Role of CO2 As a Soft Oxidant for Dehydrogenation of Ethylbenzene to Styrene over a High-Surface-Area Ceria Catalyst

Article abstract of DOI:10.1021/acscatal.5b01519

Catalytic performance and the nature of surface adsorbates were investigated for high-surface-area ceria during the ethylbenzene oxidative dehydrogenation (ODH) reaction using CO₂ as a soft oxidant. The high surface area ceria material was synt

Full text of DOI:10.1021/acscatal.5b01519

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Article
The role of CO2 as a soft oxidant for dehydrogenation of
ethylbenzene to styrene over a high-surface-area ceria catalyst
Li Zhang, Zili Wu, Nicholas C Nelson, Aaron D. Sadow, Igor I. Slowing, and Steven H. Overbury
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 22 Sep 2015
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The Role Of CO As A Soft Oxidant For Dehydrogenation Of Ethylbenzene To Styrene
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Over A HighꢀSurfaceꢀArea Ceria Catalyst
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3,4
Li Zhang , Zili Wu , Nicholas C. Nelson , Aaron D. Sadow , Igor I. Slowing and Steven H.
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Overbury *
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. Chemical Sciences Division, Oak Ridge National Lab 2. Center for Nanophase
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Materials Sciences, Oak Ridge National Lab, 3, Department of Chemistry, Iowa State
University and 4. Chemical and Biological Sciences, US DOE Ames Laboratory
Abstract
Catalytic performance and the nature of surface adsorbates were investigated for highꢀsurfaceꢀ
area ceria during ethylbenzene oxidative dehydrogenation (ODH) reaction using CO as a soft
2
oxidant. The high surface area ceria material was synthesized using a template assisted method.
The interactions between ethylbenzene, styrene and CO on the surface of ceria and the role of
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CO for the ethylbenzene ODH reaction have been investigated in detail by using activity test, in
2
situ Diffuse Reflectance Infrared and Raman spectroscopy. CO as an oxidant, not only favored
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the higher yield of styrene, but also inhibited the deposition of coke during the ethylbenzene
ODH reaction. Ethylbenzene ODH reaction over ceria followed a twoꢀstep pathway:
Ethylbenzene is first dehydrogenated to styrene with H formed simultaneously, and then CO2
2
reacts with H via the reverse water gas shift. The produced styrene can easily undergo
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polymerization to form polystyrene, which is a key intermediate for coke formation. In the
absence of CO , the produced polystyrene transforms into graphiteꢀlike coke at temperatures
2
o
above 500 C, which leads to catalyst deactivation. While in the presence of CO , the coke
2
deposition can be effectively removed via oxidation with CO2.
Key words: Highꢀsurfaceꢀarea ceria, Oxidative dehydrogenation, Ethylbenzene, Styrene, CO2,
infrared spectroscopy, Raman spectroscopy, Coking
This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with
the U.S. Department of Energy. The United States Government retains and the publisher, by accepting
the article for publication, acknowledges that the United States Government retains a non-exclusive,
paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript,
or allow others to do so, for United States Government purposes. The Department of Energy will provide
public access to these results of federally sponsored research in accordance with the DOE Public Access
Plan
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Introduction
Styrene is one of the most important chemicals in the petrochemical industry for the
production of polymers including polystyrene and styreneꢀbutadiene rubber. Styrene has been
mainly produced by ethylbenzene dehydrogenation in the presence of an iron oxide catalyst with
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excess supply of steam at high temperature (600ꢀ650 C). Typical catalysts used in the
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dehydrogenation of ethylbenzene are KꢀFe O catalysts . However, there are a few problems
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associated with the dehydrogenation of ethylbenzene such as the large amount of energy
consumption during the process, the low yield of styrene due to thermodynamic limitation of
endothermic reaction, irreversible catalyst deactivation and deposition of dehydrogenated
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carbonaceous species, i.e. coke . Therefore, an economic and efficient ethylbenzene
dehydrogenation process is in high demand. Various methods have been attempted to solve the
above mentioned problems of using steam during the dehydrogenation of ethylbenzene. The
oxidative dehydrogenation (ODH) process of ethylbenzene to styrene in the presence of oxidant
has attracted considerable attention since it can be operated at lower temperatures and
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ethylbenzene conversion would not be equilibrium limited . A variety of oxidizing agents such
as O , N O, SO and CO have been used for the oxidative dehydrogenation of ethylbenzene to
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,9ꢀ12
lower the reaction temperature
. Exploitation of CO as a soft oxidant for the commercial
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catalytic ODH of ethylbenzene to styrene has received enormous interest recently because of
several benefits of using CO compared to other oxidants. It has been reported that CO can act as
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a soft oxidant as well as diluent . In addition, CO stays in the gas form throughout the reaction
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and so does not need latent heat for vaporization. CO can also decrease the partial pressure of
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reactants more effectively than steam and it has the highest heat capacity among various gases
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.
Further, in the presence of CO , coupling between reverse waterꢀgas shift reaction
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CO2
H2
H O
CO
(1)
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and the ethylbenzene ODH reaction becomes more favorable
. However, it has been reported
that a small amount of CO inhibits the catalytic activity of ethylbenzene dehydrogenation due to
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the decomposition of the active phase of iron oxide catalyst in presence of CO2
. Thus using
CO was largely limited in the process for preparing styrene due to its property of deactivating the
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ferrite compounds. It is necessary to develop a catalyst to retain sufficient activity and selectivity
when using CO for the styrene production process.
