Structure of the active sites on H3Po4/ZrO2 catalysts for dimethyl carbonate synthesis from methanol and carbon dioxide

Article abstract of DOI:10.1021/jp0121522

The catalytic properties of H₃PO₄/ZrO₂ catalysts for dimethyl carbonate (DMC) synthesis from CH₃OH and CO₂ were investigated. The modification of ZrO₂ with H₃PO₄ promoted the activity for selective DMC formation. The characterization of H₃PO₄/ZrO₂ catalysts was performed by means of BET, XRD, ³¹P MAS NMR, LRS, DRIFT, and NH₃-TPD. From the characterization results, it was determined that ZrO₂ had Lewis acid sites and did not have Br??nsted acid sites. The presence of Br??nsted acid sites as well as Lewis acid sites was observed on H₃PO₄/ZrO₂ (P/Zr = 0.05). The acidity of the Br??nsted sites is weak, and the site's structure is a bridged OH. Furthermore, this Br??nsted site is formed by the interaction between tetragonal Zr and P. In the reaction scheme of DMC formation, the Br??nsted acid site can contribute to CH₃OH activation and to the enhancement of the catalytic activity.

Full text of DOI:10.1021/jp0121522

J. Phys. Chem. B 2001, 105, 10653-10658
10653
Structure of the Active Sites on H3PO4/ZrO2 Catalysts for Dimethyl Carbonate Synthesis
from Methanol and Carbon Dioxide
Yoshiki Ikeda, Mohammad Asadullah, Kaoru Fujimoto, and Keiichi Tomishige*
Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
ReceiVed: June 6, 2001; In Final Form: August 5, 2001
The catalytic properties of H
CO were investigated. The modification of ZrO
formation. The characterization of H PO /ZrO catalysts was performed by means of BET, XRD, P MAS
NMR, LRS, DRIFT, and NH -TPD. From the characterization results, it was determined that ZrO had Lewis
acid sites and did not have Brønsted acid sites. The presence of Brønsted acid sites as well as Lewis acid
sites was observed on H PO /ZrO (P/Zr ) 0.05). The acidity of the Brønsted sites is weak, and the site’s
structure is a bridged OH. Furthermore, this Brønsted site is formed by the interaction between tetragonal Zr
3
PO
4
/ZrO
2
catalysts for dimethyl carbonate (DMC) synthesis from CH
3
OH and
2
2
with H PO promoted the activity for selective DMC
3
4
31
3
4
2
3
2
3
4
2
and P. In the reaction scheme of DMC formation, the Brønsted acid site can contribute to CH
and to the enhancement of the catalytic activity.
3
OH activation
1
Introduction
2
CH OH + 2NO + / O f 2CH ONO + H O
3
2
2
3
2
The development of CO2-based methods for the synthesis of
carbonic acid diesters is very attractive. Dimethyl carbonate
DMC), the lowest homologue of this family, is drawing much
1
Some of the other routes to DMC synthesis from CO2 have
recently been reported. The selective DMC synthesis via the
reaction of supercritical CO2 and trimethyl orthoacetate using
the molecular catalyst Bu2Sn(OCH3)2 has been reported.9
(
attention as a safe, noncorrosive, and environmentally acceptable
substitute for the carbonylating and methylating agents phosgene
2
and dimethyl sulfate. DMC can also be used as an octane
3
CH C(OCH ) + CO f (CH O) CO + CH COOCH
3 3 3 2 3 2 3 3
booster in gasoline. Furthermore, the addition of DMC to diesel
fuel decreases the emission of particulate matter. If DMC were
used as a fuel additive, a large scale-up of current world DMC
production would be necessary.
It is known that DMC can be synthesized directly from CH3-
OH and CO2 in the presence of dialkoxydibutyltin.10 The
reactions were carried out under pressure with CO2. It is
assumed that CO2 is inserted into the Sn-O bond of Bu2Sn-
(OCH3)2 followed by alcoholysis, yielding carbonate and Bu2-
Sn(OH)2. This species is again esterified by methanol so that
the tin catalyst can be reused.
Three methods of DMC production have been developed. The
first is the stoichiometric reaction of methanol and phosgene in
4
a concentrated sodium hydroxide solution.
