Simple selective hydrogenation of benzene to cyclohexene in the presence of sodium dicyanamide

Article abstract of DOI:10.1039/c3gc36615d

A new catalyst system was used in the liquid phase hydrogenation of benzene to cyclohexene containing only an aqueous solution of Ru/La₂O ₃ and a very small quantity of sodium dicyanamide (NaDCA). This additive considerably improves the catalyst performance compared to DCA based ionic liquids. The effects of the amount of NaDCA and metal loading of the catalyst were investigated and a high initial selectivity to cyclohexene of 70% was reached.

Full text of DOI:10.1039/c3gc36615d

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ARTICLE TYPE
Simple selective hydrogenation of benzene to cyclohexene in presence of
sodium dicyanamide
*
Frederick Schwab, Martin Lucas and Peter Claus
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A new catalyst system was used in the liquid phase hydrogenation of benzene to cyclohexene containing
only an aqueous solution of Ru/La O and a very small quantity of sodium dicyanamide (NaDCA). This
2
3
additive considerably improves the catalyst performance compared to DCA based ionic liquids. The
effects of the amount of NaDCA and metal loading of the catalyst were investigated and a high initial
selectivity to cyclohexene of 70 % was reached.
1
0
consisting of an organic and an aqueous phase, hydrogen and the
1
. Introduction
solid catalyst. The latter is always based on ruthenium and a
second metal. In addition, large amounts of inorganic salts are
dissolved in the aqueous phase which increase the hydrophilicity
of the Ru catalyst. For this purpose, especially zinc salts are
commonly used. NaOH is often added as well to increase the
selectivity to cyclohexene. It is believed that a hydrate shell is
formed around the inherently hydrophobic catalyst by the
4
5
5
6
6
7
7
5
0
5
0
5
0
5
The hydrogenation of benzene is of both industrial and academic
interest, i.e. for the production of cyclohexane (CHA) and
cyclohexene (CHE) and as benchmark test for the performance of
hydrogenation catalysts. Under industrial conditions, the
intermediates of this consecutive hydrogenation, cyclohexene, is
not formed with usual hydrogenation catalysts, even at low
degrees of conversion, which could be easily explained regarding
the free standard reaction enthalpies (formation of cyclohexene: ꢀ
1
5
aqueous phase. Because of the lower solubility of cyclohexene in
3
water compared to benzene in H O (factor 6 at 150 °C, 50 bar)
2
the reꢀadsorption of cyclohexene and the consecutive
hydrogenation to cyclohexane is impeded.
1
2
0
23 kJ / mol, formation of cyclohexane: ꢀ98 kJ / mol, Scheme 1)
and by kinetic reasons (high reactivity of cyclohexene).
After initial attempts of gas and liquid phase hydrogenation of
4
benzene to cyclohexene resulting in yields < 1 % Drinkhard
introduced an aqueous phase with salts and NaOH reaching
5
yields of 32 % for the first time. Based on these experiments
ASAHI Chem. Corp. developed a catalyst system which was used
in a pilot plant with an annual production of 60,000 tons of
6
Scheme 1 Benzene hydrogenation as a consecutive reaction.
cyclohexene in 1988. In this reaction system an unsupported Ruꢀ
Zn catalyst was suspended with zirconia in the aqueous phase. In
addition, zinc sulfate was dissolved. The catalyst system used
reached a cyclohexene selectivity of 80 % (maximum yield: 56
%). However, the salt loading in the reactor was significantly
2
3
3
4
5
0
5
0
The hydrogenation of benzene to cyclohexene is of
considerable industrial interest, as a cheap synthesis of this
intermediate would be an attractive alternative route to synthesize
products like adipic acid or εꢀcaprolactam (monomeres of
polyamides). The stateꢀofꢀtheꢀart process for the synthesis of εꢀ
caprolactam consists of the complete hydrogenation of benzene to
cyclohexane with
hexanol/cyclohexanone mixture. However, this oxidation step can
only be performed at a conversion level of 5 to 10 % to reach
7
high, about approximately 50 times of the amount of Ru.
