Mesoporous Ta–W Composite Oxides: A Highly Effective and Reusable Acid–Base Catalysts for the Cycloaddition Reaction of Carbon Dioxide with Epoxides

Article abstract of DOI:10.1007/s10562-021-03725-2

Cycloadditions of epoxides and carbon dioxide into corresponding cyclic carbonates were performed over mesoporous Ta–W composite oxides prepared by a modified hydrolytic method. The best yields of styrene carbonate were obtained when the Ta/W mole ratio was 2:1 (labeled as Ta_0.67W_0.33O_s). Under optimal reaction conditions, the conversion of styrene oxide and selectivity of styrene carbonate reached 95 and 97%, respectively. These Ta–W composite oxides have been extensively characterized by several techniques. X-ray diffraction (XRD) patterns and Transmission Electron Microscope (TEM) revealed that Ta₂O₅ was completely dispersed in WO_x. Scanning electron microscopy (SEM) exhibited that the particle size distributions become more and more uniform with increment of tungsten content. CO₂ and NH₃ temperature-programmed desorption (CO₂ and NH₃-TPD) revealed that Ta_0.67W_0.33O_s catalyst had the strongest acid and base strength. X-ray photoelectron spectroscopy (XPS) shows that the strongest acid–base sites of Ta_0.67W_0.33O_s catalyst origin from its highest lattice oxygen concentration and W 4f_5/2 species with Bronsted acidity. We discussed the reaction kinetics and proposed a possible mechanism, indicating the excellent catalytic activity is attributed to the cooperative action of acidic and neighboring basic sites on the catalyst surface. Graphic Abstract: Cycloaddition reactions of carbon dioxide with epoxides into corresponding cyclic carbonates were performed over mesoporous Ta–W composite oxides prepared by a modified hydrolytic method. The best yields for cyclic carbonates were obtained when the Ta/W mole ratio was 2:1 (denoted as Ta_0.67W_0.33O_s). Acid-base synergy, specific surface area and mesoporous structure could be ascribed to the main reasons for the highest catalytic activity of Ta_0.67W_0.33O_s catalyst. Meanwhile, reaction kinetics was discussed and a possible reaction pathway was proposed.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-021-03725-2

Catalysis Letters
https://doi.org/10.1007/s10562-021-03725-2
Mesoporous Ta–W Composite Oxides: A Highly Efective and Reusable
Acid–Base Catalysts for the Cycloaddition Reaction of Carbon Dioxide
with Epoxides
Bingqian Yin1 · Gui Liu1 · Fang Wang2 · Weizhong Mao1 · Shiwen Miao1 · Junhua Liu1,3
Received: 7 April 2021 / Accepted: 2 July 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
Cycloadditions of epoxides and carbon dioxide into corresponding cyclic carbonates were performed over mesoporous
Ta–W composite oxides prepared by a modifed hydrolytic method. The best yields of styrene carbonate were obtained
when the Ta/W mole ratio was 2:1 (labeled as Ta0.67W0.33Os). Under optimal reaction conditions, the conversion of styrene
oxide and selectivity of styrene carbonate reached 95 and 97%, respectively. These Ta–W composite oxides have been exten-
sively characterized by several techniques. X-ray difraction (XRD) patterns and Transmission Electron Microscope (TEM)
revealed that Ta2O5 was completely dispersed in WOx. Scanning electron microscopy (SEM) exhibited that the particle size
distributions become more and more uniform with increment of tungsten content. CO2 and NH3 temperature-programmed
desorption (CO2 and NH3-TPD) revealed that Ta0.67W0.33Os catalyst had the strongest acid and base strength. X-ray photo-
electron spectroscopy (XPS) shows that the strongest acid–base sites of Ta0.67W0.33Os catalyst origin from its highest lattice
oxygen concentration and W 4f5/2 species with Bronsted acidity. We discussed the reaction kinetics and proposed a possible
mechanism, indicating the excellent catalytic activity is attributed to the cooperative action of acidic and neighboring basic
sites on the catalyst surface.
Graphic Abstract
Cycloaddition reactions of carbon dioxide with epoxides into corresponding cyclic carbonates were performed over
mesoporous Ta–W composite oxides prepared by a modifed hydrolytic method. The best yields for cyclic carbonates
were obtained when the Ta/W mole ratio was 2:1 (denoted as Ta0.67W0.33Os). Acid-base synergy, specifc surface area and
Bingqian Yin and Gui Liu have contributed equally to this work.
* Fang Wang
wangfang2012@njtech.edu.cn
* Junhua Liu
liujh2007@aliyun.com
1
College of Chemistry and Materials Science, Nanjing
Normal University, Nanjing 210023, China
2
College of Chemistry and Molecular Engineering, Nanjing
Tech University, Nanjing 211816, China
3
Changzhou Institute of Innovation and Development, Nanjing
Normal University, Changzhou 320411, China
Vol.:(0123456789)
1 3

B. Yin et al.
mesoporous structure could be ascribed to the main reasons for the highest catalytic activity of Ta0.67W0.33Os catalyst. Mean-
while, reaction kinetics was discussed and a possible reaction pathway was proposed.
Keywords Ta–W composite oxides · Carbon dioxide · Epoxides · Cycloaddition reaction · Basic and acidic sites
Mesoporous transition-metal oxides have been exten-
sively studied due to their wide range of applications [9].
Examples of such mesoporous transition-metal oxides
include TiO2, ZrO2, SnO2, Mn2O3, which are used as
photocatalyst [10], catalyst support [11], gas sensors [12]
and oxidation catalyst [13]. Nb-W and Ta–W composite
oxides have been used in the hydrolysis of sucrose and
Friedel–Crafts alkylation as solid acid catalysts [14, 15].
