|M.Sc Student||Harush Adi|
|Subject||Functionalizing Mesoporous Silica Surfaces and their|
Molecular-Level Characterization by Solid State
|Department||Department of Chemistry||Supervisor||Professor Asher Schmidt|
|Full Thesis text|
Ever since mesoporous materials were first synthesized in the early 1990s their broad application potential such as in catalysis, separation, and waste removal has drawn great scientific and technological attention. Particularly, the mesoporous adsorbents such as MCM-41, HMS, and SBA-15, are robust inorganic solids which possess large specific surface areas, and tunable pore volumes. The relative ease by which surface-dense hydroxyls are chemically modified makes them ideal anchoring groups for various chemical compounds that may serve many purposes. In particular, thiol-functionalized silica-based mesoporous adsorbents have attracted extensive attention since they are expected to exhibit specific binding ability toward highly toxic pollutants, and in particular Hg2, as a consequence of soft Lewis acid-base interactions.
In this work I have grafted onto SBA-15 three widely used linkers - APTES, APDMES, MPTMS (aminopropyl thriethoxysilane, aminopropyl dimethoxysilane, mercaptopropyl trimethoxysilane), and coupled L-cysteine to grafted APTES, in attempt to achieve thiol terminated functional group with Hg2 adsorption/binding potency.
As molecular-level characterization of the functionalized mesoporous silica I have employed primarily multinuclear (29Si, 13C, 1H, 2H) solid state NMR; nitrogen adsorption-desorption (BET) was employed to characterize the properties of the mesoporous SBA-15. Solid state NMR techniques enabled us confirm the successful grafting and identify the various binding modes, while BET measurements confirmed to what extent the mesoporous material characteristics were affected. Solid-state NMR confirmed the coupling of L-Cys to the grafted APTES. For SBA-15 functionalized by the thiol-terminated linker (MPTMS) the NMR shows Hg2 binding to the linker. Surprisingly, the NMR reports that Hg2 loading, results in partial linker detachment. These observations therefore highlight both the potential and limitations of mercury removal by such grafted linkers.
Following the grafting of the three linkers, as means to assess the accessibility of the pendent groups to peroform their function, we have employed 2H MAS NMR to identify their dynamic state (amines and thiol) as function of hydration. This study shows for each of the grafted linkers induced hindrance that limits access of the functional groups by water molecules; this hindrance appears more pronounced for the amine- than for the thiol- terminated propyls, hence highlighting a possible functionality limiting mechanism. Interestingly, the -ND3 groups in the samples studied in the least dehydrated state, do not report fast-limit free rotation. These observations therefore imply that a notable barrier for free rotation, most likely due to interactions of the charged amine with the surface silanols, alleviate the C3 symmetry of this group.
Solid state NMR spectroscopy, as employed in this study, proves as a unique tool to provide direct evidence on the molecular details of the tethered functional groups, which are key in the design of materials with tailored function.