A Computational Study of Lowenstein’s Rulein ZeoliteA

The Si,Al ordering in zeolite A(formula Na96Al96Si96O384)has been studied theoretically using steric energy minimization by simulated annealing.The consistent molecular mechanics force field is used.Calculations are performed on four kind of Si,Al distributions:i)the 4:0 ordered structure,in which Lowenstein's rule is obeyed and no Al-O-Al bridges occur,ii)and iii)two hypothetical structures having 3:1 framework ordering,i.e.,each silicon is connected by oxygen to three aluminum atoms and one silicon atom,iv)a structure with random Si,Al distribution and three kind of cation occupations:i)The experimental Na+ occupation;i.e.,Na+ occupy sites I ,II and III with 64,24 and 8 per unit cell,respectively;ii)and iii)Na+ occupy sites I,II and III randomly.The steric energy,the heat of formation and unit cell parameter of each refined structure are obtained.An energetic preference for the Lowensteinian Si,Al distribution and the experimental Na+ occupation are found.