production of xylenes from toluene and 1,2,4-trimethylbenzene over zsm-5 and mordenite catalysts in...

Research Article

Production of Xylenes from Toluene and 1,2,4-Trimethylbenzene over ZSM-5 and MordeniteCatalysts in a Fluidized-Bed Reactor

The production of various xylenes from toluene, heavy aromatics such as 1,2,4-trimethylbenzene (1,2,4-TMB) and their mixture was investigated over H-ZSM-5(H-Z), H-mordenite (H-M) and a dual zeolitic catalyst comprising ZSM-5 andmordenite (H-ZM). The experiments were conducted in a riser-simulator reactorunder different operating conditions to study the effect of temperature, reactiontime and feed composition on conversion and product yields. At 400 °C, the con-version of toluene over the three catalysts yielded mainly benzene and xyleneswith maximum conversion at 25 % and a xylene yield of 12.5 wt % over the H-Mcatalyst. The transformation of 1,2,4-TMB doubled the conversion level andxylene yield and suppressed benzene formation. However, a considerable portionof the 1,2,4-TMB feed was isomerized into 1,2,3-TMB and 1,3,5-TMB accompa-nied by the formation of tetramethylbenzenes (TeMBs). The conversion of anequimolar mixture of toluene and 1,2,4-TMB over the three catalysts resulted inhigher toluene conversion and double xylene yield in comparison with 1,2,4-TMB alone. The advantage of using a dual zeolitic catalyst was observed at anequimolar feed of toluene and 1,2,4-TMB, exhibiting maximum toluene conver-sion, higher xylene yield and the formation of lower levels of undesirable pro-ducts.

Keywords: Disproportionation, Dual catalyst, Isomerization, Mordenite, Toluene,Transalkylation, Trimethylbenzenes, Xylenes, ZSM-5

Received: February 25, 2010; revised: March 23, 2010; accepted: April 27, 2010

DOI: 10.1002/ceat.201000083

1 Introduction

The production of xylenes via the disproportionation of tolu-ene or the transalkylation of heavy aromatics (with or withouttoluene or benzene) offers refiners an excellent opportunity toadd value to their product streams. Other advantages includecompliance with the more stringent gasoline specifications(less heavy aromatics) and the expected decline in gasoline de-mand. The main applications of xylenes are in the productionof synthetic fibers, plasticizers as well as solvents and gasolineblending. Commercial transalkylation processes such as Trans-Plus and Tatoray have become an essential part of the modernaromatics industry [1, 2]. Currently, p-xylene accounts for ca.83 % of the global demand for xylenes, which exceeds 42 mil-

lion tons per year, followed by o-xylene at 9 % of the totaldemand [3].

The conversion of toluene and heavy aromatics takes placeon solid acid catalysts and is one part of several parallel con-secutive reversible reactions, mainly transalkylation and dis-proportionation. The list of equations in Tab. 1 summarizessome of the individual reactions and products that can beidentified during the transalkylation reaction [4, 5]. Tolueneundergoes disproportionation to produce benzene and xy-lenes. TMBs get converted into xylenes and TeMBs and alsoundergo isomerization to an almost thermodynamic equilibri-um mixture. The utilization of zeolitic catalysts in the transfor-mation reaction plays a key role in the selective production ofdesirable xylene products. Various types of medium pore zeo-lites such as ZSM-5 [6], omega, erionite [7], NU-87 [8, 9],MCM-22 [9], and large pore zeolites such as ZSM-12 [10, 11],faujasites [12–17], beta [8, 17–19], mordenite [8, 11, 17],SAPO-5 [20], and zeolite L [21], have been tested for thetransalkylation reaction. Coke formation and catalyst stabilityare among the various factors affecting the activity and selec-

Abdullah M. Aitani1

Ashraf M. Ali1

Saidu M. Waziri1

Sulaiman Al-Khattaf1

1 Center of Research Excellencein Petroleum Refining andPetrochemicals, King FahdUniversity of Petroleum andMinerals (KFUPM), Dhahran,Saudi Arabia.

–Correspondence: Prof. S. Al-Khattaf (skhattaf@kfupm.edu.sa), Centerof Research Excellence in Petroleum Refining and Petrochemicals, KingFahd University of Petroleum and Minerals (KFUPM), Dhahran 31261,Saudi Arabia.

Xylenes 1193

tivity of these zeolite catalysts. However, current transalkyl-ation catalysts not only deactivate rapidly but are also difficultto regenerate.

