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.

By using a PTC process, one can achieve faster reactions, obtain higher conversions or yields, make fewer byproducts, eliminate the need for expensive or dangerous solvents that will dissolve all the reactants in one phase, eliminate the need for expensive raw materials and/or minimize waste problems.^[1] Phase-transfer catalysts are especially useful in green chemistry—by allowing the use of water, the need for organic solvents is reduced.^[2][3]

Contrary to common perception, PTC is not limited to systems with hydrophilic and hydrophobic reactants. PTC is sometimes employed in liquid/solid and liquid/gas reactions. As the name implies, one or more of the reactants are transported into a second phase which contains both reactants.

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.^{[4][5][6][7][8]} 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.

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, but are unstable toward base, degrading to phosphine oxide.^[9]

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:

${\displaystyle {\ce {C8H17Br_{(org)}{}+ NaCN_{(aq)}->[{\ce {R4P+Br-}}] C8H17CN_{(org)}{}+ NaBr_{(aq)}}}}$

By the quaternary phosphonium cation, cyanide ions are "ferried" from the aqueous phase into the organic phase.^[10]

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.^[11]

An alternative to the use of "quat salts" is to convert alkali metal cations into hydrophobic cations. In the research lab, crown ethers are used for this purpose. Polyethylene glycols are more commonly used in practical applications. These ligands encapsulate alkali metal cations (typically Na⁺ and K⁺), affording large lipophilic cations. These polyethers have a hydrophilic "interiors" containing the ion and a hydrophobic exterior.

Chiral phase-transfer catalysts have also been demonstrated.^[12]

PTC is widely exploited industrially.^[9] Polyesters for example are prepared from acyl chlorides and bisphenol-A. Phosphothioate-based pesticides are generated by PTC-catalyzed alkylation of phosphothioates. One of the more complex applications of PTC involves asymmetric alkylations, which are catalyzed by chiral quaternary ammonium salts derived from cinchona alkaloids.^[13]

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:
  1. transfer of aqueous phase from organic phase to the external surface of solid catalyst;
  2. transfer of aqueous phase inside the pore volume of solid catalyst;
  3. transfer of the substrate from aqueous phase to the interphase between aqueous and organic phases
  4. transfer of the substrate from the interphase to the aqueous phase;
  5. mixing and diffusion of the substrate in the aqueous phase;
  6. transfer of the substrate from the aqueous phase to the external surface of solid catalyst;
  7. transfer of the substrate inside the pore volume of the solid catalyst;
  8. catalytic reaction (adsorption, chemical reaction and desorption).

In some systems, without vigorous stirring, no reactivity of the catalyst is observed in conventional catalytic system.^{[4][5][6][7][8]} 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.^[4][5][6] The active site located on the external surface of the zeolite particle were dominantly effective for the observed phase boundary catalytic system.^{[7] [14]}

Modified zeolite on which the external surface was partly covered with alkylsilane, called phase-boundary catalyst was prepared in two steps.^{[4][5][6][7][8]} 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.

Janus interphase catalyst is a new generation of heterogeneous catalysts, which is capable to do organic reactions on the interface of two phases via the formation of Pickering emulsion.^[15]

• Ionic transfer