A solution and solid state study of niobium complexes University of ...
A solution and solid state study of niobium complexes University of ... A solution and solid state study of niobium complexes University of ...
Chapter 2 Promoter or Active phase: Interactions that allow the reactants to interact simultaneously with the metal and the promoter are the most relevant to catalysis. When the catalyst consists of metal fragments covered by promoter oxide, some steps in the overall reaction may be catalyzed at the metal- promoter-liquid/gas interphase. A supported metal-promotor interaction has been observed in niobium oxide-promoted Rh/SiO2 catalysts 47 , niobia- promoted Pt/Al2O3 catalysts 48 , and silica supported NiNb2O6 catalysts 49 . Solid Acid Catalyst: Although niobium-containing catalysts were studied in various acid catalysed reactions and their acidity was assessed by various techniques, they have not been widely used as acid catalysts in practise. Datka et al. 50 examined the acidic properties of niobium oxide catalysts and found Lewis acidity in the silica-, magnesia-, titania-, and zirconia-supported systems, while Brønsted acid sites were only encountered when the niobia was supported on alumina or silica. Recent work includes a study by Silva et al. 51 on Nb-doped iron oxides that were used as heterogeneous catalysts to oxidize organic compounds in aqueous solutions containing hydrogen peroxide (H2O2). The H2O2 treatment of the solid catalyst induces important surface and structural changes to the iron oxides, essentially by formation of peroxo-niobium complexes which enhances the catalytic properties of the composite. Research on the catalytic performance of niobium in crystalline and amorphous solids in catalytic oxidation reactions were done by Ziolek et al. 52 in 2011. Bulk niobium(v) oxide materials were used as catalysts in the gas phase oxidation of methanol with oxygen, liquid phase oxidation of glycerol with oxygen and the liquid phase oxidation of cyclohexene with H2O2. The amorphous materials containing niobium were the most effective catalyst because of the strong interaction between the Nb and H2O2. That was not the case for the crystalline catalysts 47 T. Beutel, V. Siborov, B. Tesche, H. Knözinger, J. Catal., 167, 379, 1997. 48 T. Hoffer, S. Dobos, L. Guczi, Catal. Today, 16, 435, 1993. 49 K. Kunimori, H. Shindo, H. Oyanagi, T.Uchijima, Catal. Today, 16, 387, 1993. 50 J. Datka, A. M. Turek, J. M. Jehng, I. E. Wachs, J. Catal., 135, 186, 1992. 51 A. C. Silva, R. M. Cepera, M. C. Pereira, D. Q. Lima, J. D. Fabris, L. C. A. Oliveira, Appl. Catal. B, 107, 237, 2011. 52 M. Ziolek, D. I. Sobczak, M. Trejda, J. Florek, H. Golinska, W. Klimas, A. Wojtaszek, Appl. Catal. A, 391, 194, 2011. 19
Chapter 2 containing niobium as the niobium species promoted the stabilization of the active species loaded. 2.6 β-diketone complexes of niobium Metal β-diketonates are amongst the most studied coordination compounds and their chemistry has been examined for most of the metals in the periodic table. The ligand forms chelates with the metal and delocalizes the negative charge over a “metallacycle”. 53 β-diketones has the potential to be easily derivatized using well- established procedures. The steric and electronic nature of this ligand type may be varied to probe the structure and function of interest. Variation of the R groups influence the properties displayed by metal β-diketonates and the ability to form higher coordinate species can be achieved by variation of the terminal R groups on the β-diketone. It has been shown that the stepwise incorporation of electron-withdrawing trifluoromethyl groups increases the affinity of the central metal ion for further ligation. 54 Keto Enol Figure 2.1: Keto-enol tautomerism in β-diketones. The simplest β-diketone is acetylacetone (2,4-pentanedione, acacH), which tends to form neutral complexes with niobium. The adopted geometry normally reflects the preferred geometry of the metal ion involved. Slight basic conditions cause deprotonation and the resulting acetylacetone anion readily complexes a range of 53 R. C. Mehrotra, R. Bohra, D. P. Gaur, Metal β-diketonates and Allied Derivatives, Academic Press, London, 1978. 54 R. van Eldik, K. Bowman-James, Advances in Inorganic Chemistry, Academic Press, London, 3, 2006. 20
- Page 1 and 2: A solution and solid state study of
- Page 3 and 4: Table of contents Abbreviations and
- Page 5 and 6: 3.4 Infrared Spectroscopy (IR) ....
