Pathways of light compounds formation during propane and isobutane dehydrogenation on Al-Cr catalysts

Article abstract of DOI:10.1016/j.apcata.2010.04.026

Reaction pathways of the light hydrocarbons formation during propane and isobutane dehydrogenation on Al-Cr catalyst have been studied. It was determined that the majority of light compounds are formed not directly from initial paraffin, but from the main product of the paraffin dehydrogenation; olefin. The sequence of the light compounds formation reactions includes: (1) dehydrogenation of the initial paraffin with formation of targeted olefin; (2) consecutive hydrocracking of the newly produced main olefin with formation of the one carbon shorter chain olefin and methane; (3) one carbon shorter chain olefin can be converted by two parallel reactions further. The first reaction is hydrogenation of the one carbon shorter chain olefin to the formation of the one carbon shorter chain paraffin. Another parallel reaction is hydrocracking of the one carbon shorter olefin to the two carbon shorter olefin and methane. If short olefin is ethylene the final product of the hydrocracking reaction is methane. This sequence can be presented by using isobutane dehydrogenation as an example which also includes steps of light compounds formation from C₃ hydrocarbons:(1)C₄H₁₀ → C₄H₈ + H₂(2)C₄H₈ + H₂ → C₃H₆ + CH₄(3a)C₃H₆ + H₂ → C₃H₈(3b)C₃H₆ + H₂ → C₂H₄ + CH₄(4a)C₂H₄ + H₂ → C₂H₆(4b)C₂H₄ + 2H₂ → 2CH₄. Obtained reaction pathways of light compounds formation can be applied to optimize the concept of the dehydrogenation process and explain puzzling phenomena regarding the temperature profile in the fixed bed dehydrogenation process.

Full text of DOI:10.1016/j.apcata.2010.04.026

Applied Catalysis A: General 382 (2010) 139–147
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Pathways of light compounds formation during propane and isobutane
dehydrogenation on Al-Cr catalysts
V.Z. Fridman∗
Sud-Chemie Inc., P.O. Box 32370, Louisville, KY 40232, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction pathways of the light hydrocarbons formation during propane and isobutane dehydrogenation
on Al-Cr catalyst have been studied. It was determined that the majority of light compounds are formed
not directly from initial paraffin, but from the main product of the paraffin dehydrogenation; olefin.
The sequence of the light compounds formation reactions includes: (1) dehydrogenation of the initial
paraffin with formation of targeted olefin; (2) consecutive hydrocracking of the newly produced main
olefin with formation of the one carbon shorter chain olefin and methane; (3) one carbon shorter chain
olefin can be converted by two parallel reactions further. The first reaction is hydrogenation of the one
carbon shorter chain olefin to the formation of the one carbon shorter chain paraffin. Another parallel
reaction is hydrocracking of the one carbon shorter olefin to the two carbon shorter olefin and methane.
If short olefin is ethylene the final product of the hydrocracking reaction is methane. This sequence
can be presented by using isobutane dehydrogenation as an example which also includes steps of light
compounds formation from C3 hydrocarbons:
Received 16 February 2010
Received in revised form 7 April 2010
Accepted 13 April 2010
Available online 20 April 2010
Keywords:
Light compounds
Al-Cr catalyst
Dehydrogenation
Thermo-cracking
Hydrocracking
Propane
Isobutane
C4H10 → C4H8 + H2
C4H8 + H2 → C3H6 + CH4
C3H6 + H2 → C3H8
(1)
(2)
(3a)
(3b)
(4a)
(4b)
C3H6 + H2 → C2H4 + CH4
C2H4 + H2 → C2H6
C2H4 + 2H2 → 2CH4
Obtained reaction pathways of light compounds formation can be applied to optimize the concept of
the dehydrogenation process and explain puzzling phenomena regarding the temperature profile in the
fixed bed dehydrogenation process.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
formed by different cracking reactions during paraffin dehydro-
genation.
The paraffin dehydrogenation process on Al-Cr catalyst has been
known for about 70 years. There are hundreds of published stud-
ies that investigated different aspects of the technology including
the catalyst and kinetics of reactions. However, there are still some
questions regarding this process that need to be answered. One of
these questions is the reaction pathway of the light compounds
formation such as methane, ethane, ethylene and others that are
To the best of our knowledge there are no published experi-
mental studies aimed to determine the reaction pathway of light
compounds formation during the dehydrogenation process. Our
guess is that this subject has never been considered as an important
one. In spite of the absence of the experimental studies regarding
the pathway of light compounds formation during dehydrogena-
tion, some authors presented their presumptions based on light
compounds composition how these reactions might have happen.
