Conversion of heavy reformate into xylenes over mordenite-based catalysts

chemical engineering research and design 8 9 ( 2 0 1 1 ) 2125–2135

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Conversion of heavy reformate into xylenes over mordenite-based catalysts S.A. Ali a , A.M. Aitani a , C. Ercan b , Y. Wang b , S. Al-Khattaf a,∗ a

Center of Research Excellence in Petroleum Refining & Petrochemicals, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia b Research & Development Center, Saudi Aramco, Dhahran 31311, Saudi Arabia

a b s t r a c t The conversion of heavy reformate into high-value xylenes was studied over a series of H-mordenite-based catalysts in a fluidized-bed reactor at 400 ◦ C. The results show that methyl-ethyl-benzenes (MEBs) were more reactive than trimethylbenzenes (TMBs) over all the catalysts studied. Mordenite catalyst with higher acid site concentration (M1) favored dealkylation of MEBs while another mordenite catalyst with lower acid site concentration (M2) favored disproportionation of TMBs. Mixing M2 with ZSM-5 (M2Z1) enhanced MEBs conversion (69.2%) and xylenes yield (22.5 wt.%). The conversion of heavy reformate and toluene mixtures over M2Z1 catalyst increased xylenes yield to a maximum at 25.3 wt.% for a feed containing 70:30 heavy reformate and toluene. The results of converting mixtures of 1,2,4-TMB/toluene and heavy reformate/toluene indicate that catalyst acid site concentration plays a key role in promoting desirable transalkylation reactions needed to enhance xylenes yield. The amount of coke formed increased with the acid site concentration of catalysts and more coke laydown was observed during conversion of heavy reformate than 1,2,4-TMB. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Mordenite; Heavy reformate; Transalkylation; Aromatics; Xylenes; Fluidized-bed reactor; ZSM-5

1.

Introduction

The escalating demand for clean transportation fuels along with their increasingly stringent specifications continues to challenge most refiners around the world. Among the specifications of reformulated gasoline is the reduction of maximum aromatics content. Heavy reformate, containing aromatics with carbon number nine or higher (C9+), is one of the blending streams of gasoline. In response to the limitation on aromatics in gasoline, a fraction of heavy reformate is taken out from gasoline pool. Transalkylation of heavy reformate to value-added xylenes is considered a practical route to utilize the excess heavy reformate. Xylenes are important starting components for the production of synthetic fibers etc. and its global demand is increasing at 6–8% per year. Tsai et al. (2009) reported that commercial transalkylation processes such as TransPlus and Tatoray have become essential part of modern aromatics complex. With more refineries opting for refinery–petrochemical integration, transalkylation of heavy

∗

reformate is becoming one of the favorable routes to boost their operating profit margins. The C9 components of heavy reformate contains mainly (TMBs) and methylethyl benzenes (MEBs), together with some minor quantities of C10+ aromatics. Due to the variety of compounds present in heavy reformate, parallel and consecutive set of reactions takes place during the transformation of heavy reformate (Ali et al., 2011). Major reactions are illustrated in Fig. 1. The desired reaction is the transalkylation of TMBs and toluene to produce two moles of xylenes. When heavy reformate is converted, the amount of toluene required for this reaction is produced from the dealkylation of heavier aromatics. Isomerization of TMBs (1,2,3-, 1,2,4- and 1,3,5-isomers) and xylenes (ortho-, meta- and para- isomers) occur simultaneously. In addition, the disproportionation of TMBs and toluene and dealkylation of alkylbenzenes take place. The parallel and consecutive reactions system is characterized ˇ by multiple chemical equilibria. Cejka and Wichterlova (2002) reported that using different catalysts can accelerate certain

Corresponding author. Tel.: +966 3 860 1429; fax: +966 3 860 4234. E-mail address: [email protected] (S. Al-Khattaf). Received 29 September 2010; Received in revised form 23 January 2011; Accepted 30 January 2011 0263-8762/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2011.01.031

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Fig. 1 – Examples of chemical reactions during conversion of heavy reformate to xylenes. reactions while not influencing the others even under similar reaction conditions. The products from transalkylation of heavy reformate, therefore, require several separation steps and recycling of unconverted starting components to achieve optimum product yield and purity. Activity and selectivity of catalysts used in the conversion of large aromatic molecules such as TMBs, MEBs and heavier alkyl substituted (C10+) aromatics are significantly influenced by the dimensions and the pore structure of the catalysts. AlKhattaf et al. (2010) reviewed the application of zeolites as shape selective catalysts for transalkylation reactions. Several zeolite-based catalysts have been reported in laboratory studies by Tsai et al. (1999, 2008) on the transformation of toluene ˇ and TMBs. The zeolites include Y (Cejka and Wichterlova, ˇ 2002; Al-Khattaf et al., 2006; Krejci et al., 2010), L-zeolite (Cejka ˇ and Wichterlova, 2002), beta (Cejka and Wichterlova, 2002; Das ˇ et al., 1994a,b), ZSM-5 (Röger et al., 1998); MCM-41 (Cejka et al., 2001); NU-87 (Park and Rhee, 2000); SAPO-5 (Dumitriu et al., 2002); and mordenite (Tsai et al., 2004; Tsai, 2006; Wu and Leu, 1983). Serra et al. (2005) tested and optimized the performance

of multi-zeolitic catalysts with 10-, 12-, and 10-12-ring channels and metals impregnation in the transalkylation of heavy reformate. It was found that zeolite pore size and geometry have a direct influence on dealkylation and transalkylation of the different alkyl groups. In contrast to medium pore zeolites, the reaction of TMB transformation over large pore zeolites, such as zeolites Y, beta and mordenite, may proceed mainly in the zeolite channel system. Thus, the rate of the reaction is much higher comˇ pared to that of medium pore zeolites. Cejka et al. (1999, 2004) attributed this difference to the possibility to accommodate higher concentrations of reactants inside the channel system leading to the increase in the rate of bimolecular reaction. A weak hydrogenation function is introduced in the zeolitic catalyst by incorporating suitable metals. Such dual-function catalysts promote desirable reactions such as hydrogenation of coke precursors, dealkylation of certain C9 compounds, saturation of cracking products, and their subsequent alkylation. Das et al. (1994a,b) incorporated different metals such as Cu, Ni, Pt, Mo, Re, etc. into the zeolite catalysts to

