U.S. patent application number 13/265412 was filed with the patent office on 2012-02-09 for process for preparing reactive zinc by electrochemical reduction.
This patent application is currently assigned to BASF SE. Invention is credited to Daniel Breuninger, Markus Brueggemann, Steven Brughmans, Andreas Fischer, Ulrich Griesbach, Itamar Michael Malkowsky, Marc Martin, Daniela Mirk, Laszlo Szarvas, Gerrit Waters.
Application Number | 20120031771 13/265412 |
Document ID | / |
Family ID | 42226658 |
Filed Date | 2012-02-09 |
United States Patent
Application |
20120031771 |
Kind Code |
A1 |
Malkowsky; Itamar Michael ;
et al. |
February 9, 2012 |
PROCESS FOR PREPARING REACTIVE ZINC BY ELECTROCHEMICAL
REDUCTION
Abstract
The invention relates to a process for preparing reactive zinc
by electrochemical reduction, wherein iron or steel is used as
cathode material.
Inventors: |
Malkowsky; Itamar Michael;
(Speyer, DE) ; Mirk; Daniela; (Speyer, DE)
; Martin; Marc; (Lemfoerde, DE) ; Szarvas;
Laszlo; (Ludwigshafen, DE) ; Brueggemann; Markus;
(Grosskarlbach, DE) ; Brughmans; Steven;
(Mannheim, DE) ; Breuninger; Daniel;
(Bobenheim-Roxheim, DE) ; Waters; Gerrit;
(Karlsruhe, DE) ; Griesbach; Ulrich; (Mannheim,
DE) ; Fischer; Andreas; (Heppenheim, DE) |
Assignee: |
BASF SE
Ludwigshafen
DE
|
Family ID: |
42226658 |
Appl. No.: |
13/265412 |
Filed: |
April 7, 2010 |
PCT Filed: |
April 7, 2010 |
PCT NO: |
PCT/EP10/54559 |
371 Date: |
October 20, 2011 |
Current U.S.
Class: |
205/457 |
Current CPC
Class: |
C25C 5/02 20130101; C25C
1/16 20130101 |
Class at
Publication: |
205/457 |
International
Class: |
C25B 3/12 20060101
C25B003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 20, 2009 |
EP |
09158225.4 |
Claims
1-9. (canceled)
10. A process for preparing reactive zinc, the process comprising:
a) filling an electrolysis cell comprising a cathode and a zinc
anode with at least one electrolyte selected from the group
consisting of N,N-dimethylformamide, N,N-dimethylacetamide,
N-methylpyrrolidone and a tertiary amide, the electrolyte further
comprising: at least one redox mediator selected from the group
consisting of naphthalene, N,N-dimethyl-1-naphthalene, a
1-substituted naphthalene, phenanthrene, anthracene, 4,4'-bipyridyl
and 4,4'-di-tert-butylbiphenyl; and at least one electrolyte salt
selected from the group consisting of tetrabutylammonium
fluoroborate and sodium methylsulfonate, b) applying an electric
current to the cell until a 2-20% by weight strength suspension of
reactive zinc in the electrolyte is formed, wherein the cathode
comprises iron or steel and the applying of the electric current is
carried out at a temperature in a range from 20 to 60.degree.
C.
11. The process of claim 10, wherein the at least one electrolyte
is N,N-dimethylformamide.
12. The process of claim 10, wherein the at least one electrolyte
salt is tetrabutylammonium tetrafluoroborate.
13. The process of claim 10, wherein the applying of the electric
current is carried out at a temperature in a range from 35 to
45.degree. C.
14. The process of claim 10, wherein a current density of from 1 to
4 A/dm.sup.2 is employed.
15. The process of claim 10, wherein the electrolysis cell is an
undivided electrolysis cell.
16. The process of claim 10, wherein the cathode is an iron or
steel tube and the zinc anode is arranged concentrically within the
cathode.
17. The process of claim 10, wherein the process is carried out
batchwise.
18. The process of claim 10, wherein the process is carried out
continuously.