For instance, the authors [1] presented schematics of the reaction
pathways of all reactions that are observed during isobutane dehy-
drogenation indicating that light compounds are formed by direct
cracking of paraffin. Shmulevich et al. [2] studying kinetics of the
∗
Tel.: +1 502 634 7423; fax: +1 502 634 7486.
E-mail address: vladimir.fridman@sud-chemie.com.
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.04.026

140
V.Z. Fridman / Applied Catalysis A: General 382 (2010) 139–147
isopentane dehydrogenation suggested the model of the light com-
pounds formation that also says that they are formed directly from
paraffin:
After the 100 cycles of testing the catalyst was tested again for activ-
ity at standard conditions to verify if activity is stable. The results
of these tests always indicated that short testing of the catalyst at
different conditions did not change its initial performance.
The reaction pathway of the light gas formation during dehydro-
genation of paraffins was studied by analyzing the compositions
and relative selectivity of the light gas products as a function of
the reaction (contact) time. In the case of simple reactions this
method can provide preliminary information regarding the reac-
tion pathway. Furthermore the kinetic data were analyzed to obtain
dependence between the particular product reaction rates dCx/dt to
dCy/dt extrapolated to the zero reaction time, which should achieve
zero value in the case of consecutive reactions and be different from
zero and infinity in the case of parallel reactions [5]. The method of
separated reactions [5] also has been applied where conversion of
the possible intermediates such as propane, propylene, isobutane,
isobutylene, ethane and ethylene in the presence of hydrogen at
the reaction conditions was studied.
i-C5H12 → 0.5CH4 + 0.5C2 + 0.5C3 + 0.5C4H8
Authors of [3], on the basis of amount of light compounds
obtained after thermo-desorption of isobutane and isobutylene,
concluded that paraffin is the main producer of light compounds
on Al-Cr catalyst. It should be noted that it is pretty common view
on this subject.
However, our attempt to explain the main olefin selectivity pro-
file in the Al-Cr catalyst bed using this presumption did not match
to the experimental data, thus creating doubts that this assumption
is correct. Moreover, the chemistry of the cracking and hydroc-
racking reactions on the low acidity Al-Cr catalyst also does not
support the assumption that initial paraffin is a main source for
light compounds formation.
At the same time, we are convinced that understanding the
reaction pathway of light compounds formation during the dehy-
drogenation process is very important for development of a highly
selective catalyst. Furthermore, misconception regarding light
compounds pathway formation also can be detrimental to quality
of the engineering configuration of the dehydrogenation process. In
our view there was a necessity in verifying correctness of this pre-
sumption. The present study was conducted to give an experimen-
tal proof of the reaction pathway of the light compounds formation
during propane and isobutane dehydrogenation on Al-Cr catalyst.
3. Results and discussion
3.1. Reaction pathways of the light gas formation during propane
dehydrogenation on Al-Cr catalyst
During propane dehydrogenation on Al-Cr catalyst the light
compounds mixture formed by side reactions consists of methane,
ethane and ethylene. The light compounds from propane can be
formed by reaction of the thermo-cracking and catalytic cracking.
2. Experimental
3.1.1. Reactions of thermo-cracking during propane
dehydrogenation
Al-Cr catalyst prepared according to [4] with 20% of Cr2O3
has been used for the current study. The kinetic study was
conducted in the unit that simulated the cyclic dehydro-
genation process. This process operates in the cyclic mode
dehydrogenation–regeneration. The following steps were included
in one dehydrogenation–regeneration cycle: initial evacuation, the
catalyst reduction by H2, second evacuation, dehydrogenation for
10 min, purge, and regeneration for 15 min. The reaction prod-
ucts were analyzed by a HP 6890 gas chromatograph with special
configuration that provides simultaneous analysis of CO, CO2 and
hydrocarbons from methane up to butadiene. The separation of the
compounds was conducted with application of the molecular sieve
column Supelco 13061U and capillary column J&W Scientific.
The plug reactor with diameter 25.4 mm has been used. The
commercial pellets of the catalyst with a diameter of 3 mm were
loaded in the reactor. Because the fresh Al-Cr catalyst changes its
initial activity during the first few cycles (breaking in period) the
catalyst was aged to achieve stable catalyst performance. The aging
of the fresh catalyst was conducted at about 600 ◦C and for 300
cycles after which the catalyst activity achieved the constant value.
