“…These desorb at high temperatures in the form of CO and atomic iodine, respectively. Analogous behavior has been reported previously following the dissociation of CH 3 I, CH 2 I 2 , and C 2 H 5 I on the same surface, − and will therefore not be discussed further. 9 Temperature-programmed desorption spectra of 1-iodopropane adsorbed on a MoAl alloy at 150 K as a function of propylene exposure, where the exposures are marked adjacent to the corresponding spectrum, monitoring (a) 2 (hydrogen), (b) 40 (propylene), and (c) 44 (propane) amu.…”
Section: Resultssupporting
confidence: 70%
“…These desorb at high temperatures in the form of CO and atomic iodine, respectively. Analogous behavior has been reported previously following the dissociation of CH 3 I, CH 2 I 2 , and C 2 H 5 I on the same surface, [20][21][22] and will therefore not be discussed further. H 2 desorbs at ∼286 K where the desorption temperature increases monotonically with increasing 1-C 3 H 7 I exposure, so that at the highest exposure (10 L), it reaches ∼370 K. This behavior is different from that shown in Figure 9a and will be discussed in more detail below.…”
Section: Propylene On a Moal Alloysupporting
confidence: 64%
“…Since MoAl alloy surfaces are extremely effective for methylene insertion into surface-carbon single bonds to form higher, singly bonded metal−alkyl groups, , this implies that they may also be similarly active for methylene insertion into the carbon-surface bond of di-σ bonded olefin species to yield metallacycles. Metallacycles have been proposed to be the central intermediates in olefin metathesis catalysis, where the reverse of the insertion reaction forms metathesis products.…”
Section: Introductionmentioning
confidence: 99%
“…18 Annealing these films to ∼1500 K causes CO desorption through alumina reduction by the carbidic carbon, and results in MoAl alloy film formation. [17][18][19] Recently, studies of the chemistry of CH 3 I, 20 CH 2 I 2 , 21 and ethylene and C 2 H 5 I 22 on the MoAl alloy surface in ultrahigh vacuum showed that besides hydrogenation, dehydrogenation, and dissociation, extensive methylene migratory insertion 20,21 and H-D exchange reactions [20][21][22] occurred. It appears that the catalytic properties of the alloy are different from metallic molybdenum, but rather similar to those of late transition metals.…”
The adsorption of C 3 hydrocarbons propylene, 1-iodopropane, and 1,3-diiodopropane is studied in ultrahigh vacuum on a molybdenum-aluminum alloy formed by molybdenum hexacarbonyl reaction with a planar alumina film grown on a Mo(100) substrate. Carbon-iodine bond scission occurs below ∼200 K to deposit iodine, and form propyl species from 1-iodopropane and a C 3 metallacycle from 1,3-diiodopropane. Propyl species either undergo -hydride elimination to yield propylene or hydrogenate to form propane. Propylene adsorbs as both π-and di-σ-bonded species, and the di-σ form hydrogenates to yield propane, where the addition of the first hydrogen to form propyl species is slower than the second hydrogenation step to yield propane. Propylene also thermally decomposes on the surface to desorb hydrogen and deposit carbon where the methylyne group is the most, and the methyl group the least reactive. The metallacyclic intermediate reacts to give an allylic intermediate, which forms propylene, but also decomposes by C-C bond cleavage to evolve ethylene and deposit methylene species on the surface. This is a key step in the mechanism proposed for heterogeneously catalyzed olefin metathesis and this is the first time that this chemistry has been directly identified in ultrahigh vacuum.