Recent studies included a variety of second metals, various
8
support materials and new additives in the aqueous phase. Using
a
subsequent oxidation to
a
cycloꢀ
a lanthanum doped Ru/SBAꢀ15 catalyst Liu et al. achieved a
9
yield of 56 %. In addition to zinc sulfate as additive, toxic
cadmium sulfate was used as well. According to the authors the
latter should provide a direct modification of the catalyst surface,
while zinc sulfate should stabilize the cyclohexene in the aqueous
phase and thus preventing its reꢀadsorption (and the complete
hydrogenation). A catalyst without additives in the aqueous phase
2
high selectivities. If the challenging task, namely a selective
hydrogenation of benzene to cyclohexene, could be achieved, the
latter can easily be hydrated to cyclohexanol. This indicates a
significant improve to the existing process as the disadvantages
of the process would be eliminated and the production of εꢀ
caprolactam would be both energetically more efficient and
resource saving.
10
was used by Liu et al reaching a yield of 56 %. Still, the solid
catalyst consists of all parts which are used in the aforementioned
catalyst systems. It is described in the form of Ru/ZnOꢀ
ZrO (OH) . NaOH was also added in the production process via
The reaction system of the selective hydrogenation of benzene
described in literature is a very complex tetraꢀphase system,
x
y
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precipitation. The reduction with sodium borohydride is
2.2 Catalyst Testing
mentionable whereby the catalyst could be contaminated with
boron. The leaching behavior was not investigated by which ZrO2
and ZnO reached the liquid phase and then could act as the
aforementioned additives. Sun et al. reached a maximum yield
of cyclohexene of 64 % by increasing the ruthenium content and
adding small amounts of diethanolamine.
The stateꢀofꢀtheꢀart can thus be characterized as a complicated
multiꢀphase system containing benzene, hydrogen, Ru catalyst,
water and other additives. It has to be noted that without the latter
no cyclohexene formation is observed. Due to the high precious
metal content of the catalyst (up to 95 wt. %) and the large
amounts of salt additives and NaOH in the aqueous phase the
systems described are neither environmentally nor economically
viable. So there is still need for further research.
The catalyst (0.5 g), the aqueous NaDCA solution (100 mL) and
the benzene phase (50 mL) were introduced into the autoclave
(300 mL, Parr Instruments, r = 1000 rpm) and were flushed twice
with argon (10 bar). The reactor was heated to 373 K under argon
5
5
1
1
5
0
5
0
5
0
5
(2 bar). The introduction of hydrogen defined the start of the
reaction. In defined intervals samples were taken for gas
chromatography analysis (HP 6890, FID, capillary column
Agilent DBꢀWax, l=30 m, d =0.25 mm, t =0.25 ꢁm).
60
i
i
1
1
2
2
3
3
2
.3 Characterization
The catalyst was analyzed by Xꢀray photoelectron spectroscopy
VG ESCALAB 220 iXL with an Al Kα radiation source under
(
ambient conditions) to determine the oxidation state of ruthenium
and to prove the existence of ingredients of the additive on the
catalyst surface.
65
Recently, we presented a simple catalyst system, consisting of
only supported ruthenium in water with very low quantities (ppm
range) of the ionic liquids [B3MPyr][DCA]. The need of salt
additives and second metals is completely eliminated.
Nevertheless, the formation of cyclohexene occurs with high
3. Results and Discussion
12
selectivity. This catalyst system is in good agreement with the
The catalytic behavior of 1Ru/La
2
O
3
in the selective
1
3
1
2 principles of green chemistry. The needed amounts of active 70 hydrogenation of benzene was investigated with different DCA
metal and ionic liquid could be reduced dramatically as well as
separation and corrosion problems could be avoided.
In the present work this system is further improved by the
based ILs as additives. They were used in equimolar
concentration. The high initial selectivities of the experiments
with ILs were of about 60 %. However, with sodium dicyanamide
(NaDCA) as additive an initial selectivity of 66 % was reached
usage of a very small quantity of sodium dicyanamide (NaDCA)
in presence of lanthanum oxide supported ruthenium. The 75 and cyclohexene was the main product at conversions below 15
catalyst performance is compared to three ionic liquids
comprising the DCA anion and [BMIM], [BMPL] or [B3MPyr]
in the cationic part of the molecule (Scheme 2). Ionic liquids have
received by now a very wide range of applications in chemical
research, including such as solvents in organic syntheses or in the
%. This catalyst system is more selective over the whole
conversion range than these containing ILs which clearly
demonstrates a further improvement of the [B3MPyr][DCA]
based alumina supported catalyst system.
In comparison with NaDCA as additive the ILs had an
inhibiting influence on the performance of the catalyst. The
selectivity to cyclohexene was decreased in all experiments. The
imidazolium and pyridinium cations lowered the conversion from
45 % to 34 % and 13 %, respectively, after 240 min. Maybe,
80
14
SILP and SCILL concepts in catalysis research.
