XYLENE ISOMERISATION CATALYSTS

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. 

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. 

An aromatic hydrocarbon feedstock consisting mainly of m-xylene is to be isomerised catalytically in a process for the production of / -xylene. The product from the reactor consists of a mixture of p-xylene, m-xylene, o-xylene and ethylbenzene. As part of a preliminary assessment of the process, calculate the composition of this mixture if equilibrium were established over the catalyst at 730 K. 

Fig. 9.5 Transformation of m-xylene over HMORIO and NaHMORlO samples. Disproportionation / Isomerisation rate ratio (D/I) versus m-xylene conversion (X%) on fresh and on deactivated catalysts HMORIO (0), 14NaHMOR ( ), 28NaHMOR (A), 45NaHMOR (+), 63NaHMOR (o)...

New data treatment methods such as 2D correlation infrared spectroscopy (2D-COS) lead to significant improvements for in situ studies. We report here the application of 2D-COS to the in situ infrared study of orthoxylene isomerisation in H-MFI zeoiite in working conditions. This led to improve the quantitative monitoring of xylene isomers in the micropores of the solid and to the detection of traces of coke in the catalyst. A correlation was found between coke and the perturbation of specific hydroxy groups in the solid. 

Disproportionation of toluene on ZSM-5 zeolite catalyst gives benzene and a mixture of the three xylene isomers as primary products. p-Xylene is the most valuable product, and kinetically favoured because it diffuses most easily through the intracrystalline pores of the catalyst. However, secondary isomerisation of xylenes on the surface of the unmodified zeolyte gives a net non-selective catalyst. Collins et aL [64] obtained an increase in p-xylene selectivity by operating the reaction in the immediate vicinity of the solvent critical point (r/Tc(toluene) = 1.002 p = 415 bar) interpreting this as due to a solvent cluster-induced constraint on the adsorption of xylenes, which inhibits secondary reactions. As the temperature is raised, r/7c(toluene) = 1.01, both the degree of selectivity and its pressure dependence fall. 

In the di-substituted benzene series o- and m-xylene give the expected products. However, p-xylene undergoes isomerisation to furnish 2,4-dimethylbenzaldehyde. The tri-substituted benzene series i.e. mesitylene and pseudo-cumene yield normal formylation products. Tetralin and di-isopropyltetralin in the naphthalene series are reported to undergo Gattermann-Koch formylation whereas naphthalene itself is formylated in the modified reaction using HF and BF3 as catalyst. 

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