Lanthanum and Cesium-Loaded SBA-15 Catalysts for MMA Synthesis by Aldol Condensation of Methyl Propionate and Formaldehyde

Article abstract of DOI:10.1007/s10562-016-1810-x

Abstract: Aldol condensation of methyl propionate (MP) and formaldehyde (FA) is a green and sustainable route to synthesize methyl methacrylate (MMA). In this work, lanthanum and cesium-loaded SBA-15 acid–base bifunctional catalysts for aldol condensation have been prepared by wetness impregnation method to maximize the yield of MMA. With the investigation of the relationship between the properties of catalysts and the catalytic performance, the appropriate acid–base sites for this reaction were provided. The results showed that La existed as LaONO₃ on the catalysts and could constitute additional Lewis acid sites on the catalysts. The Cs-0.1La/SBA-15 (with La content of 0.1?%) exhibited higher catalytic activity than the other La-containing catalysts because of the higher density of medium base sites and medium acid sites. The catalysts calcined at 450?°C performed well on this reaction due to the formation of medium base sites on the catalysts as well as the big surface area around 400?m²/g and uniform pore size at 6?nm. When the MP/FA molar ratio was 1/1, the conversion of MP was 29.2?% and the selectivity of MMA could reach 90.4?%. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1810-x

Catal Lett
DOI 10.1007/s10562-016-1810-x
Lanthanum and Cesium-Loaded SBA-15 Catalysts for MMA
Synthesis by Aldol Condensation of Methyl Propionate
and Formaldehyde
Yanan Wang1 Ruiyi Yan2 Zhaopo Lv2 Hui Wang2 Lei Wang2
•
•
•
•
•
Zengxi Li1 Suojiang Zhang2
•
Received: 17 February 2016 / Accepted: 8 July 2016
Ó Springer Science+Business Media New York 2016
Abstract Aldol condensation of methyl propionate (MP)
and formaldehyde (FA) is a green and sustainable route to
synthesize methyl methacrylate (MMA). In this work,
lanthanum and cesium-loaded SBA-15 acid–base bifunc-
tional catalysts for aldol condensation have been prepared
by wetness impregnation method to maximize the yield of
MMA. With the investigation of the relationship between
the properties of catalysts and the catalytic performance,
the appropriate acid–base sites for this reaction were pro-
vided. The results showed that La existed as LaONO3 on
the catalysts and could constitute additional Lewis acid
sites on the catalysts. The Cs-0.1La/SBA-15 (with La
content of 0.1 %) exhibited higher catalytic activity than
the other La-containing catalysts because of the higher
density of medium base sites and medium acid sites. The
catalysts calcined at 450 °C performed well on this reac-
tion due to the formation of medium base sites on the
catalysts as well as the big surface area around 400 m2/g
and uniform pore size at 6 nm. When the MP/FA molar
ratio was 1/1, the conversion of MP was 29.2 % and the
selectivity of MMA could reach 90.4 %.
Graphical Abstract
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1810-x) contains supplementary
material, which is available to authorized users.
Keywords Lanthanum-loaded Á Aldol condensation Á Acid
and base properties
& Hui Wang
huiwang@ipe.ac.cn
1 Introduction
& Suojiang Zhang
sjzhang@home.ipe.ac.cn
Methyl methacrylate (MMA) is a high-value polymer
intermediate, which is widely used for organic glass
(polymethyl methacrylate), acrylic fibers, plastics, paint,
and lustering agent. The traditional way of producing
MMA is the Acetone Cyanohydrin process (ACH), which
is unsustainable and environmentally unfriendly. There-
fore, numerous trials had been made to prepare MMA with
1
School of Chemistry and Chemical Engineering, University
of Chinese Academy of Sciences, Beijing 100049, People’s
Republic of China
2
Beijing Key Laboratory of Ionic Liquids Clean Process, Key
Laboratory of Green Process and Engineering, State Key
Laboratory of Multiphase Complex Systems, Institute of
Process Engineering, Chinese Academy of Sciences,
Beijing 100190, People’s Republic of China
123

Y. Wang et al.
hydrocarbon routes [1, 2]. Researchers divided these routes
into C2, C3 and C4 routes according to their feedstocks. C2
route refers to the one using ethylene as the raw material
[3]. a-MMA route is the modified C2 route which involves
two steps by converting ethylene, CO, and methanol to
MMA. Carbonylation of propylene is a better way than the
ACH process, both of which are based on C3 feedstocks,
but the life of catalysts for the oxidation of isobutyric acid
is short. In the C4 route, isopropylene is transformed to
MMA, but the processing is too complicated through two
steps of oxidation [4–8]. Compared with the other routes,
the a-MMA route has distinctive advantages in rich
materials, short routes, and environmentally friendly.
