1-Butene metathesis over Mo/mordenite-alumina catalyst: Effect of sodium exchange degree in mordenite zeolite

Article abstract of DOI:10.1016/j.molcata.2013.02.015

A series of Mo/mordenite-alumina catalysts with progressive sodium exchange degrees in mordenite were prepared and their performance in 1-butene metathesis reaction was evaluated. Significant variations in product distribution and catalytic activity were observed when the exchange degree of mordenite changed. As revealed by NH₃-TPD and ¹H MAS NMR results, the acid site density and distribution over the catalysts were directly related to the degree of ion-exchange of sodium in mordenite. The change of support acidity led to different anchoring modes and dispersion of Mo species on the support which further resulted in the different reduction behavior of Mo species under olefin atmosphere. A good correlation between product distribution and properties of the catalysts was proposed.

Full text of DOI:10.1016/j.molcata.2013.02.015

Journal of Molecular Catalysis A: Chemical 372 (2013) 121–127
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
1-Butene metathesis over Mo/mordenite-alumina catalyst: Effect of sodium
exchange degree in mordenite zeolite
Xiujie Li∗, Dazhou Zhang, Xiangxue Zhu, Fucun Chen, Shenglin Liu, Sujuan Xie, Longya Xu∗
Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of Mo/mordenite-alumina catalysts with progressive sodium exchange degrees in mordenite
were prepared and their performance in 1-butene metathesis reaction was evaluated. Significant varia-
tions in product distribution and catalytic activity were observed when the exchange degree of mordenite
changed. As revealed by NH3-TPD and 1H MAS NMR results, the acid site density and distribution over the
catalysts were directly related to the degree of ion-exchange of sodium in mordenite. The change of sup-
port acidity led to different anchoring modes and dispersion of Mo species on the support which further
resulted in the different reduction behavior of Mo species under olefin atmosphere. A good correlation
between product distribution and properties of the catalysts was proposed.
Received 31 October 2012
Received in revised form 29 January 2013
Accepted 14 February 2013
Available online 27 February 2013
Keywords:
1-Butene
Propene
Olefin metathesis
Molybdenum
Mordenite-alumina
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
roles of catalyst acidity and its influence on metal location is of
great importance for regulating the catalyst design and preparation.
As one of the most important basic petrochemicals, nowa-
days the demand for propene is growing rapidly in world-wide
chemical market. However, traditional processes for propene pro-
duction cannot meet this growing demand. Several on-purpose
propene production technologies including methanol to olefin
process, steam cracking, propane dehydrogenation and catalytic
cracking of C4 alkenes have been proposed [1]. In addition, olefin
metathesis reaction, especially those catalyzed by supported het-
erogeneous catalysts, provides an alternative route to produce
propene [2–4]. Compared with the cross metathesis of ethene and
2-butene to propene, 1-butene auto-metathesis has attracted more
interest due to the absence of valuable ethene source [5–9]. How-
ever, due to the participation of isomerization reaction, the reaction
pathway of 1-butene metathesis is more complicated as shown
in Scheme 1. As propene is the target product, cross metathe-
sis of 1-butene and 2-butene is regarded as the optimal reaction
pathway for 1-butene auto-metathesis. This process requires the
initial double bond isomerization of 1-butene. It is well known
that acid sites could catalyze 1-butene isomerization reaction and
they also participate in the following metathesis process [10–12].
Metal carbene species are the active sites for olefin metathesis.
Well dispersion of metal oxide on the support will contribute to
the enhancement of metathesis activity [13]. Thus, elucidating the
Our recent study showed that Mo supported on mordenite-alumina
composites exhibited high activity in the 1-butene auto-metathesis
reaction [8,14]. However, the function of catalyst acidity, especially
the role of acid sites over mordenite zeolite on the catalyst perfor-
mance should be further revealed. In this contribution, we try to
elucidate the effect of sodium exchange degree of mordenite on
the surface acidity, position of molybdenum within the support
and redox behavior of the catalyst. A series of mordenite-alumina
composites with different sodium exchange degrees were prepared
and used as supports to prepare Mo/mordenite-alumina catalysts
with different acidity. Quantitative acidity information before and
after Mo loading was obtained from the NH3-TPD and 1H MAS NMR
spectra. H2-TPR was performed to reveal the location and redox
behavior of Mo species. The results indicated that the performance
of the catalysts in 1-butene auto-metathesis was well related to
their acidic and redox properties.
