The effect of catalyst preparation on the activity of MoO3-SiO2 catalyst in transesterification of diethyl oxalate

Article abstract of DOI:10.1134/S0023158414060032

Transesterification of diethyl oxalate (DEO) with phenol over MoO₃-SiO₂ catalysts prepared by the sol-gel technique (MoO₃-SiO₂ (SG)) and the impregnation method (MoO₃-SiO₂ (I)) was conducted to produce diphenyl oxalate (DPO), which can be used as a precursor for manufacturing diphenyl carbonate (DPC). The sample MoO₃-SiO₂ (SG) containing 12 wt % of MoO₃ showed the best performance with 71.0% conversion of DEO and 32.0% selectivity to DPO. Compared to MoO₃-SiO₂ (I), improvements in the DEO conversion and DPO selectivity with MoO₃-SiO₂ (SG) were 16.1 and 7%, respectively. Crystal structure and phase composition of MoO₃-SiO₂ (I) and MoO₃-SiO₂ (SG) catalysts with varying MoO₃ contents were investigated. The sample MoO₃-SiO₂ (SG) with a similar chemical composition to MoO₃-SiO₂ (I) has a larger specific surface area, indicating that the active component is well dispersed on the surface of the MoO₃-SiO₂ (SG) catalysts. Results of XRD and XPS measurements suggest a high degree of dispersion of MoO₃-SiO₂ (SG) catalysts that can account for an increase in DEO conversion and DPO selectivity. Coordinately unsaturated MoO₃ species play a significant role in the catalytic performance of MoO₃-SiO₂ (SG) catalysts in transesterification of DEO with phenol. In addition, IR measurements of pyridine adsorption and NH₃-TPD data indicate that the amount of acid sites on the surface of MoO₃-SiO₂ (SG) exceeds that found for the surface of MoO₃-SiO₂ (I). An enhanced concentration of surface MoO₃ species in tetrahedral coordination coupled with the presence of weak Lewis acid sites appear to be the main reason why MoO₃-SiO₂ (SG) catalysts are superior to the MoO₃-SiO₂ (I) system.

Full text of DOI:10.1134/S0023158414060032

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2014, Vol. 55, No. 6, pp. 763–769. © Pleiades Publishing, Ltd., 2014.
The Effect of Catalyst Preparation on the Activity
of MoO3–SiO2 Catalyst in Transesterification of Diethyl Oxalate1
,
b
L. Biana , S. P. Wanga, *, and X. B. Maa
aKey Lab for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin
University; Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072 P.R. China
bCollege of Science, Hebei University of Engineeing, Handan, 056038 P.R. China
*eꢀmail: spwang@tju.edu.cn
Received December 10, 2013
Abstract—Transesterification of diethyl oxalate (DEO) with phenol over MoO3–SiO2 catalysts prepared by
the sol gel technique (MoO3–SiO2 (SG)) and the impregnation method (MoO3–SiO2 (I)) was conducted
−
to produce diphenyl oxalate (DPO), which can be used as a precursor for manufacturing diphenyl carbonate
(DPC). The sample MoO3–SiO2 (SG) containing 12 wt % of MoO3 showed the best performance with 71.0%
conversion of DEO and 32.0% selectivity to DPO. Compared to MoO3–SiO2 (I), improvements in the DEO
conversion and DPO selectivity with MoO3–SiO2 (SG) were 16.1 and 7%, respectively. Crystal structure and
phase composition of MoO3–SiO2 (I) and MoO3–SiO2 (SG) catalysts with varying MoO3 contents were
investigated. The sample MoO3–SiO2 (SG) with a similar chemical composition to MoO3–SiO2 (I) has a
larger specific surface area, indicating that the active component is well dispersed on the surface of the
MoO3–SiO2 (SG) catalysts. Results of XRD and XPS measurements suggest a high degree of dispersion of
MoO3–SiO2 (SG) catalysts that can account for an increase in DEO conversion and DPO selectivity. Coorꢀ
dinately unsaturated MoO3 species play a significant role in the catalytic performance of MoO3–SiO2 (SG)
catalysts in transesterification of DEO with phenol. In addition, IR measurements of pyridine adsorption and
NH3–TPD data indicate that the amount of acid sites on the surface of MoO3–SiO2 (SG) exceeds that found
for the surface of MoO3–SiO2 (I). An enhanced concentration of surface MoO3 species in tetrahedral coorꢀ
dination coupled with the presence of weak Lewis acid sites appear to be the main reason why MoO3–SiO2
(SG) catalysts are superior to the MoO3–SiO2 (I) system.