To verify a stable catalyst performance, in the end of the aging, the
catalyst was run for 10 cycles at 567 ◦C in propane dehydrogenation
at LHSV – 1 h−1 at a total dehydrogenation pressure in the reactor
of 0.5 atm, or in the case of isobutane dehydrogenation in the end of
the aging, the catalyst was run for 10 cycles at 567 ◦C in isobutane
dehydrogenation at LHSV – 2 h−1 and at a total dehydrogenation
pressure in the reactor of 0.5 atm These conditions have been cho-
sen as standard conditions. After completion of the aging test that
provided stable catalyst performance the sample was unloaded and
used for making the catalyst composite for testing. Before load-
ing, the catalyst was mixed with alpha-alumina chips. Changing of
the catalyst loading provided different reaction time from 0.22 up
to 1.84 s. Experiments were performed in the temperature range
between 540 and 630 ◦C. Each experiment was conducted three
times. The average value for yield has been used for calculation.
Initially the reaction of propane thermo-cracking was studied
where alpha-alumina inert with surface area 0.5 m2/g was loaded in
place of the catalyst. This experiment imitated the conversion of the
paraffin between catalyst pellets. Propane with purity 99.9% with
was passed through the reactor to provide contact time from 0.22
to 1.84 s and at 600 ◦C. Similar experiments were conducted with a
mixture of propane and hydrogen where hydrogen concentration
in the feedstock mixture was 30 mol%.
There are only few theoretical possibilities for formation of the
light compounds by thermo-cracking from propane and propylene
(reactions (1) and (2)).
C3H8 → C2H4 + CH4
(1)
(2)
C3H8 → C3H6 + H2; C3H6 + H2 → C2H4 + CH4
The outlet gas after propane thermo-conversion contained only
two compounds with carbon number less than C3 such as methane
and ethylene. This strongly suggests that propane decomposes only
into ethylene and methane by reaction (1). Besides these two com-
pounds a small amount of propylene and coke were formed during
propane thermo-decomposition.
It should be pointed out that the rate of propane thermo-
decomposition was relatively low. For instance, at 600 ◦C and a
reaction time of 1.84 s, the observed propane conversion was only
2.7%, but the maximum yield of all light compounds was lower than
1% (Fig. 1a).
Addition of the hydrogen in propane has a very strong inhibiting
effect on the rate of thermo-decomposition of propane (Fig. 1a). The
yield of ethylene and methane was down by about four times when
compared with conversion of propane without hydrogen (Fig. 1a).
One of the pathways of the thermo-cracking reaction occurs
via radical mechanism where the cleavage of the C–C bond under
high temperature starts from CH3–CH*–CH3 1- or 2-propyl radi-
cal. The assumption is that hydrogen also can generate radical H*
that is probably involved in the recombination of new produced

V.Z. Fridman / Applied Catalysis A: General 382 (2010) 139–147
141
Fig. 1. Yield of the light compounds formed during of propane (a) and isobutane (b) dehydrogenation as a function of reaction time: (1) paraffin without catalyst at 600 ◦C;
(2) paraffin–hydrogen mixture without catalyst at 600 ◦C; (3) on Al-Cr catalyst at 600 ◦C.
CH3–CH*–CH3 propyl radical and inhibits the thermo-cracking
reaction.
of this pathway is hydrocracking of propylene to ethylene and
methane (reaction (4)) and then ethylene can be hydrogenated to
ethane (reaction (6)).
Based on the results of the propane dehydrogenation conducted
at different contact time on Al-Cr catalyst the relative selectivity
between ethane to ethylene and between ethylene to ethane for
product produced during dehydrogenation of propane at 600 ◦C has
been calculated (Fig. 2).
3.1.2. Reactions of light compounds formation on the Al-Cr
catalyst during propane dehydrogenation
Dehydrogenation of propane was conducted using Al-Cr catalyst
with reaction times of 0.22, 0.45, 0.9 and 1.84 s at a dehydro-
genation pressure of 0.5 bar and temperatures ranging from 570
to 620 ◦C. Fig. 2 depicts the dependence between molecular con-
centration of the light compounds in outlet gas produced at 600 ◦C
during propane dehydrogenation and different reaction time.
Besides propylene, dehydrogenation of propane on Al-Cr cat-
alysts usually leads to formation of the light compounds such as
methane, ethane and ethylene (Fig. 2).
Theoretically the light compounds generated during propane
dehydrogenation might be formed from propane and propylene by
reaction of propane thermo-cracking (reactions (1) and (2)) or by
propane and propylene hydrocracking as follows:
As seen in Fig. 3 the ratio between the rates of ethylene to ethane
formation decreases as a function of reaction time and approaches
infinity when reaction time is extrapolated to zero, which is an indi-
cation that during propane dehydrogenation ethylene is not formed
from ethane. The ratio between the rates of ethane to ethylene
formation increases as a function of reaction time and approaches
zero when reaction time is extrapolated to zero. This dependence
is typical for consecutive reactions where ethane is produced from
ethylene [5]. Growth of ethane is provided by consumption of ethy-
lene that leads to increasing relative selectivity between ethane
and ethylene. Thus, this observation strongly suggests that dur-
ing propane dehydrogenation ethane is formed from ethylene by
consecutive reaction of ethylene hydrogenation.
