M. Pang et al. / Applied Catalysis A: General 490 (2015) 146–152
147
of the Mo precursors with the supports, followed by calcination
and carburization [15,16]. The dispersion of the resultant carbides
on the supports is therefore determined by the impregnation
procedure. Unfortunately, most of the results were unsatisfactory,
exhibiting carbides either locally aggregating or unincorporated
into the framework of the supports. These carbide particles are
thermally unstable thanks to the weak contact with the supports
that under certain harsh reaction conditions they would undergo
thermal sintering, leading to activity loss and shortening of the
service life. Advanced methods have been delivered to enhance
the coalescence between the carbides and the supports in order to
improve the carbides durability in catalytic use. In this regard, Ji
et al. had taken advantage of Mo species interplaying with P123
micelles in the fabrication of SBA-15 to achieve the inclusion of
For the synthesis of MoCx incorporated into ordered meso-
porous silica framework of SBA type with varied Si/Mo molar ratios,
2 g of P123 was dissolved in 50 mL HCl (2.5 M). The temperature of
◦
the solution was kept at 40 C using a water bath. A certain amount
of the hybrids was added into the P123 solution. Exceptionally, due
◦
to the limited amount of the hybrids dissolved in HCl at 40 C, the
◦
hybrids were first dissolved in 50 mL HCl (2.5 M) at 80 C in synthe-
sizing the sample with Si/Mo molar ratio equaling 6. The solution
◦
was then used to dissolve P123 after being cooled to 40 C. After
◦
stirring at 40 C for 5 h, 5.82 g of TEOS was added in, and soon later,
the solution became opaque indicating the hydrolysis of TEOS. After
◦
stirring at 40 C for another 5 h, the opaque system was transferred
◦
into a 100 mL Teflon autoclave and isothermally treated at 100 C
for 24 h. The precipitate was collected by filtration but without
◦
MoOx into the silica framework. Followed by CH -carburization,
washing and, dried at 120 C. Pyrolyzing the precipitate in Ar/H2
4
the highly dispersive MoOx were transformed into -Mo C which
(100 mL/min, 1:1 (v/v)) led to MoCx@OMSF samples with varied
2
preserved the intimate connection of that between MoOx and the
support, featuring good thermal stability [17]. Zhang et al. had
introduced an organic-organic cooperative self-assembly approach
to imbed MoC nanoparticles (NPs) into the ordered mesoporous
carbon. The resorcinol resin, which was used as the bridge agent
in the synthesis, first assembled around F127 micelles then inter-
Si/Mo molar ratios. The temperature was increased linearly from
◦
◦
◦
RT to 650 C with a linear increase of 5 C/min and hold at 650 C
for 90 min. The gas was switched off after the temperature dropped
◦
to 300 C below to allow the slow diffusion of air back into the
quartz tube to passivate the carbide surface in order to avoid bulk
oxidation. The as-prepared samples were denoted as MoCx@OMSF-
70, 50, 30, 15, 10, 6, where the number corresponded to the Si/Mo
molar ratio. Pure OMSF was prepared for comparison. Its synthe-
sis was identical to that for the MoCx@OMSF sample but without
6−
acted with Mo7O24
groups. After the carburization of MoOx by
the resin, MoC NPs were evenly decorated into the carbon walls
derived from the pyrolysis of the resin, and strongly anchored
to the carbon support [18]. The two examples just demonstrated
how to achieve the target of finely distributing carbides within the
framework of the supports in a self-assembly manner. Yet, the key
question remains, while there is either a way to obtain molybde-
num carbides with single phase by eliminating the use of external
carbon sources, or a way to incorporate them into the framework of
the supports by the operation of self-assembly but still preserving
the carburization of MoOx by free unbonded carbon, there is a blank
of the integration of the two techniques; and those composites
with single phase carbide finely included in the support framework
remains a good solution to the aforementioned two deactivation
issues of molybdenum carbides catalysts but still a big challenge.
