Acid Zeolites: An Attempt to Develop Unifying Concepts (P. H. Emmett Award Address, 1981)

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Abstract
The recent developments of zeolite chemistry are characterized by the appearance of attempts to explain certain properties of these porous crystalline solids in terms of general chemical properties developed for other solids or even for other states of matter. Transition metal complexes in zeolites sometimes exhibit the same properties as in more conventional solvents [1]. Thus the zeolite behaves as a mono to polydentate macroligand of the transition metal ion [2]. Although numerous observations [2] underline the role of the zeolite as a solid solvent or as a common solid matrix, its unique properties in terms of structure or cage geometry should not be overlooked. Indeed, the appropriate activation of Ru(III) hexammine in faujasite-type zeolites gives a very active water gas-shift catalyst [3]. A cationic Ru-complex seemed to be formed under these conditions, having no homogeneous equivalent, and its stabilization requires the specific geometry of a faujasite supercage.