Titanium Silicalite-1 (TS-1) shows an outstanding ability to catalytically epoxidize olefins with hydrogen peroxide (H2O2), leaving only water as byproduct. 1,2 Despite the industrial use of the TS-1/H2O2 system for the production of more than one million tons of propylene oxide per year, 3 the active site structure remains elusive, although it has been studied for almost 40 years by spectroscopic and computational methods. 4-10 TS-1 is a zeotype of MFI structure in which a small fraction of Si-atoms (1-2 %) are substituted by Ti, and its catalytic properties are generally attributed to isolated Ti(IV) sites. 1 Herein, we analyze a series of highly active and selective TS-1 propylene epoxidation catalysts. By UV-Vis and Raman spectroscopy, as well as electron microscopy, we show that Ti is well-dispersed in all samples, with formation of small TiOx clusters at high Ti-loadings. Most notably, irrespective of Ti-content, all samples show a characteristic solid-state 17 O NMR signature when contacted with H2 17 O2, indicating the formation of bridging peroxo species on dinuclear Tisites. Using DFT (density functional theory) calculations, we propose a mechanism of propylene epoxidation on a dinuclear site, in which the cooperativity between two titanium atoms enables a low-energy reaction pathway where the key oxygen-transfer transition state bears strong resemblance to that of olefin epoxidation by peracids.The active species in TS-1 are commonly proposed to be isolated Ti(IV) sites bearing peroxo 11 or hydroperoxo moieties, 12 although the involvement of terminal Ti-oxo and activated H2O2 on Ti(IV) has also been discussed (Fig. 1a,b). 7 In contrast, the only homogeneous Ti-based epoxidation catalysts able to efficiently utilize H2O2 as primary oxidant are dinuclear, such as the Berkessel-Katsuki epoxidation catalyst 1 (Fig. 1c). [14][15][16][17][18][19] While the structural characterization of molecular systems is well-established and has enabled the isolation of peroxo compounds, obtaining information on the structure of Ti-sites in TS-1 with molecular-level precision has proven more challenging.Recent work by some of us has shown that solid-state 17 O NMR spectroscopy is a powerful tool for understanding and assessing the reactivity of peroxo species. 20 Oxygen-17 is an NMRactive quadrupolar nucleus whose spectroscopic properties can be readily measured by solidstate NMR and computed by DFT. The NMR signature (chemical shift and quadrupolar coupling) is highly sensitive to the symmetry and electronic structure around the oxygen atoms. We thus reasoned that 17 O NMR spectroscopy would be a valuable tool to harness the signature of the active sites in TS-1 and thereby probe their structure. In this study, we investigate five TS-1 samples prepared in the BASF laboratories (Table 1). 21,22 Two of these samples have a Ti-content of 1.9 wt%, one of which was prepared on hundred-kg scale (sample 1), the other three samples have Ti-loadings of 1.5 wt%, 1.0 wt%, and 0.5 wt%. The five samples have surface areas between 4...

Substitution effects on magnetism of shandite-type compounds have been studied by density functional theory. The decrease of the Fermi level in the novel half-metallic ferromagnet Sn2Co3S2 to higher maxima of the density of states was modeled for substitutions on the Co site by the 3d metals Fe, Mn and Cr due to a rigid band scheme. Spin-polarized energy hyper surfaces and densities of states are calculated for Sn2Co3S2, and experimentally not yet known Sn2Fe3S2, Sn2Mn3S2 and Sn2Cr3S2 with shandite-type structure. The stability of half-metallic ferromagnetic characteristics, Slater-Pauling behavior, and alternative metastable spin states are discussed.

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