Influence of Surface Silanol Structure and Sydration Forces on Alkoxide-Derived Silica Gel Structure
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EXPERIMENTAL PROCEDURE Preparation of alkoxide-derived silica gels Amorphous silica gels were prepared by mixing 0.25, 0.5 or I mol of tetraethyl-orthosilicate (TEOS, 52, 104 or 208 g) and deionized water with acid (HCI) or base (NH4OH) catalysis. The components are presented in Table 1. After stirring for 24 h, the solutions were poured into glass dish, covered by plastic film and left to gel at room temperature. The gels were allowed to contract at room temperature until they were free on the wall of glass dish. Drying was then performed at 80 °C for 24 h in an oven. Specific surface area of each dried gel was determined by BET method and was in the range from 350 to 450 m2/g. Table 1 Composition of solutions used to prepare silica gel TEOS H 20 HC1 NHaOH H 2 0/TEOS 208 g 68 g 0.4 g 4 208 g 170 g 1.0 g 10 104 g 170 g 1.0 g 20 52g 170 g 1.0 g 40 104 g 85 g 1.8 g 10
Designation Al A2 A3 A4 B1
Characterization of dried gels and surface properties The true density and relative density of each resultant gel and the pore size distribution in a dried gel were measured by helium-gas pycnometry and mercury porosimetry, respectively. The surface silanol and siloxian structure of the silica gels oven-dried at 80 °C were determined by diffuse and reflectance FT-IR and FT-nIR in flowing dry nitrogen gas. After heat treatment at various temperatures in the range from room temperature to 900 C for 10 min on the hot stage under a dry nitrogen gas flow, the FT-IR spectra also were examined on this stage. The additional hydration force and interaction of each gel was determined by atomic force microscope, AFM. The force was measured between a sharp tip of AFM and the flat surface of dried gels with adjustment at pH = 4.0. Gel samples were dried at 80 C for 24 h, and were transferred into the AFM in a laminar flow cabinet. Deionized and particle-free water was distilled once inside the cabinet before use in each experiment. RESULTS FT-IR and FT-NIR analysis on silica gel surface Figure 1 shows the influence of H 2 0 / TEOS mole ratio and catalyst on FT-IR spectra in the fundamental absorption region from 2500 to 4000 cm'. Absorption bands of isolated and hydrogenbonded silanol groups appeared. In base-catalyzed silica (B 1) and acid-catalyzed silica prepared from solution with a high H2 0 / TEOS mole ratio (A3, A4), the isolated silanol band appeared at 3750 cm-. On the contrary, in acid-catalyzed silica (Al, A2) with a low H20 / TEOS mole ratio the isolated silanol band disappeared. All samples had absorption of the hydrogen-bonded silanol band at 3660 cm' and the adsorbed H2 0 band from 3200 to 3600 cm-. Near-infrared (nIR) region spectra with > 4000 cm' are shown in Fig. 2. All samples have a hydrogen-bonded silanol band with bonded water at 4415 cm-. Base-catalyzed silica (BI) and acidcatalyzed silica prepared from solution with a high H20 / TEOS mole ratio (A3, A4) appeared the vicinal free silanol group band at 4560 cm-, however, the free silanol group band of Al samples weakened and 242
nearly disappeared. For the highes
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