Fe and B substitution in ZSM-5 zeolites: A quantum-mechanical study

Density functional theory has been used to study isomorphous substitution of Fe and B for Si in orthorhombic MFI type zeolite ZSM-5. Calculations were performed on monomer [T(OH)4], dimer [(OH)(3)-Si-O-T-(OH)(3)] and pentamer T(OSiO3H3)(4) cluster models where T = Fe or B. It has been experimentally observed that Bronsted acidity of substituted ZSM-5 increases in the order B-ZSM5 << Fe-ZSM-5 << Al-ZSM-5. Although change in acidic properties with substitution is known, the position of framework substitution with Fe and B in ZSM-5 has not yet been established. In the present work, we have done detailed density functional calculations to find out the site of framework substitution with Fe and B in ZSM-5. We have observed that the calculated properties display a substantial size dependence on the size of the model. Our calculations suggest that the preferable siting for Fe is 3 and 11 and for B it is 2 and 12 in ZSM-5 zeolite framework.