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

University Of Cincinnati Main Campus, Cincinnati OH

Investigators

Abstract

0854203 Dong Microporous 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.