2
CH OH + COCl f (CH O) CO + 2HCl
3 2 3 2
The second is the oxidative carbonylation of CH3OH with
carbon monoxide and oxygen, catalyzed by cuprous chloride
in a slurry reaction system,5 where the reaction proceeds in a
redox cycle of copper ions as follows:
2CH OH + CO f (CH O) CO + H O
3
2
3
2
2
-7
It has also been reported that DMC can be synthesized from
CH3OH and CO2 in the presence of tin(IV) and titanium(IV)
1
1
alkoxides and the metal acetates. The water produced with
DMC causes the deactivation of these alkoxide catalysts.
Recently, we have reported that DMC was selectively
1
2
CuCl + 2CH OH + / O f 2Cu(OCH )Cl + H O
3
2
2
3
2
2
Cu(OCH )Cl + CO f (CH O) CO + 2CuCl
12
3
3
2
synthesized from CH3OH and CO2 using zirconia catalysts.
With some other catalysts, dimethyl ether (DME) was formed,
The third is an excellent DMC synthesis process based on
oxidative carbonylation using a palladium catalyst and a methyl
12,13
where DMC was not detected at all.
It is characteristic that
DME formation on ZrO2 is below the detection limit. The
8
nitrite promoter.
amount of DMC formation showed the volcano-type dependence
on the calcination temperature of zirconium hydroxide.1
2,14
It
CO + 2CH ONO f (CH O) CO + 2NO
3
3
2
was found that the DMC formation rate was strongly dependent
on the acid-base bifunctional properties on ZrO2.
The methyl nitrite used in this process is synthesized by the
following reaction, which proceeds at room temperature without
any catalyst:
We have also reported that the modification of ZrO2 catalysts
with phosphoric acid promoted activity for DMC synthesis with
high selectivity and that the reactions proceeded at much lower
temperatures with modified ZrO2 catalysts than with the
*
Present address: Institute of Materials Science, University of Tsukuba,
1
5
unmodified ZrO2. In this paper, we characterized phosphoric
acid-modified ZrO2 catalysts with various phosphoric acid
1
2
-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan. Tel and Fax: +81-
98-53-5030. E-mail: tomi@tulip.sannnet.ac.jp.
1
0.1021/jp0121522 CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/05/2001

10654 J. Phys. Chem. B, Vol. 105, No. 43, 2001
Ikeda et al.
contents by means of Brunauer-Emmett-Teller (BET) surface
TABLE 1. Dependence of DMC Formation and BET
Surface Area of H PO /ZrO Catalysts on H PO Content
a
31
area, X-ray diffraction (XRD), P magic-angle spinning NMR
3
4
2
3
4
3
1
2
spectroscopy ( P MAS NMR), laser Raman spectroscopy
LRS), diffuse reflectance infrared Fourier transform (DRIFT),
molar ratio P/Zr amount of DMC (mmol) BET surface area (m /g)
(
0
0
0.08
0.20
0.26
0.30
0.29
0.05
nd
nd
nd
118
178
189
216
231
214
210
94
and ammonia temperature-programmed desorption (NH3-TPD)
in order to elucidate the promoting mechanism.
.0125
0.025
0
0
0
0
0
1
.05
.1
.3
Experimental Methods
b
.5
Catalyst Preparation. ZrO2 was prepared by calcining a
commercially available zirconium hydroxide (ZrO2‚xH2O; Na-
calai Tesque Inc.) at 673 K for 3 h in air. Phosphoric acid-
modified ZrO2 catalysts were prepared by impregnating ZrO2‚
xH2O with an aqueous H3PO4 solution (Aldrich, 85 wt %).
Water was removed by heating, and the sample was dried at
.7
11
a
Reaction conditions: CH
3
OH/CO
2
) 192/200 mmol; catalyst
b
weight, 0.5 g; reaction temperature, 403 K; reaction time, 2 h. nd )
not detected.
3
93 K for 10 h, followed by calcination at 673 K for 3 h in air.
The calcination temperature of these catalysts was also
12-15
optimized.
These catalysts are represented by H3PO4/ZrO2.
The content of H3PO4 is denoted as the molar ratio P/Zr in
parentheses [e.g., H3PO4/ZrO2 (P/Zr ) 0.05)].
