Olefin Epoxidations Using Supercritical Carbon Dioxide and Hydrogen

Shane A. Nolen, Jie Lu, James S. Brown, Pamela Pollet, Brandon C. Eason,. Kris N. Griffith, Roger Gla1ser,† David Bush, David R. Lamb,‡ Charles L...
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Ind. Eng. Chem. Res. 2002, 41, 316-323

Olefin Epoxidations Using Supercritical Carbon Dioxide and Hydrogen Peroxide without Added Metallic Catalysts or Peroxy Acids Shane A. Nolen, Jie Lu, James S. Brown, Pamela Pollet, Brandon C. Eason, Kris N. Griffith, Roger Gla1 ser,† David Bush, David R. Lamb,‡ Charles L. Liotta, and Charles A. Eckert* Schools of Chemical Engineering and Chemistry and the Specialty Separations Center, Georgia Institute of Technology, Atlanta, Georgia 30332-0100

Georg F. Thiele and Karin A. Bartels Degussa-Hu¨ ls Corporation, 379 Interpace Parkway, Parsippany, New Jersey 07054-0677

An alternative means of epoxidation is reported that uses environmentally benign supercritical carbon dioxide as both a solvent and reactant in combination with aqueous H2O2, which is made possible through the in situ formation of peroxycarbonic acid. Experiments were conducted at 40 °C and 120 bar in which cyclohexene was epoxidized to 1,2-cyclohexene oxide and 1,2-cyclohexanediol in this aqueous-organic biphasic system. Through the addition of NaHCO3 and the hydrophilic cosolvent dimethylformamide, the conversion increased from 0.4 mol % (without additives) to 12.6 mol % (with 0.1 mol % NaHCO3 and 13 mol % dimethylformamide). The results suggest that the reaction occurs within the aqueous phase, which led to investigations using the water-soluble olefin 3-cyclohexen-1-carboxylate sodium salt as a means of verifying the reaction location. Epoxidation of 3-cyclohexen-1-carboxylate sodium salt went to completion in less than 20 h at 40 °C and 120 bar with an epoxide yield of 89 mol % and diol yield of 11 mol %. Introduction Epoxides and their derivatives are widely used as intermediates in a variety of industrial applications, including syntheses of cosmetics, detergents, polymers, and curing agents.1 The most straightforward route to an epoxide is through the epoxidation of the corresponding olefin. Hydrogen peroxide is a cheap and readily available oxidant for this purpose, with innocuous decomposition byproducts of water and oxygen. Hydrogen peroxide has been used for various applications from bleaching to epoxidations,2 but it has limited application for oxidative synthesis without activation by the addition of catalysts or conversion to a more reactive peroxy acid. Metal catalysts, composed primarily of transition metals including V, Mo, Mn, Re, and Ru, have been used in conjunction with hydrogen peroxide to form epoxides.3 These catalysts provide good selectivities and yields and can be used in relatively small amounts, as little as 0.03 mol %.4 However, problems with catalyst stability and the need for postreaction separations and recycling provide additional processing challenges and expenses. Another means of activating hydrogen peroxide is through the generation of peroxy acids, which are more reactive than hydrogen peroxide. Peroxy acids are typically formed in situ, because of their unstable nature, through the acid-catalyzed reaction of a car* Author to whom correspondence should be addressed. E-mail: [email protected]. Phone: 404-894-7070. Fax: 404894-9085. † Present Address: Institute of Chemical Technology, University of Stutgart, D-70550 Stuttgart, Germany. ‡ Present Address: General Electric, Waterford, NY 12188.

boxylic acid with hydrogen peroxide.5 Epoxidations using peroxy acids are thought to occur through a bicyclic mechanism resulting in regeneration of the carboxylic acid starting material.5 These carboxylic acids must then be recovered for reuse or disposal. Additionally, the potential for explosions makes the use of traditional peroxy acids undesirable. Because of the drawbacks of both catalyst and traditional peroxy acid-catalyzed epoxidations, we developed a process that can overcome these difficulties by using the simplest possible activated hydrogen peroxide species, peroxycarbonic acid.6 It is well-known that CO2 reacts with water to form carbonic acid (reaction 1).7 By analogy, the reaction of CO2 with hydrogen peroxide should generate peroxycarbonic acid (reaction 2).8

To date, no explicit proof of the existence of peroxy-

carbonic acid has been presented. Other researchers have alluded to the existence of peroxycarbonic acid;8-11 however, because of its instability, it has not been isolated in its pure form. Nevertheless, characterizations of peroxycarbonic acid salts have been performed using X-ray crystallography12 as well as vibrational spectroscopy.13