2. Experimental
2.1. Catalyst preparation and evaluation
The mordenite-alumina composite support was prepared by
extruding a mixture of ␥-Al2O3 and NaMOR zeolite (Na2O% = 6.1%,
Si/Al = 5.6 provided by Fushun Petroleum Company, China) powder
into extrudates with a diameter of about 2 mm. The drawn extru-
dates were dried at 120 ◦C for 12 h and calcined at 540 ◦C for 2 h.
The obtained support was designed as NaM-30Al, which meant the
∗
Corresponding authors. Tel.: +86 411 8437 9279; fax: +86 411 8437 9279.
E-mail addresses: xiujieli@dicp.ac.cn (X. Li), lyxu@dicp.ac.cn (L. Xu).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.02.015

122
X. Li et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 121–127
2.2. Catalyst characterization
Temperature-programmed desorption of ammonia (NH3-TPD)
measurements were carried out in a conventional U-shaped
stainless-steel micro-reactor (i.d. = 4 mm) using flowing helium
(He) as the carrier gas. The NH3-TPD process was monitored by
an on line gas chromatograph equipped with a TCD detector. Typi-
cally 140 mg sample was pretreated at 600 ◦C for 1 h in flowing He
(25 ml/min), then cooled to 150 ◦C and saturated with NH3. After
that, the sample was purged with pure He for a certain period until
a stable GC-baseline was attained. Subsequently NH3-TPD experi-
ment was carried out in the range of 150–700 ◦C at a heating rate
of 18.8 ◦C/min.
1H MAS NMR spectra were recorded on a Varian Infinityplus-
400 spectrometer equipped with a 4 mm probe. Before the 1H MAS
NMR measurements, samples were dehydrated at 400 ◦C under
a pressure below 10−2 Pa for 20 h. Then 1H MAS NMR spectra
were collected at 399.9 MHz using single-pulse sequence with
ꢀ/4 pulse, 4 s recycle delay with a spinning speed of 10 kHz.
Chemical shifts were referenced to DSS. For the determination of
quantitative results, all samples were weighed, and the spectra
were calibrated by measuring a known amount of 1,1,1,3,3,3-
hexafluoro-2-propanol performed in the same conditions [15].
The Dmfit software was used for deconvolution using fitted
Gaussian–Lorentzian line shapes [16].
Scheme 1. Possible metathesis reaction pathways of 1-butene.
weight percent of alumina in the support was 30%. Then, NaM-30Al
was ion-exchanged with NH4NO3 solution at 80 ◦C. The exchange
degree can be controlled by varying the exchanging time. NH4NaM-
30Al was ground into 16–32 meshes after calcination at 520 ◦C for
2 h and denoted as HnNa100–nM-30Al where n (%) stands for the
exchange degree of total sodium in the support. The actual Na con-
tent was obtained by the XRF measurements. Catalysts with 6%
Mo loading were prepared by wet impregnation of HnNa100–nM-
30Al with an aqueous solution of (NH4)6Mo7O24·4H2O, then dried
at 120 ◦C and finally calcined at 600 ◦C for 2 h. The catalysts were
denoted as 6Mo/HnNa100–nM-30Al, where 6 (%) stood for the weight
percent of Mo atoms in the catalysts.
The catalysts (1.0 g) were tested in a fixed-bed down-flow stain-
less steel reactor of 7 mm in diameter with quartz sand to fix the
catalyst bed. Quartz wool was applied to separate the catalyst from
the quartz sand. In the middle of catalyst bed, a thermocouple
was applied to detect the reaction temperature. After activation
for 2 h at 550 ◦C under nitrogen to remove the moisture, the sys-
tem was cooled down to the reaction temperature at 150 ◦C. All
the reaction products were analyzed by an on-line Varian CP/3800
gas chromatograph equipped with an alumina-plot column and an
FID detector. The connecting lines between the reactor outlet and
the sampling valve were heated to prevent the condensation of the
products with high molecular weights. 1-butene feed (1-butene
95%, butane 5%) was obtained from Dalian Special Gas Company.
The 2-butene isomers (trans-2-butene and cis-2-butene) from
1-butene isomerization are considered as feed stock. The conver-
sion of 1-butene and selectivity are calculated using the following
equations:
27Al MAS NMR experiments were carried out on a Bruker
AdvanceIII-600 MHz spectrometer with a spinning rate of 15 kHz.
