Results of the last decade with respect to the selective hydrogenation of hydrocarbons with multiple unsaturation (dienes and alkynes) over heterogeneous palladium catalysts are reviewed. Factors such as metal dispersion, carbon deposits, and the use of promoters and additives controlling catalytic activities and chemo-, regio- and stereo-selectivity are discussed. A detailed treatment of the status of the selective removal of hydrocarbon impurities with multiple unsaturation from industrial feedstocks is also given.

Hydrogenation of carbon–carbon multiple bonds: chemo-, regio- and stereo-selectivity

Catalysis is a multidisciplinary science that serves a broad range of industries covering specialty, fine, intermediate, commodity and life science chemicals. Catalysts are commonly used for the hydrogenation of alkenes, alkynes, aromatics, aldehydes, ketones, esters, carboxylic acids, nitro groups, nitriles and imines. These materials may be in the form of bio-, homogeneous, heterogeneous and heterogenised homogeneous catalysts where each type has its own special properties that can be adjusted for their optimal use. Heterogeneous catalysts are commonly used in the form of powders for slurry and fluidized bed reactions or as formed bodies for fixed bed hydrogenations. The addition of promoters and adjustments in particle size and porosity allow these catalysts to be fine tuned for specific reactions. Homogeneous catalysts are also very flexible where the choice of active metal, ligands and reaction conditions can lead to highly selective hydrogenations. The separation problems associated with homogeneous catalysts have led to the development of heterogenised homogeneous catalysts via the fixation of the active complexes on organic or inorganic supports or via application in biphasic systems. While there have been some successes in this area, there still remains a considerable amount of work to be done. Biocatalysts are of particular interest for the formation of enantiopure compounds. New developments in cofactor regeneration and raised substrate tolerance help to implement enzymatic reduction reaction on industrial scale. Efficient reductases are still needed to broaden this technology for further applications. It is expected that the development of these catalysts will be accelerated through the use of high throughput screening and that the testing of catalyst libraries will allow for the rapid selection of both catalyst and conditions for targeted processes. Reactor technology will also change as continuous processes slowly replace the workhorse batch ones for the manufacture of large-scale chemicals. This paper provides an overview of the above-mentioned topics for the industrially relevant hydrogenations that have been brought on stream over the last 20 years, as well as, the upcoming processes that will set the future trends in chemical industry.

New developments in hydrogenation catalysis particularly in synthesis of fine and intermediate chemicals

Hardening of vegetable oils is reviewed from an engineering point of view. The present review focuses on kinetics of the hydrogenation and relevant transport and adsorption steps. It aims to contribute to accelerate new research to improve substantially on selectivities in general and a decrease of trans fatty acid content in particular. From a comprehensive literature review, we concluded the absence of reliable, mechanistically based kinetic rate expression. Moreover, transport limitations, both intraparticle and interfacial, cannot be excluded from the vast majority of available experimental data. Therefore, future research should focus on the development of intrinsic kinetic rate expressions, which may subsequently contribute to develop new and improved hydrogenation catalysts.

Heterogeneous hydrogenation of vegetable oils: A literature review

Revisiting the hydrogenation of sunflower oil over a Ni catalyst

Thermodynamic balance in reaction system of total vegetable oil hydrogenation

The partial hydrogenation of a rapeseed oil in the presence of a supported palladium catalyst was studied. In the temperature range 60–120°C, at which kinetic data were collected, we observed that diffusion limitations are always operative, in particular intraparticle diffusion limitations. The contribution of the mass transfer to the reaction rates was carefully evaluated by determining the mass transfer parameters according to independent routes. In spite of the masking effect of diffusion, kinetic laws suitable to describe the hydrogenation of trienic, dienic and monoenic compounds were identified. A Langmuir-Hinshelwood kinetic equation describes well the behaviour of the unsaturated molecules in the reaction, while the hydrogen reaction order was indirectly shown to be zero. A 3/2/1 ratio between the adsorption constants of trienic, dienic and monoenic compounds was assumed, corresponding to the different probability of a double bond interaction with the palladium surface. The kinetic constants obtained by fitting experimental data gave quite similar results for the polyunsaturated fatty acids and very different for the monounsaturated ones, revealing an intervention of two distinct reaction mechanisms. In the case of polyunsaturated fatty acids, double bond migration with the formation of conjugated dienes would occur before hydrogenation. Conjugated dienes are more quickly hydrogenated than isolated or single double bonds. In a second part of the paper, a more general kinetic model is developed based on a complex scheme of simultaneous and consecutive reactions where hydrogenation reactions occur together with cis-trans isomerizations. A kinetic equation with the corresponding parameters for the cis-trans isomerization of the C 18 monoenic species was determined from experimental data. Subsequently, this equation was used with the same parameters in the general kinetic model to describe the behaviour of all the other cis-trans isomerization reactions. This assumption has turned out to be reasonable. The reliability of the more general kinetic model, of the mass transfer and kinetic equations for hydrogenation processes, of the kinetic equation for cis-trans isomerizations, and of the parameters appearing in the model was verified on the basis of all the experimental data collected. A brief discussion on the factors influencing the catalyst selectivity concludes the paper.

P. Parrella

Papers

Role of mass transfer and kinetics in the hydrogenation of rapeseed oil on a supported palladium catalyst