DMC Synthesis from CH3OH and CO2. The reaction was
carried out in a stainless steel autoclave reactor with an inner
volume of 70 mL. The standard procedure is as follows: 6.1 g
of CH3OH (192 mmol; Kanto Chemical, 99.8% min) and 0.5 g
of the catalyst were placed in an autoclave, and then the reactor
was purged with CO2. A total of 8.8 g of CO2 (200 mmol;
Takachiho Trading Co. Ltd., 99.99%) was introduced, and the
initial pressure was about 4 MPa at room temperature. The
reactor was heated and magnetically stirred constantly during
the reaction. The reaction was carried out at different temper-
atures (383-443 K) for 2 h. Products in both the gas and liquid
phases were analyzed by gas chromatograph (GC) equipped with
a flame ionization detector (FID) and a thermal conductivity
detector (TCD). In the gas phase, no products were observed.
CO was below the detection limit of the FID-GC equipped with
a methanator. Under all of the reaction conditions shown in
this paper, DMC was the only product, and DME was below
the detection limit of the FID-GC.
Figure 1. Dependence of DMC formation on the reaction temperature
over ZrO
])]. Reaction conditions: CH
weight, 0.5 g; reaction time, 2 h.
2
(9) and H
3
PO
4
/ZrO
2
[P/Zr ) 0.025 (O), 0.05 (4), and 0.1
(
3
OH/CO ) 192/200 mmol; catalyst
2
NH3-TPD profiles were obtained by a mass spectrometer
(Balzers, Prisma QMS 200). The intensity of the mass signal
(m/e ) 16) was monitored. Sample pretreatment and gas
adsorption were performed in a closed circulating vacuum
system. Before the measurements of the TPD profiles, the
sample was evacuated for 1 h at room temperature. The sample
weight was 0.05 g. The heating rate was about 7 K/min.
Catalyst Characterization. BET surface area, XRD patterns,
and LRS spectra of the catalysts were measured with a Gemini
2360 (Micromeritics, N2 adsorption), a RINT-2400 (Rigaku, Cu
KR), and a LABRAM 1B (JOBIN-YBON, He-Ne laser),
respectively. 31P MAS NMR spectra were obtained at 121.65
MHz on a Fourier transform pulsed NMR spectrometer (JEOL
JNM-CMX-300). The weight of each sample for 31P MAS NMR
was about 0.4 g. The spinning speed was 5.0 kHz. Proton
decoupling and cross polarization were not employed. DRIFT
spectra were obtained using a Magna 550 (Nicolet) with an MCT
detector. An in-situ cell (Spectra-Tech) was used for high-
temperature and high-pressure observations. Catalyst powder
was placed in the sample cup in the cell. This cell was connected
to the high-pressure gas flow system. Pretreatment was carried
out in N2 flow at 673 K for 0.5 h. CH3OH (Kanto Chemical,
Results and Discussion
DMC Synthesis from CH3OH and CO2. Table 1 shows the
dependence of DMC formation and the BET surface area of
H3PO4/ZrO2 catalysts on the H3PO4 content. Both the amount
of DMC formation and the BET surface area increased with
increasing H3PO4 content at low P/Zr. The DMC amount
reached a maximum at P/Zr ) 0.05 and then decreased with
increasing H3PO4 content. Furthermore, the surface area also
increased with increasing H3PO4 content until P/Zr ) 0.1 and
then decreased. Although H3PO4/ZrO2 (P/Zr ) 0.3 and 0.5) had
a high surface area, the activity of DMC formation was very
low. This suggests that the DMC formation is not directly related
to the surface area of H3PO4/ZrO2 (Table 1).
Figure 1 shows the dependence of DMC formation on the
reaction temperature over ZrO2 and H3PO4/ZrO2 (P/Zr ) 0.025,
0.05, and 0.1). DMC formation with ZrO2 is controlled by the
reaction rate under these reaction conditions. Modification with
H3PO4 enhanced the activity at all of the reaction temperatures.
It seems that the amount of DMC formation on H3PO4/ZrO2
(P/Zr ) 0.025) at 443 K reached the equilibrium level. The
DMC amount on H3PO4/ZrO2 at 423 K became about 4 times
larger than that on ZrO2. The amount of DMC on H3PO4/ZrO2
99.8% min) was introduced by the pulsed gas under atmospheric
pressure at 443 K with N2 as a carrier gas until the coverage
reached the saturation level. Then, CO2 (Takachiho Trading Co.
Ltd., 99.99%) was introduced onto the CH3OH-preadsorbed
sample under atmospheric pressure, and CO2 was pressurized
stepwise to 5 MPa, where the sample temperature was kept at
443 K. Each spectrum was obtained 3 min after the introduction
of CO2 at each pressure. To obtain the profiles of NH3-TPD,
NH3 (Takachiho Trading Co. Ltd., 99.999%) was introduced
at 298 K by the same procedure as that of CH3OH after the
pretreatment. The sample was heated to 673 K. Each spectrum
was obtained 10 min after the target temperature was reached.

