Copper-containing SiCN precursor ceramics (Cu@SiCN) as selective hydrocarbon oxidation catalysts using air as an oxidant

Article abstract of DOI:10.1002/chem.200902836

A molecular approach to metal-containing ceramics and their application as selective heterogeneous oxidation catalysts is presented. The aminopyridinato copper complex [Cu₂(Ap^TMS)₂] (Ap^TMSH = (4-methylpyridin2-yl)trimethylsilanylamine) reacts with poly(organosilazanes) via aminopyridine elimination, as shown for the commercially available ceramic precursor HTT 1800. The reaction was studied by ¹H and ¹³C NMR spectroscopy. The liberation of the free, protonated ligand Ap ^TMSH is indicative of the copper polycarbosilazane binding. Crosslinking of the copper-modified poly(organosilazane) and subsequent pyrolysis lead to the copper-containing ceramics. The copper is reduced to copper metal during the pyrolysis step up to 1000 °C, as observed by solid-state ⁶⁵Cu NMR spectroscopy, SEM images, and energydispersive spectroscopy (EDS). Powder diffraction experiments verified the presence of crystalline copper. All Cu@SiCN ceramics show catalytic activity towards the oxidation of cycloalkanes using air as oxidant. The selectivity of the reaction increases with increasing copper content. The catalysts are recyclable. This study proves the feasibility of this molecular approach to metal-containing SiCN precursor ceramics by using silylaminopyridinato complexes. Furthermore, the catalytic results confirm the applicability of this new class of metal-containing ceramics as catalysts.

Full text of DOI:10.1002/chem.200902836

FULL PAPER
DOI: 10.1002/chem.200902836
Copper-Containing SiCN Precursor Ceramics (Cu@SiCN) as Selective
Hydrocarbon Oxidation Catalysts Using Air as an Oxidant
[
a]
[b]
[b]
[c]
Germund Glatz, Thomas Schmalz, Tobias Kraus, Frank Haarmann,
[
b]
[a]
Gꢀnter Motz,* and Rhett Kempe*
Abstract: A molecular approach to
the copper-modified poly(organosila-
zane) and subsequent pyrolysis lead to
the copper-containing ceramics. The
copper is reduced to copper metal
during the pyrolysis step up to 10008C,
presence of crystalline copper. All
Cu@SiCN ceramics show catalytic ac-
tivity towards the oxidation of cycloal-
kanes using air as oxidant. The selectiv-
ity of the reaction increases with in-
creasing copper content. The catalysts
are recyclable. This study proves the
feasibility of this molecular approach
to metal-containing SiCN precursor ce-
ramics by using silylaminopyridinato
complexes. Furthermore, the catalytic
results confirm the applicability of this
new class of metal-containing ceramics
as catalysts.
metal-containing ceramics and their ap-
plication as selective heterogeneous ox-
idation catalysts is presented. The ami-
nopyridinato copper complex [Cu2-
TMS
TMS
65
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(Ap )2] (Ap H=(4-methylpyridin-
as observed by solid-state Cu NMR
2-yl)trimethylsilanylamine) reacts with
spectroscopy, SEM images, and energy-
dispersive spectroscopy (EDS). Powder
diffraction experiments verified the
poly(organosilazanes) via aminopyri-
dine elimination, as shown for the com-
mercially available ceramic precursor
HTT 1800. The reaction was studied by
Keywords: aminopyridinato
li-
1
13
H and C NMR spectroscopy. The lib-
gands · ceramics · copper · Cu@-
SiCN · heterogeneous catalysis ·
oxidation
eration of the free, protonated ligand
TMS
Ap H is indicative of the copper pol-
ycarbosilazane binding. Crosslinking of
Introduction
ieved by the introduction of transition metals into the ce-
ramic. One approach is to mix metal powders or alloys with
the ceramic precursor and to ceramize this material.
