Polyamine-functionalized mesoporous silicas: Preparation, structural analysis and oxyanion adsorption

A series of polyamines, NH2(CH2CH2NH)n−1H (where n = 2-6, i.e. EDA-, DETA-, TETA-, TEPA- and PEHA), were attached to 3-chloropropyltrimethoxysilane (CPTMS) grafted MCM-41 (ClPrM41) and SBA-15 (ClPrS15) by the nucleophilic substitution of chlorine. The porous structure was maintained well after this synthesis. CHN elemental analysis implied that a polyamine molecule reacted with two chloropropyl chains on PrM41 while EDA, DETA and TETA (and higher amines) reacted with 1, 1.5 and 2 chloropropyl groups on ClPrS15. IR spectra revealed the presence of primary and secondary amino groups on EDA-, DETA- and on TETA-PrM41 and PrS15. Both bridge and bicrural structures are suggested for the major forms of the organic chains for polyamine-PrM41. On the other hand, the mode of binding shifted with the size of polyamine chain on PrS15. (We proposed linear structures for EDA, terminal bridge, linear and forked structures for DETA and bridged structures for TETA as surface species.) A clear even number effect (by which the amount of adsorption is enhanced in the cases of EDA, TETA and HEPA) was observed in an arsenate adsorption series on polyamine-PrM41, though the amount of adsorption decreased with the number of amino groups in the polyamine chain in polyamine-PrS15. Adsorbents prepared by the same procedure but with nonporous silica showed smaller As/N ratios in arsenate adsorption.