Molybdenum containing cage like mesoporous KIT-5 for enhanced catalytic conversion of 1-butene and ethylene to propene

Article abstract of DOI:10.1016/j.molcata.2016.02.019

Molybdenum containing three-dimensional (3D) cage-like mesoporous KIT-5 (Mo-KIT-5) materials with various Si/Mo ratios was successfully prepared by one-pot hydrothermal synthesis. The obtained materials were characterized by various techniques, such as XRD, N₂ physisorption, TEM, FTIR, UV-DRS, XPS, and NH₃-TPD. Characterization results revealed that molybdenum species could be finely dispersed in the framework of KIT-5 without disturbing long-range ordering at Si/Mo ratios higher than 15, while collapse of mesoporous structure and presence of bulk MoO₃ was observed for Mo-KIT-5-5 sample with a Si/Mo ratio of 5. Mo-contained materials were investigated in catalytic conversion of 1-butene and ethylene to propene. Among all of Mo-KIT-5 materials studied, Mo-KIT-5-40 with Si/Mo ratio of 40 exhibited best catalytic performance caused by the fact that it processed superior textural properties, substantial active Mo species, and appropriate amount of acidic sites. Further decreasing the Si/Mo ratio resulted in declined activity due to the presence of extraframework Mo species. It was found that doped Mo-KIT-5-40 showed better catalytic activity than control supported Mo/KIT-5-40 and Mo/SiO₂-40 catalysts with same Mo loading due to the highly dispersed active Mo species on the mesoporous framework. Moreover, Mo-KIT-5 was also superior to other Mo containing mesoporous materials, such as Mo-SBA-15 with two-dimensional pores and Mo-KIT-6 with three-dimensional cylindrical pores, implying the advantage of this kind of ordered 3D cage-like mesoporous materials in this reaction.

Full text of DOI:10.1016/j.molcata.2016.02.019

Journal of Molecular Catalysis A: Chemical 416 (2016) 39–46
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Molybdenum containing cage like mesoporous KIT-5 for enhanced
catalytic conversion of 1-butene and ethylene to propene
Kai Taoa, Qingxiang Mad, Noritatsu Tsubakic, Shenghu Zhoub,∗, Lei Hana,∗
a Institute of Inorganic Materials, School of Materials Science & Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, China
b School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
c Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
d State Key Laboratory Cultivation Base of Natural Gas Conversion, College of Chemistry & Chemical Engineering, Ningxia University, Yinchuan 750021,
China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 January 2016
Received in revised form 18 February 2016
Accepted 18 February 2016
Available online 22 February 2016
Molybdenum containing three-dimensional (3D) cage-like mesoporous KIT-5 (Mo-KIT-5) materials with
various Si/Mo ratios was successfully prepared by one-pot hydrothermal synthesis. The obtained mate-
rials were characterized by various techniques, such as XRD, N2 physisorption, TEM, FTIR, UV-DRS, XPS,
and NH3-TPD. Characterization results revealed that molybdenum species could be finely dispersed in
the framework of KIT-5 without disturbing long-range ordering at Si/Mo ratios higher than 15, while col-
lapse of mesoporous structure and presence of bulk MoO3 was observed for Mo-KIT-5-5 sample with a
Si/Mo ratio of 5. Mo-contained materials were investigated in catalytic conversion of 1-butene and ethy-
lene to propene. Among all of Mo-KIT-5 materials studied, Mo-KIT-5-40 with Si/Mo ratio of 40 exhibited
best catalytic performance caused by the fact that it processed superior textural properties, substantial
active Mo species, and appropriate amount of acidic sites. Further decreasing the Si/Mo ratio resulted in
declined activity due to the presence of extraframework Mo species. It was found that doped Mo-KIT-5-
40 showed better catalytic activity than control supported Mo/KIT-5-40 and Mo/SiO2-40 catalysts with
same Mo loading due to the highly dispersed active Mo species on the mesoporous framework. More-
over, Mo-KIT-5 was also superior to other Mo containing mesoporous materials, such as Mo-SBA-15 with
two-dimensional pores and Mo-KIT-6 with three-dimensional cylindrical pores, implying the advantage
of this kind of ordered 3D cage-like mesoporous materials in this reaction.
Keywords:
Mo-KIT-5
Mesoporous
Isomerization
Metathesis
Propene
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
molybdenum [6] based catalysts. Re based catalysts exhibited out-
standing catalytic performance for metathesis. But, the high price
Propene is a highly demanded basic building block in petro-
chemical industry, which is mainly obtained through steam
cracking of oil based feedstocks, such as ethane, naphtha and
gas oils. Alternatively, methanol to propene (MTP) or methanol
to olefins (MTO) producing propene from non-oil resources has
attracted great attentions [1]. However, a considerable amount of
butene is generated as byproduct. Olefin metathesis, which can
convert C4 byproducts into valuable propene, providing an effi-
cient way of utilizing C4 and increasing the yield of propene, is of
great importance for industrial application [2]. Since the first report
of olefin metathesis in 1964 [3], many supported metathesis cata-
lysts have been developed, including rhenium [4], tungsten [5] and
and limited availability restricts the application of Re. Alternatively,
W and Mo with lower cost and compatible catalytic performance
were extensively explored for metathesis of olefin.
