Effect of zirconia polymorph on the synthesis of diphenyl carbonate over supported lead catalysts

Article abstract of DOI:10.1016/j.mcat.2019.02.021

Zirconia supported Pb-based catalysts with purely tetragonal/monoclinic crystals were prepared and analyzed well by XRD, TEM-EDS, XPS, H₂-TPR, BET, Py-IR and NH₃-TPD techniques. The results indicate that zirconia polymorph has great effect on their structure and catalytic property for the synthesis of diphenyl carbonate (DPC) through methyl phenyl carbonate (MPC) disproportionation due to the differences of surface chemical properties, and tetragonal zirconia supported lead catalyst (TZ-Pb) shows bigger dispersion degree of PbO, higher surface area and Lewis acid amounts and thereby exhibits higher catalytic activity and selectivity compared to monoclinic zirconia supported catalyst (MZ-Pb). Furthermore, TZ-Pb shows better reusability due to strong metal-support interaction and may be readily recycled for at least four times without remarkable reactivity loss. This work provides a prospective reference for the facile and efficient synthesis of zirconia polymorph materials in various catalysis applications.

Full text of DOI:10.1016/j.mcat.2019.02.021

MolecularCatalysis468(2019)117–124
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Effect of zirconia polymorph on the synthesis of diphenyl carbonate over
supported lead catalysts
Songlin Wanga,b, Hongying Niua, Mengjun Guoa, Jianji Wangb, Tong Chenc,⁎, Gongying Wangc
a School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China
b Postdoctoral Programs, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal
University, Xinxiang 453007, China
c Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Zirconia supported Pb-based catalysts with purely tetragonal/monoclinic crystals were prepared and analyzed
well by XRD, TEM-EDS, XPS, H2-TPR, BET, Py-IR and NH3-TPD techniques. The results indicate that zirconia
polymorph has great effect on their structure and catalytic property for the synthesis of diphenyl carbonate
(DPC) through methyl phenyl carbonate (MPC) disproportionation due to the differences of surface chemical
properties, and tetragonal zirconia supported lead catalyst (TZ-Pb) shows bigger dispersion degree of PbO,
higher surface area and Lewis acid amounts and thereby exhibits higher catalytic activity and selectivity com-
pared to monoclinic zirconia supported catalyst (MZ-Pb). Furthermore, TZ-Pb shows better reusability due to
strong metal-support interaction and may be readily recycled for at least four times without remarkable re-
activity loss. This work provides a prospective reference for the facile and efficient synthesis of zirconia poly-
morph materials in various catalysis applications.
Diphenyl carbonate
Dimethyl carbonate
Tetragonal zirconia
Monoclinic zirconia
1. Introduction
the disproportionation. As both K1 and K2 are much small, the key of
the process is the development of efficient catalytic system. Besides, in
Polycarbonate (PC) is significant engineering thermoplastic resin
and has been widely utilized in chemical industry because of its su-
perior electrical, heat resistance, and mechanical properties [1,2]. Di-
phenyl carbonate (DPC) is the main raw feedstock for the production of
PC and is also a crucial chemical intermediate in the synthesis of fine
chemicals, polymer materials, pharmaceuticals and other fields [3,4].
So far several alternative technologies for the preparation of DPC have
been developed in the industry, and the transformation between phenol
and dimethyl carbonate (DMC) into DPC is regarded as a best alter-
native approach to avoid toxic phosgene as starting material, which
subsequently reacts with bisphenol-A to produce PC [5–9]. However,
due to severe thermodynamic constraint concerning the transester-
ification from phenol and DMC (KP = 3.0 × 10–4, 180 °C), the produc-
tion of DPC is highly unfavorable even at high temperatures, and
thereby numerous studies have focused on the process. The reaction
starts from the transesterification of phenol and DMC to methyl phenyl
carbonate (MPC) and methanol (Scheme 1-1), followed by MPC dis-
proportionation to DPC and DMC (Scheme 1-2), or MPC continues to
react with phenol to produce DPC and methanol (Scheme 1-3), but from
comparison with the two step reactions regarding Eq. (1) and Eq. (2),
the first step is the rate-determining step, thus most studies to date have
extensively focused on the first-step reaction [10–14]. Nevertheless,
only little work has been made regarding the second-step dis-
proportionation [15,16], moreover, in previously published work the
disproportionation can hardly carry out thoroughly and restrict greatly
the production of DPC [10–14]. Thus, considering the disproportiona-
tion reaction is the key step to the production of DPC, it is very valuable
to further study on this reaction.
The conversion of MPC to DPC is extremely low because the reac-
tion is kinetically and thermodynamically unfavorable. Thus, it is sig-
nificantly demanded for developing new and efficient catalysts with
high reactivity and stability. Several catalytic formulations have been
explored for MPC disproportionation, mostly based on organo-tita-
nium/tin, metal oxides and supported metal oxide catalysts [16–18].