DOI: 10.1134/S0023158414060032
1
technology is the synthesis of DPC via a green process.
Polycarbonates (PCs) are important engineering
Phosgeneꢀfree processes for DPC synthesis have been
proposed to replace the conventional phosgene proꢀ
cess [4–11].
thermoplastics with excellent mechanical, optical as
well as electrical and heat resistant properties. Thereꢀ
fore, they have been used in many fields as the substiꢀ
tutes of glass and metals. A conventional method for
PCs synthesis is the interfacial polycondensation of
bisphenolꢀA (BPA) with the highly toxic phosgene in
the presence of bases [1]. To meet increasing demand
for safe and environmentally friendly processes for
PCs synthesis an alternative route was recently proꢀ
posed. A process involves the synthesis of diphenyl
carbonate (DPC) followed by the transesterification
between DPC and bisphenol–A [2]. In this scheme,
no toxic solvents are used and the byꢀproduct phenol
could be recycled.
A promising approach is the transesterification of
dimethyl oxalate (DMO) with phenol to prepare
diphenyl oxalate (DPO), followed by the decarbonylaꢀ
tion of DPO to DPC [12, 13]. There are many reports
about the ester exchange between DMO and phenol to
produce DPO [10, 11, 14–19], followed by the decarꢀ
bonylation of DPO to produce DPC. However, there
are few reports [20] describing the preparation of DPO
by the transesterification of diethyl oxalate (DEO
)
with phenol followed by the decarbonylation of DPO
to produce DPC.
However, DPC is prepared commercially by the
reaction of phenol and phosgene in the presence of
bases such as sodium hydroxide [3]. Again, the obviꢀ
ous drawback of this traditional process for DPC is the
use of phosgene. Therefore, in order to produce PCs
without social consequences of pollution, the key
Due to low equilibrium constants a twoꢀstep proꢀ
duction of DPO was proposed for the transesterificaꢀ
tion of DEO with phenol [21]. The first step is the forꢀ
mation of ethyl phenyl oxalate (EPO) and the second
step is the production of DPO via disproportionation
of EPO:
1
The article is published in the original.
763

764
BIAN et al.
OH
COOCH2CH3
COOCH2CH3
COOCH2CH3
COO
+
+
CH3CH2OH
(I)
COO
COO
COOCH2CH3
COO
COOCH2CH3
COOCH2CH3
2
+
(II)
Safe operation and low corrosion effects on the method. As a typical procedure, the dried SiO2 supꢀ
equipment are the main factors in determining the port was contacted with an aqueous solution of
compatibility of this process with the requirements of ammonium molybdate of appropriate concentration
the green chemistry. Moreover, because DEO is a liqꢀ with stirring. The support immersed in a solution was
uid at room temperature it can be easy feedꢀin and in then stored at ambient temperature in nitrogen for
addition it is cheaper than DMO.
48 h to ensure that molybdenum compound is incorꢀ
porated and can diffuse through the surface of SiO2.
The pretreated catalysts were dried in an oven for 4 h
at 393 K and calcined in a muffle furnace at 823 K for 5 h.
