A new organo-ruthenium substituted tungstotellurate: Synthesis, structural characterization and catalytic properties

Article abstract of DOI:10.1039/c6nj01914e

Reaction of [RuC₆H₆Cl₂]₂ with TeO₂ and Na₂WO₄·2H₂O in aqueous solution (pH 4.7) yielded a novel organo-ruthenium supported tungstotellurate polyanion, [Te₂W₂₀O₇₀(RuC₆H₆)₂]^8- (Ru-1), which is composed of two [RuC₆H₆]²⁺ units linked to a [Te₂W₂₀O₇₀]^12- fragment through Ru-O(W) bonds resulting in an assembly with idealized C_2h symmetry. Furthermore, the polyanion Ru-1 was anchored on 3-aminopropyltriethoxysilane (apts)-modified SBA-15 to prepare new catalysts (SBA-15-apts-Ru-1) containing different amounts of Ru-1, which were characterized using powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N₂-adsorption measurement and Fourier transform infrared reflectance (FT-IR) spectroscopy. Finally, the catalytic activity of SBA-15-apts-Ru-1 was evaluated for the aerobic oxidation of n-tetradecane using air as the oxidant in the absence of any additives or solvents. In addition, the optimum catalytic reaction conditions were also determined.

Full text of DOI:10.1039/c6nj01914e

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A new organo-ruthenium substituted
tungstotellurate: synthesis, structural
Cite this:DOI: 10.1039/c6nj01914e
characterization and catalytic properties†
a
a
a
b
a
Da-Ming Zheng, Rui-Qiang Wang, Yu Du, Guang-Feng Hou, Li-Xin Wu and
Li-Hua Bi*
a
Reaction of [RuC
6
H
6
Cl
2
]
2
with TeO
2
and Na
2
WO
4
Á2H
2
O in aqueous solution (pH 4.7) yielded a novel
8À
organo-ruthenium supported tungstotellurate polyanion, [Te
2 20
W O
70(RuC
fragment through Ru–O(W) bonds
resulting in an assembly with idealized C2h symmetry. Furthermore, the polyanion Ru-1 was anchored on
-aminopropyltriethoxysilane (apts)-modified SBA-15 to prepare new catalysts (SBA-15–apts–Ru-1)
containing different amounts of Ru-1, which were characterized using powder X-ray diffraction (XRD),
X-ray photoelectron spectroscopy (XPS), N -adsorption measurement and Fourier transform infrared
H
6 6
2
) ]
(Ru-1), which is
2+
12À
composed of two [RuC
6
H
6
]
2 20
units linked to a [Te W O ]
70
3
Received (in Montpellier, France)
17th June 2016,
Accepted 25th August 2016
2
reflectance (FT-IR) spectroscopy. Finally, the catalytic activity of SBA-15–apts–Ru-1 was evaluated for the
aerobic oxidation of n-tetradecane using air as the oxidant in the absence of any additives or solvents.
In addition, the optimum catalytic reaction conditions were also determined.
DOI: 10.1039/c6nj01914e
www.rsc.org/njc
V
tetrahedral XO (X = B, Si, Ge, P, As ) or trigonal pyramidal XO
(
4
3
Introduction
III
III
III
IV
IV
X = As , Sb , Bi , Se , Te ) building blocks via the sharing of
Polyoxometalates (POMs) are an exceptional and distinct family corners, edges, and rarely faces, where M is called the addenda
of inorganic metal–oxygen clusters combining remarkable struc- atom and X is called the central heteroatom. A literature survey
1
–5
IV
tural variety with intriguing chemical or electronic properties,
showed that polyoxotungstates containing Te as the central hetero-
leading to potential applications in various areas including atom have been less explored than those containing other central
6
,7
8,9
10–13
14
analysis, materials science, catalysis,
biomedicine,
heteroatoms. In 1992, Yurchenko et al. revealed the first transition
1
5–18
IV
nÀ
nanotechnology
nuously attracted attention for nearly 200 years since the first (M = Cu , n = 10; M = Fe , n = 7), but no single crystal structure
POM was found in 1826. To date, a large number of novel was presented. Then, several research groups developed
etc. As is well-known, POMs have conti- metal-containing tungstotellurates [M (H O) (a-Te W O ) ]
3
2
3
9
33 2
2
+
3+
1
9
20
IV
POMs with various sizes, diverse compositions, and fantastic Te -based tungstotellurates, synthesized using routine synthetic
architectures are still being discovered because of the self- reactions in aqueous solutions. On the one hand, a series of tungsto-
assembly formation mechanism of POMs. Among the synthesis tellurates without any incorporated transition metals has been
2
2À 21
13À 21
strategies for POMs, the one-pot self-assembly route has become reported, for example, [H
2
Te
4
W
20
O
80
14À 22
]
,
[NaTeW15
O
54
]
,
12À 22
10À 22
an important method for constructing new POMs up to now.
