Preparation of Cs-La-Sb/SiO2 catalyst and its performance for the synthesis of methyl acrylate by aldol condensation

Article abstract of DOI:10.1039/c5ra05133a

A variety of cesium supported catalysts with amorphous SiO₂ as a carrier were prepared by vacuum impregnation and ultrasonic impregnation methods, which were used for the synthesis of methyl acrylate by aldol condensation of methyl acetate and formaldehyde. The as-prepared catalysts were characterized by XRD, N₂ adsorption-desorption, NH₃-TPD, CO₂-TPD, XPS, ICP and TG. The results indicated that the bifunctional Cs-La-Sb/SiO₂ showed a high conversion of methyl acetate as well as a high yield of methyl acrylate, which was attributed to the weak acid-base sites and the formation of basic Cs-O-Si species with the addition of antimony and lanthanum, respectively. Furthermore, Cs-La-Sb/SiO₂-(V) prepared by vacuum impregnation showed a high initial catalytic activity, but its activity sharply decreased due to the loss of active species. Comparatively, Cs-La-Sb/SiO₂-(U) prepared by ultrasonic impregnation showed a good stability owing to the cavitation effect of ultrasonic waves. The conversion of methyl acetate and yield of methyl acrylate remained above 20% and 9.0% (based on methyl acetate) for 100 h without obvious deactivation. Moreover, carbon deposition was the main factor for the deactivation of Cs-La-Sb/SiO₂-(U), and the coked catalyst could be regenerated by calcination in air. This journal is

Full text of DOI:10.1039/c5ra05133a

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Preparation of Cs–La–Sb/SiO catalyst and its
2
performance for the synthesis of methyl acrylate by
aldol condensation
Cite this: RSC Adv., 2015, 5, 32826
Yanan Wang, Xuewei Lang, Guoqing Zhao, Huanhui Chen, Yiwei Fan, Linqing Yu,
Xiuxiu Ma and Zhirong Zhu*
2
A variety of cesium supported catalysts with amorphous SiO as a carrier were prepared by vacuum
impregnation and ultrasonic impregnation methods, which were used for the synthesis of methyl acrylate
by aldol condensation of methyl acetate and formaldehyde. The as-prepared catalysts were
characterized by XRD, N
2
3 2
adsorption–desorption, NH -TPD, CO -TPD, XPS, ICP and TG. The results
indicated that the bifunctional Cs–La–Sb/SiO
2
showed a high conversion of methyl acetate as well as a
high yield of methyl acrylate, which was attributed to the weak acid–base sites and the formation of
basic Cs–O–Si species with the addition of antimony and lanthanum, respectively. Furthermore, Cs–La–Sb/
2
SiO -(V) prepared by vacuum impregnation showed a high initial catalytic activity, but its activity sharply
decreased due to the loss of active species. Comparatively, Cs–La–Sb/SiO
2
-(U) prepared by ultrasonic
Received 23rd March 2015
Accepted 1st April 2015
impregnation showed a good stability owing to the cavitation effect of ultrasonic waves. The conversion
of methyl acetate and yield of methyl acrylate remained above 20% and 9.0% (based on methyl acetate)
for 100 h without obvious deactivation. Moreover, carbon deposition was the main factor for the
DOI: 10.1039/c5ra05133a
www.rsc.org/advances
2
deactivation of Cs–La–Sb/SiO -(U), and the coked catalyst could be regenerated by calcination in air.
other is solid base catalyst, such as oxides or hydroxides of
alkali metals and alkaline earth metals supported on porous
As a fundamental industrial monomer, acrylic ester is widely materials (e.g. SiO , ZrO , Al O , TiO , MgO and ZSM-5).
2 2 2 3 2
Introduction
26–35
1
–5
used in the production of plastics, adhesives and coatings
owing to the conjugated unsaturated system C]C–C]O.
Nevertheless, more attentions are paid to solid base catalysts for
vapor phase aldol condensation because of the easy preparation
6
Among the acrylates, methyl acrylate (MA) is the essential and high catalytic activity. SiO
2
supported cesium catalyst was
7–9
monomer for the synthesis of PAN bres. Also, adipic acid, reported to be a good solid base catalyst for vapor phase aldol
27
which is an intermediate to manufacture nylon-66, is produced condensation. Mamoru Ai studied the vapor-phase aldol
10–12
by tail-to-tail dimerization of methyl acrylate.
Commercially, condensation of methyl propionate with formaldehyde over a
methyl acrylate is produced by esterication of acrylic acid and series of silica-supported alkali and alkaline earth metal cata-
methanol, prior to the synthesis of acrylic acid by the gas phase lysts and found that silica-supported cesium catalyst showed a
13–17
catalytic oxidation of propylene (Scheme 1).
