Isomerization Of Ethylbenzene Research Articles

Location of the Active Sites for Ethylcyclohexane Hydroisomerization by Ring Contraction and Expansion in the EUO Zeolitic Framework

Identifying the location of the active sites in a zeolite is a current challenge, impeding the design of optimal catalysts. In this work, we identify the location of the most active sites of 1-ethylcyclohexene isomerization in the EUO framework (10 MR channels, 12 MR side pockets) thanks to DFT calculations corroborated by experiments. Skeletal isomerization of cycloalkenes is a crucial industrial reaction for the bifunctional isomerization of ethylbenzene. Ethylcyclohexene is protonated by framework protons into cyclic carbenium ions, which undergo ring contraction–expansion reactions through protonated cyclopropane (PCP) like transition states. Ab initio calculations clearly show that the acid sites located at the intersection between the channel and the pocket stabilize much less the cyclic carbenium ions involved in the reaction than 12 MR pockets and 10 MR channel sites due to stronger dispersion stabilizing interactions. This computational finding is fully confirmed experimentally by the comparison ...

"Ab initio" synthesis of zeolites for preestablished catalytic reactions.

Unlike homogeneous catalysts that are often designed for particular reactions, zeolites are heterogeneous catalysts that are explored and optimized in a heuristic fashion. We present a methodology for synthesizing active and selective zeolites by using organic structure-directing agents that mimic the transition state (TS) of preestablished reactions to be catalyzed. In these zeolites, the pores and cavities could be generated approaching a molecular-recognition pattern. For disproportionation of toluene and isomerization of ethylbenzene into xylenes, the TSs are larger than the reaction products. Zeolite ITQ-27 showed high disproportionation activity, and ITQ-64 showed high selectivity for the desired para and ortho isomers. For the case of a product and TS of similar size, we synthesized a catalyst, MIT-1, for the isomerization of endo-dicyclopentane into adamantane.

Industrial Xylene/Ethylbenzene Isomerization Unit Using a Radial‐Flow Reactor and EUO‐Type Zeolite

AbstractA mathematical model based on 34 days continuous operation of an industrial isomerization unit was developed. The unit involves a radial‐flow reactor with a catalyst capable of converting xylenes and ethylbenzene to mixed xylenes. The catalyst contains EU‐1 zeolite, platinum, and alumina as binder. Two reactions are considered, i.e., ethylbenzene isomerization and xylene isomerization. The rates are based on the Hougen‐Watson model according to the literature. An optimization procedure based on the trust‐region‐reflective algorithm was carried out in order to obtain new kinetic constants that minimize the difference between the actual and the calculated values. The standard error of the parameters estimated was calculated through the deleted‐one Jackknife method.

Ethylbenzene isomerization over bifunctional platinum alumina–EUO catalysts: Location of the active sites

The transformation of ethylbenzene (EB) was carried out on intimate mixtures of Pt/Al 2O 3 (PtA) and HEUO (Si/Al = 15) catalysts with different composition under the following conditions: fixed bed reactor, temperature 683 K, pressure of hydrogen and ethylbenzene equal to 8 and 2 bar respectively, weight hourly space velocity between 5 and 60. Products result from the following transformations of ethylbenzene or/and of the reaction products: the bifunctional desired isomerization of EB into xylenes (X), the acidic reactions of disproportionation, dealkylation of EB (followed by ethylene hydrogenation) and EB–X transalkylation, the hydrogenation of EB followed by ethylcyclohexane isomerization and cracking. The selectivity to isomers, which is very low on the fresh catalysts, increases significantly during an initial period of fast deactivation, essentially at the expense of disproportionation and dealkylation by products. This deactivation as well as the increase in the selectivities to isomers were shown to result from the blockage by carbonaceous deposits of the access to the inner sites of the EUO micropores. The selectivity to isomers of EB transformation over these inner sites is very low, which could be due to the trapping of ethylcyclohexene intermediates within the monodimensional channels of EUO zeolite. In contrast, over the stabilized catalysts for which only the protonic sites of large side pockets located on the outer surface can be active, the selectivity to isomers is very high, which could be related to the environment of these sites as well as to their easy access by cycloalkene intermediates.

