Preparation of KF-La2O2CO3 solid base catalysts and their excellent catalytic activities for transesterification of tributyrin with methanol

Article abstract of DOI:10.1039/c4cy00167b

A series of 25% KF-La₂O₂CO₃ catalysts (25-KF-LOC-x, x = 673, 723 and 773) were prepared at different calcination temperatures, which were tested as basic catalysts for the transesterification of tributyrin with methanol to produce methyl butyrate and characterized by means of XRD, SEM, CO₂-TPD, FTIR, XPS and XRF. It was found that the calcination temperature greatly influences the catalytic activity, and its order is 25-KF-LOC-673 < 25-KF-LOC-723 < 25-KF-LOC-773. Especially, the 25-KF-LOC-773 catalyst exhibits a very high activity, and the conversion of tributyrin is nearly 100%, the yield of methyl butyrate reaches 94% at 308 K. The activation energy of the 25-KF-LOC-773 catalyst is as low as 55.03 kJ mol^-1. The excellent catalytic activity of the 25-KF-LOC-773 catalyst can be attributed to the largest amount of surface hydroxyl among these 25-KF-LOC-x catalysts. The results indicate that the Bronsted base is the main active site at low reaction temperature. The recycling use and stability have been investigated over the 25-KF-LOC-773 catalyst. The results indicate that the 25-KF-LOC-773 catalyst has a high stability after being stored for 90 days. The deactivation of the used 25-KF-LOC-773 catalyst at 308 K is due to the loss of hydroxyl on the surface. However, the conversion of tributyrin can reach 91%, and the yield of methyl butyrate also can achieve 64% when the transesterification reaction is performed at 338 K over the second used 25-KF-LOC-773 catalyst. It indicates that the second used 25-KF-LOC-773 catalyst still possesses good catalytic activity at the higher reaction temperature due to the unchanged strong Lewis basic sites provided with surface oxygen anions. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cy00167b

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Preparation of KF–La O CO solid base catalysts
2
2
3
and their excellent catalytic activities for
Cite this: Catal. Sci. Technol., 2014,
transesterification of tributyrin with methanol†
4, 2957
Wen Jiang, Xiaoyu Niu, Fulong Yuan,* Yujun Zhu* and Honggang Fu
A series of 25% KF–La
calcination temperatures, which were tested as basic catalysts for the transesterification of tributyrin with
methanol to produce methyl butyrate and characterized by means of XRD, SEM, CO -TPD, FTIR, XPS and
XRF. It was found that the calcination temperature greatly influences the catalytic activity, and its order is
5-KF–LOC-673 < 25-KF–LOC-723 < 25-KF–LOC-773. Especially, the 25-KF–LOC-773 catalyst exhibits a
very high activity, and the conversion of tributyrin is nearly 100%, the yield of methyl butyrate reaches
2 2 3
O CO catalysts (25-KF–LOC-x, x = 673, 723 and 773) were prepared at different
2
2
−
1
9
4% at 308 K. The activation energy of the 25-KF–LOC-773 catalyst is as low as 55.03 kJ mol . The
excellent catalytic activity of the 25-KF–LOC-773 catalyst can be attributed to the largest amount of
surface hydroxyl among these 25-KF–LOC-x catalysts. The results indicate that the Brønsted base is the
main active site at low reaction temperature. The recycling use and stability have been investigated over
the 25-KF–LOC-773 catalyst. The results indicate that the 25-KF–LOC-773 catalyst has a high stability
after being stored for 90 days. The deactivation of the used 25-KF–LOC-773 catalyst at 308 K is due to
the loss of hydroxyl on the surface. However, the conversion of tributyrin can reach 91%, and the yield of
methyl butyrate also can achieve 64% when the transesterification reaction is performed at 338 K over
the second used 25-KF–LOC-773 catalyst. It indicates that the second used 25-KF–LOC-773 catalyst still
possesses good catalytic activity at the higher reaction temperature due to the unchanged strong Lewis
basic sites provided with surface oxygen anions.
Received 9th February 2014,
Accepted 22nd April 2014
DOI: 10.1039/c4cy00167b
www.rsc.org/catalysis
Most biodiesels today are produced by transesterification
in the presence of homogeneous catalysts such as sulfuric
1
. Introduction
Nowadays, the increase in the demand for energy is at a sub-
stantial rate with the growing economy of the populous devel-
oping countries. Currently, this high energy demand mainly
depends on fossil fuel resources. Fossil fuels have been
depleting day by day, so diversifying fuel resources has been
given much more attention. Recently, biofuels (biodiesel or
ethanol), biomass, and solar energies have been wide spread.
Among them, biodiesel is a renewable and environmentally
friendly alternative for the diesel engine. Generally, trans-
esterification, the reaction of vegetable oil or animal fat with
an alcohol to form fatty acid methanol esters (FAME) and
glycerol, is the most common method for biodiesel prepara-
acids, sodium methoxide, sodium or potassium hydroxide,
which are more effective and provide much faster reaction
rates than heterogeneous catalysts. However, homogenous
catalysts have many disadvantages such as separating prob-
lems between the catalyst and the products, and sensitivity
to water and acid, which directly limits the production of
5
–7
biodiesels.
as solid base and acid,
In this case, heterogeneous catalysts, such
7
–18
19,20
and immobilized enzyme,
have
been studied to replace the homogeneous catalyst. In
previous publications, many kinds of solid base catalysts
have been investigated for the transesterification of trigly-
cerides with methanol, including alkali and alkali earth metal
1
–4
12–15,18
7,8,21–24
25
tion under acid or base catalysis.
oxides,
hydrotalcites,
zeolites,
and anion
2
6
exchanged resins. Among them, the hydrotalcite(-like) com-
pounds have attracted interest as catalysts for vegetable oil
transesterification due to their high surface area and the tun-
7
,8,21
Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang
University), Ministry of Education, School of Chemistry and Materials,
Heilongjiang University, Harbin, 150080 PR China. E-mail: yujunzhu@hlju.edu.cn,
fulongyuan2000@yahoo.com; Fax: +86 451 86609650; Tel: +86 451 86609650
able basicity using chemical composition modification.