The distribution of TMBs provides useful insight on thepore structure of zeolites during the transalkylation reaction ofheavy aromatics [22]. In zeolites with 12-ring channels, Mar-tens et al. [23] showed that zeolites with adjacent cages favorthe formation of the bulky 1,3,5-TMB isomer while zeoliteswith straight channels and side pockets at regular distances,such as mordenite, are favorable for the formation of the1,2,3-TMB isomer. The ratio of the rates of disproportionationto isomerization (D/I) is useful in providing informationabout the pore or cage size. Millini and Perego [24] carried outa computational study aimed at screening different zeolite cat-alysts, which were useful for the simultaneous disproportiona-tion and isomerization of TMB and TeMB. Serra et al. [25]tested and optimized the performance of multi-zeolitic cata-lysts with channel systems containing 10-ring, 12-ring, and10+12-ring geometries in the transalkylation of heavy refor-mate. It was found that the zeolite pore size and geometry havea direct influence on dealkylation and transalkylation of thedifferent alkyl groups. Tsai et al. [26] developed a dual-catalystsystem comprising of Pt/ZSM-12 and H-beta to improve thepurity of benzene product during transalkylation of heavy aro-matics.

The present study investigated the effect of feed, reactiontime and temperature on the transalkylation of toluene, 1,2,4-TMB, and a toluene-1,2,4-TMB mixture (50:50 mol.-% basis)over three types of zeolite-based catalysts comprising H-ZSM-5, mordenite and a dual zeolitic catalyst containing equalquantities of H-ZSM-5 and H-mordenite. The results highlightthe effect of the dual zeolitic catalyst on conversion, selectivityto different xylenes and the distribution of undesirable prod-ucts from the disproportionation and isomerization reactions.

2 Experimental

2.1 Catalysts and Materials

The ZSM-5 zeolite (MFI pentasil structure) was obtained fromCATAL International, UK, in the H-form and H-mordenite ze-olite (MOR structure) was obtained from the Tosoh Company,Japan. Both zeolites have a crystal size of 1000 nm. ZSM-5 hasa cubic crystal structure with straight 10-ring channels (5.3 ×5.5 Å). The channels are connected by sinusoidal channels(5.1 × 5.5 Å). Mordenite has an orthorhombic crystal structure

with straight 12-ring channels (6.5 x 7.0 Å) and crossed 8-ringchannels (2.8 x 5.7 Å). The two catalysts (H-Z and H-M) wereprepared by mixing the ZSM-5 and H-mordenite, separately,in dry forms with alumina binder and converting into gran-ules. The H-Z and H-M contain 66 wt % H-ZSM-5 andH-mordenite, respectively, whereas, the dual catalyst, H-ZM,contains 33 wt % each H-ZSM-5 and H-mordenite. All threecatalysts contain 33 wt % alumina binder, (Cataloid AP-3)obtained from CCIC, Japan. The binder contains 75.4 wt %alumina, 3.4 wt % acetic acid, and water as the balance. Thedual H-ZM catalyst was prepared following the procedure ofHassan et al. [27].

Analytical grade (99 % purity) pure 1,2,4-TMB and toluenewere obtained from Sigma-Aldrich. All chemicals were used asreceived and no attempt was made to further purify the sam-ples.

2.2 Catalyst Characterization

The surface area of the catalysts was measured by nitrogen ad-sorption at –196 °C using a NOVA 1200 Porosimeter fromQuantachrome. The concentration and the type of acid siteswere determined by adsorption of pyridine as probe moleculesfollowed by FTIR spectroscopy (Nicolet 6700 FTIR) using theself-supported wafer technique. Pyridine can be used withoutspatial restrictions in medium and large pore zeolites. It can si-multaneously probe Brönsted and Lewis type acidity, givingeasily distinguishable bands for protonated and coordinativelybonded forms [28]. Prior to the adsorption of probe mole-cules, self-supporting wafers of samples were activated in situby evacuation at a temperature of 450 °C overnight at a pres-sure of 1 · 10–4 Torr. The adsorption of pyridine proceeded at150 °C for 20 min at a partial pressure of 5 Torr followed by a20 min evacuation at different temperatures, i.e., 150, 250, 350and 450 °C.

The concentrations of Brönsted and Lewis acid sites were cal-culated from the integral intensities of individual bands charac-teristic of pyridine on Brönsted acid sites at 1550 cm–1 and aband of pyridine on Lewis acid sites at 1455 cm–1 by using mo-lar absorption coefficients of e(B) = 1.67 ± 0.1 cm lmol–1 ande(L) = 2.22 ± 0.1 cm lmol–1, respectively [29]1). The spectrawere recorded with a resolution of 4 cm–1 by the collection of128 scans for each single spectrum.

For the determination of Si/Al, the data after desorption at250 °C were used, due to the fact that the adsorption of pyri-dine at 150 °C is also influenced by adsorption on non-acidicOH groups and pyridine easily desorbs between 150 and250 °C. With increasing temperature of desorption, the inten-sity of the band assigned to pyridine adsorbed on Brönsted orLewis acid sites decreases, and therefore, shows the existence ofa variation of these sites with different acid strength. The val-ues of the Si/Al molar ratio provided for each catalyst sampleis based on the assumption that all sites are involved in inter-action with the pyridine molecules.

Table 1. Typical reactions of toluene and 1,2,4-TMB transforma-tions.

