Pd catalyst supported on CeO2 nanotubes with enhanced structural stability toward oxidative carbonylation of phenol

Article abstract of DOI:10.1039/c8ra10090j

Ordered CeO₂ nanotubes (CeO₂-T) were prepared via a hydrothermal synthesis process using the triblock copolymer polyethylene oxide-polypropylene oxide-polyethylene oxide (P123) as a morphology control agent. CeO₂-T characterization demonstrated the formation of single crystal structures having lengths between 1-3 μm and diameters < 100 nm. A supported Pd catalyst (Pd/CeO₂-T) was also prepared through hydrothermal means. H₂-temperature reduction profile and Raman spectroscopy analyses showed that the oxygen vacancies on the CeO₂ surface increased and the reduction temperature of the surface oxygen decreased after Pd loading onto CeO₂-T. Pd/CeO₂-T was employed as a catalyst toward the oxidative carbonylation of phenol and the reaction conditions were optimized. Phenol conversion was 53.2% with 96.7% selectivity to diphenyl carbonate under optimal conditions. The integrity of the tubular CeO₂ structure was maintained after the catalyst was recycled, however, both activity and selectivity significantly decreased, which was mainly attributed to the Pd active component significantly leaching during the reaction.

Full text of DOI:10.1039/c8ra10090j

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Pd catalyst supported on CeO2 nanotubes with
enhanced structural stability toward oxidative
carbonylation of phenol
Cite this: RSC Adv., 2019, 9, 11356
*
*
and Yanji Wang
Zengjie Fu,
Zhimiao Wang, Hongjuan Wang, Fang Li, Wei Xue
Ordered CeO2 nanotubes (CeO2-T) were prepared via a hydrothermal synthesis process using the triblock
copolymer polyethylene oxide-polypropylene oxide-polyethylene oxide (P123) as a morphology control
agent. CeO2-T characterization demonstrated the formation of single crystal structures having lengths
between 1–3 mm and diameters < 100 nm. A supported Pd catalyst (Pd/CeO2-T) was also prepared
through hydrothermal means. H2-temperature reduction profile and Raman spectroscopy analyses
showed that the oxygen vacancies on the CeO2 surface increased and the reduction temperature of the
surface oxygen decreased after Pd loading onto CeO2-T. Pd/CeO2-T was employed as a catalyst toward
the oxidative carbonylation of phenol and the reaction conditions were optimized. Phenol conversion
was 53.2% with 96.7% selectivity to diphenyl carbonate under optimal conditions. The integrity of the
tubular CeO2 structure was maintained after the catalyst was recycled, however, both activity and
selectivity significantly decreased, which was mainly attributed to the Pd active component significantly
leaching during the reaction.
Received 8th December 2018
Accepted 29th March 2019
DOI: 10.1039/c8ra10090j
rsc.li/rsc-advances
accepted that palladium is the most active species toward the
1 Introduction
oxidative carbonylation reaction, therefore,
a number of
heterogeneous palladium catalysts anchored on supports
including activated carbon, polystyrene, silicon dioxide, layered
double hydroxides, organic–inorganic hybrid materials and
mixed metal oxides have been developed.5 Zhang et al.6 reported
Diphenyl carbonate (DPC) is a signicant green organic
carbonate that is widely used to synthesize many important
organic compounds and polymer materials. Typically, DPC is an
important intermediate when producing high quality poly-
carbonates, which are one of the major thermoplastics engi-
neered to possess excellent mechanical, electrical, optical and
heat resistance performance properties.1,2 Therefore, the
synthesis of DPC has attracted considerable attention. The
industrial method to manufacture DPC is based on phosgene,
however, because of issues related to the toxicity and corrosive
nature of the materials concerned, this manufacturing process
will eventually become obsolete. As a more sustainable and
green approach, phosgene-free processes have been explored
and developed.2 Among the phosgene-free routes, oxidative
carbonylation of phenol is the most promising candidate,
demonstrating several advantages such as readily available
starting materials, high atom utilization, and water as the sole
by-product.3
that the Pd/PbO–MnFe2O4 catalyst, prepared via
a co-
precipitation method, demonstrated high catalytic activity
with a turnover frequency reaching 70.56 molDPC (molPd h)ꢀ1
toward the oxidative carbonylation of phenol. Xue et al.7
exploited a water-in-oil microemulsion nanoreactor approach
using an embedded Pd–Cu–O/SiO2 catalyst giving a DPC yield of
35.4% with reduced Pd leaching and improved stability.
