Preparation, acid modification and catalytic activity of kaolinite nanotubes in α-pinene oxide isomerization

Abstract

In this work kaolinite nanotubes (KNT) were obtained from commercial kaolin AKF-78 (Uzbekistan) by starting material sequential intercalation by DMSO and methanol, followed by treatment with a cetyltrimethylammonium chloride solution. Acid functionalization of KNT for catalytic applications was successfully performed for the first time using a two-step treatment with piranha solution (H₂SO₄-H₂O₂), which resulted in the removal of organic impurities as synthetic artifacts and an increase in specific surface area by 3.9 times (up to 159 m² g^-1), pore volume by 1.5 times (0.23 cm³ g^-1) and acidity by 4.1 times (49 μmol g^-1). The values of the porous structure parameters and concentration of acid sites in processed kaolinite nanotubes practically corresponded to those for natural halloysite nanotubes (HNT) modified in the same way. Both types of materials demonstrated catalytic activity in the model reaction of α-pinene oxide isomerization in various solvents, including green ones, with selectivity to trans-carveol up to 55-57% and campholenic aldehyde of 50-51%, depending on the medium used. A satisfactory correlation between solvent polarity and selectivity was also observed. To the best of our knowledge, this is the first example of using modified kaolinite nanotubes per se as a catalyst. Overall, treatment of KNT with piranha solution provides not only catalytic activity but also the opportunity for further functionalization and application of these nanomaterials.

Fig. 1. Schematic illustration for preparation of KNT from kaolin (modified from ref. 13).

Fig. 2. A general scheme of the catalytic isomerization of α-pinene epoxide (1) into campholenic aldehyde (2), iso-campholenic aldehyde (3), pinocarveol (4), isopinocamphon (5), trans-carveol (6), trans-sorberol (7), p-cymene (8) and pinol (9) (reproduced from ref. with permission).

Fig. 3. X-ray diffraction patterns of the studied kaolin from different deposits.

Fig. 4. SEM images of the material obtained by sequential treatment of kaolin from Sulton Uvays (a) and Alyans (b) mines with DMSO, methanol and CTACl (0.1 mol L⁻¹).

Fig. 5. SEM images of the material obtained by sequential treatment of commercial kaolin AKC-30 (a) and AKF-78 (b) with DMSO, methanol and CTACl (0.1 mol L⁻¹).

Fig. 6. SEM images of kaolin AKF-78 (a), KNT obtained from it (CTACl 1.0 mol L⁻¹, (b)) and the materials treated with piranha solution (KNT-Pir, (c)).

Fig. 7. FTIR spectra of kaolinite AKF-78 (1), KNT obtained from it (2) and KNT-Pir (3).

Fig. 8. SEM image halloysite pretreated with the piranha solution (HNT-Pir).

Fig. 9. Nitrogen adsorption–desorption isotherms of (a) kaolinite AKF-78 (1), KNT obtained from it (2), KNT-Pir (3), and (b) HNT initial (4) and HNT-Pir (5).

Fig. 10. Correlation between the dielectric constant of the solvent and selectivity for aldehydes 2 and 3 (a) as well as alcohols 6 and 7 (b).

Fig. 11. Mechanism of α-pinene oxide isomerization.