Alkynes, adsorption

A pre-exponential factor and activation energy for each rate constant must be established. All forward rate constants involving alkyne adsorption (ki, k2, and ks) are assumed to have equal pre-exponential factors specified by the collision limit (assuming a sticking coefficient of one). All adsorption steps are assumed to be non-activated. Both desorption constants (k.i and k ) are assumed to have preexponential factors equal to 10 3 sec, as expected from transition-state theory [28]. Both desorption activation energies (26.1 kcal/mol for methyl acetylene and 25.3 kcal/mol for trimethylbenzene) were derived from TPD results [1]. 

The observation of negative apparent activation energy can most simply be interpreted in terms of the competition between the adsorption and desorption of methylacetylene on the surface. This qualitative explanation is illustrated in Figure 3, where the steady-state production of trimethylbenzene is compared with the TPD trace of methylacetylene. The fall off in steady state cyclotrimerization rate matches the tail of the desorption spectrum and illustrates the role of reactant desorption at higher temperatiu-es controlling the availability of alkyne monomers and thus the overall cyclotrimerization rate in this temperatime/pressure regime. 

FIGURE 2.2 Mechanism of half-hydrogenation of alkynes showing two possible adsorption sites. 

In Ref 169, some peculiarities associated with adsorption of alkyne peroxides from DM F-water solutions onto the mercury electrode in the presence of tetraethylammonium cations have been described. Polarography and electrocapillary measurements were employed as the experimental techniques. It has been shown that interfacial activity of these peroxides was determined by the species generated as a result of associative interactions between peroxides and DMF and tetraethylammonium cations. 

The first examples of what can be categorized as [2 + 2] type cycloaddition product formed by reaction between an alkene and a silicon surface were reported in the late 1980s. Alkenes such as ethylene, as well as the related alkyne molecule acetylene, were reacted with the clean Si(100)-2 x 1 surface in vacuum [196-213]. The adsorption of these unsaturated C2 molecules (ethylene and acetylene) on Si(100)-2 x 1 is also discussed in Chapter 1. The alkenes were found to chemisorb at room temperature, forming stable species that bridge-bonded across the silicon dimers on the surface. The reaction proceeded by formation of two new a bonds between Si and C atoms, hence the bonding was referred to as di-sigma bonding. In addition, it was shown that while the bonds of the alkene and of the Si—Si dimer are... 

Alkynes have also been shown to form the [2 + 2] cycloaddition product. Acetylene (H—C=C—H), the simplest alkyne, forms an interesting adsorption case, because the specific adsorption geometries of acetylene on Si(100)-2 x 1 have been debated [11,201,207,210,224-236]. Acetylene was first found experimentally to form a [2 + 2] C=C cycloaddition product that exhibits a cyclobutene-like surface structure on Si(100)-2 x 1 [210,227]. Later STM measurements revealed that at least two different surface products were present [228,231,233], and identified a product that is oriented perpendicularly to the dimer row. From these images, it was argued that in addition to an intradimer [2 + 2] C=C cycloaddition geometry, acetylene also forms a surface adduct that bridges two dimers along a row. Several theoretical... 

In general, in Part II we apply the same pattern of analysis to the numerous published vibrational spectra derived from the adsorption of alkynes, alkanes, and aromatic hydrocarbons. In addition, we summarize recently obtained spectroscopic results characterizing hydrocarbon species obtained by thermal, photochemical, or electron-bombardment dissociation of halogen- or nitrogen-substituted alkanes on single-crystal metal surfaces. The hydrocarbon surface species so obtained are normally as anticipated from the replacement of the heteroatoms by surface metal atoms. The... 

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

The high selectivity of alkene formation is not the result of a large rate difference in the hydrogenation of the triple and double bonds. Rather, it is ensured by the strong adsorption of alkynes compared with that of alkenes. The alkyne displaces the alkene from the surface or blocks its readsorption. 

As with monoolefins, it is generally considered that, before undergoing reaction, the hydrocarbon is adsorbed at the metal surface. However, compared with monoolefins, the adsorption of alkynes and alkadienes has been little studied. 

Almost no studies of the adsorption of diolefins and higher alkynes... 

