M. Kocirik et al., INVESTIGATION OF SORPTION AND TRANSPORT OF SORBATE MOLECULES IN CRYSTALS OF MFI STRUCTURE TYPE BY IODINE INDICATOR TECHNIQUE, MICROPOROUS AND MESOPOROUS MATERIALS, 23(5-6), 1998, pp. 295-308
Iodine indicator technique (IIT) involving light microscopy has been i
ntroduced in our laboratory to investigate sorption and mass transport
phenomena in zeolites and peculiarities of crystal morphology via col
oring of zeolite crystals. The coloring of silicalite-1 90 degrees-int
ergrowths was performed using pure iodine vapors and binary solutions
of iodine in organic solvents (benzene, cyclohexane, toluene, ethylben
zene, p-xylene, and decahydronaphthalene). Coloring from the liquid ph
ase was carried out either under co- or counter-diffusion conditions.
The rate of coloring from the vapor phase, and under co-diffusion cond
itions from the liquid phase, was found to be limited by external mass
transport and the coloring was uniform. Under counter-diffusion condi
tions, the rate of coloring was to about 2 to 3 orders of magnitude lo
wer than in the latter case. Except for decahydronaphthalene, the colo
ring patterns were non-uniform, and they visualized at least in the be
ginning stage of the coloring process the interfaces of crystal sectio
ns. Also, the cracks were visualized being decorated with iodine. Ther
e is a marked difference between the development of coloring patterns
of benzene and cyclohexane on one side and their linear alkyl derivati
ves (toluene, ethylbenzene and p-ethyltoluene) on the other side. In t
he former case, the iodine enters the bulk of crystal sections via sec
tion interfaces. In the latter case, the bulk of the sections appears
to be inaccessible for iodine molecules. Other information on channel
system accessibility provides IIT for decahydronaphthalene, where the
molecular sieve effect of 10-membrane oxygen rings with respect to dec
ahydronaphthalene is indicated. (C) 1998 Elsevier Science B.V. All rig
hts reserved.