Synthesis of hydrogen peroxide over Pd/SiO2/COR monolith catalysts by anthraquinone method

Article abstract of DOI:10.1016/j.cattod.2016.03.023

In this work, a series of Pd/SiO₂/COR (cordierite) catalysts with different Pd contents were successfully prepared by impregnation method. The obtained Pd/SiO₂/COR catalyst samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), BET, hydrogen temperature programmed reduction (H₂-TPR), inductively coupled plasma-atomic emission spectrometry (ICP-AES), and X-ray photoelectron spectroscopy (XPS). X-ray photoelectron spectroscopy (XPS) show that Pd element is mainly in the state of PdO, and 1.5% Pd/SiO₂/COR catalyst with the theoretical Pd content of 1.5% has the most PdO content on the monolith channel surface when compared to other Pd content monolith catalysts in this paper. Then in the experiment it was found the optimum catalyst is 1.5% Pd/SiO₂/COR, which is consistent with XPS results. The STY can achieve 793.2 g H₂O₂ g^?1 Pd h^?1 over 1.5% Pd/SiO₂/COR catalyst.

Full text of DOI:10.1016/j.cattod.2016.03.023

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Synthesis of hydrogen peroxide over Pd/SiO2/COR monolith catalysts
by anthraquinone method
Yanyan Guoa, Chengna Daia, Zhigang Leia,∗, Biaohua Chena, Xiangchen Fangb,∗
a State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 266, Beijing, 100029, China
b Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC, Liaoning Fushun, 113001, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, a series of Pd/SiO2/COR (cordierite) catalysts with different Pd contents were success-
fully prepared by impregnation method. The obtained Pd/SiO2/COR catalyst samples were characterized
by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron
microscopy (HRTEM), BET, hydrogen temperature programmed reduction (H2-TPR), inductively cou-
pled plasma-atomic emission spectrometry (ICP-AES), and X-ray photoelectron spectroscopy (XPS).
X-ray photoelectron spectroscopy (XPS) show that Pd element is mainly in the state of PdO, and 1.5%
Pd/SiO2/COR catalyst with the theoretical Pd content of 1.5% has the most PdO content on the monolith
channel surface when compared to other Pd content monolith catalysts in this paper. Then in the exper-
iment it was found the optimum catalyst is 1.5% Pd/SiO2/COR, which is consistent with XPS results. The
STY can achieve 793.2 g H2O2 g−1 Pd h−1 over 1.5% Pd/SiO2/COR catalyst.
Received 28 October 2015
Received in revised form 9 March 2016
Accepted 11 March 2016
Available online xxx
Keywords:
Catalytic hydrogenation
H2O2
Pd/SiO2/COR
Monolith catalyst
Anthraquinone process
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
products, because it produces H2O2 in the autoxidation process.
Meanwhile, H8eAQH2 does not produce H2O2, so it can be called
Hydrogen peroxide (H2O2) is an important green chemical
raw material and product widely used as oxidant, bleach, disin-
fectant, deoxidizer, polymer initiator and crosslinking agent. Its
applications include military, textile, papermaking, chemical, phar-
maceutical, environmental protection, food industries, and others
[1,2].
non-active product [6,7,9,10–14].
The hydrogenation process requires a catalyst, and currently
palladium-based pellet catalysts are mainly used in industrial
applications [7,15–25]. Halder and Lawal [22] conducted the reac-
tion in a microreactor with inner diameter 0.775 mm and the
loading 13 mg supported Pd catalyst (1% Pd). Drelinkiewicz et al.
[26] prepared hydrogen peroxide over palladium-based pellet cat-
alyst in a turbulence reactor with the diameter 5.4 cm and the
loading 19 g (25 cm3) supported Pd catalyst (2% Pd). Edvinsson et al.
[27] reported the synthesis of H2O2 in a commercial slurry reactor
at 55 ◦C and 200 kPa. In recent years, as a kind of novel catalysts,
monolithic honeycomb catalysts are widely used in gas, liquid and
multiphase systems due to their unique characteristics such as low
pressure drop, high specific external surface area and effective-
ness factor, minimum axial dispersion, and less catalyst attrition
[28–32].