[Te W O ]
,
[Te W O (OH) ]
,
[Te W O (OH) ]
,
2
17 61
2
2À
16 58
2
2
18 62
2
3
12À 23
During the one-pot self-assembly synthesis process, the conden- [(WO ) (Te W O ) ]
and [Te W O ]
;
on the other hand,
2
4
2
15 54 4
3
21 75
sation reactions of different source ions in aqueous media result in different types of transition metal substituted tungstotellurates
8
À 24
the aggregation of octahedral [MO
6
] (M = W, Mo, V, Nb, Ta), have been disclosed, for instance, [Mn
4
(H
O) (WO
,
2
O)10(b-TeW
)(b-TeW
[(Zn(H O)
O) (a-TeW
(H O) {Fe(H
and [(UO ) (H O) (a-
9
O
9
O
33
)
33
)
2
]
,
8À 25
8À 25
[
Ni
3
(H
(V(H
Zn(H
Fe (H
2
O)
O)
O)
O)
8
(WO
(VO
0.5(b-TeW
2
)(TeW
)(WO
9
O
33
)
2
]
,
[Co
3
(H
2
8
2
2
]
,
6
À 25
[
(
[
2
3
)
2
2
2
)(b-TeW
9
O
33
)
2
]
2
3 2 2 1.5
) (WO ) -
a
College of Chemistry, State Key Laboratory of Supramolecular Structure and
Materials, Jilin University, Changchun 130012, P. R. China.
E-mail: blhytyyz@163.com
8
À 25
10À 26a
2
2
)
9
O
)
33
)
2
4
]
,
[Cu
[{Ru
3
(H
2
3
9
O
33
)
2
]
,
À 26b
4
2
2
(b-TeW
9
O
33
2
]
,
4
O
6
2
9
}
2
2 2 2
O) } (b-
b
À 26c
10À 27
Key Laboratory of Functional Inorganic Material Chemistry (MOE),
School of Chemistry and Materials Science, Heilongjiang University,
Harbin 150080, P. R. China
TeW O ) H] , [Pd (a-TeW O ) ]
TeW O ) ]
,
9
33 2
3
9
33 2
2 2
2
2
1
2À 28
.
More recently, Reinoso’s group reported two
9
33 2
tungstotellurates disubstituted with first-row transition metals and
†
Electronic supplementary information (ESI) available: IR spectra of SBA-15, apts
II
10À
2 2 2 9 33 2
(WO ) (B-b-TeW O ) ]
N,O-chelating ligands, [{M (imc)(H
(
2
O)}
M = Mn and Co; imc = 1H-imidazole-4-carboxylate). Through-
out the above literature, it can be found that the number of
and SBA-15-apts; TGA curve of Ru-2. Selected bond distances (Å) for Ru-2. CCDC
471318. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c6nj01914e
II
29
1
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transition metal-containing tungstotellurates synthesized using under a nitrogen atmosphere. After refluxing for ca. 11 h with
conventional synthesis methods is still scarce, and the most stirring, the mixture was filtered and the solid was washed with
substituted ions are from the first-row transition metals. Strikingly, toluene in order to remove ungrafted apts. Subsequently, the
tungstotellurates containing second or third-row transition metals solid was collected and heated at 100 1C for ca. 5 h in an oven,
have been mentioned rarely in the literature. Therefore, the and the amino-group decorated mesoporous silica material
synthesis and structural characterization using single crystal (SBA-15–apts) was obtained. Secondly, 0.4 g of SBA-15–apts
X-ray diffraction of novel tungstotellurates substituted with was mixed with 20 mL of water evenly, and then the pH of
second or third-row transition metals is still a great challenge the mixture was adjusted to ca. 2 by adding HCl (2 M). Finally,
and there are opportunities to discover not only their structures the protonated silica material was filtered and added to 50 mL
but also their further applications as catalysts.
of an aqueous solution containing a known amount of Ru-2
Here, we report the synthesis and structural characterization of (10 mg, 20 mg and 40 mg) to prepare the anchored samples
a novel di-organo-ruthenium-grafted tungstotellurate, Na (TMA) - with Ru-1 loadings of 1.5%, 2.2%, and 4.5%, respectively, and
6
2
[
Te W O (RuC H ) ]Á25H O (Ru-2), which reveals a Krebs’ struc- this was followed by stirring of the mixture for 10 h. Then,
2
20 70
6
6 2
2
IV
ture with Te as the central heteroatom without any disorder. the Ru-1 decorated silica materials were filtered and washed
Moreover, we used it as an efficient heterogeneous catalyst toward with water several times. The prepared heterogeneous catalysts,
the aerobic oxidation of n-tetradecane under completely green SBA-15–apts–Ru-1, were dried at 100 1C in an oven.
conditions. The development of novel catalysts with a better
Characterization
catalytic performance for n-tetradecane oxidation is being exten-
sively pursued in both industrial and academic research.