Because of the difficulty of separation and recovery between the pro-
duct and catalyst for homogeneous organo-titanium/tin as well as the
low catalytic activities of metal oxides, much more attention has been
now paid on supported metal-oxide catalysts, by contrast, zirconia
supported lead is found to exhibit better catalytic properties than others
K
2 (0.19, 25 °C) and K3 (1.2 × 10–5, 25 °C), DPC is mainly generated by
⁎ Corresponding author.
E-mail address: chentongw@sina.com.cn (T. Chen).
https://doi.org/10.1016/j.mcat.2019.02.021
Received 31 October 2018; Received in revised form 9 February 2019; Accepted 22 February 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

S. Wang, et al.
MolecularCatalysis468(2019)117–124
Scheme 1. Transesterification process between DMC and phenol into DPC.
for the disproportionation [18,19]. Nevertheless, it is well known that
zirconia exhibits various crystals such as tetragonal, monoclinic and
cubic crystal structures, where the coordination state of oxygen and
zirconium are different due to different surface chemical properties. In
the monoclinic crystal, O2− is tricoordinated/tetracoordinated and
Zr4+ is heptacoordinated, however, in the tetragonal crystal, O2− is
tetracoordinated and Zr4+ is octacoordinated [20,21]. The differences
of zirconia crystalline phase as supports, especially for metal oxide
catalysts, have usually significant influence on the dispersion, structure
and nature of active component as well as the interaction strength and
so on [22,23]. For example, several studies have reported that mono-
clinic/tetragonal zirconia as catalysts or supports show distinctly dif-
ferent catalytic activities in lots of reactions such as the direct synthesis
of HBP from CO2 and phenol, the glycerol dehydration to acrolein, the
CO hydrogenation to synthesize methanol, the n-butane isomerization
and so on [24–27].
Considering that zirconia possesses notable physical-chemical
properties and displays good catalytic properties that are also re-
markably influenced by crystal phase, the employment of different
zirconia phase is important for many materials or catalytic reactions. To
the best of our knowledge, there is no report about the effect of zirconia
polymorph on the structure-catalytic properties relationship of Pb-
based catalysts for MPC disproportionation. Thus, in the current work,
in order to understand the effect of zirconia crystallite phase on its
structure and catalytic property for MPC disproportionation, tetragonal
and monoclinic zirconia supported Pb-based catalysts were prepared
and characterized by transmission electron microscope (TEM), X-ray
photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD),
temperature programmed reduction of H2 (H2-TPR), pyridine-infrared
spectroscopy (Py-IR), N2 adsorption/desorption and temperature pro-
grammed desorption of NH3 (NH3-TPD), as well as that the stability of
the catalysts was also investigated in detail.
zirconia was prepared by calcination of as-prepared zirconium hydro-
xide at 500 and 750 °C for 5 h. The obtained sample was denoted as TZ
and MZ, respectively.
TZ and MZ supported lead catalysts were prepared using incipient
wetness impregnation process. The corresponding sample was im-
pregnated with required amount of Pb(NO3)2 aqueous solution and kept
for 24 h at room temperature. Afterwards, they were dried, and then
calcinated for 5 h at 500 °C. The as-synthesized catalyst was designated
as TZ-Pb and MZ-Pb, respectively. The PbO loading amount was 13 wt
%.
2.2. Material characterization
X-ray diffraction pattern was recorded using X’pert DX-2700B dif-
fractometer (Philip) with Ni-filtered Cu Kα radiation source
(0.15418 nm). The sample was scanned in 2θ range of 0 to 90° with a
scanning rate of 0.02° /second at the operating voltage and current of
40 kV and 30 mA. The crystal phase was identified by comparison with
reference patterns in the JCPDS database.
The textural properties of the catalysts were determined by nitrogen
adsorption/desorption isotherms at −196 °C (3H-2000PS2, BeiShiDe
Instrument). Specific surface area and pore size distribution were cal-
culated by BET equation and BJH method, respectively. Prior to mea-
surement, the sample was degassed at 250 °C for 6 h under vacuum to
remove the physiosorbed moisture.
The composition of the catalysts was performed by X-ray fluores-
cence spectroscopy (EDX1800B). The energy resolution was
165
5 eV, and the peaks of Pb and Zr were identified by analyzer
crystal. Before the measurement, the curve was firstly calibrated by
standard samples, and then the element content of the samples was
carried out.
X-ray photoelectron spectroscopy (XPS) was measured on Kratos
Model XSAM 800 spectrometer with an exciting X-rays source of Al
Kα = 1486.6 eV. The C1 s line at 284.6 eV was used for the correction
of binding energies.
2. Experimental section
2.1. Material preparation
Transmission electron microscopic photograph (TEM) was carried
out on TECNAIG2F20-S-TWIN electron microscope with 200 kV accel-
erating voltage, and the elemental maps were recorded by the energy
dispersive spectrometer (EDS). The sample was prepared by grinding
the powder, and then dispersed into an ethanol solution followed by
ultrasonication. Then one droplet of the suspension was placed on
carbon-coated copper grid, and finally evaporated for measurement.
