Selective catalytic oxidation of geraniol to citral with molecular oxygen in supercritical carbon dioxide

Article abstract of DOI:10.1016/j.apcata.2011.01.001

Selective catalytic oxidation of geraniol to citral with molecular oxygen in supercritical carbon dioxide (scCO₂) has been investigated. The catalyst used was a chromium containing mesoporous molecular sieve catalyst viz. CrMCM-41. Comparison studies were performed with CoMCM-41, PtMCM-41 and PdMCM-41 catalysts. Among the various catalysts studied, CrMCM-41 showed a high conversion of geraniol and an excellent selectivity for citral. In contrast CoMCM-41, PtMCM-41 and PdMCM-41 catalysts exhibited low conversion of geraniol. However all three catalysts compared showed similar citral selectivity to CrMCM-41. The effect of CO₂ pressure and reaction temperature geraniol oxidation was studied with CrMCM-41. Supercritical CO₂ medium was found to enhance the conversion of geraniol and/or yield of citral. It was noticed that the catalyst can be recycled with negligible loss of conversion.

Full text of DOI:10.1016/j.apcata.2011.01.001

Applied Catalysis A: General 394 (2011) 209–214
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Selective catalytic oxidation of geraniol to citral with molecular oxygen in
ଝ
supercritical carbon dioxide
Sudhir E. Dapurkara ,a ∗, Hajime Kawanamia,∗∗, Maya Chatterjee , Chandrasekhar V. Rode ,
a
b
Toshirou Yokoyama , Yutaka Ikushimaa
a
Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST),
Tohoku, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan
b
Chemical Engineering and Process Development Division, National Chemical Laboratory, Pune 411021, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 August 2010
Received in revised form
Selective catalytic oxidation of geraniol to citral with molecular oxygen in supercritical carbon dioxide
(
scCO2) has been investigated. The catalyst used was a chromium containing mesoporous molecular sieve
catalyst viz. CrMCM-41. Comparison studies were performed with CoMCM-41, PtMCM-41 and PdMCM-
1 catalysts. Among the various catalysts studied, CrMCM-41 showed a high conversion of geraniol and
2
8 December 2010
4
Accepted 2 January 2011
Available online 8 January 2011
an excellent selectivity for citral. In contrast CoMCM-41, PtMCM-41 and PdMCM-41 catalysts exhibited
low conversion of geraniol. However all three catalysts compared showed similar citral selectivity to
CrMCM-41. The effect of CO2 pressure and reaction temperature geraniol oxidation was studied with
CrMCM-41. Supercritical CO2 medium was found to enhance the conversion of geraniol and/or yield of
citral. It was noticed that the catalyst can be recycled with negligible loss of conversion.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Selective oxidation
Geraniol
Citral
Molecular oxygen
Supercritical carbon dioxide
Heterogeneous catalyst
1
. Introduction
Selective oxidation of alcohols to aldehydes and ketones is
have been reviewed [6]. Among them, use of supercritical carbon
dioxide for oxidation reactions is very appealing [7–10]. It is sta-
ble under any oxidative conditions and has excellent miscibility
with oxygen than conventional liquid solvents. It is known as an
environmentally benign reaction medium. It has liquid-like density
and gas-like viscosity, consequently leading to good solubility and
mass transfer. Furthermore, only slight changes in temperature and
an important transformation [1–4]. Traditionally, stoichiometric
amount of oxidants such as manganese dioxide or chromium tri-
oxide and/or homogeneous catalytic methods are employed [1–4].
Excessive use of these oxidants, tedious work-up of the hazardous
metal residues make such procedures undesirable. Moreover in
these reactions recyclability of homogeneous catalytic systems is
lacking. Many effective and recyclable transition metal and noble
metal based heterogeneous catalytic systems have been studied
◦
pressure around critical point of CO2 (Pc = 7.4 MPa, Tc = 31.1 C) can
affect the reaction rates remarkably, which is not the case with con-
ventional solvents. Oxidation of alcohols with molecular oxygen
(O ) in scCO2 has been studied with mostly supported noble metal
2
[
5]. However these catalytic systems are generally performed in
catalysts [11–14] and with supported CrO [15]. This method offers
3
volatile and/or toxic organic solvents.
a significant advantage in terms of yield, separation of catalysts and
simplicity of the reaction procedure.