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Table 1 – Catalyst composition and ammonia TPD results. Catalyst compositiona , wt.%

Catalyst ID

H-mordenite

Strong acid site

Total acid amount (mmol/g)

H-ZSM-5

Si/Al2 ratio

18

180

27

150

M1 M1Z1 M1Z2 M2 M2Z1 M2Z2

66 33 33 – – –

– – – 66 33 33

– 33 – – 33 –

– – 33 – – 33

a

Weak acid site

Amount (mmol/g) 0.50 0.47 0.48 0.01 0.11 0.10

Peak temp. (◦ C) 185 185 185 150 180 180

Amount (mmol/g) 1.08 0.91 0.96 0.16 0.19 0.19

Peak temp. (◦ C) 500 500 490 410 400 370

1.58 1.38 1.44 0.17 0.30 0.29

Balance: 34 wt.% alumina binder (AP-3).

hydrogenate poly-aromatics coke precursors during heavy aromatics transalkylation and thus reduce catalyst aging. Serra et al. (2005) studied the conversion of heavy reformates with toluene over seven zeolites and each zeolite was impregnated with seven metals (Re, Ni, Mo, Ga, Pt, La, and Bi). They reported that the incorporation of certain metals increase the xylene yield. Among the metals studied, molybdenum exhibited the highest positive effect followed by rhenium. Tsai et al. (2010) developed a dual-catalyst system comprising Pt/ZSM-12 and H-Beta to improve benzene product purity during transalkylation of heavy aromatics. Serra et al. (2008) described the use of catalyst system consisting of at least one zeolite with a ring of 10 oxygen atoms and at least one zeolite with a ring of 12 oxygen atoms, incorporating such metals as Mo, Re and Ga for use in transalkylation of alkylaromatic hydrocarbons. A process for the transalkylation of alkyl-aromatic hydrocarbons using two reaction zones was also patented recently (Serra et al., 2009). The inventors claim that such a process results in higher xylene yield from C10 alkyl-aromatics and benzene or toluene as feedstock. The present study investigates the conversion of heavy reformate to xylenes over H-mordenite-based catalysts with different concentrations of acid sites in a fluidized bed reactor. Two mordenite catalysts with, widely different Si/Al ratios, were the key components of the catalysts. The effects of a mixture of mordenite and ZSM-5 catalysts on C9 conversion, selectivity to xylenes and product distribution have been studied as well. The conversion of heavy reformate and toluene was compared with the conversion 1,2,4-TMB and toluene to elucidate major reactions and parameters that enhance the yield of xylenes.

2.

Experimental

2.1.

Materials

Table 1 lists the six mordenite-based catalysts with or without ZSM-5 zeolite used in this study. H-mordenite zeolite was obtained from Tosoh Company, Japan. Mordenite (MOR) has an orthorhombic crystal structure with straight 12-ring channels ˚ × 7.0 A) ˚ and crossed 8-ring channels (2.8 A ˚ × 5.7 A). ˚ The (6.5 A ZSM-5 zeolite (MFI) was obtained from CATAL International, in the H-form. ZSM-5 has a cubic crystal structure with straight ˚ × 5.5 A). ˚ The channels are connected by 10-ring channels (5.3 A ˚ ˚ Both zeolites have a crystal sinusoidal channels (5.1 A × 5.5 A). size of about 1 ␮m.

M1 and M2 catalysts contain 66 wt.% H-mordenite having a Si/Al molar ratio of 18 and 180, respectively. They were prepared by mixing H-mordenite in dry form with alumina binder and converting the mixture into granules. Two H-ZSM5 zeolites used, Z1 and Z2, have Si/Al molar ratio of 27 and 150, respectively. The mixed-zeolite catalysts (M1Z1, M1Z2, M2Z1, and M2Z2) contain 33 wt.% each of corresponding Hmordenite and H-ZSM-5. All the catalysts contain 34 wt.% of alumina binder (Cataloid AP-3) obtained from CCIC, Japan. The binder contains 75.4 wt.% alumina, 3.4 wt.% acetic acid, and water as balance. Commercial heavy reformate, used as feed, was obtained from a refinery. Analytical grade (99% purity) 1,2,4-TMB and toluene were obtained from Sigma–Aldrich. All chemicals were used as received as no attempt was made to further purify the samples.

2.2.

Catalyst characterization

Acidity measurement of the mordenite-based catalysts was carried out by temperature programmed desorption (TPD) of ammonia using Multi-Task TPD-1-AT instrument (Bel Japan Co.). The calibration of TPD was based on the measurement of strong acid sites of the standard sample (acidity of mordenite (Si/Al = 20) = 0.8 mmol g−1 ). Pretreatment of samples was conducted under helium flow at 500 ◦ C for 1 h. Ammonia adsorption was carried out at 20 Torr for 10 min at 100 ◦ C, followed by flowing helium for 30 min at 100 ◦ C. TPD was then performed under a flow of helium at 30 Torr by increasing the temperature from 100 ◦ C to 600 ◦ C at a rate of 10 ◦ C/min. The results provide the amount of weak and strong acid sites as well as the peak temperatures of desorption. The acid site concentration was measured by the amount of ammonia desorbed. The used catalyst samples were analysed for the carbon content in coke formed after reaction at 400 ◦ C for 20 s. Carbon analyzer (EA 2000, Analytikjena) was used for the purpose, which facilitates determination of total carbon by high temperature oxidation in a current of oxygen.

2.3.

Riser simulator

Catalytic tests were carried out in a riser simulator (fluidizedbed) reactor which is a bench-scale system with internal recycle unit invented by de Lasa (1992). The riser simulator consists of two outer shells, the lower section and the upper section which allow easy loading and unloading of the catalyst, as illustrated in Fig. 2. The reactor was designed in such

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Table 2 – Composition of the commercial heavy reformate.

