Rhenium oxide-modified H3PW12O40/TiO2 catalysts for selective oxidation of dimethyl ether to dimethoxy dimethyl ether

Article abstract of DOI:10.1039/c4gc01373e

An efficient rhenium oxide-modified H₃PW₁₂O₄₀/TiO₂ catalyst is found for a new synthesis of dimethoxy dimethyl ether from dimethyl ether oxidation. The effects of Re loading, H₃PW₁₂O₄₀ content and different feedstocks on the performance of Re-H₃PW₁₂O₄₀/TiO₂ were investigated. The results showed that DMM₂ selectivity was significantly improved up to 60.0%, with 15.6% of DME conversion over 5% Re-20% H₃PW₁₂O₄₀/TiO₂. NH₃-TPD, NH₃-IR, Raman spectra, H₂-TPR, XPS and TEM were used to extensively characterize the structure and surface properties of the catalysts. The introduction of H₃PW₁₂O₄₀ significantly affected the structure and reducibility of surface rhenium oxide species, in addition to increasing the acidity of the catalyst. The increased number of Lewis acid sites and weak acid sites and the optimal ratio of Re⁴⁺/Re⁷⁺ of Re-H₃PW₁₂O₄₀/TiO₂ were favorable for the formation of DMM₂ from DME oxidation. The possible reaction pathway of DME oxidation to DMM₂ was proposed.

Full text of DOI:10.1039/c4gc01373e

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Rhenium oxide-modified H PW O /TiO
3
12 40
2
catalysts for selective oxidation of dimethyl ether
Cite this: DOI: 10.1039/c4gc01373e
to dimethoxy dimethyl ether†
a
a
a,b
a
a
Qingde Zhang, Yisheng Tan,* Guangbo Liu, Junfeng Zhang and Yizhuo Han*
An efficient rhenium oxide-modified H
dimethyl ether from dimethyl ether oxidation. The effects of Re loading, H
feedstocks on the performance of Re–H 40/TiO were investigated. The results showed that DMM
selectivity was significantly improved up to 60.0%, with 15.6% of DME conversion over 5% Re–20%
40/TiO . NH -TPD, NH -IR, Raman spectra, H -TPR, XPS and TEM were used to extensively
characterize the structure and surface properties of the catalysts. The introduction of H 40 signifi-
3
PW12
O
40/TiO
2
catalyst is found for a new synthesis of dimethoxy
3
PW12O40 content and different
3
PW12
O
2
2
H
3
PW12
O
2
3
3
2
3
PW12O
Received 19th July 2014,
Accepted 28th July 2014
cantly affected the structure and reducibility of surface rhenium oxide species, in addition to increasing
the acidity of the catalyst. The increased number of Lewis acid sites and weak acid sites and the optimal
DOI: 10.1039/c4gc01373e
4+
7+
ratio of Re /Re of Re–H
3
PW12
O
40/TiO
2
2
were favorable for the formation of DMM from DME oxidation.
www.rsc.org/greenchem
The possible reaction pathway of DME oxidation to DMM
2
was proposed.
oxygenates with more carbon could be further synthesized as
fuel and diesel oil additive via directional recombination of
1
. Introduction
Energy and environment have been paid more and more atten- these basic groups. Therefore, DME direct oxidation to DMM2
tion with the rapid development of the global economy. The is one of most attractive green routes for DME utilization.
need for sustainable and environmentally friendly resources in
Numerous efforts have been made to investigate the cataly-
1
9–13
all areas has become extremely urgent. Dimethyl ether (DME, tic conversion of DME.
CH –O–CH ), which can be synthesized via a one-step process version routes has received extensive attention because of its
DME oxidation as one of the con-
3
3
at low cost from syngas generated from coal, biomass and advantages of simplicity and feasibility. However, DME oxi-
natural gas has been known as a clean fuel and a potential dation reactions are mostly carried out at above 553 K, along
2
–4
non-petroleum oil route chemical synthesis material.
with the generation of COx by-products due to the high stabi-
Because of its special structure of C–O–C and high cetane lity of the DME molecule. The activation of DME molecules at
5
number (55–60), DME is a very clean substitute for diesel fuel.
