Selective electrosynthesis of p-methoxybenzaldehyde
at spinel type CoMn
2
O
4
/titanium-composite anodes*
E. LODOWICKS, F. BECK
University of Duisburg, Fachgebiet Elektrochemie, Lotharstr. 1, D-47057 Duisburg, Germany
Received 21 May 1997; revised 11 September 1997
Activated titanium anodes with a spinel coating of CoMn
2
O
4
of about 1 lm thickness were devel-
oped. A relative stability in acid electrolytes was found. The novel anode was employed for the
anodic oxidation of p-methoxytoluene (PMT) to p-methoxybenzaldehyde (PMB), which is of
industrial interest. Batch type electrolyses with 30% theoretical conversion (4 F mol
A1
) were used for a
parametric screening. The solvent/electrolyte-system was 1
M
H
2
SO
4
/5
M
H
2
O in methanol. Optimum
results (60% selectivity, 50±60% current eciency) were obtained at low concentrations of the educt
(0.2
M
). A part of the current is consumed for the formation of the benzylmethylether, which can be
further oxidized to PMB. It was proved that the novel anode operates according to the mechanism of
heterogeneous redox catalysis with ter- and heptavalent manganese as the redox species. A turnover
factor of >2000 is unusual for a spinel in acid solution.
Keywords: organic electrosynthesis, p-methoxybenzaldehyde, p-methoxytoluene, cobalt manganese spinel, titanium composite
anodes, heterogeneous redoxcatalysis
1. Introduction
Benzaldehyde and its paraderivatives are important
organic intermediates. They can be synthesized by
direct anodic oxidation of the corresponding tolu-
enes, even at an industrial scale [1±5]. This electro-
synthesis is superior to the traditional chemical routes
via chlorine chemistry, where benzalchlorides play an
important role as intermediates. However, the full
potential of this opportunity is not yet realized at
classical anode materials like carbon, lead dioxide or
platinum due to the formation of acetales as inter-
mediates [1±5] or due to the application of dissolved
oxidants like Mn
3+
or Ce
4+
in relative dilute elec-
trolytes [2]. The optimum strategy would be the uti-
lization of a selective anode with an appropriate
surface ®xed oxidic redox system. Beer has demon-
strated titanium to be a highly feasible base material
for this purpose [6]. Ti/RuO
2
-composite anodes with
a variety of other oxides like TiO
2
, SnO
2
,Sb
2
O
3
or
IrO
2
have been employed for many years as chlorine
or oxygen anodes.
These oxides have no advantage for anodic oxi-
dation in organic chemistry due to their low O
2
- and
Cl
2
-overvoltage. But other oxides like Cr
2
O
3
[7] do
overcome this problem. Previous work on Mn-sys-
tems as speci®c redoxmediators in electroorganic
oxidations demonstrated the feasibility of ceramic or
electrodeposited MnO
2
-layers [8], of electrogenerated
MnO
2
slurries [9] and of the redox couple Mn
2+
/Mn
3+
[10, 11]. From a historical point of view, it should be
mentioned, that Eberson and Nyberg [12, 13] initi-
ated the direct acetoxylation route of toluene (deriv-
atives) at platinum and carbon anodes. Anodic
methoxylation at the same anodes in methanolic
electrolytes, containing LiBF
4
or NaOMe, was de-
scribed by Ronla
Â
n et al. [14].
Recently, the relative stability of Ti/CoMn
2
O
4
anodes of the spinel type was demonstrated for sim-
ple electroorganic model reactions such as
isopropanol 3 acetone in acid solutions [15].
CH
3
eCHOHeCH
3
À3 CH
3
eCOeCH
3
2H
2e
À
1
In the present paper the application of these novel
anodes for the more interesting oxidation mentioned
above is shown [16].
2. Experimental details
Chemicals for electrolyte preparation were analytical
grade. H
2
SO
4
(95±97%), acetonitrile, methanol and
isopropanol were from Riedel de Hae
È
n. Organic
compounds as the starting material, 4-methylanisol,
p-methoxytoluene (PMT) (Merck) and standards for
GC such as 4-methoxybenzaldehyde (PMB) (Merck),
4-tert-butylbenzaldehyde (Lancaster) and others were
generally of 98±99% purity.
The preparative anodes were 50 mm ´ 50 mm ´
1 mm sheets of titanium (Thyssen A.G., Krefeld,
Contimet 35
TM
). A current lead of 10 mm ´ 50 mm
was cut at the middle of one side. The quadratic
electrode areas were cleaned and degreased and
* Dedicated to Professor Dr. Hans Scha
È
fer on the occasion of his 60th birthday.
JOURNAL OF APPLIED ELECTROCHEMISTRY 28 (1998) 873±880
0021-891X
Ó
1998 Chapman & Hall
873