Heterogeneously Catalysed Oxidative Dehydrogenation of Menthol in a Fixed-Bed Reactor in the Gas Phase

Article abstract of DOI:10.1002/open.201900158

For the first time, the oxidative dehydrogenation of (?)-menthol to (?)-menthone and (+)-isomenthone in a marketable quality was carried out in a continuous gas phase reactor as a sustainable process using molecular oxygen as green oxidant and solid catalysts which do not contaminate the product mixture and which are easily to remove. The diastereomeric purity remained largely unchanged. Three types of catalysts were found to be very active and selective in the formation of menthone and isomenthone: AgSr/SiO₂, CuO distributed on a basic support and RuMnCe/CeO₂, where Ru, Mn and Ce exist in an oxidized state. The best overall yield of menthon/isomenthone obtained with an Ag-based catalyst was 58 % at 64 % selectivity, with a Cu-based catalyst 41 % at 51 % selectivity and with a Ru-based catalyst 68 % at 73 % selectivity. Reaction conditions were widely optimized.

Full text of DOI:10.1002/open.201900158

DOI: 10.1002/open.201900158
Full Papers
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Heterogeneously Catalysed Oxidative Dehydrogenation of
Menthol in a Fixed-Bed Reactor in the Gas Phase
[a]
[a]
[a]
[a]
[b]
Anna Kulik, Katja Neubauer, Reinhard Eckelt, Stephan Bartling, Johannes Panten,
[a]
and Angela Köckritz*
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For the first time, the oxidative dehydrogenation of (À )-
menthol to (À )-menthone and (+)-isomenthone in a market-
able quality was carried out in a continuous gas phase reactor
as a sustainable process using molecular oxygen as green
oxidant and solid catalysts which do not contaminate the
product mixture and which are easily to remove. The diastereo-
meric purity remained largely unchanged. Three types of
catalysts were found to be very active and selective in the
formation of menthone and isomenthone: AgSr/SiO , CuO
2
distributed on a basic support and RuMnCe/CeO , where Ru, Mn
2
and Ce exist in an oxidized state. The best overall yield of
menthon/isomenthone obtained with an Ag-based catalyst was
58% at 64% selectivity, with a Cu-based catalyst 41% at 51%
selectivity and with a Ru-based catalyst 68% at 73% selectivity.
Reaction conditions were widely optimized.
[
14–20]
1
. Introduction
investigated.
In general, the costs for recycling of catalysts
and solvent might prevent the application of such homoge-
neous catalysts in a larger scale.
The heterogenization of homogeneous catalysts onto the
surface of organic or inorganic supports was also described for
the aerobic oxidation of menthol. For example, TEMPO, Fe- or
Co complexes,
polyoxometalates were immobilized. But a leaching of the
active species of such catalysts into the liquid phase cannot be
avoided, even if it cannot be proved due to the small scales
described and the detection limits of ICP methods.
Heterogeneous catalysts were also applied in the liquid
phase. Using Ru/CeO2 (15% yield) or Ru(OH) /SiO @Fe O
4
(17% yield), only poor yields of menthone were produced. The
latter catalyst could be removed from the reaction mixture by
means of a magnet. Under the same reaction conditions,
carveol was converted in the presence of this catalyst to 92% of
carvone. Au/MgO could be used as catalyst without an addi-
tional base, but the yield of carvone was also low. Albadi et al.
(À )-Menthone and (+)-isomenthone are used in the fragrance
[1]
industry in synthetic peppermint oils and bases. They can be
easily epimerized via an enol tautomer using acidic catalysts.
Both terpenes are minor ingredients of various Mentha species,
together with menthol as main terpene-type component.
Therefore a manufacture from menthol by oxidation with toxic
chromic acid or catalytic dehydrogenation applying copper
[
21]
[22–25]
[26]
[27]
Pd(II) catalysts,
MoO (acac)2
or
2
[28]
[1]
chromite was carried out on a larger scale. Furthermore, a
couple of catalytic aerobic oxidations for the synthesis of
menthone/isomenthone in the liquid phase using both homo-
geneous and heterogeneous catalysts were described in the
scientific literature reported in the following part.
The application of TEMPO and TEMPO-analogue bicyclic N-
oxides as catalysts in the aerobic oxidation of menthol were
described several times,
[29]
[30]
x
2
3
[
2–8]
menthone yields of more than 90%
were obtained. Disadvantages are larger amounts of additive
needed and salt loads formed. Different Cu-, Co- und Mo-
complexes catalysed the oxidation of menthol also according to
[31]
[32]
investigated Au/CuO-ZnO or Co nanoparticles/Al O3 cata-
lysts in H O, but a semi-stoichiometric amount of Cs CO as
2
[
9–13]
a radical mechanism.
In this case, the necessary amounts of
2
2
3
bases and/or additives are disadvantageous. Furthermore, a
series of palladium- and ruthenium complexes, often together
with co-catalysts and further additives, were successfully
base was necessary. Platinum nanoparticles on mesoporous,
specially treated activated carbon were found to be very active
and selective in the conversion of menthol (90% conversion,
9
1
7% selectivity) in an autoclave in the presence of dioxane at
0 bar air. The formation of ether hydroperoxides might be
[33]
[
a] Dr. A. Kulik, Dr. K. Neubauer, R. Eckelt, Dr. S. Bartling, Dr. A. Köckritz
Leibniz Institute for Catalysis (LIKAT Rostock)
Albert-Einstein-Str. 29a, 18059 Rostock, Germany
E-mail: angela.koeckritz@catalysis.de
b] Dr. J. Panten
detrimental for an upscaling of the procedure. The polyoxome-
talate Na Fe [AlMo O ] 2H O induced 92% yield of
menthone.
menthone. Unfortunately, the good yields of the latter two
procedures could not be reproduced in own experiments. In
general, a disadvantage of application of heterogeneous
catalysts in the liquid phase is a frequently observed leaching of
the active metal.