Researchers usually just consider their acidity and never
1 Introduction
The efective conversion of hazardous wastes such as car-
bon dioxide into valuable chemicals can make important
contributions to the protection of the resources and envi-
ronment on the earth. The conversion of carbon dioxide
into cyclic carbonates with various kinds of epoxides is
a promising methodology because of its high economic
values [1, 2]. In addition, there is 100% atomic utilization
for this cycloaddition reaction (Scheme 1). In recent years,
many methods have been developed for the synthesis of
cyclic carbonates, such as noble metal catalysts [3], com-
plex [4], ionic liquids [5], organic bases [6] and co-cata-
lysts [7, 8]. However, these catalyst systems often lead to
the drawbacks of high cost, high toxicity and reusability of
catalyst is difcult. Therefore, it is really desired to explore
efective, non-noble metals and environmentally friendly
heterogeneous catalysts for this cycloaddition reaction.
Scheme 1 The conversion of carbon dioxide into cyclic carbonates
with various kinds of epoxides
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Mesoporous Ta–W Composite Oxides: A Highly Efective and Reusable Acid–Base Catalysts for the…
pay attention to their basicity. However, in this paper, we
have found that the Ta–W composite oxides contain both
acidic and basic properties.
and BJH (Barrett-Joyner-Halenda) pore size distributions
were measured at the temperature of liquid nitrogen using
a micromeritice ASAP 2050 analyzer. SEM (JSM-7600F)
instruments were used to study the morphologies of all the
obtained materials. CO2-TPD and NH3-TPD techniques were
performed on a micromeritics Auto Chem II 2920 appara-
tus. Firstly, 100 mg of catalysts were placed in the quartz
tube reactor, treated in high purity helium with 30 ml/min
at 500 °C for 30 min, then cooled down the temperature to
50 °C. The samples were adsorbed with 10% CO2(NH3)/He
mixed gas for 120 min to saturation at 50 °C, continued to
purge with high purity helium until the baseline is stable,
and the temperature is raised from 30 °C to 600 °C at a rate
of 10 °C /min in the atmosphere of high purity helium gas
fow. ICP-OES analyses were carried out to examine the
actual compositions of Ta and W for the introduced catalysts
and the leaching of Ta and W species during the recycled
experiments using a Varian ICP-OES 720ES equipment.
In this paper, we have synthesized mesoporous Ta–W
composite oxides with diferent mole ratios of Ta/W and
used them in the cycloaddition reaction. The best yields for
cyclic carbonates were achieved when the Ta/W mole ratio
was 2:1 (labeled as Ta0.67W0.33Os). The mesoporous struc-
ture of Ta0.67W0.33Os catalyst can expose more basic and
acidic sites. Compared with other Ta–W composite oxides,
Ta0.67W0.33Os catalyst has the strongest base strength and
largest BET (Brunauer–Emmett–Teller) surface area. The
base strength, specifc surface area and mesoporous struc-
ture could be the main reasons for the highest catalytic
activity of Ta0.67W0.33Os catalyst. As far as we know, this
is the frst time to notice that both the acidic and basic sites
exist in Ta–W composite oxides, and this is also the frst
time to report the cycloaddition reaction of carbon diox-
ide with epoxides into corresponding cyclic carbonates
over mesoporous Ta–W composite oxides. Obviously, the
acid–base synergistic of solid acid catalyst will provide an
efective way to explore related reactions.
2.3 Typical Catalytic Procedure
Catalytic cycloaddition of carbon dioxide to cyclic carbon-
ates were carried out in a 100 ml intermittent reactor of stain-
less steel. Typically, catalyst (50 mg), epoxides (5 mmol),
solvent (5 ml) were added to the above reactor successively.
The reactor was then purged with carbon dioxide to replace
the air in the reactor for 3–5 times, and then maintained the
carbon dioxide pressure of 1 MPa unchanged. The reactor
was heated to 160 °C by immersing in the oil bath, and then
the reaction was carried out for the 10 h with continuous stir-
ring (stirring rate: 700r/min, using the c200 type magneton).
After the reaction, the reactor was placed into the cold water
to stop the reaction immediately. A gas chromatograph (GC
9560) equipped with a SE-54 capillary column was used to
identify the products by authentic samples. For recyclability
tests, the used catalyst was separated by fltration through
Brinell funnel and washed fve times with ethanol, vacuum
drying was then carried out at 80 °C for the recycled cata-
lysts, and the recycled catalysts was placed in the same reac-
tion procedure as mentioned above.
2 Experimental Section
2.1 Preparation of Catalysts
Mesoporous Ta–W composite oxides with diferent Ta/W
mole ratios (2:1, 1:1, 1:2) were prepared from TaCl5 (Shang-
hai Macklin Biochemical Co., Ltd), WCl6 (Shanghai Mack-
lin Biochemical Co., Ltd) and the poly block copolymer
surfactant pluronic P-123 (Sigma-Aldrich Corporation)
was used as a structure-directing agent. These oxides were
synthesized through a modifed hydrolytic method [14]. In
a typical procedure, 1 g p-123 was dissolved into 10 g of
dehydrated n-propyl alcohol, and diferent molar ratios of
TaCl5 and WCl6 (The total moles remain at 6 mmol) were
added under constant stirring. Then the solution was titrated
with 30 mmol of water, the resulting black slurry was stirred
for 24 h at 40 °C, and then the black product was dried for
10 h at 80 °C. The obtained samples were calcined for 5 h
at 500 °C. The acquired catalysts are denoted as TaxW1-xOs
(x refers to the Ta/(Ta+W) mole ratio). Mesoporous pure
TaOx and non-mesoporous pure WOx were also prepared in
accordance with the procedure as described above.
