A highly efficient rod-like-CeO2-supported palladium catalyst for the oxidative carbonylation of glycerol to glycerol carbonate

Article abstract of DOI:10.1039/d1ra02187g

A rod-like-CeO₂-supported Pd catalyst (Pd/CeO₂-r) was prepared using two-step hydrothermal impregnation and used in the oxidative carbonylation of glycerol to produce glycerol carbonate. The characterization results showed that the Pd was highly dispersed on the surface of the CeO₂-r, and metallic Pd was the main species in the catalyst. The Pd/CeO₂-r exhibited good catalytic performance for the oxidative carbonylation of glycerol. Under optimized reaction conditions, the glycerol conversion and glycerol carbonate selectivity were 93% and 98%, respectively, and turnover frequency was 1240 h^?1. However, because of the leaching of Pd and the growth of Pd particles, the catalyst was gradually deactivated throughout reuse.

Full text of DOI:10.1039/d1ra02187g

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A highly efficient rod-like-CeO -supported
2
palladium catalyst for the oxidative carbonylation
Cite this: RSC Adv., 2021, 11, 17072
of glycerol to glycerol carbonate
Ziyan Wang, Shuo Guo, Zhimiao Wang, *ab Fang Li, Wei Xue *ab
a
b
ab
abc
and Yanji Wang
A
2 2
rod-like-CeO -supported Pd catalyst (Pd/CeO -r) was prepared using two-step hydrothermal
impregnation and used in the oxidative carbonylation of glycerol to produce glycerol carbonate. The
characterization results showed that the Pd was highly dispersed on the surface of the CeO -r, and
metallic Pd was the main species in the catalyst. The Pd/CeO -r exhibited good catalytic performance
2
2
Received 19th March 2021
Accepted 23rd April 2021
for the oxidative carbonylation of glycerol. Under optimized reaction conditions, the glycerol conversion
and glycerol carbonate selectivity were 93% and 98%, respectively, and turnover frequency was 1240
DOI: 10.1039/d1ra02187g
h
ꢀ1
. However, because of the leaching of Pd and the growth of Pd particles, the catalyst was gradually
rsc.li/rsc-advances
deactivated throughout reuse.
17
high atom economy of the process. The reaction equation is
shown in Scheme 1.
1
Introduction
In recent years, concerns about global warming have resulted in
The catalysts used for the OCG reaction include homoge-
increased interest in the biodiesel industry as a source of renew- neous and heterogeneous catalysts. Hu et al. explored
17
1
able energy. However, during the manufacture of biodiesel, a large a homogeneous PdCl
(phen)/KI (phen ¼ 1,10-phenanthroline)
2
2
quantity of glycerol (GL) is produced as a by-product. For every 10 catalyst system for the OCG reaction. GL conversion was 92%,
tons of biodiesel produced, approximately 1 ton of GL is produced. GLC selectivity was 99%, and the turnover frequency (TOF)
ꢀ1
As biodiesel is produced on a large scale, the use of GL has become reached 184 h . Casiello et al. studied the homogeneous OCG
16
2
an important factor affecting the development of the biodiesel reaction using CuCl as a catalyst and pyridine as a co-catalyst.
