Highly selective C=C bond hydrogenation in α,β-unsaturated ketones catalyzed by hectorite-supported ruthenium nanoparticles

Article abstract of DOI:10.1016/j.molcata.2011.12.012

Metallic ruthenium nanoparticles intercalated in hectorite (particle size ～7 nm) were found to catalyze the specific hydrogenation (conversion 100%, selectivity > 99.9%) of the carbon-carbon double bond in α,β- unsaturated ketones such as 3-buten-2-one, 3-penten-2-one, 4-methyl-3-penten-2- one. The catalytic turnovers range from 765 to 91,800, the reaction conditions being very mild (temperature 35 °C and constant hydrogen pressure 1-10 bar). After a catalytic run, the catalyst can be recycled and reused without loss of activity and selectivity

Full text of DOI:10.1016/j.molcata.2011.12.012

Journal of Molecular Catalysis A: Chemical 355 (2012) 168–173
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Highly selective C C bond hydrogenation in ␣,␤-unsaturated ketones catalyzed
by hectorite-supported ruthenium nanoparticles
Farooq-Ahmad Khana, Armelle Vallatb, Georg Süss-Finka,∗
a Institut de Chimie, Université de Neuchâtel, CH-2000 Neuchâtel, Switzerland
b Service Analytique Facultaire, Université de Neuchâtel, CH-2000 Neuchâtel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 August 2011
Received in revised form 31 October 2011
Accepted 8 December 2011
Available online 17 December 2011
Metallic ruthenium nanoparticles intercalated in hectorite (particle size ∼7 nm) were found to catalyze
the specific hydrogenation (conversion 100%, selectivity > 99.9%) of the carbon–carbon double bond in
␣,␤-unsaturated ketones such as 3-buten-2-one, 3-penten-2-one, 4-methyl-3-penten-2-one. The cat-
alytic turnovers range from 765 to 91,800, the reaction conditions being very mild (temperature 35 ◦C
and constant hydrogen pressure 1–10 bar). After a catalytic run, the catalyst can be recycled and reused
without loss of activity and selectivity
Keywords:
© 2011 Elsevier B.V. All rights reserved.
Ruthenium nanoparticles
Ruthenium-modified hectorite
Selective C C bond hydrogenation
Selective hydrogenation of unsaturated
ketones
Mesityl oxide reduction
1. Introduction
hectorite is the most important one because of its exceptional
swelling properties. It can be defined as layers of negatively charged
Chemoselective hydrogenation of ␣,␤-unsaturated carbonyl
compounds is useful in the preparation of fine chemicals, fla-
vors, hardening of fats, pharmaceutical manufacturing processes
and in the synthesis of various organic intermediates and solvents
[1]. The selectivity of the reaction is a problem and requires spe-
cific reaction conditions and catalyst systems. In heterogeneous
catalysts, the effect of metal–support interaction also plays an
important role in determining the selectivity of the reaction [2].
Therefore, the design of nanocomposites consisting of functional
metals and adequate matrices is a challenge for the fabrication
of recyclable catalysts. Highly active metallic nanoparticles must
be stabilized by a suitable support in order to prevent aggre-
gation to bulk metal [3]. Hectorite is a naturally occurring clay,
belonging to the smectite family of layered minerals. These mate-
rials are composed of individual platelets containing a metal oxide
center sandwiched between two silicone dioxide outer layers.
Included in this group of minerals are sodium hectorite, bentonite
(montmorillonite), saponite, vermiculite, kenyaite, volkonskoite,
sepiolite, beidellite, magadiite, nontronite and sauconite. Of these,
two-dimensional silicate sheets held together by cationic species
in the interlaminar space, which are susceptible to ion exchange
[4–6]. Ruthenium-supported hectorite obtained by ion exchange
has been reported by Shimazu et al. using [Ru(NH3)6]2+ cations
[7] and by our group using [(C6H6)Ru(H2O)3]2+ cations [8] or
[(C6H6)4Ru4H4]2+ cations [9] for the intercalation. These materi-
als show high catalytic activity for the hydrogenation of olefins
[7] and of aromatic compounds [10,11]. Recently, we reported the
highly selective low-temperature hydrogenation of furfuryl alco-
hol to tetrahydrofurfuryl alcohol catalyzed by hectorite-supported
nanoparticles [12].