6
.3
0.9
11 39
2
[
34]
V2O without any base produced 77% of
5
[35]
[
Symrise AG
Mühlenfeldstraße 1, 37603 Holzminden, Germany
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/open.201900158
©
2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution Non-Commercial NoDerivs License, which permits use and dis-
tribution in any medium, provided the original work is properly cited, the
use is non-commercial and no modifications or adaptations are made.
Without any catalyst in subcritical H O (50 bar total pressure,
2
[36]
5
–20 bar O ), 85% menthone could be isolated. It seems a
2
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sufficient processing from the chemical point of view but
requires special design of the equipment.
simple aliphatic or cycloaliphatic alcohols do not succeed with
menthol as substrate.
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It should be mentioned that the non-oxidative dehydrogen-
ation of menthol was also successful in the presence of
heterogeneous catalysts. Here, a reaction in the gas phase at
NiÀ Cu/Al O doped with 0.1% CeO resulted in 48% menthone
Thus, a process management at higher temperatures as
presented here might be favorable. Ag, Cu and Ru-based
catalysts were identified to be active in the menthol oxidation
as a result of a broader screening of various oxidation catalysts,
whereby the non-active catalysts should not be discussed here
in detail. For the first time, heterogeneous Ag catalysts were
used for menthol oxidative dehydrogenation. The menthone/
isomenthone yields obtained with heterogeneous Ru catalysts
in this study are much better than described for solid Ru
2
3
2
[37]
and 24% thymol beside low amounts of by-products. The
transfer dehydrogenation with styrene as H acceptor at Cu/
2
[38,39]
Al O led to a mixture of menthone and isomenthone.
But
2
3
the stoichiometric amounts of styrene are detrimental.
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[40,41]
Patents
were claimed, too, but we do not know if the
[29,30]
procedure is really being used. Also the homogeneous Shvo
catalyst or RuH CO(PPh ) were found to be suitable for the
catalysts in the liquid phase in a batch reactor.
In the
following, both the composition of the active catalysts (see
Tables 1 and 2) and reaction conditions were optimized in order
to obtain the highest possible yields of menthone/isomenthone
at the best possible selectivities.
The heterogeneous catalysts were prepared by incipient
wetness impregnation (Ag and Cu catalysts) and precipitation
with sodium hydroxide/carbonate (Ru catalysts).
2
3 3
[42]
direct dehydrogenation of menthol in mesitylene at 165°C.
Continuous processes for the synthesis of fine chemicals in
the gas phase are rare but quite possible. From a sustainable
point of view, a solvent-free process using heterogeneous
catalysts and oxygen as the sole oxidant would be highly
desirable. Such a process should be also very advantageous for
a scale-up to technical scale. Neither solvent, catalysts, additives
nor salts have to be removed and recycled, the products can be
processed directly, and a leaching of the active metal species
into the product mixture causing often deactivation is also
precluded. For the first time, the oxidative dehydrogenation of
With the exception of SiO -supported catalysts, compression
2
2
of the powdery material (2.2 t/cm ), crushing and sieving were
carried out (after deposition of active species and post-treat-
ment such as reduction or calcination) because only the catalyst
(À )-menthol to (À )-menthone and (+)-isomenthone (Scheme 1)
Table 1. Composition, BET area and pore volume of Ag and Cu catalysts
investigated in this study.
Catalyst
ICP
[calc./found]
Ag
BET
[m g
Pore volume
2
À 1
3
À 1
[wt%]
]
[cm g
]
nd
2
metal
Ag/SiO2
1.5/1.34
1.5/1.30
1.5/1.48
1.5/1.38
1.5/1.33
1.5/1.50
1.5/1.37
1.5/1.48
1.5/1.59
1.5/1.42
1.5/1.11
1.5/1.44
1.5/1.28
1.5/1.51
0.5/0.53
1.0/0.97
3.0/3.10
1.5/1.70
1.5/1.56
1.5/1.23
331.4
284.5
181.9
221.4
267.0
288.7
276.2
178.7
115.8
183.3
202.7
29.7
0.9050
0.7677
0.8339
0.8462
0.7920
0.8491
0.8445
0.8140
0.2716
0.1239
0.1509
0.0868
0.3573
0.0299
0.7652
0.7698
0.7675
0.8175
0.7776
0.7024
1
.5/4%AgSr/SiO
AgNa/SiO
AgLi/SiO
2
4.0/3.56
1.1/1.05
0.3/0.3
6.3/5.36
2.6/1.02
2.7/1.88
1.8/1.57
2
2
Scheme 1. Aerobic oxidative dehydrogenation of (À )-menthol (identified by-
products: 1-menthene, 2-menthene, 3-menthene, cymene, menth-2-enone,
thymol.
