ELSEVIER Applied Catalysis A: General 116 (1994) 71-79

applied catalysis A

Zeolite beta catalyzed C 7 and C 9 aromatics transformation

Jagannath Das a, Yajnavalkya S. Bhat a, Ashish I. Bhardwaj b, Anand B. Halgeri a'*

aResearch Centre, Indian Petrochemicals Corporation Limited, Baroda 391346, India bDepartment of Physics, Indian Institute of Technology, Kharagpur 721302, India

Received 1 October 1993; revised 8 February 1994; accepted 2 May 1994

Abstract

Transformation of commercial C 7 and C 9 aromatic streams to xylenes over zeolite beta have been studied. These results have been compared with those obtained on other zeolites like ZSM-5 and mordenite. The effect of variations in reaction parameters like temperature, weight hourly space velocity (WHSV) and feed composition on xylene formation was considered. The trimethylbenzenes became disproportionated, while ethyltoluenes underwent de-ethylation followed by disproportiona- tion of product toluene. The highest yield of xylenes was obtained with a feed consisting of nearly equal proportions of toluene and 1,3,5-trimethylbenzene. Lower WHSV and higher temperature enhanced the formation of xylenes.

Keywords: Aromatics; C7 aromatics; C9 aromatics; Transformation; Xylenes; Zeolite beta

1. Introduction

In aromatics complexes of the petrochemical industry, xylenes production is usually optimized by upgrading low value C7 and C9 aromatics (AT and A9)

streams. The xylenes are important starting materials for various processes like production of synthetic fibres and plasticizers. Although m 7 transformation is well documented in the open literature [ 1,2], transformations involving m 7 and A 9

mixtures are in the form of patents [ 3,4]. The commercial transalkylation processes employ either silica-alumina or zeolites like noble metal incorporated dealuminated mordenite (M) catalysts [ 5 ].

*Corresponding author. Tel. ( +91-265)72011, fax. ( +91-265)72098.

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72 J. Das et al. /Applied Catalysis A: General 116 (1994) 71-79

Of late considerable interest is being generated on catalytic aspects of zeolite beta (fl). This zeolite has been scarcely used in hydrocarbon transformation reac- tions. It is a high silica, large pore zeolite which possesses a three-dimensional, 12- membered ring pore system. It is the only large pore zeolite to have chiral pore intersections, fl has a near random degree of stacking faults and still maintains its full sorption capacity of about 0.2 ml /g [ 6 ]. Isomerization of meta-xylene, alkanes cracking and toluene methylation over it have been reported [7,8]. Tsai et al. [ 9,10 ] have dealt with stability enhancement during cumene disproportionation by silica deposition and steam pretreatment of ft. Disproportionation of toluene and trimethylbenzenes (TMB) and their transalkylations over/3 have been studied [ 11 ]. This paper involves the study of only three A 9 compounds and pure toluene. The performance of zeolite fl for commercial m 7 and A 9 transalkylation has not been reported in the literature. Considering this, the present work was aimed at exploring the potential of /3 for transformation of A 7 and m 9 s t r e a m s to xylenes using a commercial feed stock.

2. Experimental

fl was synthesized by a hydrothermal technique under autogenous pressure fol- lowing the procedure reported earlier [ 12]. The synthesis batch constituted 5 (TEA)20:A1203:30 SIO2:3 Na20:1500 H20. The crystallization period was for 7 days. After the crystallization the zeolite was separated from the mother liquor, washed thoroughly with hot distilled water and dried in an oven at 110°C for 6 h. The as-synthesized material was characterized by several physico-chemical tech- niques. The X-ray diffraction (XRD) pattern matched with the earlier published data. The crystals were of spheroidal shape and size was about 0.4 to 0.6/xm. Si/ A1 ratio of the zeolite was around 35.27A1 nuclear magnetic resonance indicated an absence of any extra framework aluminium.

The as-synthesized zeolite was calcined at 500°C and further ion-exchanged repeatedly with 1 M ammonium nitrate solution. The ammonium form of the zeolite was converted to the proton form by calcining at 500°C. Temperature-programmed desorption (TPD) of ammonia was carded out to obtain information on acidity of the proton form zeolite.

The reaction runs were carried out in a fixed bed, continuous flow, glass reactor at atmospheric pressure. 1-2 g of the zeolite in proton form was used in the catalytic experiments. The details of the experimental set up are reported elsewhere [ 13 ]. m 7 and A 9 streams were mixed in different proportions and fed to the reactor. In the case of the A 7 stream pure toluene and different toluene enriched feeds from the petrochemical complex were used, while the A 9 stream was obtained from naphtha reformate.

