Effect of metal oxide composite method on catalytic oxidation performance of aerogel supported Pd catalysts in oxidative carbonylation

Article abstract of DOI:10.1016/j.jorganchem.2020.121110

A series of metal manganese-cerium silicon composite aerogel supported Pd catalysts were prepared by the in-situ method, the precipitation method and the impregnation method. The catalysts were applied to synthesize diphenyl carbonate (DPC) by oxidative carbonylation. The effects of different preparation methods and composite metal oxide contents on catalytic activities were studied. The prepared catalysts were characterized by XRD, FTIR, BET, TEM, H₂-TPR and XPS. The results showed that the metal composite method had a great influence on the catalyst particle size and specific surface area; the low temperature oxidation performance and surface oxygen species content of the catalysts prepared by different methods were different. The catalyst prepared by the impregnation method has a large specific surface area and particle size, good low-temperature oxidation performance and more surface adsorption oxygen, which helps to improve the multi-step electron transfer efficiency, to promote the regeneration of the active component Pd²⁺ and to increase catalytic activity.

Full text of DOI:10.1016/j.jorganchem.2020.121110

Journal Pre-proof
Effect of metal oxide composite method on catalytic oxidation performance and
adsorption oxygen of aerogel supported Pd catalysts in oxidative carbonylation of
phenol
Meng Peng, Chao Hong, Yuhai Huang, Ping Cheng, Hua Yuan
PII:
S0022-328X(20)30011-5
DOI:
https://doi.org/10.1016/j.jorganchem.2020.121110
JOM 121110
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 13 November 2019
Revised Date: 25 December 2019
Accepted Date: 7 January 2020
Please cite this article as: M. Peng, C. Hong, Y. Huang, P. Cheng, H. Yuan, Effect of metal oxide
composite method on catalytic oxidation performance and adsorption oxygen of aerogel supported Pd
catalysts in oxidative carbonylation of phenol, Journal of Organometallic Chemistry (2020), doi: https://
doi.org/10.1016/j.jorganchem.2020.121110.
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Effect of metal oxide composite method on catalytic oxidation
performance and adsorption oxygen of aerogel supported Pd
catalysts in oxidative carbonylation of phenol
Meng Penga, , Chao Honga, Yuhai Huangb, Ping Chengb, , Hua Yuana,
a School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan
430205, PR China.
b School of Foeign Languages, Wuhan Institute of Technology, Wuhan 430205, PR China.
Corresponding
authors
E-mail
addresses:
pm201415001@163.com
(M.
Peng),
2371549169@qq.com (P. Cheng), yuanhua@wit.edu.cn (H. Yuan).
Abstract:
A series of metal manganese-cerium silicon composite aerogel supported Pd
catalysts were prepared by the in-situ method, the precipitation method and the
impregnation method. The catalysts were applied to synthesize diphenyl carbonate
(DPC) by oxidative carbonylation. The effects of different preparation methods and
composite metal oxide contents on catalytic activities were studied. The prepared
catalysts were characterized by XRD, FTIR, BET, TEM, H2-TPR and XPS. The
results showed that the metal composite method had a great influence on the catalyst
particle size and specific surface area; the low temperature oxidation performance and
surface oxygen species content of the catalysts prepared by different methods were
different. The catalyst prepared by the impregnation method has a large specific
surface area and particle size, good low-temperature oxidation performance and more
surface adsorption oxygen, which helps to improve the multi-step electron transfer
efficiency, to promote the regeneration of the active component Pd2+ and to increase
catalytic activity.
Key words: diphenyl carbonate, aerogel, oxidative carbonylation, oxygen species,
supported palladium catalyst
1. Introduction
Diphenyl carbonate (DPC) is an important intermediate for the preparation of
polycarbonate (PC), which is the second largest engineering thermal plastic. The main
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methods of its preparation are phosgene, transesterification and oxidative
carbonylation method [1,2], among which the oxidative carbonylation method is to
synthesize DPC directly with CO, O2 and phenol in the presence of catalyst. Because
of its simple process, cheap raw materials, only a single by-product H2O, and a high
atom utilization, this method is a green environmental protection method with bright
prospect in industrial application [3]. The catalysts for the synthesis of DPC by the
oxidative carbonylation are divided into two main types: homogeneous complex
catalysts and heterogeneous supported catalysts. The former has a big
disadvantage-its product is very hard to separate. As a consequence of good
performance of easy product separation, the heterogeneous catalytic oxidation method
becomes the most active research direction of synthesizing DPC by carbonylation
[4,5]. After more than a decade of active research and development, a variety of
supported catalysts have been attempted such as the activated carbon, molecular
sieves, polymers, carbon nanotubes [6-9] and so on. Unfortunately, the active
component Pd is very easy to aggregate, and Pd2+ is very poor to be recycled, which
led to the low yield of DPC. This kind of research has been confronted with a
bottleneck, with no industrialized report.
