波谱学杂志 ›› 2007, Vol. 24 ›› Issue (3): 368-370.

• 博士论文摘要 • 上一篇    下一篇

固体酸催化剂及分子筛晶化过程的核磁共振研究

作者:徐君 导师:邓风   

  1. 中国科学院 武汉物理与数学研究所, 湖北 武汉 430071
  • 收稿日期:1900-01-01 修回日期:1900-01-01 出版日期:2007-09-05 发布日期:2009-12-05

NMR Studies on Solid Acids and Crystallization of Molecular Sieves

Author: XU Jun Advisor: DENG Feng   

  1. Wuhan Institute of Physics and Mathematics, The Chinese Academy of Sciences, Wuhan 430071, Hubei, China
  • Received:1900-01-01 Revised:1900-01-01 Online:2007-09-05 Published:2009-12-05

摘要: 介绍了一些常规固体核磁共振的技术的发展状况, 并综述了固体核磁共振在多孔材料中的应用情况, 其中包括沸石分子筛酸性和结构表征以及目前在分子筛合成机理研究中的应用进展. 本文的主要内容是利用固体核磁共振结合其它表征技术研究了一类介孔固体酸催化材料的酸性以及两种微孔磷铝分子筛的合成晶化过程. 
利用有机聚合物为模板剂合成出两种介孔复合氧化物MoOx/ZrO2与WOx/ZrO2固体酸材料. 并对其进行了物理化学性质的表征. 研究发现它们在较高的焙烧温度下仍保持有较高的比表面积及规整的孔径. 利用固体核磁共振技术及DFT计算的方法发现在介孔WOx/ZrO2及MoOx/ZrO2表面存在两种类型的Br[AKo¨D]nsted酸位, 其酸强度强于传统的HZSM-5分子筛, 与典型的固体强酸硫酸锆相当, 但弱于100%的硫酸. 借助于理论计算清楚地揭示了酸性位的具体结构和酸强度信息, 且计算结果与实验结果符合得非常好. 除了弱酸性的Zr-OH基团外, 在介孔氧化锆表面存在大量的Lewis酸位(配位不饱和的Zr4+). 在引入Mo或W物种后, Mo-OH或W-OH与不饱和的Zr4+配位产生作为Br[AKo¨D]nsted酸位的桥式Mo-OH-Zr(或W-OH-Zr)羟基, 并且导致氧化锆表面Lewis酸位的大量减少. 利用理论计算证实了单聚或多聚体形式的桥式Mo-OH-Zr(或W-OH-Zr)羟基是强Br[AKo¨D]nsted酸性位产生的根源, 同时提出了酸性位形成的机理.
利用水热晶化法合成了AlPO4-5和MgAPO-36两种具有重要工业潜在应用价值的分子筛材料并用固体核磁共振技术结合X射线衍射、傅里叶红外、高分辨电子显微镜等技术研究了它们的详细晶化过程. 研究发现在HF存在条件下加热120 min后具有AFI类型的骨架晶化开始. 此时31P NMR谱中在δ -22与 δ -29处出现两个来源于骨架P-O-Al单元的信号, 19F NMR谱中在 δ -120处出现来源于骨架F-Alpen-O-P单元的信号, 证实了晶体骨架开始形成. 另外, 凝胶中五配位Al(F-Alpen-O-P)的出现是分子筛晶化开始的另一个标志. 利用27Al→31P HETCOR、 31P{27Al} TRAPDOR及1H→31P CP/MAS等双共振实验详细研究了初始凝胶相及晶化开始时凝胶相的结构, 在120 min加热凝胶中区分出具有不同化学环境的配位P原子, 提出了详细的晶化过程机理. 
对于另一种具有高活性的镁取代的MgAPO-36分子筛催化材料, XRD发现在第二阶段423 K时, 加热1.5 h后骨架晶化开始. 31P NMR证实此时凝胶相中形成了P-O-Al及Mg-O-P骨架单元, 此后(2 h)凝胶相中出现半晶相的棒状颗粒. 并最终晶化为不规则球状的MgAPO36晶体(18 h后). 利用27Al→31P HETCOR及1H→31P CP/MAS分析了中间凝胶相的微观结构并利用31P{27Al} TRAPDOR NMR技术研究骨架晶化开始时凝胶相中P(nAl) (n=1~4)的配位结构, 得出了5种具有不同配位状态的微观化学环境, 提出了详细的晶化过程机理.

