Oxidation framework positions

Abstract We briefly underline the relevance of TS-1 catalyst for industrial applications in mild oxidation reactions using hydrogen peroxide as oxidant and review the experimental works employed over last two decades for imderstanding the structme of the Ti centers in the bare TS-1 material. After an animated and controversial debate that has lasted in the literature until 1994, several works (reviewed here in depth) have definitively assessed that Ti atoms occupy framework positions substituting a Si atom and forming tetrahedral... 

The incorporation of vanadium(V) into the framework positions of silicalite-2 has been reported by Hari Prasad Rao and Ramaswamy . With this heterogeneons oxidation catalyst the aromatic hydroxylation of benzene to phenol and to a mixtnre of hydroqninone and catechol conld be promoted. A heterogeneons ZrS-1 catalyst, which has been prepared by incorporation of zirconinm into a silicalite framework and which catalyzes the aromatic oxidation of benzene to phenol with hydrogen peroxide, is known as well in the literature. However, activity and selectivity were lower than observed with the analogous TS-1 catalyst. 

These important results have stimulated many research workers in universities and industrial research laboratories in the world to investigate the particular state of aggregation and coordination that Tilv assumes when forced into framework positions of hydrophobic crystalline silicas. Researchers are also engaged in the search for other compounds containing titanium and silicon oxides with Tilv in the same coordination and environment, on the assumption that similar catalytic properties would be obtained. Relevant discoveries have been made, and additional valuable information has been obtained on this new class of materials and on their catalytic performance in many different reactions. 

The first discovered member of the group of crystalline microporous materials made of oxides of titanium and silicon is titanium silicalite-1 (TS-1). TS-1 has attracted much interest for its unique catalytic properties it is also of interest by virtue of the proposal that Tiiv assumes tetrahedral coordination in substituting for SiIV in framework positions of crystalline silica, as stated above. To clarify this point, many detailed studies of the TS-1 structure have been carried out. An outcome of the work was the discovery of new crystalline microporous titanium silicates. 

The effect of zeolite porosity on the reaction rate was also well demonstrated in liquid-phase oxidation over titanium-containing molecular sieves. Indeed, the remarkable activity in many oxidations with aqueous H2O2 of titanium silicalite (TS-1) discovered by Enichem is claimed to be due to isolation of Ti(IV) active sites in the hydrophobic micropores of silicalite.[42,47,68 69] The hydrophobicity of this molecular sieve allows for the simultaneous adsorption within the micropores of both the hydrophobic substrate and the hydrophilic oxidant. The positive role of hydrophobicity in these oxidations, first demonstrated with titanium microporous glasses,[70] has been confirmed later with a series of titanium silicalites differing by their titanium content or their synthesis procedure.[71] The hydrophobicity index determined by the competitive adsorption of water and n-octane was shown to decrease linearly with the titanium content of the molecular sieve, hence with the content in polar Si-O-Ti bridges in the framework for Si/Al > 40.[71] This index can be correlated with the activity of the TS-1 samples in phenol hydroxylation with aqueous H2C>2.[71] The specific activity of Ti sites of Ti/Al-MOR[72] and BEA[73] molecular sieves in arene hydroxylation and olefin epoxidation, respectively, was also found to increase significantly with the Si/Al ratio and hence with the hydrophobicity of the framework. 

By a careful control of the preparation parameters, it is possible to obtain the TMI successively in solution (Fig. 5. model I), on the surface in extraframework position (models II-V), then in the surface in framework position (model VI). to finally reach the bulk of the oxide (model VII). Models II IV are pertinent to the deposition of the active component precursor. 

From the electrochemical point of view, an important class of materials is that constituted by aluminosilicates incorporating cobalt, iron, etc., centers. In the case of Fe-based zeolites with Mobil Five structure (FeZSM-5) materials, different forms of iron can coexist. These include isolated ions either in framework positions (isomorphously substituting silicon centers), isolated ions in cationic positions in zeolite channels, binuclear and oligonuclear iron complexes in extra-framework positions, iron oxide nanoparticles (size <2 nm), and large iron oxide particles (FcjOj) in a wide distribution (up to 25 nm in size) located in the surface of the zeolite crystal (Perez-Ramirez et al., 2002). The electrochemistry of such materials will be reviewed in Chapter 8. 

Molybdenum-containing catalysts are known to catalyze a variety of hydrogenation, oxidation and metathesis reactions [7-9]. Incorporation of Mo in the framework is limited because of the strain involved in the incorporation of large transition metal ions in the framework position. However the incorporation of Mo in the framework of MCM-41 is likely because of the greater flexibility of the structure as well as due to the difference in the mechanism of formation of these materials. 

Another option that sometimes enables immobilization of isolated metal ions stable to leaching, and avoidance of the formation of oligomers, is the synthesis of zeolites or zeotypes containing isolated metal ions in framework positions. In these the oxidation properties of the metal atoms are associated with the main characteristics of zeolites which involve shape-selective effects and unique adsorption properties which can be tuned in terms of their hydrophobicity-hydrophi-licity, enabling selection of the proportions of reactants with different polarities that will be adsorbed in the pores. Researchers at ENI succeeded in introducing Ti into silicalite producing the TS-1 redox molecular sieve oxidation catalyst [64]. TS-1 has an MFI structure formed by a bidimensional system of channels with 0.53 nm X 0.56 nm and 0.51 nm X 0.51 nm pore dimensions. The incorporation of Ti into the framework has been demonstrated by use of several techniques-XRD, UV-visible spectrophotometry, EXAFS-XANES a good review has been published by Vayssilov [65]. 

