Biphasic liquid/solid

In conclusion, many methods are available for the immobihzation of metal complexes with diazaligands. The biphasic liquid methods seem to be more versatile - especially in the case of ionic liquids, where modification of the ligands is not needed. Very good results can also be obtained with solid supports, although the surface effects, which can be positive or negative. 

The isolated Ru(0) nanoparticles were used as solids (heterogeneous catalyst) or re-dispersed in BMI PP6 (biphasic liquid-liquid system) for benzene hydrogenation studies at 75 °C and under 4 bar H2. As previously described for rhodium or iridium nanoparticles, these nanoparticles (heterogeneous catalysts) are efficient for the complete hydrogenation of benzene (TOP = 125 h ) under solventless conditions. Moreover, steric substituent effects of the arene influenced the reaction time and the decrease in the catalytic TOP 45, 39 and 18h for the toluene, iPr-benzene, tBu-benzene hydrogenation, respectively, finally. The hydrogenation was not total in BMI PPg, a poor TOE of 20 h at 73% of conversion is obtained in the benzene hydrogenation. 

Gas-liquid-solid (G-L-S) where a reagent gas is brought into contact with a liquid solution where the reactant is dissolved and where a heterogeneous catalyst is suspended, that are triphasic systems, can be considered biphasic L-S systems in the context of this review. 

A butoxylcarbonylation reaction was conducted in a liquid-liquid biphasic system under process conditions, but the removal of the product was conducted in a liquid-solid biphasic system at a lower temperature (84). lodobenzene or 4-bromoacetophenone reacted with CO at a pressure of 1-8 atm in the presence of a palladium-benzothiazole complex catalyst in the ionic liquid [TBA]Br (m.p. = 110°C) in the presence of Et3N base. The catalyst/ionic liquid system was recycled by extractive removal of the butyl ester product with diethyl ether. The solid residue, containing the catalyst, [TBA]Br, and Et3N.HBr, remained effective in subsequent carbonylation tests. After each cycle, the yields were still close to the initial value. A slight decrease in yield was attributed to a loss of catalyst during handling. 

Figure 9,1 Representation of some biphasic systems (a) liquid/liquid systems, and (b) liquid/solid system. Dashed area organic phase white area water phase black dots biocatalyst [12].

In a liquid/liquid biphasic system (Figure 9.1a), the enzyme is in the aqueous phase, whereas the hydrophobic compounds are in the organic phase. In pure organic solvent (Figure 9.1b) a solid enzyme preparation is suspended in the solvent, making it a liquid/solid biphasic system. In a micellar system, the enzyme is entrapped in a hydrated reverse micelle within a homogeneous organic solvent... 

Boy et al. [79] used lyotropic liquid crystals (LC) for the immobilization of HfeHNL. The solid LC phase is not used because of the high viscosity. Therefore, the LC is used in a biphasic system consisting of the LC and an organic solvent. Such biphasic liquid crystal systems consist of organic solvent, water, and surfactant, where poorly soluble substrates and products are dissolved in the organic solvent and the liquid crystal matrix, which contains the enzyme, has a protective effect on it. By optimization and by virtue of the immobilization, it is possible to establish an extractive continuous process [79]. 

For many oxidation reactions, the presence of ionic liquids appears to have a beneficial effect in that increased reaction rates are observed and stabilisation of catalytically active intermediates in these solvents has been demonstrated by means of in situ spectroscopic methods in several cases. And while the oxygen solubility is very poor in common ionic liquids solid oxidants are often dissolved more efficiently, resulting in increased reaction rates, ever under biphasic conditions. 

The drawback of SILP strategy to heterogeneous catalysis lies in the weak interactions (physical adsorption) between the IL and the solid support which limit applications to gas-solid phase reactions, only. Indeed, in biphasic liquid phase conditions both mechanical forces and partitioning processes could slowly remove the IL film from the solid support. To circumvent this problem, stronger interactions must be created between the IL phase and the support. 

