Medium-pore Titanium Zeolites

A promising and cleaner route was opened by the discovery of titanium silica-lite-1 (TS-1) [1,2]. Its successful application in the hydroxylation of phenol started a surge of studies on related catalysts. Since then, and mostly in recent years, the preparation of several other zeolites, with different transition metals in their lattice and of different structure, has been claimed [3]. Few of them have been tested for the hydroxylation of benzene and substituted benzenes with hydrogen peroxide. Ongoing research on suppoi ted metals and metal oxides has continued simultaneously. As a result, knowledge in the field of aromatic hydroxylation has experienced major advances in recent years. For the sake of simplicity, the subject matter will be ordered according to four classes of catalyst medium-pore titanium zeolites, large-pore titanium zeolites, other transition metal-substituted molecular sieves, and supported metals and mixed oxides. 

Many oxidation reactions have been carried out using hydrogen peroxide and the titanosilicate, TS-1. However, this catalyst has relatively small pores and is therefore not an efficient catalyst for the oxidation of large molecules. This problem has been solved by the successful generation of a medium-pore titanium zeolite Beta-Ti [136]. Cyclododecane and cyclohexane are both oxidised selectively by H2O2 in the presence of the new titanium zeolite, favouring the ketone product. 

Since the first synthesis of TS-1 in 1983 [1], considerable efforts have been devoted to the synthesis of titanium-containing zeolites [2, 3]. Recently, Ti-beta, a large-pore molecular sieve, has been extensively studied [4, 5]. Owing to its unique large-pore channel system, Ti-beta seems to be more active than the medium-pore TS-1 catalyst for the oxidation of cyclic and branched alkenes with aqueous hydrogen peroxide. Under the usual synthesis conditions, however, Ti-beta crystallizes with some Al as a framework constituent [4], This leads to the presence of acid centers, which may have a detrimental effect on the activity or selectivity of this type of catalyst. Since 1992, the discovery of a new family of mesoporous molecular sieves has received much attention [6,7], Because of their mesoporous nature (20-100A), the Ti-MCM-41 zeolites may be useful as oxidation catalysts for larger molecules [8], In this... 

Many titanium silicates are active for this reaction, Ti-beta and Ti-HMS being the most active. The results demonstrate that in catalysis by TS-1 and by medium-pore zeolites, the reaction is limited by diffusion. TS-48, which has been found to be inactive in other oxidation reactions, is an active catalyst for the oxidation of aniline (Gontier et al, 1994 Sonawane et al., 1994). 

Titanium-containing pure-silica ZSM-48 (e.g., [71, 72]), a unidimensional medium pore zeolite, and titano-aluminosilicates with the structure of zeolite Beta [72-74] are materials which are currently scrutinized in catalytic oxidation reactions [75]. In the latter case, however, residual acidity created by framework aluminum leads to undesired side reactions. Since, so far, the direct synthesis of Al-free pure titaniumsilicate Beta was not successful, van Bekkum et al. [76] developed a special post-synthesis modification technique. The three-step procedure... 

UTD-1 may have catalytic properties such as those observed with the commercially sucsessfiil titanium silicahte TS-1 catalyst which is effective for alkane and alkene o ddation as well as phenol hydroxylation in the presence of hydrogen peroxide [8]. The large pore nature of Ti-UTD-1 should allow the reaction of large substrates such as 2,6-di-tert-butylphenol as well as the use of oxidants such as tert-butylhydroperoxide (t-BHP) which are too large for the medium pore TS-1 zeolite. Ti-UTD-1 offers an opportunity to examine reactivity in pore space greater than Ti-beta but less than the mesoporous Ti-MCM-41 type molecular sieves. In the present study results for the peroxide based oxidation of cyclohexane, cyclohexene and 2,6-di-tert-butylphenol will be presented. 

Because acidified titanium oxide is the catalyst usually employed commercially for the transformation of 1 into 2 [8] there has been much investigation of this catalytic system [9]. A 1995 paper by Stefanis et al. [10] reported an investigation of the reaction of 1 in several alumina-pillared clays (PILCs montmorillonite- and beidellite-based, and their and Ca" -exchanged congeners) under Lewis acid conditions (solid is activated by heat to remove all water). The results were compared with those obtained by use of medium-pore zeolites USY, NH4+-ZSM-5, and H-mordenite. Conversion to 2 > 50% was always observed. The aim of the work was to clarify differences between site availability and acidity for the two types of solid. 

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