Isomerization and Fragmentation of Cyclohexanone in a Heated Micro-Reactor

The thermal decomposition of cyclohexanone (C6H10O) has been studied in a set of flash-pyrolysis microreactors. Decomposition of the ketone was observed when dilute samples of C6H10O were heated to 1200 K in a continuous flow microreactor. Pyrolysis products were detected and identified by tunable VUV photoionization mass spectroscopy and by photoionization appearance thresholds. Complementary product identification was provided by matrix infrared absorption spectroscopy. Pyrolysis pressures were roughly 100 Torr, and contact times with the microreactors were roughly 100 μs. Thermal cracking of cyclohexanone appeared to result from a variety of competing pathways, all of which open roughly simultaneously. Isomerization of cyclohexanone to the enol, cyclohexen-1-ol (C6H9OH), is followed by retro-Diels–Alder cleavage to CH2CH2 and CH2C­(OH)–CHCH2. Further isomerization of CH2C­(OH)–CHCH2 to methyl vinyl ketone (CH3CO–CHCH2, MVK) was also observed. Photoionization spectra identified both enols, C6H9OH and CH2C­(OH)–CHCH2, and the ionization threshold of C6H9OH was measured to be 8.2 ± 0.1 eV. Coupled cluster electronic structure calculations were used to establish the energetics of MVK. The heats of formation of MVK and its enol were calculated to be Δf H 298(cis-CH3CO–CHCH2) = −26.1 ± 0.5 kcal mol–1 and Δf H 298(s-cis-1-CH2C­(OH)–CHCH2) = −13.7 ± 0.5 kcal mol–1. The reaction enthalpy Δrxn H 298(C6H10O → CH2CH2 + s-cis-1-CH2C­(OH)–CHCH2) is 53 ± 1 kcal mol–1 and Δrxn H 298(C6H10O → CH2CH2 + cis-CH3CO–CHCH2) is 41 ± 1 kcal mol–1. At 1200 K, the products of cyclohexanone pyrolysis were found to be C6H9OH, CH2C­(OH)–CHCH2, MVK, CH2CHCH2, CO, CH2CO, CH3, CH2CCH2, CH2CH–CHCH2, CH2CHCH2CH3, CH2CH2, and HCCH.