Showing papers in "Chemistry: A European Journal in 2011"
TL;DR: Water proved to be essential for the formation of well-ordered Zr-bdc-NH(2) and the first single-crystal structural analysis of a ZR-based metal-organic framework.
Abstract: We present an investigation on the influence of benzoic acid, acetic acid, and water on the syntheses of the Zr-based metal-organic frameworks Zr-bdc (UiO-66), Zr-bdc-NH(2) (UiO-66-NH(2)), Zr-bpdc (UiO-67), and Zr-tpdc-NH(2) (UiO-68-NH(2)) (H(2) bdc: terephthalic acid, H(2) bpdc: biphenyl-4,4'-dicarboxylic acid, H(2) tpdc: terphenyl-4,4''-dicarboxylic acid). By varying the amount of benzoic or acetic acid, the synthesis of Zr-bdc can be modulated. With increasing concentration of the modulator, the products change from intergrown to individual crystals, the size of which can be tuned. Addition of benzoic acid also affects the size and morphology of Zr-bpdc and, additionally, makes the synthesis of Zr-bpdc highly reproducible. The control of crystal and particle size is proven by powder XRD, SEM and dynamic light scattering (DLS) measurements. Thermogravimetric analysis (TGA) and Ar sorption experiments show that the materials from modulated syntheses can be activated and that they exhibit high specific surface areas. Water proved to be essential for the formation of well-ordered Zr-bdc-NH(2) . Zr-tpdc-NH(2), a material with a structure analogous to that of Zr-bdc and Zr-bpdc, but with the longer, functionalized linker 2'-amino-1,1':4',1''-terphenyl-4,4''-dicarboxylic acid, was obtained as single crystals. This allowed the first single-crystal structural analysis of a Zr-based metal-organic framework.
1,249 citations
TL;DR: In this minireview, squaramides are presented from their roots as artificial anion receptors in molecular recognition studies to their recent advances as powerful bifunctional hydrogen-bonding catalysts in asymmetric organocatalysis.
Abstract: In this minireview, squaramides are presented from their roots as artificial anion receptors in molecular recognition studies to their recent advances as powerful bifunctional hydrogen-bonding catalysts in asymmetric organocatalysis. The main features of the squaramido functionality and the direct comparison with the analogous ureas and thioureas are also discussed.
604 citations
TL;DR: Combination of base-catalyzed lignin hydrolysis with addition of boric acid protecting agent shifts the product distribution to lower molecular weight compounds and increases product yields beyond 85%.
Abstract: The products of base-catalyzed liquid-phase hydrolysis of lignin depend markedly on the operating conditions. By varying temperature, pressure, catalyst concentration, and residence time, the yield of monomers and oligomers from depolymerized lignin can be adjusted. It is shown that monomers of phenolic derivatives are the only primary products of base-catalyzed hydrolysis and that oligomers form as secondary products. Oligomerization and polymerization of these highly reactive products, however, limit the amount of obtainable product oil containing low-molecular-weight phenolic products. Therefore, inhibition of concurrent oligomerization and polymerization reactions during hydrothermal lignin depolymerization is important to enhance product yields. Applying boric acid as a capping agent to suppress addition and condensation reactions of initially formed products is presented as a successful approach in this direction. Combination of base-catalyzed lignin hydrolysis with addition of boric acid protecting agent shifts the product distribution to lower molecular weight compounds and increases product yields beyond 85%.
490 citations
421 citations
TL;DR: This review summarizes the structures, biological activities, and synthetic routes for natural compounds containing HPI, emphasizing the different strategies for assembling this motif.
Abstract: Research on natural products containing hexahydropyrrolo[2,3-b]indole (HPI) has dramatically increased during the past few years. Newly discovered natural products with complex structures and important biological activities have recently been isolated and synthesized. This review summarizes the structures, biological activities, and synthetic routes for natural compounds containing HPI, emphasizing the different strategies for assembling this motif. It covers a broad range of molecules, from small alkaloids to complex peptides.
394 citations
TL;DR: The basic functional elements needed to fabricate electronic devices are described and summarized and a high-level roadmap for device-related MOF research is proposed to stimulate thinking within the MOF community concerning the development these materials for applications including sensing, photonics, and microelectronics is proposed.
