Selective Hydrogenation of Benzene to Cyclohexene over 2Ru/La2O3-ZnO Catalyst without Additional Modifiers

Article abstract of DOI:10.1002/cctc.201600493

Selective benzene (BEN) hydrogenation to cyclohexene (CHE) has been studied in a complex four-phase system by using 2Ru/La₂O₃-ZnO as the catalyst. The catalyst is highly efficient (yield of desired cyclohexene=28 %) and stable for about five cycles. Characterizations reveal formation of La(OH)₃ on the catalyst surface and formation of Zn(OH)₃ ^? during the reaction, which play a key role in the formation of CHE. For the first time, the reaction mixture can be observed in situ, showing the emulsion of this complicated four-phase system. The droplet size of the organic compound and the rate of hydrogenation are correlated with different stirring rates excluding mass transfer limitations.

Full text of DOI:10.1002/cctc.201600493

DOI: 10.1002/cctc.201600493
Full Papers
Selective Hydrogenation of Benzene to Cyclohexene over
Ru/La O -ZnO Catalyst without Additional Modifiers
2
2
3
[
a]
Hendrik Spod, Martin Lucas, and Peter Claus*
Selective benzene (BEN) hydrogenation to cyclohexene (CHE)
has been studied in a complex four-phase system by using
which play a key role in the formation of CHE. For the first
time, the reaction mixture can be observed in situ, showing
the emulsion of this complicated four-phase system. The drop-
let size of the organic compound and the rate of hydrogena-
tion are correlated with different stirring rates excluding mass
transfer limitations.
2
Ru/La O -ZnO as the catalyst. The catalyst is highly efficient
2 3
(yield of desired cyclohexene=28%) and stable for about five
cycles. Characterizations reveal formation of La(OH) on the
3
ꢀ
catalyst surface and formation of Zn(OH)3 during the reaction,
Introduction
Sustainable economics in the production of industrial products
has attracted a lot of attention over the past few decades. For
a better comparison, respective to green processes, a standard
set of 12 principles has emerged, which can be used to assess
necessary, the use of ZrO as a dispersant, and the presence of
2
an aqueous solution of ZnSO . ZnSO acts as a modifier and
4
4
adsorbs on the catalyst surface, achieving a more hydrophilic
surface structure. Thereby, the catalyst develops a hydrate shell
that prevents re-adsorption of the formed cyclohexene that
occurs because of the inferior solubility of cyclohexene in
water compared with benzene (factor of six at 1508C,
[
1]
any process. It is not possible to implement all 12 principles,
but they help to improve existing industrial applications. The
formation of adipic acid is based on the hydrogenation of ben-
zene to cyclohexane, followed by an air oxidation of cyclohex-
ane to a mixture of cyclohexanone and cyclohexanol at low
conversions of 4 to 8%. Disadvantages of this route are the re-
sulting byproducts such as alcohols, aldehydes, and ketones,
because the treatment of these during the oxidation step is
very expensive. In addition, an oxidation using peroxides has
also been realized, which is detrimentally associated with an
[5]
50 bar). Thus, much research has focused on finding novel
and greener catalysts (less ruthenium), resulting in ruthenium-
loaded catalysts on different metal oxides. Unfortunately, many
of these catalyst systems require additional modifiers such as
ZnSO or NaOH in the aqueous phase for high selectivities and
4
yields of cyclohexene. Using inorganic additives has two disad-
vantages: they generate more waste and additives may result
in corrosion and separation problems during the process. Be-
cause of this, some research has focused on finding supported
Ru catalyst systems without additives. These novel catalysts
can be categorized into two types: those using different co-
[
2]
explosion risk. As an alternative to these routes, a selective
hydrogenation of benzene to cyclohexene followed by a hydra-
tion over H-ZSM5 zeolites to cyclohexanol in high selectivities
(
>99%) is possible. This route is favored because of its simplic-
[6]
ity and atomic economy. The formation of cyclohexene based
on benzene is a key step in the technical development, be-
cause the formation of cyclohexene is thermodynamically not
metals such as Co, Cu, or Zn with ruthenium on the support
and supported ruthenium catalysts without any additional ad-
[7]
ditives. The present work shows the performance of a simple
low loaded ruthenium catalyst, supported on La O -ZnO, in the
ꢀ
1
preferred (formation of cyclohexene: ꢀ23 kJmol , formation
2
3
ꢀ
1
of cyclohexane: ꢀ98 kJmol ). Therefore, formation of cyclo-
selective benzene hydrogenation to cyclohexene without addi-
tional additives during the reaction. For the first time, the reac-
tion mixture of the complicated four-phase system can be ob-
served in situ under the reaction conditions. Furthermore, the
catalyst performance is tested under different reaction condi-
tions and characterized in detail.
