B. Sun et al. / Applied Catalysis A: General 467 (2013) 310–314
311
be switched from py-THQ to DHQ by changing the reaction medium
from methanol to hexafluoroisopropanol using a Rh/Al O catalyst
products was done by GC–MS analysis. The GC separation was car-
ried out on a ZB-5MS column (30 m × 0.25 mm, 0.25 m) using a
2
3
◦
◦
[
22].
temperature program of 35–200 C at 5 C/min. The instrument
used was a ThermoFinnigan® Trace GC-Polaris Q. The data was col-
lected by using extracted ion chromatograms of marker m/z values
for each molecule from the total ion chromatograms (TIC).
◦
Ya-Ping Sun et al. obtained exclusively DHQ from Q at 150 C and
0 bar H2 using a hydroxyapatite-supported ruthenium catalyst
4
[
23]. On the other hand, R. A. Sánchez-Delgado et al. reported that
ruthenium nanoparticles immobilized on poly(4-vinylpyridine)
catalyze the hydrogenation of Q to give almost exclusively py-
THQ at 120 C and 40 bar H2 in toluene [24,25]. Very recently, L.
A freshly prepared suspension of nanoRu@hectorite and the
desired amount of quinoline was used. Then the autoclave was
pressurized with hydrogen (20–60 bar) and then heated to 100 C.
◦
◦
Zhou et al. reported ruthenium nanoparticles intercalated in CTAB-
modified montmorillonite to give exclusively py-THQ in methanol
at 100 C [26].
After the reaction, the pressure was released, the solution was
filtered (0.22 m, PTFE) and analyzed in order to determine the
substrate conversion and selectivity (in %). The catalytic reaction
was followed by gas chromatography coupled to a mass detector.
The products were separated on an apolar column and identified
by their retention time and their mass spectrum using the electron
impact (EI) ionization method.
◦
We have observed that ruthenium(0) nanoparticles can
be intercalated in hectorite to give
a black solid material,
nanoRu@hectorite, which is a highly efficient and reusable catalyst
for the hydrogenation of benzene [27–29]. This catalyst showed
remarkable selectivities for the hydrogenation of furfuryl alcohol
[
30] and for the hydrogenation of ␣,-unsaturated ketones [31].
2.3. Catalyst recycling
Herein, we report this catalyst nanoRu@hectorite to show a switch-
able selectivity for the hydrogenation of quinoline. Depending on
the reaction medium, the reaction gives either py-THQ (>99%) or
DHQ (>99%). Of practical significance is the selective conversion of
quinoline to py-THQ under mild reaction conditions in the benign
aqueous medium. This versatile heterogeneous catalyst may have
potential applications in the broad field of pharmaceuticals and fine
chemicals, where these structural motifs are commonly found in
numerous biologically active natural products and pharmacologi-
cally relevant therapeutic agents [32].
After the catalytic run, the nanoRu@hectorite catalyst was sepa-
rated by decantation of the centrifuged reaction mixture. Then the
catalyst was washed with diethyl ether and cyclohexane, and then
◦
reactivated in the autoclave under H pressure (50 bar) at 100 C for
2
1
4 h in the reaction medium used for the next catalytic run. After
pressure release and cooling, a new batch of quinoline was added
for the next catalytic run.
3. Results and discussion
2
. Experimental
The nanoRu@hectorite catalyst was prepared from synthetic
2.1. Syntheses
sodium0 hectorite, a pure white solid presenting an idealized
cell formula of Mg5.5Li0.5Si O (OH) Na · n H O [36] and an aque-
8
20
4
2
White sodium hectorite powder was synthesized according to
the method of Bergk and Woldt [33]. The sodium cation exchange
capacity, determined under the method of Lagaly and Tributh
ous solution of benzene ruthenium dichloride dimer containing
+
the aqua complexes [(C6H6)RuCl2(H2O)], [(C6H6)RuCl(H2O)2]
and [(C6H6)Ru(H2O)3]2+ in equilibrium [37]; the yellow material
obtained by ion exchange of the sodium cations against benzene
ruthenium aqua cations in the interlaminar space is an air-stable
catalyst precursor that can be isolated and stored. This yellow
catalyst precursor was then reduced, suspended in the appro-
priate solvent, by molecular hydrogen to give the active catalyst
nanoRu@hectorite, an air-sensitive black powder (Scheme 1) [28].
The ruthenium loading of nanoRu@hectorite was assumed to
be 3.3 wt% [27], based upon the molar ratio of [(C6H6)RuCl2]2
used (corresponding to 75% of the experimentally determined [13]
cation exchange capacity), and the presence of metallic ruthe-
nium was proven by its typical reflections in the X-ray diffraction
pattern [31]. The specific surface of nanoRu@hectorite was deter-
[
[
34], was found to be 104 mEq per 100 g. The dimeric complex
(C H ) RuCl ] was synthesized following the procedure reported
6
6
2
2 2
by Arthur and Stephenson [35].
2
.1.1. Preparation of the ruthenium(II)-containing catalyst
precursor
The neutral complex [(C H ) RuCl ] (83.8 mg, 0.17 mmol) was
6
6
2
2 2
dissolved in distilled and N -saturated water (50 ml), giving a clear
2
yellow solution after vigorous stirring for 1 h. The pH of this solution
was adjusted to 8 (using a glass electrode) by adding the appropri-
ate amount of 0.1 M NaOH. After filtration this solution was added
to 1 g of finely powdered and degassed (1 h high vacuum, then N2-
saturated) sodium hectorite. The resulting suspension was stirred
2
mined by low-temperature nitrogen adsorption to be 207 m /g,
◦
for 4 h at 20 C. Then the yellow ruthenium(II)-containing hectorite
which is significantly higher than for the unmodified sodium hec-
2
was filtered off and dried in vacuo.
torite (87 m /g), and the pore size distribution in nanoRu@hectorite
shows a maximum of 1.98 nm [37]. The size distribution of the
ruthenium(0) nanoparticles in nanoRu@hectorite was studied by
transmission electron microscopy (TEM) using the “ImageJ” soft-
ware [38] for image processing and analysis, selected area electron
diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDS).
The micrographs show the ruthenium(0) nanoparticles: at the
edges of superimposed silicate layers the nanoparticles are visible,
the lighter tone of which is typical for intercalated particles. The
mean particle size and standard deviation (ꢀ) were estimated from
image analysis of ca. 100 particles at least (Figs. 1 and 2). We have
shown earlier that ruthenium(0) nanoparticles intercalated in hec-
torite can have different shapes (hexagonal or spherical) and sizes
(4–38 nm), depending on the reaction conditions for the reduction
step [30–32]. The nanoRu@hectorite catalysts prepared here have
mean particle sizes of 10 nm (in water) and 3 nm (in cyclohexane).
2
.1.2. Preparation of the nanoRu@hectorite catalyst
The ruthenium(0)-containing hectorite was obtained by reac-
ting a suspension of the yellow ruthenium(II)-containing hectorite
(
50 mg, 0.01592 mmol Ru) in a magnetically stirred stainless-steel
◦
autoclave (volume 100 ml) under a pressure of H (50 bar) at 100 C
for 14 h using different solvents. After pressure release and cooling,
the nanoRu@hectorite catalyst was isolated as a black material.
2
2
.2. Catalysis
The selective hydrogenation of quinoline was carried out in a
magnetically stirred stainless-steel autoclave (100 ml). The air in
the autoclave was displaced by purging three times with hydrogen
prior to use. The quantitative chemical analysis of hydrogenation