Transalkylation of ethyl benzene with triethylbenzene over ZSM-5 zeolite catalyst

https://doi.org/10.1016/j.cej.2010.07.026Get rights and content

Abstract

Transalkylation of 1,3,5-triethylbenzene (TEB) with ethylbenzene (EB) has been studied over ZSM-5 zeolite using a riser simulator reactor with respect to optimizing DEB yield. The reaction temperature was varied from 350 to 500 °C with contact time ranging from 3 to 15 s to report on the effect of reaction conditions on TEB conversion, DEB selectivity and isomerization of TEB. The transalkylation of TEB with EB was compared with the reactions of pure 1,3,5-TEB and EB (disproportionation, isomerization and cracking). A synergistic effect was observed on the conversion of 1,3,5-TEB and DEB yield. The 1,3,5-TEB conversion increased from 40% to 50% with simultaneous increase in the DEB selectivity from 17% to 36% in transalkylation reaction (EB + 1,3,5-TEB) as compared with the reaction of pure 1,3,5-TEB. It was found that pure 1,3,5-TEB underwent cracking reaction to produce DEB and EB. The isomerization of 1,3,5-TEB was more active at low temperature while cracking was more active at high temperature. The temperature of 350 °C was observed as the optimum for production of maximum amount of DEB. Kinetic parameters for the disappearance of 1,3,5-TEB during its transformation reaction via cracking and isomerization pathways were calculated using the catalyst activity decay function based on time-on-stream (TOS). The apparent activation energies decrease in order Esecondary cracking > Eprimary racking > Eisomerization for ZSM-5 catalysts.

Introduction

Reactions of aromatic hydrocarbons represent the most important part of petrochemistry and zeolites are indispensable catalysts for these applications [1]. Alkylations [2] disproportionations [3], [4] isomerizations [5], and transalkylation reactions [6], [7] are of the primary interest for upgrading of less-valuable aromatics.

Considerable industrial demand recently appeared for selective production of diethylbenzene. The para-diethylbenzene (p-DEB) is a high value adsorbent commonly for p-xylene adsorptive separation in processes UOP Parex and IFP eluxyl [8], [9], [10], [11]. The p-DEB is key starting material for production of p-divinyl benzene an important monomer for the production of copolymers, such as ion-exchange resin and viscosity modifiers of lubricant oil. In contrast and similarly to the situation with xylene isomers, o-DEB and m-DEB have lower market value. Due to the fact that all three isomers of DEB posses similar physical properties their separation is a difficult task and direct highly selective preparation of p-DEB is still rather challenging.

Recently, two processes of production of p-DEB, i.e. EB alkylation with either ethanol [12], [13] or ethylene [14] and shape selective EB disproportionation were reported [15], [16], [17]. Initially, Karge et al. [18], [19] used EB disproportionation to characterize acid forms of zeolites. Large-pore zeolites exhibited an induction period but their selectivities to p-diethylbenzene were close to thermodynamic ones. On the other hand, significant increase in p-DEB selectivity was observed for ZSM-5 zeolite. Following this pioneer work, numerous studies were carried out over USY, Beta, MCM-22, ZSM-5 [20], [21], [22]. The composition of the diethylbenzene isomers in the reaction mixtures was usually limited by thermodynamics leading to an equilibrium composition with relative low amount of the p-DEB compared with the m-DEB. This resulted in the conclusion that the mechanism of zeolite-catalyzed ethylbenzene disproportionation is very similar to toluene disproportionation i.e. differences in diffusion coefficients among para, meta, and ortho diethylbenzenes play an important role [23]. Further on, proper deactivation of the external surface of zeolite crystals by Mg or P compounds enhances the selectivity up to 98–99% [23].

Detailed mechanistic study by Huang et al. [24], [25] using solid state 13C NMR evidenced that zeolite acidity and pore size strongly affect their catalytic behavior in EB disproportionation. It was observed that EB disproportionation occurred at low reaction temperatures over large-pore zeolite H-Y in contrast to medium-pore ZSM-5 zeolites. This can be the effect of diffusion as ZSM-5 zeolite as acid sites in ZSM-5 are of much higher acid strength. Generally, ethylbenzene disproportionation proceeds via alkyl-carbenium mechanism resulting in secondary reactions leading to coke formation.

