Fidalgo Alexandra, Ilharco Laura M
Centro de Química-Física Molecular, Complexo I, Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal.
Chemistry. 2004 Jan 23;10(2):392-8. doi: 10.1002/chem.200305079.
Monolithic porous silica xerogels were synthesized by the sol-gel process, and their local structure was analysed by vibrational spectroscopy. The silica alcogels were prepared by a two-step hydrolytic polycondensation of tetraethoxysilane (TEOS) in isopropanol, with a water/TEOS molar ratio of 4. The hydrolysis step was catalysed by hydrochloric acid (HCl), with different HCl/TEOS molar ratios (ranging from 0.0005 to 0.009), and the condensation step was catalysed by ammonia (NH(3)), with different NH(3)/HCl molar ratios (ranging from 0.7 to 1.7). After appropriate ageing, the alcogels were washed with isopropanol and subcritically dried at atmospheric pressure. The diffuse reflectance infrared Fourier transform (DRIFT) spectra were analysed in terms of the main siloxane rings that form the silica particles, taking into account the splitting of the nu(as)Sibond;Obond;Si mode into pairs of longitudinal and transverse optic components, by long-range Coulomb interactions. It was proven that the proportion of residual silanol groups (which correlates with hydrophilicity), and the fraction of siloxane 6-rings (which correlates with porosity) may be tailored by adequate catalytic conditions, mostly by the hydrolysis pH. This was explained in terms of the reactions' mechanisms taking place in the two-step sol-gel process followed.
通过溶胶 - 凝胶法合成了整体式多孔二氧化硅干凝胶，并通过振动光谱分析了它们的局部结构。二氧化硅醇凝胶是通过四乙氧基硅烷（TEOS）在异丙醇中进行两步水解缩聚制备的，水/TEOS摩尔比为4。水解步骤由盐酸（HCl）催化，HCl/TEOS摩尔比不同（范围为0.0005至0.009），缩合步骤由氨（NH₃）催化，NH₃/HCl摩尔比不同（范围为0.7至1.7）。经过适当老化后，醇凝胶用异丙醇洗涤并在常压下进行亚临界干燥。考虑到通过远程库仑相互作用，ν（as）Si—O—Si模式分裂为纵向和横向光学分量对，根据形成二氧化硅颗粒的主要硅氧烷环对漫反射红外傅里叶变换（DRIFT）光谱进行了分析。结果表明，残余硅醇基团的比例（与亲水性相关）和硅氧烷六元环的分数（与孔隙率相关）可以通过适当的催化条件进行调整，主要是通过水解pH值。这是根据随后两步溶胶 - 凝胶过程中发生的反应机理来解释的。
