Observed Rate Constants Research Articles

Kinetics and Mechanism of Catalyzed Oxidation of L-Cysteine by Salen Schiff Base Complexes of Co(III), Fe (III) and Cr(III)

Kinetics of oxidation of L-cysteine by new series of substituted ONNO-donor salen-type Schiff base complexes of general formula [MIII(L)Cl] (M = Co, Fe, Cr; L = Schiff base ligand) have been studied in aqueous solutions. Measurements were run at constant temperature (25º C), constant ionic strength (0.20 M), and constant pH (7.0) under pseudo-first order conditions, in which the concentration of cysteine is around two orders of magnitude greater than that of metal complex. The observed rate constant was determined by following the change in absorbance of reaction mixture at a predetermined wavelength with time. Results show that the rate of oxidation depends on the type of metal center, with Co(III) complexes were found to have the highest rates due to higher reduction potential of Co(III). The oxidation rate was also found to depend on steric factor and the electron withdrawing / releasing ability of the ligand bound to the metal ion.

Redox Reaction between an Oxo-Bridged Diiron Complex and Hydrazine: A Kinetic Study

Objective: Mechanistic investigation of the reaction between the complex ion [FeIII 2(μ-O)(phen)4(H2O)2]4+ (1) (phen = 1,10-phenanthroline) and its hydrolytic derivatives [FeIII 2(μ-O)(phen)4(H2O)(OH)]3+ (1a) and [FeIII 2(μ-O)(phen)4(OH)2]2+ (1b) which coexist in rapid equilibria in the range pH = 3.00 to pH = 5.00 (pKa1 = 3.71 ± 0.03, pKa2 = 5.28 ± 0.07) with N2H5 + ion to produce [Fe(phen)3]2+ has been covered in this study. Methods: Rise in absorbance of the product with time was measured and rates have been determined using initial rate methods. Dependence of rate on pH and concentration of N2H5 + are studied in detail. Results: In the presence of excess phenanthroline, the reaction follows simple first-order exponential profile. Interestingly, in the absence of any added phenanthroline, the reaction becomes faster and the reaction profile changes to nearly linear with curvature near the end of the reaction. Slight autocatalytic nature has been observed only for the slower reactions. The observed rate constants obtained using initial rate method show first-order dependence on the concentration of N2H5 +. An increase in rate with increasing pH has been noticed. The plot of rate versus 1/[H+] shows saturation beginning near pH 4.6, suggesting that the mono-deprotonated form of the complex (i.e. 1a) is the active oxidant in this pH range. Conclusion: Presumably, the reaction becomes faster in the absence of added phenanthroline because ligand dissociation from the parent complex becomes facilitated when there is no external phenanthroline ligand present. This produces a coordinatively unsaturated Fe(III) species which is a faster oxidant.

Consolidated 3D Co3Mn-layered double hydroxide aerogel for photo-assisted peroxymonosulfate activation in metronidazole degradation

Novel 3D Co3Mn-layered double hydroxide aerogel (Co3Mn-LDH/rGA) was synthesized for photo-assisted peroxymonosulfate (PMS) activation to degrade aqueous metronidazole (MTZ). The observed rate constant of MTZ removal in the Co3Mn-LDH/rGA/PMS/Vis system exhibited 4.3, 4.1 and 3.6 times higher than that of Co3Mn-LDH/PMS, Co3Mn-LDH/rGO/PMS and Co3Mn-LDH/rGA/PMS processes, respectively. Therein, the efficiency of PMS activation can be simultaneously promoted by the 3D graphene aerogel as extensive electron transfer channels and visible light that stimulates the separation of electron-hole pairs in Co3Mn-LDH/rGA. The photo-generated electrons and transformation of Co2+/Mn3+ to Co3+/Mn4+ on the surface of Co3Mn-LDH/rGA co-contributed to PMS activation in producing •SO4−, •OH and 1O2 for MTZ degradation. Moreover, the 3D Co3Mn-LDH/rGA was consolidated in cyclic equipment to achieve an outstanding reusability, in which MTZ could be efficiently and continuously removed for up to 10 cycles with acceptable metal leaching. This work not only takes deep insights into the reaction mechanism of visible light assisted PMS activation by 3D Co3Mn-LDH/rGA, but also provides promising method for the continuous degradation of pollutants.

