Janusz Lipkowski

Abstract:

Molecular inclusion has been in chemical practice for over two centuries but it happened in the 1950^th when the physicochemical nature of the systems became clear to science. The subset named ‘clathrates’ was the first to be characterized on structural (1948) and thermodynamic (1959) background. Presently a very large class of compounds are considered as clathrates. These are either genuine clathrates, with no bonding interactions between host and guest components, and several structurally analogous compounds with stronger interactions, namely ionic, inorganic and other ‘clathrates’.

Water is extremely important host material, able to form a variety of clathrate or clathrate-like ‘compounds’. This behaviour was first discovered by Davy in 1809 who crystallized chlorine in the form of pentahydrate (as determined a couple of years later by Faraday). Characteristic feature of these compounds is non-stoichiometry, hence the ‘penta’-hydrate was the approximate value. Very many clathrate hydrates are known recently and some are of special technological importance. Of special value is methane clathrate. Huge amounts of this material have been found in permafrost regions and even much more is  present under the sea, at depths approximately equal 500 m or more. Since 2013 these deposits are being exploited, however experimentally, by Japan and somewhat later by China.  Some details will be discussed, including the idea of methane desorption combined with CO₂ sequestration. Ecological problems associated with the processes will be signalled.

Another class of inclusion compounds known sine the end of 19^th century are cyclodextrins, obtained by a special fermentation of starch. Also in this very case the technology-oriented studies started in mid XX century. Cyclodextrins, native and chemically modified,  are presently a huge industry worldwide providing enormously large variety of products, from pharmaceutical, food additives, environment friendly additives in technological processes, It will be briefly illustrated with the use of cyclodextrin derived anti-COVID preparations.

Of historical value are technological attempts to apply the high selectivity of molecular inclusion expressed by some coordination complexes towards petrochemical mixtures[KM1] , like C₈ (xylenes and ethylbenzene). Labofina has arrived up to semicommercial scale but the process appeared non-competitive due to a trivial problem of filtering fine powders of the clathrates. It is nevertheless important to point out extremely important and controllable selectivities in these systems enabling even efficient separations of mixtures of isotopomers. On analytical scale these compounds are used in thee, so-called, clathrate chromatography.

Other examples will be shown and illustrated, including the family of calixarene hosts, cucurbiturils and cavitands. Some dynamic properties of the solids will be demonstrated on selected examples.