However, it is not possible to directly blend DME with diesel have long been the challenges in the conversion of DME.
due to the low boiling point of DME (246.3 K). Dimethoxy DME can be activated and further oxidized to synthesize
dimethyl ether (DMM , CH OCH OCH OCH ) is considered to some important chemicals, such as dimethoxymethane
a lower temperature and highly selective conversion of DME
2
3
2
2
3
be a promising diesel oil additive due to a structure similar to (DMM), 1,2-dimethoxyethane (DMET), HCHO, ethanol, methyl
that of C–O–C–O–C–O–C with DME and high content of formate (MF), over acid, base or redox sites of different func-
6
14–21
oxygen and high cetane number (41). Currently, DMM2 is tional catalysts.
Liu et al. have reported the synthesis of
mainly synthesized via the condensation of methanol and DMM from the oxidation of DME and methanol using
7
,8
14
trioxymethylene over acid catalysts. The C–O and C–H bonds
3+n n
H V Mo12−nPO40 as the catalyst. Our group has investigated
of DME molecule can be selectively broken to form CH3, DME direct oxidation to DMM over Mn-modified H SiW O
4
12 40
2
2,23
CH
3
O and CH
3
OCH
2
groups over the proper catalyst, and the and Cs-modified H
3
PW12
O
40 catalysts.
The formation of
the C–O–C–O–C chain of DMM molecule from the C–O–C
chain of the DME molecule has been achieved. Further en-
hancing the chain growth of C–O can increase the oxygen content
in oxygenates, which could further improve the thermal
efficiency of diesel and reduce the emission of the particulate
matters. However, to obtain longer chains of C–O–C–O–C by
means of DME oxidation is a very challenging work.
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry,
Chinese Academy of Sciences, Taiyuan 030001, China. E-mail: hanyz@sxicc.ac.cn,
tan@sxicc.ac.cn; Fax: +86-351-4044287; Tel: +86-351-4049747
b
University of Chinese Academy of Sciences, Beijing, 100049, China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c4gc01373e
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Rhenium oxide has been widely used in some reactions due respectively. The amount of Re in the catalyst refers to the
to its unique oxidation performance and acidic properties. Re– amount of Re O .
2
7
Sb–O catalysts exhibited good performance for the selective
2
4
2 7
oxidation of isobutane and isobutylene. The Re O catalyst
2
.2. Catalytic oxidation of DME
2
5
showed high catalytic activity in olefin metathesis. The Re
oxide/zeolite catalyst was very active in the selective oxidation
of benzene to phenol. Methyltrioxorhenium (CH ReO ) is
3 3
considered to be a very efficient catalyst for epoxidations, C–H
and Si–H oxidations and also olefin metathesis and aldehyde
The catalytic oxidation of DME was carried out in a conti-
nuous-flow type fixed-bed reactor. The catalyst (1 mL,
2
6
20–40 mesh) was diluted with ground quartz to prevent over-
heating of the catalyst due to exothermic reaction. The catalyst
was treated in flow of O
2
(30 mL min− ) for 1 h before reaction.
1
2
7–31
olefinations.
Due to sublimation under certain conditions,
Re oxides have not been widely used in selective oxidation
The reactant mixture consisted of DME, DMM and O2 with
ratio of nO –nDME–nDMM = 1.3 : 1.2 : 1. DMM was introduced
into the reactor by bubbling O through a glass saturator filled
2
with DMM (Acros Organics, purity 99.5+%) at 293 K. The reac-
tion products were analyzed by gas chromatography GC-9A
3
2
2
reactions. However, Re-based catalysts demonstrated high
activity and selectivity for the synthesis of DMM by methanol
oxidation. Yuan et al. examined SbRe O and supported ReOx
2
6
catalysts for methanol oxidation to DMM and found that these
catalysts showed high activity and high selectivity for DMM
(
0
Shimadzu Co.) equipped with flame ionization detector (30 m ×
.32 mm, PEG-40M), GC-4000A with thermal conductivity
detector (Porapak T column) and GC-4000A (TDX-01 column)
3
3–35
synthesis from methanol oxidation.