Vapor-phase aldol condensation between methyl propi-
onate and formaldehyde is the key step in the a-MMA
route, but the catalysts available for this reaction exhibit
low yields. Researchers firstly studied the activity of V-Si-
P catalysts for the aldol condensation [9]. Silica-supported
alkali and alkaline earth metal hydroxides catalysts were
prepared and the catalytic performances in the production
of MMA were investigated by Ai [10]. They found that the
CsOH/SiO2 had good catalytic activity, with the yield of
MMA reaching 13 % when MP/FA = 0.2, but the catalytic
activity decreased slowly with the time-on-stream. Li et al.
[11] chose the modified Zr–Mg–Cs/SiO2 catalysts to get a
higher yield of MMA, but the selectivity was still low
(*80 %). Using SBA-15 supported cesium catalyst,
selectivity of MMA could reach 93 %, but with low con-
version, around 26 % [12]. The mechanism of aldol con-
densation reported by Gogate [13] showed that both acid
and base sites in the catalysts were active for this reaction.
Thus, the key issue of this reaction was the development of
suitable acid–base bifunctional catalysts and balance the
acid and base sites to maximize the yield of MMA [3].
Recently, rare earth compounds, especially lanthanum
compounds, have been widely used in catalysts [14, 15]. It
is widely reported that lanthanum-containing compounds
were excellent catalysts for many condensations [16–21].
The lanthanum-containing catalysts showed excellent
activity in these reactions due to the property of alkalinity
and thermal stability of lanthanum oxide [16, 22, 23].
Kumar et al. [24] had previously synthesized ceria–lan-
thanum based catalysts for the preparation of dimethyl
carbonate. The yield was significantly high when the molar
ratio of Ce/La was 1/4, which indicated that the density of
basic sites and the total amount of acidic sites could be
enhanced by increasing the amount of lanthanum. In the
research of Nguyen [25], lanthanum orthophosphate cata-
lysts were active and selective for the dehydration of light
alcohols. The lanthanum phosphates have both Brønsted
and Lewis acidic sites. The Lewis acid sites related to the
lanthanum cations played an important role in this reaction.
In a word, the lanthanum-containing catalysts could change
the acid–base properties of catalysts and change the cat-
alytic performance further. However, the relationship
between the catalytic mechanism and the acid–base sites
formed by La had not been reported. Furthermore, lan-
thanum-supported catalysts have not been used for MMA
synthesis by aldol condensation of MP and FA so far.
Ordered mesoporous siliceous materials such as SBA-15
have recently received growing attention due to their
appealing textural properties, high surface area, apprecia-
ble thermal and hydrothermal stability, so the SBA-15 was
selected as the support.
In this work, the Cs-La-supported SBA-15 catalysts
were prepared by the impregnating method. The Cs-xLa/
SBA-15 catalysts were characterized by NH3 and CO2-
Temperature Programmed Technique, X-ray Diffraction,
Ultra-high Resolution Field Emission Scanning Electron
Microscope, Fourier Transform Infrared Spectrometer and
N2 Physisorption. The effects of La content and calcination
temperature of the catalysts on the catalytic activity for the
aldol condensation of MP and FA were studied in a fixed-
bed reactor. Based on the discussion of the relationship
between the properties of catalysts and the catalytic per-
formance, the appropriate acid–base sites for this reaction
were provided.
2 Experiment Section
2.1 Catalyst Preparation
Cesium nitrate (99.9 %) was industrial grade and pur-
chased from Hubei Baijierui Advanced Materials CO.
LTD. Poly (ethylene oxide)-block-poly (propylene oxide)-
block-poly (ethylene oxide) triblock copolymer (P123) was
purchased from Aldrich. Lanthanum nitrate hexahydrate
(corresponding
La2O3
content
C44.0 %)
and
tetraethoxysilane (TEOS) are analytical reagents. Methyl
propionate (C99.0 %), paraformaldehyde (C94.0 %) and
methanol (C99.0 %) are analytical reagents.