Elemental analysis for C, H and N was performed using a Flash
EA1112 analyzer from Thermo Quest Italia SPA. Elemental
analysis results for W, Te, Ru and Na were obtained with a
Perkin-Elmer Optima 3300 DV ICP-OES Spectrometer. The IR
spectra were obtained using a Bruker Vertex 80v spectrometer
equipped with a DTGS detector (64 scans), using a dried sample
pressed into KBr pellets. Powder X-ray diffraction (XRD) measure-
ments were performed with a Rigaku D/MAX-2500 with Cu Ka
radiation and a wavelength of 1.542 Å, operated at 40 kV and
Experimental section
Materials
All chemicals were utilized as purchased without any further
purification. Purified water was used throughout the experiments.
Purified water was prepared by passing it through a RiOs 8 unit
followed by use of a Millipore-Q Academic purification system.
40 mA in Bragg–Brentano geometry. The measurements for the
N adsorption isotherms were recorded using a Quantachrome
2
Synthesis of Na
Na WO O (0.5 g, 1.5 mmol) was dissolved in 30 mL of
Á2H
distilled water to form solution A, TeO (0.035 g, 0.2 mmol) was
6
(TMA)
2
[Te
2
W
20
O
70(RuC
6
H
6
)
2
]Á25H
2
O (Ru-2)
Autosorb-iQ analyzer. The materials were degassed at 150 1C
under vacuum for more than 4 h before measurement. The specific
surface areas were estimated using Brunauer–Emmett–Teller (BET)
2
4
2
2
dissolved in 10 mL of NaOH (0.25 M) to form solution B; and
then solution B was added to solution A dropwise and the
mixture was stirred at room temperature for 0.5 h. During this
period, the pH of the reaction mixture was adjusted to 4.7 with
0
means within a P/P range of 0.05–0.35. The distribution of the
pore sizes was evaluated using the adsorption branch of the
measured isotherms based on the nonlocal density functional
theory (NLDFT). The pore volume and pore size were acquired
from the adsorption branch of the measured isotherm curves
using the NLDFT method. X-ray photoelectron spectroscopy
glacial acetic acid. [RuC H Cl ] (0.06 g, 0.1 mmol) was added
6
6
2 2
into the mixture, and the solution was heated at 50 1C for 1 h
with constant stirring. After cooling to room temperature, the
resulting mixture was filtered, followed by addition of 1 mL of a
TMA solution (0.1 M) to the filtrate, which was then allowed to
evaporate in an open beaker at room temperature. Orange red
crystals of Ru-2 were isolated and collected after approximately
two months. Yield: 0.3 g, 48.8% based on Ru. Calcd (found) for
(XPS) spectra were obtained using an ESCALAB-250 spectro-
meter involving a monochromic X-ray source (Al Ka line,
1
486.6 eV). Thermogravimetric analysis (TGA) was accom-
À1
plished with a TGA Q500 with a heating rate of 10 1C min
under an air flow.
Ru-2: Na 2.24 (2.22), Ru 3.29 (3.21), Te 4.15 (4.22), W 59.82 X-ray crystallography
À1
(
1
6
60.24), C 3.91 (3.90), H 1.41 (1.46), N 0.46 (0.49). IR/cm
:
Data for single-crystal X-ray diffraction of the Ru-2 compound
were collected using an Oxford Diffraction Gemini R Ultra
diffractometer with graphite-monochromated Mo Ka radiation
489 (m), 1438 (m), 952 (s), 861 (sh), 818 (s), 782 (s), 747 (m),
96 (m), 665 (m), 503 (w), 477 (w).
(
l = 0.71073 Å) at 293 K. Empirical absorption corrections were
Preparation of the heterogeneous catalysts SBA-15–apts–Ru-1
applied according to the equivalent reflections. The structure of
The process of surface decoration of the SBA-15 and subsequent the Ru-2 compound was obtained from direct methods, and
2
fixing of Ru-1 is described below. Firstly, the mesoporous silica refined using full-matrix least-squares methods on F (SHELXS-97
30,31
(1 g, SBA-15), purchased commercially, was heated to 130 1C for crystallographic software package).
The refinement of all the
6
h in a Schlenk tube under vacuum conditions to remove the non-hydrogen atoms was carried out anisotropically. Hydrogen
adsorbed water, followed by the addition of a solution containing atoms for the benzene and TMA molecules were positioned in
-aminopropyltriethoxysilane (apts, 30 mL) and toluene (40 mL) calculated places and rode on their parent atoms. The results
3
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Table 1 Crystal data for Ru-2
Ru-2
Empirical formula
Fw
Crystal system
Space group
a (Å)
b (Å)
c (Å)
20 86 2 6 2 2 20
C H N Na O95Ru Te W
6147.19
Monoclinic
C2/c
32.117(2)
13.8184(4)
26.7619(12)
90.00
a (deg)
Fig. 1 Ball-and-stick (left) and polyhedral (right) representations of the
b (deg)
110.830(6)
90.00
11 100.8(9)
4
3.678
21.537
8
À
polyanion [Te
2
W
20
O
70(RuC
6
H
6
2
) ]
(Ru-1). The WO
6
octahedra are red.