Abstract: Metal-organic frameworks (MOFs) and related material classes are attracting considerable attention for applications such as gas storage, separations, and catalysis. In contrast, research focused on potential uses in electronic devices is in its infancy. Several sensing concepts in which the tailorable chemistry of MOFs is used to enhance sensitivity or provide chemical specificity have been demonstrated, but in only a few cases are MOFs an integral part of an actual device. The synthesis of a few electrically conducting MOFs and their known structural flexibility suggest that MOF-based electronic devices exploiting these properties could be constructed. It is clear, however, that new fabrication methods are required to take advantage of the unique properties of MOFs and extend their use to the realms of electronic circuitry. In this Concepts article, we describe the basic functional elements needed to fabricate electronic devices and summarize the current state of relevant MOF research, and then review recent work in which MOFs serve as active components in electronic devices. Finally, we propose a high-level roadmap for device-related MOF research, the objective of which is to stimulate thinking within the MOF community concerning the development these materials for applications including sensing, photonics, and microelectronics.
394 citations
TL;DR: The graphene nanosheets in the composite materials could act not only as lithium storage active materials, but also as an electronically conductive matrix to improve the electrochemical performance of Fe(3)O(4).
Abstract: Fe3O4-graphene composites with three-dimensional laminated structures have been synthesised by a simple in situ hydrothermal method. From field-emission and transmission electron microscopy results, the Fe3O4 nanoparticles, around 3-15 nm in size, are highly encapsulated in a graphene nanosheet matrix. The reversible Li-cycling properties of Fe3O4-graphene have been evaluated by galvanostatic discharge-charge cycling, cyclic voltammetry and impedance spectroscopy. Results show that the Fe3O4-graphene nanocomposite with a graphene content of 38.0 wt% exhibits a stable capacity of about 650 mAh g(-1) with no noticeable fading for up to 100 cycles in the voltage range of 0.0-3.0 V. The superior performance of Fe3O4-graphene is clearly established by comparison of the results with those from bare Fe3O4. The graphene nanosheets in the composite materials could act not only as lithium storage active materials, but also as an electronically conductive matrix to improve the electrochemical performance of Fe3O4.
392 citations
TL;DR: The first experimental demonstration of porous MOF as a new class of precursor for preparing ORR catalysts is reported, which has the following advantages when used to prepare non-PGM electrocatalysts: MOFs have clearly-defined three-dimensional structures.
Abstract: We demonstrate a new approach of preparing a non-platinum group metal (PGM) electrocatalyst for oxygen reduction reaction through rational design by using cobalt imidazolate framework—a subclass of metal-organic framework (MOF) material—as the precursor with potential to produce uniformly distributed catalytic center and high active-site density. MOFs represent a new type of materials, and have recently been under broad exploration of various important applications due to their amenability to rational design for different functionalities at molecular level. In particular, their high surface areas, well-defined porous structures, and building block variety not only distinguish them from the conventional materials in gas adsorption and separation, but also offer new promises in catalysis application. However, the application of porous MOFs for electrocatalysis in fuel cell has yet to be exploited. The oxygen reduction reaction (ORR) at the cathode of a proton exchange membrane fuel cell (PEMFC) represents a very important electrocatalytic reaction. At present, the catalyst materials of choice are platinum group metals (PGMs). The high costs and limited reserves of PGMs, however, created a major barrier for large-scale commercialization of PEMFCs. Intensive efforts have been dedicated to the search of low-cost alternatives. The discovery of ORR activity on cobalt phthalocyanine stimulated extensive investigations of using Co–N4 or Fe–N4 macromolecules as precursors for preparation of transition metal (TM) based, non-PGM catalysts. The ORR activity over a cobalt–polypyrrole composite was observed, of which a Co ligated by pyrrolic nitrogens was proposed as the catalytic site. Activation in an inert atmosphere of the similar TM– polymer composite through pyrolysis further improved the catalytic activity. More recently, significant enhancement in ORR activity was demonstrated in a carbon-supported iron-based catalysts, and it was suggested that micropores (width <20 ) have critical influence on the formation of the active site with an ionic Fe coordinated by four pyridinic nitrogens after high-temperature