Phase Transfer Catalysis - Publications of the IAS Fellows

modification methods to graft reactive molecules to the available
hydroxyl groups on the clay surface through coupling
agents lead to low loading capabilities. To have higher loading
possible and still retain the advantages of using clay as a
support, Akelah et al. (1994) supported phosphonium moieties
on a polymer-clay composite, which was synthesized by
grafting copolymers of styrene and chloromethylstyrene with
2% ammonium salts onto montmorillonite interlayers by a
cation exchange process.
IWechanism of Triphase Catab,sis.. Although the activity of
a supported PT catalyst is usually less than that of the corresponding
soluble catalyst, it is believed (Molinari et al., 1979;
Montanari et al., 1983, Anelli et al., 1984) that the mechanism
of the phase-transfer cycle remains the same. However,
there are certain characteristics typical of heterogeneous catalysts
that make supported PTC different from soluble PTC.
For example, in a triphase catalytic system, one does not consider
the planar phase boundary as in a classical two-phase
system. Instead, a volume element which incorporates the
catalytic active sites as well as the two liquid phases has to be
considered. Diffusion of both the aqueous and organic phases
within the solid support is important. Various mechanisms
have been proposed for triphase catalysis, some of which are
touched upon here. However, it should be noted that no single
mechanism has been verified completely, and it is quite
possible that the true mechanism involves a combination of
the various mechanisms proposed so far.
Like in traditional heterogeneous catalysis, the kinetics of
supported PTC is influenced by three fundamental processes:
(1) Mass transfer of reactants from bulk liquid phase to
the surface of the catalyst particle.
(2) Diffusion of the reactant molecules from the catalyst
particle surface to the active sites within the porous particle.
(3) Intrinsic reactivity of reaction at the active sites.
Triphase catalysis is more complicated than traditional heterogeneous
supported catalysis, because it involves not merely
diffusion of a single gaseous or liquid phase into the solid
support but requires the diffusion of both the aqueous phase
(for ion exchange to take place) and the organic phase (for
the organic reaction to take place) to the solid surface and
within the solid (Figure 12). The fact that the aqueous phase
and the organic phase interact differently with different solid
supports further complicates the issue. In a typical triphase
system, since the catalyst support is usually lipophilic, the organic
phase fills the catalyst pores and forms the continuous
phase with the dispersed aqueous phase droplets diffusing
through it to reach the quat species immobilized at the solid
surface. Also, the choice of support can sometimes alter the
reaction mechanism. For example, Tundo and Venture110
(1981) reported a mechanism for PTC reactions using silica
gel as support, which accounted for the active participation
of the gel by adsorption of reagents,
In general, any mechanism for triphase catalysis has to account
for the interaction of two immiscible phases with a solid
bound catalyst. Telford et al. (1986) suggested an alternating
shell model that requires periodical changes in the liquid
phase filling the pores of the catalyst pores. Schlunt and Chau
(1986) from the same research group tried to validate this
model using a novel cyclic slurry reactor that allowed the organic
and aqueous phases to contact the catalyst sites in con-
630 March 1998
0 0 0
0
0 0 0 0 0 0 0 organic droplets phase
0 0 OY\
0 0
0
0 Q(-&
aqueous phase 0 0 0 0
aqueous film A
catalyst
’
0 00 0
triphase
catalyst
aqueous phase
Figure 12. Plausible mechanism for liquid-liquid-solid
triphase catalysis.
Adapted from Desikan and Doraiswamy (1995).
trolled sequential steps. Their results indicated that only the
catalyst in a thin shell near the particle surface was utilized.
We do not fully comprehend the driving force for the alternative
exchange of liquids within a solid support, since a solid
support particle is either hydrophilic or lipophilic, and cannot
alternate between the two. A more realistic mechanism
(Tomoi and Ford, 1981; Hradil et al., 1988) involves the collision
of droplets of the organic (or aqueous) phase with solid
catalyst particles dispersed in a continuous aqueous (or organic)
phase. The hydrophilic (or lipophilic) nature of the
support determines which phase fills the pores of the catalyst
and acts as the continuous phase. Svec’s model (Svec, 1988)
for transport of the organic reagent from the bulk phase
through water to the catalyst particle has been developed in
terms of emulsion polymerization (Figure 13). Since free migration
of the ion pairs between phases is not possible because
the cation is part of the solid support, it is necessary
for the immobilized PT catalyst to be just at the boundary of
the two phases or to fluctuate between the two. The presence
of spacer chains is believed to help such an oscillation of the
Catalyst
particles
Organic
droplets
Aqueous phase M+Y-
(continuous +
phase)
Figure 13. Mechanism of triphase catalysis based on
emulsion polymerization.
Adapted from Svec 0988).
Vol. 44, No. 3 AIChE Journal
J