Meinusch Rafael, Ellinghaus Rüdiger, Hormann Kristof, Tallarek Ulrich, Smarsly Bernd M
Institute of Physical Chemistry, Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany.
Phys Chem Chem Phys. 2017 Jun 7;19(22):14821-14834. doi: 10.1039/c7cp01846k.
The preparation of monolithic SiO with bimodal porosity using a special sol-gel procedure ("Nakanishi process") generally shows a pronounced sensitivity towards several physico-chemical parameters of the initial solution (concentrations, precursors, pH, temperature, etc.). Thus, temporal and spatial variations of these parameters during the sol-gel reactions can affect the final meso- and macropore space with respect to the pore size distributions and homogeneity. In this study we thoroughly examine the sol-gel reaction in terms of the impact of temperature accuracy and homogeneity during the gelation and their effect on meso- and macropore space. The in-depth characterization of the macroporosity in monolithic SiO rods, prepared by utilizing a highly homogeneous and accurate temperature profile, shows that a decrease of only 1.5 °C during the reaction doubles the mean size of the macropores in the analyzed temperature ranges (22.0-28.0 °C and 33.5-36.5 °C). Rheological measurements of the gelation points and the viscosity of the starting solutions prove that a higher reaction rate is the main reason for this marked temperature-sensitivity. Furthermore, the mesoporosity is affected to a surprising extent by the applied small temperature differences during the gelation reaction. This phenomenon is shown to be mainly caused by the temperature-dependent differences in macropore and skeleton dimensions and an inhomogeneous distribution of mesopore sizes within the skeleton. In essence, our study reveals that the impact of temperature on the formation of meso- and macroscale dimensions during the sol-gel process has been underestimated so far. The impact of a poor temperature homogeneity during monolith synthesis is exemplarily demonstrated by the application of monolithic silica capillary columns in HPLC.
采用特殊的溶胶 - 凝胶法（“中西法”）制备具有双峰孔隙率的整体式二氧化硅通常对初始溶液的几个物理化学参数（浓度、前驱体、pH值、温度等）表现出显著的敏感性。因此，溶胶 - 凝胶反应过程中这些参数的时间和空间变化会影响最终的介孔和大孔空间的孔径分布和均匀性。在本研究中，我们从凝胶化过程中温度精度和均匀性的影响及其对介孔和大孔空间的作用方面，对溶胶 - 凝胶反应进行了深入研究。通过利用高度均匀和精确的温度分布制备的整体式二氧化硅棒中大孔率的深入表征表明，在分析的温度范围（22.0 - 28.0°C和33.5 - 36.5°C）内，反应过程中仅1.5°C的温度降低就会使大孔的平均尺寸加倍。起始溶液凝胶化点和粘度的流变学测量证明，较高的反应速率是这种显著温度敏感性的主要原因。此外，凝胶化反应过程中所施加的微小温度差异对介孔率产生了惊人程度的影响。这种现象主要是由大孔和骨架尺寸的温度依赖性差异以及骨架内介孔尺寸的不均匀分布引起的。本质上，我们的研究表明，到目前为止，温度对溶胶 - 凝胶过程中介观和宏观尺寸形成的影响被低估了。整体式硅胶毛细管柱在高效液相色谱中的应用示例性地证明了整体合成过程中温度均匀性差所带来的影响。
