Study of the Effect of Synthesis Conditions on the Structural Properties of Biomimetic Polysaccharide-Templated Mesoporous Silica ^†

In our work, we chose chitosan (polysaccharide) as a template for the synthesis of mesoporous silica-based material (ZChM). Both acidic and slightly alkaline synthesis conditions were tested. In the acidic method, we varied the molecular weight of chitosan (200 and 500 Da) and the time of TEOS addition.

Biomimetic silica samples (ZChM-series) were characterized using the Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Brunauer-Emmett-Teller (BET) Surface Area Measurement, Thermogravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) methods. According to the obtained data, a material with a more ordered structure was achieved by using an acidic type of synthesis with chitosan 200 Da and a controlled rate of TEOS addition. This material had a surface area of around 790 m²/g, which is only 30% less than the surface area of MCM-41-type materials. The material synthesized in alkaline conditions had a significantly lower specific surface area. In addition to the surface area, the synthesis conditions also affected the pore size of the resulting materials—the material obtained under the alkaline conditions had the largest pore diameter (6.64 nm), while the second sample synthesized in an acidic environment (ZChM-2a) had a slightly lower pore diameter than MCM-41; 2.73 vs. 3.3 nm, respectively.

The new bioinspired silica samples were tested as adsorbents of water-soluble amino acids, and both the kinetics and adsorption equilibrium were studied. It was found that synthesis conditions can significantly affect the sorption properties of mesoporous silica. The biomimetic silica that was synthesized with chitosan as a template in an acidic medium (ZChM-1a) can sorb tryptophan and phenylalanine. Otherwise, the material ZChM-1s (synthesized in an alkaline medium) does not practically sorb these amino acids.

Thus, chitosan can be successfully used as a template for the synthesis of mesoporous silica. By varying the medium conditions, the molecular weight of chitosan, and the rate of addition of TEOS, it is possible to obtain mesoporous materials with different surface and adsorption properties.