Fan Weibin, Duan Ren-Guan, Yokoi Toshiyuki, Wu Peng, Kubota Yoshihiro, Tatsumi Takashi
Catalytic Chemistry Division, Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan.
J Am Chem Soc. 2008 Aug 6;130(31):10150-64. doi: 10.1021/ja7100399. Epub 2008 Jul 10.
A new route to the synthesis of TS-1 has been developed using (NH4)2CO3 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)2CO3 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 (29)Si MAS (magic-angle spinning) NMR spectroscopy techniques. It was shown that the presence of (NH4)2CO3 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)2CO3. 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.
已开发出一种以碳酸铵((NH₄)₂CO₃)作为结晶介导剂来合成TS-1的新路线。通过这种方式，可以显著提高骨架钛含量，而不会形成骨架外钛物种。所制备的催化剂的硅钛比低至34，相比之下，埃尼化学集团（Clerici, M. G.; Bellussi, G.; Romano, U. J. Catal. 1991, 129, 159）以及Thangaraj和Sivasanker（Thangaraj, A.; Sivasanker, S. J. Chem. Soc., Chem. Commun. 1992, 123）分别建立的方法A和方法B所得到的硅钛比为58。该材料的缺陷位点比通过其他两种方法合成的样品少。因此，它对各种有机底物的氧化表现出更高的活性，如直链烷烃/烯烃、醇类、苯乙烯和苯。通过采用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以及(²⁹)Si MAS(魔角旋转)核磁共振光谱技术跟踪整个结晶过程，研究了(NH₄)₂CO₃存在下TS-1的结晶机理。结果表明，(NH₄)₂CO₃的存在不仅显著降低了pH值，减缓了结晶过程，使钛掺入骨架的过程与成核和晶体生长良好匹配，而且改变了结晶机理。虽然在晶体生长阶段有少量未凝胶化的钛转移到固体中，但在由固化凝胶的解离、重组和再聚结引发的结晶过程中，似乎固相转变机理占主导。相比之下，方法A体系中发生典型的均相成核机理。因此，虽然在方法A体系中，大部分钛阳离子在结晶几乎完成后才插入晶格，但在(NH₄)₂CO₃存在下，钛的掺入在更早的成核阶段就开始了。这有利于钛掺入骨架，导致钛在骨架中的分布更均匀。对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，钛物种的微观特性整体改善，因此随着结晶的进一步进行，催化活性大幅提高。
