Propane Conversion in the Presence of Alumina-Based Aerogel

Article abstract of DOI:10.1134/S0965544119010109

Abstract: Features of propane conversion for selective olefin production in the presence of nanofibrous aerogel materials have been studied. The effect of modification of aerogel alumina with titania and silica on the catalytic activity and the olefin selectivity in propane conversion at atmospheric pressure has been shown. Low-temperature nitrogen adsorption has revealed that these catalysts have a developed specific surface area of about 300 m²/g; water vapor adsorption at 293 K has shown that materials of this type contain 0.2–0.6?mmol/g of acid sites. X-ray diffraction analysis and transmission electron microscopy have revealed that the synthesized materials are an amorphous aerogel with tangled fibers with a thickness of ~6 nm.

Full text of DOI:10.1134/S0965544119010109

ISSN 0965-5441, Petroleum Chemistry, 2019, Vol. 59, No. 1, pp. 71–77. © Pleiades Publishing, Ltd., 2019.
Russian Text © E.B. Markova, A.G. Cherednichenko, V.N. Simonov, Yu.M. Serov, M.V. Odintsova, A.S. Lyadov, 2019, published in Neftekhimiya, 2019, Vol. 59, No. 1,
pp. 69–75.
Propane Conversion in the Presence of Alumina-Based Aerogel
E. B. Markovaa, b, c, *, A. G. Cherednichenkoa, V. N. Simonovc, d, Yu. M. Serova,
M. V. Odintsovaa, and A. S. Lyadovb
aPeoples’ Friendship University of Russia, Moscow, Russia
bTopchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, Russia
cFrumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia
dNational Research Nuclear University Moscow Engineering Physics Institute, Moscow, Russia
*e-mail: ebmarkova@gmail.com
Received July 18, 2018; Revised July 23, 2018; Accepted July 26, 2018
Abstract—Features of propane conversion for selective olefin production in the presence of nanofibrous
aerogel materials have been studied. The effect of modification of aerogel alumina with titania and silica on
the catalytic activity and the olefin selectivity in propane conversion at atmospheric pressure has been shown.
Low-temperature nitrogen adsorption has revealed that these catalysts have a developed specific surface area
of about 300 m2/g; water vapor adsorption at 293 K has shown that materials of this type contain 0.2–
0.6 mmol/g of acid sites. X-ray diffraction analysis and transmission electron microscopy have revealed that
the synthesized materials are an amorphous aerogel with tangled fibers with a thickness of ~6 nm.
Keywords: propane conversion, ethylene, propylene, alumina, titania, silica
DOI: 10.1134/S0965544119010109
INTRODUCTION
of 62% [5, 7]. In general, oxide catalyst systems are
effective catalysts in propane conversion to olefins.
Features of propane conversion in the presence of
samarium vanadite and samarium vanadate synthe-
sized by the solid-phase synthesis method were stud-
ied in [8]. It was shown that SmVO3 catalyzes mostly
the propane cracking to form methane and ethylene,
while SmVO4 equally accelerates both the cracking
and dehydrogenation of propane.
Light olefins (ethylene and propylene) are the most
important intermediates for heavy organic synthesis
and polymer production [1]. On an industrial scale,
these products are mostly synthesized by straight-run
gasoline pyrolysis [2]. Currently, the fraction of global
propane production in the composition of natural and
associated petroleum gas increases; therefore, pro-
pane is an attractive feedstock for the production of
light olefins. The catalytic conversion of propane can
occur via two routes:
The use of contact systems in which the reaction
mixture is forcibly transported through the catalyst
pores and the reaction for high-temperature processes
occurs mostly in the diffusion region leads to an
increase in the process rate [9, 10], which can signifi-
cantly decrease the probability of occurrence of side
processes, namely, the formation of tar and coke in the
catalysts. Diffusion limitations can be decreased by
the use of fibrous materials, particularly those based
on various nanowires.
(i) propane dehydrogenation to propylene:
С3Н 8 → С3Н 6 + Н 2,
ΔH ° = 124 kJ/m ol, and
(ii) propane cracking to ethylene and methane:
С3H8 → CH4 + C2H4, ΔH ° = –56 kJ/mol.