10.1021/ie0100378 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/20/2001

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To test our theory of an improved system that utilizes peroxycarbonic acid as the oxidant, we investigated the epoxidation of simple olefins with hydrogen peroxide in supercritical carbon dioxide (scCO2; Tc ) 31 °C, Pc ) 74 bar). Epoxidations of olefins in scCO2 have been performed previously using catalysts such as Mo(CO)6 and VO(acac)2 along with tert-butyl peroxide;14,15 however, this work provides the first example in which CO2 is used as a hydrogen peroxide activator without any peroxy acid or metallic catalyst additions. Furthermore, CO2 is a benign alternative used in place of halogenated or otherwise undesirable organic solvents in this system. The specific epoxidations investigated were of the olefins cyclohexene and 3-cyclohexen-1-carboxylate sodium salt at 40 °C and 120 bar to yield their corresponding epoxides and diols. As part of our investigation, the effects of sodium bicarbonate, hydrogen peroxide stabilizers, and cosolvents were explored. Although the epoxidations that we describe use carbon dioxide to form peroxycarbonic acid (hydrogenperoxymonocarbonate) as the oxidant, Richardson et al. demonstrated through similar work that epoxidations can be performed using only hydrogen peroxide and large excesses of sodium bicarbonate.10,11 The decision to work with scCO2 was made on the basis of the unique properties and subsequent advantages provided by supercritical fluids (SCFs). The physical properties of SCFs are intermediate between those of gases and liquids, making them especially attractive as solvents for industrial applications. SCFs have sufficient density to give appreciable dissolving power,16 while having viscosities similar to those of gases17 and molecular diffusivities 1-2 orders of magnitude higher than those found in liquids,18-21 which facilitates mass transfer. SCFs also have the ability to be tuned by manipulation of temperature, pressure, and cosolvent addition22-24 allowing for the optimization of equilibria, reaction rates, yields, and selectivities.16,25,26 These properties give SCFs the capacity to replace hazardous organic solvents27 while providing an ideal environment for the production of new materials. Additionally, carbon dioxide has the advantage of being nonflammable, which facilitates the design of oxidation processes that are inherently safe to potential explosion hazards. Experimental Section Materials. The chemicals used during these investigations were obtained and used without further purification. They include hydrogen peroxide (Aldrich, 30 wt %), carbon dioxide (Matheson, SFC grade), nitrogen (Air Products, High Purity grade), cyclohexene (Aldrich, >99%, 0.01% BHT), acetonitrile (Aldrich, >99.9% HPLC grade), methanol (Aldrich, 99.93% HPLC grade), sodium bicarbonate (Aldrich, >99.99% Reagent Plus grade), N,N-dimethylformamide (Aldrich, >99.9% ACS grade), N,N-dimethylacetamide (Aldrich, >99.9% HPLC grade) propylene carbonate (Aldrich, 99.7%), 1,2-cyclohexene, oxide (Fluka, 99%), 1,2-cyclohexanediol (Aldrich, 98%), 2-cyclohexen-1-one (Aldrich, >95%), 2-cyclohexen-1-ol (Aldrich, 95%), acetone (Aldrich, 99.8% HPLC grade), water (Aldrich, HPLC grade), formic acid (Aldrich, 88% ACS grade), 3-cyclohexene-1-carboxylic acid (Aldrich, 97%), and hydroquinone (Aldrich, >99%). In addition to the chemicals listed above, the following compounds were synthesized within our laboratory for monophasic epoxidation investigations: 3-cyclohexen1-carboxylate sodium salt, 3,4-epoxycyclohexan-1-car-

Figure 1. Biphasic epoxidation experimental apparatus.