[6,11–15]
Considerable research interest during the last few years has
focused on SiCN-precursor ceramics.
stability, corrosion resistance, and long-term durability are
provided by amorphous SiCN-precursor ceramics derived
from poly(organosilazanes). Because of the molecular path-
way to these ceramics, they are highly tunable and versatile
[1–5]
High-temperature
Molecular pathways to introduce a transition metal have
been described for ferrocene-functionalized ceramic precur-
sors but the general applicability is limited due to the re-
[
16–18]
stricted variety of available precursors.
teresting approach to heterobimetallic nanoparticles was de-
Recently, an in-
[6–10]
[19]
materials.
An extension of the property profiles, such as
scribed that used polymeric precursors.
A more general
improved electrical and thermal conductivity, should be ach-
molecular approach is the use of organometallic compounds
[20]
such as iron and cobalt carbonyls.
Unfortunately, metal
carbonyls are very volatile and extremely toxic and may va-
porize during the ceramic formation at high temperature.
[
a] Dr. G. Glatz, Prof. Dr. R. Kempe
Inorganic Chemistry II, University of Bayreuth
[21]
Metal alkyls such as trimethylaluminum
or early transi-
for example, amidotitanium com-
have shown a certain potential to strongly in-
9
5440 Bayreuth (Germany)
[22]
E-mail: kempe@uni-bayreuth.de
tion-metal amides,
plexes,
[
8–10,23]
[
b] Dr. T. Schmalz, Dipl. Ing. T. Kraus, Dr. G. Motz
Ceramics Materials Engineering, University of Bayreuth
teract with the ceramic precursor and increase ceramic
yield. For late transition metals (which would be interesting,
for example, for catalysis) the corresponding amido–metal
complexes are not available or only as not easily accessible
compounds. In addition, many late transition metal alkyls
are stable against acidic NÀH protons and do not necessarily
9
5440 Bayreuth (Germany)
E-mail: guenter.motz@uni-bayreuth.de
[
c] Prof. Dr. F. Haarmann
Institute of Inorganic Chemistry, RWTH Aachen University
5
2074 Aachen (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902836.
react with the ceramic precursors via alkane elimination as
Chem. Eur. J. 2010, 16, 4231 – 4238
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4231

the early transition metals do. A safe and general concept
that allows the introduction of a broad variety of transition
metals should be based on metal complexes that are able to
react with the NÀH functions of the precursor to form a co-
valent bond between the metal and the ceramic precursor.
Furthermore, these metal complexes should show high solu-
bility in a variety of organic solvents, especially in those sol-
vents that are used to dissolve the ceramic precursors. More-
over, the compounds should not contain “alien” elements,
elements that are not wanted in the ceramic. Additionally,
this class of compounds should be easy to synthesize on a
multigram scale and be available for a broad range of differ-
ent metals. These issues can be addressed by using amino-
[24–30]
pyridinato complexes.
We report herein on the synthesis
and characterization of a novel copper aminopyridinate,
which was used for metal modification of the commercially
available ceramic precursor HTT 1800 (Clariant Advanced
Materials GmbH, Sulzbach, Germany). The reaction of the
metal compound with the preceramic polymer was investi-
gated by NMR spectroscopy. After ceramization of the
metal-modified precursor, the ceramic was characterized by
using SEM, energy-dispersive spectroscopy (EDS), powder
diffraction, thermogravimetric analysis (TGA), and elemen-
tal analysis. The processing scheme for the synthesis of the
novel Cu-SiCN ceramics by molecular design is shown in
Figure 1.
The selective oxidation of simple alkanes is one of the
[31–33]
major challenges of todayꢁs chemistry.
Owing to the dif-
ficulties and high expense associated with the handling and
transportation of natural gas, large amounts remain unex-
[34,35]
ploited.
Thus, there is a great need for catalysts that are
able to convert simple alkanes into the corresponding mono-
oxidation products. The selective monooxidation of inert al-
kanes is a reaction of great industrial importance and there-
fore, this reaction type has generated much research interest
[36–38]
during recent years.