The performance of supported metathesis catalyst was pro-
foundly influenced by the nature of supports [7,8], such as textural
properties and acidity. The most widely used supports were con-
ventional SiO2 [9], Al2O3 [10], zeolite [11] and mixed metal oxide
[12]. The rapid development of mesoporous materials during the
last decades has stimulated the exploration of mesoporous molec-
ular sieves (MMS) and organized mesoporous alumina (OMA) as
metathesis catalysts [13]. Mesoporous materials possessing uni-
form mesopores, large surface area and pore volume, compared
with conventional supports. The large surface area could afford a
high amount of active sites and the uniform mesopores facilitate the
intra-pore diffusion of reactants and products. To date, many MMS
such as MCM-41 [7,14], MCM-48 [7], SBA-15 [7,14–16], HMS [17],
∗
Corresponding authors.
E-mail addresses: zhoushenghu@ecust.edu.cn (S. Zhou), hanlei@nbu.edu.cn
(L. Han).
http://dx.doi.org/10.1016/j.molcata.2016.02.019
1381-1169/© 2016 Elsevier B.V. All rights reserved.

40
K. Tao et al. / Journal of Molecular Catalysis A: Chemical 416 (2016) 39–46
and KIT-6 [18] have been employed as the supports of metathe-
sis catalysts. In brief, MMS supported W (Mo) catalysts exhibited
better catalytic performance than that of conventional silica gels
supported catalysts due to the better dispersion of active species
on MMS. In addition, the pore size and architecture of MMS also
influenced the metathesis activity. Topka et al. [7] systematically
studied the metathesis of 1-octene over three typical MMS, i.e.
MCM-41, MCM-48, and SBA-15 supported Mo catalysts and the cat-
alytic activity decreased in the order: MCM-41 > MCM-48 > SBA-15.
The different dispersion of active species over three supports was
responsible for the difference in catalytic performance. Bhuiyan
et al. [14] also compared the catalytic property of W-SBA-15 and
W-MCM-41 in 2-butene metathesis to propene, and found that W-
MCM-41 catalyst showed better activity than that of W-SBA-15. The
better activity of W-MCM-41 was attributed to the well-dispersed
active tetrahedral tungsten oxide species. Besides mesoporous sil-
ica, mesoporous alumina [19] and alumina-silica [20,21] based
catalysts also showed promising metathesis activity.
KIT-5, a novel mesoporous silica with ordered 3D cage-like
mesopores was first reported by Kleitz et al. [22]. KIT-5 was believed
to be superior to 1D mesoporous silicas, because the interconnected
3D large pores were beneficial for mass transportation of reactants
and products, and providing more accessible adsorption sites [23].
Therefore, KIT-5 was a desirable catalyst supports for versatile reac-
tions [24–26]. Herein we first report the synthesis of high active
mesoporous Mo-KIT-5 catalyst for production of propene from
metathesis of 1-butene and ethylene. The catalyst was systemat-
ically characterized by multiple characterization techniques. The
Mo-KIT-5-40 catalyst exhibited an improved catalytic performance
compared with control supported Mo/KIT-5-40 and Mo/SiO2-40
catalysts with same Mo loading due to high dispersion of active
Mo species. The Mo-KIT-5 catalyst was also advantageous to other
mesoporous catalysts, such as Mo-SBA-15 and Mo-KIT-6.
2.2.2. Preparation of control Mo/KIT-5-40 and Mo/SiO2-40
catalysts
For comparison, KIT-5 and SiO2 supported Mo catalysts were
synthesized by wet-impregnation of traditional SiO2 gel and KIT-
5 with 10 ml of ammonium paramolybdate aqueous solution. The
impregnated sample was dried at 110 ◦C overnight and calcined at
550 ◦C for 4 h with a ramping rate of 1 ◦C/min. The Mo loading of
both catalysts were fixed at 2.5 wt%, which was identical to that
of Mo-KIT-5-40, as confirmed by inductively coupled plasma opti-
cal emission spectroscopy (ICP-OES). The catalysts were denoted
as Mo/KIT-5-40 and Mo/SiO2-40, respectively, where 40 was the
nominal Si/Mo ratio.
