Recent Advances in Photocatalytic Oxidation of Methane to Methanol

Methane is one of the promising alternatives to non-renewable petroleum resources since it can be transformed into added-value hydrocarbon feedstocks through suitable reactions. The conversion of methane to methanol with a higher chemical value has recently attracted much attention. The selective oxidation of methane to methanol is often considered a “holy grail” reaction in catalysis. However, methanol production through the thermal catalytic process is thermodynamically and economically unfavorable due to its high energy consumption, low catalyst stability, and complex reactor maintenance. Photocatalytic technology offers great potential to carry out unfavorable reactions under mild conditions. Many in-depth studies have been carried out on the photocatalytic conversion of methane to methanol. This review will comprehensively provide recent progress in the photocatalytic oxidation of methane to methanol based on materials and engineering perspectives. Several aspects are considered, such as the type of semiconductor-based photocatalyst (tungsten, titania, zinc, etc.), structure modification of photocatalyst (doping, heterojunction, surface modification, crystal facet re-arrangement, and electron scavenger), factors affecting the reaction process (physiochemical characteristic of photocatalyst, operational condition, and reactor configuration), and briefly proposed reaction mechanism. Analysis of existing challenges and recommendations for the future development of photocatalytic technology for methane to methanol conversion is also highlighted.     

Fabrication of an amperometric tyramine biosensor based on immobilization of tyramine oxidase on AgNPs/l-Cys-modified Au electrode

An amperometric tyramine biosensor was constructed based on covalent immobilization of black gram tyramine oxidase onto citric acid-capped silver nanoparticles bound to surface of Au electrode through cysteine self-assembled monolayer. The enzyme electrode was characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, and cyclic voltametry. The biosensor showed optimum response within 8 s, when polarized at 0.25 V, pH 8.5, and 35 °C, with linearity from 0.017 to 0.25 mM and a detection limit of 0.01 mM. The biosensor was employed for measurement of tyramine in beer and sauce. The mean analytical recovery of added tyramine in beer at 0.36 and 0.72 mM were 97.2?±?2.7 and 95.8?±?4.1 %, respectively, and within and between batches coefficients of variation were 0.33–0.38 and 0.34–0.62 %, respectively. The enzyme electrode lost 35 % of its initial activity after its 100 uses, over a period of 2 months, when stored at 4 °C.     

Enhancing the Reaction Rates of Morita–Baylis–Hillman Reaction in Heterocyclic Aldehydes by Substitutions

The molecular origin of the experimentally observed pronounced difference in the rates of Morita–Baylis–Hillman (MBH) reaction in heterocyclic aldehydes, depending on the position of the formyl group, is investigated herein by using DFT‐based mechanistic studies and free energy computations. These calculations are based on the 1,4‐diazobicyclo[2.2.2]octane (DABCO)‐catalyzed MBH reaction of methyl acrylate with substituted 4‐ and 5‐isoxazolecarbaldehyde, which are slow‐ and fast‐reacting substrates, respectively. As a result of this study, we propose that by tailoring ring substitutions the reactivity of the formyl group for MBH reactions may be enhanced in slow‐reacting heterocyclic aldehydes. This proposition is demonstrated by enhancing the rate of the MBH reaction in 4‐isoxazolecarbaldehyde more than 10⁴‐fold by installing an ester substitution at the C‐3 position. Similarly, the reactivity of the formyl group towards the MBH reaction in substituted 3‐pyrazolecarbaldehyde and pyridinecarbaldehyde is shown to be increased several‐fold by a halo substitution. We also confirm that the reasons for different reactivities of heterocyclic aldehydes and the proposed scheme for improving the reaction rates remains valid for all the three mechanisms proposed for the MBH reaction, namely, Hill–Isaacs, McQuade, and Aggarwal.     

