Highly Catalytic Activity of Ba/Γ-Ti–Al2O3 Catalyst for Aldol Condensation of Methyl Acetate with Formaldehyde

Article abstract of DOI:10.1007/s10562-018-2535-9

Abstract: In this work, titanium-doped mesoporous Al₂O₃ (γ-Ti–Al₂O₃) was prepared by an evaporation-induced self-assembly method and used as a carrier of Ba/γ-Ti–Al₂O₃ catalyst to catalyze the aldol condensation of methyl acetate with formaldehyde to methyl acrylate in a fixed-bed reactor. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, N₂ adsorption–desorption, pyridine absorption performed via Fourier transform infrared spectroscope (Py-IR), and NH₃ and CO₂ temperature-programmed desorption (NH₃ and CO₂-TPD). Experimental results indicated that the doping of the titanium species into the frame work of mesoporous Al₂O₃ (γ-Ti–Al₂O₃) had a significant influence on the catalytic activity via modifying the acid–base surface properties of the catalyst. Furthermore, the Ba/γ-Ti–Al₂O₃ catalyst demonstrated excellent catalytic performance, with a methyl acetate conversion rate of 50% and methyl acrylate selectivity up to 90.2%. Compared with the Ba/Al₂O₃ catalyst, the Ba/γ-Ti–Al₂O₃ catalyst had better catalytic activity, stability and potential for practical application, which was likely due to an increased number of Lewis acid sites, especially the medium acid sites. Graphical Abstract: Barium supported mesoporous γ-Ti–Al₂O₃ catalyst was found to be an effective catalyst for vapor phase aldol condensation of methyl acetate with formaldehyde. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2535-9

Catalysis Letters (2018) 148:3402–3412
https://doi.org/10.1007/s10562-018-2535-9
Highly Catalytic Activity of Ba/γ-Ti–Al2O3 Catalyst for Aldol
Condensation of Methyl Acetate with Formaldehyde
Qiang Bao1 · Hui Qi3 · Chunlei Zhang2 · Chunli Ning2 · Yi Zhang2 · Ye Jiang1 · Yifan Wu1 · Wenying Gui1 · Zhenlu Wang1
Received: 22 June 2018 / Accepted: 20 August 2018 / Published online: 31 August 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
In this work, titanium-doped mesoporous Al2O3 (γ-Ti–Al2O3) was prepared by an evaporation-induced self-assembly method
and used as a carrier of Ba/γ-Ti–Al2O3 catalyst to catalyze the aldol condensation of methyl acetate with formaldehyde to
methyl acrylate in a fixed-bed reactor. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectros-
copy, N2 adsorption–desorption, pyridine absorption performed via Fourier transform infrared spectroscope (Py-IR), and
NH3 and CO2 temperature-programmed desorption (NH3 and CO2-TPD). Experimental results indicated that the doping
of the titanium species into the frame work of mesoporous Al2O3 (γ-Ti–Al2O3) had a significant influence on the catalytic
activity via modifying the acid–base surface properties of the catalyst. Furthermore, the Ba/γ-Ti–Al2O3 catalyst demonstrated
excellent catalytic performance, with a methyl acetate conversion rate of 50% and methyl acrylate selectivity up to 90.2%.
Compared with the Ba/Al2O3 catalyst, the Ba/γ-Ti–Al2O3 catalyst had better catalytic activity, stability and potential for
practical application, which was likely due to an increased number of Lewis acid sites, especially the medium acid sites.
Graphical Abstract
Barium supported mesoporous γ-Ti–Al2O3 catalyst was found to be an effective catalyst for vapor phase aldol condensation
of methyl acetate with formaldehyde.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-018-2535-9) contains
supplementary material, which is available to authorized users.
Extended author information available on the last page of the article
Vol:.(1234567890)
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Highly Catalytic Activity of Ba/γ-Ti–Al2O3 Catalyst for Aldol Condensation of Methyl Acetate…
3403
Keywords γ-Al2O3 · Ba/γ-Ti–Al2O3 catalysts · Vapor-phase aldol condensation · Methyl acrylate · Lewis acid sites
SBA-15 catalyst, and the yield and selectivity of MA could
reach 48.4 and 95.0%, respectively. Zhu et al. [25] reported
a Cs–La–Sb/SiO2 catalyst and showed the conversion of
methyl acetate (20%) as well as a small yield of methyl
acrylate (8%). However, they found that the activity of the
best catalyst sharply decreased as the reaction progressed
due to coke deposition and the irreversible loss of the Cs
species.
1 Introduction
As fundamental industrial monomers, methyl acrylate (MA)
and acrylic acid (AA) are widely used in the manufacture
of paint additives, adhesives, textiles, and leather treating
agents [1–4]. Currently, MA is produced through esterifica-
tion of methanol (MeOH) with AA, and the synthesis of AA
is completed through two-step oxidations of propylene with
air [5–10]. However, propylene comes from non-sustainable
sources in the petrochemical industry, and its price is greatly
influenced by crude oil prices. With the ever-increasing
demand for petroleum, it is becoming increasingly popu-
lar to use propylene as a feedstock. Therefore, it is highly
important to develop a novel and green route for AA and
MA synthesis.
