G Model
CATTOD-9445; No. of Pages8
ARTICLE IN PRESS
2
Y. Kuwahara et al. / Catalysis Today xxx (2015) xxx–xxx
as an ideal support owing to its pore diameter (2–20 nm) similar to
the size of the metal NPs and the defined porous arrangement that
can spatially-isolate the NPs. However, the metal NPs supported on
the conventional mesoporous silicas usually suffer from leaching
and particle aggregation during reactions and low-accessibility of
reactant molecules because of the two-dimensionally-aligned long
mesopore channel systems. One of the key issues for achieving high
catalytic activity for the supported metal NPs is to develop a new
method to highly-disperse and stabilize them by manipulating the
morphology, porous structure and surface environment of meso-
porous silicas. In regard to this issue, we have recently developed a
nels of mesoporous silica, in which the heteroatoms (especially
Zr) imbedded within the matrix of mesoporous silica provide a
productive effect to anchor and stabilize the guest species via inter-
molecular acid–base interactions [44,45]. Such effects are expected
to be beneficial for dispersing and stabilizing metal NPs as well, and
thus improve the catalytic performances of the metal NPs.
Herein, we demonstrate hydrogenation of levulinic acid and its
esters into ␥-valerolactone by using Ru NP catalysts confined in
Zr-containing spherical mesoporous silica as a bifunctional cat-
may offer an improved stability and reusability of the Ru NP cat-
alysts. Furthermore, the imbedded Zr sites may contribute to GVL
selectivity owing to their acidic property to catalyze the dehydra-
tion/dealcoholation reaction steps [46–48]. To this end, a series of
Zr-containing spherical mesoporous silica having P6mm hexagonal
mesoporous structure with varied Zr content were synthesized by
sol–gel process using cetyltrimethylammonium bromide (CTAB) as
a pore-directing agent and tetraethylorthosilicate (TEOS) as a sili-
con source. The influences of Zr incorporation on porous structures,
size of Ru NPs and catalytic performances (including activity, selec-
tivity and stability) were investigated in detail. The activities of
the catalysts were examined by the hydrogenation of levulinic acid
(LA) and methyl levulinate (ML) at ambient reaction conditions and
were compared with Ru NPs supported on the conventional silica
materials.
For comparison, a conventional mesoporous silica with large
particle size (MCM-41) was prepared by a hydrothermal synthesis
method using CTAB as an organic template and TEOS as a silicon
source with the molar ratio of Si:NH3:CTAB:H2O = 1.0:3.3:0.1:66.
2.2. Synthesis of supported Ru NP catalysts
The supported Ru NP catalyst was synthesized by a conven-
tional impregnation method and a following reduction treatment
in a flow of H2. 1.0 g of support was dispersed in 50 mL of aqueous
solution containing 3.51 g of ruthenium(III) nitrosyl nitrate solution
(Ru(NO)(NO3)x(OH)y, 1.5% as Ru, Aldrich) and subjected to ultra-
sonication for 5 min. After stirring for 24 h at room temperature,
the slurry was then placed in a rotary evaporator, and the water
was removed under vacuum to facilitate the incorporation of the
Ru(III) ions into the support. After drying at 100 ◦C, the sample was
reduced in a flow of 10% H2 in N2 (100 mL/min) at 350 ◦C for 5 h
with a ramping rate of 4 ◦C/min. The Ru loading was adjusted to
5 wt.%. The conventional mesoporous silica (MCM-41), fumed sil-
ica (SBET = 279 m2/g, Wako Pure Chemical Ind., Ltd.) and zirconia
(SBET = 112 m2/g, Daiichi Kigenso Kagaku Kogyo Co., Ltd.) were also
used as supports.
