Ruthenium(0) nanoclusters supported on hydroxyapatite: Highly active, reusable and green catalyst in the hydrogenation of aromatics under mild conditions with an unprecedented catalytic lifetime

Article abstract of DOI:10.1039/c0cc00494d

The preparation of ruthenium(0) nanoclusters supported on hydroxyapatite and their characterization by a combination of complementary techniques are described. The resultant ruthenium(0) nanoclusters provide high activity and reusability in the complete hydrogenation of aromatics under mild conditions (at 25 °C and with 42 psi initial H₂ pressure).

Full text of DOI:10.1039/c0cc00494d

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Ruthenium(0) nanoclusters supported on hydroxyapatite: highly active,
reusable and green catalyst in the hydrogenation of aromatics under
mild conditions with an unprecedented catalytic lifetimew
¨
Mehmet Zahmakıran, Yalcın Tonbulz and Saim Ozkar*
Received 19th March 2010, Accepted 10th May 2010
First published as an Advance Article on the web 21st May 2010
DOI: 10.1039/c0cc00494d
The preparation of ruthenium(0) nanoclusters supported on
hydroxyapatite and their characterization by a combination of
complementary techniques are described. The resultant ruthenium(0)
nanoclusters provide high activity and reusability in the complete
hydrogenation of aromatics under mild conditions (at 25 1C and
with 42 psi initial H2 pressure).
support itself.11 Since we have achieved unprecedented
catalytic activities in the hydrogenation of benzene by using
ruthenium(0) nanoclusters previously,6,17 the same metal was
selected as catalyst supported on the HAp.
In this communication, we report the preparation and
characterization of ruthenium(0) nanoclusters, supported
on hydroxyapatite, Ru(0)/HAp, and their superb catalytic
performance in terms of activity, selectivity, reusability and
lifetime in the hydrogenation of aromatics under mild
conditions (at 25 1C, with 42 ꢀ 1 psig initial H2 pressure).
Ru(0)/HAp were easily and reproducibly prepared by a
method18 that differs from the one previously reported for
the preparation of Ru(0)/HAp.19 Our methodology in the
preparation of Ru(0)/HAp comprises the ion-exchange20 of
Ru3+ ions with Ca2+ ions in the lattice of HAp, followed by
the reduction of the Ru3+-exchanged HAp with sodium
borohydride in aqueous solution at room temperature. After
centrifugation, copious washing with water, and drying under
vacuum (10ꢁ3 Torr) at 80 1C, Ru(0)/HAp were obtained as
dark grey powders and characterized by ICP-OES, XRD,
TEM, XPS and the N2 adsorption–desorption technique.
The XRD patterns of Ru(0)/HAp containing 0.42% wt Ru
as determined by ICP-OES are identical with that of HAp
(Fig. ESI-1w); any impurities or other phases were not
observed, indicating that the host material remains intact at
the end of the procedure without observable alteration in the
framework lattice and loss in the crystallinity.
The complete hydrogenation of aromatics is one of the
ubiquitous and challenging transformations used in both
laboratory and industrial synthetic chemistry1 and traditionally
has been carried out at high temperature (Z 100 1C) and/or
high pressure (Z 50 atm H2).2,3 Although a range of hetero-
geneous catalysts can achieve the hydrogenation of aromatics
under mild conditions (r25 1C and r3 atm H2),4–6 most of
them suffer from difficult synthetic procedures, low activity or
low stability. Therefore, the development of an easily prepared,
highly active, long-lived and reusable catalyst that operates
under mild conditions is still a paramount challenge.
Recently, hydroxyapatite (Ca10(PO4)6(OH)2, HAp)7 has
generated great interest in view of its potential usefulness
as biomaterials,8 adsorbents,9 ion-exchangers,10 and catalyst
supports.11 Of particular importance are the results of recent
studies showing that hydroxyapatite supported Pd(0),12
Ru(0),13 Ag(0)14 and Au(0)15 nanoparticles exhibit outstanding
catalytic activities in the aerobic oxidation of alcohols, oxidative
cleavage of alkenes, oxidation of phenylsilanes, and epoxidation
of styrene, respectively. These results encouraged us to focus
on the use of HAp matrix in the stabilization of transition
metal nanoclusters catalysts in the hydrogenation of aromatics.
