Water molecular catalysts

Renewed interest in this method came recently from its adaptation to the immobilization of water/ organic solvent biphasic catalysts, resulting in the so-called supported aqueous phase catalysts (SAPCs).117 The molecular catalyst is immobilized via water, which is hydrogen bonded to the surface silanol groups reactants and products are in the organic phase (Figure 11)... 

The sol-gel process involves hydrolysis of alkoxide precursors under acidic or basic conditions, followed by condensation and polycondensation of the hydroxylated units, which lead to the formation of porous gel. Typically a low molecular weight metal alkoxide precursor molecule such as tetramethoxy silane (TMOS) or tetra ethoxysilane (TEOS) is hydrolyzed first in the presence of water, acid catalyst, and mutual solvent... 

The activity of water-soluble catalysts based on the hydrophilic biden-tate phosphines is also significantly influenced by the nature of the anion [94-100]. Thus for example, using the catalyst formed in situ from [Pd(TsO)2(NCMe)2] and the mef/za-sulphonated analogue of dppp under conditions close to the ones reported above, but in water as solvent and with an excess of acid (acid/Pd = 500/1), the productivity is ca. 3900, 3600 or 2250 g polymer(gPd h)-1, when the acid is TsOH, TFA or AcOH, respectively, under usual conditions [96]. If one considers that the monomers are a little soluble in H20, very high productivity and molecular weight are obtained using [Pd(CH3)(NCCH3)(dapp-s)](OTf) (dapp-s =... 

Molecular catalyst for water oxidation has been attracting a great deal of attention not only as a model for the photosynthetic catalyst but also as a component in an artificial photosynthesis. Many structural models have been synthesized and investigated as the photosynthetic Mn complex. Tetrakis(2,2 -bipyridine)(di /i -oxo)di-Mn complex lu) and tetrakis(2,2 -bipyridine)( jJ. -oxo)di-Ru complex (2, Meyer s complex)14) are typical examples, but many of them failed to show high activity for water oxidation. 

Proton reduction is an important catalysis in water photolysis. Pt and Pt02 have been the best known catalysts for process. However, these colloidal or powder catalysts are not well suited for the construction of a conversion system based on molecules, and, moreover, incorpuration of these strongly colored materials into photochemical conversion systems should be avoided because of their possible filter effect. From this point of view it is desirable to use a molecular catalyst if a highly active one is available. 

Figure 7. Models suggested for explaining the observation of the photooxidation of water by a sensitizer S, (Ru(bpy)32+), which in the excited state S transfer an electron to a sacrificial acceptor A, [CoCNH Cl], in the presence of a colloidal catalyst (RuCy or a molecular catalyst (cwRu(bpy)2(H20)22+). See text for the significance of a, b, and c. (Reprinted with permission from ref. 32. Copyright 1983.)...

Figure 6.7 Gas purification system for removal of oxygen (BTS catalyst) and water (molecular sieves).

Yagi M, Kaneko M. Molecular catalysts for water oxidation. Chem Rev 2001 101 21-35. 

Yagi M, et al. Molecular catalysts for water oxidation toward artificial photosynthesis. Photochem Photobiol Sci. 2009 8(2) 139-47. 

Yin Q, Tan JM, Besson C, et al. A fast soluble carbon-free molecular water oxidation catalyst based on abundant metals. Science. 2010 328(5976) 342-5. 

Brimblecombe R, Koo A, Dismukes GC, Swiegers GF, Spiccia L. Solar driven water oxidation by a bioinspired manganese molecular catalyst. J Am Chem Soc. 2010 132(9) 2892-4. 

Sala X, Escriche L, Llobet A. Molecular Ru and Ir complexes capable of acting as water oxidation catalysts. In Wydrzynski TJ, Hillier W, editors. Molecular solar fuels. Cambridge, UK The Royal Society of Chemistry 2012. p. 273-87. 

Sala X, Romero I, Rodriguez M, Escriche L, Llobet A. Molecular catalysts that oxidize water to dioxygen. Angew Chem Int Ed. 2009 48 2842-52. 

Thomas J. Meyer First well-defined molecular catalyst for water oxidation... 

Alternatively, surface-mediated synthesis involves the immobilization of a mononuclear metal complex (similar to the ones described in Sect. 19.2) and the subsequent treatment in CO at different temperatures to form a supported metal carbonyl cluster, for example, [HIr (CO)jJ formed from Ir(CO)2(acac) on MgO and [Rhj(CO),5] formed from Rh(CO)2(acac) on MgO and y-Al Oj [45 7]. Synthesis of metal carbonyl clusters on oxide supports apparently often involves OH groups or water on the support surface analogous chemistry occurs in solution [13]. The synthesis of molecular catalysts from a mononuclear metal complex is likely to occur with a yield less than that associated with simple adsorption of a preformed metal cluster, and so the latter precursors are preferred, except when they do not fit into the pores of the support (e.g., a zeolite). 

Ligands and complex catalysts derived therefrom may catalyze reactions under circumstances which require aqueous or mild conditions, such as bioorganic substrates (bioorganometallic conversions cf. Section 3.3.10.2). However, the great advantage of water-soluble catalysts is that they overcome the basic problem of homogeneously catalyzed processes the separation of the product phase from the (molecular) catalyst itself, which is soluble in it. The unit operations necessary to achieve this usually include thermal operations such as distillation, decomposition, transformation, and rectification, process steps which normally cause thermal... 

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