C. Nguyen-Huy, et al.
AppliedCatalysisA,General583(2019)117125
catalysts, such as Pd, Ru, Cu, and their alloys with second transition
metals [29–40]. The catalytic conversion and selectivity are strongly
affected by the type of catalysts. Although many literatures reported
effective catalysts for FAL hydrogenation, lowering the usage of pre-
cious metals and gaseous hydrogen is an important issue to be over-
come for the commercialization of this process [31].
templates are termed m-Co3O4(p6mm) and m-Co3O4(Ia3d), respectively.
The as-prepared m-Co3O4 is further reduced under H2 (4% H2 in Ar,
flow rate = 100 mL‧min−1) at 350 or 500 °C for 1 h, and the obtained
products are labelled as m-CoO(p6mm)-350, m-CoO(Ia3d)-350, m-Co
(p6mm)-500, and m-Co(Ia3d)-500 according to the bulk crystal struc-
ture, space group, and reduction temperature.
Here, we present highly ordered mesoporous cobalt oxide catalysts
as strong candidates for noble metal-free catalytic FAL hydrogenation.
Although the catalytic activity and selectivity are greatly changed by
the surface environments, the active species on the surface of cobalt
catalysts in the presence of mesoporous structures is still under in-
vestigation [41,42]. In this works, the impacts of the structure, por-
osity, and oxidation state of mesoporous cobalt oxides are investigated
to identify catalytically active species for conversion of FAL hydro-
genation. The surface structures and adsorption properties of these
oxides after treatment in different reduction environments are clarified,
by combining catalytic experiments with surface characterizations and
density functional theory (DFT) calculations. While the mesoporous
CoO catalyst with p6mm symmetry show the highest reaction rate
among catalysts, the combined results demonstrate that CoO is a major
active phase with a distinct product selectivity toward MF via selective
hydrogenation of FAL to FA and subsequent hydrogenolysis to MF.
2.3. Catalyst characterization
The surface area, pore volume, and average pore diameter of me-
soporous cobalt oxide series are characterized by the
Brunauer–Emmett–Teller (BET) method by nitrogen physisorption in a
BEL SORP-max unit. Surface morphologies and structural information
of the fresh and used catalysts are characterized by transmission elec-
tion microscopy (TEM, JEOL JEM-1400). X-ray diffraction (XRD) pat-
terns are recorded on an X-ray diffractometer (PANalytical X’Pert PRO)
using Cu Ka as radiation source (λ = 0.154056 nm). X-ray photoelectric
spectroscopy (XPS) analysis is carried out on a K-alpha (ThermoFisher)
system equipped with an Al Kα X-ray radiation source.
Temperature programmed reduction by H2 (H2-TPR) is carried out
with an Auto ChemⅡ 2920 instrument (Micromeritics Instruments Co.).
Prior to each TPR run, the catalyst is placed in a quartz reactor and
pretreated in He flow at 30 mL min−1 and 100 °C for 30 min to purge
any residual oxygen. After cooling the reactor to room temperature, the
2. Experimental
catalyst is then heated to 800 °C at
a constant heating rate of
10 °C‧min−1 under 2% H2–He flow of 30 mL‧min−1. The hydrogen
consumption is monitored by a thermal conductivity detector (TCD,
Agilent 7820A). The amount of H2 consumed is determined by a series
of CuO/SiO2 with different Cu loading as a reference.
2.1. Materials
All chemicals used are reagent-grade. Cobalt(II) nitrate hexahydrate
(Co(NO3)2·6H2O, ≥98.0%), 1-butanol (anhydrous, 99.8%), poly(ethy-
lene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)
PEO20−PPO70−PEO20 (Pluronic P123), and tetraethoxysilane (TEOS)
are purchased from Sigma-Aldrich.
in situ powder XRD data are recorded on a SmartLab (Rigaku) X-ray
diffractometer equipped with a D/teX Ultra 250 detector using Cu Kα
radiation (λ = 1.54184 Å). The samples are placed on a kanthal
(FeCrAl) filament cavity, and then subjected to thermo-programmed
reduction from 30 to 900 °C (heating rate of 5 °C‧min−1) under a flow of
4.0 vol.% H2 in Ar (30 mL‧min−1). At defined temperatures, the in situ
diffractograms are recorded in the 2θ range from 10 to 80° with a step
of 0.05° (step time of 2 s).
