Determination of the acidic properties of zeolite by theoretical analysis of temperature-programmed desorption of ammonia based on adsorption equilibrium

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.