Catalysis Communications 4 (2003) 9196

www.elsevier.com/locate/catcom

Liquid-phase hydrogenation of benzene to

cyclohexene catalyzed by Ru/SiO2 in the presence of
waterorganic mixtures
Estevam V. Spinac
e *, Jorge M. Vaz
Instituto de Pesquisas Energ

eticas e Nucleares IPEN/CNEN-SP, Centro de Qu
mica e Meio Ambiente CQMA,
Av. Prof. Lineu Prestes, 2242, Cidade Universit

aria, S~
ao Paulo, SP 05508-900, Brazil

Received 5 August 2002; received in revised form 6 December 2002; accepted 6 December 2002

Abstract

The liquid-phase hydrogenation of benzene to cyclohexene was studied using a Ru/SiO2 catalyst prepared by re-
duction of ruthenium(III) chloride impregnated in a hydrophilic non-porous silica. In a biphasic water/benzene system
at 423 K and 5 MPa of hydrogen pressure, a 14% cyclohexene yield was obtained at 60% benzene conversion. Increased
cyclohexene yields and selectivities were observed in the presence of ethylene glycol/water and glycerol/water mixtures,
which consist mainly of hydrated organic molecules that can enhance the hydrophilicity around the ruthenium particles
favoring the cyclohexene desorption.
2003 Elsevier Science B.V. All rights reserved.

Keywords: Benzene; Cyclohexene; Partial hydrogenation; Ethylene glycol; Glycerol; Hydrophilicity

1. Introduction e-caprolactone through hydrogenation of benzene

followed by direct oxidation of cyclohexane to
Asahi Chemical Industry Co. has developed a cyclohexanol and cyclohexanone [2,3].
technology for highly selective partial hydrogena- The partial hydrogenation of benzene to cy-
tion of benzene to cyclohexene thus establishing a clohexene is performed in a batch reactor under
new process for producing cyclohexanol [1]. In this stirring using non-supported ruthenium particles
process the theoretical consumption of hydrogen as catalyst in a water/benzene biphasic system. In
is reduced to one-third and fewer undesirable by- order to obtain high cyclohexene yields (48% of
products are formed in comparison to the con- cyclohexene yield at 60% of benzene conversion)
ventional process for making adipic acid and/or large amounts of zinc sulfate have to be added to
the aqueous phase (Ru:Zn molar ratio of 1:6). This
salt is chemisorbed on the ruthenium particles
*
Corresponding author. Tel.: +55-11-3816-9333; fax: +55-11-
surface changing their characteristic from hydro-
3816-9325. phobic to hydrophilic. In this manner, a water
E-mail address: espinace@net.ipen.br (E.V. Spinac
e). layer surrounds the ruthenium particles and favors

1566-7367/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1566-7367(02)00270-4
92 E.V. Spinac
e, J.M. Vaz / Catalysis Communications 4 (2003) 9196