Fig. 2 – Schematic diagram of the riser simulator. way that an annular space is created between the outer portion of the basket and the inner part of the reactor shell. A metallic gasket seals the two chambers with an impeller located in the upper section. Upon rotation of the shaft, gas is forced outward from the center of the impeller towards the walls. This creates a lower pressure in the center region of the impeller thus inducing flow of gas upward through the catalyst chamber from the bottom of the reactor annular region where the pressure is slightly higher. The impeller provides a fluidized bed of catalyst particles as well as intense gas mixing inside the reactor. Kraemer (1991) provided a detailed description of various riser simulator components, sequence of injection and sampling. All experiments were conducted at a catalyst/reactant ratio of 5 (mass of catalyst = 0.81 g, mass of reactant feed injected = 0.162 g); residence times of 5, 10, 15 and 20 s; and temperatures of 400 ◦ C. During the investigations, some of the runs were repeated to check for reproducibility in the conversion, which was found to be within a range of ±2%. To start the experiment, the reactor was heated to the desired temperature. The vacuum box was also heated to around 250 ◦ C and evacuated (0.5 psi) to prevent any condensation of hydrocarbons inside the box. The heating of the riser simulator was conducted under continuous flow of inert gas (argon) and the process usually takes few hours until thermal equilibrium is attained. The temperature controller was set to the desired reaction temperature, in the same manner the timer was adjusted to the desired reaction time. At this point the gas chromatograph (GC) is started and set to the desired conditions. Once the reactor and the GC reached the desired operating conditions, the feed was injected directly into the reactor via a loaded syringe. After the reaction, the four-port valve opens immediately ensuring that the reaction was terminated and the entire product stream sent on-line to the analytical equipment via a pre-heated vacuum box chamber. The conversion of C9s (TMBs, MEBs) and toluene is defined as follows: conversion of C9s =

[C9s]feed − [C9s]product

conversion of toluene =

2.4.

[C9s]feed

× 100

[toluene]feed − [toluene]product [toluene]feed

Major compound

Abbreviation

Composition, wt.%

Iso-propyl benzene n-Propyl benzene 1-Methyl 2-ethyl benzene 1-Methyl 3-ethyl benzene 1-Methyl 4-ethyl benzene 1,2,3-Tri-methyl benzene 1,2,4-Tri-methyl benzene 1,3,5-Tri-methyl benzene

iPB nPB 1M2EB 1M3EB 1M4EB 1,2,3TMB 1,2,4TMB 1,3,5TMB

1.7 4.3 6.5 18.5 9.1 6.6 39.0 10.1

Total C9 aromatics n-Butyl benzene 1,4-Diethyl benzene 1,3-Diethyl benzene 1,3-Dimethyl, 5-ethyl benzene 1,4-Dimethyl, 2-ethyl benzene Others C10s

nBB 1,4DEB 1,3DEB 1,3DM5EB 1,4DM2EB

95.9 0.5 0.8 0.4 0.8 0.4 1.2

Total C10 aromatics

4.1

cross-linked polyethylene glycol with an internal diameter of 0.32 mm. The composition of heavy reformate feed, as presented in Table 2, includes about 96 wt.% C9 and 4% C10 aromatics. Among the C9 components, TMBs (55.7 wt.%) and MEBs (34.1 wt.%) were the major constituents.

3.

Results and discussion

3.1.

Characterization of the catalysts

The results of catalyst characterization by TPD are presented in Table 1. M1 catalyst possessed highest amount of weak as well as strong acid sites while M2 exhibited lowest amount of weak as well as strong acid sites. Hence the total acidity of M1 (1.58 mmol/g) was over nine times that of M2 (0.17 mmol/g). This result corresponds to the ten times difference in Si/Al molar ratio between M1 and M2. The total acid site concentration of Z1 [66 wt.% HZSM-5 (Si/Al molar ratio = 27) + 34 wt.% alumina binder] was 1.17 mmol/g. On the other hand, the total acid site concentration of Z2 [66 wt.% H-ZSM-5 (Si/Al molar ratio = 150) + 34 wt.% alumina binder] was only 0.34 mmol/g. These results indicate that the total acidity of the zeolitic components used in the catalysts is in the following order: M1 > Z1 > Z2 > M2. It is quite possible to have higher acidity of certain mordenites, such as M1, compared to ZSM-5 as reported by Lonyi and Valyon (2001). Mixing equal quantities of M1 with Z1 resulted in decrease in total acidity from 1.58 to 1.38 mmol/g and mixing M1 with Z2 caused reduction to 1.44 mmol/g. However, mixing equal quantities of M2 with Z1 or Z2 resulted in increase in total acidity from 0.17 to about 0.30 mmol/g. The differences in acid site concentration can cause wide difference in catalytic behavior.

× 100

3.2. Conversion over mordenite catalysts with different Si/Al ratios

Feed and product analyses

Heavy reformate feed and reaction products were analysed using an Agilent 6890N GC equipped with a flame ionization detector (FID) and a capillary column INNOWAX, 60-m

The conversion of heavy reformate was investigated over two mordenite-based catalysts (M1 and M2). Table 3 presents the distribution of products and conversion obtained at 400 ◦ C and reaction time of 20 s. Fig. 3 presents the composition of

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Table 3 – Product distribution and conversion of heavy reformate at 400 ◦ C and 20 s reaction time. Feed composition, wt.% Product distribution, wt.% Light gases (C1 –C4 ) Benzene Toluene Ethylbenzene para-Xylene meta-Xylene ortho-Xylene i-Propylbenzene n-Propylbenzene Indane 1,2,3-TMB 1,2,4-TMB 1,3,5-TMB 1M2EB 1M3EB 1M4EB C10 aromatics Total Grouped amounts Xylenes TMBs MEBs C9 aromatics C10 aromatics Conversion, wt.% C9s conversion TMBs conversion MEBs conversion C10s conversion Ratios Xylenes/benzene (molar) Methyl group/benzene ring p-Xylene/xylenes p-Xylene/ortho-xylene 135-TMB/123-TMB

Catalyst M1

M1Z1

M1Z2

M2

M2Z1

M2Z2

0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.7 4.3 0.0 6.6 39.0 10.1 6.5 18.5 9.1 4.2

