Epoxidation of Olefins by Molybdenum(VI) Catalysts Supported on

and Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1. 1XL, United Kingdom. Functional polyimides ...
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Epoxidation of Olefins by Molybdenum(VI) Catalysts Supported on Functional Polyimide Particulates† Jou-Hyeon Ahn,*,‡ Jong-Chan Kim,‡ Son-Ki Ihm,§ Chang-Gun Oh,§ and David C. Sherrington| Department of Chemical and Biological Engineering and Engineering Research Institute, Gyeongsang National University, 900 Gajwa-dong, Jinju 660-701, Korea, Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Taejon 305-701, Korea, and Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, United Kingdom

Functional polyimides have been prepared in a bead form by nonaqueous suspension polycondensation. A molybdenum(VI) complex was supported on the functional polyimide particulates containing a triazole group and used as a catalyst in the liquid-phase epoxidation of higher olefins with tert-butyl hydroperoxide as the oxygen source. Serious degradation of the polyimidesupported Mo(VI) catalyst does not happen in air until ∼400 °C. The polyimide-supported Mo catalyst was highly active and selective and could be recycled 10 times with no detectable loss of Mo from the support. Introduction Epoxides are very important substrates because they are starting materials to produce specialty and fine chemicals. They can be formed from corresponding olefins by oxidation with molecular oxygen or air in the presence of a catalyst, but other oxygen sources that are more selective for the catalytic oxidation are hydrogen peroxide, various alkyl hydroperoxides, peracetic acid, and sodium hypochlorite.1,2 The typical example is the industrial production of propylene oxide, which is carried out by the liquid-phase epoxidation of propylene with an alkyl hydroperoxide as the oxygen source, catalyzed by a homogeneous Mo(VI)3 or a heterogeneous titanium/silica catalyst.4 In recent years, the design and synthesis of catalytically active polymer-supported metal complexes has received considerable interest because these heterogenized catalysts offer several practical advantages over their homogeneous soluble counterparts.5 Functionalized polystyrene copolymers have been used widely as support materials. Sherrington and Simpson6 reported a supported metal catalyst by impregnating polystyrene resin with molybdenum and vanadium catalysts. These polystyrene resins, however, have a drawback in their limited thermo-oxidative stability.7,8 The scope for application is therefore restricted, particularly in polymer-supported transition metal complex oxidation catalysts.9 Consequently, there is a need for the development of polymer supports with a much higher intrinsic thermo-oxidative stability. Polybenzimidazoles and polyimides are likely candidates in this respect. Miller et al.10-12 also reported a Mo(VI) catalyst † Dedicated to Professor David C. Sherrington on the occasion of his 60th birthday. * To whom correspondence should be addressed. Tel.: +82-55-751-5388. Fax: +82-55-753-1806. E-mail: jhahn@ gsnu.ac.kr. ‡ Gyeongsang National University. § Korea Advanced Institute of Science and Technology. | University of Strathclyde.

impregnated in a thermo-oxidatively stable polymer of polybenzimidazole in a porous bead, which is employed in the epoxidation of cyclohexene. A comparative study of the polystyrene- and polybenzimidazole-supported Mo(VI) species has revealed that the polybenzimidazolesupported catalyst has a catalytic activity and stability better than that of the polystyrene-supported catalyst. In addition, Cunnington et al.13 reported an epoxidation of olefins by a Mo catalyst supported on a polybenzimidazole resin, where the catalyst showed excellent catalytic activity and selectivity in the course of preparing propylene oxide from propylene and retained catalytic activity on recycling without leaching. The catalyst, however, has revealed several shortcomings as follows: First, its catalytic activity decreases gradually when it is recycled in the epoxidation of a higher olefin such as cyclohexene; second, the polybenzimidazole resin employed as a support can only be prepared by a complex melt process at a high temperature over 200 °C; third, the monomer used for preparing the polybenzimidazole resin is very expensive. Therefore, there are strong needs for developing an inexpensive and thermo-oxidatively stable polymersupported catalyst that has a high catalytic activity and selectivity in the epoxidation of higher olefins and retains its activity after repeated use. Unlike polybenzimidazole-based thermo-oxidatively stable supports, polyimides can be prepared under relatively mild conditions from starting materials of only low or modest cost.14,15 Indeed polyimide particulates have been prepared in a bead form without a functional group16,17 or with a functional group.18,19 The presence of the functional group in the polyimides allows further chemical exploitation, particularly as a catalyst support capable of operating under rather severe oxidative conditions. In this work, functional polyimide particulates carrying a triazole group were prepared, and Mo(VI) complexes were then supported on the beads and employed as heterogeneous catalysts in the liquid-phase epoxidation of alkenes using tert-butyl hydroperoxide (TBHP) as the oxidant.