19. The process of claim 11, wherein the at least one electrolyte
salt is tetrabutylammoniurn tetrafluoroborate.
20. The process of claim 11, wherein the applying of the electric
current is carried out at a temperature in a range from 35 to
45.degree. C.
21. The process of claim 12, wherein the applying of the electric
current is carried out at a temperature in a range from 35 to
45.degree. C.
22. The process of claim 10, wherein a current density of from 1.5
to 3 A/dm.sup.2 is employed.
23. The process of claim 10, wherein a current density of from 1.5
to 2.5 A/dm.sup.2 is employed.
24. The process of claim 11, wherein a current density of from 1 to
4 A/dm.sup.2 is employed.
25. The process of claim 12, wherein a current density of from 1 to
4 A/dm.sup.2 is employed.
26. The process of claim 13, wherein a current density of from 1 to
4 A/dm.sup.2 is employed.
27. The process of claim 18, further comprising, after obtaining a
2-20% by weight strength suspension of reactive zinc, c)
discharging the electrolyte from the electrolysis cell, and
simultaneously introducing an equal amount of a fresh electrolyte.
Description
[0001] The invention relates to a process for preparing reactive
zinc by electrochemical reduction, wherein iron or steel is used as
cathode material.
[0002] There is a great need for processes for preparing reactive
zinc as starting material for functionalized metal-organic building
blocks. These building blocks serve, for example, for the
construction of pharmacologically relevant active compounds or
complex agrochemicals. Thus, zinc organyls which can be obtained
from reactive zinc can be used in transition metal-aided couplings
to form C,C bonds, with allyl, aryl, alkenyl and alkynyl halides
being able to be used as coupling participants. Furthermore, zinc
organyls can be added onto carbonyl compounds, with chiral
auxiliary reagents even making stereoselective transformations of
this type possible.
[0003] The direct synthesis of zinc organyls from elemental zinc is
possible in only few cases because of a passivating ZnO layer.
These include the Reformatsky regents which are synthesized from
commercial zinc powder and .alpha.-haloacetic esters. In addition,
reactive halogen compounds, first and foremost alkyl iodides, can
be reacted with unactivated zinc powder. A disadvantage of this
reaction is that only zinc organyls of .alpha.-haloacetic esters or
alkyl iodides and no other functionalized zinc organyls can be
obtained, so that this method of preparation is very restricted and
extremely substrate-specific.
[0004] However, the majority of zinc organyls cannot be obtained
from unactivated, elemental zinc. Various processes for the
activation of zinc and the subsequent syntheses of the
corresponding zinc organyls have been described in the prior
art.
[0005] In Handbook of Functionalized Organometallics--Applications
in Synthesis, Wiley-VCH Verlag Weinheim, 2005, P. Knochel describes
various methods of obtaining zinc organyls. These include
transmetalation, chemical activation of zinc and the preparation of
reactive zinc by chemical reduction.
[0006] For the purposes of the present invention, transmetalation
is the reaction of a metal organyl with a usually inorganic metal
salt, resulting in the organyl part being transferred from one
metal to the other. Li and Mg organyls can also be used to generate
the various corresponding zinc organyls. A great disadvantage of
this process is that it is usually only possible to prepare
unfunctionalized metal organyls since many functional groups are
not compatible with Li and Mg organyls. Functional groups such as
nitriles, carboxylic esters, ketones or tertiary amides are
attacked by addition of Li and Mg organyls and are thus no longer
available for further reactions. Other functions such as
acetylides, secondary amides or nitro compounds which comprise
moderately acidic protons can be deprotonated by strong metal
organyls, as a result of which these, too, are no longer available
for further reactions.
[0007] Zinc is chemically activated in classical processes by means
of LiCI, iodide, dibromoethane or TMSCI as auxiliaries. All these
reagents serve to overcome the passivating ZnO layer. The
disadvantage of these reactions is that the chemical auxiliaries
have to be added in substoichiometric or stoichiometric amounts and
must not interfere in subsequent reactions of the zinc organyls.
The use of these zinc organyls is therefore limited.