Nikonova et al.
reported that Re oxide-based catalysts prepared using Re
homo- and heterometallic alkoxide complexes as a new
approach showed an increased activity and selectivity for
methanol oxidation to DMM. Chan et al. investigated the
oxygen-modified Re surface for methanol oxidation to DMM.
However, the investigation on the interactions of acidic
additives (such as heteropolyacids) with surface Re species is
not sufficient. The interactions of acidic additives (such as
with thermal conductivity detectors was used to analyze H
CO, CO and CH .
The data of the entire work was calculated based on carbon
balance, and the carbon balances of most experiments were
within 95%–99%.
2
,
2
4
3
6
3
7
2
.3. Catalyst characterization
2
−
3−
SO4 , PO4 , P-containing species, etc.) with surface vanadia
2.3.1. Temperature programmed desorption (TPD). The
species have been previously reported; the vanadium species NH
3
-TPD profiles were obtained in a fixed-bed reactor system
connected with a thermal conductivity detector. The catalyst
3
8,39
are significantly affected by these acidic additives.
In this paper, we report rhenium oxide-modified PW /TiO
−
1
(100 mg) was pretreated at 673 K under N flow (40 mL min
)
1
2
2
2
catalysts and their application in a new route for the synthesis for 2 h and then cooled down to 373 K under N
2
flow. Then,
of DMM from DME oxidation. PW12 was chosen for its NH was introduced into the flow system. The TPD profiles
2
3
−
1
acidity. More importantly, the introduction of PW12 not only were recorded at a temperature rising rate of 5 K min from
increases the total amount of acid sites, but also significantly 373 to 923 K.
affects the structure and reducibility of the surface rhenium
3 3
2.3.2. NH -IR spectra. NH -IR spectra were recorded with a
oxide species. The increased number of weak acid sites, the Bruker Tensor 27 with a MCT detector in the range of
4
+
7+
−1
−1
Lewis acid sites and the optimal ratio of Re /Re of Re–PW12
TiO are favorable for the oxidation of DME to DMM . Thus scans. The catalyst powder was put in an in situ IR cell
far, there have been no reports about the direct oxidation DME equipped with KBr windows. Before adsorption, the catalyst in
/
600–4000 cm , with a resolution of 4 cm and 64 acquisition
2
2
−
1
to DMM
2
over Re-modified PW12/TiO
2
catalysts.
the IR cell was purified at 673 K in N
.5 h. The cell was then cooled down to 373 K. Next, NH
was introduced into the cell, and the desorption was carried
out in N flow at 373 K. After physically adsorbed NH mole-
cules were removed in N flow for 0.5 h, the IR spectra were
collected at 373 K. The catalyst itself was used as the back-
catalysts were prepared by the ground of the spectra.
incipient wetness impregnation method. An aqueous solution 2.3.3. Temperature programmed reduction (TPR). H
of H PW O (Shanghai Chemical Co.) was impregnated in was conducted in a fixed-bed reactor system equipped with a
2
flow (30 ml min ) for
0
3
gas
2
3
2
. Experimental
2
2
.1. Catalyst preparation
Re-modified H 40/TiO
3
PW12
O
2
2
-TPR
3
12 40
2
TiO (Shanghai Chemical Co.) at 298 K for 6 h, then dried over- thermal conductivity detector. The sample (100 mg) was pre-
night at 393 K, and calcined at 673 K for 4 h. An aqueous solu- treated in Ar at 673 K for 0.5 h and then cooled down to 323 K.
tion of ammonium perrhenate (NH ReO , Strem Chemicals, Further, a 10% H –Ar mixed gas was switched on and the
4
4
2
−
1
Inc.) was used to impregnate H
3
PW12
O
2
40/TiO , and the follow- temperature was linearly increased at a rate of 5 K min from
ing procedures were the same as those mentioned above. The 323 K to 923 K.
