Highly effective transformation of methyl phenyl carbonate to diphenyl carbonate with recyclable Pb nanocatalyst

Article abstract of DOI:10.1039/c9ra03931g

Diphenyl carbonate (DPC) is a type of versatile industrial chemical, and the disproportionation of methyl phenyl carbonate (MPC) is a key step to produce DPC. However, the design and formulation of a catalyst for the efficient synthesis of DPC is a major challenge due to its small equilibrium constant. The support material is a critical factor influencing the performance of Pb nanocatalysts. Thus, a series of Pb-based catalysts over MgO, ZrO₂, SiO₂, TiO₂ and Al₂O₃ were prepared to investigate the effect of the support materials on the physicochemical properties and catalytic performances for the conversion of MPC to effectively synthesize DPC. The catalysts were well characterized by XRD, BET, TEM, XPS, ICP-OES, H₂-TPR, Py-IR and NH₃-TPD. The results showed that the nature of the support obviously affected the structural properties and catalytic performances, and Pb was dispersed better on SiO₂, TiO₂, ZrO₂ and MgO than on Al₂O₃, and showed stronger metal-support interaction over MgO and ZrO₂. The activity results revealed that PbO/MgO and PbO/ZrO₂ exhibited higher catalytic activities because they contained higher Pb dispersion and more Lewis acid sites, and the catalytic activities followed the order PbO/MgO > PbO/ZrO₂ > PbO/SiO₂ > PbO/Al₂O₃ > PbO/TiO₂. On the contrary, PbO/MgO and PbO/ZrO₂ exhibited better reusability due to strong interaction between the highly dispersed Pb and the supports, and the activity decrease in the case of PbO/SiO₂, PbO/Al₂O₃ and PbO/TiO₂ mainly resulted from the Pb leaching loss. This work would contribute to exploiting novel catalytic materials in a wide range of applications for the efficient synthesis of organic carbonates.

Full text of DOI:10.1039/c9ra03931g

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PAPER
Highly effective transformation of methyl phenyl
carbonate to diphenyl carbonate with recyclable Pb
nanocatalyst†
Cite this: RSC Adv., 2019, 9, 20415
c
Songlin Wang,ab Hongying Niu,a Jianji Wang,b Tong Chen,
Gongying Wangc
*
and Jiamin Zhanga
Diphenyl carbonate (DPC) is a type of versatile industrial chemical, and the disproportionation of methyl
phenyl carbonate (MPC) is a key step to produce DPC. However, the design and formulation of a catalyst
for the efficient synthesis of DPC is a major challenge due to its small equilibrium constant. The support
material is a critical factor influencing the performance of Pb nanocatalysts. Thus, a series of Pb-based
catalysts over MgO, ZrO2, SiO2, TiO2 and Al2O3 were prepared to investigate the effect of the support
materials on the physicochemical properties and catalytic performances for the conversion of MPC to
effectively synthesize DPC. The catalysts were well characterized by XRD, BET, TEM, XPS, ICP-OES, H2-
TPR, Py-IR and NH3-TPD. The results showed that the nature of the support obviously affected the
structural properties and catalytic performances, and Pb was dispersed better on SiO2, TiO2, ZrO2 and
MgO than on Al2O3, and showed stronger metal-support interaction over MgO and ZrO2. The activity
results revealed that PbO/MgO and PbO/ZrO2 exhibited higher catalytic activities because they
contained higher Pb dispersion and more Lewis acid sites, and the catalytic activities followed the order
PbO/MgO > PbO/ZrO2 > PbO/SiO2 > PbO/Al2O3 > PbO/TiO2. On the contrary, PbO/MgO and PbO/ZrO2
exhibited better reusability due to strong interaction between the highly dispersed Pb and the supports,
and the activity decrease in the case of PbO/SiO2, PbO/Al2O3 and PbO/TiO2 mainly resulted from the Pb
leaching loss. This work would contribute to exploiting novel catalytic materials in a wide range of
applications for the efficient synthesis of organic carbonates.