Additionally, CaO and CaO supported catalysts have been
widely investigated because of their high activities for the
1
8,27
transesterification of triglycerides with alcohols.
However,
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00167b
a problem for the hydrotalcite(-like) and the CaO catalysts is
This journal is © The Royal Society of Chemistry 2014
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that the active species end up leaching from the solid catalyst
KF·2H O (3.186 mmol, 0.300 g) and La O CO (3.246 mmol,
2
2
2
3
28–30
and acting as a homogeneous catalyst in the reaction.
This
1.200 g) were added into 120 mL distilled water, and then the
mixture was stirred in a water bath at 313 K for 12 h to
evaporate the water and obtain the solid product. Then the
solid products were dried at 363 K overnight, calcined at dif-
ferent temperaturea (673, 723 and 773 K, respectively) for 3 h
based on the TG-DTA measurement (Fig. S1†). Thus, 25%
KF–La O CO -x catalysts were obtained and denoted as
drawback considerably limits the utility of such simple, cheap,
and environmentally benign catalysts.
In recent years, many researchers have shown much inter-
est in rare earth oxides (REOS) as catalysts for trans-
esterification or as catalyst supports due to the strong
1
2,31,32
basicity and good catalytic activity of REOS as CaO.
2
2
3
However, some important questions from a fundamental
point of view remain unclear and unsolved except the
leaching of active species. For example, most of these
reported catalysts for the transesterification of triglycerides
require high temperatures (>423 K) and long reaction times
25-KF–LOC-673, 25-KF–LOC-723 and 25-KF–LOC-773, in which
25 is the KF percentage content in the total catalyst amount
and x is the calcination temperature.
2.2. Catalyst characterizations
3
3–35
(
from 2 h to 24 h) to get higher than a 90% FAME yield.
The Thermogravimetric-differential thermal analysis (TG-DTA)
profiles were recorded on a HCT-1 thermal analyzer at a
In fact, the leaching of active metal species or catalyst deteri-
oration in the liquid-phase reaction is a particular problem
−
1
heating rate of 10 K min in static atmosphere. X-ray diffrac-
tion (XRD) patterns of the catalysts were obtained with a
D/MAX-3B X-ray Diffractometer (Rigaku Co.), using Cu Kα radia-
tion combined with a Ni-filter. The morphology was observed
by scanning electron microscopy (SEM) on a Hitachi S-4800
field emission electron microscope operating at 20 kV. The sur-
3
6,37
for most solid-base catalysts.
In addition, how to improve
the stability and recycling of solid base catalyst in the atmo-
spheres is also one of the key problems. Hence, much recent
effort has been devoted to developing the novel solid base
catalysts with high activity and stability.
In our previous work, the prepared KF–La O had a good
2
3
face area was derived from N sorption measurement at 77 K
2
catalytic performance for the transesterification of tributyrin
with a Micromeritics Tristar 3000 physisorption instrument. In
each case, the sample was degassed under vacuum at 423 K for
3
8
with methyl alcohol.
deactivated rapidly when exposed to air. Thus, in this paper,
we report a new solid base 25% KF–La CO catalyst, using
2 3
However, the KF–La O catalyst
4
h before the measurement. The FT-IR spectra were carried on
2
O
2
3
a PE Spectrum One FT-IR spectrometer in KBr disks at room
temperature. X-ray photoelectron spectroscopy (XPS) was
recorded on a Thermo ESCALAB 250 spectrometer using a
monochromatic AlKα X-ray source (15 KV, 150 W) and analyzer
pass energy of 100 eV. Binding energies (BE) are referenced to
the C (1s) binding energy of carbon taken to be 284.7 eV. The
content of K was analyzed by X-ray fluorescence spectrometer
the transesterification of tributyrin with methyl alcohol as
the probe reaction. The relationship between trans-
esterification activity and surface basicity of the catalyst is
discussed for the 25% KF–La O CO catalysts prepared at dif-
2 2 3
ferent calcination temperatures. The recycling and stability
are also investigated.
(
XRF) using a Bruker S4 Explorer instrument. Carbon dioxide
temperature programmed desorption (CO -TPD) was carried
2
2
. Experimental
out in a full automatic instrument (XQ TP-5080, China) and
performed in the following procedure. Firstly, 100 mg of the
catalyst was mounted in a quartz tube and calcined under a
All chemicals used in this work are of an analytical grade, pur-
chased from Tianjin Kermel Chemical Reagents Development
Centre, and used as received without further purification.
−1
helium stream (30 mL min ) at 573 K for 1 h. After the catalyst
was cooled down to 298 K, pure CO
2
was introduced into the
−
1
system at the rate of 20 mL min for 1 h. Then the catalyst
−
1
was flushed in He flow (30 mL min ) to remove physisorbed
2
.1. Catalysts preparation
CO
2
at 298 K. Finally the sample was gradually heated from
The La O CO (denoted as LOC) was synthesized by the homo-
2
2
3
−1
2
98 K to 873 K at a ramp of 10 K min . The CO desorption
2
geneous precipitation method using lanthanum nitrate hexahy-
drate (La(NO ·6H O) as a lanthanum source and urea to raise
was monitored by a thermal conductivity detector (TCD) and
recorded by a computer.
