Yu Yunkai, Wang Jianhao, Fang Nan, Chen Zhen, Liu Dongxu, Liu Yueming, He Mingyuan
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, P. R. China.
School of Energy and Materials, Shanghai Polytechnic University, Shanghai, 201209, China.
Phys Chem Chem Phys. 2023 May 3;25(17):12220-12230. doi: 10.1039/d2cp05483c.
The catalytic performance of titanosilicates involving hydrogen peroxide (HO) as the oxidant is strongly influenced by the solvents. Until now, there is still a lack of a universal principle that can guide the choice of a solvent. Herein, the kinetics of HO activation catalyzed by various titanosilicates in different solvents is investigated, and an isokinetic compensation effect is concluded. This indicates that the solvent participates in the HO activation process for the formation of a Ti-OOH species. Additionally, the results of isotopically labeled infrared spectra preliminarily confirm that the solvent acts as the mediator to promote the proton transfer during the HO activation process. The catalytic activities of a series of TS-1 catalysts toward 1-hexene epoxidation are compared, which include Ti(OSi)OH species with a range of densities but a constant total Ti content. This reveals that the solvent effect is closely related to the Ti active sites of these TS-1 catalysts. Based on these results, a principle for the rational choice of solvent for this catalytic process is proposed. ROH is found to be the mediator for Ti(OSi) sites, and methanol, which has a strong proton-donating ability, is the best solvent for these sites. However, for the Ti(OSi)OH sites, water (HO) is the mediator, and a weaker hydrogen bonding between HO molecules promotes proton transfer more effectively. The solvent influences the catalytic performance by perturbing the hydrogen bonds between the HO molecules, and aprotic acetonitrile, which has a strong ability to break the hydrogen bonding network between HO molecules, is the best solvent for Ti(OSi)OH sites. This study provides experimental evidence that the solvent promotes the catalytic performance of titanosilicates by assisting the proton transfer during the catalytic HO activation process, which will pave the way toward the rational choice of solvent for the titanosilicate-catalyzed oxidation systems.
以过氧化氢（HO）作为氧化剂时，钛硅酸盐的催化性能会受到溶剂的强烈影响。到目前为止，仍然缺乏一个能够指导溶剂选择的通用原则。在此，研究了不同溶剂中各种钛硅酸盐催化HO活化的动力学，并得出了等动力学补偿效应。这表明溶剂参与了HO活化过程以形成Ti-OOH物种。此外，同位素标记红外光谱的结果初步证实，溶剂在HO活化过程中作为促进质子转移的介质。比较了一系列TS-1催化剂对1-己烯环氧化的催化活性，这些催化剂包括具有一系列密度但总Ti含量恒定的Ti(OSi)OH物种。这表明溶剂效应与这些TS-1催化剂的Ti活性位点密切相关。基于这些结果，提出了该催化过程中合理选择溶剂的原则。发现ROH是Ti(OSi)位点的介质，而具有强质子供体能力的甲醇是这些位点的最佳溶剂。然而，对于Ti(OSi)OH位点，水（HO）是介质，HO分子之间较弱的氢键更有效地促进了质子转移。溶剂通过扰乱HO分子之间的氢键来影响催化性能，具有强破坏HO分子之间氢键网络能力的非质子乙腈是Ti(OSi)OH位点的最佳溶剂。本研究提供了实验证据，证明溶剂在催化HO活化过程中通过协助质子转移来促进钛硅酸盐的催化性能，这将为钛硅酸盐催化氧化体系中溶剂的合理选择铺平道路。
