Pore-size engineering of silicon imido nitride for catalytic applications

Article abstract of DOI:10.1002/1521-3773(20011119)40:22<4204::AID-ANIE4204>3.0.CO;2-D

High specific surface areas and adjustable pore sizes are outstanding characteristics of nanoporous silicon nitride based materials prepared by using oxygen-free molecular precursors in a novel template-assisted sol-gel approach. The nitrides represent a new class of shape-selective superbase catalysts (see, for example, the schematic representation of alkene isomerization).

Full text of DOI:10.1002/1521-3773(20011119)40:22<4204::AID-ANIE4204>3.0.CO;2-D

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Chem. Int. Ed. 1998, 37, 3105 ± 3108; d) F. H. Arnold, A. A. Volkov,
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Pore-Size Engineering of Silicon Imido Nitride
for Catalytic Applications**
David Farrusseng, Klaus Schlichte, Bernd Spliethoff,
Annette Wingen, Stefan Kaskel,* John S. Bradley, and
Ferdi Schüth
[
3] C. S. Chen, Y. Fujimoto, G. Girdaukas, C. J. Sih, J. Am. Chem. Soc.
982, 104, 7294 ± 7299.
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Angew. Chem. 1997, 109, 2961 ± 2963; Angew. Chem. Int. Ed. Engl.
Microporous and mesoporous materials with high internal
surface area and pore volume play a key role in the
development of new heterogeneous catalysts and solid
membranes.[ The strategy in the synthesis of materials with
well defined pore structure is to direct the networking of
molecular or ionic precursors by a templating agent, which
can be a molecule, an ion, a polymer, or a supramolecular
assembly.[ Removal of the occluded template produces
materials with patterns of characteristic pore size, shape,
and structure. Although there is a growing interest in
expanding these templating routes to materials other than
oxides, only in the case of sulfides and some super-Prussian-
blue compounds have micro- and mesoporous inorganic
nonoxide materials been synthesized.[ Nitrido-sodalites with
very small pores, probably inaccessible to organic substrates,
1
997, 36, 2830 ± 2832.
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Several methods for the synthesis of dense silicon nitride
starting from elemental silicon, silicon chloride, or even
carbodiimide have been reported. However, the first micro-
[
[5]
porous silicon imido nitrides with a mean pore size of 0.7 nm
were synthesized by Bradley and Dismukes in pyrolysis
reactions of polysilazanes.[ Mesoporous silicon imido nitride
[
[
6]
60, 105 ± 111.
15] The assay also yields a linear increase in absorption when increasing
concentrations of sodium acetate are used; see also reference [12] and
the instructions for use enclosed with the commercial test-kit for
accuracy of the method.
with a narrow pore-size distribution (d ˆ 5.6 nm) and a high
av
2
� 1
specific surface area (up to 1000 m g ) was obtained recently
using the ammonolysis of silicon tetrachloride in organic
solvents.[
7]
[
[
16] E. M. Anderson, K. M. Larsson, O. Kirk, Biocatal. Biotransform.
1998, 16, 181 ± 204.
We describe herein a template-assisted method which
allows, for the first time, tailoring the pore size of microporous
silicon nitride materials in a wide range from primary to
secondary micropores. In this procedure, tris(dimethylami-
17] Eapp values are the ratio of initial rates determined from the hydrolysis
of pure R or S enantiomers. Etrue values are derived from resolutions of
racemates and therefore also include competition between the two
enantiomers for the active site of the enzyme.
18] More than 50000 mutants can be screened per day, if the analysis time
can be reduced to about 1 min per test and faster pipetting steps are
possible.
