Organoruthenium-supported polyoxotungstate - Synthesis, structure and oxidation of n-hexadecane with air

Article abstract of DOI:10.1002/ejic.201201037

A ruthenium complex, KNa[Ru₂(C₆H₆) ₂(CH₃COO)₆] (Ru-KNa), and its polyoxotungstate derivative, Na₆[{Ru(C₆H₆)}₂W ₈O₂₈(OH)₂]·16H₂O (Ru-Na), have been successfully isolated from routine synthetic reactions and characterized by X-ray single-crystal structure analysis, IR spectroscopy and elemental analysis. A remarkable aspect of Ru-KNa is that it has two ligand types, benzene and acetate, and the acetate ligands are connected exclusively by a central Na cation to form a dimeric sandwich-type structure, which is further connected by K cations to construct the 3D structures. Based on complex Ru-KNa, the compound Ru-Na was synthesized, and it consists of two {Ru(C ₆H₆)} units linked to a [W₈O ₂₈(OH)₂]^10- fragment by three Ru-O(W) bonds to result in an assembly with idealized C₂ symmetry in which the polyanions form 3D structures by the connection of Na chains. Subsequently, the compound Ru-Na was anchored on (3-aminopropyl)triethoxysilane (apts) modified SBA-15 to prepare the solid catalysts, which were characterized by powder XRD, N₂ adsorption measurements and FTIR spectroscopy. Finally, the catalytic efficiency of Ru-Na was assessed in the oxidation of n-hexadecane with air without any additives and solvents. The results indicated that Ru-Na is a heterogeneous catalyst and exhibits higher catalytic activity than previously reported Ru-containing polyoxotungstates. Copyright

Full text of DOI:10.1002/ejic.201201037

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DOI:10.1002/ejic.201201037
Organoruthenium-Supported Polyoxotungstate – Synthesis,
Structure and Oxidation of n-Hexadecane with Air
[
a]
[a]
[a]
[a]
[a]
Rui-Qi Meng, Bin Wang, Hui-Min Sui, Bao Li, Wei Song,
[
a]
[a]
[a]
Li-Xin Wu, Bing Zhao, and Li-Hua Bi*
Dedicated to Professor Michael T. Pope on the occasion of his 80th birthday
Keywords: Ruthenium / Tungsten / Green chemistry / Polyoxometalates / Oxidation
A
(
Na
ruthenium
Ru-KNa), and
)}
complex,
its
KNa[Ru
polyoxotungstate
]·16H (Ru-Na), have been
2
(C
6
H
6
)
2
(CH
derivative,
3
COO)
6
]
2
bonds to result in an assembly with idealized C symmetry
in which the polyanions form 3D structures by the connection
of Na chains. Subsequently, the compound Ru-Na was an-
chored on (3-aminopropyl)triethoxysilane (apts) modified
SBA-15 to prepare the solid catalysts, which were charac-
6
[{Ru(C
6
H
6
2
W
8
O28(OH)
2
2
O
successfully isolated from routine synthetic reactions and
characterized by X-ray single-crystal structure analysis, IR
spectroscopy and elemental analysis. A remarkable aspect of
Ru-KNa is that it has two ligand types, benzene and acetate,
and the acetate ligands are connected exclusively by a cen-
tral Na cation to form a dimeric sandwich-type structure,
which is further connected by K cations to construct the 3D
structures. Based on complex Ru-KNa, the compound Ru-Na
2
terized by powder XRD, N adsorption measurements and
FTIR spectroscopy. Finally, the catalytic efficiency of Ru-Na
was assessed in the oxidation of n-hexadecane with air with-
out any additives and solvents. The results indicated that Ru-
Na is a heterogeneous catalyst and exhibits higher catalytic
activity than previously reported Ru-containing polyoxo-
tungstates.
