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A
BSTRACT
This study focuses on enhancing the sorption and catalytic properties of high-silica mesoporous zeolites
(HSMZs) and investigating the pyrolysis processes of the propane-butane fraction at elevated
temperatures. The HSMZ samples were decationized using a 25% ammonium chloride solution and
modified to their H-form aluminosilicate states. Various metal oxides such as CuO,
Li₂O, La₂O₃, ZnO, Fe₂O₃,
and BaO were employed as modifiers in different concentrations. Using the modified zeolites, the thermal
and catalytic pyrolysis of propane-butane hydrocarbon mixtures was carried out in a flow reactor at high
temperatures (~650
–
8
00 °C). To evaluate catalytic activity, a comparative analysis was conducted
between thermal and catalytic cracking results. The selectivity and overall yields of unsaturated C₂–C₄
hydrocarbons, including ethylene and propylene, were measured. The catalysts were prepared via
extrusion and tabletting methods, and their structures were characterised using infrared spectroscopy (IR)
Journal
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Original
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Research Article
Study of The Thermodynamics of The Propane-Butane
Fraction Pyrolysis Process
Submission Date:
March 13,
2025,
Accepted Date:
April 09, 2025,
Published Date:
May 11, 2025
Crossref doi:
https://doi.org/10.37547/ijasr-05-05-03
Normurot I. Fayzullayev
Doctor of Chemical Sciences, Professor, Samarkand State University named after Sharof Rashidov, Institute
of Biochemistry, Department of Polymer Chemistry and Chemical Technologies, Samarkand, Uzbekistan
Sanjar H. Saidqulov
Laboratory assistant, Department of Polymer Chemistry and Chemical Technologies, Institute of
Biochemistry, Samarkand State University named after Sharof Rashidov, Samarkand, Uzbekistan
Khamdam I. Akbarov
Doctor of Chemical Sciences, Professor, National University of Uzbekistan, Department of Physical
Chemistry, Tashkent, Uzbekistan
Hayitali N. Ibodullayev
Student, Institute of Biochemistry, Samarkand State University named after Sharof Rashidov, Samarkand,
Uzbekistan
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and X-
ray phase analysis. The IR spectra revealed absorption bands at 1120, 800, 560, and 460 cm⁻¹,
indicating the crystallinity and framework structure of the HSMZs. Thermodynamic calculations of
dehydrogenation, cracking, and dehydrocracking of low-molecular-weight saturated hydrocarbons
(propane and butane) were performed in the range of 200
–800 °C. Based on the changes in Gibbs
free
energy (∆G⁰) and equilibrium constants (Kₚ
), the temperature-dependent stability of each process was
assessed. The results allowed for the identification of optimal temperature intervals for obtaining valuable
unsaturated hydrocarbons from propane-butane mixtures. These findings provide insights for modelling
high-temperature thermal and catalytic processes, improving the use of modified zeolites, and increasing
the selectivity of the resulting products.
Objective
: To study the thermodynamics of the propane-butane fraction pyrolysis process.
K
EYWORDS
Propane-butane fraction, flow and pulse modes, high-temperature thermal process, propylene, HSMZ, IR
spectra.
I
NTRODUCTION
The pyrolysis of hydrocarbons at elevated
temperatures in the absence of oxygen is a widely
studied
process;
however,
comparative
experimental data concerning this process under
continuous flow conditions remain scarce in the
literature. Particularly, there is a lack of systematic
investigations that allow for the comparison of
results obtained under different flow regimes and
temperature intervals during the thermal cracking
of propane-butane hydrocarbon mixtures. To
ensure consistent conversion levels, experimental
conditions must be designed such that the reaction
proceeds in the kinetic domain and the molecular
structure of the saturated hydrocarbons remains
relatively unaltered at the onset of decomposition.
Under pulse reaction systems, it becomes feasible
to observe the “quasi
-
stationary” state of the
catalyst, in which the composition of the resulting
products becomes independent of the number of
pulses applied. This approach allows for detailed
analysis of the catalytic behaviour and product
distribution during the pyrolysis of propane-
butane mixtures at high temperatures in an inert
environment [1
–
3].
