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

Zeolites and related crystalline microporous oxides—tetrahedrally coordinated atoms covalently linked into a porous framework—are of interest for applications ranging from catalysis to adsorption and ion-exchange 1 . In some of these materials (such as zeolite rho) adsorbates 2 , ion-exchange, and dehydration and cation relocation 3 , 4 can induce strong framework deformations. Similar framework flexibility has to date not been seen in mixed octahedral/tetrahedral microporous framework materials, a newer and rapidly expanding class of molecular sieves 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 . Here we show that the framework of the titanium silicate ETS-4, the first member of this class of materials 8 , can be systematically contracted through dehydration at elevated temperatures 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-selective adsorbents 17 suitable for commercially important separations of gas mixtures of molecules with similar size in the 4.0 to 3.0 Å range, such as that of N 2 /CH 4 , Ar/O 2 and N 2 /O 2 .