The SBA-15 material was prepared according to the
method reported by Zhao et al. [26]. TEOS was used as the
silica source and P123 as the template. 2.0 g of P123 was
dissolved in 60 g of HCl (2 mol/L) and 15 g of H2O, and
the mixture was stirred for 3 h to get a clear solution (so-
lution A) at 40 °C. Then 4.35 g of TEOS was dropped to
solution A and then stirred vigorously for 24 h at 40 °C.
Afterwards it was transferred into an autoclave and aged
for 24 h. The resultant substance was filtered, washed,
dried and calcined at 550 °C for 6 h to get SBA-15.
Cs-La/SBA-15 catalysts with different La-loadings and
the same Cs-loading (the Cs element content was 15 %
which was based on weight percent of the supports [12])
were prepared by wetness impregnation method. 1.101 g
123

Lanthanum and Cesium-Loaded SBA-15 Catalysts for MMA Synthesis by Aldol Condensation of…
CsNO3 and 1.29 g La(NO3)3Á6H2O (10 wt%) were dis-
solved in 30–35 mL water to get a clear solution (solution
B) at room temperature. The solution B was dripped to 5 g
SBA-15 solid in an Erlenmeyer flask, then the mixture was
oscillated for 12 h at 40 °C. The resulting material was
dried at 100 °C in air for 12 h and calcined in air at 450 °C
for 5 h. These samples were marked with Cs-xLa/SBA-15,
where x was the amount of La in the catalysts (x = 0, 0.01,
0.05, 0.1, 0.5, 1, 5, 10 wt%).
for 90 min at 50 °C. Then the samples were purged with
pure helium at 50 °C for 30 min. When the baseline was
stable, the NH3 desorption rate was monitored and gave
one measurement every 1.0 s by the thermal conductivity
detector in helium flow at a heating rate of 10 °C/min to
500 °C. The basicity of the catalysts was measured using
10 % CO2-He mixtures and the processing was the same as
that of the NH3-TPD.
2.3 Catalytic Activity Measurement
2.2 Catalyst Characterization
The reaction was measured in a fix-bed reactor at atmo-
spheric pressure in nitrogen environment. The reactor was
made of a stainless steel tube placed vertically in an electric
furnace, which was 42 cm long, 0.8 cm outside diameter and
0.5 cm inside diameter. 0.8 mL of catalyst was put exactly in
the middle of the tube with the quartz sands filled both ends.
The mixed solution of MP, FA and methanol (the molar ratio
of MP/FA/methanol was 1/1/1.5) was fed in from the top of
the reactor by an advection pump. The methanol was used as
a solvent to depolymerize the paraformaldehyde feed. The
liquid samples were collected after 180 min on stream and
then collected every 120 min. Gas samples were analyzed
on-line and liquid products were analyzed off-line by gas
chromatography (GC) using an Agilent 7890A GC apparatus
with n-heptane as an internal standard and sample detection
was accomplished with a Flame Ionization Detector (FID).
The activity of the catalysts was measured by testing the
conversion of MP and the selectivity and yield of MMA,
which were calculated using Eqs. 1, 2, 3.
X-ray diffraction (XRD) patterns were obtained in an X’
Pert PRO MPD diffractometer operating at 45 kV and
200 mA with CuKa radiation. The intensity of diffraction
was measured by increasing angle (2h) from 5o to 90o at a
o
rate of 15 /min.
The amount of cesium and lanthanum in the Cs-La/
SBA-15 catalysts were quantified by inductively coupled
plasma (ICPE9000) from SHIMADZU.
HITACHI Ultra-High Resolution Field Emission Scan-
ning Electron Microscope (UHR FE-SEM) was used to get
the images of catalyst samples with the operation voltage
of 5.0 kV.
The BET specific surface area, the pore volume, and
pore size were obtained from nitrogen absorption isotherm
at liquid nitrogen temperature in a Micrometrics ASAP
2460 apparatus. Before measurement, the samples were
degassed at 150 °C for 4 h.
Fourier Transform Infrared Spectrometer (FT-IR Nicolet
380) was used to get infrared spectra of the catalyst sam-
ples by using potassium bromide-disk technique. Every
time about 2 mg sample was added to 200 mg KBr to
prepare the disks for testing. The surface acidity of the
catalysts was investigated by means of studying pyridine
adsorption on a Nicolet 6700 FTIR spectroscopy in situ
equipped with a cell. The samples that had been pressed
into a plate were put into the cell, and pretreated at 150 °C
under vacuum for 30 min to remove the impurity. After
cooling to 30 °C, the cell was scanned and the obtained
data was used as the background. Then pyridine vapor was
introduced into the cell for 30 min to reach equilibrium.
