Method for the production of an adsorbent made of metal-organic
framework structures (MOF)
The present invention relates to a method for the production of an
absorbent made of metal-organic framework structures (MOF), in the
case of which at least one metal salt is converted with at least one
organic ligand. The conversion is effected at a temperature greater than
1 oooc in a solvent mixture which comprises DMSO and water. In
addition, the invention relates to an absorbent produced with the
method according to the invention or to a substrate coated with such an
absorbent and also to possibilities of use of such an absorbent or
substrate.
Metal-organic network connections (metal-organic frameworks, MOFs)
are included in microporous materials. On a molecular level, they
consist of cationic metal ion clusters (Secondary Building Units, SBUs)
which are bridged by organic anions (linkers) forming polyvalent
•
2
coordinative bonds. Because of the highly symmetrical, crystalline
construction of the lattice structures and the fact that the porosity is
produced in fact at a molecular level, MOFs are included in general in
materials with the greatest inner surface (up to SBET > 4,000 m2 / g) and,
because of their chemical variety, they are of great interest for
applications such as gas storage, catalysis, sorption heat
transformation and many more where they can supplement or replace
materials, such as silica gels, zeolites or activated carbons. Because of
the numerous combination possibilities of SBUs and clusters, many
thousands of structures are in fact known.
MOFs are produced artificially, there are various possibilities for this. It
is common to all of them that metal cations and linker anions are made
to react and in fact in such a slow and controlled manner that the
desired crystalline phase can be formed. Apart from a few exceptions
(anodic synthesis or galvanic displacement), the metal cations (in the
form of a readily soluble metal salt) are thereby present in a high
concentration. The anions are produced in a low concentration in situ
from the corresponding free acid (likewise present) by means of
deprotonation in order to achieve a slow, controlled crystal formation.
The deprotonation can be achieved by several methods, for instance by
the addition of a basic compound, cathodically by reduction or, as
standard, by solvothermal syntheses, the deprotonation being effected
by heating· (equilibrium displacement or formation of basic
decomposition products of the solvent). Most MOFs to date have been
able to be produced only by solvothermal synthesis.
Solvents used frequently in the state of the art are, in pure form or as a
mixture, dialkyl formamides, low alcohols and water (i.e.: "hydrothermal
synthesis"). According to the MOF used, only a few solvents thereby
lead to the formation of the desired structure. In the case of the wrong
choice of solvent, an amorphous, non-porous or even no product at all
3
1s then obtained. Numerous MOFs, in particular highly-porous MIL
structures which are intended to be used as sorption material for heat
transformation applications, can be produced, according to the state of
the art, only when using water as solvent.
In US 2010/0226991 A1, a hydrothermal synthesis at lOOoc 1s
described. The reaction could hence be implemented theoretically at
atmospheric pressure and in an open apparatus, even if it is not
proposed in US 2010/0226991 A1 for this purpose. A suspension of
trimesic acid in aqueous solution of FeCb · 6 H20 serves as reaction
mixture. This synthesis provides MOFs with low crystallinity and a
small inner surface. Since the process takes place in suspension, the
MOFs are obtained as a mixture with unconverted linker compound.
Complex cleaning is therefore necessary. In addition, this synthesis is
not suitable for coating methods since particles of the linker compound
are incorporated without control in the layer, as a result of which
undesired material properties are produced.
In addition, US 2010/0226991 A1 describes a hydrothermal synthesis
at 130°C with the addition of hydrofluoric acid, the Fe3+ cations being
prepared in situ from metallic iron and nitric acid. However, this
synthesis must be implemented at high pressure in an autoclave.
In US 2009/0227446 A1, a hydrothermal synthesis with a similar
reaction mixture with microwave radiation at 200°C is described. This
must also be implemented at high pressure and in a closed reaction
vessel.
Considered as a whole, the synthesis methods described in the state of
: the art have the following disadvantages:
Because of the typical reaction temperatures (> 100°C), the
process must take place at high pressure and in corresponding
4
vessels (autoclaves). This impedes the reaction control (no view
into the vessels) and increases the cost expenditure enormously,
in particular if modified variants of the solvothermal synthesis are
intended to be used (e.g. coatings by means of temperature
gradients or cathodic deprotonation). Continuous synthesis
implementation, which is desirable for reasons of economy of the
process is particularly difficult to achieve in the case of syntheses
at high pressure.
