Unsaturated hydrocarbons surface complexes

Therefore, the measurement of the relative reactivities in separate and in competitive experiments will permit the evaluation of either K jK or KiK IK K depending upon whether the principal surface species are the TT-complexed multiply unsaturated hydrocarbons or the respective half-hydrogenated states. If the former situation exists, the evaluated ratios might be expected to correlate with the association constants of the hydrocarbons with silver ion (78), but not if the main surface species are the half-hydrogenated states. Apparently, it is the latter condition which prevails. 

One may inquire whether the evidence that 77-allyl complexes yield desorbed olefins when formed from dienes and hydrogen, or from alkenes, is pertinent to the question concerning the course of the exchange of such complexes formed by the adsorption of saturated hydrocarbons. The composition of the surface must be different under the two circumstances in one there must be few sites not occupied by olefin or half-hydrogenated intermediates, while in the other (the exchange of saturated hydrocarbons) many sites must be vacant. Consequently, in the absence of an excess of any unsaturated hydrocarbon, there is no driving force for the desorption (or displacement) of the unsaturated intermediates which are formed on the surface and intermediates of any degree of unsaturation remain bonded to the surface and leave it only as saturated hydrocarbon. Yet the evidence obtained from the reactions of the unsaturated hydrocarbons must indicate the paths which may be traversed under either circumstance. 

Bond [294] used comparisons between homogeneously and heterogeneously catalysed interconversions of unsaturated hydrocarbons to deduce that the reactive state of an adsorbed hydrocarbon may reasonably be assumed to be a jr-complex (see Sect. 3.2, p. 22). On this assumption, a molecular orbital model appropriate to a face-centred cubic metal was developed. By considering the direction of emergence and degree of occupation of the metal atomic orbitals at the (100), (110) and (111) faces, assuming that the atomic orbitals on the surface keep the same orientation as in the bulk metal, which may not be valid [295], he concluded that the (111) planes were least suited to the adsorption requirements of... 

The hundreds of different cuticular hydrocarbon components reported on insects can be divided into three major classes, n-alkanes, methyl-branched components and unsaturated hydrocarbons. There are reports of methyl-branched alkenes (see below), but these are rare. The hydrocarbon components on the surface of insects are usually complex mixtures comprised of anywhere from a few to up to hundreds of different components in some species. 

Here hydrocarbon conversion reactions occur wholly or at least partly on the carbonaceous overlayer on the metal and oxide surfaces, as reported by others (13,15-20). Poly-condensed EDA complexes may behave as giant alkenes in which by reversible catalytic hydrogenation/dehydrogenation occurs. This mechanism is similar to the intermolecular hydrogen transfer mechanism proposed (IS) for hydrogenation of unsaturated hydrocarbons. 

Many of the significant reactions of unsaturated hydrocarbons (hydrogenation, isomerization, carbonylation, oxidation, polymerization) are catalyzed heterogeneously by metals in or near Group VIII or homogeneously by salts and complexes of these elements. Those reactions effected in both systems are discussed in terms of probable common intermediates anomalies, where they occur, are ascribed either to the ability of surfaces to form intermediate species which cannot be stabilized by single metal atoms or to the ability of the latter to coordinate simultaneously more than one hydrocarbon molecule. 

The hydrogenation of unsaturated hydrocarbons continues to attract attention by reason both of the practical importance and of the theoretical interest of this system. This is particularly true for the selective hydrogenation of alkadienes and alkynes in alkenes rich cuts [74]. Fundamental studies [75] have shown that Pd is the most active and selective metal for these reactions. Some reasons for the better performances exhibited by Pd-based catalysts can be learned from the mechanism and the surface complexes involved in these transformations. 

The reactions occurring at the sea surface and in the euphotic zone, photo-oxidation, oxidation processes and association of some hydrocarbons with orgemic complexes such as humic or fulvic acids (Khsm and Schnitzer, 1972 Gagosian and Stuermer, 1977), tend to reduce the concentration of more labile compounds, especially the unsaturated hydrocarbons. Oxidation processes lead to the formation of alcohols, acids, alkyl and arylethers, carbonyl compounds and sulfoxides (Kawahara, 1969 Hansen, 1977). 

The presence of hydroxyl groups imparts a degree of polarity to the surface so that molecules such as water, alcohols, phenols, and amines (which can form hydrogen bonds) and unsaturated hydrocarbons (which can form ir-complexes) are adsorbed in preference to nonpolar molecules such as saturated hydrocarbons. Because of its selectivity for aromatics silica gel was used as the adsorbent in the Arosorb process for separation of aromatics from paraffins and naphthenes but by far the most important current application is as a desiccant. 

Transition States in Acid-Cataiyzed Reactions on Zeolites. A considerable number of experimental attempts were carried out to elucidate the structure of intermediates in the acid-catalyzed reactions. This information may be used to deduce the nature of elusive transition states, which are present on the catalytic surface in concentrations far below detection limit. For example, adsorption of alcohols and unsaturated hydrocarbons on acidic zeolites gives rise to the MAS NMR signal attributed to alkoxy species (135). Transition of these surface complexes to the corresponding carbenium ions, an important step in their further transformations, was shown to have low activation energy. In the synthesis of methyl-tetrabuthyl ether on zeolite Beta, features due to secondary and tertiary carbon atoms in alkoxy species have been observed. This observation along... 

Palladium-fullerene complexes exhibit catalytic activity with respect to hydrogenation of unsaturated hydrocarbons [55, 63], It has been suggested that there are two types of metal atoms in Pd C6o, and that palladium atoms bridging neighboring fullerene units are catalytically inactive [55, 63]. The palladium particles responsible for the catalytic activity are believed to be excessive metal atoms adsorbed on the surface of Ceo [55, 63], A clear picture of how multiple metal atoms arrange on the fullerene surface is, however, not available, and it is not known whether the excessive metal atoms adsorb individually or form metal clusters on the fullerene surface. 

---