Abstract:
A method for synthesizing quinone from an aromatic compound is developed that employs a paired electro-oxidation method and a undivided electrochemical cell. The electrolyte solution is a combination of an aromatic solution (aqueous or nonaqueous) and a redox mediator solution, which can be V 5+  /V 4+ , Fe 3+  /Fe 2+ , or Cu 2+  /Cu + , in an undivided electrochemical cell. The electrolyte reaction is conducted by bubbling oxygen into the bottom of the cathode, then the oxygen is reduced to hydrogen peroxide (H 2  O 2 ). Simultaneously, at the anode surface, lower valence state ions can be oxidized to higher valence states. Hydrogen peroxide then oxidizes the rest of the low valence state ions to form high valence ions, OH-free radicals, and combinations of both. These ions and radicals then react with the aromatic compound in the solution and form the resultant product, quinone.

Description:
BACKGROUND 
     This invention pertains to an effective method to electrochemically synthesize quinone compounds from aromatic compounds using a paired electro-oxidation approach in an undivided cell. 
     The two primary techniques for oxidizing an aromatic compound to a quinone compound are: (1) the catalytical oxidation method; and (2) the electro-oxidation method. The first method requires high temperature, high pressure, and catalysts. Maintaining this high temperature and pressure results in high cost, difficulty of operation, and poor selectivity of resultant products. 
     Using an electrochemical method to convert an aromatic compound to a quinone compound has several advantages including low temperature and pressure, easy to operate, and high selectivity of resultant products. 
     The electro-oxidation approach is to oxidize an aromatic solid or its organic solution in an anode of an electrochemical cell to produce a quinone. The electrolyte solution is composed of a redox mediator solution and an aromatic compound (solid or solution). The redox mediator solution can be an aqueous inorganic redox couple in solution such as Mn 3+  /Mn 2+ , Ag 2+  /Ag + , Ce 4+  /Ce 3+  V 5+  /V 4+ , Co 3+  /Co 2+ , and Tl 3+  /Tl 2+ . The low valence state metal ions of the redox mediator are oxidized first on the anode surface and become higher valence ions. Then the oxidized ions react with the aromatic compounds and form quinone and lower valence ions. The resultant product, i.e., the quinone compound, can be removed from the solution, and simultaneously the lower valence ions can be recycled into the cell for another oxidation cycle. The cycle can proceed continuously until the aromatic compound is completely exhausted. 
     Among the chemical reactions, quinone is formed on the anode surface, and the cathodic reaction involves a fine metal wire (as cathode) and the reduction reaction of water, which forms hydrogen; or alternatively, to reduce some undesired compound so that the reaction on the anode surface would not be affected. Consequently, the process becomes inefficient, resulting in a poor distribution of the system voltage and current, and the waste of electric energy on the cathode. 
     U.S. Pat. No. 4,950,368 provided a paired electro-oxidation method to synthesize ethylene glycol on the cathode from the reduction of formaldehyde in an electrochemical cell that had a dialysis membrane installed in the middle. Simultaneously, at the surface of the anode, higher valence metal ions are formed. These ions can oxidize another compound and produce a second product. This paired electro-oxidation approach can increase efficiency and save energy; however, there are still disadvantages. For example, the resultant products of both electrodes are not the same, requiring further separation. 
     In the R.O.C. Patent No. 30226 (application No. 76100506), equivalent to J. of Applied Electrochemistry 17 (1987) 753-759, the detailed procedure of a paired electro-oxidation synthesis method for the production of aromatic aldehyde is described. The method, which generated a Fenton&#39;s reagent, could be used to oxidize toluene to benzaldehyde. Simultaneously, at the anode the Mn 2+  was oxidized to Mn 3+ , and Mn 3+  could continuously oxidize toluene to benzaldehyde. The Mn 2+  ions were reoxidized at the anode again and could be used repeatedly. 
     The purpose of this invention is to develop a paired electro-oxidation method to synthesize a quinone compound effectively from an aromatic compound. An improved reactor design for the paired electro-oxidation reactions is employed, i.e., reactions take place in an undivided single cell without the presence of any membrane. 
     SUMMARY 
     An effective paired electro-oxidation synthesis method that can oxidize an aromatic compound and form a quinone compound is provided. The electrolyte solution can be a combination of an aromatic solution (aqueous or nonaqueous) and a redox mediator solution such as V 5+  /V 4+ , Fe 3+  /Fe 2+ , or Cu 2+  /Cu + , in an undivided electrochemical cell. Electrolytical reaction is carried out by bubbling oxygen into the bottom of the cathode. At the anode surface of this cell, lower valence state ions can be oxidized to higher valence state; simultaneously, oxygen is reduced to hydrogen peroxide (H 2  O 2 ) at the cathode surface. H 2  O 2  then oxidizes the rest of low valence state ions and forms high valence ions, .OH free radicals, and combinations of both, which can react with the aromatic compound in the solution to form the resultant product--quinone. 
     In the above description, optimally, the aromatic compound is anthracene or an anthracene attached with at least one or more --OH 3 , --NO 2 , C 1  -C 8  alkyl and halogen functional groups. 
     Optimally, the organic solution and the redox mediator solution is well stirred during the process. 
     Optimally, an anode plate in the electrochemical cell and the cathode is constructed in a parallel-metal-net cylindrical form. 
     Optimally, the redox mediator solution contains some supporting electrolytes, which can be H 2  SO 4 , HClO 4 , HNO 3 , HNO 2 , formic acid, and acetic acid. The adequate concentration of the supporting electrolyte is from 0.5 to 10 molar. 
     The unique advantages of this invention are: (1) redox reactions of anode and cathode take place in an undivided electrochemical cell; (2) only one kind of redox mediator is used; (3) electrochemical reactions occur on both electrodes and indirectly convert the reactant to product, then product to reactant; (4) the undivided electrochemical cell does not need any membrane; (5) the structure is simple, cost effective, and easy to maintain. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the design of the undivided electrochemical cell. 
     FIG. 2 shows a flow scheme of the continuous process for the production of anthraquinone using this invention. 
     This invention, including the rationale and the significant improvement of the cell design, can be further understood by the following description. 
    
