Search

Transport diffusivity of molecules in a porous solid is constricted by the rate at which molecules move from one pore to the other, along the concentration gradient, i.e. by following Fickian diffusion. In heterogeneous porous materials, i.e. in the presence of pores of different sizes and chemical environments, diffusion rate and directionality remain tricky to estimate and adjust. In such a porous system, we have realized that molecular diffusion direction can be orthogonal to the concentration gradient. To experimentally determine this complex diffusion rate dependency and get insight of the microscopic diffusion pathway, we have designed a model nanoporous structure, metal-organic framework (MOF). In this model two chemically and geometrically distinct pore windows are spatially oriented by an epitaxial, layer-by-layer growth method. The specific design of the nanoporous channels and quantitative mass uptake rate measurements have indicated that the mass uptake is governed by the interpore diffusion along the direction orthogonal to the concentration gradient. This revelation allows chemically carving the nanopores, and accelerating the interpore diffusion and kinetic diffusion selectivity. In heterogeneous porous systems diffusion rate is controlled by pore window sizes and concentration gradient. Here authors show that in a porous metal-organic framework system, the diffusion direction can be orthogonal to the concentration gradient. [ABSTRACT FROM AUTHOR]

Metal–organic frameworks (MOFs, also known as porous coordination polymers or PCPs) are a novel class of crystalline porous material. The tailorable porous structure, in terms of size, geometry and function, has attracted the attention of researchers across all disciplines of materials science. One of the many exciting aspects of MOFs is that through directional and reversible coordination bonding, organic linkers (chromophores with metal-coordinating functional groups) and metal ions (and clusters) can be spatially organized in a preconceived geometry. The well-defined spatial geometry of the metals and linkers is very advantageous for optoelectronic functions (solar cells, light-emitting diodes, photocatalysts) of the materials. This feature article evaluates the scope of charge transfer (CT) interactions in MOFs, involving the organic linkers and metal ion or cluster components. Irrespective of the type (size, shape, electronic property) of organic chromophores involved, MOFs provide an insightful path to design and make the CT process efficient. The selected examples of MOFs with CT characteristics do not only illustrate the design principles but render a pathway towards understanding the complex photophysical processes and implementing those for future optoelectronic and catalytic applications. [ABSTRACT FROM AUTHOR]

Adsorptive chemical separation is at the forefront of future technologies, for use in chemical and petrochemical industries. In this process, a porous adsorbent selectively allows a single component from a mixture of three or more chemical components to be adsorbed or permeate. To separate the unsorted chemicals, a different adsorbent is needed. A unique adsorbent which can recognize and separate each of the chemicals from a mixture of three or more components is the necessity for the next generation porous materials. In this regard, we demonstrate a "dynamic chemical clip" in a supramolecular framework capable of thermodynamic and kinetics‐based chemical separation. The dynamic space, featuring a strong preference for aromatic guests through π‐π and C‐H⋅⋅⋅π interactions and adaptability, can recognize the individual chemical isomers from mixtures and separate those based on thermodynamic and kinetic factors. The liquid‐phase selectivity and separation of the aromatic isomers are possible by the adaptability of the "chemical clip" and here we elucidate the prime factors in a combinatorial approach involving crystallographic evidence and detailed computational studies. [ABSTRACT FROM AUTHOR]

Storage and separation of small (C1-C3) hydrocarbons are of great significance as these are alternative energy resources and also can be used as raw materials for many industrially important materials. Selective capture of greenhouse gas, CO2 from CH4 is important to improve the quality of natural gas. Among the available porous materials, MOFs with permanent porosity are the most suitable to serve these purposes. Herein, a two-fold entangled dynamic framework {[Zn2(bdc)2(bpNDI)]⋅4DMF} n with pore surface carved with polar functional groups and aromatic π clouds is exploited for selective capture of CO2, C2, and C3 hydrocarbons at ambient condition. The framework shows stepwise CO2 and C2H2 uptake at 195 K but type I profiles are observed at 298 K. The IAST selectivity of CO2 over CH4 is the highest (598 at 298 K) among the MOFs without open metal sites reported till date. It also shows high selectivity for C2H2, C2H4, C2H6, and C3H8 over CH4 at 298 K. DFT calculations reveal that aromatic π surface and the polar imide (RNC=O) functional groups are the primary adsorption sites for adsorption. Furthermore, breakthrough column experiments showed CO2/CH4 C2H6/CH4 and CO2/N2 separation capability at ambient condition. [ABSTRACT FROM AUTHOR]