The choice of HAp as support16 is also motivated by the
following advantages: (i) HAp has nonporous structure and
doesn’t cause significant mass transfer limitation, (ii) it has
high ion-exchange ability and adsorption capacity, and (iii) its
low surface acidity prevents side reactions arising from the
The size and the morphology of the Ru(0)/HAp were
analyzed by using transmission electron microscopy. Fig. 1
shows the TEM image of the Ru(0)/HAp sample with a
ruthenium loading of 0.42% wt. This reveals the presence of
randomly distributed ruthenium(0) nanoclusters on the HAp
surface in the range of 0.9–4.0 nm with a mean diameter
of 2.6
ꢀ
0.6 nm which corresponds to Ru(0)B670
nanoclusters.21 It should be noted that the TEM observation
of the same sample from different areas confirms the stability
of ruthenium(0) nanoclusters against agglomeration.
Department of Chemistry, Middle East Technical University,
06531 Ankara, Turkey. E-mail: sozkar@metu.edu.tr
The oxidation state of ruthenium and the surface composi-
tion of Ru(0)/HAp were investigated by X-ray photoelectron
spectroscopy. The survey scan XPS spectrum of Ru(0)/HAp
indicates that ruthenium is the only element detected in
addition to the HAp framework elements (Fig. ESI-2aw).
The high resolution Ru 3d and 3p XPS spectra of
Ru(0)/HAp show two prominent bands at 281.5 and 462.3 eV,
which are readily assigned to Ru(0) 3d5/2 (Fig. ESI-2bw)
and Ru(0) 3p3/2 (Fig. ESI-2cw), respectively.22 The N2
adsorption–desorption isotherms of both HAp and Ru(0)/HAp
w Electronic supplementary information (ESI) available: Experimental
section; Table S-1: The eight catalyst systems achieving the complete
hydrogenation of neat benzene arranged chronologically, Fig. ESI-1:
XRD patterns of HAp, fresh Ru(0)/HAp and Ru(0)/HAp recovered at
the end of the third reuse from the hydrogenation of benzene, Fig. ESI-2:
Survey, Ru 3d and 3p XPS spectrum of Ru(0)/HAp, Fig. ESI-3: N2
adsorption–desorption isotherms of HAp, Ru(0)/HAp, Fig. ESI-4:
Graphs of [aromatic] consumption versus time for Ru(0)/HAp
catalyzed hydrogenation. Fig. ESI-5: TEM micrograph of recovered
Ru(0)/HAp. See DOI: 10.1039/c0cc00494d
z On leave of absence from Ziya Gokalp Faculty of Education, Dicle
¨
University, 21280 Diyarbakır, Turkey.
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Fig. 1 (a) TEM, (b) ZC-TEM image of ruthenium(0) nanoclusters
supported on hydroxyapatite.
(Fig. ESI-3a and bw, respectively) show type III isotherms,
reflecting the absence of micropores (o2 nm).23 The
Brunauer–Emmett–Teller surface area of Ru(0)/HAp was
found to be 82.5 m2
g
ꢁ1, which is larger than that of the
ꢁ1). This result accounts for
Fig. 2 Normalized substrate concentration versus time for the
Ru(0)/HAp catalyzed hydrogenation of aromatics under mild condi-
tions at 25 ꢀ 0.1 1C and 42 ꢀ 1 psig initial H2 pressure.
parent HAp (69.3 m2
g
the physical adsorption of ruthenium(0) nanoclusters on the
HAp surface.
(150 mg, with a Ru content of 0.42% wt corresponding to
6.23 mmol Ru) provides 192 600 turnovers in benzene hydro-
genation over 400 h before deactivation (Scheme 2). The
average TOF value during this lifetime experiment is 480 hꢁ1
which is smaller than the initial TOF value (610 hꢁ1), indicating
that the nanoclusters are deactivating as the catalytic reaction
proceeds. Nevertheless, this is the longest catalytic lifetime
reported to date since the highest TTO known for neat
benzene hydrogenation is 7250.6
The catalytic activity of Ru(0)/HAp was tested in the
hydrogenation of aromatics. The catalytic hydrogenation of
aromatics was started by agitation of a Ru(0)/HAp sample in
the aromatic substrate at 25 ꢀ 0.1 1C and 42 ꢀ 1 psig initial H2
pressure. The progress of the reaction could be followed by
monitoring the hydrogen uptake which was converted to the
concentration loss of aromatic substrate by using the stoichio-
metry (Scheme 1).