industry. GL can be converted into valuable chemicals by reaction, Excellent conversions (>92%) and selectivities (>93%) were ob-
+
3
4
5
6,7
such as acrolein, glycerol ether, 1,2-propanediol, lactic acid,
tained, and it was found that Cu is the intermediate species in the
8
and glycerol carbonate (GLC). GLC is an important derivative of catalytic cycle. Because of the inherent disadvantages of homoge-
GL with a high boiling point, high biodegradability, low amma- neous catalysts, such as the difficulty in separation and recovery of
bility, and low toxicity. It can be used as a green solvent in the elds catalysts, heterogeneous catalysts have been attracting increasing
of paint, cosmetics, medicine, and lithium batteries. It can also be attention. Hu et al. developed a zeolite-Y-conned Pd catalyst
used as a raw material in the synthesis of surfactants, agrochem- (PdCl (phen)@Y) for the OCG reaction. This heterogeneous cata-
icals, and polymers. Therefore, much attention has been given lyst exhibited a high activity, with a reasonable GL conversion of
to the synthesis of GLC. There are several methods for the 95%, GLC selectivity of 98%, and a TOF of 317 h . The catalyst
synthesis of GLC from GL, including phosgenation, trans- could be reused ve times without a signicant decrease of the
and the oxidative carbonyl- catalytic activity. Wang et al. studied a commercially available Pd/
ation of GL. The oxidative carbonylation of glycerol (OCG) is C catalyst for GLC synthesis via the OCG reaction with the aid of
18
2
9,10
ꢀ1
11
2
1
2,13
14,15
19
esterication,
urea alcoholysis,
16
ꢀ1
a highly efficient and attractive approach for obtaining GLC NaI. Under the optimum conditions, a TOF of 900 h was
2
because of the high availability and low cost of CO and O , and the
a
Hebei Provincial Key Laboratory of Green Chemical Technology and High Efficient
Energy Saving, School of Chemical Engineering and Technology, Hebei University of
Technology, Tianjin 300130, China. E-mail: weixue@hebut.edu.cn; wangzhimiao@
hebut.edu.cn
b
Tianjin Key Laboratory of Chemical Process Safety, Tianjin 300130, China
Scheme 1 Chemical equation for the oxidative carbonylation of GL to
GLC.
c
Hebei Industrial Technology Research Institute of Green Chemical Industry,
Huanghua 061100, Hebei, China
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8
obtained. Lei et al. reported Pd@PQP-NHC as a recyclable catalyst washed with water and ethanol until the ltrate was neutral.
ꢁ
for the OCG reaction. It could afford 96% conversion of GL with The resulting solid was then dried at 60 C and calcined at
ꢀ
1
ꢁ
ꢁ
ꢀ1
a high TOF of 1901 h
.
500 C for 4 h with a heating rate of 5 C min . Finally, a light-
The oxidative carbonylation reaction involves electron yellow powder, the rod-like CeO , was obtained, and was
2
transfer. Pd compounds are usually used as catalysts with denoted by CeO
various co-catalysts. Ceria, widely used in various oxidation PdCl (0.018 g) was dissolved in a mixture of deionized water
exhibits a good promotion effect for Pd-catalyzed (10 mL) and NH $H O (28 wt%, 5 mL) with ultrasonic treat-
oxidative carbonylation because of its high oxygen adsorption ment. CeO -r (0.36 g) was dispersed in deionized water (10 mL).
capacity and good oxygen storage and release (Ce /Ce ) Then the two mixtures were mixed together and stirred for 2 h.
2
-r.
2
20,21
reactions,
3
2
2
3
+
4+
22
performance. In our previous work, a Pd–Ce–O/SiO catalyst Next, NaBH solution (0.010 g NaBH dissolved in 10 mL H O)
2
4
4
2
was prepared using a microemulsion method. The catalyst was was added dropwise into the mixture, which was stirred for
used for the oxidative carbonylation of phenol to synthesize 24 h. The solid was ltered and washed with ethanol and dried
4
+
ꢁ
diphenyl carbonate (DPC). It was found that Ce entered the in a vacuum at 80 C. Finally, the solid was calcined in a tube
ꢁ
PdO crystal lattice, which meant that the electrons of the furnace at 300 C under owing nitrogen gas. The obtained
deactivated Pd could easily be transferred to the Ce. Therefore, catalyst was denoted by Pd/CeO
the Pd–Ce–O/SiO catalyst was easily regenerated and exhibited loading of 3.0 wt%.
