Advantageous heterogeneously catalysed hydrogenation of carvone with supercritical carbon dioxide

Article abstract of DOI:10.1039/c1gc15495h

The hydrogenation of carvone was investigated for the first time in high-density carbon dioxide. The hydrogenation over 0.5 wt% Pd, or Rh, or Ru supported on alumina was found to be generally faster in a single supercritical (sc) phase (fluid reagents) than in a biphasic system (liquid + fluid reactants). The reaction with Pd produced fully hydrogenated products (isomers of carvomenthone) and carvacrol. The Rh catalyst was more selective and favoured carvomenthone isomers with higher selectivity and carvotanacetone as a secondary product. Additionally, the rhodium catalysed reaction exhibited high > 84% selectivity of carvotanacetone with the conversion of > 25% after only 2 min of reaction. The less active Ru catalyst gave significantly lower conversion and the product variety was greater as carvomenthone isomers, carvotanacetone and carvacrol were formed. The conversion and selectivity to carvomenthone within 2 h of the reaction starting followed the order: Pd > Rh > Ru and Rh > Pd > Ru, respectively. High conversion, and diverse and high selectivity accompanied by significant reduction in reaction time depending on the catalyst were achieved in supercritical CO₂ compared with hydrogenation occurring in conventional organic solvents.

Full text of DOI:10.1039/c1gc15495h

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Green Chemistry
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PAPER
Advantageous heterogeneously catalysed hydrogenation of carvone with
supercritical carbon dioxide
Catarina I. Melo,a Rafał Bogel-Łukasik,a,b Marco Gomes da Silvaa and Ewa Bogel-Łukasik*a
Received 3rd May 2011, Accepted 26th July 2011
DOI: 10.1039/c1gc15495h
The hydrogenation of carvone was investigated for the first time in high-density carbon dioxide.
The hydrogenation over 0.5 wt% Pd, or Rh, or Ru supported on alumina was found to be
generally faster in a single supercritical (sc) phase (fluid reagents) than in a biphasic system (liquid
+ fluid reactants). The reaction with Pd produced fully hydrogenated products (isomers of
carvomenthone) and carvacrol. The Rh catalyst was more selective and favoured carvomenthone
isomers with higher selectivity and carvotanacetone as a secondary product. Additionally, the
rhodium catalysed reaction exhibited high > 84% selectivity of carvotanacetone with the
conversion of > 25% after only 2 min of reaction. The less active Ru catalyst gave significantly
lower conversion and the product variety was greater as carvomenthone isomers, carvotanacetone
and carvacrol were formed. The conversion and selectivity to carvomenthone within 2 h of the
reaction starting followed the order: Pd > Rh > Ru and Rh > Pd > Ru, respectively. High
conversion, and diverse and high selectivity accompanied by significant reduction in reaction time
depending on the catalyst were achieved in supercritical CO2 compared with hydrogenation
occurring in conventional organic solvents.
