Spray-dried porous silica microspheres functionalised by phosphonic acid groups

Porous silica-based microspheres with pore surfaces functionalized by phosphonic acids are obtained for the first time from a direct co-condensation route using two alkoxide precursors (Si(OEt)4 and Si(OEt)3(CH2)2P(O)(OEt)2), surfactant self-assembly and spray-drying methods. The synthesis employs octadecyltrimethylammonium bromide as template agent. The post-treatment of the microspheres in boiling concentrated hydrochloric acid insures the hydrolysis of the ethoxy groups initially bound to phosphorus in the organophosphonate–siloxane precursor, without destroying the morphological and textural properties. Final porous materials have the chemical structure [SiO1.81(OZ)0.38]0.91[SiO1.35(OZ)0.29(CH2)2P(O)(OH)2]0.09 (with Z  H or Et), and posses interesting textural properties: SBET = 747 m2 g−1, and dp = 2.3 nm.

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► Silica microspheres with phosphonic functional groups were made by spray-drying method.
► Using of [C18H37N(CH3)3]Br allows the formation of microspheres with dp = 2.3 nm, SBET = 747 m2 g−1.
► The post-treatment of boiling HCl allows for a complete hydrolysis of the ester phosphonate groups.
► 31P NMR MAS spectroscopy was shown that there is 15% of –CH2PO(OH)2−y(OSi)y (1 ⩽ y ⩽ 2) fragments.