Efficient production of acrylic acid by dehydration of lactic acid over BaSO4 with crystal defects

Article abstract of DOI:10.1039/c6ra28429a

BaSO₄ catalysts with different micromorphologies and crystal texture were prepared and used to investigate the structure-activity relationship in the dehydration reaction of lactic acid (LA) to acrylic acid (AA). SEM and N₂ physisorption were used to study the micromorphology. XRD and photoluminescence spectra were employed to analyze the crystal texture of samples prepared with different methods and treatments. The results revealed that BaSO₄ with smaller crystals and more defects had higher activity and selectivity to AA. It was likely that the crystal defects provided the active acid sites for dehydration of LA to AA, as evidenced by XPS and NH₃-TPD measurements. Using ethanol as the solvent and ultrasound treatment during the preparation of BaSO₄, imperfect small crystals with more defects were formed, which increased the AA selectivity to 78.8%.

Full text of DOI:10.1039/c6ra28429a

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Efficient production of acrylic acid by dehydration
of lactic acid over BaSO4 with crystal defects†
Cite this: RSC Adv., 2017, 7, 10278
*
Shuting Lyu and Tiefeng Wang
BaSO4 catalysts with different micromorphologies and crystal texture were prepared and used to investigate
the structure–activity relationship in the dehydration reaction of lactic acid (LA) to acrylic acid (AA). SEM and
N2 physisorption were used to study the micromorphology. XRD and photoluminescence spectra were
employed to analyze the crystal texture of samples prepared with different methods and treatments. The
results revealed that BaSO4 with smaller crystals and more defects had higher activity and selectivity to
AA. It was likely that the crystal defects provided the active acid sites for dehydration of LA to AA, as
evidenced by XPS and NH3-TPD measurements. Using ethanol as the solvent and ultrasound treatment
during the preparation of BaSO4, imperfect small crystals with more defects were formed, which
increased the AA selectivity to 78.8%.
Received 20th December 2016
Accepted 31st January 2017
DOI: 10.1039/c6ra28429a
rsc.li/rsc-advances
Diverse reaction pathways of LA with or without catalysis make
it challenging to increase the yield of AA in dehydration of LA.
1. Introduction
The early research on dehydration of LA focused on mixed
metal phosphates and sulfates as catalysts. In 1958, mixed
sulfates of Na2SO4/CaSO4 with molar ratio of 1 : 25 was re-
Lactic acid (LA), one of the most important biomass derived
platform chemicals, is regarded as a promising alternative
resource due to the excessive depletion of fossil resources.1,2
With two functional groups, LA has wide applications in the
food industry, commodity chemicals and biopolymers.3,4 LA can
be produced by traditional microbial fermentation5 or glycerol
dehydrogenation.6 In a recent work, LA was produced with
a high yield of 70% by catalytic conversion of biomass raw
materials such as lignocellulose.7
LA can be converted into valuable chemicals by a variety of
catalytic processes, among which the dehydration of lactic acid
to acrylic acid (AA) attracts particular attention,8 because AA is
widely used in the manufacture of paint additives, adhesives,
textiles, leather treating agents and synthetic resins. Currently,
the industrial production of AA mainly depends on petroleum-
based propylene or propane as feedstocks, which is costly and
unsustainable.6 Therefore, the development of a sustainable
process for conversion of LA to AA is of great signicance.
The main side products in dehydration of LA are acetalde-
hyde (AD) and COx from decarbonylation and decarboxylation,
propionic acid (PA) from hydrogenation, and 2,3-pentanedione
from condensation. Non-catalytic self-esterication and poly-
merization can also produce some side products.9 Further side
products such as acetic acid (AcOH) are produced from oxi-
dization of acetaldehyde or carbonylation of propionic acid.10
ported to give an AA yield of 68% at 400 C.11 However, little
ꢀ
attention was attracted at that time and the mechanisms of
LA reaction such as dehydration and decarbonylation/
decarbonylation were not studied until the last decade.
