Solid Biphasic Systems

The simplest and cheapest way to produce supported PaHNL is to use almond meal itself. The step of purification of the enzyme is avoided [43], but low loading of enzyme is the main disadvantage. Other HNLs have also been employed using this crude type of enzyme preparation, such as apple, apricot, cherry, plum [44, 45], peach and loquat [46] meal. However, these catalysts were applied to a minor extent compared with almond meal. 

Iin and co-workers [40] have developed a so-called micro-aqueous organic reaction system. In contrast to former preparations of almond meal, the almond kernels are soaked in water prior to grinding. After the defatting step, the meal contains 8-10% water (w/w), making it unnecessary to add the amount of water needed for enzyme activity. The reactions are carried out in buffer-saturated organic solvents to avoid a possible drying effect of the solvent on the biocatalyst. Further addition of water to the reaction results in lower conversions and values, 

Figu re 9.4 Enzymatic transcyanation of -bromoaldehydes and racemic ketone cyanohydrins [53]. 

The presence of the aldehyde as cyanide acceptor is essential to achieve the resolution of ketone cyanohydrins with good enantioselectivities. The unreacted (S)-ketone cyanohydrin and the (R)-co-bromoaldehyde cyanohydrin formed could be isolated in moderate to good enantiopurity (50 to 95% and 75 to 92% respectively) [53]. 

The conversion of co-hydroxyalkanals to the corresponding cyanohydrins in moderate enantioselectivities could also be accomplished by transhydrocyanation with acetone cyanohydrin as the cyanide source. These substrates are considered difficult because of their high solubility in water. Through the employment of an almond meal preparation in a micro-aqueous organic reaction system, the ee-values could be significantly improved [54]. 

---

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. 

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... 

Betzemeier et al. (1998) have used f-BuOOH, in the presence of a Pd(II) catalyst bearing perfluorinated ligands using a biphasic system of benzene and bromo perfluoro octane to convert a variety of olefins, such as styrene, p-substituted styrenes, vinyl naphthalene, 1-decene etc. to the corresponding ketone via a Wacker type process. Xia and Fell (1997) have used the Li salt of triphenylphosphine monosulphonic acid, which can be solubilized with methanol. A hydroformylation reaction is conducted and catalyst recovery is facilitated by removal of methanol when filtration or extraction with water can be practised. The aqueous solution can be evaporated and the solid salt can be dissolved in methanol and recycled. 

Lipophilicity represents the affinity of a molecule or a moiety for a lipophilic environment. It is commonly measured by its distribution behavior in a biphasic system, either liquid-liquid (e.g. partition coefficient in 1-octanol-water) or solid-liquid (retention on reversed-phase high-performance liquid chromatography or thin-layer chromatography system). 

The biphasic system was transferred to a separatory funnel (250 mL) and extracted with ether (3 x 40 mL). The organic fractions were combined. The solvent was removed using a rotary evaporator, to produce a yellow oil and a white solid (polymerized trimethoxysilane). 

It is essential that all PSs are multiphase. The easiest case to handle is the biphase system consisting of a condensed phase (solid) and a void inside porous particles or between consolidated ensembles of nonporous or porous particles. The void occupies a part of the volume, s, which is referred to as porosity. The other part of a PS volume is equal to ri=(l -e), and is termed density of packing. It is filled with the condensed phase (see Section 9.4). Generally, PSs can include various condensed phases of different structure, including combinations of solid(s) and liquid(s). 

The most frequent multiphasic systems in the literature are biphasic systems. Industrially, the most relevant are gas-solid (G-S) systems where gaseous reactants are fluxed over a solid catalyst, generating products that are collected at the outlet. The synthesis of ammoiua is an obvious example. 

Supporting ionic liquids in the pores of solid materials offers the advantage of high surface areas between the reactant phase and that containing the supported liquid catalyst. This approach is particularly useful for reactants with less than desired solubility in the bulk liquid phase. Another incentive for using such catalysts is that they can be used in continuous processes with fixed-bed reactors (26S). The use of an ionic liquid in the supported phase in addition to an active catalyst can help to improve product selectivity, with the benefit being similar to what was shown for biphasic systems. However, care has to be taken to avoid leaching the supported liquids, particularly when the reactants are concentrated in a liquid phase. 

However, none of the oxidations described in this book requires the use of solid RuO. It is generated in solution, normally in a biphasic system from a lower oxidation state compound such as RuO. nH O or RuCl3.nH30 and a co-oxidant replenishes the RuO reduced by the organic substrate (1.2.6). 

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].

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]. 

As well as biocatalysis in neat organic solvents and biphasic systems (fundamentals and synthetic applications), the present volume covers new and promising aspects of non-aqueous enzymology that have emerged in recent years, including biocatalysis with undissolved solid substrates or vaporized compounds, the use of ionic liquids as solvents, and the preparative-scale exploitation of oxynitrilases and dynamic kinetic resolutions . For the sake of completeness and comparison,... 

Lithergols, i.e. polyphase, at least biphase systems, one of the phases being liquid, another solid, e.g. carbon and liquid oxygen. 

Many experimentalists are familiar with this principle of doping a sample with a species that couples better with the microwave irradiation and so can act as a thermal dissipater. What is often less appreciated is the general nature of this process, as not only solid/liquid interfaces but also liquid/liquid biphasic systems such as emulsions show the same effects59-63. Figure 6.2 represents the heating profiles of toluene and a perfluorinated solvent first independently and then as an emulsion. A similar trend can be seen in a hexane/acetonitrile mixture, although because of the superior heating capacity of acetonitrile the effect is less pronounced. 

The representative reaction system applied in asymmetric phase-transfer catalysis is the biphasic system composed of an organic phase containing an acidic methylene or methine compound and an electrophile, and an aqueous or solid phase of inorganic base such as alkaline metal (Na, K, Cs) hydroxide or carbonate. The key reactive intermediate in this type of reaction is the onium carbanion species, mostly onium enolate or nitronate, which reacts with the electrophile in the organic phase to afford the product. 

Rothenberg, G., Royz, M., Arrad, O. and Sasson, Y. (1999) In situ generation and synthetic applications of anhydrous hydrogen fluoride in a solid-liquid biphasic system. J. Chem. Soc., Perkin Trans., 1, 1491. 

Thanks to their speed and relatively low computational cost, M D and MC simulations can be used for studying the physical properties of large systems. This is extremely useful in heterogeneous catalysis, e.g., for modeling the structure and the properties of the bulk and the surface of a solid catalyst, or the properties of the bulk and interface of liquid/liquid biphasic systems. However, since the number of particles modeled is still very small compared to real materials, the models are susceptible to wall effects. One neat trick for avoiding this problem is to apply periodic boundary conditions The volume containing the model is treated as the primitive cell of an... 

---