Reaction Mechanism of Ethylbenzene Isomerization

Abstract

Isomerization of ethylbenzene in the gaseous phase over bifunctional Pt/AI^a nd Pt-zeolite catalysts was investigated in a microcatalytic fixed bed reactor using the pulse method. Over the Pt/Al203 catalyst, where the carrier displays only weak acidity, xylenes are formed from ethylbenzene mainly via the route ethylbenzene-ethylcyclohexene-1,2-methylethylcyclopentene-1,2-dimethylcyclohexene-o-xylene, which corresponds to predictions from a bifunctional mechanism in which skeletal rearrangements of tertiary carbocations only are considered to be the rate-determining steps. Over Pt-zeolite catalysts with higher acidity, skeletal isomerizations in which secondary carbocations are involved contribute-in addition to the route mentioned above-to the measured product distributions.

Transformation of ethylbenzene-m-xylene feed over MCM-22 zeolites with different acidities

“…The authors also introduce hydroisomerisation function by loading Pt to the modified support. The ability of platinum for converting EB to xylenes is well known [29] and zeolites impregnated with this noble metal are widely studied in reactions of mixed EB-m-xylene feeds transformation [4,5,7,30]. As it was expected, the introduction of Pt into our [Al]MCM-22 leaded to higher net gain of products of EB conversion through dealkylation and transalkylation as well as isomerization to the aimed xylenes (Table 2) …”

“Ab initio” synthesis of zeolites for preestablished catalytic reactions

“…A commercial catalyst for EB isomerization is a bifunctional Pt/mordenite catalyst that can catalyze the different reaction steps shown in Figure 3A (20). Indeed, the hydrogenating metal function will partially hydrogenate the aromatic ring, which after protonation on the acid sites will form the expanded C7 protonated ring (30). Taking this into account, we have compared the ITQ-64 with two mordenite catalysts containing different Si/Al molar ratios (22 and 47, see MOR_25 and MOR_47, respectively in Table S4), being all these zeolites combined with 1%wtPt/Al2O3 in order to introduce the required hydrogenating function to the catalysts (see catalyst preparation in experimental).…”

Kinetic modeling of the ethylbenzene/xylene isomerization reaction over HZSM‐5 zeolites revisited

“…The dominant route is thought to be Pt metal assisted ethylbenzene dehydrogenation into ethylhexene, followed by skeletal rearrangements and final Pt metal assisted hydrogenation into o-xylene. 29 In the absence of a hydrogenating/dehydrogenating catalyst, however, ethylbenzene transformation into xylenes is theoretically possible in the absence of any alkene intermediate. Such a phenomenon may occur through skeletal rearrangement reactions of ethylbenzene molecular ions (Scheme 1).…”

“…27 × 10 3 h −1 g catalyst −1 ), explains why this reaction is relatively slow. Considering diffusion effects, and based on Equation(29), the apparent activation energy for this reaction is calculated to be 41.92 kJ mol −1 . This should be noted that our calculation resulted in near zero values for k9 and k 10 , showing that ethylbenzene prefers to convert into m-xylene directly, without o-xylene acting as an intermediate.…”