Structured catalysts may have great potential to improve the
conversion or selectivity during the anthraquinone hydrogenation
process [33,34]. Moreover, monolith catalyst has lower pressure
drop and better mass transfer performance than the traditional
pellet catalyst. In this work the palladium-based monolith cata-
lyst using SiO2 as the secondary carrier (i.e., Pd/SiO2/COR monolith
catalyst) was proposed for the catalytic hydrogenation of eAQ for
the synthesis of H2O2, aiming to combine the advantages of pal-
ladium active centers (high activity) and monolith honeycomb
support (low pressure drop and high capacity). Then, the prepared
Previously, hydrogen peroxide was mainly prepared by
hydrolysis of the ammonium peroxydisulfate. Today, more
than 99% of hydrogen peroxide is commercially manufactured
by the anthraquinone method, including the 2-ethyl-9,10-
anthraquinone (eAQ) hydrogenation process and the autoxidation
process of 2-ethyl-9,10-anthrahydroquinone (eAQH2, one of
products of eAQ hydrogenation process), while the catalytic
hydrogenation of eAQ process is the key step [3–8]. As
the target product of hydrogenation of eAQ process, eAQH2
is formed in the quinone-hydroquinone stage but under-
goes further hydrogenation reaction. The by-products mainly
include 2-ethyl-5,6,7,8-tetrahydroanthracene-9,10-diol and 2-
ethyl-1,2,3,4-tetrahydroanthracene-9,10-diol (H4eAQH2), and 2-
ethyl-1,2,3,4,5,6,7,8-tetrahydroanthracene-9,10-diol (H8eAQH2).
H4eAQH2 is the only desired compound among all of the by-
∗
Corresponding authors.
E-mail addresses: leizhg@mail.buct.edu.cn (Z. Lei),
fangxiangchen.fshy@sinopec.com (X. Fang).
http://dx.doi.org/10.1016/j.cattod.2016.03.023
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Guo, et al., Synthesis of hydrogen peroxide over Pd/SiO2/COR monolith catalysts by anthraquinone
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Pd/SiO2/COR monolith catalysts were characterized by XRD, SEM,
BET, H2-TPR, ICP-AES, and XPS. Finally, the influence of differ-
ent operating parameters (catalyst content, reaction temperature,
pressure, flow rates of inlet H2 and eAQ solution, and concentration
of eAQ solution) on reaction performance was investigated to get
the optimum condition for the synthesis of H2O2 over Pd/SiO2/COR
catalyst.
30 eV for high-resolution scans. The binding energy calibration of
all spectra was referenced to the adventitious carbon (C1s) signal
at 284.6 eV to reduce the charging effect of samples.
2.3. Catalytic activity measurement Pd/SiO2/COR
The catalytic hydrogenation of eAQ was performed in a stain-
less steel fixed-bed reactor (20 mm in inner diameter and 450 mm
in length) with a central thermocouple to measure the temper-
ature of reaction zone under atmospheric pressure. There are 11
elements of Pd/SiO2/COR monolith catalysts packed in the constant
temperature zone of the reactor and embedded between the pre-
treated cordierite supports in both sides of the reaction zone to
support the catalyst bed. The 11 elements were tightly installed
so that the gaps between each other, as well as between monolith
section and stainless steel tube, can be ignored. Before the hydro-
genation experiments, the catalysts were reduced in situ under
hydrogen atmosphere at 150 ◦C, and then the reactor was cooled
down to the reaction temperature. The feedstock (eAQ dissolved in
C9 aromatics and trioctyl phosphate mixture with a volume ratio
of 3:1, the concentration of eAQ solution was 60 g L−1) was then
pumped into the reactor by double-plunger pump, while hydrogen
was input through a gas mass flow meter. Then, the hydrogenated
anthraquinone working solution was pumped into the oxidation
device for oxidation reaction by the air from oil-free air genera-
tor (GC-ready SPB-5000 Automatic Air Source) to produce H2O2
at room temperature. Afterwards, the oxidated working solution
(H2O2 solution) was extracted with 25.2 wt% sulfuric acid solution
several times.