3
)
3
2
3
9
the pH of the solution and also as a source of carbon. Lantha-
num nitrate (0.010 mol, 4.330 g), urea (0.40 mol, 24.024 g) and
water (5.56 mol, 100 g) were mixed at a molar ratio of 1 : 40 : 556
and the mixed solution was heated at 353 K and then kept at
that temperature. At a predetermined time, the resulting mix-
ture was cooled to room temperature, the redundant solution
was filtered, and the resulting solid was washed with water three
times and then dried at 363 K for 4 h, and calcined at 773 K for
2
.3. Catalytic evaluation
All the catalytic experiments were carried out in a 30 mL dou-
ble layer glass reactor equipped with a reflux condenser, a
magnetic stirrer and a superthermostat. The standard condi-
tion was as follows: 1.00 mL (3.42 mmol) of tributyrin and
102.4 mmol of methanol were added into the flask. After the
mixture was heated to the desired temperature (298–338 K),
30 mg of catalyst was added, and then the reaction was car-
ried out under these conditions for 60 min.
3
h. Then, La
The KF supported on La O CO
3
2 2 3
O CO support was obtained.
catalysts (25%
KF–La O CO ) were prepared by the impregnation method.
2
2
2
2
3
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Paper
Tributyrin is successively transesterified by methanol to
yield dibutyrin, monobutyrin, glycerol and for each step 1 mol-
ecule of methyl butyrate (Scheme 1). Tributyrin, dibutyrin,
monobutyrin, glycerol and methyl butyrate in the liquid prod-
uct were identified by GC-MS (Agilent 6890/5973N). The distil-
late at 64 °C from the reaction products, the azeotrope of
1
methanol and methyl butyrate, is further identified using H
NMR spectroscopy (AVANCE III 400, BRUKER) (Fig. S2†). The
liquid products were analyzed quantitatively by gas chromatog-
raphy (SP-2100 China, FID detector) with an OV-1 capillary
column (30 m × 0.25 mm × 0.33 μm) according to the
2
2–24,27,40
literature.
The quantitative analysis of the mixture
was determined by the calibration curves, using cyclohexa-
none as the internal standard. Methyl butyrate yield is
defined as the moles of methyl butyrate produced divided by
the moles of tributyrin reacted divided by 3 (since three moles
of methyl butyrate are produced per mole of tributyrin at
complete conversion).
Fig. 1 XRD pattern of the 25-KF–LOC-x catalysts (A: LOC; B: 25-KF–
LOC-673; C: 25-KF–LOC-723; D: 25-KF–LOC-773; ● LaOF).
3
. Results and discussion
than that of LOC. Moreover, the surface area increases
slightly with the calcination temperature, which is in agree-
ment with the SEM results.
3
.1. XRD characterization
Fig. 1 shows the XRD patterns of 25-KF–LOC-x catalysts with
different calcination temperatures. It is observed that a new
phase of LaOF appears at 2θ = 26.88, 31.06, and 52.62 ° (PDF
3.3. IR characterization
0
1-077-0204) for 25-KF–LOC-x catalysts. With the increase in
calcination temperature from 673 K to 773 K, the intensities
of the characteristic peak of LaOF increase significantly,
while the peak intensities corresponding to La O CO phase
Fourier transform infrared (FTIR) spectroscopy is usually
employed as an additional probe to evidence the presence of
hydroxyl groups as well as other organic and inorganic spe-
cies. The FTIR spectra of 25-KF–LOC-x catalysts are shown in
Fig. 3. For all of the catalysts, a broad band at 3300–3700 cm
is assigned to the stretching and bending vibrations of O–H
2
2
3
decline. In addition, the diffraction peaks of KF are not
found in the XRD patterns of all 25-KF–LOC-x catalysts. It
can be inferred that the presence of an interaction between
KF and La O CO , results in the formation of the new LaOF
−
1
bond, which indicates the presence of hydroxyl on the catalyst
2
2
3
4
1
−1
phase for 25-KF–LOC-x catalysts.
surface. A prominent absorption band at around 1461 cm
can be observed for these samples. According to the litera-
4
2
−1
ture, the band centered at 1461 cm and the narrow band at
3
.2. SEM characterization
−
1
8
59 cm are ascribed to the C–O stretch mode and the bend-
Fig. 2 is the SEM of the LOC and 25-KF–LOC-773 catalysts.
From Fig. 2a it can be seen that the LOC support shows a
slippery rod structure with length varying from 2 to 5 μm and
a diameter of about 0.5–1 μm. The rod structure remains
almost unchanged after loading KF (Fig. 2b), but the surface
becomes rough, stacked by a number of nanoparticles. It
also indicates that chemical reaction may occur to the forma-
ing mode of structural carbonate, respectively. A shoulder band
−
1
around 1655 cm may be attributed to the vibration of hydro-
gen bonding between hydroxy and carbonate. The sharp band
−
1
observed around 1365 cm due to the v vibration of the CO .
3
3
−
1
The bands below 800 cm are mostly due to the metal–
4
3,44
oxygen vibration.
Compared with the intensity of the
La–O vibration, the intensity of the hydroxyl groups peak
−1
tion of LaOF phase between KF and La
2 2 3
O CO support
(
at 3300–3700 cm ) gradually enhances with increases in calci-
confirmed by the above XRD results. The BET surface area of
nation temperature for the 25-KF–LOC-x catalysts. When the
calcination temperature is 773 K, there exists a mass of
hydroxyl on the surface of the 25-KF–LOC-773 catalyst. The
order of the amount of hydroxyl groups for 25-KF–LOC-x cata-
lysts is 25-KF–LOC-773 > 25-KF–LOC-723 > 25-KF–LOC-673.
However, there is only a small amount of hydroxyl group on
the surface of the LOC support. Thus, the IR results demon-
strate that the 25-KF–LOC-773 catalyst possesses the largest
amount of hydroxyl groups among the 25-KF–LOC-x catalysts.
