Continuous synthesis of diethyl carbonate from ethanol and CO2 over Ce-Zr-O catalysts

Article abstract of DOI:10.1039/c4cy01400f

Ce_xZr_1-xO₂ (x = 0, 0.2, 0.5, 0.8 and 1.0) solids were prepared by a citrate method and characterized by various techniques such as N₂-adsorption (BET-SA), XRD, XPS, TEM, H₂-TPR, NH₃- and CO₂-TPD. The catalytic performance of these solids was evaluated for the direct synthesis of diethyl carbonate (DEC) from ethanol and CO₂ in continuous mode using a plug-flow reactor (PFR). According to thermodynamic data, the reaction is favourable at low reaction temperatures and high reaction pressures. Thus, the catalytic experiments were carried out at reaction temperatures ranging from 80 to 180 °C and at reaction pressures from 80 to 180 bar. The Ce_xZr_1-xO₂ catalysts exhibited significant differences in their performance mainly depending on (i) their Ce:Zr ratio and (ii) the different acid-base characteristics. Among the series Ce_0.8Zr_0.2O₂ (C80Z) and Ce_0.5Zr_0.5O₂ (C50Z) catalysts displayed the most efficient performance. Moreover, C80Z, pretreated at 700 °C, yielded DEC at the equilibrium conversion level of Y_DEC ~ 0.7% at 140 °C and 140 bar at a CO₂:ethanol ratio of 6:1 at a LHSV of 42 L_liq kg_cat^-1 h^-1. This journal is

Full text of DOI:10.1039/c4cy01400f

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Continuous synthesis of diethyl carbonate from
ethanol and CO2 over Ce–Zr–O catalysts
Cite this: DOI: 10.1039/c4cy01400f
*
Iuliia Prymak, Venkata Narayana Kalevaru, Sebastian Wohlrab and Andreas Martin
CexZr1−xO2 (x = 0, 0.2, 0.5, 0.8 and 1.0) solids were prepared by a citrate method and characterized by
various techniques such as N2-adsorption (BET-SA), XRD, XPS, TEM, H2-TPR, NH3- and CO2-TPD. The catalytic
performance of these solids was evaluated for the direct synthesis of diethyl carbonate (DEC) from ethanol
and CO2 in continuous mode using a plug-flow reactor (PFR). According to thermodynamic data, the
reaction is favourable at low reaction temperatures and high reaction pressures. Thus, the catalytic experi-
ments were carried out at reaction temperatures ranging from 80 to 180 °C and at reaction pressures from
80 to 180 bar. The CexZr1−xO2 catalysts exhibited significant differences in their performance mainly
depending on (i) their Ce: Zr ratio and (ii) the different acid–base characteristics. Among the series
Ce0.8Zr0.2O2 (C80Z) and Ce0.5Zr0.5O2 (C50Z) catalysts displayed the most efficient performance. Moreover,
C80Z, pretreated at 700 °C, yielded DEC at the equilibrium conversion level of YDEC ~ 0.7% at 140 °C and
140 bar at a CO2 : ethanol ratio of 6 : 1 at a LHSV of 42 Lliq kgcat−1 h−1.
Received 24th October 2014,
Accepted 29th January 2015
DOI: 10.1039/c4cy01400f
www.rsc.org/catalysis
gasoline combustion. Engine tests show that 5 wt% DEC in
diesel fuel can reduce particulate emissions by up to 50%.11
Introduction
Carbon dioxide is a non-toxic, abundant and low-cost C1 feed-
stock. It is an environmentally friendly chemical reagent and
can be understood as a phosgene substitute.1 Furthermore,
CO2 conversion into various useful chemical products is cer-
tainly very attractive from both economic and environmental
points of view. In fact, there are many methodologies to trans-
form carbon dioxide into various commercially important
products, for instance, dry reforming of CH4 with CO2 to pro-
duce syngas,2 transformation of CO2 to cyclic carbonates by
cycloaddition with epoxides2a,3 or glycols3 and via oxidative
carboxylation of olefins with CO2,3 synthesis of methanol
There are several known conventional methods for the
synthesis of DEC, such as the traditional phosgene-based
route,12 oxidative carbonylation of ethanol,13 carbonylation of
ethyl nitrite,14 catalytic alcoholysis of urea,15 transesterification
of organic carbonates,16 and carbonylation of ethanol.17
A
major drawback of these routes is the use of poisonous gases
(phosgene, ethyl nitrite, carbon monoxide). On the other
hand, only a few efforts are being made in recent times to
develop a catalyst for direct synthesis of DEC. However, the
majority of these efforts were confined to only batch pro-
cesses. To the best of our knowledge, attempts to use contin-
uous process for DEC production from ethanol are very rare.
Thus, the present approach on the continuous synthesis of
DEC from CO2 and ethanol is highly attractive and addition-
ally it would also certainly allow CO2 to be used as a valuable
and renewable low cost feedstock. The reaction route for the
direct synthesis of DEC using CO2 is shown below in
Scheme 1.
