Density-dependent formation of the pure trans-isomer of the unsaturated alcohol by selective hydrogenation of citral in supercritical carbon dioxide

Article abstract of DOI:10.1002/adsc.200700613

Selective hydrogenation of citral to unsaturated alcohol [geraniol (trans) + nerol (cis)] was carried out in supercritical carbon dioxide (scCO ₂) using an MCM-41 supported plantinum catalyst (～ 1 wt% Pt). A remarkable rate of isomerization of the unsaturated alcohol [nerol (cis) to geraniol (trans)] during the hydrogenation of citral was achieved simply by tuning the density of CO₂. Optimum reaction conditions were developed to obtain only geraniol (trans) with a selectivity of 98.8% and citral conversion of 99.8%. A significant change in the cis:trans ratio of the product (1:82.3) from the substrate (1:1.3) was observed depending on the various reaction parameters like carbon dioxide and hydrogen pressure, reactant concentration, reaction time and, particularly, the total selectivity for unsaturated alcohol [geraniol (trans) +nerol (cis)]. It has been observed that the presence of hydrogen is necessary for isomerization. Our results were explained in terms of a density-dependent, two-step model. The kinetic behaviour shows that the rate of isomerization was higher in scCO₂ compared to other organic solvents and the pure form of geraniol (trans) was obtained exclusively. A probable reaction pathway was proposed in order to explain the isomerization during hydrogenation of citral in scCO₂ medium.

Full text of DOI:10.1002/adsc.200700613

UPDATES
DOI: 10.1002/adsc.200700613
Density-Dependent Formation of the Pure trans-Isomer of the
Unsaturated Alcohol by Selective Hydrogenation of Citral in
Supercritical Carbon Dioxide
M. Chatterjee,a,* Y. Ikushima,a T. Yokoyama,a and M. Satoa
a
Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology, 4-2-1
Nigatake, Miyagino-ku, Sendai 983-8551, Japan
Fax : (+81)-22-237-5215; e-mail: c-maya@aist.go.jp
Received: December 26, 2007; Revised: February 5, 2008; Published online: February 26, 2008
Abstract: Selective hydrogenation of citral to unsa-
turated alcohol [geraniol (trans) + nerol (cis)] was
carried out in supercritical carbon dioxide (scCO2)
using an MCM-41 supported plantinum catalyst (~
1 wt% Pt). Aremarkable rate of isomerization of
the unsaturated alcohol [nerol (cis) to geraniol
(trans)] during the hydrogenation of citral was ach-
ieved simply by tuning the density of CO2. Opti-
mum reaction conditions were developed to obtain
only geraniol (trans) with a selectivity of 98.8% and
citral conversion of 99.8%. Asignificant change in
the cis:trans ratio of the product (1:82.3) from the
substrate (1:1.3) was observed depending on the
various reaction parameters like carbon dioxide and
hydrogen pressure, reactant concentration, reaction
time and, particularly, the total selectivity for unsa-
turated alcohol [geraniol (trans) +nerol (cis)]. It
has been observed that the presence of hydrogen is
necessary for isomerization. Our results were ex-
plained in terms of a density-dependent, two-step
model. The kinetic behaviour shows that the rate of
isomerization was higher in scCO2 compared to
other organic solvents and the pure form of gera-
niol (trans) was obtained exclusively. Aprobable re-
action pathway was proposed in order to explain
the isomerization during hydrogenation of citral in
scCO2 medium.
tion is the hydrogenation of a specific unsaturated
bond (C=O, C=C) leaving the other untouched. For
instance, citral is an unsaturated aldehyde, mainly ob-
tained as the mixture of cis and trans-forms with an
isolated C=C along with conjugated C=O and C=C
bonds. Hydrogenation of the conjugated C=O leads
to cis- and trans-isomers of the unsaturated alcohol
(nerol and geraniol, respectively). The trans-isomer of
the unsaturated alcohol (geraniol) having a rose-like
odour, mainly used in perfume can also act as an alle-
lochemical or secondary metabolite, which are indis-
pensable for the life process,[2] for antimicrobials, fun-
gicides and insecticides in the pharmaceutical, cos-
metic and flavour manufacturing industries.[3] In addi-
tion to that geraniol (trans) is the starting material for
the synthesis of Phomactin, which is a biological
active platelet-activating factor (PAF) antagonist and
lipid mediator.[4] Moreover, in the course of unsatu-
rated alcohol formation, a mixture of cis- and trans-
isomers was also obtained along with different types
of hydrogenated products and loss of the quality of
the desired product. Thus, the synthesis of only the
trans-isomer of the unsaturated alcohol from the cis/
trans mixture of the reactant is a challenging task.