2. Experimental
2.1. Preparation of Pd/SiO2/COR
A series of Pd/SiO2/COR monolith catalysts were prepared using
coating method according to the following four steps. 1) The pre-
treatment of cordierite monolithic support: the cordierite monolith
samples with a diameter of 20 mm and a height of 10 mm were
cut from a commercial honeycomb cordierite (ϕ 101.6 × 127 mm,
400 cpi square channels), and then pretreated using 15 wt% nitric
acid solution at 80 ◦C for 4 h. Afterwards, the pretreated cordierite
monolith samples were washed using deionized water to neutral,
dried at 100 ◦C for 4 h and calcined in a Muffle furnace at 550 ◦C
for 4 h; 2) The coating of SiO2: the cordierite samples after acid
treatment were immersed into the silicon sol solution for 5 min
and then dried at 100 ◦C. The coating procedure was repeated sev-
eral times. Then the samples (SiO2/COR) were dried at 100 ◦C for
4 h and calcined at 550 ◦C for 4 h; 3) The coating of active cata-
lyst Pd: the samples obtained in step 2) were immersed into the
6 mg·mL−1 PdCl2 aqueous solution, and then dried as the coat-
ing of SiO2. The procedure was repeated several times to achieve
the desired coating amount of Pd. Thus, a series of PdCl2/SiO2/COR
precursors can be obtained; 4) The activation of precursor: the pre-
cursor was calcined at 550 ◦C to remove the chlorine element, and
the Pd/SiO2/COR monolith catalysts were obtained. The theoretical
content means that the Pd in the solution is all supported onto the
catalyst based on theoretical calculation for loading target content.
The content of H2O2 obtained from the oxidation of eAQH2
was measured by the KMnO4 titration. The yield of H2O2
ꢀ
ꢁ
(H2O2(mol)/EAQ(mol)) · 100% and the space time yield (STY,
(H2O2(g)/Pd(g))/time(h)) were introduced to characterize the cat-
alytic performance of Pd/SiO2/COR catalysts with different Pd
contents.
2.2. Characterization
3. Results and discussion
X-ray diffraction (XRD) patterns were collected from a Bruker D8
Advance X-ray diffractometer (40 kV, 40 mA) using Cu-K␣ radiation
(␭ = 0.15418 nm). Data were recorded in the 2Â range from 5◦ to 50◦
with a step size of 5◦ and a count time of 1 min per step.
Scanning electron microscopy (SEM) micrographs were
recorded on a JEOL JSM-6701F microscope working at 5.0 kV
accelerating voltage. Before observation, the samples were subse-
quently sputter coated with a thin gold film by an ion-sputtering
instrument to make the sample conductive.
3.1. Characterization of monolith catalysts
3.1.1. XRD analysis
Fig. 1 shows the XRD patterns of the cordierite (COR), the pre-
treated COR, the SiO2/COR, and the Pd/SiO2/COR monolith catalysts
The Brunauer-Emmett-Teller (BET) specific surface areas of the
catalysts were determined by using a Micromeritics ASAP-2020
sorptometer apparatus. Before the measurement of BET, the sam-
ples were degassed at 350 ◦C for at least 5 h under vacuum. The total
surface area was calculated according to the BET method.
Hydrogen temperature programmed reduction (H2-TPR) exper-
iments of the calcined Pd/SiO2/COR catalysts were carried out on a
Thermo Electron TPD/R/O 1100 series instrument equipped with a
thermal conductivity detector (TCD). The samples were reduced in
a stream of 10% H2 flow (volume fraction, balanced by Ar) with the
flowrate of 30 mL min−1 and heating rate of 10 K min−1 from 273
to 1173 K.
