Yttrium Oxide Supported La2O3 Nanomaterials for Catalytic Oxidative Cracking of n-Propane to Olefins

Article abstract of DOI:10.1007/s10562-019-02927-z

Abstract: La₂O₃ nanorods were prepared by simple hydrothermal synthesis method. Yttrium oxide (1, 3, 5 and 7 wt%) supported La₂O₃ and SO₄^2? incorporated La₂O₃ nanorods were prepared impregnation method and used as catalysts in oxidative cracking of n-propane. The pure La₂O₃ nanorods exhibited 15% n-propane conversion with 22% olefins (ethane and propene) selectivity. Considerable improvement in n-propane conversion was observed in case of 3 wt% yttrium oxide supported on La₂O₃ nanorods (25% conversion of n-propane and 36% selectivity to olefins) at reaction temperature of 550?°C. Interestingly, 5 wt% yttrium oxide supported 10 wt% SO₄^2?/La₂O₃ nanorod sample exhibited superior performance in n-propane conversion (42%) and olefins selectivity (54%). The yttrium oxide loading and sulfation of La₂O₃ nanorods influenced the catalytic activity. The characterization of synthesized nanomaterials was performed using elemental analysis, XRD, FT-IR, N₂-physisorption, SEM, XPS and H₂-TPR techniques. The obtained results indicated that yttrium oxide was highly dispersed over the La₂O₃ nanorods because of strong interaction between the two rare earth metal oxides. Additionally, deposition of yttrium oxide to sulfated La₂O₃ nanorods increased the surface area and the amount of Lewis acid sites (for the activation of n-propane) on La₂O₃ nanorods. Yttrium oxide supported sulfated La₂O₃ catalyst showed no deactivation during the 24?h of reaction and without coke formation. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-019-02927-z

Catalysis Letters
https://doi.org/10.1007/s10562-019-02927-z
Yttrium Oxide Supported La O Nanomaterials for Catalytic Oxidative
2
3
Cracking of n‑Propane to Oleꢀns
1
1
1
Fawaz S. Al‑Sultan · Sulaiman N. Basahel · Katabathini Narasimharao
Received: 10 March 2019 / Accepted: 7 August 2019
©
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
La O nanorods were prepared by simple hydrothermal synthesis method. Yttrium oxide (1, 3, 5 and 7 wt%) supported
2
3
2
−
La O and SO incorporated La O nanorods were prepared impregnation method and used as catalysts in oxidative crack-
2
3
4
2
3
ing of n-propane. The pure La O nanorods exhibited 15% n-propane conversion with 22% oleꢀns (ethane and propene)
2
3
selectivity. Considerable improvement in n-propane conversion was observed in case of 3 wt% yttrium oxide supported on
La O nanorods (25% conversion of n-propane and 36% selectivity to oleꢀns) at reaction temperature of 550 °C. Interest-
2
3
2
−
ingly, 5 wt% yttrium oxide supported 10 wt% SO /La O nanorod sample exhibited superior performance in n-propane
4
2
3
conversion (42%) and oleꢀns selectivity (54%). The yttrium oxide loading and sulfation of La O nanorods inꢁuenced the
2
3
catalytic activity. The characterization of synthesized nanomaterials was performed using elemental analysis, XRD, FT-IR,
N -physisorption, SEM, XPS and H -TPR techniques. The obtained results indicated that yttrium oxide was highly dispersed
2
2
over the La O nanorods because of strong interaction between the two rare earth metal oxides. Additionally, deposition of
2
3
yttrium oxide to sulfated La O nanorods increased the surface area and the amount of Lewis acid sites (for the activation
2
3
of n-propane) on La O nanorods. Yttrium oxide supported sulfated La O catalyst showed no deactivation during the 24 h
2
3
2
3
of reaction and without coke formation.
Graphic Abstract
n-Propane
Light Olefins
Oxidative
cracking
Y O3
2
Y O
2 3
La O
Sul-La O3
2
2
3
Keywords Yttrium oxide · La O nanorods · Sulfated La O · n-Propane · Catalytic oxidative cracking
2
3
2
3
1
Introduction
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02927-z) contains
supplementary material, which is available to authorized users.
Light oleꢀns such as ethene and propene are important raw
materials in syntheses of alcohols, plastics, detergents and
fuels [1]. The demand for the light oleꢀns is anticipated
to surge considerably in coming years and the light ole-
*
Katabathini Narasimharao
nkatabathini@kau.edu.sa
1
Surface Chemistry and Catalytic Studies (SCCS) Group,
Department of Chemistry, Faculty of Science, King
Abdulaziz University, P. O. Box 80203, Jeddah 21589,
Kingdom of Saudi Arabia
ꢀ
ns are conventionally manufactured from natural gas and
crude oil by steam cracking, ꢁuid-catalytic-cracking, and
catalytic dehydrogenation processes [2]. Though, listed
Vol.:(0
1
123456789)
3

F. S. Al-Sultan et al.
processes are extensively studied, these processes are
suꢂering from drawbacks that the manufacturing of ole-
relate the oxidative catalytic cracking performance with their
physico-chemical characteristics.