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Experimental Section Materials. N,N′-Dimethylacetamide (DMAc; Aldrich, HPLC grade) was used without further purification. Acetic anhydride (BDH) was predried over anhydrous sodium acetate. Pyridine (Aldrich, anhydrous) was distilled from KOH prior to use. Poly(maleic anhydrideco-octadec-1-ene) (1:1, Polysciences), as a polymeric stabilizer, was used as supplied. Paraffin oil (liquid paraffin 5LT, A. J. Beveridge Ltd.) was used as a suspending medium. Pyromellitic dianhydride (PMDA; Aldrich) was recrystallized from butan-2-one before use. 3,5-Diamino-1,2,4-triazole (DAT; Aldrich) was used without further purification. Preparation of Functional Polyimide Particulates. A procedure similar to that which we have already reported was employed.16-19 This involves the preparation of a prepolymer poly(amic acid) (PAA) solution in DMAc, followed by imidization in suspension in paraffin oil. A typical procedure for the preparation of linear functionalized spherical polyimide particulates (PI-DAT) was as follows: To a 500-mL three-necked, round-bottomed flask were added 14.87 g of DAT (0.150 mol) and 197 g of DMAc, and the resulting solution was stirred under a N2 atmosphere until the diamine was completely dissolved in DMAc. While the solution was mechanically stirred, 34.03 g of finely ground PMDA (0.156 mol) was slowly added in a small quantity, while keeping the temperature low with a cold water bath. As the solution became transparent, its viscosity increased. Stirring was continued overnight at room temperature. To a polymerization reactor were added 500 g of paraffin oil with poly(maleic anhydride-co-octadec-1-ene) (1:1; 0.5 wt % in oil) as a suspension stabilizer. The PAA solution (20 wt % PAA in DMAc) as prepared above was added to the reactor and suspended for 2 h at 60 °C at a speed of 400 rpm. Following this, imidization was initiated by dropwise addition of a mixture of acetic anhydride (4.0 M excess of PMDA used) and pyridine (3.5 M excess of PMDA used). After dehydration for 24 h with constant stirring, the polyimide particulates were filtered, washed, and extracted with dichloromethane and then dried at 80 °C in a vacuum oven. Preparation of a Polyimide-Supported Mo Complex. A polyimide-supported Mo(VI) catalyst was prepared as follows: To a 50-mL round-bottomed flask were added 2.0 g of polyimide particles bearing a triazole residue (PI-DAT) and 7.0 g of molybdenyl acetylacetonate [MoO2(acac)2], and 20 mL of ethanol was further added. The reaction mixture was kept under reflux for 3 days. Upon completion, the polyimide-Mo complex catalyst was filtered off and extracted with ethanol in a Soxhlet apparatus for 72 h. The supported complex was dried thoroughly under vacuum. The Mo content was measured by inductively coupled plasma to be 1.08 mmol g-1 for PI-DAT‚Mo. Catalytic Epoxidation. A three-necked reactor was used in the epoxidation of cyclohexene, and a thermostatic circulating water bath was employed for controlling the reaction temperature precisely. Under a N2 atmosphere, 0.08 g of PI-DAT‚Mo, 7.5 mL (74 mmol) of cyclohexene, and 0.5 mL of bromobenzene as an internal standard of gas chromatography were placed in a reactor equipped with a condenser, septum cap, and stirrer, and the resulting solution was stirred at 60 °C for 20 min to reach a thermal equilibrium. Then, 2 mL of an anhydrous TBHP solution (5 mmol of TBHP) was

Figure 1. Optical photograph of PI-DAT beads (∼50-425 µm).

Figure 2. FTIR spectra of PI-DAT and PI-DAT‚Mo.