hexene in high yields requires a complex four-phase system
(
g/l/l/s), consisting of hydrogen (g), an organic (l) and an aque-
[
3]
ous phase (l), and a solid Ru catalyst (s). In 1989, Asahi Chem-
ical Industry Co., Ltd., developed a process for the selective
[
4]
benzene hydrogenation by using a Ru–Zn catalyst. The disad-
vantages of this process are the high amount of ruthenium
[a] H. Spod, M. Lucas, Prof. Dr. P. Claus
Technische Chemie II, TU-Darmstadt
Ernst-Berl-Institut fꢀr Technische und Makromolekulare Chemie
Alarich-Weiss Straße 8
Results and Discussion
Catalytic performance of Ru/La O -ZnO
2
3
6
4283 Darmstadt (Germany)
Based on previous work by our group, we tested the influence
of different parameters on the yield of cyclohexene on employ-
E-mail: claus@tc2.tu-darmstadt.de
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/cctc.201600493.
[3,8]
ing Ru/La O -ZnO as the catalyst.
The optimization was
2
3
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Table 1. Range for the parameter optimization by statistical software
Design of Experiments).
(
Parameter
Range
Units
Pressure
Temperature
10–50
100–150
0.5–3
1–5
bar
8C
g
wt%
g
m
catalyst
LoadingRu
m
NaDCA
0–10
done by a statistical software DoE (Design of Experiments)
under the given reaction conditions (see Table 1).
The software allows identification of the interactions of pa-
rameters with preferably few experiments. Thus, the number
of single experiments (16807 experiments by using a fractional
experimental plan with seven levels) for a parameter optimiza-
tion can be minimized to 35 runs. Further details can be ob-
Figure 2. Effect of pressure on the yield (empty squares) and reaction rate
(
empty triangles) of benzene conversion to cyclohexene. Reaction condi-
tions: 3 g 2Ru/La -ZnO, 50 mL benzene, 100 mL H O, 1508C, 1000 rpm.
2
O
3
2
[
9]
tained in the literature. After the implementation of the ex-
periments, the software constructs a model for the optimal re-
action conditions, including the different effects of the param-
eters. The model in Figure 1 shows an increasing yield of cyclo-
hexene up to 29% without using any additives during the
reaction with an increasing pressure at an optimum tempera-
ture of 1508C, using 3 g of 2Ru/La O -ZnO as the catalyst.