Generally, the transesterification to DPO is carried
out in the liquid phase using homogeneous catalysts
such as Lewis acids or soluble organic Pb, Sn, or Ti
compounds [12, 13]. However, in these homogeneous
transesterification systems, the separation of the cataꢀ
lysts from the products is complicated under industrial
conditions. Therefore, to make the regeneration and
Transesterification of Diethyl Oxalate with Phenol
Transesterification of DEO with phenol was carried
separation easier, the development of active solid catꢀ out in a threeꢀnecked flask (250 mL) equipped with a
alysts is highly desirable. Molybdenum oxide has distillation column and a thermometer to monitor and
found a wide application in the industrial processes control the reaction temperature. Particularly, the
such as dehydration, dehydrogenation, and isomerizaꢀ temperature in the top of distillation column was kept
tion [22–25]. Earlier the reactivities of supported at 363 K by recycling hot water in order to remove ethꢀ
MoO3 catalysts in the transesterification of dimethyl anol from the reaction system. Using this method the
oxalate with phenol were described [18]. In this work, reaction equilibrium limitation for reaction (I) was
we present results of the investigation on the synthesis avoided and the reaction was accelerated to proceed
of DPO from the transesterification of DEO with pheꢀ towards the desired direction. The reaction mixture
nol over MoO3–SiO2 catalysts prepared by different contained 0.1 mole of DEO, 0.3 mole of phenol and
methods. By comparing the catalytic performance of 1.8 g of the catalyst. After raw materials and catalyst
the catalyst prepared by the sol gel method (MoO3– were placed into a batch reactor, the reactor system
−
SiO2 (SG)) with the catalysts prepared by the impregꢀ was purged in 30 SCCM nitrogen to displace air. The
nation method (MoO3–SiO2 (I)) an attempt is made feed of nitrogen was stopped after 10 min and the
to illustrate advantages of using the solꢀgel technique. threeꢀnecked flask was heated at a rate of 10 K/min.
The reaction was conducted at 453 K and normal
pressure.
EXPERIMENTAL
Qualitative analysis of the reaction products and
Catalyst Preparation
gel method. The sol gel method was used
the distillates were conducted on a GC–MS (Agilent
HP5890–HP5971MSD) equipped with an HPꢀ5 30 m
capillary column. Quantitative analysis of the reaction
mixture was carried out using an HP 4890 (with a capꢀ
illary column HPꢀINNOWAX, 30 m length, 0.32 mm
The sol
−
−
to prepare a series of MoO3–SiO2 (SG) catalysts with
the composition ranging from 1 to 24 wt % MoO3.
Ethyl silicate (TEOS) and ammonium molybdate
i.d. and 0.25 m film thickness) with a flame ionizaꢀ
μ
(
AHM) were used as precursors of SiO2 and MoO3
tion detector. The products were composed of ethyl
phenyl oxalate and diphenyl oxalate and alcohol
emerged as distillate.
respectively. The calculated amounts of AHM and
TEOS were dissolved in 30 mL of distilled water and
60 mL of ethanol respectively. AHM solution was then
added dropꢀwise into the TEOS ethanol solution with
stirring. The sol color turned from slight green to dark
green, and a dark green gel was formed consequently.
The gel was dried under atmosphere of nitrogen for
24 h, and then ground. The catalysts were prepared by
drying the pellets in oven at 393 K for 4 h and calcining
dry pellets at 823 K for 5 h in a muffle furnace.
Catalyst Characterization
The BET surface area was determined using a volꢀ
umetric adsorption unit (CHEMBETꢀ3000) by low
temperature adsorption of nitrogen.
Xꢀray diffraction (XRD) was performed on a
The impregnation method. A series of MoO3–SiO2 Rigaku C/maxꢀ2500 diffractometer provided with
(I) catalysts with the composition ranging from 1 to graphite monochromator employing nickelꢀfiltered
24 wt % of MoO3 was prepared by the impregnation Cu
K
radiation (λ = 1.5405 Å) at 40 kV and 100 mA.
α
KINETICS AND CATALYSIS Vol. 55
No. 6 2014

THE EFFECT OF CATALYST PREPARATION ON THE ACTIVITY
765
Table 1. Catalytic activities of MoO3
with different content of MoO3 prepared by the solꢀgel
method in the transesterification of DEO with phenol
−
SiO2 (SG) catalysts
The samples were scanned at a rate of 8
°
/min over the
5
° ≤ range. The diffractograms were comꢀ
2
θ < 80
°
pared with JCPDS–ICDD references for identificaꢀ
tion purposes.
Converꢀ
sion (%)
MoO3
contents
(wt %)
Selectivity (%)
Yield (%)
Surface composition and structure of catalysts were
studied by Xꢀray photoelectron spectroscopy (XPS).
The measurements were performed using a Perkinꢀ
DEO
DPO
EPO
DPO
EPO
Elmer PHI 1600 ESCA spectrometer with Mg
K radiꢀ
α
1
3
37.8
55.2
61.6
71.0
70.8
15.4
26.8
26.5
32.0
19.4
84.6
73.2
73.5
68.0
80.6
5.8
14.8
16.3
22.7
13.7
32.0
40.4
45.3
48.3
57.1
ation (1253.6 eV) as incident beam. The samples were
mounted in a specimen holder via doubleꢀsided adheꢀ
sive tape. Spectra were recorded in steps of 0.15 eV.