Then the cell was scanned after being vacuumed for
10 min and recorded at 50, 100, 200 and 300 °C. The
background has been subtracted from the spectrum of the
corresponding catalyst adsorbed with pyridine.
MPin;mol À MPout;mol
Conversion of MP ¼
Selectivity of MMA ¼
 100%
 100%
ð1Þ
MPin;mol
MMAout;mol
MPin;mol À MPout;mol
ð2Þ
ð3Þ
Yield of MMA ¼ Conversion of MP
 Selectivity of MMA
3 Results and Discussion
3.1 Catalyst Characterization
3.1.1 Element Analysis
The acid and base properties were obtained on an Auto
Chem II 2920 Chemisorption Analyzer from Micrometrics
by the method of temperature programmed desorption. For
acidity measurements, about 30 mg samples were placed in
a quartz tube and pretreated in a helium flow at 120 °C for
30 min. Then the temperature was reduced to 50 °C. A
mixture of 10 % NH3-He was passed through the samples
The Element content of the catalysts was investigated by
ICP and the result was shown in the Table 1. The calcu-
lated contents of Cs and La were slightly greater than the
actual contents, which proved that although some loss of
elements have happened during the catalyst preparation,
123

Y. Wang et al.
Table 1 The Element content of the Cs-xLa/SBA-15 catalysts cal-
cined at 450 °C
Catalyst
Calculate value (wt%) Measured value (wt%)
Cs
La
Cs
La
Cs/SBA-15
15
0
11.2
10.0
11.6
11.0
9.9
0
Cs-0.01La/SBA-15 15
0.01
0.0072
0.080
0.69
4.0
Cs-0.1a/SBA-15
Cs-1La/SBA-15
Cs-10La/SBA-15
15
15
15
0.1
1
10
the Cs and La were both impregnated on the catalysts. The
contents of Cs were about 10–11 %, and the contents of La
increased with the calculated contents. EDS images of La
on the Cs-xLa/SBA-15 catalysts were shown in Fig. S1. La
were homogeneous distributed on the surface of SBA-15
when the content of La was less than 5 %. However, La
distributed heterogeneously when the La content was
10 %, and La aggregation was observed.
Fig. 2 XRD pattern of Cs-0.1La/SBA-15 catalysts (La content:
0.1 %) with different calcination temperatures
of CsNO3 indicating that Cs existed on the catalysts in the
form of CsNO3. It is in accordance with the literature
reporting that CsNO3 was decomposed and volatilized
above 500 °C [11].
3.1.2 XRD Analysis
The XRD patterns of the Cs-xLa/SBA-15 catalysts with
different contents of La were shown in Fig. 1. There were
no obvious diffraction peaks related to La compound [27].
Typical diffraction peaks of CsNO3 were observed, as
shown by the peaks marked by # in Fig. 1, indicating that
Cs existed on the catalysts in the form of CsNO3.
However, diffraction peaks related to La compounds
were not present in the patterns. So we calcined the raw
material La(NO3)3Á6H2O and 10La/SBA-15 (content of La
was 10 %) at 450 °C. The XRD patterns of La(NO3)3-
6H2O, calcined La(NO3)3Á6H2O, 10La/SBA-15 and Cs-
10La/SBA-15 were shown in Fig. 3. The calcined
La(NO3)3Á6H2O at 450 °C showed typical diffraction peaks
of LaONO3 (marked by @), indicating that the
La(NO3)3Á6H2O undergone a transformation into LaONO3
when calcined at 450 °C. It is in accordance with the
The XRD patterns of the catalysts with different calci-
nation temperatures were shown in Fig. 2. With the calci-
nation temperature increasing, the intensity of the typical
diffraction peaks of CsNO3 decreased. The catalysts cal-
cined under 500 °C maintained the complete crystal form
Fig. 3 XRD patterns of La(NO3)3Á6H2O, La(NO3)3Á6H2O calcined at
450 °C, 10La/SBA-15 calcined at 450 °C and Cs-10La/SBA-15
calcined at 450 °C
Fig. 1 XRD pattern of the Cs-xLa/SBA-15 catalysts calcined at
450 °C with different La contents
123

Lanthanum and Cesium-Loaded SBA-15 Catalysts for MMA Synthesis by Aldol Condensation of…
literature reporting that the thermal decomposition of
La(NO3)3Á6H2O follows the following steps: the weight
loss was about 20 % between 50 and 200 °C which was
attributed to the loss of adsorbed and crystal water;
La(NO3)3 was decomposed into La2O3 by two steps with
40 % weight loss from 400 to 650 °C. And LaONO3 was
proposed as an intermediate phase between La (NO3)3 and
La2O3 [28, 29].
decreased, corresponding to the decomposition of CsNO3
and La(NO3)3Á6H2O, in accordance with the XRD results.
