US2027144A

US2027144A - Formation of leuco compounds

Info

Publication number: US2027144A
Application number: US431984A
Authority: US
Inventors: Louis S Bake
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1930-02-27
Filing date: 1930-02-27
Publication date: 1936-01-07
Anticipated expiration: 1953-01-07

Description

Patented Jan. 7, 1936 UNITED STATES PATENT OFFICE FORMATION or LEUCO COMPOUNDS Louis S. Bake, Pennsgrove, N. J., assignor to E. I. du Pont de Nemours & Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application February 27, 1930, Serial No. 431,984

17 Claims.

This invention relates to a method of reduction of organic compounds and more particularly to the reduction of vat colors to their leuco compounds. It especially contemplates the reduction of indigo to its phenol, or leuco compound, known commercially as indigo white.

In present commercial use there are two satisfactory general. methods for the reduction of indigo to indigo white. In the first method the' The zinc hydrosulfite is in turn preliminarily produced by the interaction of sodium bisulfite or gaseous sulfur dioxide with metallic zinc as follows:

The second general method involves the reducing action of zinc-and sodium bisulfite upon the indigo probably according to the following equa- I This method has the disadvantages that the sodium bisulfite necessary to carry out the reac- 35 tion is lost as a by-product in its transformation to the normal sulfite and that the sodium bisulfite must be preliminarily prepared, ordinarily by the reaction of sodium hydroxide and sulfur dioxide as follows:

40 N aOH+S OHNaHS 03.

It is an object of this invention to produce an 7 5 improved process for the reduction of indigo to its leuco compound. Further objects are to produce an improved quality of indigo white and in general to overcome the disadvantages of the prior art. Further objects will appear herein.-

55 after.

These objects are accomplished by the present invention whereby the reduction is brought about by the use of zinc hydrosulfite. Zinc hydrosulfite may be added to the water suspension of indigo blue and a successful reduction 5 secured, but it is preferred to form the zinc hydrosulfite in the vat. Accordingly, the zinc hydrosulfite is formed directly in the indigo water suspension by'adding to it metallic zinc dust and passing in gaseous sulfur dioxide, the 10 reaction taking place as follows:

, The zinc hydrosulfite thus formed reacts immediately with the indigo to form the phenol and zinc bisulfite according to the following reaction:

The zinc bisulfite which is formed is insoluble in water, as is the indigo phenol and in order to separate them it is convenient to convert the zinc into a soluble compound. This can be readily done by treatment of the vat mixture with an acid to cause formation of zinc salts that are soluble, the indigophenol being insoluble in acid solution. ,Satisfactory acids are hydrochloric and sulfuric. The reaction taking place on the addition of the acid is as follows:

If an amount of zinc hydrosulfite theoretically necessary to reduce all the indigo were used a large quantity of. sulfur dioxide would be lost as is obvious from the above equation (Equation 3). This general process is therefore modified to avoid this loss.

According to this modification the indigo is made into a water suspension which is neutral or slightly acid and an amount of zinc slightly more than necessary to form the theoretical amount of zinc hydrosulfite necessary for the reduction ofv all the indigo is added. Because of reactions of minor importance in the vat some zinc may be lost,- therefore a slight excess is added. Thereafter, gaseous sulfur dioxide in a smaller amount than is necessary to combine with all the zinc to form zinc hydrosulfite is passed into the suspension.

The solution is then heated from to C. and approximately 0.95 of the theoretical amount of sulfuric acid necessary to convert all the zinc bisulfite (which would be formed by the zinc present) into zinc sulfate is added either continuously or in separate portions over 55 a period of 15 to 30 minutes or longer, at which time reduction is complete. A substantial excess of acid at the end of the reduction tends to cause the formation of ,indigo blue, therefore slightly less than the molecular proportion of acid is used. The acid is not all added at one time quickly as it would probably react with the metallic zinc directly. The phenol is then separated and washed in any desired manner.

The theoretical explanation of the modified process is probably as follows. The small relative amount of sulfur dioxide added in the beginning reacts with its equivalent of metallic zinc forming an equivalent amount of zinc hydrosulfite which reduces a corresponding molecular amount of indigo, forming zinc bisulfite which is reacted upon (Equation 3) by the acid present liberating an amount of sulfur dioxide equal to the amount added originally, minus of course, any small amount that may have been oxidized by oxygen which may have been present. This regenerated sulfur dioxide reacts in exactly the same manner as that originally added, by combining with its equivalent of the remaining zinc to form more zinc hydrosulfite. Thus, the above cycle continues until the reduction is complete and the zinc is in solution as zinc sulfate.

