A singe step process for synthesis of Yellow dye 11 of consistent colour tone
Technical field of invention: This invention discloses a continuous process for the manufacture of yellow-11 dye. Particularly the invention discloses a process for the manufacture of yellow dye Y 11 with no colour variations.
Background and prior art:
(Formula Removed)
Yellow 11 is a dye used widely in the dye industry for varied applications in printing, textiles and others. The synthesis of the dye using para-nitro-toluene sulphonic acid (PNTOSA) and sodium hydroxide is well known in the art. Refer US patent 4310331, BIOS Report No. 1548, page 173, 1946.
US Patent No.4310331 discloses the process for the preparation of the dyestuff Direct Yellow 11, in the form of stable concentrated solution which comprises, reacting caustic soda in aqueous solution with a solution containing PNTSA ( para-nitro toluene sulfonic acid), one or more alkanolamines and one or more alcohols containing an etheroxide at a temp of 40°C to 90°C, then neutralizing caustic soda with sulfuric acid and separating the sodium sulphate by filtration, which yields crude free acid dyestuff.

An article titled "Stilbene Dyes" Published in Colour Index International third addition; vol-4 discloses synthesis of Direct Yellow 11 by heating of 5-nitro-o-toluene sulfonic acid with aqueous NaOH. It also describes various yellow dyes or mixtures of dyes which are obtained according to the concentration of NaOH, the temperature and the duration of the reaction,
An article titled "CLXII.—The colouring matters of the Stilbene group. Part III" by Arthur George Green and Percy Field Crosland in Journal. Chem. Soc, Trans., 1906,89, 1602-1614
discloses effect of temperature and alkaline reduction in the preparation of stilbene dyes. More particularly p-nitrotoluenesulphonic acid on warming with aqueous NaOH gives a crimson red solution of the dinitrostilbenedisulphonate, which on further heating becomes orange-yellow with the production of Direct Yellow or Stilbene Yellow. Further the purer form of the dye can be obtained by alkalline reduction of dinitrostilbenedisulphonic acid.
US Patent No. 4617380 discloses a process for the preparation of a liquid dyestuff of CI Direct Yellow 11 (Paper Yellow 11) in stable form, from the tetrasodium complex of this stilbeneazo(xy) base-catalyzed condensation of 4-nitrotoluene-2-sulfonic acid using as the starting material. Further the free acid complex of said dyestuff treated with an alkanolamine/water mixture to obtain a stable liquid dyestuff Direct Yellow 11 of proper tinctorial strength. The final complex, after removal of the excess water, is adjusted to an absorbence of 24 at lambda.max 417 nm.
US Patent No.4617381 discloses a process is disclosed for the preparation of concentrated stable liquid dye solutions of CI Direct Yellow 11 starting with the product of the basic condensation of para-nitrotoluene-ortho-sulfonic acid in the form of its tetrasodium reaction mass; which is further treated with a lower alkanol/water medium; to obtain Direct Yellow 11-alkanolamine complex.

US Patent No. 5041539 discloses a batch process for preparing and concentrating storage-stable aqueous solutions of yellow azo- or azoxystilbene dyes which comprises: (a) self-condensing 4-nitrotoluene-2-sulfonic acid in an alkaline solution of an alkali metal hydroxide (NaOH, KOH); the condensation is carried out. in a pH range from about 11 to 13 and in the temperature range from 60 °C to 80°C. (b) acidifying the reaction mixture of step (a) with H2S04 after the condensation reaction is substantially complete; (c) and further conducting cation exchange reaction by using alkanolamine salts.
In spite of various literature references- both patent and non patent categories available, an unaddressed problem in the art pertains to the inconsistent colour tone of the product obtained. Uniformity and consistency in colour tone of product is of immense importance, since the colour tone uniformity determines the quality of applications where the yellow dye may be used.
Further, existing and currently implemented methods are batch methods, are very crude and yield inconsistent product color tone, thereby resulting in significant deviation of the tone of the standardized dye from its intended application. Of these drawbacks in prior art methods, inconsistency of product color tone is the most vexed problem to which solutions are being proposed, though unresolved as yet.
Typical batch process practiced in the industry is a two step process where PNTOSA is mixed with some part of NaOH (caustic lye) which leads to a temperature rise from 22.4 - 46.5 °C depending upon the quantity. Larger quantities yield much higher temperature rise and hence mixing of reactants in one step is avoided. A lot of ice is added to the reactor keep the temperature in control. After stabilizing the temperature, reaction mixture is taken to 58-62 °C and then rest of the NaOH is added. This time the temperature rise is not significant and typically it goes to as much as 68 - 70 °C. Then it is maintained in this range until reaction is complete. A brown -red dye is produced, which is applied on paper after dilution.
It was observed that the uv absorbance of the dye prepared by traditional batch processes of prior art resulted in the absorbance of the dye at 410-421 nm. Incomplete

