Diffusion of cyclohexanes in ZSM-5 zeolites

Diffusion coefficients have been measured for the diffusion of cyclohexane and alkylcyclohexanes ranging from methyl- to pentylcyclohexane in ZSM-5-type zeolites using the samples of flat plate-like crystallite. The uptake curves of Cyclohexanes by these samples are considered to represent primarily the diffusion of Cyclohexanes into the straight channel. The diffusion coefficient of cyclohexane at 377 K was 4.8 × 10−13 cm2/sec and they were 9.9 × 10−13, 2.3 × 10−13, 2.6 × 10−13, 3.8 × 10−13, and 5.4 × 10−13 cm2/sec for methyl-, ethyl-, n-propyl, n-butyl-, and n-pentylcyclohexane, respectively. The diffusion coefficients of monosubstituted n-alkylcyclohexane showed a minimum at ethylcyclohexane and increased steadily as the chain length got longer. The length of the ethylcyclohexane molecule is comparable to the intersection space and the sharp decrease in the diffusion coefficient can be interpreted in terms of the cage effect. The diffusion coefficient of t-1,4-dimethylcyclohexane was two orders of magnitude larger than that of ethylcyclohexane although they have about the same molecular length and kinetic diameter. This may be attributed to the methyl-methyl interaction resulting in a favorable orientation for diffusion through the intersection space.