Silicalite-1 molecular sieves synthesis

Recently, based on such an oxidative system, the synthesis of nitrone (12) an inhibitor of 5a-reductase has been carried out (Scheme 2.7) (50). Oxidation of amines with H2O2 can be catalyzed with peroxotungstophosphate (PCWP) (51), Se02 (52-54), and titanium silicalite molecular sieves TS-1 and TS-2 (55, 56). 

In this communication, we describe briefly the synthesis of Sn-MFI, Sn-MEL and Sn-MTW silicalites, their characterization by XRD, IR and D Sn MAS-NMR spectroscopy and their activity in the oxidation of a few organic substrates in presence of aq. H2O2 in order to differentiate the medium pore (MR and MEL) Sn-silicalites from the large pore MTW-type Sn-silicalite molecular sieves. 

Recently, there has been a growing interest into niobium- and tantalum-containing molecular sieves. The introduction of niobium into mesoporous molecular sieves has been studied by Ziolek et al [3,4], while Antonelli and Ying reported the synthesis of mesoporous niobium oxide [5], The synthesis and characterization of niobium- and tantalum-containing silicalite-1 (NbS-1 and TaS-1) was published recently [6,7,8] and some evidence has been presented for isomorphous substitution [6,8] of Nb and Ta into the silicalite-1 framework. The synthesis of NbS-2 (MEL) [9] and a new molecular sieve named NbAM-11 have been reported as well [10],... 

In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties. 

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. 

The isomorphous substitution of T atoms by other elements produces novel hybrid atom molecular sieves with interesting properties. In the early 1980s, the synthesis of a zeolite material where titanium was included in the MFI framework of silicalite, that is, in the aluminum-free form of ZSM-5, was reported. The name given to the obtained material was titanium silicalite (TS-1) [27], This material was synthesized in a tetrapropylammonium hydroxide (TPAOH) system substantially free of metal cations. A material containing low levels (up to about 2.5 atom %) of titanium substituted into the tetrahedral positions of the MFI framework of silicalite was obtained [28], TS-1 has been shown to be a very good oxidation catalyst, mainly in combination with a peroxide, and is currently in commercial use. It is used in epoxidations and related reactions. TS-1, additionally an active and selective catalyst, is the first genuine Ti-containing microporous crystalline material. 

In 1978, the same year that the structure of ZSM-5 was first described, Flanigen and her co-workers reported the synthesis, structure and properties of a new hydrophobic crystalline silica molecular sieve (Flanigen et al., 1978). The new material, named Silicalite (now generally called Silicalite-I), has a remarkably similar channel structure to that of ZSM-5 but contains no aluminium. It was pointed out by the Union Carbide scientists that, unlike the aluminium-containing zeolites, Silicalite has no cation exchange properties and consequently exhibits a low affinity for water. In addition, it was reported to be unreactive to most acids (but not HF) and stable in air to over 1100°C. 

The demonstration by Enichem workers [1] that titanium silicalite (TS-1) catalyzes a variety of synthetically useful oxidations with 30% aqueous hydrogen was a major breakthrough in the field of zeolite catalysis [2], The success of TS-1 prompted a flourish of activity in the synthesis of other titanium-substituted molecular sieves, such as titanium silicalite-2 (TS-2) [3], Ti-ZSM-48 [4] Ti-Al-mordenite [5], Ti-Al-beta [6]and Ti-MCM-41 [7]. Moreover, this interest has also been extended to the synthesis of redox molecular sieves involving framework substitution by other metals, e.g. chromium, cobalt, vanadium, etc. [8]. 

Up until the late seventies attempts to develop redox molecular sieves were mainly limited to the ion-exchange approach (see later). This situation changed dramatically with the discovery, by Enichem scientists in 1983 [6,7], of the unique activity of titanium silicalite-1 (TS-1) as a catalyst for oxidations with 30% aqueous hydrogen peroxide. Following the success of TS-1, interest in the development, and application in organic synthesis, of redox molecular sieves has increased exponentially and has been the subject of several recent reviews [8-11]. It has even provoked a revival of interest in another approach to producing redox molecular sieves the so-called ship-in-a-bottle method [12-15]. 

Following the success of TS-1 a variety of Ti-substituted molecular sieves were prepared by hydrothermal synthesis (Table 2) [28-32], Furthermore, various redox metals, e.g. V, Cr, Mn, Fe, Co, Cu, Zr, and Sn, have been reportedly incorporated into silicalites, zeolites,... 

The isomorphous substitution of Si" " by Ti and Sn" ", V"" in the silicalite-2 (ZSM-11, MEL) framework has been the subject of some recent reports [1-4]. The possibility of substituting silicon by zirconium in the framework of ZSM-5 (MFI) by hydrothermal analysis has been reported but not really substantiated by experimental evidence [5-9]. We have recently attempted the isomorphous substitution of Si" by Zr ions in the MFI and MEL framework by hydrothermal synthesis by using ZrCU as a source [10,11]. In these reports, we have shown that about 0.6 - 1.0 Zr atom per unit cell could be incorporated in the framework positions and that those well dispersed Zr ions are capable of catalyzing hydroxylation of phenol with aqueous H2O2. This report describes the hydrothermal synthesis of Al- free zirconium silicate molecular sieves with MEL structure using two different sources viz., zirconium (IV) tetrachloride and zirconium (IV) acetylacetonate and their physico-chemical characterization and a comparison of the samples. 

Si Liquid NMR studies of Sn-silicalite-1 molecular sieve during synthesis... 

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]. 

In their contribution to synthesis and characterization of a tantalum silicalite-1 molecular sieve with MFI structure, Ko and Ahn [342] regarded the occurrence of an IR band at 965 cm as an indication of tantalum incorporation into the framework. Their interpretation followed that given in [326,327]. The intensity of the 965 cm band increased with increasing Ta content, and the material exhibited strong Bronsted acidity. However, a corresponding band aroimd 960 cm" was not detected after isomorphous substitution of Al by Fe into, for instance, mordenite [343]. 

Doubly substituted analogues of TS-1 have also been reported. Trong et al. (130) synthesized bifunctional molecular sieves with titanium and various trivalent ions, for example, Ti-MFI that also contained, Al, or Ga. Tin and vanadium have also been incorporated into the titanium silicalite structure (33,131) by a primary synthesis method. The incorporation of a second metal changes the redox properties of the materials as well as their morphology. Incorporation of tin into titanium silicalite improved the epoxidation selectivity of the catalyst compared with that of (mono-substituted) TS-1. 

A wealth of techniques can be applied at the nascence of a catalyst material. This will be illustrated using two showcases, namely the synthesis of molecular sieve silicalite-1, which possesses one of the most well-known microporous frameworks. 

Zeolites are crystalline aluminosiHcates. Their unit cells are quite complex, as they have intricate microporous structures. Currently, around 200 frameworks are known for zeolites [10], and they all have one specific characteristic chaimels and pores in the size range 2 A to 1 nm, incorporated into the framework structure. This characteristic makes them appropriate for use as, for example, molecular sieves, cation-exchange materials, supports for catalytic active phases, and catalysts themselves [11, 12]. Controlled synthesis of zeoHte materials is still a challenge, and in this regard only a few selected zeolites have been studied in detail [13]. Silicalite-1 (MFI framework) has a pure-silica stmcture, but does not have active sites. The incorporation of, for example, heteroatoms such as aluminum (ZSM-5) makes it catalytically active [14, 15]. Nevertheless, silicalite-1 can be seen as an archetype system, of which its preparation has been characterized in great detail. 

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