Multiphasic Systems

A multiphasic system for a chemical reaction can be constituted by any combination of gaseous, liquid, and solid phases. If a catalyst is present, it can be homogeneous or heterogeneous, thereby adding further phases—and degrees of freedom—to the system. Extra phases add new variables to a reaction, and it is therefore necessary that this be done for an advantage, such as an easier separation of the products, improved rates and selectivity, improved catalyst stability,

Methods and Reagents for Green Chemistry: An Introduction, Edited by Pietro Tundo, Alvise Perosa, and Fulvio Zecchini

Copyright © 2007 John Wiley & Sons, Inc.

better catalytic efficiency, or improved environmental performance. It is just as obvious that there is a large number of cases where added phases generate advantages, as is testified by the growing number of articles in this area.

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

In a gas-liquid (G-L) system, a reagent gas is brought into contact with a liquid solution where reactant and homogeneous catalyst are dissolved. A typical case is that of homogeneous catalytic hydrogenation.

In a liquid-solid (L-S) system, a heterogeneous (or heterogenized) catalyst is used to promote a reaction in solution. This can be run in batch or in continuous-flow, and there are numerous examples of reactions done in an L-S system.

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.

Conversely, liquid-liquid (L-L) systems appear a little less obvious as systems for a chemical reactions, because partitioning of the species between the different phases becomes a critical issue. For example, two reactants can be in separate phases, and may need to be brought together by a phase-transfer catalyst (PTC),1,2 or by a surfactant. The advantage here lies in the possibility of carrying out a reaction between two species with opposite polarity, without the need for a solvent such as acetone, dimethyl sulfoxide (DMSO), or dimethyl formamide (DMF). The phase-transfer-catalyzed halogen exchange reaction is an example. Recently, more work has been done on L-L biphasic systems that involve "neoteric" solvents such as dense carbon dioxide, polyethylene glycols (PEGs), and ionic liquids, which are catalyst-phylic. These solvents can often aid in catalyst separation and product recovery by phase separation of the two.3

Solid-solid (S-S) systems are now being investigated in view of eliminating solvents from chemical reactions. Here the paradigm is "the best solvent is no solvent." Just mixing two solids can often lead efficiently and cleanly to a product; however, there are limitations that are mainly due to the choice of reagents and to mass and heat transport.4,5

It should be pointed out that many biphasic systems have found their way into the chemical industry, starting from PTC and continuous flow (CF) processes. The reasons are that efficiency can be increased (rates, selectivity, energy requirements, reaction intensification), making them more economic and often more environmentally compatible, in short, more sustainable.

What we highlight here are some new recent multiphasic reaction systems for catalysis. The systems described here have in common a catalyst-philic phase, which contains, or coats a catalyst (mainly heterogeneous), or in some instances is the catalyst itself (PTC). There are two or three separate phases, and a general composition that can be summarized as: liquid-liquid-solid (L-L-S), or liquid-liquid-liquid-solid (L-L-L-S). One of the Ls indicates the liquid-ionic/hydrophilic phase (water, PEG, PT agent, ionic liquid, etc.) that is rich in the active catalytic species.

Before the 1990s there was little in the literature on multiphasic L-L-S and L-L-L-S systems used for chemical reactions. There is, however, a relatively large volume of work done on other types of multiphasic systems related to the present topic: supported liquid-phase catalysis (SL-PC), and gas liquid phase transfer Catalysis (GL-PTC).6 The common denominator in both cases is the presence of an interfacial liquid layer of a hydrophilic compound between the catalyst and the bulk of the reaction.

In SL-PC, a catalyst is supported on a solid matrix in the form of the film of a nonvolatile liquid phase adsorbed on the solid. The catalytic film can be, for example, a molten salt or a molten oxide (e.g., Deacon's catalyst (CuCl2/KCl) used to oxidize HCl with oxygen for the chlorination of ethylene in the synthesis of vinyl chloride, Figure 6.1; V2O5 for the oxidation of sulphurous to sulphuric anhydride). Alternately, it can be a liquid phase (e.g., ethylene glycol, PPh3, butyl benzyl phthalate, etc.) that contains a soluble catalytic species such as a metal complex.

The reagents flow through in the gaseous phase (if they flowed in the liquid state, the catalytic species would be washed away), and the product diffuses in the gaseous stream and is collected at the outlet. The main drawback is that only relatively light compounds can be reacted, since they have to be in the gas phase, and low boiling products (and by-products) must be formed so that they can be easily recovered.

SL-PC was developed in view of industrial applications, since CF methods are largely preferred in that context. As an example, the hydroformylation of light olefins (up to C6), propylene in particular, was thoroughly studied by Sholten and co-workers to the pilot plant stage and to the calculations for a large-scale plant (20,000 ton/y). The catalyst was hydridocarbonyltris-(triphenylphosphine)-rhodium(I) dissolved in liquid PPh3 as the stationary phase. It is noteworthy that the problems of Rh leaching and stability seemed to be resolved by operating under SL-PC conditions and by using the correct CF parameters. The limitation, for the industrial

Bulk

Stationary liquid film

CuClj/KCI

Alumina support

Figure 6.1 SL-PC for the synthesis of vinyl chloride with Deacon's catalyst.

CuClj/KCI

Alumina support

Figure 6.1 SL-PC for the synthesis of vinyl chloride with Deacon's catalyst.

Bulk

R4P+ Br

Bulk

R4P+ Br

Stationary liquid film

Figure 6.2 GL-PTC for the continuous-flow Finkelstein reaction in the gas phase.

Solid salt

Figure 6.2 GL-PTC for the continuous-flow Finkelstein reaction in the gas phase.

application, was the low conversion per pass necessary to avoid the aldol condensation side reaction that ensues when the space velocity is low.

The situation changes if the reaction is not truly catalytic, that is, when the reagent bed promotes the reaction, but is also consumed, such as in the case of a halogen exchange reaction carried out under GL-PTC conditions (Figure 6.2).7 In this case, for example, a gaseous alkyl bromide can be fluxed over a reagent bed made by a quaternary phosphonium bromide impregnated on silica gel, and potassium iodide. At the reaction temperature (150°C), the phosphonium salt is molten, and the reaction takes place by diffusion of the alkyl halide into the liquid stationary phase, followed by halide exchange and diffusion of the product back to the gaseous stream. Iodide is replenished by exchange with the solid salt.

While the onium salt remains globally unchanged, the iodide is consumed during the reaction. GL-PTC was developed, among other thing, for reactions between ionic nucleophiles and gaseous electrophilic substrates, and for base mediated reactions. The PT immobilized catalyst is a liquid at the operating temperature; it dissolves the salt, it activates the anion, and promotes anion exchange with the solid bed.

Conceptually, GL-PTC and SL-PC are closely related. In fact, both involve the presence of a stationary liquid interfacial layer, between the flowing gas phase and the support, where the reaction takes place. Various examples of the preceding techniques have been reported over the years, and many are collected and cited in the monograph mentioned earlier.6

The present chapter targets multiphasic catalytic systems that can be represented in general as L-L-S and L-L-L-S systems (Figure 6.3). The liquid phases are two or three, and separate at ambient conditions. One of the Ls is a catalyst-philic liquid phase that can be either ionic or hydrophilic, the equivalent to the supported liquid film described in the previous section. Figure 6.3 shows the two different arrangements of the multiphasic systems that is considered here.

The underlying idea is that what is sometimes a separate liquid phase that holds the catalyst, can, in other conditions, be considered a supported

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  • mari
    What are multiphasic reactions?
    8 years ago

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