Synthesis, crystallization mechanism, and catalytic properties of titanium-rich TS-1 free of extraframework titanium species
Synthesis, crystallization mechanism, and catalytic properties of titanium-rich TS-1 free of extraframework titanium species
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无骨架钛物质富钛TS-1的合成、结晶机理及催化性能
DOI：
10.1021/ja7100399
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发表时间：
2008-08-06
影响因子：
15
通讯作者：
Tatsumi, Takashi
中科院分区：
文献类型：
--
作者：
Fan, Weibin;Duan, Ren-Guan;Tatsumi, Takashi
使用 (NH4)(2)CO3 作为结晶介导剂开发了一条合成 TS-1 的新路线。这样，可以显着增加骨架Ti含量，而不形成骨架外Ti物种。所制备的催化剂具有低至 34 的 Si/Ti 比，而 Enichem 小组 (Clerici, M. G.; Bellussi, G.; Romano, U. J Catal. 1991, 129,159) 和 Thangaraj 和 Sivasanker (Thangaraj, A.; Sivasanker, S. J Chem. Soc., Chem.通讯，1992，123）。该材料比其他两种方法合成的样品含有更少的缺陷位点。因此，它对各种有机底物（例如直链烷烃/烯烃和醇、苯乙烯和苯）的氧化表现出更高的活性。通过X射线衍射(XRD)、场发射扫描电子显微镜(FE-SEM)、热重/差热分析(TG/DTA)、电感耦合等离子体原子发射光谱(ICP)、傅里叶变换红外光谱(FTIR)、X射线光电子能谱(XPS)、漫反射紫外-可见光谱和Si-29 MAS（魔角旋转）核磁共振波谱技术。结果表明，(NH4)(2)CO3 的存在不仅大大降低了 pH 值，减慢了结晶过程，使 Ti 掺入骨架中与成核和晶体生长相匹配，而且还改变了结晶机制。尽管在晶体生长期间少量非胶凝Ti转变为固体，但似乎在由固化凝胶的解离、重组和再聚结引发的结晶过程中固相转变机制占主导地位。相比之下，方法A系统中发生了典型的均相成核机制。因此，尽管在方法A系统中，大部分Ti阳离子在结晶几乎完成后插入到晶格中，但Ti的包含在(NH 4 )(2)CO 3 存在下的早期成核期开始。这有利于Ti掺入骨架中，使Ti在骨架中分布更加均匀。在整个结晶过程中收集的样品上1-己烯和2-己醇的氧化表明，即使在结晶完成后，Ti-OH和Si-OH的缩合仍在进行。这导致疏水性增加和钛物质微观特征的整体改善，从而随着结晶的进一步进展，催化活性大大提高。
A new route to the synthesis of TS-1 has been developed using (NH4)(2)CO3 as a crystallization-mediating agent. In this way, the framework Ti content can be significantly increased without forming extraframework Ti species. The prepared catalyst had a Si/Ti ratio as low as 34 in contrast to the ratio of 58 achieved with the methods A and B established by the Enichem group (Clerici, M. G.; Bellussi, G.; Romano, U. J Catal. 1991, 129,159) and Thangaraj and Sivasanker (Thangaraj, A.; Sivasanker, S. J Chem. Soc., Chem. Commun. 1992, 123), respectively. The material contained less defect sites than the samples synthesized by the other two methods. As a result, it showed much higher activity for the oxidation of various organic substrates, such as linear alkanes/alkenes and alcohols, styrene, and benzene. The crystallization mechanism of TS-1 in the presence of (NH4)(2)CO3 was studied by following the whole crystallization process with X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), thermogravimetry/differential thermal analysis (TG/DTA), inductively coupled plasma atomic emission spectrometry (ICP), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), diffuse reflectance UV-vis spectroscopy, and Si-29 MAS (magic-angle spinning) NMR spectroscopy techniques. It was shown that the presence of (NH4)(2)CO3 not only drastically lowered down pH, slowing down the crystallization process and making the incorporation of Ti into the framework match well with nucleation and crystal growth, but also modified the crystallization mechanism. It seems that the solid-phase transformation mechanism predominated in the crystallization process initiated by dissociation, reorganization, and recoalescence of the solidified gel although a small amount of nongelatinated Ti shifted to the solid during the crystal growth period. In contrast, a typical homogeneous nucleation mechanism occurred in the method A system. Thus, although in the method A system most of Ti cations was inserted into the lattice after the crystallization was nearly completed, the inclusion of Ti started at the earlier nucleation period in the presence of (NH4)(2)CO3. This is favorable for the incorporation of Ti into the framework, resulting in a more homogeneous distribution of Ti in the framework. Oxidation of 1-hexene and 2-hexanol over the samples collected during the whole crystallization process indicated that condensation of Ti-OH and Si-OH proceeded even after the crystallization was completed. This resulted in an increase in hydrophobicity and an overall improvement in microscopic character of Ti species and consequently a great increase in the catalytic activity with further progress of crystallization.