Preparation of a mercaptopropyl bonded silica intermediate in supercritical carbon dioxide: Synthesis, characterisation and chromatography of a quinine based chiral stationary phase

This research examines the preparation of a mercaptopropyl bonded silica intermediate in supercritical carbon dioxide (sc-CO₂) and the subsequent conversion in sc-CO₂ to a quinine derived chiral stationary phase (CSP). The effects of reaction temperature, pressure and time on the surface coverage of the silica intermediate were investigated when porous silica particles (Exsil-Avanti, 3 μm) were reacted with 3-trimethoxymercaptopropylsilane in sc-CO₂. We present results which demonstrate that a stable mercaptopropyl bonded silica intermediate can be successfully prepared under supercritical conditions of 40 °C, 483 bar, in a substantially reduced reaction time of 1 h with superior surface coverages compared to organic solvent based methods. The further utility of this supercritical fluid technology was demonstrated by the free radical addition of a quinine derived chiral selector onto a mercaptopropyl bonded silica intermediate in sc-CO₂. This supercritical fluid generated chiral stationary phase (CSP) was utilised for the direct LC enantioseparation of a series of 3,5-dinitrobenzoyl (DNB) amino acids. Bonded silica samples were characterised using elemental analysis, diffuse reflectance infrared fourier transform (DRIFT) spectroscopy, solid state ¹³C and ²⁹Si CP-MAS NMR spectroscopy, and thermogravimetric analysis (TGA). This supercritical fluid functionalisation approach offers an efficient and cleaner alternative to existing organic solvent based approaches for the preparation of bonded silica phases.