Diffusion and adsorption of hydrocarbons in microporous crystalline molecular sieve zeolites

Abstract

Diffusion coefficients have been measured for cyclohexane and alkylcyclohexanes ranging in ZSM-5 type zeolite 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 377K was $4.8 ×10^{-13} ㎠/sec$ and it was $9.9 ×10^{-13} ㎠/sec$, $2.3 ×10^{-13} ㎠/sec$, $2.6 ×10^{-13} ㎠/sec$, $3.8 ×10^{-13} ㎠/sec$ and $5.4×10^{-13} ㎠/sec$ 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 gets 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. Zeolite ZSM-23 crystallites of platelet habit were obtained from Diquat-7 dibromide salt, which have a controlling effect to form one-dimensional channel system. The diffusion coefficients and adsorption capacities of benzene, mono substituted n-alkylbenzene and para-disubstituted benzene in ZSM-23 were measured. The diffusion coefficient of benzene at 379K was $1.1×10^{-12} ㎠/sec$, and it was $8.5×10^{-13} ㎠/sec$, $5.2×10^{-13} ㎠/sec$, $3.6×10^{-13} ㎠/sec$, $1.6×10^{-13} ㎠/sec$, $7.8×10^{-13} ㎠/sec$ and $5.8×10^{-13} ㎠/sec$ for tolune, ethylbenzene, n-propylbenzene, n-butylbenzene, p-xylene, p-ethyltolune and p-diethylbenzene, respectively. Diffusion coefficients of monosubstituted alkylaromatics are smaller than that ...