Topics in Catalysis Vol. 20, Nos. 1–4, July 2002 (# 2002)
97
Effects of catalyst phase structure on the elementary processes involved in
the synthesis of dimethyl carbonate from methanol and carbon dioxide
over zirconia
Kyeong Taek Jung and Alexis T. Bell*
Chemical Sciences Division, Lawrence Berkeley National Laboratory, and Department of Chemical Engineering, University of California,
Berkeley, CA 94720-1462, USA
E-mail: [email protected]
In situ infrared spectroscopy has been used to investigate the synthesis of dimethyl carbonate (DMC) from methanol and carbon
dioxide over tetragonal (t-ZrO2 ) and monoclinic zirconia (m-ZrO2 . While similar species were observed for both catalyst phases, the
dynamics of the elementary processes were different. The dissociative adsorption of methanol to form methoxide species was
approximately twice as fast on m-ZrO2 as on t-ZrO2 . CO2 insertion to form monomethyl carbonate, an intermediate in the synthesis
of DMC, occurred more than order of magnitude more rapidly over m-ZrO. By contrast, the transfer of a methyl group from
adsorbed methanol to monomethyl carbonate and the resulting formation of DMC proceeded roughly twice as fast over m-ZrO2 .
The observed patterns are attributed to the higher Brønsted basicity of hydroxyl groups and cus-Zr4þ O2 Lewis acid/base pairs
present on the surface of zirconia.
KEY WORDS: zirconia-catalyzed reactions; methanol and CO2 to dimethyl carbonate
1. Introduction
Several studies have shown that dimethyl carbonate
(DMC) can be synthesized by the reaction of methanol
with carbon dioxide over zirconia [1–5]:
2CH3 OH þ CO2 ! ðCH3 OÞ2 CO þ H2 O
The activity of zirconia for this reaction has been
ascribed to the presence of both basic and acidic sites on
its surface. Both in situ and infrared spectroscopy have
been used to investigate the mechanism of DMC
synthesis over zirconia [3–5]. These studies have led to
the proposed mechanism shown in figure 1 [5]. Molecular adsorption of methanol occurs via its oxygen atom
to coordinatively unsaturated Zr4þ cations present at
the catalyst surface. The dissociation of the adsorbed
methanol leads to the formation of a methoxide group
(Zr–OCH3 ) and the release of a proton, which reacts
with a surface hydroxyl group to produce water. Carbon
dioxide inserts into the Zr–O bond of the methoxide to
form a mondentate methyl carbonate group) Zr–
OC(O)OCH3 ). This process is facilitated by the interactions of the C and O atoms in CO2 with Lewis acid/
base pairs of sites (Zr4þ O2 ) on the surface of the catalyst. Methyl carbonate species can also be produced via
the reaction of methanol with carbon dioxide adsorbed
in the form of bicarbonate species, but this process is
slower than that involving the reaction of carbon dioxide with methoxide species. DMC is formed by the
*To whom correspondence should be addresses.
reaction of the methyl carbonate species with methanol,
a process that results in the transfer of a methyl group to
the carbonate and restores a hydroxyl group to the
zirconia surface. The decomposition of DMC on
monoclinic zirconia has also been investigated and has
been observed to occur via the reverse of the processes
described for the synthesis of DMC. The conclusion,
therefore, is that the synthesis of DMC over ZrO2
involves both amphoteric Zr–OH hydroxyl groups and
cus-Zr4þ O2 sites that act as Lewis acid/base pairs. The
present study was undertaken to establish the effect of
zirconia phase on the dynamics of the elementary processes presented in figure 1, using previously characterized samples of teragonal (t-ZrO2 ) and monoclinic
zirconia (m-ZrO2 ) of comparable surface area [6]. The
identity of the species formed and the dynamics of their
formation and loss were monitored by in situ infrared
spectroscopy.
2. Experimental
The preparation of tetragonal and monoclinic zirconia used in this work has been described previously [7].
