CONTROLLED MODULATION OF MESOPOROSITIES IN METAL (M=AL, FE) PHOSPHATE SOLIDS

Mesoporous solids which posses average pore diameters between 7 and 20 nm, depending on the composition, have been prepared. The solids have the general formula Al100P(x)M20 where M = Al or Fe, and x = 0, 4.5, 9, 18, 36, 72 or 144. The initial addition of phosphorus as phosphate transforms the originally crystalline oxide/oxides into amorphous solids. These amorphous materials posses a narrow pore size distribution: 80-90% of the pores lie within 1-2 nm of the average pore diameter. Subsequent incremental amounts of phosphorus transform the material into a crystalline solid whilst the pore size distribution becomes much wider and the maximum moves towards larger pore diameters. Substitution of 20% of the aluminium by iron results, at a low phosphorus content, in pores with smaller pore volumes and smaller surface areas. The data in the dV(p)/dD(p) = f(D(p)) graphs, where V(p) is the incremental pore volume and D(p) is the average pore diameter, can be approximated using an admixture of Gaussian and Lorentzian curves. For low phosphorus contents the dV(p)/dD(p) = f(D(p)) curves have a mainly Gaussian profile but the gradual addition of phosphorus transforms them to Lorentzian-type curves. An attempt to approximate the histograms dV(p) = f(D(p)) with the minimum number of distribution curves made up of the corresponding Gaussian and Lorentzian components indicates that each successive addition of phosphorus creates a dominant new pore component at a larger pore diameter. At the same time, the components at smaller pore diameters are diminished and eventually disappear as more phosphorus is added.