boxylate sodium salt, and 3,4-dihydroxycyclohexan-1carboxylate sodium salt. Characterization of the compounds was performed to ensure that the desired olefins were synthesized. As part of the characterization, melting points were measured with a Mel-Temp melting point apparatus and 1H NMR spectra were obtained using a 500-MHz Bruker DRX instrument with CDCl3 as the solvent. A description of the synthesis procedures is provided in Appendix A. Cyclohexene Epoxidation Apparatus and Procedures. Reactions were performed in a 125.6 ( 0.6 mL 316 stainless steel Parr (model 4560) high-pressure/ high-temperature stirred autoclave (Figure 1) with an internal depth of 1.75 in. and a 2 in. internal diameter. The temperature was regulated to within 1 °C of the set point using a Parr (model 4842) controller. Agitation was maintained at 600 ( 10 rpm using a four-blade 85° pitched-blade impeller, unless otherwise noted. After reaction completion, depressurization of the reactors was achieved by slowly venting through a series of solvent traps filled with acetone. This was done to collect any organic compounds that escaped from the reactor during depressurization. The reactor contents were collected and analyzed along with the depressurization vent solvents. Analysis was performed using a Hewlett-Packard (model 6890) gas chromatograph (GC) equipped with mass spectrometry (MS) and flame ionization detectors (FID). The MS detector was used for qualification, while the FID was used for quantification. External standards of known concentration of all of the products and reactants were used to calibrate the GC for quantification. Quantification of hydrogen peroxide was determined by iodometric titration with an accuracy of (0.2 wt %.28 3-Cyclohexen-1-carboxylate Sodium Salt Epoxidation Apparatus and Procedures. Reactions were conducted in a 31.4 ( 0.8 mL 316 stainless steel stirred autoclave manufactured in-house. Stirring in this autoclave was achieved using a Teflon-coated magnetic stir bar and stir plate. Temperature control was maintained to within 1 °C of the set point using an Omega (model CN8500) controller along with an Omega LUX heating band. After reaction completion, depressurization of the reactor was accomplished by slowly venting through a series of solvent traps filled with water. The reactor contents and depressurization vent solvents were collected for analysis as described for the

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Figure 2. Cyclohexene epoxidation in an aqueous H2O2/organic fluid phase.

biphasic system. Analysis was performed by COSY NMR spectroscopy with CDCl3 as the solvent. External standards of known concentration of all of the watersoluble olefin products and reactants were used to calibrate the NMR instrument for quantification. Prior to NMR analysis, all samples were treated with sodium sulfite to decompose any remaining peroxide and vacuum-dried at room temperature to remove water. In all experiments, liquid reactants were introduced into the reactors using gastight syringes, whereas solids were weighed and added prior to sealing of the reactors. Carbon dioxide was introduced to these vessels using an Isco (model 500D) syringe pump. Results and Discussion Cyclohexene Epoxidation. The research presented here focuses on the epoxidation of cyclohexene in a CO2 and aqueous H2O2 system made possible through the in situ formation of peroxycarbonic acid. The peroxycarbonic acid-catalyzed products from this reaction are cyclohexene oxide and 1,2-cyclohexane diol. It should be noted that 1,2-cyclohexane diol is formed primarily through the consecutive hydrolysis of the epoxide, but it can also be formed directly through the addition of hydroxyl radicals to cyclohexene. Although cyclohexene oxide and 1,2-cyclohexane diol are the desired epoxidation products, the production of byproducts is possible. It is conjectured that the byproducts 2-cyclohexen-1-one, 2-cyclohexen-1-ol, and an isomer of 2-cyclohexen-1-ol form through metal-catalyzed radical mechanisms via allylic hydrogen abstraction from cyclohexene.29,30 Cyclohexene epoxidation was performed in a biphasic system composed of a supercritical CO2/olefin phase and an aqueous H2O2 phase (Figure 2). The phase behavior of the system was estimated using the Patel-Teja equation of state and was visually verified at selected points with a Jerguson (model 12-T-32) view-cell. CO2 and cyclohexene become completely miscible above 82 bar at 40 °C; therefore, we performed investigations at 120 bar and 40 °C to ensure that there was a single CO2/olefin phase as well as an aqueous H2O2 phase. Formation of Peroxycarbonic Acid. This work provides further evidence consistent with the existence of peroxycarbonic acid as hydrogen peroxide is unlikely to form epoxides without activation because of its insufficient oxidizing strength.31,32 Nevertheless, hydrogen peroxide is able to form diols via a radical mechanism. To provide support for the concept that peroxy-

Figure 3. Comparison between cyclohexene epoxidations after 20 h at 40 °C and 120 bar with and without NaHCO3 in N2 and CO2. x-axis legend: 1, cyclohexene conversion based on product yields; 2, cyclohexene oxide yield; 3, 1,2-cyclohexandiol yield; 4, 2-cyclohexen-1-one yield; 5, 2-cyclohexen-1-ol isomer yield; 6, 2-cyclohexen-1-ol yield. The reactant molar ratios [cyclohexene: H2O2:H2O:NaHCO3:(CO2 or N2)] were approximately equal for each reaction and are (no additives) 1:2.6:11.3:0:23.6; (NaHCO3) 1:2.6: 11.3:0.04:23.6. Postreaction H2O2 concentration: (N2, no additives) 24.65 wt %; (CO2, no additives) 20.7 wt %; (N2, NaHCO3) 0 wt %; (CO2, NaHCO3) 0 wt %.