2.3. Catalyst characterization
Inductively coupled plasma optical emission spectroscopy (ICP-
OES) analysis was employed to determine the real Mo content of
sample using PerkinElmer OPTIMA 2100 DV optical emission spec-
troscopy spectrometer. Small-angle X-ray diffraction (SAXD) and
wide-angle X-ray diffraction (WAXD) patterns of samples were
collected on a Bruker AXS D8 Advance diffractometer with Cu K␣
radiation in the 2ꢀ angle range of 0.5–5◦ and 20–80◦, respectively.
N2 adsorption–desorption isotherm of the sample was measured
at 77 K using a Micrometrics ASAP-2020M adsorption apparatus.
Before the measurement, the sample was outgassed at 200 ◦C for
6 h. The specific surface area was determined by the multiple
Brunauer–Emmett–Teller (BET) method. The pore volume and pore
size distribution were obtained from the adsorption branch of the
isotherm using Barrett–Joyner–Halenda (BJH) method. Transmis-
sion electron microscopy (TEM) images were acquired on a JEOL
2100 transmission electron microscope operated at 200 kV. The
Fourier transform infrared spectra (FTIR) were recorded with a
Bruker Tensor 27 spectrophotometer. UV–vis diffuse reflectance
spectra (UV-DRS) were performed in PE lambda 950 equipment
using BaSO4 as reference. X-ray photoelectron spectroscopy (XPS)
studies were performed on AXIS ULTRA DLD multifunctional X-
ray photoelectron spectroscopy with an Al source. Temperature
programmed desorption of ammonia (NH3-TPD) was carried out
in a quartz micro-reactor. 0.1 g catalyst was pretreated at 550 ◦C
for 30 min in He (25 cm3 min−1) prior to NH3-TPD measurement
before the reactor was cooled down to room temperature, and
then the sample was saturated with NH3. The physically absorbed
NH3 was removed by flushing the sample with He at 120 ◦C for 2 h.
Temperature-programmed desorption of ammonia was carried out
from 120 to 650 ◦C with a ramping rate of 5 ◦C/min, and the amount
of desorbed NH3 was monitored by a thermal conductivity detector
(TCD).
2. Experimental
2.1. Chemicals
Pluronic F127 (Mw = 12,500) was purchased from Sigma–Aldrich
as the structure directing agent. Tetraethyl orthosilicate (TEOS, AR)
and ammonium paramolybdate ((NH4)6Mo7O24·4H2O, AR) were
purchased from Sinopharm Chemical Reagent Co., Ltd., as silicon
and Mo sources, respectively. Silica gel (BET surface area, 399 m2/g)
was obtained from Qingdao Haiyang Chemical Co., Ltd. All the
reagents were used as received without further purification.
2.2. Catalyst preparation
2.2.1. Synthesis of Mo-KIT-5 catalysts
2.4. Metathesis reactions
The Mo-KIT-5 catalysts with various Si/Mo were prepared using
Pluronic F127 as the structure directing agent following the proce-
dure described by Kleitz et al. [22]. Typically, 4 g F127 and required
amount of ammonium paramolybdate were dissolved in 192 g
deionized water and 8.4 g of 35 wt% hydrochloric acid under stir-
ring. To this mixture, 19.2 g of TEOS was dropwise added. The
resulting mixture was vigorously stirred at 45 ◦C for 24 h. Then, the
mixture was transferred to a Teflon-lined autoclave, and subject to
a hydrothermal treatment at 100 ◦C for 24 h. The products were col-
lected by filtering without washing, and drying at 100 ◦C overnight.
Finally, the removal of F127 templates was achieved by calcining
the sample in a muffle oven at 550 ◦C for 4 h with a heating rate
of 1 ◦C/min. The catalyst was denoted as Mo-KIT-5-x, where the x
represented the Si/Mo ratio in the synthesis gel.
Catalytic performance of as-synthesized catalysts for metathesis
of 1-butene and ethylene to propene was evaluated in a fixed-bed
stainless reactor (i.d. 10 mm) under atmospheric pressure. In each
test, 1 g of shaped catalyst (20–40 mesh) was placed at the center of
the reactor and sandwiched by inert SiO2 beads. Prior to reaction,
the catalysts was in situ activated by high pure N2 (35 ml/min) at
550 ◦C for 4 h to remove the moisture, followed by cooling down
to reaction temperature. Then, a mixture of ethylene and 1-butene
(nC2H4/n1 − C4H8 = 2) was introduced and the reaction was started.
The metathesis reaction conditions were 450 ◦C, 0.1 MPa, weight
hourly space velocity (WHSV, 1-C4H8 + C2H4) of 0.8 h−1. The efflu-
ent gases released from the reactor were analyzed by an online
gas chromatograph (GC) equipped with a flame ionization detector
(FID). The calculation of 1-butene conversion and product selectiv-
ity has been described elsewhere [15,18].