Interpenetrating metal organic frameworks are interesting functional materials exhibiting exceptional framework properties. Uptake or exclusion of guest molecules can induce sliding in the framework making it porous or non-porous. To understand this dynamic nature and how framework interaction changes during sliding, metal organic framework (MOF) 508 {Zn(BDC)( 4,4′-Bipy)0.5 · DMF(H2O)0.5} was selected for study. We have investigated structural transformation in MOF-508 under variable conditions of temperature, pressure and gas loading using Raman spectroscopy and substantiated it with IR studies and density functional theory (DFT) calculations. Conformational changes in the organic linkers leading to the sliding of the framework result in changes in Raman spectra. These changes in the organic linkers are measured as a function of high pressure and low temperature, suggesting that the dynamism in MOF-508 framework is driven by ligand conformation change and inter-linker interactions. The presence of Raman signatures of adsorbed CO2 and its librational mode at 149 cm−1 suggests cooperative adsorption of CO2 in the MOF-508 framework, which is also confirmed from DFT calculations that give a binding energy of 34 kJ/mol. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Interpenetration is a natural phenomenon frequently encountered in porous coordination polymers (PCPs) or metal-organic frameworks (MOFs). Traditionally interpenetration has been considered as a threat to permanent porosity and several strategies have been adopted to control the framework interpenetration. Recent literature reports have unveiled that interpenetration has paramount importance in several material properties particularly in storage and separation of small gas molecules. Such frameworks also show interesting structural flexibility based on shearing or movement of the nets and also reveals guest induced dynamic structural transformation for modulated specific functions. In this review, we will emphasize several interpenetration phenomena observed in coordination polymers, their intriguing structural aspects and fascinating material properties. [ABSTRACT FROM AUTHOR]

Flexible and dynamic porous coordination polymers (PCPs) with well-defined nanospaces composed of chromophoric organic linkers provide a scaffold for encapsulation of versatile guest molecules through noncovalent interactions. PCPs thus provide a potential platform for molecular recognition. Herein, we report a flexible 3D supramolecular framework {[Zn(ndc)( o-phen)]⋅DMF} n ( o-phen=1,10-phenanthroline, ndc=2,6-napthalenedicarboxylate) with confined nanospaces that can accommodate different electron-donating aromatic amine guests with selective turn-on emission signaling. This system serves as a molecular recognition platform through an emission-readout process. Such unprecedented tunable emission with different amines is attributed to its emissive charge-transfer (CT) complexation with o-phen linkers. In certain cases this CT emission is further amplified by energy transfer from the chromophoric linker unit ndc, as evidenced by single-crystal X-ray structural characterization. [ABSTRACT FROM AUTHOR]

Abstract: Porous polyimides are important class of macromolecules owing to their excellent redox behaviour, efficient capture of CO2 and H2 gases, interesting photocatalytic properties and superior thermal and chemical stabilities. Here we describe in detail, the synthesis and gas storage properties of a series of porous polyimides (Tr-NPI, Tr-PPI, Tr-CPI, Td-PPI and Td-CPI) with various network topologies derived from polycyclic aromatic hydrocarbon linkers. These polyimides are synthesized in a single step by the condensation of corresponding polycyclic aromatic dianhydrides (NDA, PDA and CDA) with structure directing amine (TAPA and TAPM) monomers, having trigonal and tetrahedral geometry. The structure of all the polymers was fully characterized by various techniques. The present work also introduces for the first time porous polyimides containing rigid polycyclic aromatic compounds such as coronene. All the polyimides presented here exhibit high thermal stability and show selectivity towards CO2 uptake at room temperature (293 K), due to the presence of aromatic clouds and CO2 phillic oxygen and nitrogen functionalities on their pore surface. Moreover these polymers also showed significant uptake of H2 gas (77 K). The present work has significant implications on the design of robust porous organic solids from small molecules for efficient capture of CO2 and H2 gases. [Copyright &y& Elsevier]

A two-fold interpenetrated microporous bi-pillared-layer framework of Fe(II), {[Fe(2,6-napdc)(4,4-bipy)](EtOH)(HO)} (1) (2,6-napdc = 2,6-naphthalenedicarboxylate; 4,4-bipy = 4,4-bipyridine) composed of mixed ligand system has been synthesized and structurally characterized. The 2,6-napdc linkers form a 2D corrugated sheet of {Fe(2,6-napdc)} by linking the secondary building unit of Fe(CO) in the ac plane, which are further connected by double 4,4-bipy pillars resulting in a bi-pillared-layer type 3D framework. The 3D framework is two-fold interpenetrated and exhibits a 3D channel structure (4.0 × 3.5, 1.5 × 0.5 and 2.2 × 2.1Å) occupied by the guest water and ethanol molecules. Framework 1 shows high thermal stability, and the desolvated framework (1 $^{\boldsymbol\prime}$ ) renders permanent porosity realized by N adsorption profile at 77K (BET surface area of ~52 m g). Moreover, the framework 1 $^{\boldsymbol\prime}$ also uptakes different solvent vapours (water, methanol and ethanol) and their type-I profile suggest strong interaction with pore surfaces and overall hydrophilic nature of the framework. Temperature dependent magnetic measurements suggest overall antiferromagnetic behaviour in compound 1. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Metal mediated in situC–N coupling between 4,4′-azobipyridine and disodium-trans-glutaconate at room temperature has formed a new multifunctional linker Z-dhpewhich subsequently self-assembles with Zn(ii) or Cd(ii) resulting in a chiral or an achiral metal–organic framework, respectively, depending on its different coordination modes. [ABSTRACT FROM AUTHOR]