Coke formation during the methanol-to-olefin conversion: in situ microspectroscopy on individual H-ZSM-5 crystals with different Brønsted acidity

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 Brønsted 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.     

Elucidating the origin of diastereoselectivity in a self-replicating system: selfishness versus altruism

We have investigated a diastereoselective self‐replicating system based on a cycloaddition of a fulvene derivative and a maleimide using a two‐pronged approach of combining NMR spectroscopy with computational modelling. Two diastereomers are formed with identical rates in the absence of replication. When replication is enabled, one diastereomer takes over the resources as a “selfish” autocatalyst, while exploiting the competitor as a weak “altruist”, resulting in a diastereoselectivity of 16:1. We applied 1D and 2D NMR spectroscopic techniques supported by ab initio chemical shifts as well as ab initio molecular dynamics simulations to study the structure and dynamics of the underlying network. This powerful combination allowed us to decipher the energetic and structural rationale behind the observed behaviour, while static computational methods currently used in the field did not.     

Solvent and TMEDA effects on the configurational stability of chiral lithiated aryloxiranes

The employment of hexane/N,N,N',N'-tetramethylethylenediamine (TMEDA) dramatically hinders the racemization of those lithiated styrene oxides (trifluoromethyl-, chloro-, and phenylthio-substituted) that have been proven to be configurationally unstable in THF on the timescale of their reactions. The barriers to inversion and the activation parameters, calculated (Eyring equation) for reactions performed in THF, THF/TMEDA, and hexane/TMEDA, suggest the intervention of particular enantiomerization mechanisms for each case. The role of TMEDA in both coordinating and noncoordinating solvents has also been questioned and discussed in light of the kinetic data gathered and a model for deprotonation in hexane/TMEDA has also been proposed. The synthetic benefits of our results became apparent on establishing an asymmetric synthesis of an industrially important antifungal agent.     

17O NMR and computational study of a tetrasiliconiobate ion, [H(2+x)Si4Nb16O56](14-x)-

Rates of oxygen-isotope exchange were measured in the tetrasiliconiobate ion [H(2+x)Si(4)Nb(16)O(56)]((14-x)-) to better understand how large oxide ions interact with water. The molecule has 19 nonequivalent oxygen sites and is sufficiently complex to evaluate hypotheses derived from our previous work on smaller clusters. We want to examine the extent to which individual oxygen atoms react independently with particular attention given to the order of protonation of the various oxygen sites as the pH decreases from 13 to 6. As in our previous work, we find that the set of oxygen sites reacts at rates that vary over approximately 10(4) across the molecule at 6相似文献   

Tuning molecular recognition in water-soluble nanogels with enzyme-like activity for the kemp elimination

The synthesis and characterization of water-soluble imprinted nanogels with enzyme-like activity in the Kemp elimination is reported together with studies that demonstrate how the recognition properties, morphology, and catalytic activity of the nanoparticles can be tuned by the use of surfactants, such as Tween 20. A detailed kinetic investigation is carried out, which shows clear evidence of saturation kinetics and rule out the effects of mass transfer. This is supported by characterization of the polymeric materials that confirms the morphological changes resulting from the use of surfactants. These results provide an important tool for the development of nanoparticle-based, new catalyst-mimicking enzymes.     

Oxidation of Zn(Cys)4 zinc finger peptides by O2 and H2O2: products, mechanism and kinetics

The reactivity of a series of Zn(Cys)(4) zinc finger model peptides towards H(2)O(2) and O(2) has been investigated. The oxidation products were identified by HPLC and ESI-MS analysis. At pH<7.5, the zinc complexes and the free peptides are oxidised to bis-disulfide-containing peptides. Above pH 7.5, the oxidation of the zinc complexes by H(2)O(2) also yields sulfinate- and sulfonate-containing overoxidised peptides. At pH 7.0, monitoring of the reactions between the zinc complexes and H(2)O(2) by HPLC revealed the sequential formation of two disulfides. Several techniques for the determination of the rate constant for the first oxidation step corresponding to the attack of H(2)O(2) by the Zn(Cys)(4) site have been compared. This rate constant can be reliably determined by monitoring the oxidation by HPLC, fluorescence, circular dichroism or absorption spectroscopy in the presence of excess ethyleneglycol bis(2-aminoethyl ether)tetraacetic acid. In contrast, monitoring of the release of zinc with 4-(2-pyridylazo)resorcinol or of the thiol content with 5,5'-dithiobis(2-nitrobenzoate) did not yield reliable values of this rate constant for the case in which the formation of the second disulfide is slower than the formation of the first. The kinetic measurements clearly evidence a protective effect of zinc on the oxidation of the cysteines by both H(2)O(2) and O(2), which points to the fact that zinc binding diminishes the nucleophilicity of the thiolates. In addition, the reaction between the zinc finger and H(2)O(2) is too slow to consider zinc fingers as potential sensors for H(2)O(2) in cells.     

