Photo-deposition preparation of supported Pd catalysts for non-phosgene one-step synthesis of diphenyl carbonate

Article abstract of DOI:10.1016/j.catcom.2018.10.017

Pd catalysts anchored on manganese oxide octahedral molecular sieves (OMS-2) were synthesized by photo-deposition method, and then used for one-step oxidative carbonylation of phenol to synthesize diphenyl carbonate (DPC). The results showed that the catalytic activity of Pd/OMS-2 catalyst prepared by photo-deposition was apparently better than those prepared by the traditional precipitation and impregnation method, the distribution of Pd particles was more uniform with the average size of 1.3 nm. This may result from the strong interaction between the carriers and the active Pd species. Irradiated for 5 h at pH 5.0, with the Pd loading of 2.5 wt%, the highest DPC yield of 18.1% was achieved.

Full text of DOI:10.1016/j.catcom.2018.10.017

Accepted Manuscript
Photo-deposition preparation of supported Pd catalysts for non-
phosgene one-step synthesis of diphenyl carbonate
Changjiang Wu, Xiaojun Yang, Qifeng Tian, Hang Bai, Xiaolu Li
PII:
S1566-7367(18)30547-8
DOI:
Reference:
https://doi.org/10.1016/j.catcom.2018.10.017
CATCOM 5528
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3 October 2018
19 October 2018
Please cite this article as: Changjiang Wu, Xiaojun Yang, Qifeng Tian, Hang Bai, Xiaolu
Li , Photo-deposition preparation of supported Pd catalysts for non-phosgene one-step
synthesis of diphenyl carbonate. Catcom (2018), https://doi.org/10.1016/
j.catcom.2018.10.017
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ACCEPTED MANUSCRIPT
Photo-deposition Preparation of Supported Pd Catalysts for Non-phosgene One-step Synthesis of
Diphenyl Carbonate
Changjiang Wua, Xiaojun Yang a, b*, Qifeng Tian a*, Hang Baia, Xiaolu Lib
(aKey Laboratory for Green Chemical Process of Ministry of Education, Hubei Key
Laboratory of Novel Reactor and Green Chemical Technology, School of Chemical
Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430205, P. R.
China
bBioproduct Sciences and Engineering Laboratory, Department of Biological Systems
Engineering, Washington State University, Richland, WA 99354, USA)
E-mail: 10100201@wit.edu.cn (X. Yang); qftian@wit.edu.cn (Q. Tian)
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Abstract
Pd catalysts anchored on manganese oxide octahedral molecular sieves (OMS-2)
were synthesized by photo-deposition method, and then used for one-step oxidative
carbonylation of phenol to synthesize diphenyl carbonate (DPC). The results showed
that the catalytic activity of Pd/OMS-2 catalyst prepared by photo-deposition was
apparently better than those prepared by the traditional precipitation and impregnation
method, the distribution of Pd particles was more uniform with the average size of 1.3
nm. This may result from the strong interaction between the carriers and the active Pd
species. Irradiated for 5 hours at pH 5.0, with the Pd loading of 2.5 wt. %, the highest
DPC yield of 18.1% was achieved.
Keywords: Manganese oxide octahedral molecular sieve; photo-deposition;
strong metal-support interaction; oxidative carbonylation.
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1. Introduction
Diphenyl carbonate (DPC) is an important environmentally benign intermediate,
which can be applied to produce pharmaceutical chemicals, pesticides, and polymer
materials [1–3]. To date, the main routes for DPC synthesis include phosgene process,
transesterification approach, and oxidative carbonylation of phenol. Among them,
phosgene process is extremely toxic followed by alkali neutralization of the
by-product hydrochloric acid, producing a large amount of waste stream, which
makes this process not sustainable and economical [4,5]. Also, the reaction
equilibrium constant of the transesterification method is too low to obtain high yield
and selectivity [6]. Oxidative carbonylation of phenol direct utilizing phenol, CO and
O2 is an attractive green approach which takes the advantage of low-cost and
available raw material with only water as by-product [7]. Considerable contributions
have been made to the development of catalytic oxidative carbonylation reactions.