It is currently believed that effective hydrocarbon
cracking catalysts are systems based on ultrastable zeo-
lites, in which the zeolite in a cation-decationated
form is stabilized at a temperature of 500–550°C and
a partial pressure of water vapors below 0.8 atm [3, 4].
Propane conversion is mediated, along with alu-
mina, by other oxides and/or their mixtures. The
oxides that are most commonly used in this reaction
A disadvantage of this method is a low SiO2/Al2O3 are titania and silica. Therefore, it was of interest to
synthesize catalytically active composite materials of
the aerogel type based on nanofibrous alumina modi-
fied with titania and silica in propane conversion and
study the features of this process in the presence of the
ratio and, accordingly, low thermal stability and cata-
lytic properties of the resulting zeolite. For example,
propane conversion in the presence of catalysts based
on a modified ZSM-5 zeolite was studied. It was
shown that these systems provide an olefin selectivity synthesized catalyst systems.
71

72
MARKOVA et al.
Parameters of the pore structure of the samples
EXPERIMENTAL
were determined from nitrogen adsorption isotherms
at 77 K, which were measured on a Micromeritics
ASAP 2020-MP automatic high-vacuum unit in a rel-
ative pressure range of 0.001–0.98. The samples were
preevacuated to a residual pressure of less than
10 mmHg.
The specific surface area of all the samples was
measured by the Brunauer–Emmett–Teller (BET)
method. The surface was calculated by the compara-
tive MP method (micropore size distribution) and the
t-plot method (determination of micro- and/or meso-
pore volumes and specific surface area of the sample
compared with the initial adsorption isotherm of a
nonporous material with the same surface chemistry).
The total specific surface area was determined from
the slope of the initial portion of the comparative MP
plot; the mesopore surface was determined from the
comparative plot in the polymolecular adsorption
region by the MP method. In addition, the ratio of the
volume and surface of micro- and mesopores was
determined by the t-plot method.
Comparative assessments of surface functional
groups and the number of primary adsorption sites
(PASs) of the nanofibrous aerogels were obtained
from the water adsorption isotherm at 293 K on a vac-
uum weighing system with a McBain quartz helical
spring balance with a sensitivity of 10 μg with a
weighed portion of up to 100 mg.
Catalytic tests were conducted in a U-shaped flow
quartz reactor at atmospheric pressure in a wide tem-
perature range (300–1273 K). Propane was used as the
feedstock; it was supplied at a rate of 55.8 μmol/s,
which was constant in all the tests. The catalyst load
was 0.05 g.
The nanofibrous aerogel was prepared by wet air
oxidation of metallic aluminum diffusing through a
mercury amalgam layer on the surface of an aluminum
plate in a specially constructed reactor [11]. Alumi-
num plates with a size of 100 × 100 × 1 mm, which
were produced by OOO Lab-3 in accordance with
Specifications TU 6-00-00205133-63-97 (purity of
99.999%), after the formation of a mercury amalgam
layer on their surface, were placed into the reactor
chamber and held at 20°C and a humidity of 70%
for 5 h. Under these conditions, the average growth
rate of the nanofibrous alumina aerogel was 1 cm h–1.
The alumina synthesized by this method contained
40–43 wt % of water and has an amorphous structure.
The density of the freshly prepared alumina-based
nanofibrous aerogel was 0.004 g/cm3.
After that, the synthesized alumina samples were
impregnated with titanium(IV) isopropoxide and
triethoxychlorosilane at 373 K for 24 h. The dopant
and basic alumina were taken in equal volume ratios.
This synthesis is based on the ability of both tita-
nium(IV) isopropoxide and triethoxychlorosilane to
undergo hydrolysis. The surface of the original nano-
fibrous aerogel contains OH groups capable of under-
going the hydrolysis reaction with the above reagents
to form the respective oxides [12]:
n –OH s + Me OR
g
(
)( ) )4 ( )
(
→ –O– Me OR
s + nROH g ,
(
(
(
)
(
)
4−n ( ) ( )
n
–O– Me OR
s + 4 – n H O g
4−n ( ) ( )
(
→
)
(
)
)
2
n
–O– Me OH s + 4 – n ROH g ,
)( ) ( )
(
)
(
)
n
where (s) is the surface and (g) is the gas phase.