In order to confirm the role of the hydroxyl group, the 25-KF–
LOC-773 catalyst was treated at room temperature using CO2,
2
−1
2
5-KF–LOC-x catalysts is only 2.3–3.5 m g , which is lower
Scheme 1 Transesterification reaction of tributyrin with methanol.
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Fig. 2 SEM of the LOC and the 25-KF–LOC-773 catalysts (a: LOC, b: 25-KF–LOC-773, c: the second used 25-KF–LOC-773).
2
Fig. 4 CO -TPD profiles of the LOC and the 25-KF–LOC-x catalysts
Fig. 3 FT-IR spectra of the LOC and the 25-KF–LOC-x catalysts
A: LOC; B: 25-KF–LOC-673; C: 25-KF–LOC-723; D: 25-KF–LOC-773;
E: 25-KF–LOC-773 after CO treatment).
(A: LOC; B: 25-KF–LOC-673; C: 25-KF–LOC-723; D: 25-KF–LOC-773).
(
2
present two desorption bands with desorption temperatures
and the FTIR spectra of post-treatment catalysts are shown in
at 300–420 K and 650–850 K, resulting from the interactions
Fig. 3E. After the 25-KF–LOC-773 catalyst is treated by CO , the
2
between CO and the weak and strong basic sites, respec-
2
−
1
band around 3370 and 1655 cm assigned to hydroxyl drops
off obviously, while the other peaks almost remain unchanged.
It indicates hydroxyl on the surface has intense interactions
tively. The weak basic sites can be attributed to the Brønsted
base, and the strong basic sites can be assigned to the Lewis
base. The weak basic sites are caused by the formation of sur-
4
4
with CO
2
which then leads to the weakening of the hydroxyl
face hydroxyl during the formation of LaOF. And the strong
basic sites result from the oxygen anions of La CO inter-
peak. Therefore, the hydroxyl group on the 25-KF–LOC-x cata-
2
O
2
3
lysts plays the role of the basic sites.
acting with the highly dispersed KF species, which result in
oxygen anions showing Lewis base sites. The TPD curve of
the 25-KF–LaOC-773 catalyst without pre-adsorption of CO
further confirms the desorption band at 650–850 K is not
assigned to the decomposition of La CO phase (Fig. S3†).
The CO -TPD data is summarized in Table 1. Based on Fig. 4
2
3
2
.4. CO -TPD
The basicity of an oxide surface is closely related to the
electrondonating properties of oxygen anions, which
increases with the electro-positive character of combined
mental ions. The value of electro-negativity of F is larger
than that of O , therefore, the negative charge of lattice
oxygen would be drawn toward F . Thus, the basicity of the
2
2 2 3 2
La O CO . CO -TPD is widely used for the determination of
the surface basicity of the solid base catalyst. Fig. 4 illustrates
2
O
2
3
2
and Table 1, the amount of weak basic sites for 25-KF–LOC-673,
−
−1
25-KF–LOC-723 and 25-KF–LOC-773 is 28, 35 and 77 μmol g ,
2
−
respectively, which increases in the order of 25-KF–LOC-673,
25-KF–LOC-723 and 25-KF–LOC-773, which agrees well with
the results of IR. However, the strong basic site shifts to low
temperature, and the desorption peak area dramatically
decreases when the calcination temperature increases from
673 K to 773 K. In other words, the order of the amount
−
5-KF–LOC-x catalyst should be higher than that of
1
the CO -TPD profiles of LOC and the 25-KF–LOC-x catalysts.
The CO
peak in the range 475–600 K, indicating its low basicity on
2
−
1
2
-TPD curve of LOC shows a small CO
2
desorption
of strong Lewis base sites is 25-KF–LOC-673 (253 μmol g ) >
−1
−1
25-KF–LOC-723 (175 μmol g ) > 25-KF–LOC-773 (65 μmol g ),
the surface. All CO -TPD profiles of 25-KF–LOC-x catalysts
which is inverse with that of the weak basic sites. Meanwhile,
2
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2
Table 1 CO -TPD data for LOC and the 25-KF–LOC-x catalysts
a
2
−1
b
c
−1
b
d
−1
e
−1
Catalysts
LOC
SA (m g
)
T
(K)
WBA (μmol g
)
T
(K)
SBA (μmol g
)
TBA (μmol g
)
5.4
2.3
2.9
3.5
4.7
—
—
28
37
77
23
542
789
784
753
742
13
253
175
65
13
281
212
142
74
2
2
2
2
5-KF–LOC-673
376
369
364
369
5-KF–LOC-723
5-KF–LOC-773
5-KF–LOC-773
f
51
a
b
c
d
e
SA: BET surface area. T: temperature of desorption peak. WBA: weak basic amount. SBA: strong basic amount. TBA: total basic
f
amount. The second used catalyst.
the quantity of weak basic sites increases obviously with
the LaOF amount shown in Fig. 5. Nevertheless, the increase
in the formation quantity of LaOF leads to the distinct
reduction of the amount of strong basic sites, which indi-
cates that the Lewis base sites attributed to oxygen anions
obviously decrease with the increase in the calcination tem-
perature. Moreover, this weakening of the strong basic site
2
−
also would be caused by the gradual loss of surface CO3 of
La CO at a high calcination temperature. Combined with
2
O
2
3
the XRD and IR results, it can be found that the more LaOF
is produced, the more hydroxyl group on the surface of the
2
5-KF–LOC-x catalyst is formed (Fig. 5 and 6). However, the
total basic amount decreases in the order of 25-KF–LOC-673,
2
5-KF–LOC-723 and 25-KF–LOC-773.
Fig. 6 Relation plots of hydroxyl relative amount versus weak basic
amount for the 25-KF–LOC-x catalysts.