The hydrogenation of ␣,␤-unsaturated ketones implies either
the olefinic C C bond or the carbonyl C O bond, or both of them.
In addition, side reactions have to be considered as well [2]. Sup-
ported metals such as platinium, rhodium, ruthenium, gold, nickel,
aluminum, copper and iron are reported to be active for the hydro-
genation of ␣,␤-unsaturated ketones [13]. However, in most cases
the selectivity for C C bond hydrogenation is only high at low
conversion [14,16]. Therefore, palladium is conventionally used to
selectively reduce C C bond in unsaturated carbonyl compounds
[15,16]. Complex metal hydrides such as potassium triphenylboro-
hydride and lithium aluminum hydride–copper(I) iodide also show
a good selectivity (upto 99%) for olefinic bond hydrogenation in
both cyclic and acyclic enones, but they result in the production of
substantial amounts of waste [17]. Some organometallic complexes
∗
Corresponding author at: Institut de Chimie, Université de Neuchâtel, Avenue
de Bellevaux 51, CH-2000 Neuchâtel, Switzerland. Tel.: +41 32 7182400;
fax: +41 32 7182511.
E-mail address: georg.suess-fink@unine.ch (G. Süss-Fink).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.12.012

F.-A. Khan et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 168–173
169
are also highly selective toward the hydrogenation of C C bond
in ␣,␤-unsaturated ketones under milder conditions [18]. These
complexes are sensitive to permanant deactivation and show all
disadvantages of homogeneous catalysts. Metal-free approaches
toward such hydrogenations are almost futile with a 75% selectivity
toward saturated ketones [19].
We have been interested in the influence of increasing steric
hinderence at the C C bond of ␣,␤-unsaturated ketones on the
selectivity of the hydrogenation using our hectorite-supported
nano-ruthenium [8–12] as catalyst. Therefore, 3-buten-2-one, 3-
penten-2-one and 4-methyl-3-penten-2-one have been studied.
Of these three substrates, 4-methyl-3-penten-2-one is the most
important industrial precursor; it is also called mesityl oxide
which, upon selective hydrogenation, gives methyl isobutyl ketone.
Methyl isobutyl ketone is an important commercial solvent with a
reported world consumption of 295 thousand metric tons in 2007
[20].
clear yellow solution after intensive stirring for 1 h. The pH of
this solution was adjusted to 8 (using a glass electrode) by adding
the appropriate amount of 0.1 M NaOH. After filtration this solu-
tion was added to 1 g of finely powdered and degassed (1 h
high vacuum, then Ar-saturated) sodium hectorite 1. The result-
ing suspension was stirred for 4 h at 20 ◦C. Then the yellow
ruthenium(II)-containing hectorite 2 was filtered off and dried
in vacuo.
2.1.2. Preparation of the ruthenium(0)-containing hectorite 3 by
reduction with molecular hydrogen
The ruthenium(0)-containing hectorite 3 was obtained by react-
ing a suspension of the yellow ruthenium(II)-containing hectorite 2
(50 mg, 0.01592 mmol Ru) in a magnetically stirred stainless-steel
autoclave (volume 100 mL) under a pressure of H2 (50 bar) at 100 ◦C
for 14 h using ethanol (5 mL) as solvent. After pressure release and
cooling for 48 h, 3 was isolated as a black material.
Traditionally, methyl isobutyl ketone is manufactured via a
three-step process in which acetone condensation gives diace-
tone alcohol which readily dehydrates into mesityl oxide. The
olefinic C C bond in mesityl oxide is then selectively hydrogenated
to methyl isobutyl ketone avoiding further C O reduction into
methyl isobutyl carbinol. Methyl isobutyl ketone production may
also be achieved by using a bifunctional catalyst to facilitate all
three reaction steps in single step. A 20–60% conversion of ace-
tone with 30–90% selectivity for methyl isobutyl ketone is observed
for these single-step processes under harsh reaction conditions
(80–160 ◦C, 10–100 bar H2) [1b,21]. Thus, the methyl isobutyl
ketone concentration in the effluent is typically less than 30 wt.%
necessitating further purification steps [1b]. The large-scale pro-
duction of methyl isobutyl ketone still follows the three-step route
[22] involving mesityl oxide hydrogenation into methyl isobutyl
ketone at 150–200 ◦C and 3–10 bar H2 using Cu or Ni catalysts [23]
or, alternatively, on a supported palladium catalyst at 80–220 ◦C
[15d–f]. It is therefore desirable to find alternative green processes,
which produce methyl isobutyl ketone under mild reaction con-
ditions. Metallic ruthenium nanoparticles intercalated in hectorite
are promising as catalysts, since they can be easily handled and
recycled.