2
However, with higher selectivity than other catalysts. SiO modied by mixed
the declining of propylene resources based on petrochemical
industry, it is crucial to develop a new synthetic process of
methyl acrylate. The development of coal chemistry industry
makes the synthesis of methyl acrylate by vapor phase aldol
condensation of methyl acetate and formaldehyde which are
the downstream products as a potential way. The mechanism
for the reaction as shown in Scheme 2 was reported by Vitcha
18
and Sims in 1966.
Two types of heterogeneous catalyst are applied for the
formation of acrylates by vapor phase aldol condensation. One
19–26
is considered as solid acid catalysts (e.g. V
2
O
5
, P
2
O
5
);
the
Department of Chemistry, Tongji University, 1239, Siping Road, Shanghai 200092,
China. E-mail: zhuzhirong@tongji.edu.cn; Fax: +86-21-65981097; Tel: +86-21-
Scheme 1 Commercial production of methyl acrylate.
6
5982563
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(Cs
2
CO
3
,
99.9%, Aladdin), lanthanum nitrate hydrate
(La(NO) $6H O, 99%, Aladdin) and antimony chloride (SbCl ,
3 2 3
99.0%, Sinopharm Chemical Reagent CO., Ltd) were used as the
precursor of cesium, lanthanum and antimony, respectively.
The concentration of the metal salt was determined by the
desired metal-loading (4.7 wt% based on the quality of the support
for Cs, 2.5 wt% for La, and 2.5 wt% for Sb) and the amount of
3
water (ethanol for SbCl ) that the support can absorb during the
impregnation. The catalysts prepared by vacuum impregnation
ꢁ
2
under 6 ꢀ 10 Pa were donated as Cs/SiO -(V) and Cs–La–Sb/
2
SiO -(V). The catalysts prepared by ultrasonic impregnation for 2 h
2
were donated as Cs/SiO -(U), Cs–Sb/SiO -(U) and Cs–La–Sb/SiO -
2
2
2
(
U). For Cs–Sb/SiO
2
2
and Cs–La–Sb/SiO catalysts, the impregna-
tion of antimony, lanthanum and cesium was nished step by
step. All the impregnated catalysts were dried overnight in air at
Scheme 2 Mechanism of vapor phase aldol condensation.
ꢂ
ꢂ
100 C and then calcined in owing air at 600 C for 1 h.
metals was developed to further improve the catalytic perfor- Catalyst characterization
mance for vapor phase aldol condensation. Zr–Mg–Cs/SiO
2
35
X-ray diffraction (XRD) patterns of the catalysts were charac-
terized by a Bruker Foucs D8 diffractometer with Cu Ka radia-
30
31
catalyst, Zr–Fe–Cs/SiO
2 2
catalyst and Bi–Cs/SiO catalyst
exhibited good reusability and long cycle life for the aldol
condensation of methyl propionate and formaldehyde.
Ordered mesoporous silica material SBA-15 now received
much attention as support for solid base catalyst used in vapor
phase aldol condensation thanks to its special two-dimensional
ꢂ
ꢂ
tion at 40 kV and 2q region from 10 to 70 .
The N adsorption–desorption isotherm of catalyst was
2
measured by a Surface Area Analyzer (NOVA 2200e, Quantach-
rome) at 77 K in order to investigate the textural properties of
ꢂ
catalysts. The samples were rstly outgassed at 50 C for 30 min
3
1,36,37
31,38
hexagonal structure,
was obtained over this kind of catalyst. Compared to SBA-15,
common amorphous SiO was easily synthesized with lower
cost. If the catalyst prepared using common amorphous SiO
and well catalytic performance
ꢂ
ꢂ
and then at 300 C (150 C for deactivated catalysts) for 3 h before
measurement. The total surface area was calculated by multi-
points BET method using thee adsorption data in the relative
pressure range from 0.05 to 0.30. The total pore volume was
determined from the amount absorbed at the relative pressure of
about 0.99. The pore size distribution curve and mean pore
diameter were calculated by the BJH model. Micropore surface
area and micropore volume were calculated by t-plot method.
The amount of cesium, lanthanum and antimony in the
catalysts was identied by inductively coupled plasma (ICP,
OPTIMA2100DV, Perkin Elmer).
2
2
shows the similar catalytic performance, the catalyst would have
higher industrial application value.
A large number of papers have reported the preparation of
26,29,34
19,27,28,30,31,33,35
methacrylic acid
and methyl methacrylate
by
vapor phase aldol condensation, while little has studied the
formation of methyl acrylate by aldol condensation of formal-
dehyde and methyl acetate. However, vanadium phosphorus
oxides as solid acid catalyst was studied for the aldol conden-
The acidity and basicity of the catalysts were measured by
temperature-programmed desorption of ammonia (NH -TPD)
3
20,25
sation of formaldehyde and methyl acetate by Ai M.
and Feng
24
X. et al. For the source of formaldehyde, formalin is much more
attractive than trioxymethylene and methylal because of the
price and available. This paper investigated the catalytic perfor-
mance of vapor phase aldol condensation of formaldehyde and
methyl acetate using the aqueous solution of 36% HCHO as the
and carbon dioxide (CO -TPD) on a PX200 TPD/TPR instrument
equipped with a TCD (CHEMBET 3000).
