Proton and water activity-controlled structure formation in zinc carboxylate-based metal organic frameworks

The contributions of terephthalic acid and Zn(2+)-coordinated water in N,N-diethylformamide (DEF) to the overall proton activity in the synthesis of MOF-5 (Zn4O(BDC)3, BDC = 1,4-benzenedicarboxylate) were quantitatively determined by combined electrochemical and UV-vis spectroscopic measurements. Structural transformations of zinc carboxylate-based metal organic frameworks due to their exposure to environments with variable water concentrations and the chemical means necessary to revert these transitions have been investigated. It was found that the water-induced structural transition of MOF-5 to the hydroxide structure Zn3(OH)2(BDC)2 x 2 DEF (MOF-69c) can be reverted by a thermal treatment of the obtained compound and its subsequent exposure to anhydrous DEF. MOF-5 syntheses from simple carboxylates as well as a water-free synthesis based on nitrate decomposition are presented. The latter demonstrates that nitrate can serve as the sole source for the oxide ion in MOF-5.     

Fragmentation reaction via an ipso-attack in the multistep synthesis of regioselectively functionalized Calix

The acid-catalyzed convergent synthesis of a calix[8]arene containing a regioselectively functionalized upper rim has been investigated through the isolation and identification of cyclic and acyclic byproducts. Theoretical calculations demonstrated that the synthetic process involves two types of reaction mechanisms, one of which leads to the favorably constructed framework, while the other causes undesirable fragmentation via ipso-substitution. A possible rationale is proposed to explain the overall reaction pathways which derive the calix[8]arene along with the byproducts.     

Synthesis of Chiral MOF-74 Frameworks by Post-Synthetic Modification by Using an Amino Acid

The synthesis of chiral metal–organic frameworks (MOFs) is highly relevant for asymmetric heterogenous catalysis, yet very challenging. Chiral MOFs with MOF-74 topology were synthesised by using post-synthetic modification with proline. Vibrational circular dichroism studies demonstrate that proline is the source of chirality. The solvents used in the synthesis play a key role in tuning the loading of proline and its interaction with the MOF-74 framework. In N,N′-dimethylformamide, proline coordinates monodentate to the Zn²⁺ ions within the MOF-74 framework, whereas it is only weakly bound to the framework when using methanol as solvent. Introducing chirality within the MOF-74 framework also leads to the formation of defects, with both the organic linker and metal ions missing from the framework. The formation of defects combined with the coordination of DMF and proline within the framework leads to a pore blocking effect. This is confirmed by adsorption studies and testing of the chiral MOFs in the asymmetric aldol reaction between acetone and para-nitrobenzaldehyde.     

Characterization of interfacial water in MOF-5 (Zn4(O)(BDC)3)--a combined spectroscopic and theoretical study

In this article we report the detection and characterization of adsorbed interfacial water within the cages of the metal-organic framework MOF-5 (Zn(4)O(BDC)(3)) by terahertz time-domain spectroscopy (THz TDS) in the frequency range from 5 to 46 cm(-1). The experimental spectra suggest a coupling of the intermolecular motions of the water molecules adsorbed to the collective vibrations of the network at 4 wt% hydration. This finding is supported by the results of MD simulations. When increasing the water content to 8 wt% we observed a non reversible decomposition of MOF-5.     

Assessing the purity of metal-organic frameworks using photoluminescence: MOF-5, ZnO quantum dots, and framework decomposition

Photoluminescence (PL) spectroscopy was used to characterize nanoscale ZnO impurities, amine-donor charge-transfer exciplexes, and framework decomposition in samples of MOF-5 prepared by various methods. The combined results cast doubt on previous reports describing MOF-5 as a semiconductor and demonstrate that PL as a tool for characterizing MOF purity possesses advantages such as simplicity, speed, and sensitivity over currently employed powder XRD MOF characterization methods.     

Highly dispersed palladium(II) in a defective metal-organic framework: application to C-H activation and functionalization

High reversibility during crystallization leads to relatively defect-free crystals through repair of nonperiodic inclusions, including those derived from impurities. Microporous coordination polymers (MCPs) can achieve a high level of crystallinity through a related mechanism whereby coordination defects are repaired, leading to single crystals. In this work, we discovered and exploited the fact that this process is far from perfect for MCPs and that a minority ligand that is coordinatively identical to but distinct in shape from the majority linker can be inserted into the framework, resulting in defects. The reaction of Zn(II) with 1,4-benzenedicarboxylic acid (H(2)BDC) in the presence of small amounts of 1,3,5-tris(4-carboxyphenyl)benzene (H(3)BTB) leads to a new crystalline material, MOF-5(O(h)), that is nearly identical to MOF-5 but has an octahedral morphology and a number of defect sites that are uniquely functionalized with dangling carboxylates. The reaction with Pd(OAc)(2) impregnates the metal ions, creating a heterogeneous catalyst with ultrahigh surface area. The Pd(II)-catalyzed phenylation of naphthalene within Pd-impregnated MOF-5(O(h)) demonstrates the potential utility of an MCP framework for modulating the reactivity and selectivity of such transformations. Furthermore, this novel synthetic approach can be applied to different MCPs and will provide scaffolds functionalized with catalytically active metal species.     

Phase selection and discovery among five assembly modes in a coordination polymerization

The combination of zinc(II) nitrate with 1,3,5-(triscarboxyphenyl)benzene (H 3BTB) leads to five different microporous coordination polymers (MCPs). Two of these were previously known (MOF-177 and MOF-39), whereas polymer-induced heteronucleation was used in the discovery of three phases that have not been previously reported ( Zn/BTB ant, Zn/BTB tsx, and Zn/BTB dia). Modification of crystallization conditions allows for the bulk-scale synthesis of each of these MCPs. Zn/BTB ant and Zn/BTB tsx are each interpentrated 6,3-connected nets composed of the basic zinc carboxylate secondary building unit (SBU) and the tritopic linker BTB. The underlying noninterpenetrated net of Zn/BTB ant is derived for the net of anatase, whereas that of Zn/BTB tsx is the previously unreported "tsx" framework. Zn/BTB dia consists of an underlying diamondoid net in which four linear, trinuclear zinc hourglass SBUs are arranged about a central mu 4-oxo anion as the tetrahedral unit in the net and BTB further links the hourglass SBUs. Zn/BTB ant, Zn/BTB tsx, and MOF-177 are here defined as polymorphic frameworks in that each is composed of the same SBU and linker but differ in topology and thus pore structure. These frameworks may be called a polyreticular series by analogy to several reported isoreticular series. The effect of linker-linker interactions are discussed.     

Room temperature synthesis of metal-organic frameworks: MOF-5, MOF-74, MOF-177, MOF-199, and IRMOF-0

Room temperature synthesis of metal-organic frameworks (MOFs) has been developed for four well-known MOFs: MOF-5, MOF-74, MOF-177, and MOF-199. A new isoreticular metal framework (IRMOF), IRMOF-0, having the same cubic topology as MOF-5, has been synthesized from acetylenedicarboxylic acid using this method to accommodate the thermal sensitivity of the linker. Despite acetylenedicarboxylate being the shortest straight linker that can be made into an IRMOF, IRMOF-0 forms as a doubly interpenetrating structure, owing to the rod-like nature of the linker.     

Enhanced Hydrostability in Ni-Doped MOF-5

Ni-doped MOF-5s were successfully synthesized for the first time via solvothermal crystallization process to enhance the hydrostability. Several characterization techniques, including X-ray diffraction (XRD), thermogravimetrical analysis (TGA), scanning electron microscopy (SEM), energy-dispersive spectroscopy instrument (EDS), inductively coupled plasma (ICP), infrared spectroscopy (IR), atomic sorption, diffuse-reflectance UV-vis spectroscopy, and gas sorption measurement, strongly support the effective incorporation of Ni(II) ions into the framework. The results demonstrated that the Ni-doped MOF-5s not only exhibit larger Langmuir specific surface areas and larger pores than the undoped MOF-5, but also significantly enhance water resistance of the framework. The H(2) uptake capacity of undoped MOF-5 drops rapidly when exposed to the ambient air, whereas the H(2) adsorptions of the Ni-doped MOF-5s remain stable for 4 days.     

