Information

Hydrous Layer Silicates (HLSs) like Kanemite, Na₄[H₄Si₈O₂₀] * 12 H₂O or Magadiite, approx. Na₂[H₂Si₁₄O₃₀] * 9 H₂O), are useful as adsorbents [1] and have been used for the preparation of composite materials by intercalating the HLS with various organic molecules and cations [2]. HLSs have, however, in particular been widely used as raw materials (silicate source) for the synthesis of new layered materials [3], mesoporous silicas [4] and various zeolites [5].

Materials obtained from HLSs and possessing micro- or mesoporous frameworks include:

• expanded framework structures by techniques of pillaring [6 - 9]
• mesoporous silicas with card-board like structures by delamination [10 - 13]
• high silica zeolites by topotactic condensation of the layers [14 - 16].

The layer silicates considered here as Hydrous Layer Silicates (HLSs) consist of a tetrahedral silicate layer of interconnected [SiO₄]-units and an inter-layer region where cations of low charge density (!) and water molecules are located.

• the silicate layer contains equal numbers of terminal silanol / siloxy groups on either side of the layer. The T-positions are nearly exclusively occupied by Si atoms with only traces of Al³⁺, B³⁺ etc.
• the inter-layer region contains the cations of low charge density which are predominantly organic cations like diethyldimethylammonium, hydrated inorganic cations like ([Na(H₂O)₆]⁺, or protons forming terminal silanol groups. This is in contrast to other layer silicates including non-hydrated inorganic cations like Na⁺, K⁺, Ca²⁺, etc. with ionic radii of ca. 1 Å. Makatite for example, a layer silicate that has strong ionic interactions between “naked” Na⁺ cations and the silicate layers is not considered a HLS. Several HLSs occlude in addition water molecules in the inter-layer region.

For more details on Hydrous Layer Silicates see for example: Marler and Gies [16], Schwieger and Lagaly [17], and Ramos et al. [18].

So far, silicate layers of 16 different layer topologies (= layer types) have been identified as being part of an HLS structure. It is obvious that some additional layer types exist, e.g. in Magadiite and Kenyaite, but the corresponding structures have not yet been solved. The HLSs can be classified according to their specific layer types. Each layer types has its unique topology. As an acronym designation such a layer type a distinct code (three underlined small characters) is used. If the particular type of layer is already known as a layer-like building unit of a framework structure the name of the corresponding framework silicate is the basis of the code, e.g. the layer-like building units of cristobalite are named cri. In most cases the layer is known from a zeolite framework. Then, the corresponding 3-letter code of the zeolite framework type [19] is used as an acronym (however, in lower case and underlined). E.g. the layer-like building units of the zeolite sodalite (framework type SOD) are named sod.

The aim of this compilation is to serve as a helpful tool to identifying a material as a known HLSs. This listing should also be an aid to solve the structure of a new and, so far, unknown HLS.

Structure analysis of HLSs is usually difficult because of limited crystallinity of the materials. The weak bonding interaction between neighboring silicate layers leads to a more or less prominent disorder of layer stacking. As a consequence, diffractions peaks at angles higher than ca. 35° 2theta (Cu Kα) are often very weak or missing at all. This is far from atomic resolution and structure solution by Direct Methods usually fails. If, however, the type of silicate layer can be identified, model building is a very promising method to establish a meaningful structure model which, subsequently, can be refined.

This “Database of Hydrous Layer Silicates“ was arranged following the stile of the very useful publications “Atlas of zeolite framework types“ [19] and “Collection of simulated powder patterns for zeolites“ [20]. The HLS layer type corresponds to the zeolite framework type.

In contrast to identifying the layer type of a HLSs, it is relatively easy (though not in all cases) to identify the framework type of a new zeolite material if the framework type is already known. A variation of the chemical composition of the material (T-sites and/or the non-framework content) leads to only slightly deviating lattice parameters and therefore the XRD diffraction peaks are observed at similar positions for a given framework type.

In the case of a new and unknown HLS it is much more difficult to identify the layer type. HLSs possessing the same layer type but having cations of different size intercalated between the silicate layers or having different water contents will show considerably deviating lattice parameters and, consequently, quite different XRD patterns. Therefore, the XRD pattern is not in all cases useful as a tool to identifying the layer types. To partly overcome this shortcoming the FTIR spectra of the HLSs are also displayed as an additional information. FTIR spectra of HLSs with identical layer types show related patterns in the range between 400 and 1300 cm-1 (range of lattice vibrations). A visual comparison of FTIR spectra of HLSs with identical layer types show similar characteristics and differ apparently from spectra of HLSs containing silicate layers of another type.

This collection of data is a first draft and will be updated on a regular basis. In addition to the HLSs although some related materials are listed.

For each HLS there are up to seven items of information:

• A table with crystallographic and chemical data,
• the powder X-ray diffraction pattern,
• a list of the diffraction peaks,
• two structure plots,
• the fractional atomic coordinates,
• a CIF-file,
• the FTIR spectrum recorded by an ATR unit in reflexion mode.