3.1.4 BET Specific Surface Area Analysis
The pore size, pore size distribution and specific surface
area of SBA-15 and La-containing catalysts were investi-
gated by N2 adsorption isotherm analyses at liquid nitrogen
temperature. The results were shown in Table 2. The BET
surface area for the catalysts containing La and Cs are
considerably lower than the area for SBA-15. The catalysts
containing La and Cs had similar BET surface area and
pore volume, thus, an increase in the lanthanum content
between 0 and 10 % had no evident influence on the pore
surface area and pore volume of the catalysts. The decrease
of BET surface area and pore volume was due to the fact
that impregnation of Cs on SBA-15 could occupy some
channels. In addition, the pore diameters of all the samples
were distributed uniformly around 6 nm.
The comparison of XRD patterns between 10La/SBA-15
and Cs-10La/SBA-15 showed that the diffraction peaks
related to LaONO3 were not present. Taking the element
analysis result into consideration, when the calculated
value of La on the catalysts was 10 %, the measured value
was only 4.0 %. Thus LaONO3 was well dispersed in the
channels of SBA-15.
3.1.3 IR Analysis
The IR spectra in the fingerprint of SBA-15 and different
La-containing Cs-xLa/SBA-15 samples were shown in
Fig. 4. The vibration band at 1079 cm-1 was assigned to
mas(Si–O–Si) and the vibration band at 802 cm-1 was
assigned to ms(Si–O–Si) [30, 31]. The two peeks occurred
in all the spectra of SBA-15 and Cs-xLa/SBA-15 catalysts
and had no evident changes, indicating the Cs and La
loaded on the SBA-15 had no influence on the skeletal
structure of SBA-15. A band at 1385 cm-1 was observed in
the Cs-xLa/SBA-15 samples, but not observed in SBA-15.
Textural properties and pore size distribution of the Cs-
0.1La/SBA-15 catalysts at different calcination tempera-
tures were obtained (Table 3 and Fig. S3). Isotherms
belonged to the ‘‘type VI’’ class in the cases of calcination
temperatures lower than 500 °C and exhibited a type ‘‘H1’’
hysteresis loop indicating that the catalysts were meso-
porous in nature (Fig. S3). The pore size was uniformly
distributed around 6 nm. The surface area of these cata-
lysts was in the range of 313–494 m2/g and decreased in
the order of 350, 400, 450, 500 °C. The pore volume of the
catalysts also decreased in the same order. The reason for
the slightly decrease was that the layer-type LaONO3 was
formed [33] and dominated the pore of SBA-15 during the
decomposition of La(NO3)3Á6H2O.
-
This band can be assigned to the NO3 antisymmetric
stretching vibrations of N–O [32], indicating the existence
of nitrate anion.
The IR spectra in the fingerprint of the Cs-0.1La/SBA-
15 samples with different calcination temperatures were
shown in Fig. S2. The band intensity of mas(N–O)
When the calcination temperature was above 500 °C,
the decomposition and volatilization of CsNO3 happened.
The resulting collapse of the SBA-15 structure led to the
formation of a bimodal pore size distribution and the swift
decline of surface area and pore volume.
3.1.5 Pyridine-FTIR Analysis
Acid species of the Cs/SBA-15 and Cs-0.1La/SBA-15
samples were investigated by the pyridine FTIR spectra
after adsorbing pyridine vapor for 30 min. Typical pyridine
adsorption peaks referring to Lewis acid sites were shown
near 1446 and 1597 cm-1 (Fig. 5). The intensity of Lewis
adsorption peak at 1446 cm-1 in the spectra of Cs-0.1La/
SBA-15 was higher than that of Cs/SBA-15. So the pres-
ence of La in the catalysts could enhance the amount of
Lewis acid sites in the catalysts.