Chemical shifts were referenced to (NH4)Al(SO4)2·12H2O at
0.4 ppm as a secondary reference. The spectra were accumulated
for 800 scans with a ꢀ/12 flip angle and a 2 s pulse delay.
Temperature-programmed reduction experiments under H2
(H2-TPR) were carried out in a conventional setup with a gas chro-
matography as the H2 detector. The catalyst (80 mg) was loaded in
a tubular quartz reactor, pretreated under flowing Ar of 20 ml/min
at 550 ◦C for 1 h, then cooled down to 100 ◦C. Under flowing 10%
H2/Ar flow (20 ml/min), H2-TPR profiles were obtained in the range
of 100–900 ◦C at a heating rate of 14 ◦C/min.
3. Results and discussions
3.1. N2 adsorption results
Table 1 lists the textural properties of 6Mo/HnNa100–nM-30Al
catalysts. At low sodium exchange degree, a relatively small spe-
cific surface area of 105 m2/g is observed for 6Mo/H38Na62M-30Al.
As the exchange degree increases, the specific surface areas of
the catalysts gradually increase to 231, 276 and 281 m2/g for
6Mo/H67Na33M-30Al, 6Mo/H82Na18M-30Al and 6Mo/H100Na0M-
30Al, respectively. It is well known that mordenite has one-
dimensional 12-MR channel (0.74 × 0.65 nm) connected with
8-MR window side pockets (window size 0.34 × 0.48 nm). Previous
reports indicated that sodium exchange first occurred selectively
within the 12-MR channels, more sodium ions would locate in the
8-MR side pockets which may block the entrance of N2 molecules
during N2-adsorption process [17,18]. As the exchange degree
increases, more and more sodium ions locating at the 8-MR win-
dow will be replaced by H+, which may increase the access of N2
to side pockets, thus results in increased specific surface area of
the catalysts. This proposition can be further testified by the fact
that the increase of specific surface areas is mainly caused by the
increase of micropore surface areas (Table 1). Moreover, the micro-
pore surface area increases significantly at relatively low exchange
degree (6Mo/H38Na62M-30Al to 6Mo/H67Na33M-30Al) and only
minor increase occurs at high exchange degree (6Mo/H82Na18M-
30Al to 6Mo/H100Na0M-30Al), which indicates that the sodium ions
[1 − butene]F − [1 − butene]p − [2 − butene]p
Xbutene
=
× 100%
[1 − butene]F
[Colefin
]
p
Solefin
=
× 100%
]
p
[ethene]p + [propene]p + [pentene]p + [hexene]p + [C7+
In the above equations, [1-butene]F represents the molar per-
centage of 1-butene in the feed; [ethene]p, [propene]p, [2-butene]p,
+
[pentene]p, [hexene]p, and [C7
]
p
are the molar percentage of
each component in products, respectively. In addition, the heavy
products having seven or more carbon atoms are denoted as C7
Carbonaceous deposits on the catalyst are not taken into account for
the selectivity calculation. Yolefin in the manuscript means the molar
yield of the designed olefin product. And the detailed calculation
method was described in Ref. [8].
+
.

X. Li et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 121–127
123
Table 1
Textural properties and acidity information over 6Mo/HnNa100−nM-30Al catalysts.
Catalyst
BET surface areaa (m2/g)
Micropore areaa (m2/g)
Pore volumea (cm3/g)
Micropore volumea (cm3/g)
Br␾nsted acidity
amountb (mmol/g)
6Mo/H38Na62M-30Al
6Mo/H67Na33M-30Al
6Mo/H82Na18M-30Al
6Mo/H100Na0M-30Al
105
231
276
281
46
0.1308
0.1955
0.2254
0.2335
0.0208
0.0694
0.0828
0.0871
–
150
178
187
0.142
0.175
0.221
a
Determined by N2 adsorption isotherms.
b
Determined by quantitative analysis of 1H MAS NMR spectra.
in 8-MR windows are the most difficult to be exchanged. The pore
volume, especially the micropore volume, increases accordingly
upon the exchange degree.
3.2. NH3-TPD and 1H MAS NMR results
Fig. 1 shows the NH3-TPD profiles of the HnNa100–nM-30Al
supports and 6Mo/HnNa100–nM-30Al catalysts. For H38Na62M-30Al
support, only one NH3 desorption peak at around 280 ◦C (nomi-
nated as l) is observed. No desorption peaks at high temperatures
are found, which indicate that only weak acid sites present in
the catalyst. After introduction of Mo species, the acid amount
decreases from 0.671 to 0.446 mmol/g (Fig. 2). In the case of
H
67Na33M-30Al, a minor broad peak at around 500 ◦C (nominated
as h) is observed in addition to the major peak at 280 ◦C (peak l).
