Mechanistic Insight into Propylene Epoxidation with H₂O₂ over Titanium Silicalite-1: Effects of Zeolite Confinement and Solvent

Abstract

Density functional theory (DFT) calculations were performed to investigate the effects of zeolite confinement and solvent on propylene epoxidation with H2O2 over the titanium silicalite-1 (TS-1) catalyst. The 144T and 143T cluster models containing typical 10MR channels of TS-1 were constructed to represent the tripodal­(2I) and Ti/defect sites. It was found that the confinement of the zeolite pore channel not only impacts the adsorption stability of guest molecules but also alters reaction barriers, as compared to the results obtained based on small cluster models. When dispersion corrections were considered, an enhancement of the adsorption stability of guest molecules was observed because of the important contribution from van der Waals interactions, especially for propylene adsorption. An explicit protic methanol molecule was introduced into the catalytic system to probe the influence of the solvent on propylene epoxidation, based on which a significant enhancement of CH3OH–H2O2 co-adsorption was obtained owing to H-bond formation. More importantly, the energy barrier for H2O2 dissociation was largely reduced by ∼13 kcal/mol because of the participation of the methanol in the H-transfer process and the formation of H-bond network, resulting in an alteration of the rate-limiting step. By comparison, adding an aprotic acetonitrile solvent did not have substantial effect on reaction path and kinetics. The calculation results clearly demonstrate the important role of the protic methanol solvent, which not only strengthens the adsorption of guest molecules but also promotes the kinetics for propylene epoxidation with H2O2 over TS-1 catalyst.

Resolving the Reaction Mechanism for Oxidative Hydration of Ethylene toward Ethylene Glycol by Titanosilicate Catalysts

“…Four candidate active sites participating alone refer to paths 1–4, whereas hydrolyzed titanium sites and perfect titanium sites coupling together refer to paths 5–6. In a previous work, based on small cluster models, ,, two types of mechanisms were proposed for olefin epoxidation with H 2 O 2 /TS-1, which are stepwise and concerted mechanisms. The stepwise mechanism contains two main steps, namely, H 2 O 2 dissociation and olefin epoxidation.…”

“…The energy barriers for the process in hydrolyzed (I) and hydrolyzed (II), including the abstraction of H from the framework and the desorption of EG, are 0.30 and 0.06 eV, respectively. In addition, there may be Si defect sites in TS-1 zeolite ,, (Figure S2c), which are similar to the hydrolysis model, so the catalytic activity of Si defect sites for this reaction was also considered here. As shown in Figure S3, the energy barrier of EO formation in first stage of reaction in the Si defect model is 1.7 eV, which is much higher than that of the hydrolyzed model.…”

“…In this work, we used cluster model to construct hydrolyzed sites and perfect sites to represent the active sites of TS-1, based on the lattice of silicalite-1 zeolite (MFI topological structure) containing six layers successively extended from the Ti center, which have been often employed in previous works. , The 35T cluster of hydrolyzed (I) and hydrolyzed (II) models were constructed with a Ti atom located at the T1 and T4 sites and represented as TiSi 34 O 47 H 48 and TiSi 34 O 46 H 50 , respectively. The isolated titanium perfect model and dinuclear titanium perfect model were correspondingly generated with Ti atoms located at the T1 and T6, T9 sites and represented as TiSi 53 O 80 H 56 (54T) and Ti 2 Si 49 O 73 H 58 (51T), respectively.…”

钛硅分子筛催化剂高效钛位点的理性构筑与调控及催化烯烃环氧化的性能

“…1 所示) [27][28][29] 。此外，该催化体系反应过程简单、可 操作性强 ；反应条件温和， 反应温度一般不高于 100 o C，反应压力多为常压，安全性能得到极大改善； 采用过氧化氢做氧化剂，产 物选择性高、环境污染较 小 [30][31][32] 。在上述体系中，钛硅分子筛中独特的选择性 氧化活性位为骨架中的钛位点 [33][34][35][36] ，因此，精 准构筑 环氧化钛活性位点成为理性设计高效 TS-1 催化剂的 关键。 图 1 TS-1 在不同反应中的应用…”

Method for introducing heteroatoms into the framework of FER-type zeolite by intercalation of sub-zeolite of Lamellar precusor PLS-3

“…Zeolites containing heteroatoms are a kind of functionally heterogeneous catalyst that can be used in a variety of industrial processes due to the heteroatom active sites they possess [1][2][3]. Many studies have been conducted on the application of metalosilicate zeolites, for example, aluminosilicate zeolites can be used for cracking [4], isomerization [5], and alkylation [6]; titanosilicate zeolites can be used for ammoximation [7] and epoxidation [8]; stanosilicate zeolites can be used for Baeyer-Villiger oxidation [9], Meerwein-Ponndorf-Verley reduction [10], isomerization of glucose to fructose [11], convert glyceraldehyde to alkyl lactates [12], and activate carbonyl groups efficiently [13]. Therefore, to further broaden the application of metalosilicate zeolites and create innovative heterogeneous catalysts with outstanding catalytic performance, it is critical to investigate a way of introducing heteroatoms into the framework of zeolites.…”