超级铝热剂Al/CuO前驱体的制备、表征、热分解机理及非等温分解反应动力学

以硝酸铜、无水乙醇、1,2-环氧丙烷和纳米铝粉为原料, 在超声振荡条件下, 采用溶胶-凝胶法制备了纳米复合含能材料——超级铝热剂Al/CuO的前驱体. 利用热重-差示扫描量热-傅里叶变换红外-质谱(TG- DSC-FTIR-MS)联用技术, 研究了纳米Al/CuO溶胶-凝胶前驱体的热行为和分解过程及机理. 利用不同升温速率下的TG-DTG分析, 研究了纳米超级铝热剂Al/CuO的溶胶-凝胶前驱体的热分解反应机理, 采用了6种动力学分析方法进行动力学参数计算, 得到前驱体分解反应的表观活化能、反应级数、频率因子等动力学参数, 纳米Al/CuO前驱体分解反应的动力学方程为: dα/dt=10^14.0×4α^3/4exp(-2.0×10⁴/T).     

HPD-100树脂吸附小檗碱的热力学及动力学研究

研究了小檗碱在HPD-100树脂上的吸附热力学和动力学行为。考察了小檗碱在HPD-100树脂上的吸附等温曲线和吸附动力学曲线,确定了吸附过程的热力学及动力学参数。结果表明,HPD-100树脂对小檗碱的等温吸附可采用Langmuir方程描述、吸附为自发进行的放热型物理吸附过程;Dunwald–Wagnen内扩散方程能较好地拟合吸附动力学数据,吸附过程以颗粒内扩散为主要控制步骤,该吸附过程的表观活化能为29.11kJ/mol。本文为HPD-100树脂分离富集小檗碱类生物碱的实际应用提供基础数据及指导。     

TiO2‐assisted Photocatalytic Degradation of Vitamin B12 in Aqueous Solution:Influencing Factors,Kinetics, and Reaction Investigations

The photodegradation (λ=365 nm) of the biomolecule vitamin B₁₂, catalyzed by the photocatalyst TiO₂ nanoparticles (NPs), has been investigated in aqueous suspension. The photodegradation process of vitamin B₁₂ has been monitored by means of electronic absorption (Abs), Fourier‐transform infrared (FT‐IR), and resonance Raman (RR) spectroscopies, respectively. The results show that only under UV illumination in the presence of TiO₂ is there effective degradation, and the photocatalytic degradation of vitamin B₁₂ is strongly influenced by the amount of TiO₂ NPs, the pH, and the initial concentration of vitamin B₁₂. The photocatalytic reaction kinetics of vitamin B₁₂ conforms to a Langmuir‐Hinshelwood isotherm model. Changes involving the three structural units of the carbon‐metal bond C–Co, the organic corrin macrocycle combined with the benzimidazole nucleotide, and the inorganic CN in the vitamin B₁₂ molecule during the photocatalytic degradation are also discussed.     

Thermal Decomposition Kinetics of Some Di‐ and Triorganotin(IV) Carboxylates Derived from para‐Nitrophenylethanoic Acid

Thermogravimetric (TG) investigations of organotin(IV) carboxylates with the general formula R_mSnL_4−m (where R=CH₃, C₂H₅, n‐C₄H₉, C₆H₅, cyclo‐C₆H₁₁, n‐C₈H₁₇, m=2, 3, and L=para‐nitrophenylethanoate anion) have been performed. Derivative thermogravimetry (DTG) and differential thermal analysis (DTA) techniques, Horowitz‐Metzger method and the fundamental thermodynamic relations are used to evaluate the thermokinetic parameters of each thermal degradation pattern. Results reveal that the thermal stability is functional to Sn C and Sn O bonds. In the case of R₂SnL₂, activation energy, reaction order and pre‐exponential factor associated with the bulk degradation processes increase as the alkane chain length increases. Hence, Oct₂SnL₂ is thermally more stable than Bu₂SnL₂, which in turn is more resistant to thermal dissociation than Et₂SnL₂. The same phenomenon is not observed for R₃SnL compounds because their degradation is highly irregular. Furthermore, R₂SnL₂ has larger values of kinetic parameters than those of corresponding triorganotin(IV) para‐nitrophenylethanotes. Thermodynamic parameters of these compounds also reinforce the above facts.