    
     DETAILED DESCRIPTION 
     A unique method that effectively synthesizes a quinone from an aromatic compound using paired redox electrochemical reaction and an undivided cell is provided. The electrochemical cell contains a redox mediator (which can be V 5+  /V 4+ , Fe 3+  /Fe 2+ , or Cu 2+  /Cu + ), an electrolyte solution, and the aromatic compound. Oxygen is bubbled into the solution at the bottom of the cathode. Among the chemical reactions, higher valence state metal ions oxidants are formed on the cathode surface. The lower valence state metal ions in solution form a Fenton reagent, which in turn is also a strong oxidant. Therefore, the aromatic compound in solution reacts with both strong oxidants at both electrodes and forms the desired quinone compound. 
     All the chemical reactions taking place in the electrochemical cell can be illustrated by the example of anthracene oxidized to anthraquinone. A mixture of anthracene and its organic electrolyte solution, a redox mediator, V 5+  /V 4+ , is well stirred in an electrochemical cell where the anodic reaction is: 
     
         V.sup.4+ →V.sup.5++ e.sup.- 
    
     the cathodic reaction is: 
     
         2 H.sup.++ O.sub.2 +2 e.sup.- →H.sub.2 O.sub.2 
    
     and the chemical reactions in the solution are: 
     
         H.sub.2 O.sub.2 +V.sup.4+ →.OH+V.sup.5++ OH.sup.- 
    
     
         C.sub.14 H.sub.10 +6 V.sup.5+ +2 H.sub.2 O→C.sub.14 H.sub.8 O.sub.2 +6 V.sup.4+ +6 H.sup.+ 
    