added to initiate the reaction. Samples were withdrawn by syringe, and the concentration of cyclohexene oxide was monitored by gas chromatography (HP5890 series II plus) with a capillary column (Ultra 2). Analytical Methods. The particle size distribution of functional polyimide particulates was determined by sieving: mesh of 38, 75, 106, 212, and 425 µm. Each size fraction was represented by weight percent. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet SX20B instrument with KBr disks. Results and Discussion The triazole-containing polyimide bead (PI-DAT) was prepared using suspension polycondensation methodology.16-19 The PAA solution in DMAc was prepared from PMDA and DAT. The PAA solution was then dispersed as droplets in paraffin oil containing poly(maleic anhydride-co-octadec-1-ene) (1:1) as a suspension stabilizer, and imidization was induced at 60 °C by the addition of a mixture of acetic anhydride and pyridine. Typically, 90-100% of mainly spherical polyimide particulates was obtained after washing and drying, as shown in the optical photograph in Figure 1. The particles were in a size range of 38-425 µm, and 76 wt % of the product was from 106 to 425 µm, which was used as the polymer support. Figure 2 shows the FTIR spectra of PI-DAT and PI-DAT‚Mo. PI-DAT shows the characteristic absorption bands typical of polyimides at 1780, 1720 (heterocyclic carbonyl vibration), 1348 (C-N stretching vibration), and 720 cm-1 (imide ring deformation).

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Figure 3. Thermogravimetric analysis of PI-DAT‚Mo.

The homogeneous Mo complex, MoO2(acac)2, was supported on PI-DAT to give PI-DAT‚Mo with a Mo content of 1.08 mmol g-1. The color of PI-DAT‚Mo was mainly dark blue. The FTIR spectrum of PI-DAT‚Mo (Figure 2) shows the typical bands of polyimides and two further bands attributable to ModO stretching modes in the region 900-1000 cm-1, indicating the presence of supported oxomolybdenum centers.21,22 It is known that the Mo complex formed by ligand exchange with MoO2(acac)2 on polymer supports can have two structures involving ModO and bridging Mo-O-Mo.6 There is no clear-cut evidence in the IR spectrum of PI-DAT‚Mo for a Mo-O-Mo bridge because the FTIR band of Mo-O-Mo appears at ∼720 cm-1, which is one of the typical IR bands of polyimides. The detailed structure of the Mo centers in this polymer-supported Mo catalyst therefore remains unclear. However, it is very likely that all of the Mo centers on the supported catalysts probably contain oxomolybdenum centers as indicated by the initial rate data and lack of an induction period (see later).23 One of the authors has already reported on the possible structures of polymersupported Mo complexes.10,20 Figure 3 shows the thermogravimetric analysis curve of the polyimide-supported Mo(VI) catalyst. Serious degradation of PI-DAT‚Mo in air does not happen until ∼400 °C. The functional polyimide therefore shows good prospects for high-temperature application as supports, certainly in reactions up to 300 °C. The polyimide-supported Mo(VI) complex catalyst was applied to the epoxidation of olefins. An excess of olefin was used in all of the reactions as normal in industrial processes. The yield of epoxide was therefore based upon the initial quantity of TBHP used in the reactions. For example, when initially 5 mmol of TBHP are used in the epoxidation of cyclohexene, the yield is 100% if 5 mmol of cyclohexene oxide are produced.

In the preliminary experiments, the concentration effect of TBHP, cyclohexene, and catalyst on the yield of cyclohexene oxide was investigated for the kinetic studies, and it was known that the reaction rate was most strongly dependent on the concentration of TBHP.

Figure 4. Effect of the TBHP concentration on catalytic epoxidation of cyclohexene using a PI-DAT‚Mo complex at 80 °C.

Figure 5. Effect of the temperature on the catalytic activity of a PI-DAT‚Mo complex in the epoxidation of cyclohexene.

Figure 4 shows the effect of the TBHP concentration on the yield of cyclohexene oxide in the epoxidation of cyclohexene catalyzed by PI-DAT‚Mo at 80 °C. The initial rate was shown to be linearly dependent on the TBHP concentration. In previous studies using supported Mo epoxidation catalysts, a first-order dependency for the TBHP concentration was observed within certain limits.11,24 As shown in Figure 4, nearly all of TBHP was used to produce cyclohexene oxide without the formation of byproducts. Figures 5 and 6 showed the temperature dependency in the epoxidation of cyclohexene using the polyimidesupported Mo catalyst. The activity increased with the reaction temperature, and no byproducts were detectable even at the higher temperatures. However, an induction period was observed at lower temperature. A linear Arrhenius plot was obtained as shown in Figure 6, and the activation energy was determined to be 21.5 kJ mol-1. This value is similar to that reported for a homogeneous Mo complex catalyzed epoxidation reaction.25 The similar activation energy seen for this supported Mo catalyst presumably reflects the facts that the active center of the immobilized catalyst is easily accessible, that in the supported Mo catalyst diffusional effects are not important, and that the environment of the active center in the resin is similar to that of the homogeneous catalyst.

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Figure 6. Arrhenius plot for the epoxidation of cyclohexene with a PI-DAT‚Mo catalyst.