The above reactions (1), (2), and (3), may be assumed empirically to proceed simultaneously or in sequence so that the reaction goes to completion without further attention.

While in the majority of cases the above reduction proceeds smoothly in neutral indigo suspension, cases have been found when a poor grade of zinc dust has been used that an activator was necessary to start the reaction, which, when once started, went smoothly. to completion. Such activators are hydrochloric and sulfuric acids, ammonium chloride and zinc chloride. These compounds work very efiiciently when present in an amount necessary to give the original indigo and water suspension a distinct acid reaction to litmus paper.

The invention will be readily understood from a consideration of the following examples:

Example I Indigo C16H10N2O2 I l H H Two hundred pounds of 'neutral 20% indigo paste were placed in the reducing tub, 250 pounds of water added and the suspension stirred thoroughly while 4 pounds of sulfur dioxide in the gaseous state were added. Fourteen pounds of zinc dust available zinc), as a slurry inwater, was then added and the temperature raised to approximately 70 C. Eighteen pounds of sulfuric acid as a 30% solution were added continuously over a period of 30 minutes. Stirring was continued until the reduction was complete, when the indigo phenol was filtered and washed free from zinc sulfate.

Example II Two hundred pounds of neutral 20% indigo paste were placed in the reduction tub, 250 pounds of water added and the agitator started. Zinc chloride solution was added until the suspension was distinctly acid to litmus paper, and then 4 pounds of gaseous sulfur dioxide were added. Fourteen pounds of zinc dust (95% available zinc), as a slurry in water, was then added and the temperature raised to approximately 70 C. Eighteen pounds of sulfuric acid as a 30% solution were added continuously over a period of 30 minutes. Stirring was continued until the reduction was complete, when the indigo phenol was filtered and washed free from zinc sulfate.

Example III Two hundred pounds of neutral 20% indigo paste were placed in the reduction tub, 250 pounds of water added and the agitator started. Ammonium chloride solution was added until the suspension was distinctly acid to litmus paper, and then 4 pounds of gaseous sulfur dioxide added. Fourteen pounds of zinc dust (95% available zinc), as a slurry in water, was then added and the temperature raised to approximately 70 C. Eighteen pounds of sulfuric acid as a 30% solution were added continuously over a period of 30 minutes. Stirring was continued until the reduction was complete, when the indigo phenol was filtered and washed free from zinc sulfate.

Example IV Two hundred pounds of neutral 20% indigo paste were placed in the reduction tub, 250 pounds of water added and the agitator started. Hydrochloric acid solution was added until the suspension was distinctly acid to litmus paper, and then 4 pounds of gaseous sulfur dioxide added. Fourteen pounds of zinc dust (95% available zinc), as a slurry in water, was then added and the temperature raised to approximately 70 C. Eighteen pounds of sulfuric acid as a 30% solution were added continuously over a period of 30 minutes. Stirring was continued until the reduction was complete, when the indigo phenol was filtered and washed free from zinc sulfate.

Example V Two hundred pounds of neutral 20% indigo paste were placed in the reduction tub, 250 pounds of water added and the agitator started. Sulfuric acid solution was added until the suspension was distinctly acid to litmus paper, and then 4 pounds of gaseous sulfur dioxide added. Fourteen pounds of zinc dust (95% available zinc), as a slurry in water, was then added and the temperature raised to approximately 70 C. Eighteen pounds of sulfuric acid as a 30% solution were added continuously over a period of 30 minutes. Stirring was continued until the reduction was complete, when the indigo phenol was filtered and washed free from zinc sulfate.

Ninety-six pounds of the vat color, Ponsol yellow G (neutral 22% paste), Colour Index #1118, were placed in the reducing tub, 200 pounds of water added, the agitation started, and 3 pounds of gaseous sulfur dioxide added. Four pounds of zinc dust were then added as a slurry in water and the temperature raised to 35-50 C. Four pounds of sulfuric acid were added continuously over a period of 15-30 minutes, when the reduction was complete. Then the insoluble leuco compound was filtered off, suspended in water and dissolved in caustic, after which it was ready for use.

