Handbook of Solvents - George Wypych - ChemTech - Ventech!

13.3 Effects of organic solvents on phase-transfer catalysis 833
13.3.2.2 Effect of solvents on the reaction in triphase catalysis
The disadvantage of using liquid-liquid phase-transfer catalysis (ll-PTC) is in the separation
of catalyst from product after reaction. This problem can be overcome using the immobilized
catalyst on a solid support (e.g., porous polymer pellet). Simple mechanical separation
processes, such as filtration or centrifugation, can be employed to separate the solid catalyst
from the product in liquid form. A detailed investigation of the effect of polymer particle on
the reaction rate was conducted by Wang and coworkers. 131,133-137 Wang and Wu 136 studied
the reaction of substitution of hexachlorocyclotriphosphazene and 2,2,2-trifluoroethanol in
an organic solvent/alkaline solution by triphase catalysis. The polymer, which was prepared
from the polymerization of styrene monomer and chloromethylstyrene monomer, served as
the support for the immobilization of the catalyst.
The effects of the structure of the polymer support, which can be related to the factors
of the degree of crosslinking, ring substitution (RS), lipophilicity of the polymer, the chloride
density and solvents on the imbibed compositions 84,118 on the reaction rate or conversion
were investigated. This imbibed composition, influenced by the internal structures of
the triphase catalyst particles, affected the reactivities. The interaction of the polymer support
pellet and the organic solvents play an important role in determining the reaction rate
and the conversion of the reactant. The reaction could be improved to obtain a high reaction
rate by using a polar solvent.
For investigating the degree of crosslinking of the polymer, the resistance of mass
transfer within the catalyst pellet is small. When a smaller degree of crosslinking of the
polymer support is used. This is due to the fact that a larger value of the swell of the polymer
was obtained when a small degree of crosslinking of the polymer was used. Wang and Yu 131
have similar observations for the reaction of allyl bromide and 2,4-dibromophenol under
triphase catalysis. A maximum value exists for the degree of swell and the imbibed composition,
as shown in Table 13.3.28 for the degree of crosslinking. 136
In Table 13.3.27, 136 the degree of swell for the polymer support with a 6% crosslinking
is larger than that for the two other degrees of crosslinking. This implies that greater
amounts of NaOCH2CF3 were imbibed into the catalyst pellet with a 6% crosslinking. The
reaction rate is directly related to the amount of the imbibed composition. Also, in Table
13.3.27, the imbibed compositions are affected by the structure of the polymer support. The
reactivity of the triphase catalysis can also be determined from the composition imbibed by
the particles. It can be observed that the reactivities were highly affected by the lipophilicity
of the catalyst pellet for the substitution reaction in the organic phase and the hydrophilicity
of the catalyst pellet of the ion exchange in the aqueous phase. For example, the reaction
rate in the organic phase was promoted by using a lipophilic polymer support catalyst when
the substitution reaction rate was slow. In Table 13.3.27, the amount of chlorobenzene and
water imbibed in the macroporous pellet was greater than that in the microporous pellets for
most cases. 136 However, the macroporous pellet with 10% degree of crosslinking had the
least lipophilicity and degree of swelling; therefore, the reactivity was the lowest for the
macroporous pellet with a 10% crosslinking among the polymer-support catalysts. The reactivity
environments which were created by the lipophilicity and the hydrophilicity of the
polymer support plays an important role in determining the reactivity.
It is known that the distribution of organic phase and aqueous phase existing in the porous
pellet is affected by a change of the ring substitution (RS) of the polymer support. 84
Wang and Wu 136 prepared three kinds of polymer supports with different numbers of ring