Received 24th May 2019
Accepted 12th June 2019
DOI: 10.1039/c9ra03931g
rsc.li/rsc-advances
(phosgene method,6,7 oxidative carbonylation,8,9 trans-
esterication,10,11 etc.), the transesterication of phenol and
1. Introduction
dimethyl carbonate (DMC) was regarded as a most likely
pathway to synthesize DPC without employing phosgene since
the starting materials presented low toxicity and high biode-
gradability. This route consisted in two-step reaction where
methyl phenyl carbonate (MPC) of the intermediate was rst
produced by the transesterication (Scheme 1-(1)), followed by
the further transesterication reaction of MPC and phenol
Due to its outstanding electrical, mechanical and heat resis-
tance properties, polycarbonate (PC), as an important engi-
neering thermoplastic for ubiquitous applications in several
industries (automobiles, office equipment, medical devices,
etc.), has attracted considerable interest in the last decades.1,2
Diphenyl carbonate (DPC),
a building block, was oen
employed for the phosgene-free synthesis of PC by melt trans-
esterication with bisphenol-A, and was also used for the
production of pharmaceuticals, polymer materials, ne chem-
icals, etc.3–5 It was, therefore, necessary to develop efficient and
environmentally benign synthetic routes to replace the highly
toxic synthesis via phosgene. Among the alternative routes
aSchool of Chemistry and Chemical Engineering, Henan Institute of Science and
Technology, Xinxiang 453003, P. R. China. E-mail: wangsonglin2009@163.com
bPostdoctoral Programs, Key Laboratory of Green Chemical Media and Reactions,
Ministry of Education, School of Chemistry and Chemical Engineering, Henan
Normal University, Xinxiang 453007, P. R. China. E-mail: jwang@htu.cn
cChengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu
610041, P. R. China. E-mail: chentongw@sina.com.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra03931g
Scheme 1 Transesterification process between DMC and phenol into
DPC.
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(Scheme 1-(2)) or the disproportionation of MPC (Scheme 1-(3)).
Concerning the second-step reaction the equilibrium constants
K2 and K3 indicated that DPC was mainly produced by the
second step disproportionation instead of the further trans-
esterication.12,13 On the contrary, the rst step was rate-
determining step since K1 was much lower than K3. Hence,
considerable studies on the rst step transesterication have
been particularly carried out,14–17 but only few efforts have been
made to the second step disproportionation.18,19 Indeed, the
disproportionation of MPC is also the key step to produce DPC.
Overall, it is very signicant to intensively study on MPC
disproportionation, which is advantageous to improving the
yield of DPC and the conversion of the starting materials in
future industrial applications.
The disproportionation reaction suffered from the low yield
and selectivity of DPC due to the relatively low equilibrium
constant and reaction rate. Therefore, the development of
highly active catalysts is crucial in this reaction. Among various
homogeneous catalytic systems reported like organo-tin/
titanium catalysts, even though they exhibited outstanding
catalytic activities,19,20 they had received little consideration
because of the separation problems between DPC and the
catalysts. Therefore, heterogeneous catalysis currently attracted
more attention because these catalysts might be facilely recov-
ered from the reaction mixture by simple ltration and could be
recycled, making the procedure more feasible.
Recently, we studied the catalytic performances of various
metal oxides including TiO2, MoO3, PbO and so on,21–23 and
found that PbO was prepared into the supported Pb catalysts or
mixed metal oxides signicantly improved the catalytic activity,
life time and stability.24,25 However, the nature and property of
the support is oen found to modify the distribution, loading,
size, shape and electronic state of the active species as well as
their interactions with support with some special physical–
chemical properties, which is well known to strongly affect the
catalytic activity and stability.26–31 Therefore, the study to
investigate the structure–function relationship of the supports
on Pb-based catalysts for MPC disproportionation is very
worthwhile, which is conducive to screen suitable and prom-
ising supports.
2. Experimental section
2.1 Material preparation
All chemicals adopted in this work were analytical purity, which
were purchased from Sinopharm Chemical Reagent Co., Ltd
and were directly used as received without further purication.
