Dajian Zhu

Huazhong University of Science and Technology, Wu-han-shih, Hubei, China

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Publications (7)37.96 Total impact

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    ABSTRACT: Al in: A new strategy was introduced to modify the electronics and steric hindrance of the Pd(II) ion in order to change its reactivity towards benzene hydroxylation. In trifluoroacetic acid, free Pd(II) ions provide dominantly biphenyl, with phenol as minor product. Ligation of bpym to the Pd(II) ion results in its deactivation with regard to benzene functionalization. The addition of the redox inactive Al(III) ion to the Pd(II) (bpym) complex recovers its catalytic activity, and alters the reactivity of Pd(II) ion from benzene coupling to hydroxylation.
    No preview · Article · May 2013 · Chemistry - An Asian Journal
  • Yujuan Wang · Song Shi · Huajun Wang · Dajian Zhu · Guochuan Yin
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    ABSTRACT: The kinetics of hydrogen abstraction by manganese(IV) species having hydroxo or oxo group reveals that they have very similar reactive characters in the transition state of hydrogen abstraction.
    No preview · Article · Aug 2012 · Chemical Communications
  • Yujuan Wang · Jiayi Sheng · Song Shi · Dajian Zhu · Guochuan Yin
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    ABSTRACT: Clarifying how versatile physicochemical parameters of an active metal intermediate affect its reactivity would help to understand its roles in chemical and enzymatic oxidations. The influence of the net charge on electron transfer and hydrogen abstraction reactions of a manganese(IV) species having hydroxide ligand has been investigated here. It was found that increasing one unit of the positive net charge from 2+ to 3+ would accelerate its electron-transfer rate by 10–20 fold in oxygenation of tris(4-methoxyphenyl)phosphine. In contrast, the hydrogen abstraction rate is insensitive to its net charge change, and the insensitivity has been attributed to the compensation effect between the redox potential and pKa, which determine the hydrogen abstraction capability of a metal ion. Similar net-charge-promoted electron transfer but not hydrogen abstraction has also been observed in intramolecular electron transfer and hydrogen abstraction reactions when using thioxanthene as substrate. Together with the previous understanding of the reactivity of the identical manganese(IV) species having MnIV–OH or MnIV=O functional groups, the relationships of the oxidative reactivity of an active metal intermediate with its physicochemical parameters such as the net charge, the redox potential and the metal–oxygen bond order (M–O versus M═O) have been discussed with this manganese(IV) model.
    No preview · Article · Jun 2012 · The Journal of Physical Chemistry C
  • Yujuan Wang · Song Shi · Dajian Zhu · Guochuan Yin
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    ABSTRACT: Clear elucidation of the oxidative relationships of the active metal hydroperoxide moiety with its corresponding metal oxo and hydroxo intermediates would help the understanding of the different roles they may play in redox metalloenzymes and oxidation chemistry. Using an Mn(Me(2)EBC)Cl(2) complex, it was found that, in t-butanol-water (4 : 1) with excess H(2)O(2) at pH 1.5, the Mn(IV)-OOH moiety may exist in the catalytic solution with a mass signal of m/z = 358.1, which provides a particular chance to investigate its oxidative properties. In catalytic oxidations, the Mn(IV)-OOH moiety demonstrates a relatively poor activity in hydrogen abstraction from diphenyl methane and ethylbenzene with TOF of only 1.2 h(-1) and 1.1 h(-1) at 50 °C, whereas it can efficiently oxygenate diphenyl sulfide, methyl phenyl sulfide and benzyl phenyl sulfide with TOF of 13.8 h(-1), 15.4 h(-1) and 17.8 h(-1), respectively. In mechanistic studies using H(2)(18)O and H(2)(18)O(2), it was found that, in the Mn(IV)-OOH moiety mediated hydrogen abstraction and sulfide oxygenations, the reaction proceeds by two parallel pathways: one by direct oxygen insertion/transfer, and the other by plausible electron transfer. Together with a good understanding of the corresponding manganese(IV) oxo and hydroxo intermediates, this work provides the first chance to compare the reactivity differences and similarities of the active metal oxo, hydroxo and hydroperoxide intermediates. The available evidence reveals that the Mn(IV)-OOH moiety has a much more powerful oxidizing capability than the corresponding Mn(IV)=O and Mn(IV)-OH functional groups in both hydrogen abstraction and oxygenation.
    No preview · Article · Mar 2012 · Dalton Transactions
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    ABSTRACT: The M n+-OH moiety in a manganese(IV) complex has more powerful electron-transfer capability than its corresponding M n+=O moiety. An M n+=O moiety can abstract hydrogen from a substrate, then rebind the OH group from its reduced M (n-1)+-OH form to the substrate radical. In contrast, the active center with an M n+-OH cannot perform similar rebound from its reduced M (n-1)+-OH 2 group (see scheme; HAT=hydrogen abstraction).
    No preview · Article · Aug 2011 · Angewandte Chemie International Edition
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    ABSTRACT: The oxidative carbonylation of ethanol to diethyl carbonate (DEC) was investigated by an efficient catalyst system comprising of Co-Schiff base complexes. Effects of Schiff base ligands, reaction time, catalyst concentration, temperature and pressure on the catalytic activity were studied. Co(salophen) [N,N′-bis(salicylidene) o-phenylenediamine cobalt] catalyst exhibited better catalytic activity compared with other Co complexes. When the oxidative carbonylation was carried out at the reaction conditions: 0.12mol/L Co(salophen), P(CO)/P(O2)=2:1, 3.0MPa, 140°C, 2.5h, the conversion of ethanol is 15.8%, the selectivity to DEC is 99.5% and the turnover number (TON) is 22.2. The corrosion behavior of Co(salophen) catalyst to the stainless steel reactor was also examined. The corrosion rate to the stainless steel by Co(salophen) catalyst is below 0.005mm/a. SEM images demonstrated that the pitting corrosion was not observed on the surface of the stainless steel.
    No preview · Article · Jun 2011 · Fuel
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    ABSTRACT: Dimethyl carbonate (DMC) has been synthesized by oxidative carbonylation of methanol over the Co−Schiff base complexes, which have been encapsulated in situ in zeolite Y by a “ship-in-the-bottle” approach. These hybrid materials Co−Schiff base/zeolite Y have been characterized by FT-IR, UV−vis, XRD, BET, and TG/DTA techniques. Analysis of the hybrid materials indicates the formation of complexes in the cavity without affecting the zeolite framework structure. The catalytic activities and corrosion behavior of the encapsulated complexes and their homogeneous analogues were examined. Zeolite-encapsulated Co complexes were found to be more active and stable than the neat Co complexes. When the reaction was carried out using 1.0 g of encapsulated catalyst, 40 mmol of methanol, CO/O2 ratio of 2:1, and at 3.0 MPa and 120 °C for 4 h, zeolite-encapsulated Co−salophen shows the highest activity; and the conversion of methanol and selectivity to DMC were 25.4 and 99.5%, respectively. It was also demonstrated that zeolite-encapsulated Co−salophen catalyst can be reused five times without loss of activity. Both neat and encapsulated Co−salophen exhibit noncorrosive behavior to the reactor made by stainless steel.
    No preview · Article · May 2009 · Energy & Fuels