Table 1 Quantitative component data of mesoporous Ta–W compos-
ite oxides obtained from ICP-OES
Sample
Ta/(Ta+W) (%)a
W/(Ta+W) (%)a
2.2 Characterization Technique
Ta0.67W0.33Os
Ta0.5W0.5Os
Ta0.33W0.67Os
0.669
0.495
0.327
0.331
0.505
0.673
The crystal phases of all the samples were characterized
by XRD (D/max 2500/PC) scanning from 5° to 90°. The
specific BET (Brunauer–Emmett–Teller) surface area
aAtomic mole ratio
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B. Yin et al.
believed that the actual compositions of our catalysts are
very similar to what we expected.
Figure 1 shows the XRD patterns of Ta–W composite
oxides and corresponding pure TaOx, WOx. Figure 1 (a)
revealed the existence of an amorphous structure of Ta2O5
for pure TaOx (JCPDS: 54–0514). For pure WOx (Fig. 1
(e)), the intensive and sharp difraction patterns at 2θ=23,
24, 26.5, 28.7, 34, 41.5, 50, 55.6 and 61.3° can be assigned
to crystallized tungsten oxide (WO3) (JCPDS: 20-1324).
Figure 1 (b, c) shows the XRD diffraction patterns of
Ta0.67W0.33Os and Ta0.5W0.5Os, respectively. Interestingly,
the XRD patterns of Ta0.67W0.33Os and Ta0.5W0.5Os com-
posite oxides do not exhibit any difraction of crystallized
tungsten oxide. It is because that the tungsten oxide phase
completely disperses into Ta2O5 due to the formation of a
strong synergistic interaction between TaOx and WOx [15].
Moreover, these two samples only have extensive difraction,
which further proves that WO3 is completely dissolved in the
Ta2O5 lattice, and WO3 and Ta2O5 form WO3-Ta2O5 solid
solution. With the increase of molar ratios of W/Ta, the dif-
fraction patterns of Ta–W composite oxides show the crys-
tallization of tungsten oxide. The crystalline structures of
these materials are completely consistent with those reported
in the literature [16].
Fig. 1 XRD patterns of (a) pure TaOx, (b) Ta0.67W0.33Os, (c)
Ta0.5W0.5Os, (d) Ta0.33W0.67Os and (e) pure WOx
3 Results and Discussions
3.1 Catalyst Characterization
To study the BET surface area and porous nature,
nitrogen adsorption–desorption measurements of all the
samples were recorded and are shown in Fig. 2. Figure 2
The compositions of Ta and W for the mesoporous
Ta0.67W0.33Os, Ta0.5W0.5Os and Ta0.33W0.67Os samples are
listed in Table 1. As can be seen from the Table 1, it is
Fig. 2 A Nitrogen adsorption–desorption profles and B BJH pore size distributions of mesoporous (a) pure TaOx, (b) Ta0.67W0.33Os, (c)
Ta0.5W0.5Os, (d) Ta0.33W0.67Os and (e) pure WOx
Table 2 Structural properties of
Samples
TaOx
Ta0.67W0.33Os
Ta0.5W0.5Os
Ta0.33W0.67Os
WOx
mesoporous Ta–W composite
oxides and non-mesoporous
WOx
SBET/m2·g–1
dpor/nm
126
4
109
5
103
5.6
87
4.6
20
22
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Mesoporous Ta–W Composite Oxides: A Highly Efective and Reusable Acid–Base Catalysts for the…
shows the nitrogen adsorption–desorption isotherms and
the BJH pore size distribution curves of pure TaOx cata-
lysts, Ta–W composite oxides and pure WOx catalysts.
As shown from Fig. 2A, all of the catalysts (a) pure TaOx,
(b) Ta0.67W0.33Os, (c) Ta0.5W0.5Os and (d) Ta0.33W0.67Os
exhibit a characteristic type IV isotherm with a type H1
hysteresis loops at p/p0 = 0.4–0.8, and the correspond-
ing pore size are located in 3–8 nm with a narrow distri-
bution (Fig. 2B (a–d)), which indicate the formation of a
typical mesoporous structure with the BET surface area of
87–126 m2·g−1 (Table 2). As observed by the Fig. 2 (e),
no mesoporous structure was obtained for pure WOx and
the mesoporous structure of Ta0.33W0.67Os (d) was also not
obvious comparing with Ta0.67W0.33Os (b) and Ta0.5W0.5Os
(c) samples, which revealed the formation ability of
mesoporous structure decreased in tungsten-enriched mate-
rials. The BET surface area decreased gradually from 109
(mesoporous Ta0.67W0.33Os) to 20 m2·g−1 (non-mesoporous
WOx) with the increase of tungsten content. The surface
area of Ta0.67W0.33Os material is larger than correspond-
ing Ta0.5W0.5Os and Ta0.33W0.67Os materials. The nitrogen
adsorption–desorption isotherm is reversible when the rela-
tive pressure is around 0.4. The isotherm curves of materials
with pore sizes of about 3 to 8 nm show triangular hysteresis
loops when the relative pressure is higher than but still close
to 0.4. The XRD indicates that the WOx in Ta0.67W0.33Os is
completely dissolved in the TaOx and is uniformly dispersed,
and its hysteresis loop widens signifcantly as the dispersion
uniformity decreases (Fig. 2B (b–d)). During desorption,
it can be expected that the narrower pore segments, which
still contain condensed nitrogen, will prevent the capillary
from evaporating from the wider pore segments, which can-
not enter the gas phase directly, leading to a “single-pore”
blocking efect [17]. This could explain why TaOx, despite
its large specifc surface area, does not work well because its
aperture is too narrow. The mesoporous structure with high
surface area was not the only factor afecting the catalytic
activity of Ta0.67W0.33Os catalyst.