Metal silicon composite aerogel has many excellent carrier properties of aerogel
such as low density, high specific surface area, high porosity, 3D network structure
and so on [10-12]. Silicon aerogel with a mesoporous structure limits the grain size of
the metal oxide, and its high specific surface area promotes the active ingredient and
the metal cocatalyst to be highly dispersed on the carrier, thereby providing more
catalytic active sites; electron transfer between metal and silicon-based aerogels, ie
metal-carrier strong interaction (SMSI effect), can regenerate high-valence metals and
improve the oxidation capacity of metal cocatalyst; in addition, silicon aerogels have
chemical stability and thermal stability, so it is destined to have a good application
prospect in the oxidative carbonylation of phenol to DPC. The preparation process of
metal / silicon-based composite aerogel is simple, the reaction raw materials is easy to
get and the reaction process is easy to control. At present, the mature preparation
methods include in-situ method, precipitation method and impregnation method [13,
2 / 19

14].
Surface modification of Pb catalysts with rare earth oxides to improve the
oxidation activity and to enhance catalytic performance is one of the hotspots [15-18].
Among them, the most important rare earth in industrial catalysis is a composite oxide
of Ce. Its special catalytic performance is manifested in the following three aspects: 1.
The ionic radius of rare earth is large, with a high charge, and easy to form a stable
new crystal structure; 2. The ionic radius of rare earth is large, with high coordination,
and between different coordination structures, the conversion energy barrier is so low,
that it is suitable for catalytic reaction; 3. Cerium is easy to convert between +3
valence state and +4 valence state, and it is easy to form flowing oxygen vacancies in
the structure, so it has better oxygen storage performance and transmission capacity
on surface, and can produce a synergistic effect with the metal interface [19], which
will contribute to the multistep electron transfer process of the oxidative
carbonylation. Based on the above three reasons, the metal Ce is inevitably a good
promoter in the carbonylation catalytic oxidation process. According to the theory of
redox potential, the metal oxides, which can play an intermediate role in the redox
chain of the oxidative carbonylation, generally have an oxidation-reduction potential
between the following two states, that is, a high-valent cation can oxidize Pd0 to form
an active component Pd2+, and the reduced state, low-valent ions are rapidly oxidized
by oxygen to high-valent ions. While the redox potential of MnO2/Mn2+ (1.224 V) just
lies between Pd2+/Pd0 (0.99 V) and O2/H2O (1.23 V). Theoretically, Mn oxides can
implement the redox cycle regeneration of the catalytic active component Pd2+.
Therefore, the composite oxide of Ce-Mn is a promising cocatalyst for oxidative
carbonylation of phenol to DPC.
In this paper, Ce-Mn oxide silicon-based composite aerogel supported Pd
catalysts were prepared with the low-energy atmospheric pressure drying technology
and the catalytic carrier reconstruction technology, which fully utilized the synergistic
promotion of the cocatalyst, Ce-Mn oxide and the carrier, silicon-based aerogel. It
investigated the different effects of in-situ, precipitation and impregnation methods on
the synthesis of DPC by oxidative carbonylation of phenol.