[
8]
[
no)silylamine [(CH ) N] SiNH 1
is ammonolyzed in a
concentrated solution of CH (CH ) NH (n ˆ 11 ± 17) in hot
3
2
3
2
3
2
n
2
acetonitrile. The ammonolysis can be considered as an
analogue of silicon oxide manufacture by sol ± gel methods
using prehydrolyzed tetramethoxysilane and water. After
cooling to room temperature, a gel forms which is dried and
heated slowly to 823 K in flowing ammonia. According to
nitrogen physisorption experiments (Figure 1), such materials
prepared using alkylamines of different chain length
[*] Dr. S. Kaskel, Dr. D. Farrusseng, K. Schlichte, B. Spliethoff,
Dr. A. Wingen, Dr. F. Schüth
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
Fax : (‡49)208-306-2995
E-mail: kaskel@mpi-muelheim.mpg.de
Dr. J. S. Bradley
Department of Chemistry
University of Hull
Cottingham Road, Hull HU6 7RX (UK)
[
**] We thank Dr. P. Šgren and U. Specht for helpful discussions on
physisorption and precursor syntheses. A Reimar Lüst Fellowship for
S.K. was provided by the Max Planck Society.
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The nanometer-sized micropores are easily discerned in
high-resolution TEM pictures (Figure 2). They reveal the
uniform channel-like pore structure of the air-sensitive
nitrides. Crystallization of the inorganic network was neither
Figure 2. TEM picture of microporous silicon nitride (template: octade-
cylamine, dav ˆ 1.7 nm).
observed in TEM pictures nor in XRD powder patterns even
after annealing the gels at 1273 K in a stream of gaseous
ammonia. Samples treated thusly maintained their high
3
� 1
micropore volume (0.35 cm g ) as well as the pore structure.
This surprisingly low sintering tendency is an attractive
feature and reveals the promising potential of the material
as a support in high temperature catalysis under nonoxidizing
conditions.
Figure 1. a) Nitrogen physisorption isotherms of microporous silicon
imido nitrides prepared using CH (CH NH
(n ˆ 11 ± 17) as templates,
b) logarithmic plot (Vˆ volume adsorbed normalized to volume at
P/P
ˆ 1).
3
2
)
n
2
0
The pore evolution mechanism is still unknown. The size of
the pores is of the same order as the size of the templating
molecules, and therefore a micellar templating mechanismÐ
in any case unlikely because of the low dielectric constant of
the organic solventÐcan be excluded. In fact, we believe that
the amines are occluded in the first condensation step and
released after thermal consolidation of the network during the
heat treatment at about 673 K.
(
CH (CH ) NH , n ˆ 11, 13, 15, 17) are all microporous and
3
2
n
2
the isotherms are of type I. The micropore volume increases
with the length of the amine alkyl chain (Table 1). Logarith-
mic plots (Figure 1b) show that for the longer amines used as
templates the nitrogen adsorption predominately occurs at
IR spectroscopy of the heat-treated samples was used to
identify silicon imido nitride [ nÄ ˆ 3408 (n(NH)), 1549
Table 1. Nitrogen physisorption results for high surface-area silicon imido
nitrides prepared using linear aliphatic amines CH
plates.
3 2 n 2
(CH ) NH as tem-
d(NH )), 1182 (d(NH)), 891 cm (n (Si N))]. The 29Si
� 1
[8]
(
2
a
s
2
Template chain
ength n
Micropore
volume [cm g ]
Average pore
diameter [Š]
MAS NMR spectrum consists of a broad line with a maximum
at d ˆ � 41 and a shoulder at about d ˆ � 50. The line covers a
region between d ˆ � 35 and � 55. Such a broadening
originates from the amorphous character of preceramic
silicon nitride powders, in which various different coordina-
tion spheres are present.[
Silicon imido nitride with high accessible surface area can
be used as a solid base catalyst in Michael addition reactions
due to the presence of basic sites. For example, moderately
3
� 1
1
1
1
1
1
1
1
2
3
4
5
7
0.32
0.34
0.37
0.38
0.42
0.48
11.8
12.4
13.6
13.7
15.7
16.5
10]
acidic substrates like malononitrile (pK ˆ 11) are activated to
a
higher relative pressures corresponding to secondary micro-
pores. The actual calculation of the pore size using Horvath ±
Kawazoe or density functional theory (DFT) methods is
difficult, since the principal interaction potentials of the
adsorbate and the adsorbent are unknown and assumptions
form addition products with substrates like acrylonitrile
(Scheme 1).
