Hydrogenation multiphase catalysis

As with classical multiphase catalysis, the organometallic catalyst is retained here in a liquid phase that is immiscible with the second phase containing substrates and/or products. For hydrogenation, the liquid/SCF system is always biphasic, whereas conventional systems are usually triphasic (liquid-1 /liquid-2/ H2). The liquid phase must provide a stable environment for the organometallic catalyst and should be insoluble in the SCF phase. Water, ILs and PEG have been used successfully for this purpose, together with scC02 as the mobile phase. Again, the products must not be too polar in order to be effectively extracted if C02 is used as the SCF. 

The use of ionic liquids has been successfully studied in many transition metal-catalyzed hydrogenation reactions, ranging from simple alkene hydrogenation to asymmetric examples. To date, almost all applications have included procedures of multiphase catalysis with the transition-metal complex being immobilized in the ionic liquid by its ionic nature or by means of an ionic (or highly polar) ligand. 

The investigations done by Claus and coworkers as part of the project Smart Solvents/Smart Ligands focussed on the selective hydrogenation of Q ,/3-unsaturated carbonyl compounds and showed the potential of aqueous multiphasic catalysis for the production of chemicals for fine chemistry, e.g., fragrance materials. 

Keywords Aqueous multiphase catalysis Regioselective hydrogenation Hydrodynamics Mass transport Kinetic modelling... 

The use of iortic hquids has been successfully studied in many transition metal ca lyzed hydrogenation reactions ranging from simple olefin hydrogenation to examples of asymmetric hydrogenation. Almost all apphcations so far include procedures of multiphase catalysis with the transition me l complex being... 

This chemical and physico-chemical behavior of the binary H2O-CO2 mixture [38] suggests that water is an attractive liquid to be combined with supercritical carbon dioxide in multiphase catalysis. CO2/H2O systems have adequate mass-transfer properties, especially if emulsions or micro-emulsions can be formed ([39] and refs, therein). The low pH of aqueous phases in the presence of compressed CO2 (pH ca. 3-3.5 [40]) must be considered and the use of buffered solutions can be beneficial in the design of suitable catalytic systems, as demonstrated for colloid-catalyzed arene hydrogenation in water-scC02 [41]. 

Onal, Y., Lucas, M., and Claus, P. (2005) Application of a capillary microreactor for selective hydrogenation of a,C-unsaturated aldehydes in aqueous multiphase catalysis. Chem. Eng. Technol, 2 (9), 972-978. 

Figure 5.45 reproduced from Centi, G., Dittmeyer, R., Perathoner, S. and Reif, M. Tubular inorganic catalytic membrane reactors advantages and performance in multiphase hydrogenation reactions. Catalysis Today, Vol. 79-80, pp. 139-149,... 

Multiphase homogeneous catalysis (continued) hydroformylation, 42 483-487, 498 hydrogenations, 42 488-491 metal salts as catalysis, 42 482-487 neutral ligands, 42 481 82 organic reactions, 42 495 0X0 synthesis, 42 483-487 ring-opening metathesis polymerization and isomerization, 42 492-494 telomerizations, 42 491-492 diols as catalyst phase, 42 496 fluorinated compounds as catalyst phase, 42 497... 

We focus on heterogeneous catalysis with single and multiple reactant phases, as these are the most common in practice. Examples include environmental catalysis, fat hardening, hydrodesulfurization of oil streams, hydrogenation of fine chemicals, and selective conversions catalyzed by immobilized enzymes or cells in biotechnology. The most popular reactors used in industry for multiphase applications are slurry bubble columns and trickle-bed reactors. They are shovm in Figure 1. 

The supercritical fluids exhibit gas-like viscosities, diffusivities, and liquid-like densities. These favorable transport properties lead to enhanced mass transfer, permeation, and wetting characteristics. The mass transfer limited multiphase reactions will benefit from reduction of a number of phases, as in the case of most oxidation, hydrogenation, or replacement of the more viscous liquid phase with a supercritical or a less viscous expanded liquid phase. The mobility combined with tunability results in effective maintenance of catalyst activity in heterogeneous catalysis. 

WangX, Nie Y, Lee JLC, Jaenicke S. Evaluation of multiphase microreactors for the direct formation of hydrogen peroxide. Applied Catalysis A General 2007 317 258-265. 

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