Sulphated mesoporous La2O3-ZrO2 composite oxide as an efficient and reusable solid acid catalyst for alkenylation of aromatics with phenylacetylene

Article abstract of DOI:10.1016/j.apcata.2015.01.023

Sulphated mesoporous La₂O₃-ZrO₂ composite oxides (SO₄^2-/meso-La₂O₃-ZrO₂) were firstly prepared by a surfactant-assisted co-precipitation/hydrothermal crystallization w

Full text of DOI:10.1016/j.apcata.2015.01.023

Accepted Manuscript
Title: Sulphated mesoporous La2O3-ZrO2 composite oxide as
an efficient and reusable solid acid catalyst for alkenylation of
p-xylene with phenylacetylene
Author: Zhongkui Zhao Jinfeng Ran
PII:
S0926-860X(15)00038-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.01.023
APCATA 15215
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
13-10-2014
27-12-2014
15-1-2015
Please cite this article as: Z. Zhao, J. Ran, Sulphated mesoporous La2O3-
ZrO2 composite oxide as an efficient and reusable solid acid catalyst for
alkenylation of p-xylene with phenylacetylene, Applied Catalysis A, General (2015),
http://dx.doi.org/10.1016/j.apcata.2015.01.023
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Sulphated mesoporous La2O3-ZrO2 composite oxide as an efficient and reusable solid
acid catalyst for alkenylation of p-xylene with phenylacetylene
Zhongkui Zhao*, Jinfeng Ran
State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and
Engineering, School of Chemical Engineering, Dalian University of Technology, 2 Linggong
Road, Dalian 116024, China
2-
SO4 /meso-La2O3-ZrO2 is an efficient solid acid catalyst for direct alkenylation.
Developed catalyst shows high selectivity and easily regeneration behavior.
La incorporation into ZrO2 significantly improves catalytic performance.
Catalytic property strongly depends on catalyst nature affected by La addition.
This work presents a robust catalyst for producing α-arylstyrenes via alkenylation.
Sulphated mesoporous La2O3-ZrO2 composite oxide as an efficient and reusable solid acid
catalyst for alkenylation of aromatics with phenylacetylene
Zhongkui Zhao*, Jinfeng Ran
State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering,
School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian
116024, China. E-mail: zkzhao@dlut.edu.cn
1
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* Corresponding author.
Tel.; +86 411 84986354.
Fax: +86 411 84986354.
E-mail: zkzhao@dlut.edu.cn (Z. K. Zhao).
ABSTRACT
2-
Sulphated mesoporous La2O3-ZrO2 composite oxides (SO4 /meso-La2O3-ZrO2) were firstly
prepared by a surfactant-assisted co-precipitation/hydrothermal crystallization with
2-
subsequent impregnation method (SACPHC-IM). The prepared SO4 /meso-La2O3-ZrO2
catalyst was employed as solid acid to catalyze direct alkenylation of p-xylene with
2
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phenylacetylene for clean production of α-arylstyrene. Various characterization techniques
such as N2 adsorption-desorption, X-ray diffraction (XRD), FT-IR, NH3
temperature-programmed desorption (NH3-TPD) were employed to reveal the relationship
2-
between catalyst nature and catalytic performance. The as-prepared SO4 /meso-La2O3-ZrO2
2-
catalyst shows much superior catalytic activity and similar selectivity to SO4 /meso-ZrO2,
ascribed to increasing acid properties, enlarging specific surface area and increasing pore
volume, reducing average crystalline size resulting from the promoting effect of adding La
2-
into ZrO2 matrix. Furthermore, the SO4 /meso-La2O3-ZrO2 catalyst was optimized by
2-
varying the atomic ratio of La/(La+Zr) from 0.05 to 0.2. The SO4 /meso-La2O3-ZrO2 catalyst
with the 0.1 of optimum La/(La+Zr) atomic ratio exhibited excellent catalytic performance,
ascribed to the increase in amount acid sites. The results for stability test demonstrated that
close to 100% of maximum conversion can be achieved, and more than 80% of conversion
can be maintained with the time on stream up to 780 min. Through simple calcination
treatment, the spent catalyst can be almost complete recovered. The developed
2-
SO4 /meso-La2O3-ZrO2 catalyst could be a potential candidate for α-arylstyrene production
via acid catalyzed direct alkenylation of aromatics with phenylacetylene.