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Good performance for dehydrogenation of ethylbenzene has been reported for a solid
catalyst containing vanadium as an active component, wherein CO serve as oxidant and without
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steam
of ethylbenzene were significant with CO in the presence of Fe O /Al O , MnO ꢀZrO
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. Others also reported that the enhancement effects of the dehydrogenation activity
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and a
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SBAꢀ15 supported CeO –ZrO catalyst . However, the reaction temperature of dehydrogenation
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over these catalysts is still high (above 500 C). Therefore, developing an efficient catalyst with
high activity at lower temperature is still highly desirable.
Ceria is a key component in the formulation of many industrially important catalysts and
currently attracts attention in the application of the ODH of ethylbenzene catalysts due to its
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oxygen storage/release capacities . It has been reported that ceria itself could be a good catalyst
for ODH of ethylbenzene to styrene by using O that helps decrease coking during the ODH
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reaction . However, there is safety concern using O , and the styrene selectivity among the
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products might be low due to over oxidation by O . CO as a soft oxidant might favor the styrene
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yield over ceria catalyst and it is much safer than O . So far the effect of CO in the reaction of the
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ethylbenzene ODH over the pure ceria catalyst has not been established. In this study, a highꢀ
surfaceꢀare ceria material was synthesized and was tested for ethylbenzene ODH to styrene using
CO . The role of CO for the catalytic performance and coking mechanism has been investigated
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in detail. We find that the highꢀsurfaceꢀarea ceria prepared by the templateꢀassisted synthesis
method exhibits higher ethylbenzene conversion and styrene yield at low temperatures (400ꢀ500
o
C) than commercial ceria nanoparticles. CO favors ODH reaction through a twoꢀstep pathway.
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Ethylbenzene is first dehydrogenated to styrene, and then CO reacts with the H byproduct via
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the reverse waterꢀgas shift. In situ DRIFTS and Raman characterization proves that CO acts as a
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soft oxidant not only favoring the low temperature ODH of ethylbenzene over the ceria catalyst,
but also inhibiting the coke deposition during the reaction.
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2
. Experimental
2.1. Catalyst preparation
Highꢀsurfaceꢀarea (HSAꢀ) CeO was synthesized by a templateꢀassisted synthesis method using a
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nonꢀ ionic block copolymer template (Pluronic P104) as described elsewhere Typically,
Ce(NO ) •6H O (4.4 g, 10 mmol) and Pluronic P104 (5 g) were dissolved in ethanol. The
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solution was stirred for 2 h and then cast into a petri dish. The dish was placed into a preꢀheated
oven at 65 °C for 24 h and then placed into an oven at 150 °C for an additional 24 h. The sample
was then calcined in air at 450 °C for 4 h. A reference commercially available CeO (Sigmaꢀ
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Aldrich) was selected for comparison with the HSAꢀCeO2.
2.2. Catalyst characterization
The surface area of the samples, using the multipoint Brunauer, Emmett and Teller (BET)
analysis method, was measured by N2 adsorption at −196 °C, using an Autosorbꢀ1
Quantachrome flow apparatus. The samples were dehydrated in vacuum at 250 °C overnight,
before surface area measurements. Pure HSAꢀCeO prepared using a Pluronic template assisted
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synthesis method yielded a surface area of 202 m /g while the surface area for the commercial
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ceria is 40 m /g.
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.3. Catalytic reaction
A micro catalysis testing unit of the laboratory scale was utilized for examining the performance
of the heterogeneous solid catalyst in the dehydrogenation of ethylbenzene with CO as an
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oxidant. There was a vertically installed Uꢀtube in the fixed bed reactor made of quartz with 4
mm of internal diameter, 350 mm of length, and the reaction temperature was adjusted within the
range of 400ꢀ500 °C by using electric heaters and programmable temperature controller of an
Zeton Altamira (AMIꢀ200) instrument. The dehydrogenation catalyst (100 mg, 60ꢀ80 mesh) was
mixed with 200 mg of quartz beads with the similar size and filled into the reactor and pretreated
for 1 hour at 400 °C with 5 % O /Ar gas prior to performing a reaction. Liquid ethylbenzene was
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supplied by means of a motorized syringe pump (New Era pump system, NE1000) into a heated
line where it vaporized in the carrier stream. Ethylbenzene was introduced as a reactant (2.2
µmol/min), CO as oxidant (5 mL/min), Ar as the carrier (15 mL/min), giving a total flow rate of
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about 20 mL/min and a CO : ethylbenzene ratio of 100:1. The reactants had been mixed and
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preheated at 150 °C before the reactant was supplied into the reactor. The reactants and products
were analyzed using a continuously sampling quadrupole mass spectrometer (QMS; Pfeifferꢀ
Balzers Instruments) followed by a gas chromatograph (Buck Scientific) equipped with a flame
ionization detector. For analysis of the products, HPꢀinnowax column (30 m long, 0.32 mm i.d,
and 0.25 µm film thickness) was employed. The main products analyzed were styrene, benzene,
toluene, and methane. Other gas products such as H , CO, and CO were monitored by the mass
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spectrometer. The total carbon balance closed to within ± 5%.
2.4. In situ spectroscopy
Infrared spectra were collected using a Thermo Nicolet Nexus 670 spectrometer in diffuse
reflectance mode (DRIFTS). A Pike Technologies HCꢀ900 DRIFTS cell with nominal cell
volume of 6 mL was used. The powder sample (50 mg) was placed in the sample cup with a
porous screen at the bottom surface, allowing the gas to uniformly pass through the catalyst from
top to bottom. A feed gas mixture, controlled using mass flow controllers, was supplied at a flow
rate of 10 mL/min. Prior to DRIFTS measurements of adsorption or reaction, the CeO samples
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were first calcined in a flow of 10 % O /He at 500 C for 0.5 h and then cooled to room
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temperature. A background spectrum of the calcined sample was recorded in flowing He at each
temperature used for analysis. These background spectra were subtracted from subsequent
sample spectra obtained at the same temperatures. The DRIFTS spectra were collected from
ꢀ1
ꢀ1
4000 to 650 cm , accumulating 100 scans at a 4 cm resolution.
