The High-Performance Hollow Silicalite-1@Titanium Silicalite-1 Core-Shell Catalyst for Propene Epoxidation

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.

The synthesis of nano-sized TS-1 zeolites under rotational crystallisation conditions can inhibit anatase formation

Hydrothermal synthesis is a typical method for the preparation of TS-1. In the current study, agitation was introduced during the crystallisation stage, and nano-sized TS-1 with little anatase TiO₂ was successfully synthesised in a short time (1-8 h). Furthermore, under rotational crystallisation conditions, a series of TS-1 samples was prepared with different crystallisation times, and the products obtained were investigated as catalysts for the oxidative desulfurisation of thiophene from a model fuel. All samples were characterised utilising X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂-adsorption, ultraviolet-visible (UV-Vis) spectroscopy and Fourier-transform infrared (FT-IR) spectroscopy techniques. It was found that under rotational crystallisation conditions, nano-sized TS-1 could be synthesised in a short time that had improved efficiency for the oxidative desulfurisation reaction of thiophene in comparison with TS-1 synthesised by static crystallisation for 24 h.

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

Hollow core-shell structured TS-1@S-1 zeolite (HCS-TS) was prepared successfully for the first time, which exhibited excellent activity in the epoxidation of alkenes. Combining TEM, UV-vis, UV-Raman, pyridine-IR, solid-state MAS NMR, XPS and so on characterization, the improvement in the catalytic performance of hollow core-shell structured TS-1@S-1 zeolite was credited to the newly formed superior active sites: defective Ti(OSi)₃(OH) species in HCS-TS and six-coordinated titanium active species in uncalcined HCS-TS (HCS-TS^P). Interestingly, these two different titanium active species in the samples could be constructed through calcination or not in the same synthesis process. A possible formation mechanism was investigated in detail; it indicated that the hollowing treatment of TS-1 in the first step was conducive to the construction of the new superior active sites in the samples, and there was a synergistic effect on the formation of these active sites between TPAOH and TEOS in the second step of the synthesis process. This strategy is feasible to enhance the catalytic performance of TS-1, and is suitable for the synthesis of TS-1 on an industrial scale.