Hydrogenation heterogeneous reactions

The solvent used in catalytic hydrogenation is chosen for its ability to dissolve the alkene and is typically ethanol, hexane, or acetic acid. The metal catalysts are insoluble in these solvents (or, indeed, in any solvent). Two phases, the solution and the metal, aie present, and the reaction takes place at the interface between them. Reactions involving a substance in one phase with a different substance in a second phase aie called heterogeneous reactions. 

Heterogeneous reaction (Section 6.1) A reaction involving two or more substances present in different phases. Hydrogenation of alkenes is a heterogeneous reaction that takes place on the surface of an insoluble metal catalyst. 

The reactants in FTS are carbon monoxide and hydrogen. The reaction may be considered a hydrogenative oligomerization of carbon monoxide in presence of a heterogeneous catalyst. 

By heterogeneous reaction of methyl cinnamate, the C=C double bond is selectively hydrogenated [26] and methyl 3-phenylpropionate is formed the ester function is not affected. 

In conclusion, we note that the appearance of hydrogen atoms in the gas volume in catalytic reaction of dehydration of alcohol at low pressures observed in [25] by the sensor technique confirms that dehydration of alcohol on the surface of the zinc oxide catalyzer yields hydrogen atoms. In other words, this heterogeneous reaction does not result in production of hydrogen molecules through the process... 

Temperature according to thermodynamics prediction, nickel oxide is reducible by hydrogen at all practical temperatures. However, this heterogeneous reaction,... 

The hydrogen-deuterium exchange H2 + D2 — 2HD is the simplest heterogeneous reaction taking place on the surface of semiconductors. This reaction has been thoroughly studied experimentally. It has been shown that under the influence of illumination (other conditions being constant) the rate of the reaction is considerably changed. 

The scope of hydrogen transfer reactions is not limited to ketones. Imines, carbon-carbon double and triple bonds have also been reduced in this way, although homogeneous and heterogeneous catalyzed reductions using molecular hydrogen are generally preferred for the latter compounds. 

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

Some gas—solid heterogeneous reactions follow a similar rate law. Under certain conditions, the rate of hydrogenation of ethene is described by the equation... 

Liquid phase hydrogenation catalyzed by Pd/C is a heterogeneous reaction occurring at the interface between the solid catalyst and the liquid. In our one-pot process, the hydrogenation was initiated after aldehyde A and the Schiff s base reached equilibrium conditions (A B). There are three catalytic reactions A => D, B => C, and C => E, that occur simultaneously on the catalyst surface. Selectivity and catalytic activity are influenced by the ability to transfer reactants to the active sites and the optimum hydrogen-to-reactant surface coverage. The Langmuir-Hinshelwood kinetic approach is coupled with the quasi-equilibrium and the two-step cycle concepts to model the reaction scheme (1,2,3). Both A and B are adsorbed initially on the surface of the catalyst. Expressions for the elementary surface reactions may be written as follows ... 

Horn, A. B., J. R. Sodeau, T. B. Roddis, and N. A. Williams, Mechanism of the Heterogeneous Reaction of Hydrogen Chloride with Chlorine Nitrate and Hypochlorous Acid on Water Ice, J. Phys. Chem. A, 102, 6107-6120(1998). 

It is a known fact that in many heterogeneous reactions propylene is more reactive than ethylene. The allylic hydrogen is labile and the dissociative (in C—H) adsorption in the allylic position is promoted by that. Possibly, the higher olefins can thus be adsorbed by two dissociative adsorptions through the vinylic or the allylic position 186). 

The procedure for getting the polymer-bound ligands is very easy to reproduce. Three jS-functionalized aromatic ketones were successfully reduced to the corresponding alcohols by heterogeneous asymmetric hydrogen transfer reaction with formic acid-triethylamine azeotrope as the hydrogen donor. One of the product alcohols (19c) is an intermediate for the synthesis of optically active fluoxetine. 

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