A reactivity index study to rationalize the effect of dopants on Brönsted and Lewis acidity occurring in MeAlPOs

Abstract

The influence of both bivalent and trivalent metal substituents from a range of metal cation (Co, Mn, Mg, Fe and Cr) on the acidic property (both Brönsted and Lewis) of metal substituted aluminum phosphate MeAlPOs is monitored. The influence of the environment of the acid site is studied both by localized cluster and periodic calculations to propose that the acidity of AlPOs can be predictable with accuracy so that AlPO material with desired acidity can be designed. A semi-quantitative reactivity scale within the domain of hard soft acid-base (HSAB) principle is proposed in terms of the metal substitutions using density functional theory (DFT). It is observed that for the bivalent metal cations Lewis acidity linearly increases with ionic size, where as the Brönsted acidity is solely dependent on the nearest oxygen environment. Intramolecular and intermolecular interactions show that once active site of the interacting species is identified, the influence of the environment can be prescribed. Mg(II)-doped AlPO-34 shows highest Brönsted acidity and whereas Cr(III)-doped species shows lowest acidity. Fe(II)/Fe(III)-doped AlPO-34 shows highest Lewis acidity, whereas Mn(III), Mg (II) shows lowest acidity.