X-ray photoelectron spectroscopy (XPS) measurements were
conducted on AXIS Ultra DLD spectrophotometer in order to
analyze the surface properties of the catalysts.
2
2
source of formaldehyde over amorphous SiO supported cesium
Thermogravimetric (TG) analysis of the deactivated catalysts
was carried out on a STA409 PC analyzer. The samples were
catalyst prepared by vacuum impregnation and ultrasonic
impregnation. Then, lanthanum and antimony species were
added to improve the catalytic activity which was the rst time
for the catalyst used in vapor phase aldol condensation. Also, the
deactivation mechanisms of the catalysts prepared by different
impregnation methods were discussed.
ꢂ
ꢂ
heated from room temperature to 800 C with a rate of 10 C
ꢁ1
ꢁ1
min under air in a ow of 30 mL min
.
GC-MS (Agilent 5973N) was used to determine the compo-
sition of soluble coke in deactivated catalysts. The framework of
deactivated catalyst was eliminated using 15% HF, and then the
2 2
soluble coke was extracted with CH Cl .
Experimental
Catalyst reaction
Catalyst preparation
Methyl acetate ($99.0%), formaldehyde (aqueous solution of
support was purchased from 36% HCHO) and methanol were analytical grade and all
Aote Catalysts Company of Jiangyan. Cesium carbonate purchased from Sinopharm Chemical Reagent CO., Ltd.
The spherical amorphous SiO
2
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The aldol condensation reaction was carried out in a single
pass, xed bed ow reactor made of a stainless steel tube
mounted vertically in the furnace at atmospheric pressure.
Formalin, an aqueous solution of HCHO (36%), was used as a
source of formaldehyde (HCHO). HCHO was dissolved in
methyl acetate (MeOAc) with a molar ratio of 1/10, and meth-
anol (HCHO/CH
3
OH ¼ 1, wt) was added into the reaction feed
in order to avoid the polymerization of formaldehyde and
prevent the formation of side product acetic acid (AA) from
hydrolysis of MeOAc. The mixed solution was injected in from
the top of reactor using a syringe pump, and nitrogen was fed in
from the top of the reactor. The reaction was carried out at
ꢂ
ꢁ1
3
80 C and 1.0 h WHSV. The product was collected and
analyzed by an HP5890 gas chromatograph equipped with a
30 m HP Innowax capillary column and an FID detector.
The activity of catalyst was measured by testing the conver-
sion of MeOAc; the selectivity and yield of MA and acetone
which were calculated by equations below.
Fig. 1 XRD patterns of SiO
2
and SiO
2
supported cesium catalysts.
C
MeOAc (conversion of MeOAc) ¼
(
MeOAcin,mol ꢁ MeOAcout,mol)/MeOAcin,mol ꢀ 100%
supported cesium catalysts, Cs/SiO -(V) and Cs/SiO -(U), increased
2
2
S
MA (selectivity of MA) ¼ MAout,mol/(MeOAcin,mol
slightly from 17.95 nm to 17.97 nm and 17.99 nm. It was because
that the cesium species blocked some micropores of SiO
ꢁ
MeOAcout,mol) ꢀ 100%
2
31
YMA (yield of MA) ¼ CMeOAc ꢀ SMA
support, which also caused the decrease of micropore surface
and micropore volume. There was a signicant reduction in the
2
surface area and pore volume for Cs–La–Sb/SiO owing to the more
modication by cesium, lanthanum and antimony. For different
catalysts prepared by vacuum and ultrasonic impregnation, cata-
lysts prepared by ultrasonic impregnation possessed slightly lower
Results and discussions
Physicochemical properties of the prepared catalysts
2
ꢁ1
2
ꢁ1
surface area (114.3 m
g
, 105.8 m
g
) and pore volume
Fig. 1 shows the XRD patterns of the SiO
catalysts and SiO support. All the SiO
2
supported cesium
3
ꢁ1
3
ꢁ1
(
0.708 cm g , 0.671 cm g ) than catalysts prepared by vacuum
2
2
ꢂ
supported cesium
2
ꢁ1
2
ꢁ1
3
ꢁ1
impregnation (114.9 m g and 106.4 m g , 0.706 cm g and
ꢂ
catalysts showed a broad peak between 17 and 38 similar to
3
ꢁ1
0
.674 cm
higher than that of Cs/SiO
analysis error. The amount of active components supported on
g
). However, the pore volume of Cs/SiO
2
-(U) was
2
parent SiO support which can be assigned to amorphous
2
-(V) due to the instrumental and
39
silica. No new diffraction peak appeared aer modied by
metal species (cesium, lanthanum and antimony oxides). This
indicated that the structure of SiO was preserved well. Also,
2
there were no new phases produced during impregnation
process. However, the intensity was slightly decreased for the
modied catalysts which indicated that the metallic species
were dispersed on the support.