Chemistry of 5(2H)-isoxazolones: Novel conversion of positional isomers

Reactions of 4-arylidene-3-methyl-5(4H)-isoxazolones with nucleophiles lead to 4-substituted-benzyl-3-methyl-5(2H)-isoxazolones. Formation of 5(4H)- and 5(2H)-isoxazolone derivatives have been observed when 4-substituted-arylidene-3-rriethyl-5(4H)-isoxazolones are reacted with methyl magnesium iodide. Oxidation of 4-substituted-benzyl-3-methyl-5(2H)-isoxazolones with manganic acetate and pyridinium chlorochromate, gives 4,4′-bis[4-substituted-benzyl-3-methyl-5(4H)-isoxazolone]. Oxidation of 4-substituted-benzyl-3-methyl-5(2H)-isoxazolones with metachloroperbenzoic acid gives 4-hydroxy-4-substituted-benzyl-3-methyl-5(4H)-isoxazolones and reactions of the same substrates with N-bromosuccinimide furnish 4-bromo-4-substitutedbenzyl-3-methyl-5(4H)isoxazolones.     

Dynamic fracture of a kane-mindlin plate

We study dynamic crack problems for an elastic plate by using Kane-Mindlin's kinematic assumptions. The general solutions of the Laplace transformed displacements and stresses are first derived. Path independent integrals for stationary cracks subjected to transient loads and steadily growing cracks are deduced. For a stationary crack in a very thin plate subjected to impact loads, the crack tip dynamic stress intensity factor (DSIF), K₁(t), is related to the far field plane stress one, K₁⁰(t), by where ν is Poisson's ratio. For a crack steadily growing with speed V, the crack tip DSIF, K₁(V), is given by where K₁⁰(V) is the plane stress DSIF and A(V) and B(V) are known functions of V. These results are applied to compute the DSIF for a semi-infinite stationary crack in an unbounded plate subjected to impact pressure on the crack faces. The results of DSIF for a finite crack in an infinite plate under uniform impact pressure on the crack surfaces show that for each plate thickness, the maximum DSIF is higher than that for the plane stress case.     

Analysis of material instability at an impact loaded crack tip

Assuming that a material point becomes unstable when the effective plastic strain there reaches a critical value, it is found that a material instability will initiate at a point on the crack-tip of an impulsively loaded prenotched plate that makes an angle of −14° to the notch-tip. This agrees well with the observed values of −5° to −15°.     

Optimum Young��s Modulus of a Homogeneous Cylinder Energetically Equivalent to a Functionally Graded Cylinder

For a functionally graded (FG) circular cylinder loaded by uniform pressures on the inner and the outer surfaces and Young??s modulus varying in the radial direction, we find lower and upper bounds for Young??s modulus of the energetically equivalent homogeneous cylinder. That is, the strain energies of the FG and the homogeneous cylinders are equal to each other. For a typical power law variation of Young??s modulus in the FG cylinder, it is shown that taking only two series terms, yields good values for bounds of the equivalent modulus. We also study two inverse problems. First, an investigation is made to find the radial variation of Young??s modulus in the FG cylinder, having a constant Poisson??s ratio, that gives the maximum value of the equivalent modulus. Second, the complementary problem of finding the radial variation of Poisson??s ratio in the FG cylinder, having a constant stiffness, that gives the maximum value of the equivalent modulus, is considered. It is found that the spatial variation of the elastic properties, that maximizes the equivalent modulus, depends strongly upon the external loading on the cylinder.     

Symmetrization of the trimer model for Si(111) 7×7

A symmetrized extension of the trimer model for the Si(111) 7×7 structure is presented. It exhibits new features such as sixfold coordinated Si atoms which are not unusual in Si chemistry but have not been considered in previous structural models. Atomic coordinates are optimized by a Keating-type strain energy minimization and the structure factor for transmission electron diffraction is calculated.     

NAA determiantion of iron and silicon in USGS standards and bauxites using fast neutrons from241Am-Be source

Fast neutron-activation methods have been developed for the determination of iron and silicon in USGS and Indian standards and bauxites. Nuclear reactions⁵⁶Fe/n, p/⁵⁶Mn and²⁸Si/n, p/²⁸Al were carried out using²⁴¹Am-Be neutron source and cutting off thermal neutrons with a Cd shield. For Si a cyclic method was adopted due to short half life of²⁸Al /2.3 min/. The methods are non destructive, fast, economic and ideal for bulk analysis of rocks and process control.A part of this work was presented at the International Symposium on Nuclear Analytical Chemistry, Slowpoke Reactor Facility, Dalhousie University, Canada, June 5–7, 1985.