Synthesis and properties of some mesoporous aluminophosphates with acidic surface sites

Abstract

Mesoporous solids of the general formula Al₁₀₀P_xFe₅-y[x= 0, 15, 60, and y(the final firing temperature in °C)= 400 or 600] have been prepared and characterised by surface area and porosity measurements, X-ray diffraction (XRD), Mössbauer spectroscopy and solid-state NMR (²⁷Al and ³¹P). The surface acidities of the solids were also determined by ammonia adsorption, and the ζ-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 γ-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 Mössbauer 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 ²⁷Al 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 ζ-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.