catalyst was designated as 5% Re–20% PW /TiO . Re/TiO and 2.3.4. Raman spectra. Raman spectra were recorded on a
catalysts were prepared accord- Jobin-Yvon Labram-HR Confocal Laser Micro Raman spectro-
1
2
2
2
other Re-modified PW12/TiO
2
ing to the above procedures. For the catalysts used in this meter with an argon-ion laser at the excitation wavelength of
study, Re and PW12 are referred to Re O and H PW O , 514.5 nm and the resolution of 1 cm− . The Raman spectra
1
2
7
3
12 40
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were obtained in the range 100–1100 cm− under ambient and redox sites. This is because a low Re loading can not offer
conditions. enough redox-active centers to improve the catalyst perform-
.3.5. X-ray photoelectron spectra (XPS). XPS were ance, while a high Re loading may reduce the dispersion of
1
2
measured on a XPS-AXIS Ultra of Kratos Co. by using Mg Kα active Re species and excessively weaken the acidity, which
radiation (hν = 1253.6 eV) with X-ray power of 225 W (15 kV, further destroys the balance of acid sites and redox sites and
1
5 mA).
.3.6. TEM. TEM images were taken on a JEM-2010 Trans-
mission electron microscope.
decreases the catalyst activity.
2
3.2. Effects of PW12 content on the activity of
2
.3.7. Element analysis. The amount of deposited carbon 5% Re–PW /TiO
12 2
on the spent catalysts after reaction was determined using an
Elementar Vario EL Cube analyzer (Germany).
Table 2 shows the selectivity to product and the conversions of
DME and DMM as a function of PW12 content in the 5% Re–
2 2
PW12/TiO . When 10% PW12 was introduced to 5% Re/TiO ,
DMM selectivity is increased to 48.7% and DMM conversion
2
increases as well, but DME conversion clearly decreases.
DMM selectivity increases with increasing PW12 content and
2
passes through a maximum at 20% PW12 and then decreases
with further increasing PW12 content. The decrease in selecti-
3
. Results and discussion
3
.1. Influence of Re loading on the performance of
Re–20% PW12/TiO
2
Table 1 shows the influence of Re loading on the performance vity was owing to the increase of CH
3
OH and HCHO selecti-
of Re–20% PW /TiO . Over unmodified 20% PW /TiO cata- vities, and concurrently, CO and CH selectivities increase as
1
2
2
12
2
4
lyst, DMM selectivity is as low as 7.7%, whereas the selectivity well. This is probably because the C–O bonds in DME mole-
2
to by-product CO is 9.1%, and the selectivities to CH
3
OH and cules are easily broken due to the strong absorption of DME
HCHO are up to 25.7% and 39.6%, respectively. These results over the acid sites of catalyst and CH O and CH species are
3
3
are in agreement with the strong acidity of PW . When 1% Re generated, which leads to the formation of CH OH and CH
4
1
2
3
2 2 3 3
is used to modify PW12/TiO , DMM selectivity is obviously through combining CH and H. CH OH can decompose to CO,
increased to 44.2% and DME conversion shows an evident and CO can be further oxidized to CO if deep oxidation reac-
2
increase as well, along with a decrease in DMM conversion. tions take place. However, in our reaction experiments, CO2
DMM selectivity reaches the highest value of 60.0% when Re was barely detected. This may be because high gas hourly
2
loading is increased to 5%. Above this loading, DMM selecti- space velocity (GHSV), relatively low temperature of the reac-
2
vity decreases slightly to 54.6% at 8% Re. As can be seen from tions and the optimum catalyst performance make it difficult
Table 1, DME conversion increases continuously, while DMM for DME to be overoxidized to CO
2
. MF may be formed
conversion constantly declines with the increase of Re loading. through the further oxidation of CH OCH OH intermediate
3
2
It can be concluded that the optimum Re loading is 5% for formed from the acetalization reaction of CH OH and HCHO
3
1
4
2
the oxidation of DME to DMM due to the balance of acid sites over acid sites.