The vast majority of these researches focus on an efficient and easily separated
supported palladium catalysts [8].
The catalytic performance of supported catalysts is closely related to the particle
size of metal loaded on the support [9]. It is generally believed that the catalytic
activity of supported catalysts can be improved by decreased size, which is known as
“size effect” of the nanoparticles. The activity improvement is caused by highly
dispersed nanoclusters with high exposed active sites and maximized metal atom
utilization [10–12]. The supported catalysts prepared by the traditional precipitation
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method and impregnation method are prone to thermal sintering during the calcination
process, resulting in low dispersity and aggregation of the metal particles on support
surface. It could make the supported catalysts deactivated or deteriorated [13]. In
recent years, photo-deposition method was found to be a fine alternative with low
metal loading, high dispersity and stable performance [14]. Aided with photo-assisted
redox reaction, the decomposition of metal precursor occurred and atomically
dispersed catalysts were obtained [13]. According to Zheng’s research [15], ethylene
glycol (EG) radical could be released on the TiO2 surface under mild UV conditions,
then a highly stable, atomically dispersed Pd catalyst was anchored on TiO2
nanosheets through a unique “PdCl2-EG radical-TiO2” interface. The catalyst shows a
very high catalytic activity in hydrogenation of C=C and C=O. In theory, the utmost
distribution of supported metal catalysts is monoatomic dispersity [16]. Therefore,
photochemical technology provides us with a tactic to obtain high-performance
supported catalysts [17–20].
A new type of molecular sieves, manganese oxides octahedral molecular sieves
(OMS-2), has been extensively investigated in catalysis, gas cleansing, separation,
batteries, and chemical sensing. OMS-2 can also be used as photocatalysts due to the
property of semiconducton [21]. Liu et al. studied the solid-phase photocatalytic
degradation of polyethylene film with OMS-2 in the ambient air under ultraviolet and
visible light irradiation [22]. Iyer et al. found that K-OMS-2 prepared by solvent-free
method showed the highest activity for selective oxidation of 2-propanol to acetone
under visible light irradiation [23]. Based on the photocatalysis performance of
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OMS-2, we aimed to prepare palladium supported catalysts on OMS-2 by
photo-deposition method. This method may afford high performance catalysts with
small sizes and high dispersity. The palladium catalysts with smaller particle size and
higher dispersity prevented the reduction and sintering of the active species [24,25].
Therefore, we prepared palladium supported catalysts by photo-deposition method,
and the properties and catalytic performance were evaluated in the oxidative
carbonylation reaction compared with that prepared by the traditional precipitation
and impregnation method.
2. Experimental
2.1 Catalysts Preparation
The manganese oxide octahedral molecular sieves (OMS-2) was prepared by a
conventional reflux method [26,27]. The supported Pd/OMS-2 catalysts were
prepared by photo-deposition (PD) (Figure S1), precipitation (Pre) and impregnation
(IM) methods. The palladium precursor solution was obtained by dissolving the solid
PdCl2 (1.68 g) in 200.0 ml deionized water with the pH being adjusted to 2.0 with
concentrated hydrochloric acid. Please see the Supplementary Materials for additional
details.
2.2 Catalysts activity evaluation
The oxidative carbonylation reaction was performed in a 250.0 ml stainless steel
autoclave equipped with a magnetic stirrer. Phenol (50.0 g), tetrabutylammonium
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bromide (TBAB, 1.0 g) for stabilizing the Pd nanoparticles [28]) and the as-prepared
catalyst (1.0 g) were introduced into the autoclave. Then the autoclave was sealed and
heated to 65 ℃. Subsequently, the reaction was carried at a pressure of 5.0 MPa and
the gas mixture of CO and O2 (CO/O2 = 47/3 molar ratio) was charged. The reaction
was allowed to proceed for 4 hours as a cycle. Then, the autoclave was cooled down
and the products were collected. The yields of final products were determined by a
capillary gas chromatography instrument (GC-8000, China) with a FID detector and a
SE-54 capillary column. The activity tests were carried out under a kinetic regime
without the impact of mass transfer limitations.