For Raman studies, the sample was pretreated in an in situ Raman catalytic reactor (Linkam
CCR1000). Raman scattering was collected via a customized ellipsoidal mirror and directed by a
fiber optics bundle (Princeton Instruments) to the spectrograph stage of a triple Raman
spectrometer (Princeton Instruments Acton Trivista 555). Edge filters (Semrock) were used in
front of the UVꢀvis fiber optic bundle to block the laser irradiation. The 442ꢀnm excitation (~5
mW at sample) was emitted from a HeCd laser (Melles Griot). A UVꢀenhanced liquid N ꢀcooled
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CCD detector (Princeton Instrument) was employed for signal detection. The Raman reactor sits
on an XY stage (Prior Scientific, OptiScan XY system) and translates in raster mode while
collecting the spectrum. The fast translation has shown been able to minimize any laser damage
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of the surface adsorbates and the samples. Cyclohexane was used as a standard for the calibration
of the Raman shifts.
For both IR and Raman in situ spectroscopy, experiments were performed to investigate the
ethylbenzene adsorption, desorption, ODH, and styrene adsorption, desorption. For each
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experiment, the sample was first calcined (5% O /Ar, 10 mL/min) at 450 C for 30 min in the in
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situ cell, and then decreased to room temperature and purged by He or Ar (10 mL/min) for 30
min before exposure to reactants. For styrene experiments, about 1 µL of neat styrene was
dropped into the catalyst at room temperature prior to reꢀestablishing the purge. A bubbler was
used to introduce gas phase ethylbenzene into the reactant stream.
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. Results and discussion
.1. Catalytic results
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Catalytic performance of HSAꢀCeO and of commercial CeO was measured under steady state
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conditions between 400ꢀ500 C using a CO /ethylbenzene molar ratio of ~100. A pretreated
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sample was held at a sequence of increasing temperatures, for 2h at each temperature, and the
reactants and products were analyzed repeatedly and averaged. In all cases, the major product in
the reaction is styrene (selectivity > 80 %), accompanied with a small amount of benzene, and
toluene as byproducts (< 20 %). Ethylbenzene conversion (X ), product selectivity (S ) and
EB
p
styrene yield (Y ), are shown in the following equations:
ST
X = (1 ꢀ Ethylbenzene out / Ethylbenzene in ) ꢁ 100
(2)
(3)
(4)
EB
Sp= (Product out / (Ethylbenzene in – Ethylbenzene out) ꢁ 100
Y = (X * S ) ꢁ 100
ST
EB
ST
Pure HSAꢀCeO (Figure 1(a)) gives a higher ethylbenzene conversion compared with the
2
commercial ceria (Figure 1(b)) in the whole temperature range. The ethylbenzene conversion
increased with temperature over the two catalysts. The highest catalytic performance was
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obtained at 500 C with ethylbenzene conversion of ~ 85 %, styrene selectivity of ~ 90 % and
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yield of ~ 80 % over the HSAꢀ CeO . The reaction rates of ethylbenzene at 450 C based on the
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surface areas are similar for the two catalysts as shown in Table 1, which indicates that the main
reason of the higher activity and yield of styrene over HSAꢀ CeO is the larger surface area of the
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HSAꢀCeO . In previous reports of K promotedꢀFe O catalysts, the ethylbenzene conversion was
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typically measured at higher temperature (580ꢀ600 °C) using steam as a diluent and yielded
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specific activity of 18 x 10 ꢂmole/(hrꢁgm). Although Kꢀ Fe O catalysts have higher specific
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activities under those conditions compared to the HSAꢀCeO (350 ꢂmole/(hrꢁgm), the lower
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temperatures and the use of CO as a diluent has advantages.
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To understand the role of CO in the dehydrogenation of ethylbenzene, the reaction was also
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conducted without CO for the HSAꢀCeO catalyst. Table 2 compares the catalytic results for the
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ethylbenzene dehydrogenation to styrene at 400ꢀ500 C under steady state in the presence and
absence of CO in the flow. In absence of CO , the dehydrogenation reaction also occurred and
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produced lower amount of styrene. This suggests that direct dehydrogenation reaction of
ethylbenzene to styrene occurred over the HSAꢀCeO catalyst. However, both the ethylbenzene
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conversion and styrene yield were increased by the addition of CO2.
Aging tests in CO were conducted as shown in Figure 2 for a freshly pretreated HSAꢀCeO at
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50 C. Figure 2 shows that although the measured ethylbenzene conversion initially decreased,
it stabilized after 2 h and the styrene yield was nearly constant over the 12 hour test. The initial
decrease in the apparent conversion may be attributed in part to initial retention of ethylbenzene
by surface adsorption including formation of poly styrene (see below). The nearly stable styrene
yield indicates the steady reaction rate without catalyst deactivation.
QMS measurements of the H produced during the reaction are shown in Figure 3, and these
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indicate that H was produced during the dehydrogenation reaction over the HSAꢀCeO in both
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the presence and absence of CO . However, less H was observed at the same temperature when
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CO is present, which might be due to the oxidation of the produced H by CO i.e., a reverse
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water gas shift reaction (rWGS). In order to further confirm this reaction route, we also tested
the reverse water gas shift reaction over the HSAꢀCeO by flowing CO and H over pretreated
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HSAꢀCeO within the same temperature range used for the ethylbenzene dehydrogenation. A
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molar ratio CO : H of 2:1 was used with a total flow rate of 25 mL/min and Ar as the carrier
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gas (100 mg sample; CO : 1.6 %). The observed conversion of CO to CO increased with
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increasing temperature, and the highest conversion of CO to CO is around 10 % at 500 C, close
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to the predicted equilibrium value for rWGS. This result proves that CO reacts with H via the
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reverse water gas shift reaction, catalyzed by HSAꢀCeO , during the ethylbenzene reaction using
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CO as a soft oxidant.