SEM micrographs of TaOx, Ta–W composite oxides, and
WOx were shown in Fig. 3. All the materials exhibit the
morphology of the spherical-type particles, but the particle
size of various materials was still diferent. The samples of
TaOx (Fig. 3a), Ta0.67W0.33Os (Fig. 3b), Ta0.5W0.5Os (Fig. 3c)
and Ta0.33W0.67Os (Fig. 3d) showed an average diameter of
around 70–80 nm. The WOx materials (Fig. 3e) exhibited
the uniform spherical particles with an average diameter of
about 50 nm. Additionally, in the case of Ta–W composite
oxides (Fig. 3b–d), the particle distribution was more and
more uniform with the increment of tungsten content.
The morphology, particle size and crystallinity of the
Ta–W composite oxides as well as single metal oxides are
shown in Fig. 4. It can be seen from Fig. 4a that the lattice
fringe of the (220) crystalline phases (JCPDS: 54-0514)
of the orthorhombic pure TaOx is 0.30 nm. In addition,
TaOx has no obvious lattice difraction fringes and can be
described as an amorphous structure, which is consistent
with the XRD data. Figure 4c shows that orthorhombic
pure WOx is primarily covered by (001) planes ascribed to
Fig. 3 SEM micrographs of a TaOx, b Ta0.67W0.33Os, c Ta0.5W0.5Os, d Ta0.33W0.67Os and e WOx samples
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B. Yin et al.
Fig. 4 TEM micrographs of a TaOx, b Ta0.67W0.33Os, c WOx samples; EDS mapping images d–h of Ta0.67W0.33Os
WO3 (JCPDS: 20-1324). It should be pointed out that the
lattice fringe of Ta0.67W0.33Os (JCPDS: 34-0380) is mainly
enclosed by the (001) crystal surface of tungsten oxide, and
did not observe the lattice fringe of Ta2O5, which is related
to the formation process of WO3-Ta2O5 solid solution, Ta
atoms are inserted into the WO3 lattice to form a homogene-
ous single-phase alloy [18]. All these analyses are consistent
with the XRD data. In addition, the energy-dispersive X-ray
spectroscopy (EDS) of Ta0.67W0.33Os (Fig. 4d–h) shows
that Ta, W, O and C elements can be detected and are well
distributed in the sample. The above results indicate that
Mesoporous Ta–W composite oxides (Ta0.67W0.33Os) was
successfully synthesized by modifed hydrolysis method.
According to previous literatures [19, 20], the cycload-
dition of carbon dioxide with various epoxides originates
from the cooperative action of the acid and basic sites
located in the neighbor on the catalyst surface. The acti-
vation of carbon dioxide by basic sites has a very impor-
tant effect on the catalytic activity and is a necessary
step for this reaction [20, 21], which can be evaluated by
CO2-TPD techniques as shown in Fig. 5A. The CO2-TPD
profles of mesoporous Ta–W composite oxides were simi-
lar (Fig. 5Ab, c and d), which exhibits a main broad peak
at 87 °C, 86 °C and 84 °C for mesoporous Ta0.67W0.33Os
(Fig. 5Ab), Ta0.5W0.5Os (Fig. 5Ac) and Ta0.33W0.67Os
(Fig. 5Ad) samples, respectively. As shown in Table 3, it is
A
B
°C
°C
a
°C
b
c
°C
°C
°C
a
°C
b
°C
c
°C
d
e
d
e
100
200
300
400
500
600
100
200
300
400
500
600
Temperature (°C)
Temperature (°C)
Fig. 5 A CO2-TPD and B NH3-TPD curves of catalysts (a) TaOx, (b) Ta0.67W0.33Os, (c) Ta0.5W0.5Os, (d) Ta0.33W0.67Os and (e) WOx
1 3

Mesoporous Ta–W Composite Oxides: A Highly Efective and Reusable Acid–Base Catalysts for the…
Table 3 Distribution of basic
Sample
Weak sites
Moderate sites
(CO2/mmol g−1
Strong sites
Total alkalinity
(CO2/mmol g−1
)
sites in the catalyst
(CO2/mmol g−1
)
)
(CO2/mmol g−1
)
TaOx
WOx
Ta0.67W0.33Os
Ta0.5W0.5Os
Ta0.33W0.67Os
0.11
0.03
0.16
0.15
0.14
0.09
0.04
0.05
0.03
0.04
0.02
0.02
0.06
0.06
0.03
0.22
0.09
0.27
0.24
0.21
Determined by CO2-TPD
obvious that the base strength of mesoporous Ta0.67W0.33Os
catalyst is the strongest comparing with other mesoporous
Ta–W composite oxides, mesoporous pure TaOx and non-
mesoporous WOx. Moreover, the mesoporous pure TaOx
has frst peak at 95 °C is related to weak basic sites and
the broad desorption peak at about 272 °C corresponds to
moderate basic sites. Additionally, the CO2-TPD profle of
non-mesoporous WOx (Fig. 5Ae) is almost regarded as a
straight line comparing with other three Ta–W composite
oxides and mesoporous pure TaOx (Fig. 5Aa, b, c and d),
suggesting the base strength of WOx sample is the weakest
among all the materials.
cooperative action of acid and basic sites. When WOx dis-
perses completely within the TaOx at the Ta/W molar ratio
of 2, the total alkalinity is at a maximum, indicating the W
content plays an important role in tuning the acid–base prop-
erties of the TaxW1-xOs catalysts (Table 3). Form Tables 3,
4, we can see it clearly that Ta0.67W0.33Os catalyst has the
highest acid and base site. These results indicate that both
weak acid and base sites contribute to catalytic performance
in the synthesis of cyclic carbonates from carbon dioxide
and styrene oxide.