2.3 Catalysts Characterization
The X-ray diffraction (XRD) patterns were recorded on a BRUKER D8
ADVANCE X-ray diffractometer with Cu Kα radiation source. The 2θ scanning range
was from 20° to 80°, and the operating voltage and current were set as 40 kV and 40
mA, respectively. Transmission electron microscope (TEM) images were obtained
with a JEOLJEM-2100 instrument at an acceleration voltage of 200 kV. X-ray
photoelectron spectroscopy (XPS) was carried out on a VG Multilab 2000 using Al Kα
radiation. The spectra were corrected by referencing the C1s peak at 284.8 eV. H2
temperature-programmed reduction (H2-TPR) profiles of the samples (ca. 100 mg)
were obtained on an AutoChem II 2920 Chemisorption Analyzer. The H2-TPR profile
of each sample was recorded during the reduction process at ramp of 10 ℃/min from
room temperature to 500 ℃ in a mixture of 5% H2+ 95% Ar (25 mL / min).
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3. Results and discussion
3.1 Characterization of catalysts
Figure 1 showed the XRD patterns of three catalysts. Diffraction peaks of all
samples at 2θ=28.9°, 37.5°, 42.1° and 50.0° were consistent with the standard
diffraction peaks of OMS-2(JCPDS 29-1020), indicating that all the as-prepared
catalysts retained the same crystal structure. After Pd loaded, the crystal structure of
OMS-2 did not change. However, no diffraction peak of palladium species appeared,
which suggested that Pd particles were highly dispersed on OMS-2 supports. It was
reported that characteristic diffraction peaks in XRD pattern cannot be observed when
the average particles size is smaller than 4.0 nm [29]. In addition, the 2.5 wt. %
loading of Pd is relatively low, so we speculate that Pd is well dispersed on the
surface of OMS-2 supports with small particle size.
TEM images in Figure 2 demonstrate the particle distribution of active palladium
species. It could be seen that the particle size of Pd prepared by photo-deposition
method is very small and well distributed even if being calcined. However, the Pd
nanoparticles prepared by precipitation and impregnation method were severely
agglomerated in Figure S2. Nagai’s reported that the sintering inhibition effect on Pt
can be controlled by the electron density of oxygen in the support through the
Pt-oxide-support interaction [30], even if being calcined. Here, we used ethylene
glycol radicals and Pd to form complexes and to enhance the interaction between Pd
species and OMS-2. Moreover, the dispersity of Pd particles was yet well with the
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mass ratio of Pd loading increasing from 2.5 wt. % to 5.0 wt. %, although Pd particle
sizes increased from 1.3 nm to 3.5 nm.
The catalysts were further characterized by H2-TPR to analyze the interaction
between Pd species and OMS-2 supports, and the spectra are displayed in Figure 3.
For each sample, there was only one wide reduction peak can be observed at below
400℃. The presence of wide plateaus in the spectra indicates multiple reduction
peaks overlap together [25,31]. Zhang [32] had reported that the hydrogen reduction
peaks of Pd/OMS-2 catalyst at 100℃~200℃ involved one reduction peak of PdO to
metallic Pd0. Compared to Pd/OMS-2-Pre and Pd/OMS-2-IM catalysts, onset
temperature of reduction peak for Pd/OMS-2-PD catalyst is the lowest at 58 ℃,
indicating the strongest redox ability at low temperature. The spectra also revealed
that the reducing properties of Pd species are strongly affected by the preparation
methods. The Pd/OMS-2-PD catalysts possess much stronger metal-support
interaction than the other two, which resulted in more highly dispersed Pd species and
better catalytic performance in the oxidative carbonylation of phenol.
