Colloids and Surfaces A: Physicochemical and Engineering Aspects
Effect of ammonium salts on dispersive and adsorptive parameters of silicas precipitated from sodium metasilicate solution
Introduction
Synthetic silicas exhibits broad application in various disciplines including, i.a., catalysis, adsorption, chromatography, etc. The application spectrum reflects the type of SiO2 synthesis as well as physicochemical and adsorptive properties of the product [1], [2], [3].
Synthetic silicas used to be obtained in several types of processes, i.a., by precipitation from solutions of sodium metasilicate, according to Stöber's reaction, by thermic hydrolysis of SiCl4 or by oxygenation of silica-organic compounds. Thus, the technique of obtaining silica exerts a significant effect on its dispersive and adsorptive characteristics. The multi-tone production of synthetic silica relies first of all on the precipitation process. Depending upon the conditions, the process can yield silicas of a gel structure or as highly dispersed sediments.
In most of cases such synthesis of silica gels in conducted at low temperatures. The gels exhibit an extensive specific surface area (300–900 m2 g−1) and used to be applied mainly as adsorbents and packing of chromatographic columns [4], [5].
Shortcomings of typical silica gels include first of all the disordered size and volume of pores. The discovery of ordered mesoporous silicas and their subsequent modification permitted to obtain substances of strictly defined properties (defined specific surface area, stable pore structure and size). The ordered mesoporous silicas represent also a very valuable chromatographic and adsorptive material [5], [6], [7], [8], [9], [10], [11]. The silicas are obtained first of all by precipitation from sodium metasilicate solution using acidic agents: mineral acids, acidic anhydrides and ammonium salts, most frequently at elevated temperatures [12], [13]. Silicas precipitated from sodium metasilicate solutions in an emulsion system exhibit specific physicochemical properties. Their particle diameter ranges from 0.3 to 1.2 μm and the specific amounts to 150–300 m2 g−1. Due to specific type in which synthesis of the silica is conducted the products exhibit high chemical affinity to organic compounds [14], [15].
In this study we decided to conduct reactions of synthesis of colloidal silica precipitated from aqueous solutions of sodium metasilicate using six various ammonium salts as precipitating agents. Effects of the applied ammonium salts was studied on the type of obtained dispersions (particle size, tendency to form agglomerate structures) and on adsorptive capacity of the product (BET surface, pore size and volume, mesoporous character). The listed above properties determine surface structure and types of interactions in multiple technological processes (during filling of plastics, preparation of paints, etc.).
Section snippets
Materials
In order to obtain highly dispersed silica several substances were used, including:
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aqueous solution of sodium metasilicate;
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precipitating agents, including ammonium chloride, ammonium sulphate(VI), ammonium hydrogen carbonate, ammonium carbonate, ammonium nitrate(V), ammonium dihydrogen phosphate(V) (POCH S.A. Poland);
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hydrophobicity-inducing agent, Rokafenol N-9 (nonylphenyl polyoxyethylene glycol ether) (ROKITA S.A., Poland).
Preparation of hydrated silicas
The reaction vessel, equipped with a propeller top stirrer, was
Results and discussion
Principal physicochemical parameters of silicas obtained in the precipitation process are presented in Table 1. In the experiments sodium metasilicate was dosed to appropriate ammonium salt at the ammonium salt:sodium metasilicate ratio of 1:2, no hydrophobicity-inducing agent was added and the system was supplemented with 1 weight part of Rokafenol N-9.
Structure and pore characteristics of the silicas, based on the data on nitrogen adsorption/desorption on their surfaces, are presented in
Conclusions
Silicas precipitated from solutions of sodium metasilicate using ammonium hydrogen salts (NH4HCO3 and NH4H2PO4) proved to be very active: they exhibited high capacity to absorb paraffin oil and high specific surface areas and the curves of nitrogen adsorption/desorption of their surface documented high volumes of adsorbed nitrogen. Optimum temperatures of silica precipitation were established for five types of the employed ammonium salts. Silicas precipitated in presence of neutral salts
Acknowledgements
This work was supported by the PUT Research Grant DS No 32/008/2002.
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