Two-phase Reaction Systems

For a single-phase chemical reaction, formation of a two-phase flow using the boiling process in a reactor will intensify the mixing of reactants along the axis of the device. 

The correlation between the longitndinal mixing ontput and dispersed particle size, in the two-phase reaction flow in tnbnlar tnrbnlent devices, is determined by the mechanical method controlling the phase contact surface. At the same time, there is 

Where applicable, two-phase processes in which one phase is water are a more attractive option than the dipolar aprotic solvents, because solvent recovery is greatly simplified. In this section some of the principles involved in the design of two-phase reaction systems will be described. The subject has been treated in great detail elsewhere [19-21]. One of the present authors has provided a short review and an introduction to the theory [22]. At least three distinct reaction mechanisms are possible. 

---

The influence of transport process in two-phase reaction systems depends on flow conditions, which change with the size of the equipment. This is the reason for the historic observation that performance changes as processes are scaled up and therefore scale-up should be done in several steps, each limited to a small increase in size. This is a slow and expensive method and still does not guarantee optimum design. 

The present method of preparing anhydrides is distinguished from other procedures by its simplicity and high yield. It avoids the two-phase reaction systems of older methods and the need... 

Recently, it has been reported that methyl 2-pyridyl sulphoxides (10) and related pyridyl derivatives (11) (see Schepie 25) are good phase transfer catalysts for SN2 reactions of various primary or secondary alkyl halides in a two-phase reaction system and for the alkylation of phenylacetonitrile or phenylacetone with alkyl halides in liquid-liquid two-... 

Aqueous-organic two-phase reaction has been widely performed [18]. One of the purposes of using two-phase reaction system is to control the substrate concentration in aqueous phase where the biocatalysts exist. Hydrophobic substrate and products dissolve easily in the organic phase, so that the concentration in the aqueous phase decreases. The merits of controlling and decreasing the substrate concentration in the aqueous phase are as follows ... 

The enantioselective reduction of alkyl 3-oxobutanoates by carbonyl reductase (SI) from C. magnoliae was also performed in organic-aqueous two-phase reaction system (Figure 8.15) [llc,d]. 

Au (or Ag) content. Decanethiol-protected AuPt alloy bimetallic nanoparticles of ca. 2.5 nm in particle size were similarly prepared [58]. The preparations of PdPt [59] and AuPd [60] bimetallic nanoparticles in water-in-oil (w/o) microemulsions can be realized in two-phase reaction system, in which a surfactant molecule itself works as a protecting agent in these cases. 

Carbonvlation of Benzyl Halides. Several organometallic reactions involving anionic species in an aqueous-organic two-phase reaction system have been effectively promoted by phase transfer catalysts(34). These include reactions of cobalt and iron complexes. A favorite model reaction is the carbonylation of benzyl halides using the cobalt tetracarbonyl anion catalyst. Numerous examples have appeared in the literature(35) on the preparation of phenylacetic acid using aqueous sodium hydroxide as the base and trialkylammonium salts (Equation 1). These reactions occur at low pressures of carbon monoxide and mild reaction temperatures. Early work on the carbonylation of alkyl halides required the use of sodium amalgam to generate the cobalt tetracarbonyl anion from the cobalt dimer(36). 

Upon variation of the stirring velocity between 500 and 1500 rpm the conversion of the olefin remained at the same high level and the selectivity to the linear aldehyde also remained constant. Obviously there is no mass transfer limitation in this two-phase reaction system. In comparison to the single-phase reaction in propylene carbonate as the only solvent [23], the selectivity decreases from 95% to 70%, which can be explained by the high concentration of the non-electron-donating solvent dodecane in the propylene carbonate phase. The presence of the dodecane leads to a decrease of the isomerization velocity, which results in a lower hnearity of the formed aldehydes. 

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

The next attempt to further improve the reaction efficiency was to reduce the volume of the acetic acid solvent and to proportionally increase the initial concentration of the alcohol substrate while keeping the total reaction volume at constant level. The main purpose of these studies was to determine the minimum amount acetic acid needed to maintain a homogeneous system until complete conversion of hexan-l-ol to 2. Since the oxidation reaction produces stoichiometric amounts water, it was felt, that the formation of a second aqueous phase along with the hydrophobic aldehyde phase would lead to the creation of a two-phase reaction system with the inevitable partition of the catalyst system between the two phases. In addition it was also important to determine the highest possible S/C ratio while maintaining a reasonable reaction rate. 

The toxic effect on biocatalytic activity and stability in two-phase reaction system media can be divided into two effects. The first one, called the molecular-toxicity effect, is a direct toxic effect of the solvent molecules, which are dissolved in the aqueous phase and interact with the biocatalyst, particularly with whole cells. The second one, which is created by the presence of an interface between the aqueous and the organic solvent phase, is called the phase-toxicity effect [2, 24]. 

The use of NBD-Cl for the fluorescence analysis of alkylamine-generating pesticides has been investigated [173]. A two-phase reaction system is employed for the hydrolysis and labeling of N-methyl- and N,N-dimethyl-carbamate pesticides. The residue is hydrolyzed in 0.1 M sodium carbonate and the liberated amine is treated with NBD-Cl in an organic phase (IBMK, isobutyl methyl ketone) above the aqueous layer. An aliquot portion of the organic layer is used for chromatography. The reactions involved are shown in Fig. 4.65. 