The compounds used as linkers, typically aromatic or olefinic di-,
tri- or tetracarboxylic acids, are generally poorly water-soluble
and only dissolve at temperatures > lOOoC in the reaction
medium. During cooling they precipitate agam. The MOF
isolated after conclusion of the reaction is therefore generally
contaminated with unconverted radicals of the linker molecule
which must be removed by complex washing with organic
solvents. Simple separation of the MOF from the synthesis
solution (e.g. by filtration) is likewise impossible, which likewise is
an obstacle to a continuous reaction implementation.
The hydrothermal syntheses known from the state of the art for
MOFs at T o: lOOoC provide MOFs with comparatively low
crystallinity and a small inner surface, in addition in the form of
very small particles, even nanoparticles. Since the process takes
place here in suspension, these MOFs are also obtained as a
mixture with unconverted linker compound. Complex cleaning is
therefore required. The corresponding syntheses are not suitable
for coating methods because particles of the linker compound are
incorporated without control in the layer, as a result of which
undesired material properties are produced.
Starting from the state of the art, it is accordingly the object of the
present invention to provide a method for the production of an
5
absorbent made of metal-organic framework structures, which can be
effected at atmospheric pressure and from a homogeneous solution and
by means of which the difficulties of the methods described in the state
of the art are overcome.
This object is achieved by a method having the features of patent claim
1 and the adsorbent having the features of claim 14. The dependent
patent claims represent advantageous developments. In claim 15, uses
according to the invention are indicated.
According to the invention, a method for the production of an absorbent
made of metal-organic framework structures (MOF) is hence provided,
in the case of which at least one metal salt is converted with at least one
organic ligand. The conversion is thereby effected at a temperature
greater than 100oc in a solvent mixture which comprises DMSO and
water.
The method according to the invention is distinguished by a zeotropic
DMSO-water mixture being used as solvent instead of water. DMSO is
thereby not a common solvent used in MOF synthesis or in general in
the synthesis of inorganic solids. By using a solvent comprising DMSO
and water, a boiling point of the reaction mixture of above 100oc can be
achieved even at atmospheric pressure. Hence, it is possible by means
of the method according to the invention to produce an MOF which is
produced in a standard hydrothermal manner at atmospheric pressure.
As a result, costs are saved since high-pressure-resistant apparatus is
not required. Furthermore, the solvent mixture comprising DMSO and
water which is used is an excellent solvent for most metal salts and
organic compounds, as a result of which the reaction mixture is present
in the form of a homogeneous solution. As a result, the implementation
of a large-scale, even continuous, production is significantly easier
because any resulting MOF can be removed by filtration from the
6
process during operation smce it ultimately concerns the single solid
suspended in the solution.
As a result of the fact that the method can be implemented at
atmospheric pressure and from a homogeneous solution, continuous
synthesis implementation is hence possible, as a result of which the
method according to the invention can be implemented in a significantly
more economical manner than the methods known from the state of the
art.
The use of water in the method according to the invention is necessary
since the MOFs to be produced generally comprise water molecules even
in the dehydrated state. At least a certain proportion of water must
therefore be present in the reaction mixture.
With the method according to the invention, MOFs with high
crystallinity and a large inner surface can be obtained. The use of a
solvent mixture comprising DMSO and water is hereby crucial for
success of an MOF synthesis effected at normal pressure and from a
homogeneous solution. When using other solvents instead of DMSO in
the solvent mixture, either merely X-ray-amorphous, non-porous
structures can thus be obtained or the solution gels immediately or no
solid at all is obtained. By using a solvent mixture comprising DMSO
and water in the method according to the invention, MOFs with high
crystallinity and a large inner surface can thus be obtained, the
synthesis being able thereby to be implemented simultaneously at
atmospheric pressure and from a homogeneous solution.
In a preferred variant of the method according to the invention, the
solvent comprises at least one chemically reducible anion. Such a
chemically reducible anion is reduced by the DMSO contained in the
reaction mixture and also the decomposition products thereof. The
7
thereby produced reduced product assists the deprotonation of the
organic ligand and hence the formation of the MOF.
The chemically reducible amon 1s selected preferably from the group
consisting of nitrate, chlorite, chlorate, perchlorate, bromite, bromate,
perbromate, iodite, iodate, periodate, sulphate, hydrogen sulphate or
mixtures hereof.
In a particularly preferred variant of the method according to the
invention, the metal salt is a metal nitrate. If the metal salt used
concerns a metal nitrate, then the nitrate is reduced, during conversion,
by DMSO and also the decomposition products thereof to form nitrite
which is more basic and thus assists the deprotonation of the organic
ligand and hence the formation of the MOF.