Transalkylation (1) 1,2,4-TMB + Toluene → 2 Xylenes

Isomerization (2) Xylene Isomers → p-, m-, o-xylene(3) 1,2,4-TMB → 1,2,3-TMB + 1,3,5-TMB

Disproportionation (4) 2 (1,2,4-TMB) → Xylenes + TeMBs(5) 2 Toluene → Xylenes + Benzene

Dealkylation (6) TMBs → Toluene + Light Gases

–1) List of symbols at the end of the paper.

1194 A. M. Aitani et al.

2.3 The Riser Simulator

The conversion of toluene and 1,2,4-TMB was carried out in ariser simulator reactor, which is a bench-scale unit with an in-ternal recycle unit, as invented by de Lasa [30]. The riser simu-lator consists of two outer shells, i.e., the lower section and theupper section, which allow the catalyst to be loaded or un-loaded easily. The reactor was designed in such a way that anannular space is created between the outer portion of the bas-ket and the inner part of the reactor shell. A metallic gasketseals the two chambers with an impeller located in the uppersection. A packing gland assembly and a cooling jacket sur-rounding the shaft provide support for the impeller. Uponrotation of the shaft, gas is forced outward from the center ofthe impeller towards the walls. This creates a lower pressure inthe center region of the impeller, thus inducing a flow of gasupward through the catalyst chamber from the bottom of thereactor annular region where the pressure is slightly higher.The impeller provides a fluidized bed of catalyst particles aswell as intense gas mixing inside the reactor. A detaileddescription of various riser simulator components, sequenceof injection and sampling can be found in the work by Krae-mer [31].

2.4 Experimental Procedure

All experiments were conducted at a catalyst/reactant ratio of5 (mass of catalyst = 0.81 g, mass of reactant feed injected =0.162 g), residence times of 5, 10, 15 and 20 s, and tempera-tures of 300, 350 and 400 °C. Three types of feeds were used,i.e., 100 % toluene, 100 % 1,2,4-TMB, and an equimolar mix-ture of toluene and 1,2,4-TMB. For the dual catalyst, two addi-tional mixtures of toluene and 1,2,4-TMB were used at30:70 % and 70:30 %, respectively. During the investigations, anumber of runs were repeated to check for reproducibility inthe conversion results, which was found to be excellent. Typicalerrors were in the range of ± 2 %.

The reactor was heated to the desired reaction temperature.The vacuum box was also heated to ca. 250 °C and evacuatedat around 0.5 psi to prevent any condensation of hydrocarbonsinside the box. The heating of the riser simulator was con-ducted under continuous flow of inert gas (argon) and theprocess usually required a few hours until thermal equilibriumwas finally attained. Meanwhile, before the initial experimentalrun, the catalyst was activated for 15 min at 620 °C in a streamof air. The temperature controller was set to the desired reac-tion temperature and the timer was adjusted to the desiredreaction time in a similar manner. At this point, the gas chro-matograph (GC) was started and set to the desired conditions.

Once the reactor and the GC reached the desired operatingconditions, the feed stock was injected directly into the reactorvia a loaded syringe. After the reaction, the four-port valveopened immediately ensuring that the reaction was terminatedand the entire product stream sent online to the analyticalequipment via a pre-heated vacuum box chamber.

The reaction products were analyzed in an Agilent 6890NGC equipped with a flame ionization detector (FID) and acapillary column INNOWAX, 60-m cross-linked polyethylene

glycol with an internal diameter of 0.32 mm. The level of cokedeposited on spent catalysts was determined by a commoncombustion method. In this method, a carbon analyzer multiEA 2000 (Analytikjena) was used. Oxygen was supplied to theunit directly. A small amount of the spent catalyst (0.35 g) wasused for the analysis. The coke deposited on the sample duringreaction experiments was burned completely, thus convertingthe carbonaceous deposit into carbon dioxide. The amount ofcoke formed was determined by measuring the moles of car-bon dioxide.

The conversion levels of toluene and 1,2,4-TMB were calcu-lated according to the following expressions, Eqs. (1) and (2),respectively:– Conversion of toluene (TOL):

xTOL � �TOL�0 � �TOL�P

�TOL�0

� 100 % (1)

– Conversion of 1,2,4-TMB:

xTMB � �TMB�0 � �TMB�P

�TMB�0

� 100 % (2)

where (TMB)0 and (TOL)0 represent the initial concentrationsof 1,2,4-TMB and toluene, respectively, while (TMB)P and(TOL)P are the respective concentrations in the product mix-ture. The apparent selectivity ratio of disproportionation (SD)to isomerization (SI) is defined by Eq. (3) [9]:

SD/SI = 2(TeMB isomers)/(1,2,3- and 1,3,5-TMB) (3)

3 Results and Discussion

3.1 Catalyst Characterization

The physical and acid properties of the three catalysts are pre-sented in Tab. 2. The H-Z and H-M catalysts exhibited typicaltextural properties with BET surface areas of 295 m2/g and411 m2/g, respectively. The dual H-ZM catalyst showed a rela-tively high surface area of 352 m2/g. The Si/Al molar ratios ofthe catalysts measured by FTIR were 29, 136 and 36 for H-Z,H-M and H-ZM, respectively. The results of acidity measure-ments showed that all FTIR spectra exhibited the followingsimilar characteristic bands:

Table 2. Results of catalyst characterization.

