[Preparation and application of chiral silica gel spheres based on L-glutamic]

Inorganic mesoporous silica gel spheres, which possess sufficient mechanical strength, thermal stability, and mobile phase endurance, are the most important and widely used materials for column packing in high performance liquid chromatography (HPLC). However, amorphous silica gel is generally reported as an inorganic chiral silica gel, and spherical all-inorganic chiral silica gel has not been reportedly used as the chiral stationary phase for HPLC. In this paper, inorganic spherical mesoporous silica gel was used in the method of polymerization-induced colloid aggregation (PICA), with silica sol as the raw material and L-glutamic acid (L-Glu) as the chiral monomer, to obtain mixed spheres of urea-formaldehyde resin and colloidal silica in a chiral environment. After high-temperature calcination (at 550 ℃) to remove the resin, inorganic mesoporous silica gel spheres based on L-Glu were prepared. Elemental analysis revealed that the prepared L-Glu chiral silica gel spheres were calcined completely, indicating that there were no organic constituents. Scanning electron microscopy (SEM) images of the silica gel spheres showed that the surface of the silica gel spheres was not smooth, with a uniform particle size of 3.0-4.5 μm. Transmission electron microscopy (TEM) images showed that the pore size distribution of the synthetic silica gel spheres was uniform due to the accumulation of pores. Nitrogen adsorption tests revealed that the specific surface area of L-Glu chiral silica gel spheres was 117.844 m²/g, the pore volume was 0.411 cm³/g, and the average pore size was 12.312 nm. All the characterizations indicated that the inorganic chiral mesoporous silicon had a regular spherical shape. The silica gel spheres possess frameworks and pore structures, providing a chiral microenvironment that is suitable as a chiral stationary phase for separating racemic compounds by HPLC. Because of the chiral pore structure and frameworks, the enantiomers were retained to different degrees and separation was achieved. The porous structure also increased the contact surface between the racemates and the active sites of the inner wall and improved the separation efficiency. Hydrogen bonding between the chiral stationary phase and the racemates, dipole interactions, and van der Waals forces were also involved in enantiomer resolution. An HPLC column was prepared with L-Glu chiral silica gel as the stationary phase and n-hexane-isopropanol (9∶1, v/v) as the mobile phase. Fifteen racemic compounds were successfully separated on the chiral HPLC column, including seven racemic compounds for baseline separation, at a flow rate of 0.1 mL/min using 254 nm as the detection wavelength at 25 ℃. The separation of 10 benzene position isomers was successfully achieved, and eight of the benzene position isomers reached baseline separation. Experimental results showed that the chiral silica gel spheres possess sufficient mechanical strength and thermal stability, along with good chiral recognition ability and the ability to separate positional isomers. Compared with ordinary silica gel, these chiral silica gel spheres afforded better chiral separation and better separation of benzene position isomers, without further modification. The preparation of this chiral stationary phase also has the advantages of being cheap, convenient, and feasible.