Synthesis of zeolite a from by-product of aluminum etching process: effects of reaction temperature and reaction time on pore volume.

Author:Hussar, Kanokorn
 
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INTRODUCTION

The properties and uses of zeolites are being explored in many scientific disciplines: modern inorganic chemistry, physical chemistry, colloid chemistry, biochemistry, mineralogy, geology, surface chemistry, oceanography, crystallography, catalysis and in all types of chemical engineering process technology. The wide variety of applications includes separation and recovery of normal paraffin hydrocarbons, catalysis of hydrocarbon reactions, drying of refrigerants, separation of air components, carrying catalysts in the curing of plastics and rubber, recovering radioactive ions from radioactive waste solutions, removing carbon dioxide and sulfur compounds from natural gas, sampling air at high altitudes and removal of atmospheric pollutants such as sulfur dioxide (Breck, 1984).

Commercial adsorbents which exhibit ultraporosity and which are generally used for the separation of gas and vapor mixtures include the activated carbons, activated clays, inorganic gels such as silica gel and activated alumina and the crystalline aluminosilicate zeolites. Zeolite molecular sieves have pores of uniform size (3-10 A) which are uniquely determined by the unit structure of the crystal. These pores will completely exclude molecules which are larger than their diameter.

Zeolites are crystalline, hydrated aluminosilicates of group I and group II elements (as formed in nature or synthesized), in particular, sodium, potassium, magnesium, calcium, strontium and barium. Structurally the zeolites are "framework"aluminosilicates which are based on an infinitely extending three-dimensional network of Al[O.sub.4] and Si[O.sub.4] tetrahedra linked to each other by sharing all of the oxygen's. The chemical formula of zeolite is best expressed for the crystallographic unit cell as: [[[M.sub.2/n]O.[Al.sub.2][O.sub.3]xSi[O.sub.2]y[H.sub.2]O (Breck, 1984). In this oxide formula, x is generally equal to or > 2 since Al[O.sub.4] tetrahedra are joined only to Si[O.sub.4]tetrahedra, n is the cation valence. The framework contains channels and interconnected voids which are occupied by the cation and water molecules as shows in Fig. 1 (Auerbach et al., 2003).

Hydrogel process of zeolites is generally synthesized from sodium aluminosilicate gel prepared from various silica and alumina ratio as raw material (Breck, 1984). The gels are crystallized in a closed hydrothermal system at temperatures varying generally from room temperature to about 175[degrees]C. The alkali metals form soluble hydroxides, aluminates and silicates. These materials are well suited for the preparation of homogenous mixture. The gel preparation and crystallization is represented schematically using the [Na.sub.2]O-[Al.sub.2] [O.sub.3]-Si[O.sub.2]- [H.sub.2]O (zeolite A) system as shows in Fig. 2 (Breck, 1984).

The gel is probably produced by the copolymerization of the individual silicate and aluminates species by a condensation-polymerization mechanism. The gel composition and structure appear to be controlled by the size and structure of the polymerizing species.

The etching process in the aluminum profile is etched by sodium hydroxide solution which produces an aluminum hydroxide (Al [OH.sub.3]) as by-product, which carry a large amount of [Al.sub.2] [O.sub.3]content. It produces a low wholesale price aluminum hydroxide (Al[OH.sub.3]) normally further used for producin alum [Al.sub.2][S[O.sub.4].sub.3].

Thus, higher valued added product such as zeolite should be considered as in this research.

This research focuses on reaction temperature and reaction time as controlling factor for synthesis of zeolite A and also on its pore volume. This study aims to achieving the following: (i) synthesis of zeolite A by hydrogel process using raw material from the byproduct of aluminium etching process, (ii) analysis of surface area is performed by the BJH method and the DR method for analysis of gas adsorption-desorption using nitrogen., (iii) [CO.sub.2] adsorption capacity.

MATERIALS AND METHODS

Materials: Synthetic zeolite was synthesized by hydrogel process, using as sodium metasilicate (from Ajax Finechem, lab grade), sodium hydroxide (from Carlo Erba, lab grade) and by-product from aluminum etching process.

Synthesis of zeolites: Preparing of the reactant sodium silicate solution; weigh sodium metasilicte about 42 g, dissolve in water at temperature 50[drgrees]C to make the mole ratio of reactants of 2 (Si[O.sub.2]/[Al.sub.2][O.sub.3] = 2, [Na.sub.2]O/[Al.sub.2][O.sub.3] = 2). Preparing of sodium aluminates solution; weigh by-product from aluminum etching process about 10.7525 g to make the mole ratio of reactants of 2 (Si[O.sub.2]/[Al.sub.2][O.sub.3] = 2, [Na.sub.2]O/[Al.sub.2][O.sub.3] = 2), dissolve into water 240 [cm.sup.3] and add 30 [cm.sup.3] of 50% NaOH at temperature 60[drgrees]C for 30 min before filtrating. The hydrogel process of the synthetic was carried out between sodium silicate solution and sodium aluminates solution at the range of temperature from 75-95[drgrees]C in stirrer tank reactor, temperature controlled by water bath with fixed stirring speed at 300 rpm for 1-3 h. The experimental design was reported in Table 1, with the crystallization products obtained by filtrating, washed with deionized water and dried in an air oven at 100[drgrees]C for 16 h.

Table 1: List of experimental design: temperature and reaction time. (The mole ratios of the starting reactants are fixed at 2 of SiO2/Al2O3, 2 of Na2O/ Al2O3 and 85 of H2O/Na2O.) Samples Time (h) Temp. ([degrees]C) ZA11 1 95 ZA12 1 85 ZA13 1 75 ZA21 2 95 ZA22 2 85 ZA23 2 75 ZA31 3 95 ZA32 3 85 ZA33 3 75 Note...

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