20th Annual Saudi-Japan Symposium
Catalysts in Petroleum Refining & Petrochemicals
Dhahran, Saudi Arabia - December 2010
Catalytic Aromatization of Alkanes
M. Naseem Akhtar, Sulaiman S. Al-Khattaf
Center of Research Excellence in Petroleum Refining and Petrochemicals
King Fahd University of Petroleum & Minerals
Dhahran 31261, Saudi Arabia
1. Introduction
The conversion of light alkanes into aromatics is an important catalytic reaction from both
industrial and academic view points. Indeed light alkanes aromatization leads to the
formation of the higher value aromatic hydrocarbons (mainly, benzene, toluene and xylene
(BTX)), which are valuable intermediates in the chemical and petrochemical industries.
This reaction is highly complex reaction which involves transformation of various
hydrocarbons into different reaction steps.
The aromatization of light alkanes over ZSM-5 catalyst [1-8] can be explained on the
bases of a three stage process as below:
1. Transformation of alkanes into alkenes
2. Interconversion of alkenes into higher alkenes
3. Aromatization of alkenes.
The objective of this study was to understand the effect of Ga/(Ga+Al) and Si/(Ga+Al)
ratios in Ga-Al-MFI zeolite on propane conversion, aromatic yield, acidity and product
selectivity during the aromatization of propane.
2. Experimental
A series of H-Ga-Al-MFI zeolites were synthesized by the hydrothermal crystallization.
The resulting zeolite catalyst samples were characterized using XRD, XRF and NH3 TPD
techniques.
The propane aromatization reaction was carried out in a fixed bed reactor system with a
continuous flow of feed gases under atmospheric pressure at 540 ºC. The reaction products
were analyzed using on-line gas chromatograph equipped with TCD and FID detectors.
3. Results and Discussion
In this study we have synthesized two series of Ga-Al-MFI zeolites. In first series we
have changed the Ga/(Ga+Al) ratio while the ratio of Si/(Ga+Al) was kept constant. In the
second series of zeolites the Si/(Ga+Al) ratio was varied while keeping Ga/(Al+Ga) ratio
almost constant.
3.1 Effect of Ga/(Ga+Al) Ratio :
In this study we have synthesized a series of six Ga-Al-MFI zeolites (Cat-1 to Cat-5)
having Ga/(Ga+Al) ratio varied from 0.0 to 0.6. The Si/(Ga+Al) ratio was kept constant
during the synthesis of these zeolites so that the direct effect of Ga/(Ga+Al) ratio on the
aromatization of propane can be studied. The results are presented in Figure-1. It has been
observed that there is a tremendous increase in propane conversion and aromatic yield
when gallium has been added to the MFI zeolite. The propane conversion was increased
from 34 % to 86 % (an increase of ~150%) when Ga/(Ga+Al) ratio was increased from 0.0
to 0.1. In the similar way the aromatic yield was also observed to increase from 14 to 67
(~390 %) when Ga/Al ratio was increased from 0.0 to 0.1. Then there was small increase
in propane conversion (from 86 to 89 %) and aromatic yield (from 67 to 72) when
Ga/(Ga+Al) ratio was increased from 0.1 to 0.3. However when Ga/(Ga+Al) ratio was
further increased from 0.3 then both propane conversion and aromatic yield were observed
to decrease. Therefore a maximum value of propane conversion and aromatic yield was
obtained at the ratio of Ga/(Ga+Al) of 0.3. It has been reported in the literature [9-12] that
when gallium is incorporated in MFI zeolite then the performance of zeolite for LPG
aromatization is increased, which support our observation. It has been observed that D/C
ratio increases with increase in the ratio of Ga/Al. This observation also support our
conclusion that amount of N-FW work gallium species increases with increase in Ga/Al
ratio. It has been reported in the literature [8] that gallium species present in the
framework of MFI zeolite is responsible for dehydrogenation, oligoemerization and other
series of reactions responsible to produce aromatics from lower alkanes. However N-FW
gallium species are mainly responsible for dehydrogenation reactions and result into
increase in D/C ratio.