AgBa/SiO2
AgMg/SiO
2
AgCa/SiO
AgK/SiO
Ag/MgO
Ag/hydrotalcite
AgSr/hydrotalcite
AgSr/La
AgSr/MgO
AgSr/Nb
2
2
was investigated in a flow reactor in the gas phase. Various
heterogeneous catalysts were tested, and the reaction con-
ditions were optimized.
4.0/3.02
4.0/3.22
4.0/3.09
4.0/3.45
4.0/3.47
4.0/3.55
4.0/3.54
1.0/1.46
3.0/2.82
10.0/8.41
2
O
3
174.3
20.5
2
O
5
0
1
.5/4%AgSr/SiO
/4%AgSr/SiO
2
276.1
277.3
283.7
312.4
289.2
240.0
0.2
2
3/4%AgSr/SiO2
2. Results and Discussion
1
1
1
.5/1%AgSr/SiO
.5/3%AgSr/SiO
.5/10%AgSr/SiO
2
2
Menthol is difficult to oxidize since all substituents are
preferably arranged in the less reactive equatorial conformation,
the OH group is not activated (Figure 1). Therefore many
catalytic systems which work well in the oxidation of more
2
CuO-0.2
CuO-1.0
CuO-1.6
1.0
1.6
Cu/SiO
CuO/SiO2
CuO/MgO
CuO/hydrotalcite
CuO/Nb
CuO/La O3
CuOSr/SiO
CuOMg/SiO
2
1.3/1.14
1.5/1.14
1.5/1.21
1.5/1.20
1.5/1.17
1.5/1.06
1.5/1.16
1.5/1.21
1.5/1.13
1.5/1.19
346.5
361.9
132.1
148.0
20.0
0.8312
0.8436
0.2231
0.1675
0.0288
0.0563
0.8190
0.8622
0.8192
0.8571
2 5
O
2
24.4
2
4.0/3.61
2.6/1.15
6.3/5.82
2.7/1.80
275.8
299.2
299.6
293.2
2
CuOBa/SiO
CuOCa/SiO2
2
Figure 1. Position of the equilibrium between axial and equatorial conforma-
tion of substituents in (À )-menthol.
nd
À 1
[a] 0.5 mmol 2 metalg support
.
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confirmed that the oxidative dehydrogenation can only occur
in the presence of a suitable catalyst.
Table 2. ICP analyses, BET areas and pore volumes of prepared Ru
catalysts.
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Starting with 1.5/4%AgSr/SiO2 as catalyst, optimal condi-
Catalyst
ICP [calc./found]
BET
Pore
2
À 1
[
m g
]
volume
tions for reaction temperature, menthol/O ratio and residence
2
3
À 1
[
cm g
]
time were determined first. An increase in reaction temperature
from 250°C to 350°C led to a nearly linear rise of both
menthone and isomenthone yield and to an increased menthol
conversion (see Figure S6). However, the proportion of by-
products such as 1-, 2- and 3-menthene, cymene and thymol
Ru
Mn
Ce
0
.2%Ru/CeO
2
0.2/
0.16
0.5/
0.35
1.0/
0.77
52.9
46.5
52.7
0.1381
0.1198
0.1233
0
.5%RuMnCe/
2
1.2/1.0
1.6/
n.d.
1.6/
n.d.
CeO
.0%RuMnCe/
CeO2
1
1.2/
1.06
(see Table S4 for yields of by-products) also grew significantly,
1
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1
1
1
1
1
1
2
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2
2
2
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2
2
2
2
3
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4
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5
5
especially in the range between 300–350°C. Considering higher
conversion and yields vs. selectivity, both 300 and 350°C were
chosen as reaction temperature for subsequent experiments.
À 1
À 1
fraction with a particle diameter between 400–800 μm was
installed in the reactor.
Lowering the flow rate from 30 mLmin to 20 mLmin (see
Figure S7), that means increasing the residence time by factor
1.5, caused an increase of menthol conversion from 49% to
81% and a rise in yield of menthone and isomenthone from
16% to 27% and 11% to 18%, respectively. Even though a
The Ag- and Cu-based catalysts were prepared in adaption
[43]
of a literature procedure. In case of Ag catalysts, an alkali or
earth alkali dopant was investigated together with the variation
of catalyst support. Since AgSr/SiO2 was very active in the
catalytic reaction, both the Ag and the Sr content were also
optimized. In the series of Cu catalysts, earth alkali species were
added as dopant, and the support was also varied.
À 1
further reduction of flow rate to 10 mLmin led to a menthol
conversion of 96%, the desired product selectivity decreased
À 1
due to more undesired by-products. A flow rate of 30 mLmin
was chosen for further experiments due to the lowest
proportion of by-products (6%), such as menthenone, 1-, 2- and
3-menthene, cymene and thymol, under these conditions. The
influence of menthol/O2 ratio on conversion and yield of
menthone/isomenthone was also investigated (see Figure S8).
Few Ru catalysts on ceria, investigated earlier by some of us
[44]
in the oxidation of different alcohols, were also prepared and
tested (Table 2).
As seen at Tables 1 and 2, the BET surface and pore volume
of the catalysts strongly depends on the morphology and
texture of the support, or in case of unsupported CuO, on the
calcination temperature.
