Microporous solids based on pillared metal(IV) phosphates and phosphonates

After a brief illustration of the structures of three-layered zirconium phosphates, some general concepts on the influence of the structural characteristics on the distribution of the pillars, as well as some general strategies used to obtain microporous pillared compounds, are discussed. The synthesis of covalently pillared alpha-zirconium phosphite diphosphonates is first described and the use of pillars with bases for the creation of microporosity in the interlayer region is reported. The structure of a microporous compound in which the pillar with bases 3,3',5,5'-tetramethyl-biphenyldiphosphonate was used (interlayer microporosity 405 m(2) g(-1), micropore volume 0.16 cm(3) g(-1), pore size 0.5 nm), is described. The controversial results reported in the literature for microporous compounds of alpha-zirconium phosphate pillared with inorganic polyoxycations are also discussed. Particular emphasisis is then given to recent results on the preparation of microporous compounds by topotactic pillaring of gamma-zirconium phosphate with organic radicals. The preparation and structure of the compound pillared with approximate to 25% of biphenyldiphosphonate groups is illustrated. A large specific surface area (approximate to 320 m(2) g(-1)), a great micropore volume (0.12 cm(3) g(-1)) and a narrow micropore distribution with an average diameter of 0.58 nm, was obtained. The topotactic pillaring of gamma-zirconium phosphate with nonrigid pillars and with crown ethers, as well as the encouraging prospects for the preparation of microporous and thermally stable compounds from gamma-ZrP pillared with polyoxycations, are then examined. Finally, the possibility of preparing microporous solids by pillaring the recently discovered ZrPO(4)Cl(dmso) with dicarboxilates, is reported.