ꢀ
ns is not up to the market demand and also high energy
requirement, formation of coke which is responsible for
catalyst deactivation [3]. The catalytic oxidative cracking
has become an alternative to steam cracking process as it is
possible to make the oleꢀns production process less energy
consuming by adding oxygen in the reactants stream in
presence of catalyst which facilitates cracking of alkanes
at low reaction temperatures [4]. Fabrication of an eꢂec-
tive catalyst technology is essential, which could oꢂer high
alkane conversion and selectivity to oleꢀns in presence of
oxygen without excessive oxidation of alkane and oleꢀns
2 Experimental
2.1 Synthesis of Catalysts
2.1.1 Bulk La O and Sulfated La O Nanorods
2
3
2 3
Hydrothermal synthesis method was utilized to synthesize
bare La O nanorods. In a typical synthesis, 2.0 g of lantha-
2
3
num nitrate was added to 30 mL of double distilled water,
to this 18 g of KOH was added and the total contents were
moved into a Teꢁon lined stainless steel autoclave and it was
placed in an electric oven and the temperature of the oven
was raised to 200 °C and kept it for 48 h. After the hydro-
thermal treatment, the autoclave was brought to temperature
of 25 °C and the obtained solid product was collected and
washed by using distilled water and with dil. HCl to elimi-
nate the excess potassium hydroxide. The washed cake was
then calcined at 600 °C for 6 h in air to obtain nanorods of
La O . Sulfation of La O nanorods was performed using
[
5, 6]. Although, many research reports were existed in
the literature on fabrication of eꢃcient catalyst for the
oxidative dehydrogenation of hydrocarbons, very few stud-
ies were existed pertaining to catalytic oxidative cracking
of hydrocarbons [7]. Researchers utilized diꢂerent types
of catalysts; acidic catalysts (dealuminated H-ZSM-5, Cu
loaded H-ZSM-5), basic catalysts (Ca-Sr-Al and W-K-Al
mixed oxides) and transition metal oxide catalysts (Cr–Al
and V-oxides) [8].
The rare earth metals could be categorized as light or
heavy (as per their electronic conꢀguration) and possessed
diꢂerent chemical, electrical and catalytic properties [9].
These are key components in ꢁuid cracking catalyst and the
results indicated that rare earth metal exchanged zeolites
are more active than the traditional transition metal oxide
catalysts in ꢁuid cracking process [10, 11]. Yoshimura et al.
2
3
2
3
diluted H SO by simple wet chemical impregnation. A cal-
2
4
culated quantity of La O nanorods sample was immersed
2
3
in 1.0 M H SO solution (for 1.0 g of La O -NR, 15 mL
2
4
2
3
of 0.1 M H SO ). The resulted H SO –La O mixture was
2
4
2
4
2
3
dried at 100 °C for 12 h and the obtained material was cal-
cined at 600 °C for 4 h to acquire the sulfated La O -NR.
2
3
[
12] indicated alkali metal doped La O catalysts oꢂers
2 3
high selectivities to light alkenes in oxidative cracking of
n-butane process. Nakamura et al. [13] synthesized Li sup-
ported rare earth oxides (La O , Y O , Sm O and Gd O )
2.1.2 Yttrium Oxide Deposited La O and Sulfated La O
2 3 2 3
Yttrium oxide supported La O and sulfated La O nano-
2
3
2
3
2
3
2
3
2
3
2
3
for oxidative cracking of n-butane and the authors observed
materials were obtained using impregnation technique. Cal-
that Li supported Y O oꢂered superior performance. Wakui
culated quantity of La O or sulfated La O nanorods sam-
2
3
2
3
2
3
et al. [14] used La modiꢀed HZSM-5 materials for n-butane
cracking and authors reported that these catalysts exhibited
mild activity. Sa et al. [15] utilized gold supported La O and
ple was immersed in an aqueous solution of yttrium nitrate
(BDH chemicals, UK) corresponding to 1, 3, 5 and 7 wt%
of yttrium oxide. The total contents were stirred for 2 h and
the resultant material was then dried at 100 °C for 12 h. The
dried materials were calcined at 600 °C for 4 h in an electric
furnace in presence of air.
2
3
sulfated La O catalysts for oxidative cracking of n-butane.
2
3
It is known that impregnation of sulfate anions over metal
oxides provides robust solid acid catalysts which possesses
strong Brönsted acid sites to enhance the rate of acid cata-
lyzed reactions [16, 17]. In our previous studies, gold sup-
ported sulfated CeZrO and La O were identiꢀed as eꢂec-
2.2 Characterization of Synthesized Materials
2
2
3
tive catalysts for oxidative cracking of n-propane [18, 19].
In this contribution, yttrium (heavy rare earth metal) oxide
supported over lanthanum (light rare earth metal) oxide and
sulfated lanthanum oxide nanomaterials were synthesized
and used as catalysts for oxidative cracking of n-propane for
the ꢀrst time. The physico-chemical properties of yttrium
oxide supported over La O nanorods were investigated
The elemental composition of the synthesized materials was
determined by using ICP-AES, Optima 7300DV (Perkin-
Elmer) instrument. The sulfur content of the catalysts was
determined by CHNS elemental analysis using Perkin-Elmer
2400 instrument. The XRD patterns of the powders were
collected by using PANalytical XpertPro diꢂractometer.
The crystallite size of obtained materials was determined
by applying the Scherer Eq. (1).