a
Table 1 Effects of Re loading on the performance of Re–20% PW12/TiO
2
for DME oxidation
Selectivity (C-mol%)
DME
DMM
Catalyst
Conversion (%)
Conversion (%)
DMM
2
CH
3
OH
HCHO
MF
CO
CH
4
CO
2
2
1
3
5
8
0% PW12/TiO
2
1.5
7.4
12.3
15.6
21.1
36.9
23.2
13.8
11.9
8.8
7.7
44.2
51.5
60.0
54.6
25.7
7.2
8.4
7.7
7.5
39.6
20.2
15.4
12.8
13.1
12.3
20.1
19.0
12.4
18.3
9.1
4.9
3.4
4.6
4.3
5.6
3.4
2.3
2.5
2.2
0
0
0
0
0
% Re–20% PW12/TiO
% Re–20% PW12/TiO
% Re–20% PW12/TiO
% Re–20% PW12/TiO
2
2
2
2
a
–nDME–DMM = 1.3 : 1.2 : 1, atmospheric pressure, 513 K, cat.: 1 mL, 3600 h−1
Reaction conditions: nO
2
.
a
Table 2 Effects of PW12 content on the activity of 5% Re–PW12/TiO
2
for DME oxidation to DMM
2
Selectivity (C-mol%)
DME
DMM
Catalyst
Conversion (%)
Conversion (%)
DMM
2
CH
3
OH
HCHO
MF
CO
CH
4
CO
2
5
5
5
5
% Re/TiO
2
25.2
15.8
15.6
15.8
3.2
7.1
11.9
15.0
46.9
48.7
60.0
44.9
10.2
10.1
7.7
15.5
14.4
12.8
16.9
20.7
19.0
12.4
14.2
4.0
5.0
4.6
6.9
2.7
2.8
2.5
3.6
0
0
0
0
% Re–10% PW12/TiO
% Re–20% PW12/TiO
% Re–30% PW12/TiO
2
2
2
13.5
a
Reaction conditions are the same as in Table 1.
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The existence of ReOx in the catalyst is very helpful for the oxidation to DMM
2
has not been clear although DMM
4
2,43
generation of CH O. Two methoxy species can be obtained carbonylation over zeolite catalysts has been reported.
3
from the oxygen in DME and the lattice oxygen in the catalyst According to the reaction results in the present work, we
2
2,40
through dissociating one DME molecule.
2
This can explain propose that DMM may be synthesized from DME oxidation
why the amount of CH is not in excess in the products.
The introduction of PW12 has a more obvious effect on
DMM conversion compared to the conversion of DME. DMM
via DMM.
Based on the reaction results, a possible synthesis route for
from DME oxidation has been demonstrated in
4
2
Additionally, the introduction of PW12 can reduce the loading Scheme 1. It has been found that CH O groups were present
3
amount of Re towards the activity and selectivity (Sel.DMM over the catalyst surface after the adsorption of DMM over 5%
4
2 2
4.2% of 1% Re–20% PW12/TiO versus Sel.DMM 46.9% of 5% Re–20% PW12/TiO (see Fig. SI-1†), which further proved that
Re/TiO ). This indicates that DMM selectivity reaches maximum the middle C–O bond of DMM molecule was broken and that
2
2
4
1
value only when the acid sites and redox sites of the catalyst are DME can be formed over acid sites. Molera et al. investigated
balanced. the gas-phase oxidation of DMM at 491 K and found that
The appropriate acidity of PW12 is necessary for the DME CH OCH OCH can be obtained after O capture of H from
3
2
2
2
4
4
oxidation to DMM
2
.
the CH
3
OCH
2
OCH
3
.
Additionally, Daly and Simmie also
studied the oxidation of DMM at 800–1200 K and 507 kPa and
3
.3. Effects of different feedstocks on the formation of
45
obtained similar conclusions.
Connecting the reaction
OCH OCH group can be
DMM over 5% Re–20% PW12/TiO
2
2
results, it is proposed that the CH
3
2
2
Table 3 shows the effects of different feedstocks on the for- obtained after the cleavage of the terminal C–H bond of DMM
mation of DMM . When DME and O
were fed into the reactor molecule over the redox sites, and the CH O group should be
packed with 5% Re–20% PW /TiO catalyst, a DMM selectivity formed over acid sites. Then, CH OCH OCH OCH may be
group combining CH
large amount of HCHO is yielded and CO selectivity also under the cooperation of the acid sites and the redox sites of
reaches 8.8%. However, when the reactants shift to DMM and the catalyst. When DMM, DME and O were altogether
, DMM conversion is 26.1% and DMM
selectivity reaches admitted to 5% Re–20% PW12/TiO , the existence of DME in
0.0%, but DME selectivity in product is as high as 54.7%. the feedstocks restrained the decomposition of DMM to DME
When DME and DMM were co-fed with O to the reactor, the and further promoted the conversion of DMM to DMM . It is
was significantly improved up to 60.0%. suggested that DMM may be the intermediate in the oxidation
DMM is easily decomposed to DME over a strong acid cata- of DME to DMM
2
2
3
1
2
2
3
2
2
3
of 42.2% is obtained and DMM
2
selectivity is 6.4%, while a synthesized via the CH
3
OCH
2
OCH
2
3
O
2
O
2
2
2
3
2
2
selectivity to DMM
2
.