was synthesized, and it consists of two {Ru(C
linked to a [W O28(OH) ]
8 2
6
H
6
)} units
1
0–
fragment by three Ru–O(W)
Introduction
aspects: (i) Ru-POMs were used as excellent organic cata-
lysts in oxidation processes of some organic substrates, for
Ruthenium-containing polyoxometalates (Ru-POMs)
have attracted increasing attention in recent years because
example, alkanes, alkenes, alcohols, cellobiose and cellu-
[20]
lose;
(ii) Ru-POMs were documented as efficient chemi-
of their structural variety and exciting catalytic proper-
cal, electrochemical and photochemical catalysts for H O
2
ties.[
1–3]
To date, different ruthenium precursors, for exam-
[21]
oxidation and CO reduction. However, for the oxidation
2
[
4]
[5]
ple, RuCl ·nH O,
[Ru(H O) ][C H SO ] ,
Ru(acac)3
3
2
2
6
7
7
3 2
of organic substrates most oxidation processes require sacri-
ficial oxidants, additives, solvents, or drastic conditions
[6]
(acac = acetylacetonate), Ru(dmso) Cl (dmso = di-
4
2
[
7]
VI
VI
[8]
[9]
methyl sulfoxide), Cs [Ru NCl ]/[nBu N][Ru NCl ],
2
5
4
4
(e.g., high temperature or pressure). Therefore, the develop-
Me
Me
[
[
RuL Cl ] (L = 1,3-dimethylimidazolidine-2-ylidene),
4 2
ment of catalytic oxidation of organic molecules under
green conditions is technologically and environmentally im-
portant. Recently, Ru-POMs were found to catalyze the oxi-
dation of n-hexadecane under environmentally friendly con-
ditions in the liquid phase with air alone, without any addi-
6
[10]
Ru(η -arene)Cl ] , [Ru(dmf) ](CF SO ) (dmf = N,N-di-
2
2
6
3
3 3
[12]
[
11]
methlyformamide),
[Ru (O CCH ) Cl],
Ru(bpy) Cl2
2
2
3 4
3
bpy = 2,2Ј-bipyridine),[ and (NH ) [RuCl ], have been
13]
[14]
(
4
2
6
used to synthesize Ru-POMs, which can be subdivided into
[
15]
two main classes, ruthenium-supported POMs
and ru-
[22]
tives. Unfortunately, the yield and turnover numbers are
thenium-substituted POMs,[
16]
such as [{Ru(η -p-
6
low. It is clearly desirable to carry out oxidation reactions
of n-hexadecane with higher yield and turnover numbers.
In this paper, we report a diorganoruthenium-supported
polyoxotungstate, Na [{Ru(C H )} W O (OH) ]·16H O
[
17]
8– [18]
MeC H iPr)} W O ], [{PW O (RuC H )} {WO }] ,
6
4
4
2
10
11 39
6
6
2
2
1
0– [19]
[
Sb W O {Ru(p-cymene)} ]
.
The studies of the cata-
2
20 70
2
lytic properties of Ru-POMs have focused on the following
6
6
6
2
8
28
2
2
(
Ru-Na), as an efficient heterogeneous catalyst with high
[
a] College of Chemistry, State Key Laboratory of Supramolecular
yields and turnover numbers for the oxidation of n-hexade-
cane under completely green conditions. The development
of catalysts with better performances for the oxidation of
n-hexadecane is being extensively pursued in both industrial
and academic research.
Structure and Materials, Jilin University,
Changchun 130012, P. R. China
E-mail: blh@jlu.edu.cn
Homepage: http://www.jlu.edu.cn/newjlu/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201201037
Eur. J. Inorg. Chem. 0000, 0–0
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Results and Discussion
(WO
3
)
3
(μ
3
-O)
3
(μ
3
-OH)]5– clusters linked together by two
equivalent cis-{WO } groups (Figure 2). The linking WO
2
2
6
Synthesis and Crystal Structures
units exhibit octahedral coordination to two terminal oxo
ligands and four doubly bridging oxo ligands with a typical
two short/two intermediate/two long bonding pattern
The compound Ru-KNa was obtained by dissolving
[
RuC H Cl ] in a sodium acetate solution (pH = 6.0) fol-
6 6 2 2
(
Table S1). The structure of the polyoxotungstate anion in
lowed by the addition of KCl; the mixed solution was kept
open to the air, which led to the isolation of the mixed po-
tassium–sodium salt as a crystalline material. However, Ru-
6
Ru-2 is similar to that in [{Ru(η -p-MeC H iPr)} (μ-OH) ] -
6
4
2
3 2
6
6
[{Ru(η -p-MeC H iPr)} W O (OH) {Ru(η -p-MeC H iPr)-
6
4
[15]
2
8
28
2
6
4
(
H O)} ] (Ru-3).
