About the hydrogenation of α-pinene in the case of turpentine hydrodesulfurization on a highly porous block cell catalyst

Hydrodesulfurization of turpentine on a block catalyst with Ni and Pd applied to it allows reducing the content of total sulfur in turpentine to values below 50 ppm. It is well known that double bonds, including the multiple bond of α-pinene, may also be hydrogenated, although to a lesser extent than the C-S bond. In order to control the content of α-pinene in the hydrodesulfurization process for turpentine in parallel with the control of sulfur content in sulfated turpentine the concentrations of α-pinene were found before and after the catalyzed process. After hydrodesulfurization of turpentine from Bratsk PPM on the catalyst 0.4 %Pd/6.0 % g-Al2O3 at 75 °C and 1.5 MPa the loss of α-pinene was 5 %, most likely due to hydrogenation. In addition, when the Pd content in the catalyst varies from 0.2 to 1 %, the tendency of decreasing the α-pinene concentration is obvious, which is quite natural: an increase in the metal concentration contributes to an increase in the reaction rate, including the α-pinene hydrogenation reaction. It is shown that at hydrodesulfurization of turpentine from Ust-Ilimsk PPM at 1.5 MPa and 120 °C during 12 h in the presence of 10 % Ni and 7 % of ZnO and the subsequent implementation of the process at 25 °C for 5 h in the presence of 0.4 % Pd and zinc oxide the hydrogenation of α-pinene does not take place. This is important because α-pinene is a starting compound in the composition of turpentine used to obtain high-quality commercially viable products - pine oil of grades MS-85, MS-95 and the perfume terpineol - by acid-catalytic hydration. Small loss takes place upon decrease of the Pd concentration to 0.2 % and the increase in process temperature; however, in this case, the sulfur content in the final product is significant. A good result on the sulfur content was obtained by increasing the Pd content to 1 %, with no preliminary desulfurization on the nickel catalyst. However, in this case there are noticeable losses, more than 30 %, of the main component of turpentine - α-pinene. The results presented in the paper confirm the selectivity of the catalytic hydrogenation reaction under mild conditions in relation to the C-S bond in comparison with the multiple C=C bond: despite the fact that the turpentine consists mainly of α-pinene, and the content of sulfur-containing compounds in terms of total sulfur is present in ppm, the C-S bond is mainly hydrogenated. Under more severe conditions it was possible to obtain a product of quantitative hydrogenation of α-pinene in turpentine from Ust-Ilimsk PPM, pre-purified from sulfur, namely, cis -pinane.