6
12
24
The C1s (284.5 eV) was used as an internal standard to
calibrate the binding energies. An estimated error of
0.1 eV can be assumed for all the measurements. The
scanning of the spectra was done at pressures less than
10–8 Torr.
Reaction conditions: phenol 0.3 mol, DEO 0.1 mol, catalyst
1.8 g, reflux temperature 363 K, reaction temperature 453 K,
reaction time 3 h.
IR measurements of adsorbed pyridine were carried
out on a Bruker Vector 22 FTꢀIR spectrometer. The
scanning range was from 4000 to 500 cm–1 and the resꢀ
olution was 4 cm–1. The sample powder was pressed
into a selfꢀsupporting wafer. Prior to each experiment,
the catalysts were evacuated (1 Pa) at 693 K for 1.5 h.
After the samples were cooled to 333 K, large portions
of pyridine were admitted to saturate the catalysts.
Subsequently, the samples were outꢀgassed for 0.5 h at
333 K and the spectra were recorded.
Table 2. Catalytic activities of MoO3
with different content of MoO3 prepared by the impregnaꢀ
tion method in the transesterification of DEO with phenol
−
SiO2 (I) catalysts
MoO3 DEO Selectivity (%) Yield (%)
contents converꢀ
(wt %) sion (%)
DPO
EPO
DPO EPO
1
3
6
12
24
32.6
53.2
54.9
54.8
44.5
18.8
24.2
25.0
18.1
13.7
81.2
75.8
75.0
81.9
86.3
6.1
12.9
13.7
9.9
26.5
40.3
41.2
44.9
38.4
NH3–TPD spectra were recorded using
a
Micromeritics Co. model 2910 chemical adsorption
instrument. The catalysts were heated to 393 K in
flowing Ar for 1 h, and then cooled to room temperaꢀ
ture. Adsorption of NH3 was carried out at 323 K for
the time period needed to saturate catalysts. After satꢀ
uration was achieved, the reactor was switched from
NH3 to Ar and the catalysts were heated to 873 K at a
rate of 10 K/min.
6.1
Reaction conditions: phenol 0.3 mol, DEO 0.1 mol, catalyst
1.8 g, reflux temperature 363 K, reaction temperature 453 K,
reaction time 3 h.
Table 3. Specific surface area of MoO3
−
SiO2 catalysts preꢀ
pared by sol−gel and impregnation methods
MoO3 contents (wt %)
1
3
6
12 24
SiO2 (SG) (m2/g) 223.9 191.8 157.1 131.1 95.3
SiO2 (I) (m2/g)
155.1 148.0 134.3 99.0 59.1
RESULTS AND DISCUSSION
Performances of Catalysts
MoO3
−
MoO3
−
The results of DEO conversions, selectivities and
yields of DPO over MoO3–SiO2 (SG) and MoO3–
SiO2 (I) are demonstrated in Tables 1 and 2, respecꢀ
tively. For the MoO3–SiO2 (SG) catalysts, the converꢀ
sion of DEO, selectivity and yield of DPO were
improved steadily from 37.8, 15.4, and 5.8% to 71.0,
32.0, and 22.7% with MoO3 contents increasing from
1 to 12 wt %, respectively. Thereafter, all parameters
characterizing the reactivity deteriorated, indicating
that MoO3–SiO2 (SG) with 12 wt % MoO3 showed
the best catalytic performance for the transesterificaꢀ
greatly improved compared with that of MoO3–SiO2
(I). It can be thus inferred that in terms of activity in
transesterification MoO3–SiO2 (SG) was superior to
MoO3–SiO2 (I).
Specific Surface Area Measurements
The specific surface area of molybdenum oxide
tion of DEO with phenol. At the same time, the catalysts prepared by sol gel and the impregnation
−
behavior of MoO3–SiO2 (I) was tested and it was methods is shown in Table 3. The specific surface area
found that trends pertinent to catalytic properties of of MoO3–SiO2 (SG) catalysts decreased gradually
MoO3–SiO2 (I) were similar to that of MoO3–SiO2 from 223.9 to 95.3 m2/g as the content of MoO3
(SG). The highest DEO conversion, DPO selectivity increased from 1 to 24 wt %. The similar trend was
and yield (54.9, 25.0 and 13.7% respectively) were observed for MoO3–SiO2 (I) catalysts prepared by the
found with 6 wt % MoO3–SiO2 (I) catalysts. It seems impregnation method. However, for the catalysts with
that reactivity of MoO3–SiO2 (SG) catalysts was comparable MoO3 content, the specific surface area of
KINETICS AND CATALYSIS Vol. 55
No. 6 2014

766
Intensity, a. u.
