title: Developing Functional Layered Materials : From Synthesis and Characterization towards Applications
Creators: name: Loch, Patrick
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: layered materials
subject: delamination
subject: exfoliation
subject: nanomaterials
subject: layered silicates
subject: layered zeolites
subject: hectorite
subject: ilerite
subject: materials science
subject: 540

description: The present thesis deals with structural features of layered compounds, resulting properties and their use for the systematic design of functional materials with regard to potential industrial applications. The focus is set on two main classes of layered structures. In particular, in the first part of the thesis, the structure and properties of 2:1 layered silicates are discussed (e.g., Na hectorite, Na+0.5[Mg2+2.5Li+0.5]O10F2), while the layered zeolite Ilerite, also known as NaRUB-18 or octosilicate (Na8[Si32O64(OH)8]‧32 H2O), was used for the design of functional materials in the second part. For negatively charged layered compounds like Na-hectorite or Ilerite, ion exchange of interlayer cations represents an easy approach to functionalize the interlayer space. For example, simple Ni-exchange of synthetic Na-hectorite could be used to obtain Ni-hectorites, which show significant crystalline swelling upon exposure to high pressures of CO2. This leads to high volumetric CCS (Carbon Capture and Storage) capacities, making Ni-hectorites attractive for the capture of anthropogenic CO2. While the swelling of 2:1 layered silicates in response to water is well understood, the swelling mechanism with CO2 still lacks explanations. To explain the swelling capability and optimize the adsorption properties concerning a potential industrial feasibility, the structure of the intercalation compound was analyzed in detail. In some cases of intercalation, ordered or random interstratified structures may be formed. Therefore, the focus was set on the identification of such potential structures. Due to the high tendency of Ni to condensate to oligocations, an ordered interstratified structure was formed during ion exchange in Ni-hectorite, consisting of interlayers of uncondensed, smectite-like, hydrated [Ni(H2O)6]2+, and condensed, chlorite-like [Ni(OH)0.83(H2O)1.17]0.37^1.17+ interlayers. Despite the presence of such an ordered interstratification, electron densities of the two interlayer species are too similar, and no typical superstructure reflection could be observed. Therefore, the electron contrast of both interlayers was improved by ion exchange with an alkylammonium cation and by thermal annealing, increasing the intensity of the superstructure reflection and helping to identify the ordered interstratification. In the next step, the obtained structure of Ni-hectorite was used to identify the underlying swelling mechanism with CO2. Combining a combination of analytical tools, the interactions of CO2 and thus the swelling could be entirely attributed to the intercalated [Ni(OH)0.83(H2O)1.17]0.37^1.17+ interlayer species. The mechanism was ascribed to the reversible formation of a carbonate species at the edges of the chlorite-like interlayer species, binding CO2 in the interlayers and leading to the observed swelling. DFT calculations supported this mechanism. Consequently, the CCS capacity of Ni-hectorite can be optimized by increasing the amount of condensed chlorite-like interlayers, rendering Ni-hectorite competitive to other adsorption materials. The second part of the thesis deals with the systematic design of functional materials, applying established mechanisms to the layered zeolite Ilerite. The production of dispersions consisting of single-layer nanosheets via repulsive osmotic swelling represents an established procedure for selected classes of layered compounds, like 2:1 layered silicates, layered titanates, or niobates. Despite the potential of their porous and catalytically active layers, nanosheet production of layered zeolites is still difficult and limited to certain examples. Contrary to 2:1 layered silicates, where the layer charge is independent of the pH, exposed OH groups at the basal surfaces of layered zeolites lead to a variation of the layer charge with pH and complicate delamination. However, the conditions to trigger osmotic swelling for 2:1 layered silicates were recently identified. Applying the appropriate conditions for osmotic swelling, the mechanism could be adapted to the layered zeolite Ilerite. Ilerite can be crystallized in a scalable and mild sol-gel process in appealing, large, square-shaped platelets. By ion exchange of interlayer sodium with the sterically demanding, hydrophilic amino sugar N methyl-D-glucamonium, functional, large aspect ratio nanosheets were obtained in a scalable and environmentally friendly process. The functionality of the zeolite nanosheets was demonstrated by doctor blading of a nematic suspension into thin, inorganic films. Due to the high aspect ratio, these films showed excellent gas barrier properties, relevant for industrial application. Moreover, the layered, square-shaped morphology makes Ilerite attractive for the application as a pearlescent pigment. Extending the color range of pearlescent pigments is still a challenge and may be achieved by using colored substrates. Therefore, Ilerite was used as starting material to synthesize one of the first synthetic blue pigments, the layered compound Egyptian blue (CaCuSi4O10), to maintain the appealing morphology of Ilerite. This pseudomorphosis was achieved in a two-step procedure, by cation exchange of Ilerite with Cu, followed by the addition of missing stoichiometric amounts of Ca and Cu, allowing the synthesis of Egyptian blue at relatively low temperatures of 1000°C. Being technically benign and allowing control of the morphology, this process is advantageous over existing hydrothermal, solid-state or melt flux synthesis routes. This work is a cumulative thesis. A detailed description of the results can be found in the attached manuscripts.