4
from CO2 and H2 etc. Among different approaches, direct
synthesis of organic carbonates from alcohols and CO2,5 is
gaining huge interest in recent times due to its high commer-
cial significance. For instance, dimethyl carbonate (DMC)
3,6
from methanol and CO2
or diethyl carbonate (DEC) from
7
ethanol and CO2 are two challenging examples of such an
approach of CO2 utilisation. DEC in particular is of special
importance due to its unique commercial applications.
For example, it is widely used in lubricants, cosmetics, as
plasticizer, in pharmaceutical applications8 and as electrolyte
in lithium ion batteries.9 Furthermore, DEC can be used as
an additive to diesel fuel due to its high oxygen content
(40.6%) and high octane booster power ((R + M)/2 = 105,
where R is the research octane number and M stands
for motor octane number)10 to improve the performance of
In spite of the obvious environmentally-friendly synthesis
route, some additional difficulties of this approach also
need to be taken into account. For instance, the activation of
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a,
18059 Rostock, Germany. E-mail: Sebastian.Wohlrab@catalysis.de
Scheme 1 One-step synthesis of diethyl carbonate from ethanol and
CO2.
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carbon dioxide is very difficult due to the fact that CO2 is
highly thermodynamically stable and kinetically inert. In
addition, this synthetic route also shows some thermody-
namic (equilibrium) limitations and therefore the yield of
DEC to be achieved is expected to be relatively low. Another
Efforts were made to investigate the effect of varying Ce : Zr
ratio on catalyst phase purity, morphologies, surface compo-
sition, reducibility, acid–base characteristics, as well as the
catalytic performance.
problem is the formation of H2O as a by-product, which Experimental
shifts the equilibrium towards the reactants side, in addition,
a reverse hydrolysis of formed DEC back into ethanol and
Catalyst preparation
CexZr1−xO2 solids with x = 0, 0.2, 0.5, 0.8 and 1.0 were prepared
CO2 is possible. Several studies described some ways to over-
come this problem, e.g. usage of certain chemical reagents or
absorbents might be helpful to remove H2O from the product
during the course of the reaction, for instance butylene
oxide,18 benzonitrile,19 acetals or ketals,20 acetonitrile21
but also inorganic materials like zeolites.22 Alternatively,
Dibenedetto et al.23 used a polymeric organic membrane PERVAP
1211 to remove the water formed during the reaction. Unfor-
tunately, this effort was not successful due to problem that
the reaction mixture cannot be directly separated as DEC
passes through the membrane since it is permeable at con-
centrations above 0.3%. Nevertheless, in the work of Li
et al.24 three types of supported membranes (mesoporous
silica, polyimide silica and polyimide–titania hybrid mem-
brane) were applied for another similar reaction, i.e. the syn-
thesis of dimethyl carbonate (DMC) from methanol and CO2.
Even though, the use of such membranes considerably
improved DMC formation, however, the capability of dehy-
dration at high pressure and temperature was reported to be
very low.
by a citric acid complexation method according to Alifanti
et al.29 ZrOIJNO3)2ĴxH2O (Sigma-Aldrich, technical grade) and
CeIJNO3)3Ĵ6H2O (Alfa-Aesar, 99.5%) in desired quantities were
dissolved in deionized water (0.1 M). Citric acid IJC6H8O7,
Sigma-Aldrich, 99%) was added in 10 mol% excess for com-
plete complexation of metal ions. The mixture was stirred for
2 hours at room temperature. The excess solvent was removed
on a rotary evaporator. The obtained solid was dried overnight
(16 h) under vacuum at 70 °C. This precursor was calcined at
450 °C for 3 h in air. Using this procedure, five different
CexZr1−xO2 catalysts with varying Ce contents of 0, 20, 50, 80,
100 mol% denoted as Z (pure zirconia), C20Z, C50Z, C80Z and
C (pure ceria), respectively, were prepared. Another batch of
C80Z was calcined at 700 °C for 3 h in air.
Catalysts characterization
The surface areas (SA) as determined by the BET equation
and pore volumes of the samples received from BJH equation
were measured using a NOVA 4200e device (Quantachrome
Instruments). Prior to each nitrogen sorption measurement,
the samples were evacuated for 2 h at 200 °C to remove
physisorbed water.
X-ray diffraction (XRD) studies were carried out on a X'Pert
Pro diffractometer (Panalytical, Almelo, Netherlands) with
CuKα radiation (λ = 1.5418 Å, 40 kV, 40 mA) and an
X'Celerator RTMS detector. The phase composition of the
samples was determined using the program suite WinXPOW
by STOE & CIE with inclusion of the Powder Diffraction File
PDF2 of the ICDD (International Centre of Diffraction Data).