The difference is that only two
2
2
KNa cannot be obtained in potassium acetate solution (pH
{
{
Ru(C H )} groups are bonded to Ru-2, whereas four
6 6
+
=
6.0), which suggests that Na counter cations influence
Ru(MeC H iPr)} groups are connected to Ru-3.
6
4
the formation of Ru-KNa. The compound Ru-Na was syn-
thesized by treating Ru-KNa with Na WO ·2H O in so-
2
4
2
dium acetate (pH = 6.0); the mixed solution was left to
slowly concentrate in air, which resulted in a red crystalline
product of the hydrated sodium salt. We tested if Ru-Na
can also be isolated in the reaction of [RuC H Cl ] with
6
6
2 2
Na WO ·2H O under the same conditions, but the results
2
4
2
indicated that a polyoxotungstate supported by four
{
Ru(C H )} groups was obtained, the structure of which
6
6
[15]
has been observed by Proust et al.
This observation al-
lows the conclusion that Ru-KNa is crucial for the isolation
of Ru-Na.
The structure of Ru-KNa was solved in the hexagonal
¯
space group R3, and it consists of two subunits of
–
[
Ru(C H )(CH COO) ] (Ru-1) linked by the Na cation,
6
6
3
3
which leads to a dimeric sandwich-type structure (Figure 1);
further, the K cation is coordinated by six Ru-1 anions [as
shown in Figure S1 (Supporting Information)] to form a
three-dimensional structure (Figure S2). In Ru-1, the Ru
coordination is completed by one benzene ligand and three
acetate ligands with Ru–C distances of 2.161(3)–2.172(3) Å
and Ru–O distances of 2.1271 (17) Å, respectively, which
gives rise to a structure with C symmetry.
3
Figure 2. Ball-and-stick (top) and polyhedral (bottom) representa-
tions of polyanion Na [{Ru(C )} 28(OH) ]·16H O (Ru-Na).
The WO octahedra are red.
6
6
H
6
2
W
8
O
2
2
6
Bond-valence sum (BVS) calculations[23] for Ru-2 re-
vealed that O8 belongs to a μ -hydroxo ligand and, there-
3
fore, the charge of the polyanion Ru-2 must be 6–. In the
solid state these charges are balanced by six sodium ions,
which was verified by elemental analysis.
Characterization of the Solid Catalyst
The surface modification procedure for SBA-15 and sub-
sequent Ru-2 anchoring is shown in Scheme 1. We per-
formed a number of analytical studies to obtain infor-
mation on Ru-2 anchored on apts-modified SBA-15 [SBA-
1
5-apts-Ru-2; apts = (3-aminopropyl)triethoxysilane] in-
Figure 1. Ball-and-stick representation of the dimeric sandwich-
type structure of Ru-KNa.
cluding its distribution, the material structure and the sur-
face species.
The structure of Ru-Na is centrosymmetric and was
FTIR Spectra
solved in the monoclinic space group P2 /n. The anion
1
6
–
[
{Ru(C H )} W O (OH) ] (Ru-2) is made up of two
The IR spectra for compounds Ru-Na and SBA-15-apts-
6
6
2
8
28
2
crystallographically equivalent cuboidal [{Ru(C H )}- Ru-2 before and after catalytic reactions compared with
6
6
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Scheme 1. Procedure for the synthesis of the solid catalyst SBA-15-apts-Ru-2.
those of compounds Ru-Na and SBA-15-apts are shown in XRD Measurements
Figure 3. The following points can be drawn from the IR
The small-angle powder XRD patterns of SBA-15, SBA-
5-apts, and SBA-15-apts-Ru-2 with different amounts of
spectra: (i) all of the characteristic vibrational frequencies
of Ru-Na were kept in SBA-15-apts-Ru-2, which indicates
that Ru-Na has been loaded on the surface of SBA-15-apts;
1
Ru-2 loaded are presented in Figure S3. The following can
be seen: (i) all samples show three distinct Bragg diffrac-
tions peaks, an intense peak assigned to reflections at (100)
and two low-intensity peaks assigned to (110) and (200),
which indicate that these samples possess well-ordered 2D
hexagonal mesoporous structures; (ii) after apts and Ru-2
were immobilized on the surface of SBA-15, the small-angle
powder XRD patterns (Figures S3b–e) did not change sig-
nificantly except for the expected peak-intensity changes,
which suggests that apts and subsequently Ru-2 are loaded
inside the channels of SBA-15; (iii) the peak intensities de-
crease as the amount of Ru-2 loaded increases, which im-
plies that catalysts with different loading were prepared. In
short, the mesostructure of SBA-15 is maintained during
modification of the channel surface, and the apts and Ru-2
were immobilized inside the channels of SBA-15. These re-
(ii) the IR spectra of Ru-Na and SBA-15-apts-Ru-2 before
and after catalytic reactions are the same, which suggests
that the solid catalysts for Ru-Na and SBA-15-apts-Ru-2
have good stability.
