Progress in Reaction Kinetics and Mechanism

Ln(III) complexes with chromone-3-carbaldehyde-(2′-hydroxy)benzoyl hydrazone as the Schiff base ligand have been synthesised and characterised by elemental analysis, molar conductivity, ¹H NMR, mass spectra, UV–Vis spectra, and IR spectra. Electronic absorption spectra, fluorescence spectra, ethidium bromide displacement experiments and viscosity measurements indicate that the free ligand and its Ln(III) complexes, especially the La(III) complex, bind strongly to calf thymus DNA via an intercalation mechanism. The intrinsic binding constants of the free ligand, La(III) complex, Sm(III) complex and Er(III) complex with DNA were (2.20±0.12) x 10⁵ M⁻¹, (8.83±0.04) x 10⁶ M⁻¹, (7.35±0.07) x 10⁶ M⁻¹ and (3.24±0.04) x 10⁶ M⁻¹, respectively. Furthermore, the antioxidant activity of the ligand and its Ln(III) complexes was determined by the superoxide and hydroxyl radical scavenging method in vitro, which indicates that the ligand and its Ln(III) complexes are efficient antioxidants and the Ln(III) complexes exhibit more effective antioxidant activity than the free ligand alone and some standard antioxidants, such as mannitol and vitamin C.

ABSTRACT

A virtual structural model has been developed for predicting the behaviour of cuprous sulfide reduction by hydrogen in the presence of lime. This model treats the complex successive reactions as a simple reaction. Then the kinetic and structural parameters of this assumed simple reaction were obtained by fitting the experimental data to the additive reaction times equation. Finally, the results predicted by the virtual model have been evaluated and validated using experimental data, and compared with those of mathematical modelling. This virtual model could be useful for predicting the behaviour of complex reactions without needing time-consuming computations.

Dealumination of the faujasite support was realised using a hydrothermal treatment at different temperatures (625 – 700°C). Then palladium catalysts were prepared by an ion exchange method. The prepared catalysts exhibited a high and constant activity up to 100% for N₂O conversion. This performance was dependent on certain parameters, such as a high Si/Al ratio, the structural properties of the zeolitic support, and the metal function provided by the palladium. In addition, it is considered that the active sites in the reaction, which are suspected to be PdO, are influenced by the acidic properties of the support.

The dehydrogenation of methylcyclohexane is a reversible reaction where methylcyclohexane is dehydrogenated to toluene and hydrogen in the forward reaction while the hydrogenation of toluene occurs is the reverse reaction. In the present work, both the forward and the reverse reactions were studied over the same catalyst: 1.0 wt% Pt/zeolite beta. The catalyst was prepared using the ion exchange method and it was characterised using ICP-OES, XRD, SEM, XPS, N₂-BET, and NH₃-TPD. The catalytic reactions were studied in a 1.02 cm internal diameter fixed bed tubular reactor under integral conditions and the effect of reaction operating conditions were studied on both the forward and reverse reactions. The kinetic modelling of the overall reactions, as well as of the by-products formed in the forward reaction, was carried out using simple power law models.

Catalytic esterification of acetic acid with methanol to produce methyl acetate in a stirred batch reactor was investigated in the presence of Amberlyst 16, Indion 180 and Amberlyst 36 catalysts. Amberlyst 36 was found to be an effective solid catalyst for this reaction compared to the other solid catalysts. The reaction was performed in the temperature range of 323.15–353.15 K and the catalyst loading in the range of 0.01–0.05 g cm⁻³. The initial reactant feed molar ratios of acetic acid to methanol were varied from 1:1 to 1:4. The influence of different parameters like temperature, catalyst loading, magnetic stirrer speed and average catalyst particle sizes on the conversion kinetics of acetic acid has been investigated. The maximum conversion of acetic acid was increased on increasing the feed molar ratio. From the experimental results, it was observed that the reaction is kinetically controlled rather than controlled by internal and external mass transfer. The experimental data were correlated with the pseudo homogeneous (PH), Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) models. The activity coefficients were calculated using UNIQUAC model to account for the non-ideal behaviour of the components. Of the different kinetic models; the LH, surface rate-determining model, fitted best with the experimental data.