of terpenes in scCO2, the effects of hydrogen pressure10 and flow
Introduction
rate11 as well as tuning the selectivity towards an intermediate
Selective hydrogenation of organic molecules plays an important
by use of solid catalysts with ionic liquid layer (SCIL) have been
role in the synthesis of fine chemicals on a multi-ton scale
studied.12
via heterogeneous catalysis.1 Supercritical fluids are considered
to be green solvents employed in a diverse range of chemical
Caraway, a major source of carvone (C10H14O, 2-methyl-5-
(1-methylethenyl)-2-cyclohexenone), is one of the oldest spices
reactions, including both homogeneous and heterogeneous
cultivated in Europe and is used as an alternative medicine in
catalytic processes.2,3 Among scF, supercritical CO2 (scCO2)
colic treatment, as a laxative, or a breath freshener.13 Carvone,
has received considerable attention as a useful replacement of
a terpenoid, found application as a fragrance and flavour,
organic solvents. The solvent power of scCO2 can be easily
potato sprouting inhibitor, antimicrobial agent, building block
tuned by changes in both temperature and pressure. scCO2 can
and biochemical environmental indicator.13 The hydrogenation
reduce the undesirable by-products yielding a higher end product
quality.4,5
of carvone has usually been carried out with heterogeneous
supported mono- and bimetallic catalysts. It can be noticed that
Terpenes and terpenoids are an important class of naturally
reaction times are very long14 (generally from 10 h up to even
occurring compounds widely employed in organic synthesis.6
more than 45 h). The main problem met is the conversion of
Our previous work published in Green Chem. discussed the
successful hydrogenation of terpenes (C10H16: limonene7 or
myrcene8) in high pressure assisted CO2 in biphasic or monopha-
sic conditions7,8 due to a favourable solubility relationship
between terpenes and carbon dioxide allowing an easy switch
from two phases to a one phase system.9 In the hydrogenation
carvone15 which can vary from 10%, after 40 h for Pt modified
by Sn, up to 100% at 20 h reaction time for Pt/C catalyst.16 The
access of reagents to the catalyst bed seems to play a crucial rule
in the liquid phase hydrogenation and can be recognized as a
limiting factor for conversion. An increase in the reaction time
or temperature (40 ◦C and 90 ◦C) did not solve the problem of
low conversion (i.e. after 45 h conversion was 30% with PtSn/C-
HP17 while the same catalyst modified by Ge after 10 h led to
minimal conversion18). For these reasons various modifications
of the reaction parameters have been tested, i.e. at 100 ◦C, high
acidic catalysts on zeolites19 or Pd–Cu.14 Even reaction at 12
MPa compared to atmospheric pressure14–19 at 10 ◦C–50 ◦C
aREQUIMTE, Departamento de Qu´ımica, Faculdade de Cieˆncias e
Tecnologia, Universidade Nova de Lisboa, 2829-516, Caparica, Portugal.
E-mail: ewa@dq.fct.unl.pt; Fax: (+351) 212948550
bLaborato´rio Nacional de Energia e Geologia, I.P., Unit of Bioenergy,
Estrada do Pac¸o do Lumiar 22, 1649-038, Lisboa, Portugal
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21
with platinum, palladium20 or RANEYꢀ nickel allowed the
reaction to reach 50% conversion. At 21 ◦C with a rhodium
supported complex catalyst a 100% conversion was obtained
after 23 h.22 Comparing with the long hydrogenation of carvone,
the use of heterogeneous hybrid diamine-Rh complex-carbon
of the homogeneous Rh(NN)Si catalyst reduced the reaction
time. At 60 min nearly a 100% conversion of carvone was
achieved.23 A test of the homogeneous Rh(NN)Si catalyst in
the hydrogenation of carvone showed a very low activity of this
complex with conversion of only 6% after 24 h.23 This negligible
conversion in a homogeneous system underlines the problem
appearing with the heterogeneous catalysts used and highlights
the difficulty in obtaining significant and sufficient conver-
sion of carvone in the heterogeneously catalysed liquid phase
hydrogenation.
R
Due to difficulties associated with the use of conventional
homogeneous and heterogeneous catalysts such as the mass
transfer limitation that frequently occur between the reactants
in the gas and liquid phase, scCO2 can be regarded as an ideal
medium for hydrogenation because, unlike organic solvents,
H2 is completely miscible in CO2. Applications of scCO2 to
hydrogenation involving heterogeneous catalysts are particularly
attractive.3,24 The present work is a continuation of our studies
on the employment of supercritical carbon dioxide in the hetero-
geneous catalysis of monoterpenes.4,7,8,10,11,12,25–28 In comparison
with conventional organic solvents, the use of scCO2 allowed us
to get higher diffusivities and lower viscosities.7 In consequence,
the hydrogen concentration at the catalyst surface was greatly
increased and high reaction rates were obtained compared to
the normal liquid phase operation. The mixture of carvone and
CO2 exhibits advantageous low critical pressure at moderate
temperatures allowing an easy switch from two phases to a
homogeneous one-phase fluid.29,30
It can be assumed that the use of scCO2 might help to
increase carvone conversion and decrease the reaction time in
hydrogenation compared to classical liquid phase hydrogena-
tion. To simplify the reaction monosupported commercially
available catalysts were used in this work. As the most active
palladium, followed by Ru and Rh were selected.8,10,27,31 As far
as we are aware, this is the first report on hydrogenation of
carvone where supercritical carbon dioxide instead of organic
solvents is used, either in an expanded-liquid solvent (biphasic)
system or in a single supercritical phase. The hydrogenation of
carvone involving the key compounds is presented in Fig. 1.
