The Initial Reactions of H₃PO₄ and NaH₂PO₄Supported on Silica: A Joint Experimental and Theoretical Study

A combination of X-ray powder diffraction, thermogravimetric analysis, diffuse reflection infrared Fourier transform, and 31P magic-angle spinning nuclear magnetic resonance techniques with density function computation was used to elucidate the products and mechanism of the reactions among silica, H₃PO₄, and NaH₂PO₄during the preparation of silica supported H₃PO₄and NaH₂PO₄catalysts. The spectral test results indicate that besides polyphosphoric acid, silicon phosphates on silica supported H₃PO₄ are also formed. On silica supported NaH₂PO₄ only sodium polyphosphates are present. Density functional theory (DFT) simulations indicate that in the initial stage, reaction of H₃PO₄ with silanol groups on the silica support is more favorable than that between H₃PO₄ itself. In contrast, dimerization and trimerization of NaH₂PO₄ are predicted to be the predominant initial reactions for the silica supported NaH₂PO₄catalyst.