The extent of sludge solubilization allows to estimate the efficacy of ozonation for removal of polycyclic aromatic hydrocarbons (PAHs) in municipal sewage sludge

Ozone treatment has been proven as an effective technology for removing PAHs in municipal sewage sludge. However, given the complex interaction of PAHs with sludge extracellular polymeric substances (EPS), effective sludge disintegration is required to make PAHs more accessible to oxidants, which also affects PAHs partitioning. Here, we investigated two treatment systems, namely ozonation (ozone) and sequential hydrodynamic cavitation and ozonation (HC+ozone), under varied conditions, to determine whether the extent of sludge disintegration can estimate a measurable removal of PAHs. The results showed that both the PAHs removal efficiency and kinetics highly depended on pH and ozone dose. Although no significant removal of PAHs occurred during HC treatment, the observed rate constants for Σ16 PAHs removal in HC+ozone system were 1.2–1.4-fold higher than those obtained in ozone system under the same condition. The accelerated PAHs removal could be attributed to the reduced flocs size (80% smaller) and increased soluble EPS concentration (5-fold higher) with HC pretreatment. Lastly, a positive correlation was noted between the PAHs removal efficiency and soluble EPS concentration (R2 = 0.85), indicating the important role of dissolved and colloidal matter (DCM) for PAHs partitioning between aqueous and particulate phases.

Heme-Thiolate Perturbation in Cystathionine β-Synthase by Mercury Compounds.

Cystathionine β-synthase (CBS) is an enzyme involved in sulfur metabolism that catalyzes the pyridoxal phosphate-dependent condensation of homocysteine with serine or cysteine to form cystathionine and water or hydrogen sulfide (H2S), respectively. CBS possesses a b-type heme coordinated by histidine and cysteine. Fe(III)-CBS is inert toward exogenous ligands, while Fe(II)-CBS is reactive. Both Fe(III)- and Fe(II)-CBS are sensitive to mercury compounds. In this study, we describe the kinetics of the reactions with mercuric chloride (HgCl2) and p-chloromercuribenzoic acid. These reactions were multiphasic and resulted in five-coordinate CBS lacking thiolate ligation, with six-coordinate species as intermediates. Computational QM/MM studies supported the feasibility of formation of species in which the thiolate is proximal to both the iron ion and the mercury compound. The reactions of Fe(II)-CBS were faster than those of Fe(III)-CBS. The observed rate constants of the first phase increased hyperbolically with concentration of the mercury compounds, with limiting values of 0.3–0.4 s–1 for Fe(III)-CBS and 40 ± 4 s–1 for Fe(II)-CBS. The data were interpreted in terms of alternative models of conformational selection or induced fit. Exposure of Fe(III)-CBS to HgCl2 led to heme release and activity loss. Our study reveals the complexity of the interactions between mercury compounds and CBS.

Discriminating between Concerted and Sequential Allosteric Mechanisms by Comparing Equilibrium and Kinetic Hill Coefficients.

Hill coefficients, which provide a measure of cooperativity in ligand binding, can be determined for equilibrium (or steady-state) data by measuring fractional saturation (or initial reaction velocities) as a function of ligand concentration. Hill coefficients can also be determined for transient kinetic data from plots of the observed rate constant of the ligand-promoted conformational change as a function of ligand concentration. Here, it is shown that the ratio of the values of these two Hill coefficients can provide insight into the allosteric mechanism. Cases when the value of the kinetic Hill coefficient is equal to or greater than the value of the equilibrium coefficient indicate concerted transitions whereas ratios smaller than one indicate a sequential transition. The derivations in this work are for symmetric dimers but are expected to have general applicability for homo-oligomers.

