Spectroscopic and chromatographic characterisation of a pentafluorophenylpropyl silica phase end-capped in supercritical carbon dioxide as a reaction solvent

Abstract

This research uses solid-state nuclear magnetic resonance (NMR) spectroscopy to characterise the nature and amount of different surface species, and chromatography to evaluate phase properties of a pentafluorophenylpropyl (PFPP) bonded silica phase prepared and end-capped using supercritical carbon dioxide (sc-CO2) as a reaction solvent. Under sc-CO2 reaction conditions (at temperature of 100°C and pressure of 414bar), a PFPP silica phase was prepared using 3-[(pentafluorophenyl)propyldimethylchlorosilane] within 1h. The bonded PFPP phase was subsequently end-capped with bis-N,O-trimethylsilylacetamide (BSA), hexamethyldisilazane (HMDS) and trimethylchlorosilane (TMCS) within 1h under the same sc-CO2 reaction conditions (100°C/4141bar). Elemental microanalysis, thermogravimetric analysis (TGA), and scanning electron microscopy (SEM) were used to provide support data to solid-state NMR and chromatographic evaluation. Results revealed a surface coverage of 2.2μmol/m2 for the non-end-capped PFPP silica phase while the PFPP phase end-capped with BSA gave a higher surface coverage (3.9μmol/m2) compared to HMDS (2.9μmol/m2) and TMCS (2.8μmol/m2). 29Si CP/MAS NMR analysis of the PFPP end-capped with BSA shows a significant decrease in the amount of Q3 (free silanols) and Q4 (siloxane groups) species, coupled with the absence of the most reactive Q2 (geminal silanols) in addition to increased amount of a single resonance peak centred at +13ppm (MH) corresponding to SiO*SiCH3 bond. 13C CP/MAS NMR shows the resonance corresponding to the propyl linkage (CH3CH2CH2) and methyl groups (Si(CH3)n) confirming successful silanisation and endcapping reactions in sc-CO2. Chromatographic evaluation of the BSA end-capped PFPP phase with Neue text mixture revealed improved chromatographic separation as evidenced in the enhanced retention of hydrophobic markers and decreased retention for basic solutes. Moreover, chromatography revealed a change in column selectivity for the BSA end-capped PFPP phase with dipropylphthalate eluting before naphthalene, indicating decreased silanol groups and increased hydrophobicity. The extend of BSA end-capping as measured by the increase in column efficiency (67,260N/m vs. 60,480N/m) on a 2.1 i.d.×50mm column, methylene group selectivity (αCH2=2.27 vs. 2.14) and decreased silanophilic interactions (S=3.7 vs. 4.10) indicate that the increase in carbon loading (3.9μmol/m2 vs. 2.2μmol/m2) and improvement in chromatography in good peak shape and symmetry is attributed to end-capping with trimethylsilyl groups.