Alumina is one of the most important catalytic materi-
als, which is widely used as a catalyst or carrier in the
automotive and petroleum industries due to its low cost,
desirable textural properties (such as high surface area,
mesoporosity and stability) and its Lewis acid sites [26,
27]. In our previous work, we prepared an Al2O3-supported
barium catalyst, which showed the conversion of methyl
acetate (43.4%) as well as a selectivity of methyl acrylate
(92.8%) when optimized [28]. After that, we prepared an
improved Ba–La/Al2O3 catalyst, which showed the highest
(43.5%) conversion of methyl acetate with 93.9% selec-
tivity for methyl acrylate. Although the stability of the
catalyst improved significantly due to the alkaline function
of La2O3, the activity of the Ba–La/Al2O3 catalyst was
not obviously increased compared with Ba/Al2O3 catalyst
[29]. Hence, development of catalyst with higher catalytic
activity for the target reaction is necessary.
In recent decades, a one-step synthesis of MA and AA via
vapor-phase aldol condensation of acetic acid (Aa) or methyl
acetate (Ma) with formaldehyde (FA) has been researched
and developed because of its simplified reaction route and
common feedstocks that can be derived readily from natural
gas, coal and biomass [11, 12].
Aldol condensation plays an important role in organic
synthesis for C–C bond formation, and has numerous
applications in the synthesis of fine chemicals [13–16].
According to previous work, the aldol condensation
reaction can be catalyzed by acid/base catalysts and can
readily occur over either acid, base, or acid–base bifunc-
tional catalysts. Ai [17–21] prepared a series of solid acid
catalysts using an impregnation method and found that
vanadium–titanium binary phosphates and V2O5–P2O5
binary oxide can effectively catalyze the aldol condensa-
tion of HCHO with acetic acid/methyl acetate to acrylic
acid and its derivatives. Based on their work, Feng et al.
[22] prepared an improved VPO catalyst and achieved an
optimal conversion of methyl acetate of 84.2% (including
30–35% acetic acid and 3–12% COx). They also stated that
the vanadyl phosphate entity appearing in the δ-VOPO4
form functions as the active component. However, Yang
et al. [23] reported that the V/P atomic ratio is an essential
factor determining the catalytic performance of the aldol
condensation reaction, and that the optimal ratio is 1:2.
The low stability of such catalysts and the difficulty of pre-
cisely controlling the V/P atomic ratio would be principal
obstacles to produce MA and AA based on acid catalysts
in large quantities. More attention has been paid to basic
catalysts, which often include the oxides or hydroxides of
alkali or alkaline earth metals supported on porous materi-
als, because of the easier preparation and higher catalytic
activity than acidic catalysts. Yan et al. [24] designed a Cs/
Combined with previous studies, we found that the
Lewis acid sites on the catalyst surface played a cru-
cial role in the aldol condensation reaction of methyl
acetate with formaldehyde. For titanium oxides, it has
been reported that although they have relatively low spe-
cific surface areas, the surface Lewis acid sites of TiO2
are multivalent and stronger than those of alumina [30].
Therefore, more surface Lewis acid sites may be obtained
by complexes of aluminum and titanium, which provide
certain possibilities for the catalytic activity of titanium-
doped mesoporous Al2O3 towards the target reaction.
In this paper, titanium was doped into the frame work of
mesoporous Al2O3 (γ-Ti–Al2O3) or loaded on the surface
of Al2O3 (TiO2/Al2O3). The obtained materials were used
as carriers to prepare γ-Ti–Al2O3- and TiO2/Al2O3-sup-
ported barium catalysts by the incipient wetness impregna-
tion method for the aldol condensation of methyl acetate
with formaldehyde. The catalytic performances of the
catalysts were carried out in a fixed-bed microreactor. The
physical–chemical properties of the catalysts were stud-
ied by N2-adsorption, XRD and XPS. The surface acidity
and basicity of the catalysts were studied by CO2-TPD,
NH3-TPD and pyridine adsorption IR. This work also
details the catalyst stability and reusability and postulates
a reaction route for this aldol condensation reaction.
1 3