DMC Synthesis from CH3OH and CO2
J. Phys. Chem. B, Vol. 105, No. 43, 2001 10655
3 4 2
Figure 2. Cu KR XRD patterns of fresh H PO /ZrO catalysts with
varying H PO contents. Crystal structure: metastable tetragonal (b)
3 4
and monoclinic (O).
at 403 K was comparable to that on ZrO2 at 443 K. While DMC
formation was not observed on ZrO2 at 383 K, about 0.10 mmol
of DMC was formed on H3PO4/ZrO2. Besides, the amounts of
DME and CO, which are expected byproducts, were also smaller
than the detection limit of H3PO4/ZrO2 under these reaction
conditions. Therefore, the selectivity of DMC formation on H3-
PO4/ZrO2 is estimated to be 100%. The amount of DMC
formation on H3PO4/ZrO2 (P/Zr ) 0.05) at 383 K for 64 h was
0
.53 mmol, where DME formation was not detected at all. The
Figure 3. 31P MAS NMR spectra of fresh H
varying H PO contents at room temperature: (*) spinning sidebands;
3
4 2
PO /ZrO catalysts with
DME formation rate was found to be very low. By modification
with H3PO4, the reaction temperature could be lowered by 40
K for the same level of DMC formation on ZrO2.
3
4
resonance frequency, 121.65 MHz; spinning rate, 5.0 kHz; pulse delay,
15 s and single pulse; reference signal, 85% H PO , NH H PO
4
3
4
4
2
We have calculated the equilibrium level of the CH3OH
conversion under our reaction conditions to be around 1% (DMC
(secondary standard).
)
ca. 0.96 mmol at 373-473 K). This value is higher than that
to both the monophosphates (Q0) and the end chain (Q1) PO4
1
7
of the experimental results, but this seems to be because our
calculation did not consider H2O as an impurity. Because the
reactor, reactants, and catalyst surface should contain H2O as
an impurity, the amount is difficult to estimate. H2O as an
impurity may decrease the equilibrium level of DMC forma-
tion.12
groups of pyrophosphates. In any case, these peaks are not
18
due to the middle chain (Q2) of polyphosphates. This indicates
that the dispersion of phosphate species on H3PO4/ZrO2 (P/Zr
) 0.0125, 0.05, and 0.1) is very high. The peaks around δ )
-14, -20 (major), and -25 ppm (P/Zr ) 0.5-1 in Figure 3)
31
19
seem to be due to the P nuclei of the zirconium phosphates,
which indicate that some salt compounds of H3PO4/ZrO2 (P/Zr
0.5, 0.7, and 1.0) are formed. This is supported by the XRD
Catalyst Characterization. XRD patterns of fresh H3PO4/
ZrO2 catalysts with various H3PO4 contents are shown in Figure
)
patterns in Figure 2 as described previously. From these results,
it is suggested that the most active species is the monophosphate
or pyrophosphate and that the zirconium phosphates do not
exhibit catalytic activity for DMC formation from CH3OH and
CO2.
2
. For ZrO2, both the metastable-tetragonal and monoclinic
phases were observed. The metastable-tetragonal phase gradually
decreased and the monoclinic phase almost disappeared with
the increasing content of H3PO4. The metastable-tetragonal
phase was predominantly formed on H3PO4/ZrO2 (P/Zr ) 0.05),
which exhibited the highest activity, as shown in Table 1. At
P/Zr > 0.3, H3PO4/ZrO2 catalysts did not show the diffraction
patterns of either the metastable-tetragonal phase or the mono-
clinic phase. With a higher H3PO4 content, a new peak appeared
around 2θ ) 20°. This peak has not been identified, but it seems
to be due to some salt compounds formed between the zirconium
and phosphate ions.
LRS spectra of fresh ZrO and H PO /ZrO catalysts are
2
3
4
2
shown in Figure 4. It has been reported that the tetragonal phase
ZrO exhibits typical Raman bands at 148, 263, 325, 472, 608,
2
-1
and 640 cm and that the monoclinic phase ZrO exhibits bands
2
at 140, 173, 185, 216, 260, 301, 328, 342, 378, 471, 499, 533,
-
1 20,21
553, 610, and 632 cm .