Property H-Z H-M H-ZM

BET Surface Area [m2/g] 295 411 352

SiO2/Al2O3 Molar Ratio 29 136 36

Total Acidity (Brönsted and Lewis(from Tab. 3)) [mmol/g]

0.50 0.13 0.49

Lewis Acid Sites [%] 68 80 73

Brönsted Acid Sites [%] 32 20 27

Xylenes 1195

– 3740–45 cm–1: silanol groups;– 3660–3670 cm–1: OH groups attached to alumina, which are

mostly extra-framework, and– 3608–3612 cm–1: acidic Si-OH-Al groups of zeolites

Typical FTIR spectra of the H-Z, H-M, and H-ZM catalystsare shown in Fig. 1. The values of Brönsted and Lewis acidityare presented in Tab. 3 and plotted in Fig. 2. The distributionof Brönsted and Lewis acidity in the H-Z catalyst is similar tothat in the H-ZM catalyst. The amount of pyridine adsorbedon Brönsted and Lewis acid sites at 150 °C is approximatelysimilar in the two catalysts. The Brönsted and Lewis acidity ofthe H-Z and H-ZM catalysts decreased rapidly with increasingdesorption temperature. However, in the case of the H-M cata-lyst, the Brönsted acidity was initially low and did not showsignificant change with increasing temperature. Among thethree zeolites, H-M showed the lowest total concentration ofacid sites at 0.15 mmol/g, while H-Z and H-ZM showed totalacid site numbers of 0.5 mmol/g each. The total acidity(Brönsted and Lewis) increased in the order of H-M < H-Z =H-ZM as seen from Tab. 3. The strength of acid sites influencesthe competition among the various aromatic transformationreactions listed in Tab. 1. Generally, transalkylation and dis-proportionation reactions occur on the medium and strongeracid sites, while isomerization of xylenes and TMBs predomi-nate on weaker acid sites [20].

3.2 Transformation Reactions

The results for the transformations of toluene, 1,2,4-TMB andtheir mixtures over H-Z, H-M, and H-ZM catalysts at 400 °Cand 20 s reaction time are compared in Tab. 4. The data in-clude conversion of toluene and 1,2,4-TMB and product yieldssuch as light gases (C1–C4), benzene, toluene, xylene isomers(p-xylene, m-xylene, and o-xylene), TMB isomers (1,3,5-TMB,1,2,4-TMB, 1,2,3-TMB) and TeMBs. The selectivity ratios inTab. 4 include results for p-xylene/xylenes, p-xylene/o-xylene,xylenes/TeMBs and 1,3,5-/1,2,3-TMB. The effect of tempera-ture and selectivity to benzene, xylenes, TMBs, and TeMB overthe three catalysts are presented in Tabs. 5–7. The effect of

reaction time on conversion and xylene yield are shown inFigs. 3–7. In general, the results for the three feeds show thatisomerization, disproportionation and transalkylation reac-tions occur to different extents for the three catalysts. Negligi-ble amounts of light gases and coke were reported over thethree catalysts due to the absence of dealkylation and second-ary reactions.

The results show that the distribution of xylenes over all ofthe catalysts was close to their equilibrium value as reported bythe ratio of p-xylene/xylenes, which was within 23–25 % andby the ratio of p-xylene/o-xylene, which was within 0.9–1.1.This indicates that under the reaction conditions used, noproduct selectivity was observed even over the 10-ring H-Zcatalyst. Among the xylene isomers, o-xylene is preferentiallyproduced by the disproportionation of 1,2,4-TMB followed byisomerization into m- and p-xylene.

The disproportionation and isomerization proceed at com-parable rates, and thus, it may be appropriate to define theapparent selectivity ratio, SD/SI, between these reactions, aspresented in Tab. 4 [9]. As seen from Tab. 1, the dispropor-tionation reaction involves the formation of xylenes andTeMBs from 2 molecules of 1,2,4-TMB. Since a bimolecularmechanism of TMB operates compared with a monomolecular1,2-methyl shift around the benzene ring, the channel diameterplays the most important role. The 12-ring geometry of mor-denite allows the formation of a bimolecular intermediate,which is sterically hindered in the 10-ring channel system ofthe ZSM-5 zeolite.