Ethylbenzene Isomerization on Bifunctional Platinum Alumina−Mordenite Catalysts. 2. Influence of the Pt Content and of the Relative Amounts of Platinum Alumina and Mordenite Components

Ethylbenzene transformation was carried out over intimate mixtures of Pt/Al2O3 (PtA) and HMOR zeolites under the following conditions: fixed-bed reactor, temperature 683 K, pressures of hydrogen and of ethylbenzene equal to 8 and 2 bar, respectively, weight hourly space velocity between 5 and 200. A significant number of these bifunctional PtA/HMOR catalysts were used, differing by the platinum content of PtA (0.5, 1.1 and 2.3 wt %), the framework Si/Al ratio of the mordenite (from 6.6 to 180), and especially by the relative proportions of the PtA and HMOR components. With all of the bifunctional catalysts, ethylbenzene undergoes isomerization into xylenes, disproportionation, dealkylation, secondary ethylbenzene−xylene transalkylation, hydrogenation followed by ethylcyclohexane isomerization, and secondary cracking of C8 naphthenes. The rate of isomerization which occurs through bifunctional catalysis was shown to depend mainly on the balance between hydrogenating and acid functions, taken here as the ratio between the concentration of accessible platinum atoms and of protonic acid sites (nPt/nH+). The rate of disproportionation per acid site does not depend on the relative proportion of zeolite and of Pt/Al2O3 in the bifunctional catalysts but increases with the density of protonic sites in the mordenite component. This suggests that this bimolecular reaction requires more than one acid site for its catalysis. Ethylbenzene dealkylation does not depend on the density of protonic sites but seems very sensitive to their strength. For these three reactions, the apparent activity of the protonic sites is significantly greater when the HMOR components of the bifunctional catalysts are dealuminated, hence present mesopores; most of the protonic sites of these dealuminated mordenites would be active, whereas for mordenites without mesopores, it would be the case only for the protonic sites of the shell of the crystallites. High selectivities to xylenes (≈75% at 35% conversion) were shown to be obtained on bifunctional catalysts with values of nPt/nH+ sufficiently high to have the acid isomerization of olefinic intermediates as the limiting step of ethylbenzene isomerization.

Ethylbenzene transformation on bifunctional Pt/Al 2O 3-NaHMOR catalysts : Influence of Na exchange on their activity and selectivity in ethylbenzene isomerization

Ethylbenzene transformation was carried out on bifunctional catalysts constituted by Pt/Al 2O 3 (PtA) associated with NaHMOR (Si/Al=10) zeolite samples. The operating conditions were the following: fixed bed reactor, temperature 683 K, pressure of hydrogen and of ethylbenzene equal to 8 and 2 bar, respectively, weight hourly space velocity (WHSV) between 5 and 150 h −1. Twelve different bifunctional catalysts were used differing by the Na exchange rate of the mordenite component or by the relative proportions of the NaHMOR and Pt/Al 2O 3 components or by the Pt content of the Pt/Al 2O 3 (0.5 or 2.3 wt.%). As expected from a bifunctional mechanism, the activity in isomerization per protonic site (turnover frequency, TOF) increases first with the ratio between the concentrations of accessible platinum sites ( n Pt) and of the protonic sites ( n H + ), then remains constant above a certain value of this ratio. It is shown that the exchange of the mordenite causes a significant increase in the selectivity to isomers of the bifunctional catalysts. This increase can be related to a decrease in the rates of the main secondary reaction (disproportionation), but also of the other ones: dealkylation, hydrocracking, transalkylation.

Ethylbenzene Isomerization on Bifunctional Platinum Alumina–Mordenite Catalysts: 1. Influence of the Mordenite Si/Al Ratio