2
3
using XRD, FT-IR, SEM, N -physisorption, XPS, acidity
2
measurements and H -TPR methods. An eꢂort was made to
2
D = B ꢀ∕ꢁ1 cos ꢂ
∕2
(1)
1
3

Yttrium Oxide Supported La O Nanomaterials for Catalytic Oxidative Cracking of…
2
3
where ‘D’ is the average crystallite size of the phase under
investigation, ‘B’ is the Scherrer constant (0.89), ‘λ’ is wave-
length of the X-ray beam used (1.5405 Å), ‘β ’ is the full
La(OH) as a main phase because of the hygroscopic nature
3
of La O and it was previously observed that La(OH)
2
3
3
generates from La O under atmospheric conditions [20].
1
/2
2
3
width at half maximum (FWHM) of the diꢂraction peak
and ‘θ’ is the diꢂraction angle. The SEM analysis of the
samples was carried out using JEOL Model JSM-6390LV
microscope. The FTIR spectra of calcined materials were
obtained using Bruker vertex 70 FTIR spectrometer. The
acidic character of the samples was investigated by pyri-
dine adsorption measurements using FTIR spectroscopy; the
analysis was performed over calculated amount of catalyst,
which was treated at 100 °C under vacuum for 5 h. Then, the
sample was treated with pyridine vapor and ꢀnally heated
at 100 °C under vacuum for 30 min to remove physically
adsorbed pyridine. FT-IR spectra were collected at room
temperature [18]. XPS spectra of the materials were col-
lected by using Kratos Axis Nova spectrometer. The textural
properties of the samples were obtained by carrying out the
The yttrium oxide supported La O -NR samples exhibited
2 3
additional reꢁections around 24°, 30° and 32°, which could
be due to the presence of cubic phase of Y O [JCPDS:
2
3
895591]. Presence of the reꢁections due to Y O in all the
2
3
samples indicating the formation of Y O crystals in these
2
3
samples. Sulfated La O -NR sample and yttrium oxide
2
3
supported sulfated La O -NR samples exhibited La O
2
3
2
3
and Y O phases and interestingly these samples have not
2
3
exhibited presence of diꢂraction peaks corresponding to
X-ray detectable lanthanum sulfate. A similar results were
observed in case of sulfated microcrystalline La O cata-
2
3
lysts. However, there is a clearly possibility for the formation
of amorphous lanthanum sulfate species, as major reꢁections
due to La(OH) phase were disappeared in case of sulfated
3
sample. The crystallite size of the materials was deter-
mined from the XRD patterns using Scherer equation. The
crystallite size of La O phase for sulfated La O -NR and
N -physisorption experiments over Quantachrome ASiQ
2
adsorption system. The reducibility of the samples was
2
3
2
3
investigated by using H -temperature-programmed reduc-
bare La O -NR samples was observed as 23 nm and 48 nm
2
2 3
tion using Quantachrome CHEMBET-3000 system.
respectively. The crystal size of the sulfated La O -NR sam-
2 3
ple is much smaller than that of the bulk La O -NR.
2
3
2.3 Catalytic Oxidative Cracking of n‑Propane
FT-IR spectra for yttrium oxide supported La O and sul-
2 3
fated La O nanorods are shown in Fig. S2. The La O -NR
2
3
2
3
Catalytic oxidative cracking of n-propane tests were per-
formed using a ꢀxed bed quartz reactor. The reactor was
loaded with weighed catalyst pellets (200 mg), which
were diluted with unreactive quartz particles. The reactant
and yttrium oxide supported La O -NR samples exhib-
2 3
−
1
ited major band at 695 cm which could be assigned to
−
1
La–O bond [21] The minor bands appeared at 860 cm ,
−
1
−1
1370 cm and 1475 cm could be attributed to the stretch-
−
1
2−
gas mixture, which contained n-propane (20 mL min ),
ing vibrations of C=O functional groups of [CO ] ions
3
−
1
2
0% oxygen—80% argon (100 mL min ) and argon
presented over the surface of the La(OH) [22]. FT-IR
3
−
1
(
40 mL min ) was used to perform the catalytic tests. Dif-
spectra of yttrium oxide supported La O -NR samples are
2
3
ferent reaction temperatures were used to investigate the
eꢂect of reaction temperature on the catalyst performance.
The composition of product gas mixture was continuously
analyzed with the help of Agilent 6890 A gas chromatograph
equipped with ꢁame ionization and thermal conductivity
detectors.
very similar as La O -NR sample except the intensity of
2 3
La–O band was decreased considerably and also a new band
−
1
appears at 563 cm , which could be attributed to Y–O
stretching vibration and the formation of Y O [23]. Sulfated
2
3
La O -NR and yttrium supported sulfated La O -NR sam-
2
3
2
3
−
1
−1
ples showed additional bands at 1065 cm , 1120 cm and
−
1
1
180 cm , which could be attributed to the IR bands due to
(
O-SO) and (O-SO ) functional moieties [24]. These results
3
3
Results and Discussion
clearly indicating that the sulphated samples possessed the
metal oxide-sulfate interactive species and these results con-
ꢀrm the eꢃciency of sulfation procedure steps. The bulk
elemental analysis of the samples was determined by ICP-
AES and CHNS techniques and the results are presented in
Table 1. The bulk sulfur present in sulfated La O sample
3
.1 Physico‑Chemical Properties of Yttrium
Supported La O Nanorod Samples
2
3
The powder XRD patterns of the samples are shown in Fig.
