Georgia Institute of TechnologySchool of Chemical & Biomolecular Engineering

Nanomaterials for Energy Applications


Formation of hybrid polymer-membrane materials using nanoporous carbon molecular sieving dispersed phases

Faculty: William J. Koros
Students: (current) John Perry (former) De Vu

This research seeks to use nanoporous carbon molecular sieves as the dispersed phase in a glassy polymer continuous phase to create a novel class of membranes. Proper adhesion must be achieved between the two phases in the membrane to facilitate efficient separation functionality. Of particular importance in addressing this issue of adhesion is the interfacial region between the sieves and the polymer.

Control in this region must be maintained at and below the nanometer scale to prevent imperfections that allow gas to bypass the selective inserts in the polymer. Surface treatment techniques including physical and chemical modification of the sieves are being used to prevent the formation of these nanoscopic defects that hinder the separation performance of the membrane. Analytical techniques such as FTIR and XPS spectroscopy are used to analyze the sieves before and after the surface has been modified, and permeation measurements and SEM are used to test the membranes that have been created and attempt to observe the adhesion between the inserts and the polymer.

Selected Recent Papers/Patents/Theses/Reports:

  • De Q. Vu, W.J Koros, Stephen J. Miller, Mixed matrix membranes using carbon molecular
    sieves I. Preparation and experimental results. Journal of Membrane Science, 2003. 211: p.
    311-334.

  • De Q. Vu, W.J. Koros, Stephen J. Miller, Mixed matrix membranes using carbon molecular sieves II. Modeling permeation behavior. Journal of Membrane Science, 2003. 211: p. 335-348.

Formation of hybrid polymer-membrane materials using nanoporous inorganic molecular sieving dispersed phases within continuous matrix phases

Faculty: William J. Koros
Students: (current) Ted Moore, Alexis Hillock, Shu Shu (former) Rajiv Mahajan

Organic-inorganic hybrid materials for gas separations, or mixed matrix membranes, offer both attractive properties over conventional polymer membranes and increased processability over potential molecular sieving membranes. These mixed matrix membranes consist of a polymer phase and a molecular sieve phase, such as pyrolyzed carbon or zeolites. Although mixed matrix membranes cast with zeolites appear promising, this technology is currently limited by poor adhesion between the zeolites and the class of polymers most suited to gas separations. This research seeks methods to promote adhesion between the two phases within the mixed matrix membrane environment.

Important gas separations investigated as model systems in this work include air (oxygen/nitrogen) and natural gas (carbon dioxide/methane). For this work, an appropriate molecular sieve is dispersed in compatible polyimides. We have demonstrated that a coupling agent can aid in the creation of a viable mixed matrix membrane. This coupling is required to prevent debonding at the polymer-sieve interface caused by a build up of stresses during polymer formation, which in turn can cause nanometer-scale voids at the interface. We have also developed methods to reduce the stresses the form at the interface during formation, for example, by casting at an elevated temperature. The Figure on the left shows a visual image of

these stresses in a mixed matrix film with the aid of a birefringence microscope.

Another problem that has been encountered is that many molecular sieves are extremely hydrophilic, leading pore blockage by adsorbed water. This problem has been solved by switching to a more hydrophobic molecular sieve. We feel that once these problems have been overcome for this complex system, the approach is likely to be easily tailorable to other polyimide-sieve combinations for the separation of these and other gas pairs. Polymer membranes will plasticize in the presence of high pressure carbon dioxide, which significantly decreases a membrane's selectivity. To counter this effect, crosslinking of membranes is being investigated to increase stability in the presence of plasticizing agents, including CO2. This work combines the two technologies of crosslinking and mixed matrix membranes to create a membrane that is both highly selective and plasticization resistant in the presence of aggressive natural gas streams. The properties of mixed matrix films can be and are characterized by a variety of analytical techniques including: gas permeation, sorption, SEM, NMR, FTIR, TGA, and AFM. The Figure on the right shows a close-up view of sieve particles embedded in the polymer matrix of a mixed matrix membrane.