Pure KIT-5 was prepared using the same procedure as Mo-KIT-5
samples except without adding of ammonium paramolybdate.

K. Tao et al. / Journal of Molecular Catalysis A: Chemical 416 (2016) 39–46
41
(111)
(200)
a
b
c
d
a
b
c
d
e
e
f
f
1
2
3
4
5
0.0
0.2
0.4
0.6
0.8
1.0
2-Theta (degree)
Relative Pressure (P/P )
0
(left)
Fig. 1. Small-angle XRD patterns of pure KIT-5 (a) and Mo containing KIT-5 samples:
(b) Mo-KIT-5-60, (c) Mo-KIT-5-40, (d) Mo-KIT-5-25, (e) Mo-KIT-5-15 and (f) Mo-KIT-
5-5.
Table 1
a
Textural properties of KIT-5 and Mo containing catalysts.
Sample
a0 (nm)a BET surface area
Pore volume
Pore size (nm)
b
(m2 g−1
)
(cm3 g−1
)
KIT-5
17.8
636.2
831.1
814.4
700.1
653.7
144.9
458.2
355.6
0.29
0.55
0.55
0.54
0.52
0.49
0.22
0.81
3.5
3.9
4
3.9
3.9
12.6
1.8
9.2
√
c
d
Mo-KIT-5-60 18.8
Mo-KIT-5-40 19.0
Mo-KIT-5-25 19.3
Mo-KIT-5-15 19.6
Mo-KIT-5-5
Mo/KIT-5-40
Mo/SiO2-40
–
–
–
e
f
a
Unit cell parameter (a0) was determined by the formula: a0 = d111 3.
1
10
100
3. Results and discussion
Pore Diameter (nm)
(right)
3.1. Characterization of catalysts
Fig. 2. N2 adsorption–desorption isotherms (left) and pore size distributions (right)
of various samples: (a) Mo-KIT-5-60, (b) Mo-KIT-5-40, (c) Mo-KIT-5-25, (d) Mo-KIT-
5-15, (e) KIT-5 and (f) Mo-KIT-5-5.
In present study, Mo containing KIT-5 catalysts (Mo-KIT-5) with
various Si/Mo ratios were prepared by one pot hydrothermal syn-
thesis from ammonium paramolybdate and TEOS in presence of
F127 under acidic condition. The real Si/Mo ratios in the final cata-
lysts were checked by ICP-OES. The result is presented in Table S1.
For all Mo-KIT-5 samples, the real Si/Mo ratios were higher than
those in the synthesis solution, as similar to the case of W-KIT-5
[27]. This could be explained by the fact that the Mo species tended
to be in cationic rather than oxo forms in an acidic medium.
Small-angle X-ray diffraction (SAXD) is an efficient tool to study
the ordered mesoporous materials. The SAXD patterns of KIT-5
and Mo-KIT-5 catalysts are shown in Fig. 1. Two resolved diffrac-
tion peaks in the 2ꢀ range of 0.5–1.0◦ indexed to (1 1 1) and (2 0 0)
plane was observed for pristine KIT-5, which were characteristics of
face centered close-packed cubic type materials with Fm3m space
group symmetry [22]. Compared with pure KIT-5, Mo-KIT-5 with
a higher Si/Mo ratio showed more intense (1 1 1) diffraction peak.
This revealed that the incorporation of an appropriate amount of
Mo were helpful for retain of the mesoporous structure. However
at a very low Si/Mo ratio (Mo-KIT-5-5), no resolved reflections are
found, which suggested that the long-range ordering was lost when
excessive Mo was involved. The unit cell parameters (a0) were cal-
culated from d111 and presented in Table 1, which were similar to
those of previously reported KIT-5 type materials [28]. The unit cell
parameter increased with Mo loading. This could be attributed to
the incorporation of Mo in the KIT-5 framework, since the atomic
The porosity of samples is studied by N2 physisorption at
−196 ◦C. The N2 adsorption–desorption isotherms and pore size
distributions of pure KIT-5 and Mo-KIT-5 catalysts are shown in
Fig. 2. KIT-5 showed type IV isotherm with a sharp capillary con-
densation and a broad H2-type hysteresis loop, indicating that the
material were ordered mesoporous and possessed large uniform
cage-like pores [22]. KIT-5 displayed a narrow pore size distribution
centered at 3.5 nm. Upon incorporating Mo, Mo-KIT-5 (Si/Mo = 60,
40, 25 and 15) catalysts showed similar isotherm profiles to that
of pristine KIT-5 but with higher amount of N2 adsorption and
capillary condensation step. This phenomenon was also observed
for Al-KIT-5 samples [29]. Besides, the capillary condensation was
shifted to higher relative pressure with the concomitant increase
of pore size. However, further increasing the Mo loading resulted
in a dramatic change of isotherm as for Mo-KIT-5-5, which sug-
gested that the ordered mesoporous structure was destroyed. The
Mo-KIT-5-5 sample also showed a very broad pore size distribution.
