This review is focused on the use of solid Lewis acids to promote catalytic oxidations. While the concept of using Lewis acids to promote the reaction of organic substrates with oxidizing reagents is widely accepted in homogeneous catalysis, this concept has not become evident and generally used in heterogeneous catalysis until recent days. Certainly the development of new Lewis acid solids active and selective for catalytic oxidations is an urgent need and a challenging scientific target for some substrates especially using environmentally friendly oxidants. Since the replacement of current stoichiometric oxidations for the production of fine chemicals by environmentally benign catalytic oxidations is one of the major tasks in green chemistry, solid Lewis acids are called to play a crucial role to accomplish this goal. In the review, we will see the still important role that stoichiometric oxidations play in our daily life, and how they are being substituted by catalytic oxidations. At this point, three general mechanisms in which Lewis acids are involved will be described, and the material has been organized starting from homogeneous and ending with solid catalysts for heterogeneous oxidations. A bridge between the two will be established by presenting catalytic systems that can fill the gap between the two systems helping to rationalize the nature of the catalytic active sites in solid systems. This review is obviously focused on solid oxidation catalysts, and the core of the review is organized to show the evolution from the simplest strategy for heterogeneizing homogeneous catalysts, i.e., supporting the active species on large surface area solids, to the more elaborate ones in which the active sites are part of the solid structure. Given the importance of metallosilicates, and more specifically titanosilicates, as catalysts in commercial processes, special attention has been paid to these types of materials. Although sufficient references are provided to early seminal work, special emphasis has been given to most recent contributions to this area, particularly of the last 10 years. Patent literature has also been extensively covered in this review. Examples to illustrate the concepts have been selected among recent publications, and an effort has been made to present a series of commercial and near commercial processes based on catalytic oxidations. Finally, two * E-mail: acorma@itq.upv.es. 3837 Chem. Rev. 2002, 102, 3837−3892

Lewis acids as catalysts in oxidation reactions: from homogeneous to heterogeneous systems.

Publisher Summary This chapter focuses on the microporous crystalline titanium silicates. Titanium containing materials have been investigated for various reactions, but selective oxidations with H2O2 as the oxidant have attracted the most interest. For these reactions, the formation of surface titanium peroxo compounds with H2O2 and the subsequent transfer of the peroxidic oxygen to the organic reactants have been proposed to explain the mechanism by which titanium participates in the catalytic cycle. H2O2 has the advantage of giving environmentally benign water as its by-product. Furthermore, H2O2 ranks first in terms of available oxygen in the list of oxygen donors.The unusual properties of titanium silicalites have been attributed to the presence of TiIV in framework positions of the SiO2 lattice. It is important to realize that there is a limit to the extent of substitution; the exact value is still under discussion, but is certainly not more than a few percent. Very likely, the structure of crystalline silica is not stable at higher degrees of substitution. The discovery of the new titanium silicates and of their catalytic properties in H2O2 oxidation reactions has had a major impact in catalytic science and its industrial applications.

Microporous Crystalline Titanium Silicates

Four series of narrow-disperse poly(N-isopropylacrylamide) (PNIPAM) with well-controlled molecular weights and with end groups of varying hydrophobicity were synthesized by room temperature atom tr...

End group effect on the thermal response of narrow-disperse poly (n -isopropylacrylamide) prepared by atom transfer radical polymerization

A multichannel membrane microreactor for continuous selective oxidation of aniline by hydrogen peroxide on TS-1 nanoparticles was successfully demonstrated. The high surface area to volume ratio that can be attained in the microreactor (3000 m(2)/m(3)) facilitates the selective removal of water by-product, which reduces the effect of catalyst deactivation during the reaction. An improvement in the product yield and selectivity towards azoxybenzene was also observed. Azobenzene was obtained as by-product and its formation was attributed to homogeneous reaction of nitrosobenzene with aniline. Increasing temperature was beneficial for both yield and selectivity, but beyond 340 K, microreactor operation was ineffective due to bubble formation and hydrogen peroxide decomposition. (c) 2004 Elsevier B.V. All rights reserved.

TS-1 oxidation of aniline to azoxybenzene in a microstructured reactor

Catalytic version of the H2O2–TS–1 combination has been shown to display a good reaction selectivity in the liquid-phase oxidation of anilines to symmetrical azoxybenzenes.

Selective catalytic oxidation of arylamines to azoxybenzenes with H2O2 over zeolites

Catalytic trans N-acylation of anilides with α-chloropropionyl chloride over zeolites+

Selective Catalytic Oxidation of Arylamines to Azoxybenzenes with H2O2 Over Zeolites.

A mild and effective method of zeolite catalyzed trans N-acylation of anilides in liquid-phase is described.

Catalytic Trans N‐Acylation of Anilides with α‐Chloropropionyl Chloride over Zeolites.

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Selective catalytic oxidation of arylamines to azoxybenzenes with H2O2 over zeolites

Catalytic trans N-acylation of anilides with α-chloropropionyl chloride over zeolites+

Selective Catalytic Oxidation of Arylamines to Azoxybenzenes with H2O2 Over Zeolites.

Catalytic Trans N‐Acylation of Anilides with α‐Chloropropionyl Chloride over Zeolites.