bin Wu

Water-mediated promotion of direct oxidation of benzene over the metal–organic framework HKUST-1

Pretreatment of a HKUST-1 catalyst with water significantly accelerated the catalytic oxidation of benzene to phenol and hydroquinone with hydrogen peroxide as an oxidant. The corresponding oxygenates had a yield of 36.5%, and the selectivity to phenol and hydroquinone was 53.2% and 35.5%, respectively. The turnover frequency (TOF) was 35.1 h−1. Comparatively, the product yield was only 2.7% over the original HKUST-1, and the TOF was 2.6 h−1. Moreover, water treatment protected HKUST-1 from decomposition due to formation of a new oxidation mode. Therefore, the catalytic system in the presence of water opened a new door towards a facile and efficient preparation of phenol and hydroquinone.

Direct synthesis of phenol by novel [FeFe]-hydrogenase model complexes as catalysts of benzene hydroxylation with H2O2

Three new [FeFe]-hydrogenase model complexes, μ-(SCH(CH2CH3)CH2S)–Fe2(CO)6 (complex 1), μ-(SCH(CH2CH3)CH2S)–Fe2(CO)5PCy3 (complex 2) and μ-(SCH(CH2CH3)CH2S)–Fe2(CO)5PPh3 (complex 3) were prepared. The structures of complexes 1–3 were characterized by FT-IR, UV-vis, 1H, 13C, 31P NMR spectra and single-crystal analyses. The electron density of these model complexes was studied by IR spectra, UV spectra and electrochemical analysis and evaluated against their respective catalytic performances. The CV (cyclic voltammetry) study of complex 2 showed a less positive oxidation event at 0.6 V and a more negative reduction event at −1.94 V, which is in accordance with the enlargement of electron density at diiron centers when CO were substituted by better electron donor ligands. Of all these three complexes, complex 2 exhibited the best catalytic activity, with a yield of phenol of up to 24.6% and selectivity up to 92%, which is consistent with its higher electron density of the Fe–Fe bond. This study revealed the correlations between the electron density of the catalytic site of catalysts and their performance in catalytic hydroxylation of benzene. Based on these experimental results, a catalytic oxidation mechanism via an Fe2+–μ-O–Fe2+ intermediate as oxygen transfer reagent has been proposed.

Ferrocene particles incorporated into Zr-based metal–organic frameworks for selective phenol hydroxylation to dihydroxybenzenes

UiO-66 with high dispersibility and a cuboctahedron morphology was synthesized by an improved solvothermal method. The morphology of UiO-66 was adjusted using benzoic acid as a modulator. UiO-66 with a regular morphology was then used as the support to load ferrocene (Fc). A series of Fc@UiO-66 composites were prepared via a facile impregnation method. The composites were characterized by powder X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), SEM/EDX mapping, FT-IR, UV/vis, TG analysis, N2 adsorption–desorption, and X-ray photoelectron spectroscopy (XPS). The results showed that Fc was incorporated into UiO-66, thus preventing the agglomeration of Fc particles in water. The Fc@UiO-66 composites with a Fc loading of 5% (FU-5) exhibited the highest catalytic activity for hydroxylation of phenol with H2O2 at room temperature in water, which gave a phenol conversion of 38.5% and 87.4% selectivity for dihydroxybenzenes (DHB). UiO-66 played a crucial role in the enhancement of catalytic performance compared with conventional supports such as γ-Al2O3, etc. A hydroxyl radical mechanism was proposed for this catalytic hydroxylation process and the high selectivity was attributed to the interaction between Fc particles and UiO-66.

Morphology effect of metal-organic framework HKUST-1 as a catalyst on benzene oxidation

An attempt was made to study whether the morphology effect of metal-organic frameworks HKUST-1 could significantly influence the chemical reaction of benzene oxidation. Four representative cupric salts, CuSO4·5H2O, Cu(OAc)2·H2O, CuCl2·2H2O and Cu(NO3)2·3H2O, were treated with 1,3,5-benzenetricarboxylic acid under ultrasound or with static method at room temperature to prepare metal-organic frameworks(12 types of HKUST-1 samples). And the as-prepared HKUST-1 materials were comprehensively investigated by X-ray diffraction, scanning electron microscopy and N2 adsorption-desorption. The HKUST-1 samples with different morphologies and characterisitcs were employed as catalysts for benzene oxidation with H2O2 as oxidant at 60 °C in acetonitrile to achieve the aromatic oxygenates and test their yields. In all the HKUST-1 samples, the HKUST-1/SA, HKUST-1/SA0 and HKUST-1/UN had the higher catalytic activities with the yields of benzene oxygenates of 15.9%, 16.6% and 11.7%, respectively, which can be ascribed to the larger pore volume, the stronger benzene adsorption and the smaller fine crystal particles. Comparatively, the HKUST-1/SN0 and HKUST-1/SC0 with more intact crystal, larger surface area, lower pore volume and weaker benzene adsorption had the lower catalytic activities with the yields of benzene oxygenates not more than 4%. Therefore, our results confirmed that employing various cupric precursors to prepare the HKUST-1 samples with different morphologies and characteristics can be considered as a worth strategy to design many more powerful heterogeneous catalysts.

A defective MOF architecture threaded by interlaced carbon nanotubes for high-cycling lithium–sulfur batteries

Metal organic frameworks (MOFs) have been deemed among the most promising sulfur hosts for lithium–sulfur (Li–S) batteries owing to their high specific surface areas, novel pore structures and open metal sites. However, their highly coordinated, electronically insulating and structurally unstable nature overshadows the merits of MOFs to a great extent. In this work, a novel UiO-66/carbon nanotube (UC) composite was initially synthesized via a facile one-pot synthesis strategy, in which abundant linker-missing defects were caused by introduced competitive coordination. Meanwhile, flexible and interlaced carbon nanotubes (CNTs) throughout mechanically stable UiO-66 nanoparticles constructed a reliable conductive network. Because of its superior structural stability, high electronic conductivity and strong polysulfide chemisorption, the UC architecture as the sulfur cathode in Li–S batteries shows stable cycling, delivering an initial capacity of 925 mA h g−1 at 0.5 A g−1 and a very low fading rate over 800 cycles of 0.071% per cycle at 1 A g−1. A strong chemical affinity between coordination defects and LiPSs was revealed by first principles calculations and apparent absorption, which indicates significant entrapment of soluble polysulfides by the UC composite, thus leading to the outstanding cycling performance of S@UC electrodes.