For an equimolar mixture of toluene and 1,2,4-TMB, theyield of TeMB decreased significantly, indicating that thedisproportionation of TMB was responsible for TeMBs forma-tion. The data from the isomerization reaction show thathigher amounts of 1,3,5-TMB were found compared with1,2,3-TMB for all catalysts and two feeds even though the1,2,3-isomer has a smaller molecular size than 1,3,5-TMB.However, 1,3,5-TMB is thermodynamically favored over the1,2,3-isomer [5]. The ratio of 1,3,5-TMB to 1,2,3-TMB wasabove 2.0 for all the catalysts. Furthermore, three different iso-mers of TeMBs were detected, i.e., 1,2,4,5-TeMB, 1,2,3,5-TeMBand 1,2,3,4-TeMB. The experiments also suggest that the distri-

4000 3800 3600 3400 3200

Wavenumber, cm-1

b 361036653743

1550 1500 1450

4000 3800 3600 3400 3200

Wavenumber, cm-1

360836713736

1550 1500 1450 1400

4000 3800 3600 3400 3200

Wavenumber, cm-1

36083668

1550 1500 1450 1400

H-MFigure 1. FTIR spectra of H-Z, H-ZM and H-M catalysts showing two regions: (A) IR spectra of the hydroxyl vibration region, (B) IR spec-tra of the pyridine region after its adsorption: (a) before adsorption, (b) desorption for 20 min at 150 °C, (c) desorption for 20 min at250 °C, (d) desorption for 20 min at 350 °C, and (e) desorption for 20 min at 450 °C.

bution of the xylenes and TeMB in the disproportionationproducts closely follow the values reported in the literature[5, 9].

3.3 Conversion of Toluene

The conversions of toluene over the three catalysts, i.e., H-Z,H-M, and H-ZM, as a function of temperature (300, 350 and400 °C) and reaction time (20 s) are presented in Tab. 5 andFigs. 3 and 4. The data in Tab. 5 show that each of the threecatalysts disproportionate toluene to benzene and xylenes atdifferent activities. The conversion increased with increasingtemperature and time-on-stream for all three catalysts, Fig. 3.The conversion was minimal (less than 5 %) at 300 °C and then

sharply increased to more than 20 % at 400 °C. The conversionat 400 °C and 20 s reaction time followed the order H-Z <H-ZM < H-M. However, this order did not match with theorder of total acidity reported in Tab. 2. The high conversionof toluene over H-M catalyst can be attributed to the relativelylarg pore size when compared with H-Z catalyst. The conver-sion of toluene over H-Z increased from 10 % at 350 °C to15 % at 400 °C compared with an increase from 5 % to 25 %for H-M, and an increase from 4 % to 23 % for H-ZM, respec-tively.

The selectivity to benzene and xylenes was between 45–50 %with minimal formation of TMBs. The yield of xylenes at400 °C and 20 s reaction time was 8 %, 13 %, 11 % for H-M,and H-ZM, respectively as shown in Fig. 4. The distribution ofxylene isomers over the three catalysts was close to their equi-

Figure 2. Plot of Brönsted (A) and Lewis (B) acidity values obtained at different desorption temperatures for (�) H-Z, (�) H-M, and (�)H-ZM catalysts.

Table 3. Changes in catalyst acidity with increasing temperature for the three catalysts.

Temperature [°C] Brönsted Acidity [mmol/g] Lewis Acidity [mmol/g]

150 250 350 450 150 250 350 450

H-Z 0.16 0.16 0.12 0.07 0.34 0.19 0.09 0.04

H-M 0.03 0.02 0.02 0.02 0.12 0.05 0.03 0.01

H-ZM 0.13 0.12 0.09 0.05 0.36 0.17 0.09 0.04

Figure 3. Effect of temperature and reaction time on the conversion of toluene over (�) H-Z, (�) H-M, and (�) H-ZM catalysts.

Xylenes 1197

Table 4. Overall results for toluene and 1,2,4-TMB conversion at 400 °C and 20 s reaction time.

Feed Toluene 1,2,4-TMB Toluene/1,2,4-TMB (50:50)

Catalyst H-Z H-M H-ZM H-Z H-M H-ZM H-Z H-M H-ZM

Conversion [%]

Toluene 14.6 25.3 23.1 – – – 31.3 34.4 44.4

1,2,4-TMB – – – 31.6 57.8 56.6 28.2 57.2 52.3

Product Yields [wt %]

