Structural and dynamical aspects of alkylammonium salts of a silicodecatungstate as heterogeneous epoxidation catalysts

Article abstract of DOI:10.1039/c2dt30492a

The structural and dynamical aspects of alkylammonium salts of a silicodecatungstate [(CH₃)₄N]₄[γ-SiW ₁₀O₃₄(H₂O)₂] [C1], [(n-C ₃H₇)₄N]₄[γ-SiW ₁₀O₃₄(H₂O)₂] [C3], [(n-C ₄H₉)₄N]₄[γ-SiW ₁₀O₃₄(H₂O)₂] [C4], and [(n-C ₅H₁₁)₄N]₄[γ-SiW ₁₀O₃₄(H₂O)₂] [C5] were investigated. The results of sorption isotherms, XRD analyses, and solid-state NMR spectroscopy show that facile sorption of solvent molecules, flexibility of structures, and high mobility of alkylammonium cations are crucial to the uniform distribution of reactant and oxidant molecules throughout the bulk solid, which are related to the high catalytic activities for epoxidation of alkenes.

Full text of DOI:10.1039/c2dt30492a

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PAPER
Structural and dynamical aspects of alkylammonium salts of a
silicodecatungstate as heterogeneous epoxidation catalysts†
Sayaka Uchida,‡ Keigo Kamata, Yoshiyuki Ogasawara, Megumi Fujita and Noritaka Mizuno*
Received 1st March 2012, Accepted 26th March 2012
DOI: 10.1039/c2dt30492a
The structural and dynamical aspects of alkylammonium salts of a silicodecatungstate
[(CH ) N] [γ-SiW O (H O) ] [C1], [(n-C H ) N] [γ-SiW O (H O) ] [C3], [(n-C H ) N] -
3 4 4 10 34 2 2 3 7 4 4 10 34 2 2 4 9 4 4
[
γ-SiW O (H O) ] [C4], and [(n-C H ) N] [γ-SiW O (H O) ] [C5] were investigated. The results of
10
34
2
2
5
11 4
4
10 34
2
2
sorption isotherms, XRD analyses, and solid-state NMR spectroscopy show that facile sorption of solvent
molecules, flexibility of structures, and high mobility of alkylammonium cations are crucial to the
uniform distribution of reactant and oxidant molecules throughout the bulk solid, which are related to the
high catalytic activities for epoxidation of alkenes.
a
Table 1 Epoxidation of various alkenes with H
2
O
2
catalyzed by C4
1. Introduction
Entry
Alkene
Time [h]
Epoxide yield [%]
H O -based catalytic epoxidation has received much attention
2
2
1
,2
from the economic and environmental viewpoints.
Homo-
b
1
2
Propene
1-Butene
1-Hexene
1-Hexene
3
6
5
9
5
12
2
19
27
1
80
88
92
85
75
80
92
90
17
95
81
>99
94
c
geneous transition metal catalysts such as titanium, iron, manga-
1,2
nese, tungsten, and rhenium show high activities,
especially tungsten-based catalysts including polyoxometalates
POMs) show a high efficiency of H O utilization and high
and
3
4
5
d
e
1-Hexene
(
2
2
6
7
8
9
trans-2-Hexene
cis-2-Hexene
1-Octene
1-Dodecene
Cyclopentene
Cyclohexene
Cyclooctene
Cyclododecene
1
–7
selectivity to epoxides. However, most of these homogeneous
systems share common drawbacks such as difficulty in catalyst/
product separation and catalyst reuse. The immobilization of
catalytically active species through ion exchange, adsorption,
1
0
1
1
2
3
13
8
–12
and covalent linkage can solve these drawbacks.
