Electrochemical production of dyes using graphite felt electrodes
View Patent ↗An electrochemical process for the preparation of water-soluble colorants, particularly triphenylmethane colorants and other water soluble technical dyes, is disclosed. In this process, the anode in the electrochemical cell comprises an open pore carbon material having specific void volume and surface area characteristics. A preferred electrode material is a carbon felt pad, preferably between about 1/16 inch and ¼ inch in thickness. This process provides faster reaction rates than electrochemical processes using standard electrodes, as well as higher yields compared to processes using chemical oxidizing agents.
1. An electrolytic process for the preparation of a water-soluble colorant by anodically oxidizing the corresponding leuco compound comprising the improvement wherein the anode comprises an open pore carbon material having a void volume of at least about 90% and a surface area of at least about 100 cm 2 /cm 3 .
2. The process according to claim 1 wherein the colorant is selected from triphenylmethanes, fluorans, xanthenes, azines and oxazines.
3. The process according to claim 2 wherein said open pore carbon material is a carbon felt pad having a thickness of from about 1/32 inch to about 1 inch.
4. The process according to claim 3 wherein the colorant is a triphenylmethane.
5. The process according to claim 4 wherein the void volume of the open pore carbon material is at least about 95%.
6. A process for the preparation of a water-soluble triphenylmethane colorant comprising anodically oxidizing the corresponding leuco compound in an electrolysis cell divided into an anode space and a cathode space, wherein the anode in said cell comprises an open pore carbon felt pad having a void volume of at least about 90% and a surface area of at least about 100 cm 2 /cm 3 and wherein said leuco compound is present in an anolyte solution.
7. The process according to claim 6 wherein the felt pad has a thickness of from about 1/32 inch to about 1 inch.
8. The process according to claim 7 wherein the anode is a graphite base covered at least in part by said carbon felt pad.
9. The process according to claim 7 wherein the anodic oxidation is carried out at an electrical potential of from about 1 to about 4 volts.
10. The process according to claim 9 wherein the anodic oxidation is carried out at a temperature of from about 0° C. to about 90° C.
11. The process according to claim 10 wherein the anodic oxidation is carried out at a temperature of from about 5° C. to about 30° C.
12. The process according to claim 10 wherein the anolyte solution contains from about 2% to about 50% of the leuco compound.
13. The process according to claim 12 wherein the anolyte solution contains the corresponding leuco compound of a colorant selected from Acid Blue 9 (C.I. 42090) [2650-18-2], FD&C Blue 1 (C.I. 42090) [3844-45-9], Wool Violet (C.I. 4265) [4129-84-4], FD&C Green No. 3 (C.I. 42053) [2353-45-9], and related dyes.
14. The process according to claim 12 wherein the anolyte solution additionally contains an ionizable salt at from about 0.5% to about 25%.
15. The process according to claim 14 wherein the ionizable salt is sodium chloride.