Cyclohexanone for Caprolactone & Specialty Intermediates is a ketone-grade feedstock with CAS Registry Number 108-94-1 and molar mass 98.15 g/mol. The material is controlled as a narrow-cut intermediate rather than a general-purpose solvent because residual cyclohexanol, water, acidity, and peroxides directly affect Baeyer-Villiger selectivity to ε-caprolactone. At 20 °C the density is 0.947 g/cm³ by ASTM D4052, the normal boiling point is 155.6 °C at 101.3 kPa, and the closed-cup flash point is 44 °C by ASTM D56. These properties place the feedstock in a storage and transfer class requiring electrostatic bonding, nitrogen inerting, and carbon steel or 316L stainless steel equipment.

In caprolactone service, the carbonyl carbon is the primary reaction locus for peroxycarboxylic acid attack, while the adjacent methylene groups allow ring expansion with minimal branching. The same specification envelope also supports cyclohexanone oxime production for caprolactam, ketone-formaldehyde resin chemistry, and specialty solvent applications where low acidity and controlled water content are required. The common technical requirement across these outlets is a carbonyl source that remains free of conjugated enones, heavy aldol condensation products, and metal residues carried from KA oil distillation.

What Impurity Profile Governs Feedstock Selection for Caprolactone?

Water is the most compositionally sensitive variable because ε-caprolactone hydrolyses to 6-hydroxyhexanoic acid under mildly acidic conditions; that acid then accelerates esterification and oligomerisation during storage. The release limit of 0.05 wt% by ASTM D1364 is set to keep the acid number of freshly distilled caprolactone below 0.5 mg KOH/g by internal titration, reducing the load on downstream neutralization and molecular weight control in ring-opening polymerization. Acidity as acetic acid is held at ≤0.01 wt% by ASTM D1613; acetic acid is the principal acidic residue carried forward from peracetic acid oxidation and must be removed before final distillation.

Cyclohexanol is limited to ≤0.20 wt% because its boiling point of 161 °C at 101.3 kPa lies within 5.4 °C of cyclohexanone. This proximity forces high reflux ratios in vacuum rectification and raises reboiler steam demand when a low residual alcohol specification is required. In the oxidation mass, cyclohexanol also forms cyclohexyl acetate and consumes peracetic acid, shifting the effective oxidant-to-ketone stoichiometry and increasing light-ends load in the caprolactone concentrator. Peroxide content is controlled at ≤5 mg/kg active oxygen by ASTM E298 because residual peroxides can initiate radical side reactions in downstream polymerizations and can produce false-positive peroxide results in finished caprolactone. Nonvolatile matter at ≤0.005 wt% by ASTM D1353 is relevant to reboiler tube fouling and color body formation in the final product.

The specification is not defined by a single standard; it is commonly a customer-specific release profile anchored to ASTM methods and an internal GC-FID method validated under ISO 17025. In production-scale batch records, the largest source of batch-to-batch variation is the residual cyclohexanol content from upstream phenol hydrogenation, which shifts the light-ends split and alters the recycle balance in the caprolactone recovery column.

When Peracetic Acid Is Dosed into the Baeyer-Villiger Reaction Mass

The Baeyer-Villiger oxidation of cyclohexanone to ε-caprolactone is typically performed in a continuous stirred-tank battery, a loop reactor with an external cooler, or a packed-column countercurrent contactor. Because the aqueous peracetic acid phase has limited solubility in cyclohexanone, the reaction is often mass-transfer limited until a high-shear rotor-stator mixer or a static mixer is installed immediately before the hold vessel. The reaction proceeds through a Criegee-type intermediate after nucleophilic attack of the peroxy acid on the carbonyl carbon; the cyclohexyl ring migration aptitude strongly favours ε-caprolactone over cyclohexyl formate byproducts.

Operating temperature is maintained between 40 °C and 60 °C. Lower temperatures reduce conversion rate, while higher temperatures accelerate hydrolytic ring-opening and peracetic acid decomposition. The reactor cooling system is typically a shell-and-tube exchanger with chilled water on the service side, sized to remove the exotherm generated by the oxidation and the simultaneous dilution enthalpy of acetic acid. Peracetic acid solution is maintained below 40 wt% because formulations above this threshold are classified and handled as organic peroxides with transport and storage restrictions; typical operating concentrations are 15–25 wt% in acetic acid to balance water inhibition and acetic acid load.