Studies on different zeolites modied with alkali/alkali-earth
metal salts revealed that weak and medium acid sites were
active sites in the dehydration reaction,12–14 while some re-
ported that a kind of cooperative acid–base catalysis also help
during the dehydration of LA to AA.12,15,16 Decarbonylation and
decarboxylation, known as the most important side reactions,
were mainly catalyzed by strong acid sites and were enhanced
under high reaction temperature.13,17 The surface hydroxyl
groups could also lead to particular adsorption mode favoring
decarbonylation/decarbonylation to AD, evidenced by later
study on the alkaline earth hydroxyapatites.18 In the early
period, extensive studies were conducted on different zeolites
modied with alkali/alkali-earth metal salts to improve the
catalysis efficiency.12,13,17,19–23 However, the durability of the
alkali/alkali-earth modied zeolites is a big problem because
of the deactivation, instability of framework and loss of
surface modiers under the atmosphere of water vapor and
high temperature.24 Recently, more attentions were paid to
phosphates and sulfates of alkaline earth metal.18,25–28 Vidhya
et al. reported a 60% selectivity over calcium hydroxyapatite
(HAP) catalysts, and proposed the possible mechanism of LA
conversion to AA and AD.25 Barium sulfate and the Ca3(PO4)2–
Ca2P2O7 (50 : 50 wt%) catalysts were reported to give high AA
selectivity of 74% and 76%, respectively.28,29 However, the
Beijing Key Laboratory of Green Reaction Engineering and Technology, Department of
Chemical Engineering, Tsinghua University, Beijing 100084, China. E-mail: wangtf@
tsinghua.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra28429a
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knowledge of the relationship between acidity of sulfate was carried out to determine the acidity of catalysts using an
catalysts and the selectivity to acrylic acid is still limited. automated adsorption system (ChemBET Pulsar TPR/TPD,
In the present work, we studied the BaSO4 catalysts to Quantachrome) with a thermal conductivity detector (TCD).
identify the reactive sites in sulfate for dehydration of LA to AA. For each NH3-TPD experiment, a sample of 600 mg was placed
ꢀ
Different preparation methods were used to change the textures in a quartz tubular reactor and pretreated at 350 C with a He
and acidities of the BaSO4 catalysts. The catalytic performance ow of 30 mL minꢁ1 for 30 min, simulating the sample
was correlated with the surface and microscopic characteristics, pretreatment in the catalytic reactor prior to the reaction. The
suggesting that crystal defects provided the required weak and sample was cooled to 80 ꢀC, and then 5% NH3/He waꢀs intro-
medium acid sites, which were the essential active sites for LA duced at a ow rate of 30 mL minꢁ1 for 30 min at 80 C, and
conversion and AA formation. A high selectivity to AA was then the sample was purged in He stream until a constant TCD
achieved over the optimized active BaSO4 catalyst.
level was obtained. The reactor temperature was programmed at
a rate of 10 ꢀC minꢁ1 to 600 ꢀC and the desorbed NH3 was
detected online by the TCD. Considering the pretreatment
temperature, the signals above 350 ꢀC could be assigned to the
evolution of NH3 from the thermal decomposition of ammo-
nium or carbonate residues in the samples, and had no effects
on the overall acidity of the samples.15
2. Experimental
2.1. Catalyst preparation
The BaSO4 catalysts were purchased or prepared by co-
precipitation using (NH4)2SO4 and BaCl2$2H2O as precursors.
Firstly, 0.03 mol (NH4)2SO4 and 0.03 mol BaCl2$2H2O were
separately dissolved in 120 mL distilled water. The obtained
BaCl2 solution was added to the (NH4)2SO4 solution dropwise to
form whiteꢀprecipitate of barium sulfate, which was washed,
2.3. Catalytic reaction
The catalysts were evaluated for the dehydration of LA in a xed-
bed quartz reactor (11 mm i.d.) using 0.25–1 g of catalyst at
350 ꢀC under atmospheric pressure. An aqueous solution of 20
wt% LA (1.2 mL hꢁ1) was fed into the reactor by a HPLC pump
(Series 3, 0.01–5 mL minꢁ1, SS, S. G. Seal Self Flush, Pulse
Damper) with nitrogen (30 mL minꢁ1) as carrier. The contact
time, decided by all these reaction conditions, were calculated
according to the literature.30 All the reactants and products were
preheated to 160 ꢀC to avoid undesired condensation.26 The
reaction products and unconverted LA were collected in a cold
ꢀ
dried at 80 C in air overnight, and then calcined at 500 C for
4 h to obtain the catalysts denoted as Co-pre. Other BaSO4
samples with different crystal defects were prepared using
different solvents (water and ethanol) and ultrasonic treatment
in the precipitation process. In a typical experiment, the ob-
tained (NH4)2SO4 solution and BaCl2 solution were added to the
solvent (e.g. ethanol) co-currently to form white precipitate of
barium sulfate, which was treated with ultrasound immediately
for 15 min. Aging for varied time, the white precipitate was also
washed, dried at 80 ꢀC in air overnight, and then calcined at
350, 500, 600 and 700 ꢀC for 4 h to obtain the catalysts denoted
as EtOH/ult-mh-T, in which EtOH represented the solvent
ethanol, m was the aging time in hour, and T was the calcination
temperature in Celsius.