Fabrication of Magnetic Nanoparticles Integrated Carbon Matrix from Chrysopogon zizanioides Roots: Strong Dye Adsorption and Persulphate Assisted Photodegradation

Activated carbon with magnetic nanoparticles was prepared from the roots of Chrysopogon zizanioides by impregnating the biomass with Fe3+ followed by carbonization in a muffle furnace. To assist the carbonization process, Zn2+ ions were also taken along with ferric ions during impregnation. The pHZPC, density of surface functional groups, surface acidity constants and advanced characterizations like FT-IR, N2 adsorption-desorption isotherms, SEM with EDAX, DLS, XRD and VSM analysis were carried out. The adsorption and natural sunlight induced photooxidation of two cationic dyes, namely, methylene blue and rhodamine B in presence of potassium persulphate were studied by varying parameters like time, dye concentration, pH of the dye solution, amount of magnetic activated carbon and concentration of persulphate ions. The adsorption capacities of methylene blue and rhodamine B were found to be 5.97 and 0.96 mg/g, respectively. Photocatalytic oxidation of the dyes was very rapid in the presence of persulphate with the observed rate constants being 5.3 × 10–3 min–1 for methylene blue and 13.0 × 10–3 min–1 for rhodamine B. The magnetic activated carbon is found to be effective even after five successive degradation experiments.

The recovery of sulfuric acid from spent piranha solution over a dimensionally stable anode (DSA) Ti-RuO2 electrode

Piranha solution is a highly acidic mixture of sulfuric acid and hydrogen peroxide. The present study aimed at developing a dimensionally stable anode (DSA), made of titanium metal foil coated with Ruthenium Dioxide (RuO2), for the electrochemical oxidation of hydrogen peroxide in the presence of strong sulfuric acid under ambient conditions. Results showed that hydrogen peroxide in the piranha solution was fully degraded in 5 h under a constant current of 2 A (or current density of 0.32 A-cm-2). The oxidation kinetics of hydrogen peroxide followed the Langmuir-Hinshelwood model. The observed rate constant was a function of applied current. The initial current efficiency was 17.5% at 0.5 A (or 0.08 A-cm-2) and slightly decreased to about 13.5% at applied current between 1.3 and 1.5 A (or current density of 0.208 and 0.24 A-cm-2). Results showed the capability and feasibility of the electrochemical oxidation process for the recovery of sulfuric acid from the spent piranha solution in semiconductor industrial installations or general laboratories.

Selective Hydrolysis of Terminal Glycosidic Bond in α-1-Acid Glycoprotein Promoted by Keggin and Wells-Dawson Type Heteropolyacids.

The reactivity of a range of Keggin and Wells-Dawson type heteropolyacids (HPAs): H3 PW12 O40 H4 SiW12 O40 , H3 PMo12 O40 , K6 P2 W18 O62 , and NaH2 W12 O4 , towards the heavily glycosylated α-1-acid glycoprotein (AGP) is reported. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) show that after incubation of the protein with HPAs at 80 °C and pH 2.8 complete hydrolysis of terminal glycosidic bond has been achieved, resulting in the removal of sialic acids with no observed destruction of the protein core or the residual glycan chains. The 1 H NMR spectroscopy confirmed that the released sialic acids preserve intact structure upon their excision from the protein, which makes the reported method suitable for the analysis of sialic acid modifications which play an important role in numerous biological processes. The presence of other sugars was not detected by 1 H NMR and HPAEC-PAD, suggesting that HPAs hydrolyze only the terminal glycosidic bond in the glycoprotein, resulting in the selective release of sialic acid from AGP. The kinetic results have shown that under equal temperature and pH conditions, the hydrolysis of the terminal glucosidic bond occurred faster in the presence of HPAs compared to conventional mineral acids. The observed rate constants were in the range 6,7×10-2 -11,9×10-2 min-1 and the complete and selective excision of sialic acids could be achieved within 60 min of incubation. The Trp fluorescence and CD spectroscopy show that non-covalent interaction between HPA and protein takes place in solution which could lead to stabilization of the sialosyl cation that is formed during the glycosidic bond hydrolysis by anionic HPA cluster.