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2. Experiment
2.1. Preparation of catalysts
2.1.1. In-situ method
According to the sum of the mass of Ce and Mn (in which the molar ratio was
1:1 ) and the mass of silica (calculated as the source of silicon in ethyl orthosilicate) ,
that is 10% , 20% , 40% and 60% respectively, manganese nitrate (Mn(NO3)2) and
cerous nitrate (Ce(NO3)3) were dissolved in ethanol, and added stepwise to the
precursor of silica-based aerogel (the final molar ratio of ethanol, TEOS , deionized
water, N, N-dimethylformamide was 3:1:6:0.8), and adjusted to the pH of 3 with nitric
acid solution. And then it was stirred at 50 ℃ for 6 h, poured into a petri dish, and
kept at room temperature to form alcoholic gel. After 12 h aging, at 40 ℃, the
mixture was soaked for 24 h respectively with a mixture of ethanol, and a mixture of
TEOS and ethanol (volume ratio of 1:1). Then the gel was washed with ethanol twice,
and dried at 40 ℃ for 12 h, and at 70 ℃ for 48 h. Finally it was calcined in a
muffle furnace at 500 ℃ for 4 h to get a metal silicon aerogel carrier.
PdCl2 was selected as the active Pd source, and the Pd loading was 1% of the
carrier. The accurately required PdCl2 was added a certain volume of deionized water
to, and then its pH value was adjusted with hydrochloric acid until PdCl2 was
completely dissolved in the mixture to form a brown solution. Next the above metal
silicon based aerogel carriers separately were added to the solution and stirred for 30
min until the mixture was evenly mixed. And then the pH value of the mixture was
adjusted to 9-10 with 1 mol/L NaOH solution, was continued to be stirred for 30 min
after the carriers were filtered off, washed with deionized water for 3 times, dried in
an oven at 110 ℃for 12 h, and finally the catalysts were prepared by calcination at
300 ℃ for 3 h in a muffle furnace. They were recorded as S-10, S-20, S-40, S-60
respectively.
2.1.2. Precipitation method
In the pure silicon aerogel powder sample (labeled as SA) prepared in the same
manner as above, a mixed solution of Mn(NO3)2 and Ce(NO3)3 was added, and the
NaOH solution was slowly added as a precipitant to until it precipitated completely,
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which was filtered and dried at 90 ℃ for 12 h. Finally it was calcined at 500 ℃ for
4 h to obtain different catalyst carriers, and then Pd was loaded by the above method
to obtain catalysts. The catalysts obtained by adjusting the mass ratio of metal and SA
were also labeled as C-10, C-20, C-40 , C-60.
2.1.3. Impregnation method
The pure silicon aerogel powder samples, which were prepared in the same
manner, were added a mixed solution of cerium nitrate and manganese nitrate
dropwise, stirred for 20 min, aged for 24 h, dried at 110 ℃ for 12 h, and finally
calcined at 500 ℃ for 4 h to obtain different carriers. And then Pd was loaded by the
above method to obtain catalysts. The catalysts obtained by adjusting the mass ratio of
metal to SA were also labeled as I-10, I-20, I-40 and I-60 respectively.
2.2. Characterization
In this study, the X-ray diffractometer (Model 50-A, SHIMADZU, Japan) was
used to analyze the structural characteristics and related phase parameters of the
samples. Fourier transform infrared spectroscopy (NEXU-470, Nicolet Corporation,
U.S.) was used to analyze the composition of the samples. The specific surface areas
of the test samples were measured by a specific surface area & pore analyzer (Model
NOVA2000e, Quantachrome, USA). With a transmission electron microscopy mirror
(a JEM-2010 the FEF type, Japan JEOL Co.) surface morphology and structure of the
samples were analyzed. With an automatic chemisorption analyzer (2920 FEF, USA
Micromeritics Corporation) the surface activities and the contents of the oxygen
species of the samples were analyzed. With an X-ray photoelectron spectrometer (VG
of Multilab 2000, Thermal Electron Corporation, U.S.), the material chemical
compositions, valence states of the elements and oxygen species from the surface of
the samples were analyzed.
2.3. Activity evaluation
Accurately 50 g of phenol, 1 g of catalyst, 2 g of 4A molecular sieve, and 1 g of
tetrabutylammonium bromide were added into a 250 ml high-pressure batch reactor.
O2 and CO were separately introduced into the reactor until a total reaction pressure
of 5 MPa (wherein the oxygen volume fraction was about 7%). The reaction
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temperature was controlled at 70 ℃, and the stirring speed was maintained at 600
r/min. After 4 h reaction, the reaction liquid was filtered. The reaction liquid was
analyzed by a GC-2014 gas chromatograph (SHIMADZU, Japan) to determine the
selectivity and yield.
3. Results and discussion
3.1. Characterization of the catalysts
Fig. 1. XRD pattern of silicon-based composite aerogel supported Pd catalyst
The XRD pattern of the catalysts prepared by the three methods is shown in Fig.