3404
Q. Bao et al.
evaluated from desorption branches of nitrogen isotherms
using the BJH model. X-ray photoelectron spectroscopy
(XPS) measurements were conducted on ESCALAB250
spectrophotometer. The electron binding energies were
referenced to the C 1s (Eb = 284.6 eV) peak. Tempera-
ture programmed desorption (TPD) of NH3 and CO2 were
used to characterize the acidic and basic sites over the
studied catalysts, respectively. The TPD experiments were
conducted using a ChemBET Pulsar TPR/TPD instrument
(Quantachrome Instruments) with a built-in TCD detec-
tor. Typically, 50 mg catalyst was used in each measure-
ment. The catalyst was first purged with He (UHP grade,
Airgas) at 600 °C for 0.5 h with a 10 °C/min heating ramp
rate, then cooled to 80 °C. A flow of CO2 (research grade,
Airgas) or NH3 (electronic grade, Airgas) was introduced
into the tubular catalyst bed for 30 min at 80 °C for CO2 or
NH3 adsorption. After purging the catalyst bed for approx-
imately 30 min with He to evacuate the physiosorbed NH3
or CO2, the catalyst was heated to 600 °C with a 10 °C/
min heating ramp rate. The change in thermal conductiv-
ity due to the concentration change of NH3 or CO2 in the
effluent was recorded on TCD detector. IR spectra were
recorded on a Nicolet Impact spectrometer at 4 cm−1 opti-
cal resolution. Prior to the measurements, 20 mg of the
catalysts were pressed into self-supporting discs and acti-
vated in the IR cell attached to a vacuum line at 250 °C for
0.5 h. The adsorption of pyridine (Py) was performed at
50 °C for 30 min. The excess pyridine was further evacu-
ated at 50 °C for 0.5 h. After elimination of the physically
absorbed pyridine, the sample was cooled down to room
temperature for collection of the IR spectrum; then, the
sample was returned to reaction conditions and heated at
a further temperature.
2 Experiment
2.1 Catalyst Preparation
Methyl acetate (≥ 99.0%), methanol (CH3OH;
≥ 99.0%), trioxymethylene (≥ 98.0%), barium acetate
(Ba(CH3COO)2; ≥ 99.0%) and d-glucose monohydrate
of analytical grade were purchased from the Sinop-
harm Chemical Reagent Company. Aluminum isopro-
poxide (C9H21AlO3 ≥ 98.0%) and tetrabutyl titanate
(C16H36O4Ti ≥ 99.0%) were purchased from the Alad-
din Industrial Corporation. Aluminum isopropoxide
[Al(OiPr)3] was used as the Al source and tetrabutyl
titanate (TBOT) as the Ti source. Mesoporous γ-Ti–Al2O3
(Al/Ti = 75) was prepared by the evaporation-induced
self-assembly method [31, 32]. In a typical procedure,
Al(OiPr)3 (32.7 g), glucose (28.8 g) and TBOT (0.73 g)
were added into 432 mL deionized water at room tem-
perature. After stirring for 6 h, the pH of the mixture
was adjusted to 5.5 by using a diluted aqueous nitric acid
(10 wt%) solution. The resulting solution was continuously
stirred for 24 h and then heated at 100 °C in open air to
remove water and all other volatiles. The final solid was
calcined at 600 °C for 6 h to obtain γ-Ti–Al2O3. Al2O3
was prepared by the above procedure in the absence of
TBOT. As references, titanium oxide loaded Al2O3 (TiO2/
Al2O3) material with an Al/Ti ratio of 75 was prepared
by the incipient wetness impregnation method. The
Ba/γ-Ti–Al2O3 catalyst was prepared as follows. Typically,
the support γ-Ti–Al2O3 (5 g) was impregnated with 7 mL
of an aqueous solution containing the desired amount of
Ba(CH3COO)2 (0.44 g), and the mixture was stirred for
approximately 2 h at room temperature. Afterwards, water
in the mixture was evaporated at a temperature of 80 °C
under atmospheric pressure. The dried materials were
next calcined in a muffle furnace at 600 °C for 5 h with
a 5 °C/min heating ramp rate. After natural cooling, the
Ba/γ-Ti–Al2O3 catalyst was obtained. The other supported
catalysts were prepared in the same way.
2.3 Catalyst Evaluation
The vapor phase aldol condensation reaction of methyl
acetate with formaldehyde was conducted in a fixed-bed
reactor under atmospheric pressure. Typically, 500 mg of
catalyst was loaded to the center (supported by a quartz frit)
of a stainless-steel tube reactor that was 30 cm long with a
0.8 cm inside diameter. The mixed solution of methyl acetate
and formaldehyde was fed into the reactor using an advec-
tion pump with a typical flow rate of 0.06 mL/min. The
products were identified and analyzed by gas chromatogra-
phy (GC-8A, FID) on a 60 m capillary column DB-WAX.
The mixed reactant consisting of a molar ratio of
1:2:2 Ma/FA/CH3OH, was fed to the reactor using a meter-
ing pump. FA was fed as trioxane, which was dissolved in
Ma. The formation of acetic acid because of the hydroly-
sis of Ma causes pipeline corrosion and reacts with FA to
form acrylic acid. Thus, the methanol was added to inhibit
the hydrolysis of Ma into acetic acid. The catalysts were
2.2 Catalyst Characterization
X-ray diffraction (XRD) patterns were measured with an
Empyrean X-ray diffractometer using a nickel filtered Cu
Kα source with a wavelength of 0.154 nm. An accelerat-
ing voltage of 40 kV and a current of 40 mA were used.
