Titanium silicalite selective oxidation reactions

The titanium silicalite is unique in catalyzing the oxidation of aromatics with H2O2 at high selectivity. For example, phenol is hydroxylated to a mixture of cathecol and hydroquinone. Conversion for H2O2 is 70%, and a selectivity for phenol is 90%. A listing of titanium silicalite catalysed oxidation reactions is shown in Table 1. These results suggest the "restricted transition state selectivity" to be effective because of the peculiar pore structure of the catalyst. 

The reaction is carried out using a titanium silicalite-1 (TS-1) zeolite catalyst [30, 122]. This type of catalyst is known to accelerate the selective oxidation of alcohols, epoxidation of alkenes and hydroxylation of aromatics. These reactions have importance for fine-chemical production. 

Oxidations of various organic substrates with aqueous hydrogen peroxide have been reported on titanium containing derivatives of silicalite-1, denoted as Titanium-Silicalite-1 or TS-1 [93-97]. Examples of reactions which are catalyzed by TS-1 with high H2O2 yields and product selectivities are listed in Table 6. The oxidations are generally carried out at atmospheric pressure and at temperatures ranging from 273 to 373 K. 

The pore diameter of zeolite beta is 7 A, larger than those of silicalite-1 and silicalite-2 (5.5 A). Titanium incorporated into zeolite beta reacts with molecules whose dimensions are too big to diffuse in the pores and be oxidized by TS-1 or TS-2. The drawback is that zeolite beta must contain Al3+ to crystallize, and this imparts strong protonic acidity to the solid, with the consequence that secondary acid-catalyzed reactions also take place. However, the acidic properties can be neutralized in several ways and highly selective oxidations can be carried out on Ti-beta (Section V.C.3.b). 

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. 

The discovery in the early 80 s of titanium silicalites [62-64] opened the new application perspective of zeolitic materials as oxidation catalysts. Several reactions of partial oxidation of organic reactants using dilute solutions of hydrogen peroxide could for the first time be performed selectively in very mild conditions. Other elements inserted in the lattice of silicalites have since been shown to have similarly interesting catalytic properties including, vanadium, zirconium, chromium and more recently tin and arsenic [65]. Titanium silicalites with both MFI (TS-1) and MEL (TS-2) structures have however been the object of more attention and they still seem to display unmatched properties. Indeed some of these reactions like the oxyfunctionalization of alkanes [66-69] by H2O2 are not activated by other Ti containing catalysts (with the exception of Ti-Al-Beta [70]). The same situation... 

In view of the current interest in environmental protection, we have studied the possibility of using transition substituted molecular sieves for catalyzing the heterogeneous oxidation of aniline and, more generally arylamines. We have recently reported that TS-l, the titanium-substituted silicalite-1, was an excellent catalyst for the selective oxidation of aniline into AZY [6], provided that the H202/aniline ratio was relatively low (< 1.6). We showed that the oxidation more likely proceeded via the formation of phenylhydroxylamine (PH) and nitrosobenzene (NSB) and that these intermediate compounds could react together to form AZY. Because of the small pore dimensions of the structure, the reaction could not be carried out with TBHP and was limited by diffusion of reagents and/or products in the channels. 

In the literature there has been much debate regarding the role of the lattice or extralattice Ti in Ti silicalite for a variety of oxidation reactions. In order to have a more precise idea of the role of the lattice or surface Ti and more specifically of the role of the coordination sphere of Ti, a series of monopodal and tripodal titanium surface complexes (i. e., =SiOTi(OR)3 and ( SiOIsTiOR) were derived by the reaction of the Ti alkyl (Structure 1) and hydride species with water, oxygen, methanol, and tert-butanol. The resulting complexes were then used in the epoxidation of 1-octene by tert-butyl hydroperoxide. Tripodal complexes, especially (=SiO)3Ti( Bu), were found to be significantly more active and more selective for the epoxidation of 1-octene than their monopodal counterparts [22]. 

