Fluoroacrylate Copolymer-Supported Rhodium Catalysts for

Zulema K. Lopez-Castillo,† Roberto Flores,† Ibrahim Kani,‡ John P. Fackler Jr.,‡ and. Aydin Akgerman*,†. Chemical Engineering and Chemistry ...
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Ind. Eng. Chem. Res. 2002, 41, 3075-3080

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Fluoroacrylate Copolymer-Supported Rhodium Catalysts for Hydrogenation Reactions in Supercritical Carbon Dioxide Zulema K. Lopez-Castillo,† Roberto Flores,† Ibrahim Kani,‡ John P. Fackler Jr.,‡ and Aydin Akgerman*,† Chemical Engineering and Chemistry Departments, Texas A&M University, College Station, Texas 77843

We have attached a homogeneous rhodium catalyst to a fluoroacrylate copolymer backbone, making it soluble in supercritical carbon dioxide. The polymer was synthesized by the polymerization of 1H,1H,2H,2H-heptadecafluorodecyl acrylate monomer (zonyl TAN) (I), available from Dupont, and N-acrylosuccinimide (NASI), the former increasing the solubility in supercritical carbon dioxide and the latter providing attachment sites for the catalyst. Diphenylphosphinopropylamine, NH2(CH2)3PPh2 (DPPA), was used to exchange the NASI groups in the polymer, which was then reacted with [RhCl(COD)]2 to obtain the catalyst. We determined that the catalyst is soluble in supercritical carbon dioxide and evaluated its hydrogenation activity using 1-octene and cyclohexene hydrogenation as model reactions. The synthesis route for the catalyst is reproducible, as shown by reaction activity studies on different batches of catalyst. The catalyst was evaluated at different substrate-to-rhodium molar ratios and at different temperatures. Most reactions were carried out at 173.4 bar. Introduction Chemistry in environmentally benign solvents is of increasing interest in areas as diverse as “green chemistry”, catalysis, and combinatorial chemistry.1-4 Many organic syntheses, especially those of high-value specialty chemicals and pharmaceuticals, involve the use of homogeneous catalysts, which are soluble metal salts or organometallic complexes. In homogeneous catalysis, the catalyst is dissolved in a suitable solvent, which also serves as the reaction medium. These solvents are usually toxic organic liquids that have come under close scrutiny because of environmental regulatory restrictions. There is a great push in industry today to replace organic solvents used in homogeneous catalysis with environmentally benign solvents such as water or supercritical carbon dioxide (scCO2).5-8 However, most homogeneous catalysts are soluble in neither water nor scCO2, and most organic syntheses are also solventsensitive. Another major problem in homogeneous catalysis is the separation and recovery of the catalyst after the completion of the reaction. Usually, this is accomplished by a complex extraction and precipitation procedure, resulting in catalyst loss. In addition, in most cases, only the expensive transition metal components of the catalysts are recovered, and it is necessary to resynthesize the catalyst, which introduces significant additional costs. A major thrust in industry is the development of homogeneous catalysts that can be recovered easily and intact after the completion of the reaction. In this vein, there is a significant amount of interest in polymersupported ligands for metal complexation in homogeneous catalysis.9-11 Here, we report on a novel idea that solves the issues of both solvent replacement and intact recovery of * To whom correspondence should be addressed Tel.: 979845-3375. Fax: 979-845-6446. E-mail: [email protected]. † Chemical Engineering Department. ‡ Chemistry Department.

catalyst. We use polymer-supported catalyst ligands that are soluble in scCO2, which is the reaction medium. We attach the homogeneous catalyst to a fluoroacrylate copolymer. This specific polymer is very soluble in scCO2. The use of a supercritical reaction medium, in addition to the fact that it is a benign solvent, has other additional advantages. The most significant and unique aspect of supercritical fluids is that they have physicochemical properties that can be tuned by the density of the reaction medium, thereby affecting reaction rates and selectivities.12-14 If the reactants are in the gas phase, the gas-phase components are fully miscible with the supercritical solvent, resulting in higher concentrations and, hence, higher reaction rates. The masstransfer characteristics are superior in comparison to liquid reaction media because of the higher diffusion coefficients and lower viscosities. Compared to gasphase reaction media, supercritical fluids have liquidlike densities. Finally, scCO2 is inert to most reactions; it is nontoxic, nonflammable, and inexpensive; and it has rather mild critical properties. Because the polymer used to support the catalyst is a very large molecule, it can be separated by a membrane. In the overall process, the reaction takes place in a membrane reactor. Whether the reactor is a batch or flow reactor, membrane separation of the effluent will always maintain the catalyst in the reactor. The products can be separated from the solvent, scCO2, by simple expansion, yielding solvent-free products. Carbon dioxide can be recompressed and recycled. Upon completion of the reaction and expansion of the solvent remaining in the reactor, the catalyst can also be obtained intact. The limited solubility of polar or nonvolatile compounds restricts the use of scCO2 as a solvent.15 Conventional homogeneous catalysts normally do not dissolve appreciably in scCO2. Several approaches have been used to overcome this problem. They include the use of “CO2-philic tails”, cosolvents, counterions, and surfactants.16 The addition of CO2-philic branches or “ponytails” increases the solubility of metal complexes