Deionized water was used in all experimental synthesis proce-
dures as needed.
Pure SiO2 was synthesized by sol–gel process. For this
preparation, the desired amounts of tetraethyl orthosilicate,
ethanol and distilled water were mixed together at room
temperature. Then the ammonium aqueous solution was added
gradually into the above solution under stirring at 50 ꢀC ꢀto
generate a transparent gel. The obtained gel was aged at 50 C
overnight, dried at 110 ꢀC for 12 h and subsequently calcinated
ꢀ
550 C for 5 h.
Pure TiO2 was prepared by hydrolysis process. Briey,
a proper amount of tetrabutyl titanate was dissolved in ethanol,
and heated up to 50 ꢀC under stirring. Then, appropriate
amount of water was slowly added to promote the hydrolysis
and was kept for 24 h. Finally, the sediment was ltered, washed
with diꢀstilled water, and dried at 110 ꢀC for 12 h and calcinated
at 550 C for 5 h.
Pure Al2O3 was prepared by a precipitation process. A certain
amount of aluminum nitrate nonahydrate were dissolved in the
deionized water and heated slowly to 90 ꢀC. Aerwards,
ammonia solution with 25% concentration was added into this
solution under stirring until that the pH of 9–10 was reached.
The as-prepared solid sediment was aged for 24 h and then
vacuum ltered off and washed repeatedly. Finally, the obtained
ꢀ
samples were dried at 110 C for 12 h and subsequently calci-
nated at 550 ꢀC for 5 h. Likewise, pure MgO and ZrO2 were
prepared by using the similar process, respectively.
The preparation of supported Pb-based catalysts was carried
out by incipient wetness impregnation process. Briey, SiO2,
TiO2, Al2O3, ZrO2 and MgO supports were homogeneously
impregnated by using the aqueous solution of lead nitrate as
precursor salt at ambient temperature for 24 h, respectively.
Aer impregnation, the samples were dried in air at 110 ꢀC for
12 h, and nally calcinated at 550 ꢀC for 5 h. And the PbO
loading used was 10 wt% for all the catalysts.
In the present work, we formulate Pb-based catalysts by
employing potential metal oxides as support phases and focus
on studying the effect of various supports on the physico-
chemical properties of the catalysts and their catalytic perfor-
2.2 Material characterization
mance to provide the role of support materials for developing The X-ray powder diffraction (XRD) analysis was carried out on
efficient catalysis materials for MPC disproportionation to a DX-2700B diffractometer with the Ni-ltered CuKa radiation
˚
effectively synthesize DPC. Herein the Pb-based catalysts are (1.5418 A). The X-ray tube was operated at 40 kV and 30 mA, and
prepared using wet-impregnation process over PbO loading the powder diffractogram was recorded at 0.02ꢀ inꢁte1rvals in the
onto SiO2, TiO2, Al2O3, MgO and ZrO2 supports. At the same range of 5–90ꢀ with the scanning rate of 1.2ꢀ min
.
time, the catalysts are thoroughly characterized by X-ray
The determination of the elemental composition was con-
diffraction (XRD), temperature programmed reduction of H2 ducted by Optima 2000 DV inductively coupled plasma-optical
(H2-TPR), transmission electron microscopy (TEM), X-ray emission spectrometer (PerkinElmer Inc., USA).
photoelectron spectrum (XPS), N2 adsorption/desorption
N2 adsorption–desorption isotherms were measured at
ꢀ
measurement, pyridine-infrared spectroscopy (Py-IR) and liquid nitrogen temperature (ꢁ196 C) using a 3H-2000PS2
temperature programmed desorption of NH3 (NH3-TPD) to adsorption instrument (BeiShiDe Instrument). Before the
clarify the support role on the Pb-based catalysts. In addition, measurements, the catalysts were pretreated at 250 ꢀC for
the stabilities of these catalysts are also investigated in detail.