g (deg)
3
V (Å )
Z
D
À3
calc (g cm
)
2+
À1
2 6 6
linked via two inner cis-WO groups and two outer [RuC H ]
m (mm
)
units leading to a structure with idealized C2h symmetry
F(000)
10 872
Reflections collected/unique
31 797/9750
0.1093
1.061
0.0624
0.1294
0.0866
0.1450
(see Fig. 1). Alternatively, this polyanion can be described as a
R(int)
12À
2+
2
2 20 70
dilacunary [Te W O ]
6 6
fragment stabilized by two [RuC H ]
GOF on F
a
units. This structural type was just described in 2015 by Reinoso’s
R
1
[I 4 2s(I)]
[I 4 2s(I)]
b
II
10À
II
wR
2
group for [{M (imc)(H
O)}
(WO
)
(B-b-TeW
O
)
]
(M = Mn
2
2
2
2
9
33
2
a
R
1
(all)
29
II
b
and Co; imc = 1H-imidazole-4-carboxylate), where a little M //W
disorder was observed at the external sites. In contrast, the
structure of Ru-1 doesn’t show any disorder at the same sites.
Such an observation allows us to conclude that the structure of
fragment definitely exists.
This structural type was first reported by Krebs et al. for
[X W O (OH) ]
wR
2
(all)
a
b
2
o
2 2
2 2 1/2
R
1
= S8F
o
| À |F
c
8/S|F
o
|. wR
2
= {S[w(F
À F
c
) /Sw(F
o
) ]}
.
1
2À
including from data collection, the crystal parameters and the dilacunary [Te W O ]
refinement for Ru-2 are given in Table 1. Selected bond lengths
are shown in Table S1 (ESI†). CCDC 1471318 (for Ru-2).
2
20 70
1
2À
III
III 32
2
22 74
2
2 22
(X W ) (X = Sb , Bi ), which is called a
Krebs’ type compound. Then, a series of first-row transition
metal disubstituted Krebs’ type compounds were presented.
Catalytic experiments
33
The compound Ru-2 alone and its immobilized solid SBA-15– Recently, Proust et al. synthesized organo-Ru disubstituted
apts–Ru-1 were utilized to detect their heterogeneous catalytic Krebs’ compounds through a one pot route, whereas Bi et al.
3
4,35
performance for the aerobic oxidation of n-tetradecane. A solvent- prepared the same compounds starting from X W .
The
2
22
free aerobic oxidation of n-tetradecane was studied using 2 mL of above results indicate that organo-Ru disubstituted Krebs’
fresh n-tetradecane in a 25 mL two-necked-round-bottom flask compounds can be synthesized using two different routes. We
containing a known amount of undissolved catalyst, without any were interested in whether an organo-Ru disubstituted Krebs’
IV
other additives. The mixtures were heated at different tempera- compound (X W ) with Te as the central heteroatom was present.
2
22
À1
12À
tures and stirred under a constant air flow (about 35 mL min ). Unfortunately, the Krebs’ type polyanion [Te W O (OH) ]
2
22 74
2
After a period of reaction, the resulting mixture was cooled to has not been isolated so far. Therefore, we tried to use a one pot
room temperature, and then a sample was taken out for measure- procedure to synthesize the target compound. As expected, the
ment by GC analysis using a 2014C GC instrument with a flame organo-Ru disubstituted Krebs’ type tungstotellurate polyanion
8
À
ionization detector, He as the carrier gas and a HP-FFAP column. [Te W O (RuC H ) ] (Ru-1) was obtained, and it exhibits a
2
20 70
6
6 2
perfect second-row transition metal disubstituted Krebs’ type
1
2À
III
III
skeleton, compared to [X
and [(WO (H O) (b-XW
2
W
22
O
74(OH)
2
]
2
(X W22) (X = Sb , Bi )
Results and discussion
nÀ
III
III
n+
2+
2
)
2
M
2
2
6
9
O
33
)
2
]
(X = Sb , Bi ; M = Mn ,
Characterization of Na
Ru-2)
Synthesis. A novel organo-ruthenium [RuC H ] disubstituted strate that there are no protonation sites on Ru-1, and thereby the
6
(TMA)
2
[Te
2
W
20
O
70(RuC
6
H
6
)
2
]Á25H
2
O
3+
2+
2+
2+ 32–37
Fe , Co , Ni , VO ).