The BET surface area, pore volume and mean pore size of
catalysts are summarized in Table 1. The BET surface area and
pore volume of pristine KIT-5 were 636.2 m2 g−1 and 0.29 cm3 g−1
,
respectively. Upon incorporation of Mo, Mo-KIT-5 catalysts showed
larger BET surface area and pore volume compared with pure KIT-
5 and the BET surface area decreased from 831.1 to 653.7 m2 g−1
radius of Mo is larger than that of Si4+
.

42
K. Tao et al. / Journal of Molecular Catalysis A: Chemical 416 (2016) 39–46
Fig. 3. TEM images showing (a) KIT-5, (b) Mo-KIT-5-60, (c) Mo-KIT-5-40, (d) Mo-KIT-5-25, (e) Mo-KIT-5-15, (f) Mo-KIT-5-5, (g) Mo/KIT-5-40 and (h) Mo/SiO2-40.
with decreasing Si/Mo ratio from 60 to 15. For Mo-KIT-5-5, the
BET surface area was as low as 144.9 m2 g−1, indicating the loss
of mesoporous ordering. Compared with one-pot synthesized Mo-
KIT-5-40, supported Mo/KIT-5-40 showed lower BET surface area,
and smaller pore volume and pore size. This was caused by the plug-
ging of partial mesopores by Mo species during the impregnation
process [30].
g
f
The morphology of materials was observed directly by TEM.
TEM images of various samples are provided in Fig. 3. As shown in
Fig. 3a, pristine KIT-5 sample displayed a cubic three-dimensional
ordered mesoporous structure similar to those reported by other
authors [22,27]. After the introduction of Mo, Mo-KIT-5-60 (Fig. 3b),
Mo-KIT-5-40 (Fig. 3c), Mo-KIT-25 (Fig. 3d) and Mo-KIT-15 (Fig. 3e)
samples also showed mesoporous structures with long-range
ordering. In a sharp contrast, the mesoporous structure disap-
peared and some bulk agglomerate appeared for KIT-5-5 as clearly
shown in Fig. 3f. The results suggested that mesoporous structure
of KIT-5 could be reserved after doping Mo with Si/Mo ratio as
low as 15, and too excessive Mo involved resulted in the collapse
of mesoporous structure, as also suggested by N2 physisorp-
tion result. Supported Mo/KIT-5-40 also showed well mesoporous
structure except some disordered region compared with Mo-KIT-
5-40. Mo/SiO2-40 displayed a typical morphology of amorphous
SiO2 with indiscernible nano particles [15].
The crystalline phases of Mo-containing catalysts were studied
by wide-angle powder X-ray diffraction. XRD patterns of vari-
ous samples are shown in Fig. 4. Only a broad diffraction peak
centered in the 2ꢀ range of 15–30◦ were found for one-pot syn-
thesized Mo-KIT-5 catalyst with Si/Mo ratio ranged from 15 to
60, which was characteristic of amorphous silica. No character-
istic peaks corresponding to MoO3 could be observed, suggesting
that the Mo species were well dispersed in the KIT-5 framework.
Further increasing the loading of Mo, intense diffraction peaks
ascribed to MoO3 obviously appeared, indicating the formation of
extraframework MoO3 species. Ramanathan et al. [27] found that
extraframework WO3 phase was formed at very high Si/W ratio
(100). In this work, no MoO3 was found at Si/Mo ratio as low as 15,
suggesting high dispersion of Mo species in the KIT-5 framework
or formation of amorphous MoO3. The supported Mo/KIT-5-40
showed similar diffraction patterns to that of one-step synthesized
Mo-KIT-5-40 catalyst. For conventional Mo/SiO2-40, the presence
of nanosized MoO3 species is confirmed by the weak diffraction
e
d
c
b
a
10
20
30
40
50
60
70
80
2-Theta (degree)
Fig. 4. Wide-angle XRD patterns of various Mo containing catalysts: (a) Mo/SiO2-40,
(b) Mo/KIT-5-40, (c) Mo-KIT-5-60, (d) Mo-KIT-5-40, (e) Mo-KIT-5-25, (f) Mo-KIT-5-
15 and (g) Mo-KIT-5-5.
peaks in 20–30◦ 2ꢀ range. XRD result indicated that the mesoporous
catalyst exhibited higher Mo distribution than conventional cata-
lyst, which could be ascribed to the superior textural properties of
mesoporous materials as shown in Table 1.
