Molecular sieves containing transition metals in the framework

Perego G, Millini R and Bellussi G 1998 Synthesis and characterization of molecular sieves containing transition metals in the framework Moiecuiar Sieves Science and Technoiogy vol 1, ed FI G Karge and J Weitkamp (Berlin ... 

Perego, G., Millini, R. and Bellussi, G. (1998) Synthesis and Characterization of Molecular Sieves Containing Transition Metals in the Framework (eds H.G. 

Synthesis and Characterization of Molecular Sieves Containing Transition Metals in the Framework... 

The synthesis of molecular sieves containing transition metals in the framework represents a reliable route for preparing materials with novel catalytic properties. In spite of the large number of papers and patents claiming the incorporation of several transition metal ions in different microporous silica frameworks, unambiguous evidence in favor of the isomorphous substitution of silicon has been achieved only for Ti and Fe, for the former limited to few framework types. Some evidence exists also for V and Cr, but the data available do not allow definite conclusions to be drawn in this regard. 

The subsequent Chapter 7 is devoted to the synthesis and characterization of molecular sieve materials containing transition metals in the framework. Authored by G. Perego, R. Millini and G. Bdlussi, this Chapter focuses on titanium-silicalite-1 which has recently been found to be a unique catalyst for selective oxidations with hydrogen peroxide. Also covered in this Chapter is the synthesis of vanadium- and iron-containing molecular sieves. 

Copper hydroxyphosphate also allows the direct hydroxylation of 2,3,6-TMP, leading to selectivities for TMHQ (1) of >80%. The main by-product is TMQ (5). Heterogeneous catalysts which have been used for the oxidation of 3 include zeolites, mesoporous materials and molecular sieves contaiiung transition metals. Molecular sieves containing transition metals such as vanadium or copper in the framework can simply be mixed with 2,3,6-TMP and H2O2 in acetoiutrile for the oxidation. ... 

Since the incorporation of transition metals into the frameworks of zeolites or micro-porous ahiminophosphates to form heteroatom-containing molecular sieves with important application values, the synthesis, structure, and characterization of microporous transition metal phosphates have been extensively studied in the last decade. In particular, because transition metal cations possess redox and coordination features, they are a kind of catalytic material with useful applications, and promise potential... 

One coordination of bare Mn2+ ions was reported by Kevan et al. in (A1)MCM-41 [4], Because Co2+ spectra of type a and (3 are very different from the spectrum of tetrahedral Co2+ ion, none Co2+ ions were incorporated into framework position. Thus, the discrepancy in the number of reported cationic sites in Ref. [4] and in this work should reflect different metal loading in molecular sieve or differences in its chemical composition (Si/Al ratio). As it was already mentioned, population of transition metals in individa) cationic sites depends on the metal loading. The effect of the Si/Al ratio was not studied for MCM-41 matrix, but is well known for pentasil containing zeolites [1]. 

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... 

Several medium pore molecular sieves were also evaluated for catalytic performance with an ethylbenzene and m-xylene feed. Again, the molecular sieves were tested with no added metal or hydrogen. Several of these aluminophosphate-based molecular sieves did contain transition metals as framework elements. The results of this study are summarized in Table 5, which shows that all molecular sieves yield paraxylene in amounts equivalent to thermodynamic equilibrium. Surprisingly, the cobalt- and manganese-containing molecular sieves with the -31... 

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 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. 

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