We have previously reported that a non-porous tetra-
12
1
3
4
−
n-butylammonium salt of [γ-SiW O (H O) ] , [(n-C H ) N] -
a
1
0
34
2
2
4
9 4
4
2
Reaction conditions:C4 (7.3 μmol), alkene (20 mmol), 60% H O2
13
[
γ-SiW O (H O) ]
[C4], is an effective heterogeneous
(1 mmol), EtOAc (6 mL), 333 K. Yield [%] = Epoxide [mol]/initial
1
0
34
2
2
b c d
2 2 2 2
H O × 100. Propene (6 atm). 1-Butene (2 atm). 30% H O
e
catalyst for size-selective epoxidation of alkenes with H O in
2
2
1
4
2 2
(1 mmol). C4 (14.6 μmol), 1-hexene (2 mmol), 60% H O (1 mmol),
ethyl acetate (EtOAc). Various kinds of structurally diverse
EtOAc (2 mL), 333 K.
alkenes could efficiently be oxidized to the corresponding
1
5
epoxides in high yields (Table 1). Non-activated C –C termi-
3
6
nal alkenes could efficiently be epoxidized. On the other hand, a
longer reaction time was required for epoxidation of 1-octene to
afford the corresponding epoxide in high yield, and the epoxide
yield for larger 1-dodecene was much lower than those for
C –C terminal alkenes. Such size-selectivities were also
observed for epoxidation of C –C cyclic alkenes. For epoxida-
5
12
tion of cis- and trans-2-hexenes, the configurations around the
CvC moieties were retained in the corresponding epoxides.
Upon the sorption of EtOAc, the crystal system of C4 changes
from tetragonal to cubic with an increase in the cell volume, and
EtOAc molecules are highly mobile in the solid bulk of C4.
The catalytic activity highly depends on the kinds of counter
cations, while the effect of these counter cations on H O -based
3
8
Department of Applied Chemistry, School of Engineering, The
University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp; Fax: +81-3-5841-7220
2
2
†
Electronic supplementary information (ESI) available: Powder XRD
oxidation is still unclear.
Recently, it has become evident that the dynamics of catalyst
analyses of C3 (Fig. S1) and C5 (Fig. S2), space filling models of the
crystal structure of alkylammonium salts (Fig. S3), powder XRD ana-
lyses of C3 (Fig. S4) and C5 (Fig. S5) immersed in EtOAc, intensities
structures and those of reactants and intermediates are related to
1
3
16,17
of the signals of C-CPMASNMR spectra as a function of contact
the heterogeneous catalytic activity.
For example, in situ
times (Fig. S6–S10). See DOI: 10.1039/c2dt30492a
scanning tunneling microscopy (STM) images revealed that the
surface of metal-supported catalysts restructures during catalytic
turnover as they bind, react, and release the adsorbed
‡
Present Address: Department of Basic Sciences, School of Arts and
Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo
53-8902, Japan.
1
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9979–9983 | 9979

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1
6,17
molecules.
The intermediates are also mobile and can
2.3. Vapor sorption measurement
EtOAc sorption isotherms of C1 and C3–C5 were measured at
17
quickly open active sites for continued catalytic turnovers.
Similarly, solid-state NMR spectroscopy and quantum chemical
calculations have shown that not only acidity but also mobility
of protons has a large influence on heterogeneous acid catalysis
2
98 K using an automatic volumetric vapor sorption apparatus
Belsorp (BEL Japan Inc.). About 0.1 g of the compounds were
evacuated at 298–303 K for >3 h to form the guest-free phases.
Sorption equilibrium was judged by the following criteria:
1
8–20
of zeolites and heteropolyacids.
The importance of
dynamics in enzyme catalysts has been well known for years:
The metal-containing active sites are surrounded by proteins
with high conformational flexibility, which allow specific mol-
ecules to interact with the active sites contributing to the highly
selective catalysis.
Based on these considerations, the structural and dynamical
aspects of alkylammonium salts of a silicodecatungstate
±0.3% of pressure change in 5 min. Vapor saturation pressure
(
P0) of EtOAc at 298 K was 12.9 kPa.
21,22
13
2
.4. Solid-state C cross polarization (CP) MAS NMR
spectroscopy
[
(CH ) N] [γ-SiW O (H O) ] [C1], [(n-C H ) N] [γ-SiW -
NMR spectra were recorded at 298 K with a Chemagnetics
3
4
4
10 34
2
2
3
7 4
4
10
13
O (H O) ] [C3], [(n-C H ) N] [γ-SiW O (H O) ] [C4], and
CMX-300 Infinity spectrometer operating at 75.6 MHz for
C
3
4
2
2
4
9 4
4
10 34
2
2
[
(n-C H ) N] [γ-SiW O (H O) ] [C5] were investigated, and
with MAS = 3 kHz and contact times of 0.01–100 ms. The spec-
trum of C1 showed four signals at 57.2, 56.6, 56.0, and
5
11 4
4
10 34
2
2
2
3
the results were related to their catalytic activities.