The FIIR spectra of KIT-5 and Mo-containing catalysts are shown
in Fig. 5. All samples displayed absorption bands around 465, 800,
1080, 1385, and 1636 cm−1. The band at 465 cm−1 corresponded to
the internal deformation vibration of Si
O Si bond [31]. The 800
and 1080 cm−1 band were assigned to symmetric and asymmet-
4−
ric stretching of bulk Si
O
Si for the tetrahedral SiO4 structure
units, respectively [32]. The characteristic band of C H deformation
band of the methyl groups32 (relevant to residual organic group)
or absorbed CO2 [33] appeared at 1385 cm−1. In this work, the
1385 cm−1 band became very weak after Mo was introduced, indi-
cating that this band was associated with absorbed CO2, since the
acidity generated by Mo incorporation (see NH3-TPD result) will
reduce the absorption of CO2. The 1636 cm−1 band observed in
all spectra can be indexed to water [31]. It should be noted that
with the increase of Mo content, a new band at 908 cm−1 became
evident, which can be ascribed to the terminal Mo = O groups in

K. Tao et al. / Journal of Molecular Catalysis A: Chemical 416 (2016) 39–46
43
908 800
1636
XPS Mo 3d
1385
1080
465
a
b
c
d
e
Mo6+
f
g
Mo5+
h
226 228 230 232 234 236 238 240 242
Binding Energy (eV)
2000 1800 1600 1400 1200 1000 800 600 400
Wavenumber (cm-1)
Fig. 7. XPS spectra of Mo-KIT-5-40 sample.
Fig. 5. FTIR spectra of various samples: (a) KIT-5, (b) Mo-KIT-5-60, (c) Mo-KIT-5-40,
(d) Mo-KIT-5-25, (e) Mo-KIT-5-15, (f) Mo-KIT-5-5, (g) Mo/KIT-5-40 and (h) Mo/SiO2-
40.
g
f
320
245
224
g
f
e
d
c
e
d
b
a
c
b
a
200
300
400
500
Temperature (oC)
200
300
400
Wavelength (nm)
500
600
Fig. 8. NH3-TPD curves of various Mo-containing catalysts: (a), Mo-KIT-5-60, (b)
Mo-KIT-5-40, (c) Mo-KIT-5-25, (d) Mo-KIT-5-15, (e) Mo-KIT-5-5, (f) Mo/KIT-5-40
and (g) Mo/SiO2-40.
Fig. 6. Diffuse reflectance UV–vis spectra of various Mo-containing samples: (a),
Mo-KIT-5-60, (b) Mo-KIT-5-40, (c) Mo-KIT-5-25, (d) Mo-KIT-5-15, (e) Mo-KIT-5-5,
(f) Mo/KIT-5-40 and (g) Mo/SiO2-40.
Mo6+ 3d3/2, respectively, while signals at 231.6 and 234.7 eV were
ascribed to Mo5+ 3d5/2 and Mo5+ 3d3/2, respectively [15]. XPS result
revealed that the Mo-KIT-5 catalyst contained both Mo6+ and Mo5+
species. The presence of a portion of Mo5+ species was considered
to be crucial for the catalytic activity of 1-butene metathesis [15].
NH3-TPD experiments were carried out to survey the acidity of
various samples. The NH3-TPD profiles of Mo-containing catalysts
are presented in Fig. 8. The negligible amount of NH3 desorbed from
pure KIT-5 (not shown here) was indicative of its neutral nature.
However, all Mo-containing samples showed broad NH3 desorp-
tion peaks in temperature range of 150 to 300 ◦C with peak maxima
centered around 190–220 ◦C. Generally, the NH3-TPD curve could
be divided into two regions, namely low temperature (<400 ◦C)
and high-temperature (>400 ◦C) regions. The former was assigned
to the desorption of NH3 from weak acid sites, and the later was
attributed to the desorption of NH3 from strong Brönsted and Lewis
type acid sites [34]. In this work, no NH3 desorption peaks could be
found in the high-temperature zone. Thus, it was suggested that all
Mo based catalysts contained only weak acid sites. For Mo-KIT-5
catalyst, the amount of acid initially increased with decreasing the
Si/Mo ratio down to 15 and then declined when the Si/Mo was fur-
ther decreased to 5, judging from the intensity of NH3-TPD band
as shown in Fig. 8a–e. The acidity of catalyst was derived from the
coordinative unsaturated Mo6+ species on the surface [37] and the
Mo
O Si vibration [34], revealing that Mo was successfully incor-
porated into the silica matrix.