Tetragonal zirconia was prepared by dropwise addition
of a 30 wt% ammonium hydroxide solution to a 0.5 M
solution of zirconyl chloride (ZrOCl2 8H2 O, Aldrich)
maintained at a pH of 10. The precipitated material was
heated in its mother liquor at 373 K and 1 atm for 240 h
while maintaining the pH at 10. The final product was
recovered by vacuum filtration. It was then redispersed
in deionized water to remove residual chlorine and then
1022-5528/02/0700-0097/0 # 2002 Plenum Publishing Corporation
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K.T. Jung, A.T. Bell/Methanol and CO2 to dimethyl carbonate over zirconia
Figure 1. Continued over.
filtered. Fifty such washings were carried out with a
total of 10 L. After each washing, the filtrate was
checked for CI by addition of a few drops of AgNO3
solution. The washed product was air dried at 373 K in a
vacuum oven for 24 h and then calcined in a tube furnace in pure O2 . The calcination temperature was raised
from 298 K to 973 K at 10 K/min and then held at that
temperature for 5 h. This procedure yielded a material
with a BET area of 187 m2 /g. The presence of tetragonal
zirconia as the only phase was confirmed by X-ray diffraction.
Monoclinic zirconia was prepared by boiling a 0.5 M
solution of zirconyl chloride (ZrOCl2 8H2 O, Aldrich)
under reflux at 373 K and 1 atm for 240 h, while maintaining the pH at 1.5. The precipitated material was
washed, dried, and calcined in a manner identical to that
used to produce tetragonal ZrO2 . A calcination temperature of 573 K was used to obtain a material with a
BET surface area of 110 m2 /g. X-ray diffraction spectra
of the calcined material confirmed that only monoclinic
ZrO2 was present in the calcined material in both cases.
In situ transmission infrared spectroscopy was performed using a 2 cm diameter catalyst disk weighing
50 mg. The catalyst disk was contained in a low deadvolume infrared cell [8]. Infrared spectra were collected
using a Nicolet Magna 750 series II FTIR spectrometer.
Signals were detected using a narrow-band MCT
detector. Satisfactory signal-to-noise ratio was obtained
by collecting 21–64 scans at 4 cm1 resolution. Electrical
resistance heaters were used to heat the cell and an
Omega series CN-2010 programmable temperature
controller was used to control the cell temperature.
All gases were purified prior to use and delivered to
the infrared cell via Tylan model FC-280 mass flow
controllers at a flow rate of 60 cm3 /min. He gas was
passed through an oxysorb (CrO2 ) trap to remove O2
and then through a molecular sirve trap (3 A Davison
grade 564) to remove water. A He stream containing
either 1% CO2 or CH3 OH was used for studies of
adsorption and reaction. DMC (0.01%) diluted in He
was used to investigate the adsorption and decomposition of DMC.
3. Results and discussion
As noted above, the synthesis of DMC from methanol and carbon dioxide over zirconia occurs in three
steps: the adsorption of methanol, the addition of CO2
to adsorbed methoxide groups to form monomethyl
carbonate groups, and the reaction of methanol with
monomethyl carbonate groups to form DMC. The
phase of zirconia affects the dynamics of each of these
processes, as well as the concentration and distribution
K.T. Jung, A.T. Bell/Methanol and CO2 to dimethyl carbonate over zirconia
99
Figure 1. Mechanism purposed for the synthesis of DMC from methanol and carbon dioxide [5].
of adsorbed species on the catalyst surface. The results
of infrared studies of all three processes carried out on tZrO2 and m-ZrO2 are presented below with the aim of
establishing the importance of the effects of different
types of active centers present on the surface of zirconia
on the dynamics of DMC synthesis.
Figure 2 shows infrared spectra obtained following
exposure of t-ZrO2 and m-ZrO2 to methanol at 298 K
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K.T. Jung, A.T. Bell/Methanol and CO2 to dimethyl carbonate over zirconia
Figure 2. Infrared spectra obtained during exposure of t-ZrO2 (solid line) and m-ZrO2 (dash-dot line) to CH3 OH at 298 K.
for 30 min. In all cases the spectra are referenced to the
spectrum of zirconia observed prior to the start of
methanol adsorption. Upon adsorption, methanol
reacts with the hydroxyl groups on the surface of zirconia to form water and adsorbed methoxide species.