Formation of carbonaceous and nitrogenous iodinated disinfection byproducts from biofilm extracellular polymeric substances by the oxidation of iodide-containing waters with lead dioxide

Lead dioxide (PbO2) is an important form of lead mineral scales in drinking water pipes. Iodide (I−) widely presents in source waters and can be thermodynamically oxidized by PbO2 to the reactive iodine species (I2/HOI). Biofilm extracellular polymeric substances (EPS) are nonnegligible precursors of disinfection byproducts (DBPs). The aim was to study the oxidation of I− by PbO2 and formation of iodinated DBPs (I-DBPs) from EPS. At a high molar ratio of PbO2 to I− (> 100), the observed rate constants of I− oxidation decreased as pH increased from 6.0 to 9.0 with an H+ dependence of 0.79, and the rate constant (k) was 1.6 × 1011 M−2.79 s−1. Most of formed I2/HOI (> 92%) was transformed to organic iodine in the presence of EPS. EPS had a lower formation potential (FP) of carbonaceous I-DBPs (C-IDBPs), while a higher that of nitrogenous I-DBPs (N-IDBPs) than HA, resulting in a higher Chinese Hamster Ovary cell cytotoxicity. Generally, the formation of I-DBPs decreased with the increase of pH due to the reduction of surface positive charge and electrochemical driving force. PbO2 dose and I− concentration also had a significant effect on the I-DBPs formation. EPS proteins had a higher FP of both C- and N-IDBPs than polysaccharides on account of more electrophilic sites and higher nitrogen content. In proteins, aspartic acid was the main contributor to triiodomethane and iodoacetic acids formation, whereas aspartic acid, asparagine and tyrosine were the major precursors of diiodoacetonitrile and diiodoacetamide. The study helps to improve the control strategy of I-DBPs when biofilm outbreaks in lead-containing water pipes.

Controlling the reactivity of [Pd(II)(N^N^N)Cl]+ complexes using 2,6-bis(pyrazol-2-yl)pyridine ligands for biological application: Substitution reactivity, CT-DNA interactions and in vitro cytotoxicity study

Four [(N^N^N)Pd(II)Cl]+ complexes [chloride-(2,2′:6′,2′′-terpyridine)Pd(II)]Cl (PdL1), [chlorido(2,6-bis(N-pyrazol-2-yl)pyridine)Pd(II)]Cl (PdL2), [chlorido(2,6-bis(3,5-dimethyl-N-pyrazol-2-yl)pyridine)Pd(II)]Cl (PdL3) and [chlorido(2,6-bis(3,5-dimethyl-N-pyrazol-2-ylmethyl)pyridine)Pd(II)]BF4 (PdL4) were synthesized and characterized. The rates of substitution of these Pd(II) complexes with thiourea nucleophiles viz; thiourea (Tu), N,N′-dimethylthiourea (Dmtu) and N,N,N′,N′-tetramethylthiourea (Tmtu) was investigated under pseudo first-order conditions as a function of nucleophile concentration [Nu] and temperature using the stopped-flow technique. The observed rate constants vary linearly with [Nu]; kobs = k2[Nu] and decreased in the order: PdL1 > PdL2 > PdL3 ≫ PdL4. The lower π-acceptability of the cis-coordinated N-pyrazol-2-yl groups (which coordinates via pyrazollic-N π-donor atoms) of the PdL2–4 significantly decelerates the reactivity relative to PdL1. Furthermore, the six-membered chelates having methylene bridge in PdL4 do not allow π-extension in the ligand and introduces steric hindrance further lowering the reactivity. Trends in DFT calculated data supported the observed reactivity trend. Spectrophotometric titration data of complexes with calf thymus DNA (CT-DNA) and viscosity measurements of the resultant mixtures suggested that associative interactions occur between the complexes and CT-DNA, likely through groove binding with high binding constants (Kb = 104 M−1). In vitro MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] cytotoxic activity data showed that PdL1 was the most potent complex against MCF7 breast cancer cells; its IC50 value is lower than that of cisplatin. The results demonstrate how modification of a spectator ligand can be used to slow down the reactivity of Pd(II) complexes. This is of special importance in controlling drug toxicity in both pharmaceutical and biomedical applications.

Simultaneous Quantification of Protein Binding Kinetics in Whole Cells with Surface Plasmon Resonance Imaging and Edge Deformation Tracking.

