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

• Ionic liquid-modified silicas were firstly used to adsorb Salvia phenolic acids.
• These phenolic acids was firstly preparatively separated on SiO2·Im+·PF6− column.
• Acidity of these phenolic acids was firstly determined for separation mechanism.
• There are systematic characterization, investigation and instructive findings.
• SiO2·Im+·PF6− shows good adsorption, desorption and reusability for phenolic acids.

In this work, six kinds of different ionic liquid-modified silica were synthesized and characterized with infrared spectroscopy, thermogravimetric analysis, elemental analysis, N2 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-methylimidazolium hexafluorophosphate (SiO2·Im+·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+·PF6− 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.

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