1. Clear diffraction peaks appear respectively at 36.5°, 41.2°, and 58.7° of the three
XRD curves, which matches the MnO standard spectra (JCPDS 07-0230) in the PDF
card library. So it describes that the samples have a metal oxide with aggregated phase
MnO. At 28.60°, 47.64°, 56.48°, etc. there are also clear diffraction peaks, and it
matches CeO2 standard spectra (JCPDS 34-0394 ) in the PDF card library, so it
describes that the samples have a metal oxide, CeO2. Therefore, the silicon-based
aerogel supported Pd catalysts are actually compounded with a cerium-manganese
composite metal oxide.
Fig 2 shows the FTIR spectra of the composite aerogel supported Pd catalysts
( S-20, C-20, I-20) prepared by the three methods. The absorption peaks at 1100 cm-1
and 800 cm-1 of the three kinds of samples respectively correspond to the asymmetric
stretching vibration and symmetric stretching vibration of Si-O-Si. It is indicated that
the main components of the three catalysts are amorphous SiO2. And S-20 (Fig 2. (a))
has obvious finger-like peaks from 500 cm-1 to 700 cm-1, which are caused by
structures of -O-Mn-O- and -O-Ce-O-. At 3480 cm-1 and 940 cm-1 absorption peaks
6 / 19

appearing, are attributed to the vibration absorption peaks of Si-OH, which describes
that the surface of carriers contains more Si-OH. Therefore cerium manganese oxide
and the active component Pd are dispersed well in the gel carriers, helping to increase
the active sites so as to enhance catalytic performance.
(a)
(c)
(b)
Fig. 2. FTIR spectrum of silicon-based composite aerogel supported Pd catalysts: (a) S-20, (b)
C-20 , (c) I-20.
(a)
(b)
(c)
Fig. 3. Nitrogen adsorption-desorption isotherm of silicon-based composite aerogel supported Pd
catalysts: (a) in-situ method, (b) precipitation method, (c) impregnation method.
The adsorption desorption curves of all catalysts prepared by in-situ method,
precipitation method and impregnation method are shown in Fig. 3. Based on the
contrast of the three figures, according to the IUPAC isotherm classification of gas
adsorption, the adsorption desorption curves of catalysts prepared by in-situ and
impregnation methods (Fig. 3(a) and (c)) belong to Type IV. There are some stairs at a
relative pressure between 0.4 and 0.8, because capillary condensation causes Type H2
hysteresis loop, whose shape indicates that the aerogel has a structure with ink bottle
pores and is a microporous material with a small pore structure. The adsorption
desorption curves of catalysts C-10, C-20, C-40 and C-60 prepared by the
7 / 19

precipitation method, belong to Type II, which indicates that there are no significant
adsorption desorption hysteresis loops at relatively low pressures, showing its
irregularly macroporous structure.
Table 1. Specific surface area, pore size, pore volume of silicon-based composite aerogel
supported Pd catalysts.
