In this project supported by the Chemical Structure, Dynamics and Mechanisms Program of the Division of Chemistry, Professors Clifford R. Bowers and Sergey Vasenkov of the University of Florida will employ advanced nuclear magnetic resonance (NMR) techniques to characterize the transport of gases in nanoporous materials. Pulsed field gradient NMR (PFG-NMR) and xenon-129 hyperpolarization techniques will be applied to materials with one-dimensional nanochannels composed of dipeptides, gallium molecular wheels, and aluminum-germanium and aluminum-silicon. The primary goals of the project are (1) to realize the conditions under which a transition between normal and single-file diffusion can be induced by one-component in a multi-component gas mixture inside one-dimensional nanochannels, and (2) to understand the relationship between diffusion and exchange of gases between the bulk and channel-adsorbed phases in such systems.
The results of the proposed research may have impacts on the development of new strategies for highly selective separations of mixtures of gases and catalysis. These are two of the most vital industrial processes of our day, with applications to renewable/clean energy. The educational plan integrated into this research project will emphasize the mentoring of undergraduate women and underrepresented minority students, and involvement of high school students in coordination with the University of Florida?s Center for Precollegiate Education and Training.
This project concerns diffusion of small molecules confined to one-dimensional channels having diameters below 1 nm. When molecules are large enough to fit inside the channels but too large to pass one-another, molecules move in a single-file – just like pearls on a string. Theory predicts single-file diffusion (SFD) to be drastically slower in comparison to normal diffusion (ND), where molecules can pass. SFD depends on (1) time (2) density of molecules in the channel (3) molecule-molecule forces (4) molecule-channel forces (5) channel defects such as partial or complete blockages. Understanding molecular diffusion in real single-file materials is a crucial step toward its potential exploitation in energy and environmentally related processes, such as gas purification/sequestration. Unfortunately, the time-scales of interest are not accessible to simulations on even the fastest supercomputers. This project succeeded in (1) identifying specific molecule-nanochannel materials with sufficiently ideal properties for fundamental study (2) development of measurement techniques to bridge the gap between theory and experiment and (3) exploration of the possibility of inducing SFD by co-adsorption of small and large molecules where only the latter fulfills the single-file criterion. Project outcomes are divided according to intellectual merits and broader impacts. (a) Intellectual Merits. Advanced nuclear magnetic resonance (NMR) methods were applied to the study of molecular diffusion in various one-dimensional nanochannel materials. NMR involves absorption and re-emission of radiowaves by nuclear spins in a magnetic field. The magnetic field dependence of the radiofrequency emission was used to track translational displacements of nuclei situated on the diffusing molecules. In pulsed field gradient (PFG) NMR, a spatial gradient in the magnetic field is applied along the channel axis. Translation is registered as a change in the radiowave emission frequency. In hyperpolarized spin tracer exchange (HTSE), nuclear spins are pre-polarized in a circularly polarized laser beam, thereby increasing their visibility 10,000-fold. The recorded uptake of pre-polarized Xe atoms by the nanochannels occurs by diffusion. Analysis of the uptake by fitting to our mathematical models affords an assessment of the diffusion time-scaling. Several types of channels were studied, including aluminosilicate nanotubes, self-assembled dipeptide nanotubes, and self-assembled bis-urea macrocycle nanotubes. Results in the dipeptide materials provided the first definitive experimental evidence for molecular single-file diffusion. Furthermore, by combining PFG and HTSE NMR data, the average channel length in a macroscopic ensemble of channels was determined. Comparison with scanning electron microscope images validated the theoretical model for single-file diffusion. (b) Broader Impacts. Education and Training of Students in Diffusion Theory and NMR Techniques. Graduate and undergraduate students received education in NMR theory in a course taught by the PI. The course was enhanced by the synergy with the NSF project. Students completed term papers on topics related to the project. Professional mentoring was provided to a postdoc supported by the grant. A total of four graduate and two undergraduate students participated and are co-authors on one or more of the 7 peer-reviewed publications and 14 conference presentations. Outreach to students in traditionally underrepresented groups was provided through participation in the STEPUP program of the UF College of Engineering, which was attended by ~40 minority freshman engineering students during a six-week summer residential program. Outreach to high school students was coordinated through the Junior Science Engineering and Humanities Symposium in 2011, 2012 and 2013. Impact on Research Infrastructure. A portion of project funds were used to upgrade the local research infrastructure. Instrumentation originally purchased with NSF/NHMFL funds for research and educational purposes was upgraded. The instrumentation will continue to provide training opportunities to UF and NSF-REU students. The purchase of a xenon-129 PFG-NMR insert extended the capability of the NHMFL-AMRIS 750 MHz NMR spectrometer. Potential Long Term Impacts. Areas of potential long-term impact of the fundamental research supported by this project include molecular separations for purification, catalysis and gas sequestration. Results of the project collectively represent a significant advancement towards practical applications of molecular SFD. Industrial Collaboration. Laser diode array (LDA) systems are suitable for high capacity production of Xe-129 and He-3 by spin exchange optical pumping (SEOP). Important applications include (1) lung imaging, where most of the volume of the porous tissue structure consists of void space, and (2) hyperpolarized xenon-129 biosensors for disease detection. These innovations provide the impetus to improve the efficiency of SEOP to achieve higher polarization and capacity. OptiGrate Corporation (Oveido, Florida) has recently commercialized a spectrally narrowed LDA incorporating a holographic reflecting volume Bragg grating. SEOP performance tests were performed in the PI’s lab using a 30W Optigrate laser. A ~1.5 to ~3.5 fold increase in the SEOP polarization level was achieved in comparison to the non-narrowed LDA. Increased SEOP performance obtained with Optigrate’s laser will improve MRI quality in lung and brain and will increase Xe biosensor sensitivity. These results are likely to lead to new markets for Optigrate, a Florida-based laser-optics firm.
