Hydrogen pressure dependence of the ring-opening reactions of propylcyclobutane over Pt/SiO₂ catalyst at different temperatures

The hydrogen pressure dependence of the ring-Opening reactions of propylcyclobutane was studied at three different temperatures over Pt/SiO₂ catalyst. At 373 and 573 K, only hydrogenative ring opening occurs, while appreciable amounts of aromatics, ring enlargement, and cracking products are formed at 673 K. At 373 K, the ring opens selectively in the sterically more hindered direction over both the clean and the working catalyst, yielding heptane as the major product. The selectivity is lost at 573 K and 673 K: 2,3 CC bond scission, i.e., formation of 3-methylhexane occurs with initial rates comparable to that of 1,2 CC bond scission. Over the working catalyst, the selectivity is close to statistical, with a slight excess of 3-methylhexane. An increase of hydrogen pressure has different effects on the ring-opening reactions, depending on the reaction temperature. The initial rate of product formation vs hydrogen pressure curves exhibit three extremes at 373 K, but they increase monotonously at 573 and 673 K. Over the working catalyst, the hydrogen pressure dependence curves display three extremes at 373 K, pass through a maximum at 573 K, and increase monotonously at 673 K. Mechanisms over both the initial and working catalysts are discussed on the basis of regioselectivity data and hydrogen pressure dependence curves.