sults were supported by N adsorption/desorption measure-
2
ments.
N Adsorption/Desorption Isotherms
2
Figure S4 presents N adsorption/desorption isotherms
2
for SBA-15, SBA-15-apts and SBA-15-apts-Ru-2 with dif-
ferent amounts of Ru-2 loaded. The following can be noted:
(i) for all the samples, the isotherms possess irreversible
type IV adsorption isotherms with H1 hysteresis loops as
defined by IUPAC; (ii) the adsorption isotherms exhibit a
sharp inflection at relative pressures of 0.50–0.95, which in-
dicates capillary condensation within uniform mesopores;
(iii) the absorption amount decreases with the reduction of
pore volume that results from apts modification and Ru-2
immobilization on SBA-15; (iv) the inflection points of the
step are shifted to lower P/P values, which is attributed to
0
smaller pore sizes (Figure S4 inset).
We used the Brunauer–Emmett–Teller (BET) method to
calculate all surface area data and nonlocal density func-
tional theory (NLDFT) to evaluate the pore parameters
based on the adsorption branch of the isotherm curve. The
detailed data are summarized in Table 1, and it can be seen
that (i) the surface area, pore volume and pore size are dis-
tinctly decreased after the loading of apts and subsequently
Ru-2, which can be explained by the apts and subsequent
Figure 3. Top: IR spectra of Ru-Na before (a) and after (b) catalytic
reactions. Bottom: IR spectra of Ru-Na (a), SBA-15-apts-Ru-2 af-
ter (b) and before (c) catalytic reactions, SBA-15-apts (d).
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Table 1. Composition, specific surface area, pore parameters of SBA-15, SBA-15-apts and SBA-15-apts-Ru-2 with different loaded
amount.
2
3 –1
Sample (Ru-2 loading)
Surface area BET [m g]
Pore volume [cm g ]
Pore diameter [nm]
SBA-15
SBA-15-apts
SBA-15-apts-Ru-2 (2.4%)
SBA-15-apts-Ru-2 (4.6%)
SBA-15-apts-Ru-2 (8.0%)
569
401
358
336
305
0.93
0.80
0.65
0.56
0.48
6.7
5.6
5.2
4.9
4.5
Ru-2 immobilization inside the mesochannels of SBA-15; Catalytic Activity of Ru-Na
ii) the surface area, pore volume and pore size are reduced
(
with an increase of loading amount, which suggests that
different amounts of Ru-2 were loaded within the mesoch- Catalyst Amount
annels of SBA-15; (iii) the obtained results match with the
To compare the catalytic activity of Ru-Na with pre-
viously reported systems, the catalytic reactions were per-
formed at 150 °C with a reaction time of 6 h and an airflow
rate of 30 mLmin . The performances and selectivities
over different amounts of Ru-Na are listed in Table 2 and
depicted in Figure S5. It is clear that Ru-Na is very active
with a 52.48% conversion for 15 mg of catalyst and a
XRD patterns very well.
Therefore, according to the FTIR, XRD and N adsorp-
tion/desorption measurements, it is concluded that the Ru-
2
–1
2
has been immobilized inside the mesochannels of SBA-15
to form solid catalysts.
4
5.31% conversion for 10 mg of catalyst. A comparison
Catalytic Studies of n-Hexadecane Oxidation Processes
with the previously reported results demonstrates that Ru-
Na has the best catalytic performance for the solvent free
oxidation of n-hexadecane to date.
