Copper/zinc L-tartrates: Mixed crystals and thermolysis to a mixture of copper oxide and zinc oxide that is catalytically active in methanol synthesis

Article abstract of DOI:10.1002/ejic.200600561

The system consisting of copper/zinc L-tartrate mixed crystals has been systematically explored in the whole range from pure copper tartrate to pure zinc tartrate. Mixed crystal L-tartrates were prepared and their thermochemical behaviour under oxygen was investigated. Oxidic precatalysts (CuO/ZnO) for catalytic tests in methanol synthesis were prepared by mild thermolysis of the mixed tartrates in air at 300°C. Catalytic tests were performed with a multi-channel parallel reactor. The catalytic activity shows a maximum at about equal amounts of copper and zinc whereas the specific surface area (BET surface) increases strongly when going from CuO to ZnO. This system offers a convenient, inexpensive route to CuO/ZnO precatalysts with adjustable compositions that avoids all other metals during preparation. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600561

FULL PAPER
DOI: 10.1002/ejic.200600561
Copper/Zinc -Tartrates: Mixed Crystals and Thermolysis to a Mixture of
L
Copper Oxide and Zinc Oxide That Is Catalytically Active in Methanol
Synthesis
[a]
[b]
[b]
[c]
Rainer Weiss, Sascha Vukojevi c´ , Christian Baltes, Raoul Naumann d‘Alnoncourt,
[c]
[a]
Martin Muhler, and Matthias Epple*
Keywords: Heterogeneous catalysis / Copper / Zinc / Thermochemistry
The system consisting of copper/zinc
tals has been systematically explored in the whole range
from pure copper tartrate to pure zinc tartrate. Mixed crystal
L
-tartrate mixed crys-
parallel reactor. The catalytic activity shows a maximum at
about equal amounts of copper and zinc whereas the specific
surface area (BET surface) increases strongly when going
from CuO to ZnO. This system offers a convenient, inexpen-
sive route to CuO/ZnO precatalysts with adjustable composi-
L-tartrates were prepared and their thermochemical behav-
iour under oxygen was investigated. Oxidic precatalysts
(
CuO/ZnO) for catalytic tests in methanol synthesis were pre- tions that avoids all other metals during preparation.
pared by mild thermolysis of the mixed tartrates in air at
00 °C. Catalytic tests were performed with a multi-channel
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
3
Introduction
that contains both metals in one solid phase (to achieve
a good interaction in the final catalyst) and which can be
decomposed at moderate temperature (to avoid sintering to
larger crystals). Mixed copper/zinc -tartrates turned out to
be well suited for this purpose because their synthesis is
straightforward and also because inorganic counterions like
alkali metals or carbonate can be avoided.
Methanol is the most important chemical base material
due to its use as a solvent or as a parent compound for
the industrial synthesis of higher organic compounds. The
industrial synthesis of methanol is typically carried out with
synthesis gas containing H , CO and CO with Cu/ZnO/
[
1]
2
2
Al O as catalyst, which is prepared by reduction of a CuO/
2
3
[2–8]
ZnO/Al O precatalyst.
These oxide composites are usu-
2
3
ally prepared by thermolysis of co-precipitated hydroxycar-
bonate precatalysts in which all three metals are present.
[
9] Results and Discussion
Despite numerous experimental and theoretical studies, the
catalytic processes on the surface of the bulk catalysts are Preparation
not yet fully understood.[
9–15]
The activities of the obtained
Metal -tartrates (MTTs) with different ratios of Cu/Zn
were prepared in aqueous solution by precipitation from
catalysts depend on many factors. To reduce the number of
factors that influence the performance of the catalyst, the
catalytic system is often limited to the system Cu/ZnO.
