A computer simulation study of vanadium substitution in the AlPO4-5 framework

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 (approximate to 835 cm(-1)), in agreement with the IR experiments.