ADSORPTION MECHANISM OF PHOSPHORIC-ACID ON GAMMA-ALUMINA

A series of P/Al2O3 catalysts with phosphorus loadings in the range 0-7 wt% P has been investigated by infrared spectroscopy. After pretreatment at 700°C, four types of hydroxyls were resolved in the OH stretching region of the IR spectrum for γ-alumina. Addition of phosphorus (as phosphoric acid) resulted in a reduction of all the different hydroxyls with the exception of the most basic, which increased to a maximum intensity at around 1 wt% P. A sharp band at 3676 cm-1 due to POH groups was also easily resolved, and this band increased in intensity with increasing phosphorus content. A mechanism for the adsorption of phosphoric acid on alumina is proposed on the basis of these results, whereby the different alumina hydroxyls react with H3PO4 preferentially depending on their basicity up to a critical surface concentration of around 10 × 1013 H3PO4 cm-2. Above this concentration H3PO4 reacts not only with AIOH groups but also with POH groups to form polyphosphate species. The intensity of the band due to the most basic alumina hydroxyls passed through a maximum since, at low phosphorus loadings, these sites may be generated by the reaction of H3PO4 with bridging and triply bridging alumina hydroxyls, but are not formed by this process at higher loadings. © 1991.