Light Gases 0.1 0.0 0.0 0.4 0.2 0.3 0.2 0.0 0.2

Benzene 6.6 11.8 10.8 0.0 0.1 0.1 2.3 2.7 2.2

Toluene 85.4 74.6 76.9 1.0 2.9 2.5 34.4 28.5 27.8

Xylenes 7.7 12.5 11.1 5.5 17.7 16.5 13.8 32.5 30.6

p-xylene 2.1 3.1 2.8 1.3 4.2 3.8 3.6 7.9 7.4

m-xylene 4.0 6.6 5.8 2.7 9.3 8.6 7.0 17.1 15.8

o-xylene 1.6 2.8 2.5 1.5 4.2 4.10 3.2 7.5 7.4

1,3,5-TMB 0.1 0.3 0.2 12.3 10.5 11.2 7.4 5.8 6.7

1,2,4-TMB 0.2 0.7 0.5 68.4 42.2 43.4 35.9 24.2 23.9

1,2,3-TMB 0.1 0.1 0.1 6.4 3.9 4.1 3.7 2.1 2.4

TeMBs 0.0 0.0 0.0 5.7 21.3 20.1 1.3 3.7 3.6

Selectivity Ratios

p-xylene/xylenes 0.27 0.25 0.25 0.23 0.24 0.23 0.26 0.24 0.24

p-xylene/o-xylene 1.3 1.11 1.12 0.87 1.00 0.92 1.13 1.05 1.00

xylenes/TeMBs – – – 0.97 0.83 0.82 10.6 1.01 8.50

1,3,5-/1,2,3-TMB 1.00 3.00 1.00 1.92 3.00 2.73 2.00 2.76 2.80

SD/SIa – – – 0.6 3.0 2.6 0.3 0.9 0.8

a SD/SI = 2(TeMBs)/(1,2,3- and 1,3,5-TMB

Table 5. Effect of temperature on toluene conversion and selectivity to benzene, xylenes and TMBs at 20 sreaction time.

Catalyst Temperature [°C] Conversion [%]Selectivity [%]

Benzene Xylenes TMBs

H-Z 350 10.1 48.5 50.5 1.0

400 14.6 45.2 52.7 2.1

450 22.9 43.7 52.0 4.3

H-M 300 4.9 45.6 43.0 11.4

350 17.4 47.5 49.8 2.7

400 25.3 46.6 49.5 3.9

H-ZM 300 4.3 50.0 50.0 0.0

350 14.4 47.7 50.4 1.9

400 23.1 46.6 47.8 5.6

librium value, i.e., 50 % m-xylene and 25 % each for p-xyleneand o-xylene. The amount of benzene produced was highestfor H-M, followed by H-ZM and H-Z catalysts and demon-strated a similar trend as the toluene conversion. The H-Z cat-alyst showed negligible activity for the formation of TMBs andno TeMBs and light gases were detected in the reaction prod-ucts for all three catalysts.

3.4 Conversion of 1,2,4-TMB

The conversion of 1,2,4-TMB undergoes either a dispropor-tionation reaction to produce xylenes and TeMBs or an isom-erization reaction to give 1,3,5-TMB and 1,2,3-TMB. Theeffects of temperature (300–400 °C) on the conversion of1,2,4-TMB over the three catalysts at 20 s reaction time, arepresented in Tab. 6 and Figs. 5 and 6. The data in Tab. 6 alsoshow the selectivity to disproportionation products (xylenesand TeMBs) and isomerization products (TMBs). The resultsin Fig. 5 show the effect of reaction time on total xylenes andp-xylene yields at 400 °C. For all three catalysts, the conversion

of 1,2,4-TMB increased with increasingtemperature and time-on-stream.However, the selectivity to xylenes andother major products such as TMBsand TeMBs did not show much differ-ence. The conversion of 1,2,4-TMBover the three catalysts (at 20 s reactiontime) decreased in the order H-Z <H-ZM < H-M. However, this order didnot match the order of total acidity re-ported in Tab. 2. The high conversionof 1,2,4-TMB over H-M is attributedto the relatively large pore size of H-Mwhen compared with H-Z. The forma-tion of very small amounts of tolueneover the three catalysts is attributed tosecondary reactions such as transalkyl-

ation of xylene and TeMB. Under the present reaction condi-tions, the portion of secondary reactions was very small andthe xylene to TeMB ratio over all of the catalysts was close tounity, as seen from Tab. 4.

The conversion over H-Z increased from 8 % at 300 °C to32 % at 400 °C compared with an increase from 42 % to 48 %for H-M, respectively. The dual catalyst showed a significantincrease in conversion from 54 % at 300 °C to 58 % at 400 °C.While H-Z showed the highest isomerization selectivity toTMBs (ca. 57 %) and ca. 16 % to xylenes over the temperaturerange tested, the other two catalysts showed similar behaviorin selectivity to xylenes (28 %), TMBs (30 %) and TeMBs(35 %). At 400 °C, the yield of TeMBs was ca. 20 % over H-ZMcompared with 17.6 % over H-M and 5.7 % over H-Z. How-ever, the dual catalyst did not minimize the formation of heavyaromatic compounds as they may be precursors for the forma-tion of coke, which reduces catalyst activity.

The disproportionation/isomerization selectivity ratio,SD/SI, of TMBs over H-M and H-ZM was greater than unityindicating TMBs undergo disproportionation, compared to aratio of 0.6 over H-Z indicating a preference to isomerization.

Figure 4. Yields of total xylenes and p-xylene from toluene conversion as a function of reac-tion time at 400 °C over (�) H-Z, (�) H-M, and (�) H-ZM catalysts.