The X-ray photoelectron spectroscopy (XPS) was used
to characterize the surface chemical valences of Ta–W
composite oxides with diferent doping concentration in
Fig. 6. For Ta 4f, all samples showed a broad overlapped
doublet in the binding energy (BE) region between 22 and
32 eV, as well as two satellite peaks at 34–36 and 37–40 eV,
respectively (Fig. 6A). We can see in Fig. 6A, the XPS sig-
nal of all TaOx sample is very similar, and the intensity of
peaks increased with increase of molar ratio of Ta/W. For
Ta0.67W0.33Os catalyst, the binding energies of Ta 4f lev-
els for the product appears at 28.29 and 26.38 eV for Ta
4f5/2, and 4f7/2 respectively, indicating that tantalum exists
as Ta5+ in these catalysts and it is same pattern as for the
bulk Ta2O5 [22]. Figure 6C depicts the W 4f spectrum of
single metal tungsten oxide and mesoporous Ta–W compos-
ite oxides. Obviously, the main peaks of W 4f7/2 and W 4f5/2
are 35 and 37 eV, respectively, corresponding to the W6+.
In addition, for pristine WO3, a symmetrical doublets of W
4f7/2 were centered at 35.4 eV with the spin orbit splitting
of the doublet of 2.2 eV (Fig. 6C), corresponding to W6+
ions. For Ta–W catalyst, a small amount of W loading is
The acid properties can be evaluated by NH3-TPD tech-
niques. The NH3-TPD profles of mesoporous Ta–W com-
posite oxides, mesoporous pure TaOx and non-mesoporous
WOx are shown in Fig. 5B. As observed by the NH3-TPD
results, the shape of NH3-TPD profles and desorption tem-
peratures of mesoporous Ta–W composite oxides were
similar (Fig. 5Bb, c and d), which indicates that the acid
strength of mesoporous Ta–W composite oxides was lit-
tle in diference. Similar to the CO2-TPD profle of non-
mesoporous WOx, the NH3-TPD profle of non-mesoporous
WOx (Fig. 5Be) can also be regarded as a straight line com-
paring with other three Ta–W composite oxides (Fig. 5Bb, c
and d) and mesoporous pure TaOx (Fig. 5Ba), suggesting the
acid strength of WOx sample is also the weakest among all
the catalysts. The results of CO2-TPD and NH3-TPD confrm
that the addition of Ta into WOx can simultaneously improve
the acidity and basicity of the catalyst. This increased acid-
ity and basicity result in the synergistic interaction of TaOx
and WOx, and such interaction can further enhance the
Table 4 Distribution of acidic
Sample
Weak sites
Moderate sites
(NH3/mmol g−1
Strong sites
Total acidity
sites in the catalyst
(NH3/mmol g−1
)
)
(NH3/mmol g−1
)
(NH3/mmol g−1
)
TaOx
WOx
Ta0.67W0.33Os
Ta0.5W0.5Os
Ta0.33W0.67Os
0.30
0.03
0.42
0.35
0.37
0.05
0.05
0.03
0.05
0
0
0.35
0.10
0.47
0.42
0.41
0.02
0.02
0.02
0.04
Determined by NH3-TPD
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B. Yin et al.
C
a
W4f
A
a
B
a
Ta4f
O1s
531.55eV
35.58eV
530.22eV
26.01eV
37.72eV
27.90eV
532.44eV
b
530.47eV
b
b
531.88eV
532.63eV
26.38eV
28.29eV
35.97eV
38.04eV
c
530.50eV
531.77eV
532.39eV
c
c
26.42eV
38.00eV
35.92eV
28.23eV
d
530.35eV
531.23eV
532.12eV
d
d
35.92eV
e
530.19eV
38.03eV
26.41eV
531.29eV
532.16eV
28.29eV
40
39
38
37
36
35
34
33
32
30
28
26
24
22
538
536
534
532
530
528
526
Binding energy(eV)
Binding energy(eV)
Binding energy(eV)
Fig. 6 X-ray photoelectron spectra of A Ta 4f for catalysts (a) TaOx,
(b) Ta0.67W0.33Os, (c) Ta0.5W0.5Os, (d) Ta0.33W0.67Os; B O 1 s for cata-
lysts (a) TaOx, (b) Ta0.67W0.33Os, (c) Ta0.5W0.5Os, (d) Ta0.33W0.67Os
and (e) WOx; C W 4f for catalysts (a) WOx, (b) Ta0.67W0.33Os, (c)
Ta0.5W0.5Os, (d) Ta0.33W0.67Os
required to appear strong Bronsted acid sites. In principle,
one type of species that will be favored during the synthe-
sis of tungsten oxide is Ta-containing heteropolytungstate,
which involves a closer interaction between the Ta and W
phases [23]. Scheithauer et al. [24] did proposed the forma-
tion of this type of substance because it produces a strong
Bronsted acidity. The tungsten oxide with Brønsted acid
sites in TaOx-WOx catalysts can catalyze many reactions,
shows that the Brønsted acid sites occurred on the tung-
sten oxide by the interactions of WOx with hydrogen atoms
formed a new species [25, 26]. Several W atoms could be
fxed by electron density to form active Hδ+(WO3) δ− sites on