2
4
1
lyst, while DME can be oxidized to DMM over bifunctional
The effects of oxygen on DME activation and oxidation to
As can be seen DMM (see Table SI-1†) have been investigated. When only
from Table 3, there exists an inter-conversion between DME pure DME was fed into the reactor packed with 5% Re–20%
and DMM over Re–20% PW /TiO . In terms of the reaction PW12/TiO catalyst, only a trace of DMM was found in the
was formed, whereas large amount of
1
4,22
catalysts with acidic and redox properties.
2
1
2
2
2
mechanism of DME oxidation to DMM, Liu and Iglesia have product, no DMM
proposed that DMM can be formed by the acetalization reac-
2
tion of methanol (formed by DME hydrolysis) and HCHO (oxi-
1
4
3
dized by CH OH). In addition to the acetalization reaction of
methanol and HCHO, we suggest that there possibly exists
another synthesis route for DMM mainly synthesized through
CH OCH (generated by O capturing H from DME) combin-
3 2 2
ing CH O (formed from the dissociation of DME) under
3
cooperation between the acid sites and redox sites of the
1
5,22
catalyst.
DME direct oxidation to DMM2 is a novel route for syn-
thesis of DMM . Until now, the reaction mechanism of DME Scheme 1 The possible reaction pathway of DME oxidation to DMM
2
2
.
2 2
Table 3 Effects of different feedstocks on the formation of DMM over 5% Re–20% PW12/TiO
Selectivity (C-mol%)
DME
DMM
Reactant
DME + O
DMM + O
DME + DMM + O
Conversion (%)
Conversion (%)
DMM
DME
DMM
2
CH
3
OH
HCHO
MF
2.9
7.2
12.4
CO
CH
4
CO
2
a
2
3.1
—
15.6
—
26.1
11.9
42.2
—
—
—
54.7
—
3.7
30.0
60.0
6.9
3.2
7.7
33.6
2.7
12.8
8.8
2.2
4.6
1.0
0
2.5
0.9
0
0
b
2
c
2
Reaction conditions: atmospheric pressure, 513 K, cat.: 1 mL, 3600 h−
.3 : 1.2 : 1.
1
.
a
nO
–nDME = 1.3 : 1.2. nO
b
–nDMM = 1.3 : 1. nO
c
2
2
2
–nDME–nDMM =
1
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3
CH OH and HCHO were yielded. This indicates that the for-
mation of DMM and DMM needs more lattice oxygen in the
2
catalyst as compared with the formation of HCHO. While
DMM was used as reactant in the absence of O , only 4.4%
2
selectivity of DMM was obtained; DME and MF were the main
2
products. This is because DMM decomposed to DME over acid
sites, and DMM was oxidized to MF over redox sites of the
1
4,41
catalyst.
species for the oxidation of DME to DMM
an important role in the DME selective oxidation to DMM
Lattice oxygen species work as active oxygen
2
, and O also plays
2
2
.
3
Fig. 2 NH -IR profiles of catalysts.
3
.4. Catalysts characterization
.4.1. NH -TPD. Fig. 1 shows the NH
Re–PW /TiO with different PW content. Re/TiO has only
3
3
3
-TPD profiles of 5%
3.4.2. NH
3
-IR. Fig. 2 shows the NH
3
-IR spectra of 5% Re/
1
2
2
12
2
TiO and 5% Re–PW /TiO with different PW contents. The
2
12
2
12
weak acid sites due to an NH desorption peak at about 425 K.