5
5.3 ppm with the respective intensity ratio of ca. 1 : 1 : 1 : 1 for
+
the C1 carbons of [(CH ) N] , probably because four
[(CH ) N] per formula existed at crystallographically indepen-
3 4
3
4
+
2. Experimental
dent positions. The spectrum of C3 showed three signals at 59.3,
6.1, and 11.3 ppm for C1, C2, and C3 carbons of
4
−
Syntheses of alkylammonium salts of [γ-SiW O (H O) ]
were carried out according to the literature procedure.
1
0
34
2
2
1
1
3,14
+
[
(n-C H ) N] , respectively. The spectrum of C4 showed four
3 7 4
The compounds were evacuated at 298–303 K before use and
characterized by elemental analyses, IR spectroscopy, and
thermogravimetry.
signals at 57.9, 24.6, 20.3, and 14.9 ppm for C1, C2, C3, and C4
+
carbons of [(n-C H ) N] , respectively. The spectrum of C5
4
9 4
showed four signals at 59.4, 28.6, 22.2, and 13.8 ppm for C1,
+
C3, C2 and C4, and C5 carbons of [(n-C H ) N] , respectively.
5
11 4
2
.1. Catalytic epoxidation with H O
2 2
3
. Results and discussion
Epoxidation of propene and 1-hexene was carried out with an
autoclave having a Teflon vessel and a 30 mL glass vessel,
respectively. Catalytic epoxidation was carried out as follows:
Catalyst (20 μmol for propene, 7.3 μmol for 1-hexene), propene
Table 2 summarizes effects of counter cations of C1 and C3–C5
epoxidation of alkenes with H O . As previously reported, the
catalytic activity using 1-hexene as a reactant changed in the
order of C1 (<1%) ≪ C3 (19%) < C4 (92%) ≈ C5 (94%), and
increased with the increase in the carbon chain lengths of the
alkylammonium cations.
2
2
(
(
6 atm) or 1-hexene (20 mmol), EtOAc (6 mL) and 60% H O2
1 mmol) were charged in the reaction vessel. The reaction solu-
2
tion was heated at 333 K and was periodically analyzed by GC.
14
The catalytic activity using the
3
+/4+
The amount of remaining H O was analyzed by Ce
titra-
2
2
smaller propene increased in the same order of C1 (6%) ≪ C3
tion. The conversion of H O was almost 100%. Therefore, the
2
2
(38%) < C4 (89%) ≈ C5 (81%).
yields were calculated based on initial H O .
2
2
The crystal structures of C1 and C3–C5 are shown in Fig. 1.
13
14
The crystal structures of C1 and C4 have been reported by
single crystal X-ray diffraction analysis and powder X-ray dif-
fraction analysis combined with molecular mechanics using
UFF, respectively. The crystal structures of C3 and C5 (Fig. S1
and S2†) were solved in the same way as that of C4. The
2
.2. Powder X-ray diffraction study
28
Powder X-ray diffraction (XRD) patterns of C1 and C3–C5
as prepared and immersed in EtOAc were measured with an
XRD-DSCII (Rigaku Corporation) and Cu Kα radiation
(
λ = 1.54056 Å, 50 kV, 300 mA) at 303 K. The diffraction data
Table 2 Effect of counter cations on epoxidation of alkenes with
H O
2 2
a
were collected in the range of 2θ = 4–25° at 0.01° point and 5 s
per step. The crystallographic parameters and the positions of the
silicodecatungstate in C3 and C5 were calculated using Materials
Studio Software (Accelrys Inc.) as follows: (i) the unit cell
indexing and space group determination by X-cell, (ii) peak
profile fitting by the Pawley refinement, (iii) structure optimiza-
tion by the simulated annealing method, and (iv) structure
refinement by the Rietveld method. The positions of the alkyl-
ammonium cations in C3 and C5 were optimized by molecular
mechanics using universal force field (UFF), since they could
not be located with the powder XRD structure analysis because
of the relatively low electron densities in the unit cell.