carbonic acid is the epoxidizing agent in reactions with CO2 and H2O2, a control experiment was performed in which CO2 was replaced with N2. The results (Figure 3) show that no epoxide and less than 0.06 mol % diol are formed in a N2 environment with H2O2 as the only oxidizing agent; however, a small amount of epoxide (0.02 mol %) and 0.09 mol % diol are formed in CO2 under analogous conditions. The error bars shown in Figure 3 and all future figures represent the 95% confidence interval from multiple experiments under analogous conditions. Although peroxycarbonic acid is the likely oxidant when CO2 is present, it contributes little to the epoxidation of cyclohexene by itself. To improve the effectiveness of peroxycarbonic acid as an oxidizer, we examined potential limitations to the epoxidation of cyclohexene. Investigations were made to improve the concentration of peroxycarbonic acid through the addition of NaHCO3. It is suggested that, by adding NaHCO3, the effective concentration of peroxycarbonic acid is increased by conversion to its ionized form (reaction 3). Even though the free peroxycarbonic acid might be more reactive than its corresponding conjugate base, a much greater concentration of the conjugate base should lead to improved conversion and yield.

Results from experiments with a small addition of NaHCO3 (0.1 mol %; 1 wt %) proved successful and provided stronger evidence for the existence of peroxycarbonic acid. Although peroxycarbonic acid can be formed directly from reaction between NaHCO3 and H2O2, as reported by Richardson et al.,11 the increase in epoxide yield from 0.02 mol % (without NaHCO3) to 1.6 mol % (with NaHCO3) in CO2 compared to the increase from 0 mol % (without NaHCO3) to 0.14 mol % (with NaHCO3) in N2 illustrates that CO2 facilitates the formation of a peroxy species (Figure 3). This result agrees with a hypothesis made by Richardson et al. that

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Figure 4. Influence of NaHCO3 concentration on cyclohexene epoxidations after 20 h at 40 °C and 120 bar CO2. The reactant molar ratios [cyclohexene:H2O2:H2O:NaHCO3:CO2] are (no NaHCO3) 1:2.6:11.3:0:23.6; (0.1 mol % NaHCO3) 1:2.6:11.3:0.04:23.6; (0.45 mol % NaHCO3) 1:2.6:11.3:0.17:23.6. Postreaction H2O2 concentration: (no NaHCO3) 20.7 wt %; (0.1 mol % NaHCO3) 0 wt %; (0.45 mol % NaHCO3) 0 wt %.

Figure 5. Influence of HEDP on cyclohexene epoxidation after 20 h at 40 °C and 120 bar CO2. The reactant molar ratios [cyclohexene:H2O2:H2O:NaHCO3:HEDP:CO2] are (no HEDP) 1:2.6: 11.3:0.04:0:23.6; (0.01 mol % HEDP) 1:2.6:11.3:0.04:0.0009:23.6. Postreaction H2O2 concentration: (no HEDP) 0 wt %; (0.01 mol % HEDP) 0.5 wt %.

CO2 should improve the formation of peroxycarbonic acid due to the pH and equilibrium relationships of the involved species.10 The pH during our investigations was not measured; however, a rough estimation of the pH can be made by analogy to work published by Holmes et al. in which the pH of a 4.2 wt % aqueous solution of NaHCO3 (207 bar CO2, 35 °C) was measured spectrophotometrically and found to be 5.42.33 The pH was found to vary by only (0.03 pH units over a 15 °C temperature range and a 138 bar pressure range.33 It is also important to note that epoxide selectivity is greatly improved when CO2 is used. The epoxide selectivity for the reaction containing NaHCO3 and CO2 is 80%, whereas it is only 10% with NaHCO3 and N2. This trend in selectivity supports the existence of peroxycarbonic acid since the byproduct species, which are the dominant products in the reactions with N2, are formed through radical pathways associated with H2O2 decomposition.29,30 Assurance that the observed byproducts are a result of a radical mechanism involving H2O2 was provided from a reaction of 1,2-cyclohexene oxide in a water/ NaHCO3/N2 system in which none of the byproducts were detected. The influence of the NaHCO3 concentration on the epoxidation was investigated to determine whether the epoxide yield would correspondingly increase for a 4-fold increase in NaHCO3. During these investigations, the NaHCO3 concentration remained 60% below the NaHCO3 solubility limit of 11.13 wt %,34 even at the highest NaHCO3 concentration investigated. The results (Figure 4) demonstrate that there is only a very slight difference in yield at the different NaHCO3 concentrations. The observation that cyclohexene epoxidation is essentially insensitive to the NaHCO3 concentration is readily explained if the decomposition of H2O2 occurs through the peroxycarbonic acid ion intermediate with the same reaction order in peroxycarbonic acid ion as the epoxidation reaction. In this case, the ratio of epoxidation rate to H2O2 decomposition rate is independent of the NaHCO3 concentration. An unfortunate disadvantage of using NaHCO3 is an accelerated decomposition of the hydrogen peroxide. Postreaction analyses of the results shown in Figures 3 and 4 demonstrate that no H2O2 remains at the end

