Hydrogenation of phenol in supercritical carbon dioxide catalyzed by palladium supported on Al-MCM-41: A facile route for one-pot cyclohexanone formation

Article abstract of DOI:10.1002/adsc.200900144

The hydrogenation of phenol has been carried out in supercritical carbon dioxide (scCO₂) under very mild reaction conditions at the temperature of 50°C over palladium supported Al-MCM-41 (metal loading ～1%). This palladium catalyst is shown to be highly active and promotes the selective formation of cyclohexanone (～98%), an industrially important compound, in a "one-pot～ way. The effects of different variables like carbon dioxide and hydrogen pressure, reaction time and also silica/alumina ratio of the MCM-41 support along with palladium dispersion are presented and discussed. The pressure effect of carbon dioxide is significantly prominent in terms of conversion and cyclohexanone selectivity. Moreover, the silica/alumina ratio was also found to be an important parameter to enhance the effectiveness of the catalyst as it exhibits a remarkable increase in phenol conversion from 20.6% to 98.4% as the support changes from only silica MCM-41 to Al-MCM-41. A plausible mechanism for the hydrogenation of phenol to cyclohexanone over the palladium catalyst has been proposed. The proposition is vali-dated by transition state calculations using density functional theory (DFT), which reveal that cyclohexanone is a favorable product and stabilized by < 19 kcal mol^-1 over cyclohexanol in ScCO₂ medium. Under similar reaction conditions, phenol hydrogenation was also carried out with rhodium, supported on Al-MCM-41. In contrast to the palladium catalyst, a mixture of cyclohexanone (57.8%) and cyclohexanol (42.2%) was formed. Detailed characterization by X-ray diffraction and transmission electron microscopy confirmed the presence of metal nanoparticles (palladium and rhodium) between 10-20 nm. Both the catalysts exhibit strikingly different product distributions in solventless conditions compared to scCO₂. This method can also be successfully applied to the other hydroxylated aromatic compounds.

Full text of DOI:10.1002/adsc.200900144

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DOI: 10.1002/adsc.200900144
Hydrogenation of Phenol in Supercritical Carbon Dioxide
Catalyzed by Palladium Supported on Al-MCM-41: A Facile
Route for One-Pot Cyclohexanone Formation
M. Chatterjee,a,* H. Kawanami,a,* M. Sato,a A. Chatterjee,b T. Yokoyama,a
and T. Suzukia
a
Research Center for Compact Chemical Process, AIST, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan
Fax : (+81)-22-237-5215; phone: (+81)-22-237-5211; e-mail: c-maya@aist.go.jp or h-kawanami@aist.go.jp
Accelrys KK, Nishishinbashi TS Bldg. 11F, 3-3-1 Nishishinbashi, Minato-Ku, Tokyo 105-0003, Japan
b
Fax : (+81)-03-3578-3873; phone: (+81)-3-3578-3860
Received: March 5, 2009; Published online: July 7, 2009
Abstract: The hydrogenation of phenol has been car- dated by transition state calculations using density
ried out in supercritical carbon dioxide (scCO2) functional theory (DFT), which reveal that cyclohex-
under very mild reaction conditions at the tempera- anone is a favorable product and stabilized by
ture of 508C over palladium supported Al-MCM-41 <19 kcalmolꢀ1 over cyclohexanol in scCO2 medium.
(metal loading ~1%). This palladium catalyst is Under similar reaction conditions, phenol hydroge-
shown to be highly active and promotes the selective nation was also carried out with rhodium, supported
formation of cyclohexanone (~98%), an industrially on Al-MCM-41. In contrast to the palladium cata-
important compound, in a “one-pot” way. The effects lyst, a mixture of cyclohexanone (57.8%) and cyclo-
of different variables like carbon dioxide and hydro- hexanol (42.2%) was formed. Detailed characteriza-
gen pressure, reaction time and also silica/alumina tion by X-ray diffraction and transmission electron
ratio of the MCM-41 support along with palladium microscopy confirmed the presence of metal nano-
dispersion are presented and discussed. The pressure particles (palladium and rhodium) between 10–
effect of carbon dioxide is significantly prominent in 20 nm. Both the catalysts exhibit strikingly different
terms of conversion and cyclohexanone selectivity. product distributions in solventless conditions com-
Moreover, the silica/alumina ratio was also found to pared to scCO2. This method can also be successfully
be an important parameter to enhance the effective- applied to the other hydroxylated aromatic com-
ness of the catalyst as it exhibits a remarkable in- pounds.
crease in phenol conversion from 20.6% to 98.4% as
the support changes from only silica MCM-41 to Al-
MCM-41. A plausible mechanism for the hydrogena- Keywords: density functional theory (DFT); one-pot
tion of phenol to cyclohexanone over the palladium process; Pd/Al-MCM-41; phenol hydrogenation;
catalyst has been proposed. The proposition is vali- Rh/Al-MCM-41; supercritical carbon dioxide
Introduction
process for the synthesis of cyclohexanone is pre-
ferred over the two-step process to avoid energy con-
Cyclohexanone is known as one of the key intermedi- sumption and also in terms of the capital cost. Gener-
ates in the synthesis of caprolactam and adipic acid, ally, the hydrogenation of phenol to cyclohexanone is
which are the main ingredients for the preparation of carried out mainly in the gas phase with Pd[3] or Pt
Nylon 6, Nylon 66 and polyamide resin.[1] Industrially, catalysts.[4] It has also been studied extensively in the
cyclohexanone is obtained by a two-step process in- liquid phase to obtain cyclohexanone[5] and also in an
volving the hydrogenation of phenol to cyclohexanol aqueous system,[6] which results in cyclohexanol or a
followed by the dehydrogenation to cyclohexanone.[2] mixture of cyclohexanol and cyclohexanone instead of
However, there are many deficiencies existing mainly only cyclohexanone.
in the dehydrogenation of cyclohexanol such as the
Despite the fact that the reaction has been studied
reaction temperature, the fact that the conversion is over several catalysts and different reaction environ-
constrained due to the thermodynamic equilibrium ments, the nature of the catalyst support is an impor-
and the poor hydrogen utilization. Thus, a one-step tant issue, which has a strong impact on the product
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Hydrogenation of Phenol in Supercritical Carbon Dioxide
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selectivity. It is generally accepted that phenol ad- Results and Discussion
sorbs on the support while Pd adsorbs H2 and conse-
quently supplies it to the aromatic ring by a spill-over Characterization of Catalysts
mechanism.[7] Depending on the acid-base properties
of the support, the adsorption geometry of phenol Pd/Al-MCM-41
varies and changes the product selectivity from cyclo-
hexanone to cyclohexanol or vice versa. So far, Pd Analysis of the calcined product confirmed that the
[9]
supported on MgO,[8] mesoporous CeO2 and ZrO2
Pd loadings of all the samples are ~1%. The change
have been considered as the most potent catalysts to of color from white (parent Al-MCM-41) to grey is
obtain cyclohexanone with high selectivity in gas taken as an initial indication of the presence of Pd2+
phase conditions. However, gas phase hydrogenation into the structure. After calcination, the material
suffers from disadvantages like harsh reaction condi- turns orange and this suggests the migration of Pd2+
ꢀ
tions, generation of by-products, which lowers the to the surface of Al-MCM-41, by reaction with Si
yield of desired product and the deactivation of the OH groups of defect sites due to insufficient substitu-
catalyst due to the coke deposition which is unavoida- tion of Pd2+ ions for both Si4+ sites and Al3+ sites in
ble.
