SYNTHESIS AND PROPERTIES OF SOME MESOPOROUS ALUMINOPHOSPHATES WITH ACIDIC SURFACE SITES

Mesoporous solids of the general formula Al100PxFe5-y [x = 0, 15, 60, and y (the final firing temperature in degrees C) = 400 or 600] have been prepared and characterised by surface area and porosity measurements, X-ray diffraction (XRD), Mossbauer spectroscopy and solid-state NMR (Al-27 and P-31). The surface acidities of the solids were also determined by ammonia adsorption, and the zeta-potential of the particles was measured. The gradual incorporation of phosphate groups into the alumina results in an increase of the surface area. The materials possess a pore-size distribution approximated by a mixed Gaussian and Lorentzian component which varies in a controllable manner according to the elemental composition. The material Al100P15Fe5 corresponds to that with the maximum surface area and pore volume. The XRD analysis showed that the materials without phosphorus possess the gamma-Al2O3 structure, but they become amorphous upon addition of 15% P. A further increase in the amount of phosphorous to 60% initiates the transformation of the solids to AlPO4. The Mossbauer spectra suggest that, in the samples without phosphorus, the iron species are segregated, but the addition of phosphorus results in a dispersion of the iron into the aluminium phosphate matrix. The Al-27 MAS NMR spectra indicate that the aluminium atoms are predominately in octahedral environments in the samples Al100P0Fe5-600 and Al100P15Fe5-600 but they become equally distributed between octahedral and tetrahedral sites in Al100P60Fe5-600. The zeta-potential of the particles in aquatic suspension indicates that this parameter is sensitive to the temperature of thermal treatment. The surfaces of the solids showed increased acidity which is linearly related to the phosphorus content.