Co-precipitation in Aqueous Medium

4 Co-precipitation in Aqueous Medium - Despite its effectiveness, the alkoxide method presents some drawbacks for industrial scale-up the high cost of the starting reactants and the requirement for water-free and oxygen-free environment for the alkoxide solutions. 

Most industrial catalysts based on mixed oxides are simply and economically prepared via co-precipitation in aqueous medium.20 For the preparation of hexaaluminates, this method was first reported by S. Matsuda and co-workers.21 La203 xAl203 samples were prepared starting from an aqueous solution of La and Al nitrates. The precipitation was carried out by the addition of NH4OH solution up to pH=8. After it was washed, filtered and dried, the precursor was calcined at different temperatures up to 1400 °C. For a La203/Al203 mole ratio 5/95, the formation of a layered-alumina phase was observed starting from 1000°C and samples with a surface areas of 30 m2/g and 8 m2/g have been obtained upon calcination at 1200 °C and 1400 °C for 2 h respectively. 

Ba(NCb) + HN03 + A1(N03)3 + Mn (N03)n in water, open flask, weak heating 

This preparation method was successfully extended to a wide compositional range including M-substituted hexaaluminates (M = Mn, Fe, Cr, Co, and Ni,)23 25 and Sr-La-based hexaaluminates.26 

The effectiveness of the precipitation method based on the use of (NH4)2C03 in the preparation of BaMnj Ab was also confirmed by Berg et al.28 

3 Thermal Evolution of the Precursor and Formation Mechanism of the Hexaaluminate Phases 

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Effective preparation methods of hexaaluminates for catalytic applications, such as the hydrolysis of alkoxides and the co-precipitation in aqueous medium, ensure high interspersion of the constituents in the precursor. This allows the formation of single phase materials with layered-alumina structure at reasonably low temperature (1100-1200 °C) and with high surface area. The hydrolysis of alkoxides was extensively studied and used for the industrial scale-up in the production of catalysts in the monolith shape. However, the co-precipitation in aqueous medium has much potential in view of the possible commercialization of these materials due to its simplicity and low cost. 

The second approach for improving the processabihty of ICPs is to prepare their colloidal dispersions in water or an appropriate solvent The colloid dispersions of ICPs can be obtained by chemical or electrochemical oxidation of the monomer in the presence of a steric stabihzer [29-31].The key parameter for such synthesis is the choice of an appropriate steric stabihzer which adsorbs or grafts onto the polymer coUoidal particles to prevent their aggregation or precipitation. Several polymers such as polyfethylene oxide) [32], poly(vinyl pyrroHdone) [33,34], poly(vinyl alcohol) [35], ethyl hydroxy cellulose [36], poly(vinyl alcohol-co-acetate) [37], poly(vinyl methyl ether) [38,39] and block copolymer stabihzer [40] have been used as steric stabihzers to produce PPy coUoidal dispersions. Surfactants are also employed for the synthesis of ICP coUoidal dispersions [41,42]. Very recently, stable PPy dispersions were prepared by Lu et al. by polymerizing pyrrole in an aqueous medium containing different anionic salts such as sodium benzoate, potassium hydrogen phthalate, and sodium succinate [43]. These authors also reported that the conductivity of PPy dispersions was enhanced when sodium benzoate was used as dopant. Chemical oxidahve polymerization in the presence of PSS in aqueous medium produces coUoidal dispersions and improves processability [44]. CoUoidal dispersions... 

Sn/V/Nb/Sb/0 catalysts were prepared with the co-precipitahon technique, developed for the synthesis of ratile Sn02-based systems claimed by Rhodia (8). The preparation involved the dissoluhon of SnCl45H20, VO(acac)2, SbCh and NbCls in absolute ethanol, and by dropping the soluhon into a buffered aqueous soluhon maintained at pH 7. The precipitate obtained was separated from the liquid by filtrahon. The solid was then dried at 120°C and calcined in air at 700°C for 3 hours. The V/Sb/0 catalyst was prepared by means of the slurry method that consists in a redox reachon between Sb203 and NH4VO3 in water medium, for 18h at 95°C. The... 

