Endalkachew Sahle-Demessie

Selective oxidation of alcohols by molecular oxygen over a Pd/MgO catalyst in the absence of any additives

Selective oxidation of alcohols to the corresponding carbonyl products using molecular oxygen is achieved over a simple and easily recyclable 1% Pd/MgO impregnated heterogeneous catalyst in the presence of trifluorotoluene. A variety of activated and non-activated alcohols are effectively oxidized without the use of any additives under relatively mild reaction conditions. Other supported Pd catalysts such as Pd/Hydrotalcite (HT), Pd/Al2O3, Pd/SiO2 and Pd/Zeolite-β are also studied for comparison. Pd/HT also shows a comparable oxidation activity to Pd/MgO whereas other supported catalysts were not found to be active for this reaction under the conditions studied. A mechanism of the reaction is also outlined.

Sn-exchanged hydrotalcites as catalysts for clean and selective Baeyer–Villiger oxidation of ketones using hydrogen peroxide

Abstract A Sn-doped hydrotalcite (Sn/HT) catalyst prepared by ion-exchange technique is found to be an active and selective catalyst for the liquid phase Baeyer–Villiger (BV) oxidation of cyclic ketones in acetonitrile using hydrogen peroxide (H 2 O 2 ) as oxidant. Different reaction parameters such as effect of solvent, Sn content, reaction temperature and catalyst-to-substrate ratio are studied. The activity of the catalyst for the selective BV oxidation of ketones is attributed to the presence of active Sn sites in the interstitial position of hydrotalcite support. Sn sites activate the carbonyl group of ketones followed by a nucleophilic attack by the active peroxide species (peroxycarboximidic acid by reaction of acetonitrile and H 2 O 2 ) to form a Criegee adduct that rearranges to give the lactone.

Transport and deposition of CeO2 nanoparticles in water-saturated porous media

Ceria nanoparticles are used for fuel cell, metal polishing and automobile exhaust catalyst; however, little is known about the impact of their release to the environment. The stability, transport and deposition of engineered CeO2 nanoparticles through water-saturated column packed with sand were studied by monitoring effluent CeO2 concentration. The influence of solution chemistry such as ionic strength (1-10 mM) and pH (3-9) on the mobility and deposition of CeO2 nanoparticles was investigated by using a three-phase (deposition-rinse-reentrainment) procedure in packed bed columns. The results show that water chemistry governs the transport and deposition of CeO2 nanoparticles. Transport is significantly hindered at acidic conditions (pH 3) and high ionic strengths (10 mM and above), and the deposited CeO2 particles may not be re-entrained by increasing the pH or lowering the ionic strength of water. At neutral and alkaline conditions (pH6 and 9), and lower ionic strengths (below 10 mM), partial breakthrough of CeO2 nanoparticles was observed and particles can be partially detached and re-entrained from porous media by changing the solution chemistry. A mathematical model was developed based on advection-dispersion-adsorption equations and it successfully predicts the transport, deposition and re-entrainment of CeO2 nanoparticles through a packed bed. There is strong agreement between the deposition rate coefficients calculated from experimental data and predicted by the model. The successful prediction for attachment and detachment of nanoparticles during the deposition and re-entrainment phases is unique addition in this study. This work can be applied to access the risk of CeO2 nanoparticles transport in contaminated ground water.

Oxidation of alcohols over Fe3+/montmorillonite-K10 using hydrogen peroxide

Abstract Oxidation of various primary and secondary alcohols is studied in liquid phase at atmospheric pressure over Fe 3+ /montmorillonite-K10 catalyst prepared by ion-exchange method at a pH of 4 in an environmentally friendly protocol using hydrogen peroxide. The catalyst and the method are found to be suitable for the selective oxidation of secondary alcohols to the corresponding ketones. Aromatic alcohols form mainly secondary or over oxidation products. Presence of CH 3 group at the OH bearing carbon is deleterious to the oxidation. The catalyst activity increases with increase in reaction time due to the slow activation of the catalyst. The reaction mechanism is expected to involve the intermediate formation of binuclear, peroxide ion-bridged iron complex by reaction of iron and the activated peroxide followed by oxygen transfer to the alcohol. The catalyst is also found to be easily separable and recyclable.

Selective oxidation of styrene to acetophenone in the presence of ionic liquidsPresented, in part, at the IUPAC CHEMRAWN XIV Conference on Green Chemistry:Toward Environmentally Benign Processes and Products, Boulder, Colorado, USA, 9–13 June 2001.