Example VII Pyranthrone C30H14O2 soluble leuco compound was then filtered off,

dissolved in dilute caustic. It was then ready for use.

In the above examples it does not matter whether the sulfur dioxide or the zinc dust is added first, nor does it matter if the solution is heated before either are added, except that of course sulfur dioxide is much less soluble in the indigo water suspension at the higher temperature, provided the zinc has not been added. The amount of water may be either increased or decreased without lowering the efficiency of the reduction, except in a mechanical manner. The amount of sulfur dioxide may be either increased or decreased without injury to the reaction. It is clearly evident that increasing the amount of sulfur dioxide over that required to complete the reduction would result only in a waste of gas, and very disagreeable operating conditions due to, excess sulfur dioxide in the building. The amount of sulfur dioxide as given in the above examples may be reduced to such an extent that there is present over and above that oxidized by any free oxygen present in the solution, only a slight excess at the end of the reduction. If nonoxidizing conditions are maintained, as for example by means of an atmosphere of inert gas such as nitrogen'or carbon dioxide, and/or the operation carried out in a closed vessel to prevent loss of sulfur dioxide during the reduction process, only a few ounces or less of sulfur dioxide are necessary. This results from the continuous regeneration feature as previously explained in detail.

While certain theories of operation have been set out for the new process, it is to be understood that they are given merely toaid in understanding the invention and applicant does not desire to be limited thereby, but only by the appended claims.

As many apparent and widely different em- 5 bodiments of this invention may be made without departing from the spirit thereof, it is to be understood that I do not limit myself to the foregoing examples or description except as indicated in the following claims. 10

'I claim:

1. In the reduction of indigo to leuco indigo, the steps of adding to an indigo suspension sufficient zinc to form with sulfur dioxide sufficient zinc hydrosulfite to reduce the indigo 15 present and adding to the indigo suspension less than the amount of sulfur dioxide required to molecularly combine with the zinc, the amount of sulfur dioxide added being sufiicient to form zinc hydrosulfite in the solution and thereafter 20 adding sulfuric acid to the reaction mixture.

2. In the reduction of indigo to leuco indigo, the steps of adding to an indigo suspension suflicient zinc to form with sulfur dioxide sufficient zinc hydrosulfite to reduce the indigo 5 present and adding to the indigo suspension less than the amount of sulfur dioxide required to molecularly combine with the zinc, the amount of sulfur dioxide added being sufficient to form zinc hydrosulfite in the solution and thereafter 30 adding a nonoxidizing mineral acid to the reaction mixture.

3. The process of treating indigo which comprises forming a water suspension of the indigo,

making the suspension acid with a non-oxidiz- 35 ing inorganic substance, adding enough zinc to form sufficient zinc hydrosulfite to reduce the indigo present to its leuco compound, and adding sulfur dioxide.

4. The process of treating indigo which com- 0 prises forming a water suspension of the indigo,

'making the suspension acid in reaction with zinc chloride, adding enough zinc to form sufficient zinc hydrosulfite to reduce the indigo present to its 55 leuco compound, adding about 10% of the amount of sulfur dioxide required to molecularly combine with the zinc, the amount of' sulfur dioxide added being sufficient to form zinc hydrosulfite in the solution, to form zinc hydro- 5 sulfite, allowing the sulfur dioxide to react with the zinc to form zinc hydrosulfite and allowing the zinc hydrosulfite to reduce the indigo.

6. The process of treating indigo which comprises forming a water suspension of the indigo, as adding less than the amount of sulfur dioxide required to molecularly combine with zinc, the amount of sulfur dioxide added being sufiicient to form zinc hydrosulfite in the solution, to form sufficient zinc hydrosulfite to reduce the indigo present, adding enough zinc to form sufficient zinc hydrosulfite to reduce the indigo present to its leuco compound, allowing the sulfur dioxide to react with the zinc to form zinc hydrosulflte and allowing the zinc hydrosulfite to reduce the indigo, and adding sufiicient hydrochloric acid to reduce all zinc bisulfite formed to zinc chloride.

