Characterization and reactivity of Fe-[Al,B]MFI catalysts for benzene hydroxylation with N2O ☆
Characterization and reactivity of Fe-[Al,B]MFI catalysts for benzene hydroxylation with N2O ☆
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DOI：
10.1016/j.apcata.2006.03.018
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发表时间：
2006-06
影响因子：
5.5
通讯作者：
G. Centi;S. Perathoner;Rosa Arrigo;G. Giordano;A. Katović;V. Pedulà
中科院分区：
文献类型：
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作者：
G. Centi;S. Perathoner;Rosa Arrigo;G. Giordano;A. Katović;V. Pedulà
研究了通过改变水热合成过程中的凝胶组成(Si/Al和Si/B比)和其他合成条件(硅源、磷酸对络合铁的使用)制备的Fe-[B，Al]MFI催化剂的特性。用UV-Vis DR光谱分析了样品的组成、结构(X射线衍射、傅立叶变换红外光谱)和形貌(扫描电子显微镜)，确定了铁物种的分布。结果与这些样品的反应性(苯酚的产率和苯与N2O羟基化过程中的选择性)相关联。Si/B比与苯酚最大产率之间存在线性关系。这表明，决定催化性能的主要因素是沸石中引入的B的量，而B的引入又取决于水热合成过程中的凝胶组成和制备方法。考虑到B的作用与其相对于Al更容易从骨架迁移到骨架外位置，从而更容易在热处理过程中产生缺陷位，可以解释这种线性关系。这些缺陷位置很重要，因为在催化反应的初始阶段，铁离子迁移到这些位置，创建高活性的铁位置，负责最初苯酚生产率的增加。凝胶中Al的存在对催化性能有负面影响，因为Al和B都以T原子的形式与B竞争进入MFI结构。此外，Al作为T原子的存在决定了苯酚在碳氧化物和碳质沉积形成方面的选择性降低。
The characteristics of Fe-[B,Al]MFI catalysts prepared by changing: (i) the gel composition during the hydrothermal synthesis (Si/Al and Si/B ratio) and (ii) other synthesis aspects (silica source, use of phosphoric acid to complex iron) were studied. The samples were investigated in terms of composition, structural (X-ray diffraction (XRD), Fourier transform-IR (FT-IR)) and morphological (scanning electron microscopy (SEM)) aspects and distribution of the iron species determined by UV–vis DR spectroscopy. The results were correlated with the reactivity of these samples (phenol productivity and selectivity during benzene hydroxylation with N2O). A linear relationship was observed between Si/B ratio and phenol maximum productivity. This indicates that the main factor determining the catalytic performances is the amount of B introduced in the zeolite which in turn depends on the gel composition during hydrothermal synthesis and the preparation method. The linear relationship could be interpreted considering that the role of B is related to its easier migration from framework to extra-framework position with respect to Al and therefore easier creation of defect sites during heat treatment. These defect sites are important, because during the initial stage of the catalytic reaction, iron ions migrate to these sites creating highly active iron sites responsible of the initial increase in the phenol productivity. The presence of Al in the gel influences negatively the catalytic performances, because both Al competes with B to be introduced in the MFI structure as T atom. Furthermore, the presence of Al as T atom determines a lowering of the selectivity to phenol with respect to both carbon oxides and carbonaceous deposits formation.