Nevertheless, the thermal pyrolysis of propane and
butane often leads to increased coke formation and
high energy consumption, posing significant
technological challenges. Currently, there are no
industrial-scale processes available for the high-
temperature, oxygen-free catalytic conversion of
C₃–C₄ saturated hydrocarbons into valuable light
olefins such as ethylene and propylene. Despite the
existence of several mechanistic models describing
the conversion of such hydrocarbons over high-
silica mesoporous zeolites (HSMZs), these models
typically focus on generalised product formation
pathways and often lack selectivity control [4
–
8].
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Moreover, only a limited number of catalysts with
high catalytic activity and selectivity have been
developed for these transformations. High-silica
mesoporous zeolites, particularly aluminosilicates,
offer promising structural and acidic properties for
such
catalytic
processes.
However,
the
characteristics and behaviour of various metal
oxide modifiers introduced into these zeolites to
enhance their performance remain insufficiently
studied.
Therefore, a comprehensive understanding of the
relationship between the acidic and catalytic
properties of modified HSMZs and their
performance i
n the conversion of C₃–C₄
hydrocarbons is essential for the development of
efficient and selective catalysts for industrial
applications [9
–
15].
M
ETHODS
In order to enhance the sorption and catalytic
performance of high-silica mesoporous zeolites
(HSMZs), a decationization process was carried out
using a 25% aqueous solution of ammonium
chloride. Specifically, 10 g of HSMZ aluminosilicate
was added to 100 mL of the ammonium chloride
solution. The mixture was stirred mechanically and
maintained
in a water bath at 90 °C for 4 hours to
ensure uniform treatment and effective ion
exchange. After completion, the treated zeolite was
filtered through a Büchner funnel to separate the
solid phase, then washed thoroughly with distilled
water to remove residual ions and dried under
ambient conditions. Finally, the dried sample was
calcined in a muffle furnace at 550 °C for 5 hours to
activate the acidic sites and stabilise the
framework structure.
The crystalline structure of the zeolites was
characterised using X-ray diffraction (XRD),
performed on a DRON-4 diffractometer equipped
with a molybdenum anode and a nickel filter. The
diffraction patterns were analysed to determine
interplanar spacings (d-values) and peak
intensities. The observed diffraction lines (peak
positions and relative intensities) were interpreted
by comparison with standard reference samples of
highly crystalline HSMZs, which were known to be
free of amorphous phases, as confirmed by
Fourier-transform infrared (FTIR) spectroscopy.
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Figure 2. Chromatogram of the liquid products formed during the conversion of low-
molecular-weight saturated hydrocarbons (C
₃
–
C
₄
fraction), showing the following
compounds: 1
–
Benzene, 2
–
Toluene, 3
–
m,p-Xylene, 4
–
o-Xylene, 5
–
Mesitylene, 6
–
Pseudocumene, 7
–
m-Diethylbenzene, 8
–
p-Diethylbenzene, 9
–
1,2-Dimethyl-3-
ethylbenzene, 10
–
Naphthalene, 11
–
α
-Methylnaphthalene, 12
–
β
-Methylnaphthalene.
The chromatographic analysis of the liquid-phase
pyrolysis products was performed to identify the
aromatic and alkylaromatic compounds formed
during the catalytic conversion of the C₃–C₄
hydrocarbon fraction. The chromatogram (Figure
2) revealed the presence of several key compounds
including: 1
–
Benzene, 2
–
Toluene, 3
–
m,p-Xylene,
4
–
o-Xylene, 5
–
Mesitylene, 6
–
Pseudocumene, 7
–
m-Diethylbenzene, 8
–
p-Diethylbenzene, 9
–
1,2-
Dimethyl-3-ethylbenzene, 10
–
Naphthalene, 11
–
α
-Methylnaphthalene, 12
–
β
-Methylnaphthalene.
These results were used for further analysis of
selectivity, reaction pathways, and the catalytic
influence of modified HSMZs.
R
ESULTS AND
D
ISCUSSION
Modification of High-Silica Mesoporous Zeolites
with Enhanced Sorption and Catalytic Properties
The modification of high-silica mesoporous
zeolites (HSMZs) exhibiting superior sorption and
catalytic characteristics was carried out by
impregnating the H-form aluminosilicate samples
(H-HSMZ) with aqueous solutions of selected metal
salts. The concentration of the metal salt solutions
was calculated according to the moisture capacity
of the zeolite sample to match the desired content
of the metal or metal oxide. The impregnation
process involved continuous stirring in a water
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bath at 90 °C until the complete evaporation of
moisture. Subsequently, the samples were dried at
110 °C for 8 hours and c
alcined in a muffle furnace
at 550 °C for 4 hours in air.
