Phase-transfer catalysis separation

Phase separation Phase structure Phase-transfer agents Phase-transfer catalysis... 

In addition to solvent-free processing, phase-transfer catalytic conditions (PTC) have also been widely employed as a processing technique in MAOS [15]. In phase-transfer catalysis, the reactants are situated in two separate phases, for example liquid-... 

A number of other cryptand-bound polymers have been synthesized using similar procedures to those discussed previously for immobilization of crown molecules. Apart from their use in phase transfer catalysis, such polymers have been studied extensively as chromatography reagents for the separation of a range of metal-ion types (Blasius Janzen, 1982) in a number of instances quite useful separations have been achieved. 

Uniquely interesting, complex and useful activities and phenomena occur at interfaces one need only to look at the interfaces between the land, the atmosphere, and the sea to find this truth. The same truth occurs in chemical interfaces, although sometimes it is the lack of activity that draws our attention. In many chemical situations where two species cannot collide and therefore cannot react because they are separated by an interface, the lack of activity has been overcome by use of the technique of PHASE TRANSFER CATALYSIS (PTC), which not only allows reaction to occur, but often to occur in very selective ways. 

Phase transfer catalysis (1,2) has become in recent years a widely used, well-established synthetic technique applied with advantage to a multitude of organic transformations. In addition to a steadily increasing number of reports in the primary literature, there are several reviews (3-6), comprehensive monographs (7-10) and an ACS Audio Course (1 ) which describe the phase transfer process and which provide extensive compilations of phase transfer agents and reaction types. While the list of applications and in many cases the synthetic results are impressive, phase transfer catalysts (PTCs) suffer some of the same disadvantages as more conventional hetero-and homogeneous catalysts — separation and... 

Supercritical fluids are benign alternatives to conventional organic solvents that may offer improvements in reaction rate, product selectivity, and product separation. We reported the first use of SCFs for phase-transfer catalysis (PTC), where these benign alternatives also offer greatly improved transport, product separation, catalyst recycle, and facile solvent removal (26-29). 

A membrane can be used in the so-called phase transfer catalysis as a separator between two immiscible liquids or a liquid and a gas. It serves as a well controlled contact surface. An interesting type of membrane reactor has been suggested in which a ceramic membrane is applied to regulate the contact between a gas and a liquid stream on the opposites of the membrane [De Vos, 1982 De Vos et al., 1982]. Hydrogenation of nitrobenzoic acid can be effectively performed with a porous calcium-aluminum silicate membrane reactor which essentially becomes a gas-liquid reactor. 

From an electrochemical point of view it is easily inferred that the solution in a cell near an electrode is separable into two parts a stagnant layer adjacent to the electrode in which no convective motions occur, and the remainder of the solution, which is homogeneous (bulk solution). Yet this is not a particularity of electrochemical methods since the same phenomena occur at any solid/liquid interface, as when metal particles (reductions by Zn or Na, for example) or any heterogeneous reagent is used in organic homogeneous chemistry, as well as in phase-transfer catalysis or related methods. 

It is therefore not surprising that it was only when suitable methods for catalyst separation from the substrates and reaction products of homogeneous catalysis were developed that the importance of this type of process grew. The successful developments (thermal separation or chemical reaction (e. g., [26]), immobilization by means of supports and thus heterogenization (e. g., [44]), phase transfer catalysis [45], biphasic processes (e. g., [46, 47]) or separation with membrane modules [48, 49]) are described in the relevant sections of this book (cf. [50]). 

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