To obtain detailed information on surface compositions and chemical state of
each element for all samples, the XPS analysis was conducted. The Pd 3d XPS spectra
of the samples are shown in Figure 4, and their quantitative analysis results are
summarized in Table S1. As shown in Figure 4, Pd0 is absent from all samples,
because the binding energy of all peaks shifts toward a higher binding energy at
337.2eV related to Pd 3d5/2. This phenomenon shows that there is no fitting peak for
Pd0 after the peak separation. The binding energy and the half peak width of Pd 3d5/2
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are negatively correlated to the size of the Pd nanoparticles, so it can be inferred that
the Pd nanoparticles in the catalyst have extremely small size and are dispersed on the
surface of the support. Compared to the Pd/OMS-2-Pre and Pd/OMS-2-IM catalysts,
the Pd 3d binding energy of the Pd/OMS-2-PD catalyst is the highest, suggesting the
size of the Pd nanoparticles is the smallest. This conclusion is consistent with the
TEM results. The content of Pd4+ in Pd/OMS-2-PD-before calcined and
Pd/OMS-2-PD is higher than Pd/OMS-2-Pre and Pd/OMS-2-IM. It suggests the
valence state of palladium species for Pd/OMS-2-PD samples remains bivalence or
higher before and after calcination. During the photo-deposition process, the presence
of palladium oxides is commonly ascribed to the surface oxidation of nanparticulate
by prolonged extensive exposure to air. Moreover, these nanoparticles prepared by the
photo-deposition method are easily oxidized at a low temperature due to small size of
Pd nanoparticles, which results in a transfer from Pd0 to active Pd2+. It can be
expected that the Pd/OMS-2-PD catalyst may have better catalytic activity than the
Pd/OMS-2-Pre and Pd/OMS-2-IM catalysts.
As shown in Figure 5, O1s XPS spectra of the samples could be deconvoluted
into two peaks of oxygen species: one lattice oxygen (Oα) peak bonded to metal
cations in a coordinatively saturated environment assigned at 529.7 eV, and another
surface adsorbed oxygen (Oβ) peak bonded to metal cations in a low-coordination
environment positioned at 531.3 eV [33]. As seen from Table S2, the Pd/OMS-2-PD
sample possesses the highest Oβ/Oα atomic ratio. Furthermore, the increased oxygen
vacancies is conducive to the migration of surface oxygen species, so the rate of
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oxygen species migration on the surface of Pd/OMS-2-PD catalyst is the fastest
among the three catalysts.
3.2 Catalytic performance in oxidative carbonylation of
phenol
Figure 6 shows the catalytic activity of supported Pd catalyst prepared by three
methods in oxidative carbonylation of phenol. The results showed that the catalytic
activity of Pd/OMS-2-PD was better than that of Pd/OMS-2-Pre and Pd/OMS-2-IM
under the same conditions with a longer catalyst life and no significant loss of
catalytic activity after 24 h, and the stability of Pd/OMS-2-PD catalyst was also
proved by the cycle experiments (Table S3). The DPC yield reached 20.4% at 24h.
Whereas, the catalyst prepared by precipitation method and impregnation method was
inactive after 20 h. The deactivation cause maybe ascribed to Pd leaching, which has
been discussed in our previous study [25]. The DPC yield (using Pd/OMS-2-Pre
catalysts) remained 15.5%. The order of the catalytic activity was: Pd/OMS-2-PD >
Pd/OMS-2-Pre > Pd/OMS-2-IM. For three kinds of catalysts prepared, the selectivity
are all above 95.0 % (Table S4).
The activity data illustrated that Pd/OMS-2-PD has a significantly higher activity,
probably related to the preparation process. For the catalyst prepared by
photo-deposition, the ethylene glycol in the solution becomes ethylene glycol radicals
-
after irradiation. Cl of PdCl42- is gradually replaced by ethylene glycol radicals to
form a complex [PdCl4-x (EG) x]-2+x. Then the coordinated ethylene glycol gradually
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transforms Pd2+ into metallic Pd. A part of the ethylene glycol and Pd continues to
coordinate, so that the Pd particles are consolidated and the aggregation can be
inhibited. The resultant Pd nanoparticles are eventually oxidized to PdO. The reaction
mechanism is shown in Figure S3. When calcinated at high temperature, the Pd
particle size of the Pd/OMS-2-PD catalyst remains almost the same as that uncalcined.