磷钼钨杂多酸修饰金属有机骨架MOF-5复合材料吸附亚甲基蓝

采用水热合成法,在合成金属有机骨架MOF-5的过程中引入活性组分磷钼钨杂多酸(H₆P₂Mo₁₅W₃O₆₂),制备出一种新型吸附剂H₆P₂Mo₁₅W₃O₆₂/MOF-5。 利用H₆P₂Mo₁₅W₃O₆₂/MOF-5材料对亚甲基蓝溶液进行了材料的吸附性能研究,探讨了亚甲基蓝溶液的初始pH值和初始浓度以及不同吸附温度对吸附量的影响。 结果表明,在较低的温度和亚甲基蓝溶液较低的初始pH值的条件下,有利于H₆P₂Mo₁₅W₃O₆₂/MOF-5对亚甲基蓝的吸附。 实验结果能够较好地符合Langmuir吸附等温式以及二级动力学模型,对亚甲基蓝的最大吸附量可达401.6 mg/g。 热力学参数ΔG<0、ΔH<0和ΔS>0,表明H₆P₂Mo₁₅W₃O₆₂/MOF-5对亚甲基蓝的吸附过程是自发、放热的。 此外,还探讨了H₆P₂Mo₁₅W₃O₆₂/MOF-5对甲基紫、孔雀石绿、罗丹明B和甲基橙等其它染料的吸附性能,结果表明,H₆P₂Mo₁₅W₃O₆₂/MOF-5对阳离子染料有较好的吸附效果。     

碱性金属修饰金属有机骨架材料MOF-5吸附位点及其常态下分离二氧化碳/甲烷的应用

利用“一步法”合成并表征了4种碱性金属修饰的金属有机骨架材料MOF-5(记作M-MOF-5,M=Li,Na,K,Mg)。 并应用理想溶液吸附理论(IAST)对样品吸附选择性进行计算比较。 结果表明,碱性金属掺入可以降低MOF-5材料骨架结构的“互穿”程度,同时,在常温常压下,M-MOF-5对CO₂/CH₄的吸附选择性相对MOF-5(选择性为3.79)有着显著提高,尤其是Li-MOF-5(选择性为7.39)。 此外,Li-MOF-5的CO₂捕获能力相对MOF-5也有提高。     

The interaction of water with MOF-5 simulated by molecular dynamics

Force field parameters for use with metal-organic framework-5 (MOF-5 or IRMOF-1) are presented. Flexibility within the framework is included in this model, so that structural changes upon interaction with adsorbate molecules can be observed and quantified. The model was validated by comparing simulated lattice parameters of pure MOF-5 with X-ray diffraction results. For the first time, molecular dynamics simulations have been performed that show how water interacts with MOF-5. The framework is stable at water contents up to 2.3% by mass, but distortion in the lattice structure is already evident. At water contents of 3.9% and higher, the framework collapses because of the replacement of MOF O atoms by water O atoms in the Zn coordination shells. As a result, inorganic MOF O atoms are no longer coordinated by four Zn ions, and benzene dicarboxylate linkers are no longer tethered to Zn centers.     

MOF-5: enthalpy of formation and energy landscape of porous materials

The first experimental thermodynamic analysis of a metal-organic framework (MOF) has been performed. Measurement of the enthalpy of formation of MOF-5 from the dense components zinc oxide (ZnO), 1,4-benzenedicarboxylic acid (H(2)BDC), and occluded N,N-diethylformamide (DEF) (if any) gave values of 78.64 ± 2.95 and 99.47 ± 3.62 kJ·[mol of Zn(4)O(BDC)(3)·xDEF](-1) for the as-made form and the desolvated structure, respectively. These as-made and desolvated enthalpies correspond to the values 19.66 ± 0.74 and 24.87 ± 0.94 kJ·(mol of Zn)(-1), respectively. The energetics of desolvated MOF-5 per mole of Zn falls in line with trends relating the enthalpy of inorganic porous materials (zeolites, zeotypes, and mesoporous materials) to molar volume. MOF-5 extends a plateauing trend first suggested by thermodynamic studies of mesoporous materials. This leveling off of the destabilization energetics as the void space swells suggests that additional void volume beyond a certain point may begin to act as a parameter "external" to the structure and not destabilize it further. This could help explain the rich landscape of large-volume MOFs and their ease of desolvation.     

Selectivity limitations in the heterogeneous epoxidation of olefins: branching reactions of the oxametallacycle intermediate in the partial oxidation of styrene

The partial oxidation of olefins on Ag surfaces has been a long standing problem in surface science and is of great commercial interest. We present a temperature-programmed reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS) study of the partial oxidation of styrene on Ag(111). The reaction products are CO2, water, styrene oxide, benzene, and benzoic acid. XPS gives evidence for two different reaction intermediates that we assign to an oxametallacycle and to benzoate. The oxametallacycle seems to be the precursor for both the formation of styrene oxide and the formation of benzoate, and thus, in addition to leading to styrene oxide, also leads to undesired byproducts. The benzoate reacts further to form CO2, benzene, and benzoic acid.     

Exploring the coordination chemistry of MOF-graphite oxide composites and their applications as adsorbents

Metal-organic frameworks (MOFs), besides being porous materials exhibit a very rich chemistry, which can be used for the synthesis of composites and/or the reactive adsorption of toxic gases. In this study, composites of MOFs (MOF-5, HKUST-1 or MIL-100(Fe)) and a graphitic compound (graphite or graphite oxide, GO) were synthesized and tested for the removal of NH(3), H(2)S and NO(2) under ambient conditions. The materials were characterized before and after exposure to the target gases by X-ray diffraction, thermogravimetric analysis, N(2) sorption measurement and FT-IR spectroscopy. The results indicate that strong chemical bonds exist between the MOF and GO as a result of the coordination between the GO oxygen groups and the MOFs' metallic centers. Depending on the structure of the MOF, such interactions induce the formation of a new pore space in the interface between the carbon layers and the MOF units, which enhances the physical adsorption capacity of the toxic gases. When unsaturated metallic sites are present in the MOFs, the target gases are also adsorbed via coordination to these centers. Further reaction with the framework leads to the formation of complexes. This is accompanied by the collapse of the MOF structure.     

Complex formation in the course of synthesis of zinc oxide from citrate solutions

Complex formation of zinc(II) with citric acid in acidic medium was studied by pH-potentiometric titration at various metal to acid ratios. The speciation diagrams of the complexes in relation to pH were plotted, and the stability constants of the complexes were determined. Precursors of the synthesis of metal oxide materials were prepared at two reactant ratios. A scheme of the precursor thermal decomposition based on data of derivatography and IR spectroscopy was suggested, and the compositions of compounds formed during decomposition were determined.     

A kinetic study of S-nitrosothiol decomposition

Under anaerobic conditions S-nitrosothiols 1a-e undergo thermal decomposition by homolytic cleavage of the S-N bond; the reaction leads to nitric oxide and sulfanyl radicals formed in a reversible manner. The rate constants, k(t), have been determined at different temperatures from kinetic measurements performed in refluxing alkane solvents. The tertiary nitrosothiols 1c (k1(69 degrees C) = 13 x 10(-3) min(-1)) and 1d (k1(69 degrees C) = 91 x 10(-3) min(-1)) decomposed faster than the primary nitrosothiols 1a (k1(69 degrees C) = 3.0 x 10(-3) min(-1)) and 1b (k1(69 degrees C) = 6.5 x 10(-3) min(-1)). The activation energies (E# = 20.5-22.8 Kcal mol(-1)) have been calculated from the Arrhenius equation. Under aerobic conditions the decay of S-nitrosothiols 1a-e takes place by an autocatalytic chain-decomposition process catalyzed by N2O3. The latter is formed by reaction of dioxygen with endogenous and/or exogenous nitric oxide. The autocatalytic decomposition is strongly inhibited by removing the endogenous nitric oxide or by the presence of antioxidants, such as p-cresol, beta-styrene, and BHT. The rate of the chain reaction is independent of the RSNO concentration and decreases with increasing bulkiness of the alkyl group; this shows that steric effects are crucial in the propagation step.     

MOF/graphite oxide hybrid materials: exploring the new concept of adsorbents and catalysts

Two types of metal-organic framework (MOF)/graphite oxide hybrid materials were prepared. One is based on a zinc-containing, MOF-5 and the other on a copper-containing HKUST-1. The materials are characterized by X-ray diffraction, sorption of nitrogen, thermal analyses, Fourier Transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). Their features are compared to the ones of the parent materials. The water stability and ammonia adsorption capacity of the hybrid materials were also evaluated. It was found that the latter compounds exhibit features similar to the ones of the parent MOF. In most cases, their porosity increased compared to the one calculated considering the physical mixture of MOF and GO. This new porosity likely located between the two components of the hybrid materials is responsible for the enhanced ammonia adsorption capacity of the compounds. However, for both the zinc-based and the copper-based materials (MOFs and hybrid materials), a collapse of the framework was observed as a result of ammonia adsorption. This collapse is caused by the interactions of ammonia with the metallic centers of MOFs either by hydrogen bonding (zinc-based materials) or coordination and subsequent complexation (copper-based materials). Whereas the MOF-5 based compounds collapse in presence of humidity, the copper-based materials are stable.     

Preparation of copper net-supported metal-organic framework-5 membranes for solid-state lasers

Continuous copper net-supported metal-organic framework (MOF-5) membranes of high quality are synthesized for the first time by vapor diffusion of an organic amine. The velocity of diffusion of the organic amine and reaction temperature directly affect the quality of the membranes formed. Furthermore, a laser dye is successfully encapsulated within such membranes in CHCl3. The dye-loaded MOF-5 membranes have potential applications in optics, especially in laser systems.     

The inconsistency in adsorption properties and powder XRD data of MOF-5 is rationalized by framework interpenetration and the presence of organic and inorganic species in the nanocavities

MOF-5 is the archetype metal-organic framework and has been subjected to numerous studies the past few years. The focal point of this report is the pitfalls related to the MOF-5 phase identification based on powder XRD data. A broad set of conditions and procedures have been reported for MOF-5 synthesis. These variations have led to materials with substantially different adsorption properties (specific surface areas in the range 700 to 3400 m(2)/g). The relatively low weight loss observed for some as synthesized samples upon solvent removal is also indicative of a low pore volume. Regrettably, these materials have all been described as MOF-5 without any further comments. Furthermore, the reported powder XRD patterns hint at structural differences: The variations in surface area are accompanied by peak splitting phenomena and rather pronounced changes in the relative peak intensities in the powder XRD patterns. In this work, we use single-crystal XRD to investigate structural differences between low and high surface area MOF-5. The low surface area MOF-5 sample had two different classes of crystals. For the dominant phase, Zn(OH)2 species partly occupied the cavities. The presence of Zn species makes the hosting cavity and possibly also adjacent cavities inaccessible and thus efficiently reduces the pore volume of the material. Furthermore, the minor phase consisted of doubly interpenetrated MOF-5 networks, which lowers the adsorption capacity. The presence of Zn species and lattice interpenetration changes the symmetry from cubic to trigonal and explains the peak splitting observed in the powder XRD patterns. Pore-filling effects from the Zn species (and partly the solvent molecules) are also responsible for the pronounced variations in powder XRD peak intensities. This latter conclusion is particularly useful for predicting the adsorption properties of a MOF-5-type material from powder XRD.