The volume flow rate of 5% O /Ar was adjusted constantly to
2
À 1
30 mLmin . According to the vapor pressure curves of
menthol, a menthol/O2 ratio of 1:2 should be achieved at
110°C and a ratio of 1:4 at 91°C. Therefore the thermostat
temperature for the saturator was lowered from 120°C
(standard reaction condition) to 110°C and 91°C to diminish
For determination of oxidation states, XPS spectra of 1.5/4%
AgSr/SiO , CuO/hydrotalcite and 1.0%RuMnCe/CeO were re-
2
2
corded exemplarily (see Figures S3–S5, respectively). Since for
silver oxides only small shifts of the binding energy are
observed, the position and shape of the Auger Ag MNN peaks
were also used for interpretation. A binding energy of 368.0 eV
was measured for the Ag3d5/2 peak of 1.5/4%AgSr/SiO . The
shape and position of Ag M5N45N45 and Ag M4N45N45 peaks
at kinetic energies of 348.3 eV and 353.9 eV, respectively,
the proportion of the vaporized menthol. The evaporated
amount was also controlled by weighing of the saturator vessel
after the reaction. A reduction of the menthol/O ratio from 1:1
2
to 1:2 led to an increase of menthol conversion from 49% to
71%, but only a very small increase of menthone/isomenthone
yields by 2% and a growing formation of by-products were
observed. The further lowering of the menthol content
2
together with the Ag3d5/2 peak point to the existence of Ag O
2
[45]
or mixed silver oxides at the surface. No significant shift of
the Ag3d5/2 peak (367.8 eV) was observed for this catalyst after
(menthol/O ratio 1:6) effected poorer menthone/isomenthone
2
yields of 14% and 10% at 71% menthol conversion. In addition,
1
1
20 h time on stream. The position of the Sr3d5/2 peak at
33.3 eV and the Sr3d peak at 135.1 eV indicates the
a lower menthol/O ratio led to reduced recovery rates.
2
Regrettably, the suspected formation of compounds with
higher molecular weight could not be proved. However, a high
proportion of oxygen favors the formation of by-products.
Therefore, a saturator temperature of 120°C and thus a
3
/2
[46]
existence of SrO. For CuO/hydrotalcite, the state of the Cu2p3/
peak at 933.5 eV together with significant satellite features
2
2
+
around 942 eV denote Cu but also mixed oxides cannot be
[
47]
excluded completely. In 1.0%RuMnCe/CeO the peak position
menthol/O ratio of about 1:1 were applied in further experi-
2
2
4
+ [48]
of Ru3d5/2 at 281 eV proves the presence of Ru
.
In the Ce4d
ments.
region, the binding energy of the Ce4d5/2 peak at 882.3 eV and
The control of diastereomeric purity confirmed that the
oxidative dehydrogenation of menthol with 1.5/4%AgSr/SiO2
leads almost exclusively to (À )-menthone (de�99.61%).
4
+
the strong peak at 916.2 eV, belonging to Ce4d3/2, reveal Ce
[49]
as the main oxidation state [5].
The investigations concerning catalytic oxidative dehydro-
genation started with a blank experiment. The reactor was only
filled with inert glass beads instead of the catalyst. An
appreciable menthol conversion (X=3%) was not observed
within a reaction time of 3 h, menthone and isomenthone
yields were only 1% and 0.3%, respectively. This result
Since 1.5/4%AgSr/SiO showed a good catalytic activity, the
2
replacement of Sr by alkali metals Li, Na, K and other alkaline
earth metals such as Mg, Ca and Ba was evaluated (see
Figure 2). The idea behind was the assumption that a basic
component in the catalyst might facilitate the hydrogen
abstraction from the CHÀ OH group of menthol and thus, might
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SiO , AgK/SiO and 1.5/4%AgSr/SiO (see Figure S2) explained
probably the reason for the excellent activity and selectivity of
the Sr-doped catalyst.
Strontium obviously enables a very fine and uniform
dispersion of silver particles, the average particle size distribu-
tion was measured to be 2 nm.
Both in the pure Ag/SiO2 catalyst and in AgK/SiO2,
numerous Ag particles larger than 5 nm were found, and an
uneven size distribution was observed. Almost identical, and in
2
2
2
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
comparison to SiO significantly higher, menthone and isomen-
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
thone yields were obtained at a reaction temperature of 300°C
if MgO and hydrotalcite were used as basic supports (Figure 3).
Figure 3. Monometallic Ag catalysts on different supports (reaction con-
À 1
ditions: 3 mL 1.5% Ag catalyst, 30 mLmin 5 vol% O
2
/Ar, menthol/O
2
ratio
~
1:1, 300°C and 350°C); numerical values and spectrum of by-products see
Table S4 in ESI; X=conversion.
Figure 2. Variation of basic dopant in Ag/SiO
2
catalysts (reaction conditions:
À 1
3
5
mL catalyst (1.5 wt% Ag, 0.5 mmol 2nd metal/g support), 30 mLmin
vol% O /Ar, menthol/O
2
2
ratio ~1:1); see Table S4 for spectrum and yields
of by-products; X=conversion.
On the other hand, all catalysts showed similar yields for
menthone/isomenthone of 41% at most if the temperature was
increased to 350°C. It is quite remarkably that Ag/MgO led to
significantly lower yields of undesired by-products (7%)
although it effected only a menthol conversion of 64%.