The metallic Pb dispersion was determined by hydro-oxygen titra-
tion pulse method on Micro TP-5080 chemisorption analyzer. First,
sample was pretreated at 400 °C for 1 h, and then the temperature was
lowered to 120 °C in flowing H2 atmosphere. Afterwards, the gas was
switched to flowing N2 (20 mL/min) and flushed for 3 h, followed by
saturating with flowing O2 at 120 °C for 30 min. Finally, the baseline
was stabilized in flowing N2, and then H2 was introduced into the N2
All reagents were supplied by Sigma-Aldrich Chemical Reagent
Company, which were used as received without further purification.
According to a modified literature procedure [28,29], zirconium
hydroxide was prepared by precipitation process. ZrOCl2·8H2O was
dissolved into the water and slowly heated to 90 °C to form an aqueous
solution. Ammonia solution with 25% concentration was added into
this solution under stirring until that the pH of 9.3 was reached. After
the precipitation completed, the solid sediment was aged at 60 °C
overnight. Subsequently, the obtained mixture was vacuum filtered off,
and the filter cake was washed repeatedly until no Cl− was detected by
using AgNO3 solution (20 mg/L) in the filtrate and then dried at 100 °C
to obtain zirconium hydroxide. Finally, tetragonal and monoclinic
118

S. Wang, et al.
MolecularCatalysis468(2019)117–124
flowing by means of the calibrated sample to titrate the O2. The amount
of chemisorbed H2 was obtained by subtracting the two isotherms, and
the dispersion was calculated by assuming H2 dissociative adsorption
on the Pb metallic atoms.
Temperature-programmed reduction of H2 (H2-TPR) was performed
using Micro TP-5076 chemisorption analyzer. Typically, catalyst was
first flushed with flowing N2 of 20 mL/min for 50 min at 400 °C and
then cooled to room temperature. Subsequently, the temperature was
heated up to 800 °C in flowing hydrogen-nitrogen mixture (containing
10 vol.% of hydrogen) at a heating rate of 10 °C/min, and the con-
sumption H2 was monitored with thermal conductivity detector. The
quantification of H2 was calculated by a calibration curve of CuO as
standard.
Ammonia temperature programmed desorption (NH3-TPD) was
conducted by the same instrument as that used for H2-TPR. Before
adsorption, the catalyst was first pretreated at 400 °C for 1.5 h under N2
flow. When the temperature cooled down to ambient temperature, NH3
was performed by
a flowing stream of 12 mL/min for 40 min.
Fig. 1. XRD patterns of pure and zirconia supported Pb-based catalysts.
Subsequently, N2 flow was introduced to remove physically adsorbed
ammonia, and then NH3 desorption was taken up to 800 °C at a heating
rate of 10 °C/min.
Pyridine-infrared spectroscopy (Py-IR) was carried out to detect the
acidic property of the catalysts. Sample was pressed into disks and
placed at a quartz cell, which was pretreated under vacuum for 3 h at
300 °C to evacuate adsorbed impurities. When the temperature cooled
down to ambient temperature, the sample was exposed to pyridine for
1 h and then outgassed for 2 h to remove the physically adsorbed pyr-
idine. Finally, FT-IR spectra were measured.
For MZ-Pb catalyst, monoclinic zirconia phases remain stable, but PbO
crystalline phases appear at ca. 29.1, 31.5 35.4, 45.6, 49.3, 55.5 and
60.1° whose partial diffraction peaks overlap with those of MZ, and this
reason is probably due to the different support properties of zirconia
polymorphs. Thus, it may be inferred that the introduction of lead has a
great influence on the crystalline organization of zirconia. The XRD
results indicate that the dispersion of PbO on TZ-Pb is higher than MZ-
Pb. For further confirmation, the lead dispersion degree is also de-
termined by hydro-oxygen titration and the result is listed in Table 1.
Clearly, TZ-Pb exhibits a higher dispersion degree than that of MZ-Pb,
which was consistent with the XRD analysis.
2.3. Catalytic performance evaluation
The disproportionation of MPC was conducted in three-necked
round-bottom flask connected to a liquid dividing head at atmosphere
pressure. Typically, under nitrogen atmosphere, 150 mmol MPC and
catalyst was added into the flask. When the reaction was heated up to
the desired temperature under stirring, the temperature was kept.
During the reaction procedure, DMC distillate was collected in a re-
ceiver flask to break the equilibrium limitation towards the formation
of DPC. After the reaction completion, the mixture was gradually
cooled down to room temperature. The catalyst was filtered, and the
products in the reaction mixture were detected by GC–MS analyses
using HP-6890/5973 gas chromatograph equipped with HP-5 capillary
column (30 m × 0.25 mm × 0.25 um). The conversion, yield and se-
lectivity were quantitatively analyzed using GC-7890 A gas chromato-
graph with HP-5 capillary column (30 m × 0.32 mm × 0.25 μm) and
FID detector.