Selective hydrogenation of citral has been widely
investigated, focusing particularly on the synthesis of
the unsaturated alcohol [geraniol (trans) + nerol
(cis)] using noble metals on different supports, bimet-
allic catalysts[5–7] or by addition of promoters and
basic catalysts[8] in various organic solvents.[9] Howev-
er, the production of the unsaturated alcohol has suf-
fered from the disadvantages of complex processes
leading to low yields, use of environmentally hazard-
ous organic solvents, transition metal catalysts, prob-
lems associated with catalyst-product separation and
side reactions such as decomposition, which take
place during distillation.
Keywords: density; cis/trans isomerization; mesopo-
rous material; selective hydrogenation; supercritical
carbon dioxide
Introduction
The selective hydrogenation of a,b-unsaturated alde-
Supercritical carbon dioxide (scCO2) regarded as a
hydes represents an important class of industrially rel- “green” reaction medium offers important advantages
evant reactions, useful in the specialty and fine chemi- over conventional organic solvents. The high miscibili-
cals fields.[1] The primary target of this type of reac- ty of scCO2 with many reactant gases can help to
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Selective Hydrogenation of Citral in Supercritical Carbon Dioxide
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avoid mass transfer limitations.[10] Moreover, the high catalytic system in scCO2 medium. From our previous
compressibility of scCO2,[11] could result in a signifi- studies[15] we have observed that citral hydrogenation
cant change in the chemo-, regio- and stereoselectiv- is strongly influenced by the nature of the metal ions
ity. Fine tuning of the solvent strength by minor and Pt is more favourable for the selective hydroge-
changes in pressure (larger density variations) can nation of C=O in scCO2. Thus, Pt-containing mesopo-
have a strong impact on chemical equilibrium and re- rous material was chosen to obtain high selectivity for
action rates in highly compressible supercritical geraniol (trans)+nerol (cis).
medium, and results in a strong control over selectivi-
ty in contrast to conventional organic solvents. Re-
cently, pressure-dependent product selectivity was Results and Discussion
achieved in the hydrogenation of citral in scCO2 using
various types of noble metals[12] and transition metal Catalytic Activity
catalysts.[13–15] However, most of the studies are based
on the feasibility of the process, performance of the Citral hydrogenation is a complex network process.
catalyst regarding selectivity to the unsaturated alco- Scheme 1 displays a possible hydrogenation product
hol (neglecting the isomerization) and comparison under the studied reaction conditions [T=35–808C;
with the conventional process.
PH =0.5–4 MPa; PCO =4–17 MPa; reaction time=4–
2
2
The objectives of this work is (1) to optimize the 6 h; amount of catalyst=0.025–0.2 g and reactant=
reaction conditions to obtain only the trans-isomer of 1.6–12.24 mmol] using a 50-mL autoclave. In the first
the unsaturated alcohol and (2) to understand the part, we will try to find out the optimum reaction con-
probable reaction pathway responsible for the signifi- ditions such as CO2 and H2 pressures, temperature,
cant stereoselectivity of the unsaturated alcohol, amount of citral, weight of the catalyst and also the
which is a difficult task especially for a heterogenous reaction time for the formation of geraniol (trans).
Scheme 1. Probable reaction pathway of citral hydrogenation in supercritical carbon dioxide.
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M. Chatterjee et al.