The average crystallite size (D) was calculated using Scherrer
equation:30
Until now, wide range of catalytic systems have been
studied for DMC synthesis from methanol and CO2. For
instance, CeO2,7a,21,25 K2CO3,7b Ce-SBA-15,17 Nb2O5/CeO2,23
Cu–Ni/AC,7c Ce–Si-MCM-41,7a Ce–H-MCM-41,7a metal tetra-
5b,20d,27
alkoxides,26 CeO2–ZrO2
are some of the most widely
used catalyst compositions so far. Among them, literature
reports indicate that Ce–Zr–O solids are somewhat more
effective catalysts.27 The effectiveness of this catalyst was
ascribed to the presence of acid–base sites on the surface,
which consist of coordinatively unsaturated metal cations M4+
(Lewis acid-electron acceptors), oxide anions O2− (Lewis base-
electron donors) and hydroxyl groups probably acting as
Brønsted base centers during water formation.6a It has been
proposed that dissociation of adsorbed ethanol leads to the
formation of ethoxide group on the acid sites of the catalyst
accompanied by a proton release, which reacts with a surface
hydroxyl group to produce water. CO2 is then inserted into the
M–O bond of the C2H5O–M species to produce the reaction
intermediate m-C2H5OCOO–M. This process is facilitated by
interactions of C and O atoms with Lewis acid–base pairs of
sites IJO2−–M4+–O2−). Monoethyl carbonate species react with
activated ethanol on the acid sites of the catalyst to produce
DEC.28 It was suggested that high selectivity of DEC formation
is due to rapid conversion of the ethoxide species to ethyl
carbonate species under high CO2 pressure.
K
 cos
D 
where λ is X-ray wavelength, K constant of proportionality
taken as 0.94, β is determined as the full width at half maxi-
mum of the peak and Θ is the diffraction angle. For crystal-
lite size calculation the first reflection between 27.5 and 32°
2Θ was evaluated.
For the determination of the elemental composition, a
Varian 715-ES ICP-OES (Inductively Coupled Plasma-Optical
Emission Spectrometer) was used. Approximately 10 mg of
the sample was mixed with 8 mL of aqua regia and 2 mL
hydrofluoric acid. Digestion was performed in a microwave-
assisted sample preparation system “MULTIWAVE PRO” from
Anton Paar at ~200 °C and ~50 bar pressure. The data
In this work, we describe the application and catalytic per-
formance of different Ce–Zr mixed oxide catalysts for the con-
tinuous synthesis of DEC under varying reaction conditions.
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analysis was performed on the Varian 715-ES software “ICP
Expert”.
X-ray photoelectron spectroscopy (XPS) was carried out
using a VG ESCALAB 220iXL instrument with monochromatic
AlKα radiation (E = 1486.6 eV). The samples were fixed by
using a double adhesive carbon tape on a stainless steel
sample holder. The peaks were fitted by Gaussian–Lorentzian
curves following a Shirley background subtraction.
Transmission Electron Microscopy (TEM) investigations
were carried out at 200 kV with an aberration-corrected
JEM-ARM200F (JEOL, Corrector: CEOS). The aberration
corrected STEM imaging (High-Angle Annular Dark Field
(HAADF) and Annular Bright Field (ABF)) were done with a
spot size of approximately 0.13 nm, a convergence angle of
30–36° and collection semi-angles for HAADF and ABF of
90–170 mrad and 11–22 mrad, respectively. Samples were
prepared by deposing without any pre-treatment on a holey
carbon supported Cu-grid (mesh 300) and transferred to the
microscope.
Fig. 1 Schematic diagram of the experimental set up.
equipped with an oil heated jacket (max. temperature =
220 °C); high pressure pumps, relief valves, product collector
and flow-meter. All tubings and fittings were made from
stainless steel 316L (Swagelok).
Temperature programmed reduction (TPR) profiles were
recorded in a temperature range from r.t. to 800 °C at a
heating rate of 5 K min−1 on a Micromeritics AC2920 instru-
ment. Prior to TPR measurement, all the samples were pre-
treated with 5% H2/Ar at room temperature for 10 min.
The total acidity and basicity (adsorbed mmol NH3 or CO2
per gram catalyst) of the solids were determined by tempera-
ture programmed desorption of ammonia (TPD-NH3) or
carbon dioxide (TPD-CO2), which was carried out in a home-
made apparatus consisting of a gas flow system, a high tem-
perature oven and a quartz reactor. For determination of acid
sites, the samples (100–200 mg) were treated in nitrogen at
400 °C for 30 min to remove moisture and cooled down to
100 °C in He of high purity (6.0) prior to NH3 adsorption,
which was carried out at 100 °C for 30 min in a flow of 5%
NH3/He. Afterwards, the TPD-NH3 experiments were carried
out from 100 to 450 °C in He flow (50 cm3 min−1) with a
heating rate of 10 K min−1. Desorption of NH3 was monitored
and evaluated by a thermal conductivity detector (TCD,
GOW-Mac Instrument Co.). For determination of basic prop-
erties, the sample (200 mg) was treated in He (50 mL min−1)
at 500 °C with a heating rate of 10 K min−1 for 15 min (for
the removal of adsorbed water) and cooled down to 100 °C in
He (50 mL min−1) prior to CO2 adsorption, which was carried
out at 100 °C for 90 min in a flow of 1.2% CO2–He mixture.
