Colour and protection: dyes, pigments, paints and coatings: From benzene to azo dye: intermediates and dye manufacture
Understanding Chemicals / Module 8: Colour and protection: dyes, pigments, paints and coatings Part 2 of 5From benzene to azo dye: intermediates and dye manufacture
Dyes are not made from crude aromatics in one step. They are assembled from intermediates: benzene, toluene or naphthalene rings carrying nitro, amino, sulphonic acid, chloro or hydroxy groups. This part follows the few unit reactions involved, checks the raw material bill for aniline against the chemistry, and ends with a real azo dye batch sheet.
2.1 Intermediates are the building blocks, and the hard part is getting the right isomer
Shreve calls the substituted aromatics used to build dyes intermediates. The chain is short: petroleum gives aromatic hydrocarbons, which give intermediates, which give dyes. The same intermediates feed pesticides, pharmaceuticals and rubber chemicals, so dye makers share raw materials with other industries.
The business has an unusual shape. Shreve counts about 2,000 intermediates, most made at 500 to 1,500 tonnes a year and selling, in the early 1980s, at around a dollar a kilogram. A few, such as aniline, nitrobenzene and phthalic anhydride, are made in hundreds of thousands of tonnes and used mainly outside dyes; with other bulk aromatics they are nearly 90% of cyclic intermediate output. Dyes depend mostly on the small remainder, made in labour-intensive batch plants.
The central technical problem is isomers. Adding one group to benzene gives one product, but a second group can go ortho (next door), meta (one carbon further) or para (opposite). Naphthalene and anthraquinone have two different positions even for the first group. Some substituents steer the next group to ortho and para (amino, hydroxy, alkyl); others to meta (nitro, sulphonic acid, carboxylic acid). Kent shows dye chemists using this: nitro groups are added partly to direct the next group, and amines are protected by acetylation before nitration so the ring is not oxidised.
Wrong isomers are costly. Shreve observes that when a 1,4 product is wanted, the 1,2 and 1,3 products may be worth only their fuel value, yet they consumed raw material and often become pollutants. A commodity plant answers with a selective catalyst; a dye intermediate plant with careful choice of temperature, acid strength and order of steps. That is know-how rather than hardware.
- Aromatic hydrocarbons from petroleum; under 1% from coal tar by the early 1980s
- Benzene
- Nitrobenzene
- Aniline
- Sulphanilic acid by baking aniline sulphate
- Dimethylaniline for triarylmethane dyes
- Aniline
- Chlorobenzene to nitrochlorobenzenes
- Nitrobenzene
- Toluene
- o- and p-Nitrotoluene mostly ortho and para isomers
- Toluidines
- o- and p-Nitrotoluene mostly ortho and para isomers
- Naphthalene
- Naphthalene sulphonic acids
- beta-Naphthol by caustic fusion
- H-acid and other aminonaphthol sulphonic acids
- Phthalic anhydride also from o-xylene
- Anthraquinone base of most vat dyes
- Copper phthalocyanine
- Naphthalene sulphonic acids
- Benzene
- Shreve (1984), Ch 39 Cyclic Intermediates and Dyes · pdf pp. 790-793
- Kent (2012), Ch 13 Dye Application, Manufacture of Dye Intermediates and Dyes · pdf pp. 516-518
2.2 Nitration and reduction: benzene to nitrobenzene to aniline
The classic way to put an amino group on a ring, the group that later becomes an azo link, is to nitrate and then reduce. Shreve explains why sulphuric acid is mixed with the nitric acid. It converts nitric acid into the nitronium ion (NO2+), the species that attacks the ring. It absorbs the water formed, which would otherwise slow nitration and favour destructive oxidation. And it is a heat sink: nitration releases 60 to 150 kJ per mole, and heat favours over-nitration, so jackets, coils and strong stirring are essential.
For nitrobenzene, C6H6 + HNO3 → C6H5NO2 + H2O. Shreve describes batch nitrators of 3,000 to 5,000 litres or small continuous units, held near 50 C by the benzene feed rate, with 15 to 20 minute reactions and 98% yields.
Check the bill against the equation. Benzene weighs 78 per mole and nitrobenzene 123, so the minimum is 78/123 = 0.634 t of benzene per tonne. Shreve lists 0.655 t, so about 97% of the benzene ends up in product. Nitric acid's minimum is 0.512 t against 0.530 t used. Power, labour and overhead are only about 14% of cost: a raw-material business.