Nonetheless, heterogeneous catalysts are still considered to
exhibit less catalytic activity compared with homogeneous
catalysts, hence further development of highly efficient
heterogeneous catalysts is crucial. The rare earth metal oxide
CeO2 possesses high oxygen storage capacity and release and
strong redox (Ce3+/Ce4+) performance, and thus is widely used
in various oxidation reactions.8,9 Interestingly, the reaction
performance of CeO2 nanostructures is inuenced by
morphology.10 Particularly, Zhou et al.11 observed that CeO2
Hitherto, the main focus of research involving oxidative
carbonylation of phenol lies in developing heterogeneous
catalysts to enhance repeatability and to facilitate the separa-
tion of catalysts from products.4 Generally, it is commonly
nanotubes exhibit
a larger surface-to-volume ratio than
conventional particles because the nanotubes offer two acces-
sible surfaces—internal and external, thus showing enhanced
reduction and oxygen storage capacity. Considering Ce as
a good redox co-catalyst toward the Pd-catalyzed oxidative
carbonylation of phenol, CeO2 nanotubes have been studied as
a support to promote this reaction. Yuan et al.12 previously
Hebei Provincial Key Laboratory of Green Chemical Technology & High Efficient
Energy Saving, Tianjin Key Laboratory of Chemical Process Safety, Hebei University
of Technology, Tianjin 300130, China. E-mail: lifang@hebut.edu.cn; weixue@hebut.
edu.cn
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prepared Pd-embedded CeO2 polycrystalline nanotubes (CeO2- spectra were measured by a Renishaw inVia Reex micro-
NT) comprising CeO2 grains, for the oxidative carbonylation of spectrometer. Thermo Scientic Escalab 250Xi photoelectron
phenol demonstrating reasonable phenol conversion (67.7%) spectrometer (14.6 kV, 200 W) with Al Ka (1486.6 eV) were used
and DPC selectivity (93.3%). Unfortunately, the structural for X-ray photoelectron spectroscopy and the number of scan-
integrity of the majority of the CeO2 nanotubes collapsed into ning times was 20 and the correction was performed with C 1s
small particles aer being subjected to reaction conditions (284.8 eV). The Pd content in the catalyst was determined by
resulting in a substantial reduction in observed catalytic
performance. Therefore, to prevent catalyst deactivation, programmed reduction by hydrogen (H2-TPR) was carried out
structurally stable CeO2 nanotubes is a prerequisite. on a Micromeritics AutoChem II-2920 apparatus. All samples
a Thermo Scientic iCAP 7400 ICP-OES. Temperature-
In view of the aforementioned issues, this research focused (0.1 g) were pretreated in the ow of Ar (50 mL minꢀ1) at room
on synthesizing heterogeneous catalysts with an enhanced temperature for 5 min. Then the owing gas was switched to
degree of stability compared with previously reported catalysts. 10% H2/Ar mixture (50 mL minꢀ1) and the sample was heated to
Herein, Pd-supported monocrystalline CeO2 nanotube (CeO2-T) 1000 ꢁC at a ramping rate of 10 ꢁC minꢀ1. The H2 consumption
catalysts possessing well-ordered structures were prepared and was monitored by a thermal conductivity detector (TCD).
their catalyst performance toward the oxidative carbonylation of
phenol was investigated.
2.3 Catalyst activity test
The catalyst activity was evaluated in 50 mL stainless steel
2 Experimental
2.1 Preparation of Pd/CeO2-T
autoclave lined with Teon. The Pd/CeO2-T catalyst, cocatalyst
(Cu(OAc)2, tetrabutylammonium bromide and hydroquinone),
desiccant (4A molecular sieve), solvent (CH2Cl2) and phenol
were added to the vessel, sealed and lled with O2 and CO. Aer
that, the vessel was heated to the predetermined temperature
for reaction. Aer the reaction was completed, the mixture was
cooled to room temperature and the catalyst and 4A molecular
sieves were removed by ltration.
