IN SITU BUILD-UP ELECTRODE FOR ELECTROLYSIS OF ORGANIC COMPOUNDS

ELECTRODE A ACCROISSEMENT IN-SITU DE LA COUCHE, SERVANT A L'ELECTROLYSE DE COMPOSES ORGANIQUES

English Abstract

Described herein is an electrode made inside an
electrochemical cell by entrapping in an open pore matrix a suspension of
essentially non-soluble catalytic particles of high specific surface area. In
preferred embodiments, the matrix is a piece of reticulated vitreous carbon
and the catalytic particles are palladium.

Claims

The embodiments of the invention in which an exclusive property
or privilege is claimed are defined as follows:
1. A process for the electrode build-up inside the
electrochemical cell such in situ build-up leading to an electrode able to
perform electrolysis of organic compounds and comprising the steps of
suspending essentially non soluble catalytic particles of high specific
surface
area in order to trap said particles in a conductive matrix having an open
pore
structure.
2. An electrode as in claim 1 where the said matrix is
constituted of a piece of reticulated vitreous carbon having a pore
concentration
varying from 60 to 400 pores per square inch.
3. An electrode as in claim 2 where the particles inserted in
the matrix are constituted of palladium dispersed on alumina (5% w/w) used for
hydrogenation of phenol in basic or acid media.
4. An electrode as in claim 2 where the particles inserted in
the matrix are constituted of palladium dispersed on alumina (5% w/w) used for
hydrogenation of phenol in basic or acid media.
5. An electrode as in claim 2 where the particles inserted in
the matrix are constituted of palladium dispersed on alumina (5% w/w) used for
hydrogenation of carboxylic acid in basic or acid media.
6. An electrode as in claim 2 where the particles inserted in
the matrix are constituted of palladium dispersed on alumina (5% w/w) used for
hydrogenation of pentachlorophenol in basic or acid media.

7. An electrode as in claim 2 where the particles inserted in
the matrix are constituted of palladium dispersed on alumina (5% w/w) used
for hydrogenation of pentachlorophenol to cyclohexanoi in basic or acid media.

Description

CA 02273688 1999-06-04
IN SITU BUILD-UP ELECTRODE FOR ELECTROLYSIS OF ORGANIC COMPOUNDS
Background of the invention
One branch of heterogeneous catalysis that have received attention in the two
last decade is electrocatalysis, the study of heterogeneous catalytic
reactions
involving reactant species that transfer electrons through an
electrolyte/catalyst
interface. Among electrocatalytic reactions, the electrocatalytic
hydrogenation
(ECH) of organic compounds appears to be very promising not only for
synthetical interests but also for environmental and economical consideration.
In such process, chemisorbed hydrogen (H)M is generated in situ on the
electrocatalyst surface by water electrolysis at low overpotential (eq. 1 )
and
reacts with the adsorbed organic substrate (S) (eq. 2-4)
H20+M+e =MH+OH [1]
R + M = R-M [2]
R-M + 2 H-M=(RHZ)-M [3]
(RHz)M = RHZ + M [4]
However hydrogen evolution reaction competes with hydrogenation (eq. 5-6)
which thus decreases the current efficiency for hydrogenation purpose
MH+Hz0+e =HZ+OH'+M [5]
2 MH = HZ + 2M [6]
This method of generating chemisorbed hydrogen allows reactions similar to
catalytic hydrogenation at much more lower temperature and pressure.
Furthermore, the concentration of chemisorbed hydrogen can be controlled by
adjusting the current density so as to decrease or eliminate hydrogen gas
evolution.
Among the parameters affecting such electrocatalytic process performances, the

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2
nature of the electrode (M) and the electrode surface are of prime importance.
There are some general guidelines to assist the choice of the technique of
electrode preparation and electrode material: the electrocatalytic activity,
the
physical and chemical stability and the ratio cost/lifetime.
Many electrode used consist of a catalytic material coating such as for
example
Raney nickel (R-Ni) or a transition metal (Pt, Pd, Rh) on a carbon support,
the
coating being formed by electrodeposition, plasma spraying, sputtering,
thermal
decomposition, impregnation, ion exchange, etc. However, in order to increase
the surface available for the electrochemical process, the active powder,
which
is characterized by a large surface area, can alternatively, be used directly
as the
electrocatalyst: either by depositing it on a horizontal metallic plate, or by
using
it in a flow-through packed bed reactor or by stirring the powder in the
presence
of a gold electrode. Such process may often suffer of the particle
agglomeration
inside the solution leading to a loss of activity during the course of
electrolysis.
Summary of the invention
Accordingly, it is an object of this invention to provide an electrode that
can
perform hydrogenation of organic compounds, in situ built-up said electrode
resulting from an active powder embedded in a material having an open pore
structure. The electrode is made by stirring the suspension, the catalyst is
introduced inside the pores of the substrate by convection and remains in the
pores throughout the electrolysis. Such electrode presents the main advantage
of giving a high specific surface electrode by using any type of catalysts in
a
powdery form for the ECH of organic compounds.
Detailed description of the invention
The electrode is directly prepared in the cell itself. It is made of a piece
of
material (the matrix) having an open-pore structure allowing the entrapment of
catalyst particles, which are suspended by a rotative move under convective
force. Any type of electrically conductive material which is inert during the

