Selectivity in Bromination of Aromatic Substrates by Molecular Bromine in the Presence of Reusable Zeolites

Bromination of aromatic substrates, namely, toluene, phenol, phenyl acetate and chlorobenzene has resulted predominantly in the formation of para bromo derivatives. While solution bromination of phenyl acetate and chlorobenzene is very slow, zeolite-mediated bromination proceeds smoothly, reflecting the catalytic activity of zeolite in this reaction. A suitable mechanism involving assistance from the sodium ion in generating the active brominating species is proposed. The paraselective bromination of these substrates is also compared with that of nitration and the observed differences are suitably explained.     

Ameliorating effect of silica addition in the anode-catalyst layer of the membrane electrode assemblies for polymer electrolyte fuel cells

Incorporation of silica particles through a sol-gel process into the anode-catalyst layer with a sol-gel modified Nafion-silica composite membrane renders easy retention of back-diffused water from the cathode to anode through the composite membrane electrolyte, increases the catalyst-layer wettability and improves the performance of the Polymer Electrolyte Fuel Cell (PEFC) while operating under relative humidity (RH) values ranging between 18% and 100% with gaseous hydrogen and oxygen reactants at atmospheric pressure. A peak power density of 300 mW cm⁻² is achieved at a load current-density value of 1200 mA cm⁻² for the PEFC employing a sol-gel modified Nafion-silica composite membrane and operating at 18% RH. Under similar operating conditions, the PEFC with a Membrane Electrode Assembly (MEA) comprising Nafion-silica composite membrane with silica in the anode-catalyst layer delivers a peak power density of 375 mW cm⁻². By comparison, the PEFC employing commercial Nafion membrane fails to deliver satisfactory performance at 18% RH due to the limited availability of water at its anode, acerbated electro-osmotic drag of water from anode to cathode and insufficient water back diffusion from cathode to anode causing the MEA to dehydrate.     

Analysis and design of dye-sensitized solar cell

This paper presents a methodology to numerically investigate the influence of design and operating parameters on the performance of dye-sensitized solar cell (DSSC). A one-dimensional steady-state model of the DSSC coupled with a model to predict global irradiance on a terrestrial surface is utilized to analyze the performance of a DSSC installation. Hourly, daily, monthly and annual performance of a DSSC installation is systematically analyzed over a wide range of design and operating parameters and the optimum configuration is identified with the objective of maximizing the power density. Overall, this paper illustrates a methodology to ascertain the optimal design and operating configurations of a DSSC which can be extended to any terrain.     

Nafion and modified-Nafion membranes for polymer electrolyte fuel cells: An overview

Polymer electrolyte fuel cells (PEFCs) employ membrane electrolytes for proton transport during the cell reaction. The membrane forms a key component of the PEFC and its performance is controlled by several physical parameters, viz. water up-take, ion-exchange capacity, proton conductivity and humidity. The article presents an overview on Nafion membranes highlighting their merits and demerits with efforts on modified-Nafion membranes. Energy security refers to various security measures that a given nation, or the global community as a whole, must carryout to maintain an adequate energy supply     

Clay-catalysed dealkylation of organic sulfides

Both K10- as well as modified montmorillonites such as cation-exchanged and surfactant-pillared clays catalyse the cleavage of carbon–sulfur single bonds resulting in dealkylation of organic sulfides. The corresponding disulfides are obtained in high yield. This revised version was published online in July 2006 with corrections to the Cover Date.     

Studies on gas–solid heat transfer during pneumatic conveying

Interactions between solids and gas during pneumatic conveying can be utilized for variety of applications including flash drying, solids preheating etc. Experiments on air–solid heat transfer were carried out in a vertical pneumatic conveying heat exchanger of 54 mm inside diameter, using gypsum as the solid material. The effect of solids feed rate (0.6–9.9 g/s), air velocity (4.21–6.47 m/s) and particle size (231–722.5 μm) on air–solid heat transfer rate, heat transfer area and air–solid heat transfer coefficient has been studied. Empirical correlations have been proposed for the prediction of Nusselt number based on the present experimental data. The proposed correlations predict Nusselt number within an error of ±15% for the present data.     

3-Methyltrimethylammonium poly(2,6-dimethyl-1,4-phenylene oxide) based anion exchange membrane for alkaline polymer electrolyte fuel cells

Hydroxyl ion (OH^?) conducting anion exchange membranes based on modified poly (phenylene oxide) are fabricated for their application in alkaline polymer electrolyte fuel cells (APEFCs). In the present study, chloromethylation of poly(phenylene oxide) (PPO) is performed by aryl substitution rather than benzyl substitution and homogeneously quaternized to form an anion exchange membrane (AEM). ¹H NMR and FT-IR studies reveal successful incorporation of the above groups in the polymer backbone. The membrane is characterized for its ion exchange capacity and water uptake. The membrane formed by these processes show good ionic conductivity and when used in fuel cell exhibited an enhanced performance in comparison with the state-of-the-art commercial AHA membrane. A peak power density of 111 mW/cm² at a load current density of 250 mA/cm² is obtained for PPO based membrane in APEFCs at 30 °C.     

SIEVE CASCADOGRAPHY FOR CHARACTERIZING PARTICLE SHAPE MIXES OF POWDERS

Particle shape has a profound influence on the behavior of the particles present in powders, controlling the manner in which they react, segregate, flow, and agglomerate. The currently available technique of Fourier transformation to characterize particle shape is cumbersome and can be used only by a skilled operator. Recently developed, the Sieve Cascadograph is able to characterize the shape of particles in the 0.1 to 1.0 mm size range. Employing the principle that the residence time of a particle on a sieve is a function of the particle shape, the Cascadograph consists of a stack of sieves of identical size in a shaking device. An essentially monosize powder sample (closely sized on a regular sieve stack) of about 2 grams is placed in the topmost sieve, and shaking commenced. The weight of powder leaving the lowest sieve is measured as a function of time, and is used to produce a signature representative of the particle shape distribution in the powder sample. Theoretical analysis demonstrates that monosize, monoshape samples leave the lowest sieve at a rate given by the gamma function with respect to time. Preliminary experimental results prove that the Cascadograph results are reproducible, and that a 20-sieve Cascadograph has high resolving power.     

Polymer electrolyte fuel cells employing electrodes with gas-diffusion layers of mesoporous carbon derived from a sol-gel route

Sol-gel derived mesoporous carbon (MC) for the preparation of gas-diffusion layer (GDL) and its ameliorating effect on the performance of polymer electrolyte fuel cells (PEFCs) is reported. MC with a specific surface area of 370 m²/g, pore diameter of 6.7 nm and pore volume of 0.45 cm³/g has been synthesized by co-assembly of a tri-block copolymer, namely pluronic-F127, as a structure directing agent, and a mixture of phloroglucinol and formaldehyde as carbon precursor. X-ray diffraction and transmission electron microscopy have been employed to examine the structural properties of the MC. Surface morphology of the GDL comprising MC has been studied by scanning electron microscopy. A peak power density of 0.53 W/cm² at a load current-density of 1.1 A/cm² is achieved for the PEFC employing electrodes with GDL of MC compared to the peak power density of 0.47 W/cm² at a load current-density of 0.93 A/cm² for the PEFC employing electrodes with GDL of commercial Vulcan XC-72R carbon, while operating at 70 ^oC with H₂ and air feeds at atmospheric pressure.