Chemistry Reference
In-Depth Information
Toluene
organic solution
1
aqueous feed
2
2
2+s
3
2
3
1+s
2 +s
4
organic feed
perforated plate
1+s
aqueous solution
5
40%
NaOH
TBAB
Fig. 10.2
Weng's Continuous Flow Mixer-Settler Reactor for
Three Liquid Phase Processes.
Fig. 10.1
Ternary Phase Diagram of TBAB/Toluene/40% NaOH
Showing the Region of Three Liquid Phases.
Yadav recently has explored the aromatic nu-
cleophilic substitution of
p
-nitrochlorobenzene with
butoxide anion under liquid/liquid/liquid PTC con-
ditions using the NaOH/TBAB combination. It was
found that upon increasing the catalyst loading a
third phase is observed when the catalyst amount
reaches a certain critical mass. At this point a very
sharp increase is observed in the reaction rate. Under
standard conditions 95% of the reaction was deter-
mined to take place in the third catalyst-rich phase
and only 5% in the organic phase. Other factors
affecting the system, such as speed of agitation,
p
-
chloronitrobenzene (PCNB) concentration, NaOH
concentration and temperature, also were analysed.
Experiments aimed at examining the reusability the
catalyst layer indicated that a certain loss of catalyst
takes place upon recycling. A kinetic model has
been proposed. A theoretical model for the three-
phase catalytic system also was proposed by Yang
[201].
Wang & Weng have applied a similar three-phase
system for the etherification of sodium phenolate
with
n
-butyl bromide catalysed by TBAB [202] and
concluded that the formation of the third phase
results in a critical increase in the reaction rate.
For the same reaction, Wang & Weng designed and
tested a five-section, vertical, continuous-flow,
stirred laboratory reactor of 163 cm
3
total volume
(5.45 cm i.d. and 22 cm height) shown in Fig. 10.2.
The central third section is the main site of the reac-
tion. The lighter organic phase was introduced to
section 4 and the heavier aqueous phase was ad-
mitted to section 2. The counter-current system
[194]. The number of liquid phases in the seven
distinct regions are shown within each section (S
represents the presence of a solid precipitate).
Similar conclusions were derived by Ido
et al
.,
who have studied the dehydrohalogenation of 2-
bromooctane with various onium salt catalysts in
the presence of aqueous KOH [195]. These authors
report that with dodecane as organic solvent TBAB,
THAB, TBPB and hexadecyltributylphosphonium
bromide all easily formed the third catalyst-rich
phase [196]. The same authors also examined the
halogen exchange between benzyl chloride and
KBr under the three-phase conditions. In this study
it was confirmed that the TBAB phase could be
recycled without any apparent loss in activity [197].
In another study this group examined the reaction
of benzyl chloride with sodium sulfide catalysed
by THAB [198]. The dramatic rate increase was mon-
itored here as well upon the appearance of a third
phase. In the presence of the third phase the reac-
tion rate increased with temperature up to 60°C. At
higher temperature the third phase dissolved in the
organic phase and the rate sharply decreased.
Correia [199] found that the addition of TBAHS to
a solution of NaBr in aqueous hypochlorite results in
the formation of a third phase composed mainly of
Bu
4
N
+
Br
3
-
and the organic solvent. This three-phase
system was used for the bromination of olefins and
acetylenes or for the oxidation of diphenylmethane
to benzophenone. A similar system was used by
Clark
et al
. for the side-chain oxidation of alkyl
aromatic compounds [200].