This method makes it possible to deposit both tita-
nia and silica in an amount of 5 wt %; it is not accom-
panied by the transition of titania and silica to the
crystalline phase with subsequent reduction to a null-
valence state.
To determine the morphology of the synthesized
catalysts, transmission electron microscopy (TEM)
micrographs were recorded on a 200-kV JEM 2100
microscope (JEOL, Japan). To this end, the samples
were placed on a substrate wetted with alcohol without
any pretreatment.
X-ray diffraction (XRD) analysis of the samples
was conducted on a high-precision PANalytical
Empyrean X-ray diffractometer using monochromatic
CuKα radiation in the reflection geometry in an angu-
lar range of 2θ = 10°–100°.
To determine the presence of ions in the
aerogel, elemental analysis was conducted on a
Clever-31 X-ray fluorescence spectrometer.
Sampling was conducted through a dosing valve;
the samples were analyzed on a Kristall 2000 M chro-
matograph equipped with a flame ionization detector
and a thermal conductivity detector; products were
separated on a Porapak Q column with a length of
1.5 m and a diameter of 3 mm; the temperature of the
column and the detectors was 373 and 473 K, respec-
tively. The main products of both the thermal and cat-
alytic conversion of propane were hydrogen, methane,
ethane, ethylene, and propylene.
Propane conversion was determined upon the
establishment of a steady state from the amount of
reacted propane:
n − n
fed
unreact
α =
,
n
fed
where nfed is the amount of the fed propane (μmol)
and nunreact is the amount of unreacted propane
(μmol).
Thermal analysis was conducted on an SDT Q600
TG/DSC/DTA synchronous thermal analyzer to
Using the derived experimental data, the activation
determine the stability of the synthesized nanofibrous energies for propane conversion in the presence of var-
aerogel in the high-temperature region.
ious catalyst systems were calculated. To this end, the
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2019

PROPANE CONVERSION IN THE PRESENCE
73
5.87 nm
6.84 nm
100 nm
×300 k 100 nm
×300 k 100 nm
(a)
(b)
(c)
Fig. 1. Transmission electron microscopy images at a resolution of 100 nm for samples: (a) Al O , (b) TiO /Al O , and
2
3
2
2 3
(c) SiO /Al O .
2
2 3
dependences of the logarithm of the reaction rate on
the reciprocal of temperature were studied.
For the comparative assessment of the surface
functional groups and the number of PASs in the syn-
thesized nanofibrous aerogel samples, water adsorp-
tion isotherms at 293 K were recorded (Fig. 3).
In the extremely low relative filling region, the iso-
therms have a convex portion. The description of these
portions by the Langmuir equation makes it possible
to determine the number of PASs for the adsorption of
water molecules (Table 2). In all the cases, the number
of PASs was determined according to the comparative
plots of water vapor adsorption.
According to the concept of PASs [13], in the initial
region of relative pressures, the adsorption of water
molecules occurs on oxygen-containing surface func-
tional groups. The presence of acidic oxygen com-
plexes in the catalysts increases the adsorption and
facilitates the micropore filling. The chemical state of
the surface plays a significant—commonly decisive—
role in the water vapor adsorption. The presence of
nonacid oxygen complexes does not affect the surface
polarity of the catalyst and does not enhance the water
vapor adsorption.
RESULTS AND DISCUSSION
Figure 1 shows TEM images of the synthesized
materials; it is evident that the synthesized aerogels are
composed of tangled nanowires with a diameter of 5–
6 nm.
The XRD patterns of the aerogel materials (Fig. 2)
do not exhibit reflections corresponding to the diffrac-
tion pattern of X-ray amorphous substances. In addi-
tion, it was found that the nanofibrous aerogel samples
contain only aluminum, titanium, and silicon ions;
other ions capable of affecting the physicochemical
and catalytic properties of the studied materials were
not detected.
According to thermal analysis, in a temperature
range of 20–1200°C, nanofibrous alumina does not
undergo phase transitions. In the low-temperature
region of 80–200°C, only a loss of adsorption water
was detected; other changes were not registered.