As a result, a series of modified HSMZ catalysts
were prepared with the following compositions:
•
2% CuO/H-HSMZ
•
1%, 2%, or 3% Li₂O/H
-HSMZ
•
2% La₂O₃/H
-HSMZ
•
2% K₂O
-ZnO/H-HSMZ
•
2% K₂O
-
Fe₂O₃/H
-HSMZ
•
2% Li₂O
-BaO/H-HSMZ
These modified zeolites were used as catalysts for
the high-temperature pyrolysis of propane-butane
hydrocarbon
mixtures
under
oxygen-free
conditions. The goal was to determine their
catalytic performance in comparison with purely
thermal
decomposition
under
identical
experimental conditions.
Table 1 presents the typical results of catalytic and
thermal pyrolysis of propane-butane mixtures
(~50% conversion) in a continuous flow regime
under oxygen-free high-temperature conditions.
Table 1. Comparison of Catalytic and Thermal Pyrolysis of Propane-Butane Hydrocarbon
Mixtures at High Temperatures in Flow Regime (~50% Conversion)
Days
τ,
с
Т,
°С
Propane-
butane
hydrocarbon
mixture
conversion,
%
Productivity in relation to transferred
raw materials, mass %
Selectivity
on
С
2
Н
4
,
%
СН
4
С
2
Н
6
С
2
Н
4
С
3
Н
6
∑С
2
-
С
4
molecularly
unsaturated
ethylene
series
hydrocarbons
1
655
14.8
2.7
1.0
3.4
7.7
11.1
23.0
680
17.6
3.8
1.0
5.7
7.1
12.8
32.4
735
84.4
28.0
8.4
31.8
16.2
48.0
37.7
750
95.0
40.4
9.7
38.6
6.3
44.9
40.6
655
14.6
2.4
1.0
2.6
8.6
11.2
17.8
2
680
35.7
8.9
3.9
9.7
13.2
22.9
27.2
725
83.4
31.2
9.0
29.6
13.6
43.2
35.5
750
95.0
37.4
9.8
37.3
10.5
47.8
39.3
665
46.6
10.7
4.4
12.1
19.4
31.5
26.0
685
72.3
20.1
6.2
25.8
20.2
46.0
35.7
3
720
90.0
26.6
9.8
33.2
20.4
53.6
36.9
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755
94.9
39.9
10.2
40.4
4.4
44.8
42.6
755
86.3
21.8
6.8
32.6
25.1
57.7
37.8
770
93.4
31.8
7.7
40.7
13.2
53.9
43.6
780
97.0
34.5
6.0
47.5
9.0
56.5
49.0
Structural and Spectroscopic Analysis of High-
Silica Mesoporous Zeolite Catalysts
It was found that the thermal decomposition of
saturated hydrocarbons in a high-temperature,
oxygen-free environment under flow conditions is
almost identical to that observed in static batch
setups. This similarity enables rapid kinetic studies
of catalytic processes across multiple systems
within a short time frame.
For catalytic testing, optimal zeolite-based catalyst
fractions were prepared using extrusion and tablet
pressing techniques. Two methods were employed
to shape the high-silica zeolite samples:
•
Method 1 (Pressing)
: Zeolite powders
were compressed into tablets with a diameter of
16 mm and a thickness of 2–
3
mm under
250 kg/cm² pressure using a steel mould. These
tablets were crushed and sieved to obtain particle
fractions in the 1.5
–2.0 mm² range using metal
mesh sieves.
•
Method 2 (Extrusion)
: Zeolite powders
were mixed with a binder (pseudoboehmite),
diluted nitric acid or ammonia solution, and
distilled water until a viscous paste was obtained.
The mass was extruded through a mould and cut to
the desired size. It was dried at 110 °C for 12 hours,
then calcined at 550 °C for 4 hours. The desired
fraction (1.5
–2.0 mm²) was separated by sieving.