The first step operates in the liquid phase with ammonia and H2O2 as the reactants and titanium-silicalite (TS-1) as the catalyst. TS-1 is a zeolite, developed by Eni, having a structure that belongs to the same MEI family as ZSM-5, but in which A1 is absent (acid sites are detrimental for selectivity) and substituted by tetravalent Ti ions, which can activate H2O2 and give selective reactions of oxidation (Eigure 2.33 see also Chapter 6 on propene oxide for further aspects). 

Clerici and Bellussi" have shown that hexane in methanol can be selectively oxidized to 2-hexanol, 3-hexanol, 2-hexanone and 3-hexanone using a mixture of oxygen and hydrogen at 25 - 30 . The reactions were run in the presence of HCl for 20 - 24 hours. Several titanium silicalite catalysts containing Pd (0.01 mol ratio to Ti02) were prepared and used in these reactions. Presumably, hydrogen plays the role of an electron-donor activating the Ti catalyst. However, no explanations were offered. 

While a simple matter of academic curiosity in past years, isomorphously substitued zeolites, gained very recently a major practical significance with the advent of T-substituted MFI structures such as boralite, ferri-silite which established themselves as efficient catalysts in para-xylene production. Not only can the substituting T atoms influence the acid strength and shape selective properties, they are also likely to act on their own as catalytic centres. Typical of such a behaviour is TS-1 brought to public interest by ENI researchers. Titanium as the substituting element in a silicalite structure is active in various mild oxidation reactions. 

The titanium silicalite is an industrial catalyst for the oxidation of olefins or aromatics with hydrogen peroxide as oxidant. It catalyses the epoxidation of both olefins and diolefins with high yields and with high epoxide selectivity (>98% for propylene). The reaction can be conducted with dilute H2O2 solution in contrast to other competing catalysts, which require nearly pure H2O2 as oxidant. 

In some cases, oxidation of double bonds does not stop at the epoxide, but proceeds further to oxidative cleavage of the double bond. It was reported that the reaction of a-methyl styrene with H2O2 in the presence of TS-1 or TS-2 produces a-methyl styrene epoxide (15%), a-methyl styrene diol (10-40%) and acetophenone (40-60%) (Reddy, J. S. et al., 1992). However, results similar to those obtained with titanium silicates were obtained for many other catalysts, such as HZSM-5, H-mordenite, HY, A1203, HGa-silicalite-2, and fumed Si02. These materials have much different properties and differ significantly from titanium silicates thus, the results cast some doubt on the role of the catalyst in this reaction. Furthermore, the oxidation of styrene is reported to proceed with C=C cleavage and formation of benzaldehyde, in contrast to previous reports of the formation of phenylacetaldehyde with 85% selectivity (Neri et al., 1986). 

The ODH of propane over titanium and vanadium containing zeolites and nonzeolitic catalysts revealed that Ti-silicalite was the most active. The addition of water caused an increase in selectivity, probably due to a competitive adsorption on the active sites. The reaction is proposed to occur on the outer surface of the Ti-silicalite crystallites on Lewis acid sites, and a sulfation of the catalyst, which increases the acidity of these sites, results in a further increase of the catalytic activity. The maximum conversion obtained was 17% with a propene selectivity of up to 74% [65]. Comparison of propane oxidation and ammoxidation over Co-zeolites shows an increase in conversion and propene selectivity during ammoxidation. For a conversion of 14%, 40% propene selectivity was obtained with ammonia, whereas, at 10% conversion the propene selectivity was only 12% with oxygen. The increase in activity and selectivity can be due to the formation of basic sites via ammonia adsorption [38]. 

Most of the interesting catalytic properties of zeolites are related to the presence of strong acid sites. Many processes have been developed taking advantage of the zeolites acidity and shape selectivity. At the end of the seventies, we have prepared a silicalite with titanium in lattice positions (ref.s 1,2). The discovery of TS-1 and its peculiar properties in oxidative processes with hydrogen peroxide started a completely new field of shape selective reactions (ref. 3). 

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