The Siting of Ti in TS-1 Is Non-Random. Powder Neutron Diffraction Studies and Theoretical Calculations of TS-1 and FeS-1

We present the first direct evidence of non random siting of Ti and Fe in TS-1 and FeS-1, nanoporous metallosilicate selective oxidation catalysts of MFI topology. This was accomplished by using Rietveld analysis of powder neutron diffraction data and exploiting the differences in neutron scattering lengths between Ti or Fe and Si. Previous spectroscopic, X-ray diffraction, and computational approaches have suggested a random substitution of Ti and Fe ions among the 12 crystallographically distinct Si sites in the framework of TS-1 and FeS-1. In contrast, our results indicate that titanium is distributed among only 4 or 5 of the 12 silicon sites with Ti occupying T3, -7, -8, -10, and -12. Of the 2.47 total Ti atoms per unit cell the Ti site occupancies and estimated standard deviation for sample B are as follows: T3 0.30(0.11), T7 0.34(0.14), T8 0.92(0.10), T10 0.41(0.14), and T12 0.50(0.14). In FeS-1 synthesized with 1.5 Fe atoms per unit cell, iron is found only at T8. Several starting models were chosen for initial refinement, and each returned the same specific, nonrandom distribution of Ti in the framework of MFI. We have examined several computational approaches that involve thermodynamic arguments to rationalize the experimental observations, and all have failed to predict the experimentally observed substitution pattern. This suggests that the kinetics of framework formation may play a role in directing the observed metal substitution.