The structure of the resulting aluminosilicate-
based zeolite catalysts was analysed using infrared
(IR) spectroscopy in the mid-IR region (400
–
2000 cm⁻¹),
which
includes
characteristic
absorption bands associated with the vibrational
modes of AlO₄²⁻ and SiO₄²⁻ crystal tetrahedra.
To prepare samples for IR analysis, 1
–2 mg of the
zeolite was mixed with 400 mg of KBr, pressed into
a ring mould, and placed into the spectrometer. The
observed absorption bands (ABs) arise from two
types of vibrational modes:
1.
Primary internal
vibrations
within the
TO₄ tetrahedra, which form the structural units of
the aluminosilicate framework. These do not
reflect changes in the external structure.
2.
External vibrations
along the linkages
between tetrahedra. These are more sensitive to
the zeolite’s topology, secondary building units,
and pore structure.
The following major absorption bands were
observed:
•
1120 cm⁻¹
–
intense band attributed to
antisymmetric stretching of TO₄ tetrahedra.
•
800 cm⁻¹
–
indicative of symmetric
stretching vibrations, primarily involving SiO₄
units.
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•
560 cm⁻¹
–
bending vibrations sensitive to
the topology and connectivity of secondary
structural units (e.g., 5-membered or 6-membered
rings).
•
460 cm⁻¹
–
bendin
g vibrations within TO₄
tetrahedra.
The presence of these bands confirms the
structural classification of all tested samples within
the HSMZ family. The intensity ratio of the
absorption bands at 560 cm⁻¹ and 460 cm⁻¹
(logI560/logI460)×100) was used to calculate the
degree of crystallinity for each sample. The
calculated crystallinity values for different
SiO₂/Al₂O₃ ratios were as follows:
Table 2. Crystallinity Degree of Modified High-Silica Mesoporous Zeolites Based on SiO
₂
/Al
₂
O
₃
Molar Ratio
No. Sample Description
SiO
₂
/Al
₂
O
₃
Ratio Crystallinity (%)
1
Modified HSMZ Catalyst
27
83
2
Modified HSMZ Catalyst
33
81
3
Modified HSMZ Catalyst
49
94
4
Modified HSMZ Catalyst
67
85
5
Modified HSMZ Catalyst
90
93
6
Modified HSMZ Catalyst
125
78
7
Ferro-silicate-based Catalyst Sample
–
85
Figure 2. IR Spectrum of Decationized H-HSMZ-70 Sample
Y-axis: Transmittance (%); X-axis: Wavenumber (cm
⁻
¹)
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Thermodynamic Aspects of the Transformation
of Light Hydrocarbons
Saturated hydrocarbons are thermally stable
compounds and decompose only at high
temperatures. The ultimate products of complete
thermal decomposition are carbon and hydrogen.
The thermal stability of a hydrocarbon can be
evaluated using the change in its Gibbs free energy
of formation:
0
0
0
G
H
T S
=
−
(1)
This value is also related to the equilibrium
constant Kp through the expression:
0
p
G
RTlnK
−
=
(2)
At temperatures up to 560 °C, the most
thermodynamically stable light hydrocarbon is
methane (as shown in Figures 3
–
5). At
temperatures exceeding 560 °C, the most probable
thermodynamic system shifts toward a mixture of
elemental carbon and hydrogen. As temperature
increases, the standard Gibbs free energy change
(∆G⁰) for all hydrocarbons (except acetylene)
increases, indicating a decrease in their thermal
stability.
In contrast, the thermal stability of unsaturated
hydrocarbons (such as ethylene and its
homologues) decreases more gradually with rising
temperature compared to saturated hydrocarbons
of similar molecular weight. For instance, ethylene
remains more thermodynamically stable than
saturated hydrocarbons with the same number of
carbon atoms
—
such as butene and propylene
—
at temperatures above 805 °C, whereas butenes
and propylene lose stability around 650
–670 °C.
This difference in thermal stability explains why
unsaturated hydrocarbons of the ethylene series
can be obtained through thermal transformations
of
saturated
hydrocarbons
under
high-
temperature conditions. The feasibility of such
transformations is rooted in the thermodynamic
disparity between these molecular species.
In addition to decomposition into elements, the
conversion of propane
–
butane fractions may
proceed via several other thermodynamically
feasible pathways, including:
•
Dehydrogenation,
•
C
–
C bond cleavage, and
•
Dehydrocracking
(combined
dehydrogenation and carbon
–
carbon bond
breaking).