Inductively coupled plasma-atomic emission spectrometry
(ICP-AES) experiments of the calcined Pd/SiO2/COR catalysts were
performed on a Shimadzu Corporation-ICP-7500.
X-ray photoelectron spectroscopy (XPS) spectra were per-
formed on an ESCALAB 250 photoelectron spectrometer (Thermo
Fisher Scientific, USA). XPS spectra were recorded using monochro-
mated Al K␣ excitation at pass energies of 200 eV for survey and
Fig. 1. XRD patterns of Pd/SiO2/COR samples with various contents of Pd. (a) COR;
(b) pretreated COR; (c) SiO2/COR; (d) 0.6% Pd/SiO2/COR; (e) 1.0% Pd/SiO2/COR; (f)
1.2% Pd/SiO2/COR; (g) 1.5% Pd/SiO2/COR; (h) 1.7% Pd/SiO2/COR; (i) 2.2% Pd/SiO2/COR.
Please cite this article in press as: Y. Guo, et al., Synthesis of hydrogen peroxide over Pd/SiO2/COR monolith catalysts by anthraquinone
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Fig. 2. SEM micrographs. (a) wall surface of COR; (b) wall surface of the pretreated COR; (c) wall surface of SiO2/COR; (d) wall surface of Pd/SiO2/COR; (e) cross section of
COR; (f) cross section of the pretreated COR; (g) cross section of SiO2/COR; (h) cross section of Pd/SiO2/COR. (i) and (j) HRTEM images of 1.5%Pd/SiO2/COR catalyst.
Please cite this article in press as: Y. Guo, et al., Synthesis of hydrogen peroxide over Pd/SiO2/COR monolith catalysts by anthraquinone
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Table 1
4
ICP-AES results of Pd actual content on different Pd/SiO2/COR catalysts.
(f)
Sample
Theoretical content of Pd
Actual content of Pd
0.6% Pd/SiO2/COR
1.0% Pd/SiO2/COR
1.2% Pd/SiO2/COR
1.5% Pd/SiO2/COR
1.7% Pd/SiO2/COR
2.2% Pd/SiO2/COR
0.6%
1.0%
1.2%
1.5%
1.7%
2.2%
0.04%
0.07%
0.08%
0.10%
0.30%
0.40%
(e)
Table 2
XPS Results of Pd 3d5/2 for different Pd/SiO2/COR catalysts.
(d)
(c)
(b)
Sample
Pd 3d5/2 binding energy (ev)
Element composition (%)
Si2p
O1s
Pd3d
(a)
0.6% Pd/SiO2/COR
1.0% Pd/SiO2/COR
1.2% Pd/SiO2/COR
1.5% Pd/SiO2/COR
1.7% Pd/SiO2/COR
2.2% Pd/SiO2/COR
337.10
335.60
335.90
335.75
336.45
336.80
32.63
30.24
31.49
28.59
31.09
34.33
56.05
61.22
62.70
61.14
61.48
61.20
0.13
0.16
0.16
0.84
0.44
0.10
0
100
200
300
400
500
600
700
800
900
T (
°C
)
Fig. 3. H2-TPR spectra of Pd/SiO2/COR samples with various contents of Pd. (a) 0.6%
Pd/SiO2/COR; (b) 1.0% Pd/SiO2/COR; (c) 1.2% Pd/SiO2/COR; (d) 1.5% Pd/SiO2/COR; (e)
1.7% Pd/SiO2/COR; (f) 2.2% Pd/SiO2/COR.
is the theoretical content of Pd) has the strongest peak at 90 ◦C
when compared to others, and also has stronger high-temperature
peaks than others. The results indicated that 1.5% Pd/SiO2/COR cat-
alyst has the most PdO content on the surface of monolith channel.