cyclohexene desorption from the catalyst surface, best cyclohexene yield was 13% under the studied
due its very low solubility in water, increasing the conditions. Nagahara et al. [15] studied the partial
cyclohexene yield [1,4,5]. One drawback is the hydrogenation of benzene in the presence of dif-
possible corrosion of the reactor due to acidic pH ferent alcohols (butanols, allyl and benzyl alcohol)
of the aqueous phase and the gradual deactivation in the aqueous phase. A high selectivity to cyclo-
of the catalyst [5,6]. Lately, the development of hexene (70%) was obtained at 30% of benzene
new catalytic materials or the addition of organic conversion by addition of small amounts of benzyl
substances in the aqueous phase have been inves- alcohol. We report here the results of liquid-phase
tigated to substitute zinc salts and increase cyclo- hydrogenation of benzene catalyzed by ruthenium
hexene yield and selectivity. Mizukami et al. [6,7] particles supported on hydrophilic non-porous
used a supported ruthenium catalyst, with small silica (Ru/SiO2 ) in the presence of waterorganic
amounts of copper, prepared by the sol-gel meth- mixtures and explain the increase of cyclohexene
od and obtained 31.4% yield of cyclohexene at yield and selectivity based on the structure of the
83.3% of benzene conversion in the absence of zinc waterorganic mixtures.
sulfate in the aqueous phase. Imamura et al. [8]
performed the reaction in liquid ammonia at 393
K and 3 MPa of hydrogen using lanthanides as 2. Experimental
catalysts. They obtained cyclohexene selectivities
higher than 90%, however, for benzene conver- 2.1. Synthesis and characterization of Ru/SiO2
sions smaller than 10%. When the reaction tem- catalyst
perature was decreased to 323 K cyclohexadienes
were preferentially formed. Deng and co-workers In order to obtain a catalyst with 2 wt% of
[911] described that ruthenium boride catalysts, ruthenium, hydrophilic fumed silica Aerosil 200
prepared by reduction of ruthenium salts with (Degussa) was impregnated with a solution of
borohydrides, were more selective for reducing RuCl3
1:5H2 O (Aldrich) in water:ethanol (1:1 v/
benzene to cyclohexene than ruthenium catalysts v), dried at 343 K for 10 h and reduced at 673 K in
obtained by hydrogen reduction. However the use a hydrogen ow for 2 h. Transmission electron
of these catalysts makes the addition of zinc sulfate microscopy (TEM) was carried out using a Carl
in the aqueous phase necessary in order to achieve Zeiss CEM 902 apparatus with a Proscan high-
high cyclohexene yields. A new supported amor- speed slow-scan CCD camera and digitalized
phous ruthenium boride catalyst was recently de- (1024  1024 pixels, 8 bits) using the AnalySis
veloped, resulting in high cyclohexene yield (33%) software.
in the absence of zinc sulfate in the aqueous phase.
The addition of a small amount of zinc to the as- 2.2. Catalytic tests
prepared catalyst enhanced cyclohexene yield and
selectivity [11]. Scholten and co-workers [12,13] The hydrogenation reactions were made in a
found for the gas-phase and liquid-phase hydro- 100 ml SS316 Parr Stirred Reactor (Parr Instru-
genation of benzene over non-supported ruthe- ment Co., USA) equipped with pressure gauge,
nium catalyst, that the addition of small quantities thermocouple, gas inlet valve, liquid sample valve,
of organic compounds containing a hydroxyl or an internal stirring system consisting of a motor drive
amine group (modiers) increase the cyclohexene magnetically coupled to an internal stirrer shaft
yield and selectivity. The action of modiers in the with attached turbine-type impeller, and electric
liquid phase was explained by the formation of a heater with controls. Hydrogen gas (99.995%,
hydrogen bond between cyclohexene and the White Martins) was supplied from the cylinder
modier [13]. Majahani and co-workers [14], using and introduced into the base of the reactor. The
Ru/Al2 O3 as catalyst, showed that small quantities entrance tube also served as a sampling tube for
of monoethanolamine in the aqueous phase give the liquid phase. In a typical experiment a know
better selectivities than zinc sulfate, however, the amount of catalyst, 25 ml of water (or water/or-
 E.V. Spinac
e, J.M. Vaz / Catalysis Communications 4 (2003) 9196 93