For example:
•
C
–
C bond cleavage in propane becomes
thermodynamically favourable above 350 °C.
•
Butane conversion to methane and
propylene occurs above 320 °C,
•
Butane to ethane and ethylene becomes
favourable above 420 °C,
•
Dehydrocracking of butane becomes
feasible at temperatures exceeding 610 °C.
The temperature dependence of the logarithmic
equilibrium constant logKp for these reactions was
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calculated using Equation (2) and is illustrated in
Figure 6.
Figure 3. Effect of temperature on the thermodynamic stability of saturated hydrocarbons.
Figure 4. Effect of temperature on the thermodynamic stability of unsaturated ethylenic series
hydrocarbons.
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Figure 5. Effect of temperature on the thermodynamic stability of acetylene and aromatic
hydrocarbons.
Figure 6. Temperature dependence of lgK
p
processes of transformation of low molecular
weight saturated hydrocarbons.
Based on thermodynamic calculations, the
equilibrium state and the change in the
composition of products of low-molecular-weight
saturated hydrocarbons such as propane and
butane at different temperatures make it possible
to assess the mechanism and reaction conditions of
very important processes.
Dehydrogenation of propane:
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When a propane molecule is dehydrogenated,
propylene (C₃H₆) and hydrogen (H₂) are formed as
the main products. With increasing temperature,
the reaction equilibrium shifts towards the
products: at 227°C, the conversion is 0.12%, at
627°C, this value increases to 47.42%, and at 827°C
to 86.99%. At the same time, the amount of H₂ at
equilibrium also increases from 18.49% → 46.52%,
which clearly demonstrates the effect of
temperature on an endothermic reaction.
Dehydrogenation of butane.
Dehydrogenation of butane (C₄H₁₀) produces
butylene (C₄H₈) and H₂. Butane achieves higher
conversions at lower temperatures than propane:
Conversion at 427°C is ~13.7%, at 627°C ~48.0%,
and at 727°C ~68.0%.
This reaction is also endothermic, with the amount
of H₂ increasing with increasing temperature.
Propane cracking reaction.
Thermal cracking of propane produces methane
(CH₄), ethane (C₂H₆), ethylene (C₂H₄) and other
low molecular weight fragments. At 327°C, the
conversion is 53.3%, and at 427°C, the conversion
is almost 100%. This indicates high reactivity and
easy cleavage of C
–
C bonds. Cracking of butane (to
propylene and methane) shows a 52.7%
conversion of butane at 227°C, and almost 100%
conversion at equilibrium at 327°C. The main
products in this reaction are methane (CH₄) and
propylene (C₃H₆).
Cracking of butane to ethane and ethylene.
In this process, butane is converted to C₂H₆ and
C₂H₄: 27.96% conversion at 327°C, ~52.83% at
427°C, and ~100% at 527°C. The main mechanism
in this reaction is the cleavage of C-C bonds without
the formation of H₂. Dehydrocarbonization
reaction of butane (with C-C bond cleavage). In this
reaction, carbon-carbon bonds are also cleaved
along with butane dehydrogenation: 0.02% at
227°C, 23.11% at 527°C, and almost 100% at
727°C.
Table 3. Equilibrium conversion of low molecular weight saturated hydrocarbons and
equilibrium concentration of products.
A lower
molecular
weight
saturated
hydrocarb
on
process
Т,
℃
Equilibriu
m of
conversio
n of lower
molecular
weight
saturated
hydrocar
bons, mol.