good performance. Yuan and Fu et al. studied the oxidative
2
-r and had a nominal Pd
2
23
24
carbonylation of phenol to produce DPC over a Pd catalyst
2.3 Catalyst characterization
supported on CeO nanotubes. This catalyst exhibited excellent
2
performance. They also found that the morphology of the CeO2 Scanning electron microscope (SEM) of FEI Nova Nano SEM 450
had a signicant effect on the performance of the catalyst. CeO type was carried out to observe the morphology of the catalyst
can also be used as a catalyst for the carbonylation of GL with particles. Transmission electron microscope (TEM) and
CO to produce GLC, as described in ref. 25. The GLC yield in selected area electron diffraction (SAED) were observed with
this study was up to 78.9%, and the CeO catalyst could be PHILIPS TECNOL 20. X-ray diffraction (XRD) patterns were
regenerated simply by calcination aer 5 cycles. recorded on a Bruker D8 FOCUS X-ray diffractometer with Cu
2
2
2
CeO is very important in the oxidative carbonylation reac- Ka radiation (40 kV) and a secondary beam graphite mono-
2
ꢁ
tion. However, as far as we know, CeO has not been used in the chromator (SS/DS ¼ 1 , RS 0.15 mm, counter SC) at the scan-
2
ꢁ
ꢁ
OCG reaction to produce GLC. In this paper, a rod-like-CeO2- ning 2q range of 5 –90 . The specic surface areas of the
supported palladium catalyst (Pd/CeO
performance in the OCG reaction was evaluated. The Pd/CeO
exhibited excellent catalytic performance with GL conversion of porosity analyzer. Thermo Scientic Escalab 250 Xi photoelec-
3% and GLC selectivity of 98%. We also discuss the stability tron spectrometer (14.6 kV, 200 W) with Al Ka (1486.6 eV) were
2
2
-r) was prepared, and its samples were calculated by BET equation using N adsorption–
2
-r desorption technique with a Micromeritics ASAP 2020M + C
9
and catalytic mechanism of the Pd/CeO
2
-r catalyst.
used for X-ray photoelectron spectroscopy (XPS) and the
number of scanning times was 20. The correction was per-
formed with C 1s (284.8 eV). Pd content in the catalyst was
determined by a Thermo Scientic iCAP 7400 Inductively
Coupled Plasma-Optical Emission Spectroscopy (ICP-OES).
2
Experimental
2.1 Experimental section chemicals
Polyethylene oxide–polypropylene oxide–polyethylene oxide
block copolymer (P123) and glycerol were purchased from
Sigma-Aldrich. CeCl $7H O, aqueous ammonia (28 wt%),
2
.4 Catalyst evaluation
3
2
The catalyst activity was evaluated in a stainless-steel autoclave
of Amtech® Slurry Phase Reactor Systems with an inner volume
of 50 mL. In a typical experiment, glycerol, Pd/CeO -r catalyst,
2
co-catalyst (NaI) and solvent (dimethyl acetamide, DMA) were
added into the autoclave. Then the autoclave was sealed and
absolute ethanol, PdCl , and NaBH4 were purchased from
2
Sinopharm Chemical Reagent Co., Ltd. NaI and dimethyl acet-
amide were purchased from Macklin. All chemicals were used
without further purication.
pressurized with a mixture of O (1.7 MPa) and CO (3.3 MPa)
and the temperature was heated to the desired value. Aer
2
2.2 Catalyst preparation
P123 (17.4 g) was dissolved in a mixture of absolute ethanol (60 a period of time, the autoclave was cooled to room temperature
mL) and deionized water (60 mL) with ultrasonic treatment in and vented. The catalyst was separated by centrifugation, and
a beaker. CeCl
With vigorous stirring, NH
3
$7H
2
O (5.58 g) was then added to the solution. the liquid phase was analyzed by gas chromatograph.
$H O (28 wt%) was added dropwise The reaction products were identied quantitatively using an
3
2
until the pH value of the solution was 10. The solution was then Agilent 7890B gas chromatograph with a KB-Wax capillary
stirred for another 30 min. The resulting suspension was column (30 m ꢂ 0.32 mm ꢂ 0.50 mm) and a ame ionization
ꢁ
rapidly transferred into a polytetrauoroethylene (PTFE)-lined detector. Column temperature started at 80 C and was
ꢁ
ꢁ
ꢀ1
stainless steel autoclave with a capacity of 200 mL, and hydro- increased to 260 C at a rate of 30 C min . Nitrogen was used
ꢁ
thermally treated at 160 C for 72 h. Aer cooling to room as the carrier gas. The quantitative analysis of the reaction
temperature, the solid was separated by centrifugation and products were carried out using n-butanol as internal standard.
©
2021 The Author(s). Published by the Royal Society of Chemistry
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2 2
Fig. 1 XRD patterns of (a) CeO -r, (b) Pd/CeO -r, and (c) recovered
Pd/CeO -r.