10.7 3.5 13.3 0.3 3.9 8.5 4.2 0.6 1.3 0.4 3.5 25.9 6.6 2.6 7.6 4.0 3.1

9.1 2.8 13.1 0.3 4.7 10.2 4.9 0.5 1.2 0.4 3.6 25.6 7.0 2.3 6.9 3.6 3.8

9.9 2.8 13.7 0.4 5.0 11.0 5.1 0.5 1.1 0.4 3.3 24.1 6.9 2.1 6.4 3.3 4.0

2.6 0.9 6.2 0.8 3.7 8.0 4.0 0.0 1.3 1.2 4.3 31.1 9.1 2.9 9.2 4.7 10.0

4.6 2.0 11.5 0.6 5.3 11.6 5.5 0.4 0.8 0.8 3.8 26.4 8.3 1.9 5.7 2.9 7.9

4.3 2.2 11.7 1.0 4.0 8.7 4.1 0.3 0.7 0.9 4.3 29.5 10.1 1.9 5.8 2.8 7.7

100.0

100.0

100.0

100.0

100.0

100.0

100.0

0.0 55.7 34.1 95.8 4.2

16.6 36.0 14.2 52.5 3.1

19.8 36.2 12.8 51.1 3.8

21.1 34.3 11.8 48.1 4.0

15.7 44.5 16.8 63.8 10.0

22.4 38.5 10.5 51.0 7.9

16.8 43.9 10.5 56.3 7.7

– – – –

45.2 35.4 58.4 26.2

46.7 35.0 62.5 9.5

49.8 38.4 65.4 4.8

33.4 20.1 50.7 –

46.8 30.9 69.2 –

41.2 21.2 69.2 –

– 2.35 – – 1.53

3.49 1.69 0.23 0.93 1.89

products obtained at different reaction times over M1 and M2 catalysts. C9 aromatic content in the heavy reformate decreased from 95.9 wt.% to 52.5 wt.% with M1 and to 63.8 wt.% with M2 after 20 s reaction time. This result indicates that mordenite having higher acid site concentration (M1) exhibited higher C9 conversion. Among the components of C9 aromatics, MEBs content decreased from 34.1 wt.% in heavy reformate to 14.2–16.8 wt.% while the TMBs content decreased from 55.7 wt.% to 36.0–44.5 wt.%. Fig. 4A and D present the conversion of C9 components at different reaction times over M1 and M2, respectively. The conversion of MEBs was 58.4 wt.% and 50.7 wt.% over M1 and M2, respectively. These values are higher compared to the conversion of TMBs (35.4 wt.% and 20.1 wt.% over M1 and M2, respectively. These results indicate that MEBs are more reactive than TMBs. Conversion of MEBs occurs mainly by dealkylation resulting in formation of toluene and ethylene (Reaction 4 in Fig. 1). On the other hand, conversion of TMBs can be either by transalkylation (Reaction 1-A or 1-B in Fig. 1) or disproportionation (Reaction 3-A in Fig. 1). The conversion of TMBs as well as MEBs was higher over M1 than M2. This difference can be attributed to significantly lower acid site concentration of M2 relative to M1.

5.20 1.76 0.24 0.96 1.94

3.2.1.

5.55 1.73 0.24 0.98 2.09

12.84 2.22 0.24 0.93 2.12

8.24 2.02 0.24 0.96 2.18

5.62 2.07 0.24 0.98 2.35

Dealkylation of MEBs

As presented in Table 3, the toluene content was 13.3 wt.% and 6.2 wt.%, while benzene content was 3.5 wt.% and 0.9 wt.% in the products obtained over M1 and M2, respectively. In addition, higher C1–C4 gaseous products (10.7 wt.%) from M1 as compared to lower C1–C4 gaseous products (2.6 wt.%) from M2 were observed. Higher content of toluene and light gases in products obtained over M1 catalyst indicate that this catalyst favors dealkylation of MEBs to toluene and ethylene (Reaction 4 in Fig. 1). Once the toluene is formed from MEB dealkylation, the transalkylation of TMBs and toluene to form xylenes (Reaction 1-A in Fig. 1) can takes place. On the other hand, the products from M2 contain lower amount of toluene and light gases indicating significantly lower dealkylation of MEBs. The difference in dealkylation reaction over M1 and M2 is more clearly shown in Fig. 5. Such a difference in dealkylation of MEBs between M1 and M2 can be attributed to acid site concentration. M1, having higher acid site concentration, exhibited significantly more dealkylation activity as compared to M2, which has lower acid site concentration.

3.2.2.

Transalkylation and disproportionation of TMBs

Another interesting difference between two mordenite catalysts is the amount of C10 aromatics in the reaction product

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100

Catalyst: M1 90

Product Composition (wt.%)

80

70

60

C9s

50

40

TMBs

MEBs 30

20

Xylenes Toluene

10

C10s Benzene

3.2.3.

Xylenes/benzene molar ratio

90

The xylenes-to-benzene molar (X/B) ratio in products is a sensitive indicator for the selectivity of transalkylation and disproportionation of toluene. The X/B ratio was 3.49 in product obtained over M1, whereas it was 12.84 in product obtained over M2. A significantly higher X/B ratio clearly suggests that M2 had higher selectivity towards transalkylation of TMB and toluene (Reaction 1, Fig. 1) than M1.

80

3.2.4.

0 0

5

10

15

20

25

Time (secs) 100

Catalyst: M2

Product Distribution (wt.%)

(M2) favors the formation of xylenes via disproportionation of TMBs. For the transalkylation reaction to occur, the presence of toluene is necessary which is obtained from dealkylation of MEBs. Moreover, when toluene and TMB are present in the reaction mixture, the preferential reaction is transalkylation as opposed to their individual disproportionation reactions (Wang et al., 1990). It should be noted that methyl transalkylation of TMBs and toluene can also result in the formation of tetramethyl benzenes and benzene. When the feedstock is equimolar mixture of TMB and toluene, almost equal amounts of benzene and TeMB will be formed. Higher amounts of benzene will be formed if the toluene content is higher than the TMB. On the other hand, higher amounts of TeMB will be formed if the toluene content is lower than the TMB. These thermodynamic limitations imply that the higher xylene yield can be obtained through the transalkylation between TMBs and toluene if the amount of toluene produced from MEB dealkylation matches with the TMB content in the feedstock (Ali et al., 2011).