     
         C.sub.14 H.sub.10 +6 .OH→C.sub.14 H.sub.8 O.sub.2 +4 H.sub.2 O 
    
     As the above reactions indicate, V 4+  ions are first oxidized on the anode surface and form V 5+  ions, which then release to the solution and react with anthracene to form anthraquinone. Simultaneously, V 5+  ions are reduced to V 4+  ions. Part of the V 4+  ions will be reoxidized on the anode surface. Another part of the V 4+  ions react with H 2  O 2  on the cathode to form V 5+  ions and .OH free radicals, which will continue to oxidize anthracene and generate anthraquinone. 
     At the cathode surface, a Fenton reagent, which contains a lower valence metal ions and H 2  O 2 , is formed. The H 2  O 2  reacts with V 4+  ions in the solution to form .OH-free radicals and V 5+  ions, which can oxidize anthracene to anthraquinone. The Fenton reagent is a strong oxidant that can oxidize many organic compounds such as anthracene and toluene. Besides V 4+  ions, Fe 2+  ions and Cu +  ions can also react with H 2  O 2  to generate .OH free radicals. 
     The major reaction on the cathode is to reduce the oxygen to H 2  O 2  so that it can react with V 4+  to form .OH-free radicals; therefore, oxygen needs to be supplied to the system. 
     The solution contains a redox mediator and supporting electrolyte so that it can conduct current and allow the reactions to occur. The supporting electrolyte can be any aqueous soluble compound; however, it should not interfere with the oxidation of the aromatics. Alkaline and alkaline earth groups of hydroxide, such as NaOH and KOH can react with the free radicals, and consequently lower the overall efficiency. Compounds of alkaline and halogen groups are not good supporting electrolytes for this system, because a high operating voltage may result in their decomposition. That is, the halogen ions can be oxidized to elemental halogen (gas form), causing poor conductivity and product contamination. Many electrolytes, such as H 2  SO 4 , HClO 4 , HNO 3 , HNO 2 , formic acid, and acetic acid, are good candidates for this system. These supporting electrolytes can perform in a wide range of concentrations; therefore, an optimal condition can be achieved. One of the most effective concentrations in the illustrated example is a 4- to 10-M sulfuric acid solution. 
     The material of the single undivided electrochemical cell is carefully selected so that it does not participate in the overall reaction. A good anodic material needs to have high oxygen overpotential, and some metal oxides such as PtO, PbO, and SbO are good representative materials. These materials can be coated on graphite, titanium, or ceramic materials. A good cathodic material needs to have a good catalytic property to enhance the reaction, i.e., to facilitate the reduction from oxygen to hydrogen peroxide. Good representative materials for the cathodic materials are platinum, mercury, and lead. These materials can be solid or just a coating on the supporting cathode surface. Common metals such as iron, aluminum, and zinc are inadequate for the design of this invention due to their high reactivities that may participate electrochemical reactions. Very often these inorganic ions can form various salts or even hydrogen; therefore, they may have an adverse effect on the formation of many aromatics. 
     The shape of both electrodes can be a flat surface, a net-like constructed, or a matrix form. Optimally, larger surface area at the cathode will enhance the reactions, i.e., more oxygen will be reduced to hydrogen peroxide. In one of the examples of this invention, a parallel-platinum-net cylindrical cathode with a sponge-like surface has been proven to be very effective. 
     The operating temperature of this invention is between 10° and 55° C. The operating pressure is the same as the atmosphere or close to 1 atm. The preferred operating conditions are 10° to 35° C. and 1 atm. 
     FIG. 1 illustrates this invention including an undivided cell, a flat anode surface, and a unique parallel-platinum-net cylindrical cathode with a sponge-like surface. Both electrodes are connected by a bus bar (16 and 17). Electricity is supplied through the bus bar to both electrodes. During the real-time operation, a solution that contains V 4+ , H 2  SO 4 , and an anthracene dissolved in chloroform (CHCl 3 ) is fed into the cell from line (14). Oxygen is bubbled into the cell through a gas dispenser (13, not shown in the Figure) and many small oxygen bubbles are formed. These oxygen bubbles transport to the bottom of the cathode, penetrate the parallel-metal-net cylinder, are reduced to hydrogen peroxide, then diffuse into the bulk solution and react with V 4+  ions. The products, V 5+  ions and the .OH-free radicals, can oxidize anthracene to anthraquinone. Simultaneously, V 4+  ions are oxidized to V 5+  ions on the anode surface, and released into the solution, to oxidize anthracene to anthraquinone. An agitation device (does not show in the Figure) is also installed to enhance the contact between aqueous and organic phases. 
