Efficient epoxidation over dinuclear sites in titanium silicalite-1

Abstract

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...

Reaction Pathway Discrimination in Alkene Oxidation Reactions by Designed Ti‐Siloxy‐Polyoxometalates

“…in 1986, [3] resulted in the emergence of heterogeneous catalysts of practical use for the liquid‐phase oxidation of organic compounds at laboratory and industrial scale, particularly propene to propene oxide using H 2 O 2 as the terminal oxidant [4–7] . The catalytic performances of crystalline microporous titanium‐silicalites TS‐1 are particularly associated to the structural peculiarities of the silica lattice, among which hydrophobicity nature and pore size of the internal channel system are the most relevant, [8] along with the formation of the reactive hydroperoxo moiety, either on isolated titanium sites [9,10] or on dinuclear sites [11] . Indeed, strong physisorption of small apolar substrates is favored whereas water remains outside the pores (in the liquid phase) thus limiting hydrolysis of the isolated active titanium center.…”

Transcriptomics-based drug repositioning pipeline identifies therapeutic candidates for COVID-19

“…By blocking dopamine (mainly D2) receptors in the brain, haloperidol eliminates dopamine neurotransmission which leads to improvement of psychotic symptoms 35 . Haloperidol can also bind to the sigma-1 and sigma-2 receptors, which are implicated in lipid remodeling and cell stress response 12 . As reported by Gordon et al 12 , the SARS-CoV-2 proteins Nsp6 and ORF9c interact with the sigma-1 receptor and the sigma-2 receptor, respectively.…”

“…Haloperidol can also bind to the sigma-1 and sigma-2 receptors, which are implicated in lipid remodeling and cell stress response 12 . As reported by Gordon et al 12 , the SARS-CoV-2 proteins Nsp6 and ORF9c interact with the sigma-1 receptor and the sigma-2 receptor, respectively. Moreover, they found that haloperidol decreased viral replication in SARS-CoV-2-infected Vero E6 cells with, based on their reported pIC50 and pCC50 values, a calculated Selectivity Index (SI) of 53.7 12 .…”

“…Drug repositioning is being extensively explored for COVID-19. High-throughput screening pipelines have been implemented in order to quickly test drug candidates as they are identified 12 – 15 . In the past, our group has successfully applied a transcriptomics-based computational drug repositioning pipeline to identify novel therapeutic uses for existing drugs 16 .…”

Direct propylene epoxidation with oxygen enabled by photo-electro-heterogeneous catalytic system

“…Recently, a propylene epoxidation process using hydrogen peroxide (H 2 O 2 ) as an oxidising agent (hydrogen peroxide-propylene oxide, HPPO) has received substantial attention because water is the only coproduct and it shows high PO selectivity (> 95%) 11 . The HPPO process itself is environmentally benign; however, current H 2 O 2 production through the anthraquinone process involves multi-step reactions and produces large volumes of organic waste 12 .…”