J. Das et al. /Applied Catalysis A: General 116 (1994) 71-79 73

3. Results and discussion

Fig. 1 compares the performance of/3 with other zeolites normally used for A 7

and A 9 transformation, ZSM-5 and dealuminated M. All the zeolites were of nearly the same Si/A1 ratio of around 35. The dealuminated M showed a high conversion for both A7 and A 9 in the beginning, which decreased with time-on-stream./3 also showed initially a higher conversion which decreased with time-on-stream up to the third hour; after that it became stabilized and exhibited a steady conversion. In the steady conversion region both A7 and A 9 conversions to xylenes were highest on zeolite/3. These results reflect the type of channel and pore dimension of the three zeolites and can be explained as follows. Mordenite possesses 12-membered ring straight channels of dimension 6.7 × 7.0 ~, which are connected by 8-membered

o

ring channels of dimension 2.9× 5.7 A. The pore system is constituted of non-intersecting channels. Deactivation occurs through pore blockage. The first coke molecules formed in the large channels are retained because of their low volatility. One coke molecule is enough to inhibit the diffusion of the reactant to the active sites of the channels [ 14]. ZSM-5 has a 10-membered ring pore system consisting of two intersecting channels of dimension 5.4 × 5.6 ,~ and 5.1 × 5.6 ,~. This channel system can easily sorb molecules like toluene and xylenes but not 1,3,5-trimethylbenzene. ZSM-5 has interconnecting channels and is without cavi- ties. Coke forms in the intersections of the zeolite [ 9,14 ]. ZSM-5 can tolerate much higher coke deposition than mordenite. The coke toxicity for ZSM-5 is lower than that for other zeolites. Three interconnecting twelve-ring pore systems constitute the channel structure of/3. The pore opening in the linear channel is 5.7 X 7.5 ,~. The tortious channel is formed by the intersection of two linear channel systems of

60 - ~ A l 9 onM

_ SOl'=x i~, A 9 on [3

30

201= _. X .~ . = ~ ~ @

J - - ~ . \ A 7 on ZS-M-5 101- x ~ \

9on ZSM-5~ OI ~ M ; 5 AT°nM

0 60 120 180 240 300 360 Time on stream (min)

Temp.=400°CsWHSV=3.61~ls H2/HC--4 Fig. |. Performance comparison of M, Z_~M-5 and/3 for transformation of A7 and A9 aromatics.

74 J. Das et al. /Applied Catalysis A: General 116 (1994) 71-79

60 12

I

40 ~ 9 C°nv / 1 0

30- ~

6

20

.

I0 P2 I f )

I I I ~)J00 0 25 50 75 1

A 9 content, ( w T . " ) Te.mp.= 400°C, WHSV= 3.6h "1, H2/HC=4

Fig. 2. A 9 content in the feed and its effect on catalytic activity. o

dimension 5.6 × 6.5 A. Hence the accessibility of m 7 and A 9 to the inside of the zeolite is easier, the catalytic activity for transalkylation becomes steady once the strong acid sites are covered by coke. This is in line with the observation of Guisnet and Magnoux [ 14] that coking and deactivation rates as well as coke composition depend on the pore structure and characteristics of the active sites, their strength and density.

Fig. 2 presents the results of the runs on/3 in which the m 9 stream was added to toluene in different proportions. The A 9 content was varied from 0 to 100 wt.-% in the feed. The xylenes formation was maximum for a feed composition around 50:50 toluene and A 9. The A 9 conversion decreased continuously, which indicated that disproportionation of trimethylbenzenes takes place to a maximum extent at nearly the same proportions of m 7 and m 9 in the feed. While with higher toluene concen- trations in the feed disproportionation takes place. Stoichiometrically one molecule of toluene reacts with one molecule of trimethylbenzene during transalkylation to give two molecules of xylene, whereas in disproportionation two molecules of either m 7 or A 9 react to yield only one molecule of xylene.

Table 1 shows the composition of different commercial A 7 and m 9 streams used in this study. The transformations of the three different A 7 streams mixed with A 9 streams in 50:50 proportion are reported in Table 2. Maximum xylene yield was obtained with a feed mixture containing 50% A 7 stream 1 and 50% m 9 stream.