For CeO2-T support preparation, typically, 17.4 g of P123 was
dissolved in a mixture of 60 mL absolute ethanol and 60 mL
deionized water with ultrasonic treatment and 5.58 g of CeCl3-
$7H2O was added to the solution aerwards. With vigorous
stirring, a certain concentration of NH3$H2O solution was
added dropwise into the mixture until pH was 10 and then
Quantitative analysis of the product was performed on
continually stirring for more than 30 min. The resulting
a
K2600 liquid chromatograph (Knauer, Germany) with
suspension was rapidly transferred to the stainless steel auto-
clave and hydrothermally treated at 160 ꢁC for 72 h. Aer
cooling to room temperature, the solids were separated by
centrifugation and washed with water and ethanol until the
ltrate was neutral. Finally, the achieved solid was dried at 60ꢀꢁ1C
a Venusil XBP C18 column (5 mL, 4.6 ꢂ 150 mm), CH3OH/H2O
(65/35, v/v) was used as the mobile phase and the detection
wavelength was 254 nm. Flow rate 0.6 was mL minꢀ1, injection
ꢁ
volume was 20 mL and column temperature was kept at 30 C.
External standard method was used to quantify diphenyl
carbonate and phenol in the reaction solution.
and calcined at 500 ꢁC for 4 h with a heating rate of 5 ꢁC min
.
The light yellow powder, CeO2 nanotubes, was obtained. The
sample was denoted as CeO2-T.
The preparation of Pd/CeO2-T catalyst was similar to the
preparation of CeO2-T except that a certain amount of aqueous
PdCl2 solution was added to the suspension before it was
transferred to autoclave. The resulting sample was denoted as
Pd/CeO2-T.
3 Results and discussion
3.1 Catalyst characterization
CeO2-T and Pd/CeO2-T morphologies were characterized by
scanning electron microscopy (SEM), transmission electron
microscopy (TEM) and selected area electron diffraction (SAED).
The results are shown in Fig. 1. The CeO2-T samples comprise
distinct tubes having a length of 1–3 mm. The tubes are 30–60 nm
2.2 Characterization
Scanning electron microscope (SEM), along with energy in outer diameter and ꢃ15 nm in inner diameter, which can be
dispersive spectrometer (EDS) was performed using Nova Nano easily recognized in Fig. 1(C). The darker regions in samples
SEM 450. Transmission electron microscope (TEM) and marked by lines a and b reveal the internal wall of CeO2 nano-
selected area electron diffraction (SAED) were observed with tubes. Some of the dark regions in TEM image of Pd/CeO2-T may
PHILIPS TECNOL 20 at an acceleration voltage of 200 kV. Energy be caused by the overlapping of CeO2 particles and CeO2-T,
dispersive analysis (EDS) of X-ray patterns were collected using illustrating the inhomogeneity of the material. Additionally, no
FEI Talos F200S transmission electron microscope (TEM). X-ray distinct changes are observed between the samples before and
diffraction (XRD) patterns were recorded on Bruker D8 FOCUS aer Pd loading. However, what is less obvious are the differ-
X-ray diffractometer with Cu Ka (40 kV) radiation and ences observed in the tubular channels that appear in the Pd-
a secondary beam graphite monochromator (SS/DS ¼ 1ꢁ, RS loaded catalyst (Fig. 1(D)), when compared with the non-Pd-
0.15 mm, counter SC) at the scanning 2q range of 10–90ꢁ. The loaded catalyst (Fig. 1(C)), indicating that the internal structure
specic surface areas of the samples were calculated by BET of the tubes may be inuenced as a result of Pd addition. The
equation using N2 adsorption–desorption technique with particles around nanotubes in Fig. 1(C) could be CeO2 particles
a Micromeritics ASAP 2020M + C porosity analyzer. Raman failed to participate in the assembly formation of nanotubes.
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Fig. 1 SEM images of: CeO2-T (A) and Pd/CeO2-T (B). TEM images of: CeO2-T (C) and Pd/CeO2-T (D), high resolution TEM images and SAED
patterns of CeO2-T (E) and Pd/CeO2-T (F).