That is to say, more immersion times of SiO2/COR into PdCl2 solu-
tion would no longer be benefit to the catalytic hydrogenation of
eAQ. It is reported that Pd plays a key role in hydrogenation stage
of anthraquinone hydrogenation reaction [36,37]. Thus, before the
hydrogenation experiments, the Pd/SiO2/COR catalysts in PdO state
must be reduced in situ under hydrogen atmosphere at 150 ◦C to
obtain the Pd state catalyst.
with different Pd contents. It can be seen that the diffraction peaks
are typical peaks of cordierite, which are consistent with the XRD
standard PDF card no. 089-1487 [35]. PdO peak is overlapped with
cordierite peak at 2Â = 33.6◦, 33.9◦, 29.3◦ and other peak of PdO
peak is at 2Â = 41.9◦, which are consistent with the XRD standard
PDF card no.06-0515. After treatment, the cordierite peaks did not
decrease and were kept constant. In addition, the intensity of the
cordierite peaks after treatment is the same as before treatment,
indicating that the acid treatment and calcination did not change
the cordierite structure.
3.1.4. ICP analysis
3.1.2. SEM and BET analysis
ICP-AES experiments were carried out to measure the actual ele-
ment contents in the bulk coating of Pd/SiO2/COR catalysts, and the
results are listed in Table 1. There is a large difference between the
theoretical and actual (ICP-AES result) contents of Pd for all tested
samples, indicating that the interaction between SiO2 coating and
Pd species is not strong enough to coat more active component Pd.
However, the theoretical content of Pd and ICP-AES results exhibit
the similar trend.
Fig. 2 illustrates the SEM micrographs on the surface and cross
section of the COR, SiO2/COR, and Pd/SiO2/COR monolith catalysts.
The macropores on the cordierite surface become bigger after acid
treatment. The cross sectional view of the SiO2/COR sample shows
that the colloidal silica washcoat tends to accumulate in the corners
of monolith structure. Fig. 2(c) shows that SiO2 has a uniform distri-
bution on the surface of COR channel wall, to provide more surface
area to coat the catalytic center Pd. However, no information has
been obtained about palladium, which is probably due to the rela-
tive low Pd content and the good dispersion of Pd. Thus, the small
Pd particles were not detected by SEM. From the HRTEM image as
shown in Fig. 2(i), it can be seen that PdO dispersed uniformly and
the interplanar spacing (0.294 nm and 0.212 nm) corresponded to
the (100) and (110) planes of PdO, while SiO2 is amorphous.
The total surface area was calculated according to the BET
method. The results show that after the acid treatment of COR, the
BET surface area increases from less than 1.0–13.5 m2 g−1. After
3.1.5. XPS analysis
The XPS technology was used to study the chemical state of
Pd species and relative contents of different elements on the
Pd/SiO2/COR catalyst channel surface. The XPS spectra of tested
samples are illustrated in Fig. 4, where the normalized intensity
data were adopted. It can be seen that Pd species is mainly in the
state of PdO, no PdCl2 being detected, indicating that chlorine ele-
ment was completely removed during the calcination process. In
this work, Pd 3d5/2 region was observed at binding energies in the
range of 335.6–337.1 eV. The binding energy of Pd 3d5/2 is consis-
tent with that as reported in Ref. [38].
The detailed element compositions of Pd/SiO2/COR catalysts are
listed in Table 2. With the increase of theoretical content of Pd, the
actual Pd element on the monolith channel surface first increases
and then decreases. This means that 1.5% Pd/SiO2/COR catalyst is
the optimum, which is consistent with the result of H2-TPR as men-
tioned above.
the coating of SiO2 the BET surface area achieves to 55.3 m2 g−1
,
while the coating of catalytic component Pd can further increase
the surface area (e.g., 60.8 m2 g−1 for 1.5% Pd/SiO2/COR) but not so
obviously, which are consistent with the SEM results.
