孔壁结构对气体在二氧化硅多孔材料内传质机理的研究

With the rapid developments in nanomaterials synthesizing techniques and characterization instruments, a significant number of innovative silica materials were invented and practically applied in the fields of biology and energy. Complementing these advances, the century-old Knudsen diffusion model and its derivative dusty gas model (DGM) are still employed as the primary tool to examine the adsorption and diffusion behaviors of gases in narrow pores. However, due to the strong van der Waals potentials exerted by the pore walls, the experimental interpretation based on the Knudsen model yields significant deviations. Therefore, this project is intended to employ sol-gel process, using Tetraethylorthosilicate (TEOS) and 1,2-bistriethoxysilyl ethane (BTESE) as the silica precursor, to synthesize several meso and microporous silica materials and its functional modifications. These materials are later characterized by pore size distribution, gas permeation, adsorption isotherms and adsorptive update dynamics, in order to determine the van der Waals interaction parameters between the fluids and pore walls, adsorption coefficient and gas transport diffusivity. These extracted parameters will be employed to validate the well-established mass transfer models to derive the dominant resistance of gas transport in the materials; the results can be used to explore the qualitative relationship between the gas separation factors and pore structure and functional groups, which in turn provides fundamental basis for optimization conditions of pore functional groups and structures in inorganic membranes for gas separation.

随着纳米材料合成技术和表征设备的快速发展，大量新型的微介孔二氧化硅材料被合成出来和应用于生物与能源的领域。当前传统的努森扩散和气体尘埃模型(DGM)仍作为研究气体分子在狭窄孔道内吸附和扩散行为的主要手段。由于孔壁对扩散分子具有显著的范德华力作用，努森扩散对气体在微介孔材料内传质扩散的实验数据解析存有较大误差。因此本项目拟采用溶胶凝胶法，利用正硅酸乙酯和1,2-二(三乙氧基甲硅烷基)乙烷为硅前体制备几种微介孔二氧化硅材料及其修饰体，通过分析其孔径分布、膜渗透数据、吸附曲线和气体吸附动态曲线，测定气体-孔壁的范德华力作用参数、气体在二氧化硅多孔介质中的吸附和扩散系数；并将所得上述参数通过与现有扩散模型相比较，推导出气体在微介孔二氧化硅材料中所受传质阻力的主导因素，以期定性探究不同孔道结构和官能团成分与气体传质分离效能的关系，为优化气体膜分离的孔道官能团和结构参数提供理论基础。

研究流体在微孔材料内的吸附和扩散机理对于合理设计新型纳米材料具有重要的意义。本项目针对常见的无定型二氧化硅类微孔材料，系统研究了几种常见的小分子气体在纯二氧化硅微孔材料和碳原子杂化二氧化硅微孔材料的吸附和扩散过程，研究结果显示碳原子掺杂作用会造成孔壁的Si-O键连接比较松散，从而使杂化二氧化硅类材料较易形成较大孔道，从而降低分离系数。通过模拟计算小分子气体在孔道内的吸附曲线，成功解析了杂化材料的兰纳琼斯物性参数，并成功应用到对孔口活化能的数据分析，结果显示，孔口阻力是筛选不同气体分子的关键性因素，并计算得到了几种关键性的孔道尺寸。为了研究小气体分子在膜和粉体材料内的扩散性，成功的将此种材料浸涂在大孔基体上，制备成微孔支撑膜，通过一系列气体渗透表征和气体吸附速率测试，结果显示小分子气体在粉体和膜材料的扩散具有相类似的扩散活化能，说明两种材料的孔道结构基本类似。此研究结果显示球形气体分子可以成功预测微孔材料的分离潜力，从而起到筛选膜材料的功能。