S1. Appearance of prominent X-ray reꢁections in all the
samples indicating that the synthesized samples are poly-
crystalline in nature. The synthesized La O -NR sample
2
3
was 1.15 wt% and a slight reduction of sulfur content was
observed with yttrium oxide deposition. The morphology of
synthesized samples was studied by SEM analysis. Figure 1
shows SEM images of representative samples. The SEM
image of La O -NR, 3Y-La-NR and 7Y-La-NR samples
2
3
exhibited major reꢁections corresponding to the La(OH)
3
phase [JCPDS: 36-1481] along with major reꢁections due
2
3
to hexagonal La O phase [JCPDS: 05-0602]. Although,
clearly shows the presence of nanorods with several microm-
2
3
the samples were calcined at 600 °C, the samples showed
eters in length and typical widths of 40–50 nm. Sulfation
1
3

F. S. Al-Sultan et al.
Table 1 Textural properties of
Sample
Bulk elemental analysis (wt%)
Textural properties
Surface area
yttrium oxide supported La O
2
3
and sulfated La O nanorods
La
Y
S
Pore volume
Pore
2
3
2
−1
−1
(
m g
)
(ccg
)
diameter
(
nm)
La O -NR
85.2
84.1
81.2
79.7
78.3
83.6
83.0
81.5
79.0
77.2
–
–
–
–
–
59
47
44
42
39
71
62
59
48
45
0.142
0.140
0.136
0.131
0.127
0.153
0.147
0.142
0.140
0.137
15.4
14.5
14.5
14.4
14.4
14.9
14.6
14.4
14.3
14.2
2
3
1Y-La-NR
3Y-La-NR
5Y-La-NR
7Y-La-NR
0.9
2.8
4.8
6.9
–
0.8
2.7
4.5
6.5
–
Sul-La O -NR
1.15
1.02
0.88
0.77
0.72
2
3
1Y-Sul-La-NR
3Y-Sul-La-NR
5Y-Sul-La-NR
7Y-Sul-La-NR
of La O -NR has altered the morphology of the samples,
isotherms with H3 hysteresis loops (IUPAC classiꢀcation)
and also the adsorption isotherms showed a sharp inꢁec-
tion in P/P° range of 0.8–1.0, indicating that the synthesized
samples possessed meso pore structure, which could have
raised due to the inter void spaces of nanorods. The pore
size distribution patterns of the samples (Fig. S4B) were
obtained from the desorption branch of the isotherms. The
speciꢀc surface area, pore diameter and pore volume values
for the investigated samples are presented in Table 1. It can
be observed that the surface area, pore size and pore volume
are steadily declined after yttrium oxide impregnation over
La O -NR sample. However, it is clear that the decrease of
2
3
breaking up of long nanorods morphologies is witnessed.
The breaking of long nanorods into small and broad rod like
structures were formed, as they can be seen in Fig. 1.
The samples with high yttrium oxide loading shows the
cumulative nature of secondary particles, which are made
up of agglomeration of Y O particles. Presence of non-
2
3
uniform structures could result a wide particle size distribu-
tion in samples with high yttrium oxide content and sulfated
La O -NR samples. Fig. S3 represents DR UV–Vis spec-
2
3
tra for yttrium oxide supported La O and sulfated La O
2
3
2
3
nanorods. As shown in the ꢀgure, distinctive absorption
2
3
La O edge could be witnessed in the spectra of yttrium
the pore volume and pore size is small after deposition of
yttrium oxide; therefore, it is possible to argue that most of
the Y O was located over the surface of La O -NR. In con-
2
3
oxide supported La O and sulfated La O -NR samples. The
2
3
2
3
La O -NR sample showed major absorption band around
2
3
2
3
2
3
2
50 nm. Impregnation of yttrium oxide over the La O -NR
trast, sulfation procedure resulted increased the surface area
2
3
sample resulted slight shift in band position to 255 nm and
also an increase in the intensity of absorption band. The shift
in the absorption could be attributed to the quantum size
eꢂect; This can be correlated with results obtained in the
SEM images that the long nanorods were broke into smaller
size nanorods, which should have increased the surface to
volume ratio, thus increasing the absorbing capability of the
samples after sulfation and yttrium oxide impregnation. It
was previously reported that UV–Vis spectra of bulk Y O
of La O -NR sample, probably due to the formation of lower
2
3
size La O nanorods and also amorphous interactive phase.
2
3
The surface composition and electronic states of dif-
ferent species presented in the synthesized samples were
measured using XPS analysis (Fig. 2). It is known that La
3d core spectrum split into 3d and 3d components due
5
/2
3/2
to a spin–orbit interaction. The calcined La O sample
2
3
exhibited XPS peaks at 835.8 eV & 839.3 eV and 852.6 eV
& 856.3 eV corresponding to La 3d and La 3d states
2
3
5/2
3/2
nanoparticles exhibits two absorption bands at 250 nm and
respectively (Fig. S5). Additionally, each line split into main
0 0 0 1
3
00 nm and the prior one was attributed to band gap of Y O
and satellite peaks corresponding to 3d 4f and 3d 4f L
conꢀguration respectively [26]. However, it was reported
that the nature of satellite peak depends on the type of La
compound [27]. For La based metallic compounds, the sat-
2
3
and the later one could be attributed to the surface defect
states [25]. It can be observed the appearance of a new weak
absorption band at 300 nm in case of yttrium impregnated
La O -NR and sulfated La O -NR samples.
