Competitive, consecutive hydrogenation

Competitive consecutive reactions are combinations of parallel and series reactions that include processes such as multiple halogenation and nitration reactions. For example, when a nitrating mixture of HN03 and H2S04 acts on an aromatic compound like benzene, N02 groups substitute for hydrogen atoms in the ring to form mono-, di-, and tri-substituted nitro compounds. 

However, the sulfurization of the surface palladium has a detrimental effect on the consecutive hydrogenation (Fig. 17) the olefin yield is lower on sulfided palladium. Such a result is not in accordance with the literature cited above and this difference can be explained by differences in experimental conditions the improved consecutive selectivities were obtained with the sulfur present in the feedstock, i.e., in adsorption competition, when the detrimental effect on the consecutive hydrogenation was pointed out on partly sulfided palladium without sulfur compounds in the feedstock. The selectivity for olefin production in the hydrogenation of iso-prene is increased by the presence of sulfur in the substrate (Fig. 18). 

Industrially relevant consecutive-competitive reaction schemes on metal catalysts were considered hydrogenation of citral, xylose and lactose. The first case study is relevant for perfumery industry, while the latter ones are used for the production of sweeteners. The catalysts deactivate during the process. The yields of the desired products are steered by mass transfer conditions and the concentration fronts move inside the particles due to catalyst deactivation. The reaction-deactivation-diffusion model was solved and the model was used to predict the behaviours of semi-batch reactors. Depending on the hydrogen concentration level on the catalyst surface, the product distribution can be steered towards isomerization or hydrogenation products. The tool developed in this work can be used for simulation and optimization of stirred tanks in laboratory and industrial scale. 

The hydrogenation of acetopnenone wnich has an aromatic unsaturated ring and a caroonyl functional group involves a sequence of several competitive parallel and consecutive reactions. 

Amidine derivatives are effective dehalogenation inhibitors for the chemoselective hydrogenation of aromatic halonitro compounds with Raney nickel catalysts. The best modifiers are unsubstituted or N-alkyl substituted formamidine acetates and dicyandiamide which are able to prevent dehalogenation even of very sensitive substrates. Our results indicate that the dehalogenation occurs after the nitro group has been completely reduced i.e. as a consecutive reaction from the halogenated aniline. A possible explanation for these observations is the competitive adsorption between haloaniline, nitro compound, reaction intermediates and/or modifier. The measurement of the catalyst potential can be used to determine the endpoint of the desired nitro reduction very accurately. 

M. Bikrani, L. Fidalgo, M. A. Garralda, and C. Ubide [J. Mol. Catal., 118, 47-53 (1997)] studied the catalytic activity of cationic iridium complexes for homogeneous hydrogen transfer from isopropanol to 2-cyclohexen-l-one to give cyclohex-anol. The reaction network contains consecutive, competitive, and reversible elements. In isopropanol these elements can be expressed as pseudo first-order processes ... 

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... 

The extent of a reaction may alter naturally for instance, parallel reactions of a higher order do exist. In connection with consecutive-competitive reactions, in many cases, secondary reaction products that react further appear (D), such as in the case of chlorination of organic substances, where hydrogen chloride is always generated. Another example is the polyesterification of dicarboxylic acids, upon which water formation takes place. Some industrially relevant multiple reaction systems are shown in Figure 3.18. 

---