The
dicyanamide based ionic liquids (IL) used here have to be water
soluble to avoid any fifth phase in the reaction system and must
decompose neither on the surface of the Ru catalyst nor in the
1
5
presence of water or reactants.
85 there is a competitive adsorption of the aromatic educt benzene
and these aromatic cations on the active sites of the catalyst.
7
6
5
0
0
0
+
[
[
[
[
B3MPyr]
+
BMPL]
+
BMIM]
+
Na]
40
Scheme 2 Structures of the DCA based ionic liquids (IL).
40
3
0
2
. Experimental
2
.1 Catalyst Preparation
20
10
0
The Ru/La O catalysts were prepared by impregnation following
a protocol described elsewhere. A certain amount of ruthenium
chloride hydrate was dissolved in 500 mL of water to create a
2
3
1
6
45
0
10
20
30
40
80
90
0
.01 wt. % solution. The corresponding amount of lanthanum
X (BEN) / %
ꢀ
oxide was suspended in the solution. The black solution changed
its color to green. After 3 h of stirring at room temperature the
catalyst was filtered off and dried under air for 15 h at 333 K. The
catalyst (5 g) was reduced in a hydrogen flow (87 mL/min) at 673
K (β = 5 K/min, 3 h isotherm, passive cooling).
2 3
Fig. 1 Effect of the cations of DCA based ILs, conditions: m (1Ru/La O )
ꢀ
=
2 2
0.5 g, n ([DCA] ) = 110 ꢁmol, T = 373 K, p (H ) = 20 bar, V (H O)/V
50
90
(BEN) = 2:1.
NaDCA is a cheap alternative to DCA based ionic liquids.
This journal is © The Royal Society of Chemistry [year]
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degrees of ca. 25 % with an initial selectivity (SCHE,max) around
5ꢀ70 %. This implies a similar Ru dispersion of the catalysts.
6
70
60
50
40
30
20
10
0
3
3
4
0
5
0
Chemisorption experiments with hydrogen gave low and
comparable values of 1.9, 2.7 and 2.8 ꢁmol H /g , respectively,
2
cat
0
0
10
ꢀ
mol
from which large Ru particle sizes can be calculated. However,
the presence of Ru particles could not be affirmed in control
experiments by TEM analysis. Keeping in mind the preparation
protocol of the catalysts which were reduced at 673 K, it must be
assumed that the ruthenium surface is covered with partially
reduced lanthanum oxide due to wellꢀknown SMSI effect (Strong
6
1
ꢀmol
ꢀ
ꢀ
mol
mol
170
17
Metal Support Interaction). Only at higher conversions the
Ru/La O catalyst was more selective than the other two with
5
2
3
lower Ru loading.
0
10
20
30
40
50
60
70
80
90
100
X (BEN) / %
ꢀ
Fig. 2
Effect of the amount of NaDCA, conditions: m (1Ru/La
2 3
O ) =
0
.5 g, add.: NaDCA, T = 373 K, p (H
2
) = 20 bar, V (H O)/V (BEN) = 2:1.
2
Because of the large gain in selectivity in comparison with the
5
ILs (up to 20 % at same conversion) the amount of the additive
was varied to analyze its influence on the reaction (Fig. 2). There
was only a small range of DCA addition in which the NaDCA
had a positive effect on the performance of the catalyst. In
comparison with the experiment without IL the selectivity to
cyclohexene was increased to 40 % by adding only 60 ꢁmol
NaDCA in combination with an increase of the reaction time
from 60 to 240 min. A further addition of NaDCA (110 and 170
1
1
2
0
5
0
ꢁ
mol) decreased the activity of the catalyst from 94 % to 42 %
and 28 %, respectively. The selectivities in these experiments
were increased further more and an excellent initial selectivity to
cyclohexene of about 70 % was achieved. It should be noted that
the addition of very small amounts of the additive has a
tremendous influence on the selectivity in the hydrogenation of
benzene. This is a similar effect as it was described before for the
influence of the IL [B3MPyr][DCA] in the catalyst system with
1
2
alumina as support.
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
1
Ru/La O
2
3
3
3
2Ru/La O
Fig. 4 N1s spectra of 1Ru/La O before and after reaction (BE of N1s =
398 eV).