served no change in selectivity to the intermediate product at
pressures between 50 and 80 bar. The same observations were
[10]
made by different groups. On closer consideration of the ini-
tial reaction rate of hydrogen, we observed an increasing rate
up to an optimum at 50 bar. It is known that the rate of hydro-
genation of benzene to cyclohexane on nickel-based catalysts
becomes zero order with respect to hydrogen whenever the
2
3
[11]
pressure exceeds 20 bar. Hu et al. reported that if the ad-
sorption processes of reactants (H , benzene, and cyclohexene)
2
on the catalyst surface competed to a certain extent, the initial
hydrogenation rate of benzene would increase gradually with
an increase in the hydrogen pressure to the maximum point,
corresponding to the equal surface coverage of benzene and
hydrogen. At higher hydrogen pressure, the rate would de-
crease owing to higher surface coverage of hydrogen com-
[12]
pared with benzene, resulting in lower reaction rates. Fur-
thermore, the maximum reaction rate is attributable to the dif-
ferent dependence on the reaction rates of r and r , see
1
2
[13]
Scheme 1. Thus, the hydrogenation of benzene (r ) and cy-
1
clohexene (r ) is a function of the surface adsorbates [H , ben-
2
2
zene (BEN), and cyclohexene (CHE)] on the catalyst surface
Eq.(1)–(2)]:
[
Figure 1. Parameter optimization by statistical software (DoE). Yield of cyclo-
hexene against pressure of hydrogen and temperature without any addi-
tional additives, using 3 g of 2Ru/La O -ZnO.
2 3
Effect of hydrogen pressure on the selective hydrogenation
of benzene
Because of the increasing yield of cyclohexene along with in-
creasing pressure, we made a detailed study of H pressures
2
(20 to 80 bar) on the yield and selectivity to cyclohexene over
2
Ru/La O -ZnO, see Figure 2. The selectivity to cyclohexene in-
2
3
creased with rising H pressure from 20 to 50 bar and a con-
2
stant level at pressures higher than 50 bar hydrogen was ach-
ieved (Figure S1 in the Supporting Information). Furthermore,
we reached a maximum cyclohexene yield of 28% and ob-
Scheme 1. Reaction routes: benzene to cyclohexene r
cyclohexane r
1
and cyclohexene to
2
.
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r ¼ k ½q ꢁ½qBENꢁ
ð1Þ
ð2Þ
of surface carbonates of the supporting material formed
during the preparation of the samples, back to the oxidic spe-
cies of lanthanum and zinc. Testing the performance of the cat-
alysts reduced at different temperatures (200, 300, 400, and
5008C) leads to an increased yield and activity for cyclohexene
with an increased temperature of reduction up to 4008C. At
temperatures higher than 4008C, the yield and activity de-
crease slightly, see Table 2. This optimum at 4008C is explained
1
1
H2
r ¼ k ½q ꢁ½qCHEꢁ
2
2
H2
Consequently, decreasing the partial pressure of H at the
2
optimum of 50 bar would result in a lower surface concentra-
tion of H , resulting in a lower effective reaction rate (Figure S2
2
in the Supporting Information) and yield of cyclohexene, see
Figure 2.
2 3
Table 2. Catalytic performance of 2Ru/La O -ZnO reduced at different
temperatures.
Influence of the reduction temperature
[a]
The catalytic performance of 2Ru/La O -ZnO for the formation
[b]
2
3
Reduction temperature
[8C]
Time
[min]
Conversion
(X) [%]
Selectivity
(S) [%]
Yield
(Y) [%]
of cyclohexene based on benzene was tested by using differ-
ent reduction temperatures of the catalyst. Figure 3 shows
200
120
20
18
56
63
68
67
29
34
39
39
16
21
27
26
a H -TPR (temperature-programmed reduction) profile of the
300
2
4
5
00
00
catalyst 2Ru/La O -ZnO and the support La O -ZnO. The peaks
2
3
2
3
35
at temperatures between 100 and 4008C are attributed to the
reduction of RuCl into the active Ru species. The peaks at
[a] Reaction conditions: 3 g 2Ru/La
508C, 50 bar, 1000 rpm. [b] Time of reaction at the maximum yield of cy-
clohexene.
2 3 2
O -ZnO, 50 mL benzene, 100 mL H O,
3
1
higher temperatures than 4008C are a result of the reduction
by the enhanced adsorption of H , because a high
2
temperature promotes the elimination of residual
chlorine on the catalysts. The decreasing performance
of the catalyst at a higher reduction temperature may
be a result of the agglomeration of ruthenium parti-
[14]
cles.
In conclusion, a reduction temperature of
4008C leads to a very active and highly effective cata-
lyst for the formation of cyclohexene without any fur-
ther additives.
