Hydrocarbon structure sensitivity

Various catalytic reactions are known to be structure sensitive as proposed by Boudart and studied by many authors. Examples are the selective hydrogenation of polyunsaturated hydrocarbons, hydrogenolysis of paraffins, and ammonia or Fischer-Tropsch synthesis. Controlled surface reactions such as oxidation-reduction reactions ° or surface organometallic chemistry (SOMC) " are two suitable methods for the synthesis of mono- or bimetallic particles. However, for these techniques. 

With higher hydrocarbons the structure sensitivity is usually much less pronounced whereas Ir shows a clear sensitivity with ethane, with higher hydrocarbons the sensitivity is much less (235). Cyclopentane shows a pronounced antipathic correlation with D on Pt but an independence of D with Ir and Pd (231). With Rh the correlation of activity with D is antipathic for cyclopentane (236) but sympathetic for n-pentane. Thus, both a metal and the structure of the molecule are likely essential for the structure sensitivity to occur. 

Fluorination of normal hydrocarbons is not difficult with the La-Mar fluorination process however, fluorination studies also have been successful with structurally unusual hydrocarbon compounds (65-67a) (see Fig. 13). These studies were undertaken to establish that direct fluorination was useful even with some of the most sensitive hydrocarbon structures. While the initial studies of the successful direct fluorination of these species often resulted in yields as low as 10%, these same experiments, after additional technical developments in our laboratory, routinely give yields of 70—95%. Such syntheses have often been repeated in our laboratories to satisfy scientific needs for such compounds in other laboratories. The sterically crowded fluorocarbon compounds prepared in... 

Except for H2 oxidation and hydrocarbon hydrogenations, most reactions are remarkably structure-sensitive over supported Au catalysts. One typical reaction is CO oxidation, which is remarkably sensitive to the junction perimeter between Au particles and support, the type of support and the size of the Au particles. 

LEED Studies of Acetylene and Ethylene Adsorption on Rh(lll). Exposing the clean Rh(lll) surface between 230 and 250 K to either C2H2 or C2H4 results in the appearance of sharp half order diffraction spots in the LEED pattern from a (2x2) surface structure. The new diffraction spots from the ordered hydrocarbon structures are sensitive to surface coverage. Although the spots are visible after a 1 L gas exposure, they do not become sharp and intense until 1.5 L and then immediately begin disordering above 1.5 L. 

Reactions Exhibiting Strong Structure Sensitivity. Reactions for which there is at least an order of magnitude difference in activity as a function either of particle size or of exposed crystal planes include the ammonia synthesis reaction and the hydrogenolysis of hydrocarbons. 

At sufficiently high temperatures, due to not too strong cohesion, the surface Pd atoms may acquire convenient positions to form a bond with reacting hydrocarbon molecule (189). This concept, called extractive chemisorption, was introduced by Burwell et al. (190, 191) as a possible cause of absence of steric hindrance in adsorption and reaction of some complex organic molecules. It was proposed that in chemisorption one or two metal atoms were displaced above the initial planar level, leading to increased bonding to the surface for low-dispersion catalysts. An extension of this concept to the problem of structure sensitivity allows one to explain several cases of the relatively mild (or absent) structure sensitivity in many reactions catalyzed by Pd catalysts. 

The efficiency of the initiation effect of triplet oxygen depends on the hydrocarbon structure, i.e., on the strength of the attacked C —H bond. For instance, the ratio of the rate constants of the reaction of oxygen with formaldehyde and methane is 1.3 x 109 at 100 °C [8]. This indicates that intermediates of oxidation may be more sensitive towards oxidation than the original substrate which may contribute to the appearance of heterogeneous regions where the oxidation takes place preferably. 

The size of the crystallites affects the selectivity of reactions with larger molecules, e.g., hydrocarbon chains. Small islands of sites inhibit the adsorption of larger molecules, because all adsorption sites for the selected configuration of the larger molecule have to be vacant. As a general rule, catalytic reactions are structure-sensitive on the scale < 10 nm (Piccolo et al., 1999). 

Catalytic Activity of Nickel-Loaded Titanates. A good test of the dispersion of the active metal is the activity and selectivity for the hydrogenolysis of n-butane. For example, it is well known (6) that the hydrogenolysis of saturated hydrocarbons, or the rupture of carbon-carbon bonds by hydrogen, are structure sensitive that is, their rates per surface metal atom (TOF s) vary with the percentage of metal exposed on the catalyst (i.e.. the dispersion). Typically,... 

This concept of asphaltenes is useful in the interpretation of the present data, and conversely, the data support the concept. First, 13C NMR data show that the saturated hydrocarbon structure, which constitutes the majority of the carbon in the fractions, is virtually identical between the asphaltenes and the maltenes, within the limited sensitivity of 13C NMR. This factor is consistent with the argument that there is a partitioning between fractions and that the appearance of a particular species predominantly in the asphaltene fraction results because of a relatively higher aromaticity or the presence of polar heteroatoms for a specified molecular weight. It is important to recognize, from a processing standpoint, that only a minor weight percent of the fraction (or molecule) may be responsible for its classification as an asphaltene. 

Acidic forms of zeolites are well suited as supports for metal functions which are employed for hydrogenation, since they can also withstand the presence of traces of sulfur compounds frequently found in feedstocks of petrochemical industry. It should be noted, however, that hydrogenation is a structure insensitive reaction so it will primarily depend upon the concentration of the accessible metal particles and the adsorption constant of the unsaturated hydrocarbon. This may offer an explanation as to why Pt catalysts, for example, are still active for hydrogenation, when their activity for dehydrocyclization or hydrogenolysis (i.e., for structure sensitive reactions) is completely lost (e.g., by poisoning). 

In short, catalytic oxidation of hydrocarbons is a structure-sensitive reaction, and its mechanism is strongly dependent on the type of catalyst and on process conditions. This means that the morphology of the active phase will affect the catalyst activity, and hence the preparation procedure will have a strong influence on catalyst performance. 

These results have profound effects for the selective catalytic dehydrogenation of cyclohexane to benzene, a prototypical hydrocarbon conversion reaction. On Pt(lll), the intermediates, cyclohexene and a species, have been identified and the rate constants for some of the sequential reaction steps measured [56]. Adsorption and reaction studies of cyclohexane [39], cyclohexene [44], 1,3-cyclo-hexadiene [48], and benzene [39] on the two Sn/Pt(lll) alloys provide a rational basis for understanding the role of Sn in promoting higher selectivity for this reaction. One example of structure sensitivity is shown in Fig. 2.7, in which a monolayer of cyclohexyl (C H ) was prepared by electron-induced dissociation (EID) of physi-orbed cyclohexane to overcome the completely reversible adsorption of cyclohexane... 

Three aspects of the performance of supported catalysts are also discussed in this Chapter. With the development of techniques, as outlined above, for the characterization of supported metal catalysts, it seems timely to survey studies of crystallite size effect/structure sensitivity with special reference to the possible intrusion of adventitious factors (Section 5). Recently there has been considerable interest in the existence of (chemical) metal-support interactions and their significance for chemisorption and catalytic activity/ selectivity (Section 6). Finally, supported bimetallic catalysts are discussed for various reactions not involving hydrocarbons (hydrocarbon reactions over alloys and bimetallic catalysts have already been reviewed in this Series with respect to both basic research and technical applications ). References to earlier reviews (including some on techniques) that complement material in this Chapter are given in the appropriate sections. It might be useful, however, to note here some topics not discussed that also form part of the vast subject of supported metal and bimetallic catalysts and for which recent reviews are available, viz, spillover, catalyst deactivation, the growth and... 

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