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

Abstract

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.

On configuration of exchanged La3+ on ZSM-5: A theoretical approach to the improvement in hydrothermal stability of La-modified ZSM-5 zeolite

“…Moreover, the incorporation of heteroatoms occurred through kinetically controlled process during the early stage of synthesis, and it has been proved that the computations based on thermodynamics failed to predict the experimental observed citing of Ti and Fe in zeolites. 30 We believe that such a behavior should also be valid to the process of Al substitution in ZSM-5 zeolite. Recently, Olson et al 17 have carried out x-ray diffraction experiments for extra-framework cesium cations and reached the conclusion that T4, 7, 11, and 12 would be the possible sites for Al substitution.…”

Density Functional Calculations on the Distribution, Acidity, and Catalysis of TiIV and TiIII Ions in MCM‐22 Zeolite

“…[51][52][53] Thus, the lattice O rather than the lattice Ti III radicals are crucial for the activation of alkane C À H bonds. The T7 site was chosen for the reactivity studies on TS-1 zeolite, [12][13][24][25] and the optimized structures are given in Figure S9 of the Supporting Information. The activation of a methane CÀH bond by the Ti7 III site requires an energy barrier of 17.8 kJ mol…”

“…There is abundant information on the distributions of triand tetravalent ions (e.g., B, Al, and Ti) in MFI-type zeolites. [16][17][18][19][20][21][22][23][24][25][26][27] Recently, the Al locations in MCM-22 zeolite were studied by Corma et al and our group. [28][29][30] It was found that these ions are not equally distributed among the various T sites.…”

“…[28][29][30] It was found that these ions are not equally distributed among the various T sites. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Herein, density functional calculations were employed to determine the Ti IV distribution in MCM-22 zeolite. The design of isolated titanium dioxide (TiO 2 ) within zeolites is currently a research focus, [31] owing to the unique photocatalytic reactivity of the Ti III sites for a variety Abstract: Isolated Ti species in zeolites show unique catalytic activities for a variety of chemical reactions.…”

Structure Analysis in Zeolite Research: From Framework Topologies to Functional Properties

“…The T sites which contain Ti are T3, T8, and T10, whereas all other sites are essentially free of Ti [ 96 ]. In a different neutron powder diffraction analysis [ 97 ], the authors suggested that T3, T7, T8, T10 or T6, T7, and T11, respectively, are preferentially occupied by Ti, with T10 showing only weak evidence [ 98 ]. However, the authors did not use samples isotopically enriched in Ti isotopes and, thus, were not able to differentiate contributions due to structural defects from those of Ti occupancy at specifi c T sites.…”