treatment. The onset potential for an Fe-based catalyst is found to be 0.1 V higher than that of a Co-based system although the latter is more stable under PEMFC operating condition. These previous studies proposed the nitrogen-ligated TM entities either as the precursors or the active centers for the catalytic ORR process. Another challenge for non-PGM ORR catalysts is their relatively low turn-over-frequency in comparison with Pt. To compensate low activity without using excessive amount of catalyst, thus causing thick electrode layer and poor mass transport, it is desirable to produce the highest possible catalytic-site density, that are evenly distributed and accessible to gas diffusion through a porous framework. Herein we report the first experimental demonstration of porous MOF as a new class of precursor for preparing ORR catalysts. Different from previous approaches, MOFs have the following advantages when used to prepare non-PGM electrocatalysts: MOFs have clearly-defined three-dimensional structures. The initial entities such as TM–N4 can be grafted into MOFs with the highest possible volumetric density through regularly arranged cell structure. The MOF surface area and pore size are tunable by the length of the linker. The organic linkers would be converted to carbon during thermal activation while maintaining the porous framework, leading to catalysts with high surface area and uniformly distributed active sites without the need of a second carbon support or pore forming agent. Furthermore, the TM–ligand composition can be rationally designed with wide selection of metal–linker combinations for systematical investigation on the relationship between precursor structure and catalyst activity. Our studies demonstrate the initial step to achieve such advantages. [a] Dr. S. Ma, Dr. G. A. Goenaga, Dr. D.-J. Liu Chemical Sciences & Engineering Division Argonne National Laboratory, Argonne, IL 60439 (USA) Fax: (+1) 630-252-4176 E-mail : djliu@anl.gov [b] A. V. Call Department of Materials Science and Engineering Northwestern University, Evanston, IL 60208 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201003080.
391 citations
TL;DR: This review summarizes the transition-metal-catalyzed cross-coupling of enol- and phenol-based electrophiles, including phosphates, sulfonates, ethers, carboxylates, and Phenolates.
Abstract: Enol and phenol functionalities are very common in organic molecules. Utilization of these materials is very appealing in organic synthesis because they are important alternatives to organohalides in cross-coupling reactions. In this review, we summarize the transition-metal-catalyzed cross-coupling of enol- and phenol-based electrophiles, including phosphates, sulfonates, ethers, carboxylates, and phenolates.
390 citations
TL;DR: In the past decades, unexpected strong chalcogen–chalcogen inter-actions or halogen bonds have captured interest as connec-tors.
Abstract: In the past decades a lot of progress has been achieved inthe design of miniaturized systems. For instance, the manu-facture of computer chips employing optical technologies,for example, phototemplating, is well established. However,optical procedures are limited to approximately 50 nm. Al-ternatively, nanostructures may be formed by means of selfassembly systems. Besides the arrangement of the molecularbuilding blocks by hydrogen bonds or inorganic metal–ligand bonds, unexpected strong chalcogen–chalcogen inter-actions or halogen bonds have captured interest as connec-tors.
373 citations
TL;DR: The photocatalytic activity of oxygen evolution on BiVO(4) is found to be proportionally correlated with the exposed surfaces of the (040) facet, which is assumed to be responsible for the high activity of O(2) evolution.
Abstract: Monoclinic BiVO(4) crystals with preferentially exposed (040) facets were hydrothermally synthesized by using a trace amount of TiCl(3) as the directing agent; this function was confirmed by X-ray diffraction patterns (XRD) and high-resolution transmission electron microscopy (HRTEM). The effects of the directing agent TiCl(3) and the pH values applied during synthesis have been studied, and the optimized BiVO(4) sample with highly exposed (040) facet could be obtained by using 1.2 at.% of TiCl(3) as the directing agent at a pH value of 2. Some complementary techniques were also applied to exclude the effects of the structural and physical property changes, such as surface area and hydrophilicity. The photocatalytic activity of oxygen evolution on BiVO(4) is found to be proportionally correlated with the exposed surfaces of the (040) facet. It is assumed that the active sites with a BiV(4) structure on the exposed (040) facet is assigned to be responsible for the high activity of O(2) evolution.
TL;DR: A simple glucose-assisted hydrothermal method to directly grow MoS2 nanosheets (NSs) on the CNT backbone (CNT@MoS2 NSs) is reported, which shows enhanced lithium storage properties with better cyclic capacity retention and a higher reversible capacity.
