Handbook of Solvents - George Wypych - ChemTech - Ventech!

828 Maw-Ling Wang
Table 13.3.24. Effect of organic solvent on the conversion and selectivity
Solvent Conversion, % Selectivity, % C.E., % Energy consumption, KWH/mole
CCl4 42 100 83 0.226
CHCl3 38 100 70 0.268
Toluene 17 81 34 3.154
Cyclohexane 12 100 23 4.662
Data obtained from Tasi and Chou; 120 Reaction conditions: 0.8 M of cyclohexanol, 15 mAcm -2 of current density,
graphite cathode and graphite anode, 0.007 M of RuO 2, pH=4, 0.99 Fmol -1 electricity passed, NaCl saturated solution
as electrolyte, 1275 rpm, 5 o C
13.3.1.6 Other effects on the phase-transfer catalytic reactions
Simple mechanical separation such as filtration, centrifugation or phase separation can be
used to separate the product and the phase-transfer catalyst by use of insoluble catalysts.
However, the more frequently encountered technical problems in use of PTC for industrial
applications is the need to separate the product and the phase-transfer catalyst by chemical
equilibrium separation method in the liquid-liquid two-phase phase transfer catalytic reaction.
The most commonly used methods for separation of products and PTC catalysts on an
industrial scale are extraction and distillation. Other separation methods include
sorption 3,33,57 and reaction. 45
The principle of extraction method used to separate PTC and product is based on solubility
of quaternary ammonium salt in alkaline aqueous solution. 2,25,104 For example,
tetrabutylammonium bromide is soluble to the extent of 27% in dilute (1% NaOH) aqueous
solutions, but when the solution is made more concentrated (15% NaOH), the solubility of
Bu4N + Br - decreases to 0.07%. When the products are obtained in PTC system, they can be
usually separated from PTC by distillation method. PTC catalyst in the distillation residue
may sometimes be reusable. With quaternary ammonium salts as catalysts, temperatures
above 100-120 o C usually result in partial or total decomposition of the quaternary salts to
trialkylamines and other products. Mieczynska et al. 70 and Monflier et al. 72 investigated the
hydrogenation and hydroformylation under phase transfer catalytic conditions. They found
that the yield of aldehydes obtained in hydroformylation of 1-hexene strongly depends on
solvent: 24% in toluene, 53-86% in toluene-water-ethanol mixture and 77-94% in water-ethanol
solution. The mixture of water-ethanol as a solvent was also found to be the best
for hydrogenation of 1-hexene (96% of hexane). Conversion of Ph2PCH(CH3)(COOH) phosphine into sodium salt Ph2PCH(CH3)(COONa) yields aldehyde in toluene, 92% in toluene-water
and 94% in toluene-water-ethanol mixture.
In principle, hydroxide anion is very difficult to transfer from aqueous to organic
phases, yet it is one of the most valuable and most commonly used anions in the PTC systems.
Addition of small amounts of alcohols to PTC systems requiring hydroxide transfer
causes a dramatic increase in rates. Therefore, addition of alcohol enhances the PTC reaction
as the cocatalytic effect. For example: formation of alkoxide anions, RO - , which are
more readily transferred than the highly hydrated hydroxide anion, and which can serve as a
strong base just as well as OH - , and solvation of the hydroxide with alcohol rather than with
water, making the hydroxide anion more organophilic and more easily transferred. 99,100