Even then, the thermolysis temperature and pressure, the
copper acetate and zinc acetate with -tartaric acid (H TT)
2
[
Equation (1)].
reduction temperature and the composition remain impor-
tant external factors.[
3–5,7,9,11,12,16]
In the case of dinuclear
xCu(OAc) + yZn(OAc) + H TT Ǟ
2
2
2
Cu
x
Zn
y
TT·nH
2
O + 2HOAc; x + y = 1 and n Յ 3 (1)
complexes that contain both copper and zinc, the thermoly-
sis is always complex due to the decomposition of the corre-
[
17,18]
sponding anions.
It was our goal to find a precursor
Note that the only counterion is acetate, which remains
quantitatively in solution. At a final tartrate concentration
of 0.05  large single crystals were obtained after slow crys-
tallisation. Unfortunately, these were always twinned and
not suitable for single-crystal X-ray structure analysis. At a
higher final tartrate concentration of 0.2  a fast precipi-
tation occurred and the product was obtained as a micro-
crystalline powder. These microcrystalline samples were
used for all experiments described below. In all cases we
obtained hydrates of the mixed tartrates in which the water
[
a] Inorganic Chemistry, University of Duisburg-Essen,
Universitaetsstrasse 5–7, 45117 Essen, Germany
Fax: +49-201-183-2621
E-mail: matthias.epple@uni-due.de
[
[
b] Max-Planck-Institute for Coal Research,
Kaiser-Wilhelm-Platz 1, 45470 Muelheim an der Ruhr, Ger-
many
c] Laboratory of Industrial Chemistry, Ruhr University Bochum,
Universitaetsstrasse 150, 44801 Bochum, Germany
4782
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2006, 4782–4786

Copper/Zinc -Tartrates
FULL PAPER
Table 1. Elemental analysis results of the anhydrous copper/zinc tartrates after removal of water of crystallisation at 130 °C. The molar
fractions of Cu and Zn can be derived from the formulae in the last column. They correspond well to the expected molar fraction (first
three columns).
Sample
Expected
mol-%]
Found
[wt.-%]
Cu
Calcd.
[wt.-%]
O
Calculated formulae from elemental
analysis (normalised to C
[
4
)
Cu
Zn
Zn
C
H
C
4
4 6 x y
H O Cu Zn
CuTT
100
90
80
70
60
50
40
30
20
10
0
0
29.25
26.86
24.29
22.11
18.88
15.21
11.22
8.42
0.00
2.62
4.95
7.47
10.79
14.96
19.24
22.18
25.44
28.47
31.10
22.28
22.06
22.33
22.46
22.51
22.28
22.29
22.20
22.29
22.30
22.21
2.22
2.20
2.21
2.16
2.03
2.00
2.03
2.00
1.98
2.01
2.03
46.26
46.28
46.23
45.81
45.80
45.55
45.23
45.21
44.59
44.28
44.67
C
C
C
C
C
C
C
C
C
C
C
4
4
4
4
4
4
4
4
4
4
4
H
4.75
H
4.75
H
4.71
H
4.59
H
4.30
H
4.28
H
4.33
H
4.30
H
4.28
H
4.30
H
4.36
O
O
O
O
O
O
O
O
O
O
O
1 0
6.24Cu Zn
Cu0.9Zn0.1TT
Cu0.8Zn0.2TT
Cu0.7Zn0.3TT
Cu0.6Zn0.4TT
Cu0.5Zn0.5TT
Cu0.4Zn0.6TT
Cu0.3Zn0.7TT
Cu0.2Zn0.8TT
Cu0.1Zn0.9TT
ZnTT
10
20
30
40
50
60
70
80
90
100
6.30Cu0.92Zn0.09
6.22Cu0.82Zn0.18
6.12Cu0.75Zn0.25
6.11Cu0.64Zn0.35
6.14Cu0.51Zn0.49
6.10Cu0.38Zn0.63
6.12Cu0.29Zn0.73
6.01Cu0.19Zn0.84
5.97Cu0.10Zn0.94
5.71
2.95
0.00
0 1
6.04Cu Zn
loss starts at room temperature. In order to obtain anhy-
drous samples with a defined stoichiometry the products
were fully dehydrated at 130 °C in air. Elemental analysis
(Table 1) showed that the compositions of the samples were
in good accordance with the intended composition, i.e. that
all metal ions present in solution were also present in the
same ratio in the precipitate. The Cu/Zn ratio was thereby
easily controlled by variation of the ratio of copper/zinc in
solution. Unfortunately, it was not possible to synthesise a
ternary precursor that also contains aluminium by precipi-
tation or crystallisation due to the high solubility of
Al TT .