A recent example is the hydrolysis of chloranil with aqueous sodium hydroxide, which is a multistep process leading to the chloranilate dianion, Fig. 5.21 [26]. In a two-phase reaction system consisting of solid chloranil and aqueous sodium hydroxide, the rate-limiting step is the initial attack of hydroxide ion on chloranil. Hydrolysis rates at the 010, 100 and 001 cleavage planes of chloranil were separately measured [27], and different reactivities were found this was rationalised by considering the exposure of the initially reactive functionality (assumed to be the carbonyl group) at the surface, as shown in Fig. 5.22. Hydrolysis at the 010 and 100 planes was shown to be a surface reaction driven by the hydroxide concentration adjacent to the interface the 001 plane was shown to react more slowly by prior dissolution of chloranil before reaction. 

Atherton, J.H. (1994) Mechanism in two-phase reaction systems coupled mass transfer and chemical reaction. Research in Chemical Kinetics, 2, 193. 

Tagaki et al. subsequently employed chiral menthyl-substituted thiazolium salts such as compound 9 in a micellar two-phase reaction system, reaching an ee of 35% and an improved yield of 20% [13]. Zhao et al. obtained moderate revalues of 47 to 57% and yields of 20 to 30% when combining the Sheehan catalysts with the Tagaki reaction conditions [14]. Based on their mechanistic model, Lopez Calahorra et al. developed bisthiazolium salt catalysts such as compound 10, yielding 21% of benzoin in 27% ee [15]. 

Mechanistic studies of this two-phase reaction system showed that the solubility properties of both ketone and alkene, as well as the reaction pH, were important factors [9]. In this context, the report by Yang in 1995 of a monophasic... 

Attempts have been made by Bennett and his co-workers [43,87] to analyse in more detail the kinetics of a two-phase reaction system. However, so far it has proved impossible in the absence of more detailed data on the concentrations in the two phases. Bennett only found that in the nitration of dinitrotoluene to trinitrotoluene the H20 H2S04 ratio in the organic phase is much higher than that in the acid phase. The proportion of acid in the organic phase is only 5-10% (in relation to this phase). 

Batch suspension reactors are, theoretically, the kinetic equivalent of water-cooled mass reactors. The major new problems are stabilization of the viscous polymer drops, prediction of particle size distribution, etc. Particle size distribution was found to be determined early in the polymerization by Hopff et al. (28, 29,40). Church and Shinnar (12) applied turbulence theory to explain the stabilization of suspension polymers by the combined action of protective colloids and turbulent flow forces. Suspension polymerization in a CSTR without coalescence is a prime example of the segregated CSTR treated by Tadmor and Biesenberger (51) and is discussed below. In a series of papers, Goldsmith and Amundson (23) and Luss and Amundson (39) studied the unique control and stability problems which arise from the existence of the two-phase reaction system. 

It is possible to prepare MCM-41 and also MCM-48 at room temperature. For the MCM-41 materials a two-phase reaction system was employed in which the silica source and water are immiscible, and utilised tetraethylorthosilicate, quaternary surfactants, ammonia and water at room temperature [5]. The MCM-48 materials, the... 

Workers from Hoffmann-La Roche have also devised a linear synthesis of racemic LTA4 methyl ester during the course of a synthesis of LTE4. They too employed a sulfonium salt (Scheme 3.22) in the epoxide-forming step to obtain a dienediyne epoxide. This step was made reasonably efficient by the use of a two-phase reaction system with a phase-transfer catalyst. Their sequence is also amenable to reasonable scale-up as they describe the preparation of 18 g of the epoxide mixture in a single reaction. A unique aspect of this synthesis is... 

Based on the inversely temperature-dependent solubility of phosphines modified with polyoxyethylene chains, TRPTC has been proposed, and applied to the aqueous/organic two-phase reaction system [11], The general principle of TRPTC is depicted in Figure 1. 

The present investigation was undertaken in order to better define the role of diffusion in the two-phase reaction system at acid concentrations more nearly comparable to those used in industrial practice. Another objective of this work was to obtain quantitative information describing the solubilities of various compounds in each phase. 

The primary task of modelling two-phase reaction systems is the estimation of the average diameter of droplets (bubbles and so on) of a dispersed phase and their size distribution in fast interface processes in diffuser-confusor devices. According to Kholmogorov s theory of isotropic turbulence, the specific kinetic energy of turbulence dissipation rates e are limiting in this case. 

The largest discrepancy between values, calculated using Equations 2.60 and 2.61 and the corresponding results of numerical calculations, does not exceed 23%, and the average discrepancy is 7%. Thus, proposed equations can be used to estimate the turbulent flow parameters of two-phase reaction systems in tubular turbulent diffuser-confusor devices. 

The components of two-phase reaction systems usually differ in density and viscosity, which is the reason for flow layering, especially at high flow rates. This is the lower limit of the output of a tubular turbulent reactor for fast chemical processes, including the interphase boundary. 

---