Isomerization ethylbenzene

The rather low concentration of the desired p-xylene component in the Parex unit feed means a large fraction of the feed stock contains other A8 components that are competing for adsorption sites in the adsorbent zeoHte cages. Due to this typically lean feed, a significant hike in the Parex unit capacity can be obtained by even a small increase in the composition of the p-xylene. Techniques to increase the p-xylene feed concentration include greater dealkylation of the ethylbenzene in the Isomar unit by converting from an ethylbenzene isomerization catalyst to... 

Ethylbenzene Isomerization Isomerization of EB requires both metal and acid function. Hydrogenation results in an intermediate naphthene. The acid function is required to isomerize the naphthene to a methyl-ethyl-substituted five-mem-bered ring species that can further convert to a dimethyl-substituted six-membered ring naphthene. This can be dehydrogenated by the metal function to a xylene isomer, OX in the example shown in Figure 14.9. 

Fig. 20. Effect of activation temperature on catalytic activity. O, ethylene-benzene alkylation (160) , toluene disproportionation (157) A, n-hexane cracking (161) O, 1-methyl-2-ethylbenzene isomerization (158).

The observed catalytic behavior in the case of l-methyl-2-ethylbenzene isomerization (158) was not so straightforwardly related to the Brdnsted site concentration. The maximum activity was observed with samples activated at temperatures where significant dehydroxylation had occurred. This reaction occurs more readily over acid catalysts than toluene disproportionation or xylene isomerization and may require fewer Brdnsted acid sites, or the reaction mechanism may involve Lewis sites. 

With bifunctional Pt acid catalysts, a third mechanism can participate in xylene isomerization, involving the same intermediates as ethylbenzene isomerization (2,11). 

The mechanism of ethylbenzene isomerization reported in Figure 9.6 was firstly proposed by Weisz (23). 

Fig. 9.6 Mechanism of ethylbenzene isomerization over bifunctional catalysts...

Fig. 9.7 Turnover frequency values TOF (h1) for ethylbenzene isomerization of the protonic sites of various bifunctional PtA/HMOR as a function of np/nH+ the ratio between the concentrations of accessible platinum sites and of protonic sites able to retain pyridine adsorbed at 423 K...

According to a recent study, the highest values of isomerization selectivity can be obtained with bifunctional Hmordenite catalysts with high values of the ratio between the concentrations of Pt and protonic sites accessible by reactant molecules, i.e. with catalysts on which the acid step is the limiting step of ethylbenzene isomerization. Furthermore, because of the reduction of the disproportionation activity, Na exchange of HMOR has a positive effect on the selectivity. 

Whereas the mutnal isomerization of xylenes appears to take place by the transfer of the CH3 group according to a conventioiial carbonium ion mechanism, ethylbenzene isomerization is more com to and requires the presence of hydrogen. To interpret this factor, it is assumed that the conversion comprises the production of C5 and Q naph> thenes as intermediates, according to the following reaction ... 

Mordenite has been used since the second half of the 1970s in industrial xylene and ethylbenzene isomerization proeesses, chiefly for the production of paraxylene. the isomer for whieh there is the highest demand (Table 2). First, the isomerization of ethylbenzene into xylenes implies a bifunetional aeid catalyst. Second, it requires a temperature of around 400°C and hydrogen pressure in the region of 1-1.5 MPa. The catalyst is. therefore, composed of acid mordenite associated with a strong hydrogenation function, supplied by Pt. 

It is believed that the isomerization of ethylbenzene to xylenes occur only on bifunctional catalysts, and hydrogenated intermediates are required (190). Selective catalysts for canying out this reaction have to maintain a good balance between the H-DM and the add function. Zeolites, with strong acid functions such as in mordenite or ZSM-5 actively isomerize cycloolefinic intermediates but also catalyze ring opening reactions which lead to a decrease in the formation of xylenes. However, the mild add nature of SAPOS make them specially useful for ethylbenzene isomerization. In this sense, 0.4-0.6 %wt Pt on SAPO-11 and SAPO-5 catalysts were used to isomerize mixtures of ethylbenzene and meta-xylene. Both catalysts produce near-complete xylene equilibration. SAPO-11 was more selective for producing xylenes than SAPO-5 (191). 

Isomerization is a reaction in which a molecule is transformed into a molecule having the same molecular formula but a different structure, i.e., isomers. Industrially important isomerization reactions are rearrangements of the carbon skeleton of C4-C8 hydrocarbons, and isomerization among alkyl benzene isomers such as xylenes and ethylbenzene.Isomerization including heteroatoms, such as propylene oxide to ally alcohol and Beckmann rearrangement of cyclohexanone oxime to e-caprolactam, are also significant industrial processes. 

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