The textural properties of all the samples determined by nitrogen
adsorption/desorption are listed in Table 1, and they exhibit great
difference in surface area, pore volume and pore size. The surface area
and pore volume of TZ-Pb remarkably decrease as comparison with
those of pure TZ support, but the pore size is almost unchanged, which
may be ascribed to the dispersion and deposition of PbO on the surface
of TZ. However, for MZ-Pb catalyst the surface area and pore volume
show nearly no change as comparison with MZ. It probably suggests
that PbO is not evenly distributed on MZ support due to relatively small
surface area, resulting in the low dispersion. Moreover, the surface
composition of TZ-Pb and MZ-Pb is determined by XRF technique, and
Table 1 displayed that the surface molar ratio of Pb to Zr over MZ-Pb is
larger than the nominal molar ratio of 0.082, further suggesting the
aggregation of PbO on MZ support, and it is well in line with the
characteristic peaks of PbO observed in the XRD patterns. Conse-
quently, the structural and textural differences for TZ-Pb and MZ-Pb are
distinctly associated with the different support properties of zirconia
polymorphs. In contrast, TZ-Pb reveals not only the higher dispersion of
PbO but also larger BET surface areas, and therefore it is favorable to
expose more active sites, contributing to the improvement of its
3. Results and discussion
3.1. Material characterization
3.1.1. Structural and textural characterization
Fig. 1 displays XRD patterns of pure and zirconia supported Pb-
based catalysts. When as-prepared zirconium hydroxide is calcined at
500 °C, the obtained TZ exhibits a purely tetragonal phase structure
(2θ = 30.3, 35.2, 50.2 and 60.2°, JCPDS No. 27-997) [30,31]. The
obtained MZ by calcination of 750 °C is predominantly monoclinic
phase structure (2θ = 24.1, 28.3, 31.5, 34.2 and 50.4°, JCPDS No. 37-
1484) and only a weak diffraction peak of tetragonal phase at
2θ = 30.3° is noticed. After lead addition, there is mostly the existence
of tetragonal phase for TZ-Pb and two weak diffraction peaks assigned
to monoclinic phase are observed at 28.3 and 31.5°, suggesting that the
introduction of lead results in the structure transformation of tetragonal
phase, while no diffraction peaks belonging to PbO crystalline phases
are detected, which suggest that the lead dispersion on TZ-Pb catalyst is
good, and lead oxide may be highly dispersed on the catalyst surface.
Table 1
The textural properties of pure and zirconia supported Pb-based catalysts.
Catalysts SBET
(m2/g)
Pore
Average
pore
diameter
(nm)
Pb/Zra Pb/Zrb Pb dispersionc
volume
(cm3/g)
TZ
TZ-Pb
MZ
43
0.127
0.099
0.126
0.101
7.0
7.0
20.0
20.0
–
–
–
34
15
13
0.082
–
0.091
–
0.016
–
MZ-Pb
0.082
0.103
0.007
a
Calculated by Pb and Zr stoichiometric molar ratio.
Measured by XRF.
Determined by hydro-oxygen titration method.
b
c
119

S. Wang, et al.
MolecularCatalysis468(2019)117–124
and high hydrogen consumption of peak α (as compared to peak β)
usually corresponds to high metal dispersed phase with small crystallite
size and the large interaction with support [32,33], which is in agree-
ment with the below TEM and XPS analysis. In contrast, TZ-Pb exhibits
the lower reduction temperature and larger hydrogen consumption of α
peak, and thus it possesses a stronger interaction between lead and
zirconia support, which would be favorable for improving the catalyst
stability during reaction. The results further testify that the difference
of reduction performance for TZ-Pb and MZ-Pb is largely dependent on
the surface property of zirconia support, which has great influence on
the surface nature of lead oxide and the interaction with zirconia.
3.1.3. TEM characterization
Fig. 3 presents the TEM pictures of TZ-Pb and MZ-Pb, and their
elemental distribution mapping images are also given. As shown in
Fig. 3, the catalysts show different morphological structures. TZ-Pb in
Fig. 3(A, B) mainly exhibits round-sheet-like morphological structures
with uniform and homogeneous particles, and most of them seem to
have smaller particle size (˜18 nm). While for MZ-Pb in Fig. 3(C, D), it
can be seen that MZ-Pb with larger particle size (˜45 nm) represents
flake-like crystals, which is remarkably different from TZ-Pb. The cat-
alytic activity is related to particle size, and a small size is beneficial to
the disproportionation of MPC. TZ-Pb has a small diameter with good
PbO dispersion, which is conducive to improving the catalytic activity
of the catalyst. The EDS spectrum further confirms the existence of Pb
on the surface of the supports (Fig. S1, Supplementary Information),
which evidently indicates that the Pb element is highly dispersed on the
surface of TZ-Pb in comparison with MZ-Pb, and this is also consist with
the result of XRD characterization.