A slit width of 0.25° was used on the source. Scans were
collected using a PIXcel3D detector. N2 adsorption/des-
orption isotherms were measured using a Micromeritics
ASAP 2010N analyzer. Specific surface areas were cal-
culated using the BET model. Pore size distributions are
1 3

Highly Catalytic Activity of Ba/γ-Ti–Al2O3 Catalyst for Aldol Condensation of Methyl Acetate…
3405
evaluated based on their catalytic performance. The defini-
tion of methyl acetate conversion and product selectivity
was as follows:
TiO2
TiO2
Al2O3
TiO2/Al2O3
-Ti-Al2O3
Al2O3
TiO2/Al2O3
-Ti-Al2O3
(
)
Ma Conversion (%) = Main − Maout ∕Main × 100%
0
10
20
30
40
50
Pore Diameter(nm)
(
)
MA Selectivity (%) = MA∕ Main − Maout × 100%
MA Yield (%) = Ma Conversion (%) × MA Selectivity (%)
3 Results and Discussion
3.1 Physicochemical Properties
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Relative Pressure(P/P )
0
The XRD patterns of the γ-Ti–Al2O3, TiO2/Al2O3, Al2O3
and TiO2 samples are presented in Fig. 1. From Fig. 1, we
can see that typical gamma phase was generated in syn-
thetic Al2O3 samples (JCPD no. 00-046-1131), designated
by γ-Al2O3. No diffraction peaks due to Ti species were
detected on TiO2/Al2O3, which deduced that TiO2 was
present in a highly dispersed state on the surface of TiO2/
Al2O3. For TiO2, diffraction peaks at 2θ =25.3° and 48.0°
are assigned to anatase, and the peaks at 2θ=27.2°, 35.9°,
41.0°, 54.1°, 56.4°, and 68.7° are attributed to rutile TiO2.
For γ-Ti–Al2O3 sample, the characteristic peak of γ-Al2O3
is retained, and a weak peak appeared at 25.3°, assigned
to anatase. The XRD patterns of the Al2O3-, TiO2-, TiO2/
Al2O3-, and γ-Ti–Al2O3-supported Ba catalysts are shown in
Fig. S1. From Fig. S1, we can see that no diffraction peaks
due to Ba species are detected, which is consistent with prior
studies [28]. Therefore, this also indicates that barium spe-
cies is highly dispersed on the surface of materials.
Fig. 2 N2 adsorption–desorption isotherms and pore-size distribution
of studied catalysts
H1-type hysteresis loop at high relative pressure, which
was due to the capillary condensation of nitrogen in the
pores [33]. The textural properties of all samples are sum-
marized in Table 1. As shown in Table 1, Al2O3 demon-
strated the highest specific surface area (SBET, 357 m2 g−1),
while TiO2 demonstrated a relatively low specific surface
area (SBET, 48 m2 g−1). The addition of titanium species, via
both surface impregnation (TiO2/Al2O3) and body doping
(γ-Ti–Al2O3), resulted in the reduction of the surface area
of the catalysts. A similar behavior was also observed when
barium acetate was impregnated on the carriers to prepare
Ba-modified catalysts, where the presence of the Ba com-
ponent decreases the surface area of the support materials.
This can be ascribed to a blockage of the pores by the accu-
mulated barium component. The pore diameters and pore
volumes of all the investigated catalysts (except for TiO2 and
Ba/TiO2) ranged from 5.4 to 6.0 nm and from 0.37 to 0.52
cc g−1, respectively.
The surface area and pore size distribution of the studied
catalysts were analyzed using N2 adsorption–desorption.
Figure 2 shows that all the samples were type IV with an
TiO2
Table 1 Textural properties of the supports and Ba-modified catalysts
Catalysts
SBET (m2 g−1
)
Pore volume Mean pore
(cc g−1
)
diameter
(nm)
-Ti-Al2O3
TiO2/Al2O3
Al2O3
TiO2
Al2O3
48
357
281
320
45
319
247
310
0.19
0.52
0.44
0.45
0.18
0.45
0.37
0.46
15.5
5.6
6.0
5.5
15.4
5.4
5.7
5.7
TiO2/Al2O3
γ-Ti–Al2O3
Ba/TiO2
Ba/Al2O3
Ba–TiO2/Al2O3
Ba/γ-Ti–Al2O3
10
20
30
40
50
60
70
2 ( )
Fig. 1 The XRD patterns of the studied catalysts
1 3

3406
Q. Bao et al.
A
Ti 2p
-Ti-Al2O3
TiO2/Al2O3
Al2O3
Ti4+
Ti3+
-Ti-Al2O3
Al-O-Ti
TiO2/Al2O3
500
1000
1500
2000
2500
3000
3500
4000
454
456
458
460
462
464
466
468
470
472
Wavenumbers (cm-1)
Binding energy (eV)
Al 2p
B
Fig. 4 FTIR spectra of γ-Ti–Al2O3, TiO2/Al2O3 and Al2O3
Al-O-Al
(BEs) of Ti4+ at 458.9 eV and Ti3+ at 457.9 eV [31]. Thus,
in the TiO2/Al2O3 catalyst, titanium mainly existed in the
form of Ti3+ on the surface of the material, while in the
γ-Ti–Al2O3, titanium existed in the form of Ti4+. Both
Al2O3 and TiO2/Al2O3 have similar Al 2p XPS spectra,
with the binding energy (BE) of Al3+ at 74.3 eV, assigned
to Al–O–Al [34]. In the γ-Ti–Al2O3 sample, however, the
Al 2p XPS spectra exhibit two BEs of Al3+ at 74.3 and
75.3 eV, assigned to Al–O–Al and Al–O–Ti respectively
[35, 36]. In addition, the peak at 1128.3 cm−1 appeared in
the FTIR spectrum of γ-Ti–Al2O3, assigned to the surface
Al–O–Ti groups (Fig. 4) [37], verifying titanium in the form
of Al–O–Ti in the framework of γ-Ti–Al2O3. The O 1s XPS
spectra of Al2O3 and TiO2/Al2O3 exhibited a common peak
at 531.4 eV, which is usually assigned to the lattice oxygen.