The oxidation of n-hexadecane to alcohols and ketones,
which are abundant as feedstocks, was selected to assess the
catalytic activity of Ru-Na. It is well known that the green
oxidation of n-hexadecane is very difficult because of the
low reactivity of the C–H bond. Therefore, intensive re-
search efforts are devoted to exploit active catalysts. Re-
Reaction Time
We investigated the effect of reaction time on the cata-
cently, Fe-, Cu-, and organo-Ru-containing polyoxotung- lytic activity. The catalytic activity and selectivity vs. dif-
states were determined to catalyze the oxidation of n-hex- ferent reaction times are summarized in Table 3 and de-
adecane under green conditions.[
22,24]
Unfortunately, the re- scribed in Figure 4. The results indicate that the n-hexade-
action conversion is lower than 40%. Thus, the exploitation cane conversion increased with prolongation of reaction
of new catalysts with high activity for the oxidation of n- time, as shown in the inset of Figure 4. However, as dis-
hexadecane constitutes an interesting research area. Here, played in Figure 4, the turnover frequency (TOF) for con-
the catalytic performance of Ru-Na in the oxidation of n- version of n-hexadecane sharply reaches a maximum and
hexadecane was tested. As the selectivity of the oxidation then decreases, which implies that the optimal reaction time
reaction of n-hexadecane is similar to previously reported is 4 h for Ru-Na catalyst, which is shorter than the pre-
results (the main products of the oxidation of n-hexadecane viously reported reaction time of 6 h.
are C16 ketones and alcohols in this reaction), and the cata-
Based on above performances it is concluded that the
lytic mechanism has been inferred to proceed through a optimal catalytic reaction conditions at 150 °C and an air-
free-radical autoxidation processes, the following dis- flow rate of 30 mLmin–1 for 5 mL of n-hexadecane are
cussions will be focused on the catalytic activity of Ru-Na. 15 mg of catalyst and a reaction time of 4 h.
Table 2. Catalytic activity of Ru-Na for the oxidation of n-hexadecane under green conditions with different catalyst amounts.[a]
–1
Catalyst
Amount [mg] Conv. [%] TOF[h ]
Product selectivity [%] and distribution
Ketones (7-one/6-one/5-one/4-one/3-one/2-one)
Alcohols (7-ol/6-ol/5-ol/4-ol/3-ol/2-ol)
Ru-Na
5
41.52
45.31
52.48
45.74
631
405
312
204
50.1
19.7
(6.4:2.3:4.6:2.3:3:1.1)
20.8
(6.6:2.5:4.8:2.6:3.1:1.3)
17.5
(5.2:2.6:4.2:2:2.5:1)
20.2
(6.3:2.4:4.4:2.5:3.1:1.5)
(
12.9:6.6:8.6:6.6:7.1:8.3)
10
50.9
(
13.1:6.8:8.7:6.6:7.4:8.3)
15
49.8
(
13.4:6:8.5:6.5:7.1:8.3)
51.4
13.6:6.2:8.5:6.7:7.8:8.6)
20
(
–
1
[
a] Reaction conditions: n-hexadecane: 5 mL; airflow rate: 30 mLmin ; 150 °C; 6 h.
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Table 3. Catalytic activity of Ru-Na for the oxidation of n-hexadecane under green conditions with different reaction times.[a]
–1
Catalyst
Reaction time [h]| Conv. [%] TOF [h ]
Product selectivity [%] and distribution
Ketones (7-one/6-one/5-one/4-one/3-one/2-one) Alcohols (7-o/6-ol/5-ol/4-ol/3-ol/2-ol)
Ru-Na
0.5
.5
0.98
4.31
105
154
443
513
452
405
376
298
34.3
30.2
(11:3.3:4.2:3.6:5.8:2.3)
25.6
(9:3.2:3.8:2.3:5.6:1.7)
26.3
(9.7:3.1:5.2:3.2:4.1:1)
24.7
(9.4:2.6:5:3.1:3.7:0.9)
21.3
(7:2.3:5.4:3:2.5:1.1)
20.8
(6.6:2.5:4.8:2.6:3.1:1.3)
17.6
(5.5:1.9:5.1:1.9:2.2:1)
17.2
(
8:5.1:4.3:5.1:6.9:4.9)
1
45.5
(
10:6.3:5.3:7.1:7.9:8.9)
3
4
5
6
7
24.80
38.26
42.19
45.31
49.12
66.90
49
(
12.4:6.7:7.8:6.4:7.6:8.1)
53.5
(
13.5:7.5:8.3:7:8:9.2)
47.1
(
12.1:6.4:7.8:6.3:6.8:7.7)
50.9
(
13.1:6.8:8.7:6.6:7.4:8.3)
52.0
(
13.2:6.8:8.9:6.9:7.4:8.8)
12
59.5
(15.1:7.6:10.6:7.8:8.7:9.7)
(4.8:1.9:5.1:1.7:2.3:1.4)
–
1
[
a] Reaction conditions: n-hexadecane: 5 mL; airflow rate: 30 mLmin ; 150 °C; catalyst amount: 10 mg.
