Valorizing natural-abundant glucose to lactic acid using a MOF-808 catalyst under green hydrothermal conditions.

Highly robust Zr-based MOF-808, featuring Lewis acid Zr sites and coordinate hydroxide ions upon the removal of the monocarboxylate capping reagent, emerges as an efficient catalyst for the hydrothermal conversion of glucose into lactic acid. A remarkable 99% glucose conversion with an impressive 76.6% yield of lactic acid can be achieved. The large pore window of MOF-808 facilitates the diffusion of glucose to the active sites within the framework. The single-site attribute of the catalytic center enables a high selectivity of lactic acid over the competitive product, 5-(hydroxymethyl)furfural, under hydrothermal reaction conditions.

Co-based metal-organic framework for photocatalytic hydrogen generation.

Hydrogen production through an artificial photocatalytic process in the solar light region using a water-stable Co-Tz (Tz = 1,2,4-triazolate) framework was demonstrated. Possessing such a high photostability and highly reactive sites at the tetrahedral cobalt centers, Co-Tz exhibits a great photocatalytic performance converting water into hydrogen of 9.32 mmol g-1 at 4 h in the presence of fluorescein (FI) and triethylamine (TEA) as a photosensitizer and sacrificial agent, respectively. In addition, the framework is reusable without losing its catalytical integrity.

Conductive Co-triazole metal-organic framework exploited as an oxygen evolution electrocatalyst.

A Co-triazole metal-organic framework (Co-trz) endowed with electrical conductivity was synthesized effortlessly via a microwave-based method. Providing a high density of catalytic centers with electrically conductive features, as suggested by DFT calculations, the framework exhibited a low overpotential for the oxygen evolution reaction (OER) with good kinetics. A mechanistic reaction pathway was proposed based on monitoring alterations in the oxidation state and local coordination environment of Co centers upon the occurrence of the OER. Due to its performance and its chemical and electrochemical robustness, the framework was highlighted as a promising MOF electrocatalyst for the OER.

Selective cyclohexene oxidation to allylic compounds over a Cu-triazole framework via homolytic activation of hydrogen peroxide.

Utilization of metal-organic frameworks as heterogeneous catalysts is crucial owing to their abundant catalytic sites and well-defined porous structures. Highly robust [Cu3(trz)3(µ3-OH)(OH)2(H2O)4]·2H2O (trz = 1,2,4-triazole) was employed as a catalyst for liquid-phase cyclohexene oxidation with hydrogen peroxide (H2O2). Possessing the porous structure together with Lewis acid attributes from the triangular [Cu3(trz)3(µ3-OH)] center, selective oxidation of cyclohexene to allylic products gives a molar yield of 31% with 87% selectivity. According to the highly selective allylic production, the reaction over the present Cu-MOF plausibly occurs via homolytic activation of H2O2. This finding elucidates the unique features of the MOF for efficient catalysis of cyclohexene oxidation.

Engineering zirconium-based UiO-66 for effective chemical conversion of d-xylose to lactic acid in aqueous condition.

Utilizing metal-organic frameworks (MOFs) as heterogeneous catalysts is an interesting and important application due to their well-controlled catalytic sites and well-defined porous structures. In this study we apply, for the first time, Zr-based UiO-66 for the catalytic hydrothermal conversion of d-xylose to lactic acid (LA). The reactions are catalyzed by the coordinatively unsaturated Zr4+, as Lewis acid sites, and the hydroxide ion (OH-) located at the defect sites. The catalytic performances of UiO-66 catalysts synthesized through a modulator-free approach (UiO-66) and an acetic acid modulator-assisted approach (UiO-66(AA)) are distinct due to the different concentrations of local defects. The UiO-66 catalyst possessing a higher defect concentration exhibits a superior LA yield of 1.17 mol from 1 mol of xylose. However, the UiO-66(AA) catalyst with higher crystallinity shows better selectivity for LA over furfural, a side product from the competitive pathway. The enhanced LA yield and excellent selectivity can be achieved by the removal of AA from UiO-66(AA) resulting in a novel MOF catalyst (UiO-66(AA)*) which provides more accessible catalytic sites with retained crystallinity. This work highlights that the structural engineering of MOF catalysts is crucial for the fine-tuning of their catalytic properties.

Unsaturated Mn(II)-Centered [Mn(BDC)] n Metal-Organic Framework with Strong Water Binding Ability and Its Potential for Dehydration of an Ethanol/Water Mixture.

An unsaturated Mn(II)-centered metal-organic framework was synthesized. The presence of an unsaturated Mn(II) center, together with a guest-responsive structural changing feature, plays a crucial role for strong binding with water, leading to its potential application for water/ethanol separation. In addition, the present framework is thermally stable up to 400 °C, which is beneficial for the regeneration process after adsorption.

A 2D Metal Oxide Nanosheet as an Efficient Additive for Improving Na-Ion Electrode Activity of Graphene-Based Nanocomposites.

An efficient way to improve the Na-ion electrode activity of graphene-based nanocomposite is developed by employing exfoliated metal oxide nanosheet as an additive. The titanate-nanosheet-incorporated Na-SnS2 -reduced graphene oxide (rG-O) nanocomposites can be synthesized by the electrostatically derived restacking of the colloidal mixture of SnS2 , rG-O, and titanate nanosheets with the Na+ cation. The incorporation of titanate into the Na-SnS2 -rG-O nanocomposites is effective in improving the nanoscale mixing of component nanosheets and the porosity of the composite structure. The resulting nanocomposites deliver superior discharge capacities and rate properties to the titanate-free nanocomposite. The universal applicability is further confirmed by MoS2 -rG-O nanocomposites upon the addition of titanate. This study highlights that the exfoliated metal oxide nanosheet can be used as an efficient additive for graphene-based nanocomposites to explore Na-ion electrode materials.