The hydrogenation reaction of ethylene in the absence of ethane
is thermodynamically favorable even at 600 ◦C where the equi-
librium ratio between ethane and ethylene at a total pressure
0.5 bar is about 3–1. Any new molecule of ethylene formed from
C3 compounds in the mixture where there is no ethane breaks the
equilibrium of the system and promotes the consecutive hydro-
genation.
C3H8 + H2 → C2H6 + CH4
C3H6 + H2 → C2H4 + CH4
(3)
(4)
Newly produced ethane and ethylene might further be con-
verted by hydrogenation or dehydrogenation reactions into
correspondent olefin or paraffin by reactions (5) and (6).
C2H6 → C2H4 + H2
C2H4 + H2 → C2H6
(5)
(6)
To verify this presumption, ethylene conversion in hydrogen
has been studied on Al-Cr catalyst at 600 ◦C using different con-
tact times. The initial ethylene concentration was 65 mol%. As seen
from Table 1, ethylene can be hydrogenated on Al-Cr catalyst at
600 ◦C with conversion up to 75%. This supports the assumption
about fast hydrogenation of ethylene to ethane at these conditions
and confirms the suggested pathway of ethane formation through
hydrogenation of ethylene.
The second unknown question was a reaction pathway of
methane formation from C2 compounds. There are two possibil-
ities for production of methane from ethane or ethylene. Methane
from C2 hydrocarbons can be formed by hydrocracking of ethane
(reaction (7)), or by hydrocracking of ethylene (reaction (8)). Table 1
presents the product composition of the conversion of ethane and
ethylene in the presence of hydrogen. As seen from Table 1 the
conversion of ethane in the presence of hydrogen led to very
small amount of methane. The amount of methane started to be
noticeable when ethylene concentration produced by ethane dehy-
drogenation was high enough to start forming methane. Unlike
There is also a possibility for hydrocracking of ethylene and
ethane by reaction conversion of ethylene or ethane to methane
as drawn below:
C2H6 + H2 → 2CH4
C2H4 + 2H2 → 2CH4
(7)
(8)
The character of the curves shown in Fig. 2 is typical for consec-
utive reactions where methane and ethane are the final products
of the initial feedstock transformation and propylene and ethylene
are intermediate compounds for formation of ethane and methane.
The first question that needs to be answered is “Is ethane
formed from ethylene or is ethylene formed from ethane?”. There
is no doubt that ethane and ethylene are interconnected. During
dehydrogenation of propane they are converted into each other
to satisfy the equilibrium. Ethane can be produced by propane
hydrocracking by reaction (3)). Newly produced ethane can be
dehydrogenated into ethylene (reaction (5)). There is another path-
way whereas ethane can be produced differently. The first reaction

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V.Z. Fridman / Applied Catalysis A: General 382 (2010) 139–147
Fig. 2. Propylene and light compounds concentrations in the outlet gas generated during propane dehydrogenation on Al-Cr catalysts at 600 ◦C as a function of the reaction
time.
Fig. 3. Relative rates of the ethylene and ethane formation generated during propane dehydrogenation at 600 ◦C and extrapolated to zero reaction time.
Table 1
Conversion of hydrogen–ethane, hydrogen–ethylene mixtures and concentration of the light compounds in outlet gas produced on Al-Cr catalyst at different reaction time.
Feed composition: hydrogen – 35 mol%, ethylene – 65 mol%
Temperature (◦C)
Contact time (s)
Concentration of products after ethylenehydro-treatment (mol%)
Ethylene conversion (%)
CH4
0.95
1.54
4.4
C2H6
C2H4
0.23
0.46
0.92
1.84
26.9
35.06
22.06
21.46
17.01
27.1
42.7
67.3
75.4
48.56
50.02
52.4
600
7.92
Feed composition: hydrogen – 35 mol%, ethane 65 mol%
Temperature (◦C)
Contact time (s)
Concentration of products after ethane hydro-treatment (mol%)
Ethane conversion (%)
CH4
C2H6
C2H4
0.23
0.46
0.92
1.84
0.06
0.17
0.37
0.75
27.24
26.34
25.96
25.3
2.15
2.73
2.84
3.05
7.3
9.4
11.3
11.5
600

V.Z. Fridman / Applied Catalysis A: General 382 (2010) 139–147
143
ethane, the conversion of ethylene in hydrogen led to about 10
times higher yield of methane than that after conversion of ethane
in hydrogen at the same conditions. This strongly suggested that
methane is formed from ethylene. It should be noted that a very
similar conclusion was reported recently for formation of methane
during propane dehydrogenation on Pt-Sn catalyst [6].
assumption that hydrocracking of propylene is the main pathway
for ethylene formation not the thermo-cracking of propane.