2
For ZrO , the tetragonal phase
seems to be the major phase from the XRD pattern in Figure 2,
but the monoclinic phase seems to be the major phase from the
LRS spectrum in Figure 4. Considering that LRS is the near-
surface sensitive method, these results indicate that the phase
composition on the surface is different from that in the bulk. A
Figure 3 shows the 31P MAS NMR spectra of fresh H3PO4/
ZrO2 catalysts with various H3PO4 contents at room temperature.
The broad peaks of each spectrum indicate that the phosphate
species on H3PO4/ZrO2 are not homogeneous. The peak at δ )
2
0
-
7.5 ppm (P/Zr ) 0.0125 in Figure 3) cannot be assigned at
similar tendency has been reported previously. On the other
hand, all of the bands observed for H3PO4/ZrO2 (P/Zr ) 0.0125,
0.025, and 0.05) can be assigned to the tetragonal phase. By
modification with H3PO4, the tetragonal phase was predomi-
nantly formed on the surface as well as in the bulk. We think
that there is an interaction between the phosphate and the
tetragonal zirconia. This is due to the result that the catalyst
present. The broad peak at δ ) -10.8 ppm (P/Zr ) 0.3 in Figure
-
3
) is thought to be assigned to (H2PO4) , according to the
1
6
-
report. This suggests that the salt of the (H2PO4) and
zirconium ion is formed. The broad peaks around δ ) -5.0
ppm (P/Zr ) 0.0125, 0.05, and 0.1 in Figure 3) are still
ambiguous. These peaks around δ ) -5.0 ppm can be assigned

10656 J. Phys. Chem. B, Vol. 105, No. 43, 2001
Ikeda et al.
Figure 4. LRS spectra of fresh ZrO
2 3 4 2
and H PO /ZrO (P/Zr ) 0.0125,
0
.025, and 0.05): Raman bands, metastable tetragonal (1) and
monoclinic (3).
Figure 6. DRIFT spectra of NH
0
2
3
adsorbed on H
.05) at various temperatures: (a) at 293 K before adsorption, (b) at
93 K after adsorption, (c) after subsequent elevation of temperature
3 4 2
PO /ZrO (P/Zr )
to 373 K, (d) at 423 K, (e) at 473 K, (f) at 573 K, and (g) at 673 K.
Pretreatment: 673 K for 0.5 h under N flow.
2
3 2 3 4
Figure 5. DRIFT spectra of NH adsorbed on ZrO (I) and H PO /
ZrO (P/Zr ) 0.05) (II) at various temperatures: (a) at 293 K after
2
adsorption, (b) after subsequent elevation of temperature to 373 K, (c)
at 423 K, (d) at 473 K, (e) at 573 K, and (f) at 673 K (subtracted
2
spectra). Pretreatment: 673 K for 0.5 h under N flow.
prepared by the impregnation of H3PO4 on monoclinic zirco-
niashowed very low activity for DMC formation.13 It is expected
that H3PO4 can react with a basic hydroxide on monoclinic
zirconia and form P-O-Zr (monoclinic); however, this is not
effective for DMC formation. It is thought that surface-tetragonal
zironia, which is stabilized by the addition of H3PO4, is very
important for DMC synthesis.
Figure 7. Profiles of TPD of NH
3 2 3 4
adsorbed on ZrO (s) and H PO /
ZrO (P/Zr ) 0.05) ()): NH adsorption, PNH3 ) 6.6 kPa at 293 K;
2
3
TPD conditions, heating rate of 7 K/min and sample weight of 0.05 g.
extremely weak Brønsted acid sites of the H3PO4/ZrO2 (P/Zr
) 0.05) surface. This band intensity decreased with the
temperature and almost disappeared at 573 K, while the bands
-
1
at 1605 and 1180-1160 cm were present even at 673 K.