Most drugs work by binding to receptors on the cell surface. Quantification of binding kinetics between drug and membrane protein is an essential step in drug discovery. Current methods for measuring binding kinetics involve extracting the membrane protein and labeling, and both have issues. Surface plasmon resonance (SPR) imaging has been demonstrated for quantification of protein binding to cells with single-cell resolution, but it only senses the bottom of the cell and the signal diminishes with the molecule size. We have discovered that ligand binding to the cell surface is accompanied by a small cell membrane deformation, which can be used to measure the binding kinetics by tracking the cell edge deformation. Here, we report the first integration of SPR imaging and cell edge tracking methods in a single device, and we use lectin interaction as a model system to demonstrate the capability of the device. The integration enables the simultaneous collection of complementary information provided by both methods. Edge tracking provides the advantage of small molecule binding detection capability, while the SPR signal scales with the ligand mass and can quantify membrane protein density. The kinetic constants from the two methods were cross-validated and found to be in agreement at the single-cell level. The variation of observed rate constant between the two methods is about 0.009 s−1, which is about the same level as the cell-to-cell variations. This result confirms that both methods can be used to measure whole-cell binding kinetics, and the integration improves the reliability and capability of the measurement.

Dechlorination of 2,4‐dichlorophenoxyacetic acid using biochar‐supported nano‐palladium/iron: Preparation, characterization, and influencing factors

In the present study, peanut shell, a green waste raw material, was used to prepare biochar (BC) and to obtain BC‐supported nano‐palladium/iron (BC‐nPd/Fe) composites for removing 2,4‐dichlorophenoxyacetic acid (2,4‐D) from water. Characterization analysis demonstrated that nPd/Fe particles were well dispersed on the BC surface with weakened magnetic properties. The average particle diameter and specific surface area of nPd/Fe were 101.3 nm and 6.7 m2 g−1, whereas the corresponding values of the BC‐nPd/Fe materials were 88.8 nm and 14.8 m2 g−1, respectively. Several factors were found to influence the dechlorination of 2,4‐D, including the weight ratio of BC to Fe, Pd loading ratio, initial solution pH, 2,4‐D concentration, and reaction temperature. Dechlorination results indicated that the 2,4‐D removal and phenoxyacetic acid (PA) generation rates were 44.1% and 20.1%, respectively, in the nPd/Fe system, and 100.0% and 92.1%, respectively, in the BC‐nPd/Fe system. The dechlorination of 2,4‐D was well described by the pseudo‐first‐order kinetic model (R2 > 0.97), and the observed rate constants kobs were 0.0042 min (nPd/Fe) and 0.0578 min (BC‐nPd/Fe), respectively. The reaction mechanism indicated that the dechlorination hydrogenation was the main process to remove 2,4‐D from water in the BC‐nPd/Fe system. In addition, BC inhibited the formation of a passivation layer on the particle surface during the reaction, thus maintaining the high reactivity of BC‐nPd/Fe. The easy preparation technique, high 2,4‐D dechlorination capacity, and mild reaction conditions suggest that BC‐nPd/Fe may be a promising alternative composite to remove 2,4‐D from water.

Benzophenone-3 degradation via UV/H2O2 and UV/persulfate reactions

The degradation of benzophenone-3 (BP3) in water via the UV/H2O2 and UV/persulfate (UV/PS) reactions was investigated. The degradation of BP3 exhibited pseudo-first-order kinetics in both reactions. The degradation efficiency of BP3 was higher in the UV/PS reaction than in the UV/H2O2 reaction. In both reactions, the observed rate constants (kobs) of BP3 degradation were highest at pH 6 and increased linearly with increasing dosage of H2O2 and persulfate. The second-order rate constants of BP3 with •OH (k•OH_BP3) and •SO4– (k•SO4−_BP3) were determined to be 1.09 (± 0.05) × 1010 and 1.67 (± 0.04) × 109 M-1 s-1, respectively. The kobs values of BP3 were affected by water components such as HCO3−, NO3−, Cl−, and Br− ions, as well as humic acid. Based on the identified transformation products (TPs), the degradation pathway of BP3 during both reactions was a hydroxylation reaction. The inhibition of bioluminescence in Vibrio fischeri due to BP3 and its TPs decreased more quickly in the UV/PS reaction than in the UV/H2O2 reaction. The results suggest that the UV/PS process is a better alternative to the UV/H2O2 process for removing BP3 and its toxicity in water.

