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20th Annual Saudi-Japan Symposium

Catalysts in Petroleum Refini ng & 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 bothindustrial 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 acontinuous 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.

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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 beenobserved 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. Asimilar behavior has been reported in the literature [14, 15]. However when the ratio of

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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 Ga-

Al-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 resultinto 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 observedto 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.

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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

(1) J. A. Biscardi, E. Iglesia, , J. Phys. Chem. 102, (1998) B, 9284-9289.

(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), 516-

523.(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.

(12) S. Phatansri, P. Praserthdam, A.Sripusitto, Korean, J. Chem. Eng., 17, (2000), 409-

413.

(13) T. V. Choudhary, A. K. Kinage, S. Banerjee, V. R. Choudhary, Microporous &

Mesoporous Mater., 87, (2005), 23-32.

(14) V. R.Choudhary, K. Mantri, C. Sivadinarayana, Microporous Mesoporous Mater.,

37, (2000), 1-8.

(15) H. You, Petroleum Science & Technology 24, (2006), 707-716.

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Table-1. Various Elemental ratios and acidity of catalyst samples

Si/Al Si/Ga Ga/Al

Si/

(Ga+Al)

Ga/

(Ga+Al)

*Acidity

(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

Figure-1. Plot of C3 total conversion and conversion to aromatics against Ga/(Ga+Al)

ratio during C3 aromatization using Ga-Al-MFI catalysts.

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6

C

o n v e r s i o n

%

Ga/(Ga+Al) Ratio

Total conversion" Copnversion to aromatics

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Figure-2. Plot of C3 total conversion and conversion to aromatics against Si/(Ga+Al) ratio

during C3 aromatization using Ga-Al-MFI catalysts.

Figure-3. Plot of C3 total conversion and aromatic yield against calcination temperature

of zeolite during C3 aromatization using Ga-Al-MFI catalysts.

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0 5 10 15 20 25 30 35 40 45 50

C o n v e r s i o n %

Si/(Ga+Al) Ratio

Total conversion"

Conversion to aromatics

0

10

20

30

40

50

60

70

80

90

400 450 500 550 600 650 700

C3 conversion

Aromatic yield

Calcination Temperature °C

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