     After various electrochemical reactions, the bulk solution is taken out through a pipeline (15), then the aqueous and organic phases are separated and the aqueous phase will be recycled to the cell. The organic phase contains chloroform, anthracene, and anthraquinone. Anthraquinone can be removed and the remaining anthracene is refluxed and fed into the electrochemical cell. 
     FIG. 2 illustrates the continuous process for the production of anthraquinone from anthracene. The reactor design of the undivided electrochemical cell, as illustrated in FIG. 1, is used in this invention. In the undivided cell (21), a mixture containing H 2  SO 4 , CHCl 3 , and anthracene is introduced into the electrochemical cell through (24) and (25). Some V 4+  ions are oxidized on the anode to form V 5+  ions. The oxygen is bubbled into the cathode (23) through pipeline (26), diffused through the parallel-metal-net cylindrical electrode, and reduced to hydrogen peroxide. The V 4+  ions in the bulk solution react with the hydrogen-peroxide-generated V 5+  ions and .OH--free radicals. The V 5+  ions that generate from both electrodes together with .OH--free radicals electrochemically convert the anthracene to anthraquinone. The solution after electrolysis and chemical reactions is guided out of the cell through (27) to a second reactor (28). In this second reactor, additional H 2  O 2  is merged into this reactor through pipeline (29), and reacted with V 4+  ions, to form V 5+  ions and .OH--free radicals. Both V 5+  ions and .OH--free radicals can oxidize anthracene to anthraquinone. After the reactions, the solution of the second reactor is transported to a gravity settling tank, where both the organic and aqueous phases can be separated. 
     The upper phase, i.e., the aqueous phase, will be recycled to the electrochemical cell through line (24), whereas the lower phase, i.e., the organic phase, will be fed into a distillation tower (34) through line (33) in order to recover the organic solvent. The vapor phase of the distillation tower passes through a condenser (36) and becomes a liquid phase, which is collected in the storage tank (37). The residue of the distillation tower is transported to a crystallization tank where the anthraquinone (which has the higher melting point) is separated from the organic phase and taken out continuously through line (46). The unreacted anthracene solution is quenched from a cooling device (42) through line, and is then transported to the starting material storage tank (44) by line (43). The fresh solvent is pumped from the solvent storage tank (37) through line (38) into the starting material tank. Anthracene is introduced into the starting material tank where it is dissolved by the organic solvent to form a saturated organic solution. The organic solution is fed into the electrolysis cell (21) through line (25). 
     The purpose of the following examples is to demonstrate the feasibility of this invention. The examples are not to limit or to narrow the use of this invention. 
     EXAMPLE 1 
     FIG. 1 illustrates the electrochemical synthesis of anthraquinone from anthracene. The undivided electrochemical cell has a platinum plate as the anode. A parallel-titanium-net cathodic cylinder is coated by lead and has a sponge-like shape. The surface areas of each cathode and anode are 110 and 20 cm 2 , respectively. During the reactions, the electrolyte is guided through a fluorometer and flow upward and fed into the cell at the lower righthand corner, and later flows out of the cell from the upper left corner. The electrolyte solution contains 5.00×10 -3  M of V 2  O 5 , 10M of H 2  SO 4 , and 1.12×10 -1  M anthracene dissolved in CHCl 3 . Temperature is controlled at 25±0.5° C. The oxygen is bubbled into the solution at the bottom of the cathode at a flow rate of 100 ml/min. After the reaction proceeds is identified, the electrolyte is pumped into a reactor where it is merged with a 3% H 2  O 2  flow at a flow rate of 10 ml/min at constant room temperature and atmosphere. The organic solution samples are taken out and analyzed for anthraquinone content. Detailed operating conditions and results are listed in Table 1. 
     EXAMPLE 2 
     Same as Example 1 except there is no H 2  O 2  added into the reactor. Detailed procedure is same as example 1. The results are listed in Table 1. 
     