All the synthesized materials can be attributed to
Analysis of the water vapor adsorption isotherms
mesoporous adsorbents; only nanofibrous alumina shows that the number of PASs slightly decreases in
contained a small amount of micropores (Table 1).
the case of deposition of titania on nanofibrous alu-
Table 1. Parameters of the pore structure of the nanofibrous aerogel catalysts studied
Vо, cm3/g
VBJH, cm3/g
desorption/adsorption
SBET
m2/g
,
SМЕ
m2/g
,
SМР,
2xо,
2xоBJH, nm
Catalyst
Al2O3
m2/g
Smicr, m2/g
nm desorption/adsorption
253
310
219
0.011/29.5
0.58/0.56
0.68/0.67
0.50/0.48
6.7
8.1
8.3
9.7/10.0
9.96/10.24
11.10/12.41
223
386
185
196
–
TiO2/Al2O3
SiO2/Al2O3
–
–
–
S
is the specific surface area; V and S
is the volume and apparent surface area of micropores in the samples, respectively;
BET
BJH
o
micr
V
is the mesopore volume; 2 is the pore size (two half-widths for the slit model 2 ) determined in the BET region and in the capillary con-
хо хо
densation region from the desorption branch in terms of the Barrett–Joyner–Halenda (BJH) model; 2x
lary condensation region determined by the BJH method; S is the mesopore surface; and S is the micropore surface area.
is the average pore size in the capil-
оBJH
МЕ
MP
PETROLEUM CHEMISTRY Vol. 59 No. 1 2019

74
MARKOVA et al.
(а)
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
(b)
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
20
40
60
80
100
(c)
0
20
40
60
80
100
2θ, deg
2θ, deg
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
20
40
60
80
100
2θ, deg
Fig. 2. X-ray diffraction analysis of the structure of nanofibrous aerogels based on alumina for samples: (a) Al O ,
2
3
(b) TiO /Al O , and (c) SiO /Al O .
2
2
3
2
2 3
mina and increases in the case of deposition of silica
(Table 2).
The freshly prepared nanofibrous aerogel samples
were studied as catalysts in the propane conversion
reaction. Nanofibrous alumina exhibited catalytic
activity only in the region of high temperatures starting
from 700 K (Fig. 4).
Analysis of water vapor adsorption suggests that
nanofibrous alumina and the modified composite
materials based on it have an extremely developed
adsorption capacity and a large number of PASs; these
systems are superior to zeolite materials in these
parameters. According to these findings, it can be
assumed that the synthesized nanofibrous aerogel cat-
alyst samples will exhibit catalytic activity.
At the same time, during the thermal conversion of
propane, the onset of decomposition occurred above
900 K; in this case, the main products were methane
and ethane. A further increase in temperature led to an
increase in propane conversion; however, the yield of
heavy hydrocarbons increased owing to the polycon-
densation of the resulting unsaturated and aromatic
hydrocarbons, which led to tar formation in the cata-
lyst. In a temperature range of 900–1100 K, coke for-
mation was observed; at temperatures above 1150 K,
tar formation took place.
1
19
17
15
13
11
9
2
In the presence of the titania- and silica-modified
composite samples, a significant increase in the pro-
pane conversion and a change in the olefin selectivity
compared with the respective parameters of nanofi-
brous alumina was observed (Table 3).
3
7
5
3
For the TiO2/Al2O3 catalyst, the propylene selec-
tivity achieved 47% at 1023 K. For the freshly prepared
SiO2/Al2O3 catalyst, in a low-temperature region of
600–823 K, the ethylene selectivity exceeds 50%;
above 823 K, the propylene content in the reaction
products increases; at 923 K, the propylene selectivity
achieves a maximum of 64%, while the commercial
1
0.6
0.7
0.8
0.9
1.0
0.5
P/P0
Fig. 3. Water vapor adsorption isotherm at 293 K for sam-
ples: (1) Al O , (2) TiO /Al O , and (3) SiO /Al O .
