Amino-modified silicate gels prepared by sol-gel processing as metal-ion sorbents

Amino-Modified Silicate Gels Prepared by Sol-Gel Processing as Metal-Ion Sorbents A N D R Z E J M. K L O N K O W S K I , * T E R E S A W I D E R N I K and BEATA G R O B E L N A Faculty of Chemistry, University of Gdansk, ul. Sobieskiego I8, 80-952 Gdansk, Poland

(Received: 16 March 1995; in final form: 19 June 1995) Abstract. Two series of amino-modified silicate gels prepared by sol-gel processing were used to absorb Cu(II), Ni(II), Co(II), Mn(II) and Cr(III) from aqueous solutions. These easily prepared sorbents with various content of primary amino groups in series (A) or primary and secondary amino groups in series (AA) have reasonable stability. The gel composition, time and concentration dependence of the uptake of the metal ions by these materials were studied systematically. These materials would be further used as supports to disperse catalytically active phases by conventional wet chemical procedures. Apart from this they demonstrate potential for the preconcentration aid for transition metal analysis.

Key words. Amino modified silicate gel, sol-gel processing, metal ion sorbent. Introduction

The preparation of some supported metal alkoxides involves a process by which an aqueous solution of a metal salt is contacted with a ceramic support having appropriate surface area and porous properties. The mechanism by which the metal ions attach to the ceramic surface is not well controlled in all systems. Some metal ions form facile equilibria among m a n y different, discrete molecular complexes in different phases, according to the solubility of the species in the aqueous solution. Methods which offer multi-element analysis with little sample preparation are much to be desired in the field of trace-metal analysis. One such technique is Xray fluorescence spectrometry. X-ray techniques are not intrinsically trace techniques, and trace metals must generally be first preconcentrated in a form suitable for analysis [1]. A m o n g other materials, inorganic-organic hybrid sorbents could be used as metal enrichment aids for catalysis and analysis. For example, the immobilization of organo-functional groups on silica gel offers some distinct advantages over immobilization on organic polymer supports. Firstly, the silica gel is readily modified by a variety of silylating agents, enabling a myriad of functional groups to be immobilized. Second, since the b o u n d e d group is on the surface of the support, high exchange rates are generally observed. Third, silica offers an excellent swelling resistance to changes in solvent composition coupled with good mechanical strength [2]. Silica surfaces are therefore excellent choices for the immobilization of analytical reagents, but their preparation is complicated and time-consuming.

* Author for correspondence. Molecular Engineering 5: 381-389, 1995. © 1995 Kluwer Academic Publishers. Printed in the Netherlands.

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Various workers have incorporated amino functional groups on SiO2 by the reaction of surface silanol groups with alkoxysilanes with --NH2. In this way Leyden et al. [1, 3] have shown that such modified silica materials quantitatively retain metal ions as Hg(II), Cu(II), and Mn(II). Fritz and King [4] also modified silica material, inter alia, by 3-aminopropyltrimethoxysilane. The material proved to be effective in removing iron and copper from hard water. The sol-gel technique has been used in recent years to prepare a wide range of materials, silicates among them. This method is attractive because it involves low temperature processing. Also, high purity and homogeneity are attainable by this preparation route in multicomponent systems [5]. The sol-gel process denotes the transition of a system of colloidal particles in a solution into a disordered, branched, continuous gel, which is interpenetrated by a liquid. This gelation process can take place by hydrolysis and condensation of organometallic precursors [6]. The wet gel, after drying by evaporation under normal conditions, is altered into a xerogel, a good sorbent with high porosity and surface area. This route affords the possibility to prepare hybrid systems such as organically modified silicate (ORMOSIL) gels in which several precursor types are combined. Schmidt and coworkers [7, 8] have developed silicate products with unique properties by combining tetraalkoxysilanes with organofunctional alkoxysilanes: Si(OR)4 + YR'Si(OR)3. R is an alkyl group, R' is an alkylene and Y is an organofunctional group, e.g. NH2. In principle such hybrid systems permit an unlimited number of chemical and structural modifications to be performed. Thus, sol-gel processing is an easy method with which to prepare good sorbents. Simultaneously, it affords the possibility to control the NH2/Si ratio over a wide range [9-11]. For this study silicate gels with simple amino and diamino functional groups were selected as a matrix. Since ORMOSILs form very stable chelates with many metal ions of interest and are simple to prepare, these sorbents with amino groups were investigated to determine their potential as a preconcentration or a sorption matrix.

Experimental REAGENTS All chemicals, including the solvents, were analytical grade quality and were used as received. Two porous gel series (A) and (AA) were synthesized by three types of alkoxides (Aldrich Co. USA): 3-aminopropyltrimethoxysilane HzN(CH2)aSi(OCH3)3 (abbreviated as A), N-(2-aminoethyl)-3-aminopropyltrimethoxysilane H2N(CHz)zHN(CHz)3Si(OCH3)3 (abbr. AA) and tetramethoxysilane Si(OCH3)4 (abbr. T). Alkoxide T plays the role of a modifier, changing the molar ratio x = [T]/[A] or [T]/[AA] in the sorbent. Aqueous solutions of transition metal ions have been prepared by dissolving the nitrate salts of Cu(II), Ni(II), Co(II), Mn(II) and Cr(III) from Fluka AG (Switzerland) in doubly distilled water.

AMINO MODIFIED SILICATE GELS AS M E T A L ION SORBENTS

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PREPARATION AND PROPERTIES OF THE SORBENTS The preparation of the silicate gel was carried out by the sol-gel method (without added catalyst) in methanol solution according to: series (A): A + xT + (3 + 4x)H20 ~ A--xSiO4/2, where A - - = I-I2N(CH2)3SiO3/2, series (AA): AA + xT + (3 + 4x)H20 ~ AA--xSiO4/2, where A A - - = H2N(CH2)2NH(CH2)3SiO3/2. The composition ratio x changed within the range 5 ~