[
9]
have to be made about the pore morphology. The numerical
values of the pore diameters calculated from the DFT method
Scheme 1.
(Table 1) are therefore not fully accurate but, more impor-
tantly, allow a comparison among members of a class of
similar materials which presumably have the same interaction
potentials.
Due to the basicity of the inner surface and its high thermal
and chemical stability, silicon nitride can be used as a solid
superbase catalyst (pK > 26) after potassium impregna-
a
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tion.[11] Solid superbases like alkali metal promoted oxides are
used in industry for the isomerization of polycyclic alkenes or
in side-chain alkylations of alkylbenzenes.[12] In the case of
silicon nitride, the covalent network is advantageous to
achieve shape selectivity through pore-size engineering, as
opposed to nonporous basic oxide supports like MgO or
Al O . Zeolites are less suitable for such reactions due to a
Summarizing, we have demonstrated that pore-size tailor-
ing of silicon imido nitride is possible. The compounds
described here represent a new class of shape-selective solid
superbase catalysts.
2
3
Experimental Section
high number of acidic sites. However, the catalytic isomer-
ization of 1-butene has been reported over sodium metal
clusters located in high-alumina zeolites.
The educt and product selectivity of our nitrides is
demonstrated by comparing the performance of microporous
All operations were performed using an argon-filled glove box or a vacuum
line and dry solvents. Alkylamines (Merck, Aldrich) were dried in vacuum
prior to use. Tris(dimethylamino)silylamine was prepared according to a
[
13]
[
8]
procedure previously reported and characterized by GC/MS. Typically,
the liquid precursor (1.00 mmol) was added to a solution of CH (CH NH
3
2
)
n
2
(
2 g, n ˆ 11 ± 17) in boiling acetonitrile (20 mL, saturated with gaseous
(
d ˆ 1.7 nm) and mesoporous (d ˆ 5.6 nm) silicon nitride
av
av
ammonia). After 1 h stirring, the reaction mixture was cooled to room
temperature and a gel formed at the bottom of the flask. After washing
with hot acetonitrile, the gel was dried in vacuum and heated slowly to
823 K in a dynamic ammonia atmosphere (1 bar) to give a yellowish, X-ray
amorphous silicon imido nitride powder.
catalysts (Table 2). The side-chain alkylation of toluene with
styrene (Scheme 2) proceeds within minutes with the meso-
porous catalyst and produces also higher addition products. In
Nitrogen physisorption measurements were performed on a Micromeritics
Table 2. Conversion [%] over potassium-promoted micro- and mesopo-
rous silicon imido nitride (reaction time in parentheses).
2
000 apparatus. The micropore volume was deduced from the total amount
nitrogen adsorbed at P/P
ˆ 0.2. The Micromeritics DFT program was used
to estimate absolute pore sizes. For Si MAS NMR experiments, a
0
[
14]
29
Reaction
Microporous
catalyst
Mesoporous
catalyst
Bruker Avance 500WB instrument was used. The spectra were recorded
using a single-pulse excitation (p/2, 4.5 ms) with a repetition time of 60 s and
a MAS frequency of 5.0 kHz.
The general procedure for potassium promotion is described elsewhere.[11]
All test reactions were carried out at 295 K. For alkene isomerizations,
1-hexene and 1-hexadecene were mixed (1:1) and 2 mL of the mixture was
added to the catalyst (100 mg). The ethylation was carried out in a 50 mL
steel autoclave at a pressure of 20 ± 30 bar loaded with 10 mL toluene. The
product composition was analyzed by means of GC/MS and 1H NMR
spectroscopy.
