Chapter 23 — Case Study 2: Side-Chain Oxidation and the PET Plastic Industry

"Polyethylene terephthalate — the PET in your water bottle, your soda bottle, and your polyester shirt — is built from terephthalic acid (TPA). The world's TPA supply (~80 million tons/year) is made by oxidizing p-xylene's two methyl groups to two carboxylic acids, using Co/Mn catalysts and oxygen. This single reaction — Chapter 23 side-chain oxidation, scaled up enormously — is the foundation of the polyester plastics industry." — paraphrase from a chemical engineering textbook

This case study explores side-chain oxidation of aromatic compounds at industrial scale, with a focus on the production of terephthalic acid (TPA) — the precursor to PET plastic. We see how Chapter 23 chemistry, generalized from KMnO₄-laboratory scale to Co/Mn-O₂ industrial scale, drives one of the world's largest chemical industries.

What is PET?

Polyethylene terephthalate (PET) is a polyester. It is one of the most widely used plastics globally:

  • Annual production: ~85 million tons (2023).
  • Markets:
  • Soda and water bottles (~30%)
  • Polyester fiber for clothing (40%)
  • Food packaging films (~10%)
  • Engineering plastic resins (10%)
  • Other (10%)

PET is made by polymerization of two monomers: - Terephthalic acid (TPA): 1,4-benzenedicarboxylic acid. (Or equivalently, dimethyl terephthalate, DMT.) - Ethylene glycol (EG): 1,2-ethanediol.

The polymer is:

$$n \text{ HOOC-C}_6\text{H}_4\text{-COOH} + n \text{ HOCH}_2\text{CH}_2\text{OH} \to [-\text{OOC-C}_6\text{H}_4\text{-COO-CH}_2\text{CH}_2-]_n + 2n \text{ H}_2\text{O}$$

The TPA monomer (~$1100/ton, 2023) is critical. The world makes 85 million tons/year. Where does it come from? From side-chain oxidation of p-xylene — Chapter 23 chemistry at industrial scale.

The starting material: p-xylene

p-xylene (1,4-dimethylbenzene) is a major petrochemical: - Annual production: ~50 million tons. - Source: catalytic reforming of naphtha (crude oil distillation cuts) → mixed xylenes (o, m, p); separation of p-xylene by adsorption (Parex process, UOP) or crystallization. - End uses: ~95% goes to TPA (this case study).

p-xylene has two methyl groups, both at benzylic positions. It is achiral and symmetrical, with high crystallinity that aids separation from o- and m-xylene isomers.

The Mid-Century process: laboratory chemistry, industrial scale

In 1954, Mid-Century Corporation (later acquired by Amoco) developed an industrial process for oxidizing p-xylene to terephthalic acid using air (O₂) + a Co/Mn/Br catalyst in acetic acid solvent at high temperature and pressure. This is now called the Mid-Century process or the Amoco process, and it is used worldwide.

The reaction (overall):

$$\text{p-xylene} + 3 \text{ O}_2 \xrightarrow{\text{Co/Mn/Br, AcOH, 200 °C, 15 atm}} \text{terephthalic acid} + 2 \text{ H}_2\text{O}$$

Conditions: - Solvent: acetic acid (AcOH) — used because it is stable to the oxidation conditions and dissolves the substrate. - Catalyst: cobalt acetate (Co(OAc)₂) + manganese acetate (Mn(OAc)₂) + sodium bromide (NaBr) — typically ~0.1-1% of each, by weight. - Oxidant: O₂ (from compressed air; ~5-15% O₂ in the gas feed). - Temperature: ~190-205 °C. - Pressure: 15-30 atm (to keep the AcOH solvent liquid; to provide O₂ partial pressure). - Reaction time: 30-180 min.

Yields: 95-98% TPA from p-xylene.

Mechanism: radical chain oxidation

The mechanism involves radical-chain autoxidation initiated by the metal/Br catalyst:

  1. Initiation: Co³⁺ (or Mn³⁺) abstracts a benzylic H from p-xylene, generating a benzyl radical and Co²⁺/Mn²⁺. Br⁻ helps by scavenging excess Co³⁺.
  2. Propagation: - Benzyl radical + O₂ → benzyl peroxyl radical (PhCH₂OO•). - Peroxyl radical abstracts another benzylic H from another p-xylene → hydroperoxide (PhCH₂OOH) + new benzyl radical.
  3. Hydroperoxide decomposition: PhCH₂OOH → PhCH=O (benzaldehyde) → PhCOOH (benzoic acid) — but wait, p-xylene has TWO methyls!
  4. Both methyl groups are oxidized: first one → 4-methylbenzoic acid (p-toluic acid); then the second → terephthalic acid.

The mechanism is conceptually identical to the KMnO₄ oxidation of toluene → benzoic acid (Ch 23.6) — but using O₂/Co/Mn instead of KMnO₄. It's the same Chapter 23 chemistry at industrial scale.

Why Co/Mn/Br?

  • Cobalt: the primary radical initiator and chain propagator.
  • Manganese: synergistic with Co; modulates the redox cycle.
  • Bromide: regenerates the Co³⁺ from Co²⁺; the Br• radical is a chain carrier; without Br, the catalyst quickly deactivates.

The Mid-Century recipe (Co + Mn + Br) was discovered empirically; modern process chemistry has refined the ratios for maximum yield, but the basic system is unchanged for ~70 years.