Furthermore, the stereochemical analysis of the menthone
produced with Ag/hydrotalcite at 350°C resulted in 4% (+)-
and 96% (À )-menthone.
be the reason for the good catalytic performance. For direct
comparison of the catalyst activity and selectivity, all catalysts
were tested at 300 and 350°C applying standard reaction
conditions. Additionally, monometallic Ag/SiO was included
2
into the test series to check if doping with a second metal
species is necessary.
At a reaction temperature of 300°C, only low overall yields
of menthone and isomenthone in the range of 19% at most,
and a medium conversion of up to 47% could be observed
with the catalysts containing Li, K, Ca and Ba. Higher
Since a monometallic Ag catalyst from this test series was
not able to exceed the best results of the bimetallic 1.5/4%
AgSr/SiO , a variation of the support was investigated for 1.5/
2
4%AgSr (Figure 4).
The comparison of Ag/MgO and Ag/hydrotalcite with AgSr/
MgO and AgSr/hydrotalcite demonstrated that doping with Sr
species did not reveal any positive effect regarding menthone/
conversions were obtained with AgMg/SiO , but this catalyst
2
formed almost exclusively the by-products 3-, 2- and 1-
menthene. The best results with 58% overall yield at 91%
menthol conversion (64% selectivity for menthone and iso-
menthone), which could not be improved by increasing the
temperature to 350°C, provided the metal combination Ag/Sr.
isomenthone yields. At a reaction temperature of 300°C, SiO
2
was identified as the best support material for the combination
of Ag (1.5%) and Sr (4%). Remarkably, the undesired formation
of by-products, which frequently occurred when the reaction
was executed at 350°C, could be significantly minimized by
La O as support.
The direct comparison of the monometallic Ag/SiO with the
2
bimetalllic AgSr/SiO showed that especially at 300°C, the pure
2
2
3
Ag catalyst preferably formed by-products. TEM images of Ag/
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Figure 5. Variation of Ag content in AgSr/SiO
2
catalysts (reaction conditions:
À 1
3
3
mL catalyst (4 wt% Sr), 30 mLmin 5 vol% O
00°C and 350°C) ); X=conversion, numerical values and spectrum of by-
2
/Ar, menthol/O
2
ratio ~1:1,
products see Table S4 in ESI.
loading of 1.5%, so that a continuous increase of the menthol
conversion could be observed from 0.5 to 1.5% Ag. The lower
selectivities at lower Ag loading could possibly be explained by
a parallel dehydration occurring at other active sites, which are
abundant even at low Ag loading. TEM analyses of the four
catalysts could not explain why 1.5/4%AgSr/SiO achieved the
2
best results. A uniform Ag dispersion as well as small particles
sizes and narrow particle size distributions were found for all Ag
contents between 0.5–3.0%.
In a second series, further experiments were carried out to
find the optimum loading of strontium. For this purpose, four
catalysts were investigated with Sr contents between 1–10 wt%
(Figure 6).
Figure 4. Variation of support for 1.5/4%AgSr as active species (reaction
À 1
conditions: 3 mL catalyst (1.5 wt% Ag, 4 wt% Sr), 30 mLmin 5 vol% O
2
/Ar,
An increased Sr loading from 1.0 to 3.0% effected growing
menthol conversion from 58% to 93% and menthone/
isomenthone yields from 35% to 52% at a reaction temperature
menthol/O
2
ratio ~1:1, 300°C and 350°C); X=conversion, numerical values
and spectrum of by-products see Table S4 in ESI.
Since 1.5/4%AgSr/SiO2 showed a sufficient activity (91%
menthol conversion) even at 300°C, and only a slight tendency
to by-product formation (6% overall yield), further investiga-
tions were directed on the optimization of the Ag and Sr
loading.
Therefore four catalysts were first examined with an Ag
content between 0.5-3.0 wt% at 300 and 350°C, and a reaction
period of 3 h (Figure 5). With an Ag loading of 0.5 and 1.0%,
only very low menthol conversions (43% max.) and total
selectivities for menthone and isomenthone (21% max.) were
achieved at 300°C. A temperature increase to 350°C led to
higher menthone/isomenthone yields (25% max.) and also to
slightly higher selectivities of up to 33%. However, the best
results were obtained using catalysts with Ag contents of 1.5
and 3.0%. As only few by-products had been formed, 1.5/4%
AgSr/SiO2 showed clearly an outstanding performance at a
temperature of 300°C. It seems that enough active sites for the
oxidative dehydrogenation are available already at an Ag
Figure 6. Influence of Sr content (reaction conditions: 3 mL catalyst (1.5 wt%
À 1
Ag), 30 mLmin 5 vol% O
2
/Ar, menthol/O
2
ratio ~1:1, 300 C and 350 C);
°
°
X=conversion, numerical values and spectrum of by-products see Table S4
in ESI.
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of 300°C. But effect of Sr content on activity and selectivity did
not play this significant role when the temperature was raised
to 350°C. In a range between 3.0-4.0% of Sr loading, its
optimum was found (58% maximum yield of menthone/
isomenthone at 64% selectivity). However, a further increase of
the Sr content to 10% caused the opposite effect and led only
to 18% menthone and 14% isomenthone selectivity at most (at
interaction may interfere with the process of oxidative dehydro-
genation. In addition, it is conceivable that a high Sr loading
might lead to the partial coverage of the active Ag centers,
resulting in the decreased activity.