In this paper, we report ruthenium nanoparticles (∼7 nm)
intercalated in hectorite to be a highly productive (conversion
100%, turnover number 765–91,800) and highly selective (selec-
tivity > 99.9%) reusable catalyst for the hydrogenation of various
industrially important ␣, ␤-unsaturated ketones under mild con-
ditions (ethanol solution, 35 ◦C, 1–10 bar H2). To the best of our
knowledge, such a high selectivity with a complete conversion of
mesityl oxide into methyl isobutyl ketone at mild conditions has
never been reported in published literature, except for a sodium
hydride containing complex reducing agent, but giving only a
turnover number of 20 [17e].
2.2. Catalysis
The hydrogenation of ␣,␤-unsaturated ketones was carried out
in a magnetically stirred stainless-steel autoclave. The air in the
autoclave was displaced by purging three times with hydrogen
prior to use. Quantitative chemical analysis of hydrogenation prod-
ucts was done by GC–MS analysis. The GC separation was carried
out on a ZB-5MS column (30 m × 0.25 mm, 0.25 ␮m) using a tem-
perature program of 35–200 ◦C at 5 ◦C/min. The instrument used
was a ThermoFinnigan® Trace GC-Polaris Q. The data were col-
lected by using extracted ion chromatograms of marker m/z values
for each molecule from the total ion chromatograms (TIC).
A
freshly prepared suspension (5 mL) of ruthenium(0)-
containing hectorite 3 was used, ethanol (15 mL) as well as
corresponding ␣,␤-unsaturated ketone (12.2 mmol) was added.
Then the autoclave was heated at 35 ◦C under constant hydro-
gen pressure (1–10 bar). After 1 h, the pressure was released, the
solution was filtered (0.22 ␮m, PTFE) and analyzed by GC–MS in
order to determine the substrate conversion and selectivity (in
%). The turnover number for 3-buten-2-one and 3-penten-2-one
was determined by adding 12.2 mmol of substrate in regular inter-
vals, until the catalyst lost its selectivity. However, in the case
of 4-methyl-3-penten-2-one, 122 mmol of substrate was added in
regular intervals, until the catalyst lost its activity.
3. Results and discussion
Synthetic sodium hectorite (1) is a white solid which presents
an idealized cell formula of Mg5.5Li0.5Si8O20(OH)4Na·nH2O. It
has a three-layer sheet-like morphology which results from the
two-dimensional condensation of silicic acid, two layers of SiO4
tetrahedra being connected by a layer of MgO6 octahedra. Par-
tial replacement of the Mg2+ cations in the octahedral layers by
Li+ cations leads to an excess of anionic charges of the layers,
which are compensated by Na+ cations in the interlaminar space
(Fig. 1). Hydratization of the interlaminar sodium cations to give
[Na(H2O)n]+ is responsible for the swelling of hectorite in water,
since the interlaminar space is widened [27].
Contrary to the magnesium and lithium cations in the octahe-
dral layer, the sodium cations in the interlaminar space are not
bound to the silicate framework. For this reason, the Na+ cations
can easily be exchanged in water by other water-soluble inorganic,
organic or organometallic cations. The dinuclear complex benzene
ruthenium dichloride dimer dissolves in water with hydrolysis
to give, with successive substitution of chloro ligands by aqua
ligands, a mixture of mononuclear benzene ruthenium complexes
being in equilibrium [28]. The 1H NMR signals of the D2O solu-
tion have been assigned to [(C6H6)RuCl2(H2O)] (ı = 5.89 ppm),
2. Experimental
2.1. Syntheses
White sodium hectorite (1) was prepared according to the
method of Bergk and Woldt [24]. The sodium cation exchange
capacity, determined according to the method of Lagaly and Trib-
uth [25], was found to be 104 meq per 100 g. The dimeric complex
[(C6H6)2RuCl2]2 was synthesized following the procedure reported
by Arthur and Stephenson [26].