In view of above alternative reaction media viz. water, super-
critical carbon dioxide (scCO ), polyethylene glycol, ionic liquids
Primary oxidation products of geraniol/nerol are Geranial (citral
A) and Neral (citral B) known together as citral (Scheme 1). Citral is
an important intermediate for production of perfumes, fragrances
and pharmaceuticals [16]. Supported noble metal catalysts like Pd,
Pt, Au [17–20], and Rh [21], RuCo–hydroxyapatite [22] have been
investigated for the oxidation of geraniol. Supported Pd has been
studied in detail for geraniol oxidation in continuous flow fixed
bed in scCO2 [18]. In this study, even though high citral selectivity
of 90% was noticed a conversion of only 20% was obtained at
2
ଝ
Supplementary information contains snapshots of phase behavior study of reac-
tion mixture in CO2. Note that the term “Supercritical” in this paper is used for a
reaction mixture exceeding the critical point of the solvent, despite several phases.
∗
Corresponding author. Present address: Tata Chemicals Limited, Innovation
Centre, Pune 412108, India. Tel.: +91 20 6654 9725; fax: +91 20 6654 9735.
∗
∗
Corresponding author.
E-mail addresses: sudhir.dapurkar@tatachemicals.com, sdapurkar@gmail.com
◦
(
S.E. Dapurkar), h-kawanami@aist.go.jp (H. Kawanami).
15 MPa of CO2 at 80 C. Metal containing mesoporous molecular
0
926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.01.001

2
10
S.E. Dapurkar et al. / Applied Catalysis A: General 394 (2011) 209–214
O2
OH
O
O
OH
Geraniol
Nerol
Geranial (citral A) Neral (citral B)
Citral
Scheme 1. Products of oxidation of geraniol.
sieves have been well reviewed as catalysts for various organic
transformations. They possess ordered pores, high surface area,
ease of recovery, regeneration, easy surface modification, etc.
tion); N2 adsorption (BEL SORP 28SA, BEL Japan); high resolution
transmission electron microscopy (HR-TEM, Tecnai 20 ST, FEG
200 kV); diffuse reflectance ultraviolet visible (DR UV–vis, Perkin
Elmer Instruments Lambda 35) spectroscopy. The metal content
was determined by atomic emission spectrometry with inductively
coupled plasma (ICP-AES, SPS 7800 Plasma spectrometer, Seiko
Instruments Japan).
[23,24]. Our group has extensively studied hydrogenation of citral
(reverse reaction) with mesoporous Pt [25a,b], Pt–Ru [25a] and Ni
[25c] catalysts in scCO2 previously.
We have investigated the selective oxidation of geraniol with
molecular oxygen over transition metal and noble metal containing
MCM-41 in scCO2.
2.4. Procedure for oxidation of geraniol
2
. Experimental
◦
Oxidation of geraniol was carried out at 80 C for 6 h in a stain-
2
.1. Chemicals
less steel high pressure reactor equipped with a specially designed
Teflon insert. In a typical experiment, 4 mmol geraniol and 30 mg
catalyst were placed inside the reactor vessel. The reactor was
flushed with 2 MPa CO2 followed by 1.0 MPa O2 (two times) and
Fumed silica (SiO , Aldrich); tetraethyl orthosilicate (TEOS,
5%, Aldrich); tetramethylammonium hydroxide (TMAOH,
6 wt.%, Wako Japan); sodium hydroxide (NaOH, Wako Japan);
2
9
2
◦
then heated to 80 C. To start the reaction, the reactor was charged
cetyltrimethylammonimum bromide (CTAB, 98%, Aldrich);
with 0.5 MPa O2 from the O2 cylinder followed by pumping CO2
using JASCO SCF-Get CO2 delivery pump to the desired pressure.
The total pressure of reaction system was maintained by SCF-
BPG/M (JASCO) back pressure regulator. After the reaction was
over, the reactor was cooled in an ice bath and the reactor slowly
depressurized through back pressure regulator. The reaction prod-
ucts were collected by simply conventional filtration of the reaction
chromium nitrate nonahydrate (Cr(NO ) ·9H O, Wako Japan);
3
3
2
cobalt nitrate hexahydrate (Co(NO ) ·6H O, Wako Japan); chloro
3
2
2
platinic acid (H PtCl ·xH O, 98.5%, Aldrich); palladium chloride
2
6
2
(
PdCl , Wako Japan); geraniol (96%, Tokyo Chemical Industry Co.,
2
Ltd. (TCI)); nerol (>98%, TCI); citral (99%, Aldrich); carbon dioxide
>99.99%) was supplied by Nippon Sanso Co. Ltd. All chemicals
were used as received.
