Acidic Property of MFI-Type Gallosilicate Determined by Temperature-Programmed Desorption of Ammonia

摘要

Temperature-programmed desorption of ammonia was applied to gallosilicate with MFI structure using the water vapor treatment method to remove the unnecessary low-temperature peak and the curve-fitting method to calculate the acid strength and its distribution based on the thermodynamics. The determined acid amount approximately agreed with the [Ga]-[Na] content in the low gallium content region, showing the stoichiometric generation of one acid site by the substitution of one gallium atom, and stoichiometric neutralization of one acid site by one sodium atom. The acid strength (adsorption heat of ammonia) due to the framework 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 A1 and Ga into the zeolitic crystal generates the similar acid strength, and the acidic strength is mainly determined by the crystal structure. The extraframework 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 additional type of acid site with the higher adsorption heat of ammonia.