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The next part will cover the probable reaction path- was subsequently suppressed and geraniol (trans)+
way to the formation of the geraniol (trans) in scCO2 nerol (cis) was obtained with significantly higher se-
using Pt-MCM-41 catalyst.
lectivity (98.8%+1.2%). Acloser look at the selectiv-
ity for the unsaturated alcohol shows a clear correla-
tion between CO2 density and the cis:trans ratio
[nerol (cis); geraniol (trans)] [Figure 1 (bottom)]. In
accordance with the result, when the density of CO2
is low (4–6 MPa) and it is in the gas phase, the cis:-
trans ratio (substrate 1:1.3) does not change. Howev-
Optimization of Reaction Conditions
Effect of CO2 Pressure
The influence of CO2 pressure on the conversion and er, the ratio decreases significantly (cis:trans=1:82.3)
selectivity for the hydrogenation of citral is illustrated when CO2 is in the liquid phase. Asimilar pressure
in Figure 1 (top). All results are under standard con- effect was obtained during the cis/trans isomerization
ditions (temp.=708C, PH =4 MPa, time=4 h) unless of olefins in scCO2.[16] This finding was explained by
otherwise noted. The pro2duct selectivity result shows the formation of the trans- isomer in the liquid phase
that the geraniol (trans)+nerol (cis) was obtained as conditions due to the restricted withdrawl of the reac-
the principal product. At low pressure (density) of tion products from the catalyst surface which leads to
CO2, the hydrogenation of the C=C bond also oc- further reaction of the adsorbed species. Moreover,
curred and consequently the partially saturated alde- Pt-containing mesoporous Si was used as catalyst.
hyde (citronellal) and the saturated alcohol (citronel- Density-dependent adsorption of CO2 with different
lol) were formed. No dihydrocitronellal, dimethyloc- solutes on the catalyst surface is a well known phe-
tanol and ring closure products like isopulegol were nomenon.[17–19] We are aware that the density of the
detected. As the pressure (density) of scCO2 in- supercritical medium changes substantially either by
creased the selectivity for citronellal and citronellol changing the pressure or temperature. For example, a
change in pressure from 4 to 14 MPa the density
changes from 1.6181 to 10.406 mol/L (NIST Chem
web book) and the solubility of the citral increases.
According to the phase behaviour studies the solubili-
ty of citral increases with CO2 pressure (density) and
the single phase was obtained above 10.0 MPa at
708C. So, the CO2 pressure above 10 MPa was favour-
able for the adsorption of the reactant to the catalyst
acitve site. As the loading of the reactant increases
with increasing pressure, the conversion as well as se-
lectivity was enhanced and consequently the cis:trans
ratio was changed. But above 14 MPa the solubility as
well as adsorption does not change much leaving con-
version and selectivity nearly constant. Dillow et al.
proposed a two-step model to explain the density re-
lated cis,trans-isomerization in scCO2.[20] According to
the model, the cis-isomer will be converted to the
trans-isomer via an intermediate (not in the classical
sense). In the first step the solvent shelled cis will un-
dergo a structural change by overcoming the energy
barrier through the formation of an intermediate spe-
cies before concominant rearrangement of the solvent
shell. In the second step, a rearrangement will occur
to stabilize the new species and which is density de-
pendent. An increase in conversion from 34.3% to
99.8% was observed with the increase in pressure and
a S-type curve was obtained (Figure 1, top). The steep
part of the S-type curve can be attributed to the
change in citral-rich two phases at lower pressure (4–
Figure 1. ACHTREUNG(top) Effect of CO2 pressure on the conversion and
8 MPa) to a single phase at higher pressure (>
10 MPa). This phase transition obviously augmented
the conversion due to the increase in concentration of
citral in the CO2 phase.[21] Hence, the density varia-
tion can have a strong impact on the conversion and
the product distribution of citral hydrogenation. (bottom)
Variation of cis/trans ratio with the selectivity of unsaturated
alcohol. Reaction conditions: T=708C; reaction time=4 h;
PH =4 MPa; amount of catalyst=0.1 g; reactant=
2
6.24 mmol.
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Selective Hydrogenation of Citral in Supercritical Carbon Dioxide
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selectivity along with the cis:trans ratio of the unsatu-
rated alcohol and the maximum trans-selectivity was
obtained above 10 MPa of CO2 pressure.
Effect of Reaction Time
Figure 2 displays the effect of time on the hydrogena-
tion of citral under the standard reaction conditions.