Abstract: As a typical layered inorganic material, molybdenum disulfide (MoS2) has a similar structure to graphite. In the crystal structure of MoS2, each Mo(IV) sits in the center of a triangular prism, and is bound to six S atoms. Each S atom is connected to three Mo centers. In this way, the triangular prisms are interconnected to give a layered structure, wherein the Mo atoms are sandwiched between two layers of S atoms. Because of the weak van der Waals interactions between the sheets, MoS2 has a low friction coefficient; this gives rise to its superior lubricating properties. It has also been found attractive in many other application, including catalysts and transistors. Additionally, the layered structure of MoS2 enables easy intercalation of metal ions, such as Li or Mg . Many different MoS2 nanostructures, such as nanoflakes, nanotubes and nanoflowers, have been reported so far as anode materials for lithium ion batteries (LIBs). Although some of them show relatively high capacities of up to 1000 mAhg , the unsatisfactory cycling stability hinders their practical application as anode materials of LIBs. Some methods have been proposed to improve the cycling performance of MoS2, for example, construction of composite materials of MoS2 and conductive carbonaceous materials, like amorphous carbon, carbon nanotubes (CNTs), or graphene. For example, Li et al. reported a hybrid material of CNTs coated with several layers of MoS2. [15] When tested for lithium storage capabilities, the CNT@MoS2 hybrid structure shows a relatively good cyclic capacity retention with a reversible capacity of only up to 400 mAhg , probably due to the low mass fraction of MoS2 in the composite. Thus, obtaining a high content of MoS2 in the CNT@MoS2 is important for a better lithium storage capability. Many CNT-based hybrid structures have been prepared for different applications. 17] Herein, we report a simple glucose-assisted hydrothermal method to directly grow MoS2 nanosheets (NSs) on the CNT backbone (CNT@MoS2 NSs). The content of MoS2 in the hybrid structure is greatly increased because the shell is composed of sheet-like subunits. At the same time, the large surface area provided by this unique hierarchical structure can perhaps help to store more lithium, and the void space between these sheet-like subunits can buffer the volume change during the charge/ discharge processes, and lead to improved cyclic capacity retention. Furthermore, the carbon derived from glucose could ensure an excellent contact between the CNT backbone and the shell of MoS2 NSs, and give rise to a good conducting network. As expected, in comparison with pure MoS2 flakes, these CNT@MoS2 NS nanocomposites show enhanced lithium storage properties with better cyclic capacity retention and a higher reversible capacity. Figure 1 shows the morphology of the as-prepared CNT@MoS2 NSs. From the scanning electron microscopy
TL;DR: The state-of-the-art of the system is given, some interesting strategies for the future developments are illustrated and the physicochemical and functional parameters relevant to electrochemical properties, that is, electrochemical stabilities, are presented.
Abstract: This paper presents an overview of the various types of lithium salts used to conduct Li(+) ions in electrolyte solutions for lithium rechargeable batteries. More emphasis is paid towards lithium salts and their ionic conductivity in conventional solutions, solid-electrolyte interface (SEI) formation towards carbonaceous anodes and the effect of anions on the aluminium current collector. The physicochemical and functional parameters relevant to electrochemical properties, that is, electrochemical stabilities, are also presented. The new types of lithium salts, such as the bis(oxalato)borate (LiBOB), oxalyldifluoroborate (LiODFB) and fluoroalkylphosphate (LiFAP), are described in detail with their appropriate synthesis procedures, possible decomposition mechanism for SEI formation and prospect of using them in future generation lithium-ion batteries. Finally, the state-of-the-art of the system is given and some interesting strategies for the future developments are illustrated.
TL;DR: It is found that thermally reduced graphene oxide offers the most favorable electrochemical performance among the different materials studied and has a profound impact for the applications of chemically modified graphenes in electrochemical devices.
Abstract: Electrochemical applications of graphene are of great interest to many researchers as they can potentially lead to crucial technological advancements in fabrication of electrochemical devices for energy production and storage, and highly sensitive sensors. There are many routes towards fabrication of bulk quantities of chemically modified graphenes (CMG) for applications such as electrode materials. Each of them yields different graphene materials with different functionalities and structural defects. Here, we compare the electrochemical properties of five different chemically modified graphenes: graphite oxide, graphene oxide, thermally reduced graphene oxide, chemically reduced graphene oxide, and electrochemically reduced graphene oxide. We characterized these materials using transmission electron microscopy, Raman spectroscopy, high-resolution X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry, which allowed us to correlate the electrochemical properties with the structural and chemical features of the CMGs. We found that thermally reduced graphene oxide offers the most favorable electrochemical performance among the different materials studied. Our findings have a profound impact for the applications of chemically modified graphenes in electrochemical devices.