Table 6. Effect of temperature on 1,2,4-TMB conversion and selectivity to xylenes, TMBs, and TeMB at 20 s reac-tion time.

Catalyst Temperature [°C] Conversion [%]Selectivity [%]

Xylenes 1,2,3-TMBand 1,3,5-TMB

H-Z 300 8.3 16.0 56.9 19.8

350 20.3 15.1 56.7 17.0

400 31.6 17.5 59.2 18.1

H-M 300 53.6 29.5 29.2 36.0

350 57.7 30.1 26.4 37.2

400 57.8 30.5 25.2 36.9

H-ZM 300 42.9 28.1 37.8 30.8

350 54.1 28.1 30.7 35.3

400 56.6 29.1 26.9 35.4

Xylenes 1199

The amount of TeMBs produced on H-M and H-ZM catalystsis higher than H-Z indicating that when the zeolite pores arespacious enough to allow the bulky aromatics to diffuse freely,the possibility to produce bulky aromatic molecules, which arethe precursor of coke, increases.

Millini and Perego [24] studiedthe simulation of 1,2,4-TMB con-version using the dimensions ofdifferent transformation moleculesthat were determined on the basisof the van der Waals radii of theatoms [32]. It was suggested thatthe isomerization reaction leads tothe formation of products that arebulkier than 1,2,4-TMB. This ob-servation indicates that mediumpore zeolites such as ZSM-5 wouldbe unsuitable for this reaction andlarge pore zeolites are preferred. Incontrast, medium pore zeolitesmay be of interest for the dispro-portionation reaction since the de-sired product (1,2,4,5-TeMB) has adimension comparable with that of1,2,4-TMB, which is smaller thanthose of the other TeMB isomers

[24]. However, the simulation studydid not provide any definitive answerto the problem in the sense that noclear indications were obtained aboutthe existence of a zeolite structure suit-able as a catalyst for the isomerizationof 1,2,4-TMB to 1,3,5-TMB and thedisproportionation to 1,2,4,5-TeMB.

3.5 Conversion of a Toluene and1,2,4-TMB Mixture(Equimolar Basis)

The conversion of an equimolar mix-ture of toluene and 1,2,4-TMB overthe three catalysts resulted in highertoluene conversion accompanied witha significant increase in the selectivityto xylenes, i.e., ca. 30 % increase, and asignificant drop in the selectivity tobenzene and TeMBs. As indicated inthe transalkylation reactions listed inTab. 1, two moles of xylenes are formedfrom the transalkylation of toluene and1,2,4-TMB compared to one mole eachof xylenes and benzene from the dis-proportionation of toluene. The resultslisted in Tab. 7 and shown in Figs. 7and 8 demonstrate an increase in con-version with increasing temperature,similar to the results reported earlierfor toluene and 1,2,4-TMB feeds alone.

The 1,2,4-TMB conversion (at 20 s reaction time) followed theorder H-Z < H-M < H-ZM, whereas for toluene conversion,the order was as follows: H-M < H-Z < H-ZM. The order oftoluene conversion matches with the order of the total acidityof the catalysts reported in Tab. 2.

Figure 5. Effect of temperature and reaction time on the conversion of 1,2,4-TMB over (�)H-Z, (�) H-M, and (�) H-ZM catalysts.

Figure 6. Yields of total xylenes and p-xylene for the conversion of 1,2,4-TMB as a function ofreaction time at 400 °C over (�) H-Z, (�) H-M, and (�) H-ZM catalysts.

Table 7. Effect of temperature on toluene and 1,2,4-TMB (50:50 mol.-ratio) conversion and selec-tivity to xylenes, TMBs, and TeMBs at 20 s reaction time.

Catalyst Temperature [°C]Conversion [%] Selectivity [%]

Toluene 1,2,4-TMB Xylenes 1,2,3-TMB and1,3,5-TMB

H-Z 300 18.0 1.2 41.8 54.8 6.1

350 22.3 14.1 44.7 44.2 4.9

400 31.3 28.2 48.8 39.5 4.5

H-M 300 23.6 56.8 49.8 14.7 5.6

350 30.6 58.6 55.0 13.8 5.8

400 34.4 57.2 56.7 13.8 6.5

H-ZM 300 29.6 44.0 50.7 22.4 6.4

350 39.5 48.8 60.6 18.9 6.7

400 44.4 52.3 58.5 17.3 6.8

The dual catalyst showed a high selectivity to xylenes at ca.60 % compared with the other two catalysts. The selectivity toundesirable TeMBs was ca. 6.5 % over all catalysts within thetemperature range tested. It is well known that the formationof TeMB is dictated by thermodynamics, and hence, tends todecrease with decreasing 1,2,4-TMB in the feed (see dispropor-tionation reactions in Tab. 1). Since a very negligible amountof toluene was found during the conversion of 1,2,4-TMBalone, the transformation of toluenecan only be attributed to the transalk-ylation reaction. The results show thatthe ratio of xylenes to TeMBs is lessthan 1, indicating the absence of sec-ondary transalkylation or dealkylationreactions, and hence, a negligibleamount of light gases [5, 9]. 1,2,4-TMB can simultaneously undergoisomerization and disproportionationreactions. The selectivity ratio of dis-proportionation to isomerization, SD/SI, was less than 1.0 indicating thatisomerization was dominant over thethree catalysts.