Supercritical carbon dioxide (scCO2) is a well-
the lattice of Al-MCM-41.[14]
Figure 1a exhibits the XRD pattern of Pd/Al-
known environmentally benign “green” reaction MCM-41 with different Si/Al ratios. All materials
medium for rapid and selective organic synthesis and show typical low angle (100) reflections along with
various other industrial processes. Over the past few the smaller Braggꢁs peaks limited only to the higher
years, several heterogeneously catalyzed reactions Si/Al ratio. For low Al containing material, the most
have been successfully carried out in scCO2, often intense (100) XRD line appeared at 2q=2.1808,
with higher reaction rates and different product distri- whereas after increasing the Al content with the
butions, as well as high selectivity, in comparison with change of the Si/Al ratio to 8.5, the main peak is shift-
those obtained in conventional organic solvents.[10] ed to the lower value of 2q=1.6608 with a decrease in
Recently, the hydrogenation of phenol has also been the intensity as well; a likely explanation is that the
reported in scCO2 using Rh/C[11a] and Rh/CNF.[11b] reduced scattering intensities of the Bragg reflections
The maximum selectivity for cyclohexanone was 17% might be caused by the introduction of scattering ma-
(10 MPa of H2, 12 MPa of CO2) and 43.3% (4 MPa of terials into the pores.[15] However, the scattering pat-
H2, 12–14 MPa of CO2), over Rh/C and Rh/CNF, re- tern of higher Al containing Pd-Al MCM-41 (Si/Al=
spectively.
16 and 8.5) does not show a series of Braggꢁs peak
Here, we demonstrate the one-step formation of cy- originating from the ordered two-dimensional hexago-
clohexanone by phenol hydrogenation under very nal structure of MCM-41. Therefore, the reduced
mild conditions using the Pd/Al-MCM-41 catalyst in peak intensity of the material could be due to the dis-
scCO2 as it has already been proved a promising ordered mesoporous structure. As the Si/Al ratio
medium to improve the efficiency of the catalyst. The changes from 100 to 8.5, the main XRD peak was
result has also been compared with that of Rh/Al- shifted towards the lower angle and the corresponding
MCM-41 to check the effect of the nature of metal on d spacing of Pd/AlMCM-41 increases from 4.05 nm to
p
the product distributions.
5.32 nm. The unit cell parameters (a0 =2d100/ 3) for
Mesoporous material seems to be an ideal host for different Si/Al ratios are 4.6 nm (Si/Al=100), 4.9 nm
metal nanoparticles due to its very high surface area (Si/Al=50), 5.7 nm (Si/Al=16) and 6.1 nm (Si/Al=
to immobilize catalytically active species on it or for 8.5). The increase in basal (100) spacing indicates
providing a nanosize confinement inside the pore that the incorporation of Al generally results in an en-
system.[12] To provide additional functions to siliceous hancement of the d100 spacing. This is consistent with
mesoporous material, Si can be easily substituted by the presence of tetrahedral Al in the framework and
ꢀ
Al, which in turn improves the catalytic activity be- is attributed to the longer Al O bond length com-
cause of the generation of acid sites, hydrothermal pared to the Si O bond.
stability and the dispersion of nanoparticles on the
[16]
ꢀ
To detect the Pd nanoparticles, XRD measure-
material. Compared to other conventional supports, ments are performed at the higher angle region (2q=
mesoporous material is exhibiting superior catalytic 308 to 708), and are displayed in Figure 1a as inset. In
activity and significant enhancement of selectivity in the calcined sample, five distinctive peaks at 2q=
scCO2 during the selective hydrogenation of different 33.8, 42.0, 54.8, 60.7 and 71.4 corresponding to PdO
organic compounds.[13]
are observed. Particle sizes are calculated from the
XRD line broadening of the peak at 2q=33.8 using
Scherrerꢁs equation.[17] There is a change of the parti-
cle size from 12 to 28 nm with the change in Si/Al
ratio from 100 to 8.5. After reaction, the diffraction
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M. Chatterjee et al.
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Figure 1. X-ray diffraction pattern of (a, left) Pd/Al-MCM-41; inset shows higher angle diffraction for Pd and (b, right) Rh/
AlMCM-41; inset shows higher angle diffraction for Rh.
lines of PdO disappear and a peak corresponding to has been shifted to the lower value for Si/Al=50 and
Pd(0) appears at 2q=40.1 along with some unidenti- consequently a0 has increased from 3.9 nm to 4.5 nm.
fied peaks marked with circles (Figure 1a; inset). No Moreover, for higher Al containing materials (Si/Al=
such peaks are observed after the reaction in solvent- 16 and Si/Al=8.5) the structures are collapsed after
less conditions. However, it is unreasonable to predict calcination, probably due to the incorporation of the
the possibility of interaction between CO2 and metal much larger Rh (ionic radii of Rh3+ =0.8 ꢂ). XRD
as no such peaks are found in the absence of phenol patterns of the higher angle region of the calcined
(catalyst-CO2-H2). Hence, it can be speculated that Rh/Al-MCM-41 sample exhibit a peak at 2q=34.38
the interaction of the catalyst with CO2 and H2 is dif- indicating the presence of Rh2O3 particles with ortho-
ferent when phenol is present in the system.
rhombic symmetry[18] and the particle sizes (10 nm–
20 nm) are calculated from the XRD pattern in a sim-
ilar manner as described for Pd/Al-MCM-41.
Rh/Al-MCM-41
The XRD pattern of calcined Rh/Al-MCM-41 of TEM of Pd/Al-MCM-41
Si/Al=100, 50 exhibits a single peak in the low angle
region (100) along with the other Bragg peaks thus Figure 2a shows a TEM image of Pd/Al-MCM-41
confirming the hexagonal structure (Figure 1b). Com- (Si/Al=100) through the pore axis. The image exhibits
pared to the Si/Al=100 material, the peak position the regular ordered hexagonal channel, characteristic
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Hydrogenation of Phenol in Supercritical Carbon Dioxide
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Figure 2. TEM image of (a, left) Pd/AlMCM-41 (Si/Al=100) and (b, right) Rh/AlMCM-41 (Si/Al=100).
of MCM-41. The Pd particles are spherical and dis-
tributed throughout the MCM-41 matrix. Calculation
of the particle size distribution confirms the presence
of mainly 10–12 nm particles, which corresponds well
with the XRD measurements of wide angle line
broadening. However, the higher Al content (Si/Al=
8.5) material reveals a disordered structure with
larger PdO particles as observed by TEM (not
shown).