The N
distribution curve of SiO
adsorption–desorption isotherm of SiO
classic type-IV according to the classication of the Interna-
2
adsorption–desorption isotherm and pore size
support are shown in Fig. 2. The N
support belonged to
2
2
2
40
tional Union of Pure and Applied Chemistry (IUPAC). There
was a clear H3-type hysteresis loop in the isotherm curve at the
relative pressure of 0.7–1.0, which indicated the mesoporosity
2 2
of the SiO support. The pore size distribution curve of SiO
support indicated a sharp peak centred at 17.95 nm. The
textural properties (surface area, pore volume and mean pore
2 2
diameter) of SiO supported cesium catalysts and parent SiO
are listed in Table 1. Aer modication of SiO support with
2
metal species, the surface area and pore volume both decreased,
which indicated that the metal oxide components were loaded
Fig. 2
of SiO support.
N
2
adsorption–desorption isotherm and pore size distribution
30
into the channels. However, the mean pore diameter of SiO
2
2
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Table 1 Textural properties of the parent SiO
2
and SiO
2
supported cesium catalysts
Pore volume (nm)
Total
2
ꢁ1
Surface area (m g
)
Catalysts
SiO
Total
Micropore
Micropore
Mean pore diameter (nm)
2
156.2
114.9
114.3
111.2
106.4
105.8
35.28
30.75
30.01
28.76
25.70
24.96
0.840
0.706
0.708
0.683
0.674
0.671
0.017
0.013
0.014
0.012
0.012
0.012
17.95
17.97
17.99
17.72
17.52
17.48
Cs/SiO
Cs/SiO
2
-(V)
-(U)
2
2
Cs–Sb/SiO -(U)
Cs–La–Sb/SiO
Cs–La–Sb/SiO
2
-(V)
-(U)
2
SiO
actual amount of active components supported on SiO
to the theoretical loading amount. It can be seen that the amount than the catalyst prepared vacuum impregnation due to more
of the active components in the catalysts prepared by ultrasonic active components supported on SiO (as shown in Table 2).
impregnation was higher than the catalysts prepared by vacuum The XPS measurement of SiO supported cesium catalyst was
2
is summarized in Table 2. The ICP results showed that the temperature. Meanwhile, the catalyst prepared by ultrasonic
2
was closed impregnation showed the desorption peak at higher temperature
2
2
41
impregnation. Wang et al. studied inuences of different investigated to determine the chemical state of metallic species
impregnation methods for Cs/HX catalyst, and reported that on SiO . The wide XPS spectra (Fig. 5A) conrmed that Cs/SiO2
2
ultrasonic impregnation beneted the sufficient diffusion of catalysts contain cesium (724 eV), silicon (103.4 eV) and oxygen
impregnation solution into the channels of support. More metal (531.6 eV). Meanwhile, lanthanum (834.9 eV) was detected on
active components supported on SiO
structural parameters of the catalysts prepared by ultrasonic photoelectron lines would exist at around 532 eV, so the XPS
impregnation. spectra of antimony was not clearly showed in the wide XPS
The acidity and basicity of SiO supported cesium catalysts spectra of Cs–La–Sb/SiO . The high-resolution Cs 3d spectra
2 2
also caused lower pore Cs–La–Sb/SiO catalysts. An overlapping for O 1s and Sb 3d5/2
43
2
2
5/2
were determined respectively by temperature-programmed of cesium-supported catalysts (Fig. 5B) showed three peaks at
4
4
desorption of NH3 and CO . Fig. 3 depicts the desorption 724.5 eV, 725.2 eV and 726 eV which can be assigned to Cs O ,
2
2 2
45
46
proles of NH
3
on different SiO
2
supported cesium catalysts. Cs
2
O
and Cs11
3
O , respectively. Moreover, all Cs 3d5/2 spectra
The amount of acid sites can be obtained from the area of of SiO
2
supported cesium catalysts showed a peak at 724 eV
ꢂ
45
desorption peak. All catalysts exhibited a peak below 300
which was assigned to desorption of NH
acid sites. The NH -TPD proles of Cs/SiO
C
2
corresponded to Cs in SiO , Yan et al. also reported Si–O–Cs
38
3
adsorbed on weak species in Cs/SBA-15 catalyst. The percentage contents of these
3
2
catalysts also cesium species determined by normalization method were lis-
ꢂ
showed a high temperature peak at around 350 C corre- ted in Table 3. From the NH -TPD prole of Cs–La–Sb/SiO2
3
sponding to strong acid sites generated by the impurities on the catalyst in Fig. 3, it can be easily seen that the strong acid site on
surface of SiO . Moreover, Cs–Sb/SiO -(U) catalyst exhibited the surface of SiO were neutralized by lanthanum species. The
2
2
2
more weak and medium acid sites than Cs/SiO
2
-(U) attributed
to the medium Lewis and Bronsted acidity possessed by anti-
4
2
mony species. While, lanthanum species of Cs–La–Sb/SiO
catalysts covered the strong acid sites and resulted in no high
temperature desorption peak of NH over Cs–La–Sb/SiO cata-
lysts. Fig. 4 shows the CO -TPD results of the SiO supported
2
3
2
2
2
ꢂ
ꢂ
cesium catalysts. The peak in the range of 150 C and 300 C,
corresponding to weak basic sites, was exhibited on cesium-
supported SiO
peak of CO
2
catalysts. It is worth noting that desorption
2
on Cs–La–Sb/SiO
2
catalyst shied to higher
Table 2 Elemental analysis of the SiO
2
supported cesium catalysts
Catalysts
Cs (wt%)
La (wt%)
Sb (wt%)
Cs/SiO
Cs/SiO
Cs–La–Sb/SiO
Cs–Sb/SiO -(U)
Cs–La–Sb/SiO
2
-(V)
-(U)
4.46
4.67
4.48
4.60
4.62
—
—
2.12
—
2.26
—
—
2.18
2.19
2.19
2
2
-(V)
2
2
-(U)
3 2
Fig. 3 NH -TPD profiles of SiO supported cesium catalysts.
