Pathways of Geraniol Transformation over a Mironekuton Catalyst

The subject of the presented work was the study of the pathways of geraniol transformation during its oxidation with molecular oxygen in the presence of a natural Japanese volcanic clay mineral—mironekuton—used as a green heterogeneous catalyst. Prior to the catalytic tests, a comprehensive characterization of mironekuton was carried out using SEM, XRD, EDX, FTIR, and UV–Vis techniques. The catalytic experiments were aimed at establishing reaction conditions enabling effective geraniol conversion and controlling the distribution of valuable transformation products under solvent-free conditions. The influence of temperature (75–100 °C), catalyst content (0.5–5.0 wt%), and reaction time (15–360 min) was systematically investigated. The obtained results demonstrated that pristine mironekuton exhibits moderate activity and limited selectivity toward low-molecular-weight oxygenated derivatives of geraniol, such as 2,3-epoxygeraniol, 2,3-epoxycitral, and citral. Instead, dehydration, isomerization, and dimerization reactions play a significant role, leading to the formation of higher-molecular-weight products, particularly thunbergol and 6,11-dimethyldodeca-2,6,10-trien-1-ol. Sulfuric acid treatment of mironekuton results in a pronounced enhancement of catalytic activity and a substantial shift in product distribution. This effect is directly related to the increased surface acidity, which promotes dehydration–dimerization pathways over epoxidation, leading to thunbergol as the dominant product with high and reproducible selectivity, while epoxidation products are no longer detected. Kinetic modeling of the geraniol transformation process revealed that epoxidation steps are kinetically disfavored and that epoxide species act only as short-lived intermediates, whereas dehydration–dimerization pathways are kinetically preferred. Overall, the results indicate that acid-activated mironekuton functions as an efficient and environmentally benign heterogeneous catalyst, favoring selective thunbergol formation under mild, solvent-free conditions, using molecular oxygen as a green oxidant.