TEM of Rh/Al-MCM-41
Scheme 1. Reaction path for the hydrogenation of phenol in
scCO2 over Pd/Al-MCM-41 and Rh/Al-MCM-41.
The structure of Rh/Al-MCM-41 is evident from the
TEM image of the material (Figure 2b; Si/Al=100).
An ordered array of channels has been observed. The
regular array was totally disrupted when the Al con- containing catalysts were chosen for the hydrogena-
tent increased. It is evident from the image that Rh tion of phenol and the results are shown in Table 1. It
particles are spherical and randomly distributed. An is evident that the Rh catalyst generally produces a
average particle size of ~10 nm is revealed from the mixture of cyclohexanone and cyclohexanol inde-
calculation of the particle size distribution.
pendent of the support and the highest conversion of
phenol (100%) is observed when the Rh/Al-MCM-41
or Rh/MCM-41 catalyst was used (Table 1; entries 3
and 4). On the other hand, Pt and Pd catalysts show
cyclohexanone as the major product with very high
Hydrogenation of Phenol
Scheme 1 exhibits the probable reaction pathway of selectivity (Table 1 entries 6–10). However, the
the hydrogenation of phenol to cyclohexanone and phenol conversion is very low over the Pt catalyst
cyclohexanol under the studied reaction conditions
ACHTUNGTREN(NUNG >10%) whereas Pd/Al-MCM-41 exhibits excellent
(PCO =12 MPa, PH =4 MPa, T=508C) in scCO2 conversion and selectivity to cyclohexanone (Table 1;
2
2
medium.
entry 10). As the aim of this study is to optimize the
reaction conditions to obtain high selectivity for cy-
clohexanone, the hydrogenation of phenol has been
carried out over Pd and the results are compared with
those from Rh/Al-MCM-41.
Effect of Different Catalysts
It is well-known that noble metals possess the ability
to generate atomic hydrogen,[19] so Pd-, Pt- and Rh-
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Table 1. Hydrogenation of phenol over various noble metal catalyst in supercritical carbon dioxide.[a]
Entry
Catalyst
Conversion [%]
Selectivity
Cyclohexanone
Cyclohexanol
1
2
3
4
5
6
7
8
Rh/Al2O3
Rh/C
Rh/MCM-41
Rh/Al-MCM-41
Pt/C
Pt/Al2O3
Pd/C
Pd/Al2O3
Pd/MCM-41
Pd/Al-MCM-41
Pd/Al-MCM-41
Pd/Al-MCM-41
Pd/Al-MCM-41
Pd/Al-MCM-41
35.1
62.1
100.0
100.0
2.5
32.1
21.1
46.5
52.8
100.0
91.0
60.2
70.1
86.4
97.8
97.8
97.9
97.7
97.8
67.9
78.9
53.5
47.2
0.0
8.4
9.0
12.0
46.0
20.0
98.4
98.6
98.2
96.4
90.2
39.8
29.9
13.6
2.2
2.2
2.1
9
10
11
12
13
14
2.3
2.2
[a]
Reaction conditions: catalyst=0.1 g, phenol=1.0 g; temperature=508C; PCO =12 MPa, PH =4 MPa, time=4 h; en-
2
2
tries 11–14 recycling data.
Effect of CO2 Pressure
Pd/Al-MCM-41
composed of 1:1 mixtures of cyclohexanone and cy-
clohexanol, whereas cyclohexanone is the sole prod-
uct at higher pressure (>10 MPa). Again, under the
solventless conditions, a mixture of cyclohexanone
Figure 3a shows the effect of CO2 pressure from and cyclohexanol was formed and the conversion de-
7 MPa to 14 MPa and compared with the solventless creased to 28.9% due to the mass transfer limitation.
conditions on the hydrogenation of phenol over
Pd/Al-MCM-41 at the fixed temperature of 508C and
hydrogen pressure of 4 MPa. A strong influence of Rh/Al-MCM-41
the CO2 pressure has been observed on the conver-
sion and the selectivity. At 7 MPa, the conversion is The scenario of the CO2 pressure effect has been
as low as 46% and the selectivity of cyclohexanone is changed completely when Rh/Al-MCM-41 is used
57.4%. The conversion and selectivity of cyclohexa- (Figure 3b). Under the similar reaction conditions of
none are enhanced from 46.4% to 98.4% and 57.4% Pd, apparently no significant influence of CO2 pres-
to 97.8%, respectively, as the CO2 pressure changes sure is observed on the conversion of the phenol hy-
from 7 MPa to 14 MPa and above 14 MPa the selec- drogenation; in each case, the conversion of phenol is
tivity remains constant. In comparison with the higher 100% within the reaction time of 4 h. However, a triv-
pressure region, a noticeable difference in the product ial effect of CO2 pressure was observed when the re-
distribution has been found at the lower pressure of action time changed to 0.5 h; the phenol conversion
7 MPa, where CO2 mainly exists in the gas phase. At changes from 68.2% to 52.1% with the increase in
the lower pressure of CO2 (<8 MPa), the product is pressure from 7–12 MPa. Again, at the lower pressure
Figure 3. Effect of CO2 pressure on the conversion and selectivity of (a, left) Pd/Al-MCM-41 and (b, right) Rh/Al-MCM-41.
Reaction conditions: catalyst=0.1 g, phenol=1.0 g; temperature=508C; PH =4 MPa, time=4 h.
2
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Hydrogenation of Phenol in Supercritical Carbon Dioxide
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of CO2, cyclohexanol is the main product with 100% slight change in the selectivity of cyclohexanone from
selectivity, while on increasing the pressure from 8– 86.5% to 97.8% along with the hydrogen pressure.
14 MPa, a mixture of cyclohexanone and cyclohexa-
nol is formed. The ratio of the product changes from
1:1.3 to 1:0.9 (cycloheanone:cyclohexanol) as the Rh/Al-MCM-41
pressure is enhanced from 8 MPa to 14 MPa. Surpris-
ingly, complete conversion and highest selectivity Variation of conversion and selectivity as a function
(100%) of cyclohexanol is achieved when the hydro- of hydrogen pressure over Rh/Al-MCM-41 under the
genation of phenol is carried out in the solventless similar reaction conditions as for Pd/Al-MCM-41 is
conditions (Figure 3b).
shown in Figure 4b. At 1 MPa, the conversion of
It is observed that, for both catalysts, the product phenol is low (46.1%). Increasing the pressure to
distribution at lower pressure follows different trends 4 MPa the conversion is enhanced to 100% and it
to that of the liquid CO2 phase condition. Hence, it is also affects the product distribution over Rh/Al-
expected that the reaction mechanism of phenol hy- MCM-41. When conversion is low (46.1%), then se-
drogenation in gas phase conditions of CO2 is similar lectivity of cyclohexanone is comparatively high at
to that of the solventless conditions but differs from 71.2%, but increasing conversion of phenol is accom-
that in the liquid phase conditions. Visual observation panied by a complete hydrogenation to cyclohexanol.
of the phase behavior, under similar conditions re- The enhancement of the activity and selectivity of cy-
vealed that in the higher CO2 pressure (>10 MPa) clohexanol attributes to the increased concentration
region, the mixture of substrate, CO2 and H2 trans- of hydrogen in the reaction mixture.
forms from a binary (liquid-gas) phase to a homoge-
Therefore, hydrogen pressure is an important pa-
neous phase. This transition is accompanied by the in- rameter to influence the conversion and selectivity for
trinsic change in product selectivity and also the con- both Pd- and Rh/Al-MCM-41.
version. Thus, elimination of the gas-liquid interface
is crucial to achieve highest performance of Pd/Al-
MCM-41 but it is not necessary for the Rh/Al-MCM- Effect of Reaction Time
41 catalyst.