reaction leaves some amount of PNTOSA unreacted which shifts the UV absorbance of product. Fast reaction by increasing temperature or slow reaction at lower temperature both lead to deviation from the required colour tone. On a run of 15 batches of synthesizing of Y 11 dye by traditional two step processes, the standard deviation of absorbance at 410-421 nm is close to 26%.
Another unaddressed issue in prior art is to have a continuous process for the synthesis of Y-11 dye resulting in consistent tone of the dye.
One of the reasons for not obtaining uniformity in the colour tone is a lack of an online analytical method to determine the end point of the reaction as well as the inability to eliminate the local temperature gradients in the batch reactor, again hitherto unaddressed in the art.
Objects of the invention
The main object of the present invention is to provide a process for the synthesis of Y-11 dye resulting in consistent tone of the dye.
Another object of the invention is to propose optionally batch as well as continuous process for the synthesis of Y-11 dye resulting in consistent tone of the dye.
Yet another object of the invention is to control various process parameters precisely such that the resultant dye is consistently of uniform tone.
One more object of the invention is to present an online analytical technique that affords to exactly indicate the end point of the reaction which enables production of dye with uniform tone consistently.
It would also be of significance, both scientifically as well as economically if the process proposed is a one step process which addresses the other issues of prior arts too.
Summary of the invention
Accordingly the present invention provides a single step process for synthesis of Yellow dye 11 of consistent colour tone wherein the process steps comprising: (a)

reacting aqueous solution of para nitro toluene sulphonic acid (PNTOSA) with aqueous solution of an alkali at a temperature ranging between 65-90° C and characterized in that the termination time of the reaction by uv monitoring, wherein disappearance of peak at 278 nm and appearance of peak at 410 nm determines consistency of colour tone, (b) terminating the reaction by addition of cold water and followed by addition of an acid to maintain pH ranging between 7 to 8.2 obtain the dye 11.
In an embodiment of the present invention wherein the alkali is selected from the group consisting of hydroxides of sodium, potassium, lithium, alone or in combinations thereof.
In another embodiment of the present invention wherein the ration of PNTOSA:alkali is in the range of 1:1 to 1:1.5
In still another embodiment of the present invention wherein the reaction time is 85-95 minutes
In yet another embodiment of the present invention, wherein the conversion of PNTOSA is 100%.
In a further embodiment of the present invention, wherein the process is selected from batch process or continuous process.
Description of figures
Figure 1: UV absorbance plot of process at different time intervals
Figure 2: UV absorbance plot of process at different temperatures
Figure 3: UV absorbance plot of process at different time intervals and varying quantities of alkali
Detailed description of invention:

In pursuit of a process to achieve consistency in colour tone, it was surprisingly found that a single condensation step batch as well as continuous process for the synthesis of Yellow 11 dye with control in process parameters resulted in consistent colour tone of the dye with the ◘max of the dye between 416-419 nm. The process of the invention resulting in consistent colour tone using online UV monitoring system comprises reacting aqueous solution of para nitro toluene sulphonic acid (PNTOSA) with aqueous solution of an alkali at reaction temperature of 65-90 °C and determining the termination time of the reaction by online uv monitoring.
The alkali of the invention are hydroxides of sodium, potassium, lithium, alone or in combinations thereof, in 1-1.5 equivalents of PNTOSA added.
At the completion of the reaction of PNTOSA with the alkali, acid is added to neutralize unreacted alkali. The acids are mineral acids selected from HCI, H2SO4 and such like.
The online uv monitoring of the reaction to determine the termination of the reaction comprises the steps of : 50 µl of reaction mixture is dissolved in 4 ml water. From this 4ml of sample only 20 µl of sample is taken in UV quartz cuvette and dissolved in 1 ml of water for UV-Vis analysis. UV absorption spectrum of the sample is recorded in the range of 200-800 nm. The reactant has an absorbance at 278 nm while the product has an absorbance at 410 nm.
Consistent colour tone of Yellow dye 11 is determined by the online monitoring system of the invention. In the uv spectrum, there should be no peak at 278 nm, rather there should be a valley at 278 nm indicating the complete consumption of the reactant. A peak at 410 nm indicates formation of Yellow 11 dye. This is determines colour consistency, since the presence of the initial reactant affects the tone of the product and will therefore not exhibit a peak at 410 nm.
The process of the invention results in 100 % conversion of PNTOSA to yellow 11 dye. With reference to Figure 3, in an embodiment of the invention the process is carried out with equivalent quantities of alkali and PNTOSA.

In another embodiment of the invention, the process is carried out with a ration of 1:1.5 for PNTOSA:alkali
The process is optionally batch or continuous process. The continuous flow synthesis employs SS tubes of length ranging from 1/16" to 1 inch in diameter, microreactors and such like devices. The reaction time in continuous flow synthesis ranges from 60-90 minutes, though optionally reaction times are increased to get varying colour tones. Tone of colour of dye Y 11 is further dependent on temperature of process of invention. In the batch process, the reaction times are greater than that of continuous flow synthesis, ranging from 60-120 minutes. One of the following two approaches were used for enhancing mixing in the tube or reducing axial dispersion to get better product quality.
The mixing along the length of the reactor is increased by placing static mixers at several places inside the. tube. Having static mixers helped achieve better mixing even through the viscosity of the fluids increase towards the end of the reaction tube.
The axial dispersion was reduced by using a segmented flow approach by passing inert gas through the reactor.
The process is carried out conventionally, optionally in microreactor and such like. The microreactor of process of the invention varies in diameter ranging from 1/16" to 1". The material of construction of the microreactor is SS316.
On an analysis of 15 batches of Y 11 synthesized by the process of the invention, the ◘maxOf the Y 11 dye is between 416-419 nm with standard deviation of 1.5%.
Advantages of the invention:
1. Uniform colour tone obtained
2. Batch as well as continuous process
3. Easy to maintain identical inlet composition

4. Energy resources used are within what is generated in the reaction of the acid and alkali
5. Process system reduced by about 60%.
Following examples are given by way of illustration and therefore should not construed to limit the scope of the invention
Examples
Example 1: Batch synthesis
The reaction mixture composition at the beginning contained LiOH.H2O = 125 mM (5.25g), H2O = 15 ml and PNTOSA = 62 mM (13.65g). 125 mM of lithium hydroxide monohydrate was dissolved in 15 ml of water. 62 mM of PNTOSA was added slowly to this mixture and rise in temperature was noted, the maximum rise in temperature was 40.5 °C. The temperature of the reaction vessel was then increased to 65 °C and solution was stirred for 1 hr. Then 10g of ice and 20g of ice cold water was added to the flask. The orange red paste formed was adjusted to pH = 7 by adding 6.5 ml of concentrated HCI slowly and then temperature was again increased to 75 °C and stirred for 2 hrs and three samples were taken during this time at 10 minute intervals up to 90 minutes. Analysis of samples was performed in conventional manner using UV-Vis spectrophotometer. With reference to figure 1, with progression of time from 10 minutes to 90 minutes, the peak at 278 nm reduced indicating conversion of PNTOSA and the peak wt 410 nm increased indicating formation of yellow 11 due. Conversion of PNTOSA: 100%
Example 2: Continuous flow synthesis
Continuous flow synthesis of Yellow-11 was carried out using 1/16" diameter SS 316 tube of 10m length. Reaction temperature was varied between 70 - 90 °C. The solution of 2.44 gm PNTOSA and 0.52 gm NaOH in 20 ml water was mixed with a solution of 1 gm NaOH in 8 ml water using a T-mixer. The reaction tube was maintained at constant temperature and the residence time in the reaction tube was maintained at 90 min by