Entry
1
Sample
S-10
S-20
S-40
S-60
C-10
C-20
C-40
C-60
I-10
BET / (m2·g-1) Pore size / (nm)
Pore volume / (cm3·g-1)
492
420
371
316
61.8
59.6
36.4
19.6
589
509
431
339
2.78
2.56
0.35
0.31
0.26
0.23
0.89
0.86
0.90
0.96
0.49
0.42
0.33
0.26
2
3
2.38
4
2.25
5
14.64
25.25
37.38
51.47
3.32
6
7
8
9
10
11
12
I-20
3.30
I-40
3.29
I-60
3.31
As Table 1 shows the comparison of specific surface areas, pore sizes and pore
volumes of the catalysts prepared by the in-situ method, impregnation method and
precipitation method, in general, the specific surface areas of the catalysts are reduced
with the loading increasing of the cerium manganese oxide, when the catalysts are
prepared by the same method. When catalysts are prepared by the in-situ method, due
to the greater presence of strong capillary forces when being dried, a part of the metal
in the solvent substitution process is to be lost. The bigger metal loading, the more
capillary forces, the stronger drying stress, the more easily leading to syneresis and
some tunnels collapse or disappearance, and the smaller specific surface areas,
apertures and pore volu remessulted in. In the precipitation preparation, the required
solution of NaOH is added as a precipitating agent, with increasing the mass content
8 / 19

of cerium and manganese, the hydroxide of cerium manganese precipitates and is
absorbed onto the carriers. The more absorption, the more hydroxide of cerium
manganese precipitating on the carriers’ channels and surface, the larger amount of
metal particles agglomerating to form metal clusters and blocking the pores, resulting
in a decrease in specific surface areas and pore volumes of the carriers. When most of
pore blockage, metal clusters deposit into larger holes, forming an stacking irregular
macroporous structure (with the same result as the adsorption desorption curve). So
the specific surface area of catalysts prepared by precipitation method, is much
smaller than that by other preparation methods. The specific surface area of C-60 is
only 19.6 m2·g-1, while the pore size is much larger, about 51.47 nm. At this time, a
large amount of the cocatalyst of cerium manganese oxide is agglomerated, but the
dispersion is the most uneven, and the active sites are the least. When prepared by the
impregnation method, the structural stability of the silicon aerogel network becomes
better with the increasing of metal loading, the average pore size does not change
substantially, and the specific surface area and pore volume decrease due to more and
more metal entering the aerogel channel. As Table 1 shows, the specific surface area
of catalysts prepared by impregnation method, is the largest. The complex carriers
have a more complete pore structure, with narrow pore size distribution.
Fig. 4 is the TEM photographs of three silicon composite aerogel support Pd
catalysts with the same mass fraction of metal as 20%, which are produced by
different methods. Since the loading of Pd is less than 1%, the metal particles
observed on the photograph are basically cerium manganese oxide. The picture
comparison shows that the catalysts of C-20 (Fig.4 (c), (d)) synthesized by
precipitation method has the most uneven dispersion of cerium manganese oxide
particles, and even small metal particles are almost non-existent. They are basically
large metal clusters, serious agglomeration formation, as shown in the portion
indicated by the yellow arrow (Fig. 4(a) and (b)). The large-area agglomeration of the
cocatalyst, cerium manganese oxide, is detrimental to the multi-step electron transfer
in the catalytic oxidation process, resulting in low regeneration efficiency of the active
component Pd2+.
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(a)
(c)
(e)
(b)
(d)
(f)
Fig. 4. TEM photographs of the catalysts: (a), (b) S-20; (c), (d) C-20; (e), (f) I-20.
Compared with the precipitation method, the cerium manganese oxide grains in
the catalysts C-20 and I-20, prepared by the in-situ method and impregnation method,
are evenly dispersed relatively, with similar sizes of particles. The cerium manganese
oxide is embedded in the three-dimensional network structure of silicon-based aerogel
in the form of small particles. However, in the S-20 catalysts there is a small amount
of metal agglomeration, as shown in the Fig. 4(a) and (b) with a red arrow indicating.
And the particle size is significantly larger than the metal oxide particles in I-20.
These large particles and a small amount of agglomeration may block the
silicon-based aerogel pores, so that the specific surface areas and pore volumes
decrease (consistent with the BET results), which is detrimental to the dispersion of
the cocatalysts, resulting in a decrease in the active sites. In contrast, in the catalyst
I-20 prepared by the impregnation method, the grain sizes of cerium manganese oxide
particles are smaller, so that it has the most uniform dispersion on the surface of the
carrier channel, and the most active sites.
3.2. Activity test of catalysts
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Table 2. DPC yield of catalysts prepared by the methods of in-situ, precipitation and impregnation.
Entry
1
Sample
S-10
S-20
S-40
S-60
C-10
C-20
C-40
C-60
I-10
Yield of DPC (%) Selectivity (%)
TOF (mol DPC/mol Pd h-1)
10.37
15.31
8.63
95.17
96.21
92.25
90.19
89.21
87.25
90.19
89.27
96.31
97.27
92.87
86.33
93.25
90.29
73.09
107.90
60.82
44.12
23.33
14.80
8.67
2
3
4
6.26
5
3.31
6
2.10
7
1.23
8
0.83
5.85
9
16.28
21.58a
13.02b
10.28c
15.63
12.74
114.74
152.09
91.76
72.45
110.16
89.79
10
11
12
13
14
I-20
I-40
I-60
a 1st run; b 2nd run; c 3rd run
All the catalysts prepared by in-situ method, precipitation method and
impregnation method were applied to synthesize DPC by oxidative carbonylation of
phenol. The results of the activity evaluation are shown in Table 2. In the synthesis of
DPC by oxidative carbonylation of phenol, the production of one molecule of DPC is
accompanied by the reduction of one molecule of Pd2+ to Pd0. The cerium manganese
oxide acts as a metal cocatalyst to participate in the multi-step electron transfer
process in the oxidative carbonylation, promoting the conversion of Pd0 to Pd2+. The
synergistic effect of bimetallic cerium manganese enhances the redox of the catalyst.