Epoxide yield [%]
Propene
b
Catalyst
1-Hexene
2
4
2
5
C1
C3
C4
C5
6
<1
19
92
94
2
6
38
89
81
2
7
a
2
8
Reaction conditions: Catalyst 20 μmol, propene 6 atm, 60% H
2
O
2
1
mmol, EtOAc 6 mL, 333 K, 300 min. Yield [%] = epoxide [mol]/
b
initial H O [mol] × 100. Data from ref. 14.
2
2
9980 | Dalton Trans., 2012, 41, 9979–9983
This journal is © The Royal Society of Chemistry 2012

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Fig. 1 Molecular structures of a silicodecatungstate and alkylammonium cations, and crystal structures of C1 and C3–C5. Light green and purple
6 4
polyhedra show the [WO ] and [SiO ] units, respectively. Black, blue, and pink spheres show the carbon, nitrogen, and hydrogen atoms of the alkyl-
ammonium cations, respectively.
silicodecatungstates in C1 and C3–C5 were packed into mono-
clinic, orthorhombic, tetragonal, and monoclinic cells, respect-
ively. The alkylammonium cations of C1 and C3 were localized
in the crystal lattice, while those of C4 and C5 were uniformly
distributed in the crystal lattice. The space filling model of the
structures (Fig. S3†) and N adsorption isotherms (77 K) showed
2
that all compounds were non-porous.
The EtOAc vapor sorption isotherms of the compounds at
2
98 K are shown in Fig. 2. Compounds C4 and C5 sorbed
EtOAc from the low relative pressures, and the amounts at P/P
0
−1
=
0.8 were ca. 1.5 mol mol and were similar to each other.
The amount of sorption by C3 at P/P = 0.8 was ca. 0.5 mol
0
−1
mol and smaller than those of C4 and C5. The amount of
sorption by C1 was ca. 0.1 mol mol at P/P = 0.8 and compar-
−1
0
able to that of the surface adsorption. Thus, the amounts of
EtOAc sorption increased in the order of C1 ≪ C3 < C4 ≈ C5,
which was in line with the order of their catalytic activities and
the hydrophobicity of alkylammonium cations (Table 2). As we
have previously shown, both reactant (1-hexene) and oxidant
Fig. 2 EtOAc sorption–desorption isotherms of C1 and C3–C5.
Diamond, triangle, circle, and square symbols show the data for C1 and
C3–C5, respectively. Solid and open symbols show the sorption and
desorption plots, respectively.
(
H O ) were sorbed into the solid bulk of C4 in the presence of
2 2
14
3 14
EtOAc solvent. Therefore, the EtOAc sorption ability is crucial
to the heterogeneous epoxidation catalyses of C1 and C3–C5.
Compounds C1 and C3–C5 were immersed in EtOAc, and
powder XRD patterns were measured to analyze the crystal
structures (Fig. 3). Amounts of EtOAc sorption were estimated
to be C1 (0 mol mol ) ≪ C3 (ca. 0.5 mol mol ) < C4 ≈ C5
(
have previously shown, the powder XRD pattern of C4 was
changed by the immersion and the cell volume was increased by
density of the liquid (162 Å ). The powder XRD pattern of C5
also changed by the immersion and the cell volume was
3
increased by ΔV/Z = +109 Å (Fig. S5†). On the other hand, the
powder XRD pattern of C3 changed little and the cell volume
was increased only by ΔV/Z = +10 Å (Fig. S4†). These results
3
−1
−1
show that C4 and C5 possess flexible structures while C3 is
rather rigid. According to Table 1, the catalytic activities of C1
and C3 for epoxidation of 1-hexene (yields of epoxides were
<1% and 19% for C1 and C3, respectively) were lower than
those for epoxidation of the smaller propene (yields of epoxides
in the presence of C1 and C3 were 6% and 38%, respectively).