3
.5. XPS characterization
The XPS of La 3d for 25-KF–LOC-x catalysts are depicted in
Fig. 7a and b. The La 3d5/2 and 3d3/2 core levels are observed
at 833.1–833.8 and 849.8–850.7 eV, respectively, and the cor-
responding satellite peaks are found to be at 836.6–837.3 and
nature of La atoms in 25-KF–LOC-x is confirmed by the char-
acteristic satellites found at higher BE for each of the 3d5/2
and 3d3/2 core-level peaks. The splitting of these lines is
believed to occur due to the transfer of the electron from
oxygen ligands to the La 4f (initially empty). In Table 2
and Fig. 5a and b, the splitting of the La 3d core-level
8
53.3–854.2 eV. Besides, it has been observed that the split-
ting energy between the core and satellite peaks is 3.5–4.1 eV,
in this work, for both 3d5/2 and 3d3/2 components. The bind-
ing energies and the multiplet splitting are agreed well with
46
3
+
45
the reported values for La compounds. The trivalent
peaks becomes more and more obvious from 25-KF–LOC-673,
2
5-KF–LOC-723 to 25-KF–LOC-773, which should be related
to the formation of LaOF resulting in the transfer of the
electron from oxygen ligands to the La 4f. It also explains the
formation of base sites on the 25-KF–LOC-x catalysts. In addi-
tion, compared with La O CO , the La 3d peaks shift to low
2 2 3
binding energy for all the 25-KF–LOC-x samples, which indi-
cates the increase in electron density of La. Thus, the pres-
2 2 3
ence of an interaction between KF and La O CO can be
inferred, which is consistent with the formation of the new
LaOF phase in the XRD results.
The O 1s core level spectra of the 25-KF–LOC-x catalysts
are presented in Fig. 7c. Two peaks can be well fitted in
the O 1s spectra with binding energy of 530.1 and 531.4 eV
for LOC. The peak in the lower binding energy side is
assigned to the oxygen of the La–O bond, while the one
appearing at the higher binding energy side can be fitted
4
5
with the oxygen of the La–O–C and C–O bonds. The O 1s
peaks of 25-KF–LOC-x catalysts became broaden, which is
attributed to the formation of the new phase that leads to
Fig. 5 Relation plots of LaOF relative amount versus hydroxyl relative
amount (a) and weak basic amount (b) for the 25-KF–LOC-x catalysts.
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Fig. 7 XPS of La 3d and O 1s for the LOC and the 25-KF–LOC-x catalysts (a: La 3d3/2; b: La 3d5/2, c: O 1s. A: LOC; B: 25-KF–LOC-673; C: 25-KF–LOC-723;
D: 25-KF–LOC-773).
Table 2 XPS data of the 25-KF–LOC-x catalysts
a
a
a
b
Catalysts
La 3d3/2
854.2
Area
RA (%)
La 3d5/2
Area
RA (%)
O 1s
Area
RA (%)
O/La
25-KF–LOC-673
25-KF–LOC-723
25-KF–LOC-773
19 938
14 165
58.5
41.5
837.3
833.8
33 745
11 388
74.8
25.2
532.9
530.9
17 214
15 121
2040
14 877
17 555
5349
11 429
15 865
5145
50.1
44.0
5.9
39.3
46.5
14.2
35.2
48.9
15.9
17.35
14.33
11.92
8
50.7
528.3
854.0
50.4
23 356
22 440
51.0
49.0
837.3
833.6
35 992
29 112
55.3
44.7
532.4
530.6
8
528.3
853.8
50.0
16 234
23 299
41.1
58.9
837.0
833.3
26 824
30 184
47.0
53.0
532.4
530.6
8
528.3
a
b
RA: relative area of each fitted peak in all the peaks for the every La 3d3/2, La 3d3/2 and O 1s peaks. The surface atom ratio of O to La.
the presence of a new environment of the surface oxygen.
For the 25-KF–LOC-x catalysts, a peak is observed at
5
28.3 eV except the two peaks around 530.1 and 531.4 eV
similar to the LOC sample, which can be ascribed to the
oxygen of the surface OH.
In Fig. 7 and Table 2, the relative peak intensity in
the lower binding energy side of La 3d3/2 and La 3d5/2 sig-
nificantly increases, which illustrates the increase in the
amount of La coordinated with O. And, the oxygen quan-
tity of surface hydroxyl obviously increases with the calci-
nation temperature varying from 673 K to 773 K for the
2
5-KF–LOC-x catalysts, which is consistent with the num-
ber of weak basic sites.
Fig. 8 presents a plot of the total base amount in Table 1
versus the surface atom ratio of O to La in Table 2, from
which there exists the linearity between the total base
Fig. 8 Relation plots of total basic amount versus atom ratio of O to La.
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amount and the quantity of O on the catalyst surface. Thus,
it can be inferred that the surface O provides the basicity of
the 25-KF–LOC-x catalysts, meanwhile, with the increase in
calcination temperature, the content of surface O declines,
and the total base amount also decreases.
butyrate were obtained over the MgZr mixed oxide with a
11 : 1 molar ratio of Mg to Zr prepared by precipitation at
333 K and a reaction time of 22 h. Moreover, the conver-
sion of tributyrin and yield of methyl butyrate decrease to
61% and 25% for the third run, respectively.