[0008] Rieke.RTM. zinc is a reactive zinc-comprising reagent which
is obtained by chemical reduction of ZnCl.sub.2 by means of lithium
metal in the presence of naphthalene. This material has a very high
reactivity compared to chemically activated zinc. This reactivity
results from generation of the zinc under oxygen- and water-free
conditions, as a result of which the formation of a passivating
oxide layer is avoided. A disadvantage of this reaction is that
lithium has to be used in stoichiometric amounts, so that high raw
material costs are incurred and it is also necessary to accept an
increased outlay for safety measures for handling the reactive
alkali metal.
[0009] In Handbook of Functionalized Organometallics--Applications
in Synthesis, Wiley-VCH Verlag Weinheim, 2005, P. Knochel also
describes the electrochemical activation of zinc. WO-A 01/02625
describes the transition metal-catalyzed electrochemical reduction.
Here, a zinc anode is dissolved anodically to generate Zn.sup.2+in
solution. At the same time, the transition metal is reduced at the
cathode, and is then inserted into the C-halogen bond and transfers
the organic radical to the Zn.sup.2+. Transition metals which can
be used are Ni, Co and Fe. A disadvantage of this method is the
presence of the transition metal in the future product. The zinc
organyl produced is inherently contaminated with the transition
metal which can then also be present in the products of downstream
stages. However, especially in the synthesis of pharmacological
active compounds, contamination by transition metals has to be
avoided and zinc organyls from the above-described method are
therefore not suitable for this purpose.
[0010] In Tetrahedron 2005, 61, 11125-11131, N. Kurono, T. Inoue
and M. Tokuda describe a further method for preparing reactive zinc
by electrochemical reduction, namely electrogenerated zinc (EGZn).
In this process, Zn is anodically dissolved in order to generate
Zn.sup.2+ions in the solution. Zn.sup.2+is subsequently reduced by
means of a redox mediator such as naphthalene or directly at the
cathode and forms elemental zinc in solution. This is extremely
reactive for insertions into C-halogen bonds since it does not
comprise any passivating oxide layer. Disadvantages of this process
are the use of Pt electrodes which cannot be used on an industrial
scale and are too expensive and low temperatures in the range from
0 to -10.degree. C. which would lead to increased costs.
[0011] It is therefore an object of the present invention to
provide a process which makes it possible to prepare reactive zinc
very inexpensively without the use of chemical reducing agents or
reagents and can also be used on an industrial scale.
[0012] This object is achieved by a process for preparing reactive
zinc, which comprises the following steps [0013] a) provision of an
electrolysis cell having a cathode and a zinc anode, [0014] b)
charging of the electrolysis cell with an electrolyte selected from
the group consisting of N,N-dimethylformamide,
N,N-dimethylacetamide, N-methyl-pyrrolidone and other tertiary
amides which comprises an electrolyte salt selected from the group
consisting of quaternary ammonium salts, organic metal salts and
inorganic metal salts, [0015] c) application of electric current to
the cell until a 2-20% strength suspension of reactive zinc in the
electrolyte has been formed, wherein an iron or steel cathode is
used as cathode and the electrochemical reduction is carried out at
temperatures in the range from 20 to 60.degree. C.
[0016] The process of the invention is advantageous when
N,N-dimethylformamide is used as electrolyte.
[0017] The process of the invention is advantageous when
tetrabutylammonium tetrafluoroborate is used as electrolyte
salt.
[0018] The process of the invention is advantageous when the
electrolyte further comprises a redox mediator selected from the
group consisting of naphthalene, N,N-dimethyl-1-naphthalene and
further 1-substituted naphthalenes and also phenanthrene,
anthracene, 4,4'-bipyridyl, and 4,4'-di-tert-butylbiphenyl.
[0019] The process of the invention is advantageous when the
temperature at which the electrochemical reduction is carried out
is in the range from 35 to 45.degree. C.
[0020] The process of the invention is advantageous when a current
density of from 1 to 4 A/dm.sup.2 is set.
[0021] The process of the invention is advantageous when an
undivided electrolysis cell is used.