flowing the solutions through the tube using syringe pumps. After complete conversion of PNTOSA the reaction terminates. Analysis of samples was performed in conventional manner using UV-Vis spectrophotometer. With reference to figure 2, it was observed that the formation of yellow dye 11 was optimum at 90 °C. Conversion of PNTOSA: 100%
Example 3: Continuous flow synthesis
Continuous flow synthesis of Yellow-11 was carried out in a ¼" diameter SS 316 tube of suitable length. Reaction temperature was maintained at 70 °C and the reaction was carried out with a residence time of 90 min by flowing the solutions through the tube using syringe pumps with the suitable reactant composition similar to that given in Example 2. Mixing was achieved using a 1.38 mm internal diameter T-mixer. Inline mixing was achieved by placing static mixers at every 0.5 m distance inside the reaction tube. The reaction tube was maintained at constant temperature. After complete conversion of PNTOSA the reaction terminates Analysis of samples was performed in conventional manner using UV-Vis spectrophotometer. Conversion of PNTOSA: 100%
Example 4: Continuous flow synthesis
Continuous flow synthesis of Yellow-11 was carried out in a ¼" diameter SS 316 tube of suitable length. Reaction temperature was maintained at 70 °C and the reaction was carried out with a residence time in the range of 90 min by flowing the solutions through the tube using syringe pumps with the suitable reactant composition similar to that given in Example 2. Mixing was achieved using a 1.38 mm i.d. T-mixer. An inert gas phase viz. Nitrogen was also passed at the inlet of the reactor tube to achieve segmented flow and reduce the axial dispersion. The reaction tube was maintained at constant temperature. The inert gas was vented off at the outlet of the reaction tube. After complete conversion of PNTOSA the reaction terminates. Analysis of samples was performed in conventional manner using UV-Vis spectrophotometer. Conversion of PNTOSA: 100%

We claim:
1. A singe step process for synthesis of Yellow dye 11 of consistent colour tone
wherein the process steps comprising: (a) reacting aqueous solution of para
nitro toluene sulphonic acid (PNTOSA) with aqueous solution of an alkali at a
temperature ranging between 65-90° C and characterized in that the termination
time of the reaction by uv monitoring, wherein disappearance of peak at 278 nm
and appearance of peak at 410 nm determines consistency of colour tone, (b)
terminating the reaction by addition of cold water and followed by addition of an
acid to maintain pH ranging between 7 to 8.2 obtain the dye 11.
2. The process according to claim 1, wherein the alkali is selected from hydroxides of sodium, potassium, lithium, alone or in combinations thereof.
3. The process according to claim 1, wherein the ration of PNTOSA:alkali is in the range of 1:1 to 1:1.5

4. The process according to claim 1, wherein the reaction time is 85-95 minutes
5. The process according to claim 1, wherein the conversion of PNTOSA is 100%.
6. The process according to claim 1, wherein the acid used for the pH maintenance
in step (b) is a mineral acid selected from a group consisting of HCI, H2SO4.
7. The process according to claim 1, wherein the process is selected from batch
process or continuous process.
8. A singe step process for synthesis of Yellow dye 11 of consistent colour tone is substantially as herein described with reference to the examples and drawings accompanying the specification.