Improvement of Na Ion Electrode Activity of Metal Oxide via Composite Formation with Metal Sulfide.

The composite formation with a conductive metal sulfide domain can provide an effective methodology to improve the Na-ion electrode functionality of metal oxide. The heat treatment of TiO2(B) under CS2 flow yields an intimately coupled TiO2(B)-TiS2 nanocomposite with intervened TiS2 domain, since the reaction between metal oxide and CS2 leads to the formation of metal sulfide and CO2. The negligible change in lattice parameters and significant enhancement of visible light absorption upon the reaction with CS2 underscore the formation of conductive metal sulfide domains. The resulting TiO2(B)-TiS2 nanocomposites deliver greater discharge capacities with better rate characteristics for electrochemical sodiation-desodiation process than does the pristine TiO2(B). The 23Na magic angle spinning nuclear magnetic resonance analysis clearly demonstrates that the electrode activities of the present nanocomposites rely on the capacitive storage of Na+ ions, and the TiS2 domains in TiO2(B)-TiS2 nanocomposites play a role as mediators for Na+ ions to and from TiO2(B) domains. According to the electrochemical impedance spectroscopy, the reaction with CS2 leads to the significant enhancement of charge transfer kinetics, which is responsible for the accompanying improvement in electrode performance. The present study provides clear evidence for the usefulness in composite formation between the semiconducting metal oxide and metal sulfide in exploring new efficient NIB electrode materials.

Unusually Huge Charge Storage Capacity of Mn3O4-Graphene Nanocomposite Achieved by Incorporation of Inorganic Nanosheets.

Remarkable improvement in electrode performance of Mn3O4-graphene nanocomposites for lithium ion batteries can be obtained by incorporation of a small amount of exfoliated layered MnO2 or RuO2 nanosheets. The metal oxide nanosheet-incorporated Mn3O4-reduced graphene oxide (rGO) nanocomposites are synthesized via growth of Mn3O4 nanocrystals in the mesoporous networks of rGO and MnO2/RuO2 2D nanosheets. Incorporation of metal oxide nanosheets is highly effective in optimizing porous composite structure and charge transport properties, resulting in a remarkable increase of discharge capacity of Mn3O4-rGO nanocomposite with significant improvement of cyclability and rate performance. The observed enormous discharge capacity of synthesized Mn3O4-rGO-MnO2 nanocomposite (∼1600 mA·h·g(-1) for the 100th cycle) is the highest value among reported data for Mn3O4-rGO nanocomposite. Despite much lower electrical conductivity of MnO2 than RuO2, the MnO2-incorporated nanocomposite at optimal composition (2.5 wt %) shows even larger discharge capacities with comparable rate characteristics compared with the RuO2-incorporated homologue. This finding underscores that the electrode performance of the resulting nanosheet-incorporated nanocomposite is strongly dependent on its pore and composite structures rather than on the intrinsic electrical conductivity of the additive nanosheet. The present study clearly demonstrates that, regardless of electrical conductivity, incorporation of metal oxide 2D nanosheet is an effective way to efficiently optimize the electrode functionality of graphene-based nanocomposites.

An Effective Way to Optimize the Functionality of Graphene-Based Nanocomposite: Use of the Colloidal Mixture of Graphene and Inorganic Nanosheets.

The best electrode performance of metal oxide-graphene nanocomposite material for lithium secondary batteries can be achieved by using the colloidal mixture of layered CoO2 and graphene nanosheets as a precursor. The intervention of layered CoO2 nanosheets in-between graphene nanosheets is fairly effective in optimizing the pore and composite structures of the Co3O4-graphene nanocomposite and also in enhancing its electrochemical activity via the depression of interaction between graphene nanosheets. The resulting CoO2 nanosheet-incorporated nanocomposites show much greater discharge capacity of ~1750 mAhg(-1) with better cyclability and rate characteristics than does CoO2-free Co3O4-graphene nanocomposite (~1100 mAhg(-1)). The huge discharge capacity of the present nanocomposite is the largest one among the reported data of cobalt oxide-graphene nanocomposite. Such a remarkable enhancement of electrode performance upon the addition of inorganic nanosheet is also observed for Mn3O4-graphene nanocomposite. The improvement of electrode performance upon the incorporation of inorganic nanosheet is attributable to an improved Li(+) ion diffusion, an enhanced mixing between metal oxide and graphene, and the prevention of electrode agglomeration. The present experimental findings underscore an efficient and universal role of the colloidal mixture of graphene and redoxable metal oxide nanosheets as a precursor for improving the electrode functionality of graphene-based nanocomposites.

A direct hybridization between isocharged nanosheets of layered metal oxide and graphene through a surface-modification assembly process.

An efficient and universal method to directly hybridize isocharged nanosheets of layered metal oxide and reduced graphene oxide (rGO) is developed on the basis of the surface modification and an electrostatically driven assembly process. On the basis of this synthetic method, the CoO2 -rGO nanocomposite can be synthesized with exfoliated CoO2 and rGO nanosheets, and transformed into CoO-CoO2 -rGO nanocomposites with excellent electrode performance for lithium-ion batteries. Also, this surface-modification assembly route is successfully applied for the synthesis of another mesoporous TiO2 -rGO nanocomposite. This result provides clear evidence for the usefulness of the present method as a universal way of hybridizing isocharged anionic nanosheets of inorganic solids and graphene.