Titanium alkylphosphate functionalised mesoporous silica for enhanced uptake of rare-earth ions

The separation of rare-earth elements (REEs) is usually carried out by a multi-stage solvent extraction process utilising organophosphorus extractants. Inspired by the structure of the solvating extractant tri-n-butyl phosphate (TBP), new sorbents were designed by covalently attaching short n-alkyl chains (ethyl, n-propyl and n-butyl) to titanium(IV) phosphate functionalised mesoporous MCM-41 silica by a layer-by-layer grafting route. Mesoporous MCM-41 silica served as a versatile porous support and the grafted titanium(IV) derivatives provided enhanced acid stability and solvating extraction capability. Various characterisation methods including solid-state C-13, Si-29 and P-31 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) and simultaneous thermogravimetry and differential scanning calorimetry-mass spectroscopy (TG/DSC-MS) were used to confirm the ligand attachment. The hybrid materials showed a better uptake of rare-earth ions from nitrate feed solution than the unmodified inorganic material. The optimal separation factor (SF) obtained for scandium-lanthanum separation is in excess of 100 000 at pH 2.1. The SFs calculated for dysprosium-neodymium are approximately 3, which is comparable to that of TBP in a typical solvent extraction setup. This study provides a new strategy to design inorganic-organic hybrid sorbents based on the structure of organophosphorus extractants via metal(IV)-O-P bonds.