[0022] The process of the invention is advantageous when an iron or
steel tube is used as cathode and the zinc anode is arranged
concentrically within the cathode.
[0023] The process of the invention is advantageous when it is
carried out batchwise.
[0024] The process of the invention is advantageous when it is
carried out continuously.
[0025] In the process of the invention, the activated zinc is
produced by electrochemical reduction of zinc ions provided by
dissolution of the zinc anode in an electrolysis cell. Any
electrolysis cell known to those skilled in the art, e.g. a divided
or undivided flow cell, capillary gap cell or plate gap cell, is
suitable for this purpose. Preference is given to the undivided
flow cell.
[0026] In the process of the invention, the electrolysis cell is
equipped with a zinc anode and an iron or steel cathode. Any shape
of an iron or steel cathode known to those skilled in the art, e.g.
rod-like, as metal sheet, as iron or steel sheet shaped to form a
tube, conically shaped iron or steel sheets, is suitable as
cathode.
[0027] The zinc anode itself can likewise have any shape known to
those skilled in the art, e.g. rod-like, as metal sheet, as cone or
as loose electrode. The zinc anode is particularly preferably in
the form of a rod, cylinder or cone.
[0028] Any arrangement of the anode relative to the cathode which
is known to those skilled in art is possible for carrying out the
process of the invention, e.g. arrangement opposite one another,
parallel arrangement or a concentric arrangement in which the anode
is positioned concentrically within the cathode. Preference is
given to the zinc anode being arranged concentrically within the
cathode.
[0029] The electrolysis cell is filled with an electrolyte. The
electrolyte is selected from the group consisting of
N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-pyrrolidone
and other tertiary amides. Preference is given to
N,N-dimethylformamide and N,N-dimethylacetamide. The use of
N,N-dimethylformamide is particularly preferred.
[0030] In the process of the invention, the electrolyte further
comprises an electrolyte salt selected from the group consisting of
quaternary ammonium salts, organic metal salts and inorganic metal
salts. Preference is given to tetrabutylammonium tetrafluoroborate,
sodium methylsulfonate and zinc chloride. Very particular
preference is given to tetrabutylammonium tetrafluoroborate.
[0031] As further additive, it is advantageous for the electrolyte
to comprise a redox mediator. This is preferably selected from the
group consisting of naphthalene, N,N-dimethyl-1-naphthalene and
further 1-substituted naphthalenes and also phenanthrene,
anthracene, 4,4'-bipyridyl and 4,4'-di-tert-butylbiphenyl.
Particular preference is given to naphthalene.
[0032] In the process of the invention, the electrolyte is heated
to temperatures in the range from 20 to 60.degree. C., preferably
in the range from 30 to 50.degree. C., very particularly preferably
in the range from 35 to 45.degree. C. The temperature is regulated
via a heat exchanger integrated into the electrolyte circuit.
[0033] In the process of the invention, a current density in the
range from 1 to 4 A/dm.sup.2 is applied at the anode and cathode.
The current density is preferably in the range from 1.5 to 3
A/dm.sup.2, particularly preferably in the range from 1.5 to 2.5
A/dm.sup.2.
[0034] The electrolysis is stopped when the solids content of
reactive zinc in the electrolyte has attained a theoretical content
of 2-20% by weight, particularly preferably in the range from 2 to
10% by weight.
[0035] The process of the invention can be operated batchwise or
continuously. In continuous operation, the electrolyte is
discharged from the cell at a content of reactive zinc in the range
from 2 to 20% by weight, preferably in the range from 2 to 10% by
weight. At the same time, an equal amount of fresh electrolyte is
introduced. This is continued until the zinc anode has to be
replaced because of virtually complete dissolution.
[0036] When a tubular cathode surrounding the anode is used in the
process of the invention, it is advantageous for the electrolyte to
be circuited by pumping during the electrolysis. Preference is
given to a pump circulation rate of from 100 to 600 l/h,
particularly preferably from 300 to 600 l/h.