6 h under a high vacuum environment to remove physically
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adsorbed water. The specic surface area was calculated reaction was decreased down to ambient temperature. The
using the Brunauer–Emmett–Teller (BET) method and the reaction mixture was analyzed by GC-MS instrument (HP-6890/
pore volume and porous distribution was derived from 5973) equipped with HP-5 capillary chromatography packed
Barrett–Joyner–Halenda (BJH) equation.
column. Subsequently, the products were quantitatively
The morphology of the catalysts was investigated by trans- analyzed with a gas chromatograph (GC-7890A) equipped with
mission electron microscopy (JEM-1011) with a eld-emission HP-5 capillary column (30 m ꢂ 0.32 mm ꢂ 0.25 mm) and
gun operating at 200 kV. The samples were prepared by drop- a ame ionization detector (FID).
ping an ethanol suspension of the powder particles on a carbon
lm supported copper grid.
3. Results and discussion
X-ray photoelectron spectrum (XPS) was employed to detect
the chemical composition and states of the atoms on the cata-
lysts, and conducted on a ESCALAB250Xi X-ray photoelectron
spectrometer with C as the internal standard (C 1s ¼ 284.7 eV).
H2-TPR experiment was carried out on a Micro TP-5076
chemisorption analyzer. Prior to the reduction, the catalyst
(about 50 mg) was pretreated in N2 stream (20 mL minꢁ1) at
300 ꢀC for 1.5 h and then cooled to room temperature. Aer
that, the owing H2–N2 mixture (10% H2 by volume) was
switched on, and the temperature was gradually increased from
50 to 800 ꢀC with a ramp of 10 ꢀC minꢁ1. The H2 consumption
was measured with a thermal conduction detector (TCD) that
had been calibrated using the standard sample of CuO.
3.1 Characterization of the catalysts
3.1.1 BET characterization. The BET surface area, pore
volume and pore size distribution determined by nitrogen
adsorption–desorption measurement for all the catalysts and
their corresponding supports were listed in Table 1. The average
pore diameters for these materials were in the mesoporous
range, and the surface area and pore volume of the catalysts
decreased in comparison to the corresponding supports. It was
noted that the SiO2 support displayed the highest BET surface
area among all support materials, moreover, aer impreg-
nating, the PbO/SiO2 catalyst also exhibited a larger BET specic
surface area and pore volume in contrast to PbO/MgO, PbO/
Al2O3, PbO/ZrO2 and PbO/TiO2, and the orders of both the
surface area and pore volume for them were as follows: PbO/
SiO2 > PbO/ZrO2 > PbO/MgO > PbO/TiO2 > PbO/Al2O3, in the
same orders as that for the pure supports. The average pore
diameters of the catalysts also decreased than those of the pure
supports. This was probably due to PbO being dispersed on the
surface of these supports or lled in the pore structure aer the
lead impregnating, which might result in the decrease of these
textural properties.
3.1.2 XRD characterization. To detect the structures of Pb
species on various supports, XRD measurement were performed
for both the catalysts and the pure supports. As could be seen in
Fig. 1, XRD patterns of pure supports exhibited their respective
characteristic peaks, while there were some differences with
their corresponding Pb-based catalysts among the character-
istic peaks. For instance, SiO2 exhibited a broad peak at around
22ꢀ, and aer PbO loading on its surface no PbO crystalline
phase diffraction peak was observed for PbO/SiO2. The diffrac-
tion peak at 25.4, 37.9, 48.1, 54.1 and 62.5ꢀ were attributed to
The number of acid sites was determined by NH3-TPD using
the same instrument as that used for H2-TPR. Before the
adsorption of NH3, the sample (about 100 mg) was rst treated
at 300 ꢀC for 1.5 h and then decreased to 50 ꢀC in a N2 ow of 20
mL minꢁ1. Aer the saturated adsorption of NH3 (10 mL minꢁ1
)
at 50 ꢀC for 30 min, the physically adsorbed NH3 was subse-
quently degassed from the sample for 1 h under the N2 ow (20
mL minꢁ1). Finally, the sample was heated to 800 ꢀC with a rate
of 10 ꢀC minꢁ1, the amounts of desorbed NH3 was monitored by
the thermal conduction detector (TCD). The TPD prole was
decomposed into Gaussian curves to quantify the weak,
medium and strong acid sites.