(%)
The composition of the system in equilibrium, mol (%)
C
3
H
8
C
4
H
10
CH
4
C
2
H
6
C
2
H
4
C
3
H
8
C
4
H
8
H
2
Dehydrog
227
0.1178
99.76
-
-
-
-
0.18
-
0.18
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enation of
propane
327
1.26
97.50
-
-
-
-
1.25
-
1.25
427
6.82
87.24
-
-
-
-
6.38
-
6.38
527
22.69
63.02
-
-
-
-
18.49
-
18.49
627
47.42
35.66
-
-
-
-
32.17
-
32.17
727
58.73
26.00
-
-
-
-
37.00
-
37.00
827
86.99
6.96
-
-
-
-
46.52
-
46.52
Dehydrog
enation of
butane
227
0.2459
-
99,50
-
-
-
-
0,25
0,25
327
2.61
-
94,90
-
-
-
-
2,55
2,55
427
13.69
-
75,92
-
-
-
-
12,04
12,04
527
40.09
-
42,76
-
-
-
-
28,62
28,62
627
48.00
-
35,14
-
-
-
-
32,43
32,43
727
68.02
-
19,04
-
-
-
-
40,48
40,48
827
-
-
-
-
-
-
-
-
-
Propane
cracking
127
1.59
96,86
-
1,57
-
1,57
-
-
-
227
18.7
68,42
-
15,79
-
15,79
-
-
-
327
53.32
30,44
-
34,78
-
34,78
-
-
-
427
~100
0
-
-50
-
-50
-
-
-
Cracking
of butane
to
propylene
and
methane
127
10.5
-
30,85
9,57
-
-
9,57
-
-
227
52.70
-
30,98
34,51
-
-
34,5
-
-
327
~100
-
0
-50
-
-
1
-
-
Cracking
of butane
to ethane
and
ethylene
127
0,3151
-
99,38
-
0,31
0,31
-50
-
-
227
5,04
-
90,40
-
4,80
4,80
-
-
-
327
27,96
-
56,30
-
21,85
21,85
-
-
-
427
52,83
-
30,86
-
34,57
34,57
-
-
-
527
~100
-
0
-
-50
-50
-
-
-
Processes
involving
the
cleavage
of the
carbon-
carbon
bond of
butane
227
0,020
-
99,94
-
-
0,04
-
-
0,02
327
0,461
-
98,62
-
-
0,92
-
-
0,46
427
4,338
-
88,03
-
-
7,98
-
-
3,99
527
23,11
-
52,57
-
-
31,62
-
-
15,81
627
85,67
-
5,47
-
-
63,02
-
-
31,51
727
~100
-
0
-
-
66,7
-
-
33,3
This process is a deep cracking reaction that occurs
at high temperatures, proceeds based on radical
mechanisms and produces C₂H₄, C₄H₈ and H₂ as the
main products. Cracking reactions of C₃
-
C₄
saturated hydrocarbons in the range of 300-400°C
give
high
conversions
at
equilibrium.
Dehydrogenation reactions reach equilibrium at
800-900°C. The composition of the products in
different types of propane and butane reactions
varies depending on the temperature, which is an
important factor in optimising the process. The
temperature regime should be selected depending
Volume 05 Issue 05-2025
28
International Journal of Advance Scientific Research
(ISSN
–
2750-1396)
VOLUME
05
ISSUE
05
Pages:
16-29
OCLC
–
1368736135
on the direction of the reaction (dehydrogenation,
cracking, dehydro-cracking).
C
ONCLUSION
In this study, methods for enhancing the sorption
and catalytic activity of high-silica mesoporous
zeolites (HSMZs) were developed through
decationization in ammonium chloride solution
and subsequent modification with various metal
oxides. As a result, catalysts based on H-HSMZ
modified with CuO, Li₂O, La₂O₃, ZnO, Fe₂O₃, and
BaO were synthesised and tested in the catalytic
and thermal cracking of propane-butane fractions
at high temperatures.
Experimental results revealed that, under high-
temperature flow conditions, catalytic cracking
exhibited higher conversion rates and better
selectivity compared to thermal cracking. The
catalytic performance was shown to depend
significantly on both the type of metal oxide
modifier and the catalyst preparation method
(extrusion or tabletting).
Structural analysis by infrared (IR) spectroscopy
and X-ray diffraction (XRD) confirmed the high
crystallinity of the zeolite samples and the
preservation of the aluminosilicate framework
structure. Thermodynamic analysis allowed for the
assessment of equilibrium conditions, product
distributions,
and
temperature-dependent
stabilities of various reaction pathways involving
propane and butane.
Based on the thermodynamic evaluation, an
optimal temperature range of 650
–800 °C was
recommended for achieving high selectivity
toward
the
production
of
unsaturated
hydrocarbons such as ethylene and propylene.
Overall, the developed HSMZ-based catalysts and
the defined operating conditions offer significant
potential for improving the efficiency of converting
propane-butane fractions into valuable light
olefins.
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