UV–vis diffuse reflectance spectra (UV-DRS) were performed
to study the chemical state of Mo species, since UV-DRS was a
very efficient in probing local molecular coordination of metal
oxide in the framework and/or in the extra framework of meso-
porous structure [35]. Fig. 6 shows the UV-DRS profiles of various
Mo-containing samples. Pure KIT-5 showed no absorption bands
(not shown here). Three absorption bands around 224, 245, and
320 nm were found for all Mo-containing samples. The bands
around 220–250 nm were assigned to the tetrahedral molybdate
species, whereas the band around 320 nm was ascribed to the octa-
hedral coordinated Mo
O Mo bridge bond [15,36]. A red-shift of
320 nm band and concomitant increase in intensity were observed
for Mo-KIT-5 catalysts as Si/Mo ratio decreased and Mo-KIT-5-5
displayed a strong band beyond 320 nm, suggesting the generation
of larger Mo species clusters as found by TEM (Fig. 3f).
XPS experiment was conducted to analyze the oxidation states
of Mo species in Mo-KIT-5. Fig. 7 shows a representative Mo
3d spectrum of Mo-KIT-5-40 sample. The spectrum consists of
two individual Mo 3d5/2 and Mo 3d3/2 doublets. Four deconvo-
luted components located at 231.6, 232.9, 234.7, and 236.1 eV
were observed from the figure. Signals at 232.9 and 236.1 eV were
assigned to Mo in a high oxidation state, namely, Mo6+ 3d5/2 and
hydroxyl groups formed by protonating the bridging Si
O Mo or

44
K. Tao et al. / Journal of Molecular Catalysis A: Chemical 416 (2016) 39–46
Scheme 1. The main reaction pathways of catalytic conversion of 1-butene and
ethylene over Mo-KIT-5 catalysts.
terminal MoO bonds on the surface of the samples [38,39]. There-
fore, the decrease of the acidity at low Si/Mo ratio (Mo-KIT-5-5)
could be caused by the formation of bulk MoO3 species, as con-
firmed by XRD (Fig. 4 g) and TEM (Fig. 3f). Besides, at the same
content of Mo, mesoporous Mo-KIT-5-40 and Mo/KIT-5-40 cat-
alysts exhibited more acidic sites than conventional Mo/SiO2-40
catalyst.
3.2. Catalytic activity
Various Mo-containing catalysts were tested in catalytic conver-
sion of 1-butene and ethylene to propene, which was an important
reaction in chemical industry. According to previous research, the
catalytic conversion of 1-butene and ethylene to propene was a
tandem reaction which involved isomerization of 1-butene to 2-
butene and the subsequent cross metathesis between 2-butene and
ethylene to propene [2]. Besides, several side reactions coexisted,
such as isomerization of 1-butene to isobutene, and oligomeriza-
tion of light olefins to high molecular weight compounds. The
high acidity of catalyst stimulated these unwanted side-reactions,
resulting in poor catalytic performance [40]. In this work, the main
products were propene and 2-butene, and only slight amount of
isobutene was detected as present in Table S2, since the weak acidic
feature of catalysts. Therefore, the reaction could be described as
shown in Scheme 1.
The effect of Si/Mo ratio on 1-butene conversion and propone
selectivity was studied and shown in Fig. 9. Pure KIT-5 showed
negligible conversion of 1-butene with no propene generation
(not shown here). As Mo was introduced, all Mo-KIT-5 catalysts
exhibited significant conversion of 1-butene and propene produc-
tion. With the Si/Mo ratio decreased from 60 (Mo-KIT-5-60) to 45
(Mo-KIT-5-45), 1-butene conversion increased from 73.9 to 77.5%.
Further reducing the Si/Mo ratio to 25, a slight decline of initial
activity (about 4.1%) was observed for Mo-KIT-5-25 catalyst com-
pared with Mo-KIT-5-40. However, the 1-butene conversion and
propene selectivity began to decline significantly when the Si/Mo
ratio was lower than 25, and 1-butene conversion and propene
selectivity were as low as 50.6 and 56.1%, respectively for Mo-KIT-
5-5 catalyst.
It was believed that a lower oxidation state of Mo and high
dispersion of surface Mo species was beneficial for the olefin
metathesis, since considerable amount of metal carbine species
could be generated [41,42], which was the prerequisite of the reac-
tion according to the well-known carbine-metallacyle mechanism
[43]. The dispersion of surface Mo species was close connected with
Mo content. At low Mo loading, the decrease of Si/Mo ratio leaded to
the improvement of surface Mo species density. Thus, Mo-KIT-5-45
exhibited better catalytic performance than that of Mo-KIT-5-60.
However, further decreasing Si/Mo ratio resulted in significantly
declined BET surface area, which was against the dispersion of Mo
species. Besides, as the Si/Mo ratio reduced, bulk MoO3 began to
appear as confirm by TEM (Fig. 3f) and XRD (Fig. 4g) results. Crys-
talline MoO3 was believed to be inactive for olefin metathesis [7].
