S.S. Ndlela et al.
Molecular Catalysis 502 (2021) 111393
formation of oxygenates and carbon oxides [7].
and testing.
Even though zeolites are used extensively in the ODH of short-chain
alkanes, quick deactivation due to coke deposition and their tendency to
promote deep oxidation is the reason why most researchers shy away
from using zeolites in ODH reactions involving medium to long-chain
paraffins. Because of the already mentioned superior properties of zeo-
lites [2,3], there is still a need to find a method of modification that can
eradicate the undesirable properties of zeolites while maintaining their
stability in the ODH of medium to long-chain paraffins. Low-value linear
medium-chain alkanes provide potentially a cheaper alternate feedstock
to chemical reactions such as olefins and aromatics which are one of the
key building blocks in the chemical industry.
2.2. Catalysts characterization
Powder X-ray diffraction patterns for structure and phase identifi-
cation were carried out using a Bruker D8 advance diffractometer
equipped with a graphite monochromatic filter operated at 40 kV and 40
mA. The radiation source was CuKα with a λ of 1.5406 nm. The data
collection was done at a step and scanning speed of 0.02◦ and 0.2 sꢀ 1
respectively and at a 2-theta range of 5◦–90 ◦. Inductively coupled
plasma optical emission spectroscopy (ICP-OES) was performed to
quantify the elemental composition of the material using a Perkin Elmer
Optima 5300 DV spectrometer. Standards of 1000 ppm Ga, Na, B, Ba,
and Si were all purchased from Fluka. N2 physisorption analysis was
performed using a Micromeritics Tristar II Surface area and porosity
analyzer operated at – 196 ◦C. Finely ground samples were degassed for
one hour at 90 ◦C, then at 200 ◦C for another hour, and then at 400 ◦C for
6 h under the flow of N2 before analysis, using a Micromeritics Flow Prep
060 instrument. A Micromeritics 2920 Autochem II Chemisorption
Analyser was used for all temperature-programmed experiments. Tem-
perature programmed reduction (TPR) profiles were obtained by a
published procedure [22]. Ammonia temperature-programmed
desorption (NH3-TPD) experiments were carried out using about 0.06
g of catalysts by a reported procedure [23]. CO2-TPD was carried out
using 0.06 g of catalyst. The sample was heated to 400 ◦C under a stream
of helium (30 mL/min) for 30 min, followed by a temperature decrease
to 80 ◦C under the same stream of helium before loading the sample with
carbon dioxide. The TPD tests were carried out by heating the samples at
10 ◦C/min until 900 ◦C in a constant He flow. Pyridine IR spectra were
collected using a Perkin Elmer Spectrum 100 FT-IR Spectrometer
equipped with a Universal ATR Sampling Accessory. Samples of 0.5 g
were treated with 1.0 mL of liquid pyridine. After 3 h the samples were
heated to 100 ◦C and 150 ◦C under vacuum to remove excess and
physisorbed pyridine. The samples were cooled to room temperature
before collecting the spectra in the region of 1400–1700 cmꢀ 1. For
morphology and surface structure, SEM images were obtained using a
Zeiss Ultra Plus field emission gun scanning electron microscope
(FEG-SEM) with Smart SEM Software. Before analysis, the samples were
coated with gold using a Q150R series high vacuum Quorum sputter
coater.
Zeolites offer a variety of methods in which they can be tuned, viz.
ionic exchange, isomorphic substitution, dealumination, and desilica-
tion. For this study, we looked at modifying a faujasite type zeolite by
isomorphic substitution of framework aluminium with gallium and
boron to yield another type of material called a silicalite. A silicalite is
defined as a silica polymorph with a structure similar to zeolite [8].
Previous studies focussed on the syntheses and modification of the MFI
and MCM type silicalites [9–13]. The use of MCM type zeolites analogs is
mainly as adsorbents, and they are studied very little in paraffin acti-
vation owing to their low stability. MFI silicalites are known and have
been used in several catalytic reactions. The most known MFI silicalite is
the titanium-based TS-1 silicalite [14,15]. Both MCM and MFI type ze-
olites have been studied for ODH of alkanes, and both systems suffer
from deactivation and production of COx as one of the main products
[16,17]. Therefore, using a relatively weak acidic material (FAU) with
relatively bigger pores, composed of boron or gallium in the framework,
which possesses relatively low acidity and better dehydrogenation ac-
tivity compared to aluminium in the zeolite matrix [18,19], could
improve the catalytic activity of this material in ODH of n-octane.