10.1021/ie011039v CCC: $22.00 © 2002 American Chemical Society Published on Web 06/01/2002

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either directly in the metal-attached phosphine ligands17-22 or in the anions, as in the case of cationic catalysts.22,23 The most CO2-philic solubilizers used so far are polysiloxanes and fluorocarbons.15 It is well-known that fluorous-phase-soluble compounds are highly soluble in scCO2. Recently, Bergbreiter’s group24,25 reported the synthesis of polymersupported fluorous-phase-soluble hydrogenation catalysts. Their activities and turnover numbers were comparable or superior to those of traditional hydrogenation catalysts. The polymer was synthesized from 1H,1H,2H,2Hheptadecafluoradecyl acrylate monomer (zonyl TAN) (I), available from Dupont, and N-acrylosuccinimide (NASI), as shown below.

Finally, the precursor III is reacted with [RhCl(COD)]2 to obtain the catalyst. In polymer II and precursor III, solubility in scCO2 increases with the fluoroacrylateto-NASI ratio. However, because each Rh in the catalyst is attached to three phosphine groups, some crosslinking would be expected. Hence, although II and III might be soluble in scCO2 in appreciable amounts, the catalyst might not have as high a solubility.

Details on catalyst preparation and characterization are given elsewhere.26 Experimental Section

The polymer was prepared with different fluoroacrylateto-acrylosuccinimide ratios, the former increasing the solubility and the latter providing attachment sites for the catalyst. Diphenylphosphinopropylamine, NH2(CH2)3PPh2 (DPPA), was used to exchange the NASI groups in polymer II via the reaction

A small 8-mL high-pressure cell with sapphire sight windows was fabricated for qualitative determinations of the solubilities of the catalyst precursors and catalysts prepared. The windows allowed the cell interior to be monitored visually and enabled us to confirm that the reaction mixture was in the single-phase supercritical region. The cell was equipped with a temperature controller and a pressure indicator. This cell was used only for qualitative measurements. Typically, 5-8 mg of the polymer, the catalysts precursor, or the catalyst was placed in the cell, and the cell was pressurized with CO2 at the desired temperature to observe whether the material dissolved and, if not, whether there was any partial solubility. Hydrogenation reactions were carried out using the same 8-mL view cell and also a 100-mL reactor. The view cell allows for the monitoring of any changes in number of phases as the reaction proceeds. The view cell was rested on a stir plate, and a stir bar provided agitation. The cell was heated using a heating tape. A typical experiment started with repeated flushing of the cell with CO2 to eliminate air. Catalysts and substrates were added, and heating was initiated. After the desired temperature was reached, the reactor was filled with CO2 at the desired pressure, and agitation was started. After 12 h, samples were taken by bubbling the cell contents through hexane in an ice bath to trap the products and unreacted substrates. The samples were analyzed using an HP 5890 GC with a flame ionization detector. Blank reactions were carried out before each

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Figure 1. Schematic diagram of the 100-mL reactor system.

experiment to ensure that there was no catalyst residue from previous runs. For the 100-mL reactor, the main part of the system consists of a 100-mL EZE-seal vessel with a magnetic drive stirrer by Autoclave Engineers that was heated with a heating tape and equipped with a temperature controller (Omega CN9000A). The temperature was monitored with a thermocouple (Omega 115KC) located in a thermowell inside the reactor. The pressure was measured using an Autoclave gauge. Liquid CO2 was pumped through an ice bath to the reactor using an LDC Analytical mini pump. The complete schematic diagram is presented in Figure 1. To ensure perfect mixing in this low-density fluid, a special stirrer design was employed; it consisted of a drive shaft with two Dispersimax turbines located in the bottom and in the top of the vessel to create more turbulence. A typical experimental run involved placing the desired amount of catalyst inside the reactor, sealing the vessel, flushing the system several times with CO2 to remove the air, injecting the hydrocarbon using a syringe, adding the required amount of hydrogen, pressurizing with CO2, and starting heating. Once the temperature approached the desired temperature, the pressure was adjusted by the addition of more liquid CO2 using the mini pump. Then, the magnetic drive stirrer was activated, and the catalyst was totally dissolved in the reaction mixture. The stirrer speed employed was 1600 rpm in each run to ensure complete dissolution of the catalyst. We also performed runs runs at various stirrer speeds and observed that mixing effects disappear at stirrer speeds above 1500 rpm. Samples were taken at different times through valve V4 and trapped in a sample loop between valves V4 and V5. The sample was then slowly released through valve V5, and the effluent was absorbed in hexane for 1-octene hydrogenation experiments or in acetone for cyclohexene hydrogenation experiments. In both cases, the solvent was immersed in an ice bath. Once the sample had been collected, the sample loop was cleaned with hexane or acetone to recover any product that might have precipitated during the supercritical phase expansion, and the loop was dried with

