Investigation of sorption and transport of sorbate molecules in crystals of MFI structure type by iodine indicator technique

Iodine indicator technique (IIT) involving light microscopy has been introduced in our laboratory to investigate sorption and mass transport phenomena in zeolites and peculiarities of crystal morphology via coloring of zeolite crystals. The coloring of silicalite-1 90 degrees-intergrowths was performed using pure iodine vapors and binary solutions of iodine in organic solvents (benzene, cyclohexane, toluene, ethylbenzene, p-xylene, and decahydronaphthalene). Coloring from the liquid phase was carried out either under co- or counter-diffusion conditions. The rate of coloring from the vapor phase, and under co-diffusion conditions from the liquid phase, was found to be limited by external mass transport and the coloring was uniform. Under counter-diffusion conditions, the rate of coloring was to about 2 to 3 orders of magnitude lower than in the latter case. Except for decahydronaphthalene, the coloring patterns were non-uniform, and they visualized at least in the beginning stage of the coloring process the interfaces of crystal sections. Also, the cracks were visualized being decorated with iodine. There is a marked difference between the development of coloring patterns of benzene and cyclohexane on one side and their linear alkyl derivatives (toluene, ethylbenzene and p-ethyltoluene) on the other side. In the former case, the iodine enters the bulk of crystal sections via section 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 decahydronaphthalene is indicated. (C) 1998 Elsevier Science B.V. All rights reserved.