Figure 8. Epoxidation of various olefins using PI-DAT‚Mo with TBHP.

Figure 7. Solvent effect in the epoxidation of cyclohexene using a PI-DAT‚Mo complex at 60 °C.

Figure 9. Recycling of a PI-DAT‚Mo catalyst in the epoxidation of various alkenes at 80 °C (yield of epoxide after 2 h).

Figure 7 shows the influence of solvent on the epoxidation reaction of cyclohexene. A reaction mixture with an overall volume of 10.5 mL containing 0.08 g of PI-DAT‚Mo, 2.3 mL of cyclohexene (23 mmol), 0.5 mL of bromobenzene as an internal standard, 2 mL of an anhydrous TBHP solution (5 mmol TBHP), and 5.7 mL of solvent was used to study the solvent effect. The epoxidation rate decreased in the order 1,2-dichloroethane > toluene > chlorobenzene > tert-butyl alcohol >ethanol. The results are in agreement with previous results for homogeneous and supported Mo catalysts.24,26,27 They are also consistent with the idea that the reaction intermediate in the rate-controlling step is polarized and that nondonor polar solvents enhance the charge separation of the dipole, whereas the reaction rate is reduced in donor polar solvents such as tert-butyl alcohol and ethanol and, in particular, in the latter solvent. The strong inhibition by these donor solvents can be interpreted in terms of competition with olefin in occupying the Mo catalytic site.26 The retardation of the epoxidation reaction by tert-butyl alcohol has been particularly well studied because tert-butyl alcohol is the main coproduct of the reaction, resulting from the decomposition of TBHP.26-28 High activity and selectivity in the epoxidation of various olefins using the supported Mo complex catalysts were observed under favorable conditions as shown

by the data in Figure 8. It is already known that the epoxidation of cycloolefins is much more favorable than that of linear olefins. Hence, the prolonged activity of the present polymer-supported Mo catalyst is probably the most important finding in this work. The deactivation of the catalyst either by degradation of the polymer support itself or by leaching of active species from the supported catalyst is minimal for cyclic olefins, as shown by the data in Figure 9. The polyimide-supported Mo catalyst was recovered and washed with toluene at the end of each run and used repeatedly under identical conditions. The catalyst shows substantial retention of activity over 10 recycles unlike an earlier polybenzimidazole-supported Mo complex,11,12 where the latter displayed rapid deactivation on recycling. The presently reported retention of activity is most encouraging and suggests that catalysts based on functional polyimide particulates might form the basis of a range of stable polymer-supported metal complex catalysts, where the support is readily synthesized and is highly costeffective. Application on both a laboratory and a technical scale also seems feasible. Conclusions Functional polyimide particulates containing a triazole group have been prepared and used as a polymer

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support to anchor a liquid-phase Mo catalyst for the epoxidation of olefins. The polyimide-supported Mo catalyst shows high thermo-oxidative stability up to ∼400 °C and provides superior catalytic activity and selectivity in the epoxidation of higher olefins, particularly cyclic olefins, with TBHP. Further, the supported catalyst can be easily isolated from the reaction product and recycled without severe loss of catalytic activity. Therefore, the present results show a novel supported catalyst for the olefin epoxidation, which can be prepared in a simple and economical manner. Literature Cited (1) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidation of Organic Compounds; Academic Press: New York, 1981. (2) Haines, A. H. Methods for the Oxidation of Organic Compounds; Academic Press: London, 1985. (3) Kolar, J. U.S. Patent 3,350,422, 1967; 3,357,635, 1967; 3,507,809, 1970; 3,625,981, 1971. (4) Wulff, H. P. British Patent 1,249,079, 1971; U.S. Patent 3,923,843, 1975. (5) Hartley, F. R. Supported Metal Complexes; Reidel: Dordrecht, The Netherlands, 1986. (6) Sherrington, D. C.; Simpson, S. Polymer-Supported Mo and V Cyclohexene Epoxidation Catalysts: Activation, Activity, and Stability. J. Catal. 1991, 131, 115. (7) Hodge, P., Sherrington, D. C., Eds. Polymer-supported Reactions in Organic Synthesis; Wiley-Interscience: Chichester, U.K., 1980. (8) Yermakov, Y. I.; Kuznetsov, B. N.; Zakharov, V. A. Catalysis by Supported Complex; Elsevier: Amsterdam, The Netherlands, 1981. (9) Sherrington, D. C. Polymer-supported metal complex oxidation catalysts. Pure Appl. Chem. 1988, 60, 401. (10) Miller, M. M.; Sherrington, D. C. Alkene Epoxidations Catalysed by Mo(VI) Supported on Imidazole-Containing Polymers. I. Synthesis, Characterisation, and Activity of Catalysts in the Epoxidation of Cyclohexene. J. Catal. 1995, 152, 368. (11) Miller, M. M.; Sherrington, D. C. Alkene Epoxidations Catalysed by Mo(VI) Supported on Imidazole-Containing Polymers. II. Recycling of Polybenzimidazole-Supported Mo(VI) in the Epoxidation of Cyclohexene. J. Catal. 1995, 152, 377. (12) Miller, M. M.; Sherrington, D. C.; Simpson, S. Alkene epoxidations catalysed by molybdenum(VI) supported on imidazole-containing polymers. Part 3. Epoxidation of oct-1-ene and propene. J. Chem. Soc., Perkin Trans. 2 1994, 2091. (13) Cunnington, M. J.; Miller, M.; Simpson, S.; Olason, G.; Sherrington, D. C. U.S. Patent 5,420,313, 1994. (14) Wilson, D., Stenzenberger, H. D., Hergenrother, P. M., Eds. Polyimides; Chapman and Hall: New York, 1990.