4
7
Fig. 10 is a plot of the tributyrin conversion versus the
surface oxygen of the hydroxyl in Table 2, which displays
linearity between the conversion of tributyrin and the oxy-
gen amount of the surface hydroxyl. According to the order
from 25-KF–LOC-673, 25-KF–LOC-723 to 25-KF–LOC-773 cat-
alyst, the oxygen amount of the surface hydroxyl increases,
and the catalytic activity is also markedly enhanced. These
results can prove that the surface hydroxyl is one of the
active base sites of the 25-KF–LOC-773 catalyst at the low
reaction temperature of 308 K. Thus, it can be inferred
that the hydroxyl provides active sites. When the XRD, IR,
3
.6. Catalytic activities
No product can be detected using the LOC as catalyst
in our experimental conditions. The catalytic activities over
the 25-KF–LOC-x catalysts are shown in Fig. 9. Apparently,
the yield of methyl butyrate was very low (10%) for the
2
5-KF–LOC-673 catalyst prepared at 673 K. Subsequently,
the calcination temperature rose to 723 K, the conversion
of tributyrin increased significantly to 90% over the
2
5-KF–LOC-723 catalyst, but the yield of methyl butyrate
was only 62%. Then, the conversion of tributyrin was nearly
00% and the yield of methyl butyrate reached 94% over
CO -TPD and XPS results are taken into consideration, it
2
1
can be inferred that a new phase LaOF is generated after
KF loading, meanwhile, increasing the calcination tempera-
ture, the amount of LaOF increases significantly. Although
the content of surface O declines, the amount of the sur-
face hydroxyl as the active sites increases. In conclusion,
the total base amount on the catalyst surface decreases,
but the conversion of tributyrin and yield of methyl buty-
rate still increase. It suggests that the surface hydroxyl is
the main active basic species at low reaction temperatures.
the 25-KF–LOC-773 catalyst. Thus, the 25-KF–LOC-773 cata-
lyst shows an excellent catalytic performance for the trans-
esterification of tributyrin. 773 K was thought to be the
ideal calcination temperature in the preparation process of
the KF–LOC catalyst.
To show the efficacy and advantages of the 25-KF–LOC-773
catalyst, a comparison with literature-reported similar
transesterification reaction of tributyrin with methanol has
been made. Different catalysts show different catalytic activ-
ities for this reaction when the molar ratio methanol to
tributyrin is 30 : 1. Mg–Al hydrotalcite is an important kind
of solid base catalyst for the transesterification reaction
of tributyrin with methanol.
the decomposed-rehydrated Mg–Al hydrotalcite catalyst
could reach over 80% tributyrin conversion and nearly 80%
yield to methyl butyrate within 400 min at 333 K. 84%
tributyrin conversion and nearly 52% yield to methyl
3.7. Kinetic studies
The kinetics tests of the transesterification of tributyrin were
carried out for the 25-KF–LOC-773 catalyst due to its excellent
catalytic performance. According to the references about the
2
2–24
Among these catalysts,
4
8,49
transesterification reaction of triglyceride with methanol,
2
2
the transesterification reaction of the tributyrin (T) with
methanol should also be to proceed in 3 steps to produce
dibutyrin (D), which further reacts with methanol to yield
monobutyrin (M) that finally reacts with methanol to produce
Fig. 9 Catalytic activities of the 25-KF–LOC-x catalysts ( conversion
of tributyrin,
yield of methyl butyrate, A: 25-KF–LOC-673; B: 25-KF–
LOC-723; C: 25-KF–LOC-773. Reaction conditions: 30 mg of catalyst,
−
1
the ratio of catalyst to tributyrin is 30 (mg g ), molar ratio of methanol
to tributyrin = 30 : 1, 308 K for 60 min).
Fig. 10 Relation plots of conversion of tributyrin versus the surface
hydroxyl amount.
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methyl butyrate (MB) and glycerol (G). The three steps are
described in the following eqn (1)–(3), respectively.
time will be linear, with a slope equal to the reaction rate
constant k.
The kinetics of the 25-KF–LOC-773 catalyzed trans-
esterification of tributyrin has been studied at a 30 : 1 metha-
nol : tributyrin molar ratio in the temperature range of
K1
T  MethanolD  MB
(1)
(2)
(3)
K2
2
73–291 K, and plotting a graph between −ln(1-yieldmethyl butyrate
)
K3
D  MethanolM  MB
vs. t as given in Fig. 11a. The plots are represented by straight
K4
lines, validating the first-order reaction model. The rate con-
stants were calculated from these plots and found to be 0.0024,
K5
M  MethanolG  MB
−1
K6
0
.0071 and 0.0105 min at 273, 285 and 291 K, respectively.
The activation energy (E ) and pre-exponential factor (A)
a
Since one mole of methyl butyrate is generated per mole of
methanol reacted at each step, in all, six rate constants are
reported in the literature for the whole reaction from tributyrin
to methyl butyrate as shown by eqn (4) given below:
for the same reaction were determined by following the
Arrhenius mode as given in eqn (6).
ln k = −E
a
/RT + ln A
(6)
K
K
where R is the gas constant and T is the reaction temperature in
Kelvins. A plot between lnk vs. 1/T is shown in Fig. 11b. The
T  3MethanolG  3MB
(4)
slopes of the lines provide the activation energy (E ) using the
a
According to the above equation, transesterification results
ultimately in the production of methyl butyrate and therefore,
all the intermediate reaction products (e.g. D and M) can be
ignored. In addition, although the transesterification reaction
is reversible, Diasakov et al. proposed that the reverse reaction
in excess methanol wase not important and could be
Arrhenius equation. In Fig. 11b, the activation energy calculated
−1
from the slope of the Arrhenius plot is 55.03 kJ mol , and the
7
−1
corresponding pre-exponential factor is 8.35 × 10 min . The
activation energy for 25-KF–LOC-773 catalyst is much lower than
−1
38
2 3
that of KF–La O catalyst (128.9 kJ mol ) in our previous study.
4
9–51
ignored.
Thus, a simple mathematical model expressing
1
8,48
the whole conversion as one step has been developed.
