The Surface Chemistry of Propylene, 1-Iodopropane, and 1,3-Diiodopropane on MoAl Alloy Thin Films Formed on Dehydroxylated Alumina

Abstract

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.

Formation and Decomposition of C3 Metallacycles from Ethylene and Methylene on MoAl Alloy Thin Films

“…The desorption of propylene at ∼330 K is characteristic for the decomposition of C 3 metallacycles. 39 However, no 220-K features were detected in these desorption spctra. Thus, to elucidate the nature of the 220-K desorption state, propylene (41 amu) desorption spectra were collected by dosing in the reverse order, that is by exposing the alloy to 5 L C 2 H 4 followed by various exposures to CH 2 I 2 .…”

“…The data of Figure 2a confirms that reaction occurs between the carbene formed from methylene iodide and the results of Figure 2b are consistent with both the decomposition products (ethylene and methane) and reaction temperatures from the decomposition of C 3 metallacycles formed from DIP. 39 The data of Figure 2b demonstrate that ethylene metathesis has occurred since the 12 C originally in the carbene has been incorporated into the ethylene that is produced. Finally, the data of Figure 2c indicate that reaction has not proceeded via methylene insertion into an ethyl species.…”

“…It has been demonstrated previously that C 3 metallacycles can decompose to form ethylene and methylene species (which desorb as methane), a central step in the He ´risson-Chauvin metathesis mechanism. 39 However, this reaction cannot be probed when metallacycles are formed from normal ethylene and diiodomethane, since the signals due to any reaction products (ethylene and methane) will be obscured by the initial reactants. TPD experiments were therefore performed by allowing isotopically labeled ethylene ( 13 C 2 H 4 ) to react with normal CH 2 I 2 to follow this reaction.…”

“…Metallacycles have also been grafted onto a MoAl alloy surface by exposure to 1,3-diidopropane. 39 It has been shown that these can decompose via an η 3 -allylic intermediate to yield propylene as found on transition-metal surfaces. 40 More importantly, in the context of the He ´risson-Chauvin mechanism, an alternative decomposition pathway was found on the MoAl alloy that resulted in the evolution of ethylene and the deposition of carbenes on the surface.…”

“…40 More importantly, in the context of the He ´risson-Chauvin mechanism, an alternative decomposition pathway was found on the MoAl alloy that resulted in the evolution of ethylene and the deposition of carbenes on the surface. 39 However, to fully confirm that metathesis proceeds in heterogeneous phase by the He ´risson-Chauvin pathway, it must be demonstrated that carbenes and alkenes react to form metallacycles and that they decompose similarly to the C 3 metallacycles grafted onto the surface from 1,3-diiodopropane, to evolve an alkene and deposit a carbene.…”