In order to understand the mechanism of A 7 and A 9 transformation over/3, the runs were carded out with individual m 9 compounds like 1,3,5-, 1,2,4-, 1,2,3- trimethylbenzene, o-ethyltoluene, m-ethyltoluene and p-ethyltoluene and also

J. Das et al. /Applied Catalysis A: General 116 (1994) 71-79

Table 1 Composition of different A 7 and A 9 streams used in the study

75

Composition A 7 Stream

1 2 3

A 9 Stream

Paraffinic C 6- 0.70 Toluene 100 87.01 80.94 Benzene 4.11 17.43 Ethylbenzene 6.78 0.11 0.12 Xylenes 0.75 0.05 0.18 n-Propylbenzene 2.03 p-Ethyltoluene 7.13 m-Ethyltoluene 13.67 o-Ethyltoluene 7.26 1,3,5-TMB 9.81 1,2,4-TMB 42.97 1,2,3-TMB 15.64 Other aromatics 1.35 0.77 1.19

mixed with toluene. It is quite clear from these experiments that in addition to disproportionation, isomerization of m 9 takes place. All three trimethylbenzenes produced xylenes through disproportionation, while o-, p- and m-ethyltoluenes formed xylenes through disproportionation of toluene generated in de-ethylation. As a result the total xylenes yield was much less than that obtained from trime- thylbenzenes. In case of the toluene-trimethylbenzene mixture, the amount of xylenes formed was higher than with either pure toluene or trimethylbenzenes. The toluene-ethyltoluene mixture produced larger amounts of xylenes than did pure

Table 2 Transformation of three A 7 streams mixed with A 9 stream in nearly 50:50 proportion over/3

Components Feed Product Feed Product Feed Product

C6- - 1.10 - 0.22 - 0.23 Benzene - 2.86 2.07 2.32 8.57 6.78 Toluene 49.40 39.11 42.54 38.93 40.65 34.75 EB 0.06 2.68 3.29 2.92 0.06 4.55 p-Xyl. 0.03 6.36 0.06 4.10 0.02 4.56 m-Xyl. 0.04 13.66 0.11 7.65 0.01 9.49 o-Xyl. 0.02 5.99 0.06 3.48 0.04 4.38 n-Pr benz. 1.03 0.02 1.35 0.29 1.32 0.25 Et Tols. 14.20 5.84 13.22 10.07 12.83 8.68 TMBs 34.62 18.70 35.80 25.46 35.43 21.28 Clo A 0.60 3.66 1.51 4.56 1.07 5.05 Total xylene 26.01 15.23 18.43 Xylene composition

p-Xyl. 24.45 26.92 24.74 m-Xyl. 52.52 50.23 52.49 o-Xyl. 23.03 22.85 23.77

Conditions: Temperature = 400°C, WHSV = 3.6 h - l, H2/HC = 4.

76 J. Dasetal./AppliedCata~sisA:Genera1116(1994) 71-79

I . DISPROPORTIONATION REACTIONS

2 C6H5--CH 3 ~ ~ C6H 6 + CH3~C6H4~CH 3

Toluene Benzene Xylenes

CH 3 CH 3 I I

2 CH3---C6H3--CH3~------ CH3--C6H4--CH 3 + CH --C H --CH 3 16 2 3

CH 3

II. DEALKYLATION / DISPROPORTIONATION

+ C6Hs--CH3~ CH3--C6H4--C2Hs~-------C2H 4 + C6H5--CH 3 ~ CH3--C6H4---~H 3

+

C6H 6

III. TRANSALKYLATION

CH 3 I

C 6 H 5 - - C H 3 + C H 3 - - C 6 H 3 - - C H 3 ~ 2 C H 3 - - C 6 H 4 - - C H 3

IV. ISOMERIZATION

1 , 3 , 5 - C6H3 (CH3) 3

1 , 2 , 4 -

1 , 2 , 3 - C6H3(CH3) 3

1 C6H3 ( CH3 ) 3

1,2-

I , 3- CH3--C6H 4-C2H 5

CH 3-C6H4~C2H 5

% 1 , 4 - CH3--C6H4--C2H 5

Fig. 3. Reaction scheme for A7 and A9 conversion over/3.

ethyltoluenes. The highest yield of xylenes was observed with toluene-l,3,5-tri- methylbenzene. These findings led us to propose the reaction scheme as shown in Fig. 3 for A 7 and A 9 aromatics transformation over/3.

Having proposed the reaction scheme, we started looking into the aspect of contact time effect on the formation of xylenes. The weight hourly space velocity (WHSV) of the feed was varied from 2 to 10 h - 1. The results are summarized in Fig. 4. With increase in WHSV, the contact time between the feed and the zeolite decreased and formation of xylenes went down due to a decrease in conversion of both toluene and m 9. The decrease in A 9 conversion with increase in WHSV was sharper than with toluene conversion. This can be explained on the basis of the dimensions of these molecules and the diffusivity inside the zeolite channel. The same has been explained by Haag and Chen [ 15]. They have reported that the completely shape-selective conversion of linear alkanes (A) in the presence of branched ones (B) with small-pore zeolites occurs when the diffusivity of the B species, DB = 0, that is when the B molecules are essentially completely excluded

J. Das et al. /Applied Catalysis A: General 116 (1994) 71-79 77

35

A9

I I I I 0 O0 2 4 6 8 10

WHSV { h -1) Ternp.= 375°C H 2 / H C = 4

Fig. 4. Influence of WHSV variation on catalytic performance.