Mo loading leads to the peak intensity losses of both l and h. In
order to get a more clear picture of the intensity changes of l and h
peaks in the spectra, all the profiles are deconvoluted using Gauss-
ian line and quantitative results are shown in Fig. 2. It is clear that
upon increasing sodium exchange degree, the total acid amount
of both the supports and catalysts increases. Moreover, the loss
of acid amount by comparing the catalysts and the supports also
increases upon the sodium exchange degree indicating that more
acid sites participate in the interaction with Mo species. Combined
with the N2 adsorption results, it could be deduced that the dis-
persion of Mo species on 6Mo/H100Na0M-30Al is better than that
of 6Mo/H38Na62M-30Al. As far as the peak intensity of l and h is
concerned, an increasingly lager difference between h-support and
h-catalyst could be observed, especially on 6Mo/H100Na0M-30Al
catalyst.
Fig. 2. Effects of sodium exchange level on the acidity amount of HnNa100−nM-30Al
supports and 6Mo/HnNa100−nM-30Al catalysts determined from the quantitative
analysis of NH3-TPD profiles.
presence of alumina, the spectrum of 6Mo/H100Na0M-30Al is a little
more complicated than that of H-mordenite zeolite [11]. As demon-
strated in Fig. 3, the peak at 1.7 ppm is assigned to the silanol groups
in H-mordenite zeolite; the signal at about 2.5 ppm may contain the
contribution of non-framework Al–OH in H-mordenite zeolite and
the acid hydroxyls in alumina [19]. The peak at 3.8 ppm is attributed
to the bridging hydroxyl groups, i.e., Brønsted acid sites in H-
mordenite. In the deconvoluted spectrum a broad line centered at
ca. 5.9 ppm could be identified. It is ascribed to a second Brønsted
acid site interacting electrostatically with the zeolite framework
since it could be significantly suppressed after Al irradiation [19].
For the 6Mo/H38Na62M-30Al catalyst, the main resonance peak
appears at 2.2 ppm which gradually shifts to low field position as
the sodium exchange degree increases. A reasonable explanation
High-resolution 1H MAS NMR is a useful and direct method to
characterize the Brønsted acid sites in the samples. Fig. 3 shows the
1H MAS NMR spectra of 6Mo/HnNa100–nM-30Al catalysts. Due to the
2.2
3.8
(a)
(b)
(c)
(d)
10
9
8
7
6
5
4
3
2
1
0
-1
-2
-3
-4
(ppm)
Fig. 1. NH3-TPD profiles of HnNa100−nM-30Al supports in solid line and
6Mo/HnNa100−nM-30Al catalysts in dashed line: (a) 6Mo/H38Na62M-30Al (b)
6Mo/H67Na33M-30Al (c) 6Mo/H82Na18M-30Al (d) 6Mo/H100Na0M-30Al.
Fig. 3. 1H MAS NMR spectra of 6Mo/HnNa100−nM-30Al catalysts: (a)
6Mo/H38Na62M-30Al (b) 6Mo/H67Na33M-30Al (c) 6Mo/H82Na18M-30Al (d)
6Mo/H100Na0M-30Al.

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X. Li et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 121–127
Fig. 5. 27Al MAS NMR spectra of (a) 6Mo/H38Na62M-30Al and (b) 6Mo/H100Na0M-
30Al.
Fig. 4. H2-TPR profiles of 6Mo/HnNa100−nM-30Al catalysts: (a) 6Mo/H38Na62M-30Al
(b) 6Mo/H67Na33M-30Al (c) 6Mo/H82Na18M-30Al (d) 6Mo/H100Na0M-30Al.