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It has been proposed that ethylbenzene dehydrogenation in the presence of CO may proceed
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by two routes, namely oneꢀstep and twoꢀstep pathways. For the one step pathway, ethylbenzene
is oxidized to styrene with CO through the direct interaction of CO and ethylbenzene (5):
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For the twoꢀstep pathway, ethylbenzene is first dehydrogenated to styrene with H formed
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simultaneously (6), and then CO reacts with H via the reverse water gas shift reaction (1):
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(6)
Ethylbenzene dehydrogenation occurs even in the absence of CO , so reaction (6) must occur.
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Addition of CO enhances the rate, either because reaction (5) becomes accessible or because of
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a forward shift in reaction equilibrium by the reverse water gas shift reaction. For the oneꢀstep
pathway (5), there will be no H produced. Based on the QMS results, H is produced in the
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ODH process, and more H is produced at higher temperatures with the increasing of the
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ethylbenzene conversion. Coupled with our testing result of the reverse water gas shift reaction,
we conclude that ethylbenzene ODH reaction with CO over ceria by the twoꢀstep pathway must
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occur. However, based upon the current data it is not possible to eliminate the possibility that
the oneꢀstep pathway, reaction (5), may occur simultaneously and the relative importance of
these two routes can not be determined.
To further clarify the exact role of CO in the ethylbenzene ODH reaction and explain how the
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CO interacts with the surface of the ceria and favors the activity and styrene yield, in situ
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DRIFTS and Raman studies were further carried out.
3.2. In situ DRIFTS
3.2.1 Styrene adsorptionꢀdesorption on HSAꢀCeO2
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Figure 4 shows the infrared spectra of styrene adsorbed onto a calcined sample of HSAꢀCeO at
2
room temperature and followed by sequential heating in He to higher temperatures. The
spectrum obtained at room temperature is consistent with adsorbed styrene as observed
2
6ꢀ29
previously
. Skeletal CꢀC stretching modes of the aromatic ring occur at 1600, 1540, 1500,
10
11
12
13
14
15
16
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60
ꢀ126ꢀ28
1
450, 1200, 1081, and 1020 cm
as indicated by markers in Figure 4. The CꢀH deformation
ꢀ1
and CꢀH bending of the aromatic ring appeared at 1176, and 1155 cm . The four bands above
ꢀ1
ꢀ1
26
3000 cm (3083 to 3010 cm ) are assigned to the CꢀH stretches in the adsorbed styrene . The
vinyl group of styrene has several characteristic bands: the CꢀH outꢀofꢀplane bending modes of
ꢀ1
the vinyl group, ꢀCH=CH and –CH=CH , at 910 and 990 cm respectively; their overtones
2
2
ꢀ1
ꢀ1 28
1
820 and 1980 cm ; and the CꢀH inꢀplane bending modes of the vinyl group at 1400 cm
.
ꢀ1
The bands at 1370 cm is due to the CH inꢀplane bending of the vinyl group. A band of 1629
2
ꢀ1
26
ꢀ1
cm is associated with the vinyl υ(C=C) of the styrene . The assignment of the 1270 cm band
is not clear, but it is present at all temperatures and so we assign it to an outꢀofꢀplane CꢀH
27,28
bending, possibly of the aryl H
.
Heating the adsorbed styrene to 200 °C leads to prominent changes in the spectrum. These
changes are due to combined effects of desorption and coupling (oligomerization) to form
polystyrene. The strongest evidence for the formation of polystyrene is observed in the CꢀH
ꢀ1
stretching region where prominent peaks at 2846 and 2931 cm due to the –CH ꢀ and CHꢀ
2
3
0,31
ꢀ1
vibrations of the polystyrene consistent with previous IR data
. A new band at 2721 cm
ꢀ1
shows the similar trend as the bands of 2931 and 2846 cm and is suggested to be CꢀH stretching
ꢀ1
ꢀ1
of polystyrene. In addition, the peak at 1600 cm for styrene shifts to 1595 cm and strengthens
31
to form a prominent peak. This feature is also typical of the polystyrene . Styrene persists on
the surface, but decreases due to desorption and its polymerization. Evidence of styrene is the
ꢀ
1
vinyl υ(C=C) (1629 cm ), the bending modes, ꢀCH=CH and –CH=CH , (910, 990) and their
2
2
ꢀ1
ꢀ1
overtones (1820 and 1980 cm ). Aromatic skeletal modes (1600, 1500, 1450 and 1200 cm )
decrease with temperature due to desorption of styrene and effects of polymerization.
As the temperature is increased to 400 °C, the CꢀH stretching bands of the polystyrene (2931,
ꢀ1
2846, and 2721 cm ) decrease in intensity. Simultaneously, bands due to vinyl (1980, 1820, 990
ꢀ1
and 910 cm ) decrease with increasing temperature and vanish at 500 °C. By 500 °C, only the
ꢀ1
bands at 3058, 1595, 1270 and 1155 cm persist and are assigned to a highly dehydrogenated
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32
carbon (coke), remaining on the surface of the ceria . All features due to vinyl groups disappear
along with all of the CH stretching modes indicative of dehydrogenation of all surface species,
consistent with coke.