The obtained data is strong evidence that the majority of the
light gas is formed not directly from propane, but from propy-
lene which is intermediate after dehydrogenation of propane by
consecutive reaction of hydrocracking.
The next question was “What is the reaction pathway of ethy-
lene formation?” During propane dehydrogenation ethylene can be
formed by hydrocracking of propylene (reaction (4)), or by thermo-
decomposition of propane (reaction (1)). Comparison of the total
amount of light compounds formed during propane dehydrogena-
tion on Al-Cr catalyst with the amount of light compounds formed
from propane by thermo-cracking without catalyst (Fig. 1) indi-
cates that thermo-cracking without hydrogen produces about six
times lower amount of light compounds than hydrocracking on Al-
Cr catalyst. In the presence of catalyst the rate of thermo-cracking
of propane to ethylene should be even lower because dehydro-
genation of propane leads to generation of hydrogen. As we already
mentioned before hydrogen strongly inhibits the propane thermo-
cracking reaction (Fig. 1).
Below is presented the pathway of the formation of light gases
during propane dehydrogenation:
C3H8 ꢀ C3H6 + H2
(1)
(2)
C3H6 + H2 → C2H4 + CH4
Ethylene by parallel reaction simultaneously with hydroge
(3)
Ethylene by parallel reaction simultaneously with hydrogena-
tion to ethane is converted by the hydrocracking reaction to
methane.
As it was already concluded before ethylene is not formed from
ethane, but it can be synthesized from propylene by hydrocrack-
ing (reaction (4)). In order to determine the relationship between
propylene and ethylene an analysis of relative selectivity was
conducted between ethylene and propylene as a function of the
reaction time. For ethylene selectivity it was necessary to take into
account the fact that a significant part of ethylene produced was
converted into ethane by the hydrogenation reaction. Thus, the
amount of the ethylene that was formed from the initial compound
is equal to the amount of ethylene left in the outlet gas plus the
amount of ethane formed from ethylene by hydrogenation to paraf-
fin. Fig. 4 depicts the ratio between rates of C2 compounds (ethane
and ethylene) and propylene formation generated during propane
dehydrogenation at 600 ◦C as a function of the reaction time.
As seen from Fig. 4, the ratio of the rates of ethylene formation
to propylene formation increases as a function of contact time and
goes to zero when reaction time is extrapolated to zero. This obser-
vation is typical for consecutive reactions where propylene is an
intermediate compound and source for formation of ethylene and
methane by reaction (4). However, this type of dependence can be
observed not only for a consecutive reaction, but also in the case
where one reaction is thermodynamically limited and another one
does not have thermodynamic limitations. This is the case where
propane dehydrogenation to propylene is limited by equilibrium,
but hydrocracking reaction does not have thermodynamic limita-
tion. Thus, the obtained results presented in Fig. 4 are not enough
to prove the presumption that ethylene is formed from propylene.
To confirm the correctness of this assumption, conversions
of propylene and hydrogen and propane and hydrogen mixtures
have been studied using Al-Cr catalyst at 600 ◦C with different
contact time. The results summarized in Table 2 showed that hydro-
treatment of propane do not produce a noticeable amount of light
gas. The main reaction during transformation of propane with an
excess of hydrogen was still dehydrogenation with production of
propylene.
3.2. Reaction pathways of the light gas formation during
isobutane dehydrogenation on Al-Cr catalyst
Isobutane dehydrogenation is accompanied by formation of
five compounds with hydrocarbon number less than C4: methane,
ethane, ethylene, propylene and propane. There are much more
theoretical possibilities for formations of these compounds from
isobutane than from propane.
3.2.1. Reactions of thermo-cracking during isobutane
dehydrogenation
Thermo-cracking of isobutane also have more options than
thermo-cracking of propane, which can happen by following reac-
tions presented below:
i-C4H10 → C3H6 + CH4
i-C4H10 → C2H4 + C2H6
i-C4H8 → C3H4 + CH4
i-C4H8 → 2C2H4
(9)
(10)
(11)
(12)
Unlike n-butane, the structures of isobutane and isobutylene
prohibit cracking of these compounds to two C2 molecules such as
ethane and ethylene that eliminates the possibility of reaction (10),
(12), and (17) but there is still possibility to produce C2 molecules
by reactions (14) and (15).