Fig. 1 The chemical structure of carvone and the hydrogenation
products: a) carvone, b) trans-carvomenthone, c) cis-carvomenthone,
d) carvotanacetone, e) carvenone, f) carvomenthol, g) carvacrol.
means of ThermoElectron Atomic Absorption Spectroscope
S Series.
Hydrogenation
The hydrogenations were performed in an apparatus with a
sapphire-windowed cell connected by a pump to a tubular
reactor which encloses a catalyst. This apparatus has already
been described in detail in the literature.7,8 The reactants (either
CO2-expanded liquid terpenoids or a single supercritical phase,
where liquid and gases produce one fluid phase) circulate
continuously from the bottom of the view cell, through the
catalyst bed, and back to the upper entrance of the cell. The flow
rate of 3.3 mL min-1 employed guarantees that during 1–2 min
the whole reaction mixture recirculates at least once.10,27 Due to
this, the feed composition that simulates the process performed
in the batch mode changes rapidly. The reaction samples were
taken at regular intervals through the HPLC valve equipped
with a 100 ml sampling loop. The contents of the loop were
dissolved in a measured amount of pentane with nonane used
as an internal standard for the sample analysis. CO2 was vented
to the atmosphere. For each reaction 1.5 g of the fresh catalyst
and 2 mL of carvone were used.
The qualitative identification of the products of reaction was
performed by GC/TOFMS to calculate retention indices (linear
retention indices - LRIs) according to Van Den Dool and Kratz
method32 using a mixture of C8 to C27 hydrocarbons (AccuS-
tandard, New Haven, CT, USA) in pentane. The obtained LRIs
were compared with the individual spectra with the NIST 2005
MS library using the ChemStation software.33
Experimental method
Chemicals
Hydrogen and carbon dioxide were supplied by Air Liquide
with a stated purity of 99.998 mol%. R-(-)carvone (purity:
>99.0%) and carvacrol (purity: 95.0%) were supplied by TCI
Europe. Carvomenthone, sum of isomers, (purity: 98.0%) was
delivered from Bedoukian Research, Inc. These substances were
used as received without any further purification. The 0.5 wt%
Pd/Al2O3, 0.5 wt% Rh/Al2O3, 0.5 wt% Ru/Al2O3 catalysts as
pellets were bought from Aldrich.
The quantitative analysis was carried out by gas chromatogra-
phy by means of Varian Chrompac CP-3800 gas chromatograph.
A 30 m ¥ 0.32 mm i.d. fused silica capillary column (CP-Sil 8
CB) from Varian Inc. was used. Hydrogen and air were used
as carrier g◦ases. The oven temperature was programmed from
◦
◦
◦
◦
50 C at 4 C min-1 to 130 C, from 130 C at 20 C min-1 to
◦
◦
240 C, and held isothermal at 240 C for 5 min. The samples
were injected in split mode, with a ratio of 1 : 10. The response
factor for the analysed terpenoids was 1.1 and for carvacrol and
carvomenthol only it equalled 1.0.
The presence of catalyst metals in the solutions was
tested using atomic absorption spectroscopy (AAS) by
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and carvotanacetone) were formed (<1 mol%) in the initial stage
of the reaction.
