Mechanism Of Hydrogenolysis Research Articles

Hydrogen Transfer Induced Cleavage of Biaryl Bonds

Biaryl bonds are the strongest carbon−carbon single bonds in fossil fuels. This paper examines hydrogenolysis and alkane cohydrogenolysis of biphenyl and dimethylbiphenyls, in detail. Biphenyl cleavage was found to be enhanced by copyrolysis and cohydrogenolysis with small amounts of 2,2,3,3-tetramethylbutane (hexamethylethane, HME). Much smaller enhancements were found for cohydrogenolysis with other alkanes. Increased biaryl cleavage rates, in HME cohydrogenolysis, were found to be a direct consequence of initiation by hydrogen atom generated during HME decomposition. Both biphenyl pyrolysis and hydrogenolysis mechanisms involve ipso hydrogen atom attack, followed by ejection of phenyl radical and the formation of benzene. Hydrogen atom is regenerated either through the phenylation of starting biphenyl or through the direct reaction of phenyl radical with H2. Both propagation reactions are very fast, leading to highly efficient chain transfer. Biaryl bond hydrogenolysis was found to be first-order in bi...

Hydrogenolysis and Homologation of 3,3-Dimethyl-1-butene on Ru/SiO 2 Catalyst: Implications for the Mechanism of Carbon-Carbon Bond Formation and Cleavage on Metal Surfaces

The reactions of 3,3-dimethyl-1-butene (neohexene) with hydrogen in the presence of zero-valent ruthenium metal particles supported on silica are reported. The predominant reaction is the hydrogenation of neohexene to neohexane. Simultaneous but slower homologation and hydrogenolysis reactions are reported. The homologation and hydrogenolysis reactions run at approximately equal rates, suggesting a mechanistic link between the two processes. The relative quantities of C 1-C 5 and C 7 products and the variation of these quantities with respect to varying temperature, neohexene/hydrogen ratio, and contact time are reported. The distribution of the primary products, neopentane, isobutene, and methane for hydrogenolysis and 2,2-dimethyl pentane, 2,2,3-trimethyl butane, and 4,4-dimethyl-1-pentene for homologation is discussed in terms of current thought on the mechanism of homologation and hydrogenolysis of alkanes. The most likely and strongly indicated mechanism for the hydrogenolysis of neohexene involves the deinsertion of a methylidene fragment from a ruthenium-neohexyl intermediate which is also an intermediate in the hydrogenation of neohexene. The distribution of C 7 homologation products does not allow one to distinguish between a simple insertion mechanism and a mechanism passing through a metallacyclic intermediate.

Generality and mechanism of homobenzylic-homoallylic hydrogenolysis of 5-aryl-4,5-dihydro-1,3-dioxepins

Generality and mechanism of homobenzylic-homoallylic hydrogenolysis of 5-aryl-4,5-dihydro-1,3-dioxepins

Generality and mechanism of homobenzylic-homoallylic hydrogenolysis of 5-aryl-4,5-dihydro-1,3-dioxepins

Birch reduction of several 5-aryl-4,5-dihydro-1,3-dioxepins gave rise to 3-arylbutanal in moderate to good yields by hydrogenolytic cleavage of the homobenzylic-homoallylic carbonoxygen bond when the benzylic-allylic carbon is tertiary. However, the reaction did not take place when the benzylic-allylic carbon is quaternary. Moreover, the reaction was found to proceed with complete racemization at the benzylic-allylic center indicating a concurrent elimination-reduction pathway to be involved.

Correlation between the surface configurations and hydrogenolysis: Aniline on the Pt(111) surface

A correlation has been developed between the structure of adsorbed reaction intermediates and their hydrogenolysis reactivity for aniline in hydrogen pressures up to 0.001 Torr on the Pt(111) surface. The composition and orientation of the species involved in aniline hydrogenolysis were characterized using temperature programmed reaction (TPR) and near edge x-ray absorption fine structure (NEXAFS) measurements. The benzene and ammonia formed by aniline hydrogenolysis during TPR on the Pt(111) surface increase substantially in the presence of hydrogen atmospheres. At reaction temperatures, the aromatic ring in aniline remains parallel to the surface in the presence of coadsorbed hydrogen, while in the absence of hydrogen the aromatic ring is tilted relative to the surface. This correlation between orientation and reactivity suggests that a parallel configuration may facilitate hydrogenolysis of the C–N bond in aniline. These in situ studies provide new insight into the structure of surface intermediates which may play a critical role in the mechanisms of aniline hydrogenolysis on the Pt(111) surface in 0.001 Torr of hydrogen.

