we examined the feasibility of using a neat catalyst phase.39 However, this approach was limited by the unrecoverable nature of the resultant catalyst tar. Current investigations are focused on the use of fluorous silica40 as a substitute for the fluorous solvent. Conclusions

Table 3. Effect of Phase Merging on TOF for the Epoxidation of Cyclohexene at 25 °Ca biphasic monophasic a

PCO2 (MPa)

TOF (h-1)

0 5.5

3.37 ( 0.06 2.25 ( 0.02

The fluorous solvent was FC-75. No organic solvent was used.

substrate converted per mole of catalyst per hour. The monophasic (CO2-expanded) system yielded an enhancement in the average TOF of 70% relative to the biphasic system. The results of the catalytic runs are shown in Table 2. The results are higher than those of Chechik and Crooks for the same catalyst in a biphasic system,22 probably because of variations in experimental conditions. In the absence of the catalyst, no product was detected after 24 h. Epoxidation Reaction. A second class of reactions examined for rate enhancement was the epoxidation of cyclohexene with gaseous O2 using pivaldehyde as a cooxidant (Scheme 3). The catalyst for these experiments was cobalt carboxylate.37 Once again, conditions were standardized for the comparison (reactant and catalyst amounts, stirring speed, temperature, and O2 partial pressure). Rates were determined from batch experiments of 3-5 h. For this reaction, the addition of enough CO2 to merge the phases increased the average TOF by 50%. In the absence of the catalyst, the biphasic reaction proceeded to only 5% completion in 15 h. Without O2, the reaction did not yield a detectable level of product after 24 h. In the absence of the pivaldehyde co-reactant, no detectable oxidation occurred in 5 h. The 50% increase in the conversion of the monophasic system relative to the biphasic system illustrates the effectiveness of the CO2 switch for this model epoxidation reaction. This provides a second important class of reactions where this process provides enhanced catalytic activity. The results are summarized in Table 3. Effect of the Stirring Rate. As the stirring speed is increased for the biphasic hydrogenation reaction, the rate approaches that of the monophasic reaction. However, even at high stirring rates (ca. 700 rpm), the monophasic rate is higher, indicating that contact between the organic reactant and the catalyst limits the biphasic reaction rate. Because an increase in the stirring speed does not affect the monophasic reaction, it appears that 250 rpm is sufficient to overcome gasliquid diffusion limitations.

Fluorous biphasic chemistry opens doors for the reuse of more active and selective homogeneous catalysts, but barriers remain to transferring it from the laboratory to industial implementation. We show the ability of CO2, acting as a cosolvent, to make a variety of fluorocarbons and hydrocarbons miscible at temperatures well below the upper critical solution temperature of the binary liquid mixture. This phenomenon is due to the high solubility of CO2 in both organic liquids and fluorocarbons. As examples, we report enhancements in TOF of 70% and 50% with a CO2-merged phase, relative to a fluorous biphasic system, for two model reactions. Because the fluorous and organic phases can be made miscible at ambient temperatures, this creates new opportunities for the use of fluorous-sequestered catalysts. The use of CO2 would ameliorate two limitations: the first are the mass-transfer limitations of a biphasic system, and second it would permit the use of less-miscible combinations of organic and fluorous solvents to reduce solvent losses. Further, using CO2 as a cosolvent allows for homogeneous catalysis without the addition of a volatile organic diluent in systems where either the catalyst or the substrate is thermally labile. As a result, this method could enable the use of recyclable homogeneous catalysts for the synthesis of a number of compounds, especially pharmaceutical and other fine chemicals, which are sensitive to high temperatures. Acknowledgment We are thankful for the advice and assistance of Dr. Philip Jessop (University of CaliforniasDavis), Dr. James Brown (Georgia Tech), and Michael Lazzaroni (Georgia Tech). Literature Cited (1) Horva´th, I. T.; Ra´bai, J. Facile Catalyst Separation Without Water: Fluorous Biphase Hydroformylation of Olefins. Science 1994, 266, 72. (2) Cornils, B. Fluorous Biphase SystemssThe New PhaseSeparation and Immobilization Technique. Angew. Chem., Int. Ed. Engl. 1995, 34, 1575. (3) Horva´th, I. T. Fluorous Biphase Chemistry. Acc. Chem. Res. 1998, 31, 641. (4) Curran, D. P. Strategy-Level Separations in Organic Synthesis: From Planning to Practice. Angew. Chem., Int. Ed. 1998, 37, 1174. (5) Barthel-Rosa, L. P.; Gladysz, J. A. Chemistry in Fluorous Media: A User’s Guide to Practical Considerations in the Applica-

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Received for review December 15, 2003 Revised manuscript received March 30, 2004 Accepted April 1, 2004 IE0308745