Norma A. Sánchez-Flores

Synthesis of silicalite-1 from organo-silicic gels

Well crystallized silicalite-1 has been obtained from three sources of amorphous silica, namely, rice hull ashes, commercial Davisil, and a fume silica from Aldrich. The silicas were first dissolved in glycerol according to a recently described reaction. This reaction transforms rapidly and efficiently large surface area silicates into poly-alkoxide gels. It can be schematized as an etherification of an alcohol function of glycerol by the weakly acid surface silanol groups. The facile hydrolysis of the alkoxide permits the preparation of relatively pure and reactive silica, keeping the mesoporous character of the parent starting material. We insist on the mesoporous character of the solids obtained upon hydrolyzing the organo-silicic gel because we believe the gel plays a role of template in the secondary synthesis of mesoporous structures. The hydrolysis is carried out in presence of a structure directing agent, namely tetra-propylammonium hydroxide, TPAOH. After aging, the residue is dried and calcined. The first advantage of using the organo-silicic gel is probably related to the high degree of depolymerization of silica, witness by the C/Si ratio. The second one, more subtle to define, is to provide an intermediate silica with hydrophilic a hydrophobic regions, interfering differently with the surfactant. After calcination at 500 degrees C, well crystallized silicalite-1 is obtained. The texture of the starting silica influences the textural characteristics of the final silicalite-1.

Influence of Textural Properties and Surface Fractal Dimensions on the Cobalt Adsorption Behavior of Rice Hull Ash Prepared via Solid Combustion

Rice hull ash was prepared via a fast chemical combustion method with ammonium nitrate, urea and milled rice hull. Urea was used as the chemical fuel. The mixture was heated to various temperatures (100 to 1000 °C), and the resulting products were compared with products obtained via calcination, in which samples were heated to the same temperatures for 2 hours. Changes in the structure, morphology, texture and surface fractal dimensions with respect to temperature were examined and found to be highly relevant for adsorption applications. The prepared powders were characterized physicochemically by X-ray diffraction and N2-physisorption measurements. The surface fractal dimensions of the prepared materials were calculated from N2-adsorptiondesorption isotherms. The results indicate that the fast combustion samples have larger surface fractal dimensions than calcined samples. A correlation between fractal dimensions and Co 2+ ion adsorption was found. The present study demonstrates the importance of textural properties and surface fractal dimensions in the adsorption performance of adsorbents.

Silicalite-1, an adsorbent for 2-, 3-, and 4-chlorophenols

The adsorption of the three chlorophenol isomers, ortho, meta and para, by silicalite-1 has been studied at 30 °C, below the solubility (at the same temperature) in water. Large differences, up to 30 times, have been observed between the adsorption of the para- vs. the ortho-isomer. The difference of behavior observed between the isomers is assigned to the tendency to self-organization of the para-isomer. It seems probable that the adsorption sites are at the intersection channels. From a technical point of view, silicalite-1 seems a competitive adsorbent for p-chlorophenol.

Selectivity of the Cd2+/Ca2+ exchange on modified rice hull silica

The rice hull ash is composed by 94% of SiO2, an agricultural waste that can be recovered and purified by a depolymerization reaction yielding an organo‐silicic gel. The purpose of this paper is to show that this silica can be used to fix Cd2+ from aqueous solution. The pH of hydrolysis of the organo‐silicic gel is the main factor modifying the distribution between the solid and the solution. The contact time between the Cd2+ solution and the solid was studied to optimize the sorption conditions. The equilibrium measurements were performed after 40 hours at room temperature. The competition with Ca2+ ions in the solutions was also studied in order to evaluate the selectivity of the Cd2+ fixation. It was found that the rice hull ash has a higher capacity to fix Cd2+ than the rice hull derivatives.

Behavior of Ni(II) and Mn(II) in single and binary aqueous solutions in the adsorption process by rice hull ashes (RHA)

ABSTRACT The retention of nickel and manganese by calcined rice hull ashes (RHA) in single and binary solutions was determined as a function of metal concentration and time. The maximum adsorptions of RHA for Ni(II) and Mn(II) were 7.27 and 6.45 mg/g from monocomponent solutions, and 4.6 and 3.8 mg/g from bicomponent solutions, respectively. The kinetic experimental data best fit a pseudo-second-order model. The k2 is higher for Mn(II) than Ni(II) considering the monocomponent solutions and this parameter is similar between them if the bicomponent solution is considered. The metallic species were partially desorbed from RHA by water.