These results indicate that Lewis acid sites are present on both
ZrO2 and H3PO4/ZrO2 (P/Zr ) 0.05) while the weak Brønsted
Figure 5 shows the DRIFT spectra of NH3 adsorbed on ZrO2
and H3PO4/ZrO2 (P/Zr ) 0.05) under NH3-TPD. When NH3
was adsorbed at 298 K, both spectra (I and II in Figure 5)
-
1
acid sites are present on H PO /ZrO (P/Zr ) 0.05).
exhibited bands at 1605 and 1180-1160 cm . These bands
can be assigned to the asymmetric and symmetric bending
modes of NH3 coordinated to Lewis acid sites.22 These intensi-
ties decreased with the temperature. Only the spectra of H3-
PO4/ZrO2 (P/Zr ) 0.05) (II in Figure 5) exhibit the band at
3
4
2
As shown in Figure 6, the NH3 band at 1100 cm- shifted to
1
lower wavenumbers. This band can be assigned to P-O
2
4
stretching modes. As the temperature was increased and NH3
was desorbed, this shift became smaller and almost recovered
at 573 K. This behavior due to temperature seems to be
correlated with that of the intensity of the band at 1470-1460
-
1
+
1
470-1460 cm . The band characteristic of the NH4 ion is
-
1
expected near 1450-1400 cm , and the peak at the lower
wavenumber generally corresponds to a stronger acid site. In
ionic solids (NH4Cl, etc.), the asymmetric NH4 bending
2
3
-1
cm (see II in Figure 5), which indicates the presence of weak
+
Brønsted acid sites on H3PO4/ZrO2 (P/Zr ) 0.05). Hence, it is
suggested that the weak Brønsted acid sites on H3PO4/ZrO2 (P/
Zr ) 0.05) are located near the P atoms.
Figure 7 shows the TPD profiles of NH3 adsorbed on ZrO2
and H3PO4/ZrO2 (P/Zr ) 0.05). NH3 was desorbed in a wide
temperature range. On average, a larger desorption amount of
NH3 on H3PO4/ZrO2 (P/Zr ) 0.05) than that on ZrO2 was
-
1
wavenumber is near 1400 cm , and the band is sharp. When
the Brønsted acid site is weak such that proton transfer is less
complete and the interaction is more of the type MO‚‚‚H‚‚‚
-
1 23
NH3, the asymmetric deformation mode lies near 1450 cm .
Therefore, the band at 1470-1460 cm- can be assigned to the
1
+
asymmetric bending modes of NH4 protonated from the

DMC Synthesis from CH3OH and CO2
J. Phys. Chem. B, Vol. 105, No. 43, 2001 10657
Figure 8. DRIFT spectra of CH
introduction at 443 K on ZrO (I) and H
OH adsorption, (b) after CO
c) at 1.0 MPa, (d) at 3.0 MPa, and (e) at 5.0 MPa (subtracted spectra).
flow.
3
OH adsorption and successive CO
PO /ZrO (P/Zr ) 0.05) (II):
introduction under 0.1 MPa,
2
Figure 9. DRIFT spectra of surface hydroxyl groups of ZrO
PO /ZrO (P/Zr ) 0.05) (II) before and after CH OH adsorption at
443 K: (a) before CH OH adsorption (s) and (b) after CH OH
adsorption ()). Pretreatment: 673 K for 0.5 h under N flow.
2
(I) and
H
3
4
2
3
2
3
4
2
(
(
a) after CH
3
2
3
3
2
Pretreatment: 673 K for 0.5 h under N
2
observed in the lower-temperature range. This indicates that the
modification of ZrO2 with H3PO4 led to an increase in the
number of weaker acid sites. These acid sites on H3PO4/ZrO2
(P/Zr ) 0.05) can be attributed to the weak Brønsted acid sites
near the P atoms, which do not exist on ZrO2, as described
previously.
Figure 8 shows the DRIFT spectra of CH3OH adsorption and
successive CO2 introduction at 443 K on ZrO2 and H3PO4/ZrO2
(
P/Zr ) 0.05). No significant differences between the two series
of spectra are observed. CH3OH adsorption led to the appearance
of the bands at 1160 and 1054 cm . These can be assigned to
-
1
the C-O stretching modes of terminal (t-OCH3) and bridged
(
b-OCH3) methoxy species, respectively.25 When CO2 was
introduced to the CH3OH-preadsorbed catalysts, the band
-
1
intensity at 1160 cm decreased and the bands at 1600, 1474,
-
1
1
370, and 1130 cm appeared. The intensity of these bands
Figure 10. Model of mechanism for DMC synthesis from CH
and CO on ZrO and H PO /ZrO (P/Zr ) 0.05): M ) P or Zr induced
by adjacent phosphoric species.