ꢀ
trap of ꢁ5 C and analyzed offline by gas chromatograph (GC
7900, Techcomp Ltd.) equipped with a FID detector and a TM-
SuperWax column (30 m ꢂ 0.25 mm ꢂ 0.25 mm, Techcomp
Ltd.). The reaction was conducted for 9 h, and the products of
the rst three hours were not taken into account because they
did not satisfy a good mass balance. A comprehensive evalua-
tion result for the sample was derived by solving the average
conversions or selectivities during 4–9 h. The LA conversion and
product selectivity were calculated by
2.2. Catalyst characterization
The morphology of the catalysts were characterized with the
eld emission scanning electron microscopy (SEM, JSM 7401F,
JEOL, Japan). The BET surface area and pore volume were
calculated from nitrogen adsorption–desorption isotherms
moles of unconverted LA
LA conversion ¼ 1 ꢁ
ꢂ 100%
moles of LA in the feed
measured at 77
K on a physical adsorption apparatus
moles of a defined product
moles of converted LA
(Quandasorb-SI-4, Quantachrome) aer degassing the sample
at 573 K. The composition and texture of the catalysts were
identied by an X-ray powder diffractometer (XRD, Bruker-AXS
D8 Advance, Germany), using Cu Ka (l ¼ 0.154178 nm) radia-
tion. The scanning of enlarged (121) diffraction peaks was
specially conducted at a rate of 0.1ꢀ minꢁ1 to provide precise
information for calculation of the interplanar spacing and
crystal size. The contents of crystal defects were characterized
Product selectivity ¼
ꢂ 100%
3. Results and discussion
3.1. Catalyst characterization
3.1.1. Morphology and N2 physisorption. The morphology
by the room temperature photoluminescence spectra (PL). The and microstructure of the BaSO4 catalysts prepared with
measurement was conducted on a Hitachi F-7000 luminescence different methods differed signicantly from each other, as
spectrometer, using a Xe lamp with an excitation wavelength of shown in Fig. S1† and summarized in Table 1. The commercial
325 nm. The surface composition and chemical states of the BaSO4 sample had large particle size of 0.2–1.2 mm and had no
samples were characterized by an X-ray photoelectron spec- pore structure. The samples prepared in water with or without
trometer (XPS, Thermal Scientic ESCALAB 250Xi). NH3-TPD ultrasonic treatment were similar in particle size and pore
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Table 1 Textural and acid properties of BaSO4 prepared with different methods
Surface area
Pore volume
Interplanar
Acid amount
Catalyst
Particle sizea (mm)
(m2 gꢁ1
)
(mL gꢁ1
)
spacingb (nm)
Crystal sizeb (nm)
(mmol gꢁ1
)
Commercial
Co-pre
Water/ult
EtOH/ult-0 h-500
EtOH/ult-1 h-500
EtOH/ult-2 h-500
EtOH/ult-24 h-500
EtOH/ult-0 h-350
EtOH/ult-0 h-600
EtOH/ult-0 h-700
0.2–1.2
0.1–0.6
0.1–0.8
0.03–0.3
0.03–0.3
0.03–0.3
0.03–0.3
0.03–0.3
0.03–0.4
0.1–0.8
4.2
10.1
9.5
20.0
18.5
17.9
17.7
17.4
15.8
5.9
0.006
0.027
0.024
0.102
0.095
0.094
0.093
0.069
0.073
0.031
0.3102
0.3104
0.3088
0.3063
0.3081
0.3096
0.3099
0.3063
0.3070
0.3072
—
—
—
60.0
66.7
72.9
79.4
84.1
87.3
—
0.21
4.08
6.83
21.15
19.04
18.22
17.92
6.92
11.22
2.22
a
Determined by SEM. b Determined by enlarged (121) diffraction peaks of XRD patterns.
