Wacker catalyst

There are many ways to produce acetaldehyde. Historically, it was produced either hy the silver-catalyzed oxidation or hy the chromium activated copper-catalyzed dehydrogenation of ethanol. Currently, acetaldehyde is obtained from ethylene hy using a homogeneous catalyst (Wacker catalyst). The catalyst allows the reaction to occur at much lower temperatures (typically 130°) than those used for the oxidation or the dehydrogenation of ethanol (approximately 500°C for the oxidation and 250°C for the dehydrogenation). 

Isobutylene glycol may also be produced by a direct catalyzed liquid phase oxidation of isobutylene with oxygen in presence of water. The catalyst is similar to the Wacker-catalyst system used for the oxidation... 

In 1969, 90% of vinyl acetate was manufactured by this process. By 1975 only 10% was made from acetylene, and in 1980 it was obsolete. Instead, a newer method based on ethylene replaced this old acetylene chemistry. A Wacker catalyst is used in this process similar to that for acetic acid. Since the acetic acid can also be made from ethylene, the basic raw material is solely ethylene, in recent years very economically advantageous as compared to acetylene chemistry. An older liquid-phase process has been replaced by a vapor-phase reaction run at 70-140 psi and 175-200°C. Catalysts may be (1) C—PdCb—CuCb, (2) PdClj—AI2O3, or (3) Pd—C, KOAc. The product is distilled water, acetaldehyde, and some polymer are... 

Acetaldehyde may be made (1) from ethylene by direct oxidation, with the Wacker-catalyst containing copper(II) and palladium(II) salts (2) from ethanol by vapor-phase oxidation or dehydrogenation or (3) from butane by vapor-phase oxidation. The direct oxidation of ethylene is the most commonly used process, accounting for 80% of acetaldehyde production. 

Higher alkenes can also be converted to methyl ketones with the Wacker catalyst, but the rates and selectivities are lower. Improved procedures use basic406,407 or alcoholic solvents 408 Tsuji and coworkers used the PdCl2/CuCl catalyst in DMF for the synthesis of a variety of natural products and fine chemicals.409 Only terminal alkenes are ketonized under these conditions, even when the substrate contains other functional groups.395... 

The synthesis of dialkyl oxalates by oxycarbonylation of alcohols in the presence of a dehydrating agent and a Wacker catalyst was first reported by Fenton in 1968 (equation 177).378,449 As for the previous oxycarