Double−Step Gas Sorption of a Two−Dimensional Metal−Organic Framework

Single-crystal to Single-crystal Transformations - Guest Removal and Substitution in a Robust Solvent-templated Metallocyclic Compound

The formation, structure, and single-crystal to single-crystal transformations of the dinuclear metallocycle [Ag2L2](PF6)2·2CH3CN (L = 2,3,5,6-tetrafluoro-1,4-bis(2-methylimidazole-1-yl-methyl)benzene) are described. The acetonitrile guest molecules can be removed without loss of single crystallinity to afford a porous material. CO2 sorption isotherms were recorded at various temperatures and the structures and sorption behaviour of the crystals are compared with those of a previously reported analogous system. Upon exposure to acetone vapour, the desolvated crystals undergo a single-crystal to single-crystal transformation to form the acetone solvate.

Free energy analysis for adsorption-induced lattice transition of flexible coordination framework

We conduct grand canonical Monte Carlo simulations and free energy analysis for a gate adsorption phenomenon, which is experimentally observed in flexible frameworks of porous coordination polymers. Our calculations demonstrate that the stabilization provided by the guest adsorption drives the structural transition, surmounting the energy cost in creating the adsorption space due to the movement of the host framework. Furthermore, the existence of an energy barrier between two local minima in the free-energy landscape is found to result in hysteretic adsorption.

Separation of CO2 from CH4 using mixed-ligand metal-organic frameworks

The adsorption of CO2 and CH4 in a mixed-ligand metal-organic framework (MOF) Zn 2(NDC) 2(DPNI) [NDC = 2,6-naphthalenedicarboxylate, DPNI = N, N'-di-(4-pyridyl)-1,4,5,8-naphthalene tetracarboxydiimide] was investigated using volumetric adsorption measurements and grand canonical Monte Carlo (GCMC) simulations. The MOF was synthesized by two routes: first at 80 degrees C for two days with conventional heating, and second at 120 degrees C for 1 h using microwave heating. The two as-synthesized samples exhibit very similar powder X-ray diffraction patterns, but the evacuated samples show differences in nitrogen uptake. From the single-component CO2 and CH4 isotherms, mixture adsorption was predicted using the ideal adsorbed solution theory (IAST). The microwave sample shows a selectivity of approximately 30 for CO2 over CH4, which is among the highest selectivities reported for this separation. The applicability of IAST to this system was demonstrated by performing GCMC simulations for both single-component and mixture adsorption.

Broadly hysteretic H2 adsorption in the microporous metal-organic framework Co(1,4-benzenedipyrazolate)

Reaction of Co(CF3SO3)2 with the new molecule 1,4-benzenedi(4'-pyrazolyl) (H2BDP) in N,N'-diethylformamide (DEF) at 130 degrees C generates the metal-organic framework Co(BDP).2DEF.H2O (1). X-ray analysis reveals the structure of 1 to contain chains of tetrahedrally ligated Co2+ ions linked through BDP2- ligands to generate a three-dimensional framework with 10 x 10 A2 channels. Thermogravimetric data shows the framework to have a high thermal stability, and complete desolvation occurs upon heating at 170 degrees C under dynamic vacuum for two days to afford 1d. X-ray powder diffraction data indicates that 1d possesses a substantially different structure, but converts back to 1 upon exposure to DEF, consistent with the presence of a flexible framework. Nitrogen adsorption isotherms measured for 1d at 77 and 87 K reveal an unprecedented five-step adsorption process and a Langmuir surface area of 2670 m2/g. In addition, high-pressure H2 adsorption data reveal hysteretic uptake and release, with hysteresis loops of width 1.1, 3.8, 13, and 27 bar that shift to higher pressures as the temperature increases from 50 to 65, 77, and 87 K, respectively. The high H2 uptake capacity of 5.5 excess wt % at 50 K suggests that such materials could potentially find utility for hydrogen storage via a kinetic trapping mechanism. Variable-temperature kinetics measurements have also allowed the first study of H2 diffusion within a metal-organic framework, revealing an energy barrier of 0.62 kJ/mol for H2 diffusing within the pores.

Challenges and breakthroughs in recent research on self-assembly

The controlled fabrication of nanometer-scale objects is without doubt one of the central issues in current science and technology. However, existing fabrication techniques suffer from several disadvantages including size-restrictions and a general paucity of applicable materials. Because of this, the development of alternative approaches based on supramolecular self-assembly processes is anticipated as a breakthrough methodology. This review article aims to comprehensively summarize the salient aspects of self-assembly through the introduction of the recent challenges and breakthroughs in three categories: (i) types of self-assembly in bulk media; (ii) types of components for self-assembly in bulk media; and (iii) self-assembly at interfaces.