70

C9s 60 50

TMBs 40

MEBs 30

20

Xylenes C10s Toluene

10

Benzene

0 0

5

10

15

20

25

Time (secs) Fig. 3 – Product distribution over M1 and M2 catalysts.

mixtures. With M1, the C10 aromatic content decreased with reaction time while with M2 it increased significantly as presented in Fig. 5. C10 aromatics, especially tetramethyl benzenes, can be formed either from transalkylation of TMBs and toluene (Reaction 1-B in Fig. 1) or from disproportionation of TMBs (Reaction 3-A in Fig. 1). The formation of significantly higher amount of C10 aromatics in products from M2 can be attributed to disproportionation of TMBs resulting in formation of xylenes and tetra methyl benzenes. This result indicates that the catalyst with lower acid site concentration

Methyl group/benzene ring ratio

The methyl group-to-benzene ring (M/R) ratio in liquid products is an indicator of dealkylation reactions as is associated with thermodynamic equilibrium of aromatic compounds. An M/R ratio of 2.0 is the favors the maximum xylene yield (Wang et al., 1990). The M/R ratio in the heavy reformate feedstock was 2.35. It decreased substantially to 1.69 in product obtained over M1. The significant decrease of M/R ratio resulted in more light gas formation. Furthermore, the thermodynamic equilibrium shifted the product distribution towards increase in benzene and toluene content and increase in C9 and C10 content. On the other hand, the M/R ratio of product obtained over M2 catalyst was 2.22. Hence the thermodynamic equilibrium shifted the product distribution towards decrease in benzene and toluene content and increase C9 and C10 content.

3.2.5.

para-Xylene/xylene selectivity

The selectivity ratio of para-xylene/xylenes over M1 and M2 after 20 s reaction time was within 23–24% as presented in Table 3. Also, the para-xylene/o-xylene ratio was same at 0.93. These results indicate that isomerization reaction of xylenes was near equilibrium. Improvement in selectivity to paraxylene could not be achieved by either M1 or M2. This is because the large pore size of the mordenite catalyst allows the isomers to freely move without diffusional constraints. The lack of shape selectivity towards xylenes isomers as demonstrated by this result was reported other researchers in their studies on transalkylation of 1,2,4-TMB such as Dumitriu et al. (2002) over aluminophosphates and Dumitriu et al. (1996) over ultra-stable Y-zeolite.

3.2.6.

1,3,5-TMB/1,2,3-TMB

The 1,3,5-TMB/1,2,3-TMB ratio increased from about 1.5 in heavy reformate to about 1.9 and 2.1 in products over M1 and

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80

B

A

70

50

MEBs

MEBs

MEBs

60

Conversion (wt.%)

C

C9s

C9s

C9s

40 30

TMBs

TMBs

TMBs

20 10 0 80

E

D

70

F

MEBs

MEBs

Conversion (wt.%)

60

MEBs

50

C9s C9s

40

C9s

30

TMBs

TMBs

20

TMBs

10 0 0

5

10

15

20

Reaction Time (secs)

0

5

10

15

20

0

5

10

15

20

Reaction Time (secs)

Reaction Time (secs)

Fig. 4 – Conversion of C9 aromatics and its components over M1 (A); M1Z1 (B); M1Z2 (C); M2 (D); M2Z1 (E); and M2Z2 (F) catalysts. M2, respectively. This indicates that M2 favors the formation of relatively more bulky 1,3,5 TMB than M1. Norval and Phillips, 1989 reported a thermodynamic equilibrium value for 1,3,5TMB/1,2,3-TMB ratio at 400 ◦ C to be 3.12. The fact that the experimental value obtained is different from the reported equilibrium value indicates that shape selectivity does play a significant role in product distribution of TMB isomers in M1 and M2. Wang et al. (1990) reported a value of 2.7 for the

1,3,5-/1,2,3-TMB ratio at 348 ◦ C over USY-zeolite, while Park and Rhee (2000) reported a value of 1.99 over NU-87 zeolite with catalytic properties falling between those of mediumand large-pore zeolites.

3.2.7.

Coke formation

The carbon content in the coke is determined and the results are presented in Table 4. It can be observed from the results that heavy reformate produces more coke for all catalysts due to the presence of ethyl groups, which produces more olefins that acts as coke precursors. Also it can be seen that mordenite catalysts with higher acidity (M1) produces more coke than mordenite catalyst with low acidity (M2). The general trend of

Table 4 – Carbon deposition on catalysts for 1,2,4-TMB and heavy reformate feed at 400 ◦ C and 20 s reaction time. Catalyst

Total acidity (mmol/g)

Carbon deposition, wt.% 1,2,4-TMB

Fig. 5 – Toluene, light gases and C10 aromatics content in products obtained from conversion of heavy reformate over M1 and M2 catalysts.

M1 M1Z1 M1Z2 M2 M2Z1 M2Z2

1.58 1.38 1.44 0.17 0.30 0.29

2.34 1.75 2.04 0.19 0.30 0.18

Heavy reformate 3.57 2.15 2.17 0.40 0.59 0.58

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coke formation in mixed catalysts is in line with the variation in acid site concentration. The deactivation of M1 will be quite fast compared to M2.

3.2.8.

Xylenes yield

From the above discussion, it can be concluded that the conversion of heavy reformate to xylenes over M1 and M2 catalysts occur via different reaction pathways. Xylene yield after 20 s of the reaction time was 16.6 wt.% and 15.7 wt.% with M1 and M2, respectively. The difference in the total xylenes yield over M1 and M2 is narrow (about 1 wt.%) despite the significant difference in acid site concentration. This result indicates that, depending on the catalyst acid site concentration, the formation of xylenes can take different reaction pathways with little difference in overall xylene yield. However, the different reaction pathways result in significant differences in other products, such as light gases and C10 aromatics.

3.3. Conversion over mixture of mordenite/ZSM-5 catalysts Fig. 4 presents the conversion of C9 aromatics and its components over different catalysts varying reaction time. Compared to M1, the M1Z1 catalyst resulted in slight increase (1.5 wt.%) in C9 aromatics conversion. A higher increase (4.6 wt.%) in C9 aromatics conversion was observed with M1Z2 (Table 3, Fig. 4A–C). Compared to M2, M2Z1 catalyst exhibited significantly (13.4 wt.%) increase in the conversion of C9 aromatics. However, M2Z2 resulted in a lesser enhancement (7.8 wt.%) of C9 aromatics conversion (Table 3, Fig. 4D–F).

3.3.1.