Summarizing, the above results show that at room temperature most of the styrene adsorbs as
0
1
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0
o
monomers, while the result of heating to higher temperatures (200ꢀ450 C) causes coupling of
the styrene to form polymer on the ceria. Styrene monomer is present on the surface throughout
this range as indicated by the persistent vinyl bands. Continued coadsorption of styrene and
polystyrene suggests that reverse decomposition of polymer to monomer continues, following
reaction (7). It has been reported that
(7)
o
polystyrene can depolymerize to styrene in He in the temperature range of 200 ꢀ 450 C during
2
9
o
heating process , which is consistent with our results. After heating at 500 C, no C=C band
and vinyl group were observed, indicating most of the surface species are polymerized and
depolymerization has stopped. The disappearance of the CH and CH stretching at 2931 and
2
ꢀ1
2
846 cm indicate nearly complete dehydrogenation of the alkyl H with possible coupling. Lack
of available H prevents desorption and depolymerization, leaving only a highly dehydrogenated
carbonaceous adsorbate typical of coke. Detail of this will be discussed in the following Raman
spectrum.
3.2.2 Ethylbenzene adsorptionꢀdesorption on HSAꢀCeO2
The activity results (Table 2) indicate that over HSAꢀCeO , the dehydrogenation reaction of
2
ethylbenzene to styrene can occur in the absence of CO . To clarify the surface interaction of
2
ethylbenzene with ceria in the absence of CO , infrared spectra were collected for ethylbenzene
2
o
adsorbed onto HSAꢀCeO at 150 C and followed by heating in He at various temperatures.
2
These are shown in Figure 5. Following adsorption at 150 °C, several bands characteristic of
ꢀ
ethylbenzene were observed. The CꢀH stretches in the aromatic ring occur at 3058 and 3020 cm
1
ꢀ1
,
while the bands at 2964, 2931, and 2873 cm are due to methyl and methylene CH stretches in
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the ethyl group . Skeletal CꢀC stretching modes of the aromatic ring are visible at 1600, 1540,
ꢀ
1 26,28
1
500 cm .
Upon subsequent heating to 200 °C, several new IR bands appeared or became
ꢀ1
sharper at 1980, 1820, 1629, 1400, 1370, 1351, 1270, 1176 1155 and 990 cm . These bands
were all seen in the spectra following styrene adsorption as discussed above and shown in Figure
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
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43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ꢀ1
4
. In particular the bands at 1980, 1820, 1629 and 990 cm are associated with the vinyl
function of styrene. Appearance of these bands confirms that the direct dehydrogenation of
ethylbenzene to styrene occurs over the surface of HSAꢀCeO catalyst, consistent with our
2
activity test results.
o
The bands due to styrene generally increased in intensity with temperature at 300 C but declined
o
o
after heating to 400 C or higher. At 500 C, the CꢀH stretching bands are strongly decreased
ꢀ1
(less so for the aromatic stretch at 3064 cm ). This dehydrogenation is accompanied by a
ꢀ1
persistent band near 1590 cm that can be associated with coke buildup. But several bands
ꢀ1
ꢀ1
remain due to the CꢀC stretches (1595, and 1056 cm ) and CꢀH bending (1176 cm ) of the
ꢀ1
ꢀ1
aromatic ring. Bands at 1351 cm and 1465 cm remain and are assignable to styrene oligomer
3
3
o
This indicates that at higher temperature (500 C), some of the styrene forms oligomers on the
ꢀ
surface of the ceria during the direct dehydrogenation of ethylbenzene. A new band of 2127 cm
1
o
34
appears at temperatures above 400 C which could be assigned to reduced ceria We may infer
that during the reaction (6) of direct dehydrogenation of ethylbenzene, some H was produced
2
with the styrene production. The resulting H can react with the surface lattice oxygen on the
2
o
35
ceria (8) leading to reduction of the ceria at temperatures as low as 500 C .
H2
O
H O
(8)
2
3.2.3 CO adsorptionꢀdesorption on HSAꢀCeO2
2
To clarify the role of CO in the ethylbenzene ODH reaction and how it interacts with
2
ethylbenzene on ceria, in situ DRIFTS spectra were collected for CO adsorption on HSAꢀCeO2
2
o
at 150 C and followed by purging and heating in He at various temperatures. The results are
ꢀ1
shown in Figure 6. Several IR bands appeared at 1616, 1390, 1290 and 1216 cm are assigned to
o
36 37
carbonate and bicarbonate formed from CO adsorption at 150 C on ceria surface
. These
2
o
bands decrease in intensity with increasing temperature and become very weak at 400 C. These
results prove that CO can reactively adsorb on ceria at low temperatures, but these decompose
2
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and desorb at higher temperatures. This loss of carbonate may contribute to the increased ODH
o
activity above 400 C since loss of carbonate releases available surface sites for ODH reaction.
3
.2.4 Reaction of ethylbenzene + CO over HSAꢀCeO
2 2.