3.1.3. H2-TPR analysis
TPR results of Pd/SiO2/COR catalysts demonstrate that in each
case four reduction peaks can be observed in the reduction region
(see Fig. 3). The low-temperature reduction peak at 90 ◦C is assigned
to the reduction of PdO species on the surface of the catalysts, which
makes the most contribution to catalytic hydrogenation of eAQ. The
high-temperature peaks at 300, 450 and 750 ◦C are assigned to the
reduction of PdO in the inner layer and other surface oxygen. It can
be seen that Pd/SiO2/COR catalyst with the content of 1.5% (which
3.2. Catalytic activity
The influence of different operating parameters (catalyst con-
tent, reaction temperature, reaction pressure, flow rates of inlet
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Pd3d3/2
PdO
PdO
Pd3d5/2
(i)
(f)
Pd3d3/2
PdO
Pd3d5/2 PdO
PdO
(e)
(d)
(c)
(b)
Pd3d5/2
Pd3d5/2
PdO
Pd3d3/2
Pd3d3/2
PdO
PdO
PdO
PdO
Pd3d5/2
Pd3d3/2
PdO
Pd3d5/2
PdO Pd3d3/2
(a)
326
328
330
332 334
336
338 340
342
344 346
BE (ev)
1000000
800000
600000
400000
200000
0
Pd3p3
O1s
Pd3p3 O1S
Pd3p1
200000
(a)
(b)
(ii)
(ii)
Si2p1
Si2p3
OKL1
OKL2
OKL2
OKL1
150000
100000
50000
0
O2s
O2S
Pd3d
Pd3p1
Si2P1
Si2p3
Pd3d
-200
0
200
400
600
800
1000
1200
1400
-200
0
200
400
600
800
1000
1200
1400
BE(ev)
BE(ev)
Pd3p3
O1S
Pd3p3 O1s
Pd3p1
OkL2
OkL1
200000
150000
100000
50000
0
(d)
200000
150000
100000
50000
0
(c)
OKL1
(ii)
(ii)
OKL2
Si
2p1
Si2p3
Si2p1
Si2p3
Pd3p3
Pd3d
O2S
Pd3d
O2s
-200
0
200
400
600
800
1000
1200
1400
-200
0
200
400
600
800
1000
1200
1400
BE(ev)
BE(ev)
1000000
800000
600000
400000
200000
0
Pd3p3
O1s
OKL2
Pd3p3
O1s
(f)
200000
150000
100000
50000
0
(e)
(ii)
(ii)
OkL1
OkL1
Si2P1
Si2p3
O2S
Pd3d
OkL2
Pd3p1
Si2p1
Si2p3
Pd3d
O2S
Pd3p1
-200
0
200
400
600
800
1000
1200
1400
-200
0
200
400
600
800
1000
1200
1400
BE(ev)
BE(ev)
Fig. 4. XPS Pd3d spectra (i) and the survey (ii) of samples. (a) 0.6% Pd/SiO2/COR; (b) 1.0% Pd/SiO2/COR; (c) 1.2% Pd/SiO2/COR; (d) 1.5% Pd/SiO2/COR; (e) 1.7% Pd/SiO2/COR; (f)
2.2% Pd/SiO2/COR.
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Fig. 5. Influence of reaction temperature on the yield of H2O2 and the STY. (Reaction conditions: atmospheric pressure, concentration of eAQ solution = 60 g L−1, flow rate of
eAQ solution = 0.7 mL min−1, flow rate of H2 = 100 mL min−1).
᭿, 0.6% Pd/SiO2/COR; ᭹, 1.0% Pd/SiO2/COR; ꢀ, 1.2% Pd/SiO2/COR; ꢁ, 1.5% Pd/SiO2/COR; ꢂ, 1.7% Pd/SiO2/COR; ꢀ, 2.2% Pd/SiO2/COR.
H2 and eAQ solution, and concentration of eAQ solution) on reac-
tion performance (i.e., the yield of H2O2 and the space time yield
(STY)) was investigated for the synthesis of H2O2 over Pd/SiO2/COR
catalyst.
chemicalreaction rate/external diffusion rate) was introduced to
determine whether the hydrogenation process is controlled by
chemical reaction or by external diffusion [39]. The external dif-
fusion rate was calculated by the correlation [40], and the chemical
reaction rate came from literature [41,42]. The results shows
that Da ≥ 10 (Da = 12–20 when the pressure is in the range of
0.1–1.0 MPa), indicating that the hydrogenation process rate is
more predominated by external diffusion than by reaction rate.