(
dav ˆ 1.7 nm)
(dav ˆ 5.6 nm)
[
a]
74.0 (60 h)[c]
0.0 (48 h)
99.3 (18 h)
0.0 (18 h)
25.0 (96 h)[d]
100.0 (10 min)
95.5 (2 min)
80.8 (2 min)
toluene ‡ ethene
[a]
toluene ‡ styrene
[
b]
1
1
-hexene
-hexadecene[
b]
[
a] Scheme 2. [b] Double bond shift. [c] 30 bar. [d] 20 bar.
Received: June 13, 2001 [Z17285]
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Schroden, Adv. Mater. 2000, 12, 1403 ± 1419; e) J. Weitkamp, Solid
State Ionics 2000, 131, 175 ± 188; f) U. Ciesla, F. Schüth, Microporous
Mesoporous Mater. 1999, 27, 131 ± 149; g) A. Sayari, P. Liu, Micro-
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C. J. Brinker, Chem. Mater. 1996, 8, 1682 ± 1701; i) H. L. Frisch, J. E.
Mark, Chem. Mater. 1996, 8, 1735 ± 1738.
contrast, the microporous catalyst gives no conversion due to
the bulky character of the products. The alkylation reaction
with ethene on the other hand (Scheme 2) proceeds smoothly
with both catalysts, and at higher ethene pressures (30 bar)
the dialkylation product forms even over the microporous
catalyst since both propylbenzene and (1-ethylpropyl)ben-
zene fit in the secondary micropores.
Similar differences between micro- and mesoporous ni-
trides are observed in alkene isomerization reactions. Using
the mesoporous catalyst, isomerization of 1-hexene and
[3] a) S. A. Schunk, F. Schüth in Molecular Sieves, Vol. 1 (Eds.: H. G.
Karge, J. Weitkamp), Springer, Heidelberg, 1998, pp. 229 ± 263;
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Weitkamp, S. Ernst, F. Cubero, F. Wester, W. Schnick, Adv. Mater.
1-hexadecene (1:1 mixture) to the more stable alkenes with
inner double bonds proceeds within minutes for both
substrates. The microporous solid converts instead only the
shorter substrate 1-hexene, and the pure 1-hexadecene can be
recovered from the mixture. These selectivity differences
confirm the results of the nitrogen adsorption measurements.
Probably the calculated pore radii are a little to high as
compared to the size of the substrates and products, which is
due to the limitations of the pore size analysis.
1
997, 9, 247 ± 248; f) N. Stock, E. Irran, W. Schnick, Chem. Eur. J. 1998,
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6] a) J. S. Bradley, O. Vollmer, R. Rovai, U. Specht, F. Lefebvre, Adv.
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preferred square-planar coordination environment for cop-
per(ii) ions with the option of one or two additional apical
[
[
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7] S. Kaskel, D. Farrusseng, K. Schlichte, Chem. Commun. 2000, 2481 ±
2
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Scheme 1. Schematic representation for the hydrogen bonding patterns
observed for dinuclear copper(ii) complexes with square-pyramidal coor-
dination geometry at the copper(ii) centers (e: equatorial position, a: apical
position).
[
10] a) E. A. Leone, S. Curran, M. E. Kotun, G. Carrasquillo, R. vanWee-
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[
[
11] S. Kaskel, K. Schlichte, J. Catal. 2001, 201, 270 ± 274.
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Sci. Catal. 1997, 108, 649 ± 656; b) H. Gorzawski, W. F. Hoelderich, J.
Mol. Catal. A 1999, 144, 181 ± 187.
ligands, these hydrogen bonds can be denoted as either
equatorial ± equatorial (e ± e) or equatorial ± apical (e ± a)
with regard to the position of the involved donor and acceptor
atoms. If one considers that the magnetic orbital at each
copper(ii) ion is defined by the short equatorial bonds, that is
[
[
13] L. R. M. Martens, P. J. Grobet, P. A. Jacobs, Nature 1985, 315, 568 ±
570.