ca. 1.0 mol mol− ) with the C NMR measurements. As we
1
13
3
ΔV/Z = +156 Å , which was comparable to the volume of the
EtOAc molecule calculated with the molecular weight and
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9979–9983 | 9981

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Fig. 3 Powder XRD patterns of C1 and C3–C5. (a) C1, (b) calculated
with the single crystal data of C1, (c) C3, (d) C3 immersed in EtOAc,
(
e) C4, (f) C4 immersed in EtOAc, (g) C5, and (h) C5 immersed in
1
3
2
7
Fig. 4 Solid-state C CPMASNMR spectra of C1 and C3–C5. (a)
C1, (b) C3, (c) C4, and (d) C5. The cross polarization contact times
from bottom to top were 0.1, 0.5, 1.0, and 3.0 ms.
EtOAc. Structural parameters were obtained by the Rietveld analysis
combined with molecular mechanics with UFF, except for C1 (single
crystal data).
2
8
The results of catalytic activities, sorption isotherms, and struc-
tural analyses, suggest that facile sorption of the solvent mol-
ecules (EtOAc) and structural flexibility of the catalysts are
important factors, especially for epoxidation of larger alkenes.
The silicodecatungstate is a rigid molecule since it is com-
posed of covalent bonds of inorganic atoms, while the alkylam-
monium cations can be considered to be much more flexible
since they are composed of single bonds between carbon, hydro-
gen, and nitrogen atoms. Thus, the flexibility of the alkylammo-
nium cations may contribute to the structural flexibility of the
catalysts. In order to investigate the flexibility (i.e., mobility) of
the cross polarization time constant (TCH) and proton spin-lattice
relaxation time (T1ρ) were obtained for each carbon atom by
using eqn (1), which describes the magnetization transfer from
1
13
29,30
H to C via the heteronuclear dipolar interactions:
IðtÞ ¼ I0ð1 ꢀ TCH=T1ρðHÞÞꢀ fexpðꢀt=T1ρðHÞÞ
1
ꢀ
expðꢀt=TCHÞg
ð1Þ
I(t), I , TCH, and T1ρ(H) represent the intensity of the signal at
0
contact time t, theoretical maximum intensity of the signal, cross
polarization time constant, and proton spin-lattice relaxation
time, respectively. The signal intensity of the less mobile carbon
reaches a maximum at relatively short contact times (i.e., shorter
TCH). The TCH, and T1ρ(H) values are summarized in Table 3.
The TCH values increased from the inner carbon atoms to the
outer carbon atoms suggesting that the outer carbon atoms are
more mobile. The TCH values of the most outer carbon atom
increased in the order C3 (C3 carbon: 0.54 ms) < C4 (C4
carbon: 0.73 ms) < C5 (C5 carbon: 1.5 ms), suggesting that the
alkylammonium cations with longer carbon chains are more
1
3
the alkylammonium cations,
MASNMR spectroscopy was utilized. Fig. 4 shows the
C CP (cross polarization)
13
C
CPMASNMR spectra of C1 and C3–C5 with different contact
times (0.1, 0.5, 1.0, and 3.0 ms). The intensities of the signals
increased with increase in contact times, reached a maximum,
and then decreased. The relative intensity of the signals changed
with the contact times: the relative intensity of the outer carbons
increased with increasing contact time. The intensities of the
signals were plotted against the contact time (Fig. S6–S10†), and
9982 | Dalton Trans., 2012, 41, 9979–9983
This journal is © The Royal Society of Chemistry 2012

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Table 3
TCH [ms] and T1ρ [ms] values of the compounds
Grant-in-Aids for Scientific Research from the Ministry of Edu-
cation, Culture, Science, Sports, and Technology of Japan.
a
Carbon in alkylammonium cation
C1
C2
C3
C4
C5
Notes and references
CH 9.1 × 10−
1
1
—
—
—
—
5.4 × 10
5.5
1.7 × 10
5.5
1.5 × 10
1.4
—
—
—
—
7.3 × 10
5.1
3.2 × 10
2.4
—
—
—
—
—
—
—
—
1.5
12
C1
C3
C4
T
T
T
T
T
T
T
T
T
T
1ρ 8.9
1 B. S. Lane and K. Burgess, Chem. Rev., 2003, 103, 2457–2473.
2 S. T. Oyama, in Mechanisms in Homogeneous and Heterogeneous Epoxi-
dation Catalysis, ed. S. T. Oyama, Elsevier, Amsterdam, 2008, pp. 1–99.
