Pt/ZnO 在室温水相无碱条件下绿色催化苯甲醇选择性氧化

醇类化合物选择性氧化是有机合成中一个非常重要的反应, 在精细化工领域具有重要应用. 而以水为绿色溶剂, 分子氧为绿色氧化剂实现醇类化合物选择性氧化是绿色化学领域的一大挑战. Pt 催化剂由于其优异的活化氧气和 C-H 键的能力在该反应中得到了广泛应用. 但是, 常规的 Pt 催化剂通常需要在较高温度和较高氧气压力以及加碱的条件下才能发挥作用, 从而引起了催化剂腐蚀等一系列问题. 从绿色化学角度出发, 进一步优化 Pt 催化剂, 让其能够在室温无碱条件下以空气为氧化剂选择性氧化醇类合成羰基化合物具有重要的研究价值和应用前景.本文通过化学还原法制备了 Pt/ZnO 催化剂, 系统研究了该催化剂在水相无碱条件下选择性氧化苯甲醇生成苯甲醛反应中的催化性能. X 射线电子衍射和透射电镜等结果表明, Pt 颗粒较小(3.2 ± 0.3 nm), 均匀分散在 ZnO 载体上; X 射线光电子能谱表明 ZnO 载体能够稳定 Pt 纳米颗粒表面的 Pt0物种. 上述催化剂在水相苯甲醇选择性氧化反应中, 在室温下即可催化空气高选择性氧化苯甲醇到苯甲醛 (选择性>99%), 并表现出比 Pt/SiO2, Pt/Al2O3, Pt/TiO2, Pt/Ca(Mg)-ZSM-5 等催化剂更为优异的催化活性. 这可归结于 Pt 和 ZnO 之间的协同作用. 该协同作用通过动力学实验和密度泛函理论计算 (DFT)得到了证实. 氧分压实验表明, 在以空气为氧化剂时, O2的活化并不是限制 Pt/ZnO 催化活性的关键因素, 而动力学同位素效应实验则证实了苯甲醇的 C-H 键活化是整个反应的决速步骤. 通过构建不同的理论模型, 分别计算了 Pt/ZnO 界面处以及纯 Pt 位点上苯甲醇选择性氧化的反应过程. 结果表明, 苯甲醇和氧气分子倾向于分别在 ZnO 和 Pt 上进行吸附, 随后由吸附的氧气分子来活化苯甲醇中的 C-H 键, 进而生成苯甲醛和水. 而当 ZnO 不参与苯甲醇的吸附活化时, 整个反应的活化能会大大提高, 表明 ZnO 和 Pt 之间的协同作用对于整个反应至关重要. 此外, Pt/ZnO 表现出非常优异的稳定性, 循环使用 4 次后, 催化剂结构以及催化活性没有显著变化.进一步向 Pt/ZnO 催化剂中引入少量 Bi 元素对 Pt 的电子结构进行修饰, 可以将 Pt/ZnO 的催化活性提高 3 倍. 所制备的 Pt/Bi-ZnO 复合物是目前报道的相同条件下催化苯甲醇选择性氧化反应转化频率 (45.1 h-1)最高的催化剂.     

Size-dependence of Fermi energy of gold nanoparticles loaded on titanium(iv) dioxide at photostationary state

TiO(2) particle-supported Au nanoparticles (NPs) with varying sizes and good contact (Au/TiO(2)) were prepared under a constant loading amount by the deposition-precipitation method. The Fermi energy of Au NPs loaded on TiO(2) at the photostationary state (E(F)') was determined in water by the use of S/S(2-) having specific interaction with Au as a redox probe. The E(F)' value goes up as the mean size of Au NPs (d) increases at 3.0 相似文献   

Coordination of divalent metal cation to amide group to form adduct ion in FAB mass spectrometry: implication of Zn2+ in enzymatic hydrolysis of amide bond

The structures of complexes between amides and metal ions were examined by FAB mass spectrometry and collision-induced dissociation (CID). Zn2+ was coordinated by the amide carbonyl oxygen atom of N-tetradecylacetamide (1). In contrast, Ca2+ and Mg2+ were coordinated by the amide group of 1 in both the keto and enol forms. The catalytic role of Zn2+ at the active site of the hydrolases might partly be explained by the effective attack of Zn2+ on carbonyl oxygen atom of the scissile amide group.     