2-
Keywords: Mesoporous materials; SO4 /ZrO2 solid acid; La modification; Alkenylation;
Heterogeneous catalysis
1. Introduction
The alkenylation of aromatic compounds, also called as hydroarylation of alkynes,
presents a simple and atom economic approach for alkenylaromatic compounds, which are
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extensively used in diverse fine chemical fields like dyes, pigments, pharmaceuticals, flavors,
agrochemicals, and natural products [1]. Recently, new and practical acid-catalyzed route
through the formation of vinyl cations from alkynyl group for alkenylation reaction of
aromatic compounds has been considered as a promising alternative to the classical precious
metal and transition metal complexes catalyzed route through the formation of σ-aryl metal
complexes between metal complex and aromatic ring [2-4].
Although the acid-catalyzed Friedel-Crafts alkylation is a very mature and extensively
industrially used technique, some problems like more easily oligomerization of alkynes due
to worse stability of vinyl cation than that of alkyl one, dissatisfactory selectivity, and narrow
substrate scope remain to be resolved. Many efforts were continuously made on the acid
catalyzed alkenylation reaction [5-7]. The traditional alkylation Lewis acid catalyst such as
AlCl3 could successfully catalyze alkenylation, but only 6% of extremely low yield with
heavy pollution was obtained [8]. By employing the metal triflates M(OTf)x, a
breakthrough has been made, and a good yield for alkenylated products was achieved .
Moreover, the FeCl3 [4], HSO3F, CF3SO3H [9], [BMIM][Sb2F11] [10], etc. were also
employed to catalyze this reaction, but there still exist the separation and equipment
corrosion issues.
Heterogeneous catalysis provides a new and practical alternative to homogeneous
catalysis in terms of its clean, easy-separation, catalyst reusability, and its applicability
towards continuous production in a large scale. Solid acid catalysts, especially H-type
zeolites, have been extensively used in Friedel-Crafts alkylation [11-15], but rare reports on
Friedel-Crafts alkenylation can be found. Sartori and his colleagues had made some
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pioneering work on solid acid catalyzed alkenylation of aromatics over a Y-type HSZ-360
zeolite, unfortunately the results are not satisfactory, and there exits an irreconcilable
contradiction between the selectivity and the catalytic activity. The zeolites calcined at
lower temperature exhibited high catalytic activity, but the considerable amount of
acetophenone (5-20%) was detected; the higher calcination temperature could efficiently
compress the formation of acetophenone, but resulted in remarkable decrease in the
catalytic activity. Moreover, the low catalytic efficiency (1.0 g catalyst just for 1.0 g
phenylacetylene) is another issue while this Y-type zeolite was used for the alkenylation,
ascribed to the reaction only taking place on the external surface of the catalyst owing to
its narrow pore channels within zeolite [16]. Although 95% of high alkenylated yield can
be achieved on HUSY zeolite (superior to Nafion SAC-13), it only can be used in the
intermolecular alkenylation of special substrates [17]. Therefore, the development of novel,
highly-efficient and robust solid acid catalysts for alkenylation with wide substrate scopes is
highly desirable.
In our NCES research group, we previously presented that the mesoporous supported
phosphotungstic acid catalyst on MCM-41 demonstrated much superior catalytic activity,
selectivity and stability to HY zeolite for the alkenylation of aromatic compounds with
phenylacetylene, and good reaction results can be obtained while the diverse substrates
were used [18]. However, there still exist many issues to be resolved. The catalytic
stability is a seirous problem, as well as the selectivity is to be improved. Moreover, the
facile calcination process cannot be used in the regeneration of the spent catalyst owing to
the resuting decomposition of phosphotungstic acid at calcining coke process. To wash the
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spent catalyst by nonsolar solvent doesn’t work on its recovery. Although the washing by
polar solvent i.e acetone can eilminate the deposited coke from catalyst, the inevitable
phosphotungstic acid leaching in polar solvent requires adding extra phosphotungstic acid
to maintain the cataytic activity. Therefore, for one thing, we would fabricate new
structured supported phosphotungstic acid catalysts to significantly improve the catalytic
stability; for another thing, to develop the robust mesoporous solid acids that can bear
high temperature for calcination regeneration process of coke-deposited catalyst, as well
as they would demonstrate inproved catalytic activity, selectivity, stability. Recently, the
mesoporous Fe-Al-MCM-41 was demonstarted to show high catalytic activity in direct
alkenylation of phenols with phenylacetylene [19], however, the selectivity is still to be
improved.