Bond valence sum calculations were performed to demon-
(
38
2+
6
6
À
8
tungstotellurate polyanion [Te
pared from a one pot reaction of [RuC
and TeO in aqueous solution (pH 4.7). On the one hand, this ions, respectively. At same time, elemental analysis was further
result indicates that the dimeric Ru-precursor [RuC
could be hydrolyzed under such conditions to form a reactive
2 20
W O70(RuC
6
H
6
)
2
]
(Ru-1) was pre- charge of the polyanion Ru-1 must be À8. The negative charge of
+
6
H
6
Cl
2
]
2
with Na
2
WO
4
Á2H
2
O
Ru-1 in the solid state is balanced by two TMA and six sodium
2
6
H
6
Cl
2
]
2
used to verify the full chemical composition.
Thermogravimetric (TG) analysis. As presented in Fig. S1
mononuclear electrophile in situ; on the other hand, this result (ESI†), the TG curve of the compound Ru-2 displays two primary
further confirms that new POMs could be obtained using the weight losses within ranges of about 50–360 and 360–425 1C,
one-pot self-assembly route.
in which the first weight loss from 50–360 1C is 8.92% and is
Crystal structure. The structure of Ru-2 was solved as being in attributed to the release of 25 lattice water molecules (calcd 7.32%),
the monoclinic space group C2/c. The polyanion Ru-1 of Ru-2 is and the second weight loss from 360–425 1C is 5.03% and was
8À
composed of two trilacunary B-b-[TeW
9 33
O ]
6 6
Keggin fragments assigned to the removal of two TMA counter ions and two C H
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ligands coordinated to Ru (calcd 4.95%). Thus, it can be seen
that the first weight loss of ca. 8.92% was more than 7.32% (the
theoretical calculation value), which may be due to loss of some
surface water on the sample of Ru-2 that was not fully dried
before the TG analysis. Such behavior has been observed in the
3
9
TG analysis of other POMs.
Characterization of the heterogeneous catalyst SBA-15–apts–Ru-1
The surface modification process for SBA-15 and the succeeding
Ru-1 immobilization are depicted in Scheme 1. We employed a
series of analytical techniques to characterize the heterogeneous
catalyst SBA-15–apts–Ru-1.
Fig. 2 (left) IR spectra of Ru-2 (a), SBA-15–apts–Ru-1 (b) and SBA-15–
apts (c); (right) IR spectra of SBA-15–apts–Ru-1 before (b) and after (d) the
catalytic reaction.
FT-IR spectra. IR spectra of Ru-2, SBA-15–apts and SBA-15–
apts–Ru-1 (with Ru-1 loading of 2.2%) are presented in Fig. 2
(left), where it can be seen that the characteristic vibrational
frequencies for Ru-2 are preserved in SBA-15–apts–Ru-1, suggest-
ing that Ru-1 has been immobilized on SBA-15–apts. Further-
more, as displayed in Fig. 2 (right), it can be observed that the
IR spectra of SBA-15–apts–Ru-1 before and after the catalytic
reaction are almost the same, implying that the heterogeneous
catalyst SBA-15–apts–Ru-1 has good stability during the catalytic
reaction.
XRD measurements. A series of small angle XRD patterns for
the powder state of SBA-15, SBA-15–apts and SBA-15–apts–Ru-1
with different loadings of Ru-1 was obtained, as shown in Fig. 3.
The following can be observed: (i) all the samples display three
characteristic Bragg diffraction peaks, and the intense peak was
Fig. 3 The low-angle XRD patterns of the samples.
N adsorption/desorption isotherms. Fig. 4 (left) displays N2
2
attributed to a reflection of the (100) plane and two low intensity adsorption and desorption isotherms of samples of SBA-15,
peaks were assigned to reflections of the (110) and (200) planes for SBA-15–apts and SBA-15–apts–Ru-1. It can be found that: (i) the
SBA-15. This observation demonstrates that all the samples main- isotherms of all the samples exhibit nonreversible type IV adsorp-
tain the well-ordered two-dimensional hexagonal mesoporous tion curves with a H1 hysteresis loop, which are defined by IUPAC;
structure of SBA-15; (ii) compared to SBA-15 alone, the intensities (ii) each adsorption isotherm shows a sharp inflection in the
of the Bragg diffraction peaks decreased with the immobilization relative pressure range of 0.50–0.95, indicating capillary conden-
of apts and of Ru-1, suggesting that apts and subsequently Ru-1 sation inside uniform mesopores; (iii) the absorption amounts are
are fixed inside the channels of SBA-15; (iii) the decrease of the reduced with decreasing pore volume owing to modification with
peak intensities with increase of the loading amounts of Ru-1 apts and the immobilization of Ru-1 on SBA-15; (iv) the inflection
further proves that catalysts containing different amounts of Ru-1 point for each sample moves to a lower P/P
0
, which is ascribed to
were obtained. In brief, the mesostructure of SBA-15 had been the decrease of the pore size (see Fig. 4, right).
retained after the modification with apts inside the channel
The Brunauer–Emmett–Teller (BET) method was employed to
surface and the immobilization of Ru-1 on the surface of calculate the surface areas and nonlocal density functional theory
SBA-15–apts. Furthermore, we performed N
desorption experiments to support the above conclusion.