Py-IR spectra were measured on a Thermo Nicolet 380 FT-IR
spectrometer to determine the type of acid sites. Before the
measurement, the impurity was removed from the sample
under 10ꢁ2 Pa condition at 200 C for 2 h, and then pyridine
ꢀ
vapor was introduced into the sample for the saturated
adsorption. Aer the evacuation at 200 ꢀC for 1 h in owing He
atmosphere, the Py-IR spectra were subsequently collected at
room temperature.
Table 1 Results of BET measurements for the catalysts and corre-
sponding supports
2.3 Catalytic reaction and the product analysis
MPC was synthesized by the reversible disproportionation of
DMC and DPC, and its purity was 99.8% determined by high
performance liquid chromatography.20 In a typical experiment,
the catalytic performance test of MPC was performed in a three-
necked glass beaker at atmospheric pressure. MPC of 150 mmol
and a proper amount of the catalysts were placed into the
beaker under a owing nitrogen and slowly heated under
vigorous stirring. When the reaction temperature was reached,
it was kept for the reaction. As the reaction processed, DMC
might be distilled out and continuously collected in a ask
beaker to promote the equilibrium reaction towards the
production of DPC. Upon the completion of the reaction, the
BET surface
area (m2 gꢁ1
Total pore volume
Average pore
diameter (nm)
Catalysts
)
(cm3 gꢁ1
)
PbO/MgO
MgO
PbO/Al2O3
Al2O3
PbO/ZrO2
ZrO2
PbO/TiO2
TiO2
34.5
36.9
20.7
24.8
43.3
44.3
30.3
33.4
67.3
95.4
0.157
0.182
0.049
0.070
0.106
0.098
0.073
0.074
0.522
0.747
9.0
11.3
7.9
11.6
8.2
8.8
9.9
11.3
11.1
12.3
PbO/SiO2
SiO2
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Fig. 1 XRD patterns of the catalysts and corresponding supports.
anatase TiO2, and analogously the characteristic peak ascribed
to PbO phases was absent for PbO/TiO2. ZrO2 displayed the
existence of monoclinic (2q ¼ 30.3ꢀ) and tetragonal (2q ¼ 24.3
and 31.5ꢀ) zirconia phases, and PbO/ZrO2 exhibited the same
structure with that of ZrO2 support. The diffraction peak at 25.5ꢀ
belonging to g-Al2O3 support was clearly observed,32,33 while
PbO crystalline phases appeared at 20.8, 25.4, 26.6, 27.7 and
29.7ꢀ for PbO/Al2O3. PbO/MgO exhibited also the same crystal-
line structure with those of MgO support, and no PbO diffrac-
tion peak was observed. The XRD results suggested that the PbO
active phases had better dispersion on the surface of SiO2, TiO2,
ZrO2 and MgO supports in an amorphous state than that on
Al2O3.
Fig. 2 H2-TPR profiles of the catalysts and corresponding supports.
reduction peak shied to a higher temperature and this was
mainly due to the poor interaction between PbO and the TiO2
support. Thus, the results suggested that the interaction
strengths between the active component and the different
supports were remarkably different.
