Acidic Properties of Alkaline-Earth Phosphates Determined by an Experimental-Theoretical Approach

Alkaline-earth
phosphates efficient in the dehydration of lactic
acid to acrylic acid were previously shown to contain a surface mono/dihydrogen
phosphate amorphous layer composed of M2+ cations and both
PO and POH groups. In this work, acidic properties of such
a layer were determined combining Fourier transform infrared (FTIR)
spectra achieved at the dehydrated state and under water vapor and
density functional theory (DFT) simulations of nondefective and defective
MPOH structure. The FTIR spectra of adsorbed pyridine and lutidine
revealed the presence of moderate Lewis acid sites (LAS) and of POH
groups interacting by H-bonding without significant protonation. DFT
calculations were key to interpret FTIR spectra after adsorption of
NH3: when solely adsorbed, NH3 interacts with
the LAS on both the nondefective surface and the defective surface,
whereas the POH for which H points up toward the gas phase are reoriented
downward. Brønsted acid sites (BAS) were shown to form under
water vapor. This phenomenon was shown by DFT to arise from a more
acidic character of HPO42– species for
the nondefective surface and casual formation of nondefective surface
leading to higher amount of H2PO4– species, which are more acidic BAS.