From the N2 adsorption results, it is clear that the micropore
area of 6Mo/H38Na62M-30Al is low, possibly due to the blockage
of sodium ions in the window of the side pockets. The loca-
tion of sodium ions may further limit the entrance of Mo species
into the channels. Thus, the dispersion of Mo species is poor on
6Mo/H38Na62M-30Al and most of the agglomerated Mo species are
prone to locate at the outer surface of mordenite zeolite. When
the sodium exchange level changes to 67%, the BET surface area
increases to 231 m2/g and more Br␾nsted acid sites are exposed
for Mo anchoring which may lead to an improved dispersion of
Mo species. Moreover, part of Mo species may immigrate into the
channels of mordenite which leads to high temperature shift of
the reduction peak. The peak at around 640 ◦C in Fig. 4(b) may
be assigned to the reduction of polymeric Mo species locating
inside the channels of mordenite zeolite. This is in well agreement
with that of Mo/ZSM-5-Al2O3 catalysts with different Si/Al ratios
of ZSM-5 [24]. Such trend is more obvious on 6Mo/H82Na18M-
30Al. It is interesting that no such phenomenon is observed on
6Mo/H100Na0M-30Al catalyst. Compared with 6Mo/H82Na18M-
30Al, the 670 ◦C peak shows a decrease in intensity in favor of an
intensity enhancement of the peak at 900 ◦C. The increasing ratio
of high-temperature reduction peak is related to the different dis-
tribution of Mo species on H100Na0M-30Al support. It should be
pointed out that the reduction peak at around 900 ◦C slightly shifts
to high temperature upon sodium exchange degree as shown in
Fig. 4. On basis of the discussion about reduction peak assign-
ments, the reducibility of Mo species was closely related to its
state and dispersion. The high-temperature shift in TPR profiles
may be linked with the improved dispersion of Mo species on the
support.
The varied distribution of Mo species on the support could be
shown further by the 27Al MAS NMR spectra. As shown in Fig. 5,
two peaks in the tetrahedral region are observed. The signal at
55 ppm is assigned to framework tetrahedral aluminum in mor-
denite zeolite and the broad one centered at ca. 67 ppm comes
from the four-coordinated aluminum in alumina. A marked peak
intensity loss of framework aluminum (55 ppm) is observed on
6Mo/H100Na0M-30Al compared with that of 6Mo/H38Na62M-30Al
which means more Mo species interact with the framework alu-
minum, i.e. Br␾nsted acid sites on the H100Na0M-30Al support. In
the case of H38Na62M-30Al, Mo species tend to locate on alumina
or aggregate on the surface of support due to the steric hin-
drance of zeolite channels from sodium ions. This is in accordance
for this phenomenon is that the acidic hydroxyls from alumina
(2.2 ppm) dominate the position of this peak when the sodium
content is high. As sodium ion is gradually replaced by hydro-
gen, the peak intensity from non-framework Al–OH (2.4 ppm) in
H-mordenite increases leading to a low field shift of the main
peak. On 6Mo/H38Na62M-30Al sample, almost no bridge hydroxyl
could be observed. This is in accordance with the NH3-TPD results
as Br␾nsted hydroxyls are the main source of strong acid sites.
Quantitative analysis of the spectra (Table 1) demonstrates that
the concentration of Brønsted acid sites increases from 0.143 to
0.221 mmol/g when the sodium exchange degree changes from 67%
to 100%.
3.3. H2-TPR and 27Al MAS NMR results
From the above results, it is clear that changes of the sodium
exchange degree in mordenite may result in variation of acid
amount and distribution in the HnNa100–nM-30Al support which
may further influence the anchoring modes and location of Mo
species on the support. Thus, H2-TPR experiments were per-
formed to obtain more information about the redox behavior of
6Mo/HnNa100–nM-30Al catalysts. As shown in Fig. 4(a), three main
reduction peaks locating at around 475, 570, and 900 ◦C could
be differentiated on 6Mo/H38Na62M-30Al catalyst. With progres-
sive exchange degree of sodium ions, the 475 ◦C peak shifts to
high temperature. Meanwhile, a reduction peak appears at 425 ◦C.
Simultaneously, the peak at 567 ◦C shifts to high temperature indi-
cating the different distribution of Mo species on the support.
The assignment to different reduction peaks is not straightforward
due to the presence of different molybdenum species interact-
ing with the surface to different extents. It is generally accepted
that Mo species in tetrahedral state are difficult to reduce because
of its strong interaction with the support, especially at low load-
ings. Octahedral and other higher polyhedral species are easy
to be reduced. On basis of the earlier reports [20–22], reduc-
tion peak at 425 ◦C may be assigned to the first step reduction
of Mo6+→Mo4+ of the octahedral Mo species, especially for that
on alumina. The high-temperature peak at around 900 ◦C may
be related to the reduction of tetrahedral species as well as for
the second step reduction of octahedral Mo species. The reduc-
tion peak at around 600 ◦C indicates the presence of polymeric
Mo species linked with zeolite proposed by Solis et al. [22,23].