3.6 Effect of Feed Composition

The data in Tab. 4 and Fig. 9 show theadvantage of adding 1,2,4-TMB to tol-uene transalkylation on the yields ofxylenes and benzene at 400 °C for thethree catalysts. As the amount of 1,2,4-TMB in the feed mixture was in-creased, the number of alkyl groupsrelative to aromatic rings increased,thereby increasing the formation ofxylenes. In the case of pure toluenefeed, the xylene yield ranged between7.7–12.5 wt % for H-Z, H-ZM andH-M catalysts. However, a maximumxylene yield was reached at an equimo-lar feed mixture of the three catalysts.Beyond this point, as the 1,2,4-TMBfraction in the feed was increased, thexylene yield began to fall as heavier al-kylbenzenes such as TeMBs were pro-duced, Tab. 4. The behavior of benzeneyield was different over the three feedcompositions, as seen in Fig. 9. Theyield decreased significantly from 7–12 wt % in the case of pure toluene asthe amount of 1,2,4-TMB was in-creased in the feed for the three cata-lysts. The addition of 1,2,4-TMB sup-pressed benzene yield and enhancedthe production of xylenes.

The effect of temperature (300 °Cand 400 °C) and the addition of 1,2,4-TMB in the feed mixture (30, 50, 70,100 %) on xylene yield were investi-

gated over the dual catalyst (H-ZM). The results in Fig. 10show that the trend in xylene yield was similar to that for theother catalysts with a maximum xylene yield of 32 wt % at anequimolar feed composition and 400 °C compared with23 wt % at 300 °C. The results show that the production ofxylenes from toluene can be enhanced and the undesired prod-ucts minimized as a result of higher temperature and the addi-tion of a heavier aromatic compound (1,2,4-TMB) in the feed.

Figure 7. Effect of temperature and reaction time on the conversion of an quimolar mixtureof toluene and 1,2,4-TMB over (�) H-Z, (�) H-M, and (�) H-ZM catalysts.

Figure 8. Yields of total xylenes and p-xylene for the conversion of an equimolar mixture oftoluene and 1,2,4-TMB as a function of reaction time at 400 °C over (�) H-Z, (�) H-M, and(�) H-ZM catalysts.

Figure 9. Yields of xylenes and benzene as a function of mol.-% of 1,2,4-TMB in toluene anda 1,2,4-TMB feed mixture at 400 °C and 20 s reaction time over (�) H-Z, (�) H-M, and (�)H-ZM catalysts.

Xylenes 1201

4 Conclusions

This study demonstrated the positive effect of using a dualzeolitic catalyst of ZSM-5 and mordenite in the conversion ofa toluene and 1,2,4-TMB mixture in a riser simulator thatmimics a fluidized-bed reactor. The dual catalyst exhibitedhigher activity for toluene disproportionation in an equimolarfeed mixture of toluene and 1,2,4-TMB at 400 °C. The conver-sion of toluene was maximized, yield of xylenes was doubledand formation of by-products (benzene and TeMBs) was mini-mized for the dual catalyst. However, at low concentrations of1,2,4-TMB in the feed mixture, the disproportionation of tolu-ene at 400 °C was the predominant reaction for the productionof xylenes over the three catalysts. Using 1,2,4-TMB feed alone,the production of xylenes was lower compared to the equimo-lar mixture and a considerable portion of 1,2,4-TMB wasisomerized into 1,2,3-TMB and 1,3,5-TMB accompanied withthe formation of TeMBs. The three catalysts showed a higherconversion of 1,2,4-TMB compared with toluene alone.

Acknowledgements

The authors express their appreciation for support from theMinistry of Higher Education, Saudi Arabia, in the establish-ment of the Center of Research Excellence in Petroleum Refin-ing and Petrochemicals (CoRE-PRP) at the King Fahd Univer-sity of Petroleum and Minerals (KFUPM).

Symbols used

SD [–] apparent selectivity ratio ofdisproportionation

SI [–] apparent selectivity ratio ofisomerization

xTOL [%] conversion of toluenexTMB [%] conversion of 1,2,4-TMB

Greek symbols

e(B) [cm lmol–1] Brönsted acid molar absorptioncoefficients

e(L) [cm lmol–1] Lewis acid molar absorptioncoefficients

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Figure 10. Yields of xylenes as a function of temperature andmol.-% of 1,2,4-TMB in the toluene and a 1,2,4-TMB feed mix-ture over dual H-ZM catalyst and 20 s reaction time.

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