Two NH desorption peaks at about 560 K and 770 K, corres-
3
ponding to weak acid sites and strong acid sites, respectively,
were observed after the introduction of PW . It can be seen
from Table 4 that the weak acid site number of 5% Re–20%
3
two bands at 1467 cm− and 1602 cm corresponding to
1
−1
4
6,47
Brønsted acid sites and Lewis acid sites, respectively.
apparent that the Re/TiO2 catalyst possesses only Lewis acid
It is
1
2
3
4
sites, which is consistent with the report. Evidently, Brønsted
acid sites appear in addition to Lewis acid sites after the intro-
duction of PW . Raising PW content from 10% to 20%
PW /TiO is largest among Re-modified catalysts. Further
1
2
2
1
2
12
increasing the content of PW12 leads to many more strong acid
sites and fewer weak acid sites. According to the reaction
results, the increased amount of weak acid sites could favor
mainly contributes to a constant increase in the number of
Lewis acid sites and Brønsted acid sites. However, further
increasing PW12 content leads to a decrease in the number of
Lewis acid sites and an increase in the number of Brønsted
acid sites, possibly because of the strong acidity of PW12. This
phenomenon can be confirmed by NH -TPD results. The low
3
temperature peak of the NH -TPD result can be assigned to the
Lewis acid sites. These results also suggest that Lewis acid
sites are more significant compared with the Brønsted acid
the DMM formation from DME oxidation. Due to the catalysts
2
being calcinated at 673 K, the desorption peak at 770 K corres-
ponding to strong acid sites may not completely reflect the
real changes of strong acidity of the catalyst. Therefore, only
the area of NH desorption peaks at about 560 K were inte-
3
grated to compare the changes of weak acid sites after PW12
introduction.
3
2
sites in the formation of DMM from DME oxidation.
3.4.3. Raman spectra. Fig. 3 shows the Raman spectra for
the Re/TiO and Re–20% PW /TiO catalysts with different Re
2
12
2
loadings. The Re/TiO
9
2
catalyst shows a Raman band at
68 cm− ascribed to the vibration of RevO. However, the
1
48
introduction of PW induces a noticeable increase of the
1
2
Raman band to 1007 cm− , which indicates that the introduc-
tion of PW12 greatly influences the structure of surface Re
oxide species.
1
3.4.4.
H
2
-TPR. Fig.
2
4 shows H -TPR profiles of 5%
Re–PW12/TiO
2
with different PW12 content. For the Re/TiO
2
3
Fig. 1 NH -TPD profiles of catalysts.
3
Table 4 Results of NH -TPD integration
Weak acid sites
Area
Catalyst
5
5
5
5
% Re/TiO
2
3601
6667
14 958
9466
% Re–10% PW12/TiO
% Re–20% PW12/TiO
% Re–30% PW12/TiO
2
2
2
2 2
Fig. 3 Raman spectra of Re/TiO and Re–PW12/TiO with different Re
loadings.
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4+
7+
Table 5 Relative amounts of Re and Re species
4
+
7+
Re
Re
Ratio
Catalyst
Area (%)
Area (%)
SRe4+/SRe7+
5
5
5
5
% Re/TiO
2
100
—
% Re–10% PW12/TiO
% Re–20% PW12/TiO
% Re–30% PW12/TiO
2
2
2
53.6
59.4
66.5
46.4
40.6
33.5
1.2
1.5
2.0
4
+
34,36,49
assigned to the Re species.
This suggests a strong
interaction between PW12 and surface Re oxide species, in line
with the results of TPR, Raman spectra and FT-IR. The relative
4
+
7+
amounts of Re and Re species are listed in Table 5. The
4
+
content of Re species in evident enhances, and the ratio of
Fig. 4
H
2
-TPR profiles of Re–PW12/TiO
2
with different PW12 content.