structure, while the ultrasound treated sample exhibited indicated the right shi of the (121) peaks for the catalysts
smaller pores. In comparison, the catalysts prepared in ethanol prepared with ultrasonic treatment both in water and ethanol
with ultrasound, EtOH/ult, had a much smaller particle size, solvents. The le and right shis of the diffraction peaks were
with most particles under 300 nm. The smaller pores of EtOH/ attributed to the defect-induced lattice expansion and contrac-
ult samples were also detected by TEM (Fig. S1e†) and tion respectively, according to previous studies.34–36 The inter-
conrmed by N2 physisorption in Table 1.
planar spacing was calculated from Bragg equation, and the
The specic surface area and pore volume for all the BaSO4 results are listed in Table 1. According to the theory of crystal,
catalysts are summarized in Table 1. The rst four rows the generation of defects is usually stimulated by heteroatom or
exhibited considerable differences in surface area (4.2 to 20.0 energy. Therefore, ultrasonic treatment could facilitate the
m2 gꢁ1) and pore volume (0.006 to 0.102 mL gꢁ1) among the generation of defects, evidenced by the reduced interplanar
samples prepared with different methods. The BET surface area
and pore volume of the commercial sample were very low due to
its big particle size and nonporous structure. The samples
synthesized in water with and without ultrasonic treatment
exhibited similar medium surface area and pore volume. In
contrast, the catalysts prepared in ethanol had much higher
surface area and pore volume because they had smaller particle
size and porous structure. These results indicated that the
solvent had more signicant effects on the particle morphology
than ultrasound treatment. Because of the lower solubility of
precursors, ethanol could reduce the particle size and restrain
the particle growth.31–33
Entries 4 to 10 compared the EtOH/ult samples with
different aging time or different calcination temperature (350 to
700 ꢀC). When the aging time increased from 0 to 24 h, the BET
surface area slightly decreased from 20.0 to 17.7 m2 gꢁ1, and the
pore volume decreased from 0.102 to 0.093 mL gꢁ1, indicating
slow crystal growth during the aging process. Entries 8 to 10
showed that the increase of calcination temperature led to
a sharp decline of surface area, especially at 700 ꢀC, which could
be attributed to the accelerated recrystallization under high
temperatures. The recrystallization increased the particle size
and decreased tꢀhe surface area. An exception was the sample
calcined at ꢀ350 C, which displayed a lower surface area than
that at 500 C due to the undesired carbonization of adsorbed
ethanol on the catalyst surface, as conrmed by its ash
appearance and the TG measurement (Fig. S2†).
3.1.2. XRD patterns. Fig. 1 shows the XRD patterns of the
samples prepared with different methods. All the peaks of the
four catalysts can be indexed to the BaSO4 (JCPDS 24-1035).
However, compared with the standard (121) position, Fig. 1(b) BaSO4 catalysts prepared with different methods.
Fig. 1 (a) XRD patterns and (b) enlarged (121) diffraction peaks of
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spacing of Water/ult and EtOH/ult catalysts. The crystal size was peak only slightly reduced with increasing calcination temper-
calculated from Scherrer's formula, and the results are listed in ature, suggesting that the calcination temperature had insig-
Table 1. Only the EtOH/ult samples have crystal size smaller nicant effect on the crystal defects. In addition, the crystal size
than 100 nm, while those purchased or prepared with other displayed a modest growth with the calcination temperature
methods were beyond the application range of Scherrer's increasing from 500 ꢀC to 700 ꢀC (Entries 4, 9 and 10 in Table 1).
formula. These small crystals had poor integrality and provided The abnormally larger crystal size observed in the sample
more crystal defects.
calcined at 350 ꢀC was likely attributed to the residue of
The enlarged XRD (121) diffraction peaks of BaSO4 aged for deposited ethanol in crystals. Combined with the discussion on
different time were compared in Fig. 2(a). With the increase of the particle size and morphology in 3.1.1, it can be concluded
aging time, the shi of the (121) diffraction peak gradually that the high-temperature calcination signicantly increased
decreased, indicating that the interplanar spacing became the particle size and reduced the pore structure, but only
closer to the standard value (entries 4–7 in Table 1) and the slightly affected the crystal textures and defects.