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2 2
Fig. 4 CO -TPD profiles of SiO supported cesium catalysts.
content of base Cs–O–Si structure remarkably increased aer
addition of lanthanum which changed the surface properties of
SiO support as shown in Table 3.
2
Aldol condensation of formaldehyde and methyl acetate
The catalytic performance of the as-prepared SiO supported
2
cesium catalysts for the vapor phase aldol condensation of
formaldehyde and methyl acetate is shown in Fig. 6. Formalin,
an aqueous solution of HCHO was chosen as the source of
formaldehyde. The hydrolysis of methyl acetate was unavoid-
able in the presence of water and the basic catalysts. As can be
Fig. 5 The wide XPS survey spectra (A) and Cs 3d5/2 spectra (B) of SiO
supported cesium catalysts.
2
seen, for Cs/SiO
2 2
catalyst, Cs/SiO -(U) catalyst exhibited better
catalytic performance than Cs/SiO -(V) owing to the more
38
2
condensation. The conversion of methyl acetate at about
20% and the yield of methyl acrylate at above 8.5% were
obtained over Cs–La–Sb/SiO2 catalysts. For Cs–La–Sb/SiO2
cesium active species supported on the SiO . The conversion of
2
methyl acetate and the yield of methyl acrylate both increased
over Cs–Sb/SiO
2
2
-(U) catalyst compared with Cs/SiO -(U) catalyst.
catalyst, Cs–La–Sb/SiO
methyl acetate owing to its high basicity. From textural prop-
erties of catalysts, the surface area of Cs–La–Sb/SiO -(U) was
lower than Cs–La–Sb/SiO -(V), so the diffusion of reactants was
more difficult in the channels of Cs–La–Sb/SiO -(U) catalyst,
2
-(U) catalyst showed higher yield of
This result comes from that Cs–Sb/SiO
2
-(U) catalyst showed
more acid–basic sites according to the acidity–basicity analysis
of catalysts. In addition, aldol condensation reaction was
facilitated by combination of Bronsted acid sites and base sites
2
2
2
19,29
on the catalyst surface,
Bronsted acid sites over the catalyst. Moreover, basic Cs–O–Si
structure was formed on Cs–La–Sb/SiO catalyst with the addi-
tion of lanthanum, which was benet for the vapor phase aldol
while antimony species brought
which resulted in a slight lower conversion of methyl acetate.
2
Deactivation and regeneration of Cs–La–Sb/SiO2
Fig. 7 shows the catalytic stability of Cs–La–Sb/SiO -(V) and
2
Cs–La–Sb/SiO -(U). As can be seen, an evident decline of cata-
2
Table 3 The percentage contents of different cesium species on SiO
supported cesium catalysts from XPS analysis
2
lytic activity over Cs–La–Sb/SiO
stream of 5 h, with the conversion of methyl acetate from
8.05% to 16.16% and the yield of methyl acrylate from 7.71%
to 5.50%. However, for Cs–La–Sb/SiO -(U) catalyst, the conver-
2
-(V) was observed aer time on
1
Cs–O–Si
Cs
2
O
2
Cs
2
O
Cs11O
3
2
Catalyst
(wt%)
(wt%)
(wt%)
(wt%)
sion of methyl acetate and the yield of methyl acrylate remained
at about 20.0% and 90.0% until 100 h, respectively. This result
Cs/SiO
Cs/SiO
Cs–Sb/SiO -(U)
2
Cs–La–Sb/SiO
Cs–La–Sb/SiO
2
-(V)
-(U)
23.34
28.75
30.78
65.19
58.01
31.29
22.60
24.51
16.58
21.79
39.86
41.93
40.22
16.40
18.91
5.51
6.72
4.49
1.83
1.29
2
suggested that Cs–La–Sb/SiO -(U) showed better stability for
2
aldol condensation of methyl acetate and formaldehyde.