4+
7+
Re /Re increases with the increase in PW12 content, which
indicates that the introduction of PW12 evidently changes the
oxidation state of Re species and markedly facilitates the for-
catalyst, the peaks for H2 consumption around at 623 K are
observed. The introduction of PW12 to Re/TiO results in an
4
+
mation of Re species. Combining the reaction results, the
2
4
+
7+
presence of both Re and Re species and the optimal ratio
evident shift of the reduction peak to a lower temperature,
suggesting that the addition of PW12 can significantly facilitate
the reduction of Re oxide species. Raman spectra and FT-IR
4
+
7+
of Re /Re in the catalysts are further proved to promote the
formation of DMM . After the reaction (see Fig. SI-5 and
Table SI-3†), the content of Re decreases, whereas the
2
7
+
(see Fig. SI-6†) also demonstrate that the structure of surface
4
+
content of Re obviously increases, showing that the redox
Re oxide species has been affected due to the addition of
PW12. When the PW12 content is 20%, the temperature of
reduction peak reaches its lowest value at 495 K, and the inten-
sity of the peak is larger than those of the other two catalysts,
7
+
4+
cycle of Re –Re plays an important role in DME oxidation to
DMM2.
3.4.6. TEM. TEM images show that there exist distinct
differences between the fresh and the 5% Re–20% PW12/TiO
2
2
which suggests that 5% Re–20% PW12/TiO exhibits strong
catalysts used. The surface of the catalyst has been covered by
carbon deposition after the DME oxidation reaction. Besides,
the element analysis of the catalyst used shows that the
amount of deposited carbon in the 5% Re–20% PW12/TiO
2
catalyst used is 0.9 wt% (Fig. 6).
redox ability (see Table SI-2†). The reducibility of Re–PW12
/
TiO2 is increased owing to the interaction of PW12 and the
surface Re species. This result is consistent with the report
3
−
2−
that the introduction of PO
4
and SO
4
affects the reducibil-
3
8
ity of VOx/TS-1.
.4.5. XPS. Fig. 5 shows the Re 4f XPS spectra for 5% Re–
PW12/TiO catalysts with different PW12 content. In contrast,
the 5% Re/TiO catalyst shows a peak at 46.6 eV, which should
be ascribed to the Re 4f7/2 level for the Re species, indicating
that Re species were mainly present on the surface of Re/
For the Re/TiO
2
catalysts, Re species mainly offer redox sites
3
and weak acid sites. NH -TPD and NH -IR show that the intro-
3
3
2
duction of 20% PW12 can significantly increase the number of
weak acid sites and Lewis acid sites. More importantly, XPS
results indicate that the addition of PW12 changes the oxi-
dation state of Re species on the catalyst surface due to the
interaction of Re species and PW12 and makes Re species exist
2
7
+
7
+
3
4,36,49
TiO2.
However, when PW12 is introduced to Re/TiO , the
2
most remarkable difference is the appearance of a peak at 41.8 eV
4
+
7+
in the main forms of Re and Re . H -TPR results also
2
exhibit that the doping of PW12 greatly enhances the redox
Fig. 5 Re 4f XPS spectra for Re–PW12/TiO
2
catalysts with different
2
Fig. 6 TEM images of the 5% Re–20% PW12/TiO catalyst before (a) and
PW12 content.
after (b) the catalytic DME reaction for 2 h.
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properties. Therefore, the higher DMM
attributed to the larger number of Lewis acid sites and weak
acid sites and the increased number of redox sites.
2
selectivity should be
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(
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CH OCH (the redox cycle of Re –Re plays an important role
3
7
+
4+
2
2
in DMM oxidative dehydrogenation to CH
3
OCH
2
OCH
2
). DMM
2
may be further synthesized through combining CH
3
O-
CH OCH and CH O under the cooperation of acid sites and
2
2
3
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3
12 40
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3
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.
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The selective oxidation of DME to DMM
conducted over Re–PW /TiO catalysts. A DMM selectivity of
2
has been successfully
1
1
2
2
2
2
2
1
2
2
2
2
6
0.0% and a DME conversion of 15.6% are obtained over 5%
Re–20% PW12/TiO due to the balance of acid sites and redox
sites. The interaction of Re species and PW12 increases the
total amount of weak acid sites and also enhances the redox
capability of the catalyst owing to the presence of Re and
Re species with the optimal ratio, which benefits the for-
mation of DMM from DME oxidation.
2
4
+
7
+
2
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21373253, 20903114 and 20773154),
Natural Science Foundation of Shanxi Province (No.
2
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