When the pressure increase, the saturation concentration of
hydrogenation in the working solution should increase which in
turn should lead to an increase in the liquid-solid mass transfer
rate at the same velocity. Nevertheless, with the increase of the
hydrogen pressure at the same mass flow rate, the actual volumet-
ric flow rate of the gas is reduced. Even if the mass transfer rate
of hydrogen has effect on the reaction rate, the increase in higher
hydrogen mass transfer rate from liquid to catalyst due to higher
concentration in the liquid could be offset by the decrease in mass
transfer rate due to decrease in velocity, resulting in the reaction
rate to be relatively independent of hydrogen pressure. Thus, for the
sake of convenience and safety, atmospheric pressure was chosen
in the subsequent experiments [22].
The influence of reaction temperature from 30 to 80 ◦C using
Pd/SiO2/COR catalysts with different Pd contents is illustrated in
Fig. 5. The result shows that temperature has an important influ-
ence on reaction performance as reported by Halder and Lawal [22].
With the increase of reaction temperature, both the yield of H2O2
and the STY increase gradually, the optimum temperature being
80 ◦C. Moreover, with the increase of Pd content in Pd/SiO2/COR
catalyst, the yield of H2O2 and the STY first increase and then
decrease. The Pd/SiO2/COR catalyst with theoretical Pd content of
1.5% exhibits the highest yield and STY, which is attributed to the
1.5% Pd/SiO2/COR catalyst having the most actual Pd content on the
catalyst channel surface as proved by XPS results. In this case, the
yield of H2O2 and STY at 80 ◦C are 53% and 793.2 g H2O2 g−1 Pd h−1
respectively.
,
The influence of H2 pressure is illustrated in Fig. 6(a). It is seen
that the increase of both the yield of H2O2 and STY is not so obvi-
ous in the range of atmospheric pressure to 1.0 MPa, indicating that
the catalytic hydrogenation of eAQ is not highly dependent on the
H2 pressure. Fig. 6(b) shows the plot of log(STY) versus log(PH2). It
appears that the dependency of rate on hydrogen concentration
is almost zero order. Damkohler number (Da) (defined as Da =
The influence of flow rate of feedstock is shown in Figs. 7 and 8.
As the inlet H2 gas flow rate increases, the increase of the yield of
H2O2 is not clear, while the STY sharply increases. As mentioned
above, external diffusion is the controlling step during the hydro-
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7
100
80
60
40
20
0
1000
800
600
400
(a)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Reaction pressure (MPa)
2.96
2.95
2.94
2.93
2.92
2.91
2.90
2.89
(b)
Y=0.05766X+2.7810
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
Log(PH2
)
Fig. 6. Influence of H2 pressure on the yield of H2O2 and the STY (a) and the plot of log(STY) vs. log(PH2) (b) (reaction conditions: temperature = 80 ◦C, concentration of eAQ
solution = 60 g L−1, flow rate of eAQ solution = 0.7 mL min−1, flow rate of H2 = 100 mL min−1, 1.5% Pd/SiO2/COR catalyst).
100 mL min−1 would be more appropriate. From Fig. 8, it can be
seen that both the yield of H2O2 and the STY increase with the
increase of eAQ solution flow rate. The higher the liquid flow rate,
the higher the mass transfer rate. When the eAQ solution flow rate
is above 0.7 mL min−1, the STY increases slowly, indicating that the
liquid flow rate 0.7 mL min−1 is more appropriate.