0
1
ellite peak is due to charge ꢁuctuation from 4f state to 4f
2
3
2
3
To determine the textural properties of synthesized sam-
state, while in other La compounds such as oxides, halides
and sulꢀdes, it arises from an electron transfer from ligand
to La 4f state. It was previously observed that bulk La O
ples, N -physical adsorption experiments were performed.
2
The N adsorption–desorption isotherms of synthesized
2
2
3
materials are presented in Fig. S4A. It is clear that all
sample shows XPS peaks at 835.8 eV and 839.3 eV corre-
synthesized samples exhibited typical type IV adsorption
sponding to La 3d states (La in +3 oxidation state) [26]. It
5/2
1
3

Yttrium Oxide Supported La O Nanomaterials for Catalytic Oxidative Cracking of…
2
3
Fig. 1 SEM images of yttrium oxide supported La O and sulfated La O nanorods
2
3
2
3
was clearly observed that yttrium oxide supported La O -NR
The Y 3d peak position and shape are in good agreement
2
3
and sulfated La O -NR samples showed XP peaks at 834.6
with the XP peaks for Y O [28]. Interestingly, the BE of
2
3
2
3
and 838.1 eV corresponding to La 3d states. These obser-
two Y 3d peaks are shifted to 157.5 eV and 159.5 eV in case
5
/2
vations indicating that the XP peak position for La 3d states
shifted to lower binding energy (BE) region after impregna-
tion of sulfate and Y O moieties. The shift in BE is mainly
of yttrium oxide supported sulfated La O -NR samples. The
2
3
shift of BE of the Y 3d doublet to higher binding energy due
to interaction between Y and electronegative sulfate species
[29]. To understand the interaction between the sulfur and
rare earth oxides, the S 2p photoelectron peak was deconvo-
luted into speciꢀc contributions in sulfated samples. It was
2
3
due to presence of interactive species (La-sulfate/Y O ). The
2
3
yttrium oxide supported La O -NR samples exhibited two
2
3
Y 3d peaks with BE of 157.3 eV and 159.3 eV correspond
to stoichiometric Y O .
reported that S 2p peak could be deconvoluted into S 2p
2
3
1/2
1
3

F. S. Al-Sultan et al.
3d
3Y-La-NR
La3d
3Y-La-NR
3d
5/2
Y3d
5/2
3d
3/2
3d
3/2
830
835
840
845
850
855
860 154 155 156 157 158 159 160 161 162 163
Binding energy (eV)
Binding energy (eV)
3d
7Y-La-NR
La3d
7Y-La-NR
3d
5/2
Y3d
5/2
3d
3/2
3d
3/2
830
835
840
845
850
855
860
154 155 156 157 158 159 160 161 162 163
Binding energy (eV)
Binding energy (eV)
3d
3Y-Sul-La-NR La3d
Y3d
7Y-Sul-La-NR
3Y-Sul-La-NR
S2p
5/2
3d
5/2
3d
3/2
3d
3/2
830
835
840
845
850
855
860 153 154 155 156 157 158 159 160 161 162 163
Binding energy (eV)
165 166 167 168 169 170 171 172
Binding energy (eV)
Binding energy (eV)
3d
7Y-Sul-La-NR La3d
Y3d
3Y-Sul-La-NR
7Y-Sul-La-NR
S2p
5/2
3d
5/2
3d
3d3/2
3/2
830
835
840
845
850
855
860 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Fig. 2 X-ray photoelectron spectra of yttrium oxide supported La O and sulfated La O nanorods
2
3
2
3
and S 2p contributions due to spin–orbit splitting [30].
[32]. Presence of two sets of doublets (for both S 2p and
3
/2
1/2
In case of sulfated La O , presence of two S 2p peaks at
S 2p contributions) could be observed in S 2p peak ꢀt-
2
3
3/2
1
68.8 eV and 170.0 eV were observed (Fig. S5).
tings for yttrium oxide deposited samples, therefore it is
possible to argue that there are two types of surface sulfate
species are existed in these samples as sulfated La O -NR
It was also reported that the S 2p peak around 168.8 eV
1/2
6
+
corresponding to the free S species [31], and the peak
2
3
around 170.0 eV could be attributed to metal-SO species
sample. The contribution of peak at 170.0 eV is high in case
4
1
3

Yttrium Oxide Supported La O Nanomaterials for Catalytic Oxidative Cracking of…
2
3
of 3Y-Sul-La-NR sample. The presence of less number of
free sulfate groups in the yttrium oxide supported sulfated
La O -NR samples explains the interaction of sulfate groups
yttrium oxide loading the total number of Lewis acid sites
were increased and the number of Brönsted acid sites were
diminished considerably, indicating that the deposition of
yttrium oxide results decreases the acidic strength of the
sulfated La O -NR samples.
2
3
to surface of the metal sites.
Two major and one minor O1s XP peaks were observed
for yttrium oxide supported La O -NR samples (Fig. S5).