Very little amount of ethane and ethylene (Table 2) was pro-
duced during conversion of propane. The discernable amount of
light compounds during hydro-treatment of propane was obtained
only when propylene concentration was about 10%. In our opinion
in this case the newly produced propylene started to produce light
compounds.
Unlike the hydro-treatment of propane, hydrocracking of propy-
lene at the same conditions resulted in five times higher yield
of methane, ethane, and ethylene (Table 2). Simultaneously with
light gas, a significant amount of propane obtained by propy-
lene hydrogenation reaction was seen. This result supported initial
i-C4H10 + H2 → C3H8 + CH4
i-C4H10 + 2H2 → C2H6 + 2CH4
i-C4H10 + H2 → C2H4 + 2CH4
i-C4H8 + H2 → C3H6 + CH4
i-C4H8 + H2 → C2H6 + C2H4
(13)
(14)
(15)
(16)
(17)
Similar to propane, isobutane thermo-cracking has been studied
in the quartz plug type reactor which was loaded with ␣-Al2O3 inert

144
V.Z. Fridman / Applied Catalysis A: General 382 (2010) 139–147
Fig. 4. Relative rates of the ethylene and ethane formation to propylene generated during propane dehydrogenation at 600 ◦C and extrapolated to zero reaction time.
only, having a surface area of 0.5 m2/g. Isobutane having a purity
of 99.7% was passed through the reactor at different reaction times
and at 600 ◦C.
The results of these experiments are summarized in Fig. 1b. As
a result of the isobutane thermo-decomposition only two prod-
ucts with an amount of carbon less than C4 were observed in the
outlet gas: methane and propylene. No C2 compounds were seen
in the products. Besides methane and propylene only isobutylene
and coke were formed during thermo-decomposition of isobutane.
Similar to propane thermo-cracking, isobutane thermo-
conversion was inhibited by hydrogen significantly (Fig. 1b).
in the outlet gas as a function of the reaction time provides the first
clue regarding the pathway of their formation. The curves of the
propylene and propane concentrations as a function of the reaction
time are typical for that obtained for consecutive reactions where
propylene is the intermediate compound and propane is the final
compound.
There are no doubts that propylene and propane are inter-
connected and can be converted into each other depending on
equilibrium. The question was “What compound is formed first
during isobutane dehydrogenation?” Propane theoretically can be
formed from isobutane by reaction (13) following dehydrogenation
to propylene. Propylene theoretically can be formed from isobutane
by thermo-cracking reaction (10) and from isobutylene by hydro-
cracking reaction (16). If propylene is formed first, because the
shift in equilibrium when propylene appears in gas phase without
propane the hydrogenation reaction to propane should follow.
Fig. 6 presents the ratio between the rates of propane to propy-
lene and propylene to propane as a function of the reaction time.
As seen in Fig. 6, the relative amount of propylene to propane
decreases with increasing of the reaction time and approaches
infinity when reaction time is extrapolated to zero. This is an indi-
cation that propylene is not produced from propane. Contrary to
that the ratio between the rates of the propane and propylene for-
mation grows as function of reaction time and approach zero when
3.2.2. Reactions of light compounds formation on the Al-Cr
catalyst during propane dehydrogenation
Reaction pathway of light compounds formation during isobu-
tane dehydrogenation was studied by testing Al-Cr catalyst at
different temperatures in the range 540–620 ◦C and different reac-
tion times at dehydrogenation pressure 0.5 atm using the same
approach that was applied for propane dehydrogenation.
The results of dehydrogenation of isobutane on Al-Cr catalyst
at 600 ◦C presented in Fig. 5 demonstrated that the outlet gas con-
tained the products of the side reactions and included methane,
ethane, ethylene, propane and propylene. The character of the
curves presenting molecular concentration of light gas compounds
Table 2
Conversion of hydrogen–propane and hydrogen–propylene mixtures and concentration of the light compounds on Al-Cr catalyst at different reaction time.