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Oxidic copper catalysts showed also catalytic activity in the
reaction of menthol to menthone. Therefore pure CuO, CuO
dispersed on several supports (SiO , MgO, hydrotalcite, Nb O ,
2
2
5
3
2% total selectivity), and significantly lower yields at both 300
La O ) and CuO doped with a second alkaline earth metal oxide
2 3
and 350°C. The TEM analysis of the catalysts could not
contribute to the clarification of the differences in activity and
selectivity. All catalysts show the existence of many small Ag
particles, and only very few larger particles could be detected.
The distribution of Sr could not be visualized by TEM. In
addition, Sr could not be detected by EDX because the SrLα
line was superimposed by the SiKα line. A strong substrate-Sr
(CuOÀ M, M=Sr, Mg, Ba, Ca) were investigated in this series of
tests.
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First, three copper oxides with different small BET surfaces
2
À 1
between 0.2–1.6 m g were applied at both 300 and 350°C
(Figure 7). The surface of the CuO had a significant influence on
the catalyst activity and selectivity. A larger surface area also
leads to an increased catalyst activity and consequently to a
growing menthol conversion from 24% up to 73% at 300°C.
In particular, the comparison of the product spectrum
shows that the tendency to form by-products increased
drastically (up to 26% yield for CuO-1.6 at 350°C) with rising
activity, resulting in diminished selectivities of menthone (16–
1
8%) and isomenthone (13–14%). CuO with the smallest BET
2
À 1
surface area of 0.2 m g achieved the best overall selectivity of
6
3
4% menthone and isomenthone (at 25% overall yield) at
50°C.
Analogously to the successful AgSr/SiO2 catalysts, SiO2-
supported CuO catalysts with a further active component such
as Sr, Mg, Ba, Ca (0.5 mmol second metal g support) were
À 1
prepared and investigated at 300 and 350°C (Figure 8).
Unfortunately, all doped CuOM/SiO catalysts induced only
2
a very low selectivity for menthone and isomenthone via
oxidative dehydrogenation. All catalysts led almost exclusively
to the formation of already above mentioned by-products (see
Table S4) with an overall yield of up to 74% (CuOMg/SiO at
2
Figure 7. Variation of BET surface of pure CuO catalysts (reaction conditions:
350°C).
À 1
3
3
mL catalyst, 30 mLmin 5 vol% O
2
/Ar, menthol/O
2
ratio ~1:1, 300°C and
50°C); X=conversion, numerical values and spectrum of by-products see
Obviously, dehydration to menthenes and dehydrogenation
to thermodynamically more stable aromatics were preferred
with these more basic catalysts.
Table S4 in ESI.
But since a positive effect of a basic dopant could not be
observed, further investigations were directed on the influence
of different supports for CuO.
Therefore five CuO catalysts with nominal 1.5% CuO
content were synthesized on SiO , MgO, hydrotalcite, Nb O
5
2
2
and La O as support materials. To find out if the catalytically
2
3
active species CuO can also be formed during the reaction, a
freshly reduced 1.2% Cu/SiO catalyst was additionally tested
2
(
Figure 9). The application of both Cu/SiO and CuO/SiO led to
2
2
comparable, but very low yields of desired products (max 7%).
Similar to Ag/SiO , the yields of menthone and isomenthone
2
could also be increased if basic MgO and hydrotalcite were
used as support materials, whereby 41% menthone and
isomenthone yield and a total selectivity of 51% were achieved.
CuO/MgO also showed the lowest yield of unwanted by-
products (4–9%) in the entire temperature range between 300–
3
50°C.
Figure 8. Variation of basic dopant in CuOM/SiO catalysts (reaction con-
2
À 1
In contrast to all the catalysts described here, the menthone
ditions: 3 mL CuOM/SiO
2
, 30 mLmin 5 vol% O
2
/Ar, menthol/O
2
ratio ~1:1,
3
00°C and 350°C): X=conversion, numerical values and spectrum of by-
synthesis with CuO/hydrotalcite led to 6% (+)- and 94% (À )-
products see Table S4 in ESI.
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Figure 10. Influence of Ru content (reaction conditions: 3 mL catalyst
À 1
(
0.2 wt% Ru/CeO
2
, 0.5 and 1.0 wt% RuMnCe/CeO
2
,), 30 mLmin 5 vol% O
2
/
Ar, menthol/O ratio ~1:1.); X=conversion, numerical values and spectrum
2
of by-products see Table S4 in ESI.
selectivities of menthone (43%) and isomenthone (30%) were
obtained with 1.0%RuMnCe/CeO2.
An observation in all experiments was the gap between
conversion and identified yields of products that means the
recovery rate was often lower than 90%. An assumption was
that the total oxidation of menthol to CO might play also a
2
role at reaction temperatures of 300–350°C. To verify this
assumption, the experiment with 1.0%RuMnCe/CeO2 was
repeated and the gas stream at the reactor outlet (after the
cold trap and sample collection) was fed into a concentrated Ba
Figure 9. Variation of support in Cu-based catalysts (reaction conditions:
(
OH) solution to perform a gravimetric determination of CO as
2
2
À 1
3 mL catalyst (1.2 wt% Cu or 1.5 wt% CuO), 30 mLmin 5 vol% O /Ar,
2
precipitated BaCO . In a second experiment, the gas stream was
menthol/O
2
ratio ~1:1, 300
°
C and 350°C); X=conversion, numerical values
3
numerical values and spectrum of by-products see Table S4 in ESI.
collected in a He-filled gas mouse and analyzed by GC. The
recovery rates (calculated as sum of moles of all products plus
moles of unreacted menthol (collected per hour)/moles of
menthol (fed into the reactor per hour)) of the gravimetric assay
were 79% (300°C) and 74% (350°C), and for the experiment
with subsequent GC analysis of the gas 84% (300°C) and 72%
(350°C). With both analytical methods, a total oxidation could
be detected as a side reaction of the oxidative menthol
dehydrogenation. Using the Ru catalyst, 6% (GC determination)
and 9% (gravimetric determination) of the substrate were
totally oxidized to CO . Of course, the CO content in the
menthone, that was the lowest diastereomeric purity of (À )-
menthone.