2
3
Crystal Structures, Powder Diffraction Patterns and
Precursor Morphology
[
19]
The structures of the hydrates ZnTT·2.5H O
and
2
[
20]
CuTT·3H O are both known. These compounds are not
2
isostructural. No structural data are available in the litera-
ture for Cu/Zn tartrate mixed crystals. The crystal struc-
tures of anhydrous copper -tartrate and zinc -tartrate are
also unknown. However, after the removal of water of crys-
tallisation all precursor samples were still crystalline ac-
cording to X-ray diffractometry. The diffraction patterns of
copper and zinc -tartrate are similar, but still distinguisha-
ble (Figure 1). The change between the two structures oc-
curs around an equimolar ratio of copper/zinc. Because the
single-crystal structures are not known, it is not possible to
draw safe conclusions about the phases that are present.
However, the powder diffractograms clearly indicate that
two structurally different phases of dimetallic tartrate pre-
cursors are present, both of which contain copper and zinc
in the same phase.
Figure 1. Powder diffraction patterns of the mixed Cu/Zn tartrates.
Top: overview; bottom: magnification of selected samples.
Thermogravimetric Analysis
Three MTT samples were heated to 500 °C under pure
–1
–1
oxygen (50 mLmin ) at 5 Kmin . The thermolysis occurs
The morphology of the Cu/Zn dimetallic tartrates was in one step and is accompanied by sharp exothermic DTA
studied by SEM (Figure 2). The particle shape depended peaks (Figure 3). Incidentally, the exothermic nature of the
on the ratio of the two metals, but all samples were micro- thermolysis (combustion) was clear from the observation
crystalline, as also supported by the narrow X-ray diffrac- that the sample glowed red in the furnace, even at 300 °C
tion peaks in Figure 1.
(autocatalytic oxidation by oxygen). The onset temperature
Eur. J. Inorg. Chem. 2006, 4782–4786
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4783

M. Epple et al.
FULL PAPER
ingly, such a mass increase was not observed for
Cu0.5Zn0.5TT, which means that this sample is directly con-
verted into CuO/ZnO during the combustion step. The ob-
tained final masses at 500 °C (CuTT: 36.7 wt.-%;
Cu0.5Zn0.5TT: 38.4 wt.-%; ZnTT: 37.3 wt.-%) correspond
well to the calculated masses (CuO from CuTT: 37.5 wt.-
%; CuO/ZnO from Cu0.5Zn0.5TT: 37.8 wt.-%: ZnO from
ZnTT: 38.1 wt.-%). The final products contained neither
carbon nor hydrogen (by elemental analysis). The external
morphology of the tartrate crystals is preserved in the as-
sembly of the product crystals (Figure 4). Note that Schmid
and Felsche have reported that the thermolysis of copper
tartrate under inert gas (argon) leads to copper metal.[
21]
Figure 2. Scanning electron micrographs of the tartrates CuTT (A),
Cu0.7Zn0.3TT (B), Cu0.3Zn0.7TT (C) and ZnTT (D).
of thermolysis increases with increasing zinc content. The
mass increase above 250 °C for CuTT can be ascribed to
an oxidation of the initially formed Cu O to CuO. Interest-
2
Figure 4. Scanning electron micrographs of the oxide samples ob-
tained after thermogravimetry of CuTT (A), Cu0.5Zn0.5TT (B) and
ZnTT (C) under oxygen (500 °C). Note the preservation of the
morphology of the original large crystals.
