Density functional theory was applied to investigate the structure of the framework titanium (Ti) species in the Ti-YNU-1 zeolite, and to evaluate its catalytic activity for 1-hexene epoxidation with H₂O₂ as the oxidant. The effects of solvent coordination on the stability and reactivity of Ti-hydroperoxo intermediates were studied. All calculations were performed at the theoretical level of B3LYP/6-31G(d,p) based on cluster models. Results suggested that a defective Ti-hydroxyl species [Ti(OSi)₃OH] at the pillar (T6) site between the interlayers in the Ti-YNU-1 zeolite was responsible for the Ti-specific vibrational frequency observed at 930 cm⁻¹, and was in good agreement with the experimental infrared spectra obtained for the Ti-rich Ti-YNU-1 zeolite. The stabilities of the possible Ti-hydroperoxo intermediates decreased in the order Ti-η²(OOH)–H₂O > Ti-η²(OOH)–CH₃CN > Ti-η²(OOH) as well as Ti-η²(OOH)–H₂O > Ti-η¹(OOH). The activation barriers of 1-hexene epoxidation over Ti6-η²(OOH), Ti6-η²(OOH)-H₂O, and Ti6-η²(OOH)-CH₃CN were 33.2, 35.0, and 43.9 kJ mol⁻¹, respectively. The Ti6-η¹(OOH) species possessed lower stability and had structural changes in the transition state for 1-hexene epoxidation, leading to the same transition-state structure as that of the bidentate Ti-η²(OOH)–H₂O species. For comparison, the reactivity of the perfect Ti(OSi)₄ species at the T1 site in the Ti-MWW zeolite was also investigated. The transition state of Ti1-η¹(OOH) was obtained with an activation barrier of 107.2 kJ mol⁻¹, which was much higher than that of Ti1-η²(OOH)–H₂O. This result revealed that the intramolecular hydrogen bonding in the monodenate five-membered ring structure did not enhance the stability and reactivity of Ti-hydroperoxo intermediates. These results also indicated that Ti-YNU-1 was more active than Ti-MWW, which was in good agreement with experimental results.

中文翻译：

---

Ti-YNU-1沸石中新提出的钛物种的结构和催化活性：密度泛函理论研究

应用密度泛函理论研究了Ti-YNU-1分子筛中骨架钛（Ti）的结构，并评价了其以H ₂ O ₂为氧化剂对1-己烯环氧化的催化活性。研究了溶剂配位对Ti-氢过氧中间体稳定性和反应活性的影响。所有计算均基于聚类模型在B3LYP / 6-31G（d，p）的理论水平上进行。结果表明，在Ti-YNU-1分子筛的中间层之间的支柱（T6）处，缺陷的Ti-羟基物质[Ti（OSi）₃ OH]导致了在930 cm ^-1处观察到的Ti比振动频率，与富钛的Ti-YNU-1沸石获得的实验红外光谱非常吻合。可能的Ti-氢过中间体的稳定性的顺序的Ti-η降低²（OOH）-H ₂ O>的Ti-η ²（OOH）-CH ₃ CN>的Ti-η ²（OOH）以及钛- η ²（OOH）-H ₂ O>的Ti-η ¹（OOH）。1-己烯的环氧化的过度TI6-η活化障碍²（OOH），TI6-η ²（OOH）-H ₂ O，和TI6-η ²（OOH）-CH ₃ CN为33.2，35.0，43.9和千焦摩尔^-1。所述TI6-η ¹（OOH）物种具有较低的稳定性并且具有在1-己烯的环氧化的过渡态的结构变化，导致相同的过渡态的结构，所述二齿的Ti-η的²（OOH）-H ₂ O类物质。为了进行比较，还研究了Ti-MWW沸石中T1位置处完美Ti（OSi）₄物种的反应性。TI1-η的过渡状态¹（OOH）与107.2千焦mol的活化能垒得到^-1，这比TI1-η的高得多的²（OOH）-H ₂O.该结果表明，单缩合物五元环结构中的分子内氢键没有增强Ti-氢过氧中间体的稳定性和反应性。这些结果还表明，Ti-YNU-1比Ti-MWW更具活性，这与实验结果非常吻合。