Finally, also some ruthenium-based catalysts were inves-
tigated that some of us earlier had developed for the oxidative
dehydrogenation of other primary and secondary alcohols in
[44]
liquid phase. Two catalysts with a Ru content of 0.5 and 1.0%
together with MnO2 and nanoparticular ceria doping were
prepared on ceria. In addition, a pure 0.2%Ru/CeO2 was
included into the tests (Figure 10). A ruthenium content in the
range between 0.5-1.0% has proven to be optimal. Remarkably,
2
2
reaction gas depends on the catalyst used and on the temper-
atures prevailing in the reactor which has to be taken into
account. Typical of continuous processing, an additional adjust-
ment of residence time will open up further opportunities for
improvement of selectivity.
0
.2%Ru/CeO2 with the lowest Ru content already showed a
significant activity, and all catalysts provoked a good selectivity
for menthone/isomenthone and a low formation of by-
products. In direct comparison with all previously used Ag- and
Cu-based catalysts, the best results with respect to menthone/
isomenthone yields (68% at most at 93% conversion) and
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À 1
3. Conclusions
0.92 mLg ) was impregnated with a solution of 0.236 g AgNO
and
3
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0
.966 g Sr(NO ) in 10 mL water. The moist catalyst precursor was
3
2
first dried at RT overnight and then at 120 C for 3 h, calcined at
°
For the first time, a sustainable oxidative dehydrogenation of
menthol to menthone was performed in the gas phase in a
continuous process. This study clearly demonstrates the great
potential of Ag-, Cu- and Ru-based catalysts. The first series of
experiments with Ag (1.5%) as the active component using
À 1
500°C for 4 h (heating rate: 2 Kmin ) and reduced at 400°C for
16 h in a H
stream. The catalyst powder was pressed in a tablet
2
press, carefully crushed in a mortar and sieved (fraction 400–
800 μm).
Compositions of precursors for further Ag-based catalysts can be
found in ESI. Their preparation was carried out by analogy with the
given procedure.
SiO , hydrotalcite and MgO as support materials revealed that
2
the monometallic SiO -supported catalyst essentially led to the
2
formation of by-products, whereas
menthone and isomenthone of up to 67% (32% overall yield at
8% conversion) could be achieved with support materials
a total selectivity of
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5
Cu-Based Catalysts
4
such as MgO and hydrotalcite. The doping with Sr(II), which
apparently prevents the sintering of the Ag particles during the
preparation and consequently leads to the formation of small
Ag particles, increases considerably both the catalyst activity
To prepare the CuO strands, 100 g of Cu(OH) were first calcined at
2
4
00°C. for 3 h. Thereafter, 80 g of the calcined material was mixed
with 37 mL of a 2% methylcellulose solution in deionized water to
form a paste. With the help of a syringe, 2 mm strands were
formed. These were heated slowly under synthetic air to 400
°
C
(91% conversion) and the selectivity to menthone and
À 1
(
heating rate: 5 Kmin ) and then faster to 900°C (heating rate of
À 1
isomenthone (64%). This effect was neither observed when
using basic supports such as hydrotalcite, MgO, La O as well as
10 Kmin ). The calcination took place at 900°C over a period of
6 h. The CuO particles were crashed carefully in a mortar and sieved
into fractions before being used in the reaction.
2
3
the acidic Nb O nor using other alkali and alkaline earth metals
2
5
as dopants. 1.5/4%AgSr/SiO2 was found to be the best Ag-
based catalyst so far.
The Cu-based catalysts on supports were prepared according to the
same procedure as applied for the Ag-based materials; 1.5%Cu/
SiO : For catalyst preparation, again homemade SiO granules were
used. Subsequently, 9.85 g of these SiO granules (pore volume:
0.92 mLg ) were impregnated with a solution of 0.456 g of Cu
[43]
The suitability of pure CuO as a catalyst in the oxidative
dehydrogenation depends on the size of the BET surface area.
The CuO with the lowest BET surface area gave the best results
with 63% selectivity for menthone/isomenthone yield at 40%
conversion at a reaction temperature of 350°C. In contrast to
the results of silver catalysts, doping with alkali and alkaline
earth metal oxides did not improve the yields of menthone and
isomenthone. Only by dispersion of CuO on basic supports such
as MgO and hydrotalcite, the overall menthone/isomenthone
yield (41% at most) and selectivity (51% at most) could be
significantly increased.
2
2
2
À 1
(NO ) in 10 mL of water. The precursor was first dried at RT
3 2
overnight and then at 120°C for 3 h. Thereafter, the reduction was
À 1
carried out in a H stream at 300°C for 3 h (heating rate: 2 Kmin ).
