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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. Bronsted 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.