Afterwards, the TPD-CO2 experiments were carried out in He
flow (50 mL min−1) at 100 °C for 30 min (for removal of
physisorbed CO2). After cooling to 70 °C for 10 min, the sam-
ple was heated up to 800 °C at a rate of 10 K min−1 in a
helium flow (50 mL min−1). The analysis of the effluent gases
was performed by Quadrupole mass spectrometer (Balzers
Omnistar).
In a standard procedure, the reactor was packed layer-by-
layer: 27 g of corundum (size: 1 to 1.25 mm), 1 g of catalyst
(size: 1 to 1.25 mm fraction), 27 g of corundum. A Gilson-
pump with a thermostatic kit was employed to deliver liquid
CO2 (99.9%, Air Liquide) to reactor at a flow rate of 0.6 mL
min−1. A Shimadzu-HPLC pump was used in order to inde-
pendently control the flow of ethanol of 0.1 mL min−1
(99.9%, H2O ≤ 0.1%, Roth). The molar ratio of EtOH : CO2
was kept constant during the mainly 1 : 6. Ethanol and liquid
CO2 (from a dip tube cylinder) were metered through a tube
that was filled with molecular sieve type 3A and placed
before the entrance of the reactor to remove moisture, if any
present in the used ethanol.
At the exit of reactor, a filter with pore diameter of 0.5 μm
was placed in order to avoid a discharge of catalyst with
flowing reagents. The system pressure was controlled by two
manually regulated relief valves in series. The first one was
set to the desired reaction pressure while the second one
was set to approximately 5–10 bar less. Such modification
was applied to reduce a possible rapid pressure drop and to
allow a more constant flow. Moreover, the second relief
valve was covered by an external heating to avoid freezing of
humidity. Afterwards, the liquid phase was separated from
the gas phase in a cold trap placed at the exit of the reactor
outlet. The samples were analysed by a gas chromatograph
(GC-2014, Shimadzu) using a capillary column (CP-PoraBOND
Q, 10 m × 0.53 mm × 10 μm) equipped with a FID detector.
The experiments were performed at pressures of 80, 110, 140,
180 bar and at temperatures of 80, 100, 120, 140, 160, 180 °C.
Results and discussion
Experimental setup and catalytic tests
Nitrogen sorption analysis
The experimental setup is shown in Fig. 1. The catalytic setup
mainly consisted of a high pressure reactor (inner volume =
32 mL, inner diameter = 12.7 mm, max. pressure = 200 bar),
The surface areas, pore volumes and average pore sizes of
mixed solids catalysts are summarized in Table 1. SAs were
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0.097 nm) with the smaller Zr4+ (ionic radius 0.084 nm).34
The results, obtained by Yashima et al.35 revealed that phase
transitions occurring in CexZr1−xO2 depend on their composi-
tion. The tetragonal CexZr1−xO2 phase appears with Ce < 50
mol% (C20Z), whereas above 50 mol%, a cubic CexZr1−xO2
(C50Z and C80Z) phase is formed.33c,34b,36 There is no evi-
dence for phase segregation in wide range of composition.
For pure ZrO2 (Z) the diffraction peaks can be assigned to the
tetragonal ZrO2 structure.32
Table 1 Effect of Ce loading on the textural properties and XRD-
determined crystal size of CexZr1−xO2 solids
x in
CexZr1−xO2
BET-SA,
m2 g−1
Vpore
,
dpore
nm
,
Crystal size,
nm
Sample
cm3 g−1
Z
0
88
79
57
42
27
0.069
0.060
0.064
0.076
0.036
2.8
2.8
4.2
6.2
5.3
3.8
4.6
4.8
6.0
9.0
C20Z
C50Z
C80Z
C
0.2
0.5
0.8
1.0
found to depend on the content of Ce in the catalysts. Pure
ZrO2 (Z) displays much higher SA compared to pure CeO2
(C). As a result, the surface areas of the Ce–Zr mixed oxides
are observed to decrease with increase in Ce content due to
the poor resistivity against sintering of the cerium rich solids
as claimed elsewhere.31
The crystallite sizes as well as the pore volumes of
mixed solids are found to vary in a narrow range from
4.6 to 6.0 nm and 0.060 to 0.076 cm3 g−1, respectively,
depending upon the content of Ce in the system. Pure CeO2
(C) exhibits the largest crystallite size (9 nm) and lowest pore
volume, while pure ZrO2 (Z) possesses the smallest crystallite
size and pore volume in the range of the mixed oxides. The
combination of zirconium and cerium enhances the stability
of mixed oxides and suppresses the crystal growth during cat-
alyst preparation. Additionally, it can be seen that mesopores
are present in all CexZr1−xO2 samples.