Melophlins P, Q, R, and S: four new tetramic acid derivatives, from two Palauan marine sponges of the genus Melophlus

Four new tetramic acid derivatives, named melophlins P, Q, R, and S (1-4), were isolated from two marine sponges of the genus Melophlus collected at Palau, together with seven known melophlins A, D, E, G, H, I, and O. The structures of the new compounds were elucidated on the basis of their spectral data. The absolute stereochemistries at the tetramic acid moieties of the new compounds were determined as 1 : 1 mixtures (racemic) by ESI-LC/MS analysis of derivatives obtained by oxidation and hydrolysis of the respective parent compounds. Melophlins P-S (1-4) showed cytotoxicity against the murine leukemia cell line L1210 with IC(50) values of 20.0, 10.5, 0.85, and 5.13 muM, respectively.     

Energetics of dioxygen binding into graphene patches with various sizes and shapes

Density functional theory (DFT) calculations were employed to investigate the electronic properties of an H-atom terminated graphene patch (hydrographene) smaller than a rhombic C96H26 structure with zigzag edges. Depending on shapes and sizes of hydrographenes, some hydrographenes have the triplet ground state where unpaired electrons are localized on their zigzag edges. The stability of the triplet spin state is diminished, decreasing the hydrographene sizes. The existence of the localized spin densities allows triplet dioxgen to bind into a hydrographene. According to the DFT calculations, the energetics of the dioxygen bindings is negatively influenced by downsizing hydrographenes, as well as depends on their shapes. The size-and shape-dependences of the dioxygen bindings reflect from the stability of the triplet state of a hydrographene, because its localized unpaired electrons can be utilized to be attached to an unpaired electron of triplet dioxygen.     

Unprecedented reversible redox process in the ZnMFI-H2 system involving formation of stable atomic Zn(0)

In its element: Zn(2+) at the M7 site of MFI-type zeolites activates H(2), via ZnH and OH species, and leads to Zn(0) species. The Zn(0) species returns to its original state, a Zn(2+) ion, upon evacuation of the zeolite at 873?K (see picture). The formation of the Zn(0) species is supported by DFT calculations.     

Cyclization reaction of n-propargyl epoxyamide to acetylenic 2-azetidinone,a precursor to thienamycin and related carbapenems

Base treatment of N-propargyl epoxyamide 5 afforded the acetylenic 3-(hydroxyethyl)-2-azetidinone 9a, which was subsequently transformed to phenyl thiolester 2, a versatile intermediate for the carbapenem synthesis.     

A new method analyzing large and strongly interacting reaction systems

A new method is presented to analyze the various interactions in reaction systems. The method is especially suited for large and strongly interacting systems where other analyzing methods are not practical. The method could isolate the particular interaction from the whole interaction by a procedure termed the partial diagonalization of the bond order matrix. The usefulness of the method is exemplified by the adsorption of CO on Pt and W surfaces. The interactions on the W surface are much stronger than those on the Pt surface, which is consistent with the experimental data. The role of individual interactions for the weakening of the C-O bond and the formation of the Pt-C and W-C bonds is discussed separately.     

Periodic surface calculations on chemisorption of H2 on a Ni surface

The periodic model of finite surfaces has been employed to investigate the localizability of the chemisorption interaction and to examine the validity of the cluster model. The adsorbate—surface interaction is estimated by the usual perturbation method. The model is applied to chemisorption of hydrogen molecule on a Ni(100) surface. As for the present calculations, the period model is shown to be advantageous over the cluster model in representing both the adsorbate-free surface state and the adsorbate—surface interaction.