of the experiments containing NaHCO3. This raises the question of whether H2O2 decomposition is a limitation to the epoxidation. For this reason, the effect of peroxide stability on the reaction was investigated. Influence of H2O2 Stabilization. Hydrogen peroxide is susceptible to radical decomposition reactions with trace metal impurities.35 To reduce this decomposition, we added 500 ppm of the metal chelating agent hydroxyethane-1,1-diphosphonic acid (HEDP) to the reaction system. The results shown in Figure 5 confirm that hydrogen peroxide decomposition is a limitation to the reaction. Unfortunately, HEDP did not prevent the decomposition catalyzed by NaHCO3 that occurs through a nonradical pathway. Although the hydrogen peroxide was almost completely degraded when HEDP was used, it was present long enough to provide the higher conversion and product yields reported. Mass Transport Limitation Investigations. In addition to limitations resulting from hydrogen peroxide decomposition, mass transport could inhibit the reaction by preventing interaction between the reactive species in the olefin/scCO2-aqueous H2O2 biphasic system. The transport of CO2 to the aqueous phase was found to be very rapid (Appendix B) and therefore is not limiting; however, the transfer of cyclohexene to the reaction interface or aqueous phase might be limiting. The solubility of peroxycarbonic acid in its ionized form is negligible in the nonpolar CO2/olefin phase; therefore, epoxidation must occur at the interface between phases or in the aqueous phase. To test for this type of transport limitation, the epoxidation of cyclohexene was studied under analogous conditions at mixing rates of 100 and 600 rpm. The results (Figure 6) show that higher conversion and product yields are achieved at the higher mixing rate, which confirms the existence of transport limitations. The results, however, do not provide any indication as to whether the reaction occurs at the reaction interface or within the aqueous H2O2 phase. Cosolvent Influence. To discern whether epoxidation occurs within the aqueous phase, hydrophilic cosolvents were used to enhance the olefin solubility within the aqueous phase. Without a cosolvent, cyclohexene solubility in the aqueous phase is very small, on the order of 10-5 mol/mol of solution. Cosolvent selection was based on aqueous solubility, chemical

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Ind. Eng. Chem. Res., Vol. 41, No. 3, 2002 Table 1. Summary of Epoxide and Diol Selectivities under All Reaction Conditionsa reaction conditions N2 CO2 N2, NaHCO3 CO2, 0.1 mol % NaHCO3 CO2, 0.45 mol % NaHCO3 HEDP, CO2, 0.1 mol % NaHCO3 100 rpm, CO2, NaHCO3 methanol, CO2, 0.1 mol % NaHCO3 propylene carbonate, CO2, 0.1 mol % NaHCO3 dimethylacetamide, CO2, 0.1 mol % NaHCO3 dimethylformamide, CO2, 0.1 mol % NaHCO3 dimethylformamide, N2, 0.1 mol % NaHCO3 acetonitrile, CO2, 0.1 mol % NaHCO3 3-CCSS, CO2, 0.1 mol % NaHCO3

Figure 6. Effect of mixing rate on cyclohexene epoxidation after 20 h at 40 °C and 120 bar CO2. The reactant molar ratios [cyclohexene:H2O2:H2O:NaHCO3:CO2] for both experiments are 2.6:11.3:0.04:0:23.6 Postreaction H2O2 concentration: (100 rpm) 0 wt %; (600 rpm) 0 wt %.