The negative bands in figure 2(A) are associated with the
loss of hydroxyl groups. For t-ZrO2 , bands at 3743 and
3676 cm1 are associated with bibridged and tribidged
hydroxyl groups, whereas for m-ZrO2 , the bands at 3768
and 3670 cm1 are associated with terminal and tribrudged hydroxyl groups [7,9–11]. A small band at
3743 cm1 is also evident on m-ZrO2 for bibridged
hydroxyl groups. Figures 2(B) and 2(C) illustrate the
bands observed for molecularly adsorbed methanol and
for methoxide species. The bands at 2949 cm1 (t-ZrO2 )/
2939 cm1 (m-ZrO2 ) and 2841 cm1 (t-ZrO2 )/2834 cm1
(m-ZrO2 ) are assigned to molecularly adsorbed CH3 OH
[12,13]. The bands at 2923 and 2816 cm 1 seen on both
phases of ZrO2 are due to C–H stretching vibrations of
monodentate and bidentate methoxide species [14–21].
In figure 2(C) the band at 1154 cm1 seen for t-ZrO2 is
due to bending vibrations for monodentate methoxide
species, whereas the bands at 1157 and 1034 cm1 are
due to bending vibrations associated with monodentate
and bidentate methoxide species, respectively. It is evident from figures 2(B) and 2(C) that while the surface
concentration of molecularly adsorbed methanol is
comparable on both phases of ZrO2 , the surface concentration of methoxide species is significantly higher on
m-ZrO2 . In addition, figure 2(c) shows that while only
monodentate methoxide species are present on t-ZrO2 ,
both monodentate and bidentate methoxide species are
present on m-ZrO2 .
The dynamics of methanol adsorption on both phases
of ZrO2 are shown in figure 3. For each phase, curves
are presented for the time evolution of the bands for
hydroxyl and methoxide groups. For both phases of
ZrO2 it is evident that the rates of hydroxyl group
consumption and methoxide group formation are
equivalent and that on a given phase of ZrO2 the different forms of hydroxyl groups react at a common rate.
Judging by the time required for the methoxide band
intensity to reach 50% of its maximum value, the rate of
methanol adsorption on m-ZrO2 is roughly twice as fast
as on t-ZrO2 .
The interactions of CO2 with adsorbed methoxide
species were investigated by exposing ZrO2 to methanol
for 30 min, flushing the infrared cell with He and then
introducing a stream containing CO2 in He. The spectra
taken after 30 min are shown in figure 4. In figure 4(B),
the bands appearing at 1600/1593, 1497, 1474/1475,
1370/1373, 1200, and 113 cm1 can be attributed to
monomethyl carbonate species (m-CH3 OCOO–Zr)
[22,23], whereas the bands at 1157 and 1032 cm1 are
due to methoxide species. Comparison of the bands
observed in figure 3(A) and 2(B) suggests that the bands
appearing at 2953 and 2847 cm1 on t-ZrO2 are due to
molecularly adsorbed methanol, whereas those appearing at 2931 and 2839 cm1 are due to methoxide species.
It is notable that the surface concentration of mCH3 OCOO–Zr species is significantly lower for t-ZrO2
than for m-ZrO2 and, correspondingly, the decrease in
CH3 O–Zr is less for t-ZrO2 .
The dynamics of m-CH3 OCOO–Zr species formation
and the corresponding dynamics for CH3 O–Zr consumption are illustrated in figure 5 for both t-ZrO2
K.T. Jung, A.T. Bell/Methanol and CO2 to dimethyl carbonate over zirconia
101
Figure 3. Intensities of OH and methoxyl features for (A) t-ZrO2 and (B) m-ZrO2 taken during the experiments in figure 2. Intensities are
normalized to those observed at the beginning of the transient for OH groups and to the value observed at the end of transient for methoxide
species.
Figure 4. Infrared spectra obtained during exposure of t-ZrO2 (solid line) and m-ZrO2 (dash-dot line) pre-adsorbed with CH3 OH to CO2 at
298 K.
(figure 5(A)) and m-ZrO2 (figure 5(B)). For both zirconia phases, the initial rates of m-CH3 OCOO–Zr formation and CH3 O–Zr consumption are comparable, but
the rates are more than an order of magnitude faster on
m-ZrO2 . For both phases only a fraction of the CH3 O–
Zr species are highly reactive. In the case of t-ZrO2 ,
about 15% of the original inventory of m-CH3 O–Zr
reacts rapidly and the rest more slowly. For m-ZrO2 ,
60% of the original inventory of m-CH3 O–Zr reacts
rapidly.