In the γ-Ti–Al2O3 sample, besides the peak at 531.4 eV, a
peak at 532.3 eV appeared, which is attributed to the chemi-
cal adsorption of oxygen at oxygen vacancies [38, 39].
Al-O-Ti
-Ti-Al2O3
TiO2/Al2O3
Al2O3
70
72
74
76
78
80
Binding energy (eV)
O 1s
C
531.4
532.3
-Ti-Al2O3
TiO2/Al2O3
Al2O3
3.2 Acid–Base Properties
The acid–base properties of the studied catalysts were char-
acterized by TPD of NH3–CO2, shown in Figs. 5 and 6.
Table 2 lists the total CO2 and NH3 uptakes on the basis
of catalyst weight and the corresponding concentrations for
the different types of acid/base sites. As shown in Fig. 5,
all NH3-TPD profiles were integrated (deconvoluted) in
four fixed temperature ranges (100–180 °C, 180–230 °C,
230–300 °C, 300–500 °C) by a Gaussian peak fitting
method in order to compare acidic sites of the same strength.
The total NH3 uptakes for TiO2, Al2O3, TiO2/Al2O3 and
γ-Ti–Al2O3 are 40.3, 91.1, 93.2 and 114.4 mmol g−1 cat.,
respectively. The NH3 uptakes at the four different tempera-
ture ranges for TiO2 are 5.7, 10.2, 9.4 and 15.0 mmol g−1
catalyst. For Al2O3, the NH3 uptakes are 14.2, 29.5, 10.2 and
524
526
528
530
532
534
536
538
Binding energy (eV)
Fig. 3 Ti 2p (a), Al 2p (b) and O1s (c) XPS spectra of γ-Ti–Al2O3,
TiO2/Al2O3 and Al2O3
To explore the existence of titanium species, XPS meas-
urements of the γ-Ti–Al2O3, TiO2/Al2O3 and Al2O3 were
performed. Figure 3 shows the Ti 2p, Al 2p and O1s XPS
spectra of different samples. According to previous research,
the Ti 2p 3/2 XPS spectra could exhibit two binding energies
1 3

Highly Catalytic Activity of Ba/γ-Ti–Al2O3 Catalyst for Aldol Condensation of Methyl Acetate…
3407
-Ti-Al
O
3
A
2
A
-Ti-Al2O3
TiO2/Al2O3
Al2O3
-Ti-Al2O3
TiO2/Al2O3
Al2O3
TiO2
TiO2
100
150
200
250
300
350
400
450
500
Temperature (ºC)
100
150
200
250
300
350
400
450
500
100
150
200
250
300
350
400
450
500
550
Temperature (ºC)
Temperature (ºC)
B
B
a
-Ti-Al2O3
a
-Ti-Al2O3
a
-Ti-Al2O3
Ba-TiO2/Al2O3
Ba/Al2O3
Ba/TiO2
Ba-TiO2/Al2O3
Ba/Al2O3
Ba/TiO2
100
150
200
250
300
350
400
450
Temperature (ºC)
100
150
200
250
300
350
400
450
500
550
100
150
200
250
300
350
400
450
500
Temperature (ºC)
Temperature (ºC)
Fig. 5 NH3-TPD patterns of the supports (a) and Ba-modified cata-
lysts (b)
Fig. 6 CO2-TPD patterns of the supports (a) and Ba-modified cata-
lysts (b)
37.2 mmol g−1 catalyst. For TiO2/Al2O3, the NH3 uptakes
are 15.7, 32.3, 10.6 and 34.6 mmol g−1 catalyst; whereas
these are 17.8, 33.5, 16.3 and 46.8 mmol g−1 catalyst for
γ-Ti–Al2O3. Overall, TiO2 showed the least amount of acid,
Al2O3 showed a moderate acid content, and the γ-Ti–Al2O3
catalyst had the most acid content. It is clearly revealed
that the doping of the titanium species into the frame work
of mesoporous Al2O3 can increase the total amount of
acid centers, especially medium acid centers (in the range
of 180–300 °C) and strong acid centers (in the range of
300–500 °C). The increment in strong acidity may be due
to the formation of bridged hetero metal–oxygen bonds (e.g.,
Al–O–Ti bonds) resulting in excessive charges [31]. Com-
pared with the catalyst carriers, the Ba-modified catalysts
had a dramatically decreased NH3 uptake at a temperature
of 300–500 °C (e.g., 46.8 mmol g−1 cat. for bare γ-Ti–Al2O3
and 16.5 mmol g−1 cat. for Ba/γ-Ti–Al2O3), indicating that
a large number of strong acid sites were reduced when BaO
was supported on the carriers. More interestingly, the NH3
uptake at a temperature of 180–300 °C for Ba-modified cata-
lysts increased significantly, indicating that adding a certain
amount of barium could increase the number of medium
acid sites of the catalyst. This increase might be the result
of an interaction between barium species and acid sites on
the catalyst surface, leading to an increase in medium acid
sites and a decrease in strong acid sites. Figure 6 shows the
desorption profiles of CO2 from the carriers and metal-mod-
ified catalysts. In Fig. 6, all catalysts exhibited a prominent
CO2 desorption peak in the range of 100–350 °C and the
basic sites on these catalysts were arbitrarily considered
weak. However, it should be noted that the maximal tem-
perature of the desorption peak for TiO2 is higher than that
of other carrier materials, indicating a higher intensity of
basic sites on the surface of TiO2. For metal-modified cata-
lysts, adding barium species markedly increased the total
CO2 uptake (e.g., 9.5 mmol g−1 cat. for bare γ-Ti–Al2O3 and
16.2 mmol g−1 cat. for Ba/γ-Ti–Al2O3), indicating some new
base sites formed on the catalyst surface.