(
◦
mixtures. Finally, the catalyst was separated and calcined at 540 C
for 6 h. The regenerated catalyst was used for recycling experi-
ments.
2.2. Catalyst preparation
CrMCM-41 catalyst was prepared as described previously [26]
with slight modification. 3.0 g SiO2 was slowly added to a mixture
of 2.63 g of aqueous TMAOH and 0.3 g NaOH dissolved in 28.0 g
distilled water under vigorous stirring for 30 min. Subsequently,
an aqueous suspension of CTAB (4.55 g in 14 g distilled water) was
added to the above mixture. Finally, desired amount of an aqueous
solution of Cr(NO ) ·9H O was slowly added to the above synthe-
2.5. Product analysis
The reaction products were analyzed by capillary gas chro-
matography using a FID Chromatograph (Varian CP 3800, column:
HP-5, 30 m (length) × 0.32 mm (inner diameter) × 0.25 ␮m (film
thickness)). The identity of the products was first confirmed by
using authenticated standards and their individual response factors
were determined by using ethylbenzene as an internal standard.
Additionally, identity of the reaction products was also confirmed
by using GC–MS (Varian CP 3800, 1200L Quadrupole MS/MS, col-
umn: HP-1, 30 m (length) × 0.32 mm (inner diameter) × 0.25 ␮m
(film thickness)).
3
3
2
sis gel and stirred for another 1 h. The final molar gel composition
is: SiO :0.15 NaOH:0.15 TMAOH:0.27 CTAB:60 H O:0.005 Cr O .
2
2
2
3
The resulting gel was transferred into Teflon-lined stainless steel
◦
autoclaves (ca. 100 ml), which were kept in an air oven at 100 C
for 24 h. The solid product obtained, designated as as-synthesized
CrMCM-41 was filtered off, washed with distilled water and dried
◦
at 80 C for 12 h. The as-synthesized sample was then calcined to
◦
remove the surfactant at 550 C for 8 h in air.
2
.6. Visual observation of the phase behavior in CO2
Similarly, CoMCM-41 were prepared according to the procedure
described above with Co(NO ) ·6H O as metal precursor. The molar
3
2
2
A high pressure view cell (JASCO; Model STR-458, internal vol-
gel composition is: SiO :0.15 NaOH:0.15 TMAOH:0.27 CTAB:60
2
ume 10 ml) was employed to observe the solubility of geraniol at
H O:0.005 Co O .
2
2
3
◦
8
0 C in CO2 at different pressures. The cell is equipped with sap-
The noble metals PtMCM-41 and PdMCM-41 were prepared
according to our earlier procedure described [25].
phire window and connected to a temperature-controlled bath. A
magnetic bar was used to mix contents in the viewing cell. Snap-
shots of phase behavior of the reaction mixture at pressures of 8, 10
and 14 MPa of CO2 are shown in supplementary information. The
solubility of geraniol increased with increasing CO2 pressure and a
2.3. Characterization
The catalysts were characterized by various techniques, viz.
X-ray diffraction (XRD, Rigaku: RINT 2200 VK/PC, CuK␣ radia-
single phase was reached above 12 MPa of CO2.

S.E. Dapurkar et al. / Applied Catalysis A: General 394 (2011) 209–214
211
(e)
(d)
(c)
(b)
(a)
2
4
6
8
10
2
θ
/ degree
Fig. 1. XRD pattern of (a) CrMCM-41, (b) CoMCM-41, (c) PtMCM-41, (d) PdMCM-41
and (e) CrMCM-41 after reaction.
2.7. Safety note
The experiments described in this paper involve use of high
pressure and require high pressure equipment.
3
. Results and discussion
3.1. Physicochemical properties of catalysts
Fig. 1 shows the powder XRD patterns of Cr, Co, Pt, Pd containing
MCM-41 catalysts. It can be seen from figure that XRD patterns
of all metal containing MCM-41 show three reflections at (1 0 0),
Fig. 3. HR-TEM of CrMCM-41: (a) calcined; (b) after reaction.
(
4
1 1 0), and (2 0 0), respectively. This is typical of mesoporous MCM-
1 hexagonal structure [27]. Furthermore, a representative XRD
isotherm as defined by IUPAC for mesoporous materials [28]. A
pattern of CrMCM-41 after the reaction is also included in Fig. 1.