- Page 7 and 8: Abbreviations and Symbols Abbreviat
- Page 9 and 10: Abstract 93 Nb NMR was successfully
- Page 11 and 12: Opsomming 93 Nb KMR is met sukses g
- Page 13 and 14: Chapter 1 metals. 3 Due to Wollasto
- Page 15 and 16: Synopsis... 2. Literature Review of
- Page 17 and 18: Chapter 2 Niobium resembles tantalu
- Page 19 and 20: 2.1.2 Uses Chapter 2 Niobium has a
- Page 21 and 22: 2.2 Separation of Nb and Ta 2.2.1 M
- Page 23 and 24: Chapter 2 Buachuang et al. 16 repor
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- Page 27 and 28: 2.4.6 Water absorption Chapter 2 Th
- Page 29: Chapter 2 been reported in literatu
- Page 33 and 34: Chapter 2 conclusions, with regard
- Page 35 and 36: Chapter 2 Figure 2.5: Structure of
- Page 37 and 38: 2.6.3.3 [NbCl3O(ttbd) - ] Chapter 2
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- Page 41 and 42: 2.7 Alkoxides Chapter 2 Specific kn
- Page 43 and 44: Chapter 2 Reactions of dialkylamid
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- Page 47 and 48: Chapter 2 In 1991 Lee et al. 83 pub
- Page 49 and 50: EtO EtO EtO EtO Cl Nb Cl Cl Nb Cl R
- Page 51 and 52: Chapter 2 The hemicarbonate formed
- Page 53 and 54: Synopsis... 3. Synthesis and Charac
- Page 55 and 56: Chapter 3 with γ = magnetogyric ra
- Page 57 and 58: Chapter 3 electromagnetic spectrum
- Page 59 and 60: 3.5.1 Bragg’s law Chapter 3 Bragg
- Page 61 and 62: 3.5.3 ‘Phase problem’ Chapter 3
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- Page 67 and 68: 3.7.2.4 Synthesis of [NbCl4(acac)]:
- Page 69 and 70: 4. Crystallographic Synopsis... Cha
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Chapter 2<br />
Promoter or Active phase: Interactions that allow the reactants to interact<br />
simultaneously with the metal <strong>and</strong> the promoter are the most relevant to<br />
catalysis. When the catalyst consists <strong>of</strong> metal fragments covered by promoter<br />
oxide, some steps in the overall reaction may be catalyzed at the metal-<br />
promoter-liquid/gas interphase. A supported metal-promotor interaction has<br />
been observed in <strong>niobium</strong> oxide-promoted Rh/SiO2 catalysts 47 , niobia-<br />
promoted Pt/Al2O3 catalysts 48 , <strong>and</strong> silica supported NiNb2O6 catalysts 49 .<br />
Solid Acid Catalyst: Although <strong>niobium</strong>-containing catalysts were studied in<br />
various acid catalysed reactions <strong>and</strong> their acidity was assessed by various<br />
techniques, they have not been widely used as acid catalysts in practise.<br />
Datka et al. 50 examined the acidic properties <strong>of</strong> <strong>niobium</strong> oxide catalysts <strong>and</strong><br />
found Lewis acidity in the silica-, magnesia-, titania-, <strong>and</strong> zirconia-supported<br />
systems, while Brønsted acid sites were only encountered when the niobia<br />
was supported on alumina or silica.<br />
Recent work includes a <strong>study</strong> by Silva et al. 51 on Nb-doped iron oxides that were<br />
used as heterogeneous catalysts to oxidize organic compounds in aqueous <strong>solution</strong>s<br />
containing hydrogen peroxide (H2O2). The H2O2 treatment <strong>of</strong> the <strong>solid</strong> catalyst<br />
induces important surface <strong>and</strong> structural changes to the iron oxides, essentially by<br />
formation <strong>of</strong> peroxo-<strong>niobium</strong> <strong>complexes</strong> which enhances the catalytic properties <strong>of</strong><br />
the composite. Research on the catalytic performance <strong>of</strong> <strong>niobium</strong> in crystalline <strong>and</strong><br />
amorphous <strong>solid</strong>s in catalytic oxidation reactions were done by Ziolek et al. 52 in<br />
2011. Bulk <strong>niobium</strong>(v) oxide materials were used as catalysts in the gas phase<br />
oxidation <strong>of</strong> methanol with oxygen, liquid phase oxidation <strong>of</strong> glycerol with oxygen <strong>and</strong><br />
the liquid phase oxidation <strong>of</strong> cyclohexene with H2O2. The amorphous materials<br />
containing <strong>niobium</strong> were the most effective catalyst because <strong>of</strong> the strong interaction<br />
between the Nb <strong>and</strong> H2O2. That was not the case for the crystalline catalysts<br />
47 T. Beutel, V. Siborov, B. Tesche, H. Knözinger, J. Catal., 167, 379, 1997.<br />
48 T. H<strong>of</strong>fer, S. Dobos, L. Guczi, Catal. Today, 16, 435, 1993.<br />
49 K. Kunimori, H. Shindo, H. Oyanagi, T.Uchijima, Catal. Today, 16, 387, 1993.<br />
50 J. Datka, A. M. Turek, J. M. Jehng, I. E. Wachs, J. Catal., 135, 186, 1992.<br />
51 A. C. Silva, R. M. Cepera, M. C. Pereira, D. Q. Lima, J. D. Fabris, L. C. A. Oliveira, Appl. Catal. B, 107, 237, 2011.<br />
52 M. Ziolek, D. I. Sobczak, M. Trejda, J. Florek, H. Golinska, W. Klimas, A. Wojtaszek, Appl. Catal. A, 391, 194,<br />
2011.<br />
19
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