1 3

3408
Q. Bao et al.
Table 2 List of the total CO2 and NH3 uptakes on the catalysts surface and the corresponding concentrations for the different types of acid/base
sites
Catalyst
TiO2
Al2O3
TiO2/Al2O3
γ-Ti–Al2O3
Ba/TiO2
Ba/Al2O3
Ba–TiO2/Al2O3
Ba/γ-Ti–Al2O3
Total (mmol NH3/g cat.)
T1 (100–180 °C)
T2 (180–230 °C)
T3 (230–300 °C)
T4 (300–500 °C)
Total (mmol CO2/g cat.)
T (100–350 °C)
40.3
5.7
10.2
9.4
15.0
13.0
13.0
91.1
14.2
29.5
10.2
37.2
6.5
93.2
15.7
32.3
10.6
34.6
9.1
114.4
17.8
33.5
16.3
46.8
9.5
20.2
1.8
6.9
8.3
3.2
21.7
21.7
77.9
15.0
24.7
21.1
17.1
12.8
12.8
75.2
14.9
24.3
20.6
15.4
15.8
15.8
87.6
15.8
33.2
22.1
16.5
16.2
16.2
6.5
9.1
9.5
-Ti-Al2O3
TiO2/Al2O3
Al2O3
-Ti-Al2O3
TiO2/Al2O3
Al2O3
A
A
1445
1445
1612
TiO2
TiO2
1612
1490
1490
1800
1750
1700
1650
1600
1550
1500
1450
1400
1800
1750
1700
1650
1600
1550
1500
1450
1400
-1
-1
Wavenumber (cm
)
Wavenumber (cm )
a -Ti-Al2O3
Ba-TiO2/Al2O3
Ba/Al2O3
B
a -Ti-Al2O3
B
Ba-TiO /Al2O3
2
1445
Ba/Al2O3
Ba/TiO2
1612
Ba/TiO
2
1612
1490
1445
1490
1800
1750
1700
1650
1600
1550
1500
1450
1400
1800
1750
1700
1650
1600
1550
1500
1450
1400
-1
Wavenumber(cm )
-1
Wavenumber (cm
)
Fig. 7 FTIR spectra of pyridine adsorbed on supports (a) and Ba-
modified catalysts (b) after degassing at 50 °C
Fig. 8 FTIR spectra of pyridine adsorbed on supports (a) and Ba-
modified catalysts (b) after degassing at 250 °C
As is known, pyridine can be used as a molecular probe
for the determination of Lewis and Brønsted acid sites of the
catalysts by monitoring the IR absorption peaks in the range
of 1400–1800 cm−1. To determine the nature of the acidic
sites, the acid properties of the carriers and metal-modified
catalysts were further characterized by FTIR spectra of
pyridine adsorption. The IR results are presented in Figs. 7
and 8. In Fig. 7, all oxide materials exhibited the pyridine
adsorption at 1445, 1490 and 1612 cm−1. Among these, the
observed IR bands at 1445 and 1612 cm−1 correspond to
different modes of vibration of pyridine coordinated to the
1 3

Highly Catalytic Activity of Ba/γ-Ti–Al2O3 Catalyst for Aldol Condensation of Methyl Acetate…
3409
Lewis acid sites. The band at 1545 cm−1 has been assigned to
the pyridine in Brønsted acid sites (not detected in figures).
The band at 1490 cm−1 is characteristic of the Lewis–Brøn-
sted acid complex. On these samples, only Lewis acid sites
were identified, although the Brønsted acid site was observed
in a former report [40]. When pyridine desorption occurred
at a relatively high temperatures (250 °C, In Fig. 8), the
peaks obviously decreased, suggesting that a large amount of
pyridine was removed from Lewis acid sites, which is attrib-
uted to the presence of a weakly interacting pyridine species.