2.1.1. Preparation of the ruthenium(II)-containing hectorite 2
The neutral complex [(C6H6)2RuCl2]2 (83.8 mg, 0.17 mmol) was
dissolved in distilled and Ar-saturated water (50 mL), giving a

170
F.-A. Khan et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 168–173
Fig. 1. Structural model according to Grim [27] of sodium hectorite Mg5.5Li0.5Si8O20(OH)4Na·nH2O, showing the anionic three-layer sheets and the interlaminar space
containing sodium cations and water molecules.
[(C6H6)RuCl(H2O)2]2+ (ı = 5.97 ppm), and [(C6H6)Ru(H2O)3]2+
micrographs show particles of a size up to 18 nm. At the edges of
superimposed silicate layers nanoparticles are visible, the lighter
tone of which is typical for intercalated particles. The mean par-
ticle size and standard deviation (ꢀ) were estimated from image
analysis of ca. 500 particles at least. We have shown earlier that
ruthenium(0) nanoparticles intercalated in hectorite can have dif-
ferent shapes (hexagonal or spherical) and sizes (nanoparticle size
4–38 nm), depending on the reaction conditions for the reduction
step [11]. The ruthenium(0)-containing hectorite 3 prepared here
has a mean particle size of 7 nm (Fig. 2).
(ı = 6.06 ppm) (Scheme 1) [28]. The dication [(C6H6)Ru(H2O)3]2+
which has been isolated as the sulfate and structurally character-
ized [29], is the major species present in the hydrolytic mixture
over the pH range from 5 to 8, according to the NMR spectrum
(Scheme 1).
,
When the yellow solution obtained by dissolving the
dinuclear complex [(C6H6)RuCl2]2 in water is added to
white sodium hectorite (1), the main hydrolysis product
[(C6H6)Ru(H2O)3]2+ intercalates into the solid, replacing the
appropriate amount of sodium cations, to give the yellow
ruthenium(II)-modified hectorite 2. This material, which can
be dried and stored in air, reacts with ethanolic solution of
2 under hydrogen pressure (50 bar) at 100 ◦C by reduction of
[(C6H6)Ru(H2O)3]2+ to give the black ruthenium(0)-modified
hectorite 3 (Scheme 2).
A comparison of the diffractogram for ruthenium(0)-containing
hectorite 3 with the powder pattern of sodium-containing hec-
torite (1) and ruthenium(II)-containing hectorite 2 is shown in
˚
Fig. 3. The d-spacing value (d0 0 1 = 17.8 A) is significantly higher
˚
for 3 than that of 1 (d0 0 1 = 13.32 A). The ruthenium(II)-containing
˚
hectorite 2 also shows a slight shift (d0 0 1 = 14.08 A) as com-
pared to that of 1. Peaks of the Ru phase are not observed,
which is presumably due to the low concentration of Ru nano-
crystallites, the peaks of which being hidden by the high hectorite
background.
These ruthenium nanoparticles intercalated in hectorite are
highly active and selective hydrogenation catalysts: Ruthenium(0)-
containing hectorite 3 efficiently reduces different ␣,␤-unsaturated
ketones to give saturated ketones under mild conditions, the for-
mation of the alcohols (saturated and unsaturated) being avoided.
The catalytic reaction is followed by gas chromatography coupled
The presence of metallic ruthenium was proven by its typi-
cal X-ray reflections. The specific surface of 3 was determined to
be 207 m2/g by adsorption techniques [8]. The ruthenium load-
ing was assumed to be 3.2 wt.%, based upon the molar ratio of
[(C6H6)Ru(H2O)3]2+ used (corresponding to 75% of the experimen-
tally determined [25] cation exchange capacity of 1); the ruthenium
loading determined by ICP-OES of 3.4 wt.% is found to be a little
too high. The size distribution of the ruthenium(0) nanoparticles
in 3 was studied by transmission electron microscopy (TEM) using
the “ImageJ” software [30] for image processing and analysis. The
Scheme 1. Hydrolysis of the dinuclear complex [(C6H6)RuCl2]2 in water to give a mixture of mononuclear benzene ruthenium complexes, the dicationic triaqua complex
[(C6H6)Ru(H2O)3]2+ being the major product.

