G. Tanimu, et al.
MolecularCatalysis488(2020)110893
siliceous carriers (SBA-15, silica foam and MCM-41) are utilized in the
synthesis of highly dispersed tungsten oxide catalyst for butene me-
tathesis reaction [24–26]. Nanoparticles of vanadium deposited on
mesoporous silica supports have been utilized for the ODH of n-butane
[10–14]. Supports of highly ordered mesoporous silica (SBA-15, silica
foam, and MCM-41) loaded with 20 wt% Ni and 30 wt% Bi, were also
utilized in our previous study [27]. The binary-oxide impregnated
catalysts were probed for n-butane ODH to butadiene and compared
with traditional silica gel catalysts. Mesoporous silicas as catalyst sup-
port are superior to the traditional silica gels, where the SBA-15 me-
soporous silica supported catalyst gave the best performance [27]. The
oxides was reported by Dumitru et al. [28]. The BiOx species were in-
corporated into the ZSM-5 pores during hydrothermal synthesis and it
was concluded that the well-dispersed and occluded BiOx clusters are
effective for the liquid-phase oxidation of hydrocarbons. The strength
and/or type of acid sites present and the redox properties of the catalyst
were controlled by the BiOx species and its interaction with the support
[28]. This was extended in a series of papers by different authors, for
reactants flow rate was kept at 31.2 ml/min. The performance of the
catalysts were evaluated at 450 °C and Oxygen to n-butane ratio
=2.0 mol/mol. Analyses of products was achieved with an online gas
chromatograph (GC) system (Agilent, 7890 N). Flame ionization de-
tector (FID) with GC-Gas Pro capillary column was utilized in oxyge-
nates and hydrocarbons identification. All the gases were identified
with a Thermal conductivity detector (TCD) having He and Ar as car-
riers for the TCD columns. Standard samples were used for confirming
the products. The conversion of n-butane and the selectivities of the
products were obtained using based on carbon mass balance.
2.3. Characterization of catalysts
The physical properties were obtained using N2 adsorption-deso-
rption isotherm with Micromeritics ASAP 2020 instrument, Norcross,
GA. BJH adsorption method was used in obtaining the pore surface
area, pore volume and average pore diameter. X-ray diffraction (XRD)
patterns of the calcined catalysts were recorded from 5° to 90° dif-
fraction angle using X-ray diffractometer (Rigaku Miniflex II) utilizing
Cu Kα radiation at λ = 1.5406 Å and 30 mA and 40 kV operating
parameters, at a speed of 0.5°/min and step size of 0.02°. High
Resolution Transmission Electron Microscopy (HRTEM), JEM-2100 F
model having an acceleration voltage of 200 kV was used in analyzing
the morphologies of the catalysts. Temperature Programmed Reduction
(TPR) was used in determining the catalysts reducibility. This was
achieved with a chemisorption instrument [27]. H2/Ar (5 vol%/95 vol
%) gas mixture at a flow rate of 50 cm3/min was used in the TPR
measurement. 100 mg of the calcined catalyst was preheated for 3 h at
300 °C under inert He and then cooled to room temperature. It was then
increased up to 1000 °C at the rate of 20 °C /min. The intake of H2 was
measured with TCD and CuO was applied as a reference for H2 con-
sumption calibration. The catalysts binding energy and bonding states
were analyzed using X-ray photoelectron spectroscopy (XPS) with a PHI
5000 Versa Probe II, ULVAC-PHI Inc. spectroscope. The samples were
disc-pelletized and put under a high vacuum prior to the XPS mea-
surement.
This manuscript reports the effect of cooperating BiOx species with
SBA-15 mesoporous silica support on the redox property of the active Ni
species and on the overall catalytic performance during n-butane oxi-
dative dehydrogenation. The metal oxides-support cooperation was
clearly studied using X-ray diffraction, X-ray photoelectron spectro-
scopy and temperature programmed reduction. Other mesoporous silica
supports including silica foam, silica sol, and the traditional silica gel
catalysts have also been utilized in this study as references.