3
OH
increased with the increase in CO2 pressure. These bands can
be assigned to the methyl carbonate-like species. CO2 intro-
2
2
3
4
2
25
-
1
duction did not affect the band at 1054 cm assigned to
b-OCH3. These results indicate that the methyl carbonate species
is easily formed from the interaction between CO2 and t-OCH3
and that the formation is more favorable under higher CO2
pressure. This coincides with the dependence of CO2 pressure
on the DMC formation amount, which has been reported on
ZrO2 before.14
between Zr (tetragonal) and P is essential to effective acid
formation. At present, the weak and bridged Brønsted acid sites
are thought to be on the bridging oxide ion in P-O-Zr
(
tetragonal) or Zr-O-Zr (-O-P), which is influenced by the
interaction with phosphate.
CH3OH adsorption led to the appearance of the bands at 2924
Figure 9 shows the DRIFT spectra of the surface hydroxyl
groups of ZrO2 and H3PO4/ZrO2 (P/Zr ) 0.05) before and after
CH3OH adsorption at 443 K. Before CH3OH adsorption, the
-1
and 2820 cm on both of the catalysts. These bands can be
assigned to the C-H stretching modes of t- and b-OCH3 species,
25
respectively.
-
1
bands at 3750 and 3680 cm were observed on both of the
catalysts. These bands can be assigned to the O-H stretching
modes of terminal (t-OH) and bridged (b-OH) hydroxyl groups,
respectively.2 After CH3OH adsorption, neither t-OH nor b-OH
on ZrO2 was observed (I in Figure 9). On the other hand, b-OH
on H3PO4/ZrO2 (P/Zr ) 0.05) was still present with significant
intensity, though t-OH disappeared (II in Figure 9). It seems
that the rest of the b-OH on H3PO4/ZrO2 (P/Zr ) 0.05) after
CH3OH adsorption corresponds to the weak Brønsted acid sites
near the P atoms, which is indicated by FTIR spectra of NH3
adsorption and NH3-TPD profiles. From these results, the
modification of ZrO2 with phosphate forms the weak and
bridged Brønsted acid sites. In this case, the direct interaction
From the previous results on ZrO2 catalysts,12,14 it is suggested
that acid-base bifunction is the key to selective DMC formation.
The methyl carbonate species is considered to be the reaction
intermediate. In this case, DMC is formed from the methyl
carbonate species and the methyl cation, which is related to the
activation of CH3OH on the acid sites. The reaction scheme of
DMC synthesis from CH3OH and CO2 can be postulated as
shown in Figure 10 and described as follows.
5
(I) For CH3OH activation on base sites, t- and b-OH are
converted to t- and b-OCH3 when CH3OH is adsorbed.
(II) For CO2 activation on base sites, t-OCH3 reacts with CO2
to form methyl carbonate, and b-OCH3 cannot react.

10658 J. Phys. Chem. B, Vol. 105, No. 43, 2001
Ikeda et al.
(
III) For CH3OH activation on acid sites, DMC is formed
(3) Pacheco, M. A.; Marshall, C. L. Energy Fuels 1997, 11, 2-29.
(4) Shaikh, A.-A. G.; Sivaram, S. Chem. ReV. 1996, 96, 951-976.
(5) Romano, U.; Tesei, R.; Mauri, M. M.; Rebora, P. Ind. Eng. Chem.
Prod. Res. DeV. 1980, 19, 396-403.
from the reaction between methyl carbonate and the methyl
cation.
It is suggested that the high selectivity of DMC formation
on ZrO2 and H3PO4/ZrO2 (P/Zr ) 0.05) is due to the rapid
conversion of t-OCH3 to the methyl carbonate species under
high CO2 pressure, which could be observed by DRIFT. Step
III, which is related to the activation of CH3OH on acid sites,
may be the rate-determining step in DMC formation. Brønsted
acid sites on H3PO4/ZrO2 (P/Zr ) 0.05) seem to be more
favorable than Lewis acid sites on ZrO2 for the CH3OH
activation step III. This can explain the promotion of catalytic
activity for DMC formation by modification of ZrO2 with H3-
PO4.
(
6) Molzahn, D.; Jones, M. E.; Hartwell, G. E.; Puga, J. U.S. Patent
,387,708, 1995.
7) King, S. S. T.; Jones, M. E.; Olken, M. M. U.S. Patent 5,391,803,
995.
5
1
(
(
8) Matsuzaki, T.; Nakamura, A. Catal. SurV. Jpn. 1997, 1, 77-88.
9) Sakakura, T.; Saito, Y.; Okano, M.; Choi, J.-C.; Sako, T. J. Org.