The FTIR spectra of pyridine desorbed at 50, 100, 200,
and 300 °C on the Cs-0.1La/SBA-15 and Cs-1La/SBA-15
catalysts were shown in Fig. 6. Generally, pyridine
Fig. 4 IR spectra of SBA-15 and Cs-xLa/SBA-15 catalysts calcined
at 450 °C with different La contents
123

Y. Wang et al.
Table 2 Textural of the catalysts calcined at 450 °C with different La contents
Catalyst
Amount of lanthanum (wt%)
Surface area (m2/g)
Pore volume (cm3/g)
Mean pore diameter (nm)
SBA-15
–
0
964
351
349
374
370
365
1.6
6.1
6.0
6.9
6.8
6.2
6.7
Cs/SBA-15
0.68
0.68
0.72
0.71
0.68
Cs-0.01La/SBA-15
Cs-0.1a/SBA-15
Cs-1La/SBA-15
Cs-10La/SBA-15
0.01
0.1
1
10
Table 3 Textural of the Cs-0.1La/SBA-15 catalysts with different calcination temperatures
Calcination temperature of Cs-0.1La/SBA-15 (oC)
Surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
350
400
450
500
550
600
494
405
374
313
120
141
0.83
0.75
0.72
0.65
0.44
0.33
6.5
6.9
6.6
6.9
12.6
7.7
3.1.6 NH3–TPD and CO2-TPD Analysis
TPD studies were performed to identify various acid and
base species on the surface of the catalysts. The NH3-TPD
and CO2-TPD patterns of the Cs-xLa/SBA-15 catalysts
with different contents of La were present in Figs. 7, 8. It is
evident from the results that varying the La content could
induce specific changes in the acid–base characters for
each La-supported catalyst. There were three apparent
desorption peaks in the CO2-TPD and NH3-TPD patterns of
Cs-xLa/SBA-15 that were different from each other by
their positions, heights and widths, indicating not only the
total acidity was different, but also the distribution of the
acid strength. The peaks ranged from 100 to 300 °C, from
300 to 480 °C and from 480 to 600 °C, respectively. The
higher the desorbed temperature, the stronger the strength
of the acid or base sites were [34–36]. The three peaks
corresponded to the weak, medium and strong base or acid
sites, respectively. The amount of the acid–base sites was
summarized in Table 4. Notably, the desorption of water or
other components at low temperatures and the decompo-
sition of residual nitrates at temperatures above the calci-
nation temperature have some influence on the TPD result.
So the blank experiments were designed to eliminate the
influence of water desorption and samples decomposition.
And these interferences have been subtracted from the
figures and dates shown in the following. The blank
experiments processing was the same as the TPD method
except that the mixture 10 % NH3-He (or 10 % CO2-He)
was replaced by He, so that NH3 or CO2 was not adsorbed
Fig. 5 Pyridine-FTIR spectra for Cs/SBA-15 and Cs-0.1La/SBA-15
catalyst calcined at 450 °C
adsorbed on weak acid sites desorbed at low temperatures
and that on strong sites desorbed at high temperatures. The
spectra of the two catalysts with pyridine being desorbed at
50 °C showed an obvious peak at 1446 cm-1. Afterwards,
this Lewis adsorption peaks disappeared at 100 °C on the
Cs-0.1La/SBA-15. However, the Lewis acid sites were
strong enough to be detected at 100 °C on Cs-1La/SBA-15.
The formation of different strength of Lewis acid sites was
considered to be caused by the different La contents. So the
lanthanum ions constituted additional Lewis acid sites, the
strength of which was slightly higher than the sites con-
stituted by cesium ions.
123

Lanthanum and Cesium-Loaded SBA-15 Catalysts for MMA Synthesis by Aldol Condensation of…
Fig. 6 Pyridine-FTIR spectra as a function of temperature for the Cs-0.1La/SBA-15 (a) and Cs-1La/SBA-15 (b) calcined at 450 °C
by samples and the TCD signal results represent for the
samples decomposition.
With the increase of the content of La-loading, the
density of medium base sites increased firstly and then
decreased. The Cs-0.1La/SBA-15 had the highest density
of medium base sites among the Cs-xLa/SBA-15 catalysts.
The density of weak base sites on the Cs-10La/SBA-15
increased and the peak of medium base sites (around
420 °C) shifted slightly to the low temperature zone
(around 370 °C), indicating that the strength of the base
sites was weakened because of the increasing of La
content.
