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 investigation4-10, 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.

中文翻译：

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钛硅沸石-1双核位点的高效环氧化

钛硅沸石-1 (TS-1) 是一种具有 MFI 骨架结构的沸石材料，其中 1% 至 2% 的硅原子被钛原子取代。由于它能够用过氧化氢 (H2O2) 对烯烃进行催化环氧化，只留下水作为副产品 1,2，因此它在工业中得到了广泛应用；使用该工艺每年可生产约 100 万吨环氧丙烷3。TS-1 的催化性能通常归因于沸石骨架中存在孤立的 Ti (IV) 位点。然而，尽管进行了近 40 年的实验和计算研究 4-10，但由于完全表征 TS-1 的挑战，这些活性 Ti (IV) 位点的结构尚未得到证实。在这里，结合光谱学和显微镜，我们详细描述了一系列具有良好分散的钛原子的高活性和选择性 TS-1 丙烯环氧化催化剂。我们发现，在与 H217O2 接触时，所有样品都表现出特征性的固态 17O 核磁共振特征，表明在双核钛位点上形成了桥接过氧物种。此外，密度泛函理论计算表明，两个钛原子之间的协同作用使丙烯能够通过低能反应途径进行丙烯环氧化，该反应途径具有类似于过酸烯烃环氧化的关键氧转移过渡态。因此，我们提出双核钛位点，而不是框架中孤立的钛原子，解释了 TS-1 在丙烯与 H2O2 环氧化中的高效率。