                       TABLE I______________________________________            Example 1                     Example 2______________________________________current density of cathode              1.2        1.5(mA/cm.sup.2)current density of anode (mA/cm.sup.2)              6.6        8.25flow rate of electrolyte (ml/min)              55          60oxygen flow rate (ml/min)              100        200temperature (°C.)              25          25electrolyte compositionH.sub.2 SO.sub.4 concentration (M)              10         10V.sub.2 O.sub.5 concentration (M)              5.00 × 10.sup.-3                         5.00 × 10.sup.-3anthraquinone concentration (M)              1.12 × 10.sup.-1                         1.12 × 10.sup.-13% H.sub.2 O.sub.2 flow rate (ml/min)              10          0electricity used (coulomb)              300        400theoretical yield of anthraquinone              0.167      0.167(mole/Farad)actual yield of anthraquinone              0.250      0.180(mole/Farad)electrolysis efficiency (%)              150        108______________________________________ 
    
     EXAMPLE 3 
     Repeat Example 1 except anthracene is substituted by 1,2-methylanthracene. Under this condition, the current efficiency of 2-methyl-9,10-anthraquinone is 0.12 mole/Farad. Operating conditions and results are listed in Table 2. 
     EXAMPLE 4 
     Repeat Example 1 except substituting anthracene with 1,2-diethyl-anthracene. Under this condition, the current efficiency of 1,2-diethyl-anthraquinone is approximately 0.1 mole/Farad. 
     
                       TABLE 2______________________________________current density of cathode (mA/cm.sup.2)                    2.0current density of anode (mA/cm.sup.2)                    11.0flow rate of electrolyte (ml/min)                     50oxygen flow rate (ml/min)                    100temperature (°C.)  25electrolyte compositionH.sub.2 SO.sub.4 concentration (M)                     5V.sub.2 O.sub.5 concentration (M)                    3.00 × 10.sup.-32-methyl-anthracene concentration                     0.12(M)3% H.sub.2 O.sub.2 flow rate (ml/min)                    8.5electricity used (coulomb)                    300theoretical yield of 2-methyl-anthraquinone                     0.167(mole/Farad)actual yield of 2-methyl-anthraquinone                     0.12(mole/Farad)electrolysis efficiency (%)                    150______________________________________ 
    
     
                       TABLE 3______________________________________current density of cathode (mA/cm.sup.2)                     1.5current density of anode (mA/cm.sup.2)                      8.25flow rate of electrolyte (ml/min)                     50.0oxygen flow rate (ml/min) 100temperature (°C.)   10electrolyte compositionH.sub.2 SO.sub.4 concentration (M)                     0.5V.sub.2 O.sub.5 concentration (M)                     5.00 × 10.sup.-31,2-methyl-anthracene concentration (M)                      0.013% H.sub.2 O, flow rate (ml/min)                     8.0electricity used (coulomb)                     300theoretical yield of 1,2-diethyl-9,10-                      0.167anthraquinone (mole/Farad)actual yield of 1,2-diethyl-9,10-anthraquinone(mole/Farad)              0.1electrolysis efficiency (%)                     150______________________________________ 
    
     Although the present invention and its advantages have been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention as defined by the appended claims.