Abstract:
A method of oxidizing an element in both compartments of an electrolytic cell is provided. The method comprises reducing O 2  to H 2  O 2  in the cathodic compartment with a reducing agent such as a cobalt porphyrin, cobalt phthalocyanine, or hydroquinone, and oxidizing the element in both compartments preferably in the presence of a halide. Yields of up to 200 percent are obtainable.

Description:
BRIEF DESCRIPTION OF THE INVENTION 
     The invention provides a method for obtaining up to twice the normal current yield by oxidizing the same element in both chambers of an electrolytic cell. For example, arsenic can be oxidized from As(III) to As(V) in the cathode chamber by means of an electrolytic cell when the arsenic is dissolved in water, or other suitable solvent containing oxygen which oxygen is reduced to hydrogen peroxide by a reducing agent alternatively referred to as a catalyst which may be physically or chemically attached to the cathode or dissolved in the catholyte. The reducing agent is characterized by having the capacity to reduce oxygen to hydrogen peroxide at a lower overpotential than at an electrode such as carbon. Typical reducing agents are cobalt porphyrins, hydroquinones and cobalt phthalocyanines. Typical examples include: cobalt tetrakis[N-methyl-4-pyridyl]porphyrin, cobalt tetrapyridylporphyrin, tetraphenylporphinecobalt, cobalt phthalocyanine, cobalt tetrasulfonated phthalocyanine, 1,4-dihydroxybenzene, and 1,4-dihydroxynaphalene. 
     The hydrogen peroxide which is produced in the cathode chamber then oxidizes the As(III) to As(V). In the anode chamber the As(III) is also oxidized, preferably directly at the electrode serving as the anode, or via an electrogenerated oxidizing agent in the anode chamber which can be used to generate oxidants from halide ions such as bromide and iodide. 
     Typical electrodes employed are carbon glass, graphite, carbon and the like. Preferably the reducing agent is adsorbed or reacted onto the electrode. The electrolytic cell can be composed of conventional materials such as glass, metal, ceramic or plastic. The particular electrolyte, pH and electrolysis conditions employed depend on the elements to be oxidized, but the determination of which is within the skill of the art. 
     As used herein, the term &#34;element&#34; is intended to include an ionic form or part of an ionic compound or molecule. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing illustrates a schematic view of an electrolytic cell that can be used with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description and examples will serve to illustrate the invention and preferred embodiments thereof. All parts and percentages in said examples and elsewhere in the specification and claims are by volume unless otherwise indicated. 
     Referring now to the drawing, a typical electrolytic cell is shown in which the anode 2 and cathode 4 are separated by a divider membrane 6. 
     The electrolyte, element to be oxidized, O 2  source, reducing agent if not adhered to the cathode, and optionally a halide are passed via conduit 8 through disperser 10 into the cathodic compartment. There the O 2  is reduced to H 2  O 2 , the element oxidized by H 2  O 2 , and the oxidized product recovered via conduit 12. 
     In some cases, if the product hits the electrode it will reverse the reaction. In those cases, the element to be oxidized is passed via conduits 14 and 16 to mixing chamber 18 where contact with H 2  O 2  is made. The optimum feed method for any particular element can be determined by simple experimentation. 
     The anodic compartment is fed via conduit 20 with electrolyte, the element to be oxidized and, optionally, halide. The product is removed via conduit 22. A controlled power source 24 and reference electrode 26 round out the typical electrolytic cell. 
     The following table illustrates examples of reactions which can be employed in accordance with the invention. 
     
                                           TABLE I__________________________________________________________________________EXAMPLES OF REACTIONSAnodic Compartment        Cathodic Compartment__________________________________________________________________________ 1.   ##STR1##                1.                        ##STR2##                        ##STR3##   2.   ##STR4##                2.                        ##STR5##                        ##STR6##   3.   ##STR7##                3.                        ##STR8##                        ##STR9##   4.   ##STR10##               4.                        (a) Same as 3 and,   ##STR11##                        ##STR12##   ##STR13##                        ##STR14##   5.   ##STR15##               5.                        Same as 2 followed by   ##STR16##                        ##STR17##__________________________________________________________________________ 
    
     In the following Table II, data are presented which demonstrates production of H 2  O 2  at high conversion efficiency using a reducing agent. 
     
                       TABLE II______________________________________ELECTROCATALYTIC YIELDOF HYDROGEN PEROXIDE       Total CHARGE   Total H.sub.2 O.sub.2                                Yield.sup.bExp. Conditions.sup.a       (Q), Coulombs  mole × 10.sup.5                                %______________________________________0.32 mM CoTMPyP.sup.e       21.56          10.68     93.4E.sub.cat = -0.010 V.sup.c0.32 mM CoTMPyP       44.05          21.68     95.0E.sub.cat = -0.010 V0.28 mM CoTPyP       27.8           13.4      92.6E.sub.cat = -0.010 V0.29 mM CoTMPyP       10.8           5.22      93.0E.sub.cat = -0.200 V0.29 mM CoTMPyP       39.4           18.4      90.0E.sub.cat = 0.200 VCoTPyP(ads.).sup.d       28.9           15.1      100E.sub.cat = +0.200 VCoTPyP(ads.).sup.d       48.9           23.3      92.0E.sub.cat = -0.100 V               average                      93.7 ± 2.2______________________________________ .sup.a 0.05 M H.sub.2 SO.sub.4 as supporting electrolyte; Tokai glassy carbon electrode with area of 11.4 cm.sup.2. .sup.b Based on Q/nF where n assumed as 2, and F equals 96,500 coulombs. .sup.c E.sub.cat is the applied potential measured versus a reference Ag/AgCl(sat&#39;d. KCl). .sup.d Highly polished Glassy Carbon electrode immersed in 0.05 M H.sub.2 S0.sub.4 solution containing dissolved cobalt porphyrin for 1/2 hr., rinsed with distilled water and then transferred to the electrolysis cell The catalyst is cobalt tetrapyridylporphyrin. .sup.e The catalyst is cobalt tetrakis [N--methyl4-pyridyl] PAR  In the following Table III data are presented that demonstrates that one can produce the product in both compartments of the cell. 
    