Slight decrease in peak intensity was observed after the reaction but
still three clear reflections are noticed (Fig. 1e). This indicates the
intactness of mesoporous MCM-41 hexagonal structure even after
the reaction. This was further confirmed by N2 adsorption and HR-
TEM studies of CrMCM-41 after reaction (shown in Figs. 2b and 3b).
Fig. 2 shows N2 adsorption isotherm for the CrMCM-41 catalyst
before and after the reaction. It is a reversible type IV adsorption
sharp inflection between relative pressure P/P of 2–3 corresponds
0
to capillary condensation within the mesopores. The sharpness of
this step reflects the uniformity of the pores. Further the isotherm
also confirms the absence of any micropore filling at low P/P0.
After reaction, slight N2 adsorption decrease was noticed but the
nature of the isotherm remains the same (Fig. 2b). The specific sur-
face area (SBET), pore volume (Vp,BJH) and pore diameter (dp,BJH)
of parent MCM-41 and catalysts are shown in Table 1. They are
in good agreement with those typical for ordered mesoporous
type materials [27,28]. Porosity data of parent MCM-41 and metal
containing MCM-41 are more or less similar and comply with
those characteristics of mesoporous type materials. For exam-
ple, for CrMCM-41 before reaction the values were found to be
6
5
4
3
2
1
00
00
00
00
00
00
(
a)
(
b)
2
−1
−1
3
−1
2
−1
1
0
225 m g , 0.99 cm g , 2.4 nm, and after reaction 1190 m g ,
3
.97 cm g , 2.4 nm, respectively. Slight reduction of specific sur-
face area and pore volume noticed for the CrMCM-41 catalyst after
reaction is in tandem with those usual for mesoporous materials.
HR-TEM image of CrMCM-41 catalyst before and after the reac-
tion is shown in Fig. 3. It can be seen from figure that TEM of
CrMCM-41 before reaction showed a regular hexagonal array of
uniform channels of MCM-41-type material (Fig. 3a). The pore
diameter of catalyst before reaction determined from TEM was
approximately 2.5 nm comparable with pore diameter determined
by N2 adsorption isotherm. Similar hexagonal array pattern was
observed for CrMCM-41 after the reaction (Fig. 3b).
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Relative pressure (P/P)
Fig. 2. N2 adsorption isotherm of CrMCM-41: (a) calcined; (b) after reaction.

2
12
S.E. Dapurkar et al. / Applied Catalysis A: General 394 (2011) 209–214
Table 1
Physicochemical properties of various MCM-41 catalysts before and after reaction.
Catalyst
Metal-content
wt.%)
Specific surface area,
Specific pore volume,
Vp,BJH (cm
Pore size,
dp,BJH (nm)
2
−1
3
−1
g )
(
SBET (m
g
)
MCM-41
–
1227
1225
1190
1140
1205
1096
0.98
0.99
0.97
0.95
0.95
0.92
2.4
2.4
2.4
2.4
2.35
2.3
CrMCM-41
CrMCM-41a
CoMCM-41
PtMCM-41
PdMCM-41
1.2
1.1
0.9
1.0
1.0
a
CrMCM-41 after the reaction.
The metal content in the various catalysts was determined by
ICP-AES and shown in Table 1. Chromium and cobalt contents in
CrMCM-41 and CoMCM-41 catalysts were 1.2 and 0.9 wt.%, respec-
tively. Similarly platinum and palladium were 1 wt.%. In addition
chromium content after the reaction was 1.1 wt.%.
O
O
Cr
O
O
Si
Si
Scheme 2. Chromium species in CrMCM-41.
3.2. DR UV–vis of CrMCM-41 and CoMCM-41
3.3. Effect of various catalysts on geraniol conversion and
product yield
Fig. shows DR UV–vis of as-synthesized, calcined and
4
after reaction CrMCM-41. As-synthesized CrMCM-41 showed
absorption bands around 620, 440 and 270 nm, correspond-
To understand the oxidative efficiency of various mesoporous
catalysts, we studied the influence of these catalysts on oxidation
ing to 4A2g(F) → T2g(F), A2g(F) → T (F) and 4A2g(F) → T1g(P)
4
4
4
4
1g
of geraniol with O in scCO . Table 2 shows the influence of various
2
2
transitions, respectively, characteristic of trivalent chromium in
octahedral coordination [29]. The calcined CrMCM-41 catalyst
showed strong charge transfer (oxygen to hexavalent chromium)
band around 375 nm and a weak shoulder at 445 nm. These are
assigned to monochromate and/or polychromate like species in
tetrahedrally coordinated environments [29–33]. Catalyst after
the reaction also shows strong charge transfer bands around
catalysts on oxidation of geraniol with O2 in scCO . It can be seen
2
from Table 2, without catalyst only 9% conversion of geraniol was
obtained. The selectivity of citral A and citral B was 54% and 46%,
respectively (entry 1). While parent MCM-41 gave 10% conversion
of geraniol and the selectivity of citral A and citral B was 63% and
3
7%, respectively (entry 2). This indicates interconversion of two
isomers without catalyst [35]. It was noted that no product other
than citral A and citral B was noticed in case of without catalyst and
parent MCM-41.