As one of the smallest carbon cages, C₂₀ and its properties are very important. The nature of these compounds, and the development of procedures for their functionalisation for end-use in nano sized devices, have become important fields. Both Diels–Alder and 1,3-dipolar cycloaddition reactions are appropriate strategies to functionalise carbon nano tubes and C₆₀ fullerenes. In this regard a comparison between two pathways of interaction between C₂₀ and 2-furan nitrile oxide has been performed here to determine which reaction, Diels–Alder or 1,3-dipolar addition, is more suitable for functionalisation of C₂₀. The results showed that 1,3-dipolar cycloaddition is faster than the Diels–Alder reaction for the functionalisation of C₂₀ fullerene by 2-furan nitrile oxide.

Based on the unique characteristics of lanthanum ion and aza-crown ethers, the lanthanum complexes of two aza-crown ether (L1: 1,10-Dioxa-4,7,13,16-tetraazacyclo-octadecane and L2: 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane) were designed and synthesised. The interaction between these two complexes and DNA was measured by UV-Vis spectroscopy and gel electrophoresis. Moreover, a series of experiments of cleavage of pUC19 DNA were conducted to illustrate the acidity, time and concentration effects. The results indicated that the two metal complexes can accelerate the breakage of DNA from its supercoiled form (form I) to the nicked form (form II) at near-physiological conditions, and the optimum acidity of DNA catalytic cleavage is pH=6.5 and pH=7.0 for LaL1 and LaL2, respectively. Furthermore, the LaL1 exhibited better cleavage activity than LaL2 under the same conditions, thus supercoiled DNA was thoroughly cleaved to the nicked form under the appropriate conditions. The hydrolytic mechanism was verified by applying several oxygen-scavengers to the DNA catalytic cleavage.

The role of ternary Mg–Nb oxides on H₂ absorption and desorption properties of ball milled MgH₂ is discussed on the basis of a kinetic model involving the formation of reaction pathways of ternary oxide species with easier splitting of H₂ that facilitate the transport of H₂ into the solid structure. The kinetic model comprised three key steps: dissociation of molecular hydrogen into H atoms, diffusion of H into Mg to form MgH₂ during adsorption, and successive recombination of H–H on the surface of the additives during desorption to make a pathway for H₂ sorption in the structure. The presence of transition metal oxides remarkably increases the H₂ sorption rates of nano-structured MgH₂, as determined by a volumetric Sievert apparatus. The H₂ desorption rate increases with increasing temperature from 593 to 673 K and under the same conditions, the absorption rate decreases. The most promising results were obtained for Mg₃Nb₆O₁₁-doped MgH₂ nanoparticles. The MgH₂/Mg₃Nb₆O₁₁ system is completely dehydrogenated at 673 K under 0.1 MPa H₂ and the samples fully rehydrogenated at 613 K under 2.5 MPa H₂ pressure. Activation energies were obtained for MgH₂/Mg₃Nb₆O₁₁ mixture, and without additive (ball milled MgH₂), to be 88 and 127 kJ mol⁻¹, respectively, underlying the effect of the ternary additive.

Abstract

A highly efficient method for the synthesis of dihydropyrazole derivatives has been established by using CaO as a solid basic catalyst. It was found that a 77.1% yield of 3,5-diphenyl-4,5-dihydropyrazole-1-carboxamide was obtained at ratio of aminourea hydrochloride/chalcone of 1.2:1 after 3 h at a reaction temperature of 65°C. The reaction efficiency was greatly enhanced, with the 3,5-diphenyl-4,5-dihydropyrazole-1-carboxamide yield reaching 91.2%, when the commercial CaO was modified by benzyl bromide in a simple way. The effects of modification and reaction conditions on the yields as well as the mechanism were studied thoroughly. From the results of FTIR and thermogravity analysis characterisation, the modifier was shown to be bonded on the surface of CaO chemically, and almost no Ca(OH)₂ was formed during the modification process. The type of chalcone has a strong influence on the yield of dihydropyrazole derivative.