Dealkylation of MEBs

Compared to M1, the M1Z1 catalyst resulted in an increase (4.1 wt.%) in MEBs conversion. Further increase (7.0 wt.%) in MEBs conversion was observed with M1Z2 (Table 3, Fig. 4A–C). It was also observed that toluene yield remained almost constant in products from M1, M1Z1 and M1Z2 catalysts. Compared to M2, both M2Z1 and M2Z2 catalysts resulted in a significant increase (18.5 wt.%) in MEBs conversion (Table 3, Fig. 4D–F). It was also observed that the toluene yield increased (5.5 wt.%) in products from M2Z1 and M21Z2 compared to M2 catalyst. An increase in C1–C4 gaseous products was also observed. It can be postulated that MEBs convert mainly by dealkylation forming toluene and ethylene. A portion of toluene thus formed reacts with TMBs to form xylenes via transalkylation (Reaction 1-A in Fig. 1). Such a sequence of reactions results in higher yield of xylenes. The increase in total xylenes yield over ZSM-5-containing catalysts, as presented in Table 3, is in line with the sequence of reactions mentioned above. Therefore, it can be stated that dealkylation of MEBs was enhanced by mixing ZSM-5 with both M1 and M2.

3.3.2.

Transalkylation and disproportionation of TMBs

Another noticeable effect of mixing ZSM-5 was in the C10 content in the product. Compared to M1, the C10 aromatic conversion decreased in products from M1Z1 and further decreased in products from M1Z2. The lower conversion of C10 can be attributed to lower acid site concentration of M1Z1 and M1Z2 compared to M1. For M2 catalyst, however, an increase in C10 aromatic content from 4.2 wt.% in feed to 10.0 wt.% is observed. As mentioned above, C10 aromatics, especially tetramethyl benzenes, are formed over M2 catalyst via disproportionation of TMBs (Reaction 3-A in Fig. 1). Compared to M2,

Fig. 6 – Xylenes and para-xylene yields obtained from conversion of heavy reformate over different catalysts. the C10 aromatic content in products from M2Z1 and M2Z2 decreased to 7.9 wt.% and 7.7 wt.%, respectively. These results indicate that mixing M2 with ZSM-5 resulted in somewhat lower disproportionation of TMBs, probably due to higher acid site concentration of M2Z1 and M2Z2 compared to M2.

3.3.3.

Xylenes/benzene molar ratio

The X/B ratio in products obtained over M1Z1 and M1Z2 catalysts was 5.20 and 5.55, respectively. These values are higher than 3.49 obtained over M1 catalyst, which indicates that addition of ZSM-5 to M1 caused higher selectivity towards transalkylation of toluene. On the other hand, the X/B ratio in products obtained over M2Z1 and M2Z2 catalysts was 8.24 and 5.62, respectively. These values are significantly lower than 12.84 obtained over M2 catalyst, which suggests lower selectivity towards disproportionation of toluene due to addition of ZSM-5 to M2.

3.3.4.

Methyl group/benzene ring ratio

The M/R ratio slightly increased in products obtained over M1Z1 and M1Z2 compared to M1. This shift, however, was towards the preferred value of 2.0. The reduction in benzene and toluene as well as increase in xylenes, and C10 in the product are in accordance with the thermodynamic equilibrium. M2Z1 and M2Z2 also exhibited lower M/R ratio compared to M2. Again, the shift was towards the preferred value of 2.0. It is should be noted that product obtained M2Z1 has an M/R of 2.02 (which is closest to the value of 2.0) and has the highest xylene yield of 22.4 wt.%.

3.3.5.

Coke formation

The carbon content in the coke is determined and the results are presented in Table 4. Addition of ZSM-5 to M1 resulted in decrease in coke formation, which is in line with the decrease in acid site concentration. On the other hand, addition of ZSM5 to M2 resulted in increase in coke formation due to the higher acidity.

3.3.6.

Xylenes yield

Xylenes yield changed significantly upon mixing mordenite with ZSM-5. The results, are presented in Table 3 and Fig. 6. Compared to M1, M1Z1 and M1Z2 catalysts exhibited 3.2 wt.% and 4.5 wt.% higher yield of total xylenes, respectively. This can be attributed to higher dealkylation of MEBs compared to M1 catalyst. This result indicates that mixing M1 with ZSM-5 had positive effect on the transalkylation reaction, with more effect due to mixing with Z2.

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35 M1Z2

M2Z1

Xylenes Yield (wt.%)

30

25

20

15

10

5

0

Fig. 7 – Xylenes yield from the conversion of heavy reformate and toluene over M1Z2 and M2Z1 catalysts. Compared to M2, M2Z1 and M2Z2 exhibited 6.7 wt.% and 1.1 wt.% higher yield of total xylenes, respectively. Since M2 has very low acid site concentration, mixing of Z1 resulted in increase in acid site strength. Hence the dealkylation of TMBs to toluene and ethylene was significantly enhanced. Subsequently, more transalkylation of toluene and TMB resulted in higher xylene yield.

3.3.7.

para-Xylene/xylenes selectivity

The yields of para-xylene followed the same trend as total xylenes over all the catalysts. Selectivity ratios of para-xylene/xylenes remained unchanged (0.24) in products obtained from mixed catalysts as shown in Table 3. Selectivity ratios of para-xylene/ortho-xylene increased slightly by mixing ZSM-5 with M1 and M2 catalysts. However, they remain close to unity indicating that the xylenes isomerization reaction was near the thermodynamic equilibrium. The result also shows that mixing with ZSM-5, despite an increase in the total xylenes yield, did not result in an improvement in selectivity towards para-xylene.

3.3.8.

1,3,5-TMB/1,2,3-TMB

The 1,3,5-TMB/1,2,3-TMB ratio increased by mixing Z1 and further increased by mixing Z2 with M1 and M2. These results indicate that the TMB isomerization reaction is favored by mixing with ZSM-5 due to increase in acid site concentration. Products from catalysts based on lower acid site concentration mordenite M2 (M2, M2Z1 and M2Z2) resulted in higher 1,3,5-TMB/1,2,3-TMB ratio. These results indicate that catalyst acid site concentration also plays a role in the isomerization of TMBs. However, the value of ratio (2.1–2.3) was below the thermodynamic limit of 3.2, probably due to short contact time in fluidized bed reactor. From the above results, it can be concluded that the performance of M2Z1 for transalkylation of heavy aromatics and xylenes production was higher than other catalysts studied.

3.4.

