Titanium-silicalite 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). 

The titanium-silicalite composihon, TS-1, has achieved commercializahon in selective oxidation processes and iron-siUcalite in ethylbenzene synthesis. 

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 isomorphous substitution of Siiv by Ti,v was claimed by Taramasso, Perego, and Notari in 1983 for a new material with the composition xTi02(l - x)Si02 (0.0 x 0.04 M). This has the crystalline structure of silicalite-1 (or MF1) with Tilv in framework positions it was named titanium silicalite-1 or TS-1 (Taramasso el al., 1983). The occurrence of isomorphous substitution was deduced from the regular increase in unit-cell parameters with the degree of substitution and from the good agreement between the observed and calculated values of the Si—O and Ti—O distances. The same type of evidence had already been obtained by the same authors in the synthesis of crystalline microporous boron silicates, where the smaller B—O distance relative to Si—O causes a decrease in unit-cell parameters (Taramasso et al., 1980). 

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. 

Titanium Silicalite-2 (TS-2), structurally similar to TS-1, could be prepared likewise using tetrabutylammonium hydroxide as the template [13, 14]. Titanium aluminum Beta (Ti,Al-[3) was prepared by hydrothermal synthesis from amorphous silica, sodium aluminate, tetraethyltitanate and tetraethylammonium hydroxide [15]. The presence of A1 was necessary for the crystallization of the product. Al-free Titanium Beta (Ti-[3) could be obtained in the presence of particular templates, such as dibenzyldimethylammonium hydroxide [16]. Titanium Mordenite (Ti-MOR), conversely, was obtained by post-synthesis insertion of Ti to dealuminated Mordenite [17]. Ti-MWW (Ti-MCM-22) was obtained by the synthesis of the lamellar precursor of Ti,B-MCM-22 followed by acid treatment to remove most of the boron and extra-framework Ti and finally calcination to burn out the template and bring about the condensation of lamellae into the three-dimensional MWW structure [18]. Ti is present in a number of different environ-... 

The common means of introducing redox catalytic activity in zeolites is by the substitution of framework atoms such as Si, A1 or P with redox-active metal cations. This has been accomplished by two different methods (1) hydrothermal synthesis and (2) post-synthesis modification. Irrespective of the method of preparation, with the notable exception of titanium silicalites, these redox metals in the framework are susceptible to leaching due to the solvolysis of M-O bonds [77]. Even the Ti silicalites suffer from leaching under basic conditions [76a]. 

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]. 

M. G. Clerici, G. Bellussi, U. Romano, Synthesis of propylene oxide from propylene and hydrogen peroxide catalyzed by titanium silicalite, J. Catal. 129 (1991) 159. 

R. B. Khomane, B. D. Kulkarni, A. Paraskar, S. R. Sainkar, Synthesis, characterization and catalytic performance of titanium silicalite-1 prepared in micellar media. Mater. Client. Phys. 76 (2002) 99. 

Improvements of already existing oxidation processes are continuously made (in MAA manufacture, with the riser reactor by DuPont, or in oxychlorination, by Montecatini Technologic and ICI). In addition, and still more clearly demonstrating the dynamism of industrial catal5rtic oxidation, completely new catalysts are discovered, especially with the titanium silicalite which permits the synthesis of hydroquinone from phenol, selective epoxidations, oxidations of alcohols to aldehydes, and the manufacture of cyclohexanoneoxime. 

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]. 

The heterogeneous epoxidation of compounds which contain carbon-carbon double bonds is an important industrial process in both the manufacture of fine chemicals and in the synthesis of natural products. A number of studies have demonstrated that alkenes can be readily epoxidised by hydrogen peroxide using the titanium silicalite TS-1 11 -3. However, it has been found that substitution of the alkene by electron withdrawing groups significantly decreases the reactivity of the carbon-carbon double bond since the decrease of the electron... 

The catalysts used in the aforementioned studies were always titanium silicates of MFI structure prepared by hydrothermal synthesis. Ti can, however, be inserted in the silica lattice by post-synthesis treatment of a dealuminated H-ZSM-5 with TiCl4 vapor [11]. Titanium silicalite-2 (TS-2), with the MEL structure of ZSM-11, was prepared shortly after the first synthesis of TS-1 [15]. Both catalysts have been used for the hydroxylation of phenol. Kraushaar-Czarnetzki and van Hooff showed that no major catalytic differences resulted from the method of synthesis of TS-1 [11]. The slow rate of reaction they observed was probably the result of large crystal size and low titanium content [7]. Tuel and Ben Taarit demonstrated there was no perceptible difference between the catalytic activity of TS-2 and TS-1 [8]. This was predictable, because of the close similarity of the Ti-site structure, chemical composition, and pore dimensions of the two titanium silicates. 

The synthesis of titanium silicalites TS-1 [10] and TS-2 [84-85], with MFI and MEL structure respectively, opened new opportunities in the oxidations with H O. TS-1 and TS-2, the former being the most studied, show similar properties in catalysis. Catalytic sites are isolated titanium atoms, incorporated into the zeolitic framework, in a channel system of about 0.55 nm average diameter [86-88], Different Ti-peroxo species, at lattice position, result from complex equilibria between TS-1, H O, and protic molecules [89-91],... 

Although titanium silicalite was discovered over a decade ago, studies on other redox zeolites are just beginning. This is a field in which the most promising results are anticipated. The synthesis of large pore zeolites, containing Ti or other redox metal atoms, can open entirely new routes to catal)4ic oxidations and new products. 

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. 

TS-2, a smaU-pored titanium silicalite with the MEL structure, was synthesized by Reddy et al. (29,132) Their synthesis method was a modification of that used for TS-1 and differed with respect to the SDAs for example. 

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