     
                       TABLE III______________________________________ELECTROGENERATION OF IODINE  Total Charge (Q)             Yield, %      TotalE.sub.cat (cathode).sup.a    Coulombs     anode.sup.b                          cathode                                 yield, %______________________________________ -0.10 V.sup.c    54.4         100      90     190-0.10 V  48.9         102      92     194 0.00 V  55.4         102      91     193 0.00 V  39.8         101      90     191+0.20 V  12.4         102      98     200+0.20 V  24.9         101      98     199    average:     101 ± 1                          93 ± 3                                 194 ± 3______________________________________ .sup.a E.sub.cat is the applied electrode potential versus a reference Ag/AgCl(sat&#39;d KCl) reference electrode. .sup.b Electrolyte was 0.5 M H.sub.2 SO.sub.4 and contained 0.1 M KI. .sup.c O.sub.2 was continuously bubbled through the cathode compartment during electrolysis. At the end of electrolysis, excess KI was added and I.sub.2 formed was analyzed by titration with Na.sub.2 S.sub.2 O.sub.3. The cathode consisted of CoTPyP adsorbed on a graphite rod and the electrolyte was 0.5 M H.sub.2 SO.sub.4. 
    
     The data presented in the following Table IV demonstrate that the total yield is improved when bromide is added to the catholyte. 
     
                       TABLE IV______________________________________ARSENIOUS ACID OXIDATION   YieldE.sub.app (cathode).sup.a     Anode, %   Cathode, %  Total Yield, %.sup.b______________________________________-0.30 V   96         51          147(3)-0.10 V   95         59          154(3) 0.00 V   95         56          151(3)+0.10 V   95         62          157(3)+0.20 V   96         70          166(3)     Avg: 95 ± 1                59 ± 5   155 ± 5-0.10 V   95         59          154(1).sup. c-0.10 V   93         76          169(1).sup. d-0.10 V   98         77          175(1).sup. d-0.10 V   95         89          184(1).sup. e______________________________________ .sup.a E.sub.app measured versus a Ag/AgCl(sat&#39;d KCl) reference electrode Cathode: CoTPyP adsorbed on graphite rod; O.sub.2 bubbled through solutio during electrolysis. Catholyte: 0.02 M HAsO.sub.2 in 0.5 M H.sub.2 SO.sub.4 ; vol. = 10 ml. .sup.b Anode: graphite rod. Anolyte: 0.02 M HAsO.sub.2 in 0.5 M H.sub.2 SO.sub.4 and 0.4 M KBr; vol. 10 ml. Number of coulombs passed through the cell varied from 20 to 45 Coulombs for each run; the number of runs at each E.sub.app are indicated in the parenthesis. .sup.c Same as above except 0.1 M H.sub.3 AsO.sub.4 added to catholyte an anolyte. .sup.d Same as a and b except 0.4 M KBr added to catholyte. .sup.e Same as a and b except 1.3 M KBr added to catholyte. 
    
     The data presented in the following Table V demonstrate that bromine can be generated in both compartments and then transferred to a separate vessel where it is reacted with cyclohexene to form dibromocyclohexane. 
     
                       TABLE V______________________________________BROMINATION OF CYCLOHEXENE     Yield, %E.sub.cat (cathode).sup.a       Anode.sup.b               Cathode.sup.c                          Total Yield, %.sup.d______________________________________-0.30 V     90      45              135(1)-0.10 V     88      64              152(3)-0.10 V       89.sup.e               64              153(1) 0.00 V     83      65              148(1)+0.10 V     87      66              153(1)                          Avg. 153 ± 5______________________________________ .sup.a E.sub.cat measured versus a Ag/AgCl(sat&#39;d KCl) reference electrode number of coulombs passed through cell varied from 40 to 120 coulombs. .sup.b Anode: graphite rod. Anolyte: 0.5 M KBr or NaBr in 0.5 M H.sub.2 SO.sub.4 ; vol. = 25 ml. .sup.c Cathode: CoTPyP adsorbed on graphite rod. Catholyte: O.sub.2 bubbled through 0.5 M H.sub.2 SO.sub.4 solution during electrolysis. After electrolysis stopped, 1 g. solid KBr or NaBr added to catholyte and the Br.sub.2 produced was transferred by purging solution with N.sub.2 or air gas streams to external reaction vessel containing cyclohexene (CCl.sub.4 at ice temperature). .sup.d Brominated cyclohexane analyzed by dissolving residue (left after CCl.sub.4 evaporated) in 25 ml of ethanol and introducing small aliquote sample into conventional gasliquid chromatograph. 1% DMF in ethanol serve as an internal reference. .sup.e Anolyte contained 1 M HClO.sub.4 and 0.5 M NaBr. 
    
     While the above examples and results are illustrative of the invention, similar results can be achieved with other materials and conditions than those described in the specification as would be apparent to one of ordinary skill in the art. Accordingly, the invention is intended to be limited only by the appended claims.