Supports multiphasic systems

Energy and natural resources processing. NSF should sustain its support of basic research in complex behavior in multiphase systems, catalysis, separations, dynamics of solids transport and handling, and new scale-up and design methodologies. 

One way in which the material can be used is illustrated by the practice at the University of Toronto. Chapters 1-8 (sections 8.1-8.4) on chemical kinetics are used for a 40-lecture (3 per week) course in the fall term of the third year of a four-year program the lectures are accompanied by weekly 2-hour tutorial (problem-solving) sessions. Chapters on CRE (11-15,17,18, and 21) together with particle-transport kinetics from section 8.5 are used for a similarly organized course in the spring term. There is more material than can be adequately treated in the two terms. In particular, it is not the practice to deal with all the aspects of nonideal flow and multiphase systems that are described. This approach allows both flexibility in choice of topics from year to year, and material for an elective fourth-year course (in support of our plant design course), drawn primarily from Chapters 9,19,20, and 22-24. 

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). 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. 

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-phihc 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. 

This section describes catalytic systems made by a heterogeneous catalyst (e.g., a supported metal, dispersed metals, immobilized organometaUic complexes, supported acid-base catalysts, modified zeolites) that is immobilized in a hydrophilic or ionic liquid catalyst-philic phase, and in the presence of a second liquid phase—immiscible in the first phase—made, for example, by an organic solvent. The rationale for this multiphasic system is usually ease in product separation, since it can be removed with the organic phase, and ease in catalyst recovery and reuse because the latter remains immobilized in the catalyst-philic phase, it can be filtered away, and it does not contaminate the product. These systems often show improved rates as well as selectivities, along with catalyst stabilization. 

Analogously, over the years, Arai and co-workers have investigated silica-supported ethylene glycol as a catalyst-philic phase, which contained a metal precursor, for C-C bond-forming reactions, such as the Heck reaction. They describe a multiphasic system with an organic phase (solvent) that contains only reactants and products without any catalyst. The products could be recovered by simple filtration, and the catalyst recycled many times without deactivation, since it did not precipitate, thus making the catalytic system stable and reusable (Figure 6.7). °... 

The scope of the multiphasic system was extended to coupling reactions—like the Heck reaction—using a heterogeneous supported catalyst, such as Pd/C. The rationale here lay in the observation that aryl halides were activated in the multiphasic system (as seen for hydrodehalogenation), and that therefore they should also be activated toward C-C coupling reactions. 

The general concept of onium salt supported reagents and catalysts is rather similar to all multiphasic systems, where one reagent (catalyst) is retained in one phase whilst substrates and forthcoming products remain in another. In our case, the OSs supported reagents have an almost exclusive affinity for the IL phase, and... 

The moments of an NDF represent some important physical properties of the underlying population of elements constituting the multiphase system under study. For this reason, they have to satisfy some simple rules. For instance, the positiveness of the density function over its support implies that the moment of order zero must be positive. Additionally, there are other simple, intuitive rules. For example, if the internal coordinate assumes only positive values (or in other words the moment is defined on a positive support) then the moment of order one (as well as all the other moments) must be positive. Another important property of the distribution is its variance (i.e. cr = m2 - m lmQ), which must be zero for a delta-function distribution, while it must be positive for polydisperse distributions. Accordingly, it has to be m2 > m lmo. For higher-order moments the mathematical constraints are less intuitive and cannot be directly related to specific global properties of the multiphase systems. Fortunately, the theory of moments provides some interesting theorems that turn out to be very useful in determining whether a set of moments is invalid. 

Another category of enzymatic transformations in multiphase systems is enzymes immobilized on the reactor wall as presented in Table 10.4. Enzymes are advantageously used in immobilized form because this strategy allows for increased volumetric productivity and improves stability. Continuous mode of operation is employed in these systems. The approaches commonly used for immobilization in conventional multiphase biocatalysis can also be employed in microreactors such as covalent methods, cross-linked enzyme aggregates (CLEA), and adsorption methods. The experimental setups can either be chip-type reactors with activated charmel surface walls where enzyme binds, or enzyme immobilized monolith reactors, where a support is packed inside a capillary tube. 

It is still discussed whether the formation of all mesostructured materials is governed by one unique mechanism or we have to turn to a different mechanism depending on the conditions. The two mechanisms previously described have both been supported by experimental evidence. However, the studied systems are complex multiphase systems that involve an important number of different species that evolve in fragile equilibria. As a result, they are difficult to analyze and rationalize and despite all the work that has been furnished some details still remain unclear. In the future, new tools should allow us to gain more insights into the formation of ordered mesoporous materials. 

Supported Organocatalysts in Multiphasic Systems 28.7.3.1 Tagged Organocatalysts... 

In this section of our review, recent developments in the synthesis of organosiloxane containing multiphase copolymers and networks will be discussed. Basic structural and physical characteristics of the copolymers (e.g. spectroscopic, thermal, molecular weight, etc.), supporting the formation of the multiphase structures will be given. Mechanical and morphological characteristics of representative systems will be discussed in Chapt. 4. 

There are many good reasons for using multiphasic methods to carry out reactions that employ homogeneous catalysts. In general, the catalyst is immobilized in one phase and the reactants and products are supported in other phases. Ideally, once the reaction is complete the products can be removed without contamination by the catalyst and the catalyst phase is ready for immediate reuse. However, a perfect system is extremely difficult to obtain in practice, although many solvent combinations and catalyst design strategies have been developed which come close to the ideal situation. 

---