Adsorption and diffusion of xylenes and cyclohexanes in ZSM-5 zeolite catalysts were studied using the samples of uniform shape and size.
The difference in diffusion characteristics of cyclohexanes studied is interpreted in terms of the unique channel structure of ZSM-5 zeolite and the difference in size and shape of the diffusing molecule.
Diffusion coefficient of ethylcyclohexane was found to be smaller than that of cyclohexane and methylcyclohexane. Diffusion coefficient of trans-1,4-dimethylcyclohexane, however, was larger than that of cyclohexane and methylcyclohexane. It seems that configuration of longer trans-1,4-dimethylcyclohexane is such that it diffuses preferentially in the straight channel.
ZSM-5 zeolite samples of different thickness gave same diffusion coefficient.
The activation energy for diffusion, in general, decreased with increasing diffusion coefficient.
The experimental uptake kinetics of o-xylene agreed well with the calculated one using the equation for diffusion in a plane, suggesting that o-xylene mainly diffuses in the straight channel. The number of the diffusing molecule in the straight channel increased in the order ; cyclohexane, methylcyclohexane, trans-1,4-dimethylcyclohexane.