These explained the loss of catalytic activity for Mo-KIT-5-15 and
Mo-KIT-5-5 catalysts with low Si/Mo ratios.
Fig. 9. 1-Butene conversions (a) and propene selectivities (b) versus time on
stream over various Mo-KIT-5 catalysts. Reaction conditions: catalyst weight = 1.0 g;
T = 450 ◦C; P = 0.1 MPa; WHSV = 0.8 h−1; C2H4/1-C4H8 = 2.
The catalytic performance of one-pot hydrothermal synthesized
Mo-KIT-5-40 catalyst was compared with supported Mo/KIT-5-40
and conventional Mo/SiO2-40 catalysts with identical Mo load-
ing prepared by impregnation method, as shown in Fig. 10.
Conventional silica supported Mo/SiO2-40 showed poor catalytic
performance with 1-butene conversion of 52.2% and propene selec-
tivity of 54.4% at steady-state. Supported Mo/KIT-5-40 exhibited
higher initial 1-butene conversion (60.7%) and propene selectiv-
ity (71.3%) than those of Mo/SiO2-40. But the activity gradually
declined to the same level of Mo/SiO2-40 after 8 h of reaction. In
contrast, the 1-butene conversion and propene selectivity of one
pot synthesized Mo-KIT-5-40 was as high as 77.5 and 84.5%, respec-
tively, and the catalyst was quite stable. The excellent catalytic
performance of Mo-KIT-5-40 was ascribed to its superior textu-
ral property since the larger BET surface area of Mo-KIT-5-40 could
lead to a higher dispersion of Mo species, facilitating the metathesis
reaction.
The catalytic performance of Mo-KIT-5 was also compared with
other mesoporous Mo-containing silica material with same Mo
content (2.5 wt%), namely Mo-SBA-15 with two-dimensional (2D)
hexagonal arrangement of cylindrical pores and Mo-KIT-6 with 3D
cylindrical pores (Fig. 11). As can be seen from Fig. 11, Mo-KIT-
5 exhibited superior catalytic activity to control Mo-SBA-15 and
Mo-KIT-6 catalysts. It was inferred that three dimensional cage type
porous networks with a high surface area was beneficial for the high

K. Tao et al. / Journal of Molecular Catalysis A: Chemical 416 (2016) 39–46
45
4. Conclusions
Various three-dimensional cage-like molybdenum containing
mesoporous KIT-5 (Mo-KIT-5) catalysts with different Si/Mo ratios
were prepared by one-pot hydrothermal synthesis and systemati-
cally characterized by multiple characterization techniques. It was
found that the ordered mesoporrous structure of KIT-5 could be
retained after incorporation of Mo, even though the Si/Mo ratio
as low as 15, as evident from TEM and N2 physisorption analysis.
However, further decreasing Si/Mo ratio to 5 resulted in collapse of
mesoporous structure and the formation of bulk MoO3 as suggested
by XRD result. XPS result suggested that Mo was successfully incor-
porated into the KIT-5 framework and sample composed of both
Mo6+ and Mo5+ species. NH3-TPD analysis illustrated that Mo-KIT-
5 samples contained weak acid sites. The Mo-containing catalysts
were applied in catalytic conversion of 1-butene and ethylene to
propene. With the decrease of Si/Mo ratio, the 1-butene conver-
sion of Mo-KIT-5 first increased due to more active molybdenum
oxides species generated, and then declined because of the destroy
of mesoporous structure and formation of inactive bulk MoO3. Mo-
KIT-5-40 exhibited an improved catalytic performance compared
with control supported Mo/KIT-5-40 and Mo/SiO2-40 catalysts due
to high dispersion of active Mo species. Moreover, Mo-KIT-5 also
exhibited superior catalytic activity to Mo-SBA-15 and Mo-KIT-6
catalysts, suggesting that this kind of 3D cage type materials was
very promising for catalytic conversion of 1-butene and ethylene
to propene and could be applied in other heterogeneous catalytic
reactions.
Acknowledgments
The authors are grateful for the financial support from
the National Natural Science Foundation of China (Grant Nos.
21471086, 51572272), Natural Science Foundation of Ningbo
(Grant. No. 2015A610052), the Social Development Foundation
of Ningbo (No. 2014C50013), and the K.C. Wong Magna Fund in
Ningbo University.
Fig. 10. 1-Butene conversions (a) and propene selectivities (b) versus time
on stream over Mo-KIT-5-40, Mo/KIT-5-40 and Mo/SiO2-40 catalysts. Reaction
conditions: catalyst weight = 1.0 g; T = 450 ◦C; P = 0.1 MPa; WHSV = 0.8 h−1; C2H4/1-
C4H8 = 2.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.02.
019.
90
1-Butene Conv.
Propene Sel.
80
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