Results
Product distribution
In the case of reactions catalysed by a 0.5 wt% Rh/Al2O3 there
is no significant difference between monophasic and biphasic
conditions. The only noteworthy difference observed is the
time necessary to achieve reaction equilibrium. Similarly to
the Pd catalysed reaction, the process in one phase reaches
equilibrium faster after 50 min of the reaction course whilst
in the biphasic conditions it is lengthened to 90 min. Reactions
catalysed by the Rh catalyst guide the reaction to the formation
of similar major products (46.03 mol% and 36.70 mol% for
trans- and cis-carvomenthone correspondingly) as in the Pd
catalysed reactions. The major difference was the incapacity
of the catalyst to complete conversion of carvone. The total
conversion achieved was slightly lower than in the case when Pd
was used (96%). In addition, the intermediate product such as
carvotanacetone was observed in relatively high quantity in the
initial stage of the reaction; however its concentration decayed
in the later stage being counterbalanced by the formation of
two major isomers of carvomenthone as presented in Fig. 3.
Contrary to the Pd/Al2O3 system, the reaction catalysed by the
Rh catalyst produced carvacrol, carvenone and carvomenthol
at trace level.
The hydrogenation of carvone performed in the presence of
supercritical CO2 was carried out at 323.15 K at low H2 pressure
(4 MPa) and at 12.5 and 16 MPa of overall pressure (CO2
+ H2). The selected conditions were comparable to previous
studies with other terpenes such as limonene4,7 and b-myrcene.8
Similarly to binary mixtures of limonene or myrcene and CO2,
the system consisting of CO2 + carvone29,30 exhibits relatively
low critical pressure (~10 MPa); however there are no data for
ternary systems containing hydrogen as a third component.
Following the previous results obtained for limonene and
myrcene and their mixtures with CO2 and H27,9 it can be assumed
that in the case of carvone at 12.5 MPa total pressure, the system
can be biphasic (gaseous CO2 + H2 and CO2-expanded liquid
carvone) while at 16 MPa the system can be clearly monophasic
(terpenoid + CO2 + H2 in the fluid phase). These assumptions
were easily confirmed by a visual inspection performed during
the progress of the reaction executed under both mentioned
conditions.
In the hydrogenation of carvone over Pd/Al2O3 it was
discovered that a relatively short time was necessary to achieve
equilibrium, i.e. 30 min, as depicted in Fig. 2. In the case of the
reaction performed in two phases, the product concentration is
already constant at 60 min from the beginning of the reaction.
The Pd catalyst was efficient to achieve a full conversion of
carvone with total selectivity to carvomenthone above 77% for
both single phase and biphasic reaction. Two stereoisomers of
carvomenthone were formed in a 45.27 mol% for trans- and
31.75 mol% for cis-carvomenthone in a monophasic system,
and in a 45.74 mol% and 32.78 mol% for the corresponding
isomers under biphasic conditions. Additionally, in the course
of the reactions carvacrol was produced as a third product
at 22.21 mol% and 21.48 mol% for monophasic and biphasic
conditions, respectively. In addition, in a single phase hydro-
genation three intermediate products (carvenone, carvomenthol
Fig. 3 Profile of the hydrogenation of carvone over Rh/Al2O3 catalyst
in biphasic conditions (4 MPa of H2, 12.5 MPa of total pressure, 323.15
K), expressed as mole percentage of carvone ( ), trans-carvomenthone
᭺
(ꢀ), cis-carvomenthone (ꢁ), carvacrol (᭜), carvomenthol ( ), car-
᭹
votanacetone (᭡) carvenone (ꢀ).
The results (shown in Fig. 4) obtained in reactions with
Ru/Al2O3 exhibit considerable difference from the reaction
catalysed by Pd and Rh catalysts. The Ru catalyst favours
carvotanacetone as a major product of the reaction with lower
formation of carvomenthone. Comparing with Pd and Rh,
trans- and cis-carvomenthone were formed at 10 mol% and
7 mol%, respectively, and the conversion reached nearly 51%.
Furthermore, the reaction over Ru is significantly slower and
reaches equilibrium after 120 min in monophasic and 3 h in
biphasic conditions.