Palladium-catalyzed oxidation of styrene (Wacker reaction) in the presence of 1,3-dialkylimidazolium cation based ionic liquids is described under relatively benign conditions using hydrogen peroxide. The effects of various reaction parameters and a comparison of this approach with the corresponding reactions under pressurized conditions have been made.

Microwave-expedited olefin epoxidation over hydrotalcites using hydrogen peroxide and acetonitrile

An efficient microwave-assisted epoxidation of olefins is described over hydrotalcite catalysts in the presence of hydrogen peroxide and acetonitrile. This general and selective protocol is extremely fast and is applicable to a wide variety of substrates.

Vanadium phosphorus oxide as an efficient catalyst for hydrocarbon oxidations using hydrogen peroxide

Calcined vanadium phosphorus oxide (VPO) prepared by an organic route is found to be an active and effective catalyst for the oxidation of various alkanes such as cyclopentane, cyclohexane, n-hexane, cycloheptane, cyclooctane, cyclodecane and adamantane in acetonitrile solvent using the environmentally benign oxidant, hydrogen peroxide, where the oxidation mechanism is believed to involve a reversible V4+/V5+ redox cycle.

Environmentally friendlier organic transformations on mineral supports under non-traditional conditions

Synthetic organic reactions performed under non-traditional conditions are gaining popularity, primarily to circumvent growing environmental concerns. A solvent-free approach that involves microwave (MW) exposure of neat reactants (undiluted) catalyzed by the surfaces of less expensive and recyclable mineral supports, such as alumina, silica, clay, or ‘doped’ surfaces, is described which is applicable to a wide range of deprotection, condensation, cyclization, rearrangement, oxidation, and reduction reactions, including rapid one-pot assembly of heterocyclic compounds from in situ-generated reactive intermediates. The strategy is adaptable to multi-component reactions for rapid assembly of a library of compounds. The application of microwaves and ultrasound as successful alternative energy sources is described for the selective epoxidation of alkenes and α,β-unsaturated ketones over hydrotalcites as catalysts. The ability of “green” solvents such as supercritical (sc) CO2 to dissolve many reactive gases like H2 and O2, and also a variety of organic compounds, can be exploited to facilitate many important industrial transformations in this medium, wherein the improved reactant solubility and minimized interphase mass-transfer limitations lead to enhanced reaction rates and unusual product selectivity. Consequently, the use of sc-CO2 as an attractive medium for the selective hydrogenation of maleic anhydride and α,β-unsaturated aldehydes, such as cinnamaldehyde, over supported metallic catalysts (Pd/Al2O3) is illustrated. Furthermore, recent developments in the areas of microwave or ultrasound-expedited reactions, and the use of supercritical CO2 in organic transformations are also reviewed. The salient eco-friendly features of these processes, namely the selectivity, the ease of experimental manipulation, and the enhanced reaction rates, are highlighted. Use of the above non-traditional methods promises to overcome many of the difficulties associated with conventional reactions and offers both process and environmental advantages in many cases.

Hydrodechlorination of chlorinated benzenes in a continuous microwave reactor

An expeditious hydrodechlorination of chlorobenzenes is observed over 0.5% Pd/Al2O3 catalyst by conducting the reaction under microwave irradiation conditions. Even though the loss of active metal surface area is substantial and identical in both microwave and conventional heating reactions, the higher rate and sustainability of the microwave reaction may be due to the selective and rapid absorption of microwaves by the polar chlorinated substrates that facilitates their relatively easy removal from the catalyst surface. It is also speculated that the rate of desorption of the products (especially the poisonous HCl) is more critical in sustaining the catalyst activity. The experiments also reveal a significant reduction in power consumption under the microwave reaction than the reaction conducted using conventional heating.

Transport of nanoparticles with dispersant through biofilm coated drinking water sand filters.

This article characterizes, experimentally and theoretically, the transport and retention of engineered nanoparticles (NP) through sand filters at drinking water treatment plants (DWTPs) under realistic conditions. The transport of four commonly used NPs (ZnO, CeO2, TiO2, and Ag, with bare surfaces and coating agents) through filter beds filled with sands from either acid washed and calcined, freshly acquired filter media, and used filter media from active filter media, were investigated. The study was conducted using water obtained upstream of the sand filter at DWTP. The results have shown that capping agents have a determinant importance in the colloidal stability and transport of NPs through the different filter media. The presence of the biofilm in used filter media increased adsorption of NPs but its effects in retaining capped NPs was less significant. The data was used to build a mathematical model based on the advection-dispersion equation. The model was used to simulate the performance of a scale-up sand filter and the effects on filtration cycle of traditional sand filtration system used in DWTPs.