Cycloalkanes are often employed
as model systems for catalysis and we used them to get a
first insight into the catalytic potential of the materials intro-
duced here. In general, there are a few approaches for this
reaction, which differ in the method (homogeneously or het-
erogeneously) or in the oxidation agent (hydrogen peroxide,
tert-butylhydroperoxide (TBHP), dioxygen or air). If hydro-
gen peroxide or TBHP are used as oxidant, the conversion
Figure 1. The approach to CuÀSiCN precursor ceramics by molecular
design. Selected bond lengths [ꢂ] and angles [8] of compound 3: Cu1ÀN1
1
.880(3), Cu1ÀN2 1.872 (3), Cu1ÀCu1 2.4143(8); N1-Cu1-N2A
177.83(12), N1-Cu1-Cu1 88.65(8), N2-Cu1-Cu1 89.18(8).
[39,40]
is usually very good to excellent,
and excellent selectivi-
(67%). The system we present herein shows both good con-
version (14%) as well as good selectivity (80%).
An ideal catalyst should be robust, recyclable, effective,
ty is observed for some substrates, for instance: cyclohexane
to cyclohexanol, using of cobalt-containing aluminophos-
[39]
[40]
phate
or titanium-containing aluminophosphate
cata-
and inexpensive. The metal modification of
a poly-
lysts. Heterogeneous catalysts that selectively oxidize cyclo-
alkanes using dioxygen as oxidant are documented to some
ACHTUNGTREN(NUNG organo)silazane by the use of a copper aminopyridinate fol-
lowed by the manufacture of a copper-containing ceramic
may yield a material that meets at least some of the charac-
teristics of an ideal catalyst. A series of Cu-SiCN ceramics
were applied as heterogeneous catalysts in the selective oxi-
dation of cycloalkanes using air as an oxidant.
[41–57]
extent.
tion of cyclohexane.
the selective heterogeneous oxidation of cyclooctane using
Most of the studies have focused on the oxida-
[42,43,45–54]
Fewer examples are known for
[
41,44,55–57]
dioxygen or air as the oxidant.
main product of these reactions with a selectivity from
Cyclooctanone is the
[44]
[56,57]
67% up to 98%.
The obtained conversion of cyclooc-
tane into its monooxidation products ranges from 6% to
[55]
[44]
1
8%
232
but better conversion causes lower selectivity
4
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Chem. Eur. J. 2010, 16, 4231 – 4238

Copper-Containing SiCN Precursor Ceramics
FULL PAPER
Results and Discussion
that the resonances 2 and 3 exchange positions relative to
each other (see Figure 2). The resonance of the proton at-
tached to the nitrogen (4) disappears in the spectrum of
complex 3. When 3 is allowed to react with the ceramic pre-
cursor, all the characteristic signals of the protonated ligand
appear, which provides evidence for the presence of the
ligand as proposed before (see Figure 2, bottom left).
Metal complex synthesis: To obtain copper aminopyridi-
nates for metal modification of poly(organosilazanes),
CuBr was allowed to react with two equivalents of lithiated
4-methylpyridin-2-yl)trimethylsilanylamine
2
TMS
(
(Ap H).
After workup in hexane, dark blue crystals of the mixed-
TMS
valent copper complex [Cu
tetrameric Cu complex [Cu ACHTGNUTERNNUNG
4
A
H
U
G
R
N
N
(Ap )4] (1) were isolated. The
The additional resonances observed in the spectrum of
the reaction mixture correspond to the ceramic precursor.
According to the spectra, the reaction goes to total comple-
3
I
TMS
4
TMS
by-product. It can be concluded that [Li AHCTUNGTNERNUG( Ap )2 ACHTUNGTERNNGNU( OEt ) ]
2 2 2
II
13
acts as a reducing agent towards Cu . Hence, the yields of 1
and 2 were moderate. To avoid reduction and to improve
the yield, CuBr was employed in a reaction with lithiated
tion. The same results can be concluded from the C NMR
spectra (Figure 2, right). This provides indirect evidence for
covalent bonding between the copper atoms and the precur-
sor. Direct evidence could be taken from a reaction of com-
pound 3 and an excess of hexamethyldisilazane, which was
chosen as a model substrate for HTT 1800. A reaction of
TMS
Ap . After workup in hexane, a light green powder of
Cu ACHTUNGTRENNUNG( Ap )2] (3) was obtained in excellent yields. All mo-
lecular structures were determined by X-ray crystal struc-
TMS
[
2
ture analysis (see the Supporting Information). [Cu2-
compound 3 with the model substrate should yield the
TMS
[59]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(Ap )2] shows a close structural relationship to the known
known complex [Cu {N
should appear in the H NMR spectrum (see Scheme 1).
