Ru-Catalyzed Polyethylene Hydrogenolysis under Quasi-Supercritical Conditions

Ru/C-catalyzed polyethylene (PE) and hydrocarbon hydrogenolysis under quasi-supercritical fluid of isopentane was kinetically and mechanistically investigated. PE hydrogenolysis with C-C and C-H cleavage showed zeroth order, suggesting strong adsorption of hydrocarbons. PE yielded broad product distribution of heavy (C_21-40) and diesel-range (C_11-20) hydrocarbons in the primary step of hydrogenolysis due to stochastic C-C cleavage over Ru surface. Catalytic hydrogenolysis of <i>n</i>-hexadecane, squalane, and light hydrocarbons such as <i>n</i>-pentane, iso-pentane, and <i>n</i>-hexane further described C-C cleavage reactivity between primary and secondary carbons, i.e., ¹C-²C and ²C-²C, which has an order of magnitude higher hydrogenolysis rate than that involving a tertiary carbon. The PE saturated Ru surface and lower C-C cleavage reactivity of tertiary carbon in iso-pentane, therefore, imited sovlent conversion during hydrogenolysis, whereas leading to selective PE conversion. Using hexadecane, we observed comparable hydrogenolysis rates between H₂ and D₂ (<i>k</i> _H <i>/k</i> _D ∼ 1), indicating the kinetically relevant step of C-C cleavage with facilitating C-H cleavage and rehydrogenation. However, the normal kinetic isotope effect between hexadecane and deuterated hexadecane (<i>k</i> _{C₁₆H₃₄} <i>/k</i> _{C₁₆D₃₄} ∼ 5) revealed that the dehydrogenation, i.e., C-H cleavage, can be kinetically involved in the hydrogenolysis kinetic. By considering the 8-fold lower H-D exchange rate with deuterated hexadecane compared to n-hexadecane, the lower rate for hydrogenolysis and H-D exchange with deuterated hexadecane can be attributed to the C-D bond dissociation energy being 3 kJ/mol higher than that of the C-H bond. Increasing H₂ pressure favors internal C-C bond cleavage over terminal one. This minimizes the formation of lower hydrocarbons, particularly methane. However, the increase in H₂ pressure increases the coverage of adsorbed hydrogen on the Ru particles due to competitive adsorption of H₂ and polyethylene, which, in turn, reduces the polyethylene conversion rates.