Peracetic acid is metered through a dedicated dosing line with a static mixer to prevent concentration spiking. The feed is interlocked with online FTIR monitoring of the carbonyl band at 1715 cm⁻¹ or with online NIR spectroscopy in the 1600–1800 nm region to confirm conversion before reactor discharge. If the temperature rise exceeds 15 °C within 10 min, the control system initiates quench with chilled water and sodium sulfite solution to consume unreacted peroxide. Conversion of cyclohexanone typically exceeds 95% at total residence times of 4–8 h in a 3-vessel continuous stirred-tank battery, while ε-caprolactone selectivity can exceed 90% when feed water is below 0.1 wt%. The main side products are adipic acid, 6-hydroxyhexanoic acid, and oligomeric esters formed by ring-opening of caprolactone. Published data for this specific reactor configuration is limited where catalyst-free operation is employed; site-specific optimisation is required to correlate residence time distribution with peroxide residual.

After the reaction reaches the target peroxide residual, the mass is cooled to below 30 °C and washed with water to remove acetic acid. Aqueous sodium carbonate solution is metered to pH 6.5–7.5, with the neutralization exotherm controlled by a recirculation cooler. The organic phase is then distilled under vacuum in a structured packed column at operating pressure 20–50 mbar and bottom temperature 110–140 °C. The caprolactone-rich cut boils at 96–98 °C at 10 mmHg, allowing separation from remaining cyclohexanone and cyclohexanol. Wiped-film evaporation is used for final polishing of high-viscosity bottoms; hot-zone residence time is kept below 30 min to limit oligomer formation and colour development. In manufacturing campaigns, the most frequent operational bottleneck is reboiler fouling in the crude caprolactone concentrator when nonvolatile matter in the feed exceeds the release limit.

Specialty Intermediate Chemistry Outside Polycaprolactone

In oxime service, cyclohexanone is condensed with hydroxylamine sulfate in a loop reactor at 80–90 °C and pH 3.5–5.0. The resulting cyclohexanone oxime is then rearranged in oleum or concentrated sulfuric acid to caprolactam. The ketone must be low in dissolved oxygen and peroxide residues because peroxide-promoted byproducts contribute color bodies that carry into caprolactam melt and require additional hydrogen peroxide or potassium permanganate purification. Production-scale oximation systems typically use external circulation with heat removal to manage the exotherm while avoiding local pH excursions that promote ketone self-condensation.

In adipic acid service, cyclohexanone is fed with cyclohexanol as KA oil to a nitric acid oxidation train. Operating temperatures of 60–90 °C and nitric acid concentrations of 45–60 wt% are typical, with copper and vanadium salts used as catalysts. The ratio of cyclohexanone to cyclohexanol influences the off-gas profile for N2O and NOx, but published data for site-specific absorbent efficiencies is limited. Ketone-rich feeds require careful control of the initial exotherm because cyclohexanone reacts more rapidly than cyclohexanol with nitric acid, and a feed imbalance can create local hot spots in the oxidation reactor.

For solvent and polymer intermediate applications, cyclohexanone dissolves PVC, nitrocellulose, and selected acrylic copolymers at solids loadings of 10–30 wt% using a Cowles high-shear disperser or rotor-stator mixer. The ketone solvency parameter supports use in gravure inks and polyurethane coating systems, but exposure to amines must be evaluated because ketone-amine condensation can form coloured imines. Storage requires nitrogen blanketing and closed transfer. Cyclohexanone can form peroxides when exposed to air and ultraviolet light; inventories older than 90 days should be tested for peroxides by ASTM E298. Under EU REACH Regulation (EC) No 1907/2006, cyclohexanone is supplied with harmonised classification Flam. Liq. 3, H226, and Acute Tox. 4, H332. Handling systems should avoid contact with concentrated nitric acid, peracetic acid, and other strong oxidisers unless under controlled reaction conditions.