Figure 7. Cosolvent influence on the epoxidation of cyclohexene after 20 h at 40 °C and 120 bar CO2. The reactant molar ratios [cyclohexene:H2O2:H2O:NaHCO3:cosolvent:(CO2 or N2)] are (no cosolvent) 1:2.6:11.3:0.04:0:23.6; (cosolvent) 1:2.6:11.3:0.04:5:∼18. Postreaction H2O2 concentration: (no cosolvent) 0 wt %; (methanol) 2.9 wt %; (dimethylacetamide) 7.6 wt %; (propylene carbonate) 1.2 wt %; (diemthylformamide) 2.8 wt %; (acetonitrile) 0.6 wt %.

inertness, and potential for improving the aqueous solubility of cyclohexene. Of several cosolvents evaluated, acetonitrile and dimethylformamide (DMF) provided the largest improvement in yield without loss of selectivity (Figure 7). A summary of the epoxide and diol selectivities for all of the reaction conditions investigated is presented in Table 1. A control experiment using the cosolvent dimethylformamide was performed as a means of decoupling the influence of CO2 from those of the cosolvents. This was achieved by replacing CO2 with N2 in a system containing NaHCO3. The results (Figure 8) show that, with N2, the epoxide and diol yields are 2.6 and 0.77 mol %, respectively; however, with CO2, the epoxide and diol yields are 10.7 and 0.62 mol %, respectively, showing that CO2 addition contributes to improved conversion and yields. In addition to increasing the mutual solubilities of the aqueous and organic compounds, cosolvents might provide other oxidants by reacting to form their own peroxy acids. In the case of acetonitrile, cosolvent oxidation is possible, resulting in the formation of peroxycarboximidic acid, the peroxy acid of acetonitrile.36 GC-MS analyses of experiments using acetoni-

epoxide diol selectivity selectivity 0 5 10 80 90 84 67 37 67 77 84 64 82 89

15 42 5 7 3 11 19 47 8 0 5 20 5 11

a Reaction conditions: 20 h at 40 °C and 120 bar CO . The 2 reactant molar ratios [cyclohexene:H2O2:H2O:NaHCO3:cosolvent: (CO2 or N2)] are (no cosolvent) 1:2.6:11.3:0.04:0:23.6; (cosolvent) 1:2.6:11.3:0.04:5:∼18.

Figure 8. Cosolvent vs CO2 influence on the epoxidation of cyclohexene after 20 h at 40 °C and 120 bar CO2. The reactant molar ratios [cyclohexene:H2O2:H2O:NaHCO3:cosolvent:(CO2 or N2)] are (N2 without cosolvent) 1:2.6:11.3:0.04:0:23.6; (cosolvent) 1:2.6:11.3:0.04:5:∼18. Postreaction H2O2 concentration: (no cosolvent) 0 wt %; (dimethylformamide and N2) 9.6 wt %; (dimethylformamide and CO2) 2.8 wt %.

trile show the presence of trace amounts (∼0.25 mol %) of acetamide, a known end product from the reaction of peroxycarboximidic acid with an olefin or H2O2, which confirms that cosolvent oxidation occurred.36 The concentration of acetamide for these experiments is considerably lower than the epoxide (14.4 mol %) and diol (0.79 mol %) yields, however, indicating that peroxycarboximidic acid did not play a major role in the reaction. It should also be noted that the other cosolvents investigated do not form peroxy acids as does acetonitrile. The difference in yield between reactions under analogous conditions with and without cosolvents demonstrates the importance of bringing the reactive species into contact with each other. Judging from the results with cosolvents, it would appear that cyclohexene epoxidation does occur within the aqueous phase. To confirm this, investigations were performed using the water-soluble olefin 3-cyclohexen-1-carboxylate sodium salt. Water-Soluble Olefin Epoxidation. 3-Cyclohexen1-carboxylate sodium salt (3-CCSS) was chosen as the olefin for investigation into whether epoxidation occurs