0
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0
The reaction between CO and ethylbenzene on the surface of HSAꢀCeO was investigated by in
2
2
situ DRIFTS. Spectra were collected at various temperatures while continuously flowing both
ethylbenzene and CO over the HSAꢀCeO2. As can be seen in Figure 7, bands due to gas phase
2
ꢀ1
o
38
CO (2362, 2319 cm ) were observed at 150 C along with features due to carbonates (1616,
2
ꢀ1
1
390, 1216 cm ) as seen in Figure 6. Several peaks due to adsorbed ethylbenzene also appeared
ꢀ1
o
(
1540, 1500, 1270, 1056, and 1020 cm ) at the same time. At temperature higher than 300 C,
ꢀ1
new bands assignable to styrene (1980, 1820, 1400, 1370, 1300, 1176, 1155, 990 and 910 cm )
3
9
appeared , demonstrating that the ethylbenzene dehydrogenated to styrene while in presence of
CO . Meanwhile with the elevation of temperature the bands due to carbonate (1616 and 1216
2
ꢀ1
ꢀ1
cm ) gradually disappeared while bands due to adsorbed ethylbenzene (1595 and 1176 cm )
became clearer. This result shows that both ethylbenzene and CO can adsorb on the HSAꢀCeO2
2
simultaneously at low temperatures, but the adsorbed carbonates might block the active ceria
o
sites, so inhibiting ODH reaction at low temperatures. At temperatures of 300 C and higher, the
carbonate decomposes and desorb from the surface of the catalyst, and then the adsorbed
ethylbenzene interacts with the ceria to produce styrene. Meanwhile, the band due to the reduced
ꢀ1
ceria at 2127 cm appeared, suggesting that H produced by the dehydrogenation of
2
ethylbenzene reacted with the surface oxygen on ceria causing its reduction. The ethylbenzene
first dehydrogenated to styrene and H , and then the H is available to react either with lattice O
2
2
or with CO via the reverse water gas reaction. In order to further clarify the coking formation
2
during the ODH reaction of ethylbenzene, and how CO interacts with the coke, in situ Raman
2
spectra was explored.
3
.3. In situ Raman
3.3.1 Styrene adsorptionꢀdesorption on ceria in Ar
Raman spectra of adsorbed styrene on HSAꢀCeO were collected at room temperature, followed
2
by desorption in Ar at various temperatures. As shown in Figure 8, the Raman bands of the pure
ꢀ1
HSAꢀCeO were observed at 345, 466, 556, 592 and 1714 cm , due to second order transverse
2
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acoustic (2TA) mode, F mode of the CeO fluorite phase, D band due to the presence of
2
g
2
oxygen vacancies of reduced ceria, defect sites induced D mode and secondꢀorder longitudinal
4
0
optical (2LO) modes of the ceria respectively. Raman bands at 1005, 1033, 1185, 1200, 1321,
ꢀ1
1
417, 1498, 1600, and 1629 cm due to the styrene adsorption were observed at room
temperature after introducing styrene in Ar while some of the ceria bands (345, 556, 592, and
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60
ꢀ
1
1
714 cm ) were attenuated by the styrene. The observed bands could be assigned to CꢀH
ꢀ1
bending of aromatic ring (1185, 1200 cm ), the =CHꢀ and =CH deformation (1321, and 1417
2
ꢀ1 41
ꢀ1
cm ) CꢀC stretch of the aromatic ring (1005, 1033, 1498, 1600 cm ), and vinyl C=C stretches
ꢀ
1
42ꢀ45
ꢀ1
(
1629 cm )
.
It is significant that the C=C stretching at 1629 cm disappeared at
o
temperatures above 200 C, since this suggests polymerization. Meanwhile, the CꢀC stretches of
ꢀ1
ꢀ1
ꢀ1
aromatic rings (1005, 1498 cm ) increased, the peak at 1600 cm shifted to 1595 cm , and a
ꢀ1
ꢀ1
42,43
new band at 1447 cm appeared. The band at 1447 cm is usually assigned to polyolefin
.
4
1
Comparison of the spectrum at 200° C to that reported polystyrene confirms the bands of
ꢀ1
1
005, 1185, 1447, 1498, and 1595 cm observed are due to the formation of polystyrene.
o
However, upon heating to 400 C, the bands of polystyrene decreased, while a new band at 1330
ꢀ1
ꢀ1
ꢀ1
cm increased in intensity and the band at 1595 cm shifted to 1580 cm , indicating the
ꢀ1
formation of new species. The bands at 1580 and 1330 cm have been reported as graphiteꢀlike
2
8
coke This suggests that the polystyrene must be the intermediate for the formation of coke on
o
o
the surface of the HSAꢀCeO at higher temperatures (> 400 C). After heating to 500 C, the
2
ꢀ1
bands due to polystyrene almost vanished and the two bands at 1330 and 1580 cm still present,
which indicates that polystyrene further dehydrogenated to coke deposited on the ceria surface.
These transformations in the adsorbed styrene layer also affected the CeO bands which became
2
clearer after heating to 200 °C due to partial desorption and polymerization of styrene, and then
diminished again above 400 °C as coke forms on the surface. Combined with our DRIFTS
results of styrene adsorptionꢀ desorption, in Figure 4, we have found that styrene is easy to
polymerize and form polystyrene, while polystyrene is also able to decompose to styrene
o
o
monomers in the temperature range of 200ꢀ450 C, while at temperatures above 500 C, the
polystyrene further dehydrogenated to produce coke, (9)..
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(
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3.3.2 Ethylbenzene adsorptionꢀdesorption on ceria in Ar
For comparison, Raman spectra of adsorbed ethylbenzene on HSAꢀCeO were collected at room
2
temperature and then heated in Ar at different temperatures as shown in Figure 9. After
introducing ethylbenzene (0.2 %) in Ar over the ceria at room temperature, new Raman bands
due to ethylbenzene appeared at 717, 833, 1005, 1167, 1265, 1321, 1391, 1498, 1540, 1567 and
ꢀ1
ꢀ1
ꢀ1
1
600 cm along with the Raman bands of ceria (345, 466, and 592 cm ). The band at 1167 cm
ꢀ1
is attributable to CꢀH bending of aromatic ring, the bands at 717 and 833 cm correspond to the
ꢀ1
CCC deformation of aromatic ring, and bands at 1600, 1567, 1540, 1498, and 1005 cm are
2
8
ꢀ1
assigned to CC stretch of the aromatic ring . Bands 1265, 1321, and 1391 cm are due to the
28,41,46
CH or CH deformation
. Most of the Raman bands associated with ethylbenzene
2
3
decreased in intensity with the increasing of temperature due to the desorption of ethylbenzene,
ꢀ1
ꢀ1
while the bands at 1321, 1498 cm and 1567 cm
initially increased in intensity with
o
o
temperature (100ꢀ300 C) and then decreased and disappeared at higher temperatures (> 400 C).