On the hydrogenolysis of five membered heteroatom rings over MoS 2

Quantum chemical calculation for some model clusters has been done. These clusters represent the important feature postulated at various stages of the hydrogenolysis reaction of thiophene on MoS 2 The calculation method used is the DV-Xα method. For these models, the chemisorptive bonding between thiophenic ring and the substrate is examined from the corresponding bond order values; the activation extent of the C S bond in the thiophenic ring is examined by the changes of its bond order value at various stages. The calculated results indicate their accordance with the hydrogenolysis mechanism proposed by Delmon and Dallons: (i) MoS 2 containing CUS-Mo with multiple vacancies of S (at least 3 unsaturations) and SH groups may be a favourable active center for the HDS of thiophene. (ii) Lying—flat adsorbedthiophene forms π-chemisorptive bond with CUS-Mo, in addition, there is still an adsorptive bond between thiophenic ring and the Mo vicinal to CUS-Mo. (iii) As a first step, semihydrogenation of thiophene at the a carbon position would promote the activation of thiophene. (iv) The presence of surface SH groups would enhance the activation considerably by the interaction between H atom of SH and α carbon and the other interactions as well.

Reactions of polycyclic alkylaromatics—VI. Detailed chemical kinetic modeling

We developed a detailed chemical kinetics model for the pyrolysis of long-chain polycyclic n-alkylarenes based on a general free-radical mechanism. The model accounts for the two major primary pathways in the pyrolysis network of polycyclic alkylaromatics. Using 1-dodecylpyrene (DDP) as an example, we show that the model qualitatively predicted the effects of time, temperature, and concentration on the product molar yields and the reaction kinetics. The model also predicted the autocatalytic kinetics associated with the cleavage of the arylalkyl bond. The model results showed that radical hydrogen transfer was the dominant hydrogenolysis mechanism during all but the very initial stages of the reaction when reverse radical disproportionation dominated. A sensitivity analysis revealed that reactions involving α-DDP radicals where the most important in determining the reaction kinetics and the product selectivities.

The mechanism of the desulfurization of benzenethiol by nickel (110)

The mechanism of desulfurization of benzenethiol on the Ni(110) surface was examined using temperature-programmed reaction, deuterium labeling, XPS, and HREELS. The reaction product distribution is coverage dependent, with molecular hydrogen being the only desorbed product at low coverage. Carbon-sulfur bond scission occurs at 100 K at low coverage, although the aromatic ring remains intact. At high coverage, a phenyl thiolate intermediate is formed at 100 K which gives benzene as the primary reaction product near 200 K. At high coverage, benzene formation accounts for 75% of the adsorbed benzenethiol. The desorption and spectroscopic data are most consistent with a hydrogenolysis mechanism, where both carbon-sulfur bond scission and hydrogen addition occur in the rate-limiting step at high coverage. The maximum coverage of sulfur obtained from benzenethiol is 20% less than from H{sub 2}S adsorption due to steric crowding of the aromatic rings. The intensity of certain vibrational modes suggests that the aromatic ring is tilted away from the surface in the phenyl thiolate intermediate. 26 refs., 14 figs., 2 tabs.

Reactions of polycyclic alkylaromatics. 4. Hydrogenolysis mechanisms in 1-alkylpyrene pyrolysis

The pyrolyses of 1-methylpyrene and 1-ethylpyrene were conducted at 400, 425, and 450°C in constant-volume batch reactors for holding times up to 300 min. The major products from methylpyrene pyrolysis were pyrene and dimethylpyrene whereas the major products from ethylpyrene pyrolysis were pyrene and methylpyrene. These results show that the aryl−alkyl C−C bond in methyl- and ethylpyrene is susceptible to facile hydrogenolysis. The relative importance of different hydrogenolysis mechanisms was examined through the development of mechanistic models

The mechanism of ethane hydrogenolysis over ruthenium catalysts and the isokinetic effect

An analysis has been made of the compensation effect observed for literature data on ethane hydrogenolysis over ruthenium catalysts. Using a previously derived expression for the isokinetic temperature it is concluded that the rate determining step is the breaking of the Ru-C bond. This is in accordance with recent views of Sinfelt. In order to substantiate this conclusion a recently published kinetics investigation is reconsidered. The results show two mechanisms, one for ethane/hydrogen pressures 3. In both cases the rate determining step seems to be the Ru-C bond breaking.

