Research Initiative


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 Jeff Frey

Copyright © 2002

Active Areas of Research in the Group

Molecular Transport in Nanostructured Materials: a Hierarchical Approach to Design Nanostructured Materials

Project Motivation
Nano-porous carbon (NPC) structures have structural properties allowing effective application as gas separation membranes and catalytic membrane reactors. Specifically, high selectivity has been observed for separation of O2/N2 (30:1), He/N2 (178:1) and H2/N2 (333:1). The molecular mechanism of separation on NPC are not yet fully understood. For example, in the case of O2/N2, conventional wisdom has it that the separation is kinetic, based on small geometric differences between the molecules. However, recent analyses of permeation data on NPC's via Transition State Theory do not support this mechanism. Rather, it is postulated that the enthalpy of the transition state is responsible. Hence, models for activated transport in NPC's must focus on the use of accurate interaction potential functions, such as those afforded by ab initio methods.

Confinement of glassy, amorphous polymeric membranes into mesoporous media have been shown to increase the separation performance of the confined membranes. The molecular mechanisms of diffusion in glassy, amorphous polymers are not well understood either, especially under the special circumstances of confinement. Developing models for the prediction of separation performance of polymeric membranes are dependent on an understanding of the atomistic-scale processes occuring during separation.

Goals & Expected Results
"The goal of this project is to provide a predictive, coherent theoretical description of configurational diffusion from first principles. A novel, hierarchical approach will connect ab initio quantum mechanical calculations to mesoscopic diffusivities and thermodynamic solubilities. Specific applications to be considered include gas separation in NPC's and permeation through polymers confined in mesoporous silica. The results will have application in a wide range of technologies, but most specifically for the rational design of membranes used in separation processes"

Hierarchical Approach and Task Plan

Information Flow: Computational quantum mechanics will be used as a basis for new and more accurate potential functions, which in turn can be used to produce far more realistic Monte Carlo and molecular dynamics simulations than are presently possible. The analysis of the trajectories provides short time diffusivities and establishes the transition state geometries. Both results can be extended to macroscopic length scales and to longer times via both generalized hydrodynamics and transition state theory.

Broader Impact
Scientific investigation of diffusion using computer modeling will motivate new experimental investigations, i.e.
  • Experimentally identifying transition states
  • Studies using molecular probes of diffusion: pulsed-field gradient NMR, Neutron Spin Echo spectroscopy, forced Rayleigh scattering.
Theoretical understanding of diffusion will ultimately lead to the rational design of novel nanostructured materials. Current materials can also be modified to tailor them for specific chemical or shape selective applications membranes, with possible applications in
  • Gas separations (i.e. O2/N2)
  • Mercury removal from methane
  • Reverse Osmosis Processes
  • Reactive Separations
  • Fuel Cells
  • Biological Membranes

Current Areas of Active Research

Ab initio Methods
Electronic structure calculations on NPC structures, such as those shown in Figures [1] and [2], allow the development of interaction potential functions that will allow accurate molecular dynamics simulation of diffusion in these materials.

Diffusion Studies
Diffusion of gases in nanostructured materials are studied with atomistic detail using molecular dynamics simulation (see Figure [3]). The molecular mechanisms of diffusion are studied by analysis of the results obtained from these simulations. Transition state theory and generalized hydrodynamics are used to bridge the results from molecular simulation with experimental results.

The synthesis of novel carbon-based porous materials are carried out by pyrolysis of polyfurfural alcohol. Novel carbon-based nanostructures can also be synthesized by templating techniques (Figure [4]).

(Images are linked to larger, more detailed images.)
Figure 1: P-minimal surface (intersection of six nanotubes)
Figure 2: Random Schwartzite structure
Figure 3: Diffusion path of a tracer He atom in a rigid glassy, amorphous polypropylene model.
Figure 4: Model of an amorphous NPC synthesized from polyfurfural alcohol