Efficient Combination of Two Industrial Processes for Sustainable Production of ε-Caprolactone

ε-Caprolactone (termed typically as only caprolactone) is an important industrial commodity chemical, produced in quantities of tens of thousands of tons annually, and is valued for its biodegradable properties. Despite this large scale application, its production still relies on petrochemical sources. The conventional synthesis route involves the Baeyer–Villiger oxidation (BVO) of cyclohexanone under aqueous conditions, which presents significant challenges. Due to caprolactone’s high water solubility, the process requires an energy-intensive product workup. In this study, we present a fully solvent-free process for caprolactone production, starting from biobased phenol and achieving high conversions and efficiencies. The process begins with the selective hydrogenation of phenol to cyclohexanone using a palladium catalyst under solvent-free conditions, reaching conversions of 99% and selectivities of up to 78%. After simple removal of the heterogeneous catalyst and isolation of the ketone (80% isolated yield), cyclohexanone then is converted to caprolactone by combining it with the industrial autoxidation of aliphatic aldehydes to carboxylic acids in a noncatalytic process. Notably, the production of carboxylic acids such as butyric, isobutyric, or valeric acid exceeds several hundred thousand tons per year, thus making this coupling of the two oxidation processes highly valuable. Accordingly, this approach combines two major value-creating processes without the need for additional reagents and under fully solvent-free conditions. By implementing an aldehyde dosing strategy, the fraction of aldehyde contributing to caprolactone formation increased from 5% under nondosing conditions to 30%, reaching a final conversion of 13% of cyclohexanone. The final product is isolated by straightforward fractional vacuum distillation, a standard technique in the chemical industry, achieving a total mass balance of 95% and an isolated yield of caprolactone of 72% relative to the maximum of 13%. Notably, no side product formation was observed during the BVO reaction, owing to the mild reaction conditions (50 °C). In contrast, industrial processes can produce side products (particularly 6-hydroxyhexanoic acid) at levels of up to 15% of the final product, significantly lowering the process efficiency and sustainability.