Transition metal-containing molecular

Synthesis of transition metal containing molecular sieves (microporous as well as mesoporous) is one of the fastest developing areas in molecular sieve science, as evidenced by recent published reviews [1,2] Several transition metals have been substituted into crystalline silica or aluminophosphate frameworks to yield the corresponding metallosilicate or metalloaluminophosphate molecular sieves, However, the location of the metal species and their state always remain uncertain, despite the employment of numerous different characterization methods comprising IR, NMR and ESR spectroscopy. 

In the present paper, we report some aspects of the oxidation of aniline with various peroxides over a series of transition metal - containing molecular sieves having different structures. The influence of the catalyst structure, of the nature and concentration of the peroxide and of the incorporated metal are discussed. 

Ni-Mo or NAt present, industrial hydrotreating processes are based on alumina supported Co-Mo or Ni-W sulfide catalysts (2,3). We expect that transition metal containing molecular... 

Transition metal-containing molecular sieves exhibit remarkable properties as catalysts for a variety of oxidation reactions with peroxides as the oxidant [1]. The potential of transition metal containing zeolites is however, limited because of the number and t e of heteroelements that can be incorporated in the framework and also the pore sizes of the resulting molecular... 

General Aspects of Transition Metal-Containing Molecular Sieves 188... 

In the present paper, we review the main results obtained in the preparation of transition metal-containing molecular sieves. In particular, the attention will be focused on the synthesis procedures and the physico-chemical evidence supporting the effective framework incorporation of the heteroatom. 

The accurate structure characterization of transition metal-containing molecular sieves is made difficult by the generally low heteroatom concentration. Due to the practical impossibility of determining the site geometry by direct structural methods, spectroscopic studies, together with the application of theoretical methods, have been widely used (Table 3). Actually, a lot of data exists, mainly concerning TS-1. In the following, we will try to summarize these results. 

Though several patents claim the synthesis of many transition-metal containing molecular sieves [ 191 ], there is a general lack of evidence supporting their effective incorporation. 

Cheng, S., Tsai, T., Chou, B., et al. (2002). Synthesis of TMBQ with Transition Metal-containing Molecular Sieve as Catalyst, US Patent US 143198. 

Currently the main interest in template reactions lies in their key role in the controlled synthesis or the self-assembly of a variety of supramole-cular entities (449). One needs a combination of intuition, conjecture, and serendipity (450) a recent example of successfully combining serendipity and rational design is provided by the silver(I)-promoted assembly of one-dimensional stranded chains (451). One also needs an understanding of mechanism in order to optimize the selection and design of building blocks and templates for the generation of yet more sophisticated supramolecular structures references cited in this present review contain at least some kinetic or mechanistic information or speculation. Template routes to interlocked molecular structures have been reviewed (452), while a discussion of switching by transition metal contains a little about the kinetics and mechanisms of this aspect of template... 

J.-P. Sauvage, Transition Metal-Containing Rotaxanes and Catenanes in Motion Toward Molecular Machines and Motors , Acc Chem. Res. 1998,31, 611-619. 

Previous studies [51,52] have shown the reliability ofB3LYP calculations in Also predictions, even for transition metal-containing systems. Therefore the identification of the most stable form could experimentally be done, considering the different values of Aiso for the Sc atom in the molecular complexes and in the insertion product. 

Balzani, V., Venturi, M., Credi, A. Molecular Devices and Machines. A Jorney into the Nanoworld, Wiley-VCH, Weinheim, 2003 b) Feringa, B.L. (ed.), Molecular Switches, Wiley-VCH, Weinheim, 2001 c) Raehm, L., Sauvage, J.-P. Molecular machines and motors based on transition metal-containing catenanes and rotaxanes, Struct. Bond. 99 (2001), 55-78. 

The oxidation of aniline was carried out in the liquid phase over a series of transition metal - substituted molecular sieves. For low oxidant/aniline ratios, azoxybenzene (AZY) was the major product formed over Ti-containing catalysts, the reaction was limited by diffusion for medium pore zeolites like TS-l and mesoporous silicas were preferred as they permitted the use of both H2O2 and tert-butyl hydroperoxide as oxidants. Higher oxidant/aniline ratios (>2) led to the formation of nitrobenzene (NB), whose selectivity was proportional to the catalyst concentration. In contrast, vanadium containing molecular sieves were only active with TBHP and aniline was converted very selectively into nitrobenzene for all oxidant concentrations. 

The as-synthesized and calcined CrAPO-5 and CrS-1 were characterized by XRD which showed that the samples were pure and had an API and MFI structure respectively. ICP analysis showed that both catalysts contained about 1 % chromium. The results observed in the decomposition of cyclohexenyl hydroperoxide over several redox active moleular sieves are presented in Table 1. CrAPO-5 and CrS-1 displayed rougly equal activity and selectivity in the decomposition of cyclohexenyl hydroperoxide. Blank reactions carried out with Silicalite-1 (S-1) and silicon incorporated Aluminophosphate-5 (SAPO-5) show low conversions confirming that the chromium was responsible for the catalysis. Other transition- metal subsituted molecular sieves showed low conversions. 

Many bridging ligands have been constructed that contain porphyrin subunits. Two examples are given to show the versatility of this structural motif. Porphyrins have been used for many years as models for the reactive site in the photosynthetic reaction center because of the resemblance to the natural components and the ability to vary the spectroscopic and redox properties of these chromophores by the use of bulky substituents and by their coordination to different transition metals.192 Molecular dyads of Znn/Irin and Auni/Irm and triads of Znn/Irni/Auin have been synthesized using the porphyrin bridging ligand (66).193-195... 

Molecular Machines and Motors Based on Transition Metal-Containing Catenanes and Rotaxanes... 

Molecular Motions in Transition Metal-Containing Rotaxanes. . 58... 

Raehm L (2001) Molecular Machines and Motors Based on Transition Metal-Containing Catenanes and Rotaxanes 99 55-78 Rakke T, see Kjekshus A (1974) 19 45-83 Rakke T, see Kjekshus A (1974) 19 85-104... 

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