With the addition of Pd, more particles were formed, shown in the (111), (200), (220), (311), (222), (400), (331) and (420) crystal
Fig. 1(D). This indicates that Pd affects the crystallization process planes, all of which are well indexed with the cubic uorite
of CeO2. SAED patterns of CeO2-T and Pd/CeO2-T (Fig. 1(E) and structure of CeO2 (JCPDS43-1002). Similar to the TEM micro-
(F)), which show the highly active CeO2 (111) crystal plane to be graphs, the characteristic peaks of Pd, or Pd compounds, are
exposed, demonstrate that the tubes comprise single crystal not observed either. Two possible reasons accounting for this
structures. Furthermore, Pd particles associated with Pd/CeO2-T phenomenon are: Pd exists in an amorphous state; or the Pd
are not observed from the TEM micrographs, likely as a result of particle loading is relatively low and highly dispersed in the
the strong interaction between Pd and CeO2. This hinders Pd support,14,15 and thus the peaks cannot be reected in the
growth and results in smaller Pd nanoparticles that are highly diffraction pattern since strong peaks cannot be formed.
dispersed within the CeO2 lattice.13
The lattice parameters of CeO2-T and Pd/CeO2-T were
SEM-EDS mapping results for CeO2-T and Pd/CeO2-T are calculated from the CeO2 (111) plane from the XRD. The lattice
displayed in Fig. 2 and Table 1. CeO2-T comprises not only O parameter of Pd/CeO2-T (0.5407 nm) is smaller than that of
and Ce, but also high content of C. For Pd/CeO2-T catalyst, the CeO2-T (0.5412 nm). Since the Pd2+/Pd4+ radii (0.84 A/0.62 A) are
˚
˚
same is true. That is because the samples were obtained by smaller than that of Ce4+ (0.99 A), it is suggested that Pd in the
˚
calcination in static air. And it was hard to remove completely CeO2 lattice replaces Ce4+, in part, and induces changes to the
for P123 under these conditions. Therefore the residual carbon internal structure of the catalyst, thereby resulting in a decrease
certainly contributed to the C signal. Moreover, part of the C of the CeO2 lattice parameter.16 A similar phenomenon on the
response displayed in SEM-EDS results may come from the Pd/Ce0.5Sn0.5O2 catalyst was observed by Vasilchenko et al.,17
conductive glue used for sample preparation. The presence of when the Ce0.5Sn0.5O2 catalyst was loaded with Pd.
Pd, in addition to C, O and Ce in Pd/CeO2-T demonstrates the
successful incorporation of Pd into CeO2-T.
Textural properties of CeO2-T and Pd/CeO2-T were deter-
mined by N2 adsorption–desorption measurements, and the
X-ray diffraction (XRD) was used to determine phase iden- results are summarized in Table 1. On the one hand, the
tication and the crystalline structure. Fig. 3(a) and (b) show the structural parameters of CeO2 change aer Pd loading on CeO2-
XRD patterns of CeO2-T and Pd/CeO2-T, respectively. The T because Pd inuences the crystal growth of CeO2. On the other
diffraction patterns show no obvious differences with respect to hand, ingress of the Pd species into the pore channels of CeO2-T
peak position and strength. The peaks at 2q ¼ 28.54ꢁ, 33.09ꢁ, plug the small pores while leaving the relatively larger pores
47.57ꢁ, 56.48ꢁ, 59.03ꢁ, 69.41ꢁ, 76.75ꢁ and 79.11ꢁ correspond to accessible, which increases the pore size of CeO2 to 22.9 nm.
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Fig. 2 SEM-EDS mapping images of: CeO2-T (A) and Pd/CeO2-T (B); TEM-EDAX mapping of Pd/CeO2-T (C).
Previous work has also demonstrated that Pd species in the
H2-temperature prole reduction (TPR) was undertaken to
mesoporous channels of CeO2 decreased the specic surface study the oxygen species reduction capacity and the results are
area and pore volume.18
shown in Fig. 4 and Table 2. For CeO2-T, the H2-TPR prole
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Table 1 The atom contents and textural properties of CeO2-T and Pd/CeO2-T
Contenta/wt%
Sample
C
O
Ce
Pd
ABET/m2 gꢀ1
Pore volume/cm3 gꢀ1
Pore size/nm
Lattice parameter/nm
CeO2-T
Pd/CeO2-T
62.60
59.57
24.35
23.08
13.05
17.06
—
0.30
56.2
35.5
0.18
0.09
13.7
22.9
0.5412(3)
0.5407(4)
a
Atomic ratio obtained by SEM-EDS analysis.
exhibits two peaks at 460 ꢁC and 723 ꢁC (Fig. 4(a)), corre- and 278 ꢁC (The total H2 consumption is 389.68 mmol gꢀ1).