Near-surface characterization by XPS
XPS was used to investigate the near-surface composition of
the samples. Fig. 3 compares the relative Ce, Zr and O con-
centrations measured by XPS with that of bulk composition
obtained by ICP. It can be seen that Zr is significantly
enriched in the near-surface-region of all mixed oxides. The oxy-
gen content is also above its stoichiometric value except for
C20Z. In case of near-surface concentration of Ce, there is no
general tendency. For instance Ce is either enriched (C80Z)
or remained more or less at the same concentration (C50Z)
compared to the bulk. But in the case of pure CeO2 and low
Ce content catalyst (C20Z), the surface concentration of Ce is
slightly decreased compared to its bulk.
TEM analysis
TEM (Transmission Electron Microscopy) studies were
performed on the pure oxides C and Z as well on C80Z. Rep-
resentative images are displayed in Fig. 4 revealing nanome-
ter sized particles. The particle size of CeO2 is about 8 nm,
which is quite close to the value calculated from Scherrer
equation (9 nm). In the case of C80Z particle sizes of about
5 nm are obtained but these particles are not uniform in
shape. TEM-HAADF images (Fig. 4 (1b, 2b and 3b)) confirm
the mesoporous nature of the samples. In some cases, larger
pores up to 40 nm in diameter can also be detected.
XRD analysis
Fig. 2 shows XRD patterns of different compositions of fresh
CexZr1−xO2 catalysts. Characteristic peaks for sample C can
be attributed to a cubic CeO2 phase.7a,32 With increase in zir-
conium loading in the solid composition, the main peaks
shift to higher diffraction angles leading to changes of unit
cell parameters and lattice deformation.33 This can be associ-
ated with a progressive substitution of Ce4+ (ionic radius
Fig. 2 XRD patterns for CexZr1−xO2 catalysts. Phase composition: +
cubic CeO2 (JCPDS 65-5923), * tetragonal ZrO2 (JCPDS 79-1769).
Inset: ■ c-Ce–Zr–O mixed phase, ● t-Ce–Zr–O mixed phase.
Fig. 3 Comparison of the relative Ce, Zr and O concentrations in the
near-surface-region as measured by XPS with bulk composition (ICP)
for CexZr1−xO2 (x = 0, 0.2, 0.5, 0.8, 1).
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Fig. 5 H2-TPR profiles of different CexZr1−xO2 solids. (→: indicates
stationary treatment of sample at 800 °C for 2 h).
by de Rivas et al. this fact suggests that the addition of Zr to
CeO2 sample remarkably causes reduction of surface and bulk
in one step at medium temperatures.41 Besides, some H2 con-
sumption can be noticed at higher temperatures for C80Z and
C50Z, but to a lower extent than for CeO2. Furthermore,
the position of surface reduction peak shifted from 452 to
508 °C with increasing of zirconium content. It was noted that
the extent of the reduction seems to be the highest for C50Z
sample. Moreover, the weak peak at 360 °C can be attributed
to subsurface Ce4+ in different chemical environment.42 The
H2 uptake of pure CeO2 (C) at 224 °C might be traced back to
an adsorptive process as reported by Fierro et al.43
The relative hydrogen consumption, expressed as mmol of
H2 per gram of catalyst and peak maxima are shown in
Table 2 as a direct measure of the amount of water evolved
from the sample under flowing 5% H2/Ar. The increase in H2
uptake from 1.15 mmol g−1 for pure ceria (C) to 1.793
mmol g−1 for C50Z provides hints to enhanced reducibility by
the addition of Zr to CeO2 solid.41a Further increase in zirco-
nium content beyond 50 mol% progressively reduces the H2
consumption. Moreover, for pure ZrO2 (Z) the H2 consump-
tion was only 0.192 mmol g−1. This denotes the Zr4+ cations
are hardly being reduced under the conditions applied. Our
findings are in a good agreement with those described by
Trovarelli et al.40 where the reducibility of CexZr1−xO2 solid
solutions is also strongly dependent on the crystal structure.
Fig. 4 ABF-STEM (a) and HAADF-STEM (b) images of (1) C, (2) C80Z,
(3) Z.