On the other hand, CrMCM-41 (entry 3) shows good geraniol
conversion of 53% and excellent citral selectivity of 98% with yield of
3
70 nm and a very weak shoulder at 440 nm indicating retention of
chromium species after reaction. The proposed chromium species
in CrMCM-41 (Scheme 2) as characterized by DR UV–vis and EPR is
discussed in detail in the previous studies [26,31,32].
Whereas CoMCM-41 shows absorption bands at 540, 580 and
5
2% at 14 MPa of CO . This clearly indicates that catalyst is respon-
2
6
20 nm in the visible region (blue color sample). They are assigned
4
4
sible for the enhancement of conversion of geraniol. Other product
was geranic acid. Recycling study with CrMCM-41 shows almost
same conversion of geraniol and selectivity of citral even after third
cycle (entry 4). Chromium analysis of CrMCM-41 after third recy-
cling shows approximately 1.1 wt.% of Cr (Table 1). This suggests
that catalyst is stable in scCO2 media, in comparison to its highly
prone leaching in organic solvent [31,32,36].
to A (F) → T (P) triplet transition of Co(II) ions in the tetrahedral
2
1
environments [34].
O
O
O
VI
Cr
O
O
O
Cr
1b
+
O2
O
Si
O
Si
O
Si
O
Si
1
a
CHR
CHR
H
H
O
HO
(
(
c)
a)
O
OH
Cr
2
RCH OH
+
1b
2
O
O
Si
Si
(
b)
1
c
300
400
500
600
700
800
-2 H O
2
Wavelength / nm
1
c
2RCHO
+
1a
Scheme 3. Possible reaction pathways with CrMCM-41.
Fig. 4. DR UV–vis of CrMCM-41: (a) as-synthesized; (b) calcined; (c) after reaction.

S.E. Dapurkar et al. / Applied Catalysis A: General 394 (2011) 209–214
213
Table 2
Effect of catalysts on oxidation of geraniol with O2 in scCO2.
a
Entry
Catalyst
Conversion (%)
Selectivity (%)
Citral A
Yield of citral (%)
Citral B
Others
1
2
3
4
5
6
7
8
Blank
9
10
53
51
35
42
25
30
54
63
90
88
90
92
84
83
46
37
8
9
8
6
13
12
0
0
2
3
2
2
3
5
9
10
52
50
34
41
24
29
MCM-41
CrMCM-41
CrMCM-41
b
CrMCM-41c
CoMCM-41
PtMCM-41
PdMCM-41
a
◦
Reaction conditions: C(geraniol) = 4 mmol; m(catalyst) = 30 mg; t = 6 h; T = 80 C; PCO2 = 14 MPa; PO2 = 0.5 MPa.
b
c
3
rd run.
Without CO2.
Furthermore, without CO2 (solventless), geraniol conversion
3.4. Effect of CO2 pressure
of 35% was noticed with excellent selectivity of citral (entry 5).
Although citral selectivity for both reactions (entries 3 and 5) is
same but the geraniol conversion is enhanced when the reaction is
carried out in CO2 medium. It should be noted that phase behav-
ior study of reaction mixture showed single phase above 12 MPa of
The effect of CO2 pressure on conversion of geraniol and the
product selectivity of citral was investigated with CrMCM-41 at
80 C for 6 h and the results are shown in Table 3. It can be seen
from Table 3, CO2 pressure dramatically influences the conversion
of geraniol and it has slight effect on selectivity of citral A and cit-
◦
◦
CO2 at 80 C (see in supplementary information).