Table 5 XPS results of W 4f for mesoporous Ta–W composite oxides, mesoporous pure TaOx and non-mesoporous WOx catalysts
Sample
W 4f7/2
W 4f5/2
W 4f5/2/W 4f7/2 +W 4f5/2
WOx
19.44
11.19
14.46
14.91
80.56
88.81
85.54
85.09
80.56
88.81
85.54
85.09
Ta0.67W0.33Os
Ta0.5W0.5Os
Ta0.33W0.67Os
1 3

Mesoporous Ta–W Composite Oxides: A Highly Efective and Reusable Acid–Base Catalysts for the…
Table 6 XPS results of O 1 s for mesoporous Ta–W composite oxides, mesoporous pure TaOx and non-mesoporous WOx catalysts
Sample
Olatt
Osur
Oads
Olatt/Osur +Oads
TaOx
WOx
Ta0.67W0.33Os
Ta0.5W0.5Os
Ta0.33W0.67Os
63.37
70.71
74.36
64.72
71.88
32.73
27.00
10.92
32.57
25.95
3.90
1.73
2.41
2.90
1.83
2.56
2.30
14.73
2.71
2.17
polytungstate domains [27]. By comparing the ftting curve
area, the surface W 4f5/2/ (W 4f7/2 + W 4f5/2) molar ratio
of Ta–W composite metal oxide was calculated. It can be
clearly obtained that the W 4f5/2/ (W 4f7/2 +W 4f5/2) molar
ratios of Ta0.67W0.33Os is 88% (Table 5), the surface atomic
concentrations of the W 4f5/2 for this catalyst is considerably
higher compared with other two Ta–W composite oxides,
indicating the existence of stronger synergistic interaction
between pure TaOx and WOx oxides.
Table 7 The optimization of reaction conditions for the insertion of carbon dioxide into styrene oxide
O
O
Certain conditions
+
O
CO2
O
Ph
Ph
Entry
Catalyst
Solvent
Press. (Mpa)
Temp. (°C)
Conv. (%)a
Sel. (%)a
1.
2.
3.
4.
5.
6.
7.
8.
TaOx
WOx
DMFb
DMF
DMF
DMF
DMF
DMF
Acetonitrile
Toluene
Dichloroethane
DMSOc
DMF
1
1
1
1
1
1
1
1
1
1
0.5
2
3
1
1
1
1
1
160
160
160
160
160
160
160
160
160
160
160
160
160
130
150
170
180
160
21
15
88
95
89
85
5
98
97
92
97
95
96
94
93
95
98
95
94
96
94
95
98
93
98
Ta0.75W0.25Os
Ta0.67W0.33Os
Ta0.5W0.5Os
Ta0.33W0.67Os
Ta0.67W0.33Os
Ta0.67W0.33Os
Ta0.67W0.33Os
Ta0.67W0.33Os
Ta0.67W0.33Os
Ta0.67W0.33Os
Ta0.67W0.33Os
Ta0.67W0.33Os
Ta0.67W0.33Os
Ta0.67W0.33Os
Ta0.67W0.33Os
Ta2O5 and WO3
3
6
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
80
90
83
85
47
73
96
97
30
DMF
DMF
DMF
DMF
DMF
DMF
DMFd
Reaction conditions: catalyst 0.05 g, styrene oxide 5 mmol, solvent 5 ml and 10 h
aDetermined by GC
bDMF refers to the N, N-dimethylformamide
cDMSO refers to the dimethyl sulfoxide
dUsing physical mixture of Ta2O5 and WO3 (Ta/W mole ratio was 2) instead of the above Ta0.67W0.33Os catalyst
1 3

B. Yin et al.
The type of oxygen species on the catalyst surface can be
measured by O 1 s XPS (Fig. 6B). The peak of 530.2 eV is
assigned to lattice oxygen (Olatt), while the peak at 532.3 eV
increasing temperature on the conversion was not obvious
when the temperature exceeded 160 °C. The conversion
rate will decrease rapidly with 30% conversion when the
mesoporous Ta0.67W0.33Os composite catalyst was replaced
by the physical mixture of Ta2O5 and WO3 under the opti-
mized reaction conditions (Table 7, entry 18). Moreover,
whether it is to magnify preparation of catalyst or increase
the concentration of styrene oxide, there is little diference
in catalytic performance (Table S1).
is associated with surface-adsorbed oxygen species (Oads
)
[28]. The values of Olatt/(Oads + Osur) are presented in
Table 6. Specifcally, the ratio of Olatt/(Osur +Oads) in pris-
tine mesoporous Ta–W composite oxides are approximately
2.90, 2.56 and 1.83% (Table 6), respectively, manifesting
that diferent amount of oxygen vacancies are generated
through tuning the diferent molar ratio of Ta/W. Some
researchers [29, 30] have shown that oxygen vacancies have
the advantage of improving the separation efciency of elec-
trons and holes through theoretical calculations and experi-
mental results, which is conducive to subsequent chemistry
reactions. Next, oxygen vacancy as reactive centers also can
optimize the adsorption and activation of reactants, reduce
the recombination of electron–hole pairs, and improve the
catalytic performance of materials. We can see that the
amount of Olatt species in Ta0.67W0.33Os catalyst is higher
than the other two ones. Typically, a higher Olatt concentra-
tion is considered benefcial for the improvement of cata-
lytic performance, just as the lattice oxygen can efectively
provide basic sites, so Ta0.67W0.33Os has the strongest basic
strength, which is good for the adsorption of styrene oxide
[31].