3.8. Recycling and stability
In this study, all the reactions were carried out with a metha-
nol : tributyrin molar ratio of 30 : 1. With such a large excess of
methanol, several studies suggested that the overall trans-
esterification reaction is in accord with the pseudo first order
The catalytic activities of 25-KF–LOC-773 catalyst were also mon-
itored over the used 25-KF–LOC-773 catalyst for the trans-
esterification reaction of tributyrin. After being used as the fresh
2
5-KF–LOC-773 catalyst, the used catalyst was dried at 353 K
1
8,48,52–57
kinetic. Thus, based on the references results,
pseudo first order kinetic model should be given as eqn (5)
the
and calcined at 773 K. For the second reused 25-KF–LOC-773
catalyst, the conversion of tributyrin rapidly decreased from
9
0
8.5% to 5.5%, while the yield of methyl butyrate only got to
.76% at 308 K shown in Fig. 12 (run 1 and run 2-1). It illus-
−ln(1-yieldmethyl butyrate) = kt
(5)
trates that there is an obvious downward trend in the catalytic
performance of the reused the 25-KF–LOC-773 catalyst in
regard to its reusability. It is inferred that the decrease in the
where yieldmethyl butyate is the yield of methyl butyrate at
time t (min). A plot of −ln(1-yieldmethyl butyate) as a function of
3
Fig. 11 (a) Plots of −ln(1-yieldmethyl butyate) versus time at different temperatures. (b) Arrhenius plot of lnk versus 10 /T for the reaction of tributyrin with
−1
methanol (reaction conditions: 15 mg of 25-KF–LOC-773 catalyst, the ratio of catalyst to tributyrin is 15 (mg g ), molar ratio of methanol to tributyrin = 30 : 1).
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A series of experiments were performed in order to analyze
the deactivation reason and improve the recyclability of the
2
5-KF–LOC-773 catalyst. The second used 25-KF–LOC-773
catalysts were characterized by XRD, SEM, FTIR, CO -TPD
2
and XRF. The XRD patterns of the fresh and the second used
25-KF–LOC-773 catalysts are shown in Fig. 13. It is found that
there is almost no difference in the characteristic diffraction
peaks between the fresh and the second used 25-KF–LOC-773
catalyst. However, the morphology of the second used of
2
5-KF–LOC-773 catalysts is totally changed, the rod structure
disappears and turns into an aggregate which is made up by
spheres with a diameter between 200 and 400 nm (Fig. 2c).
The BET surface area of the second used 25-KF–LOC-773 cata-
2
−1
2
−1
lysts increases from 3.5 m g to 4.9 m g due to the for-
mation of sphere structures (Table 1). Additionally, the FTIR
spectra of the fresh and the second used 25-KF–LOC-773 cata-
lysts are also quite different, as shown in Fig. 14. For the sec-
ond used 25-KF–LOC-773 catalyst, the intensity of the peak at
Fig. 12 Catalytic activities of the fresh and the used 25-KF–LaOC-773
catalysts at different conditions ( conversion of tributyrin,
methyl butyrate, reaction conditions: 30 mg of KF–La CO
the ratio of catalyst to tributyrin is 30 (mg g ), molar ratio of methanol
to tributyrin = 30 : 1, reaction time for 60 min, reaction temperature at
yield of
catalyst,
2
O
2
3
−
1
308 K (run 1: fresh catalyst, run 1-1: fresh catalyst delayed for 90 days,
−
1
3370 cm assigned to hydroxyl drops off obviously, while the
run 1-2: fresh catalyst after absorption of CO , run 2-1: the second
2
used catalyst) and 338 K (run 2-2: the second used catalyst)).
other peaks almost remain unchanged. It indicates that
hydroxyl groups on the surface have leaching, which leads to
the weakening of the hydroxyl peak. It also shows the obvious
difference between the fresh and the second used 25-KF–
catalytic activity has something to do with the loss of active
species on the catalyst surface when the fresh catalyst is used.
However, when the transesterification reaction was performed
at 338 K over the second used 25-KF–LOC-773 catalyst, the
conversion of tributyrin can reach 91%, and the yield of
methyl butyrate also can achieve 64%, as shown in Fig. 12
2
LOC-773 catalyst in the CO -TPD curves (Fig. 15). The peak
which is attributed to hydroxyl basic sites at 300–420 K
becomes weak for the second used 25-KF–LOC-773 catalyst,
which indicates the decrease in the amount of the weak basic
sites resulting from hydroxyl on the surface of the second
used 25-KF–LOC-773 catalyst. But the peak intensity of the
strong basic sites slightly decreases, and the strong basic
(run 2-2). It indicates the second used 25-KF–LOC-773 catalyst
still possesses good catalytic activity in the high reaction tem-
−
1
perature. Based on the CO -TPD results, it can be suggested
amount still gets to 51 μmol g (Table 2). Thus, it suggests
that the loss of hydroxyl on the surface of the second used
25-KF–LOC-773 catalyst leads to the decrease of catalytic
activity at the reaction temperature of 308 K.
2
that the catalytic activity at 338 K is provided by the strong
Lewis basic sites. It is interesting that 97.2% of the conversion
to tributyrin and 87.0% of the yield to methyl butyrate can be
obtained at 308 K after the fresh 25-KF–LOC-773 catalyst was
placed into the silica gel desiccator in the atmosphere for
The characterizations of XRF can also support our infer-
ence. The K : La mole ratio is 38.7% in fresh 25-KF–LOC-773
catalyst, the mole ratio of K : La decreases to 7.1% after being
9
0 days. Moreover, the catalytic activity was tested at the same
reaction conditions using the fresh 25-KF–LOC-773 catalyst
previously absorbed CO at the room temperature. The results
2
show that the conversion of tributyrin and the yield of methyl
butyrate get to 96.8% and 76% (Fig. 12 (run 1-2)), respectively,
which indicates that the 25-KF–LOC-773 catalyst remains the
high activity and the resistance for CO in the atmosphere.