~ 2 5

20

>

u 15

cn

5

4 3

3 > "6

2t~

from the zeolite interior as is the case with small pore zeolites. With medium- and large-pore zeolites, both DA and DB are finite. The observed selectivity in this case depends on the degree of diffusion limitation of the more bulky antiselective species B. Hence, when the space velocity is increased or the contact time decreased, the bulkier A 9 molecules diffuse at a slower rate than A 7 with the result that the conversion decreased to a greater extent.

Coming to the effect of temperature variation on catalyst performance, the results of these experiments are depicted in Table 3. With increase in reaction temperature from 350 to 400°C the conversions of toluene as well as of A 9 increased, due to the fact that the formation of xylenes also went up. It is interesting to note the relative

Table 3 Effect of temperature and feed composition on xylene isomer distribution

Feed

Temperature

Toluene Nearly 50:50 toluene + A9 A9 stream

350 375 400 350 375 400 350 375 400

Tol. conv. (%) 8.32 10.60 12.89 20.69 19.75 18.82 - - - C9 conv. (%) - - - 25.49 29.47 32.36 22.11 25.33 28.55

Total xyl. 4.90 6.36 7.82 15.22 16.85 18.49 6.75 8.74 10.75

Xylene composition p-Xyl.(23.52) 25.71 25.61 24.55 25.53 22.56 24.77 21.63 21.82 21.95

m-Xyl.(52.38) 52.86 52.95 53.07 49.80 50.78 51.81 47.85 48.37 49.30

o-Xyl.(24.10) 21.43 21.44 22.39 23.85 23.69 23.42 30.52 29.81 28.74

The values reported within brackets are the thermodynamic equilibrium compositions at 400°C. Conditions: WHSV = 3.6 h - 1 HE/H C = 4.

78 J. Das et al. /Applied Catalysis A: General 116 (1994) 71-79

80

50

A

40

~.30

¢p --~20

10

ENE

0 I I I I i I 200 3 O0 400 5 O0

Temperature (°C) WHSV=3.6hl; H2/HC=4

Fig. 5. Effect of temperature on xylene isomer distribution during dispropordonation of A9.

concentration of the xylenes at different temperatures with varying feed composi- tions. With pure toluene as the feed, the p-xylene concentration in the product was above thermodynamic equilibrium and the o-xylene concentration was below equi- librium. When the feed contained only A9, the product contained an o-xylene concentration above thermodynamic equilibrium and a p-xylene concentration below thermodynamic equilibrium. An equal mixture of toluene and A 9 showed product xylene compositions in between the two trends.

As only A 9 in the feed exhibited a higher o-xylene concentration in the product, we studied the reaction at temperatures lower than 350°C. The results are given in Fig. 5. At the lower temperature of 200°C, o-xylene was as high as 55%. A similar observation was reported by Wang et al. [ 11 ] during disproportionation of 1,2,4- trimethylbenzene on ft. They have explained the mechanism on the basis ofbiphenyl carbonium intermediate formation. It looks from the mechanism that o-xylene is the primary product of A 9 disproportionation.

4. Conclusions

Among the three zeolites of nearly the same Si/A1 ratio but with different channel structure and dimensions, fl showed a better activity for transformation of A 7 and m 9 streams. Maximum xylene formation was observed with a feed consisting of nearly the same amounts of m 9 and A 7 in the feed. The trimethylbenzenes became

J. Das et aL /Applied Catalysis A: General 116 (1994) 71-79 79

transformed on fl by disproportionation, while ethyltoluenes underwent de-ethyl- ation followed by disproportionation of the generated toluene. The highest xylene yield was obtained with a feed consisting of nearly equal proportions of toluene and 1,3,5-trimethylbenzene. Lower WHSV and higher temperature enhanced the formation of xylenes. With pure toluene as the feed, p-xylene concentrations in the xylenes was higher than thermodynamic equilibrium value, while with the feed made up of only m 9, the o-xylene concentration was above thermodynamic equi- librium, fl has all the potential to be exploited for transformation of commercial A 7

and A 9 streams.

Acknowledgements

One of the authors (AIB) thanks Prof. C.L. Khetrapal, Indian Institute of Science, Bangalore, for providing MAS-NMR facilities and helpful discussion. The authors are grateful to Dr. I.S. Bharadwaj, Director (R and D), IPCL, for his continuous encouragement during this work.

References

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