2
2
3
5
Ru/La O
2
4
5
5
5
0
5
From the decreasing conversion with increasing NaDCA
amount (Fig. 2) it can be assumed that the additive interacts with
the surface of the ruthenium catalyst. To provide evidence for the
interaction of ruthenium active sites and NaDCA the 1Ru/La O
2
3
catalyst was analyzed by Xꢀray photoelectron spectroscopy
(XPS) before and after the reaction. The N1s spectra (Fig. 4)
show that nitrogen could be observed on the surface of the
catalyst after the reaction in which the NaDCA was used. In
addition, it has to be noted that sodium was not detectable.
Therefore, it can be concluded that the DCA anions adsorb on the
surface of the catalyst because the anion is the only origin for
nitrogen in this reaction system. From analyzing the surface
composition the ratio of surface atoms (s) of nitrogen to
0
10
20
30
40
50
60
70
80
X (BEN) / %
ꢀ
Fig. 3
(
Effect of metal loading, conditions: m (Ru/La
2 3
O ) = 0.5 g, n
DCA)/n (Ru) = 2.3, T = 373 K, p (H
2
) = 20 bar, V (H O)/V (BEN) = 2:1.
2
2
5
The selectivityꢀconversion plot obtained for three lanthanum
oxide supported ruthenium catalysts with different metal loading
1, 2 and 5 wt.% Ru) showed a similar behavior up to conversion
ruthenium can be calculated to (N/Ru)
lower than the ratio deduced from the chosen feed composition in
= 0.18 which is distinctly
s
(
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the liquid phase (l) (N/Ru) = 2.3. It is evident that a part of the
14 V. I. Parvulescu, C. Hardacre, Chem. Rev., 2007, 107, 2615; A.
l
Riisager, R. Fehrmann, S. Flicker, R. van Hal, M. Haumann, P.
Wasserscheid, Angew. Chem., Int. Ed., 2005, 44, 815; J. Arras, M.
Steffan, Y. Shayeghi, D. Ruppert, P. Claus, Green Chem., 2009, 11,
DCA concentration in the batch reactor is adsorbed on the
catalyst surface whereby the most active Ru sites are poisoned
leading to a strong decrease of the second hydrogenation step
CHE → CHA.
Finally, the leaching behavior was investigated by ICPꢀOES and
mass spectroscopy. The analysis of the aqueous phase showed
neither ruthenium nor lanthanum to a detection limit of 0.2 ppm.
In the mass spectrum of the organic phase no indication of a Ru
complex were found. The absence of the typical isotope
distribution of ruthenium is evidence for no leaching of
ruthenium to the organic phase.
7
16.
5
15 M. Steffan, Dissertation, TU Darmstadt, 2008; E. T. Silveira, A. P.
Umpierre, L. M. Rossi, G. Machado, J. Morais, G. V. Soares, I. J.
Baumvol, S. R. Teixeira, P. F. P. Fichtner, J. Dupont, Chem. Eur. J.,
2
004, 10, 3734.
1
1
6
7
O. Mitsui, Y. Fukuoka (ASAHI), US4678861, 1987.
S. S. Kim, S. J. Lee, S. C. Hong, J. Ind. Eng. Chem., 2012, 18, 1263;
I. Y. Ahn, W. J. Kim, S. H. Moon, Appl. Catal., A:
General, 2006, 308, 75.
10
4
. Conclusions
The selective hydrogenation of benzene to cyclohexene was
achieved with this catalyst system (Ru/La O + NaDCA) with
15
20
25
2
3
excellent initial selectivities of 70 %. At conversions below 15 %
cyclohexene is the main product and a maximum yield of 14 % is
reached. This is a further improvement of our presented catalyst
system towards a greener route to produce cyclohexene.
Compared to other catalyst systems based on RuꢀZn type
catalysts together with ZrO and ZnSO as additives, the amounts
2
4
of the expensive ruthenium and NaDCA, used in presented
approach, are about two orders of magnitude smaller. The DCA
anion has the most important influence on the activity and
selectivity of the catalyst. From XPS analysis, it is evident that
the DCA anion is adsorbed on the catalyst surface whereby the
most active Ru sites are poisoned so that the undesired total
hydrogenation to cyclohexane is inhibited.
Notes and references
3
0
5
Ernst-Berl-Institute, Chemical Technology II, Department of Chemistry,
Technische Universität Darmstadt, Petersenstr. 20, 64287 Darmstadt,
Germany. FAX: +49 6151 164788; Tel: +49 6151 165369; E-mail:
claus@ct.chemie.tu-darmstadt.de
†
The authors are grateful to Dr. Jörg Radnik (Rostock) for performing
3
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1
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4
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