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Nanotube materials have potential application to many crucial technologies, including gas storage, catalysis, drug delivery (controlled/targeted release), and gas separation (e.g. H₂ purification). This ACS-PRF funded research has successfully explored the application of hyperpolarized xenon-129 NMR to characterize fundamental molecular transport properties inside several types of crystalline nanotube materials.

During year-1 of the project, experiments were focused on the dipeptide nanotube system L-alanine-L-valine. A new method was developed for enhancing the cross-peak signals in continuous-flow hyperpolarized 2D NMR exchange spectroscopy.  During year-2, the focus turned to supramolecular materials, which can exhibit porosity over a wide range of length scales and pore space architectures. In materials exhibiting one-dimensional channels, it may be possible to exploit single-file diffusion to increase efficiency in molecular separations. Single file diffusion, which is drastically slower than normal diffusion, is expected when the channel diameter becomes comparable to the size of the guest species, while normal (i.e. Fickian) diffusion is expected for larger channels.

While unequivocal observation of single file diffusion has been reported in macroscopic channel-particle systems, molecular single-file diffusion has been far more elusive, and the literature has progressed not without controversy. Pulsed field gradient NMR is regarded as the gold-standard for studying molecular diffusion in micro/macro -porous materials. The Leipzig group of J. Karger reported single-file diffusion of  CH₄ in zeolite AlPO₄-5. However, because the crystallite of the sample were smaller than the RMS displacement under the experimental conditions, doubts have been expressed concerning the validity of this conclusion. To unambiguously observe single-file transport, defect-free channels with precisely monodisperse diameters are required. 

NMR using hyperpolarized xenon-129 offers well-known advantages for studying fundamental diffusion processes. Firstly, the xenon-129 chemical shift range (>300ppm) is sensitive to
size and shape of pores, and the chemical shift is sensitive to loading (Xe-Xe interactions). Hyperpolarization circumvents the necessity for long recycle delays due to slow T1 relaxation at high field. Experiments that would normally require days or even weeks of signal averaging can be completed in minutes. High quality data were obtained on sample quantities in the 20-50 mg range. In continuous-flow hyperpolarized xenon-129 NMR, the hyperpolarized gas is continuously recirculated between the optical pumping cell and the sample space. This technique has been demonstrated in a wide variety of porous media. NMR signal enhancements in excess of 20k are routinely obtained. The PI’s lab has contributed substantially to the development of spin-exchange optical pumping using high power laser diode arrays.

The results of this ACS-PRF funded project during the past year have demonstrated how continuous-flow hyperpolarized (CFHP) xenon-129 NMR can be used to probe diffusion and gas exchange kinetics in one-dimensional nanochannels formed in a new class of nanoporous material. The centerpiece of this work is a comparative study of molecular diffusion within the supramolecular nanochannels formed upon crystallization of the molecular wheels [Ga₁₀(OMe)₂₀(O₂CMe)₁₀] and [Ga₁₈(pd)₁₂(pdH)₁₂(O₂CMe)₆(NO₃)₆](NO₃)₆. These materials were synthesized in the laboratory of Professor George Christou in the Chemistry Department at the University of Florida.

In agreement with expectations based on the collision diameter of the Xe atom relative to the differing internal diameters of the two types of gallium wheels, CFHP Xe-129 NMR data confirm that single-file diffusion occurs in the Ga₁₀ channels, while in the Ga₁₈ system, the data are consistent with a transition to normal, Fickian diffusion. Information about the electronic environment inside the channels was also provided by the xenon-129 NMR chemical shift. Furthermore, the kinetics of transport of gas atoms between the nanochannel and the bulk gas phases was studied by CFHP xenon-129 2D exchange NMR spectroscopy. This ACS-PRF sponsored research has helped to establish CFHP xenon-129 NMR as a viable technique to study diffusion and exchange processes in nanoporous materials with one-dimensional channels. In addition, the suitability of crystals of molecular wheel compounds as a new class of porous nanotubular materials for studying the channel diameter dependence of molecular transport on the micron length scale was demonstrated.

A no-cost extension of this type AC grant was requested and approved by ACS-PRF in Summer 2010. Due to the mechanical breakdown of the GKM-07 rotary vane gas recirculation pump (Gardner Denver, Thomas Division), experiments were unable to continue after April 2010. The pump had to be shipped to Germany for repair, and was not returned until September 2010. A suitable substitute for this pump is not available. 

In the extension year of the grant (2010-2011), the plan is to perform hyperpolarized saturation-recovery experiments and pulsed field gradient NMR studies on new samples of Ga₁₀ and Ga₁₈  molecular wheel crystals synthesized using higher-purity starting materials. The short xenon-129 T₁ relaxation observed in the original samples (c.a. T₁<1s) indicates the presence of paramagnetic metal impurities. Synthesis of a new sample of Ga₁₀ was recently completed in the Christou lab. Furthermore, the synthesis of a modified Ga₁₈ molecular wheel compound with a cavity diameter on the order of 1.2-1.5nm is being considered. Such samples should facilitate a more decisive observation of the transition from normal to single-file behavior over longer time-scales.

Finally, it is a pleasure to report that the results obtained in this ACS-PRF funded research project provided key preliminary data for a successful grant submission to NSF. Proposal CHE-0957641, "Inducing molecular single file diffusion by co-adsorption in one-dimensional channels for gas separations and catalysis," by C.R. Bowers (PI) and S.A. Vasenkov (co-PI) was recommended for funding by the Division of Chemistry. The NSF project will build on the results of the ACS-PRF grant by extending the experiments to binary gas mixtures. Professor Vasenkov (UF Chemical Engineering) is a leading expert on diffusion and on the application of pulsed field gradient NMR to the study of diffusion in porous media such as zeolites. Thus, as a direct consequence of this ACS-PRF award, new external funding has been received by the PI.