DMSO and metal nitrates, according to their safety data specifications,
are considered to be non-compatible chemicals. This can obviously be
attributed to the fact that poorly soluble DMSO-solvated metal nitrate
can be formed relatively easily, which has a potentially explosive effect.
As a result of the water proportion in the reaction mixture and also the
increased temperatures of the method according to the invention, the
risk of an explosion is reduced. Further possibilities for minimising the
risk of an explosion can be deduced from embodiment 1.
A particularly preferred variant of the method according to the invention
provides that the metal is selected from the group consisting of Mg, Ca,
Sr, Ba, Sc, Y, Ti, Zr, Hf, V, Nb, Cr, Mo, W, Mn, Re, Ru, Os, Fe, Co, Rh,
Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Si, Sn, Pb, As, Sb, Bi and
rare earth metals. The metal is thereby particularly preferably a
transition metal. Very particularly preferably, the metal is selected from
the group consisting of Fe, Zn, Cu, Co, Ru, Os, Mn, Ni and rare earth
metals.
8
In a further preferred variant of the method according to the invention,
the organic ligands have bridging oxygen-, nitrogen or sulphur atoms.
Furthermore, it is preferred that the organic ligands are selected from
the group consisting of bidentate ligands, polydentate ligands and
mixtures hereof.
A further preferred variant of the method according to the invention
provides that the organic ligands are selected from the group consisting
of dicarboxylic acids, tricarboxylic acids, imidazoles, triazoles and
mixtures hereof.
Furthermore, it is preferred that the organic ligands are selected from
the group consisting of trimesic acid, terephthalic acid, 4,4'-bipyridine,
biphenylbisulphonic acid, 2,6-naphthalenedicarboxylic acid, fumaric
acid, isophthalic acid, phthalic acid, oxalic acid and mixtures hereof.
A further preferred variant of the method according to the invention
provides that the solvent mixture comprises between 1 and 50% by
weight, preferably between 10 and 30% by weight, particularly
preferably between 15 and 25% by weight, of water and/or between 50
and 99% by weight, preferably between 70 and 90% by weight,
particularly preferably between 75 and 85% by weight, of DMSO. The
sum of the proportions by percentage weight of DMSO and water in the
solvent mixture is hereby preferably 100% by weight. The percentage
proportion of DMSO and water in the solvent mixture has an influence
on the boiling point of the solvent mixture. The desired boiling point of
the solvent mixture can thus be adjusted by choice of the corresponding
proportions of DMSO and water in the solvent mixture. For example, at
a proportion of 20% by weight of water and 80% by weight of DMSO, the
solvent mixture has a boiling point of 130°C at atmospheric pressure.
This solvent mixture can consequently be heated up to this temperature
at atmospheric pressure in the liquid state.
9
In a further preferred variant of the method according to the invention,
the conversion is effected with reflux and/ or at a temperature of 110 to
180°C, preferably of 120 to 150°C, particularly preferably of 125 to
135°C.
Furthermore, it is preferred that the converswn 1s implemented at a
pressure of 1 to 5 bar, preferably at atmospheric pressure. It 1s
particularly preferred that the method according to the invention 1s
implemented without additional application of pressure.
A further preferred variant of the method according to the invention
provides that the reaction mixture comprises at least one supplement,
which is selected from the group consisting of nitric acid, hydrofluoric
acid, hydrochloric acid, hydrobromic acid, methane sulphonic acid,
toluene sulphonic acid, sulphamic acid or sulphuric acid and mixtures
hereof. Due to the presence of such supplements in the reaction
mixture, the crystallinity of the obtained MOFs can be improved.
In a further preferred variant of the method according to the invention,
the reaction duration is 12 to 48 h, preferably 18 to 30 h, particularly
preferably 22 to 26 h.
A further preferred variant of the method according to the invention
provides that the resulting adsorbent is washed. Washing is thereby
preferably effected with a solvent mixture which comprises between 1
and 50% by weight, preferably between 10 and 30% by weight,
particularly preferably between 15 and 25% by weight, of water and/or
between 50 and 99% by weight, preferably between 70 and 90% by
weight, particularly preferably between 75 and 85% by i weight, of
DMSO, the sum of the proportions by percentage weight of DMSO and
water in the solvent mixture preferably being 100% by weight.
10
Furthermore, it is preferred that, after conversion, the reaction mixture
is reprocessed, the solid produced during the conversion of the reaction
mixture being separated, preferably being centrifuged or filtered off,
from the supernatant solution.
