Abstract

The substitution of Al³⁺ by V³⁺ and P⁵⁺ by V⁵⁺ in AlPO₄-5 microporous materials is studied by energy minimisation techniques. The incorporation of V³⁺ in aluminium sites is accompanied by a strong distortion of the framework. The least stable site has the lower V³⁺–O distance, and more deformed V³⁺–O–P angles. In contrast, for the substitution of P⁵⁺ by V⁵⁺ small deformations of the framework were obtained. For this reason we suggest the vanadium is found, fundamentally, as V⁵⁺ in the phosphorus sites. Very similar energies were obtained for the six different independent phosphorus sites of the asymmetric unit cell which suggests that the V⁵⁺ ion could occupy any of these sites with the same probability. Furthermore, the 78 configurations of two V⁵⁺ ions in the unit cell were studied. We did not find a relationship between the stability of the framework and the position of the substituted vanadium ions. An increment of around 1% in the volume of the unit cell with respect to the configurations of one V⁵⁺ ion was observed. Simulation of the VAPO-5 vibrational spectrum shows a new band (≈835 cm⁻¹), in agreement with the IR experiments.