Kinetics of the hydrogenation of α-pinene to cis- and trans-pinanes on Pd/C

The liquid-phase hydrogenation of alpha-pinene on a Pd/C catalyst at 0-100degreesC and hydrogen pressures of 1-11 atm was studied. It was found that the order of reaction with respect to pinene increased with hydrogen pressure and did not depend on temperature, whereas the selectivity of cis-pinane formation decreased with temperature and increased with hydrogen pressure. A mechanism was proposed for the hydrogenation of alpha-pinene. According to this mechanism, the selectivity of cis-pinane formation depends on the following two factors: (a) a temperature-dependent equilibrium between adsorbed alpha-pinene species, which are cis- and trans-pinane precursors, and (b) competition between the hydrogenation and beta-H-elimination of surface sigma-pinanyl complexes. The ratio between the rates of these reactions depends on the concentration of surface hydride species, and this concentration depends on the pressure of hydrogen.