3.1.4. XPS characterization
XPS spectra of the catalysts are conducted to further investigate the
element nature, and the results are revealed in Fig. 4. Fig. 4(A) displays
the deconvolution of Zr 3d of the catalysts, and TZ and MZ exhibit
clearly two characteristic peaks at around 181.8 and 184.2 eV, which
are attributed to Zr 3d5/2 and Zr 3d3/2 consecutively, which is in ac-
cordance with the literature reports [34,35], suggesting that only Zr4+
exists in the pure supports. Apparently, for TZ-Pb and MZ-Pb, the
binding energies are shifted towards higher values when the lead is
added, suggesting the formation of Pb − O−Zr bonds. Fig. 4(B) dis-
plays the deconvolution of Pb 4f in TZ-Pb and MZ-Pb, the binding en-
ergies centered at around 138.7 and 143.5 eV are observed, corre-
sponding to Pb 4f7/2 and Pb 4f5/2, respectively. According to literature
[36], the binding energy of Pb 4f7/2 and Pb 4f5/2 in PbO is 137.0 and
142.0 eV, and evidently the binding energies in TZ-Pb and MZ-Pb are
also shifted to higher values upon the Pb − O−Zr formation. Fig. 4(C)
displays the deconvolution of O 1 s of the catalysts, and two char-
acteristic peaks located at around 530.0 and 531.2 eV are exhibited in
TZ and MZ, which are ascribed to lattice oxygen of metal-oxides (such
as O2−) and adsorbed oxygen species (such as O- and O2−) on the
surface [37,38], respectively. Nevertheless, after the addition of lead,
the binding energies in TZ-Pb and MZ-Pb are shifted to the lower va-
lues. Based on these results, the reasons for the binding energy changes
of Zr, Pb and O in TZ-Pb and MZ-Pb can likely be accounted for electron
density variation due to the Pb − O−Zr formation, resulted in the
electron from Zr and Pb flowing to O because of higher electro-
negativity of O compared to Zr and Pb, indicating that there is an in-
teraction between lead and zirconia. In comparison, TZ-Pb exhibits the
larger binding energy variations, suggesting a stronger interaction be-
tween lead and zirconia support, which is in good conformity with H2-
TPR analysis.
Fig. 2. H2-TPR profiles of pure and zirconia supported Pb-based catalysts.
catalytic ability.
3.1.2. H2-TPR characterization
Fig. 2 displays the H2-TPR profiles of pure and zirconia supported
Pb-based catalysts. PbO shows a broad hydrogen consumption peak at
588 °C, which is assigned to PbO species reduction. Both pure TZ and
MZ present very weak hydrogen reduction peaks at the range from 450
to 600 °C that can hardly be observed, while TZ-Pb exhibits a broad but
well-defined peak in the range of 250–500 °C, therefore mainly PbO is
reduced. Likewise, MZ-Pb has a similar reduction pattern. For a clear
observation, they are integrated by Gaussian function fitting to calcu-
late their respective amounts in Table 2, respectively. Two peaks can be
observed at 350 °C (referred to as peak α) and 417 °C (peak β) for TZ-Pb
catalyst, corresponding to the highly dispersed lead species and bulk
lead oxide reduction [14,26]. And the hydrogen consumption of α peak
is approximately twice higher than that of β peak. Similar to that of TZ-
Pb, MZ-Pb shows also two reduction peaks at 358 °C (peak α) and
411 °C (peak β), but the hydrogen consumption of peak α is twice lower
than that of peak β. Generally, such low reduction temperature range
Table 2
The reduction performances of TZ-Pb and MZ-Pb determined by H2-TPR.
Catalysts Reduction temperature
(°C)
H2 consumption (mmol/ Total
g)
consumption
(mmol/g)
peak α
peak β
peak α
peak β
3.1.5. NH3-TPD characterization
NH3-TPD measurements are conducted to study the acid properties,
and the acid strength and acidity of the catalysts can be evaluated ac-
cording to their typical desorption peak. Fig. 5 displays the NH3-TPD
TZ-Pb
MZ-Pb
350
358
417
411
0.074
0.042
0.040
0.084
0.114
0.126
120

S. Wang, et al.
MolecularCatalysis468(2019)117–124
Fig. 3. TEM pictures of zirconia supported Pb-based catalysts: TZ-Pb (A, B) and MZ-Pb (C, D).
profiles of pure and zirconia supported Pb-based catalysts. There are
two notable NH3 desorption peaks, and the peak at low temperature of
around 160–490 °C represents weak acid site and the other at high
temperature in the range of around 500–750 °C represents strong acid
site [37,39], respectively. The areas of desorption peaks are linearly
proportional to the acid amount. For a clear comparison, the profiles
are deconvoluted by Gaussian function fitting, and the acid amount is
quantitatively calculated from the peak areas in Table 3. It can be seen
that this crystalline structure remarkably influences the acidity, and TZ
has higher surface acid amount than MZ. Interestingly, after introdu-
cing lead into TZ and MZ, the acid amounts of both TZ-Pb and MZ-Pb
decrease. The reasons are probably that PbO possesses both Lewis
acidic and basic properties, and lead could be easily combined or co-
ordinated with Zr by Lewis acid-base interactions through Zr–O–Pb
bond, resulted in the decrease of acid amount. Moreover, the larger
surface acid amount of TZ than MZ suggests that it can be more readily
interacted with Pb species after impregnating, thereby causing the
higher dispersion of PbO as well as forming the stronger interaction
between lead and TZ support, this analysis is in good accordance with
the above discussion of XRD, H2-TPR and XPS results. Although the acid
amounts decrease, the total acid amount in the TZ-Pb is remarkably
higher than that of the MZ-Pb catalyst, which is conductive to adsorb
and activate MPC molecules, improving the catalytic activity.
