Cationic, structural, and compositional effects on the surface structure of zeolitic aluminosilicate catalysts Original Research Article

The cationic, structural, and compositional influences on the structure and bonding of zeolitic aluminosilicates have been investigated with the density functional theory (DFT) method including local (VWN) and non-local spin density functionals (BLYP). Full optimization of structures has been carried out at the 6–31G∗/VWN and 6–31G∗/BLYP levels of theory for the different types of the SiOHAl unit in the secondary building unit of the zeolite cluster models [(OH)8HyAlxSi8−xO12](x−y)− (x,y = 0, 1, 2, 4) and the silica model (OH)8Si8O12. Changes in the environment of the silicon and aluminum framework atoms with a given Si/Al ratio generate new different acid sites. The validity of Loewensteinʹs AlOAl avoidance rule is confirmed but Dempseyʹs AlOAl avoidance rule does not hold with double four-membered ring aluminosilicate (D4R). The proton affinity (PA) of the silica model (OH)8Si6O12 at BLYP/6–31G∗ are evaluated to be 1403 ± 15 kJ/mol which is in good agreement with the experimentally observed value of 1390 ± 25 kJ/mol. Proton affinities of Brønsted hydroxyl groups in H forms of zeolites associated with different Si/Al ratios indicate that the higher the ratio, the less the proton is constrained which results in a stronger acid strength. Cations are found to have profound effects on the structure and bonding of zeolite clusters. The H ion has a strong perturbation on the important parameters i.e. SiO, AlO OH bond lengths while the Li(I) has a modest effect. The PA of the Brønsted OH groups in the zeolitic frameworks interacting with their cations are 1272.4 and 1279.3 kJ/mol for H2(OH)8Al2Si6O12 and LiH(OH)8Al2Si6O12, respectively, which correspond with decreasing bond lengths of the OH groups. These results indicate that the acid strength of the OH groups within the zeolitic framework is also determined by the presence of cations, in addition to compositional and structural effects.