The effect of reaction time on the hydrogenation of
phenol over Rh- and Pd/Al-MCM-41, suggests that
Effect of Hydrogen Pressure
Pd/Al-MCM-41
the rate of the reaction does not differ considerably
under the studied reaction conditions (Figure 5a and
Figure 5b). For instance, 65.6% and 52.1% conversion
are achieved within 0.5 h over the Pd and Rh catalyst,
Figure 4a shows the dependence of hydrogen pressure respectively. The conversion of phenol on Pd/Al-
on the conversion and selectivity over Pd/Al-MCM-41 MCM-41 remains unchanged after the extension of
at fixed temperature (508C) and CO2 pressure reaction time from 3 to 5 h and the deactivation of
(12 MPa). The degree of phenol conversion is very the catalyst could be suspected. However, the recy-
sensitive to the hydrogen pressure. When the pressure cling of the catalyst was straightforward (Table 1
changes from 1 to 4 MPa, the conversion increases entrie 11–14). Only after the 3rd recycle did the activi-
from 50.2% to 98.4% as the concentration of hydro- ty of the catalyst start to decrease. Moreover, the se-
gen is enhanced in the system. Nevertheless, there is a lectivity of cyclohexanone for the Pd catalyst re-
Figure 4. Variation of H2 pressure on the conversion and selectivity of (a, left) Pd/Al-MCM-41 and (b, right) Rh/Al-MCM-
41. Reaction conditions: catalyst=0.1 g, phenol=1.0 g; temperature=508C; PCO =12MPa, time=4 h.
2
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M. Chatterjee et al.
FULL PAPERS
result leads us to believe that initially cylohexanol is
formed at the expense of cyclohexanone. To verify
this result, the yield of each product versus conversion
using Rh/Al-MCM-41 was plotted and is shown in
Figure 5c. Interestingly, the yields of both cyclohexa-
nol and cyclohexanone increase with increasing
phenol conversion, attributing the independent for-
mation of cyclohexanone and cyclohexanol. Chary
et al.[20] observed a similar pattern during the vapor
phase hydrogenation of phenol over Pd/C catalyst.
Again, to confirm the independent formation of cy-
clohexanol, the hydrogenation of cyclohexanone was
carried out under similar reaction conditions as for
phenol, but no cyclohexanol was observed. Hence, it
can be inferred that the cyclohexanone and cyclohex-
anol are two independent products and not obtained
with the expense of others in scCO2 over Rh/Al-
MCM-41.
Therefore, depending on the nature of the catalystꢁs
active site, phenol can interact with the surface
through the aromatic ring, or the hydroxy group,
where the former ended up with the complete hydro-
genation.[21] Furthermore, Pd- and Rh/Al-MCM-41
catalysts both maintained their catalytic activity for at
least 20 h of reaction time. This indicates that the cat-
alysts are free from deactivation under the studied re-
action conditions.
As we have mentioned before (Figure 3a and Fig-
ure 3b), there is a considerable difference in the con-
version and selectivity as the reaction was carried out
in solventless conditions using Pd- and Rh/AlMCM-
41. The importance of scCO2 medium is evident from
the difference in the rate of phenol hydrogenation on
Pd/Al-MCM-41 catalyst. In solventless conditions, the
conversion is poor (14.3%) whereas a radical change
in the conversion (65.6%) is achieved in scCO2 within
the reaction time of 0.5 h. It seems to be reasonable
to explain the enhanced activity in scCO2 medium be-
cause of increased solubility of hydrogen in the
medium, which decreases the mass transfer limita-
tions. In addition, a drastic change in the product dis-
tribution is also evident from the Figure 3a. A mix-
ture of cyclohexanone and cyclohexanol is formed
under the solventless conditions whereas cylohexa-
none has been obtained in scCO2 medium. The differ-
ence in product distribution depending on the pres-
ence and absence of scCO2, is suggesting a change in
Figure 5. Variation of reaction time on the conversion and
selectivity of (a, top) Pd/Al-MCM-41 and (b, middle) Rh/Al-
MCM-41. Reaction conditions: catalyst=0.1 g, phenol=
1.0 g; temperature=508C; PCO =12 MPa, PH2 =4 MPa. (c,
2
bottom) Variation of the yield of cyclohexanone and cyclo-
hexanol with conversion over Rh/Al-MCM-41.
mained unaffected by the reaction time. No leaching the adsorption geometry of phenol influenced by the
of Pd has been detected in the supernatant after the electronic structure of the metal active site. Namely,
reaction.
the solvent influences the polarity of the catalyst sur-
The product distribution of phenol hydrogenation face[22] and a modification in electronic structure has
over Rh/Al-MCM-41 differs as the time changes from also been predicted in scCO2, which strongly affects
0.5 h to 2 h, but remains constant as soon as the con- the catalytic activity and the selectivity of the reac-
version reaches its maximum of 100%. The decreasing tion.[23]
selectivity of cyclohexanone and simultaneous in-
In contrast to Pd/Al-MCM-41, the rate of the reac-
creasing selectivity of cyclohexanol takes place within tion decreased in scCO2 (52. 1% conversion in 0.5 h)
2 h of reaction time and then remains constant. This compared to the solventless conditions (98.0% con-
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Adv. Synth. Catal. 2009, 351, 1912 – 1924

Hydrogenation of Phenol in Supercritical Carbon Dioxide
FULL PAPERS
version in 0.5 h) over Rh/Al-MCM-41 catalyst. Thus,
initial conversion in CO2 and solventless condition
shows marked difference and at present it is not pos-
sible to predict the exact cause behind the decrease in
conversion because of the complicated nature of ki-
netics of this solid (catalyst)-liquid (solvent+sub-
strate)-gas (H2) system. The yields of cyclohexanone
and cyclohexanol, change from 30.1% and 21.4 to
4.7% and 93.1%, respectively, as the reaction was per-
formed in the absence of solvent. This result can be
explained by considering the fact that when the sub-
strate molecule interacts with the catalyst the process
of adsorption of substrate to the catalyst surface pro-
ceeds through the interaction between the substrate
and the catalyst and consequently a surface complex
can be formed (reactant-metal). The magnitude of the
reaction rate of the heterogeneously catalyzed reac-
Figure 6. Effect of Pd dispersion and Si/Al ratio on phenol
conversion and cyclohexanone selectivity over Pd catalyst.