Fig. 2 Profile of the hydrogenation of carvone over Pd/Al2O3 in
monophasic conditions (4 MPa of H2, 16 MPa of total pressure,
323.15 K) catalyst, expressed as mole percentage of carvone ( ), trans-
᭺
Catalyst stability
carvomenthone (ꢀ), cis-carvomenthone (ꢁ), carvacrol (᭜), carvomen-
By analogy with the previous study on the hydrogenation of
myrcene over Pd, Rh and Ru catalyst supported on alumina the
thol ( ), carvotanacetone (᭡) carvenone (ꢀ). The solid lines are just a
᭹
merge fitting and are provided as indication of trends.
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Table 1 The initial reaction rate constant (10 k/min) for the hydro-
genation of the first C C double bond of carvone carried out over Pd,
Rh or Ru catalyst under one or two phase conditions
10 k/min
Pd
Rh
Ru
one phase
two phases
2.08
1.13
1.87
1.01
0.11
0.07
terpenes and good selectivity towards the fully hydrogenated
products in reactions performed in biphasic conditions.8,10,34
Based on the results obtained for the reaction catalysed by
Rh in this work and in our previous studies, it can be stated
that rhodium catalyst is less active than palladium.8 As it was
reported in the literature, the hydrogenation of unsaturated
olefins catalysed by ruthenium is generally even slower. It is
caused by at least 50 times lower activity of the ruthenium
catalyst in comparison with palladium or rhodium.31 The way
of expressing activity of the catalyst can be the initial reaction
rate constant (k). The k values, calculated using the procedure
described below, are collected in Table 1.
Fig. 4 Profile of the hydrogenation of carvone over Ru/Al2O3 catalyst
in monophasic conditions (4 MPa of H2, 16 MPa of total pressure, 323.15
K), expressed as mole percentage of carvone ( ), trans-carvomenthone
᭺
(ꢀ), cis-carvomenthone (ꢁ), carvacrol (᭜), carvomenthol ( ), car-
᭹
votanacetone (᭡) carvenone (ꢀ).
AAS analyses were performed.8 The AAS analyses showed that
the highest metal content was found in the sample taken from
the reaction carried out over Pd catalyst. The concentration of
palladium detected in the samples collected from the reactions
executed in one and two phases were similar and equalled 20.3
mg L-1. The rhodium catalyst can be recognised as slightly more
stable. The Rh content was found to be at the level of 15.7 mg L-1
in the samples from reactions carried out in single and biphasic
systems. The analyses showed that the Ru catalyst is the most
stable catalyst by indicating the lowest metal concentration in
the reaction samples. The data obtained for the samples after the
reaction in one and two phases showed that the concentration
of Ru metal reached the level of 9.8 mg L-1.
The ordinate units in the concentration profiles are mole
percentage of the total amount of products and taking into
account the solubility of CO2 in carvone at the reaction
conditions, the initial volumes of the CO2-expanded liquids were
calculated. The obtained results allow calculation of the initial
reaction rate constants and the initial volumes of the liquid
phase for the initial stage of the reactions in each experiment.
From the calculated volumes of the liquid phase and the known
total initial amount of carvone, its initial concentration (in
mol/vol) was calculated. In addition, the mol% of carvone
was used to calculate the concentration (mol/vol) of carvone
as a function of time due to the fact that each molecule of
product is produced from one molecule of carvone. Moreover,
the reactions were carried out in a semi-continuous way, and
the high recirculation rate used allowed the composition of
the feeds to alter continuously, consequential in kinetics like
in a batch process. Indeed, with the applied 3.3 mL min-1 flow
rate of the reaction mixture, the entire liquid was recirculated
at least once every 1–2 min. Postulating the disappearance of
carvone as a first order reaction, plots of ln[carvone] as a
function of time showed straight lines for the initial stages of
the reaction. In scCO2 the reaction kinetics exhibited a first-
order reaction rate i.e. for limonene,10 b-myrcene 27 and citral35
hydrogenation. Summarising, it can be stated that the pressure
of CO2 strongly affects the reaction conditions, and at lower
total pressure (12.5 MPa) reactions were carried out in biphasic
(liquid + gas) conditions while a total pressure of 16 MPa shifted
the reaction mixture towards a one-phase region. The pressure
tuning changes not only the number of phases, but also controls
the concentration of the reagents in the vicinity of the catalyst.
