ACIDIC PROPERTY OF MFI-TYPE GALLOSILICATE DETERMINED BY TEMPERATURE-PROGRAMMED DESORPTION OF AMMONIA

Temperature-programmed desorption of ammonia was applied to gallosilic ate with MFI structure using the water vapor treatment method to remov e the unnecessary low-temperature peak and the curve-fitting method to calculate the acid strength and its distribution based on the thermod ynamics. The determined acid amount approximately agreed with the [Ga] -[Na] content in the low gallium content region, showing the stoichiom etric generation of one acid site by the substitution of one gallium a tom, and stoichiometric neutralization of one acid site by one sodium atom. The acid strength (adsorption heat of ammonia) due to the framew ork gallium was ca. 130 kJ mol(-1) with the small distribution, close to that on the ZSM-5 type aluminosilicate, and was independent of the composition. This shows that the substitution of Al and Ga into the ze olitic crystal generates the similar acid strength, and the acidic str ength is mainly determined by the crystal structure. The extraframewor k gallium on the ion-exchange site, which was readily formed with the gallium content higher than 0.3-0.4 mol kg(-1), generated the addition al type of acid site with the higher adsorption heat of ammonia.