Hybrid organic–inorganic phenyl stationary phases for the gas separation of organic binary mixtures

Organic–inorganic hybrid silica xerogels are versatile materials that can be used in the separation/purification of gases and vapours. In the present study, hybrid xerogels were synthesised at pH 10 using phenyltriethoxysilane (PhTEOS) and tetraethoxysilane (TEOS) as silica precursors with 20, 40 and 70% PhTEOS molar ratios. The xerogels were characterised by helium pycnometry, FTIR, TGA–DSC, N2 (77 K) and CO2 (273 K) adsorption analyses. The retention time and selectivity of several binary mixtures were measured, including benzene–toluene, m-xylene–benzene, m-xylene–toluene, water–ethanol, and carbon tetrachloride–dichloromethane. The characterisation results reveal that the hybrid xerogels were mainly macroporous. For each adsorbate–adsorbent system, the retention time decreased with an increase in temperature, ruling out that activated diffusion was the primary mechanism of separation. The xerogel synthesised from 40% PhTEOS presented the greatest potential for the resolution of binary mixtures. It represents a trade-off between surface area and phenyl-group surface density. Aromatic substances had a stronger affinity for the hybrid xerogels than the other analytes due to the strong interaction between phenyl groups on the analyte and the stationary phase. Water–ethanol mixtures were effectively separated by the xerogel derived from 70% PhTEOS.