Hydrocarbons conversion reaction

The presence of an electron donor causes the equiHbrium to shift to the left. The acidity represented by this mechanism is important in hydrocarbon conversion reactions. Acidity may also be introduced in certain high siHca zeoHtes, eg, mordenite, by hydrogen-ion exchange, or by hydrolysis of a zeoHte containing multivalent cations during dehydration, eg,... 

Thus it is clear that both metal and acid sites can be poisoned and deactivated rather quickly in hydrocarbon conversion reactions and at very mild experimental conditions. 

Zeolites and other oxide-based solid acid catalysts are used in hydrocarbon conversion reactions with enormous economic impact. Their lifetime is, however, very... 

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. 

Protonic zeolites find industrial applications as acid catalysts in several hydrocarbon conversion reactions. The excellent activity of these materials is due to two main properties a strong Bronsted acidity of bridging Si—(OH)-Al sites (Scheme 3.4, right) generated by the presence of aluminum inside the silicate framework and shape selectivity effects due to the molecular sieving properties associated with the well defined crystal pore sizes, where at least some of the catalytically active sites are located. 

Catalytic Chemistry of Hydrocarbon Conversion Reactions on Metallic Single Crystals... 

Acetylene is a reactive molecnle with a low C H stoichiometry that can be used to evaluate the resistance of metal-based catalysts to the formation of carbonaceous residue (coking). Pt is very reactive, and the chemisorption of on Pt(lll) is irreversible under UHV conditions, with complete conversion of into surface carbon during heating in TPD. Alloying with Sn strongly reduces the amount of carbon formed during heating [49]. This is consistent with observations of increased lifetimes for commercial, supported Pt-Sn bimetallic catalysts compared to Pt catalysts used for hydrocarbon conversion reactions. 

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... 

Bimetallic Pt-Sn catalysts are useful commercially, e.g., for hydrocarbon conversion reactions. In many catalysts, Pt-Sn alloys are formed and play an important role in the catalysis. This is particularly true in recent reports of highly selective oxidative dehydrogenation of alkanes [37]. In addition, Pt-Sn alloys have been investigated as electrocatalysts for fuel cells and may have applications as gas sensors. Characterization of the composition and geometric structure of single-crystal Pt-Sn alloy surfaces is important for developing improved correlations of structure with activity and/or selectivity of Pt-Sn catalysts and electrocatalysts. 

Solid acid catalysts play an important role in hydrocarbon conversion reactions in the chemical and petroleum industries [1,2]. Many kinds of solid acids have been found their acidic properties on catalyst surfaces, their catalytic action, and the structure of acid sites have been elucidated for a long time, and those results have been reviewed by Arata [3]. The strong acidity of zirconia-supported sulfate has attracted much attention because of its ability to catalyze many reactions such as cracking, alkylation, and isomerization. Sulfated zirconia incorporating Fe and Mn has been shown to be highly active for butane isomerization, catalyzing the reaction even at room temperature [4]. 

Zeolite Catalysts. During the past 3 years, interest in zeolite catalysts has intensified greatly. Zeolites X, Y, and mordenite have been the center of most of the attention, with some recent interest in synthetic zeolites related to erionite and offiretite. Controversy still exists concerning the source of activity and the nature of the active sites in zeolites, such as the rare earth exchanged forms of zeolite Y for hydrocarbon conversion reactions. This is reviewed by Rabo (Vol. II, p. 284). At present, opinion seems to converge on the compromise that both the so-called Bronsted... 

Isomerization, carbon-carbon bond scission (cracking), and carbon-carbon bond formation (alkylation) are among the most imp>ortant hydrocarbon conversion reactions catalyzed by acids. Zeolites are often used to carry out these reactions during the refining of petroleum. Some of the zeolites are particularly active to convert olefins and cycloparaffins to paraffins and aromatics to produce jet fuel and gasoline. 

Structure Sensitivity of Hydrocarbon Conversion Reactions on Platinum Surfaces How does the reaction rate depend on the atomic structure of the platinum catalyst surface To answer this question, reaction rate studies using flat, stepped, and kinked single-crystal surfaces with variable surface structure were very useful indeed. For the important aromatization reactions of n-hexane to benzene and Ai-heptane to toluene, it was discovered that the hexagonal platinum surface where each surface atom is surrounded by six nearest neighbors is three to seven times more active than the platinum surface with the square unit cell [155, 156]. Aromatization reaction rates increase further on stepped and kinked platinum surfaces. Maximum aromatization activity is achieved on stepped surfaces with terraces about five atoms wide with hexagonal orientation, as indicated by reaction rate studies over more than 10 different crystal surfaces with varied terrace orientation and step and kink concentrations (Figure 7.38). 

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