14] DFT (TM) Vers.1.05, Micromeritics, Norcross, GA, USA. Slit pore
model, nitrogen on carbon at 77 K.
basically an orbital of d
of dz
x2� y2
type with some possible admixture
2
character, a qualitative magneto-structural correlation
would predict antiferromagnetic coupling for (e ± e/e ± e)
bridging patterns and weak interactions, which should be of
ferromagnetic nature, for (e ± a/a ± e) bridging patterns. In
addition, the magnitude of the magnetic coupling should also
be related to the strength of the involved hydrogen bonds.
This basic concept is in agreement with the observed
structural and magnetic data of relevant systems.[
To probe this qualitative magneto-structural relationship
we synthesized the copper(ii) complex with the trivalent,
pentadentate Schiff base ligand N-salicylidene-2-(bis(2-hy-
Magnetic Interactions as Supramolecular
Function: Structure and Magnetic Properties of
Hydrogen-Bridged Dinuclear Copper(ii)
Complexes**
4]
Winfried Plass,* Axel Pohlmann, and
Jens Rautengarten
droxyethyl)amino)ethylamine (H sabhea). The resulting self-
3
‡
complementary complex cation [Cu(H sabhea)] can be
2
Dedicated to Professor Ernst-Gottfried Jäger
on the occasion of his 65th birthday
isolated as hydrogen-bridged dinuclear complex 1. Complex
1 can be reversibly deprotonated in aqueous solution yielding
2
, the first triply hydrogen-bridged dinuclear copper(ii)
Hydrogen bonds play a key role in interactions in biological
structures, supramolecular chemistry, and crystal engineer-
complex.
[
1]
ing. As such they are also important to understand the
[{Cu(H
{Cu(H
2
2
sabhea)}
2
](BF
4
)
2
1
[
2]
properties of relevant magnetic materials. In particular for
some copper(ii)-containing coordination compounds it has
been shown that the variation of possible supramolecular
interactions can substantially influence the magnetic proper-
[
sabhea)}{Cu(Hsabhea)}]BF
4
2
The X-ray crystal structure analyses[5] of the isolated
compounds 1 ´ 0.5EtOH ´ 0.25H O (1a) and 2 ´ H O (2a)
[
3]
ties of related coordination polymers.
2
2
Although copper(ii) complexes have been widely studied,
only two basic patterns of hydrogen bonding have been
observed for dinuclear units (Scheme 1). In accord with the
reveal the dimeric hydrogen-bridged structure of the cationic
copper(ii) complexes (Figure 1). The prearranged coplanar
configuration for three donor sets of the H sabhea ligand (O1,
3
N1, and N2; see Figure 1) defines the orientation of the
equatorial plane of the distorted square-pyramidal coordina-
tion environment of the copper(ii) centers. This is consistent
with the observed elongation of the apical Cu�O bonds by
[
*] Prof. Dr. W. Plass, Dipl.-Chem. A. Pohlmann,
Dipl.-Chem. J. Rautengarten
Fachbereich für Chemie und Biologie, Anorganische Chemie
Universität Siegen
[6]
about 25 ± 30 pm as compared to the relevant equatorial
bonds (see Figure 1). The two hydrogen bonds in 1 involve the
equatorially coordinated alcohol group as donor and the
phenolate oxygen as acceptor; this leads to a (e ± e/e ± e)
bridging pattern (see Scheme 1). The observed O ´´´ O dis-
tance of 260 pm is within the range of strong O�H ´´´ O
Adolf-Reichwein-Strasse, 57068 Siegen (Germany)
Fax : (‡49)271-740-2555
E-mail: plass@chemie.uni-siegen.de
[
**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie. The John von Neumann-
Institut für Computing is acknowledged for its generous support with
computing time at the supercomputer center of the Forschungszen-
trum Jülich (Project: Magnetic Interactions in Transition Metal
Complexes).
hydrogen bonds.[ In 1 both apical coordinated alcohol
1f]
groups are oriented cis with respect to the hydrogen bonding
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