Development of modified layered silicates with superior adsorption properties for uptake of pollutants from air and water

• Development of Modified Layered Silicates with Superior Adsorption Properties for Uptake of Pollutants from Air and Water

Alsamman, Louay; Möller, Martin (Thesis advisor); Pich, Andrij (Thesis advisor)

Aachen (2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

The current work aimed to develop new porous materials based on layered silicates and their application as adsorbents for the purification of gasses and water by elimination of pollutants by efficient and selective adsorption. Commercially available layered silicates were modified either organically or inorganically. The organic modification is based on the replacement of interlayer cations such as Na+ or Mg2+ by alkyl ammonium surfactants leading to an increase of the interlayer distance of the silicate platelets. This process is called intercalation. This process changes the properties of solvation from hydrophilic to hydrophobic ones because of the hydrophobic characteristics of the alkyl chains. In this way, tailored interactions can be created between the hydrophobic adsorbent and a hydrophobic pollutant. The resulting organoclays are suitable for application in gas purification. Benzene was used as a model pollutant, which was eliminated from a gas phase in an efficiency of 485 mg per 1 g organoclay. Due to a limited solubility of the surfactant in water and the ionic bonding to the silicate layers, the surfactants can be leached. Alternatively, the interlayer cations can be exchanged by inorganic polycations followed by a calcination step. The obtained material is called pillared interlayered clay (PILC). The inorganic oxides are forming pillars between the silicate layers with a fixed distance between the sheets. The specific surface area (BET) of the starting materials increases due to the formation of micropores (< 2 nm) in the PILC, but the pore size distribution is not uniformly enough to generate a defined and maximum interaction between the adsorbent and the pollutant. This disadvantage can be overcome by a template method. The template method offers the possibility to synthesize porous clay heterostructures (PCH) with tunable pore sizes and significant higher specific surface areas. Based on polyalkoxysiloxane (PAOS), a hyper branched silica precursor polymer which simultaneously act as template, porous materials were synthesized closing the gap of pore sizes between the known microporous zeolites and ordered mesoporous silica in a range of 2-5 nm. After calcination the specific surface areas according to BET in a range of 1900 m2/g and a narrow pore size distribution of about 2 to 5 nm were detected. These new micro-/ mesoporous silica materials possess potential applications in many fields of catalysis and adsorption. In-situ functionalization of PAOS during its synthesis is proved to be the most efficient route to tailor the pore widths. To retain these functional groups, a chemical extraction of the template was performed, which leads to selected interactions with model pollutants according to the kind of PAOS modification. Because of the covalently bonded, non-extractable functional groups, this material was suitable to adsorb bisphenol A in a capacity of 144 mg per 1 g amine/silica/polyol/layered silicate or copper ions in a capacity of 205 mg per 1 g thiol/PAOS derived silica/layered silica from aqueous solution.