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首页> 外文期刊>The journal of physical chemistry, C. Nanomaterials and interfaces >Computational Assessment of Relative Sites Stabilities and Site-Specific Adsorptive Properties of Titanium Silicalite-1
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Computational Assessment of Relative Sites Stabilities and Site-Specific Adsorptive Properties of Titanium Silicalite-1

机译：钛硅铁 - 1的相对位点稳定性和位点特异性的计算评估

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摘要

Titanium Silicalite-1 (TS-1), because of its crystalline structure and its well-defined Ti sites, represents the prototype of a single site catalyst. According to this fundamental aspect and to the relevant role of TS-1 as selective catalyst in important industrial partial oxidation reactions, TS-1 has been widely characterized through both experimental and computational techniques. Still, several fundamental aspects of its structural and catalytic properties have to be addressed. Among these, an intriguing topic is the Ti location in the various sites of the MFI framework. The independent sites are generally considered to be 12, following the Pnma space group of TS-1 at high Ti loading. However, when Ti loading is lower than 2 atoms per unit cell, diffraction showed that the system must be described by the P2(1)/n space group, thereby allowing 24 independent sites. With respect to previous studies, this work aims to exploit this datum to give a more accurate description of the TS-1 system at low Ti loadings, adopting a state of the art methodology (all electron periodic B3LYP-D2 calculations). The relative stabilities of the 24 Ti sites have been evaluated, showing a good agreement with previous studies. The simulation of adsorption energies for ammonia (present as reactants in some of the most important industrial reactions catalyzed by TS-1) over the most stable sites have been computed as well, in order to validate the obtained models. Additionally to binding energies, adsorption enthalpies and Gibbs free energies have been obtained through an approximate reduced Hessian scheme. The improved local description of the Ti sites (in combination with the adducts stabilities given by the energetic data) allowed the deep understanding of subtle effects, such as the number of molecular ligands each Ti atom can actually host upon adsorption. These results, showing only few sites can efficiently host two ligands in the neighborhoods of STP conditions, allowed for the first time the heterogeneity in the experimental outcomes reported over the last two decades to be rationalized.
机译：由于其结晶结构及其明确的Ti位点，钛硅钛-1（TS-1）表示单个位点催化剂的原型。根据该基本方面和TS-1的相关作用作为在重要的工业部分氧化反应中的选择性催化剂，通过实验和计算技术得到了广泛的特征。仍然，必须解决其结构和催化性质的若干基本方面。其中，有趣的主题是MFI框架的各个站点中的TI位置。在高Ti负载下，在TS-1的PNMA空间组之后，独立的位点通常被认为是12。然而，当Ti负载为每单位细胞的2个原子时，衍射显示，该系统必须由P2（1）/ N空间组描述，从而允许24个独立位点。关于以前的研究，这项工作旨在利用该基准，以便在低TI负载下提供对TS-1系统的更准确描述，采用现有技术的状态（所有电子周期B3LYP-D2计算）。已经评估了24个Ti位点的相对稳定性，显示出与以前的研究吻合良好。还计算了氨的吸附能量的模拟（以TS-1催化的一些最重要的工业反应中的反应物存在于最稳定的位点，以验证所获得的模型。另外，通过近似的Hessian方案获得吸附焓和Gibbs自由能。改进Ti位点的局部描述（与能量数据给出的加合物稳定性组合）允许深入了解微妙效应，例如分子配体的数量，每个Ti原子在吸附时实际上可以宿主。这些结果仅显示少数位点可以在STP条件附近有效宿主两种配体，首次允许在过去二十年中报告的实验结果中的异质性进行合理化。

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