HYDROGEN PRESSURE-DEPENDENCE IN THE RING-OPENING REACTIONS OF PROPYLCYCLOBUTANE OVER PD SIO2 CATALYST/

Temperature dependence and the effects of hydrogen pressure on the rat e and regioselectivity were studied in the ring opening of propylcyclo butane over a low-dispersion Pd/SiO2 catalyst. At a certain compositio n (1.33 kPa of propylcyclobutane and 20 kPa of H-2) 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 t he sterically more hindered direction over the clean surface as well a s over the steady-state catalyst, yielding n-heptane as the major prod uct. The hydrogen pressure versus turnover frequency curves were of sa turation type for both products over the initial surface at 523 K. Ove r the steady-state surface, however, the reaction mechanism changed: r ing-opening rate versus hydrogen pressure curve passed through a maxim um for n-heptane, while it remained of saturation type for 3-methylhex ane. For rationalizing the high regioselectivity toward n-heptane form ation, the anchoring effect of the propyl side-chain was suggested.