In this work, we aimed to develop a robust mesoporous solid acid catalyst for
alkenylation, and the used solid acid catalyst may be regenerated by a facile calcination
process. The sulphated mesoporous solid superacids were previously reported to efficiently
catalyze diverse reactions [20-22], but no report on solid superacids catalyzed alkenylation
can be seen. Moreover, rare earth metals especially La has been found to efficiently improve
the catalytic performance of diverse solid acids like zeolite, sulfated zirconia [23-25]. From
above, we proposed that sulphated mesoporous La2O3-ZrO2 can be an efficient and robust
2-
catalyst for alkenylation. Herein, the SO4 /meso-La2O3-ZrO2 were firstly prepared by a
SACPHC-IM method and employed as solid acid to catalyze direct alkenylation of p-xylene
with phenylacetylene for clean production of α-arylstyrene. The sulphated mesoporous
2-
SO4 /meso-ZrO2 catalyst was also prepared by using ACPHC-IM method and was used in
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this reaction. Various characterization techniques such as N2 adsorption-desorption, XRD,
FT-IR, NH3-TPD were employed to reveal the relationship between catalyst nature and
catalytic performance. It was found that the sulfonated soid superacid exhibited promising
2-
catalytic performance in alkenylation, and the as-prepared SO4 /meso-La2O3-ZrO2 catalyst
2-
shows much superior catalytic activity and similar selectivity to SO4 /meso-ZrO2.
2-
Furthermore, the SO4 /meso-La2O3-ZrO2 catalyst was optimized by varying the atomic ratio
of La/(La+Zr) from 0.05 to 0.2. The results for stability test demonstrated that close to 100%
of maximum conversion can be achieved, and more than 80% of conversion can be
maintained with the time on stream up to 780 min. Through simple calcination treatment, the
2-
spent catalyst can be almost complete recovered. The developed SO4 /meso-La2O3-ZrO2
catalyst could be a potential candidate for α-arylstyrene production via acid catalyzed direct
alkenylation of aromatics with phenylacetylene.
2. Experimental
2.1. Catalyst preparation
The mesoporous La2O3-ZrO2 composite oxides with diverse atomic ratio of La to La and
Zr (nLa/(La+Zr)) and mesoporous ZrO2 were synthesized by SACPHC method. Typically, 16.2
mmol zirconium nitrate (Sinopharm Group, AR) and 1.8 mmol of lanthanum nitrate (Tianjin
Guangfu Institute, AR) were dissolved in deionizied water. A certain amount of CTAB
(Kermel, AR) was added to 25 ml of deionized water to give a clear homogeneous solution.
Then, the two solutions were mixed under continuously stirring for 2 h. Afterward,
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pre-determined amount of ammonium solution (25%) was added drop by drop under vigorous
stirring, leading to a final pH value of 11. After precipitation, the gelatinous pale yellow
mixture was stirred for further 4 h and subsequently transferred to Teflon autoclaves, which
o
were kept at 105-120 C for 60 h. The solid product was filtered, washed with deionized
water and ethanol, subsequently dried at 105 ◦C overnight, and then calcined in a muffle
furnace in air at 550 oC for 6 h. The La2O3-ZrO2 composite oxides with diverse nLa/(La+Zr) and
mesoporous ZrO2 were prepared by using the similar procedure as above but different
2-
nLa/(La+Zr) or only neat Zr. The SO4 /meso-La2O3-ZrO2 with diverse nLa/(La+Zr) and
2-
SO4 /meso-ZrO2 catalysts were prepared by impregnation method using 1M H2SO4 as
impregnating reagent and the subsequently dried at 105 ◦C for 2 h and then calcined at 500 oC
for 3 h.
2.2. Catalyst characterization
o
Nitrogen adsorption experiments at -196 C were carried out on a Quantachrome
Autosorb instrument. The specific surface area was calculated by the BET method, and the
pore size distribution was calculated by BJH model from the adsorption branch. XRD
patterns of the samples were recorded from 0 to 10 o and 10 to 80 o with a step width of 0.02 o
using a Rigaku D/max-2400 apparatus using Cu Kα radiation. FT-IR spectra were collected in
the wavenumber range of 4000-400 cm-1 on EQUINOX-55 Fourier Transform Infrared
Spectrometer (BRUKER). NH3-TPD measurements were performed to characterize the
acidity of the samples. After pre-treatment of 50 mg samples in Ar (up to 300 oC with a ramp
rate of 10 oC min-1, then kept for 0.5 h, 30 ml min-1 Ar flow), the samples were saturated with
8
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o
ammonia (10% NH3-90% Ar) at 100 C via the pulse injection of pure ammonia in a Ar
o
stream. Finally, the NH3 desorption step was performed from 100 and 700 C at a ramp rate
o
of 10 C min-1 with 30 ml min-1 of Ar flow. The NH3-TPD profiles were obtained via
monitoring the desorbed ammonia with a thermal conductivity detector.