The results indicate that there is a rapid hydrogena-
tion of citral during the first few minutes to citronel-
lal, citronellol and geraniol (trans)+nerol (cis). An
initial TOF of 0.152 sÀ1 to the formation of citronellal
was observed. However, after 2 h, a decrease in the
selectivity for citronellal and citronellol accompanied
with the substantial increase in the selectivity for ger-
aniol (trans)+nerol (cis) occurred. Initially, geraniol
(trans)+nerol (cis) was obtained with 79% selectivity
and reached a maximum during 4 h of reaction. The
high selectivity for geraniol (trans)+nerol (cis) in
scCO2 medium can be explained by a specific interac-
tion between CO2 and the Lewis base groups like the
C=O group of the reactant molecule, which favoured
Figure 2. Effect of time variation on conversion and selectiv-
ity. Reaction conditions: T=708C; PCO =14 MPa; PH
=
2
2
4 MPa; amount of catalyst=0.1 g; reactant=6.24 mmol.
À
the formation of C H–-O with the hydrogen atom at-
tached to the carbonyl carbon. Consequently, the re-
activity of the C=O bond increases followed by the
hydrogenation of the C=O bond.[22] Thus, from the
molecular point of view scCO2 medium was adding
extra advantages regarding the selective hydrogena-
tion of C=O.[23] Following the selctivity for geraniol
(trans)+nerol (cis), it was observed that geraniol
(trans) was obtained with enormous selectivity. The
reason could be the nerol (cis) is thermodynamically
unstable and prone to isomerize back to the thermo-
dynamically stable geraniol (trans), which became the
predominant product.[24]
Figure 3. cis/trans Ratio and selectivity to geraniol (trans)+
nerol (cis) as a function of H2 pressure. Reaction conditions:
T=708C; PCO =14 MPa; reaction time=4 h; amount of cat-
2
alyst=0.1 g; reactant=6.24 mmol.
Effect of H2 Pressure
The effect of hydrogen pressure is an important pa- changed along with the hydrogen pressure. Figure 3
rameter to describe the product distribution of citral shows the variation of cis:trans ratio and selectivity
hydrogenation in scCO2. The reaction was carried out for geraniol (trans)+nerol (cis) as a function of hy-
at different pressures of hydrogen keeping the other drogen pressure. It has to be mentioned that the cis:-
parameters constant. As the pressure changes from 2 trans ratio of the reactant was 1:1.3 but the ratio
to 4 MPa, the conversion of citral increases from changes dramatically to 1:82.3 as the hydrogen pres-
62.7% to 99.8% and maintains a first-order reaction sure increased to 4 MPa. Hence, hydrogen pressure
rate. Since selectivity for the consecutive reaction de- must be an important parameter obviously for hydro-
pends on the conversion; the increase in conversion genation as well as for isomerization to occur. To
with hydrogen pressure might have some effect on check the effect of hydrogen, we have carried out the
the selectivity. Asignificant increase in the selectivity
reaction without hydrogen keeping the other parame-
from 39.9% to 100% for geraniol (trans)+nerol (cis) ters (PCO =14 MPa; time=4 h and temperature=
2
was observed as the hydrogen pressure increased 708C) constant (Figure 3). No isomerization (cis to
from 0.5 to 4 MPa and remained constant until trans of citral) took place. After the addition of
4.5 MPa but decreased when the pressure reached 0.2 MPa of hydrogen, though, the conversion was sub-
6 MPa. Interestingly, the cis:trans ratio of the unsatu- stantially lower but isomerization of cis- to trans-unsa-
rated alcohol [geraniol (trans)+nerol (cis)] was also turated alcohol took place and the cis:trans ratio was
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M. Chatterjee et al.
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changed to 1:1.8. An increase in hydrogen pressure to
1 MPa results in afurther increase in conversion and
also favoured the cis-trans isomerization of the unsa-
turated alcohol (Figure 3), which confirmed that the
isomerization was closely related to the hydrogena-
tion. According to these results (Figure 3), hydrogen
pressure of 2–4 MPa promoted hydrogenation and
isomerization but at the 6 MPa of hydrogen the over-
all selectivity to geraniol (trans)+nerol (cis) decreas-
es. The decrease in selectivity could be due to the
transition of a single phase to a biphasic system. Ad-
dition of hydrogen in the citral+CO2 binary system
resulted in a shifting of the gas-liquid phase boundary
to the higher pressure. Thus, an increase in pressure
of hydrogen above 4.5 MPa is more likely to cause
the phase separation and the system was unable to
maintain its single phase status. Generally, it is accept-
ed that the concentration of the hydrogen adsorbed
on the catalyst is the factor that determines selectivity
and isomer formation. If the coverage of hydrogen on
the metal surface is high, the probability is higher
that two atoms are in a position to react with any
double bond on approach. This will nevertheless
result in low selectivity, because saturation of any
double bond approaching to hydrogen will occur
promptly. However, if there are only a few hydrogen
atoms adsorbed, it is more likely that only one hydro-
gen atom will react with the double bond, producing
the half-hydrogenation-dehydrogenation sequence
which increases the probability of isomerization.
