SORPTION PROPERTIES OF TITANIUM SILICATE MOLECULAR-SIEVES

Titanium silicates (TS-1) of MFI structure, with different Si/Ti ratio (7-99) including the silicalite analogue, were prepared by hydrothermal crystallization at 443 K. Following the XRD characterization, it was revealed that the unit cell volume of the microporous solids increased with the increase in the framework titanium content. Sorption properties of the protonic forms (obtained via ammonium exchange and subsequent deammoniation) of these samples were studied by BET specific surface areas using low-temperature (78 K) nitrogen sorption and equilibrium sorption capacities for water, n-hexane, cyclohexane, benzene, and n-butylamine (n-BA). Sorption kinetics were obtained for these sorbates at 298 K and P/P0 = 0.8 up to 2 h. Specific surface areas were found to decrease with the decrease in the framework titanium content in the solid. Equilibrium sorption capacities for n-hexane and cyclohexane were comparable to MFI aluminosilicate (ZSM-5); however, that for water in titanium silicate was found to be considerably lower than for the corresponding aluminosilicate. Isotherms of n-BA sorption in these titanium silicates over the temperature range 323-523 K yielded useful information about the sorption and acidic centers. Sorption isotherm data were analyzed in terms of Langmuir, BET, Dubinin, Sips, and Freundlich adsorption isotherm models. Sorption affinities (chemical potential) for n-BA sorption decreased sharply with coverage with the increase in titanium content. Isosteric heats of n-BA sorption also showed marginal increase with the increase in the titanium content.