(15) Lee, L. H., Ed. Adhesives, Sealants and Coatings for Space and Harsh Environments, Polymer Science and Techology; Plenum: New York, 1988. (16) Brock, T.; Sherrington, D. C. Preparation of Spherical Aromatic Polyimide Particulates. J. Mater. Chem. 1991, 1, 151. (17) Brock, T.; Sherrington, D. C.; Swindell, J. Synthesis and Characterisation of Porous Particulate Polyimides. J. Mater. Chem. 1994, 4, 229. (18) Ahn, J. H.; Sherrington, D. C. Synthesis of functional polyimide beads and use as MoVI epoxidation catalysts supports. J. Chem. Soc., Chem. Commun. 1996, 643. (19) Ahn, J. H.; Sherrington, D. C. Wacker Oxidation of Oct1-ene Using a Palladium(II) Complex Supported on CyanoFunctionalized Polyimide Beads. Macromolecules 1996, 29, 4164. (20) Leinonen, S.; Sherrington, D. C.; Sneddon, A.; McLoughlin, D.; Corker, J.; Canevali, C.; Morazzoni, F.; Reedijk, J.; Spratt, S. B. D. Molecular structural and Morphological Characterization of Polymer-Supported Mo(VI) Alkene Epoxidation Catalysts. J. Catal. 1999, 183, 251. (21) Cotton, F. A.; Wing, R. M. Properties of Metal-to-Oxygen Multiple Bonds, Especially Molybdenum-to-Oxygen Bond. Inorg. Chem. 1965, 4, 867. (22) Moore, F. W.; Rice, R. E. Physicochemical and Spectral Properties of Octahedral Dioxomolybdenum(VI) Complexes. Inorg. Chem. 1968, 7, 2510. (23) Bhadur, S.; Khwaja, H. Polymer-supported complexes. Part 3. Synthesis of a polystyreneanchored molybdenum(V) dithiocarbamato-derivative and its applications in reactions involving tert-butyl hydroperoxide. J. Chem. Soc., Dalton Trans. 1983, 415. (24) Ivanov, S.; Boeva, R.; Tanielyan, S. Catalytic epoxidation of propylene with tert-butyl hydroperoxide in the presence of modified carboxy cation-exchange resin “Amberlite” IRC-50. J. Catal. 1979, 56, 150. (25) Mimoun, H.; Roch, I. S.; Sajus, L. Epoxidation of olefines by the covalent Mo(VI) peroxy-complexes. Tetrahedron 1970, 26, 37. (26) Mimoun, H.; Mignard, M.; Brechot, P.; Saussine, L. Selective epoxidation of olefins by oxo[N-(2-oxidophenyl)salicylidenaminato]vanadium(V) alkylperoxides. On the mechanism of the Halcon epoxidation process. J. Am. Chem. Soc. 1986, 108, 3711. (27) Reichardt, C. Solvent Effects in Organic Chemistry; Verlag Chemie: Weinheim/New York, 1979; pp 17 and 270. (28) Su, C. C.; Reed, J. R.; Gould, E. S. Metal ion catalysis of oxygen-transfer reactions. II. Vanadium and molybdenum chelates as catalysts in the epoxidation of cycloalkenes. Inorg. Chem. 1973, 12, 337.

Received for review November 30, 2004 Revised manuscript received April 25, 2005 Accepted April 28, 2005 IE040287Z