integrality of crystals was improved. Meanwhile, the full width
3.1.3. Photoluminescence spectra. Photoluminescence
at half maximum (FWHM) of the (121) diffraction peak (PL) analysis was used to characterize the optical properties of
decreased from 0.169ꢀ to 0.158ꢀ, 0.151ꢀ and 0.144ꢀ when the the BaSO4 samples, which were related to the defect states. The
aging time increased from 0 to 1, 2 and 24 h respectively, relative PL intensities were employed to compare the contents
indicating that the crystal grew larger with longer aging time of crystal defects in samples characterized under the same
(Table 1). These results revealed that the defect-induced lattice conditions.35 Here all the samples were analyzed and EtOH/ult
contraction was gradually eliminated due to the improvement catalysts with different aging time were discussed as a typical
of crystal integrality and the reduction of defects during the series for its characteristic variation in crystal defects. As shown
aging process. The BaSO4 sample prepared without aging had in Fig. 3, all the samples exhibited a broad peak in the region of
the most signicant crystal defects among these four samples. 350–650 nm, including the ultraviolet emissions at around
Fig. 2(b) compared the crystal textures of the BaSO4 samples 390 nm and the green emissions centered at 517 nm. With the
calcined at different temperatures. The le shi of diffraction increase of aging time, the PL intensities of both the ultraviolet
and green emission decreased, indicating a decrease of the
amount of defects in the BaSO4 samples.35 Consistent with the
XRD patterns discussed in 3.1.2, the PL spectra in Fig. 3
conrmed that the amount of crystal defects in BaSO4 samples
was enhanced with ultrasonic treatment in ethanol solvent, and
the aging process signicantly reduced the amount of crystal
defects. Difference in the amount of crystal defects were also
found with the BaSO4 prepared by different methods (Fig. S3a†)
and EtOH/ult catalysts calcined at different temperature
(Fig. S3b†).
3.1.4. X-ray photoelectron spectrometer. The S2p and O1s
binding energies (BEs) of the BaSO4 samples with different
aging time were analyzed by XPS, and the results were listed in
Table 2. The S2p and O1s BEs increased with increasing aging
Fig. 2 Enlarged XRD (121) diffraction peaks of BaSO4 catalysts (a) aged Fig.
3
Photoluminescence (PL) spectra of BaSO4 catalysts with
with varied time length and (b) calcined at different temperature.
different aging time.
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Table 2 S2p and O1s binding energies of BaSO4 aged with varied time
Meanwhile, the peak temperatures of desorption were 192, 207
and 211 ꢀC for the Co-pre, Water/ult and EtOH/ult samples,
respectively, indicating that the catalyst acidity became slightly
stronger when using ethanol solvent and ultrasonic treatment.
The NH3-TPD results of the EtOH/ult samples with varied
aging time were compared in Fig. S4(a).† With increasing aging
time, the overall acid amount slightly decreased, as listed in
Table 1. The decrease of acidity amount was attributed to the
decrease of defects, especially the point defects, which was
revealed by the XPS and shi of XRD diffraction peaks. These
defects accompanied with local imbalance of electrons can
provide the active adsorption sites for NH3. With the increase of
aging time, the amount of defects decreased and resulted in the
reduction of acid sites.
Aging time (h)
0
1.0
2.0
24.0
S2p BE, eV
O1s BE, eV
168.82
531.85
168.82
531.90
168.86
531.96
168.89
531.98
time, indicating that aging process decreased the electron
density of the sample, which could be attributed to the lower
density of defects. According to the theory of point defects,37–39
especially the Frankel defects, it is quite common in ionic
crystal that cations escape from their normal sites, thus become
interstitial atoms and produce vacancies. Vacancies and inter-
stitial atoms always form in pairs, and provide sites with
unbalanced electrons. The escaped cations are electropositive
and act as Lewis acid, and the electronegative vacancies can
donate electrons and act as basic sites. In the EtOH/ult samples,
these vacancies caused by defects partly disappeared aer
aging, leading to a decrease in the electron density and an
increase in BEs. The EtOH/ult-0 h catalyst had the largest
amount of basic sites, which favored the generation of AA.12,15
3.1.5. Acid properties. The temperature programmed
desorption (TPD) of NH3 was carried out to evaluate the amount
and strength of acid sites of the different BaSO4 catalysts (Fig. 4
and S4†). The acid amount was calculated by integrating the
TPD peaks, and the results were listed in Table 1. The BaSO4
catalysts showed weak and medium acidity with a broad
desorption peak from 150 to 350 ꢀC, except that the commercial
BaSO4 had no adsorption of NH3.