Comparing the activity evaluation data in Table 2, generally speaking, the activity of
the catalyst prepared by the impregnation method, is the highest, and the single yield
of I-20 reaches 21.58%. This is mainly due to the fact that the pore structure of the
silicon-based aerogel in the catalyst prepared by the impregnation method remains
11 / 19

intact and has a larger specific surface area. The particle size of the cocatalyst of
cerium manganese oxide is small, highly dispersed on the carriers, and the surface of
the catalyst is relatively highly capable of good oxygen storage and surface oxygen
transfer, helping to improve multi-step electron transfer efficiency, so the catalyst has
the most active sites. While the catalyst prepared by the precipitation method is
almost inactive, the highest yield of C-10 is only 3.31%. The main reason is that a
large amount of agglomerated cerium manganese metal oxide particles in the catalyst
prepared by the precipitation method blocks the pores of the aerogel network, and the
dispersion of the cocatalyst is extremely uneven, resulting in the extremely small
specific surface area of the carriers and few active sites of the catalyst, which hinder
the multi-steps electron transfer process. In addition, the loading of the metal oxide
also affects the activity of the catalyst, even when the preparation method is the same.
For example, I-10, I-20, I-40 and I-60 prepared by the same impregnation method,
have different DPC yields, that is 16.28%, 21.58%, 15.63%, 12.74% respectively. The
main reason is that when the composite of cerium manganese oxide cocatalyst just
started, the active sites increased with the increase of its loading, which helped to
promote the multi-step electron transfer process and improved the regeneration
efficiency of the active component Pd2+. However, when the loading amount increases
to a certain degree, the cerium manganese metal oxide crystal grains gradually grow
up, and the poly groups gather and block the pores, which causes the specific surface
areas and pores volume of the carriers to decrease, so the active sites are instead
reduced. It is not conducive to the reactants’ adsorption on the surface and desorption
of the product in the catalyst, so the activity of the catalyst is reduced (this is
consistent with the BET results).
The regeneration of I-20 catalyst was done and they were used in the
carbonylation reaction with the fresh mixture. The yield decreased to about 13.02% in
the second run and 10.28% in the third run. But the selectivity remained little changed,
i.e., 92.87 to 86.33%. The main results are that the active component Pd is very easy
to aggregate and Pd2+ is very poor to be recycled.
3.3. Reasons for catalyst synergy
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Studies have shown that the redox ability of the cocatalyst and the surface
oxygen species play a crucial role in the redox regeneration of the active component
Pd2+ [8], so we studied the redox abilities of the catalysts prepared by different
methods, via H2-TPR profiles and O2-TPD profiles (Fig. 5), compared the XPS
spectrum (Fig. 7) to study the surface oxygen species of the catalysts, and explored
the reasons for the difference of catalytic activity.
(b)
(a)
Fig. 5. H2-TPR (a) and O2-TPD (b) of the silicon-based composite aerogel supported Pd catalysts.