TL;DR: Kinetic data have been obtained for the first time, with determination of the observed rate constant, k(obs), of the overall reaction.
Abstract: The mechanism of the reaction of trans-[ArPdX(PPh(3))(2)] (Ar=p-Z-C(6)H(4); Z=CN, F, H; X=I, Br, Cl) with Ar'B(OH)(2) (Ar'=p-Z'-C(6)H(4); Z'=CN, H, OMe) has been established in DMF in the presence of the base OH(-) in the context of real palladium-catalyzed Suzuki-Miyaura reactions. The formation of the cross-coupling product ArAr' and [Pd(0)(PPh(3))(3)] has been followed through the application of electrochemical techniques. Kinetic data have been obtained for the first time, with determination of the observed rate constant, k(obs), of the overall reaction. trans-[ArPdX(PPh(3))(2)] is not reactive in the absence of the base. The base OH(-) plays three roles. It favors the reaction: 1) by formation of trans-[ArPd(OH)(PPh(3))(2)], a key complex which, in contrast to trans-[ArPdX(PPh(3))(2)], reacts with Ar'B(OH)(2) (rate-determining transmetalation), and 2) by unexpected promotion of the reductive elimination from the intermediate trans-[ArPdAr'(PPh(3))(2)], which generates ArAr' and a Pd(0) species. Conversely, the base OH(-) disfavors the reaction by formation of the unreactive anionic Ar'B(OH)(3)(-). As a consequence of these antagonistic effects of OH(-), the overall reactivity is controlled by the concentration of OH(-) and passes through a maximum as the concentration of OH(-) is increased. Therefore, the base favors the rate-determining transmetalation and unexpectedly also the reductive elimination.
TL;DR: In this paper, the redox neutral strategy circumvents the use of wasteful metal oxidants and leads to a clean process, and the authors propose a new strategy for a clean manufacturing process.
Abstract: The redox neutral strategy circumvents the use of wasteful metal oxidants and leads to a clean process.
TL;DR: A molecular picture of the gate-opening mechanism underlying the unprecedented selectivity towards alkane adsorption and separation of alkanes and alkenes is proposed based on DFT calculations and a thermodynamic analysis of the advertisersorption-desorption isotherms.
Abstract: C2 and C3 alkanes are selectively adsorbed from mixtures over the corresponding alkenes on the zeolite imidazolate framework ZIF-7 through a gate-opening mechanism. As a result, the direct production of the pure alkene upon adsorption and the pure alkane upon desorption in packed columns is possible. Herein, a detailed investigation of the step-wise adsorption and separation of alkanes and alkenes is presented, together with a rigorous performance assessment. A molecular picture of the gate-opening mechanism underlying the unprecedented selectivity towards alkane adsorption is proposed based on DFT calculations and a thermodynamic analysis of the adsorption–desorption isotherms.
TL;DR: The preparation of a nickel-foam-supported graphene sheet/porous NiO hybrid film by the combination of electrophoretic deposition and chemical-bath deposition produces a highly porous NiO film made up of interconnected NiO flakes with a thickness of 10-20 nm.
Abstract: We report the preparation of a nickel-foam-supported graphene sheet/porous NiO hybrid film by the combination of electrophoretic deposition and chemical-bath deposition. The obtained graphene-sheet film of about 19 layers was used as the nanoscale substrate for the formation of a highly porous NiO film made up of interconnected NiO flakes with a thickness of 10-20 nm. The graphene sheet/porous NiO hybrid film exhibits excellent pseudocapacitive behavior with pseudocapacitances of 400 and 324 F g(-1) at 2 and 40 A g(-1), respectively, which is higher than those of the porous NiO film (279 and 188 F g(-1) at 2 and 40 A g(-1)). The enhancement of the pseudocapacitive properties is due to reinforcement of the electrochemical activity of the graphene-sheet film.