vered by filtration and reused without loss of their catalytic
activity; (iii) the loading of the catalysts can be modulated
according to the catalytic activity. Following such ideas, we
prepared three solid catalysts with loadings of 2.4, 4.6 and
8.0% based on elemental analysis and examined their cata-
lytic activities, which are listed in Table 4 and described in
Figure 5. The experimental results show that the solid cata-
lyst SBA-15-apts-Ru-2 with a loading of 4.6% is very active
with a 47.5% conversion of n-hexadecane and a high turn-
–1
over frequency (TOF; 8075 h ), which indicates that immo-
Figure 4. TOF for conversion of n-hexadecane. Reaction condi-
tions: catalyst: 10 mg; n-hexadecane: 5 mL; airflow rate:
–1
3
0 mLmin ; 150 °C.
Catalytic Activity of Ru-2 Immobilized on SBA-15
From the above discussion, it is clearly shown that the
conversion of n-hexadecane is higher while the TOF is
lower. Therefore, we immobilized Ru-2 on ordered mesopo-
rous SBA-15 to prepare the solid catalyst. The following
advantages can be seen: (i) the catalysts can be dispersed to
provide a large number of catalytically active sites to en-
Figure 5. Conversion of n-hexadecane over catalysts with Ru-2
loadings of 2.4, 4.6 and 8.0%. Reaction conditions: catalyst: 10 mg;
–
1
hance the TOF; (ii) the solid catalysts can be easily reco- n-hexadecane: 5 mL; airflow rate: 30 mLmin ; 150 °C.
Table 4. Catalytic activity of SBA-15-apts-Ru-2 for the oxidation of n-hexadecane under green conditions with different Ru-2 loadings.[a]
–1
Catalyst
Ru-2 [%] Conv. [%] TOF [h ]
Product selectivity [%] and distribution
Ketones (7-one/6-one/5-one/4-one/3-one/2-one) Alcohols (7-ol/6-ol/5-ol/4-ol/3-ol/2-ol)
SBA-15-apts-Ru-2
8.0
.6
.4
42.17
47.53
33.33
4119
8075
55.1
19
(
14.4:7.1:8.6:7.3:9:8.7)
(5.1:3.3:4.4:2.7:2.5:1)
21.4
(4.8:2.3:3.5:2:2.1:2.4:1.3)
23.5
4
51.2
(
13:6.9:8:6.8:8.3:8.2)
55.2
14.9:7.1:8.1:7.2:8.5:9.4)
2
10854
(
(8.4:3.3:4.1:3.1:3.9:0.7)
–
1
[
a] Reaction conditions: n-hexadecane: 5 mL; airflow rate: 30 mLmin ; 150 °C; catalyst amount: 10 mg.
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bilization of Ru-2 on SBA-15 results in improved activity. isotherms by using nonlocal density functional theory (NLDFT).
Pore sizes and pore volume were obtained from the adsorption
branch of the isotherm curve by the NLDFT method.
Moreover, a loading of 4.6% can be considered as the opti-
mal loaded amount of Ru-2 catalyst to make the best of the
catalytically active sites.
2 6 6 2 3 6 6 6 2 2
KNa[Ru (C H ) (CH COO) ] (Ru-KNa): [RuC H Cl ] (0.5 g,
1
2
.0 mmol) was dissolved in a sodium acetate solution (0.5 m,
0 mL, pH = 6.0). The mixture was heated to 80 °C for 1 h and
then cooled to room temperature. The solution was filtered, and
then a KCl solution (1.0 mL, 1.0 m) was added to the filtrate. This
Conclusions
An organo-Ru precursor, KNa[Ru (C H ) (CH COO) ] solution was allowed to concentrate in an open beaker at room
2
6
6 2
3
6
temperature. A red crystalline product started to appear after about
two weeks. Concentration was continued until almost all of the
(
Ru-KNa), was prepared and structurally characterized.