tion depends on the stability of the surface complex, matic change in the phenol conversion from 20%
which varies implicitly with the molecular structure of (only silica) to 98.4% (Si/Al=100) and also the selec-
the substrate.[24] Thus, one could expect a probable tivity of cyclohexanone changes from 78.4% to
change in the reaction rate between only phenol in 97.8%. The promoting effect of Al could be attribut-
solventless conditions and phenol solvated in CO2 ed to the increase of surface acidity. It is well known
which interacts with the catalyst. However, it is too that incorporation of Al to MCM-41 might serve as
early to comment on the reduction of the reaction acid sites and thus enhance the activity. The phenol
rate as it needs a detailed study of the molecular molecule can adsorbs on the acidic site either by the
structural changes of phenol in scCO2 over Rh/Al- donation of p-electrons of the benzene ring or
MCM-41.
through the OH group and the later is more favorable
From the above results, it can be concluded that under the present reaction conditions. The best result
the hydrogenation of phenol exhibits different types is obtained over Pd/Al-MCM-41 when Si/Al=100,
of product distribution even under the same reaction among the catalysts of varying Si/Al ratios. This ex-
conditions due to the difference in the nature of the pression of conversion and selectivity might be relat-
active metal ion. Furthermore, the reaction paths for ed to the acidity of the mesoporous support. It is
the hydrogenation of phenol considerably differ be- commonly assumed that the acid sites within the mo-
tween scCO2 and solventless conditions independent lecular sieve structure are essential for enhancement
of the nature of the metal. As our main concern is cy- of the catalytic activity.[26] However, when the Al con-
clohexanone, which is formed over Pd catalyst, the tent increases with the change in the Si/Al ratio from
following part will continue with a more detailed 50 to 8.5, the acidity changes from 161 to 381 mmoles/
study on combining the nature of the catalyst and its g and the conversion of phenol was decreased from
active site.
80.3% to 18.5%. Thus, the higher acidity of the sup-
From Table 1, it is evident that, depending on the port lowers the conversion of phenol. This result can
support, the catalytic activity changes. Indeed, accord- be explained by the fact that in the MCM-41 struc-
ing to the literature,[25] the hydrogenation of phenol ture, the incorporation of Al leads to the formation of
over Pd catalyst occurs when hydrogen is activated on tetrahedral Al3+ in the framework. For high Al con-
Pd and phenol is adsorbed as a phenolate species tents, most Al atoms are anchored on the outer sur-
over the support close to the metal particles. A signif- face rather than incorporated into the framework of
icant alteration in conversion is also found when Pd is MCM-41[27] and the growing part of the acid sites (tet-
supported on the Al-MCM-41 instead of only silica rahedral Al3+) might be neutralized by non-frame-
MCM-41 (entry 9; Table 1).
work cationic Al species which blocks the strong acid
sites.[28] In addition, increasing the Al content could
cause some deterioration in the pore structure as well
as in the texture of the mesoporous material,[29] which
is clearly observed from the Figure 1a and harmful
Effect of Si/Al Ratio
The effect of the Si/Al ratio on the activity and selec- for the activity of the catalyst. A similar result has
tivity over Pd/Al-MCM-41 catalyst is shown in been obtained by Li et al. during the Ullmann reac-
Figure 6. The Pd catalyst supported on Al-MCM-41 tion over a Pd-containing Al-MCM-41 catalyst.[30]
exhibits much higher activity than Pd-MCM-41 (only This observation is in contrast to the results of Neri
silica support). The introduction of Al leads to a dra- et al.[9] who observed a significant decrease in the se-
Adv. Synth. Catal. 2009, 351, 1912 – 1924
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M. Chatterjee et al.
FULL PAPERS
lectivity of cyclohexanone during the vapor phase hy- clusively when an optimum amount of Al is present
drogenation of phenol, with increasing acidity of the in the support, which is in contrast to the described
support and a preference of Pd/MgO over Pd/Al2O3, mechanism for the vapor and liquid phase hydrogena-
which could be due to the difference in reaction con- tion of phenol. There are two possible interactions: (i)
ditions.
catalyst-substrate and (ii) catalyst-substrate-solvent.
To gain an insight into the relation between the To understand the plausible reaction routes, a com-
nature of the catalyst and its activity, the effect of par- parison is made between the phenol hydrogenation in
ticle size distribution, which is related to the product scCO2 and in conventional organic solvents like etha-
selectivity, has also been studied. Figure 6 exhibits the nol and hexane under the same reaction conditions.
dispersion of Pd depending on the Si/Al ratio and Unfortunately, the phenol conversions in ethanol
also the soley silica material. The Pd metal dispersion (24.7%) and hexane (9.1%) are extremely poor. The
varies from 7.1% to 3.2% as the Si/Al ratio changes selectivity of cyclohexanone in ethanol (42.1%) is
from 100 to 8.5 and indicated that, with increasing Al strikingly low compared to those in the scCO2
content, the Pd particle size increases. Although a medium and hexane (65.1%). Hence, it is confirmed
slight change in the selectivity from 97.8% to 100% is that the mechanism of the phenol hydrogenation re-
evident, this is within the experimental error range. action must be different depending on the nature of
Hence, the selectivity to cyclohexanone seems to be the solvent. In scCO2, the solubility of H2 is higher
structure insensitive.
and the increasing phenol conversion in scCO2 corre-
lates well with the presence of a high concentration of
H2 because of the larger solubility and absence of
mass transfer limitations. According to the product
distribution, it is evident that phenol prefers a non-
planar orientation for adsorption on Pd/Al-MCM-41
Possible Mechanistic Considerations of the “One-
Pot” Formation of Cyclohexanone
Regarding the mechanism of the hydrogenation of rather than a co-planar one as cyclohexanone is the
phenol and the formation of cyclohexanone in the main product. One probable explanation could be
vapor phase over Pd/Al-MCM-41, it is reported that that phenol is a polar compound and its attraction to-
the adsorption orientation of the phenol molecule wards ethanol would be stronger as the polarity of the
changes from co-planar to non-planar depending on solvent is higher. From the DFT calculations (details
the acidity and basicity of the medium and results in are given in the Experimental Section) on the opti-
the formation of cyclohexanol and cyclohexanone, re- mized geometry of phenol in CO2 and in ethanol, it is
spectively (Scheme 2).[9] However, it is interesting to proved that phenol is less stable in CO2 by ca. 8 kcal
observe that, in scCO2, cyclohexanone is formed ex- molꢀ1 (Table 2). Thus, it can be suggested that, for
ethanol as medium, less phenol is available to the cat-
alyst active site, as it will prefer to stay solvated. So,
more space will be accessible for the phenol molecule
to adsorb co-planar on the surface. On the contrary,
as scCO2 is a quadropolar solvent and the interaction
between solvent and substrate will be less than for
substrate-catalyst, more phenol will be attracted to
the catalyst active site. As a result, the concentration
of phenol over the catalyst surface will by increased
and less space is available to adsorb on the catalyst
surface and that might be reasonable for phenol to
Scheme 2. Orientation of phenol adsorption on the catalyst
surface.