In order to clarify the deactivation mechanism, the crystal
structure and pore structure of used catalysts were
2
-(V)
-(U)
2
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Fig. 6 Catalytic performance of SiO supported cesium catalysts:
conversion of MeOAc; yield of MA.
Fig.
7
Catalytic stability of Cs–La–Sb/SiO2 catalysts for aldol
condensation of HCHO and MeOAc: (A) Cs–La–Sb/SiO
2
-(V); (B)
Cs–La–Sb/SiO -(U).
2
characterized by XRD and N adsorption–desorption analysis. It
2
can be seen from Fig. 8 that the frameworks of used Cs–La–Sb/
the nature of soluble coke on catalysts was further analyzed by
GC-MS. Fig. 10 showed the compositions of soluble coke. The
composition of soluble coke in two deactivated catalysts was
SiO catalysts were not destroyed during the reaction and no
2
new phases occurred. Table 4 lists the textural properties
(
surface area, pore volume and mean pore diameter) of used
catalysts. The surface area and pore volume of both the used Cs–
La–Sb/SiO -(U) and Cs–La–Sb/SiO -(V) catalysts reduced
2
2
because of the carbon deposition on catalysts.
From the discussion above, coke deposited on the catalysts
was probably a main factor responsible for the deactivation for
the deactivation of the catalysts. The thermo stability properties
of the coke on the deactivation catalysts were investigated by a
TG analyzer. As shown in Fig. 9, there were two distinct weight
loss steps displayed on the deactivated catalysts. The rst
ꢂ
weight loss from room temperature to 300 C was due to the
desorption of physically adsorbed water, and the second weight
ꢂ
ꢂ
loss occurred from 300 C to 550 C, which was denitely owing
to the combustion of deposited coke on deactivated catalysts.
ꢂ
The second weight loss in the temperature from 300 C to
ꢂ
5
50 C could be considered as the coke amount on the deacti-
vated catalyst. So, the amount of coke deposition for used
Cs–La–Sb/SiO -(V) was 1.4 wt%, while 2 wt% coke deposition
was obtained over used Cs–La–Sb/SiO
2
2
-(U). The coke may be
ꢂ
removed through carbon combustion under 500 C in air. Also, Fig. 8 XRD patterns of the used Cs–La–Sb/SiO catalysts.
2
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2
Table 4 Textural properties of the used Cs–La–Sb/SiO catalysts
2
ꢁ1
3
ꢁ1
Surface area (m g
)
Pore volume (cm g )
Catalyst
Total
Micropore
Total
Micropore
Mean pore diameter (nm)
Used Cs–La–Sb/SiO
Used Cs–La–Sb/SiO
2
-(V)
-(U)
96.2
84.7
6.27
5.89
0.489
0.403
0.002
0.002
9.572
9.063
2
were approximately 12.34% and 4.90%, respectively over
regenerated Cs–La–Sb/SiO -(V) catalyst. So, the deactivated
Cs–La–Sb/SiO -(U) catalyst was regenerated completely and the
2
2
2
deactivation of Cs–La–Sb/SiO -(U) mainly resulted from carbon
deposition.
However, Cs–La–Sb/SiO -(V) catalyst still exhibited unsatis-
2
factory catalytic performance aer removing the carbon depo-
sition. Besides carbon deposition, the loss of active component
was another important factor that might cause the deactivation
of catalysts. The metal contents of used Cs–La–Sb/SiO
were determined by ICP. The results were listed in Table 5,
showing the metal content of used Cs–La–Sb/SiO -(U) catalyst
2
catalysts
2
was almost in accord with the fresh catalyst. While the cesium
content of Cs–La–Sb/SiO -(V) catalyst sharply decreased aer
2
reaction. So, the loss of active component was the main factor of
the deactivation of Cs–La–Sb/SiO
vacuum impregnation. According to the studies of the water
2
-(V) catalyst prepared by
Fig. 9 TG profiles of the deactivated Cs–La–Sb/SiO
2
catalysts.
47
solubility and diffusion in alkali silicate melts, there was a
tendency to leach in the presence of water for alkali metal ions
in the silicate. The alkali oxide crystallites tended to agglom-
erate on the support and the agglomeration of cesium
mainly aromatic hydrocarbon. Toluene, DMBs, TriMBs and
hextraMB were all detected.
The deactivated catalysts were regenerated at 500 C for 3 h
under a steam of air. The catalytic performances of regenerated
ꢂ
48
compounds was quite hygroscopic and prone to dissolving in
water, which resulted in the loss of cesium. From Table 5, it
Cs–La–Sb/SiO -(U) and Cs–La–Sb/SiO -(V) catalysts were shown
2
2
2
could easily seen that Cs–La–Sb/SiO -(U) prepared by ultrasonic
in Fig. 11. It clearly indicated that activity of Cs–La–Sb/SiO -(U)
was restored. The conversion of methyl acetate was up to
2
impregnation could prevent the agglomerate of cesium
compounds. The physical and chemical effects of ultrasonic
cavitation during ultrasonic impregnation had an impact on the
property of catalysts. Ultrasonic cavitation could reduce the
agglomeration between particles and improve the dispersion of
cesium on the surface. In addition, the ultrasonic cavitation
20.36% and the yield of methyl acrylate is up to 9.24%. The
conversion of methyl acetate and the yield of methyl acrylate
2
Fig. 11 Catalytic performance of regenerated Cs–La–Sb/SiO cata-
Fig. 10 The composition of soluble coke on the used Cs–La–Sb/SiO
2
.
lysts for aldol condensation of HCHO and MeOAc.