The transformation of ethylbenzene was carried out on intimate mixtures of 0.5 wt% Pt/Al2O3 (75 wt%) with a platinum dispersion of 100% and of HMOR zeolites (25 wt%) with Si/Al ratios between 6.6 and 180. Products result from six main transformations of ethylbenzene or/and of the reaction products: the desired isomerization of ethylbenzene into xylenes (reaction 1), disproportionation of ethylbenzene (2), dealkylation followed by hydrogenation of ethylene (3), secondary ethylbenzene–xylene transalkylation (4), hydrogenation of ethylbenzene followed by ethylcyclohexane isomerization (5), and secondary cracking of C8 naphthenes (6). From the comparison of the product distributions over Pt/Al2O3, over HMOR and over the bifunctional catalysts, reactions 2–4 were confirmed to occur through acid catalysis and hydrogenation through metal catalysis, and reactions 1 and 6 through bifunctional catalysis. Whatever the catalyst, the selectivity to isomers increases with ethylbenzene conversion, which is due to thermodynamic limitations in reactions 2 and 5. A significant increase in selectivity to isomers is also observed when the Si/Al ratio of the mordenite component increases from 20 to 180. With these bifunctional catalysts, the initial rate of disproportionation is proportional to the square of the concentration of protonic sites, which suggests that this bimolecular reaction requires two protonic sites for its catalysis; furthermore, the rate of dealkylation (per protonic site) depends only on the strength of the protonic sites and the change of the isomerization rate with the balance between the metallic and acidic functions is the one expected from a classical bifunctional mechanism. For the bifunctional catalysts with HMOR6.6 and −10 as acidic components, the rates of disproportionation, dealkylation, and isomerization are lower than expected and the selectivity to isomers is higher. This can be explained by strong diffusion limitations in these zeolites which, in contrast to the other samples, do not present mesopores in addition to micropores.

Transformation of an ethylbenzene- o-xylene mixture on HMOR and Pt-HMOR catalysts. Comparison with ZSM-5 catalysts

The transformation of pure ethylbenzene and of an ethylbenzene (20wt.-%)- o-xylene (80 wt.-%) mixture is carried out on acid and bifunctional HMOR catalysts under the following conditions: fixed bed reactor, temperature 410°C, 0.96 MPa pressure of hydrogen and 0.24 MPa pressure of hydrocarbons, weight of hydrocarbon injected per weight of catalyst and per hour between 10 and 110. On both catalysts the main reaction of pure ethylbenzene is disproportionation, this reaction occurring mainly through diphenylethane intermediates. The main effect of platinum is to allow the bifunctional isomerization of ethylbenzene. In addition to disproportionation reactions, ethylbenzene in mixture with xylene is also transformed through similar mechanisms into dimethylethylbenzene (transethylation) and into ethylmethylbenzene (transmethylation). o-Xylene is rapidly transformed into m-andp-isomers. Large differences exist between MOR and ZSM-5 catalysts. ZSM-5 catalysts are very active for ethylbenzene dealkylation and C 8 naphthenes hydrocracking while MOR catalysts are more active for xylene isomerization and disproportionation, and ethylbenzene hydroisomerization. Disproportionation of ethylbenzene occurs mainly on H-ZSM-5 through a deethylation-ethylation mechanism and on HMOR through diphenylethane intermediates. Steric constraints on the formation of bulky bimolecular intermediates at channel intersections and diffusion limitations are responsible for the particular behaviour of ZSM-5 catalysts.

ChemInform Abstract: Isomerization of Ethylbenzene and m‐Xylene on Zeolites.

Simultaneous isomerizstion of ethylbenzene and m-xylene on zeolite catalysts, including Pt/mordenite, Pt/USY, Pt/ZSM-5, and Pd/ZSM-5, was studied. Experimental results indicated that Pt/ZSM-5 was the superior catalyst for these reactions. Pd/ZSM-5 is better than Pt/USY, although, both are good enough for the reactions, in comparison with Pt/mordenite. A kinetic model with plausible reaction paths was proposed for the isomerization. The estimated reaction rate constants and activation energies indicated that the transformation of m-xylene to o- or p-xylene might be limited by the mass-transfer rate of the diphenylmethane-type intermediate, and the formation of o-xylene from ethylbenzene could be restricted by the smaller protonated cyclopropane intermediate.

Isomerization of ethylbenzene and m-xylene on zeolites

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTIsomerization of ethylbenzene and m-xylene on zeolitesY. S. Hsu, T. Y. Lee, and H. C. HuCite this: Ind. Eng. Chem. Res. 1988, 27, 6, 942–947Publication Date (Print):June 1, 1988Publication History Published online1 May 2002Published inissue 1 June 1988https://doi.org/10.1021/ie00078a008RIGHTS & PERMISSIONSArticle Views507Altmetric-Citations33LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (690 KB) Get e-Alerts Get e-Alerts

Gas-phase isomerization of ethylbenzene over PT-heteropoly acid/zeolite catalysts