ganic mixture) and 25 ml of benzene (Fluka) were aluminas [4]. It was shown, for the supported ru-
added and the reactor was closed. To remove the thenium catalysts, that the dispersion and the size
air present in the reactor, it was purged three times of the ruthenium particles inuence the catalytic
with hydrogen. Initially the stirring was adjusted activity but not the cyclohexene selectivity [17,18].
to 300 rpm and heating was started. When the The selectivity is inuenced by the nature of the
temperature reached 423 K, hydrogen was charged support and the pore sizes. The larger average pore
into the reactor (5.0 MPa). After this, the stirring size of the support, the higher is the selectivity
was adjusted to 1000 rpm and the reaction was towards cyclohexene [18]. We utilized as support
considered to start. The values of hydrogen pres- the hydrophilic silica Aerosil 200, whose surface
sure and the agitation speed were used to conduct area is almost entirely external and is not derived
the experiments in the chemically controlled re- from any porosity. The transmission electron mi-
gime [4,16]. The reaction was monitored removing crograph of the Ru/SiO2 catalyst (Fig. 1) shows
samples of the benzene phase after interrupting the the ruthenium particles dispersed on the small
stirring of the slurry, which were analyzed by gas spherical-shaped silica particles with sizes in the
chromatography. range of 15 nm.
A preliminary study [19] of the liquid-phase
2.3. Chromatographic analysis hydrogenation of benzene using the Ru/SiO2
catalyst showed that in the absence of water the
The samples were analyzed in a gas chromato- reaction is very fast and only cyclohexane is
graph Shimadzu GC17A, equipped with a capillary formed. In the presence of water the reaction rate
column Carbowax 20M (30 m  0:25 mm  0:25 decreases and, under the studied conditions, the
lm) coupled to a FID detector. The quantication best cyclohexene yield (14%) was obtained at 423
of the benzene, cyclohexene and cyclohexane was K and 5 MPa of hydrogen in the absence of ad-
made using calibration curves. ditives. The addition of zinc sulfate in the aque-
ous phase (Ru:Zn molar ratio between 1:5 and
1:10) only decreases the reaction rate and does
2.4. Denitions
not increase the cyclohexene yield and selectivity
as described for the unsupported ruthenium cat-
Benzene conversion C alysts [1,4] Probably the zinc ions adsorb prefer-
mol of reacted benzene entially on the weak acidic silanol groups [20] of
 100 the support and do not modify the environment
mol of initial benzene
around the ruthenium particles. Similar results
Cyclohexene selectivity S were observed using ruthenium supported cata-
lysts, where the addition of zinc sulfate was not
mol of cyclohexene formed
 100 as eective to enhance the cyclohexene yield
mol of reacted benzene [11,14].
Cyclohexene yield R The results of liquid-phase hydrogenation of
benzene in the presence of waterorganic mixtures
mol of cyclohexene formed are shown in Fig. 2 and Table 1.
 100
mol of initial benzene It can be seen that the addition of ethylene
glycol and glycerol in the aqueous phase increases
the cyclohexene yields and selectivities. The use of
3. Results and discussion methanol results in a decrease on cyclohexene yield
even though the initial cyclohexene selectivity
In the liquid-phase hydrogenation of benzene in values are similar to the obtained with ethylene
the absence of zinc salts, good yields of cyclohex- glycol and glycerol. The addition of ethanol and
ene could be obtained using ruthenium particles triacetin (1,2,3-propanetriol triacetate) cause a
supported on hydrophilic oxides, like silicas and decrease in the cyclohexene yields and selectivities.
94 E.V. Spinac
e, J.M. Vaz / Catalysis Communications 4 (2003) 9196

Fig. 1. Transmission electron micrograph of the Ru/SiO2 catalyst.

From these results we can see that molecules an ethylene glycol/water mixture. On the other
containing more hydrophilic OH groups increase hand, the total substitution of water by ethylene
the cyclohexene yields and selectivities, while glycol decreases the cyclohexene yield to 11%.
molecules with more hydrophobic alkyl groups Thus the presence of water is essential to obtain
cause a decrease on these values. The best cyclo- good yields. The best yields are obtained using
hexene yields and selectivities were obtained using 550% (in volume) of ethylene glycol in the
aqueous phase while above 75% a decrease was
observed.
Probably these results can be explained by the
structure of the waterorganic mixtures. Ethylene
glycol has no hydrophobic groups and in water the
hydroxyl groups interact with the surrounding
water molecules thus forming hydrated organic
molecules [21]. In the case of methanol, the role of
the hydrophilic interactions is more pronounced
due to its small hydrophobic moiety. The forma-
tion of watermethanol hydration complexes is
more pronounced than larger water cavities ac-
commodating methanol molecules (hydrophobic
association) [22]. In waterethanol mixtures at
Fig. 2. Benzene conversion  cyclohexene selectivity in the li- high ethanol concentrations the water molecules
quid-phase hydrogenation of benzene catalyzed by Ru/SiO2 in become incorporated by ethanol [23]. Like zinc
the presence of waterorganic mixtures (1/1, v/v). sulfate the hydrated organic molecules in ethylene
 E.V. Spinac
e, J.M. Vaz / Catalysis Communications 4 (2003) 9196 95

Table 1
Liquid-phase hydrogenation of benzene catalyzed by Ru/SiO2 in the presence of waterorganic mixtures (25 ml of benzene, 25 ml of
water/organic (1/1, v/v), 0.100 g of catalyst, 423 K and 5 MPa of hydrogen)
System Maximum yield of cyclohexene Conversion of benzene Cyclohexene selectivity Reaction time
(mol%) (mol%) (%) (min)
Water 14 60 23 40
Ethylene glycol/water 19 60 32 100
Glycerol/water 16 60 27 240
Methanol/water 10 35 29 150
Ethanol/water 9 42 21 70
Triacetin/water 5 30 17 80

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