(
Chem. 1998, 63, 7095-7096.
(10) Sakai, S.; Fujinami, T.; Yamada, T.; Furusawa, S. Nippon Kagaku
Kaishi 1975, 10, 1789-1794.
(11) Kizlink, J.; Pastucha, I. Collect. Czech. Chem. Commun. 1995, 60,
87-692.
6
(
12) Tomishige, K.; Sakaihori, T.; Ikeda, Y.; Fujimoto, K. Catal. Lett.
1
999, 58, 225-229.
Conclusions
(13) Ikeda, Y.; Furusawa, Y.; Tomishige, K.; Fujimoto, K. ACS Symp.
Ser. 2001, in press.
(
1) Modification of ZrO2 with H3PO4 enhanced the activity
for DMC synthesis remarkably, especially at low temperatures.
2) The optimum composition of H3PO4/ZrO2 is P/Zr )
.025-0.1.
3) Both ZrO2 and H3PO4/ZrO2 (P/Zr ) 0.05) had Lewis acid
(14) Tomishige, K.; Ikeda, Y.; Sakaihori, T.; Fujimoto, K. J. Catal. 2000,
1
92, 355-362.
15) Ikeda, Y.; Sakaihori, T.; Tomishige, K.; Fujimoto, K. Catal. Lett.
000, 66, 59-62.
16) Kaneno, M.; Yamaguchi, S.; Nakayama, H.; Miyakubo, K.; Ueda,
T.; Eguchi, T.; Nakamura, N. Int. J. Inorg. Mater. 1999, 1, 379-384.
17) Caro, J.; B u¨ low, M.; Derewinski, M.; Haber, J.; Hunger, M.; K a¨ rger,
(
(
0
2
(
(
sites, though only H3PO4/ZrO2 (P/Zr ) 0.05) had weak Brønsted
acid sites. The direct interaction between Zr (tetragonal) and P
is essential to the formation of weak Brønsted acid sites.
(
J.; Pfeifer, H.; Storek, W.; Zibrowius, B. J. Catal. 1990, 124, 367-375.
(
4) When CO2 was introduced to the CH3OH-preadsorbed
(18) Segawa, K.; Nakajima, Y.; Nakata, S.; Asaoka, S.; Takahashi, H.
J. Catal. 1986, 101, 81-89.
catalysts, only t-OCH3 was decreased and the methyl carbonate
species was formed. The methyl carbonate species can be the
reaction intermediate of DMC formation from CH3OH and CO2.
(
19) Hayashi, S.; Hayamizu, K. Bull. Chem. Soc. Jpn. 1989, 62, 3061-
068.
20) Yamamoto, T.; Tanaka, T.; Takenaka, S.; Yoshida, S.; Onari, T.;
3
(
(
5) Brønsted acid sites on H3PO4/ZrO2 (P/Zr ) 0.05) can be
Takahashi, Y.; Kosaka, T.; Hasegawa, S.; Kudo, M. J. Phys. Chem. B 1999,
103, 2385-2393.
(21) Mercera, P. D. L.; van Ommen, J. G.; Doesburg, E. B. M.;
Burggraaf, A. J.; Ross, J. R. H. Appl. Catal. 1990, 57, 127-148.
(22) Yamaguchi, T.; Nakano, Y.; Tanabe, K. Bull. Chem. Soc. Jpn. 1978,
more effective than Lewis acid sites on ZrO2 for CH3OH
activation on the acid sites. The reaction between the activated
CH3OH on the acid sites and the methyl carbonate species gives
DMC.
5
1, 2482-2487.
23) Dang, Z.; Anderson, B. G.; Amenomiya, Y.; Morrow, B. A. J. Phys.
Chem. 1995, 99, 14437-14443.
24) Nakamoto, K. Infrared Spectra of Inorganic and Coordination
(
6) Acid-base bifunctional catalysis is essential in selective
(
DMC synthesis from CH3OH and CO2.
(
References and Notes
Complexes; Wiley-Interscience: New York, 1970; p 117.
(
(
1) Aresta, M.; Quaranta, E. CHEMTECH 1997, 32-40.
2) Ono, Y. Appl. Catal., A 1997, 155, 133-166.
(25) Bensitel, M.; Moravek, V.; Lamotte, J.; Sauer, O.; Lavalley, J.-C.
Spectrochim. Acta 1987, 43A, 1487-1491.