those of MZ and MZ-Pb. As a result, the results of Py-IR and NH3-TPD
suggest that TZ and TZ-Pb own much higher Lewis acidity, contributing
to MPC disproportionation because this reaction was Lewis acid cata-
lysis process.
3.2. Results of the catalytic evaluation for MPC disproportionation
Fig. 7 displays the results of MPC disproportionation over the cat-
alysts. This crystalline form strongly influences the catalytic activity of
Pb-based catalysts. Pure TZ and MZ supports are found to be weak
catalytic activities under the present experimental conditions, and the
introduction of lead after impregnating shows noticeable improved
catalytic activities, implying that lead is the active component phases
while TZ and MZ mainly act as the catalyst supports although they
possess larger BET surface area and higher acid amount compared to
supported Pb-based catalysts. For TZ-Pb and MZ-Pb, the MPC conver-
sion and DPC selectivity are 65.1%, 99.1% and 48.8%, 98.4% respec-
tively, indicating that TZ-Pb exhibits better catalytic performance.
Moreover, the TON value of TZ-Pb reaches up to 140 mol MPC/mol Pb
for MPC conversion obtained at 50 min, which is also obviously higher
than 103 mol MPC/mol Pb of MZ-Pb. The results suggest that the TZ-Pb
catalyst is more active than MZ-Pb, and thereby tetragonal zirconia is
more suitable crystalline phase structure for MPC disproportionation
compared to the monoclinic one.
In view of the characteristic results of Py-IR and NH3-TPD, the
catalytic activities for pure TZ and MZ supports are probably resulted
from the presence of the acidic sites on its surface. Moreover, TZ reveals
relatively higher catalytic activity compared to MZ because TZ has
more numbers of Lewis acid sites. As the studies in previous work [18],
the surface area and interaction of Pb and Zr play key role in the cat-
alytic activity for supported lead catalysts. Thus, from the above
characterized analysis results, TZ-Pb owns higher dispersion of PbO and
larger surface area, which is very advantageous to resulting in more
exposure of Lewis acid active sites to accessibly adsorb and activate
MPC, thereby behaving better catalytic activity.
3.1.6. Py-IR characterization
To obtain the information of Brønsted and Lewis acidity, the pyr-
idine-infrared spectroscopy of the catalysts is detected. Fig. 6 displays
the Py-IR spectra of zirconia supported Pb-based catalysts, and the
concentration ratio of Brønsted to Lewis acidity is given in Table 3. The
infrared absorptions at 1590 and 1440 cm−1 are assigned to pyridine
bound interacting with Brønsted and Lewis acid sites, respectively
[34,40,41]. TZ displays a remarkable absorption peak at 1440 cm−1
and a weak absorption peak at 1590 cm−1, while MZ exhibits two ob-
vious absorptions peak, indicating that TZ possesses mainly the pre-
sence of Lewis acidic sites, and MZ contains both Brønsted and Lewis
acid sites, which is in line with literature reported previously [21]. By
the addition of lead, the acidic centers do not change remarkably for
MZ-Pb, whereas for TZ-Pb they are affected that the peak intensities
increase obviously, leading to an increased acidity on the surface. As
can be seen in Table 3, the ratios of B/L for TZ and TZ-Pb are lower than
3.3. Recycling of the catalyst
The recycling of catalyst is a significant requirement for practical
application. Therefore, the recyclability of the catalyst is examined, and
121

S. Wang, et al.
MolecularCatalysis468(2019)117–124
Fig. 4. XPS spectra of pure and zirconia supported Pb-based catalysts.
122

S. Wang, et al.
MolecularCatalysis468(2019)117–124
Fig. 7. Catalytic results of MPC disproportionation over the catalysts. Reaction
conditions: catalyst 1.2 g, MPC 150 mmol, 200 °C, 2.5 h. Anisole is the trace by-
product existence.
Fig. 5. NH3-TPD profiles of pure and zirconia supported Pb-based catalysts.
Table 3
interaction.
The acidity of pure and zirconia supported Pb-based catalysts determined by
NH3-TPD.
Besides, TZ-Pb and MZ-Pb after the forth use are also detected by
XRD in Fig. 9. The results show that the diffraction peak of used TZ-Pb
and MZ-Pb is basically the same patterns as the respective freshly
prepared catalysts, suggesting that no structural changes occur on the
surface of the catalysts, and thereby tetragonal zirconia is more active
and stable crystalline structure for MPC disproportionation.
