Selectivity hydrogenation, competitive

The mechanism for the hydrogenation probably involves Rh-jt interaction with the unsaturated systems. The selectivity of competitive hydrogenation experiments has established the relative order of the interaction forces [3] ... 

A particularly clear example is shown in Fig. 8, the hydrogenation of 4-ler<-butylmethylenecyclohexane (64). In this instance, the most likely initial product of isomerization is the endocyclic isomer, 4-leri-butyl-l-methylcyclohexene, which would yield mainly trans-4-tert-butyl-l-methylcyclohexane. Further, the exocyclic olefin, the original substrate, is reduced selectively in competition with its endocyclic isomer so that the latter cannot he an intermediate in the hydrogenation of the former. 

Selective hydrogenation of dienes or acetylene caused by competitive adsorption of acetylene or dienes with olefins. 

Supported metal catalysts with molecular sieving properties75,76 are able to differentiate between alkenes with structures of differing steric demands in competitive hydrogenation. After an early report on the selective hydrogenation of propylene in the presence of isobutylene over a Pt-zeolite A catalyst77 new examples have recently been published78-84. 

The problem of selective hydrogenation of one double bond of a nonconjugated diene is essentially the same as that of competitive hydrogenation of alkene mixtures (see Section IV.A. 1). The less substituted double bond of the diene, consequently, can preferentially be hydrogenated under appropriate reaction conditions. The use of nonisomerizing catalysts, in general, is recommended to achieve such regioselective reaction. 

It was shown that with a Pd/C catalyst in the liquid phase terminal triple bonds were saturated faster than internal ones, and both hydrogenated faster than terminal or internal double bonds in competitive processes (Eqn. 16.5). Further, alkene isomerization generally does not take place over palladium catalysts when alkynes are present. This selective hydrogenation depends on the stronger adsorption of an alkyne compared to an alkene. It is also possible that steric factors can influence the selectivity in the competitive semihydrogenation of an acetylene and an olefmic group in the same molecule. When the double bond and the triple bond are c/s to each other as in 7, selective adsorption of the acetylene... 

As discussed in Chapter 13 the competitive hydrogenation of ketones and alkenes over ruthenium is influenced by the solvent. There is virtually no selectivity observed on hydrogenation of a mixture of cyclohexene and cyclohexanone over Ru/Si02 in water. However, when the solvent was cyclohexane, containing a small amount of water, selective hydrogenation of the ketone was observed. In this latter instance the water was attracted to the... 

Basically, the effect of composition of the reaction mixture on the hydrogenation process may be characterized by the reaction rates, rate constants, adsorption coefficients, or selectivities of competitive reactions. All these characteristics have been used in various studies. 

Other industrial processes that have taken advantage of the process intensification deriving from the introduction of reactive (catalytic) distillation are (i) production of high purity isobutene, for aromatic alkylation (ii) production of isopropyl alcohol by hydration of propylene (iii) selective production of ethylene glycol, which involves a great number of competitive reactions and (iv) selective desulfurization of fluid catalytic cracker gasoline fractions as well as various selective hydrogenations. Extraction distillation is also used for the production of anhydrous ethanol. 

Aromatic unsaturated aldehydes behave differently, since they are vinylogues of benzaldehydes. In cinnamaldehydes, reduction of the C=0 group and C=C bond are almost equally competitive. However, the selective hydrogenation to hydrocinnamalde-hydes or cynnamyl alcohols is controlled depending on the metal, on the support, on additives and on solvent. Hydrocinnamaldehydes form selectively over Pd catalysts modified with potassium salts of weak acids or FeSO/. Cobalt carbonyl catalysts under 0X0 conditions, or CojfCOlg in the presence of amines, efficiently (96.4% yield) catalyze selective reduction " ... 

The influence of reactant pressures upon selectivity can be easily explained by the above mechanism the changes of selectivity essentially concern the complementary formations of unsaturated alcohol and hydrocarbons which i the re uk of competitive 1,2 and 1,4 additions of hydrogen. An increase of the partial pressure of prenal on the initially saturated surface, is likely to induce a change of the adsorption configuration of the molecule in favor of a 1,2 di-o adsorption and consequently of a selective hydrogenation of the C=0 bond. 

At the reactant concentrations used, the rs remained virtually constant and no dependence on reactant concentration became observable. However concentration dependent changes in the adsorption mode were recently reported in selective hydrogenation of a, 3 unsaturated aldehyde [2]. It was supposed that the adsorption mode of cinnamaldehyde at high concentrations differs from that at lower concentrations, more precisely, cinnamaldehyde adsorbs perpendicular to the catalyst surface with the aromatic rings in parallel arrangements at high coverage. However, in om case the competitive adsorption of the modifier and the reactant complicates the situation. 

Poisoning can affect the selectivity as well as the rate of conversion, and mild poisoning may be beneficial. The oxidation of ethylene is carried out using silver catalysts that are deliberately poisoned with chlorine compounds, and the selectivity is improved, because the total oxidation reaction is suppressed more than the rate of ethylene oxide formation [14]. The presence of sulfur compounds changes the selectivity for competitive hydrogenation, such as the hydrogenation of acetylenes or diolefins in the olefins [15]. 

These. supramolecular catalysts showed high substrate selectivity in competition hydrogenation experiments and exceptional activity in the hydroformylation reactions. In contrast to the simple methylated P-cyclodextrin previously mentioned, even internal and cyclic olefins were converted into aldehydes. Such improvements were explained with the formation of an inclusion complex at the phase boundary, with the cylodextrin host fixing the substrate in the proximity of the catalytically active metal center (Fig. 

What is the selectivity for competitive hydrogenation of mixtures of ortho and paraxylene ... 

In the paper presented here, we will focus our interest on irreversible bimolecular consecutive reactions, where the substrate and interaediate product are in competition for the same reactant. As an example the selective hydrogenation of o-alkylphenol on a palladium catalyst in a stirred tank slurry reactor was investigated. To properly design such a system, the chemical reaction steps and prior physical steps, such as diffusion and sorption, must be considered. The rigorous description of a multiphase... 

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