Hydroxylation methods for mesoporous silica and their impact on surface functionalisation

Abstract

Silica supports used e.g. in chromatography, separation and bioassay lack complete efficacy unless they are surface functionalised. Thus, chemistries are grafted to the surface to enhance their properties and capacity in specific applications. Here, various strategies are examined for ‘cleaning’ and hydroxylation of SBA-15 mesoporous silica (as a high surface area exemplar) to sponsor efficient functionalisation through maximising surface hydroxyl groups as the surface binding sites. Cleaning process effects on the mesoporous silica were studied using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The physical properties were characterised using N 2 sorption and x-ray diffraction (XRD). The bulk and surface compositions were characterised by Fourier-transform infrared (FTIR) spectroscopy and 29 Si nuclear magnetic resonance (NMR) spectroscopy. Contact angle measurements were also taken, and the surface energy components calculated. Cleaning of mesoporous SBA-15 was carried out using acids (piranha acid solution & nitric acid), ultraviolet/ozonolysis and water. The surface area decreased after cleaning and the surface was found to be more active after cleaning by determination of new available silanol groups and by making the surface more hydrophilic. NMR showed that silica was cleaned as opposed to rehydroxylated as new silanol functional groups were not determined. Finally, the mesoporous silica was functionalised with 3-(aminopropyl) triethoxysilane (APTS). Elemental analysis along with NMR ( 13 C and 1 H) were used to determine the impact of cleaning. Cleaning influenced grafting by increasing the potential loading of the silane examined. This study provides a facile approach to prepare organosilicas for potential higher loading capacities. • Pre-treatment steps have positive effect on mesoporous silicas. • Cleaning increases the availability of hydroxyl groups at the surface. • Silane concentration can be increased significantly using cleaning methods. • Further knowledge into effects of ‘cleaning’ mesoporous silica SBA-15.