Ionic liquid-modified silica gel as adsorbents for adsorption and separation of water-soluble phenolic acids from Salvia militiorrhiza Bunge

In this work, six kinds of different ionic liquid-modified silica were synthesized and characterized with infrared spectroscopy, thermogravimetric analysis, elemental analysis, N-2 adsorption-desorption isotherms and particle size distribution. Then they were used as adsorbents for the preparative separation of water-soluble phenolic acids from Salvia militiorrhiza Bunge for the first time. The effects of various parameters including functional groups of adsorbents, adsorption time, initial concentration, solid-liquid ratio and temperature were investigated in detail. It was found that silica modified with N-methylimida-zolium hexafluorophosphate (SiO2.lm(+).PF6-) had most excellent adsorption capacity for target compounds, which reached 99.81 mg/g under the conditions of adsorption time of 30 min, initial sample concentration of 350 mg/L and solid-liquid ratio of 1:1 (mg/mL). Studies showed that the equilibrium data fitted well with Langmuir model of adsorption and the adsorption kinetics was more accurately described by pseudo-second-order model. Thermodynamics parameters including the change of enthalpy, entropy and Gibbs free energy were also analyzed. Furthermore, the water-soluble phenolic acids of Salvia militiorrhiza Bunge were successfully preparatively separated on the normal pressure chromatographic column packed with the SiO2.Im(+.)PF(6)(-) particles to obtain protocatechuic aldehyde, sodium danshensu, rosmarinic acid, lithospermic acid and salvianolic acid B successively. The separation mechanism was investigated on the basis of experimental results and the reusability of the ionic liquid-modified silica gel was also explored. This study provides an useful reference for the preparation separation of natural products and indicates that the ionic liquid-modified silica can be applied in the separation of water-soluble phenolic acids with great prospects. (C) 2015 Elsevier B.V. All rights reserved.