TL;DR: The general purpose of this review is to give an exhaustive and clear picture in heteroaryl-heteroaryl bond formation as well as its application in the synthesis of natural products, pharmaceuticals, catalyst ligands, and materials.
Abstract: The biheteroaryl structural motif is prevalent in polymers, advanced materials, liquid crystals, ligands, molecules of medicinal interest, and natural products. Many types of synthetic transformations have been known for the construction of heteroaryl–heteroaryl linkages. Coupling reactions provide one of the most efficient ways to achieve these biheterocyclic structures. In this review, four types of coupling reactions are discussed: 1) transition-metal-catalyzed coupling reactions of heteroaryl halides or surrogates with heteroarylmetals; 2) direct inter- and intramolecular heteroarylations of CH bonds of heteroarenes with heteroaryl halides or pseudohalides; 3) oxidative CH/CH homo- and cross-couplings of two unpreactivated heteroarenes; and 4) transition-metal-catalyzed decarboxylative cross-coupling reactions between haloheteroarenes or heteroarenes and heteroarenecarboxylic acids. The general purpose of this review is to give an exhaustive and clear picture in heteroaryl–heteroaryl bond formation as well as its application in the synthesis of natural products, pharmaceuticals, catalyst ligands, and materials.
TL;DR: The development of gold-catalyzed oxidative coupling reactions is discussed focusing on C-C bond-forming reactions of broad synthetic appeal, enabling the construction of C- C bonds between coupling partners that are not easily accessed using alternative catalysts.
Abstract: When reacted in the presence of external oxidants, gold complexes are capable of catalyzing oxidative homo- and cross-coupling reactions involving the formation of new C-C bonds. Over the last few years, several cascade processes have been reported in which coupling is preceded by a gold-mediated aryl C-H functionalization or nucleophilic addition. These reactions combine the unique reactivity of gold with oxidative coupling, enabling the construction of C-C bonds between coupling partners that are not easily accessed using alternative catalysts. In this Concept paper, the development of gold-catalyzed oxidative coupling reactions is discussed focusing on C-C bond-forming reactions of broad synthetic appeal.
TL;DR: Solid-state NMR spectroscopy investigations using (15)N-labeled PTI/Li(+)Cl(-) proved the absence of heptazine building blocks and NH(2) groups and corroborated the highly condensed, triazine-based structure model.
Abstract: Poly(triazine imide) with intercalation of lithium and chloride ions (PTI/Li+Cl−) was synthesized by temperature-induced condensation of dicyandiamide in a eutectic mixture of lithium chloride and potassium chloride as solvent. By using this ionothermal approach the well-known problem of insufficient crystallinity of carbon nitride (CN) condensation products could be overcome. The structural characterization of PTI/Li+Cl− resulted from a complementary approach using spectroscopic methods as well as different diffraction techniques. Due to the high crystallinity of PTI/Li+Cl− a structure solution from both powder X-ray and electron diffraction patterns using direct methods was possible; this yielded a triazine-based structure model, in contrast to the proposed fully condensed heptazine-based structure that has been reported recently. Further information from solid-state NMR and FTIR spectroscopy as well as high-resolution TEM investigations was used for Rietveld refinement with a goodness-of-fit (χ2) of 5.035 and wRp=0.05937. PTI/Li+Cl− (P63cm (no. 185); a=846.82(10), c=675.02(9) pm) is a 2D network composed of essentially planar layers made up from imide-bridged triazine units. Voids in these layers are stacked upon each other forming channels running parallel to [001], filled with Li+ and Cl− ions. The presence of salt ions in the nanocrystallites as well as the existence of sp2-hybridized carbon and nitrogen atoms typical of graphitic structures was confirmed by electron energy-loss spectroscopy (EELS) measurements. Solid-state NMR spectroscopy investigations using 15N-labeled PTI/Li+Cl− proved the absence of heptazine building blocks and NH2 groups and corroborated the highly condensed, triazine-based structure model.
TL;DR: The title reaction proceeds in the absence of transition metal catalysts, is operationally simple and tolerates a wide variety of functional groups like cyano, amide, aromatic halide, ether, ketone groups and C-C double bonds.