Table 2. Total energy, stabilization energy of reactant and products in ethanol and CO2 medium and transition state calcula-
tion to identify the reaction energy of cyclohexanone and cyclohexanol.
Molecule
Solvent
Total energy (Hartree)
Stabilization energy (kcalmolꢀ1)
8.0965040747
reactant+Pd
reactant+Pd
CO2
Ethanol
ꢀ474.6736518
ꢀ474.6865544
Energy of reac- Energy of product cy- Energy of cyclohex- Energy for transi-
Energy of reac-
Feasibility of reac-
tant (Hartree)
clohexanol (Hartree)
anone (Hartree)
tion state (Hartree) tion (kcalmolꢀ1) tion (kcalmolꢀ1)
ꢀ474.5717
ꢀ474.5717
ꢀ474.7984
ꢀ474.7630
ꢀ474.3430
ꢀ142.274
ꢀ161.301
ꢀ474.8128
ꢀ19.027
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Hydrogenation of Phenol in Supercritical Carbon Dioxide
FULL PAPERS
adsorb preferably in a non-planar way. Depending on tive addition of hydrogen to phenols, which maintains
the mode of adsorption, either co-planar or non- the resonance energy as much as possible and produ-
planar geometry results in a shifting of the selectivity. ces a series of hydrogenation intermediates. Differen-
To confirm this proposition, transition state calcula- ces in the resonance stabilization energy between the
tions have been performed and the results are shown transition states may play a significant role to deter-
in Table 2. This shows that the barrier height of cyclo- mining the product selectivity. On the other hand, the
hexanone formation is ca. 19 kcalmolꢀ1 less than that formation of the fully hydrogenated product may be
of cyclohexanol, which makes it a stable product in accounted for by the decrease in the contribution of
CO2 and hence confirms the highest selectivity of cy- the phenoxide ion or an increase in the strength of
clohexanone.
the adsorption to suppress the contribution of the res-
However, it is difficult to predict the exact reaction onance energy.[31] The hydrogenation of naphthol re-
route of phenol hydrogenation because heterogene- flects a relatively good selectivity to the tetrahydro-
ous catalysis in scCO2 is very complicated system and naphthol compound (ca. 85%) (entry 4; Table 3), pos-
the interaction of organic molecules in the channel sibly due to the generation of a cationic intermediate
system of molecular sieves are far from being under- through the kinetically favored C-1 (naphthalene is
stood completely.
also stabilized by resonance structures) that maintains
one benzene ring intact.
Hydrogenation of Other Hydroxy Aromatic
Compounds
Conclusions
To explore the scope of the application of Pd/Al- In conclusion, we have demonstrated the hydrogena-
MCM-41, this method has been extended to the other tion of phenol over Pd/Al-MCM-41, which effectively
hydroxy aromatic compounds under the similar reac- promotes the formation of cyclohexanone; an indus-
tion conditions and the results are shown in Table 3. trially important compound, in scCO2. From the re-
Interestingly, the ring hydrogenation is observed in sults it is confirmed that for this reaction scCO2 is an
every case and the partial hydrogenation products are essential medium because it enhances the catalytic ac-
formed with reasonable selectivity. Indeed, the hydro- tivity substantially by lowering the mass transfer limi-
genation of o-cresol produces methyl cyclohexanone tation between solid and gas phases. Furthermore, as
with ca. 78% selectivity and interestingly the dihy- the reaction has occurred at a mild temperature of
droxybenzene derivative produces hydroxycyclohexa- 508C it reduces the energy consumption and is also
none with 80% selectivity (entries 1–3; Table 3). Gen- cost effective. The reaction is free from any by-prod-
erally, the presence of electron-releasing substituents uct formation and long-term catalytic activity can be
on the aromatic ring influences the acidity due to the achieved. A detailed study on Pd/Al-MCM-41 has re-
resonance/or inductive effect and activates the ring. vealed a significant increase in phenol conversion
From these results (Table 3) it is obvious that the se- with the incorporation of Al on the mesoporous sup-
lectivity of the ketone product is higher. For explain- port due to the acidity of the support material. The
ing the much higher selectivity of the ketone inter- reaction is classified as a structure insensitive reaction
mediate over a Pd catalyst, it requires the regioselec- as the particle size translates no impact on selectivity.
Table 3. Hydrogenation of hydroxy aromatic derivatives with Pd/AlMCM-41.[a]
Entry
Substrate
Conversion [%]
Selectivity [%]
1
79.1
77.3:22.7
2
3
65.6
30.0
83.0
89.1:10.1
79.9:20.1
84.8:5.2
4
[a]
Reaction conditions: catalyst=0.1 g; phenol=1.0 g; temperature=508C; PCO =12 MPa; PH =4 MPa; time=4 h.
2
2
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M. Chatterjee et al.
FULL PAPERS
the reactor and placed in an oven with fan heater to main-
tain the desired temperature. At first H2 at the required
pressure was introduced into the reactor. After that liquid
A plausible mechanism of phenol hydrogenation has
been predicted over Pd/Al-MCM-41, which exhibits a
strong effect of the nature of the reaction medium.
The transition state calculations of the reaction using
DFT confirmed the stability of cyclohexanone in CO2
and thus its high selectivity. Alterations of the process
variables like CO2 and H2 pressure served to increase
the conversion of phenol but the selectivity to cyclo-
hexanone remain unaltered. An appreciable catalytic
performance in terms of conversion and selectivity is
also observed for the Rh/Al-MCM-41 catalyst. How-
ever, the product distribution differs considerably in
comparison with that of the Pd/Al-MCM-41 and the
mixture of cyclohexanone and cyclohexanol is
formed. In summary, this study provides a greener
and simpler way for the formation of cyclohexanone
from an environmental hazardous material like
phenol. The nature of the solvent medium and the
metal active site are the most important factors to de-
termine the product distribution. Successful applica-
tion of this method is also possible to the other hy-
droxy aromatic compounds.
CO2 was charged using
a high-pressure liquid pump
(JASCO) and then compressed to the desired pressure. The
reaction mixture was stirred during the reaction. The liquid
product was separated from the catalyst simply by filtration
and identified by GC-MS, followed by quantitative analysis
using a GC (HP 6890) equipped with capillary column and
flame ionization detector. For all results reported, the selec-
tivity is defined as:
concentration of the product
% selectivity=total concentration of the products ꢃ100.
For organic solvent in place of CO2, 5 mL of organic sol-
vent were used.