This in turn determines the rate of the reaction and indicates
also that the hydrogenation of the first double bond is always
faster in one phase than in two phases as presented in Table
1. Moreover, the experimental results showed that the initial
reaction rate constant depends on the activity of the catalyst
used for the reaction, and its decreasing trend was noticed with
diminishing catalyst activity.
Discussion
The catalyst activity
The hydrogenation of carvone performed in the environment of
scCO2 at milder conditions namely temperature and H2 pressure
was catalysed by three different noble metal catalysts. The
conventional solid catalyzed hydrogenation in organic solvents
is mainly H2 concentration dependent. In the hydrogenation
under supercritical carbon dioxide conditions in comparison to
hydrogenation in organic solvents: (1) the conversion of carvone
or theselectivity of desired products were found to be remarkably
increased; (2) the reaction time significantly decreased from
hours to minutes (i.e. 30 min reaction equilibrium achieved
over Pd in a one supercritical phase). This highlights the
advantageous role of CO2 as a reaction medium instead of the
solvents already employed in hydrogenation of carvone under
various conditions.14–23
Each of the catalysts applied in this study exhibits a different
attitude towards direction of hydrogenation of carvone and thus
proceeded to distinguishable results. The Pd catalyst is known as
an active metal catalyst and the present investigation confirms
this feature. The previous study from our laboratory for other
terpenes (limonene and b-myrcene) demonstrated that the Pd
catalyst is a very active metal and facilitates conversion of
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The catalyst and mechanism of the reaction
of carvone.37 The selectivity towards carvacrol was found
to be relatively low; however carvacrol was formed in the
highest quantity (>22%) among all tested catalysts. For all
results reported, the selectivity was calculated according to
Ci
The catalyst metal is responsible for the reaction kinetics but
also determines conversion and selectivity of the obtained
products. In particular, in the hydrogenation of carvone over
Pd/Al2O3 the reaction products can be formed via the classical
Horiuti–Polanyi mechanism.36,37 Klabunovskii and co-workers
postulated that such hydrogenation catalysed by Pd-Black leads
to the first intermediate product which is carvotanacetone and
in the further step carvomenthones and carvacrol are formed.
Both compounds are formed in the parallel pathways and
carvomenthones are created via hydrogenation while the second
one is formed by an irreversible catalysis.37
The literature reports38,39 showed that the mechanism of the
Rh catalysed hydrogenation of olefins is comparable to platinum
catalysed reactions. This may suggest that the hydrogenation of
terpenoid catalysed by Rh occurs via 1,2-s2 adsorption, contrary
to the formation of p-allyl complexes present in the case of
the palladium catalysed hydrogenation.36,39 Referring to the Ru
catalysed hydrogenation of carvone it should be underlined that
investigations presenting the mechanism of the hydrogenation
of terpenes and terpenoids are scarce.40 Our previous study8
showed that there is no agreement in this matter. This proves
that further investigations in this field should be performed.
Additionally, another important parameter influencing the
reaction outcome is the support of the catalyst. It was found
in the literature that in the hydrogenation under supercritical
conditions alumina plays an important role as it prevents de-
activation of the catalyst in the hydrogenation of cyclohexene.41
Furthermore, Baiker and co-workers showed that in the water
gas shift reaction formed under the high pressure CO2-assisted
hydrogenation alumina protects the catalyst from deactivation.42
Generally, based on experience with different catalyst
supports7,8,10,11 it can be stated that alumina is one of the most
mechanically stable supports under high pressure conditions.
S% =
×100%
the following equation
, where Ci was
Cp
∑
the concentration of component “i” and Cp was the total
concentration of the hydrogenation products. At the same time
the selectivity to fully hydrogenated products reached more
than 77% with total conversion in the case of the Pd catalysed
reaction. In the case of the Rh/Al2O3 catalyst, a higher selectivity
of 86.4% was achieved. These values of selectivities indicate
that the Rh catalyst promotes the formation of fully saturated
product leaving the conversion of carvone not complete but at
approximately 96%. The conversion of carvacrol is defined as
Cp
∑
un
C% =
×100%
, where C
follows
was the
carvone
un
carv one
Cp +C
∑
concentration of the unreacted carvone.