BIAN et al.
Intensity, a. u.
Crystal MoO3
Crystal MoO3
e
e
d
d
c
c
b
a
b
a
10
20
30
40
50
60
70
10
20
30
40
50
60
70
2
θ
, degree
2
θ
, degree
Fig. 1. XRD patterns of MoO –SiO (SG) catalysts with
Fig. 2. XRD patterns of MoO –SiO (I) catalyses with difꢀ
3 2
3
2
different content of MoO prepared by the sol
−
gel
ferent content of MoO prepared by the impregnation
3
3
method: 1 (a), 3 (
b), 6 (
c
), 12 (
d), 24 wt % (
e
).
method: 1 (a), 3 (b), 6 (c), 12 (d), 24 wt % (e).
MoO3–SiO2 (SG) was much larger than that of vated for catalytic transformations [27, 28]. Accordꢀ
MoO3–SiO2 (I), suggesting that the active component ingly, the surface MoO3 in tetrahedral coordination
in the MoO3–SiO2 (SG) catalysts has a higher degree had a higher catalytic performance than the MoO3 in
of dispersion than that in the MoO3–SiO2 (I) cataꢀ octahedral coordination. The improvement in cataꢀ
lysts.
lytic performance observed when MoO3 content
increased from 1 to 12 wt % for MoO3–SiO2 (SG) and
from 1 to 6 wt % for MoO3–SiO2 (I), respectively, may
be caused by the increasing fraction of amorphous
MoO3. By contrast, the appearance of crystalline
MoO3 on the catalyst surface brings about deterioraꢀ
tion effects on DEO conversion and DPO selectivity.
The reason may lie in the reduction of the surface speꢀ
cies in tetrahedral coordination. Therefore, the coorꢀ
dinately unsaturated MoO3 plays a significant role in
the catalytic transesterification of DEO with phenol.
XRD Measurements
Structure and chemical state of Mo species in
MoO3–SiO2 (SG) and MoO3–SiO2 (I) with various
MoO3 content were investigated by XRD (Figs. 1, 2).
The MoO3 phase could be detected by XRD only in
MoO3–SiO2 (SG) samples with a MoO3 content
above 12 wt %, indicating that in the samples with a
lower MoO3 molybdenum oxide occurs as a finely disꢀ
persed species. Strong diffraction peaks could be
detected at a loading of 24 wt %, indicating the formaꢀ
tion of crystalline MoO3. However, as shown in Fig. 2,
the diffraction peaks corresponding to MoO3 were
XꢀRay Photoelectron Spectroscopy
In order to obtain more detailed information
observed in the 12 wt % MoO3–SiO2 (I) catalysts preꢀ regarding the structure of the surface and dispersion of
pared by the impregnation method. These observaꢀ the Mo species of the catalysts for both preparation
tions provide further evidence that the extent of disꢀ methods, MoO3–SiO2 (SG) and MoO3–SiO2 (I) catꢀ
persion of the active components in MoO3–SiO2 (SG) alysts (12 wt %) were characterized by XPS. The O1
s
catalysts is higher than that in MoO3–SiO2 (I). Zhao spectra of MoO3–SiO2 (SG) and MoO3–SiO2 (I) catꢀ
et al. proposed [26] that there are two kinds of surface alysts (12 wt %) are shown in Fig. 3. It can be noticed
MoO3 species on SiO2 support. The hexavalent Mo that the O1s XPS spectrum of MoO3–SiO2 (I) catalyst
cation can occur in oxygen coordinations 4 or 6. In the was deconvolved to three binding energy profiles of
amorphous MoO3 predominant coordination is tetraꢀ 530.5, 528.9, and 532.8 eV, which can be ascribed to
hedral whereas in the crystalline MoO3 molybdenum Mo–O–Si, Mo–O and Si–O–Si, respectively [29].