These three bands can be found in the Raman spectra of styrene adsorption in Figure 8. In
ꢀ1
43,47
addition, the band at 1567 cm has also been reported to relate with conjugated olefin
,
indicating that new species of styrene might have formed during the heating process of
o
ꢀ1
ꢀ1
ethylbenzene. At 300 C and above, the band at 1600 cm shifted to 1595 cm , and the bands
ꢀ
1
related with polystyrene, as we discovered in Figure 8 at 1167 and 1498 cm were present
,
implying the formation of polystyrene. Some amount of styrene was formed in the heating
process during the ethylbenzene desorption. This is consistent with our DRIFTS results in Figure
5
. The produced styrene polymerized and formed some polystyrene on the surface of ceria.
o
However, less coke was formed at 500 C compared with the results from heating styrene shown
in Figure 8. This might be due to the lower amount of polystyrene produced during the
ethylbenzene desorption. Finally, the effect of heating the ethylbenzene layer causes the Raman
ꢀ1
ꢀ1
D band at 592 cm to decrease compared to the 556 cm band, the band due to oxygen vacancy
sites on reduced ceria. This relative intensity change implies that the H evolved during the
2
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ethylbenzene dehydrogenation process causes the reduction of ceria. This is consistent with our
DRIFTS results in Figure 5.
3
.3.3 Ethylbenzene dehydrogenation reaction on ceria in CO or Ar
2
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60
To further investigate the role of CO in the ethylbenzene ODH reaction, in situ Raman spectra
2
were collected during ethylbenzene reaction with or without CO over HSAꢀCeO . Figure 10
2
2
presents the Raman spectra from ethylbenzene interacting with CO at various temperatures.
2
o
Raman bands appeared, after introducing the ethylbenzene and CO mixture at 100 C, that were
2
similar to those seen following ethylbenzene adsorption at 25 °C (Figure 9). But, in addition,
ꢀ1
o
two new bands at 1238 and 1651 cm were observed at 100 C that are assigned to carbonates.
ꢀ1
ꢀ1
These bands are related with DRIFTS bands observed at 1216 cm and 1616 cm (Figure 7), and
indicate that both ethylbenzene and CO are coꢀadsorbed on the surface. In addition, these two
2
o
Raman bands vanished upon heating to 200 C, consistent with our DRIFTS results. During the
ꢀ1
temperature elevation process, the bands due to ethylbenzene (717, 833, and 1265 cm )
ꢀ
decreased, while some bands related with styrene (1167, 1321, 1391, 1447, 1498, and 1567 cm
1
42
o
) increased. As the temperature was increased to 400 C, the bands assigned to polystyrene at
ꢀ1
ꢀ1
1
005, 1167, 1321, 1391, and 1498 cm were significant while the band at 1600 cm shifted to
ꢀ1
1
595 cm , signaling the formation of polystyrene. However, no significant graphiteꢀlike coke
was observed. Also, as found when heating ethylbenzene in Ar, the band intensity due to the
ꢀ1
ꢀ1
defects sites on ceria (592 cm ) shifted to the oxygen vacancy band (556 cm ) with the
temperature increase, indicating that the ceria was reduced by the dehydrogenation process.
For comparison, the in situ Raman spectra during the ethylbenzene dehydrogenation reaction in
absence of CO on HSAꢀCeO at different temperatures was collected and the results shown in
2
2
o
Figure 11. After introducing ethylbenzene/Ar over the ceria at 100 C, several Raman bands due
ꢀ1
to ethylbenzene appeared (711, 821, 1005, 1167, 1265, 1321, 1391, 1498, 1567 and 1600 cm )
ꢀ1
along with the bands of ceria (345, 466, and 592 cm ). These Raman bands of ethylbenzene
o
decreased in intensity with the reaction temperature. When the temperature rises to 400 C,
ꢀ1
several bands related to polystyrene (1005, 1167, 1321, 1391 cm ) were observed, and the band
ꢀ1
ꢀ1
at 1600 cm shifted to 1595 cm which indicates that some styrene might be produced and
further polymerize to form polystyrene during the heating process. These bands of polystyrene
o
ꢀ1
vanished at temperatures above 500 C, while two new bands at 1330 and 1580 cm appeared at
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o
higher temperatures (≥ 500 C). The two bands have been observed in Figure 8 during styrene
4
8
desorption and are reported as graphiteꢀlike coke . They prove that dehydrogenation of
ethylbenzene over ceria in the absence of CO results in coke deposited on the surface of the
2
ꢀ1
HSAꢀCeO . In addition, the peak intensity of ceria at 466 cm decreased, possibly due to the
2
0
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ꢀ1
ꢀ1
buildup of coke, while the band at 592 cm shifted to 556 cm indicative of reduction of ceria.
In contrast, no significant coking was observed in the presence of CO during the ODH reaction
2
over HSAꢀCeO at high temperature (Figure 10), which indicates that CO helps inhibit
2
2
deposition of coke on the catalyst.
