Supported Metals and Mixed Oxides

Supported noble metals catalyze the hydroxylation of benzene to phenol, under mild conditions, with H2/O2 and CO/O2 mixtures as oxidants [59,60]. Yields of practical value were obtained on noble metals supported on silica modified by various metal oxides. The highest productivity was shown by PtA 20s/Si02, at 45 °C [60]. Under analogous conditions, cresols with minor amounts of benzaldehyde and benzyl alcohol were produeed by toluene, a- and /S-naphthols by naphthalene, hydroxybiphenyls by biphenyl. 

High activity and selectivity, mildness, and cleanliness are typical aspects of catalysis by redox molecular sieves. Their potential, however, in the hydroxylation of large aromatic molecules is still undefined. The latest studies still concentrate on simple aromatic compounds, chiefly phenol, benzene, and their alkylated derivatives, with limited relevance to fine chemicals, and largely involve the use of mi-croporous zeolites. Few studies relate to large-pore and mesoporous catalysts, potentially valuable in the oxidation of complex molecules. Reasons might possibly be the relative youth of redox molecular sieves, mostly at the stage of material optimization, and the lower activity of mesoporous catalysts. The latter is a major problem, as already shown for Ti-molecular sieves, for which only partial solutions have been proposed. 

A final comment is necessary on the stability of redox molecular sieves, because in this chapter emphasis was placed on studies in which the heterogeneity of catalysis was most plausible (but in many cases not rigorously proven). Yet, catalysts for which metal leaching was once detected should be not automatically excluded from further consideration, because the extent of metal solubilization depends, inter alia, on the kind of reaction performed [44]. 

Bellussi and M. S. Rigutto, in Advanced Zeolite Science and Applications (J. C. Jansen, 

Stocker and H. G. Karge, Eds.), Studies in Surface Science and Catalysis, Vol. 85, p. 177. Elsevier, Amsterdam, 1994. 

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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. 

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