Perspectives Lecture
Composition of Gas Hydrates New Answers to an Old Problem George H. Cady University of Washington, Seattle, WA 98195 Gas hydrates are nonstoichiometric crystalline solids classed as clathrate compounds. They deserve attention in elementary chemistry courses because they are interesting and are of increasing importance. Many gases of small molecular size form hydrates by becoming trapped in cavities in solid water. The crystal is then held together by hydrogen bonding between water molecules and by van der Waals' attraction between the water lattice and trapped molecules. This concept was first stated by M. von Stackelberg in 1949 (1) (Cady's translation) "The water framework of the gas hydrate can be r~. e"~~~ ~ n r d e das an 'ice modification' which is stable only if its (.;witie; are iilkd nit11,liemi~illl\inert. 33 ne3rly :I.;pc.+ih.e s i ~ e . "'l'hi:. atrtl~.turt.Illenti; snherical. mul~.;.~lesi i the rinht " that the substances are clathrates (from clathratus-enclosed or protected by cross bars of a grating) as defined by H. M. Powell in 1948 (2). Chemical Abstracts now hsts these substances as inclusion comwounds. clathrates. Much work has been done with gas hydrates and an excellent review has been written bv D. W. Davidson ( 3 ) .The present article deals largely with the composition of the solids. Topics of practical importance include the following: 1) A cold uioeline carrvine moist natural gas may become plugged with'solid gas hidrate (mostly methane hydrate). 2) Gases mav be stored under relatively low pressures as their hydrates. ~ o ; e x a m ~ l echlorine , hydrate may serve as the stored source of oxidizing agent (chlorine) in an electrochemical cell. 3) Through formation of their hydrates, some gases may be separated from mixtures. 4) Desalination of water may he accomplished by: (a) forming a gas hydrate in saline water, (h) removing the solid hydrate from the remaining saline water, and (c) decomposing the hydrate to obtain fresh water and recover the pas. ' l ~ h cnwsl impc~r1311r]ma.l:,-31 t t ~ p ki n d v l G the hydra11 u i natur.~lga. ~mgldlym~~tlinne hydmtt I . 'l'his sdid is k~wwlt lo t,wm i n ahallc~u$,IS mt,ll< i n u e r n i ~ t r ~ ~ ;arms . i t j t the 1S R . ~~~~~-~ Alaska, and Canada. It is also known to occur in the bottom of the Caspian and Black Seas and in the deep ocean bottom. While the extent of this resource is not well known, it may be sufficient to supply the world with fossil fuel for many years ( 4 , 5 ) .Production of gas from the solid deposit of hydrate is a problem, because decomposition of the hydrate to give gas, plus water or ice, is a reaction which absorbs heat. The discovery of naturally occurring methane hydrate was made in the USSR in the 1960's. A Discovery Certificate (a great honor) was awarded by the Soviet Commission of Inventions to the team of persons responsible: N. Chersky, F. Makogon, F. Trebin, A. Trofimuk, and V. Vasiliev (6). Over a period of several years Stackelberg and his coworkers a t the Universitv of Bonn studied structures using X-ray crystallography and came to recognize two types of crystals: 1)"Die Gas hvdrate" formed bv small molecules such as HzS, Sb2, Clz, etc.; 2) "Die ~lussi~keithydrate" formed by larger molecules such as C2HsC1, C H d , CHClR,etc. ( I ) . The commonly used names for these, structure I and structure 11, were introduced later bv Stackelberg and Miiller (7). The deter~ S I r ~ mthe work d t h r w min:ttim o f ccmw I S I ~ J V I U ~re+111f.d s < m r ~ ~I sI :Stat k1.11)~r:'s ~ n t I , ~7 t htypv-, ahile I ~ G ~ I . , . I I ~ ~ . in the r.,nct. I ;~rtt slwcttd I,! i i r intu "larxe" " -LO to:ihwr i . t i . but not "small" cavities. The unit cell of structure I1 is a cube of side lenpth close to
in stkcture I1 hydrates of single gases range in diameter from about 5.6 to 6.6 A. One may expect only the 16-hedral cavities to he occupied by molecules of this size. If all of these cavities were to be filled by guest molecules, the hydration number would be 13618 = 17. If structure I1 solid containing no guest molecules could be obtained, its density would be about 0.786 glml. The above discussion makes it appear that gas hydrates are likely to have empirical formulas M.5.75 H20, M.I213 H20, and M.17 H20. While the latter formula is close to that for structure I1 hvdrates. the first two are not good re~resentationsof actual compositions. Only a part of the available cavities are filled. and the extent of filline" is deuendent uuon temuerature and gas pressure and also upon size of the guest molecules.
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A Bit ol History
The history of gas hydrates begins with chlorine hydrate. In 1823 a paper by M. Faraday, chemical assistant in the Royal Institution, was entitled "On Hydrate of Chlorine" (13). Faraday credited Sir Humphry Davy with recognizing the substance to be a compound of chlorine with water. Faraday prepared crystals of the hydrate, dried them by removing
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This DaOer was oresented as the Persoectives Lecture, under the sponsorship of thk~ivisionof Chemical 'Education, at t h e American Chemical Society Meeting held in St. Louis. March 1983. ~
Volume 60
Number 11 November 1983
915
liquid solution with paper at 32OF, added them to water in a weighed vessel, reduced the chlorine to chloride ion, added silver nitrate, and then weighed the resulting silver chloride. From the weight of the latter and weight of hydrate he ohtained an average composition corresponding to CI~12.7HzO. The individual value which he favored corresponded to Clz 70.3- H-0. -.
~ a v y 'paper i was his famous Bakerian Lecture on Nov. 15, 1810 (14) in which he showed that "oxymuriatic gas" did not contain oxygen and that it could he regarded as an element. He orooosed the name "chlorine". Davv's statement referred to dy ~ ' a r a d a ywas, "It is generally stated in chemical books, that oxvmuriatic gas is cavahle of heina condensed and crvstallizedat a low timpera
