The selective oxidation of benzyl alcohols in a membrane reactor
G. Grigoropoulou,a J. H. Clark,*a D. W. Hallb and K. Scottb
a Centre for Clean Technology, Chemistry Department, University of York, UK YO10 5DD. E-mail: jhc1@york.ac.uk
b Department of Chemical and Process Engineering, University of Newcastle, Metz Court, Newcastle upon Tyne,
UK NE1 7RU
Received (in Liverpool, UK) 14th November 2000, Accepted 29th January 2001
First published as an Advance Article on the web 28th February 2001
The selective oxidation of benzyl alcohols by hypochlorite
and a phase transfer catalyst has been successfully carried
out in a membrane reactor.
hydrophobic microporous PTFE (0.2 mm) membrane was used
and a slight pressure was applied to the aqueous side to avoid
any transport of the organic phase into the aqueous phase. The
hydrophobic PTFE membrane kept the aqueous phase from
entering into the organic phase. The reaction could be run static
or continuous with counter flowing aqueous and organic
phases.
The influence of several parameters has been investigated so
as to optimise the performance of the system and in particular to
avoid further oxidation of the benzaldehyde to benzoic acid
[eqn. (1)].
Selective oxidation is one of the most important synthetic
transformations in organic chemistry based on hydrocarbon
resources. It is essential that these transformations are based on
the principles of green chemistry and avoid heavy metal
reagents, toxic solvents and the generation of large volumes of
hazardous waste. The major source of waste in many organic
reactions is commonly aqueous or salt effluent produced at the
work-up stage when the organic and inorganic components are
separated, often via an aqueous quench.1
Phase transfer catalysis (PTC) has been widely used to
transfer inorganic anions into organic media by using catalytic
amounts of lipophilic quaternary ammonium or phosphonium
salts.2 Sodium hypochlorite is a non-toxic oxidant, which has
been reported to be transferred into the organic phase in the
presence of quaternary ammonium salt3–6 and is capable of
oxidising alcohols including benzyl alcohol although it can be
difficult to control selectivity.7–9
Most PTC reactions are carried out on an industrial scale in
stirred tank reactors, which require the separation of products
afterwards. Key criteria required in a PTC reactor are high
interfacial area with little emulsification to enable easy phase
separation. The use of a membrane reactor for a PTC reaction
could fulfil these criteria as well as offering the advantages of
easy product recovery. Furthermore, the organic-free reduced
aqueous phase can be electrochemically regenerated while the
organic phase remains uncontaminated with aqueous species
thus facilitating continuous reactions. The use of a membrane
reactor in a phase transfer catalysed reaction has been
theoretically modelled before10 and tested in a simple anion
displacement reaction. It has been shown that the reactor
performance depends on the flow rate ratio and conversions are
slower than predicted due to mass transfer resistance. The
membrane acts as a stable interface and no emulsification
problems are involved.
(1)
The effect of benzyl alcohol concentration on the reaction
profiles was first investigated (Fig. 2). As the concentration of
benzyl alcohol increases the rate of conversion increases
slightly. For each reaction a small induction period is observed,
due to slow mass transfer of hypochlorite although this is not
observed if the system is allowed to equilibrate before the
alcohol substrate is added. The rate of the reaction does not
depend as much on the concentration of benzyl alcohol as
expected, however the concentration of PTC increases the rate
due to easier mass transfer. Selectivity to the aldehyde is also
reduced at low concentrations of the PTC. Changing the counter
anion in the PTC (HSO42, Cl2, Br2) has little effect on reaction
rate or selectivity but little reaction occurs in the absence of a
PTC. While the hypochlorite concentration has only a small
effect on the rate of conversion of benzyl alcohol, it strongly
influences the selectivity of the reaction. The amount of
hypochlorite transferred into the organic phase is dependant on
both the concentration of quaternary ammonium in the organic
phase and that of the hypochlorite in the aqueous phase. The
optimum concentration of hypochlorite at which we obtain a
good rate of conversion and very high selectivity to the
aldehyde is 13%.
Toluene can be used as a solvent for the reaction, although it
decreases the rate of conversion due to slower mass transfer and
benzyl chloride was produced from its oxidation under the
standard reaction conditions.
Here we report the selective oxidation of benzyl alcohols
using a porous PTFE membrane to separate the aqueous and
organic phases. A flat sheet diaphragm reactor shown in Fig. 1,
was used to carry out the reaction. In a typical reaction, a 13%
aqueous solution of hypochlorite (25 ml), was adjusted to pH 9
and then added in the aqueous side of the reactor. Benzyl
alcohol (3 mmol), and tetrabutylammonium hydrogen sulfate
(0.3 mmol), were dissolved in DCM on the organic side. A
Fig. 2 Influence of the concentration of benzyl alcohol on the conversion of
Fig. 1 Schematic diagram of flat sheet diaphragm cell.
the reaction.
DOI: 10.1039/b009178m
Chem. Commun., 2001, 547–548
This journal is © The Royal Society of Chemistry 2001
547
Grigoropoulou