Accelerated oxidation of microcystin-LR by Fe(II)-tetrapolyphosphate/oxygen in the presence of magnesium and calcium ions

Fe(II)-tetrapolyphosphate complexes are known to activate molecular oxygen (Fe(II)-TPP/O2) to produce reactive oxidants (most likely, Fe(IV)-TPP complexes) that are capable of degrading refractory organic contaminants in water. This study found that magnesium and calcium ions (Mg2+ and Ca2+) accelerate the degradation of micfrocystin-LR (MC-LR), the most toxic and abundant cyanotoxin, by the Fe(II)-TPP/O2 system. The addition of Mg2+ and Ca2+ increased the observed rate constant of MC-LR degradation by up to 4.3 and 14.8 folds, respectively. Mg2+ and Ca2+ accelerated the MC-LR degradation in the entire pH range, except for the alkaline region with pH > ca. 10. The addition of Mg2+ and Ca2+ also reshaped the pH-dependency of the MC-LR degradation, greatly increasing the rate of MC-LR degradation at neutral pH. It was found that Mg2+ and Ca2+ accelerate the reaction of Fe(II)-TPP complexes with oxygen, resulting in faster production of reactive oxidants. The findings from cyclic voltammetry and potentiometric titration suggest that Mg2+ and Ca2+ form ternary complexes with Fe(II)-TPP, which exhibit higher reactivity with oxygen. Due to the effects of Mg2+ and Ca2+, the rate of MC-LR degradation by the Fe(II)-TPP/O2 system was even higher in natural water than in deionized water.

Electrochemical degradation of three phthalate esters in synthetic wastewater by using a Ce-doped Ti/PbO2 electrode

A Ce-doped Ti/PbO2 electrode was prepared in a deposition solution containing Ce3+ and Pb2+ ions by electrodeposition, and the surface morphology, crystal structure and elemental states were characterized by SEM, XRD and XPS. The electrode was used to investigate the simultaneous degradation of three phthalate esters (PAEs), i.e., dimethyl phthalate (DMP), diethyl phthalate (DEP) and dibutyl phthalate (DBP) in synthetic wastewaters. The results showed that the electrode exhibited excellent electrocatalytic activity and good reusability and stability, and the removal efficiencies of 5 mg L−1 DBP, DMP and DEP in 0.05 M Na2SO4 (pH 7) reached 98.2%, 95.8% and 81.1% at current density of 25 mA cm−2 after 10 h degradation, respectively. The degradation processes followed pseudo first-order kinetic model very well, and the observed rate constants of DBP, DEP and DMP were 0.42, 0.40 and 0.29 h−1, respectively. The energy consumption in three PAEs degradation was also assessed. The main degradation products of the three PAEs were identified by using liquid chromatography-tandem mass spectrometry, and the possible degradation pathways mainly included dealkylation, hydroxyl addition, decarboxylation and benzene ring cleavage. This work is a promising candidate for efficient treatment of multiple PAEs in wastewater and protection of the aquatic ecological environment.

Removal of 17β-Estradiol Using Persulfate Synergistically Activated Using Heat and Ultraviolet Light

Among all female sex hormones, 17β-estradiol (E2) has been most often detected in discharge water from animal farms. The objectives of this study were to evaluate the performance of UV/heat-activated persulfate to degrade E2 and the technical feasibility to use this system for treating real wastewater. As an individual persulfate (PS) homogeneous activator, UV-activated PS removed E2 better than using low heat (40 °C) and solar irradiation to activate PS. When both UV and heat (25–65 °C) were used to activate PS, the Arrhenius equation represented well the observed rate constant, with activation energy of 118.07 kJ mol−1. Inorganic ion concentrations increased the degradation rate. These ions included Cl− (3500 mg L−1), which increased rates by 18.1%, HCO3− (250 mg L−1) by 4.6%, and NO3− (5 mg L−1) by 7.9%. However, lesser impacts on degradation kinetics were observed at higher concentrations for all constituents due to SO4·− scavenging by the formed radicals such as Cl2·−, Cl·, HCO3·, CO3·−, and NO3·. The E2 degradation observed rate constant (kobs) was highest at pH 3. Although both synthetic wastewater and real wastewater showed inhibitory effects due to UV blocking from turbidity and the existence of the –COOH and –OH functional groups that acted as radical scavengers, E2 degradation was still observed. The overall results provided proof-of-concept that UV/heat-activated PS can be applied to treat E2 in wastewater containing a high organic content and can minimize the chemical and operating costs, as solar irradiation provides the heating source.