The reaction catalysed by Ru/Al2O3 is characterised by the
lowest conversion which is associated with the highest selectivity
towards the intermediate product such as carvotanacetone. The
mentioned conversion at equilibrium is only 50.6% of the initial
concentration of carvone and selectivity to carvotanacetone
reaches a 50.7%.
Analysing the data presented in Table 2 and results depicted
in Fig. 2–4 it can be stated that either carvomenthone or
carvotanacetone are obtained with high selectivities; however
in the case of an intermediate product such as carvotanacetone
a high selectivity of 84.7% can only be achieved with low
conversion of carvone in the reaction with Rh/Al2O3. It has to
be mentioned that no conversion of carvone occurred without a
catalyst under the investigated biphasic or monophasic system.
The reasonablely high conversions and selectivities were
obtained at equilibrium e.g. in the supercritical phase system
with Pd after 30 min, with Rh after 50 min and with Ru after
120 min (Table 2).
Product distributions
The major products of the reactions carried out are either fully
saturated isomers of carvomenthone (Pd and Rh catalysts),
or carvacrol (Pd catalyst), or carvotanacetone which is an
intermediate obtained through direct hydrogenation of carvone
(Rh and Ru catalysts).
The reaction catalysed by the Pd catalyst gave mainly
fully saturated isomers of carvomenthone but also significant
quantity of carvacrol – a product of irreversible catalysis
Conclusions
The hydrogenation of carvone catalysed by palladium, rhodium
and ruthenium in carbon dioxide atmosphere was performed
in supercritical carbon dioxide in biphasic conditions, at lower
CO2 pressure as well as at higher pressure of CO2 corresponding
Table 2 The table contains the catalyst, reaction time, conversion of carvone (C (%)) and selectivities (S (%)) towards particular products at certain
reaction time as well as turnover frequencya (TOF/min-1) for biphasic and monophasic conditions for each of the catalyst
Catalyst
time/min
120
C (%)
S
carvomenthone(%)
S
carvotanacetone(%)
Scarvacrol (%)
TOF/min-1
Pd/Al2O3
Rh/Al2O3
Ru/Al2O3
99.9
76.7
0.2
22.2
610b
304c
345d
218e
74f
2
120
120
25.4
95.7
50.6
14.5
86.4
32.1
84.7
12.5
50.2
0.4
0.9
7.8
25g
a TOF is the number of moles of carvone that a mole of catalyst can convert per unit of time (at equilibrium); b after 30 min (equilibrium) in a one
phase reaction; c after 60 min (equilibrium) in a two phase reaction; d after 50 min (equilibrium) in a one phase reaction; e after 90 min (equilibrium)
in a two phase reaction; f after 120 min (equilibrium) in a one phase reaction; g after 360 min (equilibrium) in a two phase reaction.
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to supercritical conditions. Generally it can be stated that CO2
improved the conversion of carvone and significantly reduced
the reaction time. The catalysed reaction in a single supercritical
system reached equilibrium faster than in the biphasic system
The final composition of the reaction mixture was strongly
dependent on the noble metal catalyst used for the reaction. The
palladium catalyst promoted complete conversion of carvone
to two fully hydrogenated isomers of carvomenthone as well as
to carvacrol. The less active Rh catalyst favoured production of
only fully saturated stereoisomers of carvomenthone with a 96%
conversion. The least active Ru catalyst led to significantly lower
conversion and favoured the production of a larger variety of
products at the equilibrium conditions.
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Jablonski and O. A. Scelza, Catal. Today, 2001, 66, 289–295.
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in enhancing conversion at mild conditions together with the
significant reduction in reaction time.
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2830 | Green Chem., 2011, 13, 2825–2830
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