2
2
Aer the OCG reaction, the Pd/CeO -r catalyst was recovered
by centrifugation, washed thoroughly with ethanol, and dried at
ꢁ
8
0 C for 12 h. It was then reused directly in OCG reaction.
Fig. 3 TEM images of (a) CeO
recovered Pd/CeO -r.
2 2
-r, (b and c) Pd/CeO -r, and (d)
2
3
Results and discussion
3.1 Catalyst characterization
diffraction peak positions and strengths in the patterns of the two
Fig. 1 shows the XRD (X-ray diffraction) patterns of the CeO -r
2
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
samples. The peaks at 2q ¼ 28.51 , 33.10 , 47.62 , 56.53 , 59.07 ,
and Pd/CeO -r. There are no obvious differences between the
2
ꢁ ꢁ ꢁ
6
9.45 , 76.74 , and 79.21 are attributed to the (111), (200), (220),
(
311), (222), (400), (331), and (420) crystal planes of CeO with
2
a cubic structure (JCPDS65-5923). Peaks corresponding to Pd metal
or Pd compounds were not observed in the XRD patterns of the Pd/
CeO
highly dispersed on the CeO
The CeO -r and Pd/CeO
scanning electron microscopy) and TEM (transmission elec-
2
-r, which may be because a small amount of Pd species was
26,27
2
.
2
2
-r were characterized using SEM
(
tron microscopy), and the images are shown in Fig. 2 and 3,
respectively. Fig. 2(a) is the SEM image of the CeO -r, which
2
shows that most of the CeO -r has a rod-like structure, with
2
lengths from 300 nm to 2 mm, and diameters from 30 to 100 nm.
Additionally, there are some small particles attached to the
rods, with widths of less than 50 nm. The Pd/CeO
2
-r catalyst was
prepared by loading Pd species on the CeO -r, and its SEM
2
image is shown in Fig. 2(b). It can be seen from the gure that
the loading has a small effect on the morphology of the CeO -r.
2
Upon loading, although the rod structure remains dominant,
the morphology becomes less regular and the length decreases.
Fig. 3 shows the TEM images of the CeO
the conclusions are similar to those drawn from the SEM images.
Both samples comprise rod-like-shaped CeO , with some small
particles. Additionally, no obvious Pd species were observed in the
Pd/CeO -r. It is generally believed that most Pd particles on a CeO
2 2
-r and Pd/CeO -r, and
2
2
2
28
support cannot be observed, even using HR-TEM. This may be
because the high electron density of CeO prevents Pd particles
2
29
from being observed. An alternative explanation is that the strong
interaction between Pd and CeO inhibits the growth of Pd parti-
2
23
cles, and smaller Pd particles cannot be easily observed using
TEM. This is consistent with the XRD results.
The textural properties of the CeO
2 2
-r and Pd/CeO -r were
2 2
Fig. 2 SEM images of (a) CeO -r, (b) Pd/CeO -r, and (c) recovered Pd/
CeO -r.
2
analyzed using N adsorption–desorption measurements at
2
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Table 1 Textural properties and Pd content of CeO
2
-r and Pd/CeO
2
-r
2
ꢀ1
3
ꢀ1
a
Sample
CeO -r
A
BET (m g
)
Pore volume (cm g
)
Pore diameter (nm)
Pd content (wt%)
2
55.2
31.6
0.13
0.08
9.8
14.1
—
2.0
Pd/CeO
2
-r
a
Determined by ICP-OES.
the reaction temperature, pressure, catalyst loading, CO/O
ratio, and reaction time, were optimized.
.2.1 Effect of reaction temperature. The inuence of the
reaction temperature on the OCG reaction over the Pd/CeO -r
2
3
2
catalyst is shown in Fig. 4. The GL conversion was 55% when the
ꢁ
reaction temperature was 100 C. The GL conversion increased
markedly when the temperature was increased, and reached
ꢁ
9
3% at 140 C. Further increase of the reaction temperature
2
resulted in a small decrease of the GL conversion. The Pd/CeO -
r-catalyzed OCG reaction is a gas–liquid–solid multiphase
reaction, and the solid catalyst is located in the liquid-phase
2
solvent. The reactants, GL, CO and O , are dissolved in the
solvent at rst, and then react under the action of the catalyst.