2
3
2
2
3
2
2 3
PETROLEUM CHEMISTRY
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2019

PROPANE CONVERSION IN THE PRESENCE
75
Table 2. Surface characteristics determined by the water adsorption method
Specific adsorption
capacity, μmol/m2
Adsorption capacity, Amount of PASs,
Average number of water
molecules in the cluster
Catalyst
mmol/g
mmol/g
Al2O3
19.678
10.403
18.654
0.305
0.241
0.621
64
43
30
78
34
85
TiO2/Al2O3
SiO2/Al2O3
Pt/Al2O3 catalyst provides an ethylene selectivity of as range of 873–1023 K. However, an increase in tem-
perature above 1123 K led to an almost complete deac-
tivation of the catalysts within 5 h (Fig. 5).
Up to a temperature of 873 K, the amount of car-
bon after 400 h on stream was about 5 wt % with
respect to the loaded catalyst, whereas at 1123 K, after
5 h on stream, the carbon content was 59 wt %.
The removal of the formed carbon with an air
stream at 923 K for 10 h leads to the complete resto-
ration of the catalytic activity.
Thus, aerogel composite catalyst systems based on
alumina impregnated with titania or silica are an
amorphous aerogel with tangled 6-nm thick fibers.
The laced structure provides an increase in the perme-
ability of the reagents to the catalytic sites. Catalysts of
this type have a developed specific surface area of
about 300 m2/g and contain 0.2–0.6 mmol/g of acid
sites.
The catalytic activity and olefin selectivity of the
synthesized nanofibrous aerogel catalysts in the pro-
pane cracking reaction have been determined. The
effect of the modification of alumina with titania and
silica on the catalytic activity and olefin selectivity of
the catalysts in propane cracking at atmospheric pres-
sure has been first shown. The nanofibrous aerogel
composite catalysts exhibit a high olefin selectivity at a
total olefin selectivity of up to 90%. The stability of the
catalysts and the possibility of their regeneration as a
low as 35% [14].
A change in selectivity in the case of modification
of nanofibrous alumina with titania or silica is
attributed to a change in the structure of the catalytic
site. For unmodified nanofibrous aerogel alumina,
this site is an –OH group chemisorbed on the alumi-
num atom, where the adsorption and catalytic conver-
sion of propane take place, whereas in the case of
modification with titania or silica, the resulting new
catalytic site involving titania or silica facilitates the
dissociative adsorption of propane to form hydrogen
and propylene, as evidenced by lower values of activa-
tion energy for catalytic cracking (Table 4).
It was found [15] that the order of the reaction is
within the unit, with some upward and downward
deviations from the integer value. The following rela-
tionship is observed: with an increase in temperature,
the order of the reaction deviates downwards. Based
on the fact that propane cracking is a first-order reac-
tion, the activation energies for the process were calcu-
lated for the test catalysts using the Arrhenius equa-
tion. It was found that the activation energy for the
thermal conversion of propane is 143 kJ/mol, which is
close to the published data [16]; for reaction in the
presence of the catalyst based on nanofibrous aerogel
alumina, the activation energy was 85–105 kJ/mol
depending on the catalyst type (Table 3).
In the presence of nanofibrous catalysts, conver-
sion occurs mostly by the carbenium ion-mediated
mechanism and the contribution of the thermal com-
ponent is negligible. It is known that there is a correla-
tion between the bond dissociation energy and the
activation energy of the elementary reaction [13]. It is
also known [13] that, according to the Bell–Evans–
Polanyi principle, with an increase in the exothermic-
ity of the radical process, the activation energy of the
process Ea decreases. In alkanes, the primary C–H
bonds are the strongest (100 kcal/mol), while the ter-
tiary C–H bonds are the least strong (86 kcal/mol).
Accordingly, the calculated apparent activation energy
should not exceed the above values. The results are in
good agreement with this concept.
1.0
4
3
2
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
700
800
900 1000 1100
600
It was found that the on-stream stability of the
studied catalysts is fairly high. For these catalysts, in
the catalytic conversion of propane, the on-stream
time without any change in activity was 400 h at tem-
peratures of up to 873 K and 150 h in a temperature
Temperature, K
Fig. 4. Temperature dependence of propane conversion:
(1) without a catalyst, (2) Al O , (3) TiO /Al O , and
2
3
2
2 3
(4) SiO /Al O .
