Application of [hmim][BF4] ionic liquid in reversed-phase simulated moving bed for the separation of 3- and 4-hydroxybenzoic acid

−In reversed-phase liquid chromatography (RP-LC), the separation of ionized compounds at neutral pH of the mobile phase is ineffective because these compounds easily become ionized and too hydrophilic to adsorb on the hydrophobic C18 stationary phase. The common method to separate ionized compounds in RP-LC is the usage of the ion pairing agent and the control of pH in the mobile phase. In this work, [hmim][BF4] ionic liquid was adopted as a mobile phase additive in reversed-phase simulated moving bed chromatography (RP-SMB) to overcome the problem mentioned above. The results showed that when 1.5 ml/l [hmim][BF4] was added to mobile phase, 3-hydroxy benzoic acid (3-HBA) and 4-hydroxy benzoic acid (4-HBA) were separated at neutral pH in the RP-SMB. In particular, purities of 3-HBA and 4-HBA products were 97.62% and 100%, respectively, and had a good agreement with simulation results.