m-Xylene reactions over zeolites with unidimensional pore systems

Reactions of m-xylene are performed over a series of unidimensional, high-s ilica zeolites. It is determined that zeolites with a unidimensional pore s tructure have a unique reaction selectivity when compared to multidimension al zeolites of the same pore site. Unlike multidimensional zeolites with 12 member ring (MR) pores that give a paralortho p/o selectivity of >1, zeoli tes with unidimensional pores bounded by 14 MRs or large 12 MRs give a plo <1 due to the influence of a bimolecular isomerization mechanism between a xylene and trimethybenzene. The unidimensional, parallel pore zeolites (UTD -1, SSZ-31, CIT-5, SSZ-24, ZSM-12, ZSM-48) gave products with the lowest pl o ratio for the given pore size. Multidimensional zeolites and unidimension al zeolites with internal cavities larger than the pore openings all gave h igher plo selectivities. It is shown that at lower flow rates, where extern al diffusion becomes important, plo selectivity can be lowered due to an in creased amount of bimolecular isomerization occurring at or near the cataly st external surface. While the selectivity of the reactions of In-xylene gi ve useful information for characterizing zeolite structures, it is imperati ve that all zeolites be compared at the same flow rate and that the flow ra te be sufficiently high to negate significant external diffusion effects on reaction selectivity. Under the proper conditions, the reactions of m-xyle ne can give information enabling the characterization of medium, large and extra-large parr zeolites. (C) 1999 Elsevier Science B.V. All rights reserv ed.