Catalyst Characterization
Powder X-ray diffraction pattern was recorded on a Rigaku
RAD-X system using monochromatized Cu Ka radiation
(l=1.542 ꢂ). In general, the diffraction data were collected
using the continuous scan mode with a scan speed of 2 deg/
min over the scan range 2q=1.5–158 and for wide-angle
(2q=30–708) measurements.
Transmission electron microscopy (TEM) of the material
was recorded on Philips Technai operating at 200 kV. The
powder was suspended in ethanol by an ultrasonic method.
A drop of this solution was placed on a grid with a holey
carbon copper film and then allowed to dry, covered by a
watch glass.
Experimental Section
Catalyst Synthesis
Materials: Tetraethyl orthosilicate (TEOS) as silica source,
sodium aluminate as aluminium source and sodium hydrox-
ide were purchased from Wako Pure Chemicals. Cetyltrime-
thylammonium bromide (CTAB), PdCl2 and RhCl3 were
from Aldrich Chemical Co. All materials were used as re-
ceived. CO2 (>99.99%) has been provided by the Nippon
Sanso Co. Ltd.
Hydrothermal Synthesis of Pd/Al-MCM-41: Typically,
sodium hydroxide and CTAB were added to de-ionized
water and stirred until dissolved. After that, TEOS along
with the required amount of sodium aluminate are intro-
duced slowly under the stirring condition and with contin-
ued stirring for another 1 h. A 1 wt% solution of Pd salt was
introduced in to the gel and again stirred for 2 h. The final
gel composition is : 1 TEOS: x Al: 0.45 Na2O: 0.12 CTAB:
118 H2O, where x=0.01–0.11. The gel was then autoclaved
at 1408C for 48 h. The solid product was filtered, washed
thoroughly with de-ionized water followed by oven drying
at 608C. To remove the template, the material was calcined
at 5508C for 8 h in air.
Phase Behavior Studies
Visual observation of phase behavior of phenol under the
studied reaction condition was studied with a 10-mL high
pressure view cell fitted with a sapphire window. The cell is
placed over a magnetic stirrer for stirring the content and
connected to a pressure controller, to regulate the pressure
inside the view cell. In addition to that a temperature con-
troller was also used to maintain the desired temperature of
508C. Phenol was introduced into the view at a constant hy-
drogen pressure of 4 MPa while the CO2 pressure was
varied in the range of 7–14 MPa and the phase behavior of
phenol-H2-CO2 system was monitored.
Computational Methodology
All the calculations related to the transition state were per-
formed with density functional theory (DFT)[32] using the
DMol3 code[33] of Accelrys Inc. BLYP exchange correlation
functional[34a] and DNP basis set.[34b] In order to consider the
effect of solvent, we employed the conductor-like screening
model (COSMO) solvation method within the DFT formal-
ism as in the program DMol3 of Accelrys Inc.[35] In this
method, the solute molecules form a cavity within the die-
lectric continuum of permittivity e that represents the sol-
vent. The charge distribution of the solute polarizes the die-
lectric medium. The response of the dielectric medium is de-
scribed by the generation of screening (or polarization)
charges on the cavity surface. The cavity surface is obtained
as a superimposition of spheres centered at the atoms, dis-
carding all parts lying on the interior part of the surface.
For the sake of comparison, Rh/Al-MCM-41 has also
been synthesized in the similar manner. Finally, the product
was characterized by X-ray diffraction (XRD) and transmis-
sion electron microscopy (TEM). These two catalysts have
been termed as Pd/Al-MCM-41 and Rh/Al-MCM-41, re-
spectively.
Catalytic Activity
All the reactions were carried out in a 50-mL stainless steel
batch reactor and the details are given elsewhere.[13] Briefly,
0.1 g of catalyst and 1 g of the reactant were introduced into
1922
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Hydrogenation of Phenol in Supercritical Carbon Dioxide
FULL PAPERS
The spheres are represented by a discrete set of points, the
so-called basic points; eliminating the parts of the spheres
that lie within the interior part of the molecule thus
amounts to elimination of the basic grid points that lie in
the interior of the molecule. The radii of the spheres are de-
termined as the sum of the van der Waals radii of the atoms
of the molecule and of the probe radius. The surviving basic
grid points are then scaled to lie on the surface generated
by the spheres of van der Waals radii alone. The basic
points are then collected into segments, which are also rep-
resented as discrete points on the surface. The screening
charges are located at the segment points. The transition
state calculations were performed using the synchronous
transit methods as included in the DMol3 module of Accel-
rys Inc.
Complete linear synchronous transit (LST)/quadratic syn-
chronous transit (QST) begins by performing an LST/opti-
mization calculation. The TS approximation obtained in that
way is used to perform QST maximization. From that point,
another conjugate gradient minimization is performed. The
cycle is repeated until a stationary point is located or the
number of allowed QST steps is exhausted. DMol3 uses the
nudged elastic band (NEB) method for minimum energy
path calculations. The NEB method introduces a fictitious
spring force that connects neighboring points on the path to
ensure continuity of the path and projection of the force so
that the system converges to the minimum energy path
(MEP), which is as well called intrinsic reaction co-ordinate
if the co-ordinate system is mass weighted. We also calculat-
ed the vibration mode to identify the negative frequency to
confirm the transition state.
[7] a) G. Neri, A. M. Visco, A. Donato, C. Milone, M. Mal-
entacchi, G. Guibitosa, Appl. Catal. A, 1994, 110, 49;
b) Y. Z. Chen, C. W. Liaw, L. I. Lee, Appl. Catal. A
1999, 177, 1.
[8] S. Narayanan, K. Krishna, Catal. Today 1999, 49, 57.
[9] S. Velu, M. P. Kapoor, S. Inagaki, K. Suzuki, Appl.
Catal. A Gen. 2003, 245, 317.
[10] a) A. Baiker, Chem. Rev. 1999, 99, 453; b) A. Fꢄrstner,
L. Ackermann, K. Beck, H. Hori, D. Koch, K. Lange-
man, M. Liebl, C. Six, W. Leitner, J. Am. Chem. Soc.
2001, 123, 9000; c) K. Wittmann, W. Wisniewski, R.
Mynott, W. Leitner, C. L. Kranemann, T. Rische, P. Eil-
bracht, S. Kluwer, J. M. Ernsting, C. J. Elsevier, Chem.
Eur. J. 2001, 7, 4584.
[11] a) C. V. Rode, U. D. Joshi, O. Sato, M. Shirai, Chem.
Commun. 2003, 1960; b) W. Wang, F. Zhao, S. Fujita,
M. Arai, Catal. Commun, 2008, 9, 362.
[12] a) R. Kçhn, M. Fçrba, Catal. Today 2001, 68, 227;
b) M. Morey, A. Davidson, H. Eckert, G. D. Stucky,
Chem. Mater. 1996, 8, 486.