As shown in Table 1 and Fig. 4, the EtOH/ult BaSO4 sample
had the highest acid amount of 21.15 mmol gꢁ1, indicating that
ethanol solvent and ultrasound treatment enhanced the
formation of surface acid sites. In contrast, the acid amount of
Water/ult sample was only 6.83 mmol gꢁ1, mainly because the
change of solvent signicantly affected the particle size and
surface area. Nevertheless, both the EtOH/ult and Water/ult
samples had larger acid amount than the Co-pre sample,
which conrmed the positive effect of ultrasonic treatment.
The NH3-TPD proles of the BaSO4 samples calcined at
different temperatures are shown in Fig. S4(b).† The calcination
temperature had signicant effects on the acidic properties. The
ꢀ
catalyst calcined at 350 C was not considered because of the
undesired carbonization of the deposited ethanol on the
surface. The peak temperature of desorption decreased from
211 to 195 ꢀC when the calcination temperature increased from
500 to 700 ꢀC. In addition, the acid amount signicantly
decreased with increasing calcination temperature, and the
largest acid amount was obtained with the catalyst calcined at
500 ꢀC. One reason was the notable decrease of surface area
which directly reduced the acid amounts, because NH3 mole-
cules and reactants were only adsorbed on the surface.
However, this was not the only reason. Note that the acid
density also decreased, some acid sites should also disappeared
under high temperature, consisting with the unstable feature of
crystal defects with a higher electron density. This result further
conrmed that these crystal defects acted as acid sites and they
were unstable under high temperatures.
3.2. Catalytic evaluation
3.2.1. Effect of catalyst preparation method. The dehydra-
tion reaction of lactic acid was conducted to compare the
catalytic performance of the BaSO4 samples prepared with
different methods. The reaction conditions and results were
summarized in Table 3, in which some typical catalysts in the
literature were also included for a better comparison. The
EtOH/ult-0 h-500 catalyst was evaluated for the dehydration of
LA to AA from 300 ꢀC to 400 ꢀC. As shown in Table S2,† the best
ꢀ
catalyst performance was obtained at 350 C, which was used
when evaluating other catalysts and comparing with the results
in the literature. The conversion of lactic acid and selectivity to
acrylic acid were plotted as a function of the time on stream, as
shown in Fig. S5.† An induction period existed in the rst 3 h,
during which a considerable change of selectivity was observed
for all the four catalysts. For better comparison, the average
selectivity and conversion were calculated using the data during
4–9 h of reaction.
The commercial BaSO4 catalyst showed a low LA conversion
of 29.8% and a low AA selectivity of 28.8%. In contrast, the other
three catalysts prepared in this work had enhanced LA conver-
sion and AA selectivity. The Co-pre and Water/ult catalysts
Fig. 4 NH3-TPD profiles of BaSO4 catalysts prepared with different
methods.
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Table 3 Catalytic performances of BaSO4 prepared with different methods in this work and typical catalysts in the literature
Reaction conditions
Selectivity (%)
Conversion
(%)
Catalyst
T (ꢀC)
N2 ow (mL minꢁ1
)
Space velocity (hꢁ1
)
Contact time (s)
AA
AD
Ref.
Commercial
Co-pre
350
350
350
350
350
350
360
400
30
30
30
30
30
30
15.5
1
2.5 (WHSV)
2.5 (WHSV)
2.5 (WHSV)
2.5 (WHSV)
5.0 (WHSV)
2.7 (LHSV)
1.4 (WHSV)
2.1 (WHSV)
0.19
0.19
0.19
0.19
0.10
0.61
—
29.8
69.5
70.1
77.6
66.1
78.3
84
28.8
65.8
69.7
78.9
76.7
72.3
74
41.2
23.2
18.0
12.7
16.4
5.6
This work
Water/ult
EtOH/ult
EtOH/ult
Na2HPO4/NaY
HAP1.62-360
BaSO4
30
21
28
18
—
0.5
100
74
showed similar LA conversion of 69.5% and 70.1%, respectively, reaction. As a result, both the LA conversion and AA selectivity
while Water/ult had a slightly higher AA selectivity (69.7%) than were enhanced. Furthermore, the synchronous increase of acid
Co-pre (65.8%). Among all the prepared catalysts, EtOH/ult had sites and crystal defects indicated that the formation of acidity
the best performance with 77.6% LA conversion and 78.9% AA could be attributed to the escaped cations in crystal defects.