Selected Recent Papers/Patents/Theses/Reports:

  • Mahajan, R. and W.J. Koros, Factors Controlling Successful Formation of Mixed-Matrix Gas Separation Materials. Ind. Eng. Chem. Res., 2000. 39(8): p. 2692-2696.

  • Mahajan, R. and W.J. Koros, Mixed matrix membrane materials with glassy polymers. Part 2. Polymer Engineering and Science, 2002. 42(7): p. 1432-1441.

  • Moore, T.T., R. Mahajan, D.Q. Vu, and W.J. Koros, Hybrid Membrane Materials Comprising Organic Polymers with Rigid Dispersed Phases. AIChE Journal, 2004. 50(2): p. 311-21.

Formation of asymmetric hollow fiber polymer-membranes using nanoporous inorganic molecular sieving dispersed phases within non-crosslinked continuous matrix phases

Faculty: William J. Koros
Students: (current) Shabbir Husain

As new trends in membrane technology develop in the processing of hollow fiber membranes, emphasis is being placed on blending the high selectivities of alumino-silicate based zeolites with the processibility of polymers. However, to attain economic feasibility, these membranes must be capable of high productivity while also meeting product purity specifications. Hollow fiber mixed-matrix membranes provide an excellent route to achieve both these objectives.

This research will pursue the formation of practical mixed matrix asymmetric hollow fiber membranes based on a defect-free membrane spinning process. The use of commercially available polyimide, Matrimid® and polyetherimide, Ultem® 1000 with and without silane coupling agents present on the dispersed sieve phase will be investigated. The nano-scale interfacial region between each sieve particle and the bulk is believed to be particularly important in determining performance. The asymmetric membrane will be characterized using dry O2/N2 and CO2/CH4 feeds as model systems. Optimization of dope formulations (with and without zeolites) for macroscopic properties will be done using syringed solid fibers using SEM.

Transport-model-independent experimental description of transport in nanoporous membranes by optical and thermo-optical spectroscopy

Faculty: Sankar Nair
Students: (current) Weontae Oh [postdoctoral fellow]

Nanoporous molecular sieving membranes have potential to revolutionize several industrially important separations. Considerable research has hence been aimed at relating the atomic-scale and micro-scale structure of these membranes to their transport properties. The knowledge of membrane composition (including guest molecules) and transport properties as a function of depth is of prime interest. Quantitative probes of cross-sectional structure (electron microscopy, energy-dispersive X-ray analysis, and confocal microscopy) are destructive and/or ex situ.

We are developing non-destructive, in situ methods based on infrared reflectance and photoacoustic spectroscopy to probe the depth dependence of membrane composition. In recent work (see Figure), we have applied step scan photoacoustic spectroscopy (SSPAS) to depth-profile MFI zeolite membranes, made by seeded growth on porous alumina substrates in the presence of a tetrapropylammonium (TPA) structure directing agent. Our results indicate that SSPAS, when applied to membranes under a concentration gradient, can allow measurement of the concentration profile and the thickness. An important application is the transport-model-independent characterization of membrane transport, by simultaneous measurement of the concentration profile, membrane thickness and membrane flux.

Selected Recent Papers/Patents/Theses/Reports:

  • W. Oh and S. Nair, Concentration Profiling of a Molecular Sieve Membrane by Step Scan Photoacoustic Spectroscopy (Letter), Journal of Physical Chemistry B (2004, in press).

Nanoporous materials and membranes for sorption heat pumping and refrigeration applications

Faculty: Sankar Nair (ChBE), Samuel Graham (ME)
Students: (current) Yeny Hudiono

Sorption cycle-based heat pumping and refrigeration systems based on hydrophilic nanoporous adsorbents are a potentially attractive alternative to vapor compression cycle systems. Sorption based systems can target a large number of heat sources (e.g. solar energy, ground/lakes/sea, spent industrial/utility streams). They are noiseless with
few moving parts, use direct heat input instead of electricity, and employ water vapor as the working fluid thus requiring no toxic refrigerants. Making these systems commercially attractive requires progress on several fronts: (1) analysis of the heat pump performance in terms of underlying physical parameters (thermal conductivity, diffusivity, adsorption strength, system configuration as packed bed or membrane module); (2) reliable characterization of heat and mass transport properties; (3) a fundamental understanding of coupled mass and heat transport in nanoporous materials; (4) generation and evaluation of new and existing candidate sorbent materials.