H2-TPR analysis
The redox properties of the CexZr1−xO2 solids were evaluated
by temperature-programmed reduction (TPR) with 5% H2/Ar
(50 mL min−1). Fig. 5 depicts the TPR profiles of the CexZr1−xO2
catalysts, where the peak maximum indicates the temperature
that corresponds to the maximum rate of reduction. It is
known, that cerium can exist in CexZr1−xO2 solid solutions as
Ce3+ and Ce4+ ions while zirconium exists as Zr4+ only.37 Pure
CeO2 (C) has a high oxygen storage capacity and possess a
larger number of oxygen vacancies.38 These redox properties
can strongly be enhanced when Zr4+ cations are introduced into
the CeO2 lattice by higher oxygen ion mobility and increased
vacancy sites inside the modified lattice.37,39
The reduction process of pure CeO2 (C) involves two main
steps. The first region is located between 350 and 600 °C with
Tmax around 452 °C and second region starts from 600 °C
with Tmax around 792 °C. The low-temperature peak is due
to the most easily removable surface capping oxygen of CeO2,
while the high-temperature signal at 792 °C is caused by the
removal of bulk oxygen.40 The TPR profiles of the mixed
CexZr1−xO2 oxides show a main broad reduction in the region
between 500–530 °C with different Tmax values. As reported
NH3- and CO2-TPD
The concentration and strength of the acid sites was evalu-
ated by NH3-TPD (i.e. expressed as an amount of NH3
desorbed per gram of catalyst) and presented in Table 3. The
Table 2 H2 uptake and peak maxima value of CexZr1−xO2 solids
Sample
H2 uptake, mmol g−1
Peak max, °C
Z
0.192
582
C20Z
C50Z
C80Z
C
0.704
1.793
1.482
0.015, 0.271, 0.869
508
530
503
224, 452, 792
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Table 3 Acid and base characteristics of CexZr1−xO2 solids
Sample
NH3 desorbed, mmol g−1
CO2 desorbed, mmol g−1
Z
0.185
0.130
0.118
0.094
0.034
0.112
0.235
0.117
0.104
0.052
C20Z
C50Z
C80Z
C
acidity characteristics of the solids is strongly affected by the
addition of zirconium into ceria lattice.44 It is evident from
Table 3 that pure ZrO2 (Z) is more acidic (i.e. 0.185 mmol
NH3 g−1) than pure CeO2 (C). It can be seen that the total
acidity was the lowest for solid C (0.034 mmol g−1), which is
however considerably improved by the addition of zirconium.
The NH3-TPD profiles of the CexZr1−xO2 solids are shown
in Fig. 6. The results demonstrate the presence of acid sites
of different strength in these solids. The desorption peaks
of TPD profiles located at 100–200 °C, 200–400 °C and 400–
450 °C can be assigned to weak, moderate and strong acid
sites, respectively.45 Both the weak and moderate acid sites
were observed for all catalysts. However, the strong acid sites
with characteristic desorption temperature of about 450 °C
were observed in Z, C20Z and C50Z solids only.
To investigate the effect of Ce content on basic properties,
CO2-TPD experiments were also carried out. Fig. 7 illustrates
the CO2-TPD profiles and Table 3 lists the amount of CO2
desorbed during temperature programmed desorption mea-
surements of different CexZr1−xO2 solids. All Ce-based cata-
lysts exhibit two broad desorption peaks at varying tempera-
tures indicating that different types of basic sites are present
with weak (100–230 °C), moderate (230–500 °C) and strong
(above 500 °C) basic strengths.18a,46 It can be seen, that the
total concentration of CO2 desorption from sample C is very
low. Among all prepared mixed oxides, C20Z exhibited the
highest concentration of basic sites (0.235 mmol g−1).
Fig. 7 CO2-TPD profiles of CexZr1−xO2 solids.
Influence of the reaction conditions on catalytic performance
The critical points (pc, Tc) for the C2H5OH–CO2 binary system
were reported by various researchers in the past, for instance
by Baker,47 Takishima,48 Lim,49 Yeo,50 and Galicia-Luna
et al.51 The supercritical region for C2H5OH–CO2 binary system
is reached maximum near 160 bar and temperature in a range
of 120–160 °C with mixtures whose initial CO2 molar fraction
are between 0.7 and 0.9. Moreover, the larger the ethanol con-
centration in the feed mixture is, the higher is the required
temperature to reach the critical point. Based on such reports,
we suppose that these trends are still valid for quaternary sys-
tem, due to the low DEC amount (predicted x ~ 0.004 at reac-
tion equilibrium) and H2O (predicted x ~ 0.004 at reaction
equilibrium) productions. One can also expect that above
160 bar the reaction mixture might be in the liquid or super-
critical state, depending upon the system temperature.
According to literature,23,52 the use of supercritical conditions
allow to reach a higher ethanol conversion with respect to
using liquid ethanol pressurized by CO2. Such conclusion was
made based on fact that under supercritical conditions, EtOH
and CO2 are in a single phase and the effect of solubility-
dependent concentration of CO2 in the liquid phase is can-
celled out. Cai et al.53 and Leino et al.5b assumed in a theoreti-
cal study for DMC synthesis that the high performance had
been partly attributed to the fact that reaction becomes ther-
modynamically favourable as the system pressure increases.
The present work was focused on transferring the knowl-
edge from batch approaches to a continuous process, thus
the optimal flow rate of reagents is a very important parame-
ter to achieve a highest DEC yield and space-time-yield as
well. From preliminary tests performed at 140 °C and 140 bar,
a maximum possible flow rate (characterized by no significant
A main conclusion from these studies is that surfaces of
CexZr1−xO2 catalysts possess both acidic and basic sites. Inter-
estingly, the acid-to-base site ratio was about 1 for C80Z and
C50Z catalysts, which seemed to be optimum for improved
catalytic properties.