X. Li et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 121–127
125
Fig. 6. Butene conversion and product selectivity over 6Mo/HnNa100−nM-30Al catalysts with different sodium exchange degrees in the support: (ꢀ) 6Mo/H38Na62M-30Al
(ꢁ) 6Mo/H67Na33M-30Al (ꢀ) 6Mo/H82Na18M-30Al (ꢁ) 6Mo/H100Na0M-30Al (reaction temperature: 150 ◦C, pressure: 0.1 MPa, WHSV of 1-butene: 2.0 h−1).
with the H2-TPR results. In the octahedral region, a broad signal
centered at ca. 10 ppm is observed, which can be attributed to
the six-coordinated aluminum in alumina. This signal is also over-
lapped with that from the non-framework Al in mordenite zeolite.
Besides, a small peak appears at −11 ppm, corresponding to the
nonhydrated forms of the aluminum molybdate phase [11,25,26].
Therefore, a chemical reaction between the Mo species and the
support happens, which results in the formation of aluminum
molybdate.
3.4. Metathesis activity of 6Mo/HnNa100−nM-30Al catalysts
1-Butene double-bond isomerization as well as metathesis may
occur on 6Mo/HnNa100−nM-30Al catalysts. No skeletal isomeriza-
tion product i.e. iso-butene is detected. A schematic representation
of possible reaction pathways of 1-butene over 6Mo/HnNa100−nM-
30Al catalysts is shown in Scheme 1. It is clear that propene,
pentene, ethene and hexene are the main metathesis products.
Propene may come from metathesis reaction of 1-butene and

126
X. Li et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 121–127
Table 2
1-Butene metathesis and isomerization performance over 6Mo/HnNa100−nM-30Al catalysts (TOS: 4 h, temperature: 150 ◦C, pressure: 0.1 MPa, WHSV: 2.0 h−1).
Catalyst
Yethene (%)
Ypropene (%)
Ypentene (%)
Yhexene(%)
Y2-butene (%)
n1-butene/n2-butene
6Mo/H38Na62M-30Al
6Mo/H67Na33M-30Al
6Mo/H82Na18M-30Al
6Mo/H100Na0M-30Al
8.8
11.0
8.2
2.9
19.1
24.4
27.4
2.6
16.4
19.6
19.8
10.0
11.8
8.6
5.1
8.2
18.9
30.6
14.0
3.9
0.94
0.25
4.2
6.7
2-butene (Metathesis I) or metathesis of ethene and 2-butene
(Metathesis III). Ethene and hexene are the products from 1-butene
self-metathesis (Metathesis II).
6Mo/H67Na33M-30Al. On 6Mo/H38Na62M-30Al, hexene selectivity
is a little higher than that of ethene. Such phenomenon is more
obvious on 6Mo/H100Na0M-30Al as more ethene is consumed by
Metathesis III.
Catalytic performances of 6Mo/HnNa100−nM-30Al catalysts in
1-butene metathesis reaction are shown in Fig. 6. For all the
catalysts, their initial activity increases with time until the peak-
point is attained. This is ascribed to the induction period during
which metathesis active intermediates are generated in situ under
olefin atmosphere. This is common to a number of catalysts, such
as Mo/Al2O3 and Mo/SiO2 catalysts used in propene metathe-
sis [20,27,28]. It should be pointed out that a longer induction
period is observed on 6Mo/H38Na62M-30Al which lasts about 4 h.
That means the state of Mo species or the low acid density on
6Mo/H38Na62M-30Al is not beneficial for the rapid formation of
active metathesis sites. As shown in Fig. 6(a), butene conversion is
around 25% on 6Mo/H38Na62M-30Al, while 60% for the other three
samples. A little higher conversion is observed on 6Mo/H82Na18M-
30Al than that on 6Mo/H100Na0M-30Al. This may be linked with
the isomerization activity of the catalysts as shown in Fig. 6(b).
On 6Mo/H100Na0M-30Al, the initial molar ratio of 2-butene to 1-
butene in the product is up to 6.6 and it remains above 2.0 after
24 h, much higher than that over other catalysts. High 2-butene
concentration in the feed is not favorable for the metathesis of
1-butene and 2-butene, especially for 1-butene self-metathesis.
