Study of Molecular Diffusion in Zeolites by Time-Resolved Microscopic Laser Refractometry

0854203 Dong微孔沸石材料在开发新一代高效催化剂、吸附剂和膜方面引起了越来越大的兴趣。分子在沸石孔中的扩散系数是影响反应和分离的选择性和速率的关键因素。然而，在不同的物理条件下，通过不同的机制操作的各种宏观和微观方法测量的扩散系数存在很大的差异，有时甚至存在质的差异。这严重阻碍了分子筛催化剂、吸附剂和膜的理论发展和合理设计的实现。目的：本项目的目标是利用一种新的显微激光折射法来理解非平衡条件下的分子扩散，该方法是通过一种独特的沸石薄膜纤维集成微型装置来实现的。扩散率测量是基于超灵敏和真实的时间监测沸石折射率随扩散引起的吸附物浓度分布变化的变化。研究的具体技术目标包括：（i）建立新的显微激光折射测量的实验方法，并发展从实验光学信息计算扩散率的物理和数学模型;（ii）研究现有的宏观和微观技术获得的扩散率之间的异常差异的根本原因;以及（iii）了解选定分子的分子扩散系数的浓度和温度依赖性，包括强吸附的大芳烃和弱吸附的小气体，其扩散系数在科学界仍有争议。分子筛中分子扩散系数的严重差异已成为实现新一代微孔催化剂和微孔膜的合理设计以及反应优化的根本障碍分离过程。该项目还将澄清一些小分子的扩散率，其扩散难以通过现有技术测量。这些小分子包括H2、CO2、CO、CH 4和He，它们在当前全球通过催化转化和分子分离从煤、天然气和生物质生产H2的奋进中具有前所未有的重要性。实现本研究的目标依赖于建立一种新的激光折射测量方法，该方法通过物理和功能集成的沸石薄膜光纤微器件来实现。新方法允许微观和宏观测量，同时原位监测沸石结构的变化，同时避免了现有技术的主要局限性。独特的沸石纤维装置可以在现有的微观和宏观方法无法达到的宽温度和浓度范围内操作。新方法具有较高的检测灵敏度（如甲苯蒸气的10-7 bar增量）和时间分辨率（即以1 us-1观测频率连续监测）。因此，它是能够研究观察时间尺度上的微观测量结果的影响，并确定作为浓度的函数的传输扩散率通过连续测量与浓度的小阶梯变化。此外，薄膜折射法本质上避免了进行宏观测量时的外表面电阻的影响，并且可以消除有问题的吸附热效应。新方法的研究结果将通过与平行宏观实验和分子模拟工作以及文献中的NMR和QENS测量结果进行比较来评估。广泛的影响：该项目的广泛影响是科学和教育。所获得的知识沸石（主机）吸附（客人）的动态相互作用是从根本上相关的许多其他纳米多孔系统，如微孔H2存储材料，纳米管，和生物分子运输通道。对客体-主体体系光学性质的理解对于光化学传感器、光催化剂和新型光电子器件等前沿领域也具有重要价值。通过在化学工程高级实验室课程中开设微孔分子筛中分子扩散系数的光学测量实验，为化学工程本科教育做出了直接贡献。微孔介质中的分子扩散对许多当代化学技术很重要，但在化学工程本科课程中，特别是在实验室课程中没有得到充分的解决。这一努力将大大改善这种状况。

0854203DongMicroporous zeolitic materials are attracting growing interest in developing new generation of high efficiency catalysts, adsorbents, and membranes. The molecular diffusivity under confinement in the zeolitic pores is a key factor affecting the selectivity and rate of reaction and separation. However, large discrepancies and sometimes even qualitative differences exist in diffusivities measured by various macroscopic and microscopic methods which operate by distinct mechanisms under different physical conditions. This has seriously impeded the theoretical advancement and the realization of rational design of zeolite catalysts, sorbents and membranes.Objective: The goal of this project is to understand molecular diffusion under non equilibrium conditions using a new microscopic laser refractometry approach, which is realized by a unique zeolite thin film fiber integrated micro device. The diffusivity measurement is based on ultra sensitive and real time monitoring of the zeolite refractive index variation with the diffusion caused change in sorbate concentration distribution. The specific technical objectives of research include: (i) to establish the experimental methodology for the new microscopic laser refractometry measurements and develop physical and mathematical models for diffusivity computation from the experimental optical information; (ii) to investigate the fundamental causes of the anomalous discrepancies among diffusivities obtained by existing macroscopic and microscopic techniques; and (iii) to understand the concentration and temperature dependences of molecular diffusivities forselected molecules including strongly adsorbing large aromatics and weakly adsorbing small gases of which the diffusivities are still controversial in the scientific community.Intellectual Merit: The proposed research aims to resolve the issues of seriously discrepant molecular diffusivities in zeolites that have posed fundamental barriers to the realization of rational design of new generation microporous catalysts and membranes and optimization of reaction and separation processes. The project will also clarify the diffusivities for a number of small molecules of which the diffusion are difficult to be measured by existing techniques. These small molecules include H2, CO2, CO, CH4, and He which are of unprecedented importance in the current global endeavor to produce H2 from coal, natural gas and biomasses by catalytic conversion and molecular separation. Achieving the goal of this research relies on the establishment of the new laser refractometry approach that is realized by a physically and functionally integrated zeolite thin film fiber micro device. The new method allows both microscopic and macroscopic measurements with simultaneous in situ monitoring of zeolite structural changes while avoiding the major limitations of the existing techniques. The unique zeolite fiber device can operate in wide ranges of temperature and concentration inaccessible to the existing microscopic and macroscopic methods. The new method possesses ultrahigh detection sensitivity (e.g. 10-7 bar increment for toluene vapor) and temporal resolution (i.e. continuous monitoring at 1 us-1 observation frequency). Thus, it is capable of studying the effect of observation time scale on the microscopic measurement results and determining the transport diffusivity as a function of concentration by continuous measurement with small step staircase changes in concentration. Also, the thin film refractometry approach intrinsically avoids the influence of external surface resistance when performing macroscopic measurement and the problematic adsorption heat effect can be eliminated. The findings by the new method will be assessed by comparing with parallel macroscopic experiments and molecular simulation works and results of NMR and QENS measurements in the literature.Broad Impacts: The broad impact of this project is both scientific and educational. The obtained knowledge of zeolite(host) adsorbate(guest) dynamic interactions is fundamentally relevant to many other nanoporous systems such as microporous H2 storage materials, nanotubes, and biological molecular transport channels. The understanding of optical properties of guest host systems is also valuable to the frontier areas like optical chemical sensors, photocatalysts, and novel optoelectronic components. A direct contribution will be made to the chemical engineering undergraduate education by establishing a new experiment of optical measurement of molecular diffusivity in microporous zeolite for advanced ChE Lab courses. Molecular diffusion in microporous media is important to many contemporary chemical technologies but is inadequately addressed in ChE undergraduate curriculum especially in lab courses. This effort will greatly improve this situation.