2
Thus, these results indicate that the 25-KF–LOC-773 catalyst
has high stability.
In addition, the third and the forth recycling experiments
were carried out over the 25-KF–LaOC-773 catalyst, and their
results are shown in Fig. S4† (run 3-1 and run 4-1). The conver-
sion of tributyrin and yield of methyl butyrate decrease to 34%
and 66% for the third run (run 3-1), respectively. However,
when the forth recycling experiment was performed, the cata-
lyst exhibited an almost complete deactivation, whith the con-
version of tributyrin rapidly decreasing from 34% to 4%, while
the yield of methyl butyrate only got to 0.57%, as shown in Fig.
S4† (run 4-1). The results indicate a gradual loss of the basic
sites with the increase in the recycling experiment.
Fig. 13 XRD patterns of the fresh and the second used 25-KF–LOC-773
(A: fresh, B: the second used).
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anion by methanol adsorbed on the Lewis base sites at the
high temperature. Therefore, we suggest that the Lewis base
sites play a key role in the catalytic activity at high reaction
temperature (338 K).
4. Conclusions
The 25% KF supported on La O CO catalysts (25-KF–LOC-x,
2
2
3
x = 673, 723 and 773 K) were prepared by the impregnation
method. The 25-KF–LOC-x catalysts obtained at different
calcination temperatures varying from 673 K to 773 K show
different physical and chemical properties, especially the sur-
face basicity. A LaOF phase was observed in the 25-KF–LOC-x
catalysts, furthermore, the amount of LaOF increases with
Fig. 14 FT-IR of the fresh and the second used 25-KF–LOC-773
2
the calcination temperature. The results of XRD, IR and CO -
(
A: fresh, B: the second used).
TPD indicate that the formation of LaOF leads to the genera-
tion of the basic sites. Two kinds of basic sites, Brønsted base
and Lewis base, are ascribed to the hydroxyl groups and the
oxygen anions of La O CO interacting with KF, respectively.
2
2
3
Moreover, the amount of basic sites is different from that of these
two kind of basic sites. The order of the amount of the weak
base sites is 25-KF–LOC-673 < 25-KF–LOC-723 < 25-KF–LOC-773.
On the contrary, the amount of the strong basic sites of the
2
5-KF–LOC-673 catalyst is the largest, followed by the 25-KF–
LOC-723 catalyst and the 25-KF–LOC-773 in the ascending
order. However, the total basic amount decreases with the
order of 25-KF–LOC-673, 25-KF–LOC-723 and 25-KF–LOC-773.
The more LaOF is produced, the more surface hydroxyl
groups of the 25-KF–LOC-x catalyst are formed. There exists
a linearity relation between the conversion of tributyrin
and the amount of surface hydroxyl oxygen. According to the
order from 25-KF–LOC-673, 25-KF–LOC-723 to 25-KF–LOC-773
catalyst, the catalytic activity is markedly enhanced. The
2
5-KF–LOC-773 catalyst exhibits such an excellent catalytic
2
Fig. 15 CO -TPD profiles of the fresh and the second used 25-KF–
LOC-773 (A: fresh, B: the second used).
activity that the conversion of tributyrin is nearly 100% and the
yield of methyl butyrate reaches 94% at 308 K, which can prove
that hydroxyl is one of the active basic sites at low reaction tem-
perature. Furthermore, the 25-KF–LOC-773 catalyst has such a
high stability that the high activity is still kept after 90 days. It
suggests that the loss of hydroxyl on the surface of the used
25-KF–LOC-773 catalyst leads to the decrease in catalytic activ-
ity at a low reaction temperature (308 K). However, the conver-
sion of tributyrin can reach 91%, and the yield of methyl
butyrate also can achieve 64% when the transesterification
reaction is performed at 338 K over the second used 25-KF–
LOC-773 catalyst. It indicates that the second used 25-KF–
LOC-773 catalyst still possesses good catalytic activity at the
higher reaction temperature due to the unchanged strong
basic sites given by surface oxygen anions.
second used. The results suggest that the decrease in activity
had a striking correlation with the loss of hydroxys on the
surface. When K was leached, in order to keep the balance of
charge, the amount of hydroxyl on the surface would also
decrease. Therefore, the loss of active sites gives rise to the
deactivation of the catalyst during the recycle reaction.
However, the strong Lewis base sites attributed to the surface
oxygen anions is unchanged. For the forth used 25-KF–LaOC-
5
8
7
2
73 catalyst, the CO -TPD result (Fig. S5C†) displays that the
peak at about 750 K attributed to the strong basic sites
disappeared. Therefore, it suggests that the loss of Lewis
basic sites on the surface of the forth used 25-KF–LOC-773
catalyst leads to the deactivation of the forth used 25-KF–
LaOC-773 catalyst. It also confirms that the Lewis basic sites
are the active sites in the recycling experiments.
Acknowledgements
This work is supported by the Program for New Century
Excellent Talents in Heilongjiang Provincial University (1155-
NCET-014), the Natural Science Foundation of Heilongjiang
Province (B200805), the Major Foundation of Educational
It is well known that the methoxyl anion is the key species
in the transesterification of triglyceride with methanol using
2
solid base as the catalyst. It is beneficial to form methoxyl
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Commission of Heilongjiang Province of China (11531Z11),
the Foundation of Educational Commission of Heilongjiang
Province of China (11531286), the Postdoctoral Science-
research Developmental Foundation of Heilongjiang Province
of China (LBH-Q12022), the Innovative Research Team in
Heilongjiang University (Hdtd2010-10) and the Program for
Innovative Research Team in University (IRT-1237). The Project
sponsored by SRF for ROCS, SEM and Heilongjiang Province.
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