Titanium silicalite-1 (TS-1) is a zeolitic material with MFI framework structure, in which 1 to 2 per cent of the silicon atoms are substituted for titanium atoms. It is widely used in industry owing to its ability to catalytically epoxidize olefins with hydrogen peroxide (H2O2), leaving only water as a byproduct1,2; around one million tonnes of propylene oxide are produced each year using this process3. The catalytic properties of TS-1 are generally attributed to the presence of isolated Ti(iv) sites within the zeolite framework1. However, despite almost 40 years of experimental and computational investigation4,5,6,7,8,9,10, the structure of these active Ti(iv) sites is unconfirmed, owing to the challenges of fully characterizing TS-1. Here, using a combination of spectroscopy and microscopy, we characterize in detail a series of highly active and selective TS-1 propylene epoxidation catalysts with well dispersed titanium atoms. We find that, on contact with H217O2, all samples exhibit a characteristic solid-state 17O nuclear magnetic resonance signature that is indicative of the formation of bridging peroxo species on dinuclear titanium sites. Further, density functional theory calculations indicate that cooperativity between two titanium atoms enables propylene epoxidation via a low-energy reaction pathway with a key oxygen-transfer transition state similar to that of olefin epoxidation by peracids. We therefore propose that dinuclear titanium sites, rather than isolated titanium atoms in the framework, explain the high efficiency of TS-1 in propylene epoxidation with H2O2. This revised view of the active-site structure may enable further optimization of TS-1 and the industrial epoxidation process.
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All data that led us to the conclusions presented here are available with the paper or from the corresponding author on reasonable request. Source data are provided with this paper.
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C.P.G. thanks the Scholarship Fund of the Swiss Chemical Industry (SSCI) for funding. H.E. thanks the Fonds der Chemischen Industrie for a Kekulé doctoral fellowship. We thank F. Gaida for recording the UV–vis spectra of compounds 1, 1-O2 and the salalen ligand. We acknowledge B. Hinrichsen and P. Müller from BASF for the XRD and TEM measurements, and S. S. Lee from ScopeM at ETH Zürich for assistance with Raman measurements. We acknowledge J. Sauer, A. Comas-Vives, W.-C. Liao, K. Yamamoto, D. Mance, G. Noh, C. Ehinger, E. Lam, L. Lätsch and J. Meyet for discussions.
The authors declare no competing interests.
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Extended data figures and tables
a–e, Solid-state 17O NMR spectra of the five TS-1 samples: 1.9 wt% Ti (sample 1; a), 1.9 wt% Ti (sample 2; b), 1.5 wt% Ti (c), 1.0 wt% Ti (d) and 0.5 wt% Ti (e). f, Solid-state 17O NMR spectrum of H217O (red line, DFT-calculated spectrum).
a–e, STEM-EDX images of the five TS-1 samples: 1.9 wt% Ti (sample 1; a), 1.9 wt% Ti (sample 2; b), 1.5 wt% Ti (c), 1.0 wt% Ti (d) and 0.5 wt% Ti (e).
a, b, SEM image (left) and HR-TEM image (right) of the TS-1 samples with 1.9 wt% Ti (a, sample 1; b, sample 2). c, d, HR-TEM images of the TS-1 samples with 1.0 wt% Ti (c) and 1.5 wt% Ti (d).
a–e, Powder XRD of the five TS-1 samples (after calcination): 1.9 wt% Ti (sample 1; a), 1.9 wt% Ti (sample 2) with signs of an anatase phase marked by arrows (b), 1.5 wt% Ti (c), 1.0 wt% Ti (d) and 0.5 wt% Ti (e).
The Raman spectra were measured at a wavelength of 266 nm. At this wavelength, the signals associated with TiOx clusters are not resonance-enhanced, so similar features are observed in all spectra.
The spectra are shifted in steps of 20% on the vertical axis to enable direct comparison.
a, Calculated potential-energy surface for propylene epoxidation on a mononuclear Ti site, via a bis-hydroperoxo species. b, Analogous mechanism via a hydroperoxo species. c, Potential-energy surface for propylene epoxidation from a hydroperoxo species on an isolated Ti site with an adjacent vacancy. Relative electronic energies (E) are given in kcal mol−1.
a, Cluster models used to calculate the NMR signatures shown in Fig. 2. b, Structure and simulated spectrum (red) of a mononuclear peroxo species with adjacent vacant site; the experimental spectrum of TS-1 impregnated with H217O2 is shown in blue. c, Structure and simulated spectrum (red and purple) of a mononuclear hydroperoxo species with an adjacent vacant site; the experimental spectrum is shown in blue. d, Structure and simulated spectrum (red and purple) of mononuclear hydroperoxo species, in which the hydroperoxo ligand is oriented differently compared to the structure used to calculate the spectrum shown in Fig. 2; the experimental spectrum is shown in blue.
a, Solid-state 17O NMR spectrum of TS-1 (1.9 wt%, sample 1) contacted with H217O. b, Solid-state 17O NMR spectrum of Ti-free MFI contacted with H217O2.
UV–vis spectra of the Berkessel ligand (blue), the Berkessel–Katsuki catalyst 1 (red) and the corresponding peroxo species 1-O2 (green) are shown. The spectra were acquired in dichloromethane solution, with concentrations of 0.049 mmol l−1, 0.022 mmol l−1 and 0.022 mmol l−1, respectively.
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Gordon, C.P., Engler, H., Tragl, A.S. et al. Efficient epoxidation over dinuclear sites in titanium silicalite-1. Nature 586, 708–713 (2020). https://doi.org/10.1038/s41586-020-2826-3