Compositional Heterogeneity of CrAPO-5 with Neutral Framework: Effect on Sorption Properties in Comparison to Other MeAPO-5 with Charged Frameworks and Analytical Evaluation of Adsorption Potentials

Abstract

Substitution of Cr3+ for Al3+ in AlPO4-5 results in a material with a heterogeneity of composition and no framework charge. That compositional heterogeneity affects sorption properties generally in a similar way as the charged centers of the other substituted metal ions. A different influence of the neutral Cr centers can mainly be noticed for the sorbates with nonpolar molecules. Both types, i.e., neutral and charged heterocenters, cause a considerable increase of heterogeneity in relation to the inert sorbates. The controlling factors for the substitution of all investigated metals are the amount of metal, i.e., the number of heterocenters, and the distribution of the centers throughout the crystals. The latter has a higher importance for sorption of nonpolar sorbates. The kind of metal seems to play a secondary role. Adsorption potentials of low substituted materials are lower than that of AlPO4-5 likely due to strong effects of localized sorption. The potentials follow the rising metal contents and achieve the value equal to AlPO4-5 at the substitution level within the range between 0.5 and 0.65 Me/u.c. A further increase of the substitution leads to both the increase of adsorption potential and suppression of the effect of localized sorption on the isolated metal centers.