Xylene isomerisation

Thibault-Starzyk, F., Vimont, A., and Gilson, J.P. (2001) 2D-COS IR smdy of coking in xylene isomerisation on H-MFl zeolite. Catal. Today, 70, 227-241. 

Figure 9.11 (a) Diffusion shape selectivity in xylene isomerisation, (b) The M-forming process for gaso-... 

For the conversion of m-xylene, the activity of H-faujasite depends strongly on its degree of dealumination. Maximum catalytic activity is obtained for aluminum T-atom fractions equal to 0.10. Surprisingly, in contrast to both theoretical predictions and to the behaviour of H-ZSM-5, for dealuminated H-faujasites the turnover frequency per protonic aluminum site exhibits a pronounced maximum when the aluminum T-atom fraction is 0.09. The present results can be rationalized if, besides the classical predictions on zeolite acidity, a new concept of "hidden acid sites" is handled. Changes of the m-xylene isomerisation and disproportionation selectivities with the degree of dealumination of faujasite are in agreement with this concept. 

Fig. 4. Xylene isomerisation on Pt/Al203. The relative specific activities are plotted as a function of the time of residence in a single-flow reactor Adapted after Refs. 65, 66. A in the presence of H2 B in the presence of N2.

Reacting ring-labelled and side chain labelled ethylbenzene and m-xylene on industrial Pt/A Os gave further information on the possible pathways of xylene isomerisation discussed in the previous section. The reaction of labelled ethylbenzene gave a considerable amount of [ C] label in the aromatic ring of xylene products, indicating the importance of ring... 

Figure 8.9 Shape selectivity in zeolite pores. From top to bottom reactant selectivity in alkanol dehydration diffusion selectivity in alkylation of aromatics diffusion selectivity in xylene isomerisation transition state selectivity in xylene isomerisation.

The isomerisation of xylenes within H-ZSM-5 provides the textbook example of transition state shape selectivity (Table 8.2). Xylenes isomerise on zeolite H-Y to an equilibrium distribution of o-, m- and p-xylenes within the large cavities. Under the same conditions they also undergo disproportionation to give toluene and a mixture of trimethylbenzenes. This (unwanted) side reaction proceeds via a bimolecular reaction. Over the medium-pore H-ZSM-5, however, only xylenes are observed, enriched in the pnra-isomer by product shape selectivity. Little or no disproportionation is observed. This is a result of transition state selectivity, since there is no space within the ZSM-5 structure for the bimolecular transition state of the disproportionation reaction. ... 

Substituted aromatics are essential chemical feedstocks. Among the xylenes, for example, p-xylene is in great demand as a precursor to terephthalic acid, a polyester building block. The pura-isomer is therefore more valuable than the o- and m-xylenes, so there is a powerful incentive for conversion of o- and m-xylene to p-xylene. Isomerisation over solid acids occurs readily as a result of alkyl shift reactions of the carbenium-ion-like transition state. The initial protonation occurs by interaction of the Bronsted acid site with the aromatic 71 system, by an electrophilic addition. Over non-microporous solid acids, at high conversion, xylenes are produced at their thermodynamically determined ratios, which favour the meta rather than the ortho or para isomers. In addition, unwanted transalkylation reactions occur, giving rise, for example, to toluene and trimethylbenzenes. Zeolite catalysts can be much more selective. 

The first examples of molecular shape-selective catalysis in zeolites were given by Weisz and Frilette in 1960 [1]. In those early days of zeolite catalysis, the applications were limited by the availability of 8-N and 12-MR zeolites only. An example of reactant selectivity on an 8-MR zeolite is the hydrocracking of a mixture of linear and branched alkanes on erionite [4]. n-Alkanes can diffuse through the 8-MR windows and are cracked inside the erionite cages, while isoalkanes have no access to the intracrystalline catalytic sites. A boom in molecular shape-selective catalysis occurred in the early eighties, with the application of medium-pore zeolites, especially of ZSM-5, in hydrocarbon conversion reactions involving alkylaromatics [5-7]. A typical example of product selectivity is found in the toluene all lation reaction with methanol on H-ZSM-5. Meta-, para- and ortho-xylene are made inside the ZSM-5 chaimels, but the product is enriched in para-xylene since this isomer has the smallest kinetic diameter and diffuses out most rapidly. Xylene isomerisation in H-ZSM-5 is an often cited example of tranSition-state shape selectivity. The diaryl type transition state complexes leading to trimethylbenzenes and coke cannot be accommodated in the pores of the ZSM-5 structure. 

Formulate a continuous component model for xylene isomerisation. (Xylene is known to have three distinct isomers ortho-, meta- and paraxylene and the transition between them is usually described by the triangle reaction, cf. Aly et al. (1965).)... 

Aly, A. F., Rope, B. W. Wise, J. J. (1965). Kinetics of xylene isomerisation, 77th AlChe National Meeting, Paper No. 566. 

Daramola M O, Burger A J, Giroir-Fendler A, Miachon S and Lorenzen L (2010), Exctractor-type catalytic membrane reactor with nanocomposite MFI-alumina tube as separation unit prospect for ultrapure para-xylene production from m-xylene isomerisation over Pt-HZSM-5 catalyst , Appl Catal A-Gen, 386, 109-115. 

Haag S, Hanebuth M, Mabande G T P, Avhale A, Schiwieger W and Dittmeyer R (2006), On the use of a catalytic H-ZSM-5 membrane for xylene isomerisation , Micropor Mesopor Mater, 96,168-176. 

---