Solvent role

Acetic acid is the workhorse solvent: - Dissolves p-xylene and TPA at reaction temperature. - Stable to the radical/oxidation conditions (the methyl group of acetate is not benzylic; so it is much less reactive than p-xylene). - Provides a polar medium that supports the Co/Mn/Br catalyst.

Some AcOH is consumed (~2-5% of feedstock) due to side reactions, but it is recycled.

Industrial-scale considerations

A single TPA plant produces 500,000 - 1,000,000 tons/year. The reactor is a stirred tank or column type, typically 100-500 m³. Continuous operation with regular catalyst replenishment.

Key engineering challenges: - Heat removal: the oxidation is highly exothermic (~600 kJ/mol); cooling is needed. - Corrosion: Br⁻ + acetic acid + high T = corrosive; reactors are titanium-lined. - Catalyst recovery: Co/Mn are recycled in the AcOH solvent stream. - Purification: TPA is purified by recrystallization from water (TPA is a high-melting solid: m.p. >300 °C; sublimes; doesn't melt cleanly).

Output: pure terephthalic acid

The crude TPA is purified by: 1. Filtration (TPA is insoluble; precipitates from the AcOH). 2. Hydrogenation (H₂/Pd catalyst, mild conditions) to reduce trace 4-carboxybenzaldehyde (4-CBA, an oxidation intermediate that is the major impurity). 3. Recrystallization from water at high temperature.

Final purity: >99.99% TPA, with <50 ppm 4-CBA. This is purified terephthalic acid (PTA), the form sold for PET production.

From p-xylene to PET: 3 steps total

  1. p-xylene (cracked from petroleum) + O₂/Co/Mn/Br catalyst + AcOH → TPA (Chapter 23 side-chain oxidation).
  2. TPA + ethylene glycol → bis(2-hydroxyethyl) terephthalate (BHET) (esterification at high T, ~250 °C).
  3. BHET + heat + catalyst (Sb or Ge) → PET polymer (transesterification + condensation; release of EG gas).

The whole chain — from petroleum to your soda bottle — runs about 4-5 weeks from refinery to retail.

Other side-chain oxidations at industrial scale

Beyond p-xylene → TPA, other industrial side-chain oxidations include:

  • m-xylene → isophthalic acid (precursor to alkyd resins, plasticizers): same chemistry as TPA but on the meta isomer. ~5 million tons/year.
  • o-xylene → phthalic anhydride (via phthalic acid; precursor to plasticizers like phthalates, polyester resins): ~5 million tons/year.
  • Toluene → benzoic acid (food preservative, plasticizer): ~1 million tons/year.
  • Cumene → cumene hydroperoxide → phenol + acetone (Hock process): the world's main route to phenol; ~12 million tons/year of phenol made this way.

Each uses similar Co/Mn/Br catalysis or related radical autoxidation chemistry, scaled to industrial reactors. Chapter 23 chemistry is one of the most economically important chemistry sectors in the world.

Environmental considerations

PET plastic has well-known environmental problems: - Single-use bottles are a major component of plastic pollution in oceans. - Recycling: PET is one of the more easily recycled plastics (recyclable infinitely, in principle), but actual recycling rates are 30-40% globally. - Production emissions: TPA production releases ~2 tons CO₂ per ton TPA (energy + side reactions); aggregated globally, ~150-200 million tons CO₂ from TPA production alone.

Modern initiatives: - Bottle-to-bottle recycling: closed-loop recycling of post-consumer PET back to new bottles. - Bio-based PET: ethylene glycol can be made from bio-ethanol (Coca-Cola's "PlantBottle"); efforts to make TPA from biomass (e.g., from limonene, p-cymene) are active research. - Alternative polymers: polyethylene furanoate (PEF) from biomass; potential PET replacement.

The chemistry is well-established; the sustainability challenge is in the lifecycle (recycling, reuse, alternatives).

Other industrial applications of Chapter 23

Beyond p-xylene oxidation, Chapter 23 chemistry shows up in:

  • Caprolactam production (nylon-6 precursor): cyclohexane is partially oxidized by air over Co catalysts; some routes use radical chemistry.
  • Adipic acid production (nylon-6,6 precursor): cyclohexanone + air + Cu catalyst.
  • Aspirin synthesis: salicylic acid is acetylated; precursor (salicylic acid) involves Kolbe-Schmitt reaction (Ch 21) on phenol.
  • Coal tar refining: benzylic-position oxidation to clean up coal-tar derived aromatics.

Industrial chemistry's reliance on Chapter 23 (and 21, 22) is enormous.

Take-home

  • PET plastic (~85 million tons/year) starts with terephthalic acid (TPA), made from p-xylene.
  • TPA synthesis = Chapter 23 side-chain oxidation: p-xylene + O₂/Co/Mn/Br catalyst + AcOH at ~200 °C → TPA.
  • The mechanism is radical autoxidation (analogous to KMnO₄ oxidation but using O₂ + transition metal catalysts).
  • Mid-Century / Amoco process has been the standard since 1954; minor refinements but same chemistry.
  • Industrial scale: ~80 million tons TPA/year, ~100-500 m³ reactors, continuous operation.
  • Other side-chain oxidations: m-xylene → isophthalic acid; o-xylene → phthalic anhydride; toluene → benzoic acid; cumene → phenol.
  • Environmental challenges: production emissions, recycling rates, bio-based alternatives.
  • Mastery of Chapter 23 is not just academic — it is the foundation of the global polyester and plastics industry.