2.3. Catalytic performance test
The experiments on the alkenylation of p-xylene with phenylacetylene were performed in
a stainless steel fixed-bed continuous-flow reactor. 1.0 g of catalyst suppressed into tablets
and sieved into 20-60 mesh was loaded into the reactor for all the reaction tests, and the
remaining space of the reactor tube was filled with 20-60 mesh quartz granules. Before the
introduction of feedstock, the catalyst was in situ preactivated for 1 h in N2 flow at 30 ml
min-1. The liquid stream was introduced into the fixed-bed reactor by a syringe pump. N2
(99.999% purity) was used to maintain system pressure. Quantitative analysis of the collected
reaction mixture from the fixed-bed reactor was performed on a FULI 9790 II GC equipped
with an HP-5 column, 30 m×0.32 mm × 0.25 lm, and an FID detector. As an evaluation
standard of alkenylation reaction, the phenylacetylene conversion was calculated by weight
percentage of the transformed phenylacetylene in the total amount of introduced
phenylacetylene in the feed. Based on the GC-MS data and our previous work [10], besides
the main product -arylstyrene (I), a series of byproducts like acetophenone (II),
-(2,5-dimethylphenyl) styrene (III) -(2,5-dimethylphenyl) ethylbenzene (IV), and
oligomers (V) can be detected, indicating the alkenylation reaction of p-xylene with
phenylacetylene is a quite complex competition process. Besides the alkenylation, many
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sidereactions like hydration of phenylacetylene, hydrogenation of -arylstyrene, the
thermodynamic control alkenylation and the oligomerization reactions take place
simultaneously (Scheme 1). In the end, the catalyst stability and regeneration performance
were measured. The used catalyst was recovered by calcination process, and no extra fresh
catalyst was added for the regeneration performance test.
Scheme 1 The alkenylation reaction process of p-xylene with phenylacetylene over the
2-
SO4 /meso-La2O3-ZrO2 solid acid catalyst.
3. Results and discussion
3.1 The effect of La-doping
It was previously established that mesoporous solid acid exhibited much superior
catalytic performance in alkenylation reaction containing activity, selectivity, and stability
[10]. But the catalytic stability and regeneration performance are definitely required to be
improved. Therefore, to develop a novel solid acid with improved catalytic stability and
regeneration performance besides catalytic activity and selectivity is highly desirable.
Sulfonated ZrO2 has been employed to catalyze diverse reactions [26-30], but no report on
alkenylation can be found. It was previously reported that the catalytic performance of solid
acids can be efficient improved by introduction of rare earth metals, especially by La [31-35].
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Therefore, in this work, we firstly investigated the catalytic properties of La-modified and
2-
unmodified SO4 /meso-ZrO2 on the alkenylation of p-xylene with phenylacetylene, and the
reaction results are depicted in the Table 1.
2-
2-
From Table 1, in comparison of SO4 /meso-ZrO2, the SO4 /meso-La2O3-ZrO2 catalyst
exhibits higher catalytic activity with similar main product (I) percentage, and 82.7% of
phenylacetylene conversion can be obtained. Moreover, more oligomers but less
2-
-(2,5-dimethylphenyl) styrene isomer (III) over SO4 /meso-ZrO2 than that over
2-
SO4 /meso-La2O3-ZrO2 can be observed. In contrast to well-established alkylation, solid acid
catalyzed alkenylation remains huge challenge to be resolved, and biggest difficulty is to
efficiently inhibit the oligomerization of alkynes. Besides the high activity and main product
2-
percentage, the compressed oligomerization by adding La allows the SO4 /meso-La2O3-ZrO2
solid acid to become a promising catalyst for alkenylation. Then various characterization
techniques such as FT-IR, NH3-TPD, nitrogen adsorption-desorption, XRD, and were
employed to reveal the relationship between catalyst nature and catalytic properties.
The acidic properties play a decisive role in the catalytic performance of solid acid
catalyst. The introduction of SO42- group on the surface ZrO2 can form surface acid sites. Fig.