−1
YDEC loss) of 42 Lliq kgcat h−1 (τ = 68.6 s) was identified and
selected for the study. The influence of EtOH : CO2 ratios
on ethanol conversion into DEC was already described by
Dibenedetto et al.23 They found a correlation between DEC
formation and EtOH : CO2 ratio, i.e. increasing the ratio leads
to decreased EtOH conversion. Therefore, the ratio of EtOH :
CO2 = 1 : 6 was selected for our experiments.
Fig. 6 NH3-TPD profiles of CexZr1−xO2 solids.
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The influence of the total reaction pressure and tempera-
ture on DEC formation was studied over C, C80Z, C50Z, C20Z
and Z catalysts in continuous running plug flow reactor (see
Fig. 8). The areas in the three-dimensional figures reflect the
catalytic behaviour of the CexZr1−xO2 catalysts depending on
their composition. We observed an increase of DEC yield
with rise of pressure for all CexZr1−xO2 catalysts. The influ-
ence of reaction temperature is more complex. For C, C80Z
and C50Z, where the yield of DEC gradually increased with
elevating reaction temperature it reaches a maximum at 140 °C
and then drops to lower values at higher temperatures. It is
well-known from literature18a,27 that carboxylation of alcohols
is an exothermic reaction and from thermodynamic point of
view the high reaction temperature is unfavourable for organic
carbonates formation. The activity of pure ZrO2 (Z) and C20Z
slowly increased with rise in temperature in the pressure region
of 140–160 bar and reached the highest value at 180 °C. Highest
DEC yields were obtained for C50Z and C80Z possessing max-
ima at a reaction pressure of 160 and 140 bar, respectively.
ZrO2 (Z). However, CexZr1−xO2 catalyst with x = 0.8 and 0.5
exhibited better DEC yields. Thus, a relation between DEC
formation and BET-SA cannot be derived. Possible reasons
for more efficient performance of C80Z and C50Z solids com-
pared to other Ce : Zr ratios could be the differences in their
crystal structure, surface composition and acid–base proper-
ties. The cubic structure was formed for Ce-rich samples (e.g.
Ce ≥ 50 mol%), whereas a tetragonal phase was found to
be predominant in the solids with lesser Ce contents. Based
on this, we can conclude that the most active phase for
DEC formation is cubic CexZr1−xO2 (C, C80Z, C50Z). This
result is well consistent with previous works where pure
27
6b
CeO2
and mixed oxides such as Ce0.6Zr0.4O2
and
54
Ce0.8Zr0.2O2 existed in cubic phase were the most active
catalysts in direct DEC/DMC batch syntheses. In contrast,
Zhang52 and Wang27 have shown that the tetragonal
Ce0.5Zr0.5O2 and Ce0.07Zr0.93O2 phases exhibited the highest
DMC/DEC formations among other Ce–Zr mixed oxides. We
assume that the high performance of tetragonal CexZr1−xO2
phase claimed by Zhang et al. can be related to different
synthesis protocols applied. The incorporation of Zr into
the cubic CeO2 lattice remarkably affects the amount of
oxygen vacancies and the basic properties of the materials
therewith.41a Besides, XPS revealed that there is a clear
enrichment of Zr in the near-surface-region of all samples.
Interestingly, such enrichment is much more pronounced in
the case of C80Z and C50Z samples, which seemed to be one
Materials properties affecting the catalyst activity
According to literature the activity of pure CeO2 and
CexZr1−xO2 catalysts towards DMC and DEC formation had
been related to its specific surface areas.20d,25b In our contri-
bution, the BET-SA of the CexZr1−xO2 solids increase with
higher Zr content and show a maximum value for pure
Fig. 8 Effect of reaction pressure and temperature on the yield of DEC over different CexZr1−xO2 catalysts. Reaction conditions: catalyst weight
1 g, EtOH : CO2 = 1 : 6, 1 h time-on-stream, LHSV = 42 Lliq kgcat−1 h−1, τ = 68.6 s.
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of the major reasons for their improved performance. In
addition, the concentration of acid sites in CexZr1−xO2 cata-
lysts also depends on Zr content, which in turn led to the dif-
ferent catalytic behaviour. However, a large amount of strong
acid and base sites has a negative effect on the DEC forma-
tion. As a result, the pure ZrO2 (Z) and C20Z having stronger
acid–base properties showed rather poor performance. How-
ever, Wang et al. showed the absence of strong basicity and
acidity on the surface of the CexZr1−xO2 solid with the high Zr
content (0.93) whereas samples with low Zr (0.2) concentra-
tion demonstrated the strong strength of acid–base sites. In
contrast, our C80Z catalyst that exhibits weak to medium
strength of acidic sites seems to be a good balance between
the acidity and catalytic activity. As stated above, it is also
known from the literature27,55 that the catalytic activity of
CexZr1−xO2 is reduced to some extent by the presence of
strong acid–base sites on the surface of the catalysts. This
might be another reason for the requirement of higher reac-
tion pressure (supercritical CO2) and higher temperature for
mobility and also an increase of the number of anion vacan-
cies on the oxide surface.41a These oxygen vacancies can also
act as CO2 adsorption sites, which were however confirmed by
CO2-TPD analysis of the present study. Due to the significant
increased adsorption of CO2, an improved catalytic perfor-
mance could be achieved on C50Z and C80Z solids compared
to pure CeO2. In view of the highest YDEC obtained on C80Z at
possibly lowest temperatures and pressures, this solid was
further used to check the long-term stability of the catalyst.