Figure 4. ACHTREU(NG top) Effect of temperature on citral conversion
with time. (bottom) Arrhenius plot Reaction conditions:
PCO =14 MPa; PH =4 MPa; amount of catalyst=0.1 g; reac-
2
2
tant=6.24 mmol.
Hence, there must be an optimum hydrogen concen- ry to enhance the activation of the C=O bond.[25] Ac-
tration (4–4.5 MPa) that governs the highest rate of cordingly, the increasing selectivity to geraniol
hydrogenation and isomerization. Below that pressure (trans)+nerol (cis) changes the cis:trans ratio from
(<4 MPa) the conversion is low but selectivity to the 1:1.3 to 1:82.3. The product distribution does not
trans-isomer of unsaturated alcohol was prominent. If change much as the temperature increased to 808C.
the hydrogen pressure is higher than 4.5 MPa, hydro- From the above results, it could be suggested that an
genation dominates the reaction pathway.
optimum temperature of 708C is necessary to achieve
an excellent trans selectivity. Therefore, we have se-
lected this temperature to study the other parameters
affecting the conversion and selectivity. From the ini-
tial reaction rate an Arrhenius plot was obtained and
Effect of Temperature
To check the effect of temperature, citral hydrogena- the activation energy of 2.5 kJmolÀ1 was calculated
tion was conducted at 358C, 508C, 708C and 808C at from the slope of the plot (Figure 4, bottom).
14 MPa of CO2 with the constant hydrogen partial
To summarize, the results of the reaction time, tem-
pressure of 4 MPa (Figure 4, top). As expected the re- perature and hydrogen pressure indicate that the high
action was slow at 358C and a conversion of ca. 45% conversion leads to the high selectivty for geraniol
and the 57.5% selectivity for unsaturated alcohol [ger- (trans)+nerol (cis). Acomparison was made between
aniol (trans)+nerol (cis)] were observed. When the the selectivity (57.5% at 358C; 67.9% in 15 min and
reaction temperature was increased to 708C a consid- 39.9% at 0.5 MPa of hydrogen) obtained at the lowest
erable improvement in the conversion (99.8%) and conversion level to check the influence of the each of
the selectivity of 98.8% to geraniol (trans) was ob- these individual parameters on the selectivity. These
tained. These results indicate that at the lower tem- data indicates that the lowest selectivity was found at
perature the hydrogenation of C=C was favoured but lower hydrogen pressure. Thus, hydrogen pressure is
at higher temperature C=O hydrogenation predomi- an important parameter, which strongly affects the se-
nates as the gas phase bond dissociation energy of the lectivity for geraniol (trans).
C=O (174 kcalmolÀ1) is higher than that of C=C
(141 kcalmolÀ1). Thus, higher temperature is necessa-
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Selective Hydrogenation of Citral in Supercritical Carbon Dioxide
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Figure 5. Reaction order with respect to the citral concentra-
tion. Reaction conditions: T=708C; PCO =14 MPa; PH
=
Figure 6. Effect of the catalyst amount on the hydrogenation
2
2
4 MPa; amount of catalyst=0.1 g.
of citral in scCO2. Reaction conditions: T=708C; PCO
=
2
14 MPa; PH =4 MPa; reaction time=4 h; reactant=
2
6.24 mmol.
