Ring opening of decalin via hydrogenolysis on Ir/- and Pt/silica catalysts

The catalytic conversion of cis-decalin was studied at a hydrogen pressure of 5.2 MPa and temperatures of 250–410 °C on iridium and platinum supported on non-acidic silica. The absence of catalytically active Brønsted acid sites was indicated by both FT-IR spectroscopy with pyridine as a probe and the selectivities in a catalytic test reaction, viz. the hydroconversion of n-octane. On iridium/silica, decalin hydroconversion starts at ca. 250–300 °C, and no skeletal isomerization occurs. The first step is rather hydrogenolytic opening of one six-membered ring to form the direct ring-opening products butylcyclohexane, 1-methyl-2-propylcyclohexane and 1,2-diethylcyclohexane. These show a consecutive hydrogenolysis, either of an endocyclic carboncarbon bond into open-chain decanes or of an exocyclic carboncarbon bond resulting primarily in methane and C9 naphthenes. The latter can undergo a further endocyclic hydrogenolysis leading to open-chain nonanes. All individual C10 and C9 hydrocarbons predicted by this “direct ring-opening mechanism” were identified in the products generated on the iridium/silica catalysts. The carbon-number distributions of the hydrocracked products C9−C9− show a peculiar shape resembling a hammock and could be readily predicted by simulation of the direct ring-opening mechanism. Platinum on silica was found to require temperatures around 350–400 °C at which relatively large amounts of tetralin and naphthalene are formed. The most abundant primary products on Pt/silica are spiro[4.5]decane and butylcyclohexane which can be readily accounted for by the well known platinum-induced mechanisms described in the literature for smaller model hydrocarbons, namely the bond-shift isomerization mechanism and hydrogenolysis of a secondary-tertiary carboncarbon bond in decalin.

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► Decalin was converted on Ir/silica or Pt/silica at a hydrogen pressure of 5.2 MPa.
► On iridium, the six-membered rings are opened without prior isomerization.
► A mechanistic network for the hydroconversion of decalin on iridium is presented.
► The distribution of the hydrocracked products formed on iridium is simulated.
► Pt is less active than Ir, it catalyzes skeletal isomerization and hydrogenolysis.