sponding to surface oxygen and bulk oxygen reduction of CeO2, From Table 2, the total hydrogen consumption associated with
respectively.19 However, signicant changes were observed in the three reduction peaks at 125 ꢁC, 278 ꢁC and 435 ꢁC is 425.64
the H2-TPR curve of Pd/CeO2-T (Fig. 4(b)). Regarding the bulk mmol gꢀ1, which is similar to the total hydrogen consumption of
oxygen reduction temperature at 747 ꢁC, the peak position and pure PdO when the reduction temperature is 50 ꢁC (H2
intensity are similar, however, the reduction temperature of consumption isꢁ188 mmol gꢀ1
) and surface oxygen on CeO2-T
20
surface oxygen exhibited a weak peak at a decreased tempera- reduced at 460 C (H2 consumption is 238.18 mmol gꢀ1), indi-
ture of 435 ꢁC (H2 consumption is 35.96 mmol gꢀ1), and was even cating that the appearance of the low-temperature reduction
accompanied by two low-temperature reduction peaks at 125 ꢁC peaks is the result of the combined effect of PdO and CeO2-T
surface oxygen reduction.21 Furthermore, there is an inverted
peak centered at 64 ꢁC, which demonstrates the high degree of
Pd dispersion on CeO2-T. Highly dispersed PdO is reduced by
H2 to form metallic Pd. Thereaer, the Pd crystallites adsorb H2,
resulting in the formation of palladium hydride (PdHx), which
decomposes as a function of increased temperature releasing
H2, giving rise to the negative peak.22
Oxygen vacancies present on the CeO2 surface were investi-
gated by Raman spectroscopy, and the tted results are shown
in Fig. 5. A strong Raman peak located at 445 cmꢀ1 and a rela-
tively weak peak at 572 cmꢀ1 were observed for CeO2-T (Fig. 5(a))
and Pd/CeO2-T (Fig. 5(b)), which correspond to the F2g mode of
the ceria uorite structure and localized vibrations induced by
the presence of oxygen vacancies, respectively.23 Besides,
a shoulder peak at 335 cmꢀ1 appears on the side of the uorite
band, but its belonging is unable to determine. Similar cases
were reported that a shoulder peak at about 400 cmꢀ1 was
observed for ceria nanoparticles,24,25 and the assignment is also
not clear. The density of the oxygen vacancies associated with
CeO2 in CeO2-T and Pd/CeO2-T can be expressed by the ratio of S
(S ¼ S1/S2),26 in which S1 and S2 correspond to the peak areas
associated with the oxygen vacancies and the F2g vibration. The
Raman spectra show that S(Pd/CeO2-T) (0.75) is greater than
S(CeO2-T) (0.40), which suggests that the Pd addition enhances
the number of oxygen vacancies in CeO2. For Pd/CeO2-T,
a portion of Pd is incorporated into the CeO2 lattice, resulting in
a further enhancement to the number of oxygen vacancies on
the surface of the support, and molecular oxygen, O2, binds to
the vacancies easier, which is conducive to the catalytic process.
Fig. 3 XRD patterns of CeO2-T (a) and Pd/CeO2-T (b).
Table 2 H2-TPR results of CeO2-T and Pd/CeO2-T
Sample
Peak temperature (ꢁC)
H2 uptake (mmol gꢀ1
)
CeO2-T
—
125/278
50
460
435
723
747
—
389.68
188
238.18
35.96
591.71
592.42
Pd/CeO2-T
PdO20
Fig. 4 H2-TPR profiles of CeO2-T (a) and Pd/CeO2-T (b).
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temperatures reduce the solubility of O2 and CO, which reduces
their contact with the reagents.
The inuence of Pd/CeO2-T amount on the oxidative carbon-
ylation of phenol is shown in Fig. 6(C). Phenol conversion
increased rapidly with increased catalyst loading, reaching
a maximum conversion of 53.2% at 1/425(3/1275) Pd/phenol in
molar ratio. Further increasing the catalyst loading does not
further improve conversion. Catalysts speed up the rate of reac-
tion via providing an alternative reaction pathway, nevertheless,
an excessive catalyst amount does not offer any benet because of
the mass transfer resistance. DPC selectivity increases rst,
reaching 96.7% when the system comprises a Pd catalyst/phenol
molar ratio of 1/425. Further increase the catalyst loading results
in a decrease in DPC selectivity. Lower catalyst concentrations
can enhance the active sites to promote catalysis of the main
reaction, whereas at high catalyst concentrations, the interac-
tions between the additives and active ingredients are hindered.
