Phase Wacker-type oxidation

PdS04/H9PV6Mo604o/CuS04 in the presence of chemically modified P-cyclodextrins were also used as CPTC systems in the Wacker-type oxidation of higher a-olefins (Cg-Cje) to the corresponding 2-ketones (Equation 18) with high yields (90-98%) in an aqueous/organic two phase system.545,571... 

Monflier, E., Mortreux, A. Wacker-type oxidations. Aqueous-Phase Organometallic Catalysis 998, 513-518. 

R] Monflier, E. Mortreux, A. Wacker-type oxidations. In Aqueous-Phase Organometallic Catalysis (2nd Edition) Comils, B. Herrmann, W. A. (Eds.), Wiley-VCH Weinheim, Germany, 2004, pp. 481-487. 

Palladium catalysts are widely used in liquid phase aerobic oxidations, and numerous examples have been employed for large-scale chemical production (Scheme 8.1). Several industrially important examples are the focus ofdedicated chapters in this book Wacker and Wacker-type oxidation of alkenes into aldehydes, ketones, and acetals (Scheme 8.1a Chapters 9 and 11), 1,4-diacetoxylation of 1,3-butadiene (Scheme 8.1b Chapter 10), and oxidative esterification of methacrolein to methyl methacrylate (Scheme 8.1c Chapter 13). In this introductory chapter, we survey a number of other Pd-catalyzed oxidation reactions that have industrial significance, including acetoxylation of ethylene to vinyl acetate (Scheme 8. Id), oxidative carbonylation of alcohols to dialkyl oxalates and carbonates (Scheme 8.1e), and oxidative coupling of dimethyl phthalate to 3,3, 4,4 -tetramethyl biphenylcarboxy-late (Scheme 8.1f). 

Although the Wacker-type oxidation of olefins has been applied since the early 1980s, processes involving higher olefins are stiU the subject of investigations due to their poor solubility in water. Particularly interesting in this context is the inverse phase-transfer catalysis using water-soluble host molectdes. Indeed, upon a careful choice of the substituent, these receptor molecules avoid the isomerization into internal olefins or make it possible to perform substrate selective oxidations that cannot be achieved a biphasic medium with conventional transition metal catalysts. 

What do we mean by oxygen activation A reasonable working definition is catalysis of the oxidation of a hydrocarbon substrate by 2 not involving a classical free radical autoxidation mechanism or direct oxidation by a metal salt. The latter stipulation is needed to exclude Wacker-type oxidation processes in which the oxygen in the product is, initially at least, derived from water. On the other hand, it should not matter whether dioxygen complex formation precedes or follows the oxidation of the substrate by an oxometal complex (see Figure 4). The former pertains to liquid and the latter to gas phase processes. 

Supported liquid-phase catalysts (SLPCs) combine the salient features of both homogeneous and heterogeneous catalysis for enhanced catalytic and/or process efficiency (337). SLPC catalysts, in which a liquid-phase (homogeneous) catalyst is dispersed within a porous support, have been used in Wacker-type ethylene oxidation for acetaldehyde and vinyl acetate production (337, 338). In the former case, a traditional homogeneous Wacker catalyst (vide supra) consisting of a chlorinated solution of Pd and Cu chlorides retained on a support with monomodal pore size distribution... 

Quite a surprising reaction has recently been reported [74]. With a catalyst of palladium metal on carbon in aqueous phase, propene is oxidized with oxygen to give acrylic acid, probably via allyl alcohol in a allylic-type oxidation (for allylic oxidation see Section 3.3.14). In the presence of chloride or oxidants the normal Wacker-type reaction product acetone arises. 

An interesting approach to overcome these limits and thus combine the advantages of homogeneous and heterogeneous catalysis is that of supported liquid phase catalysts (SLPC or SLP). In SLPC the organometallic complex active components are dissolved in a small quantity of liquid phase dispersed in the form of an isle or film on the surface of supports. A SLPC has been applied successfully for several chemical transformations [113], particularly in the Wacker-type ethylene oxidation to acetaldehyde and vinyl acetate production by ethylene acetoxylation [114], and in other reactions catalyzed by Pd-complexes such as the Heck reaction [115]. 

Exploration of Basic Catalyst Components The study of direct oxidative acetoxyla-tion of 1,3-butadiene began with the use of Wacker-type homogeneous catalyst Pd(OAc)2-CuCl2 [10]. This catalyst system gave low l,4-diacetoxy-2-butene selectivity, and there was a problem in separating the catalyst. After that, liquid-and vapor-phase methods using a Pd-based catalyst were studied in parallel. Catalyst activity was greatly improved by the addition of Bi or Sb to the Pd catalyst in the gas-phase reaction [11]. However, catalyst activity was reduced by the adhesion of resin by-product derived from unsaturated aldehydes on the catalyst surface. Various improvements have been tried in the gas phase, but catalyst robustness has never met industrial requirements. 

The maintenance of a supported liquid layer in gas-phase reactions is also important in other heterogeneous catalytic applications, such as the Bayer/Hoechst process for vinyl acetate manufacture. However, in these systems, the catalytic metal is reduced to the metallic state, leading to significant mechanistic differences from the formally related homogeneous Wacker-type alkene oxidation/acetoxylation processes (section 11.7.7.3). 

Otsuka K, Shimizu Y, Yamanaka I, Komatsu T (1989) Wacker type and 7c-allyl type oxidations of propylene controlled by fuel cell system in the gas phase. Catal Lett 3 365-370... 

Catalytic Oxidation of Ethene to Acetaldehyde and Acetic Acid. -Evnin et al120 studied Pd-doped V2 Os catalysts for the vapor-phase oxidation of ethene to acetaldehyde in a heterogeneous type of Wacker process. From a mechanistic study they establish a redox mechanism with Pd both as the site of the ethene oxidation and of the reoxidation of the catalyst. On the basis of the role of the V4+ ions proposed by these authors, Forni and Gilardi121 substantiated this mechanism by adding tetra- and hexa-valent dopants to the V2 05 and studying the effects on the catalytic reaction. 

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