Fig. 4 Effect of reaction temperature on the oxidative carbonylation
The solubilities of CO and O decrease as the reaction temper-
2
of glycerol. Reaction conditions: GL (25 mmol), Pd : GL ¼ 1 : 2210
ꢀ
ature is increased. Therefore, from this perspective, high
temperature is unfavorable for the reaction. When the reaction
temperature is too high, the effect of solubility reduction is
more obvious, and the reaction performance and GL conversion
(
molar ratio), NaI (I /Pd ¼ 10 : 1 in molar ratio), DMA (15 mL), CO (3.3
2
MPa), O (1.7 MPa), 100 min.
19
are reduced. In the temperature range investigated in this
study, the GLC selectivity was maintained above 98%.
liquid N
2
temperature. The results are shown in Table 1. Aer
-r
Pd loading, the surface area and pore volume of the CeO
2
3.2.2 Effect of reaction pressure. The reaction pressure is
decreased markedly, while the average pore size increased. This
one of the most important factors affecting the OCG reaction.
The effect of the pressure was studied, and the results are
shown in Fig. 5. GL conversion increased linearly with reaction
pressure. When the reaction pressure was increased from
may be because the Pd species entered the CeO -r pore channels
2
and blocked some of the smaller pores, leaving larger pores
30
open. Wu et al. obtained similar results when loading Pd
species on CeO nanotubes.
2
3
9
.0 MPa to 5.0 MPa, the GL conversion increased from 72% to
3%. This is because the OCG reaction is a reaction in which the
3
.2 Oxidative carbonylation of GL over the Pd/CeO
2
-r
gas volume is reduced, and so increasing the pressure is
-r to produce favorable for the reaction. Additionally, a high pressure
The oxidative carbonylation of GL over the Pd/CeO
2
GLC was investigated, and the reaction conditions, including
Fig. 5 Effect of reaction pressure on the oxidative carbonylation of Fig. 6 Effect of reaction time on the oxidative carbonylation of
glycerol. Reaction conditions: GL (25 mmol), Pd : GL ¼ 1 : 2210 (molar glycerol. Reaction conditions: GL (25 mmol), Pd : GL ¼ 1 : 2210 (molar
ꢀ
ꢀ
ratio), NaI (I /Pd ¼ 10 : 1 in molar ratio), DMA (15 mL), CO : O
2
¼ 2 : 1, ratio), NaI (I /Pd ¼ 10 : 1 in molar ratio), DMA (15 mL), CO (3.3 MPa), O
2
ꢁ
ꢁ
1
00 min, 140 C.
(1.7 MPa), 140 C.
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Table 2 Comparison of the activities of different Pd-catalysts for
oxidative carbonylation of glycerol
ꢀ1
Catalyst system Conversion (%) Selectivity (%) TOF (h ) Ref.
Pd/CeO
Pd/CeO
2
-r
-r
93
77
98
99
99
98
99
99
1240
3075
184
317
900
This work
This work
17
18
19
8
2
PdCl
PdCl
Pd/C
2
(phen)/KI 92
(phen)@Y 95
2
82.2
Pd@PQP-NHC 96
1901
2 2
reaction. When the CO/O ratio is increased, the amount of O is
2
Fig. 7 Effect of the CO/O molar ratio on the oxidative carbonylation
decreased, which means that the in situ oxidation regeneration
of glycerol. Reaction conditions: GL (25 mmol), Pd : GL ¼ 1 : 2210
21
process, Pd(0) / Pd(II), is hindered. This means that the
ꢀ
(
1
molar ratio), NaI (I /Pd ¼ 10 : 1 in molar ratio), DMA (15 mL), 5 MPa,
ꢁ
catalyst activity and the GL conversion decrease.
00 min, 140 C.
3.2.5 Effect of amount of catalyst. Fig. 8 shows the inu-
ence of the amount of Pd/CeO -r catalyst on the OCG reaction.