oxide adopts octahedral coordination. The surface On the other hand, the deconvolution procedure of
MoO3 species in tetrahedral surrounding has lower the O1s XPS spectrum of the MoO3–SiO2 (SG) cataꢀ
oxygen coordination than the “coordinately unsaturꢀ lyst yielded only two binding energy profiles, correꢀ
ated” bulk MoO3 in octahedral coordination. The sponding to MoO–Si and Si–O–Si. In other words,
coordination sphere of a surface atom can be comꢀ no binding energy profile of 528.9 eV attributable to
pleted by adsorbed molecules, which can also be actiꢀ Mo–O was observed. This result suggests that ammoꢀ
KINETICS AND CATALYSIS Vol. 55
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THE EFFECT OF CATALYST PREPARATION ON THE ACTIVITY
Absorbance, a. u.
767
Ols
1445
1490
1540
1600
MoO3–SiO2(SG)
a
b
c
Si–O–Si
d
e
Si–O–Mo
2000 1900 1800 1700 1600 1500 1400 1300
Wevenumber, cm–1
536
534
532
530
528
526
Fig. 4. IR spectra of pyridine absorbed on MoO –SiO
3
2
(SG) catalysts with different content of MoO : 1 (a), 3 (b),
3
6 (c), 12 (d), 24 wt % (e).
Ols
MoO3–SiO2(I)
of the formation of crystalline MoO3 in the 12 wt %
MoO3–SiO2 (I). It seems that a higher extent of disꢀ
persion of the active components in the 12 wt %
MoO3–SiO2 (SG) compared to that found for the
12 wt % MoO3–SiO2 (I), results in improved DEO
conversion and DPO selectivity.
Si–O–Si
IR Measurements of Adsorbed Pyridine
Si–O–Mo
Mo–O
IR measurements of adsorbed pyridine were perꢀ
formed to determine the nature of acidic sites (Lewis
and Brönsted) on the surface of the MoO3–SiO2 cataꢀ
lysts. Figures 4 and 5 show IR spectra of pyridine
adsorbed on MoO3–SiO2 (SG) and MoO3–SiO2 (I)
catalysts, respectively. For all samples three absorption
bands at 1490, 1445 and 1600 cm–1 were detected.
MoO3–SiO2 (SG) catalysts exhibited also the band at
1540 cm–1. Absorption frequencies of 1445 and
1600 cm–1 correspond to pyridine chemisorbed on the
Lewis acid sites, while the absorption band at
1540 cm–1 is indicative of Brönsted acidity. The band
at 1490 cm–1 can be attributed to the presence of Lewis
and Brönsted acid sites. The spectra in Figs. 4 and 5
suggest that the absorption bands detected on the surꢀ
face of MoO3–SiO2 (SG) and MoO3–SiO2 (I) were
predominantly caused by the interaction of pyridine
with Lewis acid sites. It can be therefore inferred that
the Lewis acid sites were responsible for the formation
of DPO in the transesterification of DEO and phenol.
Using IR measurements of adsorbed pyridine, the
amounts of acid sites in the MoO3–SiO2 (SG) and the
MoO3–SiO2 (I) catalysts with different content of
MoO3 were calculated (Tables 4 and 5 respectively). It
can be readily seen that the amount of acid sites
536
534
532
530
528
526
Binding energy, eV
Fig. 3. O1s XPS spectra of MoO –SiO (SG) and MoO –
3 2 3
SiO (I) catalysts (12 wt % of MoO ).
2
3
nia molybdate as molybdenum source and ethyl siliꢀ
cate as silica source were adequately dispersed during
the process of the catalyst preparation by sol gel. It
−
can be concluded that molybdenum ions on the surꢀ
face of 12%MoO3–SiO2 (SG) catalyst occur in Mo–
O–Si rather than in Mo–O species. Combined with
the results of XRD shown in Fig. 1 these data suggest
that MoO3 particles can not be found in the 12 wt %
MoO3–SiO2 (SG) catalyst, indicating that the high
degree of dispersion of MoO3 in MoO3–SiO2 (SG)
catalysts. On the other hand, XRD patterns and the
O1s XPS spectra of MoO3–SiO2 (I) provide evidence decreases with the increasing MoO3 content on both
KINETICS AND CATALYSIS Vol. 55
No. 6 2014

768
Absorbance, arb. units
BIAN et al.