In addition, hydrotrope solutions have been involved in reactions concerning solid particles. As examples may be mentioned the template-free synthesis of microtubules [34], important materials in nano-technology, and the more sophisticated role of hydrotropes to concurrently optimize the interfacial tension and the colloidal stabilization of rhodium particles in biphasic liquid-liquid alkene hydrogenation catalysis [35], Finally reaction kinetics has been used as a means to follow the association of hydrotrope molecules in aqueous solutions [36],... 

The oxidation of hydrocarbon compounds in aqueous hydrogen peroxide, without added organic solvent (triphase catalysis solid TS-1 + two immiscible liquids) has been found to be faster than in organic solution (biphase catalysis solid TS-1 -I- one liquid). Similarly, the initial rate of epoxidation of propylene was also the highest when conducted in totally aqueous hydrogen peroxide (M. G. Clerici, unpublished results). 

There are several different methods to separate PNIPAM-supported catalysts from the reaction mixtures. Both liquid-solid separations and liquid-liquid separations can be used. The most frequently used liquid-solid separation method takes advantage of the varying solubility of polymers in different solvents. For example, PNIP AM can be precipitated from THF into hexanes. PNI-PAM copolymers also exhibit lower critical solution temperature (LCST) behavior. Specifically, PNIPAM and its copolymers can be prepared such that these polymers are soluble in water at low temperature but precipitate when heated up. This property may be used as either a purification method or a separation tech-nique.[l 1] A thermomorphic system is a liquid-liquid biphasic system developed in our group. It uses various solvent mixtures with temperature-dependent miscibility to effect separation of catalysts from substrates and products, as shown in Figure 2. 

Polyester 9a, which has amino sulfonic acid moieties, was obtained by a liquid/solid biphase polycondensation of terephthalyl chloride and fV,A-bis(2-hydroxyethyl)-2-aminoethane sulfonic acid in trimethylphosphate using triethylamine as an acid acceptor [29]. A thin film of blends was prepared by casting a solution containing 9a, poly(vinyl alcohol) and Ni or salts. 9a and the metal salts formed an interpenetrating network as shown in 9b. The blends exhibit an electrical conductivity up to 10 S cm (for an experiment see Section 5.4, Experiment 5-3). 

Liquid-liquid-solid reactors are commonly used for biphasic reactions catalyzed by immobilized phase-transfer catalysts (which form the third, solid phase). Certain basic aspects of such reactors were considered in Chapter 19. Three-phase reactions of this type are also encountered in biological reactions, for example, the enzymatic synthesis of amino acid esters in polyphasic media (Vidaluc et al., 1983), and the production of L-phenylalanine utilizing enzymatic resolution in the presence of an organic solvent (Dahod and Empie, 1986). Predictably, the performance of these reactors is influenced by the usual kinetic and mass transfer aspects of heterogeneous systems (see Lilly, 1982 Chen et al., 1982 Woodley et al., 1991a,b). Additionally, performance is also influenced by the complex hydrodynamics associated with the flow of two liquids past a bed of solids (Mitarai and Kawakami, 1994 Huneke and Flaschel, 1998). It is noteworthy, for instance, that phase distribution within the reactor is different from that in the feed and is also a function of position within the reactor and within the voids of each pellet in the bed. More intensive research is needed before these reactors can be rationally designed. 

Ultrasonic cavitation has particularly important effects on biphasic systems, emulsification of immiscible liquid-liquids, particle breakage and dispersion, and surface cleaning in liquid-solid mixtures. These mechanical effects, even if sometimes oversimplified, appear to be understandable and predictable by nonexperts in the field, who exploited them in a variety of heterogeneous reactions. Sonochemistry of biphasic systems then developed rapidly, and synthetic applications were reviewed in recent articles. In many cases, the presence of a phase transfer catalyst becomes unnecessary, and sonication can be considered as a "physical substitute to PTC", according to the expression of Ando. a... 

---