The catalytic performance and availability of heleropoly compounds in vapor phase isomerizalion of elhylbenzene to xylene have been studied. 12-molybdophosphoric acid and its salts of four first row transition metals (Cr. Fe, Co and Ni) were chosen as heteropoly compounds, and they were supported together with platinum over Y-zeolite. The experiments were carried out at 400‡C, 18 kg/cm2 of H2 pressure, 4.6 of H2/ethylbenzene mole ratio and 1.5 gr-feed/hr gr-cal of LWHSV (Liquid Weight Hourly Space Velocity). As the results of experiments, heleropoly acid enhances the activity and selectivity of isomerization and the activity of isomerization is strongly affected by the method of catalyst preparation. In case of 12-molybdophosphates, their acidities vary with the kind of countercalions and it seems that the acidity is closely related to the electro negativity of the corresponding countercation. The pretrealment with hydrogen shows an echanced activity and selectivity.

Isomerization of ethylbenzene and xylenes over mordenite- and faujasite-based catalysts

La vitesse d'isomerisation de l'ethylbenzene sur un catalyseur Pt/Al 2 O 3 +mordenite est 4 fois superieure a celle de la meme reaction sur un catalyseur Pt/Al 2 O 3 +faujasite. L'isomerisation de l'σ-xylene sur les 2 catalyseurs donne des taux de conversion similaires. La selectivite de l'isomerisation de l'ethylbenzene en xylenes est changee par desalumination de la mordenite

Ethylbenzene isomerization on bifunctional platinum/alumina product yields and pseudo mass action kinetics

Ethylbenzene isomerization on bifunctional Pt/Al 2O 3 catalyst was investigated in the gas phase using a micro-catalytic reactor. Product yields were determined as functions of temperature for ethylbenzene, o-, m-, and p-xylene in the temperature range 513 – 673 K and 181.8 kPa. At temperatures ≦ 553 K, p-xylene was the sole product of the isomerization reaction. o-Xylene became significant from 563 K and went through a maximum at 539 K where p-xylene went through a maximum: m-xylene was produced at the higher temperatures. The Wei-Prater method was used in the calculation of absolute rate constants for the twelve single reactions of the network. Estimated activation energies appear to account for the high yield of ortho from meta-xylene.

Influence of crystalline phase of zeolitic catalysts on efficiency of m-xylene and ethylbenzene isomerization

Influence of crystalline phase of zeolitic catalysts on efficiency of m-xylene and ethylbenzene isomerization

Reaction Mechanism of Ethylbenzene Isomerization

The reaction mechanism of gas-phase ethylbenzene isomerization over bifunctional alumina- or zeolite-supported 0.5% by wt platinum catalysts was studied in a fixed-bed pulse microreactor by determining the product distributions in the conversion of ethylbenzene and of eight probable C/sub 8/ intermediates at 300/sup 0/-400/sup 0/C and 1.8 atm. Over the Pt/Al/sub 2/O/sub 3/ catalyst, xylenes are formed from ethylbenzene mainly via ethylcyclohexene to 1,2-methylethylcyclopentene to 1,2-dimethylcyclohexene to o-xylene, which corresponds to predictions from a bifunctional mechanism, in which only skeletal rearrangements of tertiary carbocations are considered to be rate-determining. Over Pt/zeolite catalysts with higher support acidity, skeletal isomerizations via both tertiary and secondary carbocations contribute to the measured product distributions. In ethylbenzene isomerization, the yield of C/sub 8/ naphthenes decreased from 60 mole % at 300/sup 0/C to zero at 400/sup 0/C and those of C/sub 8/ aromatics and xylenes passed through maxima of approx. 90% at 380/sup 0/C and 20% at 340/sup 0/C, respectively.

Aromatization of n-octane on an aluminum-chromium-potassium catalyst

1. The composition of catalyzates of the aromatization of n-octane on an aluminum-chromium-potassium catalyst at 528° and volume velocity 0.3 h−1 was studied by the method of capillary gas-liquid chromatography. 2. Under the conditions of the aromatization reaction, there is no isomerization of ethylbenzene and xylenes. The appearance of m- and p-xylenes in the reaction products is explained by dehydrocyclization of isooctanes and isooctenes, produced by isomerization and dehydroisomerization of the original n-octane. 3. Under the conditions of the aromatization of n-octane, there is a dealkylation of ethylbenzene and xylenes. From the data obtained it follows that the dealkylation of ethylbenzene proceeds according to two mechanisms: two-step — with intermediate formation of toluene, and one-step — directly to benzene.