Preparation of thin plate-like calcium phosphate particles with slit-shaped micropores and their molecular sieve effect

Abstract

The formation of plate-like calcium phosphate particles with slit-shaped micropores produced from the forced hydrolysis reaction of 10 mM Ca(OH)2, 3.2 mM triphosphate(tripolyphosphate, tpp; $$ {{\text{P}}_3}{\text{O}}_{{10}}^{{5 - }} $$ ) and 20 mM HCl mixed solution at 100 °C for 2 h were investigated. The molecular sieve effect of produced particles was also explored by using five kinds of adsorptives with various molecular diameters. The large thin plate-like particles as prepared exhibited a large and sharp peak at 2θ = 6.929° with small diffraction patterns of β-Ca2P2O7. This characteristic peaks at 2θ = 6.929° suggested that the particles possess a layer structure with ordered basal plane and the large thin plate-like particles are made up of thin sheets. However, the large peak at 2θ = 6.929° shifted to higher 2θ value after evacuated the particles at 50∼200 °C, and these characteristic peak was diminished after treated over 300 °C. The interlayer distances of basal plane were estimated to be 1.280 to 0.980 nm. TG and FTIR measurements disclosed that large amounts of water molecules exist in the particles. It was also revealed that tpp molecules are not completely hydrolyzed to phosphate (orthophosphate, op; $$ {\text{PO}}_4^{{3 - }} $$ ) ions but still remain tpp and diphosphate (pyrophosphate, pp: $$ {{\text{P}}_2}{\text{O}}_7^{{4 - }} $$ ) ones, and are adsorbed in slit-shaped micropores. The N2 adsorption isotherms for the particles were of Type II and they did not change significantly by changing the evacuation temperature from 25 °C to 300 °C. On the contrary, the amount of adsorbed H2O molecules on the particles after evacuated at 25 and 50 °C steeply increased at ca. 0.3 < p/p 0 < 0.6 to provide a step-like adsorption isotherm, but it disappeared after evacuated over 75 °C. From the molecular adsorption experiments employing adsorptives with different molecular diameters, it was indicated that the slit-shaped micropores possess a width less than 0.353 nm being exhibits a high selective adsorption of H2O molecules into slit-shaped ultramicropores that are accessible to H2O molecules but not to N2 and other adsorptive ones. It was indicated that ultramicropores can be formed by the narrowing effect of residual tpp and pp ions unhydrolyzed in the slit-shaped micropores. The residual tpp and pp ions may serve for the adsorption of H2O molecules by their strong hydrogen bonding nature.