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Isomerization of α-pinene oxide was studied at 20–50 °C in the presence of commercial montmorillonites K-10 and K-30, acid-treated illite clay as well as synthetic aluminosilicate AS-36 using cyclohexane as a solvent. The catalysts were characterized by XRD, XRF, FTIR, MAS NMR, thermal analysis and N2 low-temperature sorption methods The main reaction products at 99.9% conversion of the substrate are campholenic and iso-campholenic aldehydes, the largest amount of which (68.1 wt.%) is formed on illite clay treated by 10% HCl. The iso-campholenic/campholenic aldehydes ratio decreases from 0.7 to 0.25 with an increase of acidity of the catalyst from 47 to 153 μmol/g. The content of products with p-menthane structure (trans-carveol, trans-sorberol, p-cymene, pinol) reaches a maximum (33.0 wt.%) in the presence of medium-acid (104.0 μmol/g) K-10 clay. The amount of campholenic aldehyde in reaction mixture increases from 34.3 to 48.2 wt.%, and content of iso-campholenic aldehyde decreases from 33.7 to 17.7 wt.% with increasing the drying (calcination) temperature of illite clay from 50 to 600 °C respectively. The scheme for the formation of reaction products on clays was proposed.

A number of acid-modified clays (halloysite, montmorillonite, illite) were studied as catalysts for isoprenol cyclization with isovaleraldehyde to tetrahydropyranols used in perfumery as a mixture of cis- and trans- isomers. The main attention was paid to properties of halloysite nanotubes after treating them with 5% HCl at temperatures from 30 to 100 °C. The catalysts were characterized by XRD, EDX, MAS NMR, SEM, and N2 adsorption-desorption methods. The acid modification of halloysite allows an efficient increase of its acidity and specific surface area, while preserving morphology of nanotubes. On relatively weakly acidic clays, the formation of the target product occurs through a hemiacetal as a precursor. On the contrary, over strongly acidic resin Amberlyst-15, the yield of tetrahydropyranol was significantly lower because of a direct formation of tetrahydropyranols and dehydration by-products. The yield of tetrahydropyranol in the presence of clays was 65–72 %, slightly increasing with an increase in their acidity. The selectivity towards the cis- isomer did not practically depend on the acid sites concentration in clays, decreasing slightly for the trans-isomer with decreasing acidity due to formation of the dehydration by-products. In general, acid-modified clays are promising catalysts for the preparation of tetrahydropyranols under mild conditions and in the absence of any solvent.