Catalysts Acid strengths (°C)
Acid amounts
(mmol/g)
Total acid
amounts
(mmol/g)
B/L ratio
Weak
Strong
Weak
Strong
TZ
TZ-Pb
MZ
394
346
404
421
671
707
716
725
0.054
0.068
0.056
0.019
0.114
0.047
0.040
0.034
0.168
0.115
0.096
0.053
0.410
0.983
1.166
1.104
MZ-Pb
4. Conclusions
In this paper, zirconia polymorph inflicts great difference on the
structural properties of zirconia supported Pb-based catalysts.
Moreover, the catalytic activities are also strongly influenced for the
disproportionation of MPC, and tetragonal zirconia is more active and
stable crystal phase compared to the monoclinic one. The conversion of
MPC and yield of DPC reach 65.1% and 64.5% over TZ-Pb, which is
significantly higher than that of MZ-Pb. And the TON value over TZ-Pb
is also obviously higher than that of MZ-Pb. The characterization results
indicate that the superior catalytic performance of TZ-Pb is related to its
good dispersion degree of PbO, large specific surface area and high
Lewis acidic active sites, which are relevant and together contribute to
the improvement of catalytic activity. Additionally, TZ-Pb shows also
better stability due to stronger metal-support interaction that after four
consecutive runs there is no significant change in MPC conversion and
DPC yield. Thus, this work has provided a prospective reference for the
extensive utility of zirconia polymorph materials in broad catalysis
areas.
Fig. 6. Py-IR spectra of zirconia supported Pb-based catalysts.
Acknowledgments
This work was supported by the National High Technology Research
and Development Program of China (863 program, No.
2013AA031703), the National Natural Science Foundation of China
(No. 21808048), the Key Project of Science and Technology Program of
Henan Province (182102210050), the Project funded by China
Postdoctoral Science Foundation (2018M632782), the Science Research
Start-up Fund of Henan Institute of Science and Technology (2015031),
the Key Scientific Research Project of Colleges and Universities in
Henan Province (17A150026), the Postdoctoral Start-up Fund of Henan
Institute of Science and Technology, the Project funded by Postdoctoral
Science Foundation of Henan Province, and the Project funded by
Natural College Student Innovation and Entrepreneurship Training
(S201810467007).
after each run completion the used catalyst is recovered by filtrating
and washing with DMC, and then dried at 110 °C overnight. The results
are illustrated in Fig. 8. The MPC conversion and DPC yield have no
significant change over TZ-Pb in the four consecutive runs, never-
theless, for MZ-Pb, the catalytic activity decreases slightly from 48.8%
to 43.5%, suggesting that TZ-Pb shows better stability compared to MZ-
Pb. Elemental analysis of used TZ-Pb and MZ-Pb obtained by XRF
presents that 0.8% and 1.5% Pb are leached out from fresh TZ-Pb and
MZ-Pb after using four times, respectively, suggesting that TZ-Pb is
more intact and there is little leaching loss of Pb during the reaction.
Combining with the H2-TPR and XPS analysis, the excellent recycl-
ability of TZ-Pb is probably resulted from its stronger metal-support
123

S. Wang, et al.
MolecularCatalysis468(2019)117–124
Fig. 8. Recycling of TZ-Pb and MZ-Pb for MPC disproportionation.
(2018) 779–784.
[11] S. Han, M. Luo, X.L. Zhou, Z. He, L.P. Xiong, Ind. Eng. Chem. Res. 51 (2012)
5433–5437.
[12] Y.T. Kim, E.D. Park, Appl. Catal. A Gen. 356 (2009) 211–215.
[13] Y. Gao, Z.H. Li, K.M. Su, B.W. Cheng, Chem. Eng. J. 301 (2016) 12–18.
[14] W.Q. Zhou, X.Q. Zhao, Y.J. Wang, J.Y. Zhang, Appl. Catal. A Gen. 260 (2004)
19–24.
[15] W.B. Kim, J.S. Lee, J. Catal. 185 (1999) 307–313.
[16] S.L. Wang, Y.Z. Zhang, Y. Chen, R.Z. Tang, T. Chen, G.Y. Wang, Chem. J. Chinese U.
35 (2014) 2177–2181.
[17] W.B. Kim, Y.G. Kim, J.S. Lee, Appl. Catal. A Gen. 194/195 (2000) 403–414.
[18] S.L. Wang, C.G. Li, Z.L. Xiao, T. Chen, G.Y. Wang, J. Mol. Catal. A Chem. 420 (2016)
26–33.
[19] Q. Fu, Y.C. Ou, Y. Zeng, G.Y. Wang, Chem. Ind. Eng. Prog. (Chinese) 36 (2017)
2748–2755.
[20] K.H. Jacob, E. Knözinger, S. Benier, J. Mater. Chem. 3 (1993) 651–657.
[21] Y.B. Zhao, W. Li, M.H. Zhang, K.Y. Tao, Catal. Commun. 3 (2002) 239–245.