Effect of Amount of Citral
Citral hydrogenation reactions were also conducted
with varying amounts of citral (1.6, 3.12, 6.24 and
12.24 mmol) to examine the differences in the product
distribution and to study the dependence of the initial
TOF on reactant concentrations. Irrespective of the
amount of the citral, geraniol (trans)+nerol (cis) was
the main product. However, according to the conver-
sion (99.8%) and selectivity (100%), 6.24 mmol of
citral shows the best result. With 12.24 mmol of citral,
the rate was slowest. To evaluate the reaction order
with respect to citral, ln
[citral], where [citral] is the initial citral amount and
the slope of the plot gives the reaction order Figure 7. Effect of organic solvents and no solvent condi-
ACHTRE[UNG TOF] was plotted against ln
ACHTREUNG
tions on the conversion and selectivity of citral hydrogena-
(Figure 5). Under the studied reaction conditions, the
system exhibits the reaction order of 0.9715Æ0.1,
which is in contrast to the result of Singh et al.[26]
They have reported a zero order reaction with respect
to citral concentration for citral hydrogenation in
hexane using Pt/SiO2 catalyst. This result seems to
tion. Reaction conditions: T=708C; PCO =14 MPa; PH
=
2
4 MPa; time=4 h; organic solvent=10 mL2; amount of cata-
lyst=0.1 g; reactant=6.24 mmol.
show that, compared to the mesoporous catalyst in niol (trans)+nerol (cis) changed from 48.8% to 100%
scCO2, the deactivation process is faster for a conven- as subsequently did the cis/trans ratio.
tional catalyst in an organic solvent medium.[27,28]
Supercritical Carbon Dioxide vs. Organic Solvent
(h3)
Effect of Catalyst Amount
To check the effect of the catalyst amount, on conver- Figure 7 represents the results of citral hydrogenation
sion and selectivity, citral hydrogenation was carried in different organic solvents and also in the solvent-
out for four different weights (0.025, 0.05, 0.1 and less condition. Compared to scCO2, conversion was
0.2 g) of the catalyst. Figure 6 illustrates that the con- lower in all organic solvents. Except benzene, mainly
version and selectivity change with the amount of cat- citronellal was obtained with high selectivity in each
alyst. For 0.025 g of catalyst ca. 15% conversion was solvent. Amaximum selectivity of 70% of the [gera-
obtained after 4 h of reaction suggesting that the rate niol (trans)+nerol (cis)] at ca. 50% conversion was
of the reaction decreased and more time was necessa- achieved in benzene and the cis:trans ratio changed
ry to reach complete conversion of citral. As the to 1:2.8. Considering the data obtained at 8 MPa of
amount of catalyst was changed from 0.025 g to 0.1 g, CO2 (because of the same conversion of ca. 50%), it
the product distribution and the selectivity for gera- was observed that the rate of isomerization in scCO2
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M. Chatterjee et al.
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(9.76”10À4 mmolsÀ1 gÀ1) is greater than that in ben-
zene (3.2”10À4 mmolsÀ1 gÀ1). It might be suggested
that, in the studied reaction conditions and al though
this type of isomerization was also observed during
hydrogenation in conventional organic solvents like
benzene, the cis:trans ratio of the product (1:2.8) does
not differ much from that of the reactant (cis:trans=
1:1.3). Hence, the formation of the unsaturated alco-
hol and consequently the pure trans-isomer is pre-
ferred in scCO2 under the studied reaction system.
Figure 8. Dependence of hydrogenation rate with each
Effect of cis:trans Ratio
isomer concentration. Reaction conditions: T=708C; PCO
=
2
14 MPa; PH =4 MPa; amount of catalyst=0.1 g; reactant=
6.24 mmol.
2
To check the effect of the cis:trans ratio of the sub-
strate on the product distribution, a series of citral
samples with different cis:trans compositions was hy-
drogenated in scCO2 and the results are shown in
Table 1. Independent of the cis:trans ratio of the sub- ture of 708C in scCO2 within the reaction time of 4–
strate, geraniol (trans) was the main product. From 6 h using Pt-MCM-41 as catalyst.
the entry 1, where mainly cis-isomer was taken, a
70% selectivity for the trans-product was observed
and the cis:trans ratio changes substantially. Similarly, Probable Reaction Path of Geraniol (trans-Isomer)
the cis:trans ratio=1:1 (entry 3) of citral also changes Formation
significantly to 1:49.1 in the product after the hydro-
genation in scCO2. Thus, cis- to trans-isomerization of It is well known that the product distribution pattern
the unsaturated alcohol occurred successfully during is associated with the stability of intermediate species
citral hydrogenation in scCO2 using Pt-MCM-41 as on the catalyst surface at a particular pressure.[29]
catalyst.