“…These desorb at high temperatures in the form of CO and atomic iodine, respectively. Analogous behavior has been reported previously following the dissociation of CH 3 I, CH 2 I 2 , and C 2 H 5 I on the same surface, − and will therefore not be discussed further. 9 Temperature-programmed desorption spectra of 1-iodopropane adsorbed on a MoAl alloy at 150 K as a function of propylene exposure, where the exposures are marked adjacent to the corresponding spectrum, monitoring (a) 2 (hydrogen), (b) 40 (propylene), and (c) 44 (propane) amu.…”
Section: Resultssupporting
confidence: 70%
“…These desorb at high temperatures in the form of CO and atomic iodine, respectively. Analogous behavior has been reported previously following the dissociation of CH 3 I, CH 2 I 2 , and C 2 H 5 I on the same surface, [20][21][22] and will therefore not be discussed further. H 2 desorbs at ∼286 K where the desorption temperature increases monotonically with increasing 1-C 3 H 7 I exposure, so that at the highest exposure (10 L), it reaches ∼370 K. This behavior is different from that shown in Figure 9a and will be discussed in more detail below.…”
Section: Propylene On a Moal Alloysupporting
confidence: 64%
“…Since MoAl alloy surfaces are extremely effective for methylene insertion into surface-carbon single bonds to form higher, singly bonded metal−alkyl groups, , this implies that they may also be similarly active for methylene insertion into the carbon-surface bond of di-σ bonded olefin species to yield metallacycles. Metallacycles have been proposed to be the central intermediates in olefin metathesis catalysis, where the reverse of the insertion reaction forms metathesis products.…”
Section: Introductionmentioning
confidence: 99%
“…18 Annealing these films to ∼1500 K causes CO desorption through alumina reduction by the carbidic carbon, and results in MoAl alloy film formation. [17][18][19] Recently, studies of the chemistry of CH 3 I, 20 CH 2 I 2 , 21 and ethylene and C 2 H 5 I 22 on the MoAl alloy surface in ultrahigh vacuum showed that besides hydrogenation, dehydrogenation, and dissociation, extensive methylene migratory insertion 20,21 and H-D exchange reactions [20][21][22] occurred. It appears that the catalytic properties of the alloy are different from metallic molybdenum, but rather similar to those of late transition metals.…”
The adsorption of C 3 hydrocarbons propylene, 1-iodopropane, and 1,3-diiodopropane is studied in ultrahigh vacuum on a molybdenum-aluminum alloy formed by molybdenum hexacarbonyl reaction with a planar alumina film grown on a Mo(100) substrate. Carbon-iodine bond scission occurs below ∼200 K to deposit iodine, and form propyl species from 1-iodopropane and a C 3 metallacycle from 1,3-diiodopropane. Propyl species either undergo -hydride elimination to yield propylene or hydrogenate to form propane. Propylene adsorbs as both π-and di-σ-bonded species, and the di-σ form hydrogenates to yield propane, where the addition of the first hydrogen to form propyl species is slower than the second hydrogenation step to yield propane. Propylene also thermally decomposes on the surface to desorb hydrogen and deposit carbon where the methylyne group is the most, and the methyl group the least reactive. The metallacyclic intermediate reacts to give an allylic intermediate, which forms propylene, but also decomposes by C-C bond cleavage to evolve ethylene and deposit methylene species on the surface. This is a key step in the mechanism proposed for heterogeneously catalyzed olefin metathesis and this is the first time that this chemistry has been directly identified in ultrahigh vacuum.
“…Organic iodides have been extensively used to examine the surface chemistry of hydrocarbons on surfaces since these tend to decompose by scission of the C−I bonds at relatively low temperatures to deposit hydrocarbon moieties, along with chemisorbed iodine. , The chemistry of methyl and methylene species was studied on MoAl alloy thin films using this strategy, where it was found that, besides H 2 and methane formation, ethylene, ethane, propylene (from both iodomethane and diiodomethane), and butene (just from diiodomethane) are produced. These products were suggested to form via methylene insertion into alkyl-metal bonds, followed by β-hydride or reductive elimination reactions , suggesting that the molybdenum−aluminum alloys are extremely effective for catalyzing methylene insertion into surface-carbon single bonds.…”
The reaction between adsorbed ethylene and methylene species has been investigated on a molybdenum-aluminum alloy grown from Mo(CO)(6) on a planar alumina film formed on a Mo(100) single crystal in ultrahigh vacuum. Di-sigma-bonded ethylene reacts with carbene species, formed on the surface from methylene iodide, to form a C(3) metallacycle. This predominantly decomposes to yield propylene, while a smaller portion yields cross-metathesis products since (12)C(13)CH(4) is formed from reaction between (13)C(2)H(4) and (12)CH(2). This work demonstrates for the first time that the reaction proceeds in heterogeneous phase via a C(3) metallacycle as proposed in the Hérisson-Chauvin mechanism.
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