Olefin hydrogenation, catalysis

The effects of added triphenylphosphine and changing temperature on ligand dissociation and equilibria were studied also. The above dimer was an active hydrogenation catalyst. The equilibrium concentration of the dimer and the rate of olefin hydrogenation catalysis by the system depend inversely on the concentration of excess phosphine ligand. 

Toshima [162, 164, 165] has studied a series of PVP protected colloidal bimetallic PtPd catalysts in olefin hydrogenation catalysis. The highest activity for the hydrogenation of 1,3-cyclooctadiene to cyclooctene was observed at a Pd Pt radio of 4 1. On the basis of EXAFS analysis [165] it was concluded that the distribution of the two metals in the particles is nonuniform, and that at the most active composition the colloid particles had a platinum core coated with palladium. A less active 1 1 Pd/Pt catalyst, with the same particle size, apparently had a more uniform distribution of metals, with both platinum and p ladium atoms at the surface. 

At least for ethylene hydrogenation, catalysis appears to be simpler over oxides than over metals. Even if we were to assume that Eqs. (1) and (2) told the whole story, this would be true. In these terms over oxides the hydrocarbon surface species in the addition of deuterium to ethylene would be limited to C2H4 and C2H4D, whereas over metals a multiplicity of species of the form CzH D and CsHs-jD, would be expected. Adsorption (18) and IR studies (19) reveal that even with ethylene alone, metals are complex. When a metal surface is exposed to ethylene, selfhydrogenation and dimerization occur. These are surface reactions, not catalysis in other words, the extent of these reactions is determined by the amount of surface available as a reactant. The over-all result is that a metal surface exposed to an olefin forms a variety of carbonaceous species of variable stoichiometry. The presence of this variety of relatively inert species confounds attempts to use physical techniques such as IR to char-... 

In catalysis, adsorbed CO may retard some reactions such as olefin hydrogenation, fuel cell conversion, and enantioselective hydrogenation. For instance, Lercher and coworkers observed the deactivation of Pt/Si02 in the liquid-phase hydrogenation of crotonaldehyde, and ascribed this deactivation to the decomposition of crotonaldehyde on platinum surface to adsorbed CO [138]. Blaser and coworkers found that the addition of a small amount of formic acid decreases the rate of liquid-phase hydrogenation of ethyl pyruvate on cinchonidine-modified Pt/Al203 catalyst, which they explained as the decomposition of formic acid on the catalyst to adsorbed CO. Interestingly, the addition of acetic acid does not decrease the reaction rate, but whether acetic acid decomposes on the catalyst as formic acid does was not mentioned [139]. 

Niobia-supported MTO has been prepared either by the deposition of sublimed MTO onto the support, or by the impregnation of the support by a solution of MTO, and has been well characterised [54]. A large variety of oxidation reactions were efficiently performed with niobia-supported MTO, such as olefin metathesis catalysis [53,54], reactions of ethyl diazoacetate, heteroatom oxidation (amine and phosphine oxidations) and olefin epoxidation with hydrogen peroxide [55] (Scheme 13). 

Hydrogenation Catalysis of Olefins by a Rhodium Hydride Complex of PhP(CH2 CH2 CH2 PPh2 )2... 

Ethylene, HF and HsO+ have been used as a model system in the ab initio closed-shell SCF calculation of the acid-catalysed hydrogenation of olefins. While catalysis by HF exhibits bifunctional character, catalysis by HsO+ proceeds via initial formation of a carbocation72. 

A particularly thoughtful computational study of chain transfer mechanisms in olefin polymerization catalysis suggests that two distinct pathways exist for (1-hydrogen of a hydrogen from the growing chain to a second olefin, the classical path involving aM-H interaction and a direct transfer in which the metal does not participate directly. Whether a catalyst will display one or the other mechanism is determined by subtle effects that are discussed in detail in the paper.91... 

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