The X-ray photoelectron spectroscopy (XPS) spectra
of the catalyst 2Ru/La O -ZnO reduced at 4008C show
2
3
ꢀ
no peak for Cl . Thus, all the chlorine of the precursor
was removed during the washing and reduction pro-
cess. Figure 4 shows the Ru3d spectra. There are two
problems associated with determining the oxidic
state of ruthenium: the peak of Ru3d is overlapped
by the peak of C1s and we observe different Ru spe-
Figure 3. H
effect of reduction temperature on the maximum yield of cyclohexene. Reaction condi-
tions: 3 g 2Ru/La -ZnO, 50 mL benzene, 100 mL H O, 1508C, 50 bar, 1000 rpm.
2
-TPR profile of the supporting material and unreduced catalyst with the
0
2
O
3
2
cies on the catalyst surface from Ru (279.91 eV) up to
2 3
Figure 4. XPS spectra of Ru3d of 2Ru/La O -ZnO reduced at 4008C.
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2
the number of droplets at an area of 25 mm plotted
against the stirring rate. We observe an increasing ini-
tial rate and amount of droplets up to 1000 rpm and
a steady state at stirring rates higher than 1000 rpm.
Moreover, there is no change in the selectivity and
yield of cyclohexene at stirring rates higher than
1000 rpm (Supporting Information, Figures S3, S4,
and S5). We see that the maximum dispersion of the
organic compound in the aqueous phase is 150 drop-
2
lets per 25 mm . Probably, a higher dispersion of the
organic phase might result in an increased reaction
rate, resulting in higher yields of cyclohexene. There-
2
fore, the formation of 150 droplets per 25 mm could
be a result of a mass transfer limitation of the organic
phase into the aqueous phase. In this case, Nagahara
et al. describes the use of different metal oxides as
possible dispersing agents for the organic compound
2 3
Figure 5. XPS spectra of La3d of 2Ru/La O -ZnO reduced at 4008C.
+
x
[15]
Ru , because of the oxidation of Ru particles during the XPS
sampling procedure. It is not clear which are the active species
for the selective hydrogenation of benzene. The La3d doublet
or for the stabilization of the catalyst. Odenbrand et al. de-
termined the droplet size to be between 0.05 and 0.14 mm
[16]
and the particle size of catalyst to be between 3 and 30 mm.
(Figure 5) at 834.98 and 838.11 eV is indicative of the formation
of La O , La(OH) , and La (CO ) . Hereby, we detect the forma-
2
3
3
2
3 3
tion of an oxidic species of lanthanum. The doublet of the
Zn2p in Figure 6 (1013.66 and 1036.05 eV) indicates that the
oxidation state of Zn is +2, implying the formation of ZnO or
Zn(OH)2.
Droplet size of the organic phase and mass transfer limita-
tions
Because of the dominant role of the mass transport in the se-
lective liquid-phase hydrogenation of benzene, we correlate
the activity of the catalyst and the droplet size of the organic
phase with different stirring rates. An increasing stirring rate
has two influences: a better dispersion of the catalyst particles
and of the organic compound in the aqueous phase, resulting
in an enlarged contact area between the gas, solid, and liquid
phases. Figure 7 shows the initial rate of hydrogenation and
Figure 7. Effect of stirring rate in the liquid phase on the initial rate of hy-
drogenation of benzene to cyclohexene and amount of droplets at an area
2
of 25 mm .
2 3
Figure 6. XPS spectra of Zn2p of 2Ru/La O -ZnO reduced at 4008C.
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Figure 10. Recycle tests of 2Ru/La
conditions: 3 g 2Ru/La -ZnO, 50 mL benzene, 100 mL H
000 rpm.
2
O
3
-ZnO without any additive. Reaction
Figure 8. Average droplet size at a stirring speed of 1000 rpm, 50 bar H
508C, 0.5 g 2Ru/La -ZnO, 50 mL benzene, 100 mL H O.