The effect of reaction pressure on the oxidative carbonylation
of phenol was examined (Fig. 6(D)). The CO partial pressure was
xed at 92 vol% (O2/CO ¼ 1/11) to prevent an explosive atmo-
sphere from forming. Phenol conversion increased as a func-
tion of increased gas pressure, reaching 56.4% at 7.7 MPa. DPC
selectivity decreased slowly from 98.2% at 4.4 MPa to 92.3% at
7.7 MPa. By increasing the gas pressure, the adsorption of CO
and O2 onto the catalysts is strengthened. The oxidative
carbonylation reaction, as a volume-reduced reaction, is easily
promoted at higher pressures, however, increased pressures
favor undesired side reactions resulting in the transformation
of CO to CO2, prompting DPC selectivity to decrease.
Fig. 5 Raman spectra of CeO2-T (a) and Pd/CeO2-T (b).
3.3 Catalyst stability
3.2 Oxidative carbonylation of phenol catalyzed by Pd/CeO2-T
The reusability of the Pd/CeO2-T catalyst was examined. Aer
the reaction, the catalyst was recovered, washed with ethanol
and dried at 60 ꢁC, and referred to as Pd/CeO2-TD. Subse-
The oxidative carbonylation of phenol was investigated as
a function of reaction conditions, with the inuence of indi-
vidual parameters such as reaction time, reaction temperature,
catalyst amount and reaction pressure shown in Fig. 6.
ꢁ
quently, the dried sample was calcined at 500 C for 1 h and
termed Pd/CeO2-TC. TEM micrographs (Fig. 7) and catalytic
performance data (Fig. 8) of Pd/CeO2-T, before and aer the
reaction, provide insight into the catalyst stability. The struc-
tural integrity of the single crystal nanotubes is maintained in
the recovered catalyst. Phenol conversion and DPC selectivity on
the recovered catalysts, regardless of the treatment methods,
were signicantly reduced. X-ray photoelectron spectroscopy
(XPS) was performed on the three catalysts to determine the Pd
state, and the results are summarized in Fig. 9. The Pd 3d5/2
binding energy of Pd/CeO2-T is 337.4 eV, ascribed to PdO,
Phenol conversion increased as a function of reaction time
(Fig. 6(A)). Particularly, the rate of phenol conversion was
observed to increased sharply between a reaction time of 7–8 h.
Conversely, DPC selectivity decreased gradually throughout the
entire process. Reactant consumption increases as a function of
reaction time, however, water formation also increases, which
stimulates the aggregation of Pd nanoparticles resulting in
decline of the catalytic activity in target reaction to manufacture
DPC.27,28 Additionally, decomposition, or self-polymerization, of
the resulting DPC occurs as a function of reaction time.29
The oxidative carbonylation of phenol increases as a func-
tion of reaction temperature (Fig. 6(B)), reected by increased
whereas the peak located at 338.3 eV is assigned to PdxCe1ꢀx
-
30,31
O2.
The formation of PdxCe1ꢀxO2 results from the electron
transfer from Pd to Ce, giving rise to Ce having a surplus elec-
tron density (Cedꢀ) and an electron-decient Pd species (Pdd+),
that induces a strong interaction between Pd and CeO2, which
increases the binding energy of PdO.32 In the Pd/CeO2-TD
spectrum, the peaks corresponding to PdO and Pdd+ are very
weak, while the 3d5/2 peak at 334.8 eV, corresponding to Pd0 is
detected, indicating the reduction of Pd(II) to Pd(0). This
transformation from Pd(II) to Pd(0) leads to catalyst deactiva-
tion.5 Additionally, the peaks corresponding to PdO and Pdd+ in
ꢁ
phenol conversion up to 50.7% at 100 C. DPC selectivity was
observedꢁto increase when increasing the reaction temperature
ꢁ
from 80 C to 90 C. Further increasing the reaction tempera-
ture resulted in a decrease in DPC selectivity. Since oxidative
carbonylation of phenol to DPC is an exothermic reaction, the
rise in temperature is unfavorable for the reaction to proceed in
the desired direction. Furthermore, higher reaction
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Fig. 6 Oxidative carbonylation of phenol over Pd/CeO2-T as a function of reaction conditions: (A) reaction time, (B) reaction temperature, (C)
catalyst amount, and (D) reaction pressure. Co-catalysts amounts: Pd/Cu(OAc)2/TBAB/H2BQ ¼ 1/5/40/40 in molar ratio.