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within the aqueous phase due to the chemical similarity 3-CCSS has to cyclohexene. Unlike the investigations with cyclohexene, 3-CCSS and peroxycarbonic acid are both in the aqueous phase, allowing them to readily react with each other. Although there is still a twophase system consisting of scCO2 and an aqueous H2O2/ olefin phase during the 3-CCSS studies, transport of CO2 to the aqueous phase is very rapid and not limiting to the reaction (Appendix B). For this reason, the reaction is treated as a single-phase reaction occurring within the aqueous phase. The epoxidation of 3-CCSS at 40 °C and 120 bar at a reactant ratio of 1:17.4:76.6:0.25:1:122.7 corresponding to cyclohexene:H2O2:H2O:NaHCO3:hydroquinone:CO2 resulted in ∼100 mol % conversion, with an epoxide yield of 89 mol % and a diol yield of 11 mol %. Unlike cyclohexene, 3-CCSS is susceptible to radical decomposition through attack of the carboxylate group; therefore, hydroquinone was used during these investigations to prevent decomposition. The dramatic improvement in epoxide yield observed with 3-CCSS supports the hypothesis that epoxidation occurs within the aqueous phase. Unfortunately, 3-CCSS has a carboxylate functionality, and therefore, could have formed a peroxy acid species that catalyzed the reaction. A reaction in which 3-CCSS is replaced with 3-cyclohexen-1-sulfonate sodium salt would not be susceptible to peroxy acid formation and would therefore decouple these effects, however, this investigation is left for future research. In light of this, a comparison of Richardson et al.’s work on the reactions of 4-vinylbenzenesulfonic acid sodium salt (monophasic) and the biphasic reaction of styrene is provided as support for the hypothesis that epoxidation occurs within the aqueous phase.11 In this example, the olefins do not form peroxy acids. Richardson et al. found that the conversion of 4-vinylbenzenesulfonic acid sodium salt was >99% with 90% epoxide selectivity in 15 h, whereas the conversion of styrene was only 40% with 99% epoxide selectivity in 24 h.11 Conclusions Olefin epoxidations have been achieved using scCO2 and H2O2 with no metallic catalyst or peroxy acid additions while maintaining high epoxide and diol selectivities. This is possible because of the probable in situ formation of peroxycarbonic acid. Additional benefits provided by scCO2 include the ability to readily tune reactions, facile separation opportunities, and a benign nonflammable solvent. Although epoxidations using scCO2 have not been completely optimized, improvement of epoxide yield from 0.02 mol % without any additives to 1.6 mol % has been achieved by increasing the concentration of peroxycarbonic acid or its conjugate base with a 0.1 mol % addition of NaHCO3. This was done while using NaHCO3 concentrations considerably lower than those reported by Richardson et al. for similar epoxidations. The epoxide yield was further improved to 10.7 mol % by increasing the aqueous solubility of the olefin by use of the hydrophilic cosolvent dimethylformamide, suggesting that the epoxidation occurs within the aqueous phase. This hypothesis was confirmed by the aqueous phase epoxidation of 3-cyclohexen-1-carboxylate sodium salt, which resulted in an epoxide yield of 89 mol %. A summary of these findings is presented in Figure 9. Through these studies, a successful and industrially interesting epoxidation technique using CO2 and H2O2 is reported; however, the

Figure 9. Summary of improvement in epoxide yield. Reaction conditions: 20 h at 40 °C and 120 bar CO2. The reactant molar ratios [cyclohexene:H2O2:H2O:NaHCO3:dimethylformamide:hydroquinone:CO2] are (cyclohexene without additives) 1:2.6:11.3:0: 0:0:23.6; (cyclohexene and NaHCO3) 1:2.6:11.3:0.04:0:23.6; (cyclohexene, NaHCO3, and dimethylformamide) 1:2.6:11.3:0.04:4.9:0: 18.2; (3-CCSS and NaHCO3) 1:17.4:76.6:0.25:0:1:122.7. Postreaction H2O2 concentration: (cyclohexene without additives) 20.7 wt %; (cyclohexene and NaHCO3) 0 wt %; (cyclohexene, NaHCO3, and dimethylformamide) 2.8 wt; (3-CCSS and NaHCO3) not quantified.

practical use of this technique is limited by slow mass transfer of the olefin to the aqueous phase and by H2O2 decomposition. This technique might find application through the use of a more reactive olefin or through improvements in the interaction between the olefin and peroxycarbonic acid obtained by using better cosolvents, surfactants, microemulsions, or water-soluble olefins. Acknowledgment The authors gratefully acknowledge financial support from Degussa-Hu¨ls. R.G. thanks the German Research Association (Deutsche Forschungsgemeinschaft) for a research stipend. Appendix A: Synthesis of Water-Soluble Olefins 3-Cyclohexen-1-carboxylate Sodium Salt. 3-Cyclohexen-1-carboxylic acid (24.94 g, 198 mmol) was dissolved in methanol (50 mL), and the mixture was stirred for 30 min at 0 °C. Aqueous sodium hydroxide (15.82 g, 1.0 equiv) was then added dropwise over 30 min. The reaction mixture was stirred at room temperature overnight. The solvent was removed by vacuum distillation, and the remaining solid was recrystallized from hot 2-propanol to give 28.43 g of white crystalline 3-cyclohexen-1-carboxylate sodium salt: 97 mol % yield; mp > 300 °C (dec); 1H NMR (CD3OD, 500 MHz) δ 1.59 (m, 1H), 1.93 (dd, J ) 3.4 Hz, 1H), 2.05 (m, 2H), 2.17 (m, 2H), 2.32 (m, 1H), 5.64 (m, 2H). 3,4-Epoxycyclohexan-1-carboxylate Sodium Salt. 3,4-Epoxycyclohexan-1-carboxylic acid was synthesized from 3-cyclohexen-1-carboxylic acid according to the literature method of Fort et al.37 It was purified by sublimation, resulting in a 46 mol % yield. An aqueous suspension (10 mL) of the 3,4-epoxycyclohexan-1-carboxylic acid (0.70 g, 4.93 mol) was then treated with NaHCO3 (0.41 g, 1.0 equiv) while the mixture was stirred. After 5 min, the water was evaporated giving 3,4-epoxycyclohexan-1-carboxylate sodium salt in quantitative yield: mp 110 °C; 1H NMR (CD3OD, 500 MHz)