Table 1. Qualitative Solubility Measurements of Two Different Precursors and Catalysts in Supercritical CO2 catalyst

solubility (mole fraction)

conditions

TAN10DPPA1 Rh(TAN10DPPA1)Cl TAN20DPPA1 Rh(TAN20DPPA1)Cl

1.46 × 10-3 not soluble 9.26 × 10-4 5.65 × 10-4

58 °C, 153 bar 24-80 °C, up to 210 bar 24 °C, 125 bar 24 °C, 139 bar

CO2 for the next sample. After an experiment, the reactor was carefully washed with acetone to remove any hydrocarbon residue, FC113 to remove any remaining catalyst traces, and HNO3 to dissolve any metallic deposits that could be formed during the experiment, and a mechanical cleaning was performed in all of the metallic parts. After that, a blank reaction test was performed to verify complete cleanness (no conversion). Results and Discussion Table 1 gives qualitative solubilities of two precursors and the corresponding catalysts. As can be seen from the table, although the catalyst precursor with the 10:1 fluoroacrylate-to-NASI ratio is soluble in supercritical carbon dioxide, the catalyst prepared from it is not. In contrast, when a precursor with a 20:1 ratio is prepared, both the precursor and the catalyst are soluble, although less solubility is obtained for the catalyst under more severe conditions. Because the catalysts are Wilkinson’s catalyst analogues, we evaluated them in the hydrogenation reactions of 1-octene and cyclohexene. Figure 2 shows the reproducibility results obtained with the catalyst Rh(TAN20DPPA)Cl with a rhodium dimer/polymer ratio of 1:3. (In the nomenclature used here, TAN is the fluoroacrylate component, and the rhodium dimer/ polymer ratio refers to the number of phosphine groups attached to Rh, as shown in the synthesis above). These reactions were carried out in the view cell. Three different batches of catalysts were tested to verify reproducibility. The experiments were performed at 343 K and 173.4 bar for 12 h. The molar ratio of hydrogen

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Figure 4. Effect of 1-octene-to-Rh molar ratio on conversion at 343 K and 173.4 bar. Figure 2. 1-Octene hydrogenation/isomerization using catalyst Rh(TAN20DPPA)Cl with a rhodium dimer/polymer ratio of 1:3. All runs were performed at 343 K and 173.4 bar for 12 h with a 1-octene/H2 molar ratio of 1:3.

Figure 5. Effect of temperature on 1-octene conversion at a constant pressure of 173.4 bar and a 1-octene/Rh molar ratio of 630. Figure 3. 1-Octene hydrogenation using catalyst Rh(TAN7DPPA)Cl with a rhodium dimer/polymer ratio of 1:3. All runs were performed at 343 K and 173.4 bar for 12 h with a 1-octene/H2 molar ratio of 1:8.

to 1-octene was 3:1 and kept constant in all experiments. As can be seen from the figure, the reproducibility is excellent considering that, for each batch, new catalyst was synthesized starting with the polymer. n-Octane is the major hydrogenation product; however, we also observed the isomerization products (E)-2-octene and (Z)-2-octene. Under reaction conditions, the catalyst was soluble, and the reaction mixture was in the supercritical phase throughout the reaction, which was verified visually. We also evaluated the catalytic activity and reproducibility for the catalyst with the same rhodium/polymer ratio but with a different fluoroacrylate/NASI ratio. Figure 3 shows the results obtained with the catalyst Rh(TAN7DPPA)Cl with a rhodium dimer/polymer ratio 1:3 in the hydrogenation of 1-octene. Under reaction conditions, this catalyst is partially soluble. At this stage, we are not sure whether the undissolved catalyst is also active. Again, different lots were tested to verify the reproducibility of the synthesis procedure. The reaction conditions were the same for all experiments, total pressure of 173.4 bar and temperature of 343 K, with substrate/Rh molar ratios of 1056.8, 1324.2, and 1075.82 gmol of 1-octene/gmol of Rh for lots 1-3, respectively. The reaction time was kept constant for all experiments at 12 h. Conversion to n-octane was almost 100% with negligible fractions of (E)-2-octene, (Z)-2-octene, and 3-methylheptane.