X-ray absorption spectroscopy (XAS) analysis is conducted at the
beamline 6D of the Pohang Accelerator Laboratory (PAL) in South
Korea. The electron beam energy and current of the incident X-ray were
3.0 GeV and 300 mA, respectively. To filter the incident photon energy,
a Si(111) double-crystal monochromator is used. The energy is detuned
by 30% to remove high-order harmonics. X-ray absorption near edge
structure (XANES) spectra of Co K-edge region are only obtained in
transmission detection mode. Background removal and normalization
of the XAS spectrum are done using IFEFFIT (Athena) software. For
reference materials, Co3O4 and CoO are purchased from Sigma-Aldrich
(99.99%) and metallic Co foil is used.
2.2. Catalyst preparation
2.2.1. Synthesis of SBA-15 and KIT-6 silicas
Two types of mesoporous silicas, SBA-15 and KIT-6, are synthesized
using TEOS as silica source and Pluronic P123 as a structure-directing
agent [4–8,43–45]. For the synthesis of SBA-15, 16 g of Pluronic P123 is
dissolved in 120 g of water and 480 g of 2 M HCl solution with stirring
at 40 °C [43,44]. Then, 34 g of TEOS is added into that solution with
stirring at 40 °C for 20 h. Afterwards, the mixture is aged at 80 °C
overnight without stirring. The solid product is recovered, washed, and
dried in an ambient environment. Calcination is carried out at 550 °C
for 6 h. In the typical preparation of KIT-6, 27 g of P123 and 43.5 mL of
concentrated HCl (37%) are dissolved in 980 mL of water in a poly-
propylene bottle, and 33.3 mL of n-butanol is added to the solution at
35 °C with vigorous stirring [45]. After 1 h, 58 g of TEOS is added to the
solution, followed by stirring at this temperature for 24 h. The capped
bottle is stored at 40 °C for another 24 h in an oven. The solid is filtered,
dried at 90 °C overnight, and calcined at 550 °C for 6 h.
2.4. Catalytic FAL hydrogenation
Catalytic reaction is carried out in a 100-mL stainless-steel high-
pressure reactor (Parr 5500) containing 1 g of FAL (Acros, 99%), 5 mg
of catalyst, and 20 mL of isopropanol (Aldrich, 99.5% anhydrous). The
reactor is initially flushed and then pressurized with hydrogen to
20 bar. The reactor is heated to 180 °C and maintained for 5 h with
stirring at 600 rpm. The liquid product is analyzed by a flame ionization
detector equipped with a capillary column (DB-Wax, 30 m length,
0.32 mm internal diameter, 0.25 μm film thickness) in gas chromato-
graphy (GC) [46]. The following program is repeated: hold at 50 °C at
3 min, heating to 50–100 °C at a rate of 10 °C‧min−1 and hold for 3 min,
heating to 100–200 °C at a rate of 25 °C‧min−1 and hold for 3 min. To
test the products, the following GC reference reagents are used: FA
(Aldrich, 98%), THFA (Aldrich, 99%), FR (Aldrich, 99%), THF (Alfa
Aesar, 99%), and MF (Aldrich, 99%). Cyclopentanol and FAL dimers are
detected in small quantities as residual products. The product
2.2.2. Synthesis of mesoporous cobalt oxides
Mesoporous cobalt oxides are prepared via the nanocasting ap-
proach using KIT-6 and SBA-15 [4–8]. Briefly, 24 mmol of cobalt nitrate
is dissolved in 6 mL of deionized water. The cobalt precursor solution is
mixed with 6 g of SBA-15 (or KIT-6) and 30 mL of toluene at 65 °C
under stirring. Toluene is slowly evaporated and the remaining pre-
cipitate is dried at 60 °C overnight and then calcined at 300 °C for 5 h.
The resulting solid is poured into 2 M aqueous NaOH solution and he-
ated to 90 °C. The silica templates is dissolved into the NaOH solution,
remaining a mesoporous Co3O4 (m-Co3O4) replica. After centrifugation
and decanting the NaOH solution containing the silica residue, the
purified m-Co3O4 is obtained. Washing of the m-Co3O4 is repeated with
the NaOH solution and the final product is rinsed twice with water and
dried at 50 °C. The catalyst samples prepared from SBA-15 and KIT-6
2