Supposing that 2-butene self-metathesis happens during the pro-
cess, the product could not be differentiated from the 2-butene
isomers. Over 6Mo/H82Na18M-30Al catalyst, the molar ratio of
2-butene to 1-butene maintains around 1.0 during the total pro-
cess, which is equal to the stoichiometry value of 2-butene and
1-butene metathesis reaction. On the other hand, the ratio is below
0.5 on 6Mo/H67Na33M-30Al sample. Such ratio is favorable for the
1-butene self-metathesis reaction. Poor isomerization activity is
observed on 6Mo/H82Na18M-30Al due to its low acidity.
Propene selectivity is closely related to sodium exchange degree
of the support. As shown in Fig. 6(c), the highest propene selectiv-
ity of about 47% is obtained on 6Mo/H100Na0M-30Al followed by
6Mo/H82Na18M-30Al, 6Mo/H67Na33M-30Al and 6Mo/H38Na62M-
30Al with propene selectivity of about 40%, 30% and 15%,
respectively, which following the same order of sodium contents.
As shown in Scheme 1, propene could be obtained from Metathe-
sis III besides Metathesis I. High 2-butene concentration in the
feed is beneficial for Metathesis III which may well explain the
trend of propene selectivity change. As far as the yield is con-
cerned, 6Mo/H100Na0M-30Al gives the highest propene yield of
about 27.4% as shown in Table 2. Due to the various routes for
propene generation, it is rational that the selectivity for pentene
is a little lower than that of propene on the same catalyst.
Combining the characterization results with the catalytic per-
formances, it is obvious that acid sites in mordenite zeolite of
HM-30Al supports play two important roles in the 1-butene
metathesis reaction. One is that the exposure degree of acid sites,
especially strong acid sites may influence the location and state of
Mo species on the support. As pointed out by Debecker and Hand-
zlik et al., the existence of suitable number of Br␾nsted acid site is
beneficial for the genesis of active metathesis centers [12,29]. Thus,
high sodium exchange degree will contribute to the formation of
metathesis active Mo species. The other role of acid site lies in its
catalytic activity in 1-butene isomerization. Substantial amount of
acid sites on the catalyst is required to guarantee the double bond
isomerization which is the requisite step of the following objec-
tive metathesis reaction. To maximize propene yield, the rate of
1-butene double bond isomerization should match with that of the
cross-metathesis of 1-butene and 2-butene. As shown in Table 2,
1-butene self-metathesis dominates on 6Mo/H38Na62M-30Al due
to the limitation of 1-butene isomerization. On 6Mo/H82Na18M-
30Al, the molar ratio of 2-butene to 1-butene amounts to 1.0 and
the number of metathesis active site becomes the pivotal factor
in determining the metathesis activity. Highest propene yield is
obtained on 6Mo/H100Na0M-30Al due to its high acid density and
well dispersion of Mo species.
4. Conclusions
Effect of sodium exchange degree on the performance of
6Mo/HnNa100−nM-30Al catalyst in 1-butene metathesis was inves-
tigated. It was found that the metathesis product distribution
could be tuned through adjusting the surface acidity of the sup-
port which can be controlled by changing the sodium exchange
degree of mordenite. Ethene (hexene) was the main product at high
sodium content and propene gradually became the dominant prod-
uct upon increasing sodium exchange degree. 6Mo/H100Na0M-30Al
with highest sodium exchange degree exhibited the best metathe-
sis performance, and high propene yield of 27.4% was obtained.
Well dispersion of Mo species and high density of Br␾nsted acid
site, as evidenced by H2-TPR and 1H MAS NMR results, benefit
catalytic performance of 6Mo/HM-30Al.
Acknowledgments
We are grateful for the financial support of the National Natural
Science Foundation of China (Grant No. 20903088 and 21006104).
6Mo/H38Na62M-30Al exhibits the highest ethene and hex-
ene selectivities which indicates that 1-butene self-metathesis
dominates in the reaction network. In this case, Metathesis I is
suppressed due to the low molar ratio of 2-butene to 1-butene.
Although ethene is the dominant product on 6Mo/H38Na62M-30Al,
its yield (8.8%) is lower than that of 6Mo/H67Na33M-30Al (11.0%).
It could be concluded that the metathesis activity is closely linked
with the catalyst acidity and location of the Mo species. Exposure
of more acid sites in H67Na33M-30Al benefits the formation of dis-
persed Mo species which contributes to the high ethene yield on
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