This reaction was carried out on an NMR scale, and indeed,
A
H
U
G
R
N
N
(SiMe ) } ],
for which signals
4
3 2 4
1
copper aminopyridinate [Cu2
lylamido)-6-methylpyridine).
A
H
U
G
R
N
U
G
2
[
58]
TMS
scaled up to multigram scale without loss of yield. Hence
compound 3 is an ideal candidate for metal modification of
poly(organosilazanes).
(beside the appearance of the liberated ligand Ap H) the
signal of [Cu {N
A
H
U
G
R
N
N
(SiMe ) } ] appeared (see the Supporting In-
4
3 2 4
formation).
Transmetalation: Complex
3
was employed as the transmeta-
lation agent in a reaction with
HTT 1800. If transmetalation
with the poly(organosilazane)
takes place, the protonated
TMS
ligand Ap H should be liber-
ated (aminopyridine elimina-
tion). The driving force of this
reaction could be the low coor-
dination number of copper in 3.
Additionally, the ceramic pre-
cursor provides an excess of N
donors to which copper can
bind datively and covalently
1
13
(
see Figure 1). H and C NMR
experiments were carried out to
verify the transmetalation reac-
tion, and for comparability all
spectra were recorded in
[
D ]THF. There are some char-
8
acteristic differences between
the NMR spectra of the proton-
ated ligand and those of com-
pound 3. Resonance 1 belongs
to the proton located in the 6-
position of the pyridine ring.
This signal, which occurs at d=
7.83 ppm in the protonated
ligand, shifts significantly up-
field to d=7.42 ppm in complex
3
. Comparison of the ligand
and complex spectra reveals 1800 (from top to bottom).
1
13
TMS
Figure 2. H (left) and C NMR spectra (right) of Ap H, compound 3, and after reaction of 3 with HTT
Chem. Eur. J. 2010, 16, 4231 – 4238
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4233

G. Motz, R. Kempe et al.
TMS
Scheme 1. Reaction of [Cu
2
A
H
U
G
E
N
N
(Ap
2
) ] with hexamethyldisilazane.
Crosslinking and ceramization: First, the metal-modified
precursor (Cu/Si ratio of 1/5) was crosslinked by using dicu-
mylperoxide (DCP) as the initiator for hydrosilylation and
polymerization reactions at 1308C. Subsequent pyrolysis at
1
0008C for 30 min in a N atmosphere led to the Cu-SiCN
2
ceramic. A ceramic yield of 61% was determined by ther-
mogravimetrical measurements. This indicates the possibility
of ceramization of the metal-modified precursor. If it is as-
TMS
Figure 3. SEM image (top left), corresponding Cu EDS-mapping (top
right), Si EDS-mapping (middle right), overlap of both mappings
sumed that the liberated ligand (Ap H) is completely
eliminated during ceramization and only the copper remains
in the ceramic, a theoretical yield of 53% would be expect-
ed. The higher ceramic yield confirms not only the disposi-
(
bottom right), and X-ray powder diffraction pattern of the ceramic
(bottom left). All magnitudes of the images are 2000ꢃ.
tion of copper in the ceramic material, but it indicates also
TMS
that the Ap
ligand contributes significantly to the ceram-
ture of 10008C, which is close to the melting temperature of
copper (10838C). Hence, the copper atoms are quite mobile
and can agglomerate. As can be seen in Figure 3, the Cu
EDS-mapping fits perfectly to the particles that can be
found in the SEM image. The Si EDS-mapping shows the
presence of silicon-free areas exactly at the position of the
copper particles, as can be seen by the overlap mapping pic-
ture. It can be concluded that the particles do not consist of
any copper compound (e.g. nitride, carbide, or silicide) but
of elemental copper. Powder X-ray diffraction studies were
carried out the milled ceramic powder to substantiate the
hypothesis of the presence of crystalline elemental copper in
the ceramic material (Figure 3). The three prominent peaks
can clearly be assigned to fcc-centered elemental copper (2q
ic yield. Elemental analysis of the ceramic was performed to
quantify the ceramic composition (see Table 1).