2
adsorption and (NLDFT) was chosen to assess the pore parameters according to
the adsorption branch of the isotherm. Detailed information is
Scheme 1 Synthesis procedure for the heterogeneous catalyst SBA-15–apts–Ru-1.
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XPS experiments, it was deduced that the novel polyanion Ru-1
has been fixed inside the mesochannels of SBA-15 to produce a
novel heterogeneous catalyst SBA-15–apts–Ru-1.
Study of the catalytic oxidation reaction of n-tetradecane
We chose the catalytic oxidation of n-tetradecane to corresponding
alcohols and ketones to evaluate the catalytic performance of
the catalysts, Ru-2 alone and SBA-15–apts–Ru-1. N-Tetradecane
includes inert C–H bonds, which make n-tetradecane exhibit a
lower reactivity in the oxidation reaction. Accordingly, the oxida-
tion of n-tetradecane normally requires appropriate solvents, extra
additives, sacrificial oxidants and strict control of the experi-
Fig. 4
2
N adsorption/desorption isotherms (left) and DFT pore size dis-
tribution plots (right) of SBA-15 (a), SBA-15–apts (b), SBA-15–apts–Ru-1
1.5%) (c), SBA-15–apts–Ru-1 (2.2%) (d), and SBA-15–apts–Ru-1 (4.5%) (e).
(
4
0
Table 2 Composition, specific surface area, and pore parameters of SBA-15, mental conditions such as a high temperature and pressure.
SBA-15–apts and SBA-15–apts–Ru-1 with different loading amounts
To date, very few n-tetradecane oxidations have been performed,
4
1–43
and a high pressure and temperature were used.
Therefore,
Sample
Ru-1 loading)
Surface area Pore volume Pore diameter
2
À1
the development of catalysts for n-tetradecane oxidation via a
green reaction process is of paramount importance. The purpose
of the present work is to find stable and highly effective catalysts,
which can make the n-tetradecane oxidation occur under mild
experimental conditions with air as the oxidant and without any
solvents and additives. Herein, the hybrid compound SBA-15–
apts–Ru-1 is used as a heterogeneous catalyst to investigate
(
BET [m g]
[cc g
]
[nm]
SBA-15
SBA-15–apts
SBA-15–apts–Ru-1 (1.5%) 379
SBA-15–apts–Ru-1 (2.2%) 360
SBA-15–apts–Ru-1 (4.5%) 353
499
488
0.87
0.78
0.74
0.69
0.67
7.1
7.0
6.4
6.3
6.1
listed in Table 2. It can be found that the pore size, pore volume the oxidation reaction of n-tetradecane under environmentally
and surface area decrease after modification with apts and sub- benign conditions. Our investigation will be dedicated to deter-
sequent immobilization of Ru-1 on SBA-15, demonstrating that the mining the best catalytic reaction conditions and comparing the
apts and Ru-1 have been anchored inside the mesochannels of catalytic performance of the as-prepared catalyst SBA-15–apts–Ru-1
SBA-15. Moreover, the pore size, pore volume and surface area all under different reaction conditions.
decrease with an increased loading amount of Ru-1, indicating
Influence of the loading amount. As is well known, the loading
that different amounts of Ru-1 have been loaded inside the amount of the active constituent on a heterogeneous catalyst
mesochannels of SBA-15. These conclusions are consistent with will influence its catalytic performance. Thus, we synthesized
the XRD patterns.
three heterogeneous catalysts containing different amounts of
XPS spectra. We carried out XPS experiments to verify the Ru-1 (1.5%, 2.2%, 4.5%), which were confirmed using elemental
composition of the as-prepared SBA-15–apts–Ru-1 heterogenerous analysis. The catalytic activities of the three catalysts were
catalyst. Fig. 5 shows the characteristic Ru3d, Te3d, W4f, N1s evaluated. As shown in Fig. 6, we could see that: (i) the hetero-
and C1s peaks for SBA-15–apts–Ru-1. Components of Ru-1 and geneous catalyst containing 2.2% Ru-1 was very active, thus this
SBA-15–apts exist in the composition of SBA-15–apts–Ru-1.
can be considered as the optimal loading of Ru-1 for the SBA-15–
As a result, combining the results of the FT-IR, XRD, and apts–Ru-1, which maximizes the catalytic active sites; (ii) com-
N adsorption and desorption experiments with those of the pared to the catalytic capacity of Ru-2 alone (see Table 3), Ru-1
2
immobilized on SBA-15 leads to an enhanced catalytic perfor-
mance and higher turnover frequency. Accordingly, the above
comparative study illustrates that the catalytic performance of
Fig. 6 Conversion of n-tetradecane over the catalysts SBA-15–apts–Ru-1
with different Ru-1 loadings of 1.5, 2.2 and 4.5%. Reaction conditions: catalyst:
À1
4
mg, n-tetradecane: 2 mL, airflow rate: 35 mL min , temperature: 150 1C,
Fig. 5 XPS spectra of SBA-15–apts and SBA-15–apts–Ru-1.
time: 6 h.