A V-ZSM-5 sample with a Si/V o 42 was synthetized outgoing from V0(C00)2 and Q-brand sodium silicate using TPA-Br as template. ESR spectroscopy proved that vanadium(IV) ions in the zeolitic framework exhibit a distorted square planar symmetry. Upon heat treatment a part of the framework vanadium ions migrate to extra-framework positions. After dehydration no Bronsted acidity was found. Treatment in oxygen and hydrogen above 570 K revealed the redox character of the V-ZSM-5 sample. In oxidation of n-bu-tane (as catalytic test reaction) the V-ZSM-5 zeolite exhibits selective dehydrogenation and aromatization activity. 

G) to octahedrally coordinated Fe(III) [13]. Significantly, in the spectrum of Fe-BEA and Fe-FER is missing a broad signal at g 2,3 assigned to undefined iron oxide species [14, 15], while it is apparent to some extend only in the Fe-MFI sample. As shown later, most of the iron was in the Fe(II) state and so the signal of isolated Fe(III) cations in tetrahedral coordination at g 5,6 and 6,0 (H 50 G) [16-18] was absent for all three samples. Accordingly, as indicated by EPR, a small amount of iron oxide could be present in Fe-MFI, while in all toee iron-zeolites a small amount of iron is present in framework positions. 

Two vanadium siiicate moiecular sieves, VS-2 and V-NCL-1 with medium and iarge pore dimensions, respectively, have been synthesised and their cataiytic activity in oxidation reactions evaluated. Isolated vanadium ions, probably in framework positions, possesss unique catalytic activity and shape seiectivity in oxidation reactions. So far, such behaviour has been found oniy in the case of titanium silicaiites. Our studies demonstrate that vanadium siiicates aiso possess such features. The iatter differ from the former in their ability to oxidise even the primary carbon atoms (in paraffins and side chain aikyl groups of aromatic hydrocarbons), and effect further secondary oxidation to a greater extent. V-NCL-1, with its large pore dimensions, enables the oxidation of bulky molecules like o- and m-xylenes and 1,3,5-and 1,2,4-trimethylbenzenes. 

Micro- and mesoporous materials containing niobium in the framework or extra framework positions were studied in the oxidation of dibutyl sulphide with H2O2. Leaching of Nb from the solid to the liquid phase was considered. Some of the catalysts prepared via the impregnation with Nb-salts show some leaching of Nb to the liquid phase and the oxidation partially occurs homogeneously in the liquid phase. The reaction proceeds mainly on the catalyst surface when the mesoporous molecular sieves containing Nb in the framework are used. 

Two types of water molecules are present in VOPO4.2H2O one water molecule is coordinated to vanadium, in a trans position to the V = O group [17]. The other is situated between the layers, and both water molecules hold the layers together by hydrogen bonding to the oxide framework. Based on these considerations, the first mass loss can be attributed to the release of interlayer water molecules, and the second mass loss to the coordinated water. 

XPS has also attracted attention as a tool for quality assessment of isomor-phously substituted zeolites. The interest arises from the expectation that nonincorporated entities of the substitutional element are deposited near the external crystallite surface or form separate particles that should be detectable as well. The experimental basis for such quality assessment would be most reliable if BE differences allowed the substitutional element to be traced both in intra- and extra-framework positions. It appears now that such discrimination is possible for several elements (Ti [135-140], V [106,141], Fe [142,143], Co [144]) due to increased BE of the elements substituted in silica-rich lattices as compared to their oxides. 

Shetty et al. (24i) a.nd Kulkami et al. (242) evaluated the structure, bonding, and acidity of tin-substituted BEA-type material using a periodic approach based on DFT. The results demonstrate that the incorporation of tin into the BEA framework decreases the cohesive energy and is an endothermic process. Hence, it is clear that the amount of tin that can be incorporated into the BEA structure is limited. This Hmitation is the Hkely reason for the decrease in the turnover number in the BV oxidation reaction as the tin content is increased in BEA samples namely, at some concentration, additional tin will be located in extra-framework positions and these sites have distinctly different Lewis acid strength than the framework tin sites. 

The synthesis of silicates that contain metal cations other than aluminium in framework positions results in solids with modified adsorptive and catalytic properties. The criteria for successful incorporation of cations into tetrahe-drally coordinated silicate frameworks are that they should exhibit solubility under synthesis conditions without the formation of an insoluble oxide or oxyhydroxide and that the substituting species should be able to adopt tetrahedral coordination. Preparation of a phase that is shown to contain other metals (by selected area chemical analysis in an electron microscope, for example) is no guarantee that the metal has adopted a framework site. Physicochemical methods must be used to determine whether this is the case. This is complicated when the metal adopts more than one site or is present at very low levels. Confirmatory evidence for framework substitution may be available from unit cell size determination (substitution of a cation larger than Si" will in general result in an increase in unit cell dimension), and determination of coordination geometry by NMR, UV-visible and EXAFS spectroscopies (Chapter 3), which are able to distinguish whether the metal is within the... 

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