Effect of pH and chitosan concentration on precipitation and morphology of hierarchical porous silica

Understanding the interaction between chitosan molecules and sodium silicate is one of the principal objectives for the development of new environmentally benign strategies for the synthesis of low-cost porous silica with hierarchical structure. In this work, we have explored two essential factors simultaneously: chitosan/silica ratios (0.1, 0.4, 0.8 and 1.6) and pH values of the mixture (3, 5 and 6). The obtained silica–chitosan composite and the calcined products were investigated by using thermal gravimetric analysis, N₂-physisorption, scanning electron microscopy, and transmission electron microscopy. Rapid precipitation of silica–chitosan composite occurred because of the initial pH values of the chitosan and sodium silicate solutions. The amount of precipitating silica–chitosan composite was increased with increasing pH value of the mixture and chitosan/silica ratios but it did not change with the decrease of pH value of the mixture. The as-synthesized products prepared at pH 3 and 6 showed macroscopic two-solid phases (consisting of silica-rich phase and chitosan-rich phase), while those prepared at pH 5 displayed liquid and solid phases. At pH 3, a unimodal porous structure was found for the calcined products obtained from the silica-rich phase while a hierarchical structure was found for those obtained from the chitosan-rich phase; meanwhile, a hierarchical structure occurred for all calcined products prepared at pH 6. Different hierarchical structures of the silica products could be obtained by changing pH value of the mixture and chitosan/silica ratios.