In order to explore the variation of DEC formation with
time, the reaction was performed at 140 °C, 140 bar for 20
hours over C80Z catalyst. The experimental results revealed
that the formation of DEC slightly increased with reaction
time and levelled off after 6 hours (YDEC = 0.55%). Beyond,
an influence of calcination temperature of CexZr1−xO2 cata-
lysts was already reported by Tomishige et al. for the cyclic
carbonate synthesis from glycol and carbon dioxide.46
Accordingly, an even higher catalytic performance could
be achieved with C80Z which was pretreated at 700 °C and
tested at 140 °C and 140 bar at different EtOH : CO2 ratios
(Fig. 9a) as well as different flow rates (Fig. 9b).
C50Z sample compared to C80Z to display maximum YDEC
.
Moreover, previous reports20d,56 have shown that the presence
of weak to medium acidity on Ce–Zr surfaces is the key factor
in a selective DEC/DMC syntheses. On strong acid sites the for-
mation of diethyl ether/dimethyl ether (DEE/DME) is favoured.
Furthermore, DEE/DME together with H2O suppresses the for-
mation of DEC/DMC. In addition, Tomishige and co-authors20d
found that during the reaction of EtOH with CO2 ethylene as
by-product also being formed. Zhang et al.52 investigated the
influence of the Ce/Zr ratio on the formation of by-products
in the direct synthesis of DMC from methanol and CO2.
The catalyst with a Ce : Zr molar ratio of 1 showed the
highest activity without forming by-products except H2O. In
addition to this, in all our continuous mode experiments no
by-products were detected revealing the advantage of the
continuous process.
Here, the EtOH : CO2 ratio was also at its optimum at 1 : 6
in accordance to Dibenedetto et al.23 Furthermore, up to a
As mentioned above, the catalytic activity was primarily
related to the strength of acid–base sites located on the Ce–Zr
surface. Interestingly, both C80Z and C50Z have shown
nearly an equal amount of acid–base sites and the highest
catalytic performance as well. This is also in accordance to
Tomishige et al. who claimed that an equal number of
neighbouring acid–base sites is required for optimal catalyst
performance, whereas they found such an effect in the direct
synthesis of DMC over ZrO2 catalysts.28a
Besides acid–base strength and the ratio of such sites,
carbon dioxide activation, which is the most difficult part
of the reaction, needs to be evaluated in a systemic manner.
According to the reaction mechanism proposed by Wada
et al.57 for the formation of DMC over Cu–CeO2 catalyst, the
carbon dioxide adsorption was related to oxidation state of
surface cerium. It was speculated that oxygen vacancies are
defect sites, which can adsorb CO2. The population of O
vacancies might increase by the reduction in H2 and/or by
the presence of Cu sites in catalysts. In case of CexZr1−xO2
solid solutions, the presence of Zr4+ in the CeO2 lattice causes
distortion in the ceria lattice resulting in an increase of oxygen
Fig. 9 Effect of different a) EtOH : CO2 ratios and b) LHSV's on DEC
yield over C80Z calcined at 700 °C. Reaction conditions: catalyst
weight 1 g, 1 h time-on-stream, T = 140 °C, p = 140 bar.
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LHSV of 42 Lliq kgcat h−1 (τ = 68.6 s) an increase of the DEC-
for BET-SA, Dr. M. Schneider for XRD measurements, Ms. A.
Simmula for ICP-OES, Dr. J. Radnik for XPS measurements,
Dr. M.-M. Pohl for TEM results, Dr. H. Atia for H2-TPR, NH3-
and CO2-TPD.
−1
yield up to YDEC ~ 0.7%. was observed. However, a further
increase in LHSV, e.g. 62 Lliq kgcat h−1 (τ = 46.5 s), caused a
−1
dramatic drop of the DEC yield (YDEC ~ 0.1%). The change of
total CO2–EtOH flow greatly affects the contact time, which
however was found to be unexpectedly very low for this con-
tinuous reaction mode.
To get a better assessment of catalytic activity of C80Z,
the predictive Soave–Redlich–Kwong equation58 was used
to calculate the equilibrium DEC yield of ~0.7% under these
selected reaction conditions (T = 140 °C, p = 140 bar). The
comparison shows that a continuous process for the direct
formation of DEC from carbon dioxide and ethanol can be
operated at the reaction equilibrium level.
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CexZr1−xO2 catalysts have been tested in the direct and con-
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