Amount of desorbed NH3 (mmol NH3/g catalyst)
0.257
1490
1445
1600
0.244
0.25
0.20
0.15
0.10
0.05
0
a
b
c
0.136
0.087
3%
0.053
1%
d
e
6%
12%
24%
2000 1900 1800 1700 1600 1500 1400 1300
Wevenumber, cm–1
Fig. 6. Amount of NH desorbed from MoO –SiO (SG)
3
3
2
catalysts with different content of MoO .
3
Fig. 5. IR spectra of pyridine absorbed on MoO –SiO (I)
from MoO3–SiO2 (SG) samples are higher than those
removed from MoO3–SiO2 (I) catalysts. As a result,
MoO3–SiO2 (SG) samples were more active as cataꢀ
lysts than MoO3–SiO2 (I).
3
2
catalysts with different content of MoO : 1 (a), 3 (b), 6 (c),
3
12 (d), 24 wt % (e).
Comparing the data collected in Tables 4 and 5, it
can be readily recognized that the number of acid sites
is not consistent with amounts of NH3 desorbed. As a
possible reason a large size of pyridine molecule can be
evoked that prevents diffusion of the adsorbate inside
the pores. Correspondingly, using pyridine as probe
molecule only acid sites on the outer surface of cataꢀ
lysts could be detected. On the other hand, smaller
NH3 molecules could move freely inside the catalysts
and the amount of acid sites on the surface and in
internal pores of catalysts could be measured. It is
therefore conceivably that the amounts of NH3 and
pyridine desorbed are different.
MoO3–SiO2 (SG) and MoO3–SiO2 (I) samples.
Notably, a marked reduction of the amount of acid
sites can be observed for the samples with an enhanced
concentration of MoO3, resulting in a poorer catalytic
performance of the catalysts. Comparing the samples
with similar chemical compositions the conclusion
can be made the amount of acid sites in MoO3–SiO2
(SG) exceeds that of MoO3–SiO2 (I). Accordingly, the
catalytic performance of MoO3–SiO2 (SG) is superior
to MoO3–SiO2 (I).
NH3–TPD Measurements
NH3–TPD characterizations of MoO3–SiO2 (SG)
and MoO3–SiO2 (I) catalysts with different contents
of MoO3 were conducted. The results showed that the
ammonia is desorbed at low temperatures, implying
that only weak acid sites exist on the surface of all catꢀ
alysts. Therefore, weak Lewis acidity is the source of
the active sites in the transesterification between DEO
and phenol. However, amounts of NH3 desorbed from
MoO3–SiO2 (SG) and MoO3–SiO2 (I) catalysts with
varying MoO3 contents are different, as illustrated in
Figs. 6 and 7, respectively. Amounts of NH3 desorbed
Amount of desorbed NH3 (mmol NH3/g catalyst)
0.14
0.128
0.12
0.110
0.095
0.10
0.08
0.06
0.04
0.02
0
0.086
0.059
Table 4. Acidity of MoO3⎯SiO2 (SG) catalysts with differꢀ
ent MoO3 contents
MoO3
contents (wt %)
−
SiO2
1
3
6
12
24
1%
3%
6%
12%
24%
Number of acid sites* 1096.7 1009.8 909.7 891.3 500.4
* Calculated from the ratio of the integrated area of IR absorpꢀ
tion peak to sample mass.
Fig. 7. Amount of NH desorbed from MoO –SiO (I)
3
3
2
catalysts with different content of MoO .
3
KINETICS AND CATALYSIS Vol. 55
No. 6 2014

THE EFFECT OF CATALYST PREPARATION ON THE ACTIVITY
769
Table 5. Acidity of MoO3−SiO2 (I) catalysts with different
MoO3 contents
(No. 1121104074ꢀ4, 1121106090ꢀ6) are acknowlꢀ
edged for financial support.
MoO3⎯SiO2 (I)
1
3
6
12
24
(wt %)
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ACKNOWLEDGMENTS
National Natural Science Foundation of China
(20506018), the Program for New Century Excellent
Talents in University (NCET–13–0411), the Program
of Introducing Talents of Discipline to Universities
(Grant B06006) and Handan scientific and technoꢀ
logical
research
development
project
KINETICS AND CATALYSIS Vol. 55
No. 6 2014