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.
The goal of this project is to develop a new fiber optic laser refractometry method for measuring molecular diffusion in nanoporous zeolites under non-equilibrium states that relevant to industrial conditions. The project has a particular interest in understanding the diffusion behaviors for a number of small molecules, which are of unprecedented importance in the current global endeavor to produce H2 from fossil fuels and biomasses with simultaneous CO2 capture by catalytic reaction and molecular separation processes. The educational goal of this project is to train graduate and undergraduate students by involving them in the technical research and through knowledge exchange in national and international conferences. The project also makes special efforts to engage undergraduate students from underrepresented groups and introduce research findings into classroom teaching. The main outcomes of this project are briefly summarized below. 1. Technical Outcomes: 1.1 Developed a new fiber optic sensor based optical refractometry method for investigation of molecular diffusion in zeolites. The optical method of diffusion measurement is achieved by a zeolite-coated fiber optic interferometer (FOI). The FOI has the ability to perform measurement over broad ranges of temperature, pressure, and adsorbate concentrations that are relevant industrial processes. It has the potential to perform both microscopic and macroscopic measurement. 1.2 Fabricated MFI-type zeolite films on both straight cut endface FOI and side surface of long-period fiber gratings (LPFG) for sensing and monitoring the dynamic process of gas adsorption and determining transport diffusivity in broad ranges of operation conditions. The zeolite-coated LPFG (Z-LPFG) achieved high sensitivity for monitoring the shift of the resonant wavelength upon molecular adsorption in the coated zeolite. These offer the opportunity to conveniently determine the transport diffusivity as functions of material chemistry and temperature. 1.3 Synthesized MFI type zeolite membranes and investigated their high temperature transport behavior for gases including H2, CO2, CO, CH4, H2O, N2 and He etc., which are relevant to the development of clean energy systems involving conversion of coal and biomass to hydrogen with CO2 capture capability. Based on the understanding of pore size-dependent, size-exclusion (steric) effect enabled high selectivity of H2 over other gases, zeolite membranes have been modified and used for construction of water gas shift (WGS) membrane reactors that can operate at high temperatures (up to 600oC). The research has led to the first zeolite membrane reactor for high temperature WGS reaction which achieved significantly improved CO conversion, methanation inhibition, and reactor throughput. 1.4 Discovered proton-selective ion transport in nanoporous zeolites and explored the utilization of zeolite membranes as a new class of high-efficiency ion exchange membrane for redox flow batteries. These important findings have led to other NSF supported research efforts in developing novel ion exchange membranes for Redox Flow Batteries which are very promising for electrical energy storage in the renewable power systems (such as solar and wind power generation systems) and future smart grids. 2. Educational Outcomes 2.1 Graduate student Training: Four graduate students, including three Ph.D. students and one MS student, were trained through this project. Two Ph.D. students and one MS student have graduated with targeted degrees in Chemical Engineering. The two Ph.D. graduates are now research scientists in U.S. universities and the MS graduate is working for a major U.S. company. The third Ph.D. student is scheduled to graduate in Summer 2015. 3.1 Undergraduate student Training: Three undergraduate students at University of Cincinnati were trained in this project, including one female African American student from the Chemistry Department and two from the Chemical Engineering program. All three have graduated with honor. One of them is pursuing graduate degree in another university and the other two are employed by major U.S. companies. 3.2 Contribution to Classroom Teaching: (1) One chapter on zeolite membranes has been developed and incorporated into the dual level course "ChE 6059 - Inorganic Membranes" which has been regularly offered in Fall semester; (2) Two new experiment modules have been developed for the lab course "CHE-437: ChE Lab – V" at University of Cincinnati and were taught in 2010 and 2011.
