Multiphase liquid/supercritical

A summary of the research activities of the last four years reveals three different important trends (a) The design of new ionic ligands for excellent catalyst immobilisation in ionic liquids and high regioselectivity (b) the successful application of cheap, halogen-free ionic liquids in the biphasic Rh-catalysed hydroformylation (c) the successful development of unusual multiphasic reaction concepts for Rh-catalysed hydroformylation, namely catalysis in ionic liquid/supercritical C02 and SILP-catalysts. 

Gas-liquid reactions are one of the areas where the use of equations of state is particularly attractive. In these reacting systems equations of state can handle subcritical and supercritical components under a wide range of conditions. Multiphase and supercritical conditions can be described with a proper equation of state, going from the heterogeneous to the critical and homogeneous regions in a continuous way. Baiker et have stressed the importance of... 

Flows are typically considered compressible when the density varies by more than 5 to 10 percent. In practice compressible flows are normally limited to gases, supercritical fluids, and multiphase flows containing gases. Liquid flows are normally considerea incompressible, except for certain calculations involved in hydraulie transient analysis (see following) where compressibility effects are important even for nearly incompressible hquids with extremely small density variations. Textbooks on compressible gas flow include Shapiro Dynamics and Thermodynamics of Compre.ssible Fluid Flow, vol. 1 and 11, Ronald Press, New York [1953]) and Zucrow and Hofmann (G .s Dynamics, vol. 1 and 11, Wiley, New York [1976]). 

In a supercritical extraction process a solvent is contacted with a solute at conditions near a critical point of the solvent plus solute mixture. The mixture may exhibit multiphase behavior invoving vapor, liquid, and solid phases, depending on the mixture composition and temperature and pressure conditions. 

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. 

A pure compound may be in the state of a gas, sohd or liquid (or even multiphase) depending on the pressure and temperature, the interrelationship being shown in Figure 23.6. Following the vapour pressure curve which separates the gas and liquid states in the direction of increased pressure and temperature leads to an area in which the densities of both phases are identical. A phase that is neither gas nor liquid follows on from the critical point P (shaded area) and this is known as the fluid or supercritical... 

The development of SCF processes involves a consideration of the phase behavior of the system under supercritical conditions. The influence of pressure and temperature on phase behavior in such systems is complex. For example, it is possible to have multiple phases, such as liquid-liquid-vapor or solid-liquid-vapor equilibria, present in the system. In many cases, the operation of an SCF process under multiphase conditions may be undesirable and so phase behavior should first be investigated. The limiting case of equilibrium between two components (binary systems) provides a convenient starting point in the understanding of multicomponent phase behavior. 

Heterogeneous catalysis takes place in multiphase systems. If a new phase, the supercritical phase, is selectively introduced, remarkable change can be expected. The effects caused by the introduction of a supercritical phase depend on many parameters, such as fluid properties, reaction conditions, and affinity. Successful supercritical phase heterogeneous catalytic reactions can be realized, as long as these parameters are carefully controlled. With greater activities, catalyst lifetimes, or selectivities than for gas phase or liquid phase reactions, some supercritical phase reactions have industrial potentid. 

Following the logic of this tree, the multiphase processes on the left-hand side belong among the operations with immobilized catalysts but on liquid supports . The topics of this book are the processes with the liquid supports water, supercritical fluids, ionic liquids, organic liquids, soluble polymers, and fluorous liquids among these, only two processes (Ruhrchemie/Rhone-Poulenc and Shell SHOP) are operative industrially so far. The more important leaves of the family tree are shaded in gray. 

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]. 

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