2
The GL conversion increases gradually as the amount of catalyst
increases, reaching a maximum of 93% at a Pd : GL molar ratio
of 1 : 2210. Increasing the amount of catalyst further did not
improve the GL conversion. Although increasing the amount of
catalyst is benecial to the reaction, too much solid catalyst
affects the stirring efficiency and increases the mass transfer
resistance. This has negative effects on the reaction. In other
words, there is an optimal amount of catalyst. Additionally, the
TOF decreased as the amount of catalyst was increased, and was
2
improves the solubility of CO and O in the solvent. This can
also accelerate the reaction and improve the GL conversion. All
the GLC selectivities were higher than 98% and were less
affected by pressure than the conversion.
3.2.3 Effect of reaction time. The effect of the reaction time
on the OCG reaction over the Pd/CeO -r catalyst is shown in
2
Fig. 6. As the reaction time is increased, the GL conversion
increases gradually, reaching 93% at 100 minutes with a TOF of
ꢀ
1
1
240 h . Upon further increase of the reaction time, the GL
ꢀ
1
1240 h at the maximum GL conversion, which was higher
conversion changes little. Within the time range of study, the
GLC selectivity remained at a high level (not less than 98%).
than those reported for most catalysts for this reaction (Table 2),
8
except the Pd@PQP-NHC catalyst from the latest reports. For
3
.2.4 Effect of CO/O ratio. The effect of the CO/O molar
2
2
the smallest amount of catalyst used in this experiment, the GL
ratio on the OCG reaction was investigated and the results are
shown in Fig. 7. The GL conversion increased from 89% to 93%
ꢀ1
conversion was 77%, and the TOF was 3075 h , which is the
highest value observed in this experiment.
when the CO/O
Upon further increase of the CO/O
conversion decreased quickly to 40%. Meanwhile, the GLC 3.3 Catalyst stability
2
molar ratio was increased from 1 : 1 to 2 : 1.
2
molar ratio to 8 : 1, the GL
selectivity changed very little with the CO/O
role of O is to ensure that the Pd(0) 4 Pd(II) cycle occurs, so
that the OCG reaction catalyzed by the Pd species is a catalytic
2
molar ratio. The
The reusability of the Pd/CeO -r catalyst was examined. Aer the
2
2
reaction, the catalyst was recovered by centrifugation, washed
ꢁ
thoroughly with ethanol, and dried at 80 C for 12 h. It was then
re-evaluated for the OCG reaction to produce GLC; the results
are shown in Fig. 9. The more times the catalyst had been
Fig. 8 Effect of the amount of Pd/CeO
2
-r on the oxidative carbon-
ꢀ
ylation of glycerol. Reaction conditions: GL (25 mmol), NaI (I /Pd ¼
1
1
0 : 1 molar ratio), DMA (15 mL), CO (3.3 MPa), O
40 C.
2
(1.7 MPa), 100 min,
ꢁ
2
Fig. 9 Reusability of Pd/CeO -r.
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reused, the lower the GL conversion. The GL conversion was The binding energies of the Pd 3d peaks increased and the
1% when the catalyst was used for the third time. peaks became wider and asymmetric, which indicates the
To clarify the cause of the catalyst deactivation, the recovered existence of Pd species with high valence. Wang et al. believed
Pd/CeO -r catalyst was characterized using XRD, and the results that some Pd(0) was oxidized to PdI during the OCG reaction.
6
19
2
2
are shown in Fig. 1(c). In addition to the diffraction peaks However, the change of the Pd valence was not the main reason
ꢁ
assigned to CeO
2
, a new diffraction peak is present at 40.15 . for the Pd/CeO
2
-r catalyst deactivation, because Pd(II) is also
8,19
This corresponds to the (111) crystal plane of metallic Pd. This active in the reaction. To demonstrate this, the PdCl
2 2
/CeO -r
might be present as a result of the aggregation and growth of Pd catalyst, without reduction by NaBH , was prepared and evalu-
4
particles during the reaction. The aggregated Pd would reduce ated for the OCG reaction. Under the same conditions as those
the contact between the reactants and the active centers, thus used for the highest conversion with the Pd/CeO -r catalyst, the
2
reducing the activity of the catalyst.