Therefore, we now report on these studies.
2. Materials and experimental methods
2.1. Catalysts preparation
Three of the studied catalysts (BBaY-S, GaBaY-S, GaNaY-S) were
prepared using a modified sol-gel method [20]. Catalysts coding was
based on the constituents used for the synthesis, where B = boron, Ba =
barium, Ga = gallium, Na = sodium, Y = faujasite type and S = Silicalite.
For BBaY-S synthesis, a 500 mL sealable Teflon beaker was used to
dissolve 14.53 g of BaCl2.2H2O (Merck NT Laboratory Suppliers) in 37.5
g of distilled water. This was followed by the addition of 1.144 g of boric
acid (Merck Chemicals) under agitation for 2 h to yield a borate solution.
After 2 h, 30 wt% colloidal silica solution (Sigma Aldrich - 26.44 g) was
slowly poured into the barium borate solution under vigorous stirring.
2.3. Catalytic testing
Catalysts testing was conducted at a temperature of 450 ◦C in a
laboratory-scale continuous-flow fixed-bed, gas phase reactor. The
reactor tube was stainless steel with an inner diameter of 10 mm.
Different ratios of Air and N2 were delivered to the reactor as the oxidant
and diluent gas respectively to achieve iso-conversion. Reactions were
performed with n-octane (Merk, assay > 98 %) and concentration in a
gaseous mixture (v/v) above n-octane’s upper flammability limit. For all
the reactions the concentration of n-octane was kept at 6 %. A calibrated
Lab Alliance Series II HPLC Pump was used to feed n-octane into the
system and the mass delivered was weighed using an electronic balance.
The n-octane fed was maintained in the gaseous phase by the heated feed
lines at 130 ◦C using heating tape. The temperature was controlled using
a CB-100 RK temperature control unit with an internal replay and
monitored using K-type thermocouples. All the reactions were carried
out using 1.0 mL of a pelletized catalyst (pellet sizes were between 600
◦
The beaker was tightly sealed and transferred to a 25 C oil bath and
aged for 24 h with constant stirring. Thereafter the solution was aged at
40 ◦C for 24 h, and finally at 80 ◦C for 48 h with no stirring. The
recovered solid was filtered under vacuum and washed with double
distilled water until the pH was 8–9 and dried at 110 ◦C overnight. The
remaining catalysts were prepared using a similar procedure as
described above. Synthesis of GaBaY-S and GaNaY-S used 3.358 g of
gallium nitrate (Sigma Aldrich) in place of boric acid. For GaNaY-S,
sodium chloride (Sigma Aldrich - 20.28 g) was used in place of
barium chloride as a charge balancing cation. The Si/M (M = Ga or B)
was kept constant at 2.7 ± 0.3 for all the prepared catalysts. The fourth
and last catalyst was prepared by ionic exchange [21], where 8 g of the
previously prepared BBaY-S was mixed in a round bottom flask with a
100 mL solution of gallium nitrate (0.90 g) dissolved in double-distilled
water. The mixture was then stirred at 80 ◦C for four hours, and then the
catalyst was filtered under vacuum and washed repeatedly with double
distilled water and then dried at 110 ◦C overnight. The resulting cata-
lysts were coded as GaBBaY-S(IE). The prepared catalysts were all
calcined under flowing air at 550 ◦C for six hours before characterization
and 1000 μm) between two thin layers of glass wool positioned at the
hottest zone of the calibrated reactor block. The spaces in the reactor
tube were packed with 24 grit carborundum and stoppered by glass-
wool on both ends. Total gas flow was measured using a Ritter drum
type wet gas flow meter. All liquid products and unreacted feed were
collected in a cylindrical stainless-steel vessel, cooled to ≈ 3.0 ◦C. A
Perkin Elmer Clarus 400 GC fitted with a 30 m ×530 μm Supelco Car-
boxen 106 PLOT column and a thermal conductivity detector was used
2
Ndlela, Siyabonga S.