Thermolysis in Air and Morphologic Studies
Larger amounts of the oxide samples for catalytic studies
were prepared by thermolysis in air at 300 °C for 24 h,
followed by slight mechanical grinding. Note that a ther-
molysis at 250 °C for 48 h gave very similar results. The
lowermost temperature necessary for complete thermolysis
was chosen in order to avoid sintering of the reaction prod-
ucts to larger crystals. Note that a small crystallite size is
[
22]
important for the catalytic performance. A high specific
copper surface area is also important for a good catalytic
[
2,23]
performance.
The diffraction pattern of the product
from CuTT shows the presence of Cu O and CuO. In con-
2
trast, all mixed oxide samples that contain copper contain
only CuO and ZnO. ZnTT leads to ZnO after thermolysis
Figure 3. Top: Thermogravimetric measurements of CuTT,
Cu0.5Zn0.5TT and ZnTT under a dynamic oxygen atmosphere. Bot-
tom: The mass loss is accompanied by sharp exothermic DTA
peaks in all three cases, indicating local combustion processes.
(Figure 5). Under these conditions, the precursor mor-
phology is better conserved and the surface of the former
tartrate crystals is smoother (Figure 6).
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Eur. J. Inorg. Chem. 2006, 4782–4786

Copper/Zinc -Tartrates
FULL PAPER
chromatography (RFC). The content of 50 mol-% Cu was
confirmed by TPR. However, the specific copper surface
2
–1
area (by RFC) was only 0.34 m g (note that the accuracy
2
–1
of values below 1 m g is limited), which is surprisingly
small given the considerable catalytic activity of this sample.
For comparison, the specific copper surface area is
2
–1
1
7.0 m g for the aforementioned hydroxycarbonate-based
2
–1
catalyst and 23.4 m g for the industrial reference cata-
[
18]
lyst.
Figure 5. Powder diffraction patterns of oxide mixtures obtained
by thermolysis in air at 300 °C (24 h) and containing CuO (x;
tenorite; JCPDS no. 481548) and ZnO (o; wurtzite; JCDPS no.
361451).
Figure 7. Catalytic activity of CuO/ZnO samples after thermolysis
of mixed tartrates (300 °C; air; 24 h) after reduction to Cu/ZnO in
situ in a multireactor with H
2
, measured at 245 °C and 4.5 MPa
(
᭹; left scale). The activity of a technical methanol catalyst was set
to 100%. The BET surface area of the oxide precatalysts (i.e. before
reduction) is shown on the right (+).
A possible explanation for the lower activity compared
to hydroxycarbonate-derived catalysts could be the fact that
the thermolysis of the tartrates is associated with an exo-
thermic effect, in contrast to the endothermic decomposi-
tion of hydroxycarbonates. This exothermic effect may lead
to microscopic self-heating and thereby to a growth of oxide
crystals. This is supported by the X-ray diffraction data
Figure 6. Scanning electron micrographs of the oxide samples ob- (Figure 5), which show narrow peaks for the oxides, i.e. no
tained by thermolysis of CuTT (A), Cu0.7Zn0.3TT (B),
Cu0.3Zn0.7TT (C) and ZnTT (D) in air at 300 °C (24 h).
nanocrystalline particles that would give rise to diffraction
peak broadening. These CuO particles form the active
centres of the final reduced catalyst (Cu on ZnO), and it is
Catalytic Activity
known that larger copper crystals are catalytically less
[
2,22,23]
active than smaller copper crystals.
This is corrobo-
The oxide samples were tested regarding their ability to
catalyze the formation of methanol from synthesis gas (CO,
CO and H ). The CuO/ZnO mixture was reduced directly
rated by the small observed specific copper surface area for
the 50:50 sample.