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δ 1.28 (m, 1H), 1.44 (m, 2H), 1.86 (m, 1H), 1.93 (m, 1H), 2.11 (m, 1H), 2.19 (m, 1H), 3.30 (m, 2H). 3,4-Dihydroxycyclohexan-1-carboxylate Sodium Salt. An aqueous solution of hydrogen peroxide (30 wt %, 2.61 mL) and formic acid (88 wt %, 17.40 mL) was slowly added to 3-cyclohexene-1-carboxylic acid (4.35 g, 34.00 mmol) at 0 °C according to the procedure described by Grewe et al.38 The temperature was then increased to 40 °C and held there for 15 min, after which it was decreased to room temperature. After 30 h, the reaction mixture was tested for unreacted H2O2 using Quantofix peroxide test sticks (Aldrich). After the reaction mixture was ensured to be free of peroxide, the solvent was removed under vacuum (T e 40 °C). Solvent must not be removed if any peroxide remains, as organic peroxides can concentrate, leading to explosion hazards. Unreacted H2O2 should be decomposed with a reductant. Following solvent removal, water (34.80 mL) was added to the residue, and the mixture was refluxed for 2 h. The reaction medium was again reduced under vacuum, and the remaining residue was dissolved in a minimum amount of ethyl acetate. The resulting solution was refrigerated overnight, forming crystals that were filtered to yield pure 3,4-dihydroxycyclohexan-1carboxylic acid (46 mol % yield). An aqueous solution of the 3,4-dihydroxycyclohexan-1-carboxylic acid (0.86 g, 0.054 mol) was stirred with NaOH (0.22 g, 1.0 equiv) at 0 °C for 2 h to form the salt adduct. Stirring was maintained overnight at room temperature. After removal of the solvent under vacuum, pure 3,4-dihydroxycyclohexan-1-carboxylate sodium salt was obtained in 84 mol % yield; mp > 250 °C; 1H NMR (CD3OD, 500 MHz) δ 1.56-1.48 (m, 3H), 1.79 (m, 1H), 1.96 (q, J ) 8.7 Hz, 1H), 2.19 (m, 1H), 2.49 (m, 1H), 3.41 (sext, J ) 3.6 Hz, 1H), 3.66 (sext, J ) 3.6 Hz,1H). Appendix B: Study of CO2 Transport into the Aqueous Phase An investigation was made to determine whether CO2 transport into the aqueous phase is limiting to the epoxidation of cyclohexene under our reactive conditions. This investigation was performed by adding aqueous NaOH to a reactor pressurized with CO2 (T ) 40 °C, P ) 92 bar). As CO2 is transported into the aqueous NaOH phase, it reacts to form NaHCO3 or Na2CO3. CO2 transport was monitored by watching the change in pressure, since it will decrease as the reaction proceeds. We found that the pressure stabilized after ∼3 min while stirring at only 260 rpm. These results show that CO2 transport is very rapid even while stirring at only one-half of the speed used during the epoxidation studies; therefore, it is not a limitation to the reaction. Literature Cited (1) Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J., Howe-Grant, M., Eds.; John Wiley and Sons: New York, 1991; Vol. 9, pp 730-753. (2) Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; Elvers, B., Hawkins, S., Ravenscroft, M., Eds.; VCH: New York, 1991; Vol. A13, pp 443-464. (3) Jorgensen, K. A. Transition Metal Catalyzed Epoxidations. Chem. Rev. 1989, 89, 431-458. (4) Berkessel, A.; Sklorz, C. A. Mn-Trimethyltriazachclononane/ Ascorbic Acid: A Remarkably Efficient Catalyst for the Epoxidation of Olefins and the Oxidation of Alcohols with Hydrogen Peroxide. Tetrahedron Lett. 1999, 40, 7965-7968.

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Received for review January 11, 2001 Revised manuscript received May 10, 2001 Accepted May 11, 2001 IE0100378