Recently Al-Khattaf et. al. [26] performed catalytic study of EB disproportionation over ZSM-5 zeolite using a novel riser simulator reactor system. It was observed that disproportionation was a dominant reaction at temperatures 350–400 °C while cracking reaction became more significant above 400 °C. Two reaction mechanisms leading to different product distributions proceeding at low and high temperatures were postulated. The DEBs can also be produced by transalkylation of triethylbenzene (TEB) with ethylbenzene (EB), however, very rare literature data are available [1]. Al-Khattaf et al. [27] reported on a similar reaction of transalkylation of toluene and trimethylbenzene. Transalkylation proceeded with a higher reaction rate resulting into the production of larger amount of xylene as compared with transformation of pure toluene or trimethylbenzene. Consequently, Akhtar and Al-Khattaf [28] performed transalkylation of EB, toluene and benzene with 1,3,5-TEB over USY steamed zeolite. Conversion of TEB was observed to depend upon the nature of alkyl substituent on the benzene ring of the partner reactant. With increasing length of the chain on the benzene ring the conversion of TEB increased.

The objection of this investigation was the transalkylation of 1,3,5-TEB with EB over ZSM-5 zeolite in a fluidized-bed reactor along with its kinetic study. The study focuses on the effect of contact time and reaction temperature related to the DEB yields and isomer distributions of DEB and TEB. Based on the experimental data, kinetic model of TEB transalkylation with EB is also proposed.

Section snippets

Catalysts

The ZSM-5 zeolite used in this study was obtained from Davison Grace while ultrastable Y zeolite (USY) from Tosoh. The protonic forms of both zeolites were prepared by ion-exchange with NH4NO3 to replace the sodium cations followed by calcination. Finally, the H-ZSM-5 and H-Y zeolites were spray-dried using kaolin as the filler and silica sol as the binder. The resulting catalyst particles of size 60 μm have the composition: 30 wt% zeolite, 50 wt% kaolin, and 20 wt% silica sol. Sodium was removed

Results and discussion

The transalkylation of 1,3,5-TEB with EB at different reaction temperatures and contact times using ZSM-5 zeolite was studied and results were compared with the transformation of pure 1,3,5-TEB and EB under identical reaction conditions (cf. Table 2, Table 3). The transalkylation of 1,3,5-TEB with EB was carried out at a molar ratio of EB:TEB, 1:1 (equivalent to 40:60 wt%, respectively). The results of transalkylation reaction has shown that the main transalkylation reaction of EB with 1,3,5-TEB

Model formulation

The transformation reaction of 1,3,5-TEB over zeolites occurs through two main reaction pathways as shown in Scheme 1 below and discussed in Section 3.2.

The first reaction pathway is cracking of TEB resulting in the formation of DEB as primary reaction products and light hydrocarbons mainly ethylene. The second reaction pathway is isomerization leading to 1,2,4-TEB and 1,2,3-TEB. Third possible mechanism includes 1,3,5-TMB disproportionation, although tetraethylbenzenes were not detected under

Conclusions

The following conclusions can be drawn from the transalkylation reaction of ethylbenzene (EB) with 1,3,5-triethylbenzene (TEB) over ZSM-5 zeolite catalyst and subsequent comparison with the reactions of pure ethylbenzene and 1,3,5-triethylbenzene under similar conditions.

  • A synergistic effect in the reaction of 1,3,5-TEB has been observed during the transalkylation reaction with ethylbenzene as compared with the transformation reaction of pure 1,3,5-TEB.

  • The selectivity to DEB (%) produced during

Acknowledgments

We would like to express our appreciations for King Abdullah University of Science and Technology (KAUST) for their financial support. The support of King Fahd University of Petroleum and Minerals (KFUPM) is also highly appreciated.

References (36)

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