The mechanism of hydrogenolysis and isomerization of oxacycloalkanes on metals IX. structure sensitive hydrogenolysis and isomerization of methyloxirane over well-characterized Pt/SiO 2 Catalysts

The hydrogenolysis of methyloxirane (propylene oxide) was investigated over five Pt/SiO 2 catalysts with different degrees of dispersion (7–100%), at 393 K, with a pulse technique coupled with GC. The number of turnovers per surface atom per pulse (turnover yield) was found to depend on the degree of dispersion. The rate of the ring-opening reaction passed through a maximum at approximately 60% D, which corresponds to a maximum in edge sites on fcc octahedra, or to 2 M sites. The regioselectivity of the ring opening did not depend upon the degree of dispersion, but the formation of the two main products (acetone and 2-propanol) displayed some structure sensitivity. The paper also deals with the interpretation of a recently observed example of structure sensitivity.

Metallacarboranes in catalysis. 8. I: Catalytic hydrogenolysis of alkenyl acetates. II: Catalytic alkene isomerization and hydrogenation revisited

Part I of this study describes the facile hydrogenolysis (and deuteriolysis) of alkenyl acetates, such as isopropenyl acetate (D) and 1-phenylvinyl acetate (E), with rhodacarborane catalyst precursors to yield acetic acid and the corresponding alkene. The catalyst precursors employed were (closo-3,3-(PPh{sub 3}){sub 2}-3-H-3,1,2-RhC{sub 2}B{sub 9}H{sub 11}) (I), (closo-2,2-(PPh{sub 3}){sub 2}-2-H-2,1,7-RhC{sub 2}B{sub 9}H{sub 11}) (II), and (closo-2,2-(PPh{sub 3}){sub 2}-2-H-2,1,12-RhC{sub 2}B{sub 9}H{sub 11}) (III). The hydrogenolysis of alkenyl acetates D and E produced propene and styrene, respectively, along with acetic acid in essentially quantitative yields. Deuterium at Rh was demonstrated not to enter the hydrogenolysis reaction. These results suggest a reaction mechanism for hydrogenolysis that is based upon the relatively slow formation and decomposition of a very reactive rhodium(III) monohydride formed through the regioselective oxidative addition of Rh{sup 1} (in the exo-nido tautomer of the rhodacarborane) to terminal B-H bonds. This new information predicated part II of this study, which is devoted to a modification of previously advanced proposals for the mechanisms of alkene isomerization and hydrogenation with rhodacarborane precursors.

Electronic effects in Ru-metal oxide catalysts on hydrogenolysis of ethane or propane

Highly dispersed ruthenium particles were prepared on different metal oxides by thermal decomposition of Na[Ru 3H(CO) 11], and used as catalysts for hydrogenolysis of ethane or propane. Both the electronic properties and dispersions of the supported Ru catalysts were markedly affected by the nature of the metal oxide support. The specific activity of hydrogenolysis of ethane or propane increased with a decrease in the Ru electron density, but was almost independent of the Ru particle size. The effect of Ru electron density on the specific activity is discussed on the basis of the kinetic mechanism of ethane hydrogenolysis.

The mechanism of hydrogenolysis and isomerization of oxacycloalkanes on metals: Part VII . Study of transformation of stereoisomeric α, α'-dimethyloxacycloalkanes

The transformations (hydrogenolysis and isomerization) of stereo-isomeric α, α'-dimethyloxacycloalkanes ( 1–8) were studied on Pt and Ni catalysts. On Pt, the cis oxirane is transformed at a much higher rate than the trans isomer, whereas there is scarcely any difference between the isomers of the 4 - 6-membered cyclic ethers; nor was the difference in reaction rate on Ni very considerable for the oxiranes. The experimental results have led to new conclusions on the absorption of the oxacycloalkanes, the mechanism of their hydrogenolysis and the stereochemistry.

Sulphide catalysts on silica as a support: VIII. Peculiarities of thiophene hydrogenolysis and probable nature of “synergetic effect”

The main kinetic features of thiophene hydrogenolysis on mono- and bimetallic sulphide catalysts were analyzed. Sulphide molybdenum and tungsten catalysts were shown to differ from nickel catalysts in that they exhibited higher hydrogenation activity. According to the data from the temperature-programmed reduction (TPR) the amount of sulphur removed at TPR was not functionally dependent on the activity of bimetallic sulphide catalysts in thiophene hydrogenolysis. A new reaction mechanism of thiophene hydrogenolysis is proposed with consideration given to the structure of the active component of bimetallic catalysts. The adsorption and activation of a thiophene molecule is supposed to occur on nickel atoms stabilized on the side planes of MoS 2 (WS 2) single slabs, while hydrogen activation is achieved on the Mo(W) atoms. A high rate of chemical reaction seems to be achieved by synchronous interaction of thiophene and hydrogen molecules in the coordination sphere of a bimetallic sulphide species in the rate-determining reaction step.