The complete hydrogenation of aromatics was also checked
by taking the 1H NMR spectra of the solution at the end of the
reaction. Fig. 2 shows the normalized substrate concentration
vs. time for the hydrogenation of benzene, toluene, p-xylene,
m-xylene, o-xylene and mesitylene catalyzed by Ru(0)/HAp at
25 ꢀ 0.1 1C and 42 ꢀ 1 psig H2. For all of the substrates the
hydrogenation starts immediately without induction period as
a preformed catalyst is used. The linear hydrogenation of
benzene, toluene, p-xylene, m-xylene, o-xylene and mesitylene
continues until the consumption of substrate with a TOF
value of 705, 519, 477, 423, 384 and 300 hꢁ1, respectively
(see Fig. ESI-4w).24 The observed trend in TOF values of
different substrates is attributed to an electronic effect25 of the
methyl substituents on the aromatic ring. Besides, the steric
hindrance also appears to be responsible for the slow rate
of o-xylene hydrogenation compared to p- and m-xylenes.26
Ru(0)/HAp catalyzing the hydrogenation of neat benzene
is an important achievement as performing the reactions
in a solventless system is one of the requirements for
‘‘Green Chemistry’’.27 There exist only seven catalyst systems
reported for the complete hydrogenation of neat benzene at
r25 1C (see Table 1 in ESIw).5,6 The new Ru(0)/HAp catalyst
The isolability and reusability of Ru(0)/HAp, two crucial
measures in heterogeneous catalysis, were also tested in the
hydrogenation of benzene. After the complete hydrogenation
of benzene, Ru(0)/HAp was isolated as dark-grey powders by
suction filtration and dried under vacuum at room tempera-
ture. The Ru(0)/HAp sample can be bottled and stored under
ambient conditions. Furthermore, when reused Ru(0)/HAp
are still active catalysts in the hydrogenation of benzene, they
retain 96% of their initial catalytic activity with the complete
conversion of benzene to cyclohexane even at the third run.
The slight decrease in the catalytic activity of Ru(0)/HAp in
the third run may be attributed to the decrease in the number
of active surface atoms by the increase of the size of
ruthenium(0) nanoclusters. Formation of clumps is also
evidenced by the TEM image of the Ru(0)/HAp recovered at
the end of the third run of benzene hydrogenation (Fig. ESI-5w),
which shows an increase of the average size to 3.1 ꢀ 0.8 nm
(Ru(0)B1100 nanoclusters). It is also noteworthy that XRD
analysis of the same sample (Fig. ESI-1w) reveals no loss in the
crystallinity of the host material.
Taking all the results together one can conclude that
Ru(0)/HAp are isolable, bottleable and repeatedly usable as
active catalysts in the hydrogenation of aromatics. That no Ru
was detected in the filtrate by ICP (with a detection limit
Scheme 1 Ru(0)/HAp catalyzed hydrogenation of aromatics in cyclo-
Scheme 2 Ru(0)/HAp catalyzed hydrogenation of neat benzene
hexane under mild conditions.
under mild conditions.
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of 24 ppb for Ru) confirms the retention of ruthenium within
the HAp matrix (no Ru passes into the solution during the
suction filtration). A control experiment was also performed to
show that the hydrogenation of benzene is completely stopped
by the removal of Ru(0)/HAp from the reaction solution.
In summary, Ru(0)/HAp could be reproducibly prepared
from readily available reagents following the procedure
reported herein. They exhibit exceptional catalytic activity in
the hydrogenation of aromatics under mild conditions and
provide a record catalytic lifetime (TTO = 192 600) in
the hydrogenation of neat benzene at 25 ꢀ 0.1 1C and
42 ꢀ 1 psig initial H2 pressure. Moreover, Ru(0)/HAp catalyzed
hydrogenation of neat benzene at room temperature is
‘‘relatively green’’ in terms of its environmental impact as it
fulfils 7 of the 12 requirements of green chemistry27 including
that (i) it is 100% selective and minimizes by-products or
waste, (ii) it maximizes the incorporation of all reactants into
the products, (iii) it is solventless (i.e., uses neat aromatics as
the substrate/solvent), (iv) it requires relatively low energy as it
occurs under mild conditions of 25.0 ꢀ 0.1 1C and r3 atm
pressure, (v) it is catalytic not stoichiometric, (vi) it does not
use any blocking, protecting/deprotecting group, (vii) real-
time monitoring is easy by measuring the H2 uptake, 1H NMR
or GC-analysis, for example. The high catalytic activity,
easy preparation, isolability, bottleability, and reusability of
Ru(0)/HAp raise the prospect of using this type of simply
prepared material for the hydrogenation of aromatics in industrial
applications as well as in small scale organic synthesis.
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18 See ESI for details of the methodw.
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21 Using the equation N = N0pV/101.1, where N0 = 6.022 ꢃ 1023
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corrected for the amount of metal that is on the surface of the
catalyst and/or the actual number of active sites. The initial TOF
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4790 | Chem. Commun., 2010, 46, 4788–4790