Chercher de l'eau: The switching mechanism of the rotary switch ethyl-2-(2-(quinolin-8-yl)hydrazono)-2-(pyridin-2-yl)acetate

The E/Z switching mechanism of the rotary switch ethyl-2-(2-(quinolin-8-yl)hydrazono)-2-(pyridin-2-yl)acetate was studied by NMR, UV–Vis, and ultrafast spectroscopy and modeled by advanced quantum-chemical calculations. Three possible mechanisms were considered theoretically: out-of-plane rotation, in-plane inversion and proton transfer. Neither of them correctly describes the experimental data by using implicit solvation. Taking into account the existence of water in the used solvents, which influence the measured rate constants, an explicit solvation was attempted in the quantum-chemical calculations. According to a simplified model, the water molecules form a wire, being able to transfer the proton from Z to E form of the switch. This leads to substantially lower transition states and corresponds to the experimentally observed rate constants. This information shines new light on the mechanism of isomerization in the rotary switches and on the understanding of the mechanism of hydrazone tautomerism as a whole.

Conversion of fatty acid methyl ester to epoxy plasticizer by auto-catalyzed in situ formation of performic acid: Kinetic modeling and application of the model

Fatty acid methyl esters (FAMEs) can be converted to epoxy plasticizer by in situ auto-catalyzed formation of performic acid (PFA) in a bi-phase reaction system. However, mass transfer effect could be neglected in a well-stirred reaction system. A pseudo-homogenous kinetic model was thus developed to describe the kinetics of FAME epoxidation. The decomposition of PFA could not be neglected. All of the observed rate constants could be well correlated with temperature and concentration ratio of formic acid to hydrogen peroxide (r = CFA/CHP), indicating that increasing temperature and H+ concentration could promote the reactions. The determined activation energies for PFA synthesis, hydrolysis, decomposition, epoxidation of double bonds and epoxide group ring-opening reactions were 56.4, 54.9, 66.9, 57.1 and 45.0 kJ/mol, respectively. Model predication suggested that the reactions should be performed at moderate temperature (323–333 K) in order to increase the consumption selectivity of PFA and hydrogen peroxide and achieve satisfying reaction rates of epoxidation. The results of this work also have provided a strong support to the mechanism interpretation that the protonation of the neutral PFA is the first step for epoxidation of double bond of FAMEs followed by a similar process to that proposed by “butterfly mechanism”.

Kinetic modelling of acid-catalyzed liquid-phase dehydration of bio-based 2, 3-butanediol considering a newly identified by-product and an updated reaction network

Study on the acid-catalyzed dehydration of bio-based platform chemical 2, 3-butanediol (BDO) to produce methyl ethyl ketone (MEK) has a relatively long history. However, the reaction networks of the system have not been completely elucidated yet. In this work, a new by-product formed during acid-catalyzed liquid-phase dehydration of BDO was detected, which was wrongly identified by the GC–MS NIST database or previous research works. The new compound was finally identified as 2-isopropyl-4, 5-dimethyl-1, 3-dioxolane (IDMD), formed by acetalization between 2-methyl propanal (MPA) and BDO under the catalysis of Bronsted acids. MPA was an intermediate product of acid-catalyzed dehydration of BDO, but its life was short primarily due to the formation of IDMD. Prepared IDMD with high purity was further characterized by FTIR, MS and NMR spectroscopy, providing data which have not been previously reported in literatures. The reaction mechanisms of acid-catalyzed dehydration of BDO were thus updated with consideration of the new side reaction. New kinetic models were further developed based on the updated reaction network. Observed kinetic constants were fitted with experimental data, and the observed rate constants were correlated with reaction temperature and acid concentration by an extended Arrhenius equation. The results revealed that the acetalization between BDO and MPA had much higher observed activation energy (191.5 kJ/mol) than the ketalization between BDO and MEK (59.7 kJ/mol). The observed reaction equilibrium constants was also affected by reaction temperature as well as acid concentration. Increasing reaction temperature was beneficial for the formation of ketalization and acetalization products, while increasing acid concentration seems to more significantly accelerated the hydrolysis of the ketalization and acetalization products.