1 shows the FT-IR spectra of meso-La2O3-ZrO2 before and after sulfated. Comparing with the
pristine meso-La2O3-ZrO2 composite oxide, four new bands at 1225, 1137, 1043, and 995
2-
cm-1 on the FT-IR of SO4 /meso-La2O3-ZrO2 sample assigned to characteristics of a bidentate
2-
2-
SO4 coordinated to Zr4+ can be observed. Such a chelating bidentate SO4 structure could
strongly withdraw electrons from its neighboring metal cations, resulting in a number of
electronic deficient metal centres on the metal cations that act as strong Lewis acid sites [36],
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and partial Lewis acid site converts to the Bronsted acid site when it absorbs water [37]. Both
B and L acid sites can catalyze the alkenylation reaction. The introduction of La into ZrO2
lattice can affects the electronic structure, and therefore hopefully changes the surface acidic
properties. NH3-TPD experiments were performed on the La-modified and unmodified
2-
SO4 /meso-ZrO2 to reveal the effect of La addition into ZrO2 on the catalytic properties. The
NH3-TPD profiles are presented in Figure 2.
From Fig. 2, the NH3-TPD profiles can be divided into four regions. The low
temperature region in the range of 150-350 oC (centred at 250 oC) is believed to be caused by
o
weak acidic sites, and second region between 350 to 450 C can be assigned to medium
o
strong acidic sites, as well as the region of 450 to 600 C can be appointed as strong acidic
sites. Moreover, the last peak appearing at 638 oC should be superacid sites. In comparison of
2-
2-
unmodified SO4 /meso-ZrO2, the modified SO4 /meso-La2O3-ZrO2 solid acid demonstrates
much more weak and medium acid sites owing to the change in electronic features of ZrO2
led by the La addition [38], which are active sites for alkenylation [18]. Therefore,
2-
SO4 /meso-La2O3-ZrO2 solid acid catalyst exhibits superior catalytic activity to the
2-
2-
unmodified SO4 /meso-ZrO2. Moreover, the more strong acid sites on SO4 /meso-ZrO2 leads
to more oligomers [13], the further polymerization of these oligomers can lead to coke
formation, which further results in deactivation of catalyst.
Furthermore, nitrogen adsorption-desorption and XRD measurements were performed to
reveal the effect of La addition on the catalyst textural feature, structure, and catalytic
performance. Fig. 3 presents nitrogen adsorption-desorption isotherms and pore size
distributions from adsorption branch. From Fig. 3, according to the IUPAC classification, the
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isotherms with type IV is the typical feature of mesoporous materials [39], indicating the
existence of mesoporous structure in the two solid acids. The mesoporous
2-
2-
SO4 /meso-La2O3-ZrO2 and SO4 /meso-ZrO2 solid acids have been successfully fabricated
by SACPHC-IM method. Furthermore, we can see that adding La can lead to an obvious
increase in pore volume from 0.23 to 0.28 ml g-1 and in specific surface area from 31.8 to
62.4 m2 g-1, which may result in more acid sites that improves catalytic activity. Moreover,
the introduction of La into ZrO2 can result in narrower pore distribution, which may be
favorable for alkenylation reaction.
The effect of adding La on the catalyst structure was furthermore investigated by XRD
2-
2-
experiments. The XRD patterns of as-prepared SO4 /meso-La2O3-ZrO2 and SO4 /meso-ZrO2
solid acid catalysts are presented in Fig. 4. From the low-angle XRD patterns of the two
catalysts presented in Fig. 4a, the resolved diffraction peaks in the low-angle XRD pattern of
2-
SO4 /meso-La2O3-ZrO2 can be observed, indicating the ordered mesopores in this material,
2-
while no well-resolved diffraction peaks in the low-angle XRD pattern of SO4 /meso-ZrO2,
suggesting its disordered mesoporous structure. The wide-angle XRD patterns of the
2-
La-modified and unmodified SO4 /meso-ZrO2 are shown in Fig. 4b. For the unmodified
2-
SO4 /meso-ZrO2, the well-resolved diffraction peaks assigned to tetragonal (JCPDS:
050-1089) and monoclinic (JCPDS: 037-1484) phases of ZrO2 can be observed [35].
