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

微孔沸石材料在开发新一代高效催化剂、吸附剂和膜方面正引起人们越来越大的兴趣。分子筛孔道中的分子扩散系数是影响反应和分离选择性和速度的关键因素。然而，各种宏观和微观方法测量的扩散系数存在很大的差异，有时甚至是定性的差异，这些方法在不同的物理条件下以不同的机理工作。这严重阻碍了沸石催化剂、吸附剂和膜的理论进步和合理设计的实现。目的：本项目的目标是利用一种新的显微激光折射测量方法来了解非平衡条件下的分子扩散，该方法是通过独特的沸石薄膜光纤集成微型器件来实现的。扩散系数的测量是基于超灵敏和实时监测分子筛的折射率随扩散引起的山梨酸盐浓度分布的变化而变化的。研究的具体技术目标包括：(I)建立新的显微激光折射率测量的实验方法，并根据实验光学信息建立扩散系数计算的物理和数学模型；(Ii)研究现有宏观和微观技术获得的扩散系数异常差异的根本原因；以及(Iii)了解所选分子的分子扩散系数的浓度和温度依赖关系，包括强吸附大芳烃和弱吸附小气体，其扩散系数在科学界仍有争议。智力优势：拟议的研究旨在解决沸石分子筛中分子扩散系数严重不一致的问题，这些问题对实现新一代微孔催化剂和膜的合理设计以及反应和分离过程的优化构成了根本障碍。该项目还将阐明一些小分子的扩散系数，这些小分子的扩散很难用现有技术来测量。这些小分子包括H2、CO2、CO、CH4和He，它们在当前全球通过催化转化和分子分离从煤炭、天然气和生物质中生产H2的努力中具有前所未有的重要性。实现本研究的目标有赖于建立一种新的激光折射率测量方法，该方法是由物理和功能集成的沸石薄膜光纤微器件实现的。新方法允许同时进行微观和宏观测量，同时在线监测沸石结构的变化，同时避免了现有技术的主要限制。这种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温度和浓度范围内工作。新方法具有超高的检测灵敏度(例如甲苯蒸气的10-7bar增量)和时间分辨率(即以1us-1的观测频率连续监测)。因此，它能够研究观测时间尺度对微观测量结果的影响，并通过连续测量浓度阶梯小的变化来确定输运扩散系数作为浓度的函数。此外，薄膜折射法在进行宏观测量时，本质上避免了外部表面电阻的影响，可以消除存在问题的吸附热效应。新方法的发现将通过与平行的宏观实验和分子模拟工作以及文献中的核磁共振和QENS测量结果进行比较来评估。广泛的影响：这个项目的广泛影响既是科学的，也是教育的。所获得的沸石(主体)吸附(客体)动态相互作用的知识与许多其他纳米孔系统基本相关，如微孔储氢材料、纳米管和生物分子传输通道。了解客体体系的光学性质对于光学化学传感器、光催化剂和新型光电子器件等前沿领域也具有重要意义。为化学实验高级课程开设微孔沸石分子扩散系数的光学测量实验，为化学工程本科教学做出直接贡献。分子在微孔介质中的扩散对许多现代化学技术是重要的，但在CHE本科生课程中，特别是在实验室课程中，却没有得到充分的解决。这一努力将极大地改善这种情况。

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.