2
,
2
O
3
2
O, 1508C, 50 bar,
1
2
O
3
2
1
We determined the average droplet size of the organic phase
under the reaction conditions to be 0.27 mm at a stirring rate
of 1000 rpm, see Figure 8, and the associated picture is shown
in Figure 9. The particle size of the catalyst was determined to
be between 0.05 and 40 mm (Supporting Information, Fig-
ure S6).
ing of Zn from the supporting material into the aqueous
phase [molar ratio of support (n /n ) before experiment=
La Zn
10.56; after experiment=11.36].
Figure 11 shows XRD patterns of the supporting material
(La O -ZnO), the fresh catalyst (2Ru/La O -ZnO-b), and the used
2
3
2
3
catalyst after the reaction (2Ru/La O -ZnO-a). The freshly cal-
2
3
cined support (La O -ZnO) at 9008C shows only peaks associat-
2
3
ed with La O3 and ZnO. Thus, a calcination temperature of
2
Recycling of the catalyst
9008C forms the oxide of lanthanum and zinc in high crystal-
Recycling tests (Figure 10) for the selective hydrogenation of
linity. As a result of the high dispersion of the Ru particles,
there are no peaks associated with ruthenium, expected at
2q=448, for the reduced catalyst before the reaction. Further-
more, we observe peaks associated with lanthanum oxide hy-
benzene, using 3 g 2Ru/La O -ZnO catalyst at 1508C and
2
3
50 bar hydrogen without further additives, show a slight de-
crease in activity, but no change in the selectivity to cyclohex-
ene and conversion of benzene, resulting in yields of about
droxide (LaOOH), lanthanum oxide carbonate [La
O (CO )], lan-
2 3
2
28% for five cycles. After five times of reuse, the catalyst
thanum hydroxide [La(OH) ], and zinc oxide (ZnO). The reason
3
shows no leaching of Ru, measured by inductively coupled
plasma optical emission spectroscopy (ICP-OES) techniques
for the formation of the hydroxides and carbonates is the
basic character of the lanthanides, which absorb atmospheric
water and carbon dioxide, resulting in the formation of car-
(Supporting Information, Table T1). However, we detect leach-
Figure 9. Picture of the complex four-phase system at a stirring speed of 1000 rpm, 50 bar H
2 2 3 2
, 1508C, 0.5 g 2Ru/La O -ZnO, 50 mL benzene, 100 mL H O using
a reactor with a sapphire glass (500 mL autoclave, PARR) and a photo set-up (Canon, macro zoom lens, shutter speed 1/2000).
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Figure 11. XRD of the supporting material (La
2
O
3
-ZnO), the catalyst before
-ZnO-a).
2 3
Figure 12. FTIR of the supporting material (La O -ZnO) and the catalyst
(
2Ru/La -ZnO-b), and after the reaction (2Ru/La O
2
O
3
2 3
before (2Ru/La
2
O
3
2 3
-ZnO-b) and after the reaction (2Ru/La O -ZnO-a).
[17]
2ꢀ
bonate anions, surface carbonates, or hydroxyl carbonates.
asymmetric stretching mode of n(CO)3 of the lanthanum
The presence of water and the prevailing reaction conditions
oxide carbonate [La O (CO )]. Observation of hydroxyl groups
2
2
3
lead to formation of lanthanum hydroxide [La(OH) ], lantha-
on the supporting material and the catalyst can be seen,
owing to the rapid hydroxylation of the metal oxides to form
3
num carbonate hydroxide (LaCO OH), and zinc hydroxide
3
[20]
[
Zn(OH)2] on the catalyst surface (2Ru/La O -ZnO-a), see
a stable hydroxide. In particular, the presence of hydroxyl
groups on the surface is important for the hydrophilic charac-
ter of the catalyst. Formation of La(OH) starting from La O
3
2
3
Figure 11. In particular, the formation of zinc hydroxide
Zn(OH) ] plays an important role in the selective hydrogena-
[
2
3
2
tion of benzene. The amphoteric character of zinc hydroxide
after oxidation of the supporting material can be confirmed by
thermogravimetric (TG)-MS measurements (Figure 13). Two
well-resolved peaks can be found at temperatures of 328 and
4748C. These two peaks are in accordance with the formation
2ꢀx
allows the formation of zincate anions [Zn(OH)x , x=3, 4, 6],
which effectively retards the further hydrogenation of cyclo-
hexene. This is the reason for the leaching of Zn during the re-
[
10f]
[20]
cycling tests. Wu et al.
studied the effect of small amounts
of LaOOH and La O .