Reduction has two routes. The old Bechamp route uses iron and hydrochloric acid: Shreve lists 1.6 t of iron per tonne of aniline, against 1.35 t in theory from Dryden's equation 4 C6H5NO2 + 9 Fe + 4 H2O → 4 C6H5NH2 + 3 Fe3O4. The iron oxide sludge must find a use, such as pigments, to keep the route affordable. The modern route hydrogenates nitrobenzene vapour; Dryden gives 270 C, 20 psi, three times the theoretical hydrogen and a copper-on-silica catalyst. By 1982 most US aniline was made this way, and only 10% of it went into dyes and intermediates.
mixed acid, about 50 C, 15-20 min; cooling water then warm water
2 Separatorcrude nitrobenzene decanted; spent acid sent to recovery
3 Washerwater and dilute alkali remove acid
4 Reducernitrobenzene (1,390 kg per t aniline) + iron + acid, heated with steam
5 Separationcrude aniline separated from aniline water and iron oxide sludge
6 Aniline stilldistillation to finished aniline
Outputs Aniline1 t Spent acidto recovery Iron oxide sludgefrom about 1.6 t ironPower, labour and overhead are 3.2%, 2.4% and 8.3% of nitrobenzene cost. Modern plants reduce with hydrogen instead of iron.
Source: Shreve (1984), Ch 39, Fig 39.1, pdf p. 794- Shreve (1984), Ch 39 Cyclic Intermediates and Dyes · pdf pp. 793-796
- Dryden (1997), IVF Chemicals from Aromatics (including Dyes) · pdf pp. 548-549
2.3 Sulphonation, chlorination and fusion steer groups onto the ring
Sulphonation puts a sulphonic acid group (-SO3H) on the ring. Kent gives three reasons: it makes the eventual dye water-soluble, gives it fibre affinity, and directs later groups. Sulphuric acid is cheap, so it is used wherever possible, but Shreve notes that each compound has a critical acid strength below which sulphonation stops, so the water formed must be removed or an expensive excess of acid used.
Temperature can choose the isomer. Sulphonating naphthalene at 40 C gives 96% of the alpha (1-) sulphonic acid; at 160 C, 85% beta (2-). Shreve then shows a neat separation: steam at 160 C hydrolyses the alpha acid back to naphthalene, which distils off, leaving pure beta acid. Fusing that with caustic soda at 200 to 325 C swaps -SO3H for -OH to give beta-naphthol, a leading coupling component. Aniline, by contrast, is sulphonated by baking its dried sulphate salt under vacuum, giving one product, sulphanilic acid.
Chlorination is the cheapest halogenation. Chlorobenzene is made by passing dry chlorine through benzene over ferric chloride; below 60 C mono outnumbers dichloro by 8.5 to 1. Shreve warns that chlorine, water, acid and heat together destroy most metals, so equipment is costly.
Anthraquinone, the base of most vat dyes, is built by a Friedel-Crafts reaction: phthalic anhydride reacts with benzene or chlorobenzene over aluminium chloride, and sulphuric acid closes the ring. Here chemical intensity soars. Shreve says some anthraquinone dyes need over 75 kg of chemicals per kg of dye. His recipe for 235 kg of 2-chloroanthraquinone uses 148 kg phthalic anhydride, 600 kg chlorobenzene (80% recovered), 800 kg aluminium chloride and 1,500 kg sulphuric acid: about 13 kg of inputs per kg of product before any dye is made.