o
However, only five well-resolved diffraction peaks at 30.1, 34.7, 50.1, 59.5, and 73.7
corresponding to the (011), (110), (112), (121), and (123) planes of tetragonal phase of ZrO2
can be observed, suggesting the addition of La can efficiently induce the formation of
tetragonal structure rather than monoclinic phase, which is in good agreement with the results
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in the reference [41]. The diffraction peaks of ZrO2 become weaker and wider as the La is
introduced, which indicates the decrease in average crystalline sizes resulted from the
introduction of La. Moreover, no diffraction peak corresponding to La2O3 on the XRD pattern
2-
of SO4 /meso-La2O3-ZrO2 can be observed although the 0.1 of higher nLa/(La+Zr) is used,
suggesting the possible insertion of La into ZrO2 matrix, consisting with the reported results
[42]. Moreover, the shift of diffraction peaks corresponding to ZrO2 phase to smaller angle
can be observed in the presence of La, indicating an enlargement in ZrO2 unit cell. The
expansion of ZrO2 lattice appears owing to the replacement of Zr4+ (0.84 Å) in the tetragonal
phase structure by the La3+ (1.06 Å) with a larger ion radius. This result further confirms the
La insertion into ZrO2 matrix, which can strengthen the La-Zr interaction.
Correlated the diverse characterization results and reaction results, it can be found that the
2-
unique structural characteristic of La modified SO4 /meso-ZrO2 may be responsible for its
higher specific surface area, larger pore volume, strengthened La-Zr interaction, and their
resulting more weak and medium acid sites, which can remarkably affect the catalytic
performance of solid acid catalyst for alkenylation reaction of p-xylene with phenylacetylene.
Now that the addition of La can significantly improve the catalytic properties of
2-
SO4 /meso-ZrO2 for alkenylation, hopefully the catalytic performance can be further
2-
improved by varying the atomic ratio of La to La and Zr in the SO4 /meso-La2O3-ZrO2
catalyst. Therefore, the followings we investigated the effect of La/(La + Zr) atomic ratio.
3.2 Effect of La/(La + Zr) atomic ratio
2-
As is shown as above, the catalytic performance of mesoporous SO4 /ZrO2 for
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alkenylation of p-xylene is significantly dependent on the acidic properties, surface area, pore
volume, pore size distribution, catalyst structure affected by the addition of La. Herein, we
2-
prepared a series of SO4 /meso-La2O3-ZrO2 solid acids with diverse nLa/(La+Zr) (0.05, 0.10,
0.15, 0.20), and measured their catalytic performance for alkenylation of p-xylene. Table 2
presents the reaction results. From Table 2, the phenylacetylene conversion increases with the
rise of nLa/(La+Zr). It reaches a maximum value (82.7%) as the nLa/(La+Zr) increases up to 0.10,
and then decreases with the further increase in nLa/(La+Zr). Moreover, it can be found that the
increase in nLa/(La+Zr) results in a continuously decrease in the percentage of main product I,
but an increase in the percentage of both sideproduct III and V. Especially, while the nLa/(La+Zr)
was increased from 0.15 to 0.20, significantly increase in percentage of both sideproduct III
2-
and V can be observed. From the reaction results, the SO4 /meso-La2O3-ZrO2 solid acid with
0.10 of nLa/(La+Zr) can be established for alkenylation reaction.
The catalytic performance of solid acids is remarkably dependent on their acid properties,
and therefore, the NH3-TPD techniques direct corresponding to acid properties were
employed to reveal the relationship between catalytic properties and catalytic performance of
2-
the as-prepared series of SO4 /meso-La2O3-ZrO2 solid acids with different molar ratios of La
to La and Zr. The NH3-TPD profiles are shown in Fig. 5.
From Fig. 5, the acid properties are significantly affected by nLa/(La+Zr), and the amount of
weak and medium strong acid sites increases with the increase of nLa/(La+Zr), and reaches the
maximum as the nLa/(La+Zr) is 0.10. The further increase in nLa/(La+Zr) leads to a decrease in the
amount of weak and medium strong acid sites. Correlated to the acid nature to the reaction
results of alkenylation, it can be concluded that the weak and medium strong acid sites are
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responsible for the alkenylation reaction. Furthermore, the continuously strengthened super
acid sites lead by the increasing nLa/(La+Zr) can be observed, which can promote the
oligomerization of phenylacetylene and the thermaldynamic controlled alkenylation [13].
Therefore, the increase in nLa/(La+Zr) results in the increased percentage of both
-(2,5-dimethylphenyl) styrene (III) and oligomers (V). The increase in oligomers may
further lead to heavy coke deposition which contributes to deactivation of solid acid catalyst.
The future work would be focused on compressing the super acids on the new mesoporous
solid acid catalysts.
3.3. Effect of reaction parameters
Based on previously reported results for the effect of eaction conditions on the
alkenylation of p-xylene over HPA/MCM-41 catalyst, herein we investigated the effect of
reaction temperature (Tr) and volume hourly space velocity (VHSV) on the alkenylation
2-
reaction over the developed SO4 /meso-La2O3-ZrO2 solid acid catalyst. The reaction results
are listed in Table 3 and 4, respectively.
