Clathrates, organic

Urea has the remarkable property of forming crystalline complexes or adducts with straight-chain organic compounds. These crystalline complexes consist of a hoUow channel, formed by the crystallized urea molecules, in which the hydrocarbon is completely occluded. Such compounds are known as clathrates. The type of hydrocarbon occluded, on the basis of its chain length, is determined by the temperature at which the clathrate is formed. This property of urea clathrates is widely used in the petroleum-refining industry for the production of jet aviation fuels (see Aviation and other gas-TURBINE fuels) and for dewaxing of lubricant oils (see also Petroleum, refinery processes). The clathrates are broken down by simply dissolving urea in water or in alcohol. 

Several alternative methods have been considered in order to increase the energy density of natural gas and facilitate its use as a road vehicle fuel. It can be dissolved in organic solvents, contained in a molecular cage (clathrate), and it may be adsorbed in a porous medium. The use of solvents has been tested experimentally but there has been little improvement so far over the methane density obtained by simple compression. Clathrates of methane and water, (methane hydrates) have been widely investigated but seem to offer little advantage over ANG [4]. Theoretical comparison of these storage techniques has been made by Dignam [5]. In practical terms, ANG has shown the most promise so far of these three alternatives to CNG and LNG. 

Clathrate Hydrates of Natural Gases, E. Dendy Sloan, Jr. Catalysis of Organic Reactions, edited by Dale W. Blackburn Fuel Science and Technology Handbook, edited by James G. Speight... 

The range of inclusion adducts formed by the organophosphazenes is very broad, the guest species varying from aliphatic and aromatic hydrocarbons to ethers, ketones and alcohols42. Some of the hosts [e.g., tris(o-phenylenedioxy)cyclotriphosphazene 12)] form clathrates not only when recrystallized from organic solvents but also... 

Systems that react in this manner fall into two classes. In the first of these the framework that dominates the crystal structure scarcely participates in the reaction. This is the case, for example, in the reaction of an organic molecule intercalated in graphite or a clay, or of a guest molecule held in a clathrate of urea or thiourea. Some cases of this sort will be treated in the next section. 

Natural gas (methane) can be obtained from gas hydrates. Gas hydrates are also called clathrates or methane hydrates. Gas hydrates are potentially one of the most important energy resources for the future. Methane gas hydrates are increasingly considered a potential energy resource. Methane gas hydrates are crystalline solids formed by combination of methane and water at low temperatures and high pressures. Gas hydrates have an iee-hke crystalline lattiee of water molecules with methane molecules trapped inside. Enormous reserves of hydrates can be foimd imder eontinental shelves and on land under permafrost. The amount of organic... 

Clathrate hydrates can be formed not only with neutral organic molecules but also with alkylammonium salts with small anionic counteranions [37] and with guests involving polymeric anions [38]. 

Depending on conditions, frozen substances in comet nuclei can be crystalline ices, amorphous ices, and clathrate hydrates (compounds in which cages in the water-ice lattice can host guest molecules). Compositions of the ices and associated organic materials in comets have been determined from both telescopic and spacecraft observations. Spectral line measurements of gases in a comet s coma allow the identification of molecules and radicals. An inherent difficulty in spectral measurements is that volatiles in the coma are commonly broken... 

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