It has been observed that when Ga/(Ga+Al) ratio is zero then C1, C2, C2” and C3” are the
dominant product fractions as compared to the aromatics. However when gallium has been
incorporated in the MFI zeolite then major product fraction is aromatic as compared to all
other products. It is well established that aromatization of propane is accompanied with
several side reactions like cracking and dehydrogenation reactions. It has been reported in
the literature [10,13] that number of strong acid sites decreases when aluminum species
are replaced with gallium species. Therefore an MFI zeolite without any gallium exhibited
more cracking and dehydrogenation reactions. However when gallium was incorporated in
MFI zeolite then aromatization reaction became predominant over other side reactions.
3.2 Effect of Si/(Ga+Al) Ratio :
In this study we have synthesized a series of seven Ga-Al-MFI zeolites (Cat-6 to Cat-12)
having Si/(Ga+Al) ratio varied from 10 to 44. We have kept the Ga/(Ga+Al) ratio constant
during the synthesis of these zeolites so that we can see the direct effect of Si/(Ga+Al)
ratio on the aromatization of propane. The results are presented in Figure-2. It has been
observed that propane conversion and aromatic yield increases with increase in Si/(Ga+Al)
ratio from 10 to 14. However after this both propane conversion and aromatic yield
continuously decrease with increase in Si/(Ga+Al) ratio of MFI zeolite. It has been
observed that acidity of MFI zeolite decreases linearly with increase in Si/(Ga+Al) ratio. A
similar behavior has been reported in the literature [14, 15]. However when the ratio of
Si/(Ga+Al) further increases from this optimum value then there is a continuous drop in
both propane conversion & aromatic yield. A similar trend has been reported by Phatansri
et. al [12]. In their work Si/(Ga+Al) ratio was changed by changing the gallium amount,
while the Si/Al ratio was fixed. It was observed that C3 conversion and aromatic yield
were increased with decrease in Si/(Ga+Al) ratio.
3.3 Effect of Calcination temperature :
In this study we have subjected the Ga-Al-MFI zeolite to various calcination temperatures
to see the effect of calcination temperature on the aromatization of propane. In this case
we have selected four different temperatures ranging from 500 C to 650 ⁰C.
It has been observed that propane conversion was increased from 60 % to 76 % with
increase in calcination temperature from 500 to 550 ⁰C. The aromatic yield was also
observed to increase from 43 to 63 % with increase in calcination temperature from 500 to
550 ⁰C. However when the calcination temperature was further increased from 550 ⁰C
then there was continuous drop in both propane conversion and aromatic yield.
It has been observed that total acidity of MFI zeolite decreases linearly with increase in
calcination temperature due to dehydroxylation of zeolite. In a similar study by Choudhary
et. al [14] the effect of calcination temperature on gallium impregnated HZSM-5 was
studied. It was observed that when calcination temperature was increased from 600 to 800
⁰C the acidity of zeolite was dropped along with drop in aromatization activity. It has been
reported in the literature [13] that degalliation take place during thermal treatment of GaAl-MFI zeolite. The degalliation results into the conversion of some of the frame work
(FW) gallium species to the non-frame-work (N-FW) gallium species. Whenever
calcination temperature is increased the degalliation process is further enhanced and result
into the increase in N-FW gallium species. The gallium species present in the frame-work
of MFI zeolite are generally involved in dehydrogenation and dehydrocylization reactions,
whereas protonic acidity of zeolite is involved in oligomerization and dehydrocylization
reactions. Therefore overall propane aromatization is controlled by both protonic acidity
of zeolite as well as by gallium species present in MFI zeolite.
4. Conclusion :
We can draw following conclusions based on this research work:
1. The Ga/(GA+Al) and Si/(Ga+Al) ratios of Ga-Al-MFI zeolite have been observed
to have profound effect on the aromatization of propane.
i. An optimum ratio of Ga/(Ga+Al) of 0.3 and Si/(Ga+Al) of ~13-18 resulted
into maximum C3 conversion and aromatic yield.
ii. The acidity of Ga-Al-MFI zeolite was observed to decrease linearly with
the increase in Si/(Ga+Al) ratio.
iii. The product distribution was observed to be strongly affected by both
Ga/(Ga+Al) and Si/(Ga+Al) ratios.