the Pd/CeO2-TC spectrum become stronger, whereas the 3d5/2 respectively. Signicant Pd leaching is believed to be the main
peaks associated with Pd(0) almost disappear, which is as factor in catalyst deactivation of Pd/CeO2-T.
a result of Pd0 re-oxidation during calcination. Notably, the
In addition to the observed enhanced reaction rate and
catalytic activity toward the oxidative carbonylation of phenol of selectivity, the nanotube structural integrity of the obtained
the recycled Pd/CeO2-TC catalyst is a little higher than the Pd/ single crystal Pd-supported CeO2 nanotubes is retained aer
CeO2-TD catalyst even though the majority of Pd0 was reac- being subjected to the oxidative carbonylation of phenol reac-
tivated into bivalent Pd species. We inferred that valent varia- tion, which is a distinguishing feature from the polycrystalline
tion of Pd is not the main factor for catalyst deactivation. The Pd Pd–O/CeO2-NT catalyst.12 However, the reutilization of Pd/CeO2-
content was determined by inductively coupled plasma (ICP) T is not satisfactory, as only a 35.6% phenol conversion was
analysis. The results show that the Pd content in Pd/CeO2-T, Pd/ observed with DPC selectivity at 42.4%—lower than the DPC
CeO2-TD and Pd/CeO2-TC is 1.87 wt%, 0.13 wt% and 0.23 wt%, conversion and selectivity observed for Pd–O/CeO2-NT (29.3%
Fig. 7 TEM micrographs of Pd/CeO2-T (A) and Pd/CeO2-TC (B).
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a co-catalyst and to enhance catalytic performance. Therefore,
monocrystalline CeO2 nanotubes with ordered structures
demonstrate great potential as catalysts and warrant further
research to design new materials.
4 Conclusions
In summary, CeO2-T with uniform tubular morphology of 1–3
mm were synthesized via a hydrothermal method using P123 as
a surfactant to control morphology. The CeO2 (111) crystal plane
was observed to be exposed in the CeO2-T support, suggesting
the formation of single crystal structures. The Pd/CeO2-T cata-
lyst was prepared based on the CeO2-T support. Characteriza-
tion data showed that active palladium species were
incorporated into the CeO2 lattice that resulted in a synergistic
effect with CeO2. Catalytic performance was observed to
improve aer Pd loading as a result of increased oxygen
vacancies on the CeO2 surface together with a decrease in the
surface oxygen reduction temperature. The oxidation carbon-
ylation of phenol was performed in the presence of Pd/CeO2-T,
with phenol conversion reaching 53.2% and DPC selectivity at
96.7% when subjected to the following reaction parameters:
100 ꢁC, 8 h, a Pd/phenol molar ratio of 1/425, and a CO pressure
of 6.6 MPa. Although signicant Pd leaching from Pd/CeO2-T
was observed that resulted in activity loss, the structural integ-
rity of the tubular monocrystalline Pd/CeO2-T was maintained
during the reaction, which is a key factor to enhanced catalytic
performance.
Fig. 8 Reusability of Pd/CeO2-T catalysts. 100 ꢁC, 8 h, O2 0.6 MPa, CO
6.6 MPa, Pd/phenol ¼ 1/425 (molar ratio), co-catalyst amounts: Pd/
Cu(OAc)2/TBAB/H2BQ ¼ 1/5/40/40 in molar ratio.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This study was supported by the National Natural Science
Foundation of China (21776057), Natural Science Foundation of
Tianjin (17JCYBJC20100, 18JCYBJC21500), Natural Science
Foundation of Hebei (B2015202228) and the Fund Program for
the Scientic Activities of Selected Returned Overseas Profes-
Fig. 9 X-ray photoelectron spectroscopy Pd 3d spectra of fresh and
recovered Pd/CeO2-T catalysts. Pd/CeO2-T (a); Pd/CeO2-TD (b); Pd/
CeO2-TC (c).
phenol conversion with 62.3% DPC selectivity).12 Pd loss is not sionals in Hebei Province (CL201605).
a negligible factor in catalyst deactivation from the observed
relationship between weak activity and signicant Pd leaching
in recycled Pd/CeO2-T. Therefore, to improve catalyst perfor-
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