selectivity under the same reaction condition. By decreasing the
3.2.2. Effect of catalyst aging time. In order to compare the
space velocity, the LA conversion increased to 97.5% while performance of the catalysts, a higher space velocity (WHSV ¼ 5
maintaining the selectivity to AA (Table S3†). As shown in Table hꢁ1) was used, considering that the catalyst activity could be
S1,† the EtOH/ult catalyst had carbon balance close to 100%, better distinguished at a lower conversion. The aging time of
indicating that the reaction data were reliable and the amount EtOH/ult samples signicantly affected the catalyst perfor-
of other unknown side products was very low. However, some mance. As listed in Table 4, both the LA conversion and AA
other catalysts had lower carbon balance, due to the poor selectivity decreased with the increase of aging time. The EtOH/
selectivity to AA and formation of heavier products that could ult-24 h catalyst had the lowest LA conversion and AA selectivity,
not be detected by GC. Considering the typical catalysts shown and EtOH/ult-0 h had the best catalytic performance (66.1% LA
in Table 3, the HAP1.62-360 catalyst was evaluated within TOS ¼ conversion and 76.7% AA selectivity). The LA conversion, AA
6–8 h,21 and the Na2HPO4/NaY and BaSO4 catalysts were evalu- selectivity, acid amount and crystal size were correlated for the
ated within TOS ¼ 2 h.28,30 Overall, the EtOH/ult catalyst had EtOH/ult-mh samples in Fig. 5. It was found that the increase of
a higher efficiency of AA production than the typical catalysts in crystal size distinctly decreased the acid amount, which in turn
the literature.21,28,30 In the work of Peng et al., the BaSO4 catalyst decreased both the LA conversion and AA selectivity. As illus-
was reported to have an AA selectivity of 74% and LA conversion trated in the analysis of XRD patterns, the growth of crystals
of 100% at a contact time of 0.5 s and temperature of 400 ꢀC. In improved the crystal integrality and reduced the crystal defects,
this work, the EtOH/ult BaSO4 catalyst had a LA conversion of which were also evidenced by XPS (Table 2) and PL measure-
77.6% at a much shorter contact time (0.19 s) and much lower ment (Fig. 3). Thus, the increase of crystal size caused reduction
temperature (350 ꢀC), suggesting EtOH/ult had a higher activity. of crystal defects. These results revealed that the decrease of
In addition, EtOH/ult showed slightly higher selectivity to AA.
crystal defects in the EtOH/ult catalysts decreased the amount
The difference in the catalyst activity could be attributed to of acid sites that were active for LA dehydration, thus decreased
the difference in acid amounts. With increasing crystal defects the LA conversion and AA selectivity. This relationship was
(Fig. S3a†) and surface area (Table 1), much more acid sites were consistent with the results of catalysts prepared with different
observed with the EtOH/ult catalyst, which were the active sites methods.
of LA conversion.12–14 In addition, the EtOH/ult catalyst showed
a higher AA selectivity (78.9%) than the Co-pre (65.8%) and
Water/ult catalyst (69.7%), due to their difference in weak and
Table 4 Effect of aging time on catalytic performance of EtOH/ult-
500 catalystsa
medium acid amount. In this work, the acid sites of all BaSO4
catalysts were of weak to medium strength, which were suitable
for the dehydration of LA to AA.14,40 Few strong acid sites were
observed, indicating that no undesired active sites were
provided. Thus, most of the side reactions occurred without
catalysis or on other sites such as the surface hydroxyl
groups.9,13,17,18 The increase of suitable acid sites enhanced the
effective adsorption and subsequent dehydration of LA to AA,
while the side reactions such as the decarbonylation/
decarboxylation and polymerization were therefore main-
tained or suppressed by the promotion of the main dehydration
Selectivity (%)
Conversion
(%)
Catalyst
AA
AD
AcOH
PA
EtOH/ult-0 h-500
EtOH/ult-1 h-500
EtOH/ult-2 h-500
EtOH/ult-24 h-500
66.1
56.2
52.6
51.7
76.7
70.5
65.7
58.6
16.4
18.2
17.2
15.2
2.9
3.0
3.2
2.7
2.0
3.0
2.7
1.4
a
Conditions: reaction temperature 350 ꢀC, BaSO4 catalyst: 0.25 g, feed
ow rate: 1.2 mL hꢁ1, WHSV ¼ 5 hꢁ1, LA feedstock: 20 wt% in water,
carrier gas N2: 30 mL minꢁ1, TOS ¼ 4–9 h.