2
3
ZnO as an additive on the selective hydrogenation of benzene.
They found an increased selectivity to cyclohexene because of
Because of the correlation between the acid properties of
the catalyst and the performance of the catalyst for the selec-
2ꢀ
[21]
the solubility of Zn(OH)
in water. It is possible that
tive hydrogenation of benzene described by Zhou et al.,
4
2ꢀ
Zn(OH)4 in hydrous solution will be adsorb on the surface of
the catalyst, which would result in the coverage of some active
sites that would be unsuitable for the formation of cyclohex-
ene and the improvement of the hydrophilicity of the catalyst.
NH -TPD (temperature-programmed desorption) characteriza-
3
tion was implemented (Figure 14). After the loading of the
supporting material with 2 wt% Ru and reduction, the acidity
ꢀ
1
decreased from 21 to 19 mmolg .
[
18]
Reichle et al. reported the increased solubility of
Zn in an aqueous phase with increasing pH and tem-
perature of the solution. Because of the alkaline char-
acter of the aqueous phase (pH after reaction=12.3),
ꢀ
formation of Zn(OH)3 in the presence of ZnO is fa-
vored. The absorbance at 290 nm at the UV/Vis spec-
trum of the aqueous phase after the reaction (Sup-
porting Information, Figure S7) confirms the presence
of a zinc species in the aqueous solution.
The FTIR spectra of the supporting material La O -
2
3
ZnO and the catalyst before the reaction (2Ru/La O -
2
3
ZnO-b) are nearly same (Figure 12). Thus, the rutheni-
um has no influence on the surface structure of the
ꢀ
1
catalyst. We observe a sharp band around 3608 cm
attributed to the n(OꢀH)ꢀ stretching mode of
[
17]
La(OH)3.
The absorption bands at 632 and
ꢀ
1
4
60 cm are the corresponding stretching modes of
[
19]
n(LaꢀO) and n(ZnꢀO).
The bands at 1557 and
Figure 13. TG-MS of the calcined supporting material La
2
O
3
-ZnO. Heating rate:
ꢀ
1
ꢀ1
1
451 cm in 2Ru/La O -ZnO-a are attributed to the 108Cmin
.
2
3
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Conclusions
A simple 2Ru/La O -ZnO catalyst was synthesized for the selec-
2
3
tive hydrogenation of benzene to cyclohexene without any ad-
ditional additives during the reaction, which resulted in a maxi-
mum cyclohexene yield of 28%. We determined for the first
time the droplet size of the organic phase (0.27 mm) in the
complex four-phase system under the reaction conditions. The
reusability of the catalyst over five cycles, showing the high
stability of this catalyst system. It was found that leaching of
ZnO forms a zincate species in the aqueous phase during the
reaction and this plays a key role in the formation of cyclohex-
ene. By obviating the need for additional additives such as
ZnSO or NaOH during the reaction, we generate a greener
4
catalyst system for this reaction.
Figure 14. NH
the reaction (2Ru/La
3
-TPD of supporting material La
2 3
O -ZnO and catalyst before
ꢀ1
2
O
3
-ZnO-b). Heating rate: 58Cmin .
Experimental Section
Catalyst preparation
The Ru/La O -ZnO catalysts were prepared by a co-precipitation
2
3
method and an incipient wetness method by following a protocol
[12]
described elsewhere.
