A SIMULATED ANNEALING STUDY OF SI,AL DISTRIBUTION IN THE FAUJASITE FRAMEWORK

Several important properties of faujasite are concerned with the Si,Al distribution in its framework. In the present study, a starting confi guration of the distribution is generated with randomly positioned Si, Al. After the simulated annealing procedure, some orderings appear in the optimized configurations of the system and lead to a series of con sequences, some of which have already been obtained by pioneer studies concerning ordering schemes. In optimized configurations, a certain d egree of disorder reasonably is maintained because of the use of stand ard Monte Carte finite temperature techniques, which bring about a rea listic employ of the numerical simulation. The calculated Al distribut ion on the first shell of the Si site is assessed by Si-29 MAS n.m.r. experiments to demonstrate the rationality of the present theory. The calculated Al distribution on the second shell of the Al site is relat ed to the acidity, which leads to the proposition that the acid amount of faujasite is in proportion to the number of single Al four rings. The electrostatic energy calculation provides possible explanations of the observed discontinuity in the plot of unit cell parameter against Al content.