Feed composition: hydrogen – 60 mol%, propane – 40 mol%
Temperature (◦C)
Contact time (s)
Concentration of products after propane hydro-treatment (mol%)
CH4
C2H6
C2H4
C3H6
0.23
0.46
0.92
1.84
0.02
0.08
0.22
0.51
0.01
0.04
0.11
0.26
0.02
0.03
0.06
0.07
3.76
6.41
9.99
10.92
580
Feed composition: hydrogen – 60 mol%, propylene – 40 mol%
Temperature (◦C)
Contact time (s)
Concentration of products after propylene hydro-treatment (mol%)
CH4
C2H6
C2H4
C3H8
0.23
0.46
0.92
1.84
0.2
0.07
0.24
0.54
1.43
0.09
0.18
0.45
0.76
7.3
9.4
11.3
11.5
0.55
0.11
2.47
580

V.Z. Fridman / Applied Catalysis A: General 382 (2010) 139–147
145
Fig. 5. Isobutylene and light compounds concentrations in the outlet gas generated during propane dehydrogenation on Al-Cr catalysts at 600 ◦C as a function of the reaction
time.
reaction time is extrapolated to zero. This is a clear indication that
propane formation is the consecutive reaction to the propylene for-
mation [5] and that propane is formed by consuming propylene by
reaction hydrogenation.
The next question was “What compound propylene is formed
from?” It can be formed from isobutane by reaction (10) or from
isobutylene from reaction (16) where isobutylene is an inter-
mediate compound formed by dehydrogenation of isobutane. To
answer this question relative selectivity of formation of propylene
to isobutylene was studied. The rate of propylene production was
calculated taking into account that propane produced was pro-
duced from propylene.
Fig. 7 depicts the ratio between of the rates of propylene and
isobutylene formation at 570 ◦C at different reaction times extrapo-
lated to a zero. This temperature is chosen because at this condition
the contribution of the reaction of conversion of propylene to C2
compounds is still very little, which makes it easier to interpret the
results.
tive reactions of isobutylene hydrocracking. However, this data is
not enough to prove this point. This type of dependence between
relative selectivity of propylene to isobutylene can be observed in
the case whereas propylene is formed from isobutane by thermo-
cracking (reaction (10)) and hydrocracking (reaction (13)), which
can grow without thermodynamic restrictions as a function of reac-
tion time and where isobutane dehydrogenation to isobutylene is
restricted by equilibrium.
To have an additional argument to determine whether propy-
lene is formed from isobutylene or isobutane hydrocracking of
isobutane with 40 mol% of hydrogen and isobutylene with 40 mol%
of hydrogen was studied at similar conditions and the results were
compared to each other. The results of this study presented in Fig. 8
show that the yield of C3 compounds is about 2.5 times higher when
using isobutylene feedstock instead of isobutane feedstock.
The observation that isobutane conversion in the presence
of hydrogen still leads to the production of C3 products can be
explained by the high rate of reaction of isobutane dehydrogena-
tion to isobutylene even at a high partial pressure of hydrogen.
Because this reaction is still very fast, following hydrocracking of
The character of the curves presented in Fig. 7 indicates that
there is a high probability that propylene is produced by consecu-
Fig. 6. Relative rates of the propylene and propane formation generated during isobutane dehydrogenation at 600 ◦C and extrapolated to zero reaction time.

146
V.Z. Fridman / Applied Catalysis A: General 382 (2010) 139–147
(3)
(4)
Thus, unlike previously expressed common view on the light
compounds formation pathway [1–3,7] stating that the main
source of the light compounds formation during paraffin dehy-
drogenation is initial paraffin, the current study showed that light
gas is mostly formed from targeted olefin such as propylene or
isobutylene by consecutive reaction of hydrocracking and hydro-
genation.
The suggested reaction pathway of the light gas formation on Al-
Cr catalyst is in a very good agreement with the carbonium ionic
theory of the reaction cracking and hydrocracking, which postu-
lates that the first step of the reaction is a formation of a carbonium
cation. As follows from review of Pines [8], weak acids can cat-
alyze the conversion of olefins in which the double bond is attacked
resulting in formation of a carbonium cation. The strong super acid
besides double bond can attack stable alkanes [8]. However, these
types of acid sites do not exist on the surface of Al-Cr dehydrogena-
tion catalyst that is designed to suppress strong acid sites by alkali
metal.
The information regarding light compounds formation path-
ways is extremely useful for understanding of the existing paraffin
dehydrogenation technology and optimization of the process.
For instance, the fixed bed dehydrogenation process on Al-Cr
catalyst has a puzzling phenomenon that did not have clear
explanation before such as hot spot formation on the bottom
of the catalyst bed. Knowing correct reaction pathway of light
gas formation helps to understand it much better. An analy-
sis of the thermo-chemistry of the light compounds formation
reactions presented in this article indicated that the majority of
them are exothermal (Table 3). The reaction of ethylene hydro-
cracking to methane is very exothermic (Table 3). No doubts
Fig. 7. Relative rates of the propane and propylene formation to isobutylene gener-
ated during isobutane dehydrogenation at 570 ◦C and extrapolated to zero reaction
time.
isobutylene leads to propylene production. The rate of the hydro-
cracking reaction with formation of methane and propylene is a
function of partial pressure of isobutylene. During dehydrogena-
tion of isobutane on Al-Cr catalyst the partial pressure of newly
formed isobutylene in these experiments increased from 0 to
25 mol%, which further is converted to propylene. However, dur-
ing isobutylene hydrocracking the isobutylene partial pressure was
significantly higher, and that provided the higher yield of C3 prod-
ucts.
