Chiral mesoporous silica prepared with supramolecular templated and their separation properties

Supramolecular templates represent a unique class of pore forming agent that can result in the transcription of chiral properties to the mesopore surface, during the synthesis of mesoporous silica. To design new chiral mesoporous silica materials it is important to understand the supramolecular interactions governing the self-assembly process. Chiral mesoporous materials offer opportunities in asymmetric catalysis and separation applications, with potential to improve enantioselectivity. The work presented in this thesis is organized around three themes: (i) the supramolecular synthesis of chiral mesoporous silica and the characterisation of chiral properties, (ii) their application for chiral separation, (iii) and mechanistic studies of chiral adsorption and enantioselectivity. 

Mesoporous silica particles are prepared using templates guanosine monophosphate (GMP) and folic acid (FA) resulting in NGM-1 and NFM-1 materials, respectively. Both of these have 2D hexagonal structure with p6mm symmetry, in their calcined form. We study and describe the interaction between the template and the different silica precursors: 3-aminopropyltriethoxy silane and tetraethyl orthosilicate in order to obtain optimal synthetic parameters, such as reactant composition, pH, stirring rate, etc. This thesis investigates any solvophobic effects on the self-assembly process and chirality of the mesopores. Characterization methods such as powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption isotherms, circular dichroism and thermogravimetric analysis are employed to follow the development of morphological, textural and chirality changes of the materials prepared.

Optimized NGM-1 and NFM-1 are used to assess the enantioseparation of D- and L- enantiomers by a range of model molecules, without the need for surface functionalization, i.e. relying solely on the surface properties. The chiral selectivity and kinetics of the mesoporous materials are measured by CD, UV-Vis spectroscopy and TGA. Results confirm that the NGM-1 and NFM-1 have opposite enantiomeric selectivity. 

Finally, the mechanism of chiral adsorption within mesoporous silica NGM-1 and NFM-1, are investigated. A model is proposed that accounts for the different pathways of individual enantiomers to form an adsorbed monolayer as well as to fill the pores of NGM-1 and NFM-1.