2
2 3
PETROLEUM CHEMISTRY Vol. 59 No. 1 2019

76
MARKOVA et al.
Table 3. Effect of temperature on the product selectivity in propane conversion
Selectivity, %
C2H6
Temperature,
K
Propane
conversion, %
Catalyst
CH4
C2H4
C3H6
In the absence
of a catalyst
723
823
0.1
1.2
92
65
37
18
11
8
27
19
19
28
–
8
–
–
923
2.1
31
8
13
55
56
1023
1123
9.0
39.2
5
Al2O3
723
823
0.3
1.0
94
67
37
17
6
6
17
20
16
25
–
8
–
8
923
2.3
14
14
6
29
53
63
1023
1123
9.5
79.1
TiO2/Al2O3
723
823
1.7
2.7
80
64
55
15
23
12
28
11
29
52
8
8
6
9
7
–
–
923
3.5
28
47
18
1023
1123
48.0
85.0
SiO2/Al2O3
723
823
0.1
0.89
19.7
60.7
94.2
48
20
8
52
58
26
36
53
–
22
2
–
–
923
64
46
15
1023
1123
13
25
5
7
function of the contact time and temperature have catalytic activity is 150–400 h depending on the oper-
been studied. The on-stream time of nanofibrous and ating temperature. It has been found that these cata-
nanocrystalline aerogel catalysts without any loss of lysts can be regenerated with an air stream.
(а)
(b)
(c)
Propane conversion, %
Propane conversion, %
Propane conversion, %
100
100
80
60
40
20
0
100
80
60
40
20
0
80
60
40
20
0
1123
1000
873
1123
1000
873
1123
1000
873
150 400
Time, h
5
150
400
5
150
400
5
Time, h
Time, h
Fig. 5. Dependence of the catalyst activity on the time and temperature of the test: (a) Al O , (b) TiO /Al O , and
2
3
2
2 3
(c) SiO /Al O .
2
2 3
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PROPANE CONVERSION IN THE PRESENCE
77
Table 4. Activation energy for propane conversion, kJ/mol
4. A. V. Glazov, O. I. Dmitrichenko, N. V. Korotkova,
et al., RU Patent No. 2127632 (1999). http://www.
freepatent.ru/patents/2473385.
Еа, kJ/mol
Catalyst
Temperature, K
5. N. Rahimi, D. Moradi, M. Sheibak, et al., Micropo-
rous Mesoporous Mater. 234, 215 (2016).
Without a catalyst
Al2O3
823–1123
823–1123
673–1123
673–1123
143
105
85
6. J. Lva, Z. Huaa, T. Gea, et al., Micropor Mesopor Mat
247, 31 (2017).
7. S. Asadi, L. Vafi, and R. Karimzadeh, Microporous
Mesoporous Mater. 255, 253 (2018).
TiO2/Al2O3
SiO2/Al2O3
8. E. B. Markova, A. S. Lyadov, and V. V. Kurilkin, Russ.
J. Phys. Chem. A 90, 1754 (2016).
95
9. N. A. Pakhomov and R. A. Buyanov, Kinet. Catal. 46,
669 (2005).
10. A. F. Bedilo, E. V. Il’ina, I. V. Mishakov, and
A. A. Vedyagin, Khim. Interesah Ustoich. Razvit. 19,
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11. O. K. Krasil’nikova, A. S. Pogosyan, N. V. Sere-
bryakova, et al., Prot. Met. Phys. Chem. Surf. 44, 362
(2008).
ACKNOWLEDGMENTS
This work was supported by the Ministry of Educa-
tion and Science of the Russian Federation under the
Program for enhancement of the competitiveness of
the Peoples’ Friendship University of Russia among
the leading research and educational centers of the
world in 2016–2020.
12. J. Aarik, A. Aidla, V. Sammelselg, et al., Thin Solid
Films 370, 163 (2000).
13. R. Sh. Vartapetyan, A. M. Voloshchuk, M. M. Dubi-
nin, et al., Russ. Chem. Bull. 31, 1078 (1982).
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Neftepererab. Neftekhim., No. 3, 8 (2013).
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sertation in Chemistry (Moscow, 2015).
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Translated by M. Timoshinina
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