Thus, the observation that isobutylene feedstock produces much
more propylene and propane on Al-Cr catalyst can be an argument
that isobutylene is a main source of these compounds formation.
The further transformation of propane and propylene on Al-Cr cat-
alyst was already discussed and determined before.
Below is presented the pathway of the formation of light gases
during isobutane dehydrogenation:
C4H10 ꢀ C4H8 + H2
(1)
(2)
C4H8 + H2 → C3H6 + CH4
Fig. 8. Concentration of the C3 products (propylene and propane) generated during conversion of the isobutylene–hydrogen and isobutane–hydrogen mixtures on Al-Cr
catalysts at 600 ◦C at different reaction time.

V.Z. Fridman / Applied Catalysis A: General 382 (2010) 139–147
147
Table 3
Heat of the reactions dehydrogenation and cracking occur during isobutane and propane dehydrogenation.
Reaction
Name and type of reaction
Heat of reaction at 600 ◦C (J/mol)
Main reaction
i-C4H10 → i-C4H8 + H2
Dehydrogenation–endothermic
Dehydrogenation–endothermic
121,800
128,290
C3H8 → C3H6 + H2
Reaction of the light gas formation
i-C4H10 → C3H6 + CH4
i-C4H8 + H2 → C3H6 + CH4
C3H6 + H2 → C3H8
Thermo-cracking–endothermic
Hydrocracking–exothermic
Hydrogenation–exothermic
Hydrocracking–exothermic
Hydrogenation–exothermic
Hydrocracking–exothermic
78,470
−43,330
−128,290
−47,200
−141,900
−211,000
C3H6 + H2 → C2H4 + CH4
C2H4 + H2 → C2H6
C2H4 + 2H2 → 2CH4
that these reactions contribute to the above described phe-
nomenon.
and methane. The last reaction is hydrocracking of ethylene to
methane.
4. Conclusions
References
[1] F. Buonomo, D. Sanfilippo, F. Trifiro, G. In, H. Ertl, J. Knözinger, Weitkam (Eds.),
Handbook of Heterogeneous Catalysis, vol. 5, Wiley-VCH, 1997, pp. 2140–2151.
[2] E.A. Shmulevich, M.E. Basner, D.A. Bolishakov, V.I. Korobov, A.I. Saltukov, A.I.
Nefedova, N.F. Opechova, VINITI, No. 841-74, Moscow, 1974, pp. 1–22.
[3] G.G. Abbasova, V.S. Gadzhi-Kasymov, B.A. Dadashev, VINITI, No. 3662-84,
Moscow, 1984, pp. 1–14.
From above presented results the following conclusions have
been made:
•
The main sources for production of light compounds during
dehydrogenation of propane and isobutane are propylene and
isobutylene. The contribution of the direct conversion of paraffin
to light compounds is much smaller.
[4] E. Cornelius, S.H. Milliken, US Patent 2,956,030.
[5] N.N. Lebedev, M.N. Monakov, V.F. Shvez, Chemistry, Moscow, 1984.
[6] G.G. Siddigi, P. Sun, V. Galvita, A. Bell, Dehydrogenation of paraffin using Pt-
Sn/Mg(Al)O: catalyst design and characterization, in: Preceedings at 21st North
American Catalysis Society Meeting, San Francisco, CA, USA, June 7–12, 2009.
[7] S.D. Jackson, I.M. Mathenson, M.L. Naeye, P.C. Stair, V.S. Sullivan, S.R. Watson, G.
Webb, in: F.V. Melo, S. Mendioroz, J.L.G. Fierro (Eds.), Studies in Surface Science
and Catalysis, 130, Elsevier Science B.V., 2000, pp. 2213–2218.
•
The main pathway of the light compounds formation during
propane and isobutane dehydrogenation includes hydrocrack-
ing of propylene and isobutylene to the one carbon shorter chain
olefin and methane. The newly formed one carbon atom shorter
olefin further by parallel reactions is hydrogenated to paraffin,
or by hydrocracking is converted to two carbon shorter olefin
[8] H. Pines, The Chemistry of Catalytic Hydrocarbon Conversion, Academic Press,
New York, 1981.