Clathrate compounds of silica

Abstract

A review on silica clathrate compounds, which are variants of pure silica zeolites with relatively small voids, is presented. Zeolites have found many uses in industrial and domestic settings as materials for catalysis, separations, adsorption, ion exchange, drug delivery, and other applications. Zeolites with pure silica frameworks have attracted particular interest because of their high thermal stability, well-characterized framework structures, and simple chemical compositions. Recent advances in new synthetic routes have extended the structural diversity of pure silica zeolite frameworks. Thermochemical analyses and computational simulations have provided a basis for applications of these materials and the syntheses of new types of pure silica zeolites. High-pressure and high-temperature experiments have also revealed diverse responses of these framework structures to pressure, temperature, and various guest species. This paper summarizes the framework topologies, synthetic processes, energetics, physical properties, and some applications of silica clathrate compounds.

Supramolecular Organization in Confined Nanospaces

“…Some silicate minerals are structurally very close to zeolites. These polymorphs of SiO 2 , known as clathrasils, have cavities of sub‐nanometer size like zeosils (all‐silica zeolites), but with such small openings to prevent diffusion in and out of the pores;, they might thus be pictured as inert hosts for nanomaterials of zero‐dimensionality. Although melanophlogite (Figure ), contains only very small molecules, like CH 4 , CO 2 or N 2 ,,, other hydrocarbons are found in clathrasils with larger cavities:, indeed, the recently discovered chibaite hosts methane, ethane, propane and isobutane ,,…”

“…Although melanophlogite ( Figure 4) [125,[272][273][274] contains only very small molecules, like CH 4 , CO 2 or N 2 , [272,275,276] other hydrocarbons are found in clathrasils with larger cavities: [119,277] indeed, the recently discovered chibaite [278] hosts methane, ethane, propane and isobutane. [119,277,278] These minerals ( Figure 10) are silica analogs of the wellknown natural gas-storage media methane hydrates. [279,280] Melanophlogite, for example, is isostructural with "structure I" (sI) hydrates (Figure 10a), which have two kinds of cages: two pentagonal dodecahedra [5 12 ] (with 12-sides) and six tetrakaidecahedra [5 12 6 2 ] (with 14-sides) in the unit cell ( Figure 11a).…”

“…This interplay holds for clathrates as well: structure H hydrate was first created in laboratory, and then found in nature . Rare silica minerals similar to zeolites and isostructural with gas hydrates, the clathrasils, contain organic molecules – methane and other simple hydrocarbons – which recall the templates of synthetic zeolites (Figure ) . Some natural zeolites exhibit chirality (goosecreekite) or extraordinary structural complexity, like paulingite (Figure ).…”

High–temperature Raman spectroscopic study of CO₂–containing melanophlogite

“…Melanophlogite is one of rare silica minerals with a clathrate structure (e.g., Momma, 2014). One of notable features of this natural clathrasil is the existence of two large cages which can host small guest molecules such as CH 4 , CO 2 , N 2 , and H 2 S (Gies, 1983;Tribaudino et al, 2008).…”

Structural Evolution of CO2-Filled Pure Silica LTA Zeolite under High-Pressure High-Temperature Conditions

“…There exist a large number of synthetic porous silica structures [11]. Zeolites are a class subdivision consisting of structures with channel-like voids with windows large enough for guest species to pass through [12,13].…”