3
.3.4 Oxidation of coke on ceria by CO2
In situ Raman spectra of the interaction between CO and the coked ceria sample were further
2
explored to clarify the exact role of CO in ODH reaction of ethylbenzene. Figure 12 gives the
2
Raman spectra of the coked ceria sample (after ethylbenzene dehydrogenation reaction in Ar at
o
6
00 C) that was oxidized at different temperatures by using CO as an oxidant. The bands due to
2
ꢀ
1
o
coke (1330 and 1580 cm ) disappeared completely after heating in CO at 450 C.
2
o
Simultaneously, Raman bands due to HSAꢀCeO appeared to be completely restored at 450 C,
2
indicating that the catalyst structure is regenerated by coke removal. This demonstrates that CO2
effectively oxidizes the coke during the reaction and thus inhibit coke buildꢀup on the ceria
catalyst.
We may conclude that, in absence of CO , coke is formed during the ethylbenzene
2
dehydrogenation reaction over ceria. Introduction of CO may enhance the reaction in two ways.
2
First, CO may react with the H evolved in the ethylbenzene dehydrogenation, by reverse water
2
gas shift reaction, removing H from the reaction stream and favoring the dehydrogenation
2
reaction. Second the CO removes coke from the surface of ceria thereby opening ceria sites.
2
CO might interact with ceria and release CO and atomic O, which could oxidize the coke at high
2
4
9
temperatures. Pulse experiments indicate that CO could also contribute to catalyst oxidation .
2
Alternatively, the CO may more rapidly form carbonates as intermediates to the oxidation of the
2
ꢀ1
coke. DRIFTS during reaction (Figure 7) showed that the peak at 2127 cm , characteristic of
o
reduced ceria, is observed at 400ꢀ500 C. This result indicates that although CO mainly
2
oxidizes the produced H and the coke, it does not completely oxidize the reduced ceria under
2
reaction conditions.
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. Conclusions
HSAꢀCeO synthesized via a templateꢀassisted synthesis method has been shown to be an active
2
and selective catalyst for ODH reaction of ethylbenzene to produce styrene by using CO as a
2
soft oxidant. In the absence of CO , ethylbenzene dehydrogenation reaction occurred over the
2
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HSAꢀCeO , but graphiteꢀlike coke was formed on the surface of ceria. In the presence of CO ,
2
2
ethylbenzene ODH reaction must follow a two steps pathway, in which ethylbenzene is first
dehydrogenated to styrene and H , and then CO reacts with H via the reverse water gas shift
2
2
2
reaction, although the direct ODH of ethylbenzene by CO can not be eliminated. Addition of
2
CO not only enhanced the ethylbenzene ODH reaction rate and styrene yield, but also helped
2
mitigate the coke deposition during the ethylbenzene ODH reaction. The coke deposit is formed
from the polymerization of styrene and further graphitization in absence of CO , while in the
2
presence of CO , the coke is oxidized by CO and the deposition is effectively inhibited.
2
2
Acknowledgments
This research/work is supported by the Critical Materials Institute, an Energy Innovation Hub
funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy,
Advanced Manufacturing Office. A portion of this research including the in situ IR and Raman
was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of
Science User Facility.
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Table 1. Ethylbenzene ODH reaction rate over HSAꢀceria and commercial ceria at 450 C.
o
2
Reaction rate ꢀ450 C
Sample
BET surface area (m /g)
2
(µmol/m /h)
CeO ꢀꢀHSA
202
40
1.73
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Ceriaꢀcommercial
2.39
Table 2. Ethylbenzene conversion, styrene yield and selectivity of ethylbenzene dehydrogenation
reaction over HSAꢀceria catalyst in presence of CO or without CO .
2
2
Temperature Ethylbenzene conversion
Styrene Selectivity
(%)
o
Styrene Yield (%)
(
C)
(%)
Ar
76
CO2
84
Ar
54
CO2
78
Ar
CO2
5
00
71
90
475
41
15
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Figure1. Catalytic performance of pure HSAꢀceria (a) and commercial ceria (b). Reaction
conditions: EB: 2.204 µmol/min; CO : 5mL/min, Ar: 15 mL/min as the carrier, total flow rate:
2
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0mL/min. Catalyst: 200 mg mixed with 200mg quartz sands (60ꢀ80 mesh). CO : EB molar ratio
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Reaction conditions: EB:2.204 µmol/min;CO : 5 mL/min, Ar as the carrier 15mL/min, total flow
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rate:20 mL/min. Catalyst:100 mg mixed with 200 mg quartz sands (60ꢀ80 mesh).
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Figure 3. Mass spectra intensity of the produced H during the ethylbenzene dehydrogenation
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2
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temperatures. Reaction conditions: styrene adsorbed on the catalyst at room temperature, and
then followed purging by He (10 mL/min) at various temperatures.
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Figure 5. DRIFTS spectra of ethylbenzene adsorbed on HSAꢀceria at 150 C and followed by
desorption in He at various temperatures. Reaction conditions: 0.2 % ethylbenzene, He as carrier
o
gas adsorbed on the catalyst at 150 C for 30 min and then followed purging by He (10 mL/min)
at various temperatures.
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2
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various temperatures. Reaction conditions: 0.2 % ethylbenzene, CO 2.5 mL/min, He 7.5
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o
mL/min as carrier gas adsorbed on the catalyst at 150 C for 30 min and then followed by heating
in reactants at various temperatures.
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Figure 9. Raman spectra of HSAꢀceria and ethylbenzene adsorbed on HSAꢀceria at room
temperature followed by desorption in Ar at different temperatures. Reaction conditions: 0.2 %
o
ethylbenzene, Ar 10 mL/min as carrier gas adsorbed on the catalyst at 25 C for 30 min and then
followed purging by Ar (10 mL/min) at various temperatures.
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