Browsing by Author "Haas, Andreas"

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Ring opening of mono- and bicyclic naphthenes via hydrogenolysis on noble metal catalysts
(2012) Haas, Andreas; Weitkamp, Jens (Prof. Dr.-Ing.)
In general the catalytic activity of the noble metals decreased in the order Ir > Rh > Pt > Pd, resulting in an increasing tendency of dehydrogenation to aromatics at the high reaction temperatures that are required. All metals were active in the cis/trans-isomerization of perhydroindan or decalin which preceded the other reactions, viz. hydrogenolysis, skeletal isomerization and dehydrogenation. Other feed hydrocarbons were n decane, ethylcyclohexane, butylcyclohexane and methyldecalin. Iridium was the best noble metal for purely metal-catalyzed ring opening, especially if naphthenes were converted that consist of two six-membered rings. For example, in the hydroconversion of cis-decalin a maximal yield of open-chain decanes of 22 % was obtained on 2.73Ir/silica at a reaction temperature of 310 °C and a hydrogen pressure of 8.0 MPa. Iridium was not active in skeletal isomerization and cleaved preferentially unsubstituted C-C bonds, i.e. primary-secondary and secondary-secondary bonds. One result of this regioselectivity was a high degree of branching in the open-chain alkanes with the same number of carbon atoms as the reactant that are formed by cleaving one C-C bond in each naphthenic ring. In the group of hydrocracked products mainly methane and C9 products, smaller amounts of ethane and C8 products and very small amounts of C3 to C7 hydrocarbons were formed in the hydroconversion of decalin. Such a distribution, denoted as hammock-type curve, seems to be typical also for the hydroconversion of other bicyclic naphthenes like perhydroindan and methyldecalin on non-acidic iridium catalysts. This type of distribution curve was simulated by assuming the following reaction sequence that was also deduced from the dependence of product selectivities on conversion: One ring in decalin is opened by hydrogenolysis under the formation of the three direct ring-opening products (direct ROPs) butylcyclohexane, 1-methyl-2-propylcyclohexane and 1,2-diethylcyclohexane. Subsequently, hydrogenolysis occurs either in the alkyl side-chains of direct ROPs (exocyclic hydrogenolysis), resulting in the formation of hydrocracked products (C9-), or in the six-membered ring (endocyclic hydrogenolysis) under the formation of seven constitutional decane isomers (direct open-chain decanes, direct OCDs). For the simulation it is assumed that also direct OCDs undergo complete hydrogenolysis under the formation of C9- products. To obtain a good agreement between the simulated and the experimentally obtained curve it turned out that the catalytic property of iridium to cleave preferentially unsubstituted C-C bonds is important. Spiro[4.5]decane was more reactive on Pt/silica in comparison with decalin but at low conversions mainly decalin was formed by skeletal isomerization, presumably via the bond-shift mechanism. Similar yields of ring-opening products were obtained at comparable reaction temperatures in the conversion of decalin and spiro[4.5]decane. It was concluded that, with both reactants, at first a similar mixture of skeletal isomers is generated which undergoes a consecutive hydrogenolysis under the formation of ROPs. On Ir/silica a direct ring-opening mechanism occurred: The first reaction is a hydrogenolytic ring opening of the five-membered ring, followed by hydrogenolysis of the six-membered ring in the respective ring-opening products. Iridium cleaves with a strong preference the bisecondary C-C bonds and conserves the bonds between the quaternary and the neighbored carbon atoms. High-performance ring-opening catalysts (HIPEROCs) are zeolitic, iridium- or platinum-containing catalysts with a small concentration of Brønsted acid sites. They were tested in the hydroconversion of n-decane, butylcyclohexane and spiro[4.5]decane. By a detailed examination of the product distribution it turned out that on these zeolitic iridium- or platinum-containing catalysts with a small concentration of Brønsted acid sites C-C bond cleavage occurs virtually only on the noble metal via hydrogenolysis. By comparing the results of butylcyclohexane conversion on the non-acidic catalysts with those on HIPEROCs evidence for the following reaction sequence on HIPEROCs was obtained: Via bifunctional catalysis the six-membered ring is isomerized into a five-membered ring. Subsequently, a fast opening of the five-membered ring occurs by hydrogenolysis on the noble metal. On iridium-containing HIPEROCs the six-membered ring is in addition also opened directly by hydrogenolysis. On Pt/silica a similar reaction sequence was observed since skeletal isomerization occurs also on platinum by metal catalysis. However, on this non-acidic support the isomerization activity is much lower compared to catalysts that contain Brønsted acid sites.