Controlled modulation of mesoporosities in metal (M  Al, Fe) phosphate solids

Abstract

Mesoporous solids which possess average pore diameters between 7 and 20 nm, depending on the composition, have been prepared. The solids have the general formula Al 100P χM 20 where M  Al or Fe, and χ = 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 possess 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 d V p/d D p = > 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 d V p/d D p = ( 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 d V p =( 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.