Figure 6 shows infrared spectra recorded during the
steady-state exposure of t-ZrO2 and m-ZrO2 to a mixture of methanol and carbon dioxide. Both at 298 and
423 K the strongest features are those for mCH3 OCOO–Zr (at 1600, 1497, 1474, 1370, 1200, and
1113 cm1 ). Bands are observed for m-CH3 O–Zr at
1153 cm1 on both t- and m-ZrO2 and for b-CH3 O–Zr
at 1040 cm1 on m-ZrO2 . For the same conditions, the
bands for m-CH3 OCOO–Zr are a factor of three to four
more intense on m-ZrO2 and the band for m-CH3 O–Zr
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K.T. Jung, A.T. Bell/Methanol and CO2 to dimethyl carbonate over zirconia
Figure 5. Intensities of m-CH3 O–Zr and m-CH3 OCOO–Zr features for (a) t-ZrO2 and (B) m-ZrO2 taken during the experiments in figure 3.
Intensities are normalized to those observed at the beginning of the transient for m-CH3 O–Zr and to the value observed at the end of transient for
m-CH3 OCOO–Zr.
is a factor of two more intense on m-ZrO2 . With an
increase in the temperature from 298 to 423 K, the
intensities of the bands for m-CH3 OCOO–Zr decrease
somewhat, whereas the intensity of the band for CH3 O–
Zr increases.
Following the experiments shown in figure 6, the
infrared cell was purged with He and the intensity of the
1371 cm1 band for m-CH3 OCOO–Zr was monitored
with time while He, CO2 /He, or CH3 OH/He was passed
through the cell. In He there is a slow decline in the band
intensity due to the decomposition of m-CH3 OCOO–Zr.
While not shown, this process is accompanied by an
increase in the intensity of the band at 1553 cm1 for
CH3 O–Zr. When a mixture CO2 in He is passed over the
catalyst, the intensity of the band for m-CH3 OCOO–Zr
increases slowly due to the formation of additional mCH3 OCOO–Zr from the remaining adsorbed m-CH3 O–
Zr. The passage of a He stream containing CH3 OH results
in a rapid decline in the intensity of the band for mCH3 OCOO–Zr. This change is attributed to the formation
of DMC and is accompanied by the adsorption of additional CH3 OH on the sites vacated by m-CH3 OCOO–Zr.
Comparison of figures 7(A) and 7(B) shows that the
reaction of m-CH3 OCOO–Zr with CH3 OH is a factor of
about two faster on m-ZrO2 than t-ZrO2 .
Infrared spectra recorded when a He stream containing DMC is passed over t- and m-ZrO2 are shown in
figure 8. The spectrum for DMC adsorbed on m-ZrO2 is
qualitatively similar to that observed when a mixture of
methanol and carbon dioxide is passed over this material. Hence the bands observed at 1602, 1466, 1358,
1200, and 1113 cm1 are attributed to m-CH3 OCOO–
Zr, whereas the bands at 2923, 2816, 1157, and 1054
cm1 are attributed to m- and b-CH3 O–Zr. The
appearance of bands at 2947 and 2834 cm1 suggests
that molecularly adsorbed CH3 OH is present as well. A
very different spectrum is observed for t-ZrO2 . In this
case, the bands at 2956, 2935, 2850, 2837, 1743, 1726,
1676, 1475, 1461, and 1433 cm1 are due to colecularly
adsorbed DMC [24]. Some evidence for a small amount
of m-CH3 OCOO–Zr is indicated by the appearance of
bands at 1598, 1358, and 1200 cm1 . A small amount of
m-CH3 O–Zr is also evident through the appearance of
the band at 1157 cm1 . The results presented in figure 8
clearly demonstrate that the decomposition of DMC
upon adsorption occurs much more readily on m-ZrO2
than on t-ZrO2 .
The results presented in figures 2–8 demonstrate that
while similar species are observed on t- and m-ZrO2
there are significant differences in the relative concentrations of these species and the dynamics of their
formation and destruction. These differences are consistent with the differences that have been reported in
the concentration and strength of hydroxyl groups and
Lewis acid/base centers present on t- and m-ZrO2 [6].
