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[资源] 非硅基介孔分子筛（过渡金属、稀土）的合成

非硅基介孔分子筛（过渡金属、稀土）的合成导读、评论

缘由
前段时间看文献，无意中看见德国应用化学杂志上一篇文章——VOCs在Au负载介孔MnOx催化剂上的催化燃烧，文章报道该催化剂对常见VOCs具有室温削除效果，活性特别高！由于本人目前也做VOCs催化燃烧，而且是做Mn相关的，于是乎对文章进行了跟踪、扩展，觉得我也可以尝试一下介孔金属氧化物的催化活性。接下来，又于是乎跟踪了一下非硅基介孔氧化物的合成现状，尤其是Mn以及稀土，看了几篇综述性文章，收获颇多。分享一下自己的感知：

正文
介孔分子筛自从1992年被发现、表征后一直备受关注（其实早在几年前日本人就合成出来了，只不过他们没有给与完整的表征，没有发现其独特的结构，看样子做科研除了点子多、经费足、设备好之外，运气也是必不可少的），我在硕士期间也赶了一把大趋势、凑了一把热闹，对稀土介孔分子筛（MCM-41、SBA- 15）的合成做了浅显的研究，并进行了某些反应的“实地考察”！最后得出与大家基本一致的结果：

硅基介孔分子筛目前在催化方面的应用非常有限

原因：

*
   自身无明显活性（基本无酸性、不掺杂金属的分子筛无氧化、还原等活性）；
*
   热稳定性差（虽然介孔分子筛具有相当大的表面积，按理是很好的催化剂载体，但是其热稳定性太差，在很多严苛的催化反应下结构基本塌陷）；
*
   水热稳定性更差（还是作为载体结构容易塌陷）。
*
   ........................................................

   但是有一点必须承认：介孔分子筛在某些性质上确实有独到之处，尤其在催化应用上有其用武之地（如大表面积、孔径可调等等）。于是合成自身具有催化活性、表面积比较大的非硅基介孔分子筛（过渡金属、稀土等等）在催化上具有广泛的应用前景。

过渡金属介孔分子筛合成未受到广泛关注的原因：
(1) The pioneer groups in mesoporous materials have a scientific background related to zeolites; therefore, they are mainly familiar with silicon and aluminum chemistry.
(2) The high reactivity toward hydrolysis and condensation of transition metal oxide precursors increases the extent of uncontrolled phase separation between organic and inorganic components, yielding non-mesostructured materials, but porous gels.
(3) The redox reactions, the possible phase transitions, and the crystallization processes are often accompanied by the collapse of the structural integrity.
(4) Synthesis procedures are extremely sensitive to many external parameters, leading in some cases to irreproducible results.

硅基介孔分子筛合成与过渡金属介孔分子筛合成之间的区别：

(1) Metallic (TM, Al, Sn) alkoxide precursors are more reactive toward hydrolysis and condensation than Si alkoxides.
(2) Because of this high reactivity, inorganic polymerization has to be partially blocked. In some cases, clusters or nanoparticles may result, which aggregate upon drying. Thermal treatment of these randomly ordered aggregates can lead to textural mesoporosity, even if the overall solid presents little or no order at the mesoscopic scale.
(3) The mesostructure quality and the extension of the organized domains are often lower for TM than for silica-based systems.
(4) The obtained structures are often unstable toward template removal. This is particularly marked for metals capable of presenting various oxidation states (V, W).
(5) After hydrolysis and condensation, metal centers mostly present a higher coordination than tetrahedral silicon.

介孔分子筛的合成途径：

由此可见，对于由+价金属离子合成介孔分子筛的途径可以从S-I+、S+X-I+两种基本路线进行，另外也可以从改变金属前躯体，制备带负电荷的前躯体，例如氢氧化物，控制溶液pH（等电点）。

过渡金属介孔分子筛合成关键控制步骤：

Control of the Precursor Reactivity

Inorganic hydrolysis and condensation have to be mastered, to avoid the instantaneous formation of an inorganic network, which would irreversibly “freeze” an ill-organized structure.

The reactivity of the precursors can be efficiently controlled by different means:

(1) by carefully adjusting the pH and dilution of metal salt solutions,

(2) by using alkoxides or other salts in the presence of condensation inhibitors (acids, complexing agents),

(3) by working in nonaqueous solvents and limited quantities of water,

(4) by resorting to evaporation-induced self-assembly,

(5) by modifying the redox state, or

(6) by using preformed nanoobjects.Some of the reported synthesis methods use a combination of two or more of these concepts;

Coordination Polyhedra, Flexibility, and Curvature.

¥ Limit the size of the first oligomers obtained after hydrolysis, rendering them less compact and more linear, by means of inhibitors or mono bridging anions.
¥ Resort to nonhydrolytic condensation processes in the first stages of texturing; typically, these processes favor lower coordination polyhedra.
¥ Perform synthesis in extreme pH conditions, in which the connectivity between coordination polyhedra may be more versatile.
¥ Combine metal cations with metal centers that accept tetrahedral coordination [Si, Al, V(IV), Ga], which can give rise to more flexible species.

¥ Insert other type of anions in the inorganic network, these anions being polyatomic (phosphate, carboxylate, ...) or being forced to bridge a limited number of cations.

机理之一：

几篇值得看看的文章：

Anion binding in self-assembled monolayers in mesoporous
http://sciencenet.cn/blog/admin/images/upfiles/200710318201946999.pdf

Chemical Strategies To Design Textured Materials
http://sciencenet.cn/blog/admin/images/upfiles/200710318520578546.pdf

Facile Synthesis and Characterization of Novel Mesoporous
http://sciencenet.cn/blog/admin/images/upfiles/2007103185020875845.pdf

Harnessing the Sol–Gel Process
http://sciencenet.cn/blog/admin/images/upfiles/20071031852246175.pdf

Manganese Oxide Mesoporous Structures Mixed-Valent Semiconducting Catalysts
http://sciencenet.cn/blog/admin/images/upfiles/2007103185723953128.pdf

Ordered Meso- and Macroporous Binary and Mixed Metal Oxides
http://sciencenet.cn/blog/admin/images/upfiles/2007103184525281992.pdf

Organization of Organic Molecules with Inorganic Molecular Species into Nanocomposite Biphase Arrays
http://sciencenet.cn/blog/admin/images/upfiles/200710318202857876.pdf

过渡金属体系有序介孔材料研究进展
http://sciencenet.cn/blog/admin/images/upfiles/2007103181329609813.pdf

本文引用地址：http://sciencenet.cn/blog/user_content.aspx?id=9989

[ Last edited by daiqiguang on 2007-10-31 at 11:34 ]

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