2
3
H -temperature programmed reduction (H -TPR) analysis
2
3
2
2
The peak at 528.8 eV BE (32.1%) could be due to the lat-
tice oxygen in La O /Y O . The strongest peak (64.2%) at
was used to study the reducibility of the yttrium oxide sup-
ported La O and sulfated La O nanorods and the patterns
2
3
2
3
2
3
2
3
higher BE at 531.4 eV could be assigned to either adsorbed
are shown in Fig. 3. The La O -NR sample exhibited a main
2 3
−
H O species [33], –OH groups or O surface atoms [34]
peak at 650 °C and a minor peak at 740 °C. It is known
that La O is reducible at high temperatures and it is clear
2
−
Presence of high concentration of H O/OH/O species indi-
2
2
3
cating that these samples underwent high surface hydration.
that La O was reduced in two stages from bulk La O to
2 3 2 3
The appearance of the small XP peak in the O1s signal at
La metal. The La O -NR sample exhibited two TPR peaks
2 3
5
33.8 eV can be related to the surface interactive species
after sulfation, however the temperature maximum of the
TPR peaks was shifted to 615 and 670 °C respectively. This
observation indicating that impregnation of sulfate groups
enhanced the reducibility of La O .
such as La–O-Y species. Yttrium oxide impregnated sul-
fated La O -NR samples also showed three O1s species; the
2
3
two XP peaks corresponding to lattice oxygen and H O/OH/
2
2
3
−
O species as similar to non-sulfated samples. However, the
The yttrium oxide impregnated samples showed a broad
minor peak at 525 °C and another incomplete peak at about
800 °C. These new peaks show signiꢀcant broadening and
this phenomenon could be due to the random distribu-
tion of La and Y ions in the samples. Sasikala et al. [35]
observed similar results in case of cerium-yttrium mixed
third peak shifted slightly to lower BE value (532.7 eV) and
the intensity of the peak increased signiꢀcantly, indicating
that the concentration of interactive species is higher in the
yttrium oxide supported sulfated La O samples, revealing
2
3
that the sulfation procedure is enhancing the generation of
interactive species.
FTIR spectra of synthesized samples after pyridine
adsorption were collected to study the nature of acidic
sites (Fig S5). The obtained results indicated that synthe-
sized samples exhibited peaks due to Brönsted (B) and
Lewis acid sites (L) [18, 19]. The quantiꢀcation of acid
sites was performed using the FTIR spectra and the data is
depicted in Table 2. Deposition of yttrium oxide resulted
loss of Brönsted acid sites of the bare La O -NR sample.
7Y-Sul-La-NR
3Y-Sul-La-NR
7Y-La-NR
2
3
The results also revealed that impregnation of sulfate ions
over La O -NR increased the number Brönsted acidic sites.
2
3
It was previously revealed that Brönsted and Lewis acidic
sites are useful for n-alkane activation. With the increase of
3Y-La-NR
Table 2 Acidity of the catalysts from pyridine adsorption measure-
ments
Catalyst
Number of active sites
B/L ratio Number
of total
sites
Lewis (L) Brönsted (B)
Sul-La O -NR
2 3
La O -NR
10.2
11.1
12.5
13.3
14.2
4.3
3.5
0.421
0.315
0.240
0.203
0.140
0.943
0.685
0.409
0.350
0.268
14.5
14.6
15.5
15.7
16.2
20.8
22.7
24.1
23.5
22.2
2
3
1Y-La-NR
3Y-La-NR
5Y-La-NR
7Y-La-NR
3.0
2.7
2.0
10.1
9.2
7.0
6.1
4.7
La O -NR
2 3
Sul-La O -NR 10.7
2
3
2
25 300 375 450 525 600 675 750
1Y-Sul-La-NR 13.5
3Y-Sul-La-NR 17.1
5Y-Sul-La-NR 17.4
7Y-Sul-La-NR 17.5
o
Temperature ( C)
Fig. 3 H -TPR patterns of yttrium oxide supported La O and sul-
2
2
2
3
fated La O nanorods
3
1
3

F. S. Al-Sultan et al.
oxide catalysts. The yttrium oxide impregnated sulfated
La O -NR samples showed similar TPR peaks as yttrium
La O nanorods support. Increase of reaction temperature
2
3
from 450 to 650 °C resulted gradual increase in n-propane
conversion for all investigated catalysts. A similar behavior
was observed in our previous studies over Ce Zr O and
2
3
oxide impregnated over La O -NR samples, however the
2
3
intensity of the lower temperature reduction peak was sig-
niꢀcantly increased; probably due to existence of interac-
tion between metal (La/Y) and sulfate ions. This observation
indicating that promotion of yttrium oxide-La O nanorods
0
.5 0.5
2
La O based catalysts.