A further preferred variant of the method according to the invention
provides that at least one substrate which is to be coated with the
adsorbent is immersed in the reaction mixture as cathode together with
a counter-anode and subsequently the conversion is implemented, the
adsorbent being deposited on the substrate in the form of a layer, and,
during conversion, a voltage being applied between the electrodes which
effects a current density of 200- 1,000 mA/dm2, particularly preferably
300 to 500 mA/dm2, at the beginning of the conversion.
By means of such a combination of cathodic coating and themal
gradient method, the formation of a functional layer on substrates is
possible, and m fact at high speed. The speed-determining
deprotonation process of the thermal gradient method can be
accelerated in this way, namely electrochemically.
reactions hereby take place on the cathode for example:
H20 + e Yz H2 + OW
--7 N02- + 2 OW
The following
The resulting hydroxyl anions can deprotonate the organic ligands or be
incorporated directly in the MOF, as a result of which crystalline layers
can be produced rapidly.
Even with respect to a continuous process for the production of free,
carrier-free adsorbent, the just described variant 'of the method
according to the invention is of particular interest. Thus it is possible
that the MOF formed on the cathode is continually wiped off, and
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merely the compounds removed from the reaction mixture need be
replenished. Thus a very economical production can be achieved.
The substrate which is used is preferably electrically conductive.
Particularly preferably, it concerns a substrate made of electrically
conductive ceramic, electrically conductive plastic material, copper,
aluminium and/ or steel, very particularly preferably made of stainless
steel of the types 1.4301 and/ or 1.4401.
Furthermore, it is preferred that, before coating, the reaction mixture is
heated without applying a voltage, this heating being effected preferably
for 20 to 90 min, particularly preferably for 40 to 50 min.
In particular, the layer has a layer thickness of 120 to 180 !liD,
preferably 140 to 160 !liD.
The present invention relates in addition to an adsorbent or to a
substrate coated with an adsorbent, which is producible with the
method according to the invention. An adsorbent produced with the
method according to the invention or a substrate coated with an
adsorbent differs consequently from products produced in the state of
the art by having traces of DMSO and traces of decomposition products
of DMSO which originate from the synthesis process. Such
decomposition products are for example, methane thiol, formaldehyde,
dimethylsulphide or dimethylsulphone.
In addition, the present invention relates to the use of an adsorbent
according to the invention or of a substrate coated with adsorbent
according to the invention for gas separation, gas storage, catalysis or
sorption-based heat transformation.
12
The present invention is explained in more detail with reference to the
following examples, without restricting the invention to the specially
illustrated parameters.
Embodiment 1
A solution of 21.25 mmol (4.48 g) of trimesic acid in 160 ml of
dimethylsulphoxide was heated to 90°C in a 250 ml three-neck flask,
then a room-temperature solution of 32 mmol (12.96 g) of Fe(N03)3 •
9H20 in 32 ml of H20 was added all at once with agitation. The yellowgreen
solution was refluxed with agitation for a duration of 24 h
(internal temperature: 131°C).
The resulting solid was centrifuged off and the supernatant solution,
still hot, was tipped into 1 I of water, in order to avoid precipitation of
potentially explosive Fe(N03)3 • 6Me2SO as by-product. After cleaning
(washing with DMF [90°C, 5 h], ethanol [60°C, overnight] and water
[90°C, 5 h]), 4.49 g of a brown-red solid was obtained. The single
crystalline phase was determined by means of X-ray powder
diffractometry as MIL-100.
Embodiment 2
The process took place as in embodiment 1, however with reflux only for
a duration of 45 min. The thus obtained, brown and slightly cloudy
liquid was placed in a double-wall vessel. The outer wall was
thermostatically controlled at a temperature of 45°C.
Then two sheets of 1.4301 stainless steel ( 1.4016 or aluminium can
also be used) of the dimensions 50 x 50 x 1.5 mm were rubbed down,
degreased with acetone and mounted on a specially prepared heating
element, which was provided with counter-electrodes respectively at a
10 mm spacing. The blank surface of the sheets was 15 cm2 per side.
13
The test structure was immersed in the cooled original solution and the
heating power was controlled (typically 210 W) such that the
temperature just below the surface of the sheets to be coated was 135°C
(determined by a thermoelement inserted in a boring). Then a voltage
was set, which at the start of the test effected a current intensity of 100
rnA (current density: 10 A/dm2). After 25 minutes, the current
intensity had fallen to 50 rnA, the test was ended, the introduced sheets
were disassembled and placed in DMF and ethanol for respectively 24
h. The formed red-brown layer was determined by powder
diffractometry as MIL-l 00.
The thus produced sheet is approx. 150 /Jill thick, stable per se, but
only loosely joined to the substrate. A firmly adhering layer can be
produced if for example 1.4568 stainless steel (trade name 17-7 PH®) is
used as substrate.