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Table 5 Elemental analysis of the used Cs–La–Sb/SiO
2
catalysts
Sb (wt%)
5 I. Degirmenci, T. F. Ozaltın, O. Karahan, V. Van Speybroeck,
M. Waroquier and V. Aviyente, J. Polym. Sci., Part A: Polym.
Chem., 2013, 51, 2024–2034.
Catalysts
Cs (wt%)
La (wt%)
6
P. Vl ˇc ek and L. Lochmann, Prog. Polym. Sci., 1999, 24, 793–
873.
Used Cs–La–Sb/SiO
Used Cs–La–Sb/SiO
2
-(V)
-(U)
2.84
4.53
2.06
2.11
2.14
2.14
2
7
E. Hauptman, S. Sabo-Etienne, P. S. White, M. Brookhart,
J. M. Garner, P. J. Fagan and J. C. Calabrese, J. Am. Chem.
Soc., 1994, 116, 8038–8060.
could enhance the interaction between the cesium species and
the silica. This indicated that the Cs–La–Sb/SiO -(U) prepared
by ultrasonic impregnation had a good reusability.
8
9
M. Brookhart and S. Sabo-Etienne, J. Am. Chem. Soc., 1991,
2
113, 2777–2779.
A. Behnia, B. Tamaddoni Jahromi and A. Nemati Kharat, J.
Coord. Chem., 2014, 67, 3018–3025.
1
0 E. A. Morris, M. C. Weisenberger, S. B. Bradley,
M. G. Abdallah, S. J. Mecham, P. Pisipati and
J. E. McGrath, Polymer, 2014, 55, 6471–6482.
Conclusions
Vapor phase aldol condensation of methyl acetate and formal-
1
1 S. Kim, Y.-S. Kuk, F.-L. Jin and S.-J. Park, Bull. Korean Chem.
Soc., 2014, 35, 407–414.
2 Y. Wang, S. Wang and J. Liu, Key Eng. Mater., 2014, 575–576,
dehyde was investigated over Cs/SiO
lyst in a xed-bed reactor. Better catalytic performance was
obtained over the bifunctional Cs–La–Sb/SiO catalyst than
Cs/SiO catalyst. The addition of lanthanum resulted in the
2 2
and Cs–La–Sb/SiO cata-
1
2
151–155.
2
13 M. M. Lin, Appl. Catal., A, 2001, 207, 1–16.
formation of basic Cs–O–Si structure which was benecial for
the aldol condensation by changing the surface properties of
14 E. Balcells, F. Borgmeier, I. Grißtede, H. G. Lintz and
F. Rosowski, Appl. Catal., A, 2004, 266, 211–221.
SiO support. Also, lanthanum species covered strong acid sites
2
15 S. Ishikawa, X. Yi, T. Murayama and W. Ueda, Catal. Today,
on catalysts and antimony provided weak acid sites especially
for Bronsted acid site. Vacuum impregnation and ultrasonic
2014, 238, 35–40.
1
6 M. H ¨a vecker, S. Wrabetz, J. Kr ¨o hnert, L.-I. Csepei,
R. Naumann d'Alnoncourt, Y. V. Kolen'ko, F. Girgsdies,
R. Schl ¨o gl and A. Trunschke, J. Catal., 2012, 285, 48–60.
7 R. Naumann d'Alnoncourt, L.-I. Csepei, M. H ¨a vecker,
F. Girgsdies, M. E. Schuster, R. Schl ¨o gl and A. Trunschke,
J. Catal., 2014, 311, 369–385.
8 J. F. Vitcha and V. A. Sims, I&EC Prod. Res. Dev., 1966, 5, 50–53.
9 J. J. Spivey, M. R. Gogate, J. R. Zoeller and R. D. Colberg, Ind.
Eng. Chem. Res., 1997, 36, 4600–4608.
impregnation methods were applied to prepare Cs–La–Sb/SiO
catalyst. Cs–La–Sb/SiO -(V) prepared by vacuum impregnation
2
2
rapidly deactivated because of the loss of cesium species in
reaction. However, the catalytic performance was obtained with
1
a good stability over Cs–La–Sb/SiO -(U) prepared by ultrasonic
2
impregnation. The cavitation effect of ultrasonic wave could
1
1
enhance the interaction between the cesium species and SiO
2
support in addition to improving the diffusion of cesium
species in silica pores. The conversion of methyl acetate and the
yield of methyl acrylate remained at above 20% and 9.0%,
respectively. The reason for deactivation of Cs–La–Sb/SiO -(U)
2
was carbon deposition formed during the reaction. Further-
2
2
0 M. Ai, J. Catal., 1987, 107, 201–208.
1 M. R. Gogate, J. J. Spivey and J. R. Zoeller, Catal. Today, 1997,
36, 243–254.
