物理化学学报 >> 2010, Vol. 26 >> Issue (03): 679-684.doi: 10.3866/PKU.WHXB20100233

催化和表面结构 上一篇    下一篇

Nb2O5/γ-Al2O3表面铌氧物种的分散状态与酸性特征

何杰, 范以宁   

  1. 安徽理工大学化学工程学院, 安徽 淮南 232001; 南京大学化学化工学院, 介观化学教育部重点实验室, 南京 210093
  • 收稿日期:2009-08-28 修回日期:2009-12-03 发布日期:2010-03-03
  • 通讯作者: 何杰 E-mail:jhe@aust.edu.cn

Dispersion and Acidity of Niobia on Nb2O5/γ-Al2O3

HE Jie, FAN Yi-Ning   

  1. School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, Anhui Province, P. R. China; Key Laboratory of Mesoscopic Chemistry of the Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China
  • Received:2009-08-28 Revised:2009-12-03 Published:2010-03-03
  • Contact: HE Jie E-mail:jhe@aust.edu.cn

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摘要：

负载型Nb2O5是多种催化反应的有效催化剂. 以草酸铌为前驱物, γ-Al2O3为载体, 通过浸渍法制备不同负载量的Nb2O5/γ-Al2O3催化剂. 采用粉末X射线衍射(XRD)、激光拉曼光谱(LRS)和吡啶吸附傅立叶变换红外(Py-IR)光谱方法对催化剂表面铌氧(NbOx)物种的分散特征、酸性特征进行表征, 通过异丁烯(IB)与异丁醛(IBA)缩合生成2,5-二甲基-2,4-己二烯(DMHD)反应评价催化剂表面酸催化活性. 结果表明, Nb在γ-Al2O3表面的单层分散容量(ΓNb)为7.6 μmol·m-2, 与“嵌入模型”理论分析Nb5+分散在γ-Al2O3优先暴露晶面(110)上八面体空位中的单层分散容量值7.5 μmol·m-2接近, 即分散的Nb5+离子键合在γ-Al2O3表面八面体空位中. 在低负载量下, 分散在γ-Al2O3表面的Nb2O5主要以孤立的NbOx物种形式通过Nb—O—Al键与载体表面键合, 与LRS结果一致. 处于孤立状态下的NbOx物种使表面Lewis 酸位量下降. 随负载量的增加, 孤立的NbOx物种通过Nb—O—Nb键连接而聚集, 并形成表面Bronsted酸位, 随着NbOx聚集度增加, 表面Bronsted 酸密度增加, 酸性增强, 对IBA与IB缩合反应催化活性增加. 当负载量超过单层分散容量时, NbOx物种呈现三维聚集状态, DMHD的转化频率(TOF)降低, 同时表面Bronsted 酸性增强, 导致目标产物DMHD 的选择性降低. Nb2O5/γ-Al2O3催化剂表面Bronsted 酸特征与NbOx物种聚集状态密切相关.

关键词: 表面酸性, Nb2O5/γ-Al2O3, 表面NbOx物种, 烯醛缩合反应

Abstract:

Supported niobium pentoxide materials are effective catalysts for a variety of reactions. Nb2O5/γ-Al2O3 catalysts with different Nb2O5 loadings were prepared by aqueous solution impregnation using niobium oxalate as a precursor on γ-Al2O3. The samples were characterized with respect to the dispersion state of the niobium oxide species on γ-Al2O3 by X-ray power diffraction (XRD) and laser Raman spectroscopy (LRS). The nature of the surface acidity was investigated using Fourier-transform infrared spectroscopy of pyridine adsorption (Py-IR). The catalytic activity of the as-prepared catalysts was evaluated by the condensation reaction of iso-butene (IB) and iso-butyraldehyde (IBA) to form 2,5-dimethyl-2,4-hexadiene (DMHD). Results reveal that the dispersion capacity (ΓNb) of Nb on γ-Al2O3 is about 7.6 μmol·m-2. This value is almost identical to the density of the octahedral vacant sites of the preferentially exposed (110) plane (7.5 μmol·m-2) on the surface of the γ-Al2O3 support. Additionally, the“incorporated model”suggests that Nb5+ cations are located on the vacant sites of the (110) plane on γ-Al2O3. These results suggest that isolated niobia (NbOx) species are present and are bound to the surface of the γ-Al2O3 support through Nb—O—Al bonds at a loading well below that corresponding to monolayer dispersion. This is consistent with the result from LRS. The formation of isolated NbOx species, which binds to the surface of the support through Nb—O—Al bonds, causes a decrease in the amount of surface Lewis acid sites (LAS) on the Nb2O5/γ-Al2O3 catalysts. With an increase in Nb2O5 loading, polymeric NbOx species are formed by the Nb—O—Nb bridging of neighboring isolated NbOx species and Bronsted acid sites (BAS) are generated. We found that the catalytic activity towards the condensation reaction of IB and IBA to form DMHD increased because the amount and strength of the Bronsted acid sites increased as the number of polymeric niobia species increased. When the loading exceeds the monolayer dispersion capacity, the catalytic activity (turnover frequency (TOF) of DMHD) decreased because of the formation of the three-dimensional NbOx species. Additionally, the selectivity of DMHD decreased because of an increase in the strength of the Bronsted acid sites. We suggest that the strength of the Bronsted acid sites are related to the state of NbOx on the surface of the Nb2O5/γ-Al2O3 catalysts.

Key words: Surface acidity, Nb2O5/γ-Al2O3, NbOx surface species, Olefin-aldehyde condensation