Figure 2 reveals that ca. 10–15% of citronellal and cit-
The conversion profile of cis- and trans-isomers of ronellol were obtained (the reaction was carried out
citral during hydrogenation in scCO2 medium shows at 14 MPa) in the beginning of the reaction and di-
that the concentrations of the cis- and trans-isomers minish at the end of the reaction. Hence, there might
decrease with time. The cis- and trans-isomers show be the possibility of the dehydrogenation of citronel-
first-order kinetics with respect to the citral concen- lal or citronellol. To confirm this we have carried out
tration. Alinear trend describes the monotonic de-
the direct hydrogenation of citronellal and citronellol
crease in the concentration (Figure 8). The decrease separately at the higher pressure (14 MPa) at 708C
of selectivity for nerol (cis) selectivity and increase for 4 h. Interestingly, at higher pressure the direct hy-
for geraniol (trans) was observed after the complete drogenation of citronellal to citronellol was observed
conversion of citral and attributes the high rate of until 1 h and then it was dehydrogenated to citral. For
conversion of the cis-isomer to the trans-isomer.
citronellol also dehydrogenation to citronellal fol-
From the above discussion it was observed that the lowed by citral was confirmed. Although we do not
unsaturated alcohol, mainly geraniol (trans), was ob- have any molecular level explanation at this stage, it
tained with high selectivity from the hydrogenation of might be suggested that the hydrogenation of citral to
citral at optimum conditions of 4 MPa of hydrogen, citronellal and citronellol is a reversible process and
14 MPa of CO2, 6.24 mmol of citral and the tempera- thermodynamically controlled. On the other hand, at
short reaction time a higher selectivity for geraniol
(trans) was observed [Figure 2; selectivity of geraniol
(trans) is about 70% within 15 min]. It seems that the
geraniol (trans) is formed from the direct hydrogena-
Table 1. Effect of cis:trans ratio of the substrate on the pro-
duct.[a]
tion of the reactant and the reaction path is kinetical-
Entry
cis:trans (Substrate)
cis:trans (Product)
ly controlled We hypothesize here that the trans-
isomer could be formed from the (i) direct hydrogena-
tion of the reactant and also (ii) as the time increases
the probability of concentration of hydrogen increases
in the medium and disturbed the equlibrium between
citral, citronellal and citronellol, consequently the ki-
netically controlled path predominates.
1
2
3
1:0.07
1:10.9
1:1
1:0.65
No cis product
1:49.1
[a]
Reaction conditions: reactant=6.24 mmol; catalyst=
0.1 g; T=708C; PCO =14 MPa; PH =4 MPa; reaction
2
2
time=4 h.
630
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 624 – 632

Selective Hydrogenation of Citral in Supercritical Carbon Dioxide
UPDATES
study demonstrates the versatility of the medium in-
stead of being a simple “substitute solvent” to the
conventional organic solvents.
Experimental Section
Materials
Citral (95% cis:trans=1:1.3), citronellal (90%), citronellol
(96%), geraniol (98%) and nerol (97%) were purchased
from Aldrich and used as received. Citral samples contain-
ing a cis:trans ratio of 1:0.07, 1:10.9 and 1:1 were obtained
from Kuraray Chemicals Co. Carbon dioxide (>99.99%)
was supplied by Nippon Sanso Co. Ltd. Tetraethyl orthosili-
cate (TEOS) and chloroplatinic acid were obtained from
Wako Japan and Aldrich, respectively. Cetyltrimethylammo-
nium bromide (CTAB) from Aldrich, was used as template
for the synthesis of the catalyst to obtain the mesoporous
structure.
Scheme 2. Probable reaction path of cis to trans isomeriza-
tion during citral hydrogenation in scCO2.
