1996
Junrui Li et al. / Chinese Journal of Catalysis 36 (2015) 1995–2003
than 30 years ago [7]. Activated carbon is typically used as a
carbon support material for Pt or Pd nanoparticles. With the
development of novel carbon materials, new forms of activated
carbon such as activated carbon fibers (ACF) and activated
carbon cloths (ACC) were prepared and applied in hydrogena‐
tion processes. For instance, ACC‐supported Pt catalysts were
applied for the hydrogenation of nitrobenzene by Solano and
coworkers in 1997 [8]. The results showed that the Pt/ACC
catalysts presented two singular advantages over granular
activated carbons (for which data had been published previ‐
ously): (1) they showed a remarkable ease of reduction, in that
the Pt was reduced completely at 393 K; and (2) they were
stable at temperatures as high as 623 K, and after long reduc‐
tion time. In addition, there was a good linear relationship be‐
tween the dispersion of the Pt and its activity, and the disper‐
sion of the Pt could be controlled by changing the surface area
of the ACC supports; the dispersion of the Pt was in turn re‐
sponsible for the catalytic activity.
Carbon nanotubes (CNTs) are allotropes of carbon with a
cylindrical nanostructure. CNTs have unusual properties that
are valuable for nanotechnology, electronics, optics, and other
materials science and technology fields. Owing to their extraor‐
dinary thermal conductivity, and their unique mechanical and
electrical properties, CNTs can be applied in a wide variety of
fields, including catalysis. In 2005, Li et al. [11] prepared
CNT‐supported Pt catalysts using an impregnation and reduc‐
tion‐precipitation method, and investigated their catalytic per‐
formance toward nitrobenzene hydrogenation under atmos‐
pheric pressure and ambient temperatures. The Pt/CNT cata‐
lyst showed superior activity for the hydrogenation of nitro‐
benzene, compared with Pt/AC (activated carbon) catalysts,
and the authors ascribed the extraordinary activity of the
Pt/CNT catalyst to the mesoporous structure of the ac‐
id‐oxidized CNTs and the highly dispersed Pt. In 2010, Sun et al.
[14] prepared multi‐walled CNT (MWCNT)‐supported ultrafine
Pt nanoparticles via a facile route, with the aid of tip sonication.
The loading of Pt on the MWCNTs reached 50%, and the size of
the Pt particles could be controllably tuned in the range of
1.9–3.5 nm, with a narrow size distribution. The Pt/MWCNT
catalyst was applied to catalyze the hydrogenation of nitro‐
benzene under a constant hydrogen pressure of 4.0 MPa, and in
solvent‐free conditions. The Pt/MWCNTs showed a turnover
frequency (TOF) of 66900 h−1, and superior selectivity for ani‐
line, because of the strong interaction between the Pt nanopar‐
ticles and the mesoporous MWNTs support.
1173 K [27]. CMK‐3 OMCs have been proven stable even under
acidic conditions, and they exhibit a unique hydrophobic affin‐
ity toward organic reactants and solvents [28]. In our previous
work, a Pt/CMK‐3 catalyst was successfully applied for the liq‐
uid‐phase hydrogenation of benzaldehyde and its derivatives
[23], and was also proven to be active and enantioselective in
the asymmetric hydrogenation of α‐ketoesters after modifica‐
tion with cinchona alkaloids [25].
Encouraged by these achievements, we aimed to extend the
applications of the Pt/CMK‐3 catalyst to include the liq‐
uid‐phase hydrogenation of nitrobenzene and its derivatives
under 4.0 MPa hydrogen pressure, and at ambient tempera‐
ture. Notably, the TOFs achieved using the Pt/CMK‐3 catalyst
were greater than 26.6 s−1 for the hydrogenation of most ni‐
troarene compounds in ethanol, and a TOF of 43.8 s−1 was
achieved for 2‐methyl‐nitrobenzene. To the best of our
knowledge, this TOF value is the highest determined to date for
the hydrogenation of nitrobenzene using Pt‐related catalysts.
Moreover, the Pt/CMK‐3 catalyst could be easily recycled, and
could be reused at least fourteen times without loss in activity
or selectivity for the hydrogenation of nitrobenzene in ethanol.
The Pt nanoparticles were stable on the CMK‐3, and the
amount of Pt that had leached into the filtrate after the reaction
was below the detection limit of ICP‐AES.
2. Experimental
2.1. Materials
Hydrogen
hexachloroplatinate
(IV)
hexahydrate
(H2PtCl6·6H2O) and other chemicals were of analytical grade.
Pluronic 123 (EO20PO70EO20, MW = 5800) was purchased from
Sigma‐Aldrich. Nitrobenzene and its derivatives were pur‐
chased from Aladdin, and were used as received. The commer‐
cial Pt/C (5%) catalyst and sodium formate were purchased
from Alfa Aesar. Sucrose, tetraethoxysilane (TEOS), hydrochlo‐
ric acid (HCl, 37%), concentrated sulfuric acid (H2SO4, 98%),
and ethanol were purchased from Sinopharm Chemical Rea‐
gent Co., Ltd.
2.2. Preparation and characterization of the catalyst
The CMK‐3 OMCs were synthesized using mesoporous silica
SBA‐15 as the hard template, sulfuric acid as the carbonization
catalyst, and sucrose as the carbon source [27]. 5% Pt/CMK‐3
catalyst was prepared according to the method reported by Li
et al. [25,29]. Specifically, the CMK‐3 was impregnated with an
aqueous solution of H2PtCl6 for 6 h, under magnetic stirring.
The mixture was then evaporated to remove the excess solvent,
and this was followed by a drying at 353 K overnight. The cata‐
lyst precursor was subsequently reduced in an aqueous solu‐
tion of sodium formate at 363 K for 2 h. The mixture was then
washed using plenty of water, to remove chlorine ions, and
dried at 353 K overnight. The obtained catalyst was denoted as
Pt/CMK‐3.
Ordered mesoporous carbon materials (OMCs) have at‐
tracted increasing interest, because of their large surface area,
uniform pore size, unique porous architecture, and high ther‐
mal, chemical, and mechanical stabilities [15,16]. These fea‐
tures mean that OMCs are promising candidates for electrode
materials [17], adsorbents [18], catalyst supports [19–25], and
catalysts [26]. Among OMCs, CMK‐3 OMCs with p6mm sym‐
metry are particularly widely known and widely applied
[21–23,25,27,28]. CMK‐3 OMCs can be synthesized using a
classic two‐step nano‐casting method, using mesoporous silica
SBA‐15 as a template, sucrose as a carbon source, sulfuric acid
as a carbonization catalyst, and carbonization performed at
X‐ray diffraction (XRD) patterns of the samples were col‐
lected using a Bruker D8 Advance instrument, using Cu Kα ra‐