A simple method for determining the relative significance of the unimolecular and bimolecular pathways of xylene isomerization over HY zeolites

The transformation of pure xylenes, of pure trimethylbenzenes (TMB), and of mixtures of m-xylene (greater than or equal to 95 mol%) and 1,2,4-trimethylbenzene was carried out at 623 K on a HY zeolite with a framework Si/Al ratio equal to 16. TMB isomerization was 4 to 10 times faster than xylene isomerization. The disproportionation of 1,2,4-TMB was 4 to 7 times faster than the disproportionation of p-xylene and of the other TMB isomers and 30 and 55 times faster than that of m-xylene and of o-xylene. The addition of 1,2,4-TMB to the m-xylene reactant caused a significant increase in the formation of xylene isomers and particularly of o-xylene owing to a rapid transalkylation between 1,2,4-TMB and m-xylene. The high rate of the latter reaction rendered possible, besides the unimolecular isomerization process, a direct isomerization of m-xylene through two successive bimolecular reactions, m-xylene disproportionation followed by transalkylation between the trimethylbenzene produced and m-xylene. From a simple model it was shown that the para/ortho selectivity of the bimolecular m-xylene isomerization was very different from (much lower than) that of the unimolecular isomerization. This explains why in a series of HY zeolites with different framework Si/Al ratios the greater the disproportionation to isomerization rate ratio (DIT) the lower the para/ortho selectivity. The value of the para/ortho selectivity of the unimolecular process was determined accurately from the extrapolation at D/I = 0 of the para/ortho ratio. Low values of D/I were obtained by inhibiting selectively m-xylene disproportionation by adding methylcyclohexane to the reactant. It is shown that the relative proportion of unimolecular and bimolecular mechanisms can be estimated from the experimental values of the para/ortho ratio. (C) 1996 Academic Press, Inc.