Ethene, hydrogenation mechanism

Criteria for differentiation of ethene hydrogenation mechanisms over metals and metal oxides... 

Soma s excellent infrared and kinetic study of ethene hydrogenation catalyzed by Pt/Al203 (423) showed clearly the dominant role played by the 77-adsorbed ethene species and by the reversibly adsorbed hydrogen that occurs at higher pressures in the form of on-top PtH. It also pointed to a Langmuir-Hinshelwood mechanism as the 77-adsorbed ethene was shown to compete with adsorbed H atoms for surface sites. 

In these case studies, in addition to a brief discussion of the catalytic applications, representative reactions are discussed with the aim of illustrating in detail the relationships between surface structures (as inferred from investigations with probe molecules) and catalytic activity. The following topics are discussed in detail (i) MgO as a model catalyst for base-catalyzed reactions (ii) the mechanism of ethene hydrogenation on ZnO (iii) Cu20 as an oxidation catalyst for the conversion of methanol to formaldehyde, with... 

Based on isotope labeling experiments (TPD, IRRAS) [36-39] it was concluded that the catalytic ethene hydrogenation reaction on surfaces proceeds as a step wise process of hydrogen incorporation. This step wise general mechanism is called the Horiuti-Polanyi mechanism [40] and is shown in Eqs.2.11-2.15 [41]. ... 

In summary, the mechanisms of the catalyzed ethene hydrogenation reaction (even on simple model surfaces) is far from being settled. A better control on the reaction, i.e. by means of a well defined catalyst might help to shed light on some of these... 

Causes of deactivation are basically three-fold chemical, mechanical or thermal— hereby six different routes of deactivation of catalyst material are described (some have been introduced before, without further explanation) poisoning (i.e. CO on Pt), fouling (i.e. coke formation during ethene hydrogenation on Pt), thermal degradation, vapor compound formation accompanied by transport, vapor-solid and/or solid-solid reactions, and attrition/crushing [162, 163]. 

Second, the well known chemisorption behavior of ethene is characterized with the same combination of EES and TPD on surfaces and serves as a future comparison for the study of the chemisorption behavior of ethene on size-selected Pt clusters by means of EES. The reactivity of ethene towards the hydrogenation reaction is probed by TPR and also further preliminary experiments (AES and IRRAS) are shown in order to investigate the mechanism of the ethene hydrogenation reaction on size-selected clusters. 

When NBS is used to brominate non-alkenyl substrates such as alkanes, another mechanism, involving abstraction of the hydrogen of the substrate by the succinimidyl radical " 14 can operate. " This mechanism is facilitated by solvents (such as CH2CI2, CHCI3, or MeCN) in which NBS is more soluble, and by the presence of small amounts of an alkene that lacks an allylic hydrogen (e.g., ethene). 

The first steps involve coordination and cycloaddition to the metal. Insertion of a third molecule of ethene leads to a more instable intermediate, a seven-membered ring, that eliminates the product, 1-hexene. This last reaction can be a (3-hydrogen elimination giving chromium hydride and alkene, followed by a reductive elimination. Alternatively, one alkyl anion can abstract a (3-hydrogen from the other alkyl-chromium bond, giving 1-hexene in one step. We prefer the latter pathway as this offers no possibilities to initiate a classic chain growth mechanism, as was also proposed for titanium [8]. The byproduct observed is a mixture of decenes ( ) and not octenes. The latter would be expected if one more molecule of ethene would insert into the metallocycloheptane intermediate. Decene is formed via insertion of the product hexene into the metallo-cyclopentane intermediate followed by elimination. 

Studies in deuterated water have shown that the hydroxyl proton does not end up in the ethanal formed. The decomposition of the 2-hydroxyethyl is not a simple P-elimination to palladium hydride and vinyl alcohol, which then isomerises to ethanal. Instead, the four protons stemming from ethene are all present in the initial ethanal product [6] (measured at 5 °C in order to suppress deuterium/hydrogen exchange in the product) and most authors have therefore accepted an intramolecular hydride shift as the key-step of the mechanism (see Figure 15.2). There remains some doubt as to how the hydride shift takes place. 

High-temperature flow-reactor studies [60,61] on benzene oxidation revealed a sequence of intermediates that followed the order phenol, cyclopentadiene, vinyl acetylene, butadiene, ethene, and acetylene. Since the sampling techniques used in these experiments could not distinguish unstable species, the intermediates could have been radicals that reacted to form a stable compound, most likely by hydrogen addition in the sampling probe. The relative time order of the maximum concentrations, while not the only criterion for establishing a mechanism, has been helpful in the modeling of many oxidation systems [4,13]. 

It is seen that Horiuti and Polanyi s 1934 mechanism for hydrogenation (426) has stood up very well to detailed spectroscopic scrutiny, with the proviso that the reactive ethene surface species is to be identified as of a n rather than of a di-a type. This mechanism is now as follows ... 

Ethene does not polymerize by the cationic mechanism because it does not have sufficiently effective electron-donating groups to permit easy formation of the intermediate growing-chain cation. 2-Methylpropene has electron-donating alkyl groups and polymerizes much more easily than ethene by this type of mechanism. The usual catalysts for cationic polymerization of 2-methylpropene are sulfuric acid, hydrogen fluoride, or a complex of boron... 

Methylpropene reacts with ethene and hydrogen chloride under polar conditions to yield 1-chloro-3,3-dimethylbutane. Show a mechanism for this reaction that is consistent with the reactants, conditions, and product. Give your reasoning. 

---