Synthesis of methyl methacrylate by aldol condensation of methyl propionate with formaldehyde over acid-base bifunctional catalysts

Article abstract of DOI:10.1007/s10562-013-1040-4

Supported cesium catalysts with various carriers (SiO₂, Al ₂O₃, TiO₂, MgO) were prepared and characterized by X-ray diffraction, BET nitrogen adsorption-desorption, NH₃ and CO₂-TPD methods and thermogravimetric analysis. Experimental results showed that the Zr-Mg-Cs/SiO₂ catalyst exhibited moderate activity for aldol condensation of methyl propionate with formaldehyde (FA) to produce methyl methacrylate. Though the activity of Zr-Mg-Cs/SiO₂ catalyst decreased with the time-on-stream, the deactivated catalyst was completely regenerated by calcination. The catalyst was regenerated 16 times and total operation time was over 500 h, its activity was identical with that of the fresh catalyst. Graphical Abstract: Gas aldol condensation reaction of methyl propionate with FA to produce MMA is an environmentally friendly technology; it doesn't need toxic raw materials. Cesium-impregnated silica exhibited better catalytic activity and high selectivity for this reaction, and this catalyst had good reusability and long cycle life. So it is a promising catalyst for industrial application. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-013-1040-4

Catal Lett (2013) 143:829–838
DOI 10.1007/s10562-013-1040-4
Synthesis of Methyl Methacrylate by Aldol Condensation
of Methyl Propionate with Formaldehyde Over Acid–Base
Bifunctional Catalysts
•
Bin Li Ruiyi Yan Lei Wang Yanyan Diao
•
•
•
•
Zengxi Li Suojiang Zhang
Received: 22 April 2013 / Accepted: 2 June 2013 / Published online: 14 June 2013
ꢀ Springer Science+Business Media New York 2013
Abstract Supported cesium catalysts with various carri-
ers (SiO2, Al2O3, TiO2, MgO) were prepared and charac-
terized by X-ray diffraction, BET nitrogen adsorption–
desorption, NH3 and CO2-TPD methods and thermogravi-
metric analysis. Experimental results showed that the Zr–
Mg–Cs/SiO2 catalyst exhibited moderate activity for aldol
condensation of methyl propionate with formaldehyde
(FA) to produce methyl methacrylate. Though the activity
of Zr–Mg–Cs/SiO2 catalyst decreased with the time-on-
stream, the deactivated catalyst was completely regener-
ated by calcination. The catalyst was regenerated 16 times
and total operation time was over 500 h, its activity was
identical with that of the fresh catalyst.
paints and coating [1]. Most of MAA and MMA are pro-
duced by the traditional Acetone Cyanohydrin process
(ACH). In this route, acetone and hydrogen cyanide react
to product cyanohydrin, which subsequently react with
sulfuric acid to form methacrylamide sulfate. The meth-
acrylamide sulfate is either hydrolyzed to MAA and
ammonium sulfate or converted to a mixture of MAA and
MMA in a combined hydrolysis–esterification step with
methanol. However, the ACH route needs to utilize highly
toxic hydrogen cyanide and corrosive, concentrated sulfu-
ric acid that are extremely harmful to human and envi-
ronment. In recent years, the environmental pressures
against shipment of HCN have forced many manufacturers
to abandon utilizing HCN. Besides, disposal of spent sul-
phuric acid/ammonium sulphate has been another source of
headaches which needs to the installation of device for
thermal decomposition of the salt and recovery of sulphuric
acid for recycling [2, 3]. Therefore, many researchers make
a lot of efforts to develop an environmentally friendly
product of MMA. Therefore, much patented work has been
devoted to investigation on the aldol-condensation reaction
of propionate derivatives (propionic acid and methyl pro-
pionate) with formaldehyde to produce MAA and MMA
[4–6].
Keywords Aldol condensation ꢀ Catalyst ꢀ Methyl
methacrylate ꢀ Methyl propionate ꢀ Formaldehyde
1 Introduction
Methyl methacrylate (MMA) and methacrylic acid (MAA)
are high-value polymer intermediate, which are widely
used for producing polymethyl methacrylate (so-called
organic glasses) or producing polymer dispersions for
CH3CH2COOH + HCHO ! CH2=C(CH3ÞCOOH + H2O
B. Li ꢀ Z. Li (&)
College of Chemistry and Chemical Engineering,
University of Chinese Academy of Sciences, Beijing 100049,
People’s Republic of China
CH3CH2COOCH3 + HCHO ! CH2=C(CH3ÞCOOCH3
+ H2O
e-mail: zxli@home.ipe.ac.cn
It is known that the synthesis of MAA and MMA is
much more difficult than that of acrylic acid and methyl
acrylate obtained by aldol condensation of acetic acid and
methyl acetate, respectively, with HCHO. Both acid and
base catalysts are known to be active in the reaction. About
R. Yan ꢀ L. Wang ꢀ Y. Diao ꢀ S. Zhang (&)
Beijing Key Laboratory of Ionic Liquids Clean Process,
Institute of Process Engineering, Chinese Academy of Sciences,
Beijing 100190, People’s Republic of China
e-mail: sjzhang@home.ipe.ac.cn
123

830
B. Li et al.
the reaction between PA and HCHO, plenty of papers and
patents have been published [7–16]. On the other hand,
there are limited reports on the reaction between MP and
HCHO. Gogate et al. [17] reported a 10.5 % yield of MMA
(based on MP) in the aldol condensation of MP with
HCHO over a V–Si–P ternary catalyst. Ai [18] reported the
conversion reached 14.2 % (based on MP) and 85.9 % of
selectivity to MMA over a silica-supported cesium
hydroxide catalyst. The mechanism for the reaction is as
follows [17]:
cesium nitrate under continuous stirring at room tempera-
ture for 5 h and then dried at 110 ꢁC for 12 h. The prepared
precursor was calcined in flowing air at 450 ꢁC. The Zr–
Mg–Cs/MgO catalyst was prepared by the similar process
with Zr–Mg–Cs/TiO2 catalyst. Adding elements magne-
sium and zirconium, can effectively improve the perfor-
mance of the catalyst. For all these catalysts, the content of
zirconium and magnesium were 0.033 and 0.025 wt%,
respectively [13]. The loading amounts of the catalysts
were based on the quality of the carriers.
•
Activation of reactant by basic sites of the catalyst
2.2 Catalyst Characterization
RꢁCH2ꢁX + B ! RꢁꢁCHꢁX + BHþðB = Oꢁ or OHꢁ;
X-ray diffraction (XRD) patterns of catalysts were recor-
ded by X’Pert PRO MPD diffractometer operated at an
accelerating voltage of 40 kV and an emission current of
40 mA with Cu Ka radiation. The angle (2h) was measured
in steps of 0.41778o/s between 5o and 90o.
R = CH3; X = COOCH3Þ
•
Activation of HCHO by acidic sites of the catalyst
HCHO + Hþ ! H2CþOH
Weight loss and temperature associated with phase
transformation were determined by thermogravimetry and
differential thermal analysis (TG/DTA) on a Shimadzu
DTG-60H analyzer. The fresh samples of catalysts before
calcination were heated from room temperature to 800 ꢁC
at rate of 10 ꢁC/min in nitrogen flow (30 mL/min).
The BET specific surface area and the pore size were
obtained from nitrogen absorption and desorption isotherm
method at liquid nitrogen temperature on a Quanta Chrome
Instrument NOVA 2000. Prior to analysis, samples were
degassed at 300 ꢁC for 10 h.
•
Reaction of two intermediate molecules to form an
aldol followed by dehydration
RꢁꢁCHꢁX + H2Cþ OH ! H2C(OH)ꢁC(H)RꢁX
! H2C = CRꢁX + H2O
Although the condensation route involves fewer steps
than the ACH process, a major disadvantage is that the
single-pass yield of MMA in the condensation reaction is
relatively low, resulting in extensive recycle of the unreacted
starting materials. Here our contribution is aimed to prepare
more effective catalysts for the production of MMA.
The adsorption of CO2 and NH3 was measured on an
Autochem II 2920 apparatus from Micromeritics. In a
typical experiment, 40 mg of samples were placed in a
quartz tube and was pretreated in a helium flow at 450 ꢁC
at a heating rate of 10 ꢁC/min and the sample was kept at
this temperature for 1 h. The temperature was then reduced
to 50 ꢁC prior to adsorption. 10 % NH3–He or 10 % CO2–
He passed over the samples for 30 min at 50 ꢁC. After
purging with pure helium at 50 ꢁC for 1 h until the baseline
was stable, the desorption profile was measured by the
thermal conductivity detector in helium flow at a heating
rate of 10 ꢁC/min to 500 ꢁC.
2 Experimental
2.1 Catalyst Preparation
Silica and c-alumina were obtained from commercial
companies. For the Zr–Mg–Cs/SiO2 catalyst, silica was
dried first at 500 ꢁC for 3 h and then impregnated with an
aqueous solution containing desired amount of zirconium
nitrate, magnesium nitrate and cesium nitrate. This mixture
was oscillated at 40 ꢁC for 24 h and then dried in an oven.
The resulting solid was calcined in flowing air at 350, 400,
450, 500, 550 and 600 ꢁC for 3 h, respectively. Cesium-
impregnated alumina (Zr–Mg–Cs/Al2O3) was prepared in a
similar method and calcined in flowing air at 450 ꢁC. For
Zr–Mg–Cs/TiO2 catalyst, aqueous ammonia (28 %) was
added dropwise with vigorously stirring into a solution of
0.2 mol/L Ti(SO4) to obtain hydroxides until pH reached 8.
The solution containing precipitation was kept in a water
bath warmed at 70 ꢁC for 5 h, and then filtered and washed
with deionized water, and dried at 110 ꢁC for 12 h. The
resulting powder was impregnated with a mixed aqueous
solution of zirconium nitrate, magnesium nitrate and
The cesium, magnesium and zirconium in the Zr–Mg–Cs/
SiO2 were measured using inductively coupled plasma
(ICP). A SHIMADZU ICPE-9000 was used for the
measurement.
2.3 Reactivity Measurement
SiO2 (20–30mesh) wasobtainedfromQingdaoHailangSilica
Gel Drier Factory. Methyl propionate (C99.0 %) and para-
formaldehyde (C94.0 %), Magnesium nitrate (C99.0 %) and
zirconium nitrate (C99.0 %), ammonia solution (25 %) and
ammonium sulfate (C99.0 %) are analytical grade.
123

Synthesis of Methyl Methacrylate by Aldol Condensation
831
Titanium(IV) sulfate (C96.0 %) is chemical grade. Cesium
nitrate (99.9 %) is industrial grade.
Yield of MAA ¼ Conversion of MP
ꢂ Selectivity of MAA
ð5Þ
The reaction was carried out in a fixed-bed reactor at
atmospheric pressure. The reactor was made of a stainless
steel tube, 42 cm long and 0.8 cm inside diameter, mounted
verticallyinthefurnace. The amountofcatalystwas4 mL and
was placed in the middle of reactor. Quartz sands were placed
both under and above the catalyst sample. The reaction tem-
perature was in the range of 300–400 ꢁC. The mixed solution
of MP, FA and methanol was fed in from the top of the reactor
by an advection pump with the feed rate of 0.1 mL/min. The
MP/FA/CH3OH molar ratio was fixed at 1/2/2, unless other-
wise indicated. Nitrogen was fed in from the top of the reactor
with a fixed feed rate. The contact timewas 1.9 s. The samples
were collected after 100 min on stream.
3 Results and Discussion
3.1 Catalysts Characterization
3.1.1 XRD Results
Figure 1a presents the XRD patterns of the prepared cat-
alysts with different supports (SiO2, Al2O3, TiO2 and
MgO). The catalysts were prepared with 15 wt% cesium-
loading and calcined at 450 ꢁC for 3 h. The cesium nitrate
with hexagonal were observed in all catalysts, but few
peaks of cesium nitrate were exhibited on the Zr–Mg–Cs/
Al2O3 catalysts. Figure 1b presented the XRD patterns of
Zr–Mg–Cs/SiO2 catalyst at different calcination tempera-
tures. The cesium nitrate with hexagonal phase was
observed in all catalysts, but the intensity of cesium nitrate
decreased with increasing calcination temperature. This
was attributed to cesium nitrate decomposed and volatil-
ized with increase of the calcination temperature; this
phenomenon was also confirmed by TG/DTA analysis.
In order to reduce the impact of water in the reaction and
avoid the hydrolysis of MP and MMA, paraformaldehyde
was dissolved in methanol to obtain mixture of formalde-
hyde and methanol, which was used as a source of HCHO
instead of formalin.
The effluent gas from the reactor was led into a con-
denser and the liquid samples were collected per 45 min
interval. After 100 min on run, the reactants and products
of gas phase and liquid phase were analyzed by gas chro-
matography (Agilent 7890A GC). N-Heptane was added to
the products as an internal standard. The activity of catalyst
was measured by testing the conversion of MP; the selec-
tivity and yield of MMA and MAA which were calculated
in Eqs. 1–5.
3.1.2 Thermal Analysis (TG/DTA)
The thermostability of the Zr–Mg–Cs/SiO2 catalysts was
investigated by TG/DTA method. The thermal stability of
cesium nitrate was studied and Fig. 2 showed the TG/DTA
curves of CsNO3. The first endothermic DTA peak turned up
at 158 ꢁC with no weight loss, which attributed to melt of
impurities in the cesium nitrate. The second big endothermic
DTA peak at 408 ꢁC with no weight loss was due to melting
of CsNO3. It was closed to the melting point of CsNO3 which
was 414 ꢁC reported in the literature. The weight loss was
97 % between 500 and 800 ꢁC accompanied with the two
broad endothermic DTA peak, which was attributed to
decomposition and volatilization of CsNO3. For the sample
of Zr–Mg–Cs/SiO2 calcined at 450 ꢁC (Fig. 3a), the weight
MPin;mol ꢁ MPout;mol
Conversion of MP ¼
Selectivity to MMA ¼
Selectivity to MAA ¼
ꢂ 100
ð1Þ
ð2Þ
ð3Þ
MPin;mol
MMAout;mol
ꢂ 100
ꢂ 100
MPin;mol ꢁ MPout;mol
MAAout;mol
MPin;mol ꢁ MPout;mol
Yield of MMA ¼ Conversion of MP
ꢂ Selectivity of MMA
ð4Þ
Fig. 1 XRD patterns of the
catalysts with different supports
(a) and calcined at different
temperatures (b): (hash)
CsNO3, hexagonal, (asterisk)
Al2O3, rhombohedral (dollar)
MgO, cubic, (ampersand) TiO2,
tetragonal
123

832
B. Li et al.
area sharply declined over 500 ꢁC. This phenomenon may
be due to the destruction of pores of SiO2 at higher tem-
perature. In Table 3, the specific surface area of the cata-
lysts decreased with increasing the content of cesium
nitrate. It indicated that cesium nitrate loaded in SiO2
occupies some channels.
3.1.4 Acidity and Basicity Analysis
The quantitative analysis of base and acid sites on the
surfaces of catalysts were determined by stepwise tem-
perature-programmed desorption (TPD) of CO2 and NH3.
The peaks showed in each of the profiles correspond to
desorption of CO2 and NH3 related to the base and acid
sites of the catalysts surface. The desorption temperature
indicates the base and acid strength of the catalysts. The
higher temperature of desorption, the stronger base and
acid strength of the catalysts [19, 20].
Fig. 2 TG/DTA curves of CsNO3
loss was 1.5 % between 50 and 150 ꢁC with a small endo-
thermic DTA peak at 55 ꢁC, which was attributed to the loss
of physical adsorbed water molecules. The second weight
loss was 5 % between 500 and 650 ꢁC accompanied a broad
endothermic DTA peak at 550 ꢁC, which was attributed to
decomposition and volatilization of CsNO3 in the Zr–Mg–
Cs/SiO2 catalyst. For the sample of Zr–Mg–Cs/SiO2 cal-
cined at 600 ꢁC (Fig. 3b), there was no visible weight loss
between 500 ꢁC and 800 ꢁC, this indicates the Zr–Mg–Cs/
SiO2 catalyst calcined at 600 ꢁC, the CsNO3 had completely
decomposed. Thus the thermogravimetry studies revealed
that the Zr–Mg–Cs/SiO2 catalyst should be prepared and
used below 500 ꢁC.
The characteristic CO2 and NH3 TPD patterns for each of
the four catalysts in Table 1 had been compared in Fig. 4.
Figure 4a gave the CO2 TPD patterns (measuring basicity).
The four catalysts had a distinct CO2 desorption peak between
50 and 250 ꢁC which corresponded to the weak strength base
sites. The number density of base sites was greater on the
c-Al2O3 andTiO2 samplesthanonthe SiO2 sample. Figure 4b
showed the acidity of catalysts by NH3 TPD patterns. The
TiO2 sample presented a desorption peak at 130 ꢁC; the SiO2
and Al2O3 samples presented desorption peaks at 150 ꢁC; and
the desorption peak of MgO sample was at 220 ꢁC; which all
corresponded to the weak strength acid sites, however, the
MgO sample showed a little stronger acidity than the other
ones. This may be due to the following reasons. The MgO
carrier had minimum specific surface area (14.7 m2/g); which
could be coated by cesium nitrate, so the base sites of MgO
were completely covered by cesium nitrate. Cesium nitrate
presented a weak acidic substance. Therefore, the acidity of
the Zr–Mg–Cs/MgO sample corresponded to the acid sites of
cesium nitrate. The c-Al2O3 sample presented another NH3
desorption peaks (300–500 ꢁC), which corresponded the
middle strength acid sites. The acid sites density on the TiO2
sample was greater than the other samples.
3.1.3 BET Specific Surface Area Analysis
The physical properties (BET specific surface area, pore
volume and mean pore diameter) of the catalysts were
presented in Tables 1, 2 and 3, respectively. Table 1
showed the commercial SiO2 and c-Al2O3 had large spe-
cific surface area, while the specific surface area of pre-
pared TiO2 and MgO were relatively small. The specific
surface area of Zr–Mg–Cs/SiO2 decreased with increasing
the calcination temperature, while the mean pore diameter
gradually increased (see Table 2). The specific surface area
slightly decreased below 500 ꢁC, but the specific surface
Fig. 3 TG/DTA curves of the
Zr–Mg–Cs/SiO2 calcined at:
a 450 ꢁC; b 600 ꢁC
123

Synthesis of Methyl Methacrylate by Aldol Condensation
833
Table 1 Textural and acid–
Catalysts
SBET
(m2/g)
Pore volume
(cc/g)
Mean pore
diameter (nm)
Total base
amount (lmol/g)
Total acid
amount (lmol/g)
base property of the catalysts
with different supports
Zr–Mg–Cs/SiO2
275.3
233.4
58.6
0.706
0.537
0.126
0.349
10.3
9.2
4.63
33.09
18.45
5.28
0.43
0.48
0.67
0.30
Zr–Mg–Cs/c-Al2O3
Cs content was determined by
the X-ray fluorescence
(PANalytical Axios)
Zr–Mg–Cs/TiO2
Zr–Mg–Cs/MgO
8.6
14.7
95.0
Table 2 Textural and acid–
base property of the Zr–Mg–Cs/
SiO2 catalysts calcined at
different temperatures
Calcination
temperature (ꢁC)
SBET
(m2/g)
Pore volume
(cc/g)
Mean pore
diameter (nm)
Total base
amount (lmol/g)
Total acid
amount (lmol/g)
350
400
450
500
550
600
291.3
294.4
275.3
254.4
114.2
76.5
0.724
0.723
0.706
0.724
0.641
0.642
9.9
9.8
5.39
4.71
4.63
4.34
3.70
3.02
0.28
0.35
0.43
0.50
0.66
0.87
10.3
11.4
22.5
33.6
Table 3 Textural and acid–
base property of the Zr–Mg–Cs/
SiO2 catalysts loaded with
different amount of cesium
Amount of
cesium (wt%)
SBET
(m2/g)
Pore volume
(cc/g)
Mean pore
diameter (nm)
Total base
amount (lmol/g)
Total acid
amount (lmol/g)
0
414.3
363.5
345.3
292.6
275.3
230.3
0.958
0.872
0.866
0.763
0.706
0.596
9.2
9.6
1.09
1.78
3.02
4.26
4.63
5.27
0.99
0.27
0.31
0.36
0.43
0.46
3
5
10.0
10.4
10.3
10.4
10
15
20
Fig. 4 TPD curves of the
catalysts with different supports
in atmosphere of: a CO2; b NH3
The characteristic CO2 and NH3 TPD patterns for Zr–
Mg–Cs/SiO2 catalysts at different calcination temperature
were compared in Fig. 5. Figure 5a showed the desorption
profiles of CO2 from Zr–Mg–Cs/SiO2 catalysts. All cata-
lysts exhibited a prominent CO2 desorption peak in the
range of 50–250 ꢁC, the basic sites on these catalysts were
considered to be weak. Increasing calcination temperature
of catalyst decreased the CO2 adsorption capacity, indi-
cating some base sites were disappeared on the catalyst
surface. Figure 5b showed the desorption profiles of NH3
from Zr–Mg–Cs/SiO2 catalysts. The NH3 adsorption
capacity increased with increasing calcination temperature.
Figure 6 compared the desorption profiles of CO2 and
NH3 from SiO2 supported different loadings of cesium
catalysts. Figure 5a gave the CO2 TPD patterns. Adding
more cesium to the SiO2 increasing its CO2 adsorption
capacity significantly, indicating new base sites were
formed on the surface. Figure 5b gave the NH3 TPD pat-
terns. Adding cesium only slightly affect the NH3 adsorp-
tion capacity. The total amount of acid and base sites on the
123

834
B. Li et al.
Fig. 5 TPD curves of the Zr–
Mg–Cs/SiO2 catalyst calcined at
different temperatures in
atmosphere of: a CO2; b NH3
catalysts surface were summarized in Tables 1, 2 and 3,
respectively. From the view of acid–base analysis, the
presence of the low-temperature peak in the range of
50–250 ꢁC directly related to the MMA yield, the amount
of base and acid sites directly corresponded to the MP
conversion.
constant of residence time, the volume of catalyst should
be uniform, so 4 mL of catalysts were used for all reaction.
However, due to differences in density of the carriers, the
qualities of 4 mL catalysts with different carriers were
diverse. Their quality and cesium-loading were summa-
rized in Table 5. For the sake of comparing catalytic
activity of active component (Cs) in different carriers, the
conversion of MP was also calculated by the unit mass
active component (Cs) of the catalyst and the results were
showed in Table 5. The conversion of MP on per gram
cesium was 0.084 mol/h in the SiO2 catalyst, which was
higher than the other ones. It was indicated that the Zr–
Mg–Cs/SiO2 catalyst had the best catalytic activity. From
the results of BET, the SiO2 and Al2O3 samples had large
specific surface area and mesoporous about 10 nm; while
the specific surface area of TiO2 and MgO samples was
relatively small. It indicated the large specific surface area
corresponded to the high yield. From the acid/base analy-
sis, these four catalysts had almost the same strength of
base sites, but the strength of acid sites was significantly
different. Compared with the Zr–Mg–Cs/SiO2 sample, the
acid strength of Zr–Mg–Cs/TiO2 sample was weak, while
the acid strength of Zr–Mg–Cs/MgO sample was stronger.
The mechanism of condensation clearly implied that acidic
and basic properties were both needed for a moderate
catalyst; and the condensation reaction may be facilitated
by a combination of a Brønsted acid site and a base site on
the catalyst surface [2]. Our results suggested there may be
3.2 Aldol Condensation of MP with FA
3.2.1 Effects of the Supports
The effects of the supports on the catalytic activity were
summarized in Table 4. The catalysts were prepared with
15 wt% cesium-loading and reaction was performed at
360 ꢁC. The samples were collected after 100 min on
stream. In this paper, 15 wt% cesium-loading were based
on the quality of the carriers. In fact, the theoretical content
of cesium was 12.3 % in the entire catalyst. The actual
cesium-loading in different carriers were measured by XRF
and the results were presented in Table 5. The actual
amounts of Cs contained in the SiO2 and MgO samples
were 11.5 and 12.1 %, respectively. These were closed to
the theoretical content of cesium (12.3 %).
The Zr–Mg–Cs/SiO2 catalyst exhibited the best activity
for the reaction, and the Zr–Mg–Cs/Al2O3 catalyst showed
moderate activity. Although the Zr–Mg–Cs/TiO2 catalyst
was active for the conversion of MP, its selectivity to
MMA was very poor. The Zr–Mg–Cs/MgO catalyst
exhibited little activity for the reaction. In order to maintain
a
possible correlation between the acid–base site
Fig. 6 TPD curves of the Zr–
Mg–Cs/SiO2 catalysts with
different amount of cesium in
atmosphere of: a CO2; b NH3
123

Synthesis of Methyl Methacrylate by Aldol Condensation
835
Table 4 Effect of the supports on the aldol condensation of MP with
FA
Catalysts
XMP
(%)
SMMA
(%)
SMAA
(%)
YMMA
(%)
YMAA
(%)
Zr–Mg–Cs/SiO2
Zr–Mg–Cs/c-Al2O3
Zr–Mg–Cs/MgO
Zr–Mg–Cs/TiO2
55.4
34.4
16.5
56.9
75.3
62.2
10.2
13.5
1.3
0.6
2.3
1.5
41.7
21.4
1.7
0.7
0.2
0.4
0.8
7.7
Reaction was performed at 360 ꢁC, Molar ratio of MP/HCHO was ‘,
feed rate of 0.1 mL/min
X conversion, S selectivity, Y yield
distribution and the condensation yield, the acid sites at
description temperature of 50–300 ꢁC was desired for the
selectivity to MMA. In the cases of TiO2 and MgO cata-
lysts, large amounts of MP were consumed without form-
ing MMA. So we used silica as support to conduct the
research on the aldol condensation of MP with FA.
Fig. 7 Effects of calcination temperature on the catalytic perfor-
mance. Reaction was performed at 360 ꢁC; molar ratio of MP/HCHO
was 1/2; feed rate of 0.1 mL/min
factor for the decrease of catalytic activity on the conver-
sion of MP.
3.2.2 Effects of Calcination Temperature
3.2.3 Effects of the Amount of Cesium Supported on Silica
Calcination temperature was an important factor in pre-
paring catalysts since it significantly affected the structure
and catalytic properties of catalysts. The Zr–Mg–Cs/SiO2
catalysts with 15 wt% cesium-loading were calcined at
350, 400, 450, 500, 550 and 600 ꢁC for 3 h, respectively.
The reaction was performed at 360 ꢁC.
The reaction was performed on the Zr–Mg–Cs/SiO2 cata-
lysts with six different cesium loading, namely, 0, 3, 5, 10,
15 and 20 wt%. All catalysts were calcined at 450 ꢁC. The
catalytic activity of the silica catalysts with different
amounts of cesium were showed in Fig. 8. The conversion
of MP increased rapidly with increasing the loading of the
cesium when the cesium loading was below 15 wt%. It
appeared to reach a plateau at 15 wt% loading. The con-
version of MP increasing slightly when the cesium loading
from 15 wt% up to 20 wt%. However, Bailey et al. [21]
reported that the catalytic activity for aldol condensation of
PA with FA went through a maximum at a cesium loading
about 5 % by weight. Tai and Davis [2] also observed that
the conversion of PA over the Cs-loaded silica catalysts
appeared to reach a plateau at 4 wt% loading for the aldol
condensation of PA with FA. This difference in loading of
cesium was attributed to the following reasons. First, the
silica which we used had a larger surface area than those of
Bailey and Tai. Second, it was believed that the aldol
condensation reaction with MP was more difficult than that
with PA. From the textural, acidity and basicity analysis,
The effect of calcination temperature on catalytic
activity was showed in Fig. 7. The conversion of MP
changed slightly at the calcination temperature below
500 ꢁC and was around 55 %, but more than 500 ꢁC, the
conversion of MP decreased rapidly. The selectivity of
MMA was not affected by the calcination temperature and
was around 73 %. From the textural analysis, the specific
surface area decreased slightly on the catalysts at the cal-
cination temperature below 500 ꢁC, while it decreased
sharply over 500 ꢁC. From the view of TG/DTA, acidity
and basicity analysis, cesium nitrate will decompose when
the calcination temperature was over 500 ꢁC, and the total
amount of base sites on the catalysts surface decreased
when increasing the calcination temperature. The decline
of surface area and the total amount of base sites with
increasing of the calcination temperature might be main
Table 5 Property of the
catalysts with different carriers
Catalyst
Cs
content (%)
Weight of 4 mL
catalyst (g)
Cs content in 4 mL
catalyst (g)
Conversion
(molMP/gCs h)
Zr–Mg–Cs/SiO2
Zr–Mg–Cs/Al2O3
Zr–Mg–Cs/TiO2
Zr–Mg–Cs/MgO
11.5
9.8
2.1
2.7
4.2
3.4
0.24
0.26
0.57
0.41
0.084
0.048
0.034
0.015
Cs content was determined by
the X-ray fluorescence
(PANalytical Axios)
13.5
12.1
123

836
B. Li et al.
the amount of acid/base sites increased with the increasing
of cesium. It provided more active sites for the reaction.
These may be the main reason of high conversion for
catalysts with high cesium content. The specific surface
area decreased with increasing cesium loading, while the
catalytic activity gradually strengthened. It indicated the
specific surface area was not a key factor for the reaction in
our experimental condition.
that the more by-products were produced at the higher
reaction temperature, so the selectivity of MMA decreased.
The maximum yield of MMA was 32.9 % at 380 ꢁC and
the selectivity of MMA was 70.3 %.
3.2.5 Effects of the MP/HCHO Molar Ratio
The reaction was performed at 360 ꢁC at Zr–Mg–Cs/SiO2
catalysts with 15 wt% cesium. The effect of the MP/HCHO
molar ratio on the reaction was summarized in Table 6.
With the increasing of the MP/HCHO molar ratio, the
conversion of MP increased from 36.2 to 55.4 %. When the
MP/HCHO molar ratio was 1/1, the selectivity to MMA
was the best and achieved to 80.3 %.
3.2.4 Effects of Reaction Temperature
The reaction was performed on the Zr–Mg–Cs/SiO2 cata-
lyst which was calcined at 450 ꢁC with 15 wt% cesium.
The molar ratio of MP/FA/CH3OH was 1/1/1.5; the reac-
tion temperature range from 300 to 400 ꢁC. The result was
showed in Fig. 9. The conversion of MP increased with
increasing of the reaction temperature, while the selectivity
proceeded with the opposite trend. This was attributed to
3.3 Catalyst Stability and Reusability
The stability and reusability of Zr–Mg–Cs/SiO2 catalyst
(cesium loading: 15 wt% and calcined: 450 ꢁC) was also
investigated. The reaction was performed at 320 ꢁC; the
molar ratio of MP/FA was 1/1. Figure 11a showed the
yield of MMA based on the MP feed as function of the
time-on stream. The activity dropped by nearly 50 % after
reacting of 32 h. The deactivation mechanism may be due
to the formation of carbonaceous deposits on the surface of
the catalyst. In fact, the catalyst turned black after use [22,
23]. The carbon content of the fresh and used catalysts was
Table 6 Effect of the MP/HCHO molar ratio on the aldol conden-
sation of MP with FA
Molar ratio
of MP/HCHO
XMP
(%)
SMMA
(%)
SMAA
(%)
YMMA
(%)
YMAA
(%)
1/0.5
1/1
36.2
37.7
55.4
73.7
80.3
75.3
0.7
2.1
1.3
26.5
30.2
41.7
0.2
0.8
0.7
Fig. 8 Effects of the amount of cesium on the catalytic performance.
Reaction was performed at 360 ꢁC; molar ratio of MP/HCHO was
1/2; feed rate of 0.1 mL/min
1/2
Reaction was performed at 360 ꢁC, feed rate of 0.1 mL/min
X conversion, S selectivity, Y yield
Fig. 9 Effects of reaction temperature on the catalytic performance.
molar ratio of MP/HCHO was 1/1; feed rate of 0.1 mL/min
Fig. 10 TG/DTA curves of the used Zr–Mg–Cs/SiO2 catalyst after
reaction
123

Synthesis of Methyl Methacrylate by Aldol Condensation
837
Fig. 11 Yield of MMA on the
fresh (a) and regenerated
catalyst (b). Reaction was
performed at 320 ꢁC; molar
ratio of MP/HCHO was 1/1;
feed rate of 0.1 mL/min
Table 7 Element content of the fresh and regenerated catalyst
cesium-impregnated TiO2 and MgO carriers presented little
activity for the reaction. The conversion and selectivity
over the Zr–Mg–Cs/Al2O3 catalyst were relatively low.
The cesium-impregnated SiO2 exhibited better catalytic
activity and high selectivity for the reaction. The charac-
terization results suggested that the calcination temperature
and content of cesium were main factors to affect the
textural properties, surface acidity and basicity and cata-
lytic activity of the catalysts. Although the single life of
this catalyst was slight short due to carbon deposition on
the surface of catalyst, its catalytic activity was completely
regenerated by a simple calcination. The Zr–Mg–Cs/SiO2
catalyst which had good reusability and long cycle life was
easily to prepare. Therefore, the Zr–Mg–Cs/SiO2 catalyst
was a promising catalyst for the aldol condensation of MP
with FA to produce MMA.
Catalyst
Cesium
(%)
Magnesium
(%)
Zirconium
(%)
Fresh catalyst
11.73
11.31
0.046
0.065
0.036
0.040
Regenerated catalyst
Regenerated catalyst: The 16th regeneration of catalyst
measured by carbon/sulfur analyzer (CS-344, US LECO
Corporation). The carbon content of the fresh catalyst was
0.07787 %, while the carbon content of the used catalyst
was 3.8461 %. The used catalyst was characterized by TG/
DTA (Fig. 10), different from the fresh catalyst, the used
catalyst had a distinct weight loss from 300 to 450 ꢁC
accompanied with a big exothermic DTA peak at 405 ꢁC,
which mainly ascribed to the coke combustion with O2.
In order to investigate the reusability of the catalyst, the
used catalyst was regenerated by calcination in a stream of
mixture of oxygen and nitrogen at 450 ꢁC for 3 h, the
volume of oxygen was 1 %. The regenerated catalyst was
investigated according to the same conditions with the
fresh catalyst by reaction 32 h. The catalyst was repeatedly
regenerated 16 times and total operation time was over
500 h. Figure 11b showed the catalytic activity of the
repeatedly regenerated catalyst. The catalytic properties
can be completely restored after regeneration. From the
ICP analysis, the content of cesium, magnesium and zir-
conium in the fresh catalyst and regenerated catalyst (after
the 16th regenerated) was showed in Table 7. It indicated
that the components of regenerated catalyst did not lose
over 500 h. So after the sixteenth regeneration, its activity
was still the same as that of the fresh catalyst, which
demonstrated that the Zr–Mg–Cs/SiO2 catalyst had good
reusability.
Acknowledgments This work was financially supported by the
National natural Science Foundation of China (No. 21076221) and
(No. 21276267); National Research Program of China (863 Program)
(No. 2012AA062903).
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