Biodiesel production via esterification of oleic acid catalyzed by MnO2@Mn(btc) as a novel and heterogeneous catalyst

The main objective of this study is to develop efficient and environmentally benign heterogeneous catalysts for biodiesel production. For this purpose, a heterogeneous MnO₂@Mn(btc) catalyst was prepared by the solvothermal method, and the prepared catalyst was tested for the esterification of oleic acid. Various techniques such as X‐ray diffraction, scanning and transmission electron microscopy, Brunauer–Emmett–Teller (BET) method, infrared spectroscopy, thermogravimetry, and NH₃‐TPD (temperature programmed desorption) analysis were employed for the characterization of the solid catalyst. The solid catalyst with MnO₂@Mn(btc) loading of 15% showed high catalytic activity and long durability in the esterification of oleic acid, in which the fatty acid methyl ester yield reached 98% consecutively for at least five cycles under mild conditions.     

Semi-analytical modeling of non-premixed counterflow combustion of metal dust

In this study, a semi-analytical model is developed for non-premixed combustion of metal dusts in counterflow configuration. Combustion domain is divided into three separate zones, each of which possesses corresponding mass and energy conservation equations as well as boundary and jump conditions. Metal dust, assumed to be aluminum, undergoes an Arrhenius-type reaction with oxidizer, when it is heated enough to reach the ignition temperature. Dimensionless forms of conservation equations are derived and utilized to elucidate the combustion characteristics. The effects of oxidizer Lewis number and fuel mass concentration on the flame position and temperature are discussed thoroughly. In addition, temperature distribution of the whole domain is calculated by numerically solving the system of partial differential equations. In order to track particles through combustion domain, Lagrangian equations of motion are solved either mathematically or numerically, considering thermophoretic, weight, buoyancy and drag forces. The effects of thermophoretic force on the particle path are investigated, and the deviation of particle from carrier neutral gas direction is obtained. The results showed a great agreement with the data reported in the literature highlighting the fact that the presented model is an efficient one to accurately model the non-premixed counterflow combustion of metal dust.

     

Theoretical investigation of dielectric corona pre-ionization TEA nitrogen laser based on transmission line method

This paper models the dielectric corona pre-ionization, capacitor transfer type of flat-plane transmission line traveling wave transverse excited atmospheric pressure nitrogen laser by a non-linear lumped RLC electric circuit. The flat-plane transmission line and the pre-ionizer dielectric are modeled by a lumped linear RLC and time-dependent non-linear RC circuit, respectively. The main discharge region is considered as a time-dependent non-linear RLC circuit where its resistance value is also depends on the radiated pre-ionization ultra violet (UV) intensity. The UV radiation is radiated by the resistance due to the surface plasma on the pre-ionizer dielectric. The theoretical predictions are in a very good agreement with the experimental observations. The electric circuit equations (including the ionization rate equations), the equations of laser levels population densities and propagation equation of laser intensities, are solved numerically. As a result, the effects of pre-ionizer dielectric parameters on the electrical behavior and output laser intensity are obtained.     

Computational Investigation of Solvent Effect on Stability,Electronic and Thermochemical Properties of Iron-Substituted Borirene and Boryl Isomers

This study investigates solvent effect on several electronic structure features, i.e. structural stability, orbital energies, HOMO-LUMO gaps and hardness of an iron aminoborirene complex [(η⁵-C₅H₅)(OC)₂Fe{μ- BN(SiH₃)₂C=C}Ph] (closed-isomer) and its isomer the boryl complex [(η⁵-C₅H₅)(OC)₂FeBN(SiH₃)₂C≡CPh] (open-isomer) through polarizable continuum model. Results revealed that the closed isomer is less stable than the open isomer, in solvent. Further, influence of the solvent on the frontier orbitals energies, HOMOLUMO gap, electrophilicity and chemical potential energies of the isomers was studied. Thermochemical analysis was conducted to study closed-open equilibrium and thermochemical parameters (ΔG and ΔH) were computed.     

Organocatalytic approach toward the green one-pot synthesis of novel benzo[<Emphasis Type="Italic">f</Emphasis>]chromenes and 12<Emphasis Type="Italic">H</Emphasis>-benzo[5,6]chromeno[2,3-<Emphasis Type="Italic">b</Emphasis>]pyridines

An efficient tandem reaction approach is described to prepare novel benzo[f]chromenes from 2,3-dihydroxynaphthalene, malononitrile and aldehydes using 10 mol% guanidine hydrochloride as the catalyst under solvent-free conditions. The method was also extended to the preparation of novel 12H-benzo[5,6]chromeno[2,3-b]pyridines from 2-aminoprop-1-ene-1,1,3-tricarbonitrile instead of malononitrile under the same reaction conditions. The described one-pot three-component reaction is characterized by short reaction times, high-product yield, mild reaction conditions, simple workup procedure, and simple purification.     