FRAUNHOFER-GESELLSCHAFT .. e.V.

Patent Claims
1. Method for the production of an absorbent made of metal-organic
framework structures (MOF), in the case of which at least one
metal salt is converted with at least one organic ligand,
characterised in that the conversion is effected at a temperature
greater than 100°C in a solvent mixture which comprises DMSO
and water.
2. Method according to the preceding claim, characterised in that
the metal salt comprises at least one chemically reducible anion,
the chemically reducible anion being selected preferably from the
group consisting of nitrate, chlorite, chlorate, perchlorate,
bromite, bromate, perbromate, iodite, iodate, periodate, sulphate,
hydrogen sulphate or mixtures hereof, and the metal salt being
particularly preferably a metal nitrate.
3. Method according to one of the preceding claims, characterised in
that the metal is selected from the group consisting of Mg, Ca, Sr,
Ba, Sc, Y, Ti, Zr, Hf, V, Nb, Cr, Mo, W, Mn, Re, Ru, Os, Fe, Co,
Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Si, Sn, Pb, As,
Sb, Bi and rare earth metals, the metal being selected preferably
from the group consisting of Fe, Zn, Cu, Co, Ru, Os, Mn, Ni and
rare earth metals.
4. Method according to one of the preceding claims, characterised in
that the organic ligands have bridging oxygen-, nitrogen or
sulphur atoms and/or the organic ligands are selected from the
group consisting of bidentate ligands, polydentate ligands and
mixtures hereof.
5. Method according to one of the preceding claims, characterised in
that the organic ligands are selected from the group consisting of
dicarboxylic acids, tricarboxylic acids, imidazoles, triazoles and
mixtures hereof, and/or the organic ligands are selected from the
group consisting of trimesic acid, terephthalic acid, 4,4'bipyridine,
biphenylbisulphonic acid, 2,6-
naphthalenedicarboxylic acid, fumaric acid, isophthalic acid,
phthalic acid, oxalic acid and mixtures hereof.
6. Method according to one of the two preceding claims,
characterised in that the solvent mixture comprises between 1
and 50% by weight, preferably between 10 and 30% by weight,
particularly preferably between 15 and 25% by weight, of water
and/ or between 50 and 99% by weight, preferably between 70
and 90% by weight, particularly preferably between 75 and 85%
by weight, of DMSO, the sum of the proportions by percentage
weight of DMSO and water in the solvent mixture being preferably
100% by weight.
7. Method according to one of the preceding claims, characterised in
that the conversion is effected with reflux and/ or at a
temperature of 110 to 180°C, preferably of 120 to 150°C,
particularly preferably of 125 to 135°C.
8. Method according to one of the preceding claims, characterised in
that the conversion is implemented at a pressure of 1 to 5 bar,
preferably at atmospheric pressure.
9. Method according to one of the preceding claims, characterised in
that the reaction mixture comprises at least one supplement,
which is selected from the group consisting of nitric acid,
hydrofluoric acid, hydrochloric acid, hydrobromic acid, methane
sulphonic acid, toluene sulphonic acid, sulphamic acid or
sulphuric acid and mixtures hereof.
10. Method according to one of the preceding claims, characterised in
that, after conversion, the reaction mixture is reprocessed, the
solid produced during the conversion of the reaction mixture
being separated, preferably being centrifuged or filtered off, from
the supernatant solution.
11. Method according to one of the preceding claims, characterised in
that at least one substrate which is to be coated with the
adsorbent is immersed in the reaction mixture as cathode
together with a counter-anode and subsequently the conversion is
implemented, the adsorbent being deposited on the substrate in
the form of a layer, and, during conversion, a voltage being
applied between the electrodes which effects a current density of
200- 1,000 mA/dm2, particularly preferably 300 to 500 mA/dm2,
at the beginning of the conversion.
12. Method according to one of the preceding claims, characterised in
that the substrate is a substrate made of electrically conductive
ceramic, electrically conductive plastic material, copper,
aluminium and/ or steel, preferably made of stainless steel of the
types 1.4301 and/ or 1.4401.
13. Method according to one of the two preceding claims,
characterised in that the layer has a layer thickness of 120 to 180
J..lm, preferably 140 to 160 J..!m.
14. Adsorbent or substrate coated with an adsorbent, which is
producible according to a method according to one of the
preceding claims.
15. Use of an adsorbent or of a .substrate coated with adsorbent
according to the previous claim for gas separation, gas storage,
catalysis or sorption-based heat transformation.