Active sites alkene isomerization

A Stern-Volmer plot obtained in the presence of donors for the stilbene isomerization has both curved and linear components. Two minimal mechanistic schemes were proposed to explain this unforeseen complexity they differ as to whether the adsorption of the quencher on the surface competes with that of the reactant or whether each species has a preferred site and is adsorbed independently. In either mechanism, quenching of a surface adsorbed radical cation by a quencher in solution is required In an analogous study on ZnS with simple alkenes, high turnover numbers were observed at active sites where trapped holes derived from surface states (sulfur radicals from zinc vacancies or interstitial sulfur) play a decisive role... 

Hydrogenation of alkenes on ZnO was studied by means of IR spectroscopy17. The interaction of -adsorbed ethylene and ZnH species was concluded to yield adsorbed ethyl, which reacts with ZnOH to form the product ethane. Higher alkenes adsorb as allylic species. Active sites for hydrogenation and those for exchange and isomerization are independent of ZnO. As a result, the main product in the deuteration of alkenes is the d2 isotopomer. 

The discussion to this point has emphasized kinetics of catalytic reactions on a uniform surface where only one type of active site participates in the reaction. Bifunctional catalysts operate by utilizing two different types of catalytic sites on the same solid. For example, hydrocarbon reforming reactions that are used to upgrade motor fuels are catalyzed by platinum particles supported on acidified alumina. Extensive research revealed that the metallic function of Pt/Al203 catalyzes hydrogenation/dehydrogenation of hydrocarbons, whereas the acidic function of the support facilitates skeletal isomerization of alkenes. The isomerization of n-pentane (N) to isopentane (I) is used to illustrate the kinetic sequence associated with a bifunctional Pt/Al203 catalyst ... 

The corresponding reaction path for the hydrogenation of an alkene on an adatom, 14, is shown in Scheme 4.3. Here the 3mH2 site, 15, can adsorb the alkene to give the adsorbed species, 16 or 17, which are then saturated using the two hydrogen atoms on the active site. The adsorbed 7t-allyl, 20, may also take part with a palladium catalyst. The formation of this species provides the pathway for the facile double bond isomerization observed over palladium catalysts. ... 

In conclusion, partially dehydroxylated oxide surfaces exhibit a large inventory of surface OH groups and water molecules together with Lewis acidic and Lewis basic sites with coordinative unsaturation (structures II and III of Scheme 1). The hydroxyl population is the souree of protons that cause enhanced surface electrical conductivity and catalytic activity. It is significant that the increase in the conductivity value is paralleled by increases in either the amount of weakly bound protons or their mobility [48]. Almost all metal oxides are active in catalytic isomerization of alkenes, which is one of the least demanding reactions in terms of the requirements for the acid strength of active sites [34]. Studies on several oxide systems show that the activity is lost after extensive dehydration and is partially restored by... 

Alkali Metals Supported on Metal Oxides. Alkali metals loaded on supports by deposition of the metal vapor have been reported as highly active catalysts for the isomerization of alkenes and the related compounds (31). For example, sodium metal deposited on alumina (Na/Al203) isomerizes 1-butene and 1-pentene at room temperature (31). Sodium metal deposited on MgO (Na/MgO) showed a high-catalytic activity for the isomerization of alkenes at 293 K and gave the basic sites stronger than = 35 (32). 

The activity of OH groups on the alumina surface can be markedly enhanced by the proximity of Cl ions. In the commercial catalysts, 7-alumina is treated with HCl to make it a highly active catalyst. 7-Alumina cannot readily catalyze the skeletal isomerization of alkenes because of its weak acidity. On the other hand, chlorinated alumina is highly active for skeletal isomerization and other strong-acid catalyzed reactions which are desirable in reforming. The strength of acid sites can be controlled by the extent of chlorination. If the Cl content is too low, the reactions which occur on the acid centers slow down and the octane number of reformate drops. If excess Cl ion is present, the extent of hydrocracking increases relative to dehydrocyclization. 

It has been suggested [21,22] that the presence of Cu and K increases the rates and extent of Fe304 carburization during reaction and the FTS rates, by providing multiple nucleation sites that lead to the ultimate formation of smaller carbide crystallites with higher active surface area. In the present investigation, Cu- and K-promoted iron catalysts performed better than the unpromoted catalysts in terms of (1) a lower CH4 selectivity, (2) higher C5+ and alkene product selectivi-ties, and (3) an enhanced isomerization rate of 1-alkene. 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

It is generally agreed that surface dehydration, typically at 600—800 K in vacuo, is a prerequisite for activity. There is still some debate as to whether ZnO must be non-stoicheiometric or not for activity to be observed, but it will become clear that it is now generally accepted that Zn-O pair sites are the active surface centres. As expected for a weak Bronsted acid, reactions of non-cyclic alkenes are limited to non-skeletal isomerizations even at 573 K isobutene does not isomerize to n-butenes. ... 

However, ammonia adsorption experiments in our laboratories ( 5) demonstrated an enhanced acidity after reduction (Figure 1). So a dual function mechanism (9, 10), in which metal sites are responsible for the (de)hydrogenation of (alkanes) alkenes and acid sites isomerize the alkenes via a carbocation mechanism, may also explain the high isomerization activity. 

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