The NH3-TPD patterns were similar when the content of
La was less than 1 %. However, with the content of La
Fig. 7 CO2-TPD patterns of Cs-xLa/SBA-15 catalysts calcined at
450 °C with different La contents
reaching 10 %, the strength of the acid sites reduced and
the density of weak acid sites increased. The increasing
density of weak acid sites can be attributed to the presence
of Lewis acid sites, in agreement with the pyridine-FTIR
analysis above. The density of medium acid sites was
slightly reduced when La was present in the catalysts. The
density of strong acid sites decreased and then increased,
and the Cs-0.1La/SBA-15 catalyst had the lowest density
of strong acid sites among the Cs-xLa/SBA-15 catalysts.
The CO2-TPD and NH3-TPD patterns of catalysts cal-
cined at different temperatures were shown in Figs. 9, 10.
The intensity of acid–base sites related to the releasing
temperature of CO2 or NH3. When the catalysts were cal-
cined below 450 °C, the amount of medium acid sites
changed slightly. While the density of medium base sites
was increased, which was due to the formation of LaONO3
from the decomposition of La(NO3)3. When the calcination
temperature was higher than 450 °C, medium acid sites and
base sites was gradually decreased. It is reported in the
Fig. 8 NH3-TPD patterns of Cs-xLa/SBA-15 catalysts calcined at
450 °C with different La contents
123

Y. Wang et al.
Table 4 Acid–base property of the catalysts calcined at 450 °C with different La contents
Catalyst
Amount of
lanthanum
(wt%)
Amount of base site (mmol/g)
Amount of acid site (mmol/g)
Weak strength
(100–300 °C)
Medium strength Strong strength Weak strength Medium strength Strong strength
(480–600 °C)
(300–480 °C)
(480–600 °C)
(100–300 °C)
(300–480 °C)
Cs/SBA-
15
0
0.136
0.074
0.472
0.135
0.111
0.441
0.135
Cs-
0.01
0.310
0.124
0.035
0.396
0.200
0.01La/
SBA-15
Cs-0.1a/
SBA-15
0.1
1
0.067
0.057
0.229
0.684
0.343
0.402
0.113
0.175
0.169
0.015
0.060
0.215
0.469
0.267
0.314
0.022
0.104
0.120
Cs-1La/
SBA-15
Cs-10La/
SBA-15
10
literatures [11, 28, 29] that LaONO3 was decomposed to
La2O3 when calcined higher than 500 °C and CsNO3 was
decomposed too, thus the reason for the density reduction
of medium acid sites and base sites was the decomposition
of CsNO3 and LaONO3.
3.2 Effects of Reaction Temperature on the Aldol
Condensation of MP with FA
Temperature had an evident influence on the reaction
activity. The performance of Cs/SBA-15 was probed by a
fix-bed reactor with the same conditions except the tem-
perature and the results were shown in Fig. 11. The con-
version of MP increased by two stage as the temperature
increased, while the selectivity of MMA increased firstly
and then decreased, reaching plateau value at 330–360 °C.
In order to produce MMA in high yield and energy-
Fig. 9 CO2-TPD patterns of Cs-0.1La/SBA-15 with different calci-
nation temperatures
Fig. 10 NH3-TPD patterns of Cs-0.1La/SBA-15 with different cal-
cination temperatures
Fig. 11 Catalytic activity of Cs/SBA-15 under different reaction
temperatures
123

Lanthanum and Cesium-Loaded SBA-15 Catalysts for MMA Synthesis by Aldol Condensation of…
efficiently, the catalytic performance was investigated at
catalysts as shown in the TPD results. The relationship
350 °C in the following studies.
between the catalytic behavior and the acid and base sites
on the catalysts was investigated, which provided evidence
for understanding the catalytic behavior of Cs-xLa/SBA-15
catalysts in the aldol condensation.
3.3 Effects of La Content in the Cs-xLa/SBA-15
Catalysts on the Aldol Condensation of MP
with FA
As shown in Figs. 7, 8, the Cs-0.1La/SBA-15 had the
highest density of medium base sites and lowest density of
strong acid sites than the other catalysts, accounting for
maximum selectivity of MMA on Cs-0.1La/SBA-15. It is
reported that La loaded on the catalysts could improve both
the dispersion of active element and the base property of
catalysts [37]. In this reaction, the large amount of base
sites and small amount of strong acid sites formed by the
La on the catalysts enhanced the activation of MP and
weakened the decomposition of MP, thus the selectivity
increased. The density of medium acid sites slightly
reduced with the introduction of La into catalysts, and there
was a little decrease on the conversion of MP. It is con-
cluded that the medium acid sites play an important role on
the maintenance of conversion of MP.
