phase-transfer catalyst

In chemistry, a phase-transfer catalyst or PTC is a catalyst that facilitates the transition of a reactant from one phase into another phase where reaction occurs. Phase-transfer catalysis is a special form of catalysis and can act through homogeneous catalysis or heterogeneous catalysis methods depending on the catalyst used. Ionic reactants are often soluble in an aqueous phase but insoluble in an organic phase in the absence of the phase-transfer catalyst. The catalyst functions like a detergent for solubilizing the salts into the organic phase. Phase-transfer catalysis refers to the acceleration of the reaction upon the addition of the phase-transfer catalyst. PTC is widely exploited industrially.Marc Halpern "Phase-Transfer Catalysis" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. Polyesters for example are prepared from acyl chlorides and bisphenol-A. Phosphothioate-based pesticides are generated by PTC-catalyzed alkylation of phosphothioates.

In ideal cases, PTC can be fast and efficient, minimizing the need for expensive or dangerous solvents and simplifying purification Phase-transfer catalysts are especially useful in green chemistry—by allowing the use of water, the need for organic solvents is lowered.

Phase-boundary catalysis (PBC) is a type of heterogeneous catalytic system which facilitates the chemical reaction of a particular chemical component in an immiscible phase to react on a catalytic active site located at a phase boundary. The chemical component is soluble in one phase but insoluble in the other. The catalyst for PBC has been designed in which the external part of the zeolite is hydrophobic, internally it is usually hydrophilic, notwithstanding to polar nature of some reactants.H. Nur, S. Ikeda and B. Ohtani, ''Phase-boundary catalysis: a new approach in alkene epoxidation with hydrogen peroxide by zeolite loaded with alkylsilane-covered titanium oxide'', '' Chemical Communications'', 2000, 2235 – 2236
Abstract
/ref>H. Nur, S. Ikeda and B. Ohtani, ''Phase-boundary catalysis of alkene epoxidation with aqueous hydrogen peroxide using amphiphilic zeolite particles loaded with titanium oxide'',
Journal of Catalysis
', 2001, (204) 402 – 408
Abstract
/ref>S. Ikeda, H. Nur, T. Sawadaishi, K. Ijiro, M. Shimomura, B. Ohtani, ''Direct observation of bimodal amphiphilic surface structures of zeolite particles for a novel liquid-liquid phase boundary catalysis'', ''Langmuir'', 2001, (17) 7976 – 7979. H. Nur, S. Ikeda and B. Ohtani, ''Phase-boundary catalysts for acid-catalyzed reactions: the role of bimodal amphiphilic structure and location of active sites'',
Journal of Brazilian Chemical Society
', 2004, (15) 719–724 – 2236
Paper
/ref>H. Nur, S. Ikeda, and B. Ohtani, ''Amphiphilic NaY zeolite particles loaded with niobic acid: Materials with applications for catalysis in immiscible liquid-liquid system'',
Reaction Kinetics and Catalysis Letters
', 2004, (17) 255 – 261
Abstract
/ref> In this sense, the medium environment in this system is close to that of an enzyme. The major difference between this system and enzyme is lattice flexibility. The lattice of zeolite is rigid, whereas the enzyme is flexible.

Types

Phase-transfer catalysts for anionic reactants are often quaternary ammonium salts. Commercially important catalysts include benzyltriethylammonium chloride, methyltricaprylammonium chloride and methyltributylammonium chloride. Organic phosphonium salts are also used, e.g., hexadecyltributylphosphonium bromide. The phosphonium salts tolerate higher temperatures.

An alternative to the use of "quat salts" is to convert alkali metal cations into hydrophobic cations. Crown ethers are used for this purpose on the laboratory scale. Polyethylene glycols and their amine derivatives are common in practical applications. One such catalyst is tris(2-(2-methoxyethoxy)ethyl)amine. These ligands encapsulate alkali metal cations (typically and ), affording lipophilic cations. Polyethers have a hydrophilic "interiors" containing the ion and a hydrophobic exterior.

Chiral phase-transfer catalysts have also been demonstrated.
Asymmetric alkylations are catalyzed by chiral quaternary ammonium salts derived from cinchona alkaloids.

A variety of functionalized catalysts have been evaluated for PTC. One example is the Janus interphase catalyst, applicable to organic reactions on the interface of two phases via the formation of Pickering emulsion.

Limitations

Quaternary ammonium cations degrade by Hofmann degradation to amines, especially at higher temperatures preferred by process chemists. The resulting amines can be difficult to remove from the product. Phosphonium salt are unstable toward base, degrading to phosphine oxide.