From this, an organo-Ru unit supported isopolytungstate,
Na [{Ru(C H )} W O (OH) ]·16H O (Ru-Na), was syn-
thesized, and it consists of two {Ru(C H )} units linked to
a [W O (OH) ]
solvent had evaporated. Yield 0.38 g, 64%. C24
774.72): calcd. C 37.2, H 3.9, K 5.1, Na 3.0 Ru 26.1; found C 37.5,
H 4.1, K 4.9, Na 3.2, Ru 25.8.
H30KNaO12Ru
2
6
6
6
2
8
28
2
2
(
6
6
1
0–
fragment by three Ru–O(W) bonds,
8
28
2
Na [{Ru(C )} 28(OH) ]·16H
6
6
H
6
2
W
8
O
2
2
O (Ru-Na): Ru-KNa (0.3 g,
which results in an assembly with idealized C symmetry.
2
0
.4 mmol) and Na WO ·2H O (0.5 g, 1.5 mmol) were dissolved in
2
4
2
Further, Ru-Na was anchored on (3-aminopropyl)trieth-
oxysilane (apts) modified SBA-15 to obtain the solid cata-
lyst SBA-15-apts-Ru-2, which was characterized by powder
XRD, N2 adsorption measurements and FTIR spec-
troscopy.
a sodium acetate solution (20 mL, 0.5 m, pH = 6.0). The solution
was heated to 80 °C for 1 h and then cooled to room temperature.
The solution was filtered and allowed to slowly concentrate in an
open beaker at room temperature. An orange crystalline product
started to appear after about one month. Concentration was con-
tinued until almost all of the solvent had evaporated. Yield 0.41 g,
We have also conducted oxidation reaction studies with
Ru-Na and SBA-15-apts-Ru-2 as heterogeneous catalysts 80%. C H Na O Ru W (2737.28): calcd. C 5.3, H 1.7, Na 5.1,
1
2
46
6
46
2
8
for the solvent-free aerobic oxidation of n-hexadecane to Ru 7.4, W 53.7; found C 5.0, H 1.8, Na 5.3, Ru 7.3, W 53.9. IR: ν˜
alcohols and ketones under ambient conditions. For Ru- = 1639 (w), 1439 (w), 927 (s), 882 (s), 813 (s), 761 (s), 672 (w), 596
–
1
(
m), 570 (m) cm .
Na, the catalyst amount and reaction time were investigated
to indicate the optimal catalytic reaction conditions, which Preparation of Solid Catalyst: The surface modification of SBA-15
were 4 h and 15 mg in our catalytic system. For SBA-15- and subsequent Ru-Na anchoring were achieved as follows: Firstly,
apts-Ru-2, different loaded amounts of Ru-2 were exam- in a Schlenk tube, the commercially purchased mesoporous silica
ined, and a high turnover frequency (TOF) of 8075 h–1 was
exhibited for a loading of 4.6% at 150 °C with a reaction
time of 6 h. The conversion of n-hexadecane for Ru-Na
SBA-15 (1 g) was heated to 130 °C under vacuum for 5 h to remove
the adsorbed water. Then, under nitrogen a solution of (3-
aminopropyl)triethoxysilane in toluene (apts; 1 wt.-%, 30 mL) was
added to the silica. The mixture was stirred and heated to reflux
for 10 h, and then the solids were collected by filtration and washed
with toluene to remove unanchored apts. The solids were heated
in an oven at 100 °C for 5 h to obtain the amino group modified
mesoporous silica (SBA-15-apts). To immobilize Ru-Na thereon,
(10 mg, 45.31%) and SBA-15-apts-Ru-2 (10 mg, 47.5%)
were significantly higher than for other POMs. Compared
with previous results, Ru-Na demonstrates the best catalytic
performance for the solvent-free oxidation of n-hexadecane
to date, which suggests that the catalysts possess great po- SBA-15-apts (0.5 g) was mixed with water, and HCl (2 m) was
tential for practical application for this type of reactions.
added to adjust the pH to ca. 2. Then, the solids were collected by
filtration and added to water (50 mL) containing a certain amount
of Ru-Na (the anchored samples with Ru-Na loading of 2.4, 4.6
and 8.0% were obtained by using 12.5, 25 and 50 mg of Ru-Na,
respectively), and the mixture was stirred for 10 h. Then, the solids
were collected by filtration and washed with water three times. Fi-
nally, the product (SBA-15-apts-Ru-2) was dried in an oven at
100 °C.
