Relative stabilities of zeolitic aluminosilicates

The question is addressed as to whether there is a theor. basis to an apparent relation that has been exptl. obsd. between the lattice topol. and the Al/Si ratio of zeolites. Calcns. are presented for 3 lattices formed with very different Al/Si ratios: faujasite, mordenite, and ZSM-5. The relative stabilities of these lattices were studied as a function of Al/Si ratio in the presence or absence of adsorbed water. Na+ ions are taken as the compensating ions. Two types of approaches were followed to study the changes in heat of formation as a function of Al2O3 content. In one approach, for a certain zeolite structure the Madelung and polarization energies were calcd. for fixed lattice positions. The heat of formation due to ionic bonding was calcd. both for the zeolitic aluminosilicate with varying amts. of Al and Na ions for the zeolite silica, with the same framework distances as in the Al-contg. material. The difference between the 2 heats of formation is assumed to represent the contribution to the total heat of formation due to bonding from the presence of Al and cations in a particular zeolite structure. This difference is added to the heat of formation due to covalent bonding of the silica polymorph as calcd. by the extended Hueckel method. The effect of hydration on the relative stabilities is estd. using empirical data on hydration with water. In the 2nd approach rigid ion lattice energy minimization calcns. were used, with potentials parameterized to fit a-quartz. This technique was particularly useful to det. the stability of the structures whose energy had been calcd. by the method discussed earlier. The presence of Al ions and cations as well as hydration has a considerably larger influence on the relative stabilities of zeolites than covalent contributions. The predictions for the order of stability of faujasite, mordenite, and ZSM-5 as a function of Al/Si ratio is in qual. agreement with the results of synthesis expt