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Ind. Eng. Chem. Res. 2009, 48, 2836–2843
Ethylbenzene Transformation over a ZSM-5-Based Catalyst in a Riser Simulator S. Al-Khattaf,*,† N. M. Tukur,‡ and S. Rabiu† KAUST Center in DeVelopment, King Fahd UniVersity of Petroleum & Minerals, Dhahran 31261, Saudi Arabia, and SABIC E&PM, P.O. Box 11425, Jubail Industrial City 31961, Saudi Arabia
The transformation of ethylbenzene has been studied over a ZSM-5-type catalyst in a riser simulator that mimics the operation of a fluidized-bed reactor. The study was conducted at 350, 375, 400, 450, and 500 °C for reaction times of 3, 5, 7, 10, 13, and 15 s. The effect of reaction conditions on the ratio of cracking to disproportionation products (C/D), the distribution of diethylbenzene (DEB) isomers (m-DEB and p-DEB), and the ratio of benzene/diethylbenzenes (B/DEB) are reported. The experimental results were modeled using quasi-steady-state approximation. Disproportionation was determined to dominate at low temperatures (350-400 °C), while cracking reaction becomes significant as higher temperatures (>400 °C). Thus, two mechanisms were postulated to represent the disappearance of ethylbenzene during the transformation reaction (one mechanism for low temperature, and another for the complete temperature range considered). Kinetic parameters that were used for the disappearance of ethylbenzene during the transformation reaction, and in its conversion into cracking and disproportionation products, were calculated using the catalyst activity decay function, based on the time-on-stream (TOS). The apparent activation energies were determined to decrease: Ecracking > Edisproportionation. 1. Introduction The catalytic disproportionation of ethylbenzene over zeolitebased catalysts to produce isomers of diethylbenzene, among other products, has gained tremendous attention recently. This is due to the wide range of applications of these compounds in a variety of industrial processes. For example, para-diethylbenzene (p-DEB), which is the most important of the three diethylbenzene isomers, is an important desorbent in adsorptive separation processes such as UOP Parex, which is used in the separation of xylene isomers. Moreover, it is an important monomer for the production of copolymers, such as ionexchange resins and viscosity modifiers of lubricant oil.1 Ethylbenzene disproportionation was initially studied by Karge and co-workers,2,3 to characterize zeolites in their acid form. During the disproportionation of ethylbenzene on largepore zeolites, the authors observed an induction period, when the conversion of ethylbenzene increased with the time-onstream (TOS) until a maximum conversion was reached. Furthermore, the selectivity of p-DEB in the product was generally poor. On the other hand, when medium-sizedmicropore zeolites were used, no induction period was observed and significant p-DEB selectivity was observed. Following this pioneer work, numerous studies of the reaction were conducted over USY, Beta, MCM-22, and ZSM-5 zeolites.4-6 In most cases, the composition of the different diethylbenzene isomers in the reaction product was limited by thermodynamics, which led to an equilibrium composition with a relatively lower amount of the para-isomer, compared to the meta-isomer (meta-diethylbenzene, abbreviated as m-DEB). This led researchers to the conclusion that the mechanism of zeolitecatalyzed ethylbenzene disproportionation is same as that of toluene disproportionation (i.e., shape selectivity for the diffusion difference among p-DEB, m-DEB, and ortho-diethylbenzene (oDEB) has an important role).7 Therefore, catalyst modification * To whom correspondence should be addressed. Tel.: +966-3-8601429. Fax: +966-3-860-4234. E-mail address:
[email protected]. † KAUST Center in Development, King Fahd University of Petroleum & Minerals. ‡ SABIC E&PM.