2
2
2
2 M. Ai, Appl. Catal., 1989, 54, 29–36.
3 M. Ai, J. Catal., 1990, 124, 293–296.
4 X. Feng, B. Sun, Y. Yao, Q. Su, W. Ji and C.-T. Au, J. Catal.,
more, the catalytic activity of the deactivated Cs–La–Sb/SiO -(U)
2
ꢂ
could be regenerated completely by calcination at 500 C for 3 h
in air.
2014, 314, 132–141.
2
2
5 M. Ai, Appl. Catal., 1989, 48, 51–61.
6 M. Ai, H. Fujihashi, S. Hosoi and A. Yoshida, Appl. Catal., A,
Acknowledgements
2003, 252, 185–191.
2
2
2
3
7 M. Ai, Appl. Catal., A, 2005, 288, 211–215.
8 M. Ai, Catal. Today, 2006, 111, 398–402.
9 J. Tai and R. J. Davis, Catal. Today, 2007, 123, 42–49.
0 B. Li, R. Yan, L. Wang, Y. Diao, Z. Li and S. Zhang, Catal.
Lett., 2013, 143, 829–838.
This work was nancially supported by National Natural
Science Foundation of China 20873091.
Notes and references
3
1 B. Li, R. Yan, L. Wang, Y. Diao, Z. Li and S. Zhang, Ind. Eng.
1
2
3
4
X. Xu, J. Lin and P. Cen, Chin. J. Chem. Eng., 2006, 14, 419–
27.
A. Niesbach, J. Daniels, B. Schr ¨o ter, P. Lutze and A. G ´o rak, 33 P. Wierzchowski and L. Zatorski, Catal. Lett., 1991, 9, 411–
Chem. Eng. Sci., 2013, 88, 95–107. 414.
A. J. Straathof, S. Sie, T. Franco and L. M. van der Wielen, 34 O. H. Bailey, R. A. Montag and J. S. Yoo, Appl. Catal., A, 1992,
Appl. Microbiol. Biotechnol., 2005, 67, 727–734. 88, 163–177.
Chem. Res., 2014, 53, 1386–1394.
32 J. Li, J. Tai and R. J. Davis, Catal. Today, 2006, 116, 226–233.
4
J. Dziczkowski, V. Dudipala and M. D. Soucek, Prog. Polym. 35 J. S. Yoo, Appl. Catal., A, 1993, 102, 215–232.
ˇ
Sci., 2012, 73, 308–320.
36 R. Mart ´ı n-Aranda and J. Cejka, Top. Catal., 2010, 53, 141–153.
This journal is © The Royal Society of Chemistry 2015
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View Article Online
RSC Advances
Paper
37 Z. Y. Wu, Q. Jiang, Y. M. Wang, H. J. Wang, L. B. Sun, 42 M. Baca, A. Pigamo, J. L. Dubois and J. M. M. Millet, Catal.
L. Y. Shi, J. H. Xu, Y. Wang, Y. Chun and J. H. Zhu, Chem.
Mater., 2006, 18, 4600–4608.
8 J. Yan, C. Zhang, C. Ning, Y. Tang, Y. Zhang, L. Chen, S. Gao,
Commun., 2005, 6, 215–220.
43 T. Honma, R. Sato, Y. Benino, T. Komatsu and V. Dimitrov, J.
Non-Cryst. Solids, 2000, 272, 1–13.
3
Z. Wang and W. Zhang, J. Ind. Eng. Chem., DOI: 10.1016 / 44 L. Soriano, L. Gal ´a n and F. Rueda, Surf. Sci., 1991, 251–252,
j.jiec.2014.11.014 1075–1080.
9 H. Wang, M. Yu, C. Lin, X. Liu and J. Lin, J. Phys. Chem. C, 45 P. A. W. van der Heide, Surf. Sci., 2000, 447, 62–72.
3
4
2007, 111, 11223–11230.
46 C. C. Phillips, A. E. Hughes and W. Sibbett, J. Phys. D: Appl.
Phys., 1984, 17, 611.
0 K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou,
R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure 47 M. G. Mesko and J. E. Shelby, Phys. Chem. Glasses: Eur. J.
Appl. Chem., 1985, 57, 603–619. Glass Sci. Technol., Part B, 2001, 42, 173–178.
1 W. Qunlong, Z. Guoqing, L. Junhui, W. Yanan, C. Huanhui 48 J. R. Monnier, J. L. Stavinoha Jr and R. L. Minga, J. Catal.,
and Z. Zhirong, Acta Pet. Sin., 2014, 30, 785–791. 2004, 226, 401–409.
4
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