2
3
The selectivity to light oleꢀns was also improved after
impregnation of yttrium oxide loading over La O -NR sup-
2
3
2
3
with sulfate ions lead to an increase in reducibility.
port. It is well reported that C–H bond activation is the rate
limiting stage in oxidative cracking of n-alkanes [18, 19];
existence of yttrium oxide on the surface of La O -NR is
3.2 Oxidative Cracking Activity of Catalysts
2
3
the reason for the improvement in catalyst performance in
oxidative cracking process. There is a clear possibility for
the existence of synergetic eꢂect between La O -NR and
Previously, we observed that micro crystalline La O exhib-
2
3
ited higher oxidative cracking activity than other bulk oxide
2
3
supports tested [19]. Au et al. [36] indicated that surface
yttrium oxide due to formation of interactive species. With
increase of reaction temperature, the selectivity to oleꢀns
increased initially (from 450 to 550 °C), however further
increase of reaction temperature resulted decrease in selec-
tivity to oleꢀns. It is known carbonization of oleꢀns occurs
at high reaction temperature [37].
−
2−
O and O
species from basic and non-reducible La O
2 3
2
are responsible for its superior activity. It is well known that
lowering the size of catalyst particles from micro to nano
size could greatly enhance the catalytic activity. To study
the role of the particle size, in this study nanosized La O
2
3
sample was prepared and investigated its ability in catalys-
ing oxidative cracking of n-propane. The catalytic activity
results of synthesized materials tested at diꢂerent reaction
temperature are presented in the Fig. 4.
Previously, it was also observed that sulfation of La O
2
3
resulted an enhancement in oxidative cracking activity. The
previous results indicated that 10 wt% sulfate ions loading is
optimum to obtain better oxidative cracking activity in case
of gold supported La O samples [19] and it is possibly due
Clearly, the bare La O -NR sample oꢂered better perfor-
2
3
2
3
mance compare to micro sized La O (15.8% conversion and
to the existence of La O -sulfate interactive species. Also,
2
3
2 3
1
8.4% selectivity to oleꢀns at 600 °C) under identical reac-
in this work, La O -NR sample was loaded with 10 wt% sul-
2 3
tion conditions [37]. Impregnation of 1 wt% yttrium oxide
over La O -NR resulted slight improvement in the catalytic
fate ions to take advantage of beneꢀcial role of sulfate ions.
Figure 5 represents the catalytic oxidative cracking activity
of yttrium oxide supported sulfated La O -NR at diꢂerent
2
3
oxidative cracking activity. Increase of yttrium oxide loading
to 3 wt% increased both conversion of n-propane and selec-
tivity to oleꢀns to 35% and 27.2% at 600 °C respectively
2
3
reaction temperatures. The results clearly indicated that sul-
fation of La O -NR increased the oxidative cracking ability
2
3
[
Fig. 4]. However, further increase of yttrium oxide loading
of La O -NR support. Considerable alteration in catalytic
2 3
to 5 and 7 wt% resulted decrease in n-propane conversion,
indicating that yttrium oxide loading inꢁuences the oxidative
cracking ability and 3 wt% appears to be optimum for the
activity was observed after impregnation of yttrium oxide
2
−
on the surface of 10SO /La O -NR support. Both conver-
4
2
3
sion of n-propane and selectivity to oleꢀns were increased
4
0
35
30
25
20
15
10
5
0
La O -NR
2 3
(A)
(B)
La O -NR
2
3
1
Y-La-NR
3Y-La-NR
Y-La-NR
Y-La-NR
35
1
Y-La-NR
Y-La-NR
5Y-La-NR
Y-La-NR
3
5
30
7
7
25
20
15
10
5
450
500
550
600
650
450
500
550
600
650
o
o
Reaction temperature ( C)
Reaction temperature ( C)
Fig. 4 Catalytic activity patterns of synthesized catalysts at diꢂerent reaction temperatures
1
3

Yttrium Oxide Supported La O Nanomaterials for Catalytic Oxidative Cracking of…
2
3
40
35
30
25
20
15
10
5
0
Sul-La O -NR
Sul-La O -NR
2 3
2
3
5
0
1
Y-sul-La-NR
Y-sul-La-NR
Y-sul-La-NR
Y-sul-La-NR
1Y-Sul-La-NR
3Y-Sul-La-NR
5Y-Sul-La-NR
7Y-Sul-La-NR
3
5
7
40
30
20
10
4
50
500
550
600
650
450
500
550
600
650
o
o
Reaction temperature ( C)
Reaction temperature ( C)
Fig. 5 Conversion of n-propane and selectivity to oleꢀns at diꢂerent reaction temperatures over yttrium oxide supported sulfated La O nanorod
2
3
samples
Table 3 Oxidative cracking of n-propane activity of yttrium
Y O sample oꢂered 18.3%, conversion of n-propane with
2 3
oxide supported La O and sulfated La O nanorods at 650 °C and
2
3
2
3
20.3% oleꢀns selectivity, which is slightly lower than bare
La O -NR sample.
−
1
GHSV=48,000 h
Catalyst
2
3
Con.