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Fig. 6 Catalyst stability of EtOH/ult-0 h-500 within 48 h. Temperature
350 ꢀC, WHSV ¼ 2.5 hꢁ1, LA feedstock: 20 wt% in water, carrier gas N2:
Fig. 5 Relationship between catalytic performances, crystal size and
acid amounts.
30 mL minꢁ1
.
better than the HAP and alkali/alkali-earth modied zeolite
catalysts. For example, over Na2HPO4/NaY the AA selectivity
decreased by more than 20% in 8 h because of the deactivation,
instability of framework and loss of surface modiers;24,30 over
HAP1.62-360 the conversion of LA decreased by about 10% in
8 h.21 The stability of the EtOH/ult catalyst in this work was
similar to the BaSO4 catalyst reported by Peng et al.28
3.2.3. Effect of catalyst calcination temperature. The effects
of the calcination temperature on the catalytic performance of
EtOH/ult-0 h catꢀalyst were summarized in Table 5. The sample
calcined at 350 C, which gave a higher conversion and lower
selectivity because of the carbonate residues, was not taken into
account for its residue of deposited ethanol. The LA conversion
signicantly decreased from 66.1% to 45.8% to 28.2% with
increasing calcination temperature from 500 ꢀC to 600 ꢀC to
ꢀ
700 C. Meanwhile, the AA selectivity decreased from 76.7% to
59.2% when the calcination temperature increased from 500 ꢀC
to 600 ꢀC. The reason for this decrease in catalytic efficiency was
the decrease of acid sites from 21.15 mmol gꢁ1 at 500 ꢀC to 11.22
mmol gꢁ1 at 600 ꢀC and 2.22 mmol gꢁ1 at 700 ꢀC, which was also
due to the reduction of crystal defects (Fig. S3b†) and surface
area (Table 1). The AA selectivity was almost unchanged when
the calcination temperature increased from 600 to 700 ꢀC,
because the sharp reduction of surface area resulted in both the
reduction of dehydration and other competing side reactions.
Thus, the results identify EtOH/ult-0 h-500 as the most efficient
catalyst for the LA dehydration.
3.2.4. Stability and carbon deposition. The stability of the
EtOH/ult catalyst was evaluated for 48 h and the results were
shown in Fig. 6. The conversion of LA and selectivity to AA
decreased by about 10% and 20%, respectively, which was much
Table 5 Effect of calcination temperature on catalytic performance of
EtOH/ult-0 h catalystsa
Selectivity (%)
Conversion
Catalyst
(%)
AA
AD
AcOH
PA
EtOH/ult-0 h-350
EtOH/ult-0 h-500
EtOH/ult-0 h-600
EtOH/ult-0 h-700
72.9
66.1
45.8
28.2
66.4
76.7
59.2
59.4
17.4
16.4
20.5
21.3
3.1
2.9
3.5
6.6
1.0
2.0
2.3
3.8
a
Conditions: reaction temperature 350 ꢀC, BaSO4 catalyst: 0.25 g, feed
ow rate: 1.2 mL hꢁ1, WHSV ¼ 5 hꢁ1, LA feedstock: 20 wt% in water,
carrier gas N2: 30 mL minꢁ1, TOS ¼ 4–9 h.
Fig. 7 TG-DTA results of the fresh and spent EtOH/ult-0 h-500
catalysts.
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To study the variation of the catalysts during the reaction,
RSC Advances
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were shown in Fig. 7. The weight loss of the spent catalyst was
relatively small due to the slow carbon deposition (Table S1†).
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ꢀ
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4. Conclusions
The structure–activity relationship of the BaSO4 catalyst was
studied for the dehydration reaction of LA to AA. Specic
conditions such as ethanol solvent and ultrasonic treatment
were employed in the co-precipitation preparation of BaSO4
catalysts, and the obtained catalyst had a much smaller
crystal size and more crystal defects. These catalysts, denoted
as EtOH/ult, also had much higher surface area and acid
amount. Further studies of the catalyst texture and binding
energy revealed that the crystal defects with unbalanced
electrons provided acid and basic sites, which acted as the
active sites in LA dehydration. This was further conrmed by
changing the catalyst preparation conditions such as the
aging time and calcination temperature, in which the catal-
ysis performance had a good correlation with the amounts of
crystal defects and acid amounts. The highest surface area
and acid amount were obtained over the EtOH/ult-0 h-500
catalyst, which consequently showed enhanced reaction
activity (88.9% LA conversion) and selectivity (78.8% AA
selectivity).
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