Experimental Section
Reagents and Materials: All reagents were used as purchased with-
out further purification. Pure water was used throughout. It was
obtained by passage through a RiOs 8 unit followed by a Millipore-
Q Academic purification set.
X-ray Crystallography: Single-crystal X-ray diffraction data for
compounds Ru-KNa and Ru-Na were collected with a Rigaku R-
AXIS RAPID imaging plate diffractometer with graphite-mono-
Instrumentation: Elemental analyses of C, H and N were performed
with a Flash EA1112 analyzer from Thermo Quest Italia SPA. Ele-
mental analyses of W, Ru, Na and K were performed with a Per-
kin–Elmer Optima 3300 DV ICP-OES spectrometer. IR spectra
were recorded with a Bruker Vertex 80v spectrometer equipped
with a DTGS detector (64 scans) with dried samples pressed into
KBr pellets. Powder XRD analyses were performed with a Rigaku
α
chromated Mo-K (λ = 0.71073 Å) at 293 K. Empirical absorption
corrections based on equivalent reflections were applied. The struc-
tures of Ru-KNa and Ru-Na were solved by direct methods and
2
refined by full-matrix least-squares methods on F by using the
SHELXS-97 crystallographic software package.[
25,26]
All non-hy-
D/MAX-2500 diffractometer with Cu-K
length of 1.542 Å operated at 40 kV and 40 mA in Bragg–Brentano
geometry. N adsorption isotherms were measured with a Quant-
achrome Autosorb-iQ analyzer. The samples were degassed at
50 °C under vacuum for more than 4 h prior to the measurements.
The specific surface areas were evaluated with the Brunauer–Em-
mett–Teller (BET) method in the P/P range 0.05–0.35. Pore size
α
radiation with a wave-
drogen atoms were refined anisotropically. The hydrogen atoms of
benzene molecules were placed in calculated positions and treated
as riding on their parent atoms. The crystal parameters, data collec-
tion and refinement results for Ru-KNa and Ru-Na are summa-
rized in Table 5. Selected bond lengths and angles are listed in
Table S1. CCDC-900792 (for Ru-KNa) and -900793 (for Ru-Na)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cam-
2
1
0
distributions were calculated from the adsorption branch of the
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
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Ru-KNa
Ru-Na
Empirical formula
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Crystal system
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b [Å]
c [Å]
α [°]
β [°]
C
774.71
hexagonal
R3
9.9840(10)
9.9840(10)
23.477(5)
90
90
120
2026.7(5)
3
1.904
1.352
1164
24
H
30KNaO12Ru C H46Na O46Ru W
2 12 6 2 8
2737.37
monoclinic
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3.613
18.953
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0.0338
1.093
0.0246
0.0613
0.0268
0.0620
2
[
a]
R
l
[IϾ2σ(I)]
[
b]
wR
2
[IϾ2σ(I)]
(all)
[a]
R
l
[
b]
wR
2
(all)
o c o o c o
a] R1 = Σ||F | – |F ||/Σ|F – F
|. [b] wR2 = {Σ[w(F 2 2)2/Σw(F ) ]}1/2.
2 2
[
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Pages: 9
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FULL PAPER
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[
Published Online: ᭿
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POM Catalysts
A ruthenium complex and its polytungstate
derivative have been synthesized and
characterized. A solid catalyst of Ru-con-
taining polytungstate anchored on SBA-15
modified with (3-aminopropyl)triethoxy-
silane was prepared, and its catalytic ef-
ficiency was assessed in the oxidation of n-
hexadecane with air without any additives
or solvents.
R.-Q. Meng, B. Wang, H.-M. Sui, B. Li,
W. Song, L.-X. Wu, B. Zhao,
L.-H. Bi* .......................................... 1–9
Organoruthenium-Supported
Polyoxo-
tungstate – Synthesis, Structure and Oxi-
dation of n-Hexadecane with Air
Keywords: Ruthenium / Tungsten / Green
chemistry / Polyoxometalates / Oxidation
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim