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