3.1.3 H2-TPR characterization. H2-TPR measurement was
conducted to investigate the reduction behaviors of PbO species
over the catalysts. The H2-TPR proles of both the catalysts and
pure supports were shown in Fig. 2. Firstly, for SiO2, ZrO2 and
MgO supports, there were no evident reduction peak appear-
ances in the temperature ranges of 100–800 ꢀC. By contrast, the
reduction characteristic peaks at 730 and 781 ꢀC were observed
for pure TiO2 and Al2O3, respectively. Secondly, for various Pb-
based catalysts, only one distinct reduction characteristic
peak was observed in PbO/SiO2, PbO/TiO2, PbO/ZrO2 and PbO/
MgO respectively, suggesting that mainly PbO was reduced. As
reported in the literatures,25,34 pure PbO revealed a clear
Thirdly, for PbO/Al2O3, ve reduction characteristic peaks
ꢀ
appeared. The rst reduction peak at 595 C corresponded to
the surface PbO specie reduction, and the second reduction
peak at 676 ꢀC might result from the reduction of the bulk PbO,
as the XRD results displayed the existence of PbO crystalline
diffraction peaks, which was primarily assigned to the weak
interaction between PbO and the Al2O3 support so that the PbO
species was difficult to reduce at a higher reduction tempera-
ture. The following three peaks with reduction temperatures at
702, 739 and 760 ꢀC were probably associated with the reduction
of the Pb–Al mixed species formed via the interaction at high
ꢀ
ꢀ
temperature (>700 C) of partially reduced Pb-species and the
reduction peak of 588 C, which was assigned to PbO species
support.35,36 Overall, according to the reduction temperatures of
PbO species, it could be inferred that the interaction strengths
between PbO and the supports were in the following order of
PbO/ZrO2 > PbO/MgO > PbO/SiO2 > PbO/TiO2 > PbO/Al2O3,
which might lead to different catalytic properties.
reduction.ꢀTherefore, these peaks in PbO/SiO2 (552 ꢀC), PbO/
ꢀ
ꢀ
TiO2 (591 C), PbO/ZrO2 (348 C) and PbO/MgO (410 C) were
typically ascribed to the reduction of highly dispersed surface
PbO species, nevertheless, it was noteworthy that there were
also obvious differences in the peak positions for them, sug-
gesting that PbO loading onto different supports had a great
inuence on the reduction temperature of PbO species. Notably,
the reduction temperature of PbO species on SiO2, ZrO2 and
MgO supports were shied to lower temperatures. This was
mainly due to the strong interaction between PbO species and
the supports which was favorable for the stability of the cata-
lysts during reaction. Whereas in the case of PbO/TiO2 the
3.1.4 TEM characterization. Fig. 3 displayed the TEM
images of the catalysts. As shown in Fig. 3(A and a), PbO/SiO2
mainly represented the spherical-like structure, and the PbO
particles might be clearly observed, indicating that they were
highly dispersed on the surface of the SiO2. For PbO/TiO2, PbO/
ZrO2, PbO/Al2O3 and PbO/MgO displayed in Fig. 3(B, b)–(E, e),
their morphology were remarkably different from each other.
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Fig. 3 TEM pictures of PbO/SiO2 (A and a), PbO/TiO2 (B and b), PbO/
ZrO2 (C and c), PbO/Al2O3 (D and d) and PbO/MgO (E and e).
The diameters of all these catalysts were also not uniform, and
the dissociation of PbO particles and the supports could not be
detected by visual inspection. Comparatively speaking, PbO/
ZrO2 exhibited a smaller particle size than the other catalysts.
3.1.5 XPS characterization. XPS analyses were employed to
identify the chemical composition and states of the catalysts.
The XPS survey spectrum veried the presence of Pb in the
supported catalysts. As shown in Fig. 4(A), the denitive binding
Fig. 4 XPS spectra of the catalysts.
the O 1s spectra was different in each catalyst, which was
probably due to the different support properties for the sup-
ported Pb-based catalysts.
3.1.6 NH3-TPD characterization. The process of MPC
disproportionation to produce DPC was mainly dependent on
the acidity of the catalyst. Herein NH3-TPD was carried out for
measuring the acidic properties of the catalysts. The NH3-TPD
proles were displayed in Fig. 5. According to the literatures,38,39
energy peak located at approximately 142.9–144.0 eV and 137.9–
23,37
139.2 eV corresponded to Pb 4f5/2 and Pb 4f7/2
,
indicating
that Pb2+ species existed on the surface of these catalysts.
Fig. 4(B) showed that the O 1s spectra of the catalysts (except
PbO/Al2O3 catalyst) could be deconvoluted into two Gaussian
peaks (OI and OII). In general, OI was ascribed to the lattice
oxygen in the literature,28,34 and OII was related to the adsorbed
oxygen species on the catalyst surface. The binding energy of
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acidity of the catalysts was correlated with the activity, thus the
high acidity of the catalyst was benecial for the activity.