While the Pd particles of Pd/OMS-2-Pre and Pd/OMS-2-IM samples are easily
aggregated without consolidation with EG radical. The corresponding aggregation
was observed in Figure 2.
Numerous studies have shown that the pH value of the precursor solution is a
crucial factor influencing the catalytic performance of the Pd catalyst. Considering
that the isoelectric point (IEP) of OMS-2 is about 6.2 [34], the net charge of OMS-2
surface at pH <6.0 can be positive and can absorb anion. PdCl42- can be combined
with OMS-2 through electrostatic attraction. However, the electrostatic force between
the support and PdCl42- is relatively low, so Pd species are easy to sinter or fall off
after calcination. Table S5 shows the effects of pH value on catalytic activity during
preparation. There is no significant disagreement on DPC yield with varied pH value,
suggesting that it is limited for electrostatic force to immobilize Pd species, which
further explains the importance of EG radical in photo-deposition method.
Besides, the performance of the catalysts of Pd/OMS-2-PD prepared with
different Pd loadings was studied. As shown in Table S5, the reaction rate rises with
the increase of Pd loading from 0.5 wt. % to 2.5 wt. %. We propose that the strength
of the Mn-O bond is weakened due to the strong interaction between ethylene glycol
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and the support, which favors the adsorption and activation of CO and phenol by
active Pd2+ sites on the manganese oxide [35]. Therefore, the catalytic activity is
enhanced. However, the high Pd loading causes the aggregation of Pd noble metals
into larger particles, and then the catalytic activity is reduced.
4. Conclusion
Compared with traditional precipitation method and impregnation method,
photo-deposition method can be used to prepare small-size, high dispersity, and
long-life catalyst. The excellent properties were confirmed by TEM images, H2-TPR
analysis and activity test data. The particle size of catalysts prepared by the
photo-deposition method is 1.3 nm. The excellent performance is related to the
2-
coordination of ethylene glycol radicals and PdCl4 , which facilitates the
consolidation and homogenous dispersion of small-size Pd particles. There is no
significant loss of catalytic activity after 24 hours, and the DPC yield was up to 20.4%
at 24 hours.
Acknowledgements
We are grateful for the financial support from Hubei Provincial Natural Science
Foundation of China (Grant No.2016CFA079) and the Graduate Innovative Funds of
Wuhan Institute of Technology (No.CX2017132).
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Figures and scheme Captions
Figure 1. XRD patterns of Pd/OMS-2 by different methods: Pd/OMS-2-PD,
Pd/OMS-2-Pre, and Pd/OMS-2-IM
Figure 2. TEM images of (a) Pd/OMS-2-PD with 2.5wt% loading and (b)
Pd/OMS-2-PD with 5wt % loading, and distribution histograms of the Pd particles in
Pd/OMS-2-PD with 2.5wt% loading and Pd/OMS-2-PD with 5wt % loading
Figure 3. H2-TPR profiles of the catalysts prepared by different methods:
Pd/OMS-2-PD, Pd/OMS-2-Pre and Pd/OMS-2-IM
Figure 4. Pd 3d spectra of Pd/OMS-2-PD-before calcined, Pd/OMS-2-PD,
Pd/OMS-2-Pre and Pd/OMS-2-IM
Figure 5. O 1s spectra of Pd/OMS-2-PD-before calcined, Pd/OMS-2-PD,
Pd/OMS-2-Pre, and Pd/OMS-2-IM
Figure 6. Catalytic activity of different catalysts: Pd/OMS-2-PD, Pd/OMS-2-Pre, and
Pd/OMS-2-IM
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Highlights
 Photo-deposition can prepare high dispersity, small-size and long-life
catalyst.
 The Pd particles were homogenously dispersed and consolidated on the
OMS-2.
 The yield of DPC can reach 18.1% by using the catalyst of
photo-deposition method.
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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6