This postulate has several implications regarding the mechanism of alkyne hydrogenation these will be discussed in Sect. 4.3. It should be noted, however, that there is as yet little or no direct evidence for structure L, although analogous structures are known to exist with organometallic complexes [161], Such a structure is also consistent with the positive surface potentials observed for acetylene adsorption on evaporated nickel films [88]. 

Early attempts to establish the existence or otherwise of a geometric factor were based upon the assumption that the surfaces of metal particles consisted of extensive arrays of atoms arranged in well-defined low index planes the optimum metal—metal distances for the strain-free adsorption of the reactant hydrocarbon were calculated [84,157]. As noted in Sect. 4.2 (p. 50) such an approach led to the conclusion that only certain crystal planes should be active in alkene and alkyne hydrogenation... 

Moreover, contrary to alkyne hydration where no adsorption of the carbonyl compound was detected, the problem is complicated here by the saturation of the strong acidic sites by the formed amide, the concentration of which shows a rapid stabilization against time (Fig.3). Consequently the reaction selectivity greatly depends on the ester percentage. The behaviour of the amide itself over the studied zeolites confirms this observation the conversion of the amide into ester goes faster on the HY2 g zeolite than on the Hg and on the HMg zeolites. This later point, together with the comprehension of the different mechanisms in relation with the zeolite properties, will be discussed in a further paper. 

In this article (Part I) we have comprehensively reviewed the structural implications of the vibrational spectroscopic results from the adsorption of ethene and the higher alkenes on different metal surfaces. Alkenes were chosen for first review because the spectra of their adsorbed species have been investigated in most detail. It was to be expected that principles elucidated during their analysis would be applicable elsewhere. The emphasis has been on an exploration of the structures of the temperature-dependent chemisorbed species on different metal surfaces. Particular attention has been directed to the spectra obtained on finely divided (oxide-supported) metal catalysts as these have not been the subject of review for a long time. An opportunity has, however, also been taken to update an earlier review of the single-crystal results from adsorbed hydrocarbons by one of us (N.S.) (7 7). Similar reviews of the fewer spectra from other families of adsorbed hydrocarbons, i.e., the alkynes, the alkanes (acyclic and cyclic), and aromatic hydrocarbons, will be presented in Part II. 

Smit and co-workers reported one of the first breakthroughs in improving the reaction by employing dry state adsorption conditions (DSAC) in an intramolecular variant of the reaction.9 It was demonstrated that by adsorbing the cobalt-alkyne complex onto silica-gel, the reaction could be performed in the absence of solvent and at lower reaction temperatures (and times) to afford cyclopentenone products (6->7 - Scheme 3). 

It was shown that with a Pd/C catalyst in the liquid phase terminal triple bonds were saturated faster than internal ones, and both hydrogenated faster than terminal or internal double bonds in competitive processes (Eqn. 16.5). Further, alkene isomerization generally does not take place over palladium catalysts when alkynes are present. This selective hydrogenation depends on the stronger adsorption of an alkyne compared to an alkene. It is also possible that steric factors can influence the selectivity in the competitive semihydrogenation of an acetylene and an olefmic group in the same molecule. When the double bond and the triple bond are c/s to each other as in 7, selective adsorption of the acetylene... 

The nature of the solvent in liquid-phase alkyne hydrogenations and the extent to which it can influence the competitive adsorption factors needed to attain selectivity should also be considered. The semihydrogenation of 1-octyne over a series of Pd/Nylon-66 catalysts of varying metal load gave 1-octene with a selectivity of 100% over a wide range of metal loads when the reaction was run in heptane.38 n-propanol, however, reaction selectivity increased with decreasing metal load. Apparently the alcohol interacted with the catalyst to modify the active sites and influenced the relative adsorption characteristics of the acetylenic and olefinic species. This can affect reaction selectivity particularly if reactant diffusion assumes some importance in the reaction. 

The partial reduction of substrates containing triple bonds is of considerable importance not only in research, but also commercially for stereoselectively introducing (Z)-double bonds into molecular frameworks of perfumes, carotenoids, and many natural products. As with catalytic hydrogenation of alkenes, the two hydrogen atoms add syn from the catalyst to the triple bond. The high selectivity for alkene formation is due to the strong absorption of the alkyne on the surface of the catalyst, which displaces the alkene and blocks its re-adsorption. The two principal metals used as catalysts to accomplish semireduction of alkynes are palladium and nickel. 

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