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 desorp tion (TPD) of ammonia to determine the acid amount and strength and it s distribution from a one-time experiment is proposed on the basis of the equilibrium between gaseous and adsorbed ammonia, i.e., free reads orption of ammonia. The entropy change was assumed to consist of the c onstant phase-transformation term and the gaseous mixing term as a fun ction of gaseous concentration of ammonia. The enthalpy change, namely adsorption heat, was assumed to have several kilojoules per mole of t he distribution. Thus a simulated TPD curve could be fitted well with the experimental data observed on mordenite and ZSM-5 zeolites. From t he parameter set that gave the best fitted curve, the acidic propertie s 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 si licate 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 k J mol(-1), of the adsorption heat of ammonia, respectively, with sever al kilojoules per mole of the distribution, irrespective of the acid a mount. Another simple principle is therefore proposed: the acid streng th of zeolite is determined by the crystal structure.