2
1.5%CuO/SiO
: The preparation was performed as described for
2
1
.5%Cu/SiO2 with an additional calcination of the catalyst in
synthetic air after reduction step at 400 C for 4 h (heating rate:
°
À 1
2 Kmin ).
CuO catalysts on several supports were prepared similarly (see ESI).
In direct comparison to the Ag and CuO catalysts, Ru
Ru-Based Catalysts
catalysts of the type RuMnCe/CeO gave the best results of this
2
study. Experiments to vary the Ru content showed that a Ru
loading of 0.5–1.0% was optimal. Menthone/isomenthone
yields of up to 68% at 73% overall selectivity could be
observed. That means that the oxidation with Ru on ceria in a
continuous reactor resulted in significantly better yields than it
The Ru-based catalysts were prepared following a literature
procedure: 1.0%RuMnCe/CeO : To a suspension of 10.0 g CeO in
210 mL deionized water, a solution of 0.223 g RuCl
Mn(NO3)2 4H O and 0.496 g Ce(NO ) 6H O in 40 mL water was
added. Then, using a syringe pump, a solution of 0.930 g of NaOH
and 0.704 g of Na CO in 20 mL of water was continuously added
dropwise while stirring the suspension over a period of 5 h. After
completion of the addition, the reaction suspension was heated to
65°C and stirred for 18 h. The solid was separated by centrifuga-
tion, washed three times with 20 mL of water and dried overnight
at 90°C in a drying oven. The active components Ru, Mn and Ce are
present on the support as oxides and/or hydroxides.
[44]
2
2
H O, 0,548 g
2
3
2
3 3
2
[29]
2
3
was already reported for discontinuous processing in a batch
reactor. One of the advantages of continuous processing is the
opportunity of adjusting and influencing the product selectivity
by kinetic control of the reaction. In addition, a Ru-based
catalyst was used to prove total oxidation as a side reaction of
the oxidative dehydrogenation in the investigated temperature
range. And not unimportant, the quality of the menthone/
isomenthone product is marketable.
Other compositions of Ru-based catalysts were prepared accord-
ingly, amounts of used reagents can be found in ESI.
Catalyst Characterization
Experimental Section
All catalysts were characterized by ICP-OES analysis, BET surface
and pore volume. Additionally selected catalysts were characterized
by TEM and XPS (see ESI).
Ag-Based Catalysts
The Ag-based catalysts were prepared in analogy to a literature
[43]
procedure; 1.5/4%AgSr/SiO : For catalyst preparation, homemade
2
SiO2 (synthesis see ESI) was fractionally sieved (315–710 μm).
Subsequently, 9.45 g of the support material (pore volume:
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Scheme 2. Flow sheet of the reactor set-up.
was calculated by removing and weighing the saturator after each
test.
Catalytic Reaction
All reactions were conducted in a home-made set-up for the
oxidative dehydrogenation in the gas phase (see Scheme 2). It
consisted of a stainless steel fixed-bed tubular reactor (inside
diameter 1.0 cm) enclosed by a heating oven, a saturator (filled Acknowledgements
with menthol) placed in a thermostat, digital MFCs for gas dosing
and control, a vessel for collection of product samples (filled with
The authors gratefully acknowledge the support of Dr. M.-M. Pohl,
6 mL of o-xylene) placed in an oil bath, and gas lines heated by
heating tapes. The set-up was monitored by control elements and
thermocouples. To carry out the experiments, first the reactor was
filled with 3 mL of the previously granulated catalyst. The saturator
filled with menthol was then immersed in the thermostat solution,
which was heated to the desired temperature (usually 120°C). With
the exception of the gas line to the saturator, the temperature of all
other lines was set to 200°C by heaters (heating bands) equipped
with thermocouples and control units, and kept constant. During
Dr. C. Kreyenschulte, Dr. H. Lund and Dr. J. Radnik for analytical
measurements and discussion and R. Bienert for technical
assistance.
Conflict of Interest
The authors declare no conflict of interest.
the heating phase, the complete test apparatus was flushed with N
via a gas line connected to the feed line after the saturator.
2
À 1
Subsequently, a gas mixture of 5/95 vol% O /Ar (e.g. 30 mLmin )
Keywords: oxidative dehydrogenation · gas-phase reactions ·
2
was passed into the saturator, which was heated to 120°C, when
supported silver catalyst
· supported copper catalyst ·
the N purge gas feed had been closed. The evaporated volume
2
supported ruthenium catalyst
fraction of menthol was determined on the basis of its vapour
pressure curves and was about 4% by volume. Thus, by using 5%
by volume O /Ar as reaction and carrier gas, a small excess of
2
oxygen (theoretical menthol/oxygen ratio 0.8:1.0) was adjusted for
oxidative dehydrogenation. The reaction was started by opening
the gas valve from the saturator to the reactor. Each catalyst was
investigated at 300 and 350°C. In order to prevent crystallization of
the unreacted menthol (mp: 41–45°C) at the reactor outlet, the gas
line from the reactor outlet to the collecting vessel was heated to
100°C. In addition, 6 mL of o-xylene was placed into the collecting
vessel serving as capture solution, and heated with an oil bath to
[1] H. Surburg, J. Panten, Common Fragrance and Flavor Materials, Wiley-
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