Sinou and co-workers also studied the allylation of uracil 190 and thymine 191, but in an aqueous solvent (water/acetonitrile in a ratio 17/2) and in the presence of trisodium triphenylphosphine trisulfonate (tppts, 199) as water-soluble phosphine. Their results are summarized in Table VI. Cinna-myl acetate was used with one equivalent of diazabicycloundecene (DBU) as a base instead of mixed carbonate. Under these conditions, good regiose-lectivities at N-l (to 193 and 185) were observed, as well as lack of diallyla-tion products for uracil and thymine, even with an excess of cinnamyl acetate. It seems that the nonformation of diallylated products is related to the precipitation of the N-l isomers 193 and 185 in the aqueous medium (94TL7085). 

Photostimulation of the catalytic activities of bio-imprinted enzymes was studied by Willner and co-workers [25]. The goal of this study was to control the enzymatic activity with light. They linked a photoactive group such as nitropyran to a-chymo-trypsin. The resulting conjugated enzyme was allowed to interact with A -acetyl-L-phenylalanine (as template) in an an aqueous medium. After precipitating the enzyme-template complex, the template was carefully eluted from the enzyme. The catalytic activities of the non-imprinted and imprinted photoactive enzymes were... 

Acrylonitrile-co-itaconic acid (AN-IA) copol5miers were S5mthesized in an aqueous medium, changing the monomer feed ratios. - Copol5mierizations were achieved with high conversion owing to water-phase precipitation pol5mierization. 

This is a simple two-step process for the fabrication of nanoparticles. The first step is polymeric emulsification in an aqueous medium. The second step is solvent evaporation from the polymer and nanoparticle precipitation. Finally, the nanoparticles are collected by ultracentrifugation and washed with distilled water for the removal of excess stabilizer and lyophUization [23]. The various types of polymer used include poly(lactic-co-glycolic acid) (PLGA), poly-D,L-lactic acid (PLA), poly(E-caprolactone) (PCL), ethyl cellulose, poly(P-hydroxybutyrate), cellulose acetate phthalate, etc. 

Fresh natural rubber latex contains about 30-40% of rubber hydrocarbon that is normally referred to as dry rubber content (DRC). However, the total solid content (TSC) is higher than the DRC due to the presence of non-rubbers in the latex, at around 5%. The DRC and non-rubber content may change due to many factors such as clone, soil and climate conditions, season, type of fertilizers used, and tapping frequency. Most of these non-rubbers are dissolved or suspended in the aqueous serum or adsorbed on the surface of rubber particles. They become trapped, tenaciously held, or co-precipitated during coagulation of the rubber probably due to their poor solubility in the aqueous medium or strong entanglement with the rubber molecule. The major non-rubbers are lipids, proteins and amino acids, minerals, inositols and carbohydrates, as shown in Table 3.1. 

The co-precipitation method involves the simultaneous precipitation of a selected pair of metal ions from their mixed aqueous solution by dilute NaOH and/or NaHCOs, Na2C03, or NH4OH solution. The pH of the reaction medium is maintained in the range 8-10, depending on the nature of metal ions. Hydrothermal treatment of the final suspension is usually carried out to obtain well-crystallized samples. Detailed descriptions of the process are available in numerous reports available in the literature [25-29]. 

The cellulose phosphate was widely applied in the removal process of divalent and trivalent (Cu ", Zn ", Co, Ni, Pb ", Mn ", Fe, Fe , Cr ) ions because of the rapid adsorption of the metals ions in comparison with the synthetic polymers. Also it was observed that the cellulose phosphate functions as an ion exchanger for lanthanide ion removal from aqueous solution. In order to evaluate the adsorbent characteristics of each support in the removal process of metal ions from aqueous solution, the most widely used method by researchers has been the batchwise one. In any adsorption process the most important parameter is the pH of the medium, which depends on the nature of the support used and also on which metal ions are to be removed. In most cases, researchers worked with solutions that have an acid pH value in order to avoid the possible precipitation of the studied metal ions. A complex study of the pH influence upon the efficiency of Fe ", Cu ", Mn, Zn, Co ", and lanthanide(III) ions removal from aqueous solutions with PBC was made by Oshima et al. Their conclusion was that the lanthanide ions are adsorbed when the pH value of the aqueous solution is less than 3, and in the case of transition metal ions the adsorption percentage increases with increasing aqueous pH and reaches over 90% at a pH value of around 4.5. 

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