'7. The process of reducing vat dyes to their leuco compounds which comprises, under nonoxidizing conditions passing into a water suspension of the dye, a small quantity of sulfur dioxide thereafter adding to the suspension approximately the theoretical amount of zinc required to form suificient zinc hydrosulfite to reduce the dye present, and then slowly adding to the mixture approximately the theoretical quantity of non-oxidizing mineral acid required to convert any zinc bisulfite formed tozinc sulfate and sulfur dioxide and finally separating the leuco compound.

8. The process of reducing vat dyes to their leuco compounds which comprises, in a nitrogen atmosphere passing into a water suspension of the dye a small quantity of sulfur dioxide thereafter adding to the suspension slightly more than the theoretical amount of zinc required to form sufficient zinc hydrosulfite to reduce the dye present, and then slowly adding to the mixture slightly less than the theoretical quantity of non-oxidizing mineral acid required to convert any zinc bisulfite formed to a soluble zinc salt and sulfur dioxide and finally separating the leuco compound.

9.The process of reducing vat dyes to their leuco compounds which comprises, in a carbon dioxide atmosphere passing into a water suspension of the dye a small quantity of sulfur dioxide thereafter adding to the suspension approximately the theoretical amount of zinc required to form sufficient zinc hydrosulfite to reduce the dye present, and then slowly adding to the mixture approximately the theoretical quantity of non-oxidizing mineral acid required to convert any zinc bisulfite formed to zinc sulfate and sulfur dioxide and finally separating the leuco compound.

10. A process of reducing a reducible organic compound which comprises mixing the organic compound with metallic zinc and an amount of sulfur dioxide in water insufficient to interact with all the zinc present, and during the reaction adding an amount of sulfuric acid to form and maintain the presence of zinc hydrosulfite from the zinc bisulfite formed.

11. The process of reducing indigo blue to indigo white which comprises forming a suspension from 200 parts of neutral 20% paste and 250 parts of water, then during agitation passing in 4 parts of gaseous sulfur dioxide, thereafter adding a slurry of zinc dust and water containing 14 parts of zinc of 95% availability and adjusting the temperature to 0., then adding 18 parts of 30% sulfuric acid continuously over a period of 30 minutes, stirring until the reaction is complete and separating the indigo white.

12. The process which comprises reducing vat dyes in an open vessel by adding slightly more sulfur dioxide to the dye than will be oxidized during the reaction, the amount of sulful dioxide added being sufiicient to form zinc hydrosulfite in the solution, thereafter adding sufficient finely divided zinc to form sufficient zinc hydrosulfite to reduce the dye to its leuco form, heating the resultant mixture to a temperature below which the sulfur dioxide will be expelled, then passing into the reaction mass continuously at a rate insufiicient to react with the zinc present, a quantity of sulfuric acid sufficient to release sulfur dioxide by reaction with any zinc bisulfite present.

13. The process which comprises reducing vat dyes in an open vessel by adding slightly more sulfur dioxide to the dye than will be oxidized during the reaction, the amount of sulfur dioxide added being sufficient to form zinc hydrosulfite in the solution, thereafter adding suflicient finely divided zinc to form sufficient zinc hydrosulfite to reduce the dye to its leuco form, heating the resultant mixture to a temperature below which the sulfur dioxide will be expelled, then passing into the reaction mass intermittently at a rate insufficient to react with the zinc present, a quantity of sulfuric acid sufficient to release sulfur dioxide by reaction with any zinc bisulfite present.

14. A process of reducing a vat color to its leuco which comprises mixing the vat color with metallic zinc and an amount of sulfur dioxide in water insufficient tointeract with all the zinc present, and during the reaction continuously adding an amount of sulfuric acid to form and maintain the presence of zinc hydrosulfite from the zinc bisulfite formed.

15. A process of reducing a vat color to its leuco which comprises mixing the vat color with metallic zinc and an amount of sulfur dioxide in water insufficient to interact with all the zinc present, and during the reaction intermittently adding an amount of sulfuric acid to form and maintain the presence of zinc hydrosulfite from the zinc bisulfite formed.

16. A process of reducing a vat color to its leuco which comprises mixing the vat color with metallic zinc and an amount of sulfur dioxide in water insufficient to interact with all the zinc present, and during the reaction adding an amount of a non-oxidizing mineral acid to form and maintain the presence of zinc hydrosulfite from the zinc bisulfite formed.

17. The process of treating indigo, which comprises forming a water suspension of the indigo,

adding to the said suspension a substance from