GL conversion was 82% and the GLC selectivity was 98% over
The morphology of the recovered Pd/CeO -r catalyst was the PdCl /CeO -r catalyst. This indicates that the activity of
2
2
2
observed using SEM and TEM, and the results are shown in Pd(II) was slightly lower than that of Pd(0). Therefore, the main
Fig. 2(c) and 3(d), respectively. The images show that the reason for the deactivation of the Pd/CeO
morphologies of the catalyst before and aer the reaction had species.
no obvious differences, and the rod-like shape was maintained.
2
-r is the leaching of Pd
2
The Pd content in the Pd/CeO -r catalyst was determined
using ICP-OES (inductively coupled plasma optical emission
spectrometry). The results showed that the Pd content was
3
.4 Proposed mechanism for the OCG reaction over Pd/
CeO -r
Based on the current understanding of the mechanism of Pd-
2
2
.0 wt% and 1.1 wt% in the fresh and recovered Pd/CeO -r
2
19,23,32
catalyzed oxidative carbonylation
results in this paper, we propose a reaction mechanism for
glycerol oxidative carbonylation over the Pd/CeO -r catalyst,
which is shown in Scheme 2. In this catalytic cycle, I rst reacts
with O to form I , and then Pd(0) is transformed into Pd
under the action of I . Pd is the active center for the OCG
reaction. GLC is then formed from GL and CO, catalyzed by
Pd , and Pd is reduced to Pd(0) at the same time. Addition-
ally, some Pd is deposited on the CeO -r support in the form of
2
Pd(II) species. Finally, H and O , generated in the previous
steps, react to form water to complete the catalytic cycle.
Notably, the Pd(II) species redeposited on the support are also
active in the OCG reaction – this is not indicated in the mech-
anism diagram. In addition, CeO -r support can also promote
the Pd-catalyzed OCG reaction. Oxygen can be adsorbed on the
2
oxygen vacancy of CeO -r surface and transformed into lattice
oxygen O ; at the same time, Ce is oxidized to Ce , while
Ce can promote the conversion of Pd(0) generated in the
reaction to Pd(II), thus regenerating the activity.
and the experimental
catalyst, respectively. Pd leaching during the reaction was also
19
a cause of catalyst deactivation. Wang et al. studied the Pd/C
catalyzed OCG reaction. They found that with the aid of NaI, Pd
species rst dissolved in solvents, and aer a period of time, were
deposited on the support, with less leaching. They also conrmed
that the dissolved Pd species were active in the reaction. The
leaching of Pd in our work was much higher than that reported by
2
ꢀ
2
+
2
2
2
+
2
2
+
2+
1
9
Wang et al. This may be because the CeO -r support had a smaller
2
2+
2
ꢀ1
specic surface area (31.6 m g ) than that of the Pd/C used by
Wang et al. (662.7 m g ). A smaller specic surface area would
not favor the re-deposition of Pd species.
+
2ꢀ
2
ꢀ1
XPS (X-ray photoelectron spectroscopy) measurements of the
fresh and recovered Pd/CeO -r catalysts were used to determine
2
the Pd state, and the results are shown in Fig. 10. There are two
2
peaks in the spectrum of the fresh Pd/CeO -r with binding
2
energies of 341.2 eV and 335.2 eV, which correspond to the Pd
2
ꢀ
3+
4+
3d3/2 and Pd 3d5/2 states, respectively. These peaks are assigned
4
+
31
to zero valent Pd species. This indicates that almost all the Pd
species were reduced by NaBH . Aer the reaction, the chemical
state of the Pd on the recovered catalyst changed signicantly.
4
To understand the heterogeneity of the Pd/CeO -r catalyst,
2
a hot ltration test was carried out under the optimized
conditions (GL (25 mmol), Pd : GL ¼ 1 : 2210 (molar ratio), NaI
Scheme 2 Proposed mechanism for the oxidative carbonylation of
Fig. 10 Pd 3d XPS spectra of fresh and recovered Pd/CeO
2
-r.
2
glycerol over the Pd/CeO -r catalyst.
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ꢀ
(
(
I /Pd ¼ 10 : 1 in molar ratio), DMA (15 mL), CO (3.3 MPa), O
2
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