Figure 4 shows reactions at different 1-octene/Rh molar ratios. These reactions were done in the 100-mL autoclave reactor with on-line sampling as a function of time. The catalyst in all reactions was Rh(TAN20DPPA)Cl, with a rhodium dimer/polymer ratio of 1:3 and a rhodium content of 2.22 mg of Rh/g of catalyst. All reactions were carried out at 343 K and 173.4 bar with a H2/1-octene molar ratio of 33. The concentrations of H2 and 1-octene were 417.15 and 9.5748 mM, respectively; the concentration of Rh was varied according to the 1-octene/Rh molar ratio studied. Conversion was low at high 1-octene/Rh molar ratios and high at low ratios; there was no appreciable difference in conversion at intermediate ratios. Figure 5 summarizes experiments at the three temperatures 318, 343, and 373 K. All experiments were carried at the same pressure, 173.4 bar, and the catalyst was Rh(TAN20DPPA)Cl at 373 K but Rh(TAN15DPPA)Cl at 318 and 343 K, with a rhodium dimer/polymer ratio of 1:3 at all temperatures. The H2/1-octene molar ratio was 33, and the 1-octene/ Rh molar ratio was 630 in all runs. The initial mole fractions of all components were kept constant at all temperatures; this means that, at lower temperatures where the density of scCO2 is higher, the concentrations of H2 and 1-octene were greater. The initial mole fractions of H2 and 1-octene were 0.0334 and 0.0010, respectively. The amount of catalyst added to the reactor was varied according to the 1-octene concentration. We also evaluated the catalyst for cyclohexene hydrogenation in the 100-mL reactor. The catalyst used was Rh(TAN15DPPA)Cl with a hodium dimer/polymer ratio of 1:3 and a rhodium content of 1.95 mg of Rh/g of catalyst. A H2/cyclohexene molar ratio of 28 was used. The experiments were performed at 173.4 bar. Figure

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ous Substance Research Center, The Scientific and Technical Research Council of Turkey (TUBITAKNATO), and Consejo Nacional de Ciencia y Tecnologia (CONACyT) of Mexico. The authors are thankful for partial support from each organization. Literature Cited

Figure 6. Cyclohexene hydrogenation at 343, 368, and 393 K at 173.4 bar.

Figure 7. Effect of cyclohexene-to-Rh molar ratio on conversion at 368 K and 173.4 bar.

6 shows the conversions obtained at 343, 368, and 393 K. The cylohexene/Rh molar ratio was 400 at 368 and 393 K and 700 at 343 K. All experiments were performed twice to verify reproducibility; the points represent the averages, and the lengths of the error bars indicate the standard deviation. The initial mole fractions of H2 and cyclohexene were 0.0561 and 0.0020, respectively. Again, these mole fractions were kept constant in all of the experiments, and the amount of catalyst in the system was changed according to the cyclohexene concentration and the required substrate/ catalyst molar ratio. Conversion increases with temperature, and for the experiments at 368 and 393 K, it levels off after 12 h. The maximum conversions reached at 368 and 393 K were 39 and 51%, respectively. Figure 7 shows the effect of the cyclohexene/Rh ratio on conversion for the same conditions: 368 K, 173.4 bar, 18.373 mM cylohexene, and 500.6 mM H2. As expected, the lower the ratio, the higher the conversion. In cyclohexene hydrogenation, the only product observed was cyclohexane; there were no side reactions. In summary, we have synthesized a rhodium catalyst, a Wilkinson’s catalyst analogue, attached to a polymer backbone that is soluble in scCO2. We have shown that the synthesis procedure is reproducible and that the catalyst is active in the hydrogenation reactions of 1-octene and cyclohexene. Acknowledgment This work was supported by the U.S. Environmental Protection Agency (U.S. EPA), the Gulf Coast Hazard-

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(26) Kani, I.; Omary, M. A.; Rawashdeh-Omary, M. A.; LopezCastillo, Z. K.; Flores, R.; Akgerman, A.; Fackler, J. P. Homogeneous Catalysis in Supercritical Carbon Dioxide with Rhodium Catalysts Tethering Fluoroacrylate Polymer Ligands. Tetrahedron, in press, 2002.

Received for review December 30, 2001 Revised manuscript received April 26, 2002 Accepted April 30, 2002 IE011039V