2
2
in the catalytic reactor to Cu/ZnO with H . The Cu/Zn ratio
2
had a remarkable influence on the catalytic activity, show-
ing a clear maximum for an approximately equimolar ratio
of Cu and Zn (Figure 7). Although the thermolysis was per-
Conclusions
Metal -tartrates that contain both copper and zinc as a
formed at moderate temperatures, the catalytic activities are mixed crystal are well-suited precursors to prepare CuO/
low in comparison to an industrial benchmark catalyst (set ZnO precatalysts that can then be reduced to Cu/ZnO, i.e.
to 100%). However, the maximum activity (17%) was of the an active methanol catalyst. The maximum activity is found
order of a Cu/ZnO catalyst prepared from co-precipitated at about equimolar amounts of copper and zinc, although
hydroxycarbonate (about 25%).[ The specific surface area the specific surface area increases strongly with increasing
of the oxide precatalysts shows an almost linear increase, zinc content. The presence of this activity maximum reflects
18]
2
–1
starting from almost zero (0.46 m g ) for pure CuO up to the interplay between metal (Cu) and support (ZnO) and
2
–1
3
0–40 m g for samples with high zinc content. The sam- the necessity for a sufficiently high specific surface area.
ple Cu0.5Zn0.5TT was also subjected to temperature-pro- The preparation by precipitation easily allows control of the
grammed reduction (TPR) and to reactive frontal ratio of Cu/Zn in the precursor, and thereby of the catalytic
Eur. J. Inorg. Chem. 2006, 4782–4786
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4785

M. Epple et al.
FULL PAPER
activity. Except for carbonate, other ions are avoided during Acknowledgments
the synthesis. This synthetic method presents an alternative
to classical hydroxycarbonate precursors.
This work was supported by the Deutsche Forschungsgemeinschaft
within the scope of the Collaborative Research Center SFB 558
(
metal–substrate interactions in heterogeneous catalysis). We are
Experimental Section
grateful to F. Schüth, Mülheim, for helpful discussions.
General: Deionised water was used for all preparations.
Preparation of the Precursor Samples: A solution of 50 mmol of
copper acetate and/or zinc acetate in the appropriate ratio in
[
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solution of the acid was rapidly poured into the solution of the
metal acetates with vigorous stirring. Approximately 30 s after the
addition, the precursor began to precipitate. The mixture was
stirred at about 50 °C for another 15 min. After sedimentation of
the precipitate, the precipitate was filtered and washed three times
with water and then once with ethanol. The tartrates were first
dried at 70 °C for 2 h and then at 130 °C for 12 h. With the excep-
tion of ZnTT, all products were obtained as fine powders. CuTT
was slightly blue, ZnTT was white, and the mixed crystals showed
a continuous row of intermediate colours.
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the procedure for the commercial benchmark catalyst ICI Katalco
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at 245 °C according to
2
003, 115, 3945–3947; Angew. Chem. Int. Ed. 2003, 42, 3815–
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7
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2
2
[
(
HP GC 6890) equipped with a methanizer FID was used for on-
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.53 mm column and CO, CO and CH were separated on a Car-
3
3
,
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3
3
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–
1
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45 °C and 4.5 MPa].
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instrument. Samples were heated from 30 to 500 °C at a rate of
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–1
5
Kmin–1 under dynamic O
2
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AXS D8 Advance instrument with Cu-K radiation. Atomic absorp-
1
[
[
α
3
tion spectroscopy (AAS) was performed with a Thermo Electron
Corporation instrument (M series) to determine the contents of cop-
per and zinc in the samples. The contents of carbon and hydrogen
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[
[
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(
TPR) was carried out on the oxide product from the sample
Cu0.5Zn0.5TT. About 0.1 g of this sample was placed in a quartz reac-
tor. The reduction was performed under flowing diluted H (4.2 vol%
in He) by increasing the temperature to 240 °C at a rate of
Kmin–1 and maintaining that temperature for 1 h. The active sur-
O reactive frontal chromatog-
2
[
[
H
1
2
2
000, 11, 956–959.
face area of Cu was determined by N
raphy at 300 K on the same sample of Cu0.5Zn0.5TT.[
2
Received: June 16, 2006
Published Online: October 12, 2006
24]
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Eur. J. Inorg. Chem. 2006, 4782–4786