Highly Selective Liquid-Phase Oxidation of Cyclohexane to KA Oil over Ti-MWW Catalyst: Evidence of Formation of Oxyl Radicals

Various types of Ti-containing zeolites, i.e., Ti-MWW, TS-1, Ti-MOR, and Ti-BEA, have been evaluated as candidates for the liquid-phase oxidation of cyclohexane using t-butyl hydroperoxide (TBHP, 7–8 wt %) as model oxidant. Ti-MWW zeolite displayed the highest activity for cyclohexanol and cyclohexanone (KA oil) with an overall selectivity higher than 90% at 80 °C, making this catalyst a candidate of choice for industrial KA oil production by deperoxidation of cyclohexyl hydroperoxide. The effect of the reaction temperature, reaction time, catalyst amount, and catalyst stability on Ti-MWW was surveyed in detail. The Ti-MWW catalyst showed a stable performance and could be recycled at least four times without detectable Ti leaching and loss of structural stability. The active sites for cyclohexane oxidation appeared to be located near external 12-ring cups in the Ti-MWW framework as suggested by a series of position-selective poisoning tests with tripropyl- and triphenylamine, impelling cyclohexane diffusion within the internal 10-ring channels. EPR experiments supported by DFT calculations suggested the coexistence of both Ti(IV)-OO• (peroxyl) and Ti(IV)-O• (oxyl) species generated through bimolecular pathways, implying simultaneously (SiO)3Ti(OOtBu) species and tBuOOH. The catalytic activity was strongly inhibited in the presence of alkenes, leading to the preferential formation of the epoxidation product with no detectable formation of radicals. Notably, this is the first time that oxyl species have been detected particularly with the help of DFT calculations. Predicted differences of g tensors between peroxyl and oxyl species at various hydration levels in the presence of cyclohexane were consistent with the EPR spectra.