From Table 3, the conversion of phenylacetylene increases with the rise of Tr up to 170
oC and reaches 98.4% of maximum at 170°C. The further increased reaction temperature
results in a decrease in conversion, ascribed to the rapid deactivation at higher temperatures
[43]. The change in Tr only leads to slight vibration in the percentage in sideproducts. 170 oC
2-
is the optimum Tr for the alkenylation reaction over SO4 /meso-La2O3-ZrO2 solid acid
catalyst.
VHSV plays a significant role in the continuous-flow fixed bed reaction. As is shown in
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Table 4, the conversion is enhanced with the increase of VHSV, and reaches 98.4 as VHSV
rises up to 6 ml h-1 g-1cat, but further increase in VHSV lead to a decrease in conversion. The
lower VHSV means a longer time that the reactant and product molecules stay in the catalyst
layer, which more easily leads to more serious oligomerizaiton and even the generation of
high polymers for coke deposition. In addition, the long-time stay of the reactant is more
favorable to the production of isomer which rises from 5.0% to 11.0% while the VHSV
decreases from 6 to 4 ml h-1 g-1cat. On the other hand, too high VHSV stands for a large
number of reactants flowing through the catalyst layer in the unit time. The catalyst cannot
provide adequate active sites for so many reaction substrates. In addition, it can also be
speculated that at high VHSV part of the reactants did not have enough time to access the
acid sites inside the pores [44]. From above, a VHSV of 6 ml h-1 g-1cat is appropriate.
3.4. Catalytic stability and regeneration performance
The catalytic stability and regeneration behavior are crucial for a solid acid catalyst in
practical application. Therefore, we investigated the stability and recovery performance of the
2-
developed SO4 /meso-La2O3-ZrO2 solid acid in alkenylation reaction. Fig. 6 presents the
conversion and selectivity as a function of time on stream for the alkenylation of p-xylene
2-
with phenylacetylene over the fresh and calcining regenerated SO4 /meso-La2O3-ZrO2 solid
acid catalysts.
As illustrated in Fig. 6, more than 80% conversion can be maintained for up to 780 min
of time on stream. In comparison with our previously reported HPA/MCM-41 catalyst, the
2-
developed SO4 /meso-La2O3-ZrO2 catalyst in this work demonstrates higher catalytic
17
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stability, and about 70% conversion can be maintained for up to 1500 min of time on stream
2-
over SO4 /meso-La2O3-ZrO2 catalyst, while less than 40% conversion over HPA/MCM-41
catalyst can be observed. Furthermore, from Fig. 6, the catalytic performance of the spent
2-
SO4 /meso-La2O3-ZrO2 catalyst can be almost complete recovered by a facile and low cost
calcination process without an extra addition of fresh catalyst, which allows it to be
promising catalyst for α-arylstyrene production. The almost complete recovery of the used
solid acid also demonstrated that the deactivation can be ascribed to coke deposition but not
2-
the leaching of SO4 , now that the extra fresh catalyst is not required. In comparison with our
previously result [10], although the developed catalyst demonstrates a little lower activity, the
higher selectivity and easier regeneration of spent catalyst by facile calcination process
allows it to be a promising solid acid catalyst for alkenylation reactions.
4. Conclusions
2-
In summary, we developed a novel and efficient SO4 /meso-La2O3-ZrO2 catalyst for
alkenylation through a SACPHC-IM method. The catalyst demonstrates excellent catalytic
performance, and 98.4% conversion with 92.5% selectivity can be obtained under optimized
reaction conditions. Especially the used catalyst can be easily but complete recovered by a
facile and low-cost calcination process, in which neither organic solvent nor extra addition of
2-
fresh catalyst is required. The SO4 /meso-La2O3-ZrO2 solid acid should be a promising
catalyst for alkenylation, of course the catalytic activity, selectivity and stability are required
to be further improved. Moreover, it was found that the catalytic performance of
2-
SO4 /meso-La2O3-ZrO2 strongly depends on the acid properties, texture and structure nature
18
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affected by insertion of La with optimum amount.
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China
(grant no. 21276041) and the Chinese Ministry of Education via the Program for New
Century Excellent Talents in University (Grant NCET-12-0079).
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Figure Captions
2-
2-
Fig. 1. FT-IR spectra of the as-prepared SO4 /meso-La2O3-ZrO2 and SO4 /meso-ZrO2 solid
acid catalysts.