Conversion of heavy reformate and toluene

The transalkylation of TMBs and toluene is desired reaction for the production of xylenes. However, toluene, required for this reaction, has to be produced from dealkylation reac-

tion as illustrated in Fig. 1. Tsai et al. (2009) reported that in order to enhance the transalkylation of TMBs, some refiners add a certain percentage of toluene to the heavy reformate feed. To study the effect of such conversion, different amounts of toluene were mixed with heavy reformate. The four mixed feedstocks used for conversion over M1Z2 and M2Z1 catalysts contain heavy reformate and toluene in the following molar ratio: (i) 20:80; (ii) 40:60; (iii) 60:40; and (iv) 80:20. The effect of heavy reformate/toluene feed on xylene yield is presented in Fig. 7. For 100% toluene feed, xylenes yield over M2Z1 was 13.7 wt.%. It increased with addition of heavy reformate and reached a maximum of 25.3 wt.% when the feed was a 70:30 mixture of heavy reformate/toluene. This indicates that the conversion of heavy reformate and toluene over M2Z1 has significant advantages. Further increase in the heavy reformate/toluene ratio beyond 70:30 resulted in a decrease in xylene yield. With M1Z2 catalyst, the xylenes yield increased with the increase in heavy reformate/toluene ratio in feed. However, an increase of heavy reformate/toluene ratio above 80:20 did not change the xylene yield. This difference in the behavior between M2Z1 and M1Z2 can be attributed to the wide difference in their acid site concentration. M1Z2, having higher acid site concentration, was able to produce the required amount of toluene for transalkylation of TMBs by dealklyation of MEBs. While M2Z1, having lower-acidity, was unable to do so. Therefore, in the case of M2Z1, conversion of toluene and heavy reformate was quite advantageous. Xylene yield obtained in the reaction of toluene and heavy reformate was compared with the yields from toluene and 1,2,4-TMB at their 1:1 molar mixture (Fig. 8) as well as from the conversion of heavy reformate and toluene over M1Z1 and M1Z2. Comparison of xylene yields obtained over M2Z1 indicates that 1:1 molar mixture of toluene and 1,2,4-TMB yielded 31.3 wt.% xylenes, which was much higher than for pure toluene (13.7 wt.%) or 1,2,4-TMB alone (18.2 wt.%). Conversion of heavy reformate resulted in a xylenes yield of 22.5 wt.%, which was also higher than the xylenes yield of 18.2 wt.% from conversion of 1,2,4-TMB. The conversion of 1:1 molar mixture of heavy reformate and toluene resulted in only marginal (from 22.5 wt.% to 25.3 wt.%) increase in xylenes yield

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Fig. 8 – Comparison of xylenes yield from conversion of toluene, 1,2,4-TMB and heavy reformate over M1Z2 and M2Z1 catalysts. compared to significant (from 18.2 wt.% to 31.3 wt.%) increase in the conversion of 1:1 molar mixture of 1,2,4-TMB and toluene. These results indicate that, for processing of heavy reformate, toluene required for transalkylation reaction was produced from dealkylation of MEBs and hence higher yield of xylenes was obtained from 1:1 molar mixture of heavy reformate and toluene than from 1,2,4-TMB alone. The results obtained from the conversion of heavy reformate and toluene over M1Z2 catalyst with higher-acid sites, were somewhat different. Comparison of xylenes yields obtained from toluene and 1,2,4-TMB indicates that 1:1 molar mixture of toluene and 1,2,4-TMB yielded 23.5 wt.% xylenes, which was higher than toluene alone (12.4 wt.%) or 1,2,4TMB alone (19.6 wt.%). Conversion of heavy reformate alone resulted in a xylenes yield of 19.8 wt.%, which was almost same as the xylenes yield of 19.6 wt.% from conversion of 1,2,4-TMB alone. The conversion of 1:1 molar mixture of heavy reformate and toluene resulted in marginal (1.1 wt.%) decrease in xylenes yield compared to 3.9 wt.%) increase in the conversion of 1:1 molar mixture of 1,2,4-TMB and toluene. These results indicate that the rate of dealkylation of MEBs to produce toluene was much higher over higher-acidity M1Z2 catalyst. This resulted in the production of required amount of toluene from heavy reformate alone. Therefore, addition of toluene to heavy reformate did not result in an increase in xylenes yield. In fact, a slight decrease in xylenes yield was observed possibly due to reduction of methyl groups in the reaction system.

4.

Conclusions

This study showed that the composition of heavy reformate can significantly alter the reaction rates and product yields over mordenite-based catalysts. Among the C9 components, MEBs were found to be more reactive than TMBs. Mordenite with higher acid site concentration (M1) favored dealkylation reactions while mordenite with lower acid site concentration (M2) favored disproportionation of C9 aromatics. The yield of total xylenes increased significantly by mixing mordenite with ZSM-5 zeolite. While mixing ZSM-5 and mordenite has shown appreciable advantage, mixing lower-acidity M2 with higher-acidity Z1 resulted in the highest xylenes yield (22.5 wt.%). It appears that such a mixture strikes the balance in catalytic properties for enhancing desirable reactions (such as dealkylation of MEBs to toluene, disproportionation of TMBs and transalkylation of TMBs and toluene, etc.), while limiting undesirable reactions (such as dealkylation of toluene etc.). The conversion of toluene and heavy reformate over lower-acidity M2Z1 increased xylenes yield to a maximum of 25.3 wt.% from a feed mixture containing 70:30 heavy reformate and toluene. Comparison of results for heavy reformate and 1,2,4-TMB indicates that toluene required for transalkylation was produced by dealkylation of MEBs; hence more xylenes yield was obtained from heavy reformate compared with 1,2,4-TMB alone. The results from the conversion of toluene/1,2,4-TMB and toluene/heavy reformate indicate that

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catalyst acidity plays a key role in promoting different reactions and hence the yield of total xylenes.

Acknowledgements This work is conducted at KFUPM-RI under the project No. CRP02238 sponsored by Saudi Aramco. The authors would like to thank KFUPM and Saudi Aramco for permission to publish this paper. Acknowledgement is also due to the Ministry of Higher Education for establishing the Center of Research Excellence in Petroleum Refining and Petrochemicals (CoREPRP) at KFUPM.

