Một vài thông tin về Polyoxometalates

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Polyoxomatalates được tạo thành trong dung dịch acid và có thể phân lập dưới dạng chất rắn nguyên chất. Có thể chia polyoxometalates thành 2 nhóm chính là isopoly anions và heteropoly anions: [MmOy]p- : isopoly anion [XxMmOy]q- (x≤m) : heteropoly anion Có rất nhiều cấu trúc khác nhau của polyoxometalates đã được khám phá: Keggin, Anderson, Dawson… Các polyoxometalates có cấu trúc Keggin được quan tâm nhiều nhất vì chúng khá bền và dễ tổng hợp, đây là những tính chất cần thiết để ứng dụng làm xúc tác. Một cách tổng quát, cấu trúc Keggin có thể được mô tả bằng công thức và có đường kính vào khoảng 1.2nm. Dạng acid của các Keggin anions thể hiện cả tính acid và tính oxy hóa nên được quan tâm nghiên cứu và ứng dụng nhiều trong xúc tác đồng thể và dị thể

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Polyoxometalates form by a self-assembly process, typically in acidic aqueous solution as the follow equations and can be isolated as solids with an appropriate counter cation ( H+, alkaline metal cation, NH4+…):

Generally, two types of polyoxometalates are distinguished as based on their chemical composition: isopoly anions and heteropoly anions. These anions may be represented by general formulas: [M¬mOy]p- : isopoly anion [XxMmOy]q- (x≤m) : heteropoly anion Where X is heteroatom and M is addenda atom. Dozen of structural types and stochiometries of polyoxometalates are known up to date. Among of which, Keggin structure is best-known and adopted by many polyoxometalates since it’s the most stable and easily available; these are among the most important characteristics for catalysis. The Keggin heteropoly anions are typically represented by the formula , where X is the hetero atom, x is its oxidation state, M is addenda atom (usually Mo+6 or W+6). The Keggin anion has diameter of ca. 1.2nm and is composed of a central tetrahedron XO4 surrounded by 12 edge- and corner-sharing metal oxygen octahedra MO6. The octahedra are arranged in four M3O13 group. Each group is formed by three octahedra sharing edges and having a common oxygen atom which is also share with the central tetrahedral XO4. The total assemblage contains 40 closed-packaged oxygen atoms. The oxygen atoms are of four types: • 12 terminal M==O • 12 edge bridging angular M—O—M shared by octahedra within M3O13 group • 12 corner bridging quasi-linear M—O—M connecting two different M3O13 group • 4 internal X—O—M Each of M3O13 group can rotate 60o about its 3-fold axis to form geometrical isomers. The structure shown above is the most common  isomer of Keggin type. Rotation of one, two, three or all four M3O13 groups produces beta, gamma,delta, epsilon isomer respectively. When being isolated in solid type with H+ as counter ion, heteropoly acids have found numerous applications as catalysts due to their acidity and redox properties. Among of which, Silicomolybdic acid recently attracts attention from researchers as heterogeneous catalyst, especially for gas-solid reaction. I. Synthesis, stability, characterization of SMA:

  1. Synthesis:

Free SMA can be synthesized by following methods:

  1. extraction with ether from acidified aqueous solution,
  2. ion exchange from salts of heteropolyacids,
  3. mixing appropriate quantities of the simple acids. The SMA exists in two forms in solution. The alpha form (the commonly known compound) is produced in water solution when there are less than 1.5 equivalents of acid per mole of molybdate in solution during formation. With more than 2 equivalents of acid per mole of molybdate in solution, the beta form is produced [9]. Recently, considerable works have been published on the formation of silicomolybdic acid (SMA) by the solid state reaction between MoO3 and SiO2 in the presence of water vapor. However, specific area of SMA is low; supporting SMA on solids with high surface areas (such as SiO2, US-Y, MCM 41, alumina…) is a useful method for improving catalytic performance. Since then, there are many methods for carrying SMA onto supporters which lead to differences in catalytic performances. • Impregnation method: this is the most popular method for preparation of catalyst. Usually, supporters were suspended into SMA solution. • SMA is also synthesis by the solid state reaction between MoO3 or molybdic acid and SiO2 on the surface of supporters with the presence of water vapor.
  1. Stability:

It’s clear that pure SMA is decomposed into MoO3 and SiO2 at high temperature. According to Kozhevnikov [2], thermal stability of Keggin HPAs is defined as temperature at which all acidic protons are lost. To this definition, SMA is stable until 350oC (623K); however, thermally decomposed SMA on silica surface becomes reconstructed under exposure to water vapor. The thermal stability of supported SMA was not much improved since SMA/SiO2 began to decompose at 563K slightly depending on the concentration of Mo and the decomposition of SMA supported on SiO2 happens more slowly than the unsupported one. Moreover, the thermal treatment induces the formation of MoO3 only in the case of unsupported SMA and high loaded catalyst. In the case of supported catalyst with 9%wt Mo, MoO3 is never observed, whatever the temperature and the duration of thermal treatment. They also reported that SMA stability was much improved up to 873K when supported on SiO2 due to carefully elevating temperature; however, they didn’t mention their temperature programmed [3]. 3. Acidity and redox properties:

The relative activity of Keggin HPAs primarily depends on their acid strength. Solid HPAs possess purely Bronsted acidity and are stronger than conventional solid acids as HX, HY zeolite, alumino silicate, H3PO4/SiO2. Another property, oxidation potential, which determines the reducibility of HPA by reaction medium, is also important. These properties (include thermal and hydrolytic stability) for the most common HPAs are summarized as below [2]: acid strength: PW > SiW  PMo > SiMo oxidation potential: PMo > SiMo >> PW > SiW thermal stability: PW > SiW > PMo > SiMo hydrolytic stability: SiW > PW > SiMo > PMo The oxidation potential of Keggin type SMA is controlled by factors such as, the nature of isomers. The oxidizability decreases in the sequences alpha, beta, gamma isomer respectively. ( to be continued) References

  1. Tran Mai Huong, Doctor of Engineering Thesis, 2006, Toyohashi University of Technology.
  2. Ivan V. Kozhevnikov, Catalysis by polyoxometalates, 2002, Wiley.
  3. S. Kasztelan, E. Payen, J.B. Moffat, J. Catal., 1988, 112, 320.
  4. Rocchiccioli and C.R.Deltcheff, J. Catal., 1996, 164, 16.
  5. Miguel A.Banares, Hangchun Hu, Israel E.Wachs, J.Catal., 1995, 155, 249.
  6. G.Mestl and T.K.K.Srinivasan, Catal.Rev.-Sci.Eng., 1998, 40(4), 451.
  7. P.C.Bakala, E.Briot, L.Salles, J.M. Bregeault, Appl.Catal.A, 2006, 300, 91.
  8. J. Piquemal, E Briot, G. Chottard, P. Tougne, J. Manoli, J. Bregeault, Microporous and Mesoporous Material, 2003, 58, 279.
  9. G.A.Tsigdinos, Heteropoly compounds of Molybdenum and Tungsten, 1978, Springer-Verlag.
  10. M.S.Kasprzak, G.E.Leroi and S.R.Crouch, Appl.Spec., 1982, 36, 285.
  11. Miguel A.Banares, Hangchun Hu, Israel E.Wachs, J.Catal., 1994, 150, 407.
  12. Wei Li, George D. Meitzner, Richard W. Borry III, and Enrique Iglesia1, J.Catal., 2000, 191, 373