Selectivity of products (%)
n-propane
3
.3 Stability of Yttrium Oxide Supported La O ‑NR
Oleꢀns
CH4
C H
COx
2 3
2
6
(
%)
Catalysts
La O -NR
23.1
29.2
30.1
35.3
29.3
29.2
32.3
38.4
34.1
33.2
18.3
22.3
23.2
23.4
27.1
23.1
23.3
24.2
29.3
25.1
24.1
20.3
3.5
0.7
1.2
1.5
2.3
2.4
1.0
2.9
2.0
2.3
2.7
7.2
1.5
1.8
2.4
2.9
3.2
1.2
2.0
2.5
2.8
5.0
67.0
74.6
73.6
69.0
71.7
71.1
73.6
65.0
70.4
70.8
72.0
2
3
Sul-La O -NR
The stability of synthesized yttrium oxide supported La O
2 3
2
3
1
3
5
7
1
3
5
7
Y-La-NR
Y-La-NR
Y-La-NR
Y-La-NR
Y-Sul-La-NR
Y-Sul-La-NR
Y-Sul-La-NR
Y-Sul-La-NR
and sulfated La O materials in oxidative cracking of n-pro-
2 3
pane was studied at 650 °C. Catalytic oxidative cracking
activity of 3Y-La O -NR and 3Y-Sul-La O -NR catalysts
2
3
2
3
was tested for 24 h continuously, and the results were pre-
sented in Fig. 6. During the durability tests, the 3Y-La O -
2
3
NR catalyst exhibited a slight decreasing trend in n-propane
conversion and oleꢀns selectivity levels. It is possible that
aggregation of nano rods/particles of La O /Y O could
2
3
2
3
Y O
occur due to continuous operation of oxidative cracking at
2
3
5
50 °C and responsible for decreasing activity trend. Inter-
estingly, the superior catalyst, 3Y-Sul-La O -NR exhibited
2
3
with increase of yttrium oxide loading from 1 wt% to 3 wt%
over sulfated La O -NR support. However, it was noticed
a stable catalytic oxidative cracking activity as shown in
the ꢀgure.
2
3
that further increase of yttrium oxide loading from 3 wt%
to 7 wt% resulted decrease of conversion of n-propane
and oleꢀns selectivity. A maximum n-propane conversion
The results obtained from the catalyst characterization
techniques could be helpful to understand the reasons for
superior activity of yttrium oxide supported over sulfated
La O -NR samples in oxidative cracking of n-propane pro-
(
38.4%) and oleꢀns selectivity (50.3%) was observed over
2
3
2
−
3
wt% Y O on SO /La O -NR sample. It is clear that at
cess. The SEM image analysis indicated that all the synthe-
sized samples possessed nano-sized tubes and particles. It
is well known that nano-sized and highly dispersed metal
oxides could oꢂer a better catalytic activity than catalyst
with micro-sized and agglomerated particles [38]. In recent
years, morphology-dependent nanocatalysis with metal and
metal oxides was reported [39]. It was observed that mor-
phology-controlled nanomaterials with more reactive crystal
planes exposed are highly eꢃcient for diꢂerent catalytic pro-
cesses. In the present research the sulfated and non-sulfated
2
3
4
2
3
high temperature, catalysts are producing higher amounts of
undesired carbon oxides, therefore lower reaction tempera-
tures are better to obtain high oleꢀns selectivity. A similar
behavior was observed in case of oleꢀns selectivity patterns
at studied reaction temperatures. Table 3 presents the con-
version of n-propane and product distribution values over all
the investigated catalysts at 650 °C. The catalytic activity of
bare Y O was also tested at 650 °C to compare its oxida-
2
3
tive cracking activity with other synthesized catalysts. Bare
1
3

F. S. Al-Sultan et al.
5
0
50
40
30
20
10
0
3
Y-Sul-La-NR
40
3
Y-Sul-La-NR
3Y-La-NR
30
3
Y-La-NR
20
10
0
0
2
4
6
8
10 12 14 16 18 20 22 24
0
2
4
6
8
10 12 14 16 18 20 22 24
Reaction time (h)
Reaction time (h)
Fig. 6 Time on stream analysis of yttrium oxide supported La O -NR catalysts
2
3
catalysts exhibited same morphology (nanorods), however
the sulfated samples possessed lower size compared to non-
sulfated samples. It is well known that reduction of the size
of particles contribute to increase the activity, so-called
size-dependent catalytic chemistry. The observations from
wt% yttrium oxide supported catalyst. Diꢂerent techniques
such as XRD, FTIR, N -physisorption, SEM, XPS and
2
H -TPR were used to study the physico-chemical properties
2
of samples, and the results indicated that yttrium oxide was
dispersed over the La O nanorods due to strong interac-
2
3
H -TPR experiments revealed that yttrium oxide impreg-
tion between the two rare earth metal oxides. Deposition
2
nated sulfated La O materials are easily reducible than
of yttrium oxide to the sulfated La O nanorods resulted
2
3
2
3
non-sulfated La O samples and it was also reported that
enhancement in the surface area and the amount of Lewis
acid sites in the catalysts, which are assisting for the activa-
tion of n-propane. Further, the investigated catalysts exhib-
ited stable catalytic oxidative cracking activity for 24 h with-
out signiꢀcant deactivation.
2
3
the sulfated metal oxides possessed more oxygen storage
capacity compared to non-sulfated metal oxides [40]. And
it was also reported that presence of sulfate-metal oxide
interactive species improves the mobility of lattice oxygen
[
41]. The SEM, N -physisorption, XPS, H -TPR and acidity
2
2
Acknowledgements The authors thank colleagues at Chemistry
measurement studies indicating that the yttrium oxide sup-
ported sulfated La O catalysts possessed small particle size,
Department, King Abdulaziz University, Jeddah for their support.
2
3
surface area, more number of Lewis acid sites and easily
reducible species; all these characteristics together assisting
for the superior activity of catalysts.
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