[13] a) M. Chatterjee, Y. Ikushima, F. Zhao, New J. Chem,
2002, 27, 510; b) M. Chatterjee, F. Y. Zhao, Y. Ikushi-
ma, Adv. Synth. Catal. 2004, 346, 459; c) M. Chatterjee,
A. Chatterjee, Y. Ikushima, Green Chem, 2004, 6, 114;
d) M. Chatterjee, Y. Ikushima, T. Yokoyama M. Sato,
Adv. Synth. Catal. 2008, 350, 624; e) M. Chatterjee, T.
Yokoyama, H. Kawanami, M. Sato, T. Suzuki Chem.
Commun. 2009, 701.
[14] a) M. H. Lim, C. F. Blanford, A. Stein, Chem. Mater.
1998, 10, 467; b) M. Frçba, R. Kçhn, G. Bouffaud,
Chem. Mater. 1999, 11, 2858; c) J. Sauer, F. Marlof, B.
Spliethoff, F. Schꢄlth, Chem. Mater. 2002, ##14##36,
217.
[15] R. Mokaya, W. Jones, Phys. Chem. Chem. Phys. 1999,
1, 207.
[16] T. Kawabata, I. Atake, Y. Ohishi, T. Shishido, Y. Tian,
K. Takaki, K. Takehira Appl. Catal. B Environmental
2006, 66, 151.
[17] H. P. Klug, L. E. Alexander, X-ray Diffraction Proce-
dures for Polycrystalline Amorphous Materials, 2nd
edn., Wiley, New York, 1974.
[18] R. S. Mulukutla, K. Asakura, T. Kogure, S. Namba, Y.
Iwasawa, Phys. Chem. Chem. Phys. 1999, 1, 2027.
[19] J. M. Chapuzet, A. Lasia, J. Lessard, in: Electrocataly-
sis, (Eds.: J. Lipkowski, P. N. Ross), Wiley-VCH, New
York, 1998, Chapter 4.
[20] K. V. R. Chary, D. Naresh, V. Viswanathan, M. Sada-
kane, W. Ueda, Catal. Commun. 2007, 8, 471.
[21] E. J. Shin, M. A. Keane, J. Catal. 1998, 173, 450.
[22] F. J. Mcquilin, W. O. Ord, P. L. Simpson, J. Chem. Soc.
1963, 5996.
[23] a) M. Arai, Y. Nishiyama, Y. Ikushima, J. Supercrit.
Fluids 1998, 13, 149; b) B. M. Bhanage, Y. Ikushima,
M. Shirai, M. Arai, Catal. Lett. 1999, 62, 175.
References
[1] B. M. Nagaraja, V. Siva Kumar, V. Shashikala, A. H.
Padmasri, S. S. Reddy, B. D. Raju, K. S. Rama Rao, J.
Mol. Catal. A: Chem. 2004, 223, 339; World Nylon 6
and 66 Supply/Demand Report, PCI fibers and raw Ma-
terials, Seaford, UK, 1998.
[2] a) V. Z. Fridman, A. A. Davydov, K. Titievsky, J. Catal.
2000, 195, 20; b) V. Z. Fridman, A. A. Davydov, K. Ti-
tievsky, J. Catal. 2004, 222, 545.
[3] a) S. Narayanan, K. Krishna, Appl. Catal. A: Gen,
1996, 147, L253; b) N. Mahata, V. Vishwanathan, Catal.
Today 1999, 49, 65; c) S. G. Shore, E. Ding, C. Park,
M. A. Keane, Catal. Commun. 2002, 3, 77; d) A. K. Ta-
lukdar, K. G. Bhattacharyya, S. Sivasanker, Appl. Catal.
A: Gen. 1993, 96, 229; e) N. Itoh, W.-C. Xu, Appl.
Catal. A: Gen. 1993, 107, 83.
[4] a) S. T. Srinivas, L. J. Lakshmi, P. K. Rao, Appl. Catal.
A: Gen. 1994, 110, 167; b) S. T. Srinivas, P. K. Rao, J.
Chem. Soc. Chem. Commun. 1993, 33.
[5] a) L. Zhuang, H. Li, W. Dai, M. Qiao, Chem. Lett.
2003, 32, 1072; b) U. M. Tang, M. Y. Huang, Y. Y.
Jiang, Macromol. Rapid Commun. 1994, 15, 527; c) M.
Higashijima, S. Nishimura, Bull. Chem. Soc. Jpn. 1992,
64, 2955; d) S. G. Shore, E. Ding, C. Park, Catal.
Commun. 2002, 3, 77.
ˇ
´
[24] P. Kacer, L. Cerveny, Appl. Catal. A: Gen. 2002, 229,
193.
[25] a) S. Scirꢅ, C. Crisafulli, R. Maggiore, S. Minicꢆ, S. Gal-
vagno, Appl. Surf. Sci. 1998, 136, 311; N. Mahata, V.
Vishwanathan, J. Mol. Catal. A 1997, 120, 267; b) S.
Galvagno, A. Donato, G. Neri, R. Pietropaolo, J.
Chem. Technol. Biotechnol. 1991, 51, 145.
[6] Y. Xiang, L. Ma, C. Lu, Q. Zhang, X. Li, Green Chem.
2008, 10, 939.
Adv. Synth. Catal. 2009, 351, 1912 – 1924
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1923

M. Chatterjee et al.
FULL PAPERS
[26] a) M. Jayamurthy, S. Vasudevan, Ber. Bunsen-Ges.
1995, 99, 1521; b) F. Haase, J. Sauer, J. Am. Chem. Soc.
1995, 117, 3780; c) S. R. Blaszkowski, R. A.van Santen,
J. Phys. Chem. 1995, 99, 11728.
[27] R. Ryong, H. K. Chang, F. H. Russ, Chem. Mater. 1997,
9, 1607.
[31] M. Higashijima, S. Nishimura, Bull. Chem. Soc. Jpn.
1992, 65, 824.
[32] W. Kohn, L. J. Sham, Phys. Rev. A, 1965, 140, 1133.
[33] B. Delley, J. Chem. Phys. 1990, 92, 508; B. Delley, J.
Chem. Phys. 1991, 94, 7245.
[34] a) B. Delley, J. Chem. Phys. 2000, 113, 7756; B. Delley,
J. Phys. Chem. 1996, 100, 6107; b) A. D. Becke, J.
Chem. Phys. 1988, 88, 2547.
[28] K. Busio, J. Janchen, J. H. C. van Hooff, Microporous
Mater. 1995, 5, 211.
[29] J. Michalik, D. Brown, J. S. Yu, M. Danilczuk, J. Y.
Kim, L. Kevan, Phys. Chem. Chem. Phys. 2001, 3, 1705.
[30] H. Li, J. Chen, Y. Wan, W. Chai, F. Zhang, Y. Lu,
Green Chem. 2007, 9, 273.
[35] A. Klamt, G. Schꢄrmann, J. Chem. Soc. Perkin Trans. 1
1993, 2, 799.
1924
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ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 1912 – 1924