Journal Article

Efficient epoxidation over dinuclear sites in titanium silicalite-1

Plodinec, Milivoj, Tragl, Amadeus Samuel, Teles, Joaquim Henrique, Parvulescu, Andrei-Nicolae, Copéret, Christophe, Gordon, Christopher P., Berkessel, Albrecht, Engler, Hauke, Lunkenbein, Thomas

2020

Overview

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 (H 2 O 2 ), leaving only water as a byproduct 1 , 2 ; around one million tonnes of propylene oxide are produced each year using this process 3 . The catalytic properties of TS-1 are generally attributed to the presence of isolated Ti( iv ) sites within the zeolite framework 1 . However, despite almost 40 years of experimental and computational investigation 4 – 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 H 2 17 O 2 , all samples exhibit a characteristic solid-state 17 O 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 H 2 O 2 . This revised view of the active-site structure may enable further optimization of TS-1 and the industrial epoxidation process. A combination of spectroscopy, microscopy and theoretical calculations shows that the reactivity of titanium silicalite-1 as an epoxidation catalyst is due to the presence of dinuclear titanium sites.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 119/118 / 140/131 / 140/133 / 639/166/898 / 639/638/77/885 / 639/638/898 / Alkenes / Analysis / Atomic properties / Binding sites (Biochemistry) / Catalysts / Computer applications / Density functional theory / Epoxidation / Ethylene oxide / Humanities and Social Sciences / Hydrogen / Hydrogen peroxide / Influence / Ligands / Magnetic resonance / Metal catalysts / multidisciplinary / NMR / Nuclear magnetic resonance / Optimization / Oxygen / Oxygen transfer / Peracids / Production processes / Propylene oxide / Science / Science (multidisciplinary) / Signatures / Silicalite / Spectroscopy / Spectrum analysis / Structure / Titanium / Titanium compounds / Transmission electron microscopy / Zeolites