We are addressing these issues by means of (a) continuum coupled mass and heat transport simulations of a sorption heat pump to identify desirable ranges of material properties, (b) measurements of the thermal conductivity behavior of candidate materials deposited as thin films, and (c) theoretical modeling and molecular simulation of coupled nanoscale thermal transport and mass transport. For example, the Figure shows the measured temperature dependence of the thermal conductivity of a nanoporous zeolite membrane by a recently developed three-omega method.

Continuous hydrothermal synthesis of inorganic nanoparticles (including battery electrode materials)

Faculty: Amyn Teja
Students and Postdocs: Chunbao Xu and Dr. Jaewon Lee
Past Students: Yalin Hao and Linda Cote
Collaborations with: John Zhang (Chem) and Angus Wilkinson (Chem)

Fine magnetic particles have found use in many applications such as recording media and ferrofluids. New uses of these particles have also been proposed in high-density information storage and in a number of biomedical applications. These new applications require excellent control of properties such as particle size and size-distribution, and variables that affect these properties are the focus of an increasing number of studies.

We are investigating the formation of magnetic oxide nanoparticles in a continuous hydrothermal reactor that allows us to essentially separate the effects of nucleation, growth, and agglomeration. Factors that affect the size, size-distribution, and morphology of nanoparticles can then be isolated. We are particularly interested in using near-critical and supercritical water in the continuous hydrothermal environment because of the tunability of their properties by small changes in temperature and pressure. We have produced nanoparticles of iron oxide (a-Fe2O3), cobalt oxide (Co3O4), cobalt iron oxide (CoFe2O4), and lithium iron phosphate (LiFePO4) in our apparatus. LiFePO4 is of particular interest as a cathode material for lithium-ion rechargeable batteries.

Selected Papers/Patents/Theses/Reports:

  • Lee J, Teja A. S. "Characteristics of lithium iron phosphate (LiFePO4) particles synthesized in compressed and supercritical water", J. Supercritical Fluids, submitted (2004)

  • Cote L. J., Teja A. S., Wilkinson A. P., Zhang Z. J., "Continuous hydrothermal synthesis and crystallization of CoFe2O4 nanoparticles," Fluid Phase Equil. 210, 307-317 (2003).

  • Hao Y., and Teja A. S., "Continuous hydrothermal crystallization of Fe2O3 and Co3O4 nanoparticles," J. Mat. Res. 18, 415-422 (2003).

  • Teja A. S. and Holm L. J., " Production of magnetic oxide nanoparticles" Chapter 9 in Supercritical Fluid Technology in Materials Science and Engineering: Synthesis, Properties, and Applications, Sun Y-P (Editor), Elsevier, pp. 327-349, (2002).

  • Cote L. J., Teja A. S., Wilkinson A. P., Zhang Z. J., "Continuous hydrothermal synthesis and crystallization of magnetic oxide nanoparticles," J. Mat. Res. 17, 2410-2416 (2002).

  • Teja A. S., and Furuya T., "Supercritical Fluid Crystallization" in Encyclopedia of Separation Science, Academic Press, London, vol. III, pp. 4301-4307, (2000).

  • Furuta S., Rousseau R. W., and Teja A. S., "Production of fine particles by rapid expansion of amino acid solutions at high temperatures and pressures," Proc. AIChE Symp. Separations, 1, 702-706, (1992).

  • Linda J. Holm, PhD 2001, Thesis Title: "Continuous hydrothermal technique for the synthesis and crystallization of magnetic oxide nanoparticles" (Dr. Cote (Holm) is currently with ExxonMobil, Houston, TX).

  • Yalin Hao, M. S. 2002, Thesis Title: "Continuous hydrothermal production of iron oxide and cobalt oxide nanoparticles" (Ms. Hao is currently at UC Davis)