Article pubs.acs.org/cm
Energetics of Formation and Hydration of a Porous Metal Organic Nanotube Sulata K. Sahu,† Daniel K. Unruh,‡ Tori Z. Forbes,*,‡ and Alexandra Navrotsky† †
Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit (NEAT ORU), University of California, Davis, Davis, California 95616, United States ‡ Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States S Supporting Information *
ABSTRACT: Hybrid materials, such as metal organic nanotubes (MON), are of interest because of their chemical tunability and permanent porosity. While an increasing number of compounds is being reported, very little is known about their thermodynamic stability. Herein, the energetics of a MON, (C4H12N2)0.5[(UO2)(Hida)(H2ida)]·2H2O (UMON, C10H21N3UO12) (ida = iminodiacetate), that possesses unique water exchange and uptake has been investigated by acid solution calorimetry, thermal analysis, and water adsorption calorimetry. The enthalpy of formation of UMON, C10H21N3UO12 (ΔHf,rxn), from the dense components (uranium oxide (UO3), piperazine (C4H10N2), and iminodiacetic acid (C4H7NO4) was −55.3 ± 0.9 kJ/mol, which was similar to values for other metal organic framework materials. The dehydration enthalpy to form an anhydrous UMON and gaseous H2O at 37 °C from thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) experiments was 57.8 ± 1.9 kJ/mol of water. This value is somewhat higher than the vaporization enthalpy of water (44 kJ/mol) and suggests modest bonding interactions of H2O with the inner walls of the nanotubes. Water adsorption calorimetry of (C4H12N2)0.5[(UO2)(Hida)(H2ida)]·2H2O indicated that the water molecules are confined inside the UMON material in two thermally distinct positions. The ice-like arrangement of the confined water molecules inside the nanotube impacts the energetics of the material and adds to the stabilization of the structure.
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INTRODUCTION
While MOFs and related materials are the most developed group of hybrid materials, one-dimensional (1D) metal organic nanotubes (MONs) are also of interest.17−25 A majority of the 1D MONs reported are built by either inducing curvature of a hybrid two-dimensional (2D) sheet or terminating the 3D MOF lattice through “bent” linkers.20,25 Effective crystal engineering of 1D MON materials is difficult because of the need for weaker non-directional bonding to order the nanotubes into an ordered crystalline array.22 As a result of these difficulties, relatively few hybrid 1D MONs have been reported in comparison to the 3D MOF materials.22 We have recently reported a novel MON that is built using a non-traditional route, namely, “soft” supramolecular interactions.26 (C4H12N2)0.5[(UO2)(Hida)(H2ida)]·2H2O (UMON, C10H21N3UO12) (ida = iminodiacetate) contains hexavalent uranium that strongly bonds to two O atoms, forming a uranyl (UO22+) cation.26 The uranyl moiety is chelated in a tridentate fashion through the equatorial plane by one ida molecule. The metal center is then further coordinated by ida linkers to form anionic hexameric macrocycles that are charge-balanced by piperazinium cations (Figure 1a). Significant corrugation is
Hybrid materials are built from both inorganic and organic components and offer significant flexibility in the design of technological systems for industrial processes and medicine.1−6 In general, hybrid materials are composed of individual or clusters of metal centers that can bond through a large number of organic linkers to form extended solids.7 Interest in hybrid materials is in part due to the potential for the rational design of materials and permanent porosity, which could lead to enhanced gas storage, greater catalytic activity, and novel drug delivery strategies.1,4,7−11 Metal organic frameworks (MOFs) are the most extensively studied hybrid materials, and thousands of compounds have been previously reported.1,6 These materials are characterized by strong bonds between the metal centers and the organic linkers to create an extended three-dimensional (3D) network.7 Both the organic and inorganic components can be varied, providing unique structural topologies and tunable pore sizes and shapes.12,13 Because of the strength of bonding between the building units and the rigidity of the organic linker, permanent porosity can often be maintained upon removal of the solvent molecules, providing enhanced gas uptake and storage and higher diffusion rates for incorporation of guest molecules.8,14−16 © XXXX American Chemical Society
Received: July 2, 2014 Revised: August 6, 2014
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dx.doi.org/10.1021/cm5024053 | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
Figure 1. (a) (C4H12N2)0.5[(UO2)(Hida)(H2ida)]·2H2O (UMON, C10H21N3UO12) contains links through ida molecules to form a macrocylic array (yellow represents uranium; blue represents nitrogen; and black represents carbon). Oxygen atoms associated with the UMON structure have been removed for clarity, but the blue spheres represent nanoconfined water molecules. Piperazinium cations are also present in the structure for charge balance.26 (b) Water molecules (blue spheres) are present throughout the interior channels of the UMON compound in well-ordered arrangements that resemble the Ice-I structure.
Figure 2. Powder X-ray diffractogram of the UMON (C10H21N3UO12) sample compared against the pattern predicted from the structure solution to confirm purity of the bulk product.
temperature (37 °C) has been observed, with the ordered water array forming upon rehydration. In addition, the nanotubes are selective for water and do not adsorb common solvents, such as methanol, ethanol, dimethyl sulfoxide (DMSO), or hexane, even after removal of the water molecules from the pore space.26 Solvent selectivity and low-temperature rehydration have not been reported for other hybrid MON or MOF materials or other nano- and microporous materials, such as single-walled carbon nanotubes or zeolites. We are currently investigating the chemical and physical properties of the UMON material to provide a detailed understanding of the structural aspects that result in the unique water storage and transport properties. In the present work, the formation enthalpy (ΔHf,rxn) of UMON (C10H21N3UO12) at 25 °C from dense components has been measured by acid solution
observed within the macrocycle, leading to the formation of nanotubular arrays with an interior diameter of 1.2 nm. This design approach has been used for the self-assembly of strictly organic materials, including hydrazide-appended pillar arene derivatives and cyclic polypeptides,27,28 but has yet to be observed in a hybrid nanotubular systems. Beyond the unusual building blocks of the UMON material, unique water transport and storage properties have also been observed.26 Two crystallographically unique water molecules that are relatively well-ordered and resemble the Ice-I structure have been located within the nanotubes (Figure 1b). The material also displays permanent porosity upon dehydration, which is somewhat surprising given the relatively weak supramolecular forces that occur between the macrocycles and the large channel diameter. Reversible dehydration at a low B
dx.doi.org/10.1021/cm5024053 | Chem. Mater. XXXX, XXX, XXX−XXX
Chemistry of Materials
Article
the sample was degassed under vacuum (
