Structural investigation of TS-1: Determination of the true nonrandom titanium framework substitution and silicon vacancy distribution from powder neutron diffraction studies using isotopes

The first complete investigation of the structure of the selective oxidatio n catalyst TS-I is presented. Constant wavelength powder neutron diffractio n data collected on isotopically substituted titanium silicalite (TS-1) sam ples, with a Si:Ti molar ratio of 39:1, have been studied using a combinati on of single and multiple data set Rietveld analysis exploiting the scatter ing length contrast between the different titanium isotopes and silicon. Th is has allowed both the silicon vacancy and titanium site substitution dist ributions to be determined, which has not been possible previously. Both di stributions are found to be nonrandom with the titanium preferentially subs tituting on 3 of the 12 crystallographically independent framework sites-T8 , T10 and T3 (in order of decreasing titanium content)-and silicon vacancie s on 2 framework sites-T1 and T5. This study illustrates the power of isoto pic substitution in powder neutron diffraction experiments to yield enhance d structural information in complex systems.