Abstract: Title reaction proceeds in the absence of transition metal catalysts, is operationally simple and tolerates a wide variety of functional groups like cyano, amide, aromatic halide, ether, ketone groups and C—C double bonds.
TL;DR: Chemical and thermal stabilities of isotypic metal-organic frameworks (MOFs), after purification to remove uncoordinated organic linkers, have been compared to understand the effect of the central metal ions on theStability of the porous MOF-type materials.
Abstract: Chemical and thermal stabilities of isotypic metal-organic frameworks (MOFs) like Al-BDC (Al-benzenedicarboxylate called MIL-53-Al), Cr-BDC (MIL-53-Cr) and V-BDC (MIL-47-V), after purification to remove uncoordinated organic linkers, have been compared to understand the effect of the central metal ions on the stabilities of the porous MOF-type materials. Chemical stability to acids, bases, and water decreases in the order of Cr-BDC>Al-BDC>V-BDC, suggesting stability increases with increasing inertness of the central metal ions. However, thermal stability decreases in the order of Al-BDC>Cr-BDC> V-BDC, and this tendency may be explained by the strength of the metal-oxygen bond in common oxides like Al(2)O(3), Cr(2)O(3), and V(2)O(5). In order to evaluate precisely the stability of a MOF, it is necessary to remove uncoordinated organic linkers that are located in the pores of the MOF, because a filled MOF may be more stable than the same MOF after purification.
TL;DR: Quick quantitative HSQC (QQ-HSQC) was applied to quantitative evaluation of different inter-unit linkages in an array of milled softwood and hardwood and technical lignins by using the guaiacyl C2 and syringyl C2-C6 signals as internal standards and was found to be highly reproducible and comparable with earlier literature reports.
Abstract: Quick quantitative HSQC (QQ-HSQC) was applied to quantitative evaluation of different inter-unit linkages in an array of milled softwood and hardwood and technical lignins by using the guaiacyl C2 and syringyl C2–C6 signals as internal standards. The results were found to be highly reproducible and comparable with earlier literature reports. The advantage of QQ-HSQC NMR analysis of lignin is contemporary detection and quantification of lignin inter-unit linkages with a direct, non-destructive method requiring short acquisition times.
TL;DR: The results presented herein provide new insights into core-shell materials as high-performance visible-light photocatalysts and their potential use in environmental protection.
Abstract: A facile solvothermal epitaxial growth combined with a mild oxidation route has been developed for the fabrication of a magnetically recyclable Fe3O4/WO3 core–shell visible-light photocatalyst. In this core–shell structured photocatalyst, visible-light-active WO3 nanoplates (the shells) with high surface area are used as a medium to harvest absorbed photons and convert them to photogenerated charges, while conductive Fe3O4 microspheres (the cores) are used as charge collectors to transport the photogenerated charges. This is a new role for magnetite. The Fe3O4/WO3 core–shell structured photocatalysts possess large surface-exposure area, high visible-light-absorption efficiency, stable recyclability, and efficient charge-separation properties, the combination of which has rarely been reported in other visible-light-active photocatalysts. Photoelectrochemical investigations verify that the core–shell structured Fe3O4/WO3 has a more effective photoconversion capability than pure WO3 or Fe3O4. At the same time, the visible-light photocatalytic ability of the Fe3O4/WO3 photocatalyst has significantly enhanced activity in the photodegradation of organic-dye materials. The results presented herein provide new insights into core–shell materials as high-performance visible-light photocatalysts and their potential use in environmental protection.
TL;DR: The progress made in developing reactions where an olefin is subjected to an asymmetric halogenation and the current state of the art, scope, and limitations of these processes are discussed.
Abstract: This Minireview discusses the progress made in developing reactions where an olefin is subjected to an asymmetric halogenation. It aims to serve as a reference for the studies reported to date, including preliminary work and mechanistic studies. The current state of the art, scope, and limitations of these processes are discussed.
TL;DR: Analysis of the reaction kinetics indicates that an increased Brønsted acid site density facilitates the formation of larger coke species and enhances their formation rate, and Polarization-dependent UV/Vis spectroscopy measurements illustrate that extended coke Species are predominantly formed in the straight channels of H-ZSM-5.