Hydrogenolysis Mechanism of Five Membered Heteroatom Rings Over MoS2‐Based Catalysts

AbstractThis paper describes a concerted mechanism for the hydrogenolysis of the carbon‐heteroatom bond of thiophene, pyrrole and furan (and homologs) on molybdenum or tungsten sulfided catalysts (promoted by Co, Ni or Fe). The five‐membered rings would adsorb by a π bond on a coordinatively unsaturated Mo (or W), with their heteroatom (S, N or O) interacting, as electron acceptors, with the sulfur atoms of two vicinal sulfhydryl groups. After a 2,3 semi‐hydrogenation, a concerted mechanism would bind the hydrogen atoms of the sulfhydryl groups to the carbon atoms in the 1 and 4 positions, while the hydrogen atoms initially attached to these 1 and 4 carbon atoms would form H2S with the thiophenic sulfur (or NH3 or H2O). The mechanism explains most published features of the reactions and explains the special selectivity of sulphide catalysts, especially with furan.

ChemInform Abstract: Mechanism of Hydrogenolysis and Isomerization of Oxacycloalkanes on Metals. Part 8. New Results on the Mechanism of Hydrogenolysis of Oxiranes(I) on Platinum and Palladium

ChemInform Abstract: Mechanism of Hydrogenolysis and Isomerization of Oxacycloalkanes on Metals. Part 8. New Results on the Mechanism of Hydrogenolysis of Oxiranes(I) on Platinum and Palladium

Homologation, hydrogenolysis, and dehydrogenation of ethane on SiO 2- and SiO 2Al 2O 3-supported reduced molybdenum oxide catalysts

Kinetics and reaction mechanisms of homologation, hydrogenolysis, and dehydrogenation of ethane were studied on reduced molybdenum oxides supported on SiO 2 and SiO 2Al 2O 3. It was found that homologation of ethane to propane occurs via (1) dehydrogenation of ethane to ethylene, (2) homologation of ethylene to propylene, and (3) hydrogenation of propylene to propane. The rate-determining step was found to be hydrogenation of adsorbed propylene or that of adsorbed C 2H 5 species with an activation energy of 10.7 kcal/mole. In addition to homologation products, significant amounts of hydrogenolysis and dehydrogenation reaction products were also found. Dehydrogenation of ethane to ethylene had an activation energy of 25 kcal/mole, and it was quite similar to that for desorption of ethylene. Consequently, the dehydrogenation rate was assumed to be controlled by desorption of ethylene formed.

Catalytic hydrogenolysis of esters: a comparative study of the reactions of simple formates and acetates over copper on silica

Previous studies of the gas phase hydrogenolysis of simple esters on a copper-on-silica catalyst have indicated differences in the kinetics and mechanism of formate hydrogenolysis as compared to the reaction of larger esters. The present studies have focused on the reaction of methyl and ethyl formate compared to that of methyl and ethyl acetate in order to quantify these differences. Formates are found to react some 1000 times faster than acetates. The detailed kinetics of formate and acetate hydrogenolysis have been measured and the results correlated with predictions of Langmuir-Hinshelwood models. Experiments with deuterium have been used to explore the mechanisms of the reactions. The results suggest that acetate hydrogenolysis proceeds via dissociative adsorption of ester followed by slow hydrogenation of the acetyl fragment so formed. Formates, on the other hand, appear to involve the reaction of undissociated ester molecules with hydrogen. The latter reaction is controlled, at least in part, by the packing density of formates on the active surface.

The mechanism of hydrogenolysis and isomerization of oxacycloalkanes on metals. Part 8.—New results on the mechanism of hydrogenolysis of oxiranes on platinum and palladium

The transformations of methyloxirane and cis- and trans-2,3-dimethyloxirane have been studied on Pt/C and Pd/C catalysts at 373 K in the hydrogen pressure range 2–70 kPa in a circulation reactor. The reactions of the dimethyloxirane isomers proceed by similar mechanisms, the rate-determining step in both cases presumably being cleavage of the C—O bond. The mechanism for methyloxirane differs from that for the dimethyloxiranes. One mechanism involves edgewise adsorption of the oxirane (probably via two non-bonding electron pairs), while the other involves flat-lying adsorption. The change in regioselectivity with increase in hydrogen pressure is caused by supression of the adsorption of oxygen via two electron pairs.