Establishment of Aromatic Pairs at the Surface of Chondroitinase ABC I: the Effect on Activity and Stability

Removal of chondroitin sulfate glycosaminoglycan (GAG) chains with chondroitinase ABC I (chABC I) in CNS injury models promotes both saxon regeneration and plasticity. It has been suggested that direct interaction between an aromatic pair appears to contribute about ??1.3 kcal/mol to the stability of a folded protein, so introducing an aromatic pair by point mutation might increase the enzyme activity and thermal stability as in the case of mesophilic xylanase, although using this approach destabilized T4 lysozyme. In this study, we used site-directed mutagenesis to investigate the effect of new aromatic pairs on activity and stability of chABC I. We replaced Ile295, Ser581, and Gly730 adjacent to pre-existing aromatic residues with Tyr to obtain new aromatic pairs, i.e., Tyr295/His372, Tyr576/Tyr581, and Tyr623/Tyr730. Results showed that K_m values of S581Y and G730Y variants decreased relative to wild-type enzyme while their catalytic efficiency (k_cat/K_m) increased but I295Y variant was inactive. Also, long-term and thermal stability of the active mutants was decreased. Fluorescence and circular dichroism studies showed that these mutations resulted in a more flexible enzyme structures: a finding which was confirmed by thermal and limited proteolytic studies. In conclusion, the activity of chABC I can be improved by introducing appropriate aromatic pairs at the enzyme surface. This approach did not provide any promising results regarding the enzyme stability.     

Relationship between cation–π and anion–π interactions: individual binding energies in the π–Mz+–π–X−–π system

The mutual influence of cation–π and anion–π interactions in the π–M^z+–π–X^?–π system (M^z+ = Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, Ca²⁺ and X^? = F^?, Cl^?) has been studied by quantum mechanical calculations. Both geometric parameters and energy data reveal that cation–π and anion–π interactions enhance each other in the π–M^z+–π–X^?–π system. Individual binding energies (E_ion···π) have been estimated in the quintuplet system using a simple new method from electron charge densities calculated at the bond critical points (BCPs) of the ion···π interaction by the atoms in molecules (AIM) method at the M062X/6-31+G(d) level of theory. With respect to the obtained individual binding energies, the strength of an ion···π interaction depends on the cooperative effects of other components.     

Electrocatalytic determination of sumatriptan on the surface of carbon-paste electrode modified with a composite of cobalt/Schiff-base complex and carbon nanotube

The electrochemical oxidation of sumatriptan on the surface of carbon paste electrode modified with multi-walled carbon nanotube and cobalt methyl-salophen complex is studied by using cyclic voltammetry and polarization studies. The results indicate that the drug is irreversibly oxidized in a one electron oxidation mechanism. It was found that the peak potential shifted negatively with increasing pH, confirms that H(+) participate in the oxidation process. The electrode is shown to be very effective for the detection of sumatriptan in the presence of other biological reductant compounds. The prepared modified electrode exhibits a very good resolution between the voltammetric peaks of sumatriptan, ascorbic acid and uric acid, which makes it suitable for the simultaneous detection of sumatriptan in the presence of these compounds in clinical and pharmaceutical preparations. It can be concluded that multi-walled carbon nanotube and Shiff base complex have synergic effect on electroacatalytic oxidation of sumatriptan. A linear range of 1-1000μM and detection limit of 0.3μM was obtained for sumatriptan from DPV measurements using this electrode in 0.1M acetate buffered solution of pH 5.0. The electrode has been applied successfully for the determination of sumatriptan in synthetic serum and commercial tablets.     

Immobilization of flavine adenine dinucleotide onto nickel oxide nanostructures modified glassy carbon electrode: fabrication of highly sensitive persulfate sensor

A simple method was used to fabricate flavin adenine dinucleotide (FAD)/NiOx nanocomposite on the surface of glassy carbon (GC) electrode. Cyclic voltammetry technique was applied for deposition nickel oxide nanostructures onto GC surface. Owing to its high biocompatibility and large surface area of nickel oxide nanomaterials with immersing the GC/NiOx-modified electrode into FAD solution for a short period of time, 10–140 s, a stable thin layer of the FAD molecules immobilized onto electrode surface. The FAD/NiOx films exhibited a pair of well-defined, stable, and nearly reversible CV peaks at wide pH range (2–10). The formal potential of adsorbed FAD onto nickel oxide nanoparticles film, E ^o′ vs. Ag/AgCl reference electrode is −0.44 V in pH 7 buffer solutions was similar to dissolved FAD and changed linearly with a slope of 58.6 mV/pH in the pH range 2–10. The surface coverage and heterogeneous electron transfer rate constant (k _s ) of FAD immobilized on NiOx film glassy carbon electrode are 4.66 × 10⁻¹¹ mol cm⁻² and 63 ± 0.1 s⁻¹, indicating the high loading ability of the nickel oxide nanoparticles and great facilitation of the electron transfer between FAD and nickel oxide nanoparticles. FAD/NiOx nanocomposite-modified GC electrode shows excellent electrocatalytic activity toward S₂O₈²⁻ reduction at reduced overpotential. Furthermore, rotated modified electrode illustrates good analytical performance for amperometric detection of S₂O₈²⁻. Under optimized condition, the concentration calibration range, detection limit, and sensitivity were 3 μM–1.5 mM, 0.38 μM and 16.6 nA/μM, respectively.     

Structure and electronic properties of amino acid ionic liquids

The interactions between eight amino acid based anions and four imidazolium-based cations have been investigated by density functional theory. The electronic and structural properties of the resulting amino acid ionic liquids (AAILs) have been unveiled by means of the atoms in molecules framework. The calculated interaction energy was found to increase in magnitude with decreasing alkyl chain length at imidazolium ring. Moreover, AAILs composed of an amino acid with some functional group such as aromatic ring had decreased interaction energy. Finally, several correlative relationships between glass transition temperature and interaction energy as well as density at bond critical point have been checked for 1-ethyl-3-methylimidazolium based ILs. Although the obtained correlations do not show excellent fits, a preliminary estimation of the glass transition temperature of different AAILs can be achieved by use of their electronic properties.