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Table 3 Catalytic activity comparison of the catalysts Ru-2 and SBA-15–apts–Ru-1 for n-tetradecane oxidation with air
Product selectivity [%] and distribution
POM
Loading [%]
À1
Catalyst
Conv. [%]
TOF [h
]
Ketones (7-one : 6-one : 5-one : 4-one : 3-one)
Alcohols (7-ol : 6-ol : 5-ol : 4-ol : 3-ol)
Blank
Ru-2
SBA-15–apts–Ru-1
—
100
2.2
9.84
44.01
47.90
—
52.6
27.7
802
68.8 (20 : 18.2 : 16.4 : 7.3 : 6.9)
61.1 (19.9 : 18.3 : 14.5 : 4.2 : 4.2)
16.3 (10.7 : 1.9 : 0.9 : 1.1 : 1.7)
22.2 (5.9 : 4.3 : 2.1 : 4.7 : 5.2)
39 162
SBA-15–apts–Ru-1 definitely depends on the concentration of
Ru-1 in the heterogeneous catalyst.
Determination of the reaction temperature. Based on the
results above, the heterogeneous catalyst SBA-15–apts–Ru-1 with
a loading amount of 2.2% was chosen as an example to study the
relationship between the catalytic activity and reaction tempera-
ture. From Fig. 7, the optimal reaction temperature was deter-
mined to be 150 1C, due to the high catalytic activity of the
SBA-15–apts–Ru-1 at this temperature.
Study of the catalyst amount. We selected the catalyst with the
optimal loading to investigate the catalytic performance versus
different catalyst amounts at the optimal reaction temperature. As
depicted in Fig. 8, it is clear that the conversion of n-tetradecane
Fig. 9 Conversion (left) and the TOF for conversion (right) of n-tetradecane
at reaction times of 0.5, 1.5, 3, 4, 5, 6, 7 and 12 h. Reaction conditions: catalyst
SBA-15–apts–Ru-1 with a loading of 2.2%: 4 mg; n-tetradecane: 2 mL;
À1
airflow rate: 35 mL min ; temperature: 150 1C.
increases with increase of the catalyst amount, below 4 mg regards to the loading amount for the catalyst, the temperature
of catalyst, whereas the conversion decreases with increase of of the reaction system and the amount of catalyst. The change
the catalyst amount above 4 mg of catalyst. Consequently, the trend of the catalytic activity against the reaction time is shown
suitable catalyst amount is 4 mg for this catalytic reaction.
in Fig. 9, which illustrates that the conversion of n-tetradecane
Reaction time. We tested the influence of the reaction time on increases with prolonged reaction time (see Fig. 9 (left)). However,
the catalytic performance using the optimized conditions with the turnover frequency (TOF) for the n-tetradecane conversion
reaches a maximum at 6 h, and then decreases (see Fig. 9 (right)),
suggesting that the optimal reaction time is 6 h for this catalytic
reaction.
From the above investigation, it was found that the conversion
for n-tetradecane oxidation under the optimal catalytic reaction
conditions reaches 47.90%. Comparison with the previously
reported results to date reveals a better performance of the
heterogenerous catalyst SBA-15–apts–Ru-1 (2.2%) for the solvent
4
4
free oxidation of n-tetradecane.
Reuse and regeneration of the catalyst. With heterogeneous
catalysts, a reduction in the catalytic activity is usually encountered
Fig. 7 Conversions of n-tetradecane at reaction temperatures of 100, owing to leakage of the active species from the surface. Thus,
1
20, 140, 150 and 160 1C. Reaction conditions: catalyst SBA-15–apts–Ru-1
the recyclability of the heterogeneous catalyst SBA-15–apts–Ru-1
2.2%) was evaluated via five successive cycles of n-tetradecane
oxidation. The tested heterogeneous catalyst was recovered via
À1
with a loading of 2.2%: 4 mg; n-tetradecane: 2 mL; airflow rate: 35 mL min
time: 6 h.
;
(
Fig. 8 Conversions of n-tetradecane with catalyst amounts of 1, 2, 4 and
Fig. 10 Evaluation of the recyclability of the SBA-15–apts–Ru-1 towards
the n-tetradecane oxidation reaction over five runs. Reaction conditions:
À1
8
mg. Reaction conditions: n-tetradecane: 2 mL; airflow rate: 35 mL min
;
À1
temperature: 150 1C; time: 6 h; catalyst: SBA-15–apts–Ru-1 with a loading
catalyst: 4 mg, n-tetradecane: 2 mL, airflow rate: 35 mL min , temperature:
of 2.2%.
150 1C, reaction time: 6 h.