References Ali, S.A., Ali, M.A., Al-Nawad, K., Ercan, C., Wang, Y., 2011. Parametric study of dealkylation–transalkylation reactions over mordenite-based bi-functional catalysts. Applied Catalysis A: General 393 (1–2), 96–108. Al-Khattaf, S., Tukur, N.M., Al-Amer, A., Al-Mubaiyedh, U., 2006. Catalytic transformation of C7 –C9 methyl benzenes over USY-based FCC zeolite catalyst. Applied Catalysis A: General 305 (1), 21–31. Al-Khattaf, S., Ali, M., Cejka, J., 2010. Recent development in transformations of aromatic hydrocarbons over zeolites. In: Cejka, J., Corma, A., Zones, S. (Eds.), Zeolites and Catalysis, Synthesis, Reactions and Applications. Wiley-VCH, v.2, pp. 623–648. ˇ Cejka, J., Wichterlova, B., 2002. Acid-catalyzed synthesis of mono-and dialkylbenzenes over zeolites: active sites, zeolite topology, and reaction mechanisms. Catalysis Reviews Science and Engineering 44 (3), 375–422. ˇ ˇ A., Hanika, J., 1999. The effect of zeolite structure Cejka, J., Krejcí, on the disproportionation of trimethylbenzene to xylenes and tetramethylbenzenes. Studies in Surface Science and Catalysis 125, 351–358. ˇ ˇ ˇ A., Zilková, ˇ cek, ˇ Cejka, J., Krejcí, N., Dede J., Hanika, J., 2001. Alkylation and disproportionation of aromatic hydrocarbons over mesoporous molecular sieves. Microporous Mesoporous Materials 44–45 (1–3), 499–507. ˇ ˇ A., 2004. Disproportionation of Cejka, J., Kotrla, J., Krejcí, trimethylbenzenes over large pore zeolites: catalytic and adsorption study. Applied Catalysis A: General 277 (1–2), 191–199. Das, J., Bhat, Y.S., Bhardwaj, A.I., Halgeri, A.B., 1994a. Zeolite beta catalyzed C7 and C9 aromatics transformation. Applied Catalysis A: General 116 (1–2), 71–79. Das, J., Bhat, Y.S., Halgeri, A.B., 1994b. Transalkylation and disproportionation of toluene and C9 aromatics over zeolite beta. Catalysis Letters 23 (1–2), 161–168. de Lasa, H.I., 1992, Riser simulator for catalytic cracking studies. U.S. Patent No. 5,102,628. Dumitriu, E., Hulea, V., Kaliaguine, S., Huang, M.M., 1996. Transalkylation of the alkylaromatic hydrocarbons in the presence of ultrastable Y zeolites transalkylation of toluene with trimethylbenzenes. Applied Catalysis A: General 135 (1), 57–81.

2135

Dumitriu, E., Guimon, C., Hulea, V., Lutic, D., Fechete, I., 2002. Transalkylation of toluene with trimethylbenzenes catalyzed by various AFI catalysts. Applied Catalysis A General 237 (1), 211–221. Kraemer, D., PhD Thesis, University Western Ontario, London, Canada, 1991. ˇ Krejci, A., Al-Khattaf, S., Ali, M.A., Bejblova, M.J., Cejka, J., 2010. Transalkylation of toluene with trimethylbenzenes over large-pore zeolite. Applied Catalysis A: General 377 (1–2), 99–106. Lonyi, F., Valyon, J., 2001. On the interpretation of NH3 -TPD patterns of H-ZSM-5 and H-mordenite. Microporous and Mesoporous Materials 47 (2), 293–301. Norval, G.W., Phillips, M.J., 1989. Alkylation of toluene with ethylene: an equilibrium approach. Reaction Kinetics and Catalysis Letters 38 (2), 261–264. Park, S.H., Rhee, H.K., 2000. Shape selective conversion of 1,2,4-trimethylbenzene over zeolite NU-87. Catalysis Today 63 (2–4), 267–273. Röger, H.P., Möller, K.P., O’Connor, C.T., 1998. The reaction network in the conversion of 1,2,4-trimethylbenzene over HZSM-5. Journal of Catalysis 176 (1), 68–75. Serra, J., Guillon, E., Corma, A., 2005. Optimizing the conversion of heavy reformate streams into xylenes with zeolite catalysts by using knowledge base high-throughput experimentation techniques. Journal of Catalysis 232 (2), 342–354. Serra, J.M., Corma, A., Guillon, E., 2008, Catalyst comprising a 10MR zeolite and a 12MR zeolite, and its use in transalkylation of alkylaromatic hydrocarbons, U.S. Patent No. 7419931. Serra, J.M., Corma, A., Guillon, E., 2009, Process for transalkylation of alkyl-aromatic hydrocarbons used in two reaction zones, US Patent No. 7,629,499. Tsai, T.S., 2006. Reactivation of acidic sites in mordenite used in toluene disproportionation. Applied Catalysis A: General 301, 292–298. Tsai, T.C., Liu, S.B., Wang, I., 1999. Disproportionation and transalkylation of alkylbenzenes over zeolite catalysts. Applied Catalysis A: General A 181 (2), 355–398. Tsai, T.S., Chen, W., Lai, C., Liu, S., Wang, I., Ku, C.S., 2004. Kinetics of toluene disproportionation over fresh and coked H-mordenite. Catalysis Today 97 (4), 297–302. Tsai, S.T., Chao, P., Liu, C., Tsai, T.C., 2008. Synergism of acidic zeolite and Pt/zeolite in heavy aromatics transalkylation. Studies in Surface Science Catalysis 174B, 1183–1186. Tsai, T.C., Liu, S., Wang, I., 2009. Supported zeolite for heavy aromatics transalkylation process. Catalysis Surveys Asia 13 (2), 94–103. Tsai, S.T., Chao, P.H., Wang, I., Tsai, T.C., 2010. Dual-bed catalyst system for improving product purity and catalytic stability in alkylbenzene transalkylation. Applied Catalysis A: General 385 (1), 73–79. Wang, I., Tsai, T.C., Huang, S.T., 1990. Disproportionation of toluene and of trimethylbenzene and their transalkylation over zeolite beta. Industrial Engineering Chemistry Research 29 (10), 2005–2012. Wu, J.C., Leu, L.J., 1983. Toluene disproportionation and transalkylation reaction over mordenite zeolite catalysts. Applied Catalysis 7 (3), 283–294.