[22] A. Ulgen, W. Hoelderich, Catal. Lett. 131 (2009) 122–128.
[23] W. Stichert, F. Schüth, S. Kuba, H. Knözinger, J. Catal. 198 (2001) 277–285.
[24] Z.H. Li, K.M. Su, J. Mol. Catal. A Chem. 277 (2007) 180–184.
[25] S.R. Ginjupalli, S. Mugawar, N.P. Rajan, P.K. Balla, V.R.C. Komandur, Appl. Surf.
Sci. 309 (2014) 153–159.
[26] Z.Y. Ma, C. Yang, W. Wei, W.H. Li, Y.H. Sun, J. Mol. Catal. A Chem. 231 (2005)
75–81.
Fig. 9. XRD patterns of fresh and used catalysts after four times run.
[27] J.C. Yori, J.M. Parera, Catal. Lett. 65 (2000) 205–208.
[28] H.R. Mahmoud, J. Mol. Catal. A Chem. 392 (2014) 216–222.
[29] M. Wu, C.L. Li, J. Zhang, C.C. Miao, Y.P. Zheng, Y.M. Sun, Ind. Eng. Chem. Res. 51
(2012) 6304–6309.
Appendix A. Supplementary data
[30] J.F.S. de Oliveira, D.P. Volanti, J.M.C. Bueno, A.P. Ferreira, Appl. Catal. A Gen. 558
(2018) 55–66.
[31] S.V.H.S. Bhaskaruni, S. Maddila, W.E. van Zyl, S.B. Jonnalagadda, Catal. Today 309
(2018) 276–281.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.02.021.
[32] S. Tada, K. Larmier, R. Buchel, C. Coperet, Catal. Sci. Technol. 8 (2018) 2056–2060.
[33] X.L. Zhang, D.F. Wang, G.D. Wu, X.L. Wang, X.X. Jiang, S.S. Liu, D.G. Zhou,
D.M. Xu, J. Gao, Appl. Catal. A Gen. 555 (2018) 130–137.
[34] S. Pyen, E. Hong, M. Shin, Y.W. Suh, C.H. Shin, Mol. Catal. 448 (2018) 71–77.
[35] H.T. Wu, Y. Yang, H.Y. Suo, M. Qing, L. Yan, B.S. Wu, J. Xu, H.W. Xiang, Y.W. Li, J.
Mol. Catal. A Chem. 396 (2015) 108–119.
[36] Z.H. Li, Y.J. Wang, X.S. Ding, X.Q. Zhao, J. Nat. Gas Chem. 18 (2009) 104–109.
[37] B.H. Jia, J.X. Guo, H.D. Luo, S. Shu, N.J. Fang, J.J. Li, Appl. Catal. A Gen. 553
(2018) 82–90.
References
[1] M.C. Figueiredo, V. Trieu, S. Eiden, J. Heijl, M.T.M. Koper, ACS Catal. 8 (2018)
3087–3090.
[2] E.R. Baral, J.H. Lee, J.G. Kim, J. Org. Chem. 83 (2018) 11768–11776.
[3] S. Bahrami, S.N. Khorasani, A. Abdolmaleki, S.M. Davoodi, A. Farzan, J. Elastom.
Plast. 50 (2018) 107–123.
[4] G. Fiorani, A. Perosa, M. Selva, Green Chem. 20 (2018) 288–322.
[5] A. Gawdzik, A. Mederska, T. Mederski, Polym. Bull. 74 (2017) 1553–1571.
[6] G. Contreras-Zarazúa, J.A. Vázquez-Castillo, C. Ramírez-Márquez, G.A. Pontis,
J.G. Segovia-Hernández, J.R. Alcántara-Ávila, Energy 165 (2017) 637–649.
[7] R. Kaneg, H. Ogihar, I. Yamanaka, Catal. Sci. Technol. 6 (2016) 6002–6010.
[8] S.H. Huang, B. Yan, S.P. Wang, X.B. Ma, Chem. Soc. Rev. 44 (2015) 3079–3116.
[9] G.Z. Fan, H.T. Zhao, Z.X. Duan, T. Fang, M.H. Wan, L.N. He, Catal. Sci. Technol. 1
(2011) 1138–1141.
[38] X.Y. Zhang, X.X. Cheng, C.Y. Ma, Z.Q. Wang, Catal. Sci. Technol. 8 (2018)
3336–3345.
[39] D.F. Wang, X.L. Zhang, X.S. Cong, S.F. Liu, D.G. Zhou, Appl. Catal. A Gen. 555
(2018) 36–46.
[40] W. Hertl, Langmuir 5 (1989) 96–100.
[41] M. Massa, A. Andersson, E. Finocchio, G. Busca, F. Lenrick, L.R. Wallenberg, J.
Catal. 297 (2013) 93–109.
[10] C.Z. Wang, T. Guo, M.M. Sun, C.H. Li, M. Guo, C.L. Li, Q. Wang, Sci. Adv. Mater. 10
124