3.3 Catalytic Activity Measurements
To further understand the catalytic activity of mesoporous
Ta0.67W0.33Os catalyst, the cycloaddition reaction of carbon
dioxide and various kinds of epoxides over mesoporous
Ta0.67W0.33Os catalyst was carried out, as shown in Table 8.
Various kinds of epoxides were successfully coupled with
carbon dioxide to generate corresponding cyclic carbonates
under the optimized reaction conditions, while cyclopen-
tene oxide and cyclohexene oxide need more time (Table 8,
entries 4–5), which could be due to the existence of larger
steric hindrance in their structure. In a typical reaction, 10 h
for styrene oxide and 3, 4-epoxy-1-butene exhibited 92 and
82% yield respectively (Table 8, entries 1 and 7). When
2-hexyloxirane and 1,2-epoxypropane were used as sub-
strates, the corresponding cyclic carbonates were achieved
with 85 and 80% yield at 18 h and 12 h, respectively
(Table 8, entries 2–3). While epichlorohydrin with electron-
withdrawing group exhibited 92% yield to corresponding
cyclic carbonate at 5 h (Table 8, entry 6), this is the highest
yield to cyclic carbonate comparing with other substrates,
which can be attributed to the electron-withdrawing group
are benefcial to the nucleophilic attack of carbonate species
on epoxides, as discussed in the mechanism investigation.
The reusability of catalysts is of great signifcance. The
mesoporous Ta0.67W0.33Os catalyst was used sequentially
fve times for this cycloaddition reaction to investigate its
recyclability, and it was also found that there was no sig-
nifcant decrease in catalytic activity (Table 9). ICP analy-
sis indicated that the amount of Ta and W species leached
from the initial catalyst after fve cycles was negligible. In
addition, we characterized the recycled catalyst and found
that there is not much deviation from previous measure-
ment, and the acid–base test shows that acid and basic sites
of the recycled catalyst and the fresh catalyst are almost
the same (Figure S1, Table S2, Table S3).
3.2 Optimization of Reaction Conditions
The cycloaddition of carbon dioxide with styrene oxide was
used as a probe reaction to determine the optimum reaction
conditions (Table 7). Firstly, the cycloaddition reaction was
carried out over TaOx, WOx, Ta0.75W0.25Os, Ta0.67W0.33Os,
Ta0.5W0.5Os and Ta0.33W0.67Os catalysts using DMF solvent
under 1Mpa of carbon dioxide, and the reaction temperature
was 160 °C (Table 7, entries 1–6). In the case of little dif-
ference in selectivity, the Ta0.67W0.33Os catalyst exhibited
the highest conversion (95%) comparing with Ta0.75W0.25Os
(88%), Ta0.5W0.5Os (89%) and Ta0.33W0.67Os (85%) sam-
ples (Table 7, entry 3–6). Additionally, both pure TaOx and
WOx exhibited 21 and 15% conversion respectively (Table 7,
entries 1 and 2), which indicates poor catalytic activity com-
paring with corresponding Ta–W composite oxides. Then
the efects of diferent solvents were also investigated for
this cycloaddition reaction using Ta0.67W0.33Os as cata-
lyst (Table 7, entries 7–10), DMF could be acted as a best
solvent, there were no more than 80% conversion for the
rest of solvents. Through investigating the efect of pres-
sure (Table 7, entries 11–13), 1Mpa of carbon dioxide was
emerged as the best reaction pressure, the conversion rate of
other pressure was no more than 90%. As observed by the
efects of temperature (Table 7, entries 14–17), high tem-
perature is benefcial to this reaction. We selected 160 °C
as optimal reaction temperature, just because the efect of
3.4 Kinetic Study
Reaction kinetics for cyclic carbonate synthesis performed
in a batch reactor suggested that rate of reaction is con-
trolled by the rate of CO2 mass transfer from the gaseous
1 3

Mesoporous Ta–W Composite Oxides: A Highly Efective and Reusable Acid–Base Catalysts for the…
O
Table 8 Cycloaddition
results of carbon dioxide and
O
Mesoporous Ta0.67W0.33Os catalyst
1Mpa CO2, DMF, 160°C.
various kinds of epoxides over
mesoporous Ta0.67W0.33Os
catalyst
+
CO2
O
O
R
R
Entry
1
Substrate
O
Product
O
Time (h)
10
Conv. (%) a
Yield (%) a
92
95
Ph
O
O
Ph
O
O
2
18
85
85
(CH2)5
O
O
(CH2)5
O
O
3
4
12
22
88
80
80
78
O
O
O
O
O
O
O
O
5
22
75
75
O
O
O
O
O
Cl
6
7
5
100
90
92
82
O
O
Cl
O
O
O
10
Reaction conditions: Ta0.67W0.33Os catalyst 0.05 g, substrate 5 mmol, DMF 5 ml, carbon dioxide pressure
1Mpa and 160 °C
aDetermined by GC
phase to the reaction mixture [32, 33]. In order to study
1 3

