Hydrogen pressure dependence in the ring-opening reactions of propylcyclobutane over Pd/SiO₂ catalyst

Temperature dependence and the effects of hydrogen pressure on the rate and regioselectivity were studied in the ring opening of propylcyclobutane over a low-dispersion Pd/SiO₂ catalyst. At a certain composition (1.33 kPa of propylcyclobutane and 20 kPa of H₂) the reaction rate versus temperature curve was found to pass through a maximum. At each temperature the ring opened selectively (or exclusively at 423 K) in the sterically more hindered direction over the clean surface as well as over the steady-state catalyst, yielding n-heptane as the major product. The hydrogen pressure versus turnover frequency curves were of saturation type for both products over the initial surface at 523 K. Over the steady-state surface, however, the reaction mechanism changed: ring-opening rate versus hydrogen pressure curve passed through a maximum for n-heptane, while it remained of saturation type for 3-methylhexane. For rationalizing the high regioselectivity toward n-heptane formation, the anchoring effect of the propyl side-chain was suggested.