As the unsturated alcohol was the predominate
product under the studied reaction conditions it could
be suggested that the citral molecules adsorb strongly
through the C=O using the most preferable adsorp-
Catalyst Synthesis
tion geometry of di-sCOh2 mode[30,31] at the expense of Pt-MCM-41 catalyst was prepared by an in situ addition of
1 wt% solution of Pt salt (H2PtCl6·xH2O) in a mixture of
surfactant and silica precursor as described elsewhere.[33]
Total amount of metal loading was ca. 1%.
formation of a bond with surface Pt atoms. Adsorbed
hydrogen occupied all the catalyst surface. Hence, the
activated C=O penetrate along the Pt H through the
surface. Then an intermediate is formed between sur-
face Pt and the citral molecule. Addition of a second
hydrogen atom and subsequent desorption of product
À
Procedure for Citral Hydrogenation in scCO2
The hydrogenation of citral was conducted in a stainless
steel batch reactor (50 mL). The detailed method was de-
scribed previously.[15] Briefly, 0.1 g of catalyst and 6.24 mmol
of citral were loaded into the reactor and placed inside the
oven at the desired reaction temperature. Hydrogen was
first loaded into the reactor followed by the liquid CO2
using a high-pressure liquid pump. Products were identified
by GC/MS and analyzed quantitatively by GC (HP 6890)
equipped with flame ionization detector (FID). The selectiv-
ity to each product was calculated by the following expres-
sion Si =Ci/ꢀCp, where Ci is the concentration of the prod-
uct ꢁiꢂ and ꢀCp is the total concentration of the product.
Citral hydrogenation in an organic solvent was carried out
under the same reaction conditions as of scCO2 except that
10 mL of solvent were used instead of CO2.
À
followed by the rotation of C C bond results in iso-
merization[32] (Scheme 2).
Conclusions
In conclusion, we have demonstrated that scCO2 is an
attractive medium to carry out the selective hydroge-
nation of citral (cis:trans=1:1.3) to produce only ger-
aniol (trans) with remarkable selectivity of ca. 98.8%
at a very high conversion of 99.8%. The selectivity for
geraniol (trans) strongly depends on the several pa-
rameters such as CO2 and hydrogen pressure, reaction
time, citral concentration and the reaction tempera-
ture. To study the possible reaction path to the forma- Separation of Geraniol (trans) from Nerol (cis)
tion of geraniol (trans) a significant effect of CO2
pressure (density) was observed. At higher density of
As the separation of isomers was beyond the scope of this
work, primarily, a very small-scale separation of geraniol
CO2, where the solubility of citral was high the hydro-
genation of C=O occurred and the unsaturated alco-
hol was obtained with high selectivity. It has also
been observed that hydrogen has an essential part in
the isomerization. In scCO2 the reaction kinetics ex-
hibited a first-order reaction rate on citral concentra-
tion as well as hydrogen pressure suggesting that the
catalyst is free from deactivation. Compared to other
organic solvents the rate of isomerization was higher
in scCO2. The formation of several kinds of by-prod-
ucts that usually occur in organic solvents can be inva-
(trans) from nerol (cis) was tried using the obtained product
in ether+anhydrous CaCl2. Awhite solid of the geraniol·-
CaCl2 complex was formed at low temperature while nerol
(cis) remained in solution. The resultant white solid was
then washed with ether and decomposed in warm water fol-
lowed by the formation of geraniol (trans) and CaCl2. A l-
though the result of separation was satisfactory, a lot of
work is still necessary to optimize the separation conditions.
Phase Behaviour Studies
3
A10-cm high-pressure view cell equipped with sapphire
lidated by using scCO2 as reaction medium. This window was used to determine the solubility of the reactant
Adv. Synth. Catal. 2008, 350, 624 – 632
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
631

M. Chatterjee et al.
UPDATES
at 708C in scCO2 at different pressures. The method is the
same as that described previously.[15] This process was re-
peated at the intervals of 2 MPa of CO2 pressure. The solu-
bility of the substarte increased with increasing CO2 pres-
sure and a single phase was reached above 10 MPa of CO2.
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[14] M. Chatterjee, A. Chatterjee, Y. Ikushima Green
Chem. 2004, 6, 114.
[15] a) M. Chatterjee, F. Y. Zhao, Y. Ikushima Adv. Synth.
Catal. 2004, 346, 459; b) M. Chatterjee, F. Y. Zhao, Y.
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Acknowledgements
The authors would like to thank Kuraray Co. Ltd (Japan) for
supplying the sample of citral.
[17] P. Subra, A. Vega-Bancel, E. Reverchon, J. Supercrit.
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632
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 624 – 632