A titanosilicate molecular sieve with adjustable pores for size-selective adsorption of molecules

Zeolites and related crystalline microporous oxides-tetrahedrally coordinat ed atoms covalently linked into a porous framework-are of interest for appl ications ranging from catalysis to adsorption and ion-exchange(1). In some of these materials (such as zeolite rho) adsorbates(2), ion-exchange, and d ehydration and cation relocation(3,4) can induce strong framework deformati ons. Similar framework flexibility has to date not been seen in mixed octah edral/tetrahedral microporous framework materials, a newer and rapidly expa nding class of molecular sieves(5-16). Here we show that the framework of t he titanium silicate ETS-4, the first member of this class of materials(8), can be systematically contracted through dehydration at elevated temperatu res to 'tune' the effective size of the pores giving access to the interior of the crystal. We show that this so-called 'molecular gate' effect can be used to tailor the adsorption properties of the materials to give size-sel ective adsorbents(17) suitable for commercially important separations of ga s mixtures of molecules with similar size in the 4.0 to 3.0 Angstrom range, such as that of N-2/CH4, Ar/O-2 and N-2/O-2.