techniques such as partial deactivation of external surface acid sites, narrowing of pore channels, increasing diffusion path length (crystal size), etc., which are commonly used to enhance para-xylene selectivity during toluene disproportionation have also been used to enhance p-DEB selectivity during ethylbenzene disproportionation. However, note that that diffusion affects the selectivity of the individual isomers of diethylbenzene formed from ethylbenzene disproportionation more than the selectivity of xylene isomers in toluene disproportionation. This is due to the greater diffusion difference between the diethylbenzene isomers, compared to the xylene isomers. Because p-DEB is the most important of the three isomers, many research groups have reported efforts to improve its selectivity during ethylbenzene disproportionation. Velasco et al.8 studied the effect of aluminum content and crystal size on the selectivity for p-DEB during ethylbenzene disproportionation on a ZSM-5 MFI zeolite. Their results showed that a p-DEB selectivity of up to 90% can be achieved with MFI samples that have high aluminum contents and a crystal size of at least 40 µm.The best results were obtained using an MFI crystal size of 14 µm and a Si/Al ratio of >40. Similarly, Sabine and Ferdi9 conducted an extensive study of ethylbenzene disproportion on H-ZSM-5 zeolite in which they explained the effects of both external acidity and particle size of the catalyst on ethylbenzene conversion and product distribution. Modification of the chemical properties of zeolite catalysts is also considered to be an important way of improving p-DEB selectivity during ethylbenzene disproportionation and much research has been reported in this area. Zhu et al.1 investigated p-DEB selectivity during ethylbenzene disproportionation over ZSM-5 catalyst samples modified by a chemical liquid deposition of silica (SiO2-CLD), a chemical vapor deposition of silica (SiO2-CVD), loading with MgO and SiO2, and loading with MgO. It was determined that the MgOmodified ZSM-5 catalyst improves the p-DEB selectivity up to 98.6% while maintaining appreciable ethylbenzene conversion of 28.1%. Improving p-DEB selectivity via modification of the ZSM-5 surface has also been reported by Wichterlova and Cejka.10
10.1021/ie801609x CCC: $40.75 2009 American Chemical Society Published on Web 02/02/2009
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Park and Rhee have also investigated ethylbenzene disproportionation, using a sample of MCM-22 and its dealuminated form at low temperatures. Among the diethylbenzene isomers, p-DEB was determined to form within the micropore as the primary product. The m-DEB and o-DEB were mainly produced via the secondary isomerization of the para-isomer on the external surface of the MCM-22 catalyst. Furthermore, the paraselectivity of MCM-22 was observed to increase with the degree of dealumination, because this treatment preferentially removes the Bro¨nsted acid sites on the external surface that are responsible for the further isomerization of the para-isomer, rather than the acid sites within the micropores. Unlike the study of catalyst development, little regarding mechanistic and kinetic studies of ethylbenzene disproportionation has been reported in the open literature. A very important mechanistic investigation of the reaction by Arsenova et al.12 over ZSM-5 catalyst showed that the reaction scheme follows a consecutive path. These workers reported that the first step in ethylbenzene disproportionation occurs in the interior of the zeolite crystal without diffusion limitations, yielding essentially p-DEB as a primary product. The latter is then converted in a secondary isomerization mainly to m-DEB. In addition, crystal size was determined to have a very important role on the product distribution, with large crystals producing higher yields of p-DEB than those of smaller crystals. These observations are usually explained based on two wellknown models. The first model, which is related to steric constraints of the various products, suggests that the isomerization of p-DEB to m-DEB is strictly restricted to active sites on the outer crystallite surface. The second model, which considers the diffusion of the various diethylbenzene isomers, suggests that isomerization occurs on active sites both on the external surface and in the interior of the zeolite. However, the relatively smaller kinetic diameter of the para-isomer enables it to diffuse more rapidly toward the gas phase than the bulkier m-DEB. Catalytic and sorption studies that were conducted by Arsonova-Hartel et al.13 suggest that the formation of o-DEB within the pores of the ZSM-5 catalyst at low temperatures ( Edisproportionation. Acknowledgment The author would like to express his appreciation to King Abdullah University of Science and Technology (KAUST) for their financial support. Also, the support of the King Fahd University of Petroleum and Minerals is highly appreciated. Mr.
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Mariano Gica also is acknowledged for his help during the experimental work. Nomenclature Ci ) concentration of species i in the riser simulator (mol/m3) CL ) confidence limit Ei ) apparent activation energy of ith reaction (kJ/mol) k ) apparent kinetic rate constant (m3/(kg-cat s)); k ) k0′ exp[-ER/ {R[(1/T) - (1/T0)]} k0′ ) pre-exponential factor in the Arrhenius equation defined at an average temperature (m3/(kg-cat s); units based on the firstorder reaction) MWi ) molecular weight of species i r ) correlation coefficient R ) universal gas constant; R ) 8.314 kJ/(kmol K)) t ) reaction time (s) T ) reaction temperature (K) T0 ) average temperature of the experiment TOS ) time-on-stream V ) volume of the riser; V ) 45 cm3 Wc ) mass of the catalysts; Wc ) 0.81 g-cat Whc ) total mass of hydrocarbons injected in the riser; Whc ) 0.162 g yi ) mass fraction of component i (wt %) Greek Letter R ) apparent deactivation constant (TOS model) (s-1)
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ReceiVed for reView October 23, 2008 ReVised manuscript receiVed December 21, 2008 Accepted December 31, 2008 IE801609X