EXAMPLES
[0037] a) Reactive zinc in a glass beaker cell: current yield
(9120-155)
[0038] 0.65 g of Bu.sub.4NBF.sub.4 and 1.25 g of naphthalene
together with 61.00 g of DMF are placed in a glass beaker cell
having a zinc anode and an iron cathode (electrode dimensions in
each case 70.times.20.times.3 mm, immersed area 45.times.20 mm,
spacing: 9 mm). After heating the electrolyte to 40.degree. C., the
electrolysis is started at a current of 0.2 A (corresponding to a
current density of 2 A/dm.sup.2). During the course of the
electrolysis, the electrolyte darkens strongly and the voltage
drops from 9.0 V to 5.5 V. After a running time of 12 hours, the
electrolysis is stopped. A dark suspension of finely divided zinc
is obtained.
[0039] Elemental analysis of the electrolyte gives a zinc (0)
content of 2.7%, corresponding to a current yield of about 60%.
[0040] b) Reactive zinc in a glass beaker cell: reactivity
(9120-172)
[0041] 0.64 g of Bu.sub.4NBF.sub.4 and 1.30 g of naphthalene
together with 61.00 g of DMF are placed in a glass beaker cell
having a zinc anode and an iron cathode (electrode dimensions in
each case 70.times.20.times.3 mm, immersed area 45.times.20 mm,
spacing: 9 mm). After heating the electrolyte to 40.degree. C., the
electrolysis is started at a current of 0.2 A (corresponding to a
current density of 2 A/dm.sup.2). During the course of the
electrolysis, the electrolyte darkens strongly and the voltage
increases from 8.0 V to 8.7 V. After a running time of 3.4 hours,
the electrolysis is stopped. A dark suspension of finely divided
zinc is obtained.
[0042] To test the reactivity of the electrochemically generated
zinc, 1.00 g of 2-bromopyridine is added to the electrolysis output
and the reaction mixture is heated at 80.degree. C. for 0.5 h.
After cooling, 2 ml of the reaction mixture is admixed with 4 ml of
water and extracted with 4 ml of MTBE (methyl tert-butyl ether).
Gas-chromatographic analysis of the organic phase indicates a
conversion of 2-bromopyridine into pyridine of 86% (at a current
yield of 60% as per example a) this corresponds to a reactivity of
70%).
[0043] c) Reactive zinc in a tube cell: current yield at full pump
power (9120-169)
[0044] An electrolyte comprising 25 g of Bu.sub.4NBF.sub.4 and 50 g
of naphthalene in 2425 g of DMF is circulated by pumping at a pump
rate of 600 l/h at 40.degree. C. in an electrolysis cell having a
steel tube as cathode (O=5.0 cm, l=55 cm, active electrode area 864
cm.sup.2) and an internal zinc rod arranged concentrically thereto
as anode (O=3.7 cm, l=55 cm, active electrode area 639 cm.sup.2).
12.8 A are applied to the electrolysis cell for 8.4 h, with the
voltage increasing from 6.0 V to 6.8 V. A dark suspension of finely
divided zinc is obtained.
[0045] Elemental analysis of the electrolyte gives a zinc (0)
concentration of 3.4%, corresponding to a current yield of 68%.
[0046] d) Reactive zinc in a tube cell: current yield at half pump
power (9120-190)
[0047] An electrolyte comprising 25 g of Bu.sub.4NBF.sub.4 and 50 g
of naphthalene in 2425 g of DMF is circulated by pumping at a pump
rate of 300 l/h at 40.degree. C. in an electrolysis cell having a
steel tube as cathode (O=5.0 cm, l=55 cm, active electrode area 864
cm.sup.2) and an internal zinc rod arranged concentrically thereto
as anode (O=3.7 cm, l=55 cm, active electrode area 639 cm.sup.2).
12.8 A are applied to the electrolysis cell for 8.4 h, with the
voltage initially increasing from 6.5 V to 9.9 V and dropping to
1.2 V over the further course of the electrolysis. A dark
suspension of finely divided zinc is obtained. Elemental analysis
of the electrolyte gives a zinc (0) concentration of 4.1%,
corresponding to a current yield of 82%.
* * * * *