Organofunctional silica mesostructures with improved accessibility and applications as heavy metal ion adsorbents

Effective approaches to the removal of toxic metals from contaminated water have involved the use of solid adsorbents, such as activated carbons, ion exchange resins, and functionalized silica-based materials including silica gels, clays and mesoporous silicas. The advantages of functionalized mesoporous silicas for environmental remediation are their high surface areas, well-defined pore size, and the ability to covalently link organic groups to the framework to allow for the selective adsorption of specific toxic heavy elements.;The overall goal of this work is to design functionalized mesostructured silica materials for use as heavy metal ion (i.e. Pb2+) adsorbents. The approaches investigated include the incorporation of organosilanes, heteroatoms, and surfactants into the framework structure. The hydrogen bonded supramolecular assembly of a neutral alkylamine surfactant and a nonionic silica sources was used to form wormhole mesostructures.1,2 The key objective was to improve the accessibility of the functional sites within the mesostructured silica framework.;Mesoporous amine-functionalized organosilicas were synthesized via several routes. A hydrophobic tBoc-protected amine group or a nitrile group was incorporated into the framework by direct assembly of an organosilane and tetraethylorthosilicate (TEOS) in the presence of dodecylamine as a surfactant. The amine groups were formed inside the mesopores of organosilica framework by Boc deprotection or nitrile reduction. The obtained mesostructure showed a much higher structural order and much higher accessibility of the amine groups in comparison to derivatives synthesized by the conventional direct co-condensation of TEOS and an amino-functional organosilane. The resulting mesostructures had a Pb2+ trapping capacity of 0.23--0.48 mmol g-1.;An attempt also was made to utilize the structure-directing surfactant for the removal of heavy metal ions from solution. As-made mesophases containing intercalated tallow amine surfactants showed good affinity towards Pb 2+ ions, having a maximum trapping capacity of 1.2 mmol g-1. However, it remained a concern that the surfactant was not irreversibly bound inside the mesopores. This problem was solved by introducing an epoxide organic moiety into the silica synthesis process in order to immobilize the amine surfactant inside the mesopores through the formation of covalent bonds. This modification also reduced the hydrophobic nature of the mesopore environment. A mesophase containing 5 mol % immobilized dodecylamine surfactant had a Pb2+ trapping capacity of 0.33 mmol g-1.;In addition, the alumination of mesostructured silica using lithium aluminum hydride, in-situ generated sodium aluminate and sodium aluminate solution as aluminum sources also was investigated. The tetrahedrally coordinated Al sites incorporated into the mesophases served as the cation exchange sites. The resultant adsorbents showed excellent Pb2+ trapping capacities of 0.75--1.0 mmol g-1.;(1) Pauly, T. R.; Liu, Y.; Pinnavaia, T. J.; Billinge, S. J. L.; Rieker, T. P. J. Am. Chem. Soc. 1999, 121, 8835--8842. (2) Kim, Y.; Lee, B.; Yi, J. Sep. Sci. Technol. 2004, 39, 1427--1442.