The xylene isomer diffusivity in ZSM zeolites was determined via vapor sorption measurements under constant pressure of the adsorbate. The diffusivities of p-xylene and m-xylene at 313K in HNa-ZSM-5 were 1.91 x 10-11 and 1.06 x 10-11cm2/s, respectively. Taking account of the diffusion data and other sorptive properties, attempts were made to correlate the crystalline characteristics of the ZSM samples with their catalytic activity for m-xylene isomerization. In particular, large ZSM-5 crystalline particles were found to be more selective (to p-xylene) but less active than the smaller isostructural particles. The enhanced shape selectivity was attributed to a screening effect due to a silica binding deposit within the large particles.

Xylene Isomer Diffusivity and Shape Selectivity in ZSM Zeolites. R.L.Mao, V.Ragaini, G.Leofanti, R.Fois: Journal of Catalysis, 1983, 81[2], 418-28