However, compared with other carrier materials, γ-Ti–Al2O3
has more obvious pyridine absorption peaks, indicating that
the catalyst surface has more relatively strong Lewis acid
sites. This finding is exactly consistent with the characteri-
zation results of NH3-TPD. These results suggest that the
doped titanium in the framework of Al2O3 altered the surface
acidity of the alumina surface, particularly increasing the
medium Lewis acid sites.
conversion of methyl acetate was only 16.8% and the selec-
tivity of methyl acrylate was 61.9%, which is the worst cata-
lytic activity compared with other carrier materials. Al2O3
and TiO2/Al2O3 catalysts showed similar activity in methyl
acetate conversion, but different product distributions were
observed. However, the highest yield in MA (33.2%) was
obtained over the γ-Ti–Al2O3 sample, corresponding to an
Ma conversion of 51.5% and an MA selectivity of 64.5%.
BET analysis showed that the surface area order of the car-
rier materials was as follows: Al2O3 > γ-Ti–Al2O3 > TiO2/
Al2O3 > TiO2. IR results of pyridine adsorption revealed
that only Lewis acid sites were found on the surface of car-
rier catalysts. From the NH3-TPD analysis, it was clear that
TiO2 gave the least amount of acid, Al2O3 and TiO2/Al2O3
showed moderate acid content, and γ-Ti–Al2O3 catalyst had
the most acid content. Combining the catalytic activity and
selectivity with the results of characterization, we can draw
the following conclusions:
3.3 Activity Tests
1. The reason for the worst catalytic activity of TiO2 cata-
lyst is the low acid content and low specific surface area
of titanium dioxide.
The aldol condensation of methyl acetate with formaldehyde
catalyzed by TiO2, Al2O3, TiO2/Al2O3, γ-Ti–Al2O3 and Ba-
modified catalysts was carried out at 390 °C in a fixed-bed
reactor. In this work, methyl acrylate was detected as the
main product. Acrylic acid and acetic acid were detected as
byproducts. The conversion of methyl acetate and the selec-
tivities of acetic acid, acrylic acid and methyl acrylate based
on methyl acetate were taken as the parameters to evaluate
the catalytic performances.
2. Al2O3 and TiO2/Al2O3 showed similar activity in aldol
condensation of methyl acetate with formaldehyde,
which is largely because the catalysts have a similar
amount of acidity and basicity on their surfaces.
3. The doping of the titanium species into the frame work
of mesoporous Al2O3 (γ-Ti–Al2O3) had significant influ-
ence on the acidic properties of the catalyst, increasing
the total amount of acid centers (especially medium acid
centers and strong acid centers), which leads to the obvi-
ous increase of catalyst activity.
The evaluation results of the carrier materials are
depicted in Fig. 9. From Fig. 9, all the catalysts showed
a certain catalytic activity towards the aldol condensation
of methyl acetate with formaldehyde. For TiO2, the initial
In our previous work, doping of alumina with BaO
could be an efficient way to improve the catalytic activity
and stability [28]. Therefore, a series of Ba-modified cata-
lysts were prepared by the incipient wetness impregnation
method for the aldol condensation of methyl acetate with
formaldehyde to methyl acrylate. The evaluation results of
the related catalysts are depicted in Fig. 10. From Fig. 10,
with the addition of barium species to the supports, all
the catalysts showed a slight decrease in catalytic activ-
ity to methyl acetate. However, the selectivity for methyl
acrylate was significantly increased. The catalytic per-
formances order of Ba-modified catalysts was as follows:
Ba/γ-Ti–Al2O3 > Ba–TiO2/Al2O3 ≈ Ba/Al2O3 > Ba/TiO2.
Among them, the Ba/γ-Ti–Al2O3 catalyst demonstrated the
highest catalytic activity, where the initial conversion to Ma
reached 50.0% and the selectivity to MA reached 90.2%.
From the XRD analysis, it was clear that the barium spe-
cies is highly dispersed on the surface of γ-Ti–Al2O3. IR
results of pyridine adsorption revealed that only Lewis acid
sites were found on the surface of Ba/γ-Ti–Al2O3, and the
100
TiO2
Al2O3
TiO2/Al2O3
-Ti-Al2O3
90
80
70
60
50
40
30
20
10
0
MA Selectivity (%)
Ma Conversion (%)
1
2
3
4
5
6
7
8
9
10
Time on stream(h)
Fig. 9 The conversion of Ma and the selectivity to MA on the carrier
materials
1 3

3410
Q. Bao et al.
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
Ba/TiO2
Ba/Al2O3
Ba-TiO2/Al2O3
Ba/ -Ti-Al2O3
MA Selectivity (%)
Ma Conversion
MA Selectivity
Ma Conversion (%)
1
2
3
4
5
6
7
8
9
10
0
50
100
150
200
250
300
Time on stream(h)
Time on stream(h)
Fig. 11 The conversion of Ma and the selectivity to MA on the
Fig. 10 The conversion of Ma and the selectivity to MA on the Ba-
Ba/γ-Ti–Al2O3 catalyst
modified catalysts
addition of barium species obviously reduced the number of
Lewis acid sites. The TPD analysis of NH3–CO2 showed that
the addition of barium species modified the acid–base sur-
face properties of γ-Ti–Al2O3, leading to an obvious increase
in medium acid sites and a decrease in strong acid sites,
while providing more weak basic sites for the Ba/γ-Ti–Al2O3
catalysts. The dramatic change in the acid–base surface
properties may be the main reason for the good catalytic
activity of the Ba/γ-Ti–Al2O3.
effective components of the regenerated catalyst were not
destroyed in the process of regeneration, and the activity of
the Ba/γ-Ti–Al2O3 catalyst was relatively stable after a total
reaction time of 300 h.