Fig. 5(a) is a H2-TPR spectrum of a silicon-based composite aerogel supported
Pd catalysts obtained by different preparation methods. It can be seen from the graph
that the curves of reduction processes of the three catalysts show approximate peak
shapes, mainly including the reduction of two reduction processes, MnO2 → MnO
and CeO2 → Ce2O3. At low temperatures, 150-250 ℃ is mainly caused by Mn4+→
Mn3+ reduction process [8]; 250-400℃ is mainly caused by Mn3+→ Mn2 reduction
process. At high temperatures, above 500℃is mainly caused by the reduction of CeO2
in the bulk phase (Ce4+→Ce3+) [20]. In the temperature-programmed reduction
process, the initial reduction temperature indicates the level of surface activity of the
sample and the strength of structural stability. Comparing the three curves, it is found
that the initial reduction temperatures of the silicon-based composite aerogel
supported Pd catalysts obtained by different preparation methods are different. The
initial reduction temperatures of the catalyst I-20 prepared by the impregnation
method, is the lowest (150 ℃), and the initial reduction temperature of the catalyst
C-20 prepared by the precipitation method is the highest (the first H2 reduction peak
almost disappears). It indicates that the average manganese valence in I-20 is
13 / 19

increased, that is, the Mn3+ and Mn2+ in the low-valent state are oxidized, while the
Mn4+ in the high valence state is increased. The catalytic process is a multi-step
electron transfer process, and the cerium-manganese composite oxide cocatalyst in
this process plays a crucial role, with multi-step electron transfer schematically shown
in Fig 6. The catalytic mechanism is as follows: when the active component Pd2+
reacts with phenol and CO to form DPC, it is reduced to Pd0 by itself; then Mn4+ in
the composite aerogel carrier, oxidizes the reduced Pd0 to Pd2+, to promote
regeneration of the active ingredients, while itself is reduced to Mn2+; Next Mn2+ can
be oxidized by Ce4+ to Mn4+, while Ce4+ is reduced to Ce3+; Finally in an oxygen
atmosphere, Ce3+ is further oxidized by O2 to Ce4+. The flexible transition between
different valence states of Ce3+ and Ce4+, makes it easy to form flowing oxygen
vacancies while maintaining structural integrity, and has better ability to store oxygen
and to transport oxygen on the surface [19], and can interface with metals generating
synergies. Thus Mn4+ increased, can effectively promote the efficiency of multi-step
electron transfer, so the surface activity of I-20 is the highest. Compared with TEM
photographs, the main reason is that I-20 particles have a small average grain size and
many surface defects, so they have good low-temperature catalytic oxidation activity
in the catalytic oxidation process (this is consistent with the activity evaluation
experiment results), so the impregnation method is good for preparation of
silica-based composite aerogel supported Pd catalyst in the synthesis of DPC by
oxidative carbonylation of phenol.
Fig. 6. multi-step electron transfer process.
The order of desorption of oxygen species on oxides is: oxygen molecule O2→
-
oxygen molecule anion O2 → oxygen atom anion O- → lattice oxygen O2- [21].
-
Surface adsorbed species of O2 and O-, belonging to the active oxygen on the catalyst
14 / 19

surface (referred to as α oxygen), are more easily desorbed, while the lattice oxygen
-
O2 (abbreviated as β oxygen) is difficultly desorbed and its desorption peak shape is
asymmetry. Desorption peaks below 500 ℃ are generally classified as desorption of
-
surface active adsorbed oxygen species (O2 and O-) on the catalyst, and desorption
peaks above 500 ℃are assigned to desorption of lattice oxygen on the catalyst (O2-)
[22]. The desorption temperature and desorption strength of different oxygen species
are closely related to the oxidation performance of the catalyst: the temperature of the
desorption peak of the surface active adsorbed oxygen species and the size of the peak
area represent the oxidation activity of the catalyst [23]. As shown in Fig. 5(b), in the
range of less than 200 ℃, oxygen desorption peaks appeared in all three catalysts, but
the start desorption temperature and the desorption peak area were different. Wherein
the start desorption temperature of adsorbe oxygen species of catalyst I-20 prepared
by impregnation method is the lowest, and its desorption peak area is the largest;
while the desorption peaks of adsorbe oxygen species of catalysts C-20 hardly
appeared. Therefore, it can be determined that the surface of the I-20 catalyst is rich in
adsorbed oxygen species, while the surface oxygen species of the catalyst C-20 is the
least. At the same time, the O2-TPD curve of the catalyst I-20 has a large desorption
peak at a temperature of 300-700 ℃, corresponding to the desorption peak of the
mobile oxygen species [24]. And the desorption peak area of the S-20 in this range is
small, while C-20 has almost no desorption peak in this range, indicating that the
surface of the catalyst I-20 has abundant mobile oxygen species. The higher the
content of mobile oxygen species in the carrier, the more oxygen vacancies which can
be provided in the redox process to adsorb and transfer adsorbed oxygen species [25].
