Transition metal-containing molecular sieves

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. 

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. 

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. 

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... 

The catalytic properties of the aluminophosphate molecular sieves are also influenced by chemical composition. The introduction of transition metals into framework positions enhances the activity and selectivity for olefin isomerization relative to the silicoaluminophosphates. The transition metal containing aluminophosphates are also surprisingly more selective for Cs aromatic rearrangements than the corresponding SAPO molecular sieves, an effect which can not be attributed solely to improved shape selectivity. 

X. He and D. Antonelli, Recent Advances in Synthesis and Applications of Transition Metal Containing Mesoporous Molecular Sieves, Angew. Chem. Int. Ed., 41,214-29(2002). 

He, X. Antonelli, D. Recent advances in synthesis and applications of transition metal containing mesoporous molecular sieves. Angew. Chem. Int. Ed. 2002, 41, 214. 

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... 

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. 

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 ... 

Nickel aluminate, a spinel, has long been known to trap nickel. Metals like arsenic(19), antimony(20-21) and bismuth(20) are known to passivate transition elements and can be used to decrease and coke make. Sulfur is also a known inhibitor for nickel therefore, higher sulfur-containing crudes may be a little less sensitive to nickel poisoning. In our work we also found that nickel at low concentrations is actually a slight promoter of the cracking reaction when incorporated into a molecular sieve (Figure 17). 

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]. 

Physico-chemical and catalytic properties of MCM-41 mesoporous molecular sieves containing transition metals (Cu, Ni, and Nb)... 

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

Most crystalline aluminosilicates have little intrinsic catalytic activity for hydrogenation reactions. However, a considerable amount of data has recently accumulated on the use of zero-valent metal-containing zeolites in many hydrocarbon transformations. Thus noble and transition metal molecular sieve catalysts active in hydrogenation (7,256-760), hydroisomerization (161-165), hydrodealkylation (157, 158,165-167), hydrocracking (168,169), and related processes have been prepared. Since a detailed discussion of this class of reactions is beyond the scope of this review, only a few comments on preparation and molecular-shape selectivity will be made. 

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