3 R. Neumann, Prog. Inorg. Chem., 1998, 47, 317–370.
4 I. V. Kozhevnikov, Catalysts for Fine Chemical Synthesis: Catalysis by
Polyoxometalates, Wiley, Chichester, 2002.
5 M. T. Pope, in Comprehensive Coordination Chemistry II, ed.
J. A. McCleverty and T. J. Meyer, Elsevier Pergamon, Amsterdam, 2004,
vol. 4, pp. 635–678.
CH 1.5 × 10− 2.8 × 10
−1
−2
−2
−2
−1
−1
−1
−1
1ρ 4.0
CH 7.5 × 10− 8.5 × 10
1ρ 6.3 5.5
CH 5.5 × 10− 6.5 × 10
1ρ 1.5 2.1
CH 3.5 × 10− 3.5 × 10
1ρ 5.8 7.5
4.2
2
−1
−1
−1
2
C4·EtOAc
C5
1
3.5 × 10
8.7
3.5 × 10
7.5
6
7
8
C. L. Hill, in Comprehensive Coordination Chemistry II, ed.
J. A. McCleverty and T. J. Meyer, Elsevier Pergamon, Amsterdam, 2004,
vol. 4, pp. 679–758.
N. Mizuno, K. Kamata, S. Uchida and K. Yamaguchi, in Modern Hetero-
geneous Oxidation Catalysis, ed. N. Mizuno, Wiley-VCH, Weinheim,
2009, pp. 185–216.
a
Carbon atom which is adjacent to the nitrogen atom is denoted as C1.
+
mobile in the solid bulk. The TCH values of [(n-C H ) N] in C4
4
9 4
J. T. Rhule, W. A. Neiwert, K. I. Hardcastle, B. T. Do and C. L. Hill,
J. Am. Chem. Soc., 2001, 123, 12101–12102.
decreased with the sorption of EtOAc, suggesting that the highly
mobile EtOAc molecules (TCH: ca. 50 ms) interact with the
alkylammonium cations and push their way through the solid
9 K. Yamaguchi, C. Yoshida, S. Uchida and N. Mizuno, J. Am. Chem.
Soc., 2005, 127, 530–531.
10 R. Neumann and H. Miller, J. Chem. Soc., Chem. Commun., 1995,
+
bulk. While the mobility of [(CH ) N] in C1 was relatively
3
4
2277–2278.
high (C1 carbon: 0.91 ms), C1 could not sorb EtOAc so that
reactants and oxidants could not enter and diffuse into the solid
bulk for the catalytic reaction to occur there.
1
1 B. Sels, D. De Vos, M. Buntinx, F. Pierard, A. Kirsch-DeMesmaeker and
P. Jacobs, Nature, 1999, 400, 855–857.
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1
1
1
1
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4. Conclusions
In conclusion, all these results show that facile sorption of
solvent molecules (EtOAc), flexibility of the crystal structures of
POM catalysts, and high mobility of alkylammonium cations are
crucial to the uniform distribution of the reactant and oxidant
molecules throughout the solid bulk of the catalyst and the high
catalytic activity of heterogeneous catalysts of alkylammonium
salts of a silicododecatungstate. These results suggest that not
only the atomic structures of the active sites but also the struc-
tures and dynamics of the surroundings is important for the
design and synthesis of highly active heterogeneous catalysts.
1
4
been reported, the substrate scope of C4-catalyzed epoxidation with
in EtOAc has not been investigated. Therefore, we extend
the scope of the present catalytic system by carrying out epoxidation of
–C12 terminal, internal, and cyclic alkenes with H (Table 1).
2 2
H O
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Dalton Trans., 2012, 41, 9979–9983 | 9983