| Reaction | Reagent | Typical conditions | What it does | Example |
|---|---|---|---|---|
| Nitration | Nitric + sulphuric acid | Cooled; about 50 C for benzene, near 20 C for toluene, near 0 C for phenol | Adds -NO2, later reduced to -NH2 | Nitrobenzene |
| Reduction | Fe + HCl; H2 + catalyst; Na2S; Na2S2O4 | Vapour phase 270 C for aniline | Turns -NO2 into -NH2 | Aniline |
| Sulphonation | Sulphuric acid or oleum | 40 C favours alpha, 160 C beta on naphthalene | Adds -SO3H for solubility and fibre affinity | Naphthalene sulphonic acids |
| Chlorination | Cl2 + FeCl3 | Below 60 C for chlorobenzene | Adds -Cl, later a leaving group | Chlorobenzene |
| Alkali fusion | Molten NaOH or KOH | 200-325 C | Replaces -SO3H with -OH | beta-Naphthol |
| Alkylation | Methanol + a little H2SO4 | About 200 C, 3.7 MPa, 5-6 h | Turns -NH2 into -N(CH3)2 | Dimethylaniline |
| Friedel-Crafts and ring closure | AlCl3, then H2SO4 | Corrosive; residues hard to dispose of | Builds the anthraquinone ring | 2-Chloroanthraquinone |
- Kent (2012), Ch 13 Dye Application, Manufacture of Dye Intermediates and Dyes · pdf pp. 516-521
- Shreve (1984), Ch 39 Cyclic Intermediates and Dyes · pdf pp. 792, 795-801, 808-809
2.4 Diazotisation and coupling: how an azo dye is made
Azo dyes are made in two steps from two intermediates. First a primary aromatic amine is diazotised: treated with nitrous acid, made in the pot from sodium nitrite and hydrochloric acid, to give a diazonium salt (Ar-N2+). Then the diazonium salt is coupled with a phenol, naphthol, aromatic amine or 'active methylene' compound, joining the halves through -N=N-. In outline: ArNH2 + NaNO2 + 2 HCl → ArN2+ Cl- + NaCl + 2 H2O; then ArN2+ Cl- + Ar'OH → Ar-N=N-Ar'OH + HCl.
Kent gives the operating rules. Diazotisation runs at 0 to 10 C because most diazonium salts decompose when warmer, which is why ice appears on dye batch sheets. Two moles of acid per mole of amine are needed in theory, 2.2 to 2.5 in practice. Phenols and naphthols couple at pH 8 to 9, where they are ionised; amines at pH 5 to 6. Couplers such as H-acid, carrying both -OH and -NH2, can couple twice, first in acid and then in alkali, giving disazo dyes. Shreve notes that coupling is nearly quantitative.
The figure shows Shreve's batch sheet for Chrome Blue Black U (C.I. Mordant Black 17), a 5 kg-mole batch. The 1,200 kg of amine (molar mass about 239) is exactly 5 kmol; the 730 kg of beta-naphthol (144) is 5.1 kmol and the 365 kg of sodium nitrite (69) 5.3 kmol, slight excesses. Five kmol of dye (molar mass about 416) is 2,080 kg, or roughly 1,700 kg at 80 to 85% yield. Add 1,700 kg of salt, put in so that every lot has the same strength, and the batch makes the 3,500 kg sold. About half of what the customer buys is common salt.
beta-naphthol dissolved in caustic and water at 60 C, cooled overnight in a 2 m x 2 m vat
2 Amine slurry1,2,4 acid paste in water with salt, cooled with ice to 15 C
3 Diazotisationsodium nitrite in 750 kg water run in slowly over 2 h, temperature held at 15 C, in a 3 m x 2 m vat
4 Couplingdiazo cooled to 5 C and run into the naphthol solution over 2 h
5 Filtrationwooden plate-and-frame press
6 Dryingbelow 75 C
7 Grinding and standardisingdye powdered and blended with salt to standard strength
Outputs Chrome Blue Black U, standardised3,500 kg for sale Filtratesalty, coloured wastewaterYield about 80-85%. Quantities read from the page image of the batch sheet; steps follow its order of entry.
Source: Shreve (1984), Ch 39, Fig 39.9, pdf p. 806- Kent (2012), Ch 13 Dye Application, Manufacture of Dye Intermediates and Dyes · pdf pp. 497-498, 525-529, 1129
- Shreve (1984), Ch 39 Cyclic Intermediates and Dyes · pdf pp. 806, 809-810
- Kent (2012), Ch 29 Pigments, Paints, Polymer Coatings, Lacquers, and Printing Inks · pdf p. 1129
See it in the table
H₂O WaterCu CopperAl AluminiumNaCl SaltH2SO4 Sulphuric acidHNO3 Nitric acidHCl Hydrochloric acidNaOH Caustic sodaCl2 ChlorineH2 HydrogenBz BenzeneRC Rubber chemicalsDye DyesPig Organic pigmentsCheck your understanding
Why is concentrated sulphuric acid mixed with nitric acid for nitration?
Shreve lists these roles: sulphuric acid generates NO2+, the attacking species, removes water that would slow the reaction and favour oxidation, and carries away heat.Why are diazotisations run at 0 to 10 C?
Kent states that diazo compounds are usually unstable when warmer, so dye works use ice to hold the temperature down until coupling.In Shreve's batch for Chrome Blue Black U, roughly what share of the 3,500 kg sold is standardising salt?
1,700 kg of the 3,500 kg is salt added so each lot has the same strength. The dye itself is roughly 1,700 to 1,800 kg.