Synthesis of nano-sized metal oxide powders and their application in separation technology

Abstract

Nano-sized inorganic oxides that find application in various fields (such as, in catalysis, sensors, pigments, membranes etc.) are also reported to be excellent reagents in various separation technologies through selective adsorption. Depending on the chemistry of the constituent atoms, they are known to exhibit remarkable adsorption properties due to their enhanced surface area and large interface volume. The nano-sized inorganic oxides can be produced by various reported physical and chemical methods. In general, the chemical methods offer the advantages of easy preparation of nearly any composition, maintaining compositional homogeneity and high purity. However, the chemical routes generally involve complex techniques and an improved level of skill is required to realize the benefits. We have been successful in standardizing the preparation of a variety of nano-sized inorganic oxides through thermolysis of a polymeric based aqueous precursor solution of the desired inorganic ions. The developed method is less cumbersome, more versatile, more cost effective and hence economically more viable than the existing ones for large-scale production of nano-sized inorganic oxides. In this paper we present the preparation of nano-sized powders of Fe 3 O 4 , Al 2 O 3 and ZrO 2 . The obtained nano-sized oxide powders were sonicated in water and incorporated in the matrix of activated charcoal through adsorption. The oxide incorporated charcoal materials have been used as the adsorbing bed for the removal of trace amounts of fluoride/arsenite and arsenate ions from industrial wastewater and aldehyde from the perfume grade alcohols. The oxides have been prepared through thermolysis of precursor solutions that contained the respective cations homogeneously dispersed in the polymeric matrix of sucrose and polyvinyl alcohol. Calcination of the fluffy, carbonaceous precursor powder at 200-500°/2 h resulted in the desired inorganic oxide phase with particle diameters ranging between 20 and 38 nm and BET surface area ranging between 120 and 200 m 2 /g. The charcoal embedded fine powders of the inorganic oxides have been able to remove fluoride/ arsenite and arsenate ions from industrial wastewater upto 0.01-0.02 ppm levels. Many of the perfume grade alcohols are produced by reduction of aldehyde by M.P.V. reduction using aluminum alkoxid or catalytic reduction using hydrogen. However, a ppm amount of residual aldehydes are perpetually retained in the alcohol even through the most efficient reduction by the mentioned process and changes the flavour of the perfume formulations. Here we report an efficient separation of cinnamaldehyde from cinnamyl alcohol using nano-sized Fe 3 O 4 powders. For this, the adsorbing bed has been further modified by impregnating it with 3-aminopropyltriethoxysilane in ethanol medium. The amino groups impregnated in the matrix readily undergoes reaction with the adsorbed aldehydes and results in the formation of Schiff bases thereby reducing the concentration of residual aldehyde contained in the alcohol to levels <0.2 ppm. The prepared adsorbent has been also tested for the removal of heptaldehyde from heptanol and octaldehyde from octanol. The major advantage of the process is that the adsorbent and the adsorbed aldehyde can be easily separated by applying an external magnetic field, thus avoiding the filtration process.