关键词: 固体核磁共振技术, 多孔材料合成, 固体酸位表征, 分子筛, 晶化机理

Abstract: In the chapter 1 of this dissertation, various solid-state NMR techniques and their applications to the structure and acidity characterization of zeolites as well as the synthesis mechanism of molecular sieves were briefly introduced. Chapter 2 focus on the synthesis of mesoporous solid acids and their NMR characterization. The characterization of crystallization process of two kinds of molecular sieves is the subject of the later two chapters.
Mesoporous ZrO2, MoOx/ZrO2 and WOx/ZrO2 materials were prepared and their solid acidity was thoroughly studied by solid-state NMR techniques and DFT calculations. Two distinct types of Br[AKo¨D]nsted acid sites with acid strength stronger than zeolite HZSM-5, comparable to sulfated zirconia, but still weaker than 100% H2SO4, were found to be present on the mesporous MoOx/ZrO2 and WOx/ZrO2 materials. With the help of theoretical calculation, the detailed structures of Br[AKo¨D]nsted sites formed on the surface of the mesoporous catalyst were revealed and the predicted acid strengths of these sites were in good agreement with experimental observations. Besides the weak acidic Zr-OH groups, Lewis acid sites (coordinatively unsaturated Zr4+ sites) are present on the surface of mesoporous ZrO2. After the introduction of Mo or W species, the coordination of Mo-OH or W-OH to the unsaturated Zr4+ sites leads to the appearance of bridging Mo-OH-Zr (or W-OH-Zr) hydroxyl groups that act as Br[AKo¨D]nsted acid sites, and a remarkable decrease in the concentration of Lewis acid sites present on the surface of ZrO2. The bridging Mo-OH-Zr or W-OH-Zr hydroxyl groups in the form of monomer and oligomer states are responsible for the strong Br[AKo¨D]nsted acidity of the MoOx/ZrO2 and WOx/ZrO2 materials. Based on our NMR experimental and theoretical calculation results, a possible mechanism was proposed for the formation of acid sites on these mesoporous materials.
Two types of industrially important molecular sives AlPO4-5 and MgAPO-36 were prepared under the hydrothermal condition. Multinuclear solid-state NMR techniques, combined with powder X-ray diffraction (PXRD), infrared (IR) and SEM spectroscopy, were employed to monitor their crystallization process. For AlPO4-5, the crystallization process is characterized by the evolution of intermediate gels, in which the long-rang ordering arrangement is probed by PXRD, revealing the threshold of the crystallization around 120 min. The appearance of 31P signals at ca. δ -22 and δ -29 due to the structural P-O-Al unit and 19F signal at δ -120 due to the structural F-Alpen-O-P unit in the NMR spectra of the series intermediate gel indicates that the crystalline framework is starting to form. The onset of the crystallization is also evidenced by the presence of the pentacoordinated Al in the structural F-Alpen-O-P unit which is considered to be associated with the ordered framework. More information about the local ordering of the gels is obtained from two-dimensional 27Al→31P HETCOR and 31P/27Al doubleresonance experiments. In combination with 1H→31P CP/MAS experiments, two micro-domains can be identified in the 120 min heated gel. A possible evolution mechanism consisting of three successive stages is proposed for the crystallization process.
For the magnesium substituted aluminophosphate MgAPO-36, the long-range ordering arrangement of the aged as well as heated intermediate gels were probed by PXRD, revealing that the crystallization of the framework begins at about 1.5 h at 423 K, when the structural Al-O-P and Mg-O-P units from the ATS framework were proved to be formed by 31P NMR. After the stick-like crystallites start to form in the semi-crystalline phase, they consequently aggregate into irregular sphere crystals. More information about the local ordering of the gels is obtained from two-dimensional 27Al→31P HETCOR and 1H→31P CP/MAS experiments. Different micro-domains can be identified with varied condensation degree of the P coordinating sphere, in which five type of P(nAl) (n=1~4) units were determined by 31P/27Al double-resonance experiments. A possible evolution mechanism consisting of three stages is proposed for the crystallization process.

Key words: solid state NMR technique, porous materials preparation, solid acid characterization, molecular sieves, crystallization mechanism