The adsorption of CO2 on the samples of t- and m-ZrO2
used in this study shows striking differences. At 298 K,
the adsorption capacity of m-ZrO2 is 3.48 mol/m2 ,
while that for t-ZrO2 is 0.07 mol/m2 . Infrared spectroscopy shows that for m-ZrO2 , CO2 adsorbs in the
form of bicarbonate and mono- and bidentate carbonate
species, whereas for t-ZrO2 , CO2 adsorbs in the form of
bidentate and polydentate carbonate species. Significant
differences in the thermal stability of adsorbed CO2 are
K.T. Jung, A.T. Bell/Methanol and CO2 to dimethyl carbonate over zirconia
103
Figure 6. Infrared spectra obtained during exposure of t-ZrO2 (solid line) and m-ZrO2 (dash-dot line) to a mixture of CO2 and CH3 OH at (A)
298 K and (B) 423 K.
also observed for the two phases of ZrO2 . The temperature at which 50% of the initially adsorbed CO2 has
desorbed is 475 K for m-ZrO2 but 375 K for t-ZrO2 . The
observed differences in the manner and strength of CO2
adsorption are attributed to the higher concentration
and basicity of hydroxyl groups, as well as the stronger
Lewis acidity of Zr4þ cations and strong Lewis basicity
of O2 anions on m-ZrO2 .
As noted in figure 1, the molecular adsorption of
CH3 OH occurs in such a way that the O and H atoms of
the hydroxyl group interact with Zr4þ and O2 centers
of a cus-Lewis acid/base pair. Dissociation of the O–H
bond involves the reaction of the weakly acidic H atom
of the methanol OH group with an adjacent Bronsted
basic OH group. The more rapid dynamics of methoxide
formation over m-ZrO2 observed in figure 3 and the
higher concentration of these groups on this phase at
steady state can be ascribed directly to the higher concentration and basicity of the OH groups present on the
surface of m-ZrO2 [6].
It is evident from figure 6 that the dynamics of
monomethyl carbonate formation exhibit a much
greater difference than the dynamics of methoxide formation over the two phases of zirconia. The greater then
ten-fold more rapid formation of monomethyl carbonate over m-ZrO2 is ascribable to the significantly higher
strength of the Lewis acid/base pairs of m-ZrO2 . This is
consistent with the significantly higher capacity and
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K.T. Jung, A.T. Bell/Methanol and CO2 to dimethyl carbonate over zirconia
Figure 7. Intensities of m-CH3 OCOO–Zr for (A) t-ZrO2 and (B) m-ZrO2 taken after switching from a mixture of CO2 and CH3 OH to a mixture
of CH3 OH, CO2 , and He at 298 K.
Figure 8. Infrared spectra obtained during exposure of t-ZrO2 (solid line) and m-ZrO2 (dash-dot line) to DMC at 298 K.
strength of m-ZrO2 for CO2 adsorption relative to tZrO2 . By contrast, the rates of reaction of monomethyl
carbonate with methanol to form DMC are more nearly
the same over the two phases of zirconia, as can be seen
in figure 7. This suggests that the rate of DMC formation over t-ZrO2 is limited primarily by the rate at which
CO2 inserts into the Zr–O bond of methoxide species to
form monomethyl carbonate species.
The strength of the cus-Zr4þ O2 Lewis acid/base
centers also affects the interactions of DMC with zirconia. As seen in figure 8, DMC adsorbs molecularly on
t-ZrO2 , but decomposes to form monomethyl carbonate
and methoxide species on m-ZrO2 . In the light of the
mechanism presented in figure 1, these differences can be
ascribed to the higher Lewis acidity/basicity of the cusZr4þ O2 centers present on m-ZrO2 .
K.T. Jung, A.T. Bell/Methanol and CO2 to dimethyl carbonate over zirconia
4. Conclusions
The present study confirms that both Brønsted basic
hydroxyl groups and cus-Zr4þ O2 Lewis acid/base pairs
serve as active centers for the synthesis of DMC from
methanol and carbon dioxide over zirconia. The higher
strength of these centers on m-ZrO2 is responsible for
the more rapid dynamics of methanol adsorption on this
phase and most particularly the significantly more rapid
insertion of CO2 into the Zr–O bond of methoxide
species to form monmethyl carbonate. The strength of
the active centers on zirconia also influences the
decomposition of DMC. Consistent with this, DMC
adsorbs and decomposes on m-ZrO2 to form monomethyl carbonate and methoxide species, but adsorbs
almost exclusively on t-ZrO2 in its molecular form.
Acknowledgement
This work was supported by the Director of the
Office of Energy Sciences, Chemical Sciences Division,
of the US Department of Energy under Contract DEAC03-SF00096.
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