Abstract: Coke formation during the methanol-to-olefin (MTO) conversion has been studied at the single-particle level with in situ UV/Vis and confocal fluorescence microscopy. For this purpose, large H-ZSM-5 crystals differing in their Si/Al molar ratio have been investigated. During MTO, performed at 623 and 773 K, three major UV/Vis bands assigned to different carbonaceous deposits and their precursors are observed. The absorption at 420 nm, assigned to methyl-substituted aromatic compounds, initiates the buildup of the optically active coke species. With time-on-stream, these carbonaceous compounds expand in size, resulting in the gradual development of a second absorption band at around 500 nm. An additional broad absorption band in the 600 nm region indicates the enhanced formation of extended carbonaceous compounds that form as the reaction temperature is raised. Overall, the rate of coke formation decreases with decreasing aluminum content. Analysis of the reaction kinetics indicates that an increased Bronsted acid site density facilitates the formation of larger coke species and enhances their formation rate. The use of multiple excitation wavelengths in confocal fluorescence microscopy enables the localization of coke compounds with different molecular dimensions in an individual H-ZSM-5 crystal. It demonstrates that small coke species evenly spread throughout the entire H-ZSM-5 crystal, whereas extended coke deposits primarily form near the crystal edges and surfaces. Polarization-dependent UV/Vis spectroscopy measurements illustrate that extended coke species are predominantly formed in the straight channels of H-ZSM-5. In addition, at higher temperatures, fast deactivation leads to the formation of large aromatic compounds within channel intersections and at the external zeolite surface, where the lack of spatial restrictions allows the formation of graphite-like coke.
TL;DR: The similar values of U(eff) for the two magnetically diluted samples imply nearly the same distribution of low-lying states for their Dy(III) centers, which is consistent with the slight axial contraction observed for 1 and 2 and further corroborated by ligand-field analysis.
Abstract: Single-ion magnets 1 and 2 and their diamagnetic analogues 3 and 4 for magnetic-site dilution were obtained through substitution of the coordinated water molecules of [Ln(TTA)(3)-(H(2)O)(2)] (Ln=Dy (1, 2), Y (3, 4); TTA = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionate) by 2,2'-bipyridine (1, 3) and 1,10-phenanthroline (2, 4) capping ligands. Their structures and magnetic properties were investigated with the goal of identifying features relevant to modulating relaxation dynamics of single-ion magnets. The metal ions in all complexes adopt an approximately square-antiprismatic (SAP) O(6)N(2) coordination environment. The SAP polyhedrons for both 1 and 2 show slight longitudinal compression, while the coordination sphere of 1 deviates more from an ideal SAP than that of 2, as indicated by the skew angles of the SAP environment. The similar values of U(eff) for the two magnetically diluted samples imply nearly the same distribution of low-lying states for their Dy(III) centers, which is consistent with the slight axial contraction observed for 1 and 2 and further corroborated by ligand-field analysis. The fast quantum tunneling rate tau(QTM) of 1, which is about ten times faster than that of 2, can presumably be associated with the larger rotation of the SAP surroundings. This distortion may result in a significant transverse anisotropy terms, and thus strongly affect the dynamic behavior of the system.
TL;DR: It is suggested that the acid-base properties of the support and the size of Au nanoparticles are two key factors controlling the alcohol dehydrogenation catalysis.
Abstract: National Natural Science Foundation of China[20625310, 20773099, 20873110, 20923004]; National Basic Research Program of China[2010CB732303]; Research Fund for the Doctoral Program of Higher Education[20090121110007]; Key Scientific Project of Fujian Province[2009HZ0002-1]
TL;DR: Potassium (trifluoromethyl)trimethoxyborate is introduced as a new source of CF(3) nucleophiles in copper-catalyzed trifluorsation reactions in mild, base-free conditions in the presence of catalytic quantities of a Cu(I)/1,10-phenanthroline complex.
Abstract: Potassium (trifluoromethyl)trimethoxyborate is introduced as a new source of CF(3) nucleophiles in copper-catalyzed trifluoromethylation reactions. The crystalline salt is stable on storage, easy to handle, and can be obtained in near-quantitative yields simply by mixing B(OMe)(3), CF(3)SiMe(3), and KF. The trifluoromethylation reagent allows the conversion of various aryl iodides into the corresponding benzotrifluorides in high yields under mild, base-free conditions in the presence of catalytic quantities of a Cu(I)/1,10-phenanthroline complex.