Redox molecular sieves

A well-known example of the latter type is titanium silicalite-1 (TS-1), a redox molecular sieve catalyst [7]. 

An extremely versatile catalyst for a variety of synthetically useful oxidations with aqueous hydrogen peroxide is obtained by isomorphous substitution of Si by Ti in molecular sieve materials such as silicalite (the all-silica analogue of zeolite ZSM-5) and zeolite beta. Titanium(IV) silicalite (TS-1), developed by Enichem (Notari, 1988), was the progenitor of this class of materials, which have become known as redox molecular sieves (Arends et al., 1997). 

Oxidation of thiophene and its derivatives was studied using hydrogen peroxide (H2O2), t-butyl-hydroperoxide and Ti-Beta redox molecular sieve as selective oxidation catalysts. A new reaction pathway was discovered and investigated using C-13 NMR, GC, GC-MS, HPLC, ion chromatography, and XANES. The thiophene oxidized to thiophene-sesquioxide [3a,4,7,7a-tetrahydro-4,7-epithiobenzo[b]-thiophene 1,1.8-trioxide] and the sesquioxide oxidized mostly to sulfate. 2-Methyl-thiophene and 2,5 dimethylthiophene also oxidized to sulfate and sulfone products. The Benzothiophene oxidation product was sulfone. This proposed new reaction pathway is different from prior literature, which reported the formation of thiophene 1,1-dioxide (sulfone ) as a stable oxidation product... 

Organic sulfur compounds are present in gasoline and diesel. With the increased emphasis on the requirement for more environmentally friendly transportation fuels [1], oxidative desulfurization, using H202 and redox-molecular sieves [2,5,6,7], has been studied and shown to significantly reduce the sulfur content of gasoline and diesel. The reaction of thiophene and its derivatives were successfully converted to oxidized compounds, but the identification of oxidized compounds was not simple because the concentrations of individual sulfur compounds were low. Most of the previous literature has reported sulfone formation. 

Arends, I.W.C.E., Sheldon, R.A., Wallau, M. and Schuchardt, U. (1997). Oxidative transformation of organic compounds mediated by redox molecular sieves. Angew... 

R. A. Sheldon, Redox molecular sieves as heterogeneous catalysts for liquid phase oxidations. Stud. Surf. Sci. Catal. 110, 151-175 (1997). 

Table 4.1 illustrates a number of redox molecular sieves which have been successfully prepared and employed as oxidation catalysts with hydrogen peroxide, alkyl hydroperoxides and molecular oxygen. 

Table 4.1 Pore size and structural topographic species of various redox molecular sieves...

Whilst the majority of the discussion thus far has been concerned with metallo-substituted redox molecular sieves, it is important to note that proto-nated zeolite forms can also be employed for selective oxidation with aqueous hydrogen peroxide. An excellent example of this is the study conducted by the Mobil Oil Corporation.52 Their work has shown that a number of protonated zeolites such as H-ZSM-5 or zeolite-/ can be used with hydrogen peroxide to catalyse the oxidation of cyclic ketones to lactones or the co-hydroxycarboxylic acids (Figure 4.12). 

TS-1 was the prototype of a new generation of solid, recyclable catalysts for selective liquid phase oxidations, which we called redox molecular sieves [76]. A more recent example is the tin(IV)-substituted zeolite beta, developed by Cor-ma and coworkers [77], which was shown to be an effective, recyclable catalyst... 

Figure 13. (a, b) Schematic representation of the oxidation pathways using redox molecular sieves (a) homolytic free radical autoxidation and (b) heterolytic oxygen transfer, (c) Oxidation of styrene to styrene oxide and transformation to 2-phenylacetaldehyde using a bifunctional Ti-silicalite catalyst. 

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]. 

Conversion and selectivities of various redox molecular sieves in the decomposition of cyclohexenyl hydroperoxide. 

The finding of an active solid redox system resulted in a flourish of activity in the development and application of diverse redox molecular sieves containing titanium (IV) and other metal ions [378-380]. Like the earlier ion-exchanged zeolites, many of the resulting catalysts, however, also suffered from loss by leaching, even when the redox element was substituted in the framework [102]. Ti-substituted zeolites remain special because of then stability. 

Redox Molecular Sieves as Heterogeneous Catalysts for Liquid Phase Oxidations... 

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]. 

Why use a redox molecular sieve Although the major motivation stems fh)m the expectation of producing a catalyst with unique activity we note that, in many cases, a stable. 

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