Scheme 3 shows the hypothetical reaction pathway of oxidation
ral B. Without CO , conversion of 35% was obtained (entry 1). As
2
of geraniol with O2 catalyzed by CrMCM-41 in scCO . Muzart [37]
CO pressure increases (entries 2–5), geraniol conversion decreases
2
2
reported reaction pathways for oxidation of alcohols catalyzed by
gradually from 64 to 53%. At 8–10 MPa of CO2 pressure, conversion
was high and the ratio of citral A/citral B was increased from 6.66
to 9.33. Under this biphasic (l/g) system, CO2 allows substantial
quantities of oxygen to solubilize, providing for high rate of reac-
tion. Previous work [38–40] has shown that one can achieve high
gas solubility and hence high reaction rate in such moderate pres-
sure system. Above 12 MPa, system transforms from two-phase to
one-phase (sc phase). This was observed by phase behavior study of
the reaction mixture (given in supplementary information). A mod-
erate conversion of 53% and good ratio of citral A/citral B (11.25)
was obtained at 14 MPa. Thus reaction parameters were studied at
this CO2 pressure.
However, further increase in CO2 pressure from 16 to 18 MPa,
conversion decreases drastically from 40 to 20%, respectively
(entries 6 and 7). However, the ratio of citral A/citral B was
high (15.33). At high CO2 pressure, the reaction system is diluted
as a consequence there is less interaction between the reac-
tant/intermediate and the catalyst leading to decrease in reaction
rate. Similar trend of decrease in activity and an increase in prod-
uct selectivity has been observed in other oxidation reactions using
CrO with tert-butylhydroperoxide (TBHP). This involves a peroxo-
3
metal pathway which preserves the Cr(VI) oxidation state. Based on
our earlier reports Cr in CrMCM-41 remains as a Cr(VI) oxidation
state (Scheme 2) [30,31]. Accordingly, we have proposed a reac-
tion pathway. A Cr(VI) species (1a) in CrMCM-41 (Scheme 2) may
interact with O2 to produce active oxidative Cr(VI) peroxo-species
(
1b). Similar type of Cr(VI) peroxo-species has been proposed as the
active species [36]. The latter reacts with RCH OH (for simplicity
2
geraniol denoted as RCH OH) and forms alkyl peroxochromium(VI)
2
intermediate (1c). Similar species have been proposed by differ-
ent groups [26,37]. The resulting species (1c) is then oxidized
to corresponding aldehyde (RCHO) with the elimination of
water.
It is important to note here that CoMCM-41 also showed good
conversion of 42% and selectivity of citral of 98% (entry 6). Noble
metal containing mesoporous catalysts (entries 7 and 8) showed
low conversion than CrMCM-41. Although selectivity of citral (cit-
ral A + citral B) was almost same citral A selectivity was less than
the latter. Geranic acid (3–5%) was obtained indicating that geraniol
oxidation with noble metal mesoporous catalysts is more sus-
ceptible to further oxidation [18]. The difference in conversion is
probably due to the different reaction pathways. As noble metal
catalysts following oxidation via dehydrogenative oxidative route
heterogeneous catalyst in scCO /O2 system [19,41].
2
3.5. Effect of reaction temperature
[
[
18], while transition metal catalysts follow peroxometal pathway
5]. Furthermore, deactivation of noble metal catalyst especially
Fig. 5 shows the effect of reaction temperature on the conversion
of geraniol with CrMCM-41 at 14 MPa of CO2 pressure. It can be
seen from figure that the geraniol conversion increases from 27 to
PdMCM-41 was noticed.
Table 3
a
Effect of CO2 pressure on oxidation of geraniol over CrMCM-41 with O2.
Entry
CO2 (MPa)
Phasesb
Conversion (%)
Selectivity (%)
Citral A
Citral A/citral B
Citral B
Others
1
2
3
4
5
6
7
0
8
s/l
35
64
60
56
53
40
20
90
80
84
87
90
92
92
8
12
9
9
8
2
8
7
4
2
2
2
11.25
6.66
9.33
s/l/g
s/l/g
s/sc
s/sc
s/sc
s/sc
10
12
14
16
18
9.66
11.25
15.33
15.33
6
6
a
◦
Reaction conditions: C(geraniol) = 4 mmol; m(CrMCM-41) = 30 mg; t = 6 h; T = 80 C; PO2 = 0.5 MPa.
b
s – solid phase (catalyst); l – liquid phase; g – gas phase; sc – supercritical phase.

2
14
S.E. Dapurkar et al. / Applied Catalysis A: General 394 (2011) 209–214
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[
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774–781.
1
00
8
6
4
2
0
0
0
0
0
Conversion of geraniol
Selectivity of citral A
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Acknowledgments
The financial support for this work was provided by the Japan
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.01.001.
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