The key objective of our research has been to combine those two
routes together in one way. To accomplish this goal, we applied
VI
the addition of the Mo –melamine hybrids. The reference sample
free of silica was prepared according to the preparation procedure
for the MoCx@OMSF samples but without adding TEOS. After the
VI
Mo –melamine hybrids were dissolved in the solution of P123,
the mixture was transferred into a 100 mL Teflon autoclave and
◦
isothermally treated at 100 C for 24 h. The solvent was removed
◦
through rotary evaporation at 60 C. The collected gel was dried in
◦
oven at 120 C before subjected to thermal treatment identical to
that in the preparation of the MoCx@OMSF samples.
The Mo/OMSF sample was prepared through the wet impregna-
tion method. Pure OMSF was impregnated in the aqueous solution
of AHM for 12 h. The collected powder was subjected to thermal
treatment in Ar/H2 (100 mL/min, 2:3 (v/v)) after dried in oven at
◦
◦
120 C. The temperature was increased linearly from RT to 700 C
VI
◦
◦
the Mo –melamine hybrids reported in our earlier work as the
with a linear increase of 5 C/min and hold at 700 C for 240 min.
single-source precursors for single phase molybdenum carbides.
The hybrids were arranged with the building blocks for mesoporous
silica in a designate order through the self-assembly effect to form
the MoCx@OMSF preform. Followed by a simple thermal treating
procedure without introducing external carbon sources, the pre-
form was turned into the final composite with MoCx species evenly
embedded in the ordered mesoporous silica framework (OMSF).
Bulk -Mo C was obtained by directly pyrolyzing the
2
MoVI–melamine hybrids in Ar/H (100 mL/min, 1:1 (v/v)). The tem-
2
◦
perature was increased linearly from RT to 650 C with a linear
increase of 5 C/min and hold at 650 C for 90 min [14].
◦
◦
2.2. Catalyst characterizations
X-ray diffraction measurements were taken on a Rigaku
D/MAX 2400 diffractometer with Cu K␣ radiation. Nitrogen
adsorption–desorption isotherms were measured at 77 K by
using ASiQC0000-4 (Quantachrome Instrument Co.). Samples were
2
. Experimental
2.1. Catalysts preparation
◦
degassed in a vacuum at 200 C for 12 h. The total pore vol-
Poly(ethylene oxide)–Poly(propylene oxide)–Poly(ethylene
oxide) (Aldrich, pluronic, P-123). Ammonium heptamolybdate
AHM), melamine (MA), and tetraethyl orthosilicate (TEOS) were
umes were estimated from the adsorbed amount of N2 at a
relative pressure P/Po of 0.95. CHN elemental analysis was per-
formed on an Elementar Vario EL system. Fourier transform
infrared (FT-IR) spectra were collected at room temperature on
(
purchased from Sinopharm Chemical Reagent Co., Ltd. All the
reagents were used as achieved without further purification.
−
1
a Nicolet Impact 410 spectrometer with a resolution of 4 cm
.
VI
The Mo –melamine hybrids (Mo19O66(C H7N ) ·12H O)
Raman spectra were recorded on a DXR Raman Microscope with
excitation line at 532 nm. The molar ratios of Si to Mo were
determined by inductively coupled plasma-atomic emission spec-
troscopy (ICP-AES) after the samples were dissolved in 10 M NaOH.
Field-emission scanning electron microscopic (FE-SEM) observa-
tions were achieved by using a NOVA NanoSEM 450 system.
Dynamic light scattering (DLS) measurements were made on a
ZETASIZER nano series Nano-ZS90 (Malvern Instrument Co.) and
zeta potentials were determined using the same instrument with
3
6
18
2
were synthesized following the procedure reported in the pre-
vious literature [14]. In a typical synthesis, the hybrids were
obtained by mixing the aqueous solutions of AHM and MA, and
◦
collecting the white precipitate. Heating H O to 80 C was neces-
2
sary to completely dissolve MA. The aqueous solution of MA was
allowed to cool to RT naturally because either a dramatic decline
of temperature or any mechanical disturbance would induce the
precipitation of MA from water.