A new method of theoretical analysis for temperature-programmed desorption (TPD) of ammonia to determine the acid amount and strength and its distribution from a one-time experiment is proposed on the basis of the equilibrium between gaseous and adsorbed ammonia, i.e., free readsorption of ammonia. The entropy change was assumed to consist of the constant phase-transformation term and the gaseous mixing term as a function of gaseous concentration of ammonia. The enthalpy change, namely adsorption heat, was assumed to have several kilojoules per mole of the distribution. Thus a simulated TPD curve could be fitted well with the experimental data observed on mordenite and ZSM-5 zeolites. From the parameter set that gave the best fitted curve, the acidic properties of zeolite were determined. The determined acid amount was close to the difference between the aluminum and sodium contents, [Al] − [Na], in most cases. This confirms a simple principle that one acid site is generated by isomorphous substitution of one aluminum atom into the silicate matrix, and one sodium atom blocks one acid site. On the other hand, the mordenite and ZSM-5 had the acid strength, ca. 145 and 130 kJ mol-1, of the adsorption heat of ammonia, respectively, with several kilojoules per mole of the distribution, irrespective of the acid amount. Another simple principle is therefore proposed:  the acid strength of zeolite is determined by the crystal structure.

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 Al 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. IntroductionZeolite, which consists of SiO 2 and Al 2 O 3 , has widely been used as a heterogeneous catalyst. 1-6 In place of the aluminum, various kinds of metals can be incorporated into the framework, and the obtained metallosilicates are being considered as novel functionalized materials. 7 Especially, gallosilicate with MFI structure has a remarkable catalytic activity for the conversion of alkane into aromatics. [8][9][10][11][12][13][14][15][16][17][18] Generation of acidity by isomorphous substitution of silicon by gallium is presumed to be the origin of the catalytic functions. Analysis of the acidic property, namely, the number, strength, and nature (Brønsted or Lewis) of the acid sites, is therefore important to interpret the catalytic functions and, moreover, must be important also from the physicochemical viewpoint.The acidic property of gallosilicate has been investigated by means of the infrared (IR) spectroscopy of surface hydroxide, 19-22 the adsorbed pyridine, 23 carbon monoxide 24,25 and nitrogen, 25 and the nuclear magnetic resonance of proton ( 1 H NMR), 26 supported by quantum chemical calculations. 27-30 These spectroscopic methods mainly deal with the quality of the acid site, but the quantitative measurement is difficult. Traditional indicator 31 and amine titration 32 methods to quantify the acid amount have a disadvantage, because they are confused by the slow diffusion of liquid molecules in the micropore. Test reactions have widely been used, but they cannot be related simply to the acidic property. Measurement of adsorption heat of such a small base as ammonia by calorimetry 33 is one powerful method to evaluate the number and strength of acid sites. By this method, the interaction between the acidic and basic materials is directly evaluated, and this gives an absolute measure of the amount and strength of acid.Temperature...