2. Experimental
2.1. Catalyst preparation
SBA-15 mesoporous silica carrier was prepared using tri-block co-
polymer as a structure directing agent. 4 g of Pluronic P123 was in-
troduced into 30 ml of de-ionized water. Stirring continued until clear
solution was achieved. 70 g of 0.28 M HCl was added to the solution
with continuous stirring for 2 h. 9 g of Tetraethyl orthosilicate (TEOS)
was introduced and stirring continued for 24 h at 40 °C and finally
heated for 48 h at 100 °C. Filtration was utilized in recovering the solid
product, de-ionized water was used several times for washing the re-
sidue and then dried at 100 °C overnight. Calcination was done at
550 °C for 6 h for template removal [36]. The other mesoporous silicas
used were silica gel Q10; procured from Fuji Silysia Chemicals Limited,
Japan. MCM-41 was prepared following the method reported by Palani
et al. [37]. Silica foam was prepared using the method obtained in the
literature [38], and silica sol was purchased from Sigma Aldrich.
supported catalysts to obtain enhanced cohabitation of mixed oxide
nanoparticles [39]. Ni and Bi precursors utilized were respectively Ni
(NO3)2·6H2O (99 %, Fisher Scientific) and Bi(NO3)3·5H2O (98 %, Fluka
Garantie). In a typical synthesis, 990 mg of Ni precursor was dissolved
in 80 ml of de-ionized water. 700 mg of Bi precursor was introduced
after dissolution with continuous stirring. 1000 mg of the support was
then introduced. The mixture was left overnight for Bi species equili-
brium adsorption and followed by evaporative drying at 80 °C for en-
forced deposition of Ni species. The product was dried further for 3 h at
120 °C and two steps calcination at 350 °C for 1 h and 590 °C for 2 h at
the rates of 10 °C/min and 15 °C/min, respectively, was carried out.
3. Results and discussion
3.1. Catalyst property
3.1.1. Surface area and porosity
The dispersion of active sites is mainly determined using the BET
surface area and catalysts pore structure [40]. The surface areas cal-
culated using the BET equation in the linear region of the N2 adsorp-
tion-desorption isotherms (P/Po = 0.05-0.3), all the catalysts pore
properties calculated using the BJH method, are presented together
with the support values in Table 1. A decrease in surface area was
observed for all the catalysts compared to that of the support. The same
trend was also observed in the catalysts pore structure.
The N2 adsorption-desorption isotherm of the 20 Ni-30 Bi-O/SBA-15
catalyst (normalized to support weight) and SBA-15 support is pre-
sented in Fig. 1(a) while the pore size distribution (normalized to
support weight) using the adsorption branch isotherm is shown in
Fig. 1(b). The isotherms are typical of type IV of the IUPAC classifica-
tion due to the existence of the type H1 hysteresis loop, which is
common for mesoporous materials having a well-defined cylindrical-
like pore channel [40]. Adsorption and desorption paths of the isotherm
coincide at low relative pressure (up to P/Po ≈ 0.5) which is an in-
dication that there exists monolayer-multilayer adsorption [41]. The
beginning of the hysteresis loop signifies capillary condensation within
the pores, and its end corresponds to the filling of the pores. Metal
loading resulted in the decrease in the height of the loop due to pore
volume decrease caused by the introduction of metal species within the
support mesopore [42]. The pore size distribution gave a sharp peak for
both the support and the catalyst, averaged around 8−10 nm, which
2.2. Catalytic testing
ODH reaction of n-butane was conducted in an automated fixed-bed
reactor obtained from Microtrac Bel Company, Japan. 300 mg of the
synthesized catalyst was introduced into the reactor to form the catalyst
bed. The contact time of n-butane was fixed at 0.42 h·g/mol. Total
2