Unsaturated hydrocarbons, surface structure

The fact that surface structure, in particular steps and coordinatively unsaturated sites, has an influence on the state and reactivity of carbon monoxide is entirely in keeping with the empirical correlation (Fig. 6) between heat of adsorption, electron binding energies, and molecular state. Elegant studies by Mason, Somorjai, and their colleagues (32, 33) have established that with Pt(lll) surfaces, dissociation occurs at the step sites only, and once these are filled carbon monoxide is adsorbed molecularly (Fig. 7). The implications of the facile dissociation of carbon monoxide by such metals as iron, molybdenum, and tungsten for the conversion of carbon monoxide into hydrocarbons (the Fischer-Tropsch process) have been emphasized and discussed by a number of people (32,34). 

The electronic structure of the surface chemical bond is discussed in depth in the present chapter for a number of example systems taken from the five categories of bonding types (i) atomic radical, (ii) diatomics with unsaturated -systems (Blyholder model), (iii) unsaturated hydrocarbons (Dewar-Chatt-Duncanson model), (iv) lone pair interactions, and (v) saturated hydrocarbons (physisorption). 

The principles described above may serve as a useful guide for unsaturated hydrocarbons, but substituents other than alkyl groups may involve attractive interactions with a catalytic site. A compound in which a phenyl group is attached to a double bond in a five- or six-membered cycloalkene and is also vicinal to an alkyl substituent yields a larger fraction of the cis stereoisomer than is formed from the related dialkylcycloalkene. Cis isomers are formed preferentially on Pd and Rh, and are believed to involve structures in which the phenyl group is bound to the surface through its ir-electrons. ... 

Avery, N. R., and N. Sheppard. The use of thermal desorption and electron energy loss spectroscopy for the determination of structures of unsaturated hydrocarbons chemisorbed on metal single-crystal surfaces I. Alk-l-enes on Pt(lll), Proc. Roy. Soc. London A, 405. 1-25 (1986). 

Unsaturated hydrocarbons can add to the deposit-forming cyclic structures with five or six C-atoms through Diels-Alder reactions. The successive dehydrogenations produce polycyclic aromatic structures. In the third reaction class, aromatic species add to macro-radicals of the surface. Note that these radicals are greatly stabilized by the aromatic resonance. Thus, the third reaction class is more important at lower temperatures, typically, in TLE, visbreaking and delayed coking units. 

For writing the mechanism of HDS (of thiophene) and HYD over the created sites, we took into account data from literature (5,18). It was assumed that the adsorption of hydrogen is rapid, so that the hydrogen atoms ne ed in HYD and HDS reactions are already on the metal sites where thiophene or unsaturated hydrocarbons are sorbed or, alternatively, on the sulfur sites direcdy surrounding these metal sites. In addition, we took into consideration the experimental observations that active sites exist even in the absence of a remote control. This is due to the defect structure of the catalyst surface. These are "original" sites, for which no creation process is needed. 

ITQ-2, a novel zeolitic structure prepared by swelling and delaminating a MWW precursor, has been studied by IR spectroscopy. The same precursor yields, when calcined, the zeolite MCM-22. Bronsted acidity has been measured as the propensity either to engage in H-bonds or to transfer the proton to unsaturated hydrocarbons. Comparison with MCM-22 shows that dealumination accompanies the process of delamination, but no appreciable change in residual Bronsted acidity takes place. Reaction of propene with Bronsted sites to branched oligomers occurs mainly on the external surface. Oligomers show no tendency to evolve to allylic cationic species, in contrast with MCM-22. 

Stereochemistry, of unsaturated hydrocarbon hydrogenation, 16, 123 Structure, of cracking catalysts, 4, 87 Surface area measurements, for studying contact catalysts, 1, 66 Surface barrier effects, in adsorption, 7, 269... 

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... 

Koestner RJ, Frost JC, Stair PC, et al Evidence for the formation of stable alkyhdyne structures from C3 and C4 unsaturated hydrocarbons adsorbed on the pt(lll) single crystal surface. Surf Sci 116(1) 85-103, 1982. 

When fuel contains heavier hydrocarbons than methane, or it is biofuel, or contains alcohols, the feedstock often contains additional compounds such as sulphur and phosphorus, that is, fertiliser impurities. In the petrochemical industry, gas-borne reactive spedes (i.e., sulphur, arsenic, chlorine, mercury, zinc, etc.) or unsaturated hydrocarbons (i.e., acetylene, ethylene, propylene and butylene) may act as contaminating agents (Deshmukh et al, 2007). These impurities cause catalyst deactivation by poisoning. The effect of a poison on an active surface is seen as site blockage or atomic surface structure transformation (Babita et a/., 2011). Therefore, it is important to choose poisoning-resistant catalyst materials. For example, nickel is not the most effective MSR catalyst although it is widely used in industry due to its low market price compared to ruthenium and rhodium. Both Ru and Rh are more effective in MSR and less carbon is formed in these systems, than in the case of Ni. However, due to the cost and availability of precious metals, these are not widely used in industrial applications. 

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