Green synthesis of surfactant-free mesoporous silica with strong hydrophilicity via metal salt modifications for moisture adsorption

In this work, the green synthesis of mesoporous silica (MPS) was carried out using low-cost sodium silicate as a precursor. To customize the moisture adsorption capacity of MPS, alkaline (sodium chloride (NaCl), potassium chloride (KCl)) and alkaline earth metal salts (magnesium chloride (MgCl2) and calcium chloride (CaCl2)) with different concentrations (0.00-0.500 g salt per g SiO2) were employed during the sol-gel synthesis process. The textural and structural properties, indicating the hydrophilic features (Si-OH, Si-O-, SiO6, and SiO4) of the MPS, were observed. Alkaline metal salts (NaCl and KCl) can enhance the formation of small mesopores, leading to an increase in the specific surface area and tortuosity of the MPS. Consequently, this improves the hydrophilic functional groups and moisture adsorption capacity. MPS_0.500NaCl exhibits a surface functional group content 1.18 times that of unmodified mesoporous silica. The NaCl-modified MPS displayed outstanding moisture adsorption capacity, ranging from 1.16 to 1.30 times higher than the unmodified MPS and silica gel. In contrast, among all alkaline and alkaline earth metal salt modifications, MPS_0.125MgCl(2) demonstrated the lowest vaporizing temperature, indicating superior moisture-releasing performance. These findings suggest promising applications of hydrophilic mesoporous silica in food and agriculture, especially for rapid moisture adsorption and creating moisture-controlled environments.