New silica clathrate minerals that are isostructural with natural gas hydrates

Abstract

silica clathrate compounds (clathrasils) and clathrate hydrates are structurally analogous because both materials have framework structures with cage-like voids occupied by guest species. The following three structural types of clathrate hydrates are recognized in nature: cubic structure I (sI); cubic structure II (sII); and hexagonal structure H (sH). In contrast, only one naturally occurring silica clathrate mineral, melanophlogite (sI-type framework), has been found to date. Here, we report the discovery of two new silica clathrate minerals that are isostructural with sII and sH hydrates and contain hydrocarbon gases. Geological and mineralogical observations show that these silica clathrate minerals are traces of lowtemperature hydrothermal systems at convergent plate margins, which are the sources of thermogenic natural gas hydrates. Given the widespread occurrence of submarine hydrocarbon seeps, silica clathrate minerals are likely to be found in a wide range of marine sediments.

Supramolecular Organization in Confined Nanospaces

“…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 ,,…”

Physics of Hydrogen

“…Without the guest molecule, the clathrate structure is thermodynamically unstable. There are three different types of polyhedral cages, normally formed by hydrogen-bonded water molecules: cubic sI-hydrate, cubic sII-hydrate, and hexagonal sH-hydrate (Figure 24.17) [87,88]. The size and the number of the trapped molecules (N 2 , H 2 , THF, THF + H 2 , CH 4 , CO 2 + H 2 , TBAF + H 2 , HQ + H 2 , MTBE + H 2 ) determine the structure type as well as the temperature and pressure of clathrate formation [89].…”

“…The hydrogen capacity of the materials varies between 3.8 wt% (31 g l 1 ) for the sII-hydrate fully loaded with H 2 and 0.37 wt% (3.8 g l 1 ) for a sI-hydrate with ethylene oxide and H 2 as guest molecules [89]. Illustration taken from Reference [88]. Nature publishing group.…”

Clathrate Hydrates

“…The traditional use of the word clathrate in science and engineering is in reference to materials comprised of hosts that are polymers trapping guests that are atoms or molecules. However current use of this word now extends to many other molecular host systems such as calixarenes [6], cyclodextrins [7], and even to inorganic polymeric systems such as zeolites [8] and chibaites [9]. Hofmann compounds [10] and metal organic frameworks (MOFs) [11] can also serve as clathrate hosts for small aromatic guest molecules such as carbon tetrachloride, benzene, toluene, and xylene.…”