F.-A. Khan et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 168–173
171
Scheme 2. Ion exchange of Na+ cations in sodium hectorite 1 (white) against [(C6H6)Ru(H2O)3]2+ cations to give ruthenium(II)-modified hectorite 2 (yellow) and reduction
of [(C6H6)Ru(H2O)3]2+ in 2 by molecular hydrogen gives ruthenium nanoparticles in the ruthenium(0)-containing hectorite 3.
Fig. 2. TEM micrograph with SAED (a) histogram (b) and EDAX analysis (c) of ruthenium(0) nanoparticles in 3.
to mass detector. The products are separated on an apolar column
and are identified by their retention time and mass spectrum using
electron impact (EI) ionization method.
can be used up to 12 times until it becomes inactive. The highly
productive reduction of mesityl oxide to methyl isobutyl ketone
by hectorite-supported ruthenium nanoparticles is striking, espe-
cially, since no traces of further C O reduction (methyl isobutyl
carbinol) observed. This highly selective C C bond hydrogenation
is also observed for other ␣,␤-unsaturated ketones. Low catalyst
loading is capable of reducing only the olefinic double bond. The
molar ratio of converted substrate to catalyst decreased in the
order 4-methyl-3-penten2-one > 3-penten-2-one > 3-buten-2-one
in the direction of decreasing steric hinderance. The increasing
steric hinderance in the substrate requires increased hydrogen
For 3-buten-2-one and 3-penten-2-one hydrogenation, catalyst
3 can be used only 2 times without loss of selectivity. However,
in the case of 4-methyl-3-penten-2-one hydrogenation, catalyst 3

172
F.-A. Khan et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 168–173
Fig. 3. XRD pattern for sodium-containing hectorite (1), ruthenium(II)-containing hectorite (2) and Ru(0)-containing hectorite (3).
Table 1
Selective hydrogenation of different ␣,␤-unsaturated ketones by metallic ruthenium nanoparticles intercalated in hectorite.
Substrate
Pressure (bar)
Temp (◦C)
Timea (h)
Conversion (%)
Selectivity (%)
TOFb (h−1
)
TONc
765
3825
91,800
3-Buten-2-one
3-Penten-2-one
4-Methyl-3-penten-2-one
1
7
10
35
35
35
2
1
1
100
100
100
>99.9
>99.9
>99.9
822
1254
1212
a
Time required for 100% conversion of 12.2 mmol unsaturated ketones into saturated ketones.
Turnover frequency calculated as moles of saturated ketone per mol ruthenium per hour for 12.2 mmol substrate hydrogenation after 25 min.
Total turn over number (until the catalyst loses its selectivity or activity).
b
c
pressure: while 3-buten-2-one is hydrogenated under 1 bar hydro-
gen pressure, for 3-penten-2-one a hydrogen pressure of 7 bar is
required, and for the bulkiest substrate 4-methyl-3-penten-2-one
10 bar. However, the selectivity for C C bond hydrogenation is
not reduced by the high pressure (Table 1). It is likely that the
absence of bulky substituents on the conjugated C C double bond
of 3-buten-2-one favors stable adsorption of the product on cat-
alytic sites. The high selectivity for the C C bond hydrogenation
of these ␣,␤-unsaturated ketones can be tentatively attributed to
the activation of the C C bond by the metal–support interaction
[31]. It can be assumed that hectorite probably modifies the elec-
tronic properties of ruthenium, which in turn leads to an increase
in the hydrogenation selectivity for the C C bond. Thus, the spe-
cific hydrogenation tendency of ␣,␤-unsaturated ketones can be
interpreted in terms of an exclusive adsorption of C C bonds at
the surface of these nanoparticles. The same metal–support effect
was observed in the highly selective C C bond hydrogenation of
furfuryl alcohol by hectorite-supported ruthenium nanoparticles
[12].
thank the Johnson Matthey Research Centre for a generous loan of
ruthenium(III) chloride hydrate.
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