2. The thermal pretreatment of Ga-Al-MFI zeolites has been observed to have strong
influence on the aromatization of propane.
i. The calcination temperature of 550 ⁰C was observed to be the optimum for
C3 conversion and aromatic yield.
ii. The total acidity of Ga-Al-MFI zeolites was observed to decrease linearly
with increase in calcination temperature.
5. Acknowledgment :
We would like to express our appreciations to King Abdulaziz City for Science &
Technology (KACST) for their financial support under project # 12-3-MT. We also want
to extend our appreciation to the support from the Ministry of Higher Education, Saudi
Arabia in establishment of the Center of Research Excellence in Petroleum Refining and
Petrochemicals at KFUPM.
6. References
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(2) N. S. Gnep, J. Y Doemet, A. M. Seco, R. F. Ramoa, M. Guisent, App. Cat., 35,
(1987), 93-108.
(3) M. Guisnet, N. S. Gnep, App. Cat. A., 146, (1996) 33.
(4) W. O. Haag, (1984), Proceeding of 6th international zeolites conference,
Butterworths, Surrey, UK, 466-475.
(5) H. Kitagawa, Y.Sendoda, Y.Ono, J. Cat., 101, (1986), 12-18.
(6) B. S. Kwak, W. M. H. Sachtler, J. Cat., 145, (1994), 456-463.
(7) B. S. Kwak, , W. M. H. Sachtler J. Cat., 149, (1994), 465-473.
(8) D. B. Lukyanov, N. S Gnep, M. R. Guisnet Ind. Eng. Chem. Res., 34, (1995), 516523.
(9) D. B.Lukyanov, T. Vahnova, Appl. Catal. A. Gen., 316, (2007), 61-67.
(10) G.Giannetto, A. Montes, N. S. Gnep, A. Florentino, P. Cartraud, M. Guisnet, J.
catal., 145, (1993), 86-95.
(11) N. Viswandham, G. Muralidhar, T. S. R. P. Rao, J Mol Catal A: Chem. 223, (2004),
269-274.
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(13) T. V. Choudhary, A. K. Kinage, S. Banerjee, V. R. Choudhary, Microporous &
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Table-1. Various Elemental ratios and acidity of catalyst samples
Si/
Ga/
*Acidity
Si/Al Si/Ga Ga/Al (Ga+Al) (Ga+Al) (mmol/g)
17.8
0.0
0.0
17.8
0.0
1.26
18.7 114.7
0.2
16.1
0.1
1.28
21.0 51.3
0.4
14.9
0.3
1.32
32.8 38.5
0.9
17.7
0.5
1.26
46.8 35.3
1.3
20.1
0.6
1.17
14.6 37.6
0.4
10.5
0.3
1.45
17.8 45.8
0.4
12.8
0.3
1.28
27.1 55.1
0.5
18.2
0.3
1.2
29.7 69.3
0.4
20.8
0.3
1.15
37.6 87.7
0.4
26.3
0.3
1.02
41.3 101.2
0.4
29.3
0.3
0.90
62.3 145.3
0.4
43.6
0.3
0.78
Total conversion"
Copnversion to aromatics
100.0
90.0
80.0
Conversion %
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Ga/(Ga+Al) Ratio
Figure-1. Plot of C3 total conversion and conversion to aromatics against Ga/(Ga+Al)
ratio during C3 aromatization using Ga-Al-MFI catalysts.
Total conversion"
Conversion to aromatics
100.0
90.0
80.0
Conversion %
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
0
5
10
15
20
25
30
35
40
45
50
Si/(Ga+Al) Ratio
Figure-2. Plot of C3 total conversion and conversion to aromatics against Si/(Ga+Al) ratio
during C3 aromatization using Ga-Al-MFI catalysts.
90
C3 conversion
80
Aromatic yield
70
60
50
40
30
20
10
0
400
450
500
550
600
650
700
Calcination Temperature °C
Figure-3. Plot of C3 total conversion and aromatic yield against calcination temperature
of zeolite during C3 aromatization using Ga-Al-MFI catalysts.