100
80
60
40
20
0
180
160
140
120
100
80
From Fig. 9(a), it can be seen that the increase in the eAQ con-
centration brings about the increase in the yield of H2O2 and the
STY. The higher the eAQ concentration, the higher the yield of
H2O2. However, when the concentration is higher than 60 g L−1, the
increase becomes slower than before due to the limiting active Pd
sites. If the active sites are not saturated, the yield of H2O2 increases
significantly with the increase of eAQ concentration. However, if
the Pd active sites are saturated, the yield of H2O2 almost does not
change. As a result, the eAQ concentration 60 g L−1 is chosen for the
eAQ catalytic hydrogenation. Fig. 9(b) shows the plot of log(STY)
versus log(CeAQ), and the slop (0.95) means that the reaction rate
has appropriately a first-order dependence on the eAQ concentra-
tion, which is consistent with the conclusion by Halder and Lawal
[22] and Santacesaria et al. [43] that the reaction rate has a first-
order dependency on the EAQ concentration in both microreactor
and conventional reactors.
20
40
60
80
100
120
140
Inlet H2 gas flow rate (mL·min-1)
Fig. 7. Influence of inlet H2 gas flow rate on the yield of H2O2 and the STY. (Reaction
conditions: atmospheric pressure, temperature = 80 ◦C, concentration of eAQ solu-
tion = 60 g L−1, flow rate of eAQ solution = 0.7 mL min−1, 1.2% Pd/SiO2/COR catalyst).
genation process because Da ≥ 10 when the gas and liquid flow
Furthermore, stability test was done for the hydrogenation reac-
tion of eAQ. Fig. 10 shows that the yield of H2O2 keeps around 52%
during 48 h.
rates are in the range of 20–140 mL min−1 and 0.3–1.3 mL min−1
,
respectively. As the flow rate of H2 increases, the gas has better
dispersion into the eAQ solution, leading to a higher mass trans-
fer rate. Furthermore, with more H2 gas, the reactants H2 and eAQ
on the active Pd sites will contact sufficiently. Considering the bal-
ance between the yield of H2O2 and the STY, the H2 flow rate at
Please cite this article in press as: Y. Guo, et al., Synthesis of hydrogen peroxide over Pd/SiO2/COR monolith catalysts by anthraquinone
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100
80
60
40
20
0
200
150
100
50
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Liquid flow rate (mL·min-1)
Fig. 8. Influence of eAQ solution flow rate on the yield of H2O2 and the STY. (Reaction conditions: atmospheric pressure, temperature = 80 ◦C, concentration of eAQ
solution = 60 g L−1, flow rate of H2 = 100 mL min−1, 1.2% Pd/SiO2/COR catalyst).
80
70
60
50
40
30
20
10
0
240
200
160
120
80
(a)
40
20
40
60
80
100
120
eAQ concentration (g·L-1
)
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
(b)
y=0.95058x+2.64971
-1.2
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
Log(CEAQ
)
Fig. 9. Influence of eAQ concentration on the yield of H2O2 and the STY (a) and the plot of log(STY) vs. log(CEAQ) (b). (Reaction conditions: atmospheric pressure, tempera-
ture = 80 ◦C, flow rate of eAQ solution = 0.7 mL min−1, flow rate of H2 = 100 mL min−1, 1.2% Pd/SiO2/COR catalyst).
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9
100
80
60
40
20
0
0
5
10
15
20
25
30
35
40
45
50
Time (h)
Fig. 10. Stability test for Pd/SiO2/COR catalyst. (Reaction conditions: atmospheric pressure, temperature = 80 ◦C, concentration of eAQ solution = 120 g L−1, flow rate of eAQ
solution = 0.7 mL min−1, flow rate of H2 = 100 mL min−1, 1.5% Pd/SiO2/COR catalyst).
4. Conclusions
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In this work, a series of Pd/SiO2/COR catalysts were successfully
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SEM, HRTEM, BET, H2-TPR, ICP-AES, and XPS. The results showed
that Pd element is mainly in the state of PdO in the presence of
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It was found that the STY can achieve 793.2 g H2O2 g−1 Pd h−1
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the appropriate conditions for the synthesis of H2O2: temperature
80 ◦C, atmospheric pressure, the inlet H2 and eAQ flow rates of 100
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native to pellet catalysts for the catalytic hydrogenation of eAQ for
the synthesis of H2O2.
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