The surface of catalyst I-20 has more oxygen vacancies and stronger oxidation ability,
can play an important role for the connection of the oxidation auxiliaries in the
oxidation carbonylation of phenol, thus improving the catalyst catalytic activity. The
oxygen desorption peak above 800 ℃ is attributed to the desorption peak of oxide
decomposition. The O2-TPD curve of C-20 indicates that the surface of the material
has almost no adsorbed oxygen, and therein oxygen appears mainly in the form of
lattice oxygen, so the catalyst has poor oxidation performance and low catalytic
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activity.
(a)
(b)
Fig. 7. XPS spectrum of a silicon-based composite aerogel supported Pd catalyst: (a) Pd3d , (b)
O1s.
In order to study the loading of the active component Pd and to compare the
surface structure of the catalyst, X-ray photoelectron spectroscopy was performed on
the sample, and the XPS spectra of the Pd3d and O1s orbits were compared. The
results are shown in Fig. 7. After peak-differenating analysis, the binding energy of
Pd3d3/2 and Pd3d5/2 in the XPS photoelectron spectroscopy of Pd on the surface of
catalyst I-20, is basically at 342.3 eV and 337.0 eV, which is basically consistent with
the XPS binding energy of standard PdO [26], indicating that the active component
supported on the surface of the catalyst is mainly in the form of Pd2+, which will be
very beneficial for the oxidative carbonylation of phenol to DPC. However, the Pd3d
tracks of the other two catalysts S-20 and C-20, shift to a lower binding energy
position significantly, corresponding to Pd0 after peak-differenating analysis. The
results show that the content of active components Pd2+ in these two catalysts is
decreased, which is not conducive to the catalytic reaction.
Two different oxygen species, adsorbed oxygen (Oα) and the lattice oxygen (Oβ)
respectively correspond to two emission peaks of similar binding energy: a high
binding energy (about 531.6 eV) showing strongly adsorbed oxygen (Oα), low binding
energy (about 529.5 eV) representing lattice oxygen (Oβ) [27]. Comparing the XPS
spectra of O ls tracks of catalysts prepared by three methods (Fig. 7 (b)), it can be
found that these surface oxygen species of three aerogel catalysts mainly exist in the
form of lattice oxygen [28], but with different content of adsorbed oxygen species.
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Among them, the content of adsorbed oxygen species in catalyst I-20 was the highest,
and the content of adsorbed oxygen species in catalyst C-20 was the lowest, which
was consistent with the results of O2-TPD. The main reason is that the average grain
sizes of the material particles are different, the lattice distortions are also different,
and the number of oxygen vacancies are different, ultimately leading to different
numbers of adsorbed oxygen. The adsorbed oxygen species present on the surface of
the catalysts can effectively promote the redox cycle of the active component Pd2+.
Thus in the oxidative carbonylation of phenol to DPC, catalyst I-20 has relatively high
catalytic activity, so the impregnation method are more suitable to produce
silicon-based composite aerogel support Pd catalysts.
4. Conclusion
A series of metal cerium manganese silicon-based composite aerogel supported
Pd catalysts were prepared by the in-situ method, precipitation method and
impregnation method respectively, and these catalysts were applied to the synthesis of
DPC by oxidative carbonylation of phenol. The effects of different preparation
methods and the content of cerium manganese oxide on the catalytic activity were
investigated. The results show that the catalyst prepared by the impregnation method
has relatively good catalytic activity, mainly due to: 1. the catalyst prepared by
impregnation has a large specific surface area and a small particle size, contributing to
the dispersion of active centers and the increase of active sites, improving the catalytic
activity; 2. the catalyst prepared by the impregnation method has better low
temperature oxidation performance and more surface adsorption oxygen, which will
help to improve the multi-step electron transfer efficiency and promote the
regeneration of the active component Pd2+, thereby increasing the catalytic activity.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are grateful for the financial support from Hubei Provincial Natural Science
Foundation of China (Grant No.2019CFB318) and Scientific Research Fund of
Wuhan Institute of Technology (Grant No.K201941).
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Highlights
1. Catalysts were prepared by the in-situ method, the precipitation
method and the impregnation method.
2. The composite method affect the particle size and specific surface area.
3. The composite method affect the surface oxygen species.

Declaration of Interest Statement
The authors declared that they have no conflicts of interest to this
work. We declare that we do not have any commercial or associative
interest that represents a conflict of interest in connection with the work
submitted.