Sol-gel synthesis of hierarchically porous TiO 2 beads using calcium alginate beads as sacrificial templates

Meso-macroporous millimetre-sized TiO 2 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 TiO 2 matrix. The TiO 2 properties were varied by altering the alginate bead synthesis conditions (concentration of the sodium alginate, NaAlg, and calcium ions, Ca 2+, in the gelling bath) and the calcination temperature. The surface area of the TiO 2 beads reached maxima using template beads prepared in a 0.27 M Ca 2+ bath, for a given concentration of NaAlg. TiO 2 (anatase) beads displayed high surface areas of 170 ± 10 m 2 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 m 2 g -1. The post-functionalization of the TiO 2 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.