The catalytic performance of Cs-xLa/SBA-15 catalysts
with different La loadings was investigated. In our previous
study, it is reported that the conversion of MP increased
rapidly with the loading of Cs and reached a plateau at 15
wt% [11], thus, the Cs content was fixed at 15 wt%. The La
content ranged from 0.01 to 10 wt% to investigate the
influence of La content on the catalytic activity for the
aldol condensation.
The catalytic performances were summarized in Fig. 12.
As the La content increased, the selectivity of MMA
increased firstly and then decreased, while the conversion
remained stable firstly and then decreased. With the La-
loading increased from 0 to 0.1 wt%, the selectivity of
MMA increased from 86.3 to 90.4 %. If more La was
added to catalyst, the selectivity decreased. The conversion
of MP would decrease when the content of La was more
than 1 wt%. The Cs-0.1La/SBA-15 had the best catalytic
activity on this reaction.
When the content of La was higher than 0.1 %, the
selectivity decreased owing to the increase of the density of
strong acid sites and the decrease of the density of medium
base sites on the catalysts, which could weaken the acti-
vation of MP and enhance the secondary reaction of MP
decomposition. At the same time, the conversion of MP
decreased rapidly. There were two reasons for the decrease:
Firstly, the increase of the density of strong acid sites led to
inappropriate acid–base ratio and carbon deposition would
occur, which would decrease the efficiency of the catalysts.
Secondly, the La tended to aggregate as shown in the EDS
analysis and changed the acid–base property of the
catalysts.
Spivey [3] had reported that both acid and base sites
were needed for the aldol condensation, and a balance
between acid and base sites is also necessary to maximize
the reaction yield. MP was activated by basic sites of the
catalyst and HCHO by acidic sites, and then a reaction of
the two intermediate molecules occurred to form MMA
followed by dehydration [13]. Besides, MP could be con-
verted to by products by strong acid sites [10]. In this
study, we found that both acid and base sites existed on the
In conclusion, when the La content was 0.1 %, the Cs-
0.1La/SBA-15 showed superior activity for the aldol con-
densation. The higher density of medium base sites and
smaller density of strong acid sites formed by La on the Cs-
0.1La/SBA-15 catalysts enhanced the selectivity of MMA.
The adequate density of medium acid sites could improve
the conversion of MP.
3.4 Effects of Calcination Temperature of Catalysts
on Aldol Condensation of MP with FA
Calcination temperature was an important factor for
preparing catalysts since it significantly affected the
structure and catalytic properties of catalysts. The Cs-
0.1La/SBA-15 were calcined at 350, 400, 450, 500, 550,
600 °C, respectively for 5 h.
Figure 13 showed the evaluation result of the catalyst
performance. The conversion of MP increased slightly
when calcination temperature was below 400 °C from 25.4
to 29.8 %, and then kept constant around 29 %, however, it
Fig. 12 Catalytic performance of Cs-xLa/SBA-15 catalysts calcined
at 450 °C with different La contents
123

Y. Wang et al.
the La loading dosage and the calcination temperature on
the acid–base properties and catalytic performance of these
catalysts in the aldol condensation of MP and FA. The
pyridine-IR results indicated that the introduction of La to
the catalysts could enhance the Lewis acid sites on the
catalysts. The higher density of medium base sites and
lower density of strong acid sites formed by La on the Cs-
0.1La/SBA-15 catalysts enhanced the selectivity of MMA
from 86.3 to 90.4 %. In addition, the Cs-0.1La/SBA-15 (La
content was 0.1 %) calcined at 450 °C exhibited higher
selectivity of MMA due to the existence of LaONO3,
which could form high density of the medium base sites on
the catalysts. When MP/FA = 1/1, the conversion of MP
was 29.2 % and the selectivity of MMA could reach
90.4 %.
Fig. 13 The effect of calcination temperature of Cs-0.1La/SBA-15
on the performance of the catalysts
Acknowledgments This work was financially supported by National
Natural Science Foundation of China (NO. 21406244, 21276267), the
Key Program of National Science Foundation of China (NO.
91434203) and the Key Laboratory of Applied Surface and Colloid
Chemistry of Shaanxi Normal University.
decreased rapidly when calcined higher than 500 °C to
around 24 %. The selectivity of MMA increased from 83.4
to 90.4 % and then dropped to 83.6 %. The highest
selectivity of MMA was obtained at 450 °C.
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