Sol–gel synthesis of hierarchically porous TiO2 beads using calcium alginate beads as sacrificial templates

Abstract

Meso-macroporous millimetre-sized TiO₂ beads were synthesized using sol–gel chemistry within preformed alginate beads. The alginate beads acted as sacrificial templates, responsible for providing the spherical shape and controling structural properties of the TiO₂ matrix. The TiO₂ properties were varied by altering the alginate bead synthesis conditions (concentration of the sodium alginate, NaAlg, and calcium ions, Ca²⁺, in the gelling bath) and the calcination temperature. The surface area of the TiO₂ beads reached maxima using template beads prepared in a 0.27 M Ca²⁺ bath, for a given concentration of NaAlg. TiO₂ (anatase) beads displayed high surface areas of 170 ± 10 m² g⁻¹ using a calcination treatment of 450 °C and samples calcined at 700 °C remained anatase with a decrease in surface area to 46 ± 1 m² g⁻¹. The post-functionalization of the TiO₂ beads was studied using alendronate, an amino-bisphosphonate, with a maximum loading (30 ± 2 mg g⁻¹) obtained at pH 2.01. The loading was highly dependent on surface area. These large (millimetre-sized) porous inorganic beads have potential in applications such as catalysis and adsorption-separation due to their ease of recovery and handling.