Laboratory examples

For example, the nucleophilic substitution reaction of an aqueous sodium cyanide solution with an ethereal solution of 1-bromooctane does not readily occur. The 1-bromooctane is poorly soluble in the aqueous cyanide solution, and the sodium cyanide does not dissolve well in the ether. Upon the addition of small amounts of hexadecyltributylphosphonium bromide, a rapid reaction ensues to give nonyl nitrile:
:C8H17Br_ + NaCN_ -> ceC8H17CN_ + NaBr_
By the quaternary phosphonium cation, cyanide ions are "ferried" from the aqueous phase into the organic phase.

Subsequent work demonstrated that many such reactions can be performed rapidly at around room temperature using catalysts such as tetra-n-butylammonium bromide and methyltrioctylammonium chloride in benzene/water systems.

Design of phase-boundary catalyst

Phase-boundary catalytic (PBC) systems can be contrasted with conventional catalytic systems. PBC is primarily applicable to reactions at the interface of an aqueous phase and organic phase. In these cases, an approach such as PBC is needed due to the immiscibility of aqueous phases with most organic substrate. In PBC, the catalyst acts at the interface between the aqueous and organic phases. The reaction medium of phase boundary catalysis systems for the catalytic reaction of immiscible aqueous and organic phases consists of three phases; an organic liquid phase, containing most of the substrate, an aqueous liquid phase containing most of the substrate in aqueous phase and the solid catalyst.

In case of conventional catalytic system;
*When the reaction mixture is vigorously stirred, an apparently homogeneous emulsion is obtained, which segregates very rapidly into two liquid phases when the agitation ceases. Segregation occurs by formation of organic bubbles in the emulsion which move downwards to form the aqueous phase, indicating that emulsion consists of dispersed particles of the aqueous phase in the organic phase.
*Due to the triphasic reactions conditions, the overall reaction between aqueous phase and organic phase substrates on solid catalyst requires different transfer processes. The following steps are involved:
*# transfer of aqueous phase from organic phase to the external surface of solid catalyst;
*# transfer of aqueous phase inside the pore volume of solid catalyst;
*# transfer of the substrate from aqueous phase to the interphase between aqueous and organic phases
*# transfer of the substrate from the interphase to the aqueous phase;
*# mixing and diffusion of the substrate in the aqueous phase;
*# transfer of the substrate from the aqueous phase to the external surface of solid catalyst;
*# transfer of the substrate inside the pore volume of the solid catalyst;
*# catalytic reaction (adsorption, chemical reaction and desorption).
In some systems, without vigorous stirring, no reactivity of the catalyst is observed in conventional catalytic system. Stirring and mass transfer from the organic to the aqueous phase and vice versa are required for conventional catalytic system. Conversely, in PBC, stirring is not required because the mass transfer is not the rate determining step in this catalytic system. It is already demonstrated that this system works for alkene epoxidation without stirring or the addition of a co-solvent to drive liquid–liquid phase transfer. The active site located on the external surface of the zeolite particle were dominantly effective for the observed phase boundary catalytic system.S. Ikeda, H. Nur, P. Wu, T. Tatsumi and B. Ohtani, ''Effect of titanium active site location on activity of phase boundary catalyst particle for alkene epoxidation with aqueous hydrogen peroxide'',
Studies in Surface Science and Catalysis
'', 2003, (145) 251–254.

Process of synthesis

Modified zeolite on which the external surface was partly covered with alkylsilane, called ''phase-boundary catalyst'' was prepared in two steps. First, titanium dioxide made from titanium isopropoxide was impregnated into NaY zeolite powder to give sample W-Ti-NaY. In the second step, alkysilane from n-octadecyltrichlorosilane (OTS) was impregnated into the W-Ti-NaY powder containing water. Due to the hydrophilicity of the w-Ti-NaY surface, addition of a small amount of water led to aggregation owing to the capillary force of water between particles. Under these conditions, it is expected that only the outer surface of aggregates, in contact with the organic phase can be modified with OTS, and indeed almost all of the particles were located at the phase boundary when added to an immiscible water–organic solvent (W/O) mixture. The partly modified sample is denoted w/o-Ti-NaY. Fully modified Ti-NaY (o-Ti-NaY), prepared without the addition of water in the above second step, is readily suspended in an organic solvent as expected.

See also

* Ionic transfer

References

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Catalysts