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course of electrolysis and which have an open pore structure is appropriate as
matrix. The specific pore concentration on the matrix can be varied from 60 to
400 pores per square inch (ppi) depending on the size of the particles to be
introduced into the pores. It has been round that a piece of reticulated
vitreous
carbon was particularly suitable due to the inertness of such material in a
lot of
media and under various working conditions.
Any particle having the appropriate catalytic activity and specific surface
area
and which is not substantially dissolved in the course of the
electrocatalytical
process can be used as dispersed particles. The particles should not react
with
the matrix. The mass ratio of particles used to organic substrate to be
hydrogenated can be varied from 0.05 to 1Ø The usefulness of particles on
substrates, for example palladium (or rhodium) dispersed on a substrate such
as
charcoal or alumina has been introduced into the matrix without problem.
After introduction of particles into the cell where the matrix was installed,
a
magnetic bar was allowed to rotate. After less than half an hour, almost al I
the
particles were entrapped in the matrix. The electrode is then ready for use.
After the electrolysis, the particles and the matrix can be separately
recuperate
to be reused. The data of Table 1 illustrate the typical results obtained for
electrolysis of some organic compounds.
Use of the invention
The invention has been particularly useful in producing electrodes of high
specific area, with particulate material that could not be agglomerated or
only
with difficulty. With these large surface area electrodes successful
electrolysis
of organic compounds could be performed with excellent yields.
The following example is given to illustrate the scope rather than limit the
present invention.

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Example 1
A two-compartment jacketed glass H-cell (volume of each compartment 50 ml)
having a Nation-324 (E.1. duPont de Nemurs & Co) membrane (1.8 cm2) as
separator and equipped with a condenser, was connected to a constant
temperature circulating bath. The cathode was a piece of reticulated vitreous
carbon (from ESC, 2.5 x 2 x 0.6 cm3 100 ppi) to which a nickel stripe for
electrical contact was fixed and into which the catalyst powder was embedded
in the following way. The active powder tested was Rh (5°I°) /
A1203. The
amount of active particles used was 0.2 g. The active particles were
introduced
in the cathodic compartment containing 30 ml of 1 M NaOH. The anodic
compartment was filled with a 1 M NaOH solution (30 ml). The catholyte was
stirred (magnetic bar) prior to the application of a constant current. The
counter
electrode was a graphite rod. After less than half an hour of polarisation,
almost
all the active particles were entrapped in the RVC matrix, and the
pentachlorophenol (200 mg, 7.5 x 10~ mol, 8.3 x 10-3 M solution) was then
added to the catholyte. After the electrolysis, the compounds were analyzed
by gas liquid chromatography (G LC) and high pressure liquid chromatography
(HPLC). The ECHsis of pentachlorophenol in 1 M aqueous NaOH leads to the
formation of 92°/° of cyclohexanol (determined by GLC).

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Tablc 1.
Ele~rodGS Electrolysis
Q I T Starting organic Final organic
substrate Product and yield
Examples Particles Matrix Electrolyte / F / mA / °C
1 Rh (5%) Reticulated 1M NaOH 10 25 25 ow off
/AIz03
vitreous cut~on I ~ 97
i
2 Rh (5%) Reticulated 1M N80H 6.5 100 23 °°°"
o°°"
/AlzOj vitreous carbon
84
a~ a~
3 Rh (5%) Reticulated 1M HZSO, 18 25 50
/A1z03 vitreous carbon
- 100%
i
4 Rh (5%) Reticulated 1M NaOH 7 100 25 0°°" off
/Alb vitrcous carbon axH o"
73 %
i
Rh (5%) Reticulated 0.2 M 18 25 60 °" off
IAlzOj vitreous carbon H3H03 off off
50 ~/~
0.4 M
6 Rh (5%) Reticulated IM NaOH 18 25 50 0"
/A1z03 vitreous aeon
100%
7 Rh (5%) Reticulated 1M NaOH 23 25 25 °" °"
/AlzOj vitreous carbon a a
92
a a
a
8 Pd (5%) Reticulated 1M NaOH 16 25 75 °~'~ °"
/AlzOj vitreous capon a
90%
i
a a
9 Pd (5%) Reticulated IM NaOH 6 25 50 ~ \ °"
/C vitreous cafion
I ~ o" I ~ 40 %
off
9 R-Ni Reticulated 1M NaOH 6 25 50
vitreous capon I w
I ~ off ( ~ 52
i
9 Pd (5%) Retlculatod 1M NaOH 6 25 50
/A1z03 vitreous carbon
I ~ off I ~ 62 %
i
Ru~C%) Spongious Nickel 1 M NaOH 6 25 50 ~ o \ °"
I ~ off I ~ X