As clathrate compounds

Although not classifiable as clathrate compounds, many other crystalline substances have holes, channels, or honeycomb structures which allow inclusion of foreign molecules, and many studies have been made in this field. Urea is an example of an organic compound which in the crystal has parallel continuous uniform capillaries it may be utilized to separate straight-chain hydrocarbons from branched-chain ones, the latter being unable to fit into the capillaries. 

A series of silica end-members of zeolites (zeosils) as well as clathrate compounds with framework composition Si02 (clathrasils) have been synthesized in the recent past [1, 2). It has been concluded that the organic guest species alone act as templates for the formation of the open framework and that these guest molecules have a structure directing role. In a detailed study it has been shown that the size and dimensionality of the pore system in porous tectosilicates is dependent on the properties of the guest species used for synthesis [3]. 

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