The supporting material La O -ZnO with
2 3
a molar ratio of n(La O )/n(ZnO)=5:1 was prepared by the precipi-
2
3
tation method. An aqueous sodium carbonate solution (65 mL, c=
ꢀ
1
1
molL ) was added quickly at 708C to a solution of nitrate salts
of lanthanum and zinc in distilled water (250 mL). After aging for
h, the precipitate was filtered and washed thoroughly with dis-
1
tilled water. The white powder was dried overnight at 1008C in an
oven. To form the oxide, the sample was calcined in an oven from
ꢀ1
room temperature to 9008C at 108Cmin and held at 9008C for
h. The catalysts were prepared by the incipient wetness method.
3
The calcined support was impregnated with the desired amount of
an aqueous solution of RuCl ·3H O and dried overnight at 1008C.
3
2
Afterwards, the catalyst was reduced in
a
hydrogen flow
ꢀ1
ꢀ1
(100 mLmin ) at 4008C for 3 h with a heating rate of 58Cmin .
Catalyst testing
The desired amount of catalyst, benzene (50 mL), and H O
2
(100 mL) were placed in the reactor (300 mL autoclave, PARR, stir-
ring speed 1000 rpm). After sealing, the reactor was flushed three
times with argon (10 bar, Air Liquide) and heated to the desired
temperature under an argon atmosphere (2 bar). The addition of
H (Air Liquide) defined the start of the reaction (constant pres-
2
sure). In defined intervals, samples were taken for gas chromato-
graphic analysis (Shimadzu GC 2010 Plus, capillary column DB-Wax,
l=30 m, d =0,25 mm, t =0,25 mm).
Figure 15. Particle size distribution histogram with Gaussian analysis fittings
and TEM images of the Ru particles of the catalyst before (2Ru/La -ZnO-
b (A)) and after (2Ru/La -ZnO-a (B)) the reaction.
2
O
3
i
i
2
O
3
TEM images and the particle size distribution histogram with
Gaussian analysis fittings of the Ru particles of the catalyst
before (2Ru/La O -ZnO-b) and after (2Ru/La O -ZnO-a) the reac-
Recycling tests
Recycling tests were performed as follows. The organic phase was
separated from the aqueous phase after cooling the reactor to
room temperature and benzene (50 mL) was re-introduced into
the reactor. After sealing, the reactor was flushed three times with
argon (10 bar, Air Liquide) and heated to the desired temperature
2
3
2
3
tion are given in Figure 15. The Ru particle size of 2Ru/La O -
2
3
ZnO increased slightly during the reaction from 1.07 nm (2Ru/
La O -ZnO-b) to 2.54 nm (2Ru/La O -ZnO-a). Nevertheless, the
2
3
2
3
increased Ru particle size during the reaction has no influence
on the yield of cyclohexene.
under an argon atmosphere (2 bar). The addition of H (Air Liquide)
defined the start of the new run of the recycling test.
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Full Papers
Characterization
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Figure 16. 500 mL reactor with sapphire glass and photo set-up for measur-
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Acknowledgements
The authors would like to thank the BASF SE for financial support
and are grateful to Dr. Jçrg Radnik (LIKAT, Rostock) for perform-
ing the XPS analyses.
Keywords: benzene · hydrogenation · multiphase systems ·
lanthanum · ruthenium
Received: April 26, 2016
Published online on && &&, 0000
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ÝÝ These are not the final page numbers!

FULL PAPERS
Benzene hydrogenation: A simple 2Ru/
H. Spod, M. Lucas, P. Claus*
& – &&
La O -ZnO catalyst was tested in the se-
2
3
&
lective hydrogenation of benzene. The
droplet size of the organic compound
in the complex four-phase system (g/l/l/
s) was determined for the first time in
situ under the reaction conditions.
Selective Hydrogenation of Benzene
to Cyclohexene over 2Ru/La O -ZnO
Catalyst without Additional Modifiers
2
3
ChemCatChem 2016, 8, 1 – 9
www.chemcatchem.org
9
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