2.3. Characterization
X-ray diffraction (XRD) patterns were recorded on a Bruker
AXS D8 Advance X-ray diffractometer with CuK␣ radiation
˚
(ꢀ = 1.54056 A, 40 kV to 40 mA). Nitrogen adsorption–desorption
isotherms were measured at –196 ◦C by using Micromeritics
ASAP2020. The samples were degassed at 300 ◦C under vacuum
for 4 h prior to the measurements. The specific surface area was
calculated by the BET (Brunauer–Emmett–Teller) method by using
adsorption data ranging from p/p0 = 0.05–0.30. The pore size dis-
tributions were obtained from the adsorption branches of the
isotherms by the BJH (Barret–Joyner–Halenda) method. Trans-
mission electron microscope (TEM) observations were performed
with a FEI TITAN80-300 operated at 200 kV. Infrared spectra were
recorded on a JASCO FTIR-6300 instrument in the spectral range
2000–400 cm−1 under vacuum with a resolution of 4 cm−1 using
samples diluted with KBr. Dynamic light scattering (DLS) study
was carried out on a Malvern Zetasizer Nano ZS at 25 ◦C in ethanol
solution.
2. Experimental
Temperature programmed desorption of NH3 (NH3-TPD) was
performed by using a BELCAT-B system (BEL Japan, Inc.) equipped
with an on-line thermal conductivity detector. The samples were
pretreated under a He flow (50 mL/min) at 600 ◦C for 1 h, allowed
to cool to 50 ◦C and subsequently exposed to flowing 5% NH3/He
gas (50 mL/min) for 1 h. After purging at 50 ◦C for 30 min with He,
NH3-TPD was carried out between 50 and 600 ◦C under a He flow
(30 mL/min) with a ramping rate of 10 ◦C/min.
The Zr-containing spherical mesoporous silicas (ZrSMS) with
different Zr contents were synthesized according to the reported
method with minor modifications [49,50]. Into an aqueous solu-
tion containing cetyltrimethylammonium bromide (CTAB, Wako
Pure Chemical Ind., Ltd.) and aqueous ammonia (28%, Wako
Pure Chemical Ind., Ltd.), tetraethylorthosilicate (TEOS, 95%,
Wako Pure Chemical Ind., Ltd.) diluted in ethanol (1.0 M) was
added rapidly at 50 ◦C under stirring. After stirring for 1 h, a
mixture of TEOS and zirconium(IV) tetrapropoxide (Zr(OnPr)4,
70% in 1-propanol, Tokyo Chemical Ind. Co., Ltd.) in ethanol
(0.2 M) was added dropwise under vigorous stirring and the
mixture was continuously stirred for another 2 h at 50 ◦C.
The molar ratio of the initial synthesis solution was adjusted
to Si:Zr:NH3:CTAB:H2O = 1.0:(0–0.1):28:0.25:2540. The solution
was then transferred to a teflon bottle, sealed and hydrother-
mally treated at 100 ◦C for 24 h under static conditions. Then,
the resulting particles were collected by centrifugation at
18,000 rpm, washed several times with deionized water and
ethanol, dried overnight and finally calcined at 550 ◦C for 6 h
to remove the organic template. The samples were denoted as
ZrxSMS, where x represents the Zr/Si molar ratio in the initial
solution.
2.4. Procedures for catalytic reactions
Hydrogenation of LA and ML was performed in a 60 mL cylin-
drical stainless steel autoclave reactor (EYELA, Inc.) equipped
with a bourdon pressure gauge. In a typical reaction, catalyst (Ru
0.5 mol%), substrate (5 mmol) and solvent (water or methanol,
10 mL) were introduced into the reactor, which was then sealed,
purged and pressurized with 0.5 MPa of H2 and then heated at 70 ◦C
with magnetic stirring. After the allotted reaction time, a portion
of the reaction mixture was withdrawn by filtration and then ana-
lyzed by a gas chromatograph (Shimadzu GC-14B) with a flame
ionization detector equipped with a capillary column (ULBON HR-
20M; 0.53 mm × 30 m; Shinwa Chemical Ind., Ltd.). Conversion of
substrate and yields of products were quantified by using bis(2-
methoxyethyl)ether as an internal standard. To study the catalyst
Please cite this article in press as: Y. Kuwahara, et al., Ru nanoparticles confined in Zr-containing spherical mesoporous sil-
ica containers for hydrogenation of levulinic acid and its esters into ␥-valerolactone at ambient conditions, Catal. Today (2015),