B. Yin et al.
Table 9 Recyclability of the mesoporous Ta0.67W0.33Os catalyst for
the insertion of carbon dioxide into styrene oxide
Cycle
1st
91
2nd
91
3rd
91
4th
90
5th
90
Yield (%)a
Reaction conditions: mesoporous Ta0.67W0.33Os catalyst 0.05 g,
styrene oxide 5 mmol, DMF 5 ml, carbon dioxide pressure 1 Mpa,
160 °C and 10 h
aDetermined by GC
the synthesis mechanism and industrial performance of
synthetic styrene carbonate using mesoporous Ta–W
composite metal oxide as catalyst, the reaction kinetics
was studied in detail. As shown in Scheme 2, under the
mesoporous Ta–W composite metal oxide catalyst system,
the overall rate equation for the synthesis of styrene car-
bonate from styrene oxide (SO) and CO2 is Eq. (1):
Fig. 7 Arrhenius plots for styrene carbonate synthesis in the presence
of mesoporous Ta–W composite metal oxide as catalyst
−rso = k[SO]a[CO2]b[Ta − W]c
(1)
−rso = −d[SO]∕dt
(5)
−rso = kapp[SO]a
Suppose a frst-order equation set, and integrate Eq. (5)
(2)
over time to obtain Eq. (6):
kapp = k[CO2]b[Ta − W]c
− ln[SO] = kapp
t
(3)
(6)
We used the Arrhenius Eqs. (7) and (8) to determine
the activation energy of styrene carbonate synthesis in the
entire temperature range (130–180 °C). Where k is the
rate constant, Ea activation energy (kJ mol−1), R the gas
constant (8.3143 J mol−1 k−1) and T is the temperature(k):
Take the natural logarithms on both sides of formula (3)
to obtain formula (4), which can be used to determine the
reaction order (b–c):
ln kapp = ln k + b ln[CO2] + c ln[Ta − W]
(4)
(
)
Ea
kapp = A. exp −
(7)
where [SO], [CO2] and [Ta–W] are the concentrations of
styrene oxide, carbon dioxide and mesoporous Ta–W com-
posite metal oxide catalyst respectively and a–c are the
orders of reaction with respect [SO], [CO2] and [Ta–W].
The assumption of constant CO2 concentration in the study
of cyclic carbonate synthesis kinetics has also been reported
before [34–36]. Assuming that the cycloaddition reaction is
an elementary frst-order reaction [37], the fnal rate equa-
tion can be simplifed as:
RT
Ea
ln kapp = ln A −
(8)
RT
ln k is equal to the reciprocal of absolute temperature (1/T).
From the slope of the straight line, the activation energy
is calculated to be 52.58 kJ mol−1. The ftted value of R is
approximately equal to 1, indicating that the reaction rate
constant has a linear relationship with the concentration of
Scheme 2 The chemical
reaction for styrene carbonate
synthesis from styrene oxide
and CO2
1 3

Mesoporous Ta–W Composite Oxides: A Highly Efective and Reusable Acid–Base Catalysts for the…
Scheme 3 Mechanism of the cycloaddition reaction of carbon dioxide with epoxides catalyzed by mesoporous Ta0.67W0.33Os catalyst
styrene oxide, so the reaction is a frst-order reaction, as
can be demonstrated by the fact that the catalytic activity
of the mesoporous Ta0.67W0.33Os composite oxide catalyst
was signifcantly higher than that of physical mixture of the
Ta2O5 and WO3 (Table 7, entry 18).
shown in Fig. 7 [38].
3.5 Mechanism Investigation
Enlightened by the cycloaddition reaction mechanisms
of carbon dioxide and epoxides was proposed by Kaneda
[20]. Here we propose a possible reaction mechanism for
this cycloaddition reaction over mesoporous Ta0.67W0.33Os
catalyst and illustrate them in Scheme 3. Many studies have
shown that epoxides and carbon dioxide are activated by
acid and basic sites respectively in cycloaddition reactions
[39–41]. Firstly, the carbon dioxide was converted to the
carbonate species by adsorbing to the basic sites, and then
the epoxides are coordinated on the adjacent acid sites on
the mesoporous Ta0.67W0.33Os catalyst surface. The carbon-
ate species are coordinated to the epoxides and then the
coordination species are activated for ring-opening by a
nucleophilic attack, which leads to the formation of an oxy-
gen anion. The corresponding cyclic carbonate is generated
through subsequent ring-closure. At last, the resulting cyclic
carbonate is desorbed from the catalyst surface to regenerate
mesoporous Ta0.67W0.33Os catalyst. This mechanism dem-
onstrates that the catalytic activity of Ta0.67W0.33Os catalyst
originates from the cooperative action of both the acidic
and neighboring basic sites on the catalyst surface, which
4 Conclusions
In conclusion, we have prepared a series of mesoporous
Ta–W composite oxides, mesoporous pure TaOx and non-
mesoporous WOx through a modifed hydrolytic method.
These materials were used in the cycloaddition reaction of
carbon dioxide and epoxide without any co-catalysts, and
some favorable yields are obtained. The Ta–W compos-
ite oxides with the Ta/W mole ratio of 2:1 (Ta0.67W0.33Os)
exhibited best catalytic activity. Under optimal reaction
conditions, carbon dioxide was added to various kinds of
epoxides with high yields over mesoporous Ta0.67W0.33Os
catalyst. This is the frst time to found that the Ta–W com-
posite oxides have both acidic and basic sites, and this is
also the frst time to report this kind of cycloaddition reac-
tion over mesoporous Ta–W composite oxides. The higher
concentration of lattice oxygen (basic sites) and W 4f5/2
with Bronsted acidity improves catalytic performance of
this reaction. The cooperative action of acid (activate the
1 3

B. Yin et al.
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(2016) A diferent reaction pathway for the reduction of aromatic
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found after the mesoporous Ta0.67W0.33Os catalyst was used
for fve consecutive times. This paper provides an efective
and environmentally friendly acid–base catalytic system for
the synthesis of cyclic carbonates in the presence of carbon
dioxide and epoxides.
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Natural Science Foundation of China (21703101, 21303085) and the
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