Nanochannels of Two Distinct Cross-Sections in a Porous Al-Based Coordination Polymer

Abstract

A new aluminum naphthalenedicarboxylate Al(OH)(1,4-NDC) x 2 H2O compound has been synthesized. The crystal structure exhibits a three-dimensional framework composed of infinite chains of corner-sharing octahedral Al(OH)2O4 with 1,4-naphthanedicarboxylate ligands forming two types of channels with squared-shape cross-section. The large channels present a cross-section of 7.7 x 7.7 A(2), while the small channels are about 3.0 x 3.0 A(2). When water molecules are removed, no structural transformation occurs, generating a robust structure with permanent porosity and remarkable thermal stability. 2D (1)H-(13)C heteronuclear correlation NMR measurements, together with the application of Lee-Goldburg homonuclear decoupling, were applied, for the first time, to porous coordination polymers revealing the spatial relationships between the (1)H and (13)C spin-active nuclei of the framework. To demonstrate the open pore structure and the easy accessibility of the nanochannels to the gas phase, highly sensitive hyperpolarized (HP) xenon NMR, under extreme xenon dilution, has been applied. Xenon can diffuse selectively into the large nanochannels, while the small ones show no substantial uptake of xenon due to severe restrictions along the channels that prevent the diffusion. Two-dimensional exchange experiments showed the exchange time to be as short as 15 ms. Through variable-temperature HP (129)Xe NMR experiments we were able to achieve an unprecedented description of the large nanochannel space and surface, a physisorption energy of 10 kJ mol(-1), and the chemical shift value of xenon probing the internal surfaces. The large pore channels are straight, parallel, and independent, allowing one-dimensional anisotropic diffusion of gases and vapors. Their walls are composed of the naphthalene moieties that create an unique environment for selective sorption. These results prompted us to measure the storage capacity toward methanol, acetone, benzene, and carbon dioxide. The selective adsorption of methanol and acetone vs that of water, together with the permanent porosity and high thermal stability, makes this compound a suitable matrix for separation and purification.

Polarized Micropores in a Novel 3D Metal–Organic Framework for Selective Adsorption Properties

“…7 Indeed a remarkable CO 2 uptake observed on aluminum naphthalenedicarboxylate MOF, Al(OH)(1,4-ndc)·2H 2 O, was also explained by favorable interactions of CO 2 with the π-electron clouds present near the polar OH groups protruding toward the host channels. 8 In our case, one can, for example, imagine that the polarized δ − charged oxygens induce an electric field in the channels. The polarizable CO 2 molecule that is approaching this negatively charged center is therefore also polarized as O δ− −C−O δ+ by these electron clouds.…”

129Xenon NMR: Review of recent insights into porous materials

“…Comotti et al [199] 129 Xe NMR experiments the authors were able to achieve a fine description of the large nanochannel space and surface area. From the fitting of the chemical shift data as a function of temperature, the enthalpy of adsorption of 10 kJ/mol has been evaluated.…”

Fibrous Architectures of Porous Coordination Polymers–Alumina Composites Fabricated by Coordination Replication

“…As shown in Figure S8, the Al(NO 3 ) 3 PMGI and calcined alumina nanofibers were prepared as a large-scale self-standing film. The obtained alumina nanofiber mat was then heated at 180°C for 1 min under microwave irradiation in a 50 mM aqueous solution of H 2 ndc or H 2 bdc to form the corresponding aluminum-based PCPs, [Al(OH)(bdc)] n 15 or [Al(OH)(ndc)] n , 16 respectively. As seen in Figure 3a, a macroscopic fiber mat was obtained and could be recognized as "PCP textile" on a millimeter scale.…”

“…This change in morphology can be illustrated by the molecular structure of [Al(OH)(L)] n ; a 1D inorganic chain structure of AlO 4 (OH) 2 is connected by dicarboxylate to form a three-dimensional framework. 15,16 The high pH condition (pH 5.4) increases the population of deprotonated dicarboxylate and promotes crystal growth in the direction of the dicarboxylate linkage, leading to a less anisotropic crystal morphology. On the other hand, needletype crystal morphology would be produced when there is a lack of deprotonated dicarboxylate ligands in the acidic condition (pH 3).…”