Hollow core-shell structured TS-1@S-1 as an efficient catalyst for alkene epoxidation

TS-1@MCM-48 Core-Shell Catalysts for Efficient Oxidation of p-Diethylbenzene to High Value-Added Derivatives

To expand the market capacity of p-diethylbenzene (PDEB), core-shell zeolite (TS-1@MCM-48) is designed as a catalyst for PDEB oxidation. TS-1@MCM-48 catalyst is synthesized by in-situ crystallization method and characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), N2 adsorption-desorption, in-situ electron paramagnetic resonance (EPR) and 29Si nuclear magnetic resonance (29Si MAS-NMR). Oxidation of PDEB by H2O2 was investigated systematically in liquid phase. The conversion of PDEB over TS-1@MCM-48 was 28.1 % and the total selectivity was 72.6 %, where the selectivity of EAP (p-ethylacetophenone) and EPEA (4-ethyl-α-methylbenzyl alcohol) was 28.6 % and 44.0 %, respectively. Compared with TS-1 and MCM-48 zeolite, the conversion rate of reactants and the selectivity of products have been significantly improved. The catalytic performance of TS-1@MCM-48 is derived from its well-crystallized microporous core and mesoporous shell with regular channels, which make active sites of TS-1 zeolite in the catalyst be fully utilized and mass transfer resistance be largely reduced. Further through theoretical calculation, we propose that the oxidation of PDEB is the result of the combination and mutual transformation of free radical process and carbocation process. Core-shell structure ensures the conversion rate of raw materials and improves the selectivity of products.

Review and perspectives on TS-1 catalyzed propylene epoxidation

Titanium silicate zeolite (TS-1) is widely used in the research on selective oxidations of organic substrates by H2O2. Compared with the chlorohydrin process and the hydroperoxidation process, the TS-1 catalyzed hydroperoxide epoxidation of propylene oxide (HPPO) has advantages in terms of by-products and environmental friendliness. This article reviews the latest progress in propylene epoxidation catalyzed by TS-1, including the HPPO process and gas phase epoxidation. The preparation and modification of TS-1 for green and sustainable production are summarized, including the use of low-cost feedstocks, the development of synthetic routes, strategies to enhance mass transfer in TS-1 crystal and the enhancement of catalytic performance after modification. In particular, this article summarizes the catalytic mechanisms and advanced characterization techniques for propylene epoxidation in recent years. Finally, the present situation, development prospect and challenge of propylene epoxidation catalyzed by TS-1 were prospected.

NMR Signatures and Electronic Structure of Ti Sites in Titanosilicalite-1 from Solid-State 47/49Ti NMR Spectroscopy

Although titanosilicalite-1 (TS-1) is among the most successful oxidation catalysts used in industry, its active site structure is still debated. Recent efforts have mostly focused on understanding the role of defect sites and extraframework Ti. Here, we report the ^47/49Ti signature of TS-1 and molecular analogues [Ti(OTBOS)₄] and [Ti(OTBOS)₃(OⁱPr)] using novel MAS CryoProbe to enhance the sensitivity. While the dehydrated TS-1 displays chemical shifts similar to those of molecular homologues, confirming the tetrahedral environment of Ti consistent with X-ray absorption spectroscopy, it is associated with a distribution of larger quadrupolar coupling constants, indicating an asymmetric environment. Detailed computational studies on cluster models highlights the high sensitivity of the NMR signatures (chemical shift and quadrupolar coupling constant) to small local structural changes. These calculations show that, while it will be difficult to distinguish mono- vs dinuclear sites, the sensitivity of the ^47/49Ti NMR signature should enable distinguishing the Ti location among specific T site positions.

Nano-Cavities within Nano-Zeolites: The Influencing Factors of the Fabricating Process on Their Catalytic Activities

Titanium silicalite-1 (TS-1) is a milestone heterogeneous catalyst with single-atom tetrahedral titanium incorporated into silica framework for green oxidation reactions. Although TS-1 catalysts have been industrialized, the strategy of direct hydrothermal synthesis usually produces catalysts with low catalytic activities, which has still puzzled academic and industrial scientists. Post-treatment processes were widely chosen and were proven to be an essential process for the stable production of the high-activity zeolites with hollow structures. However, the reasons why post-treatment processes could improve catalytic activity are still not clear enough. Here, high-performance hollow TS-1 zeolites with nano-sized crystals and nano-sized cavities were synthesized via post-treatment of direct-synthesis nano-sized TS-1 zeolites. The influencing factors of the fabricating processes on their catalytic activities were investigated in detail, including the content of alkali metal ions, the state of titanium centers, hydrophilic/hydrophobic properties, and accessibility of micropores. The post-treatment processes could effectively solve these adverse effects to improve catalytic activity and to stabilize production. These findings contribute to the stable preparation of high-performance TS-1 catalysts in both factories and laboratories.