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Title: Hyperpolarized 129Xe Nuclear Magnetic Resonance Studies of Isoreticular Metal-Organic Frameworks

Abstract

The pore environments of a series of isoreticular metal-organic frameworks (IRMOF) have been studied using hyperpolarized (HP) 129Xe nuclear magnetic resonance (NMR) spectroscopy. Xenon gas behaved as an efficient probe molecule for interrogating the variability of adsorption sites in functionalized IRMOF materials through variations in the NMR chemical shift of the adsorbed xenon. The xenon adsorption enthalpies extracted from variable temperature HP 129Xe NMR were found to be lower than published values for the physisorption of xenon. The low heats of adsorption were corroborated by xenon adsorption measurements that revealed two atoms per pore under one atmosphere of pressure at 19ºC. Average pore diameters estimated from the empirical chemical shift and pore size correlations based on a geometrical model were compared with x-ray crystallography data. The exchange processes of xenon in IRMOFs also were explored using 2D EXSY 129Xe NMR. It was found the exchange of xenon from adsorption sites within the IRMOF to the free gas space is much slower than that between the adsorption sites within the lattice. Cross-polarization experiments showed that the preferred adsorption sites were spatially removed from the phenylene rings of the network. This agrees with previous spectroscopic, structural and computational studies of gas adsorptionmore » (H2, N2, Ar) in IRMOFs that indicate the preferred binding sites reside near the carboxylate groups of the inorganic clusters.« less

Authors:
; ; ; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
912505
Report Number(s):
PNNL-SA-51427
Journal ID: ISSN 0022-3654; JPCHAX; KC0201050; TRN: US200801%%903
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Physical Chemistry, 11(16):6060 -6067; Journal Volume: 11; Journal Issue: 16
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ADSORPTION HEAT; CHEMICAL SHIFT; CRYSTALLOGRAPHY; NUCLEAR MAGNETIC RESONANCE; XENON 129; ORGANOMETALLIC COMPOUNDS; PORE STRUCTURE; MOLECULAR STRUCTURE

Citation Formats

Pawsey, Shane, Moudrakovski, I. L., Ripmeester, J. A., Wang, Li Q., Exarhos, Gregory J., Rowsell, Jesse L., and Yaghi, Omar M. Hyperpolarized 129Xe Nuclear Magnetic Resonance Studies of Isoreticular Metal-Organic Frameworks. United States: N. p., 2007. Web. doi:10.1021/jp0668246.
Pawsey, Shane, Moudrakovski, I. L., Ripmeester, J. A., Wang, Li Q., Exarhos, Gregory J., Rowsell, Jesse L., & Yaghi, Omar M. Hyperpolarized 129Xe Nuclear Magnetic Resonance Studies of Isoreticular Metal-Organic Frameworks. United States. doi:10.1021/jp0668246.
Pawsey, Shane, Moudrakovski, I. L., Ripmeester, J. A., Wang, Li Q., Exarhos, Gregory J., Rowsell, Jesse L., and Yaghi, Omar M. Tue . "Hyperpolarized 129Xe Nuclear Magnetic Resonance Studies of Isoreticular Metal-Organic Frameworks". United States. doi:10.1021/jp0668246.
@article{osti_912505,
title = {Hyperpolarized 129Xe Nuclear Magnetic Resonance Studies of Isoreticular Metal-Organic Frameworks},
author = {Pawsey, Shane and Moudrakovski, I. L. and Ripmeester, J. A. and Wang, Li Q. and Exarhos, Gregory J. and Rowsell, Jesse L. and Yaghi, Omar M.},
abstractNote = {The pore environments of a series of isoreticular metal-organic frameworks (IRMOF) have been studied using hyperpolarized (HP) 129Xe nuclear magnetic resonance (NMR) spectroscopy. Xenon gas behaved as an efficient probe molecule for interrogating the variability of adsorption sites in functionalized IRMOF materials through variations in the NMR chemical shift of the adsorbed xenon. The xenon adsorption enthalpies extracted from variable temperature HP 129Xe NMR were found to be lower than published values for the physisorption of xenon. The low heats of adsorption were corroborated by xenon adsorption measurements that revealed two atoms per pore under one atmosphere of pressure at 19ºC. Average pore diameters estimated from the empirical chemical shift and pore size correlations based on a geometrical model were compared with x-ray crystallography data. The exchange processes of xenon in IRMOFs also were explored using 2D EXSY 129Xe NMR. It was found the exchange of xenon from adsorption sites within the IRMOF to the free gas space is much slower than that between the adsorption sites within the lattice. Cross-polarization experiments showed that the preferred adsorption sites were spatially removed from the phenylene rings of the network. This agrees with previous spectroscopic, structural and computational studies of gas adsorption (H2, N2, Ar) in IRMOFs that indicate the preferred binding sites reside near the carboxylate groups of the inorganic clusters.},
doi = {10.1021/jp0668246},
journal = {Journal of Physical Chemistry, 11(16):6060 -6067},
number = 16,
volume = 11,
place = {United States},
year = {Tue Apr 03 00:00:00 EDT 2007},
month = {Tue Apr 03 00:00:00 EDT 2007}
}
  • Classical molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) simulations were used to generate self-diffusivities, adsorption isotherms and density distributions for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in five isoreticular metal-organic frameworks (IRMOFs), which varied in the cage size and in the presence and location of amine groups. These simulations were performed at room temperature (300 K) and low pressures (up to 1 ppm RDX). The atomic charges required for MD and GCMC simulations were calculated from quantum mechanical (QM) calculations using two different charge generation methods - Loewdin Population Analysis and Natural Bond Orbital Analysis. Both charge sets show that the presencemore » of amine groups increases the amount of RDX adsorbed. The cage size and the location of amine groups also affect the loading of RDX. The amount of RDX adsorbed is correlated with the energy of adsorption. The activation energy for diffusion of RDX is not positively correlated with the energy of adsorption. The density distributions identify the location of the adsorption sites of RDX-exclusively in the big cage around the metal complex vertices and between benzene rings. In the absence of amine groups on the framework, one of nitro groups on RDX interacts closely with the metal complex. In the IRMOFs functionalized with amine groups, a second nitro group of the RDX interacts with an amine group, enhancing adsorption. With regard to the application as a smart nanoporous preconcentrator, these IRMOFs are found to concentrate RDX up to 3000 times compared to the gas phase, on a volumetric basis. From a simple Langmuir estimation, the selectivity of RDX over butane is up to 5000. The diffusion of RDX is sufficiently high for real time sensor applications. These results indicate IRMOFs can be tailored with functional groups to be highly selective nanoporous preconcentrators.« less
  • Metal-organic frameworks (MOFs) are newly emerging porous materials. Owing to their large surface area and tunable pore size and geometry, they have been studied for applications in gas storage and separation, especially in hydrogen and methane storage and carbon dioxide capture. It has been well established that the high-pressure gravimetric hydrogen-adsorption capacity of an MOF is directly proportional to its surface area. However, MOFs of high surface areas tend to decompose upon activation. In our previous work, we described an approach toward stable MOFs with high surface areas by incorporating mesocavities with microwindows. To extend this work, we now presentmore » an isoreticular series of (3,24)-connected MOFs made from dendritic hexacarboxylate ligands, one of which has a Langmuir surface area as high as 6033 m 2 g -1. In addition, the gas-adsorption properties of this new isoreticular MOF series have been studied.« less