Hydrogenolysis cycloalkanes

The catalytic properties of this silica-supported tantalum hydrides are noteworthy. First, H/D exchange in D2/CH4 mixture is fast (0.2 mol/mol/s at 150 °C), which shows that these systems readily cleave and reform the C-H bonds of alkanes (Scheme 36(a)). Second, it also converts alkanes into its lower homologs and ultimately methane in the presence of H2 (hydrogeno lysis) at relatively low temperatures (150 °C). " The key step of carbon-carbon bond cleavage probably corresponds to an a-alkyl transfer on a Ta(m) intermediate followed by successive hydrogenolysis steps (Schemes 36(b) and 37). In the case of cycloalkanes, hydrogenolysis yields smaller cycloalkanes, but deactivation is very fast. This phenomenon has been associated with the rapid formation of cyclopentane and, thereby, with the formation of cyclopentadienyl derivatives videsupra Scheme 35), which are inactive for the hydrogenolysis of alkanes. 

Table XV summarizes kinetic parameters for hydrogenolysis reactions of alkanes and cycloalkanes over film catalysts and over supported catalysts for which it can be reasonably assumed that reaction is confined to the metallic phase. These kinetic parameters refer to the overall reaction, i.e., to the rate of disappearance of the parent molecule. It will be evident from Table XV that the catalytic activity of a given metal with a given...

Hydrogenolysis reactions of hydrocarbons on metal catalysts have been investigated in some detail. Extensive studies have been conducted on both alkanes and cycloalkanes. While a number of questions still remain with regard to mechanistic and kinetic details of the reactions, the general features seem reasonably clear. 

C complexes, 32 185-186 CFjHCFjH, 39 340 chemisorption complexes, 32 170-172 CjH, enthalpies, 37 141, 143 "C-labeling studies, 25 166-172 commercial, 6 197 complex molecules, 30 58-72 medium-sized rings, 30 68-72 polymethylcycloalkanes, 30 59-65 substituted aromatics, 30 65-68 cyclic-acyclic product ratio, 30 8-9 cycloalkanes, 30 68-69 function, hydrogen pressure, 30 12, 15-16 hydrocarbon reaction models, 32 202-205 hydrogenolysis and, 23 93, 103 interconversion, 30 81-82 isopentane, 30 17 label scrambling, 30 7, 12-13 mechanism, 30 5-16 bifunctional, 30 4 catalyst particle size and, 30 72-85 concerted, 30 20... 

Figure 3.8 Conversion with time in the hydrogenolysis of cycloalkanes (19Torr, 14.5 equiv.) catalyzed by (=SiO)2TaH (3) at 160°C under hydrogen (470Torr) cycloheptane ( ), methylcyclohexane ( ), cyclohexane ( ), methylcyclopentane (A) and cyclopentane (x).

Adsorption modes and hydrogenolysis were also correlated with other metal-catalyzed reactions. Gault noticed striking similarities in product distributions of isomerization and ring opening of cycloalkanes. Kinetic and tracer studies provided useful data252 268 to arrive at the conclusion that a common surface intermediate is... 

During studies of the hydrogenolysis and isomerization of the 2-Me-oxa-cycloalkanes on transition metal catalysts, it was found that different metals have different regioselectivities (refs 1,2). On Cu and Ni catalysts, primarily the C-0 bond adjacent to the substituent is split, leading to the formation of a primary alcohol or aldehyde (ref. 3), while on Pt and Pd catalysts mainly the more distant C-0 bond undergoes cleavage (ref. 4) yielding a secondary alcohol or ketone (Scheme 1). 

In this section we shall consider the results recorded in the literature that pertain to the structures of the adsorbed species. Kinetic or catalytic aspects, as could be relevant to hydrogenation, hydrogenolysis, or metathesis processes, will be treated in Part 11. Spectra of the much-investigated alkenes are discussed in detail in Part I. The spectra of the other principal types of hydrocarbon adsorbates, viz. alkynes, alkanes, cycloalkanes, and aromatics, will be analyzed in Part II. Most results are available for the type-molecules ethene, ethyne, ethane, and benzene as well as for the metals, Pt, Pd, Ni, Rh, and Ru. 

MF5 and MCI5 are strongly electrophilic see Electrophile and Electrophilic Reaction) and catalyze Friedel-Crafts reactions. The HF/TaFs system is a superacid catalyst and has been used in the selective acid-catalyzed isomerization and hydrogenolysis of cycloalkanes. Oligomerization and polymerization of alkynes with Nb and Ta halides as catalysts have been reported see Oligomerization Polymerization by Homogeneous Catalysis) ... 

Silica-supported Ta hydride (=SiO)2Ta-H (93a) presents unusual properties in the activation of alkanes. It catalyzes the metathesis reaction of alkanes to give higher and lower molecular weight alkanes, and the hydrogenolysis of alkanes such as ethane to methane. This hydride also activates the C H bonds of cycloalkanes to form the corresponding surface metal-cycloaUcyl complexes, and catalyses the H/D exchange reaction between CH4 and CD4, prodncing the statistical distribution of methane isotopomers. ... 

Hydrogenolysis of bicyclo [n.1.0] alkanes (IS) takes place primarily by breaking one of the exocyclic bonds to give the methyl cycloalkane product (Eqn. 20.6). 2 When the cyclopropane ring is substituted, alkylcycloalkanes are... 

As developed in the introduction, a number of important features in hydrogenolysis of cycloalkanes on platinum-charcoal catalysts emerges from the work of the Soviet school of catalysis. In a different approach, the hydrogenolysis of methyl- and 1,3-dimethylcyclopentanes was investigated on a series of platinum-alumina catalysts with various metal loadings (0.2-20%) (84, 85). It was found that the product distribution changed substantially with the percentage of platinum on the carrier. An almost selective... 

On platinum, the a, -dicarbene mechanism which accounts for the hydrogenolysis of cycloalkanes (Scheme 34) is no longer predominant in the hydrocracking of acyclic alkanes. It has already been emphasized that the internal fission of isopentane and n-pentane is related to the metallocyclobutane bond shift mechanism of isomerization (see Section III, Scheme 29), and that in more complex molecules, the favored rupture of the C-C bonds in a p position to a tertiary carbon atom is best explained by the rupture of an a,a,y-triadsorbed species (see Section III, Scheme 30). The latter scheme can account for the mechanism of hydrocracking of methylpentanes on platinum. Finally, the easy rupture of quaternary-quaternary C-C bonds in... 

There is extensive work to report on dehydrogenation of cycloalkanes on bimetallic catalysts, mainly in the supported form. Its motivation is quite clear to minimise parasitic reactions such as carbon deposition and hydrogenolysis, so as to have a catalytic system capable of working for long periods of time and at high selectivity. 

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