2-
2-
Fig. 2. NH3-TPD profiles of the as-prepared SO4 /meso-La2O3-ZrO2 and SO4 /meso-ZrO2
solid acid catalysts.
22
Page 22 of 34

2-
Fig. 3. Nitrogen adsorption-desorption isotherms of the as-prepared SO4 /meso-La2O3-ZrO2
2-
and SO4 /meso-ZrO2 solid acid catalysts. Insets: Barrett-Joyner-Halenda (BJH) pore diameter
distribution of the two samples from adsorption branch.
Fig. 4. a) Small-angle and (b) wide-angle XRD patterns of the as-prepared
2-
2-
SO4 /meso-La2O3-ZrO2 and SO4 /meso-ZrO2 solid acid catalysts.
2-
Fig. 5. NH3-TPD profiles of the as-prepared SO4 / meso-La2O3-ZrO2 solid acid catalysts with
diverse nLa/(La+Zr).
Fig. 6. The catalytic stability and regeneration performance of the developed
2-
SO4 /meso-La0.1Zr0.9O2 catalyst for the alkenylation of p-xylene with phenylacetylene.
Tables:
Table 1
2-
Effect of La-doping on the catalytic performance of SO4 /ZrO2 solid acid catalysts for the
alkenylation of p-xylene with phenylacetylene.a
23
Page 23 of 34

Product distribution (%)
II III IV
Catalyst
Con. (%)
I
V
2-
SO4 / meso-ZrO2
71.9
93.9 0.4 1.7 0.1 3.8
2-
SO4 /meso-La2O3-ZrO2 82.7
93.6 0.2 4.6 0.1 1.5
a
o
Reaction conditions: catalyst 1.0 g, nAr/Phen. 25:1, Tr 150 C, Ps 1.0 MPa, VHSV 6 ml h-1 g-1
cat, TOS 8 h.
Table 2
Catalytic performance in alkenylation of p-xylene with phenylacetylene over the as-prepared
2-
a
SO4 / meso-La2O3-ZrO2 solid acid catalysts with diverse nLa/(La+Zr)
.
Product distribution (%)
nLa/(La+Zr) Con. (%)
I
II
III IV
V
24
Page 24 of 34

0.05
0.10
0.15
0.20
73.5
82.7
78.8
71.0
95.9 0.4 2.3 0.1 1.3
93.6 0.4 4.6 0.1 1.5
93.2 0.2 4.5 0.1 1.9
88.3 0.3 6.5 0.3 4.3
a
o
Reaction conditions: catalyst 1.0 g, nAr/Phen. 25:1, Tr 150 C, Ps 1.0 MPa, VHSV 6 ml h-1 g-1
cat, TOS 8 h.
Table 3
Effect of Tr on the alkenylation of p-xylene with phenylacetylene over the developed
2-
a
SO4 /meso-La2O3-ZrO2 solid acid catalyst with 0.10 of the nLa/(La+Zr)
.
Product distribution (%)
Tr (oC) Con. (%)
I
II
III IV
V
25
Page 25 of 34

150
160
170
180
190
82.7
90.4
98.4
89.0
85.8
93.6 0.2 4.6 0.1 1.5
93.2 0.3 3.8 0.1 2.6
92.5 0.3 5.0 0.3 1.9
93.9 0.4 3.4 0.1 2.1
91.8 0.8 4.7 0.2 2.5
a
Reaction conditions: catalyst 1.0 g, nAr/Phen. 25:1, Ps 1.0 MPa, VHSV 6 ml h-1 g-1 cat, TOS 8
h.
Table 4
2-
Effect of VHSV on the alkenylation of p-xylene with phenylacetylene over the SO4 /meso-
La0.1Zr 0.9O2 solid acid catalyst.a
VHSV
Product distribution (%)
II III IV
Con. (%)
(ml h-1g-1cat)
I
V
26
Page 26 of 34

4
6
8
93.9
98.4
74.5
86.5 0.1 11.0 0.1 2.4
92.5 0.3 5.0 0.3 1.9
91.9 0.2 5.3 0.1 2.4
Figures:
27
Page 27 of 34

2-
2-
Fig. 1. FT-IR spectra of the as-prepared SO4 /meso-La2O3-ZrO2 and SO4 /meso-ZrO2 solid
acid catalysts.
28
Page 28 of 34

2-
2-
Fig. 2. NH3 TPD profiles of the as-prepared SO4 /meso-La2O3-ZrO2 and SO4 /meso-ZrO2
solid acid catalysts.
29
Page 29 of 34