AMORPHOUS ALPO4 AS CATALYST SUPPORT .2. CHARACTERIZATION OF AMORPHOUS ALUMINUM PHOSPHATES

Three different amorphous aluminum phosphates have been characterized by their specific surface area and pore volume and by XPS and FTIR studies. Three types of A1 surface hydroxyl groups at 3793.8, 3768.5, and 3743.2 cm-1 and only one type of P surface hydroxyl groups at 3677.5 cm-1 are found in FTIR spectra after evacuation at 800°C. The ratio of the integrated absorbance of the IR bands from AIOH and POH surface groups is a sensitive tool to study the surface of AlPO4 directly. A linear relation between the electronegativity of the metal ion and the stretching vibration of surface hydroxyl groups is proposed for Al, Si, P, and B tetrahedral surface ions. The three Al surface hydroxyl groups are assigned to four-coordinated (tetrahedral), five-coordinated, and six-coordinated surface Al ions. Amorphous AlPO4 contains four- and five-coordinated Al ions and AlPO4 with high specific surface area also has six-coordinated Al ions at the surface. CO is adsorbed on the phosphorus surface hydroxyl groups at low temperature, giving an IR band at 2170 cm-1 while the IR band from the POH group is shifted to 3495 cm-1. On AlPO4 of high surface area, CO is also adsorbed on coordinatively unsaturated, five-coordinated Al surface ions. Four-coordinated Al ions are also found, but none with the coordination number three. Impregnation with chromium compounds in ethanol does not change the specific surface area, whereas impregnation in water resulted in a decreased specific surface area, to less than half of its previous value. © 1991.