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filtration, thoroughly washed with distilled water and ethanol,
and then dried at 100 1C for ca. 5 h after each cycle. As depicted
in Fig. 10, no significant change of the n-tetradecane conversion
occurred after the five cycles, specifically, the catalytic perfor-
mance of the SBA-15–apts–Ru-1 was similar to that of the fresh
catalyst, implying that the catalyst SBA-15–apts–Ru-1 could be
recycled at least five times without obvious loss of its catalytic
activity. Moreover, the selectivity for the ketones and alcohols
remained constant, within the range of experimental error, during
the five catalytic cycles.
4 Y. Matsuki, T. Hoshino, S. Takaku, S. Matsunaga and
K. Nomiya, Inorg. Chem., 2015, 54, 11105–11113.
5 K.Y. Wang, B. S. Bassil, Z. G. Lin, I. Romer, S. Vanhaecht,
T. N. Parac-Vogt, C. S. de Pipaon, J. R. Galan-Mascaros,
L. Y. Fan and J. Cao, Chem. – Eur. J., 2015, 21, 18168–18176.
6 J. W. Zuo, N. Gao, Z. G. Yu, L. Kang, K. P. O’Halloran,
H. J. Pang, Z. F. Zhang and H. Y. Ma, J. Electroanal. Chem.,
2015, 751, 111–118.
7 (a) B. Wang, R. Q. Meng, L. X. Xu, L. X. Wu and L. H. Bi,
Anal. Methods, 2013, 5, 885–890; (b) B. Wang, R. Q. Meng,
L. H. Bi and L. X. Wu, Dalton Trans., 2011, 40, 5298–5301.
8
J. S. Li, X. J. Sang, W. L. Chen, L. C. Zhang, Z. M. Zhu,
T. Y. Ma, Z. M. Su and E. B. Wang, ACS Appl. Mater.
Interfaces, 2015, 7, 13714–13721.
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Chem. C, 2015, 3, 1732–1737.
Conclusions
A new organo-Ru grafted tungstotellurate(IV) Na (TMA) [Te W -
6
2
2
20
9
O (RuC H ) ]Á25H O (Ru-2) was successfully isolated using a
7
0
6
6 2
2
one-pot self-assembly synthesis procedure and structurally char-
8
À
1
10 H. J. Lv, Y. N. Chi, J. van Leusen, P. Kogerler, Z. Y. Chen,
J. Bacsa, Y. V. Geletii, W. W. Guo, T. Q. Lian and C. L. Hill,
Chem. – Eur. J., 2015, 21, 17363–17370.
acterized. Its polyanion [Te (Ru-1) consists
2À
2 20 70
of two {RuC } units linked to a [Te W O ] fragment via
2
W
20
O
6 6 2
70(RuC H ) ]
H
6 6
three Ru–O(W) bonds, leading to a perfect Krebs’ type polyanion
IV
11 S. S. Wang and G. Y. Yang, Chem. Rev., 2015, 115,
2
(X W
22) with Te as the central heteroatom without any disorder.
Furthermore, Ru-1 was fixed on 3-aminopropyltriethoxysilane
apts)-modified SBA-15 to produce a heterogeneous catalyst,
SBA-15–apts–Ru-1, with good stability. Characterization of SBA-
5–apts–Ru-1 using FT-IR, XPS, N -adsorption and XRD measure-
4893–4962.
1
1
1
2 J. J. Walsh, A. M. Bond, R. J. Forster and T. E. Keyes, Coord.
Chem. Rev., 2016, 306, 217–234.
3 X. Q. Du, Y. Ding, F. Y. Song, B. C. Ma, J. W. Zhao and
J. Song, Chem. Commun., 2015, 51, 13925–13928.
4 A. Saad, W. Zhu, G. Rousseau, P. Mialane, J. Marrot,
M. Haouas, F. Taulelle, R. Dessapt, H. Serier-Brault and
E. Riviere, Chem. – Eur. J., 2015, 21, 10537–10547.
(
1
2
ments indicated not only that the prepared heterogeneous
catalyst retains the hexagonal mesoporous architecture of
SBA-15 but also that Ru-1 was mostly fixed inside the channels
of SBA-15. Compared with Ru-2 alone, the catalyst SBA-15–apts–
Ru-1 revealed an enhanced catalytic performance and improved
turnover frequency for the solvent-free aerobic oxidation of
n-tetradecane under the environmentally-friendly conditions.
Furthermore, a series of experiments was carried out to obtain
the optimal experimental conditions for the catalytic reaction.
Altogether these observations suggest that the catalyst SBA-15–
apts–Ru-1 has potential for application in the solvent-free aerobic
catalytic oxidation of n-tetradecane.
15 C. Yvon, A. J. Surman, M. Hutin, J. Alex, B. O. Smith,
D. L. Long and L. Cronin, Angew. Chem., Int. Ed., 2014, 53,
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1
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7 H. N. Kim, T. W. Kim, K. H. Choi, I. Y. Kim, Y. R. Kim and
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1
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