Table 1. Elemental composition of the Cu-containing precursor and of
the resulting CuÀSiCN ceramic (Cu/Si ratio of 1/5). All values are given
in [wt%].
Metal-containing
precursor (calcd)
CuÀSiCN ceramic
(after pyrolysis at 10008C)
C
H
N
Si
Cu
O
34.0
7.5
17.4
29.8
11.3
–
15.0
0.7
21.8
44.4
13.7
3.4
[60]
values [8]: exp.: 43.46, 50.59, 74.25; lit. : 43.32, 50.45,
4.13). The ceramic matrix is amorphous and thus, cannot
7
be “seen” in the diffracting pattern. Thus, both SEM and
powder diffraction reveal the presence of elemental copper
particles in the ceramic material.
As shown in Table 1, the copper content increases signifi-
cantly during ceramization. The pyrolysis process of sila-
zanes is characterized by the release of methane and hydro-
gen, which reduces the C and H content in the resulting ce-
ramic significantly. A possible formation of volatile copper
compounds during the ceramization is prevented due to the
covalent copper–precursor bond, which was formed during
the transmetalation reaction.
SEM images of the Cu-SiCN ceramic (Figure 3) show the
presence of particles of different sizes (from some microme-
ters down to sizes of about hundred nanometers) formed
during the ceramization. The associated EDS-mappings indi-
cate that these particles consist of elemental copper. The in-
situ reduction of copper(I) to elemental copper results from
the reductive atmosphere consisting of the pyrolysis gases
methane and hydrogen. The unexpected agglomeration to
Cu particles in the mm-scale is due to the pyrolysis tempera-
Variation of the metal content: The presented molecular ap-
proach was employed to obtain copper-modified SiCN pre-
cursor ceramics that contain different amounts of copper.
Copper/silicon ratios of 1/5, 1/10, 1/25, 1/50, and 1/100 were
generated in the metal-modified preceramic polycarbosila-
zane (Table 2). Silicon was used as the reference for adjust-
ing the ratios due to the fact that silicon remains most likely
completely in the material during pyrolysis. The ceramiza-
tion behavior was investigated by thermogravimetric meas-
urements. The ceramic yields range from 61% to 74%
(Figure 4). The metal-free polycarbosilazane provides 75%
ceramic yield. Despite the liberation of large amounts of
free ligand during the reaction of the copper complex with
the SiCN precursor, quite good ceramic yields are achieva-
4234
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Copper-Containing SiCN Precursor Ceramics
FULL PAPER
Table 2. Elemental composition of the different copper-containing SiCN
ceramics. All values are given in wt% (10008C, N ).
2
Cu/Si
1/5
1/10
1/25
1/50
1/100
C
H
N
Si
Cu
O
15.0
0.7
21.8
44.4
13.7
3.4
15.1
0.6
23.8
47.8
7.3
16.6
0.7
22.8
51.0
4.6
18.4
0.6
24.1
51.7
2.4
17.6
0.5
23.8
52.9
1.8
3.2
3.4
3.1
2.2
6
5
Figure 5. Solid-state Cu NMR measurements of ceramics of different
composition.
ed, to a first approximation, by the ratio of the signal inten-
65
sities. Concerning the 1/100 ceramic, the solid-state Cu
NMR experiments indicate the absence of metallic copper.
The observed signals, which cover a broad frequency range,
indicate a “molecular” distribution of copper in the ceramic
material.
Figure 4. Thermogravimetrical measurements of the ceramics containing
different amounts of copper (10008C, N ). In all cases 2 wt% of DCP
2
were added.
SEM investigations were performed to obtain information
about the particle-size distribution. All ceramics contain
copper particles, and the number of particles as well as the
size increases with the copper content. In the case of high
amounts of copper, a broad distribution of particle sizes was
found (Figure 6, top left). Nanometer-scaled as well as mi-
crometer-scaled particles were obtained. Only small (nano-
meter-scaled) particles could be obtained at low copper con-
tent (Figure 6, bottom right). This finding can be explained
by the lower tendency of agglomeration (because less
copper is present). Thus, less and smaller particles are
formed. Beside the particles, a texture of the ceramic matrix
is obtained (Figure 7). The 1/5 ceramic shows an intense tex-
ble. This is due to the contribution of the free ligand to the
ceramic yield. But in general, as the metal content increases,
the ceramic yield decreases.
Elemental analyses of each copper-containing ceramic
were carried out to determine the final copper content in
the resulting ceramic. The higher the amount of copper in
the precursor, the higher the copper content is in the corre-
sponding ceramic. Thereby, the amount of silicon, carbon,
and nitrogen declines with increasing metal content. These
results show that the metal content of the ceramic materials
is tunable by the addition of metal aminopyridinate and can
be varied in a wide range. The detected small amount of
oxygen can be traced back to the fact that the metal-modi-
fied precursor was exposed to air for a short time when put-
ting the precursor into the furnace.
The lower copper-containing ceramics are found to be
completely amorphous by X-ray diffraction, indicating that
6
5
copper is not “visible” yet. Solid-state Cu NMR investiga-
tions of the ceramics 1/10, 1/25, 1/50, and 1/100 were per-
formed to investigate whether elemental copper or copper
compounds are present in the samples (Figure 5). The re-
sults showed the presence of metallic copper besides varying
copper compounds in the 1/10, 1/25, and presumably also in
the 1/50 ceramic. The NMR signal is identical to that of
metallic copper, as revealed by recording the NMR spectra
of fine commercial Cu particles, which provides evidence for
the presence of metallic copper in these samples. The ratio
of metallic copper besides other copper compounds increas-
es with the copper content of the samples, as can be estimat-
Figure 6. SEM images of different-sized copper particles (ceramics 1/5
top left; 1/10 top right; 1/10 bottom left, higher magnification; 1/100
bottom right; 10008C, N ).
2
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4235

G. Motz, R. Kempe et al.
the copper-containing SiCN precursor ceramics (see
Table 3). Neither TBHP only, nor TBHP in combination
with copper-free SiCN precursor ceramics were able to cata-
lyze the oxidation of cyclooctane. Furthermore, no conver-
sion was observed without the use of TBHP. Thus, it can be
concluded that TBHP and a copper-containing ceramic are
needed as the catalyst system.
Table 3. Conversion [%] of cyclooctane into monooxidation products
[
a]
[
%] observed in control experiments.
Catalyst
without TBHP
2 mol% TBHP
none
–
–
–
–
–
13.5
[
b]
copper-free ceramic
copper-containing ceramic (1:50)
[b]
Figure 7. SEM images of different ceramics (top left: 1/5; top right: 1/10;
bottom left: 1/50; bottom right: 1/100; 10008C, N ).
[a] Reaction conditions: cyclooctane (1 mL), 2 mol% TBHP, 20 bar air,
808C, 75 h. [b] An amount of 10 mg of ceramic material was used.
2
ture (Figure 7, top left), whereas less intense textures are
observed for the 1/10 and 1/50 ceramic (Figure 7, top right
and bottom left, respectively). In the case of the 1/100 ce-
ramic, no texture could be detected (Figure 7, bottom right).
EDS-mappings of the 1/5 ceramic showed that the particles
consist of elemental copper. It can clearly be seen that the
amount and size of the particles decreases with decreasing
copper content (Figure 8). Due the spatial resolution of
The selectivity observed on applying the 1/50 ceramic
[which gave 13.5% conversion (Table 3)] is rather low, since
less than 45% selectivity was observed in the cyclooctanone
formation. The advantage of the molecular design approach
is the variability in the copper content. The selectivity to-
wards monooxygenation can be tuned by changing the Cu/Si
ratio. With increasing copper content in the ceramic, the se-
lectivity rises gradually up to 80% for the monooxygenation
product cyclooctanone (Figure 9).
Figure 8. SEM images and corresponding Cu EDS-mappings (left hand:
1
2
/5; middle: 1/10; right hand: 1/100; 10008C, N ).
Figure 9. Selectivities [%] of different catalysts. The copper to silicon
ratios vary from 1:5 to 1:100. Reaction conditions: 1 mL cyclooctane,
1
0 mg ceramic, 2 mol% TBHP, 20 bar air, 808C, 75 h.
EDS, nanometer-scaled particles could not be detected in
the 1/100 ceramic (Figure 8 right), but the 1/10 and 1/5 ce-
ramics contain copper particles that can be detected by
EDS-mappings (Figure 8, middle and left, respectively),
which is due to the increasing size of the particles as the
copper content increases.
The Cu-SiCN ceramics are recyclable, when new substrate
and new activator is added (see Table 4). However, the se-
lectivity oscillates somewhat and the conversions decrease
slightly with increasing catalytic cycles. This might be due to
catalyst loss by filtration. To get evidence how general the
applicability of these ceramics is with respect to selective ox-
idation reactions, other cycloalkanes were employed as sub-
strates (Table 5). The smallest cycloalkane showed only
minor conversion (Table 5, entry 1), but with increasing size
of the rings, the conversion increases. The observed selectiv-
ity is almost constant for all cycloalkanes and is around
70% for the corresponding ketones.
Catalytic studies: The objective of the synthesis of metal-
containing SiCN precursor ceramics was to gain access to a
material that can be employed as a robust, efficient, hetero-
geneous, and recyclable catalyst. We employed air as the ox-
idant for the oxidation of the alkanes in this work, and cy-
clooctane was chosen as substrate. Control experiments
were carried out first to investigate the catalytic activity of
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Copper-Containing SiCN Precursor Ceramics
FULL PAPER
Table 4. Conversions [%] and selectivities [%] achieved by recycling of
the Cu/Si 1:10 catalyst.
obtained metal-modified ceramics are suitable as recyclable
and selective heterogeneous catalysts. The general potential
of the catalyst class (other metals and other reactions) needs
to be explored in further studies.
[
a]
Cycle
Conversion [%]
Selectivity [%]
alcohol epoxide
ketone
others
1
2
3
12.0
11.7
10.1
78
58
67
18
18
18
1
3
4
10
21
11
Experimental Section
[
a] Reaction conditions: cyclooctane (1 mL), 2 mol% TBHP, 20 bar air,
8
08C, 50 h. After every cycle, it was filtered and new substrate and acti-
General: All manipulations with air- and moisture-sensitive compounds
and materials were performed by using either standard Schlenk or glove-
box techniques. Detailed descriptions of all procedures are provided in
vator was added.
the Supporting Information.
TMS
Table 5. Conversions [%] and selectivities [%] achieved by oxidation of
different cycloalkanes using the Cu/Si 1:10 catalyst.
Synthesis of [Cu
64 mmol) in Et
.6m, 64 mmol) at 08C and stirred for 30 min. After this mixture had
2
A
H
U
G
R
N
N
(Ap
2
) ] (3): A solution of ApTMSH (11.541 g,
[
a]
2
O (100 mL) was treated with nBuLi in hexane (40 mL,
1
Entry
Substrate
Conversion [%]
Selectivity [%]
been stirred for 1 h at room temperature, it was added to a suspension of
CuBr (9.184 g, 64 mmol) in THF (100 mL) at 08C, and the color changed
from dark green to pale green. The mixture was stirred overnight at
room temperature and the solvent was then removed. The residue was
continuously extracted with hot hexane (100 mL) and filtered. Concen-
tration of the mother liquor and storage in a freezer at À308C yielded a
beige powder. Crystalline material was obtained after slow evaporation
ketone
alcohol
others
[
b]
[b]
[b]
1
2
3
4
cyclohexane
cycloheptane
cyclooctane
cyclodecane
1.0
6.0
13.9
16.0
n.d.
68
73
n.d.
n.d.
7
11
7
25
16
19
74
[
a] Reaction conditions: substrate (1 mL), 1/10 ceramic (10 mg), 2 mol%
1
of a solution of 3 in hexane. Yield: 14.802 g (61 mmol, 95%). H NMR
TBHP, 20 bar air, 808C, 75 h. [b] Not determined.
(
400 MHz, [D
8
]THF): d=0.31 (s, 18H, TMS), 2.11 (s, 6H, ar-CH3), 6.09
(
d, J=7.6 Hz, 2H, Ar-H), 6.39 (s, 2H, Ar-H), 7.42 ppm (d, J=7.6 Hz,
1
3
2
1
H, Ar-H); C NMR (400 MHz, [D
8
]THF): d=2.15, 21.40, 111.61,
16.41, 147.99 148.53, 170.13 ppm; Si NMR (300 MHz, [D ]THF) d=
Si (M
85.72): C 44.30, H 6.56, N 11.49; found: C 44.51, H 6.23, N 11.53.
Transmetalation: The poly(organosilazane) HTT 1800 was allowed to
2
9
Conclusion
8
À0.58 ppm; elemental analysis calcd (%) for
C
18
H
30Cu
2
N
4
2
r
=
4
Novel copper aminopyridinato complexes were synthesized
and characterized. The dinuclear compound 3 was employed
to metal-modify the polycarbosilazane HTT 1800 due to its
easy and inexpensive availability. This reaction was investi-
gated by NMR spectroscopy and runs smoothly to comple-
tion. Covalent bonding between the copper atoms and the
polycarbosilazane can be assumed. In conclusion, aminopyr-
idinates can be regarded as suitable coordination com-
pounds to metal-modify poly(organosilazanes).
TMS
react with [Cu
2
A
H
U
G
R
N
U
G
2 2
) ] (3) in Et O, then dicumylperoxide was added
for crosslinking of the metal-modified precursor. The solvent was re-
moved in vacuo to yield a viscous brown oil. The NMR experiments
were performed without addition of dicumylperoxide.
Crosslinking and ceramization: The viscous brown oil obtained was
placed into a furnace under nitrogen gas for crosslinking. The tempera-
ture was raised to 10008C during ceramization. The resulting ceramic ma-
terial was milled for catalysis experiments. Some fragments were taken
out before milling and embedded in an epoxy resin for SEM investiga-
tions. The powder diffractogram was recorded from the milled ceramic
A series of copper-modified SiCN ceramics could be syn-
thesized from these copper-modified precursor polymers
after cross linking. Good to high ceramic yields were ob-
served. High copper amounts up to a Cu/Si ratio of 1 to 5
still allow the synthesis of a stable ceramic material in a ce-
ramic yield of about 60%. The copper content is tunable
over a broad range. The copper-containing ceramics were
Catalytic tests: The milled ceramic was mixed together with the substrate
and a catalytic amount of TBHP. At elevated temperature the reaction
mixture was stirred under an air pressure of 20 bar. The conversion and
selectivity were determined by GC analyses.
6
5
characterized by SEM, EDS, powder diffraction, and Cu
solid-state NMR spectroscopy as well as by TGA and ele-
mental analysis. The results show the formation of crystalli-
tes and particles of different sizes, which consist of elemen-
tal copper. The size of the particles depends on the copper
content. In conclusion, copper-coordinated poly(organosila-
zanes) can be crosslinked and converted into copper-modi-
fied SiCN ceramics (Cu@SiCN). The molecular approach
allows a tailoring of the amount of copper and to some
extent (also if not fully understood at this stage) the particle
size.
Acknowledgements
Financial support from the DFG (SPP 1181 “Nanoskalige anorganische
Materialien durch molekulares Design”) is acknowledged. We thank
A. M. Dietel for laboratory assistance, Dr. S. Demeshko for measuring
magnetic susceptibilities, Dr. Ute Hçrmann for preparing the samples for
SEM investigations, and Dr. Wolfgang Milius for powder diffraction
measurements. Furthermore, we thank Clariant Advanced Materials
GmbH for providing the precursor HTT 1800. Frank Haarmann grateful-
ly acknowledges the use of the NMR facilities of the Max-Planck-Institut
fꢄr Chemische Physik fester Stoffe in Dresden, Germany.
The obtained ceramics can be employed as catalysts for
the selective oxidation of cycloalkanes under mild condi-
tions. Air was employed as the oxidant. By tailoring the
copper content as well as the nature of the metal loading,
the selectivity issues can be addressed. In conclusion, the
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Received: October 14, 2009
Published online: March 5, 2010
4238
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 4231 – 4238