3.1.7 Py-IR characterization. The type of acid sites was
performed by pyridine adsorption FT-IR spectroscopy. Fig. 6
showed the Py-IR spectra of the catalysts. As shown in Fig. 6,
PbO/SiO2, PbO/TiO2, PbO/ZrO2 and PbO/MgO exhibited two
remarkable absorption peaks at around 1438–1446 and 1584–
1596 cmꢁ1, which were attributed to Lewis and Brønsted acidic
sites, respectively.40,41 However, for PbO/Al2O3, only one
absorption peak was displayed at 1446 cmꢁ1 that suggested the
presence of only Lewis acidic sites. Meanwhile, the concentra-
tion ratio of Lewis to Brønsted acid sites was calculated by
semiquantitative analysis based on the relative peak area cor-
responding to Lewis and Brønsted acidity (AL/AB) in Table 2.
Apparently, the AL/AB ratios were higher for PbO/ZrO2 and PbO/
MgO compared to others, and combined with the acid amount
results determined by NH3-TPD, which indicated that PbO/ZrO2
and PbO/MgO contained much more Lewis acidic sites.
3.2 Results of the catalytic performance evaluation
The catalytic performance evaluation results for MPC dispro-
portionation over the catalysts were revealed in Table 3. Pure
supports were basically inactive under the current studies, while
PbO loading on these supports exhibited considerably different
catalytic performances. The catalytic activities of PbO/MgO and
PbO/ZrO2 were close and obviously higher than those of PbO/
SiO2, PbO/Al2O3 and PbO/TiO2, and the sequence of the activi-
ties for the MPC conversion was as follows: PbO/MgO > PbO/
ZrO2 > PbO/SiO2 > PbO/Al2O3 > PbO/TiO2. Previous studies
displayed that the BET surface area and acidity of catalyst
played signicant role in the catalytic activity and product
selectivity. Nevertheless, this order of the activity was not
correlated with the sequence of BET surface area, indicating
that the surface area might be not a main factor determining
the activity. Therefore, the catalytic activity might be closely
related to the acidity. Combining with the characteristic results
of Py-IR and NH3-TPD, the relationship between the activity and
Lewis acidity was displayed in Fig. 7. It was clear that the activity
depended on the acidic site, and the more Lewis acid site was,
Fig. 5 NH3-TPD profiles of the catalysts.
the acid sites of the catalysts were usually classied based on
desorption temperature range such as weak (around 100–200
ꢀC), medium (around 200–400 ꢀC), and strong (above 400 ꢀC)
acidic sites, respectively. As shown in Fig. 3, the acidity of the
catalysts depended on the nature of the supports, and PbO/SiO2,
PbO/TiO2 and PbO/ZrO2 exhibited remarkable and broad NH3
desorption peaks, suggesting the wide distribution of acid
strengths from weak, medium and strong acid sites respectively.
For PbO/MgO catalyst, only one strong high-temperature NH3
desorption peak at the range of 250 to 700 ꢀC was detected,
suggesting a strong acid property. PbO/Al2O3 showed much
stronger NH3 desorption peak in the high-temperature region,
indicating more strong acidic sites existed on the surface.
Besides, the acidity data was also calculated and the results
were given in Table 2. Clearly, the total acid amount in PbO/
MgO was remarkably higher than those of other catalysts, and
followed the sequence of PbO/MgO > PbO/ZrO2 > PbO/SiO2
>
PbO/TiO2 > PbO/Al2O3. According to previous studies, the
Table 2 The acidity of the catalysts determined by NH3-TPD
Acid amounts (mmol
gꢁ1
)
Total acid amounts
Catalysts
Weak Medium Strong (mmol gꢁ1
)
AL/AB ratio
PbO/MgO
PbO/Al2O3
PbO/ZrO2 0.004 0.019
PbO/TiO2 0.002 0.004
PbO/SiO2 0.002 0.014
0
0
0.030
0.001
0.048 0.078
0.009 0.010
0.029 0.052
0.017 0.023
0.020 0.036
0.582
—
0.610
0.579
0.489
Fig. 6 Py-IR spectra of the catalysts.