A probable reaction route is proposed for the vapor
phase aldol condensation over the Ba/γ-Ti–Al2O3 catalyst
in Fig. 12. First, the base sites attacked the carboxyl group
of methyl acetate absorbed on the catalyst surface to form a
carbanion. Next, the acid sites attacked the carbonyl group
of the adsorbed formaldehyde, increasing the electrophilic-
ity of the carbon atom. Then, the activated methyl acetate
and formaldehyde reacted to form an intermediate, probably
Combined with the catalytic performances with the char-
acterization results, we can conclude that the Ba/γ-Ti–Al2O3
catalyst had the highest catalytic activity compared with
other Ba-modified catalysts, which was likely due to more
Lewis acid sites on the surface of Ba/γ-Ti–Al2O3 catalyst,
especially the medium Lewis acid sites.
3.4 Catalyst Stability and Reusability
The stability and regeneration of the Ba/γ-Ti–Al2O3 catalyst
were also investigated at 390 °C for approximately 300 h,
and the results are shown in Fig. 11. The used catalyst was
regenerated by calcining in a stream of air at 500 °C for 5 h
and studied under the same conditions. From Fig. 11, we
can see that Ma conversion declined by nearly 70% after
reaction at 60 h. In fact, the catalyst turned brown and black
after the reaction. According to the previous research [41,
42], the deactivation of catalyst can be due to the formation
of carbon deposition on the surface of the catalyst. Although
the activity (based on the conversation of Ma) decreased
gradually with reaction time as the result of the formation
of carbonaceous deposits, the regenerated catalyst, after
removal of carbonaceous deposits at high temperatures in
a stream of air, showed nearly the same initial catalytic
activity as the fresh catalyst. This result indicated that the
Fig. 12 Reaction mechanism for the vapor phase aldol condensation
of Ma with FA over the Ba/γ-Ti–Al2O3 catalyst
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Highly Catalytic Activity of Ba/γ-Ti–Al2O3 Catalyst for Aldol Condensation of Methyl Acetate…
3411
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Finally, the adjacent acid sites catalyzed the rapid dehydra-
tion of the intermediate to form methyl acrylate.
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Mesoporous γ-Ti–Al2O3 was successfully prepared by an
evaporation-induced self-assembly method and used as a
carrier of Ba/γ-Ti–Al2O3 catalyst to catalyze the aldol con-
densation of methyl acetate with formaldehyde to methyl
acrylate in a fixed-bed reactor. The physicochemical prop-
erties of the catalysts were surveyed by XRD, N2 adsorp-
tion–desorption and XPS. The acid–base properties of the
catalysts were characterized by TPD of NH3–CO2 and FTIR
spectra of pyridine adsorption. The characterization results
showed that titanium was successfully incorporated into
the framework of γ-Al2O3. In addition, the doping of the
titanium species had a significant influence on the acidic
properties of the catalyst, increasing the total amount of
acid centers (especially medium acid centers and strong
acid centers), which was attributed to the formation of
Al–O–Ti linkages in the framework of γ-Ti–Al2O3. Then, the
γ-Ti–Al2O3 was used as a carrier to prepare a Ba/γ-Ti–Al2O3
catalyst by the incipient wetness impregnation method to
catalyze the target reaction. The experimental results showed
that the Ba/γ-Ti–Al2O3 catalyst exhibited the best catalytic
activity and selectivity on the aldol condensation of methyl
acetate with formaldehyde, and the conversion of Ma
achieved 50.0% with a selectivity of Ma of 90.2%, which
was likely due to the more Lewis acid sites on the surface of
Ba/γ-Ti–Al2O3 catalyst, especially the medium Lewis acid
sites. After a total reaction time of 300 h, the activity of
the Ba/γ-Ti–Al2O3 catalyst still retained its initial activity
after regeneration, indicating a good reusability. Compared
with the Ba/Al2O3 catalyst, the Ba/γ-Ti–Al2O3 catalyst had
better catalytic activity, stability and potential for practical
application.
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Affiliations
Qiang Bao1 · Hui Qi3 · Chunlei Zhang2 · Chunli Ning2 · Yi Zhang2 · Ye Jiang1 · Yifan Wu1 · Wenying Gui1 · Zhenlu Wang1
2
* Zhenlu Wang
Shanghai Huayi (Group) Company Technology Research
Institute, Longwu Road 4600, Shanghai 200241,
People’s Republic of China
wzl@jlu.edu.cn
1
Key Laboratory of Surface and Interface Chemistry of Jilin
3
The Second Hospital of Jilin University, Changchun 130041,
People’s Republic of China
Province, College of Chemistry, Jilin University, Qianjin
Road 2699, Changchun 130012, People’s Republic of China
1 3