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Table 3 Catalytic performance results of MPC disproportionation
over the catalystsa
Yield/%
Catalysts
MPC conversion/% DPC Anisole DPC selectivity/%
PbO/MgO
PbO/ZrO2
PbO/SiO2
PbO/Al2O3 30.2
PbO/TiO2
60.1
56.3
47.7
56.0 4.1
55.2 1.1
46.2 1.6
26.5 3.7
22.9 1.4
93.2
98.0
96.9
87.7
94.2
24.3
a
Reaction conditions: MPC (150 mmol), catalyst (1.2 g), 200 ꢀC, 2.5 h.
Anisole was a small amount of byproduct.
Fig. 7 MPC conversion and Lewis acid sites on the catalysts.
the higher the activity was. And the decrease sequence of MPC
conversion was in accordance with the order of the Lewis acid
site, suggesting that the Lewis acid was the critical factor to
determine the catalytic activity. What's more, although the
highest DPC yield was obtained over PbO/MgO due to the
acidity of this catalyst being the highest among all the catalysts,
yet it was also noteworthy that a strong acid strength could
induce the decarboxylation decomposition of MPC into the
byproduct anisole. Therefore, it could be ascertained that the
higher anisole content was mainly due to its excessively strong
acidity.20 Moreover, PbO/Al2O3 with more strong acid strength
exhibited the higher anisole content, which further testied the
reason. Thus, the results also suggested that medium and
moderately strong acidic sites were favorable for the higher DPC
selectivity.
3.3 Recyclability of the catalysts
Recycling experiments of the catalysts were performed to check
the stability. Aer each run, the catalysts were separated by
ltration, washed thoroughly with DMC and dried in vacuum at
110 ꢀC overnight, and then reused for next cycle reaction. As
displayed in Fig. 8, there was no remarkable change in the MPC
conversion and yield DPC aer three cycles over PbO/MgO and
PbO/ZrO2. However, for PbO/SiO2, PbO/Al2O3 and PbO/TiO2
catalysts, the catalytic activities decreased gradually in the
Fig. 8 Reusability of the catalysts. Reaction conditions: MPC (150
mmol), catalyst (1.2 g), 200 ꢀC, 2.5 h.
succeeding three runs, suggesting that PbO/MgO and PbO/ZrO2
possessed much better reusability than PbO/SiO2, PbO/Al2O3
and PbO/TiO2.
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In heterogeneous catalysis, the leaching of active component Institute of Science and Technology (2015031) and the Project
was oen the main reason which decreased the activity for funded by Postdoctoral Science Foundation of Henan Province.
further reuse. Therefore, the recovered catalysts were detected
by ICP-OES analysis (Table S1†), and the content of Pb in the
fresh and the third reused catalyst showed that Pb content had
no signicant loss over PbO/MgO and PbO/ZrO2, but dropped
Notes and references
markedly over PbO/SiO2, PbO/Al2O3 and PbO/TiO2, respectively.
Hence, the leaching loss of Pb might lead to the decrease of
catalytic activities for the recovered PbO/SiO2, PbO/Al2O3 and
PbO/TiO2 catalysts. Combined with the characteristic results of
H2-TPR, the excellent recyclability of PbO/MgO and PbO/ZrO2
was possibly attributed to their strong interactions of the
supports with highly dispersed Pb, which was potentially
important for practical applications. Besides, the recovered
catalysts aer the third use were also detected by XRD charac-
terization (Fig. S1†). The result indicated that the diffraction
peaks of the third reused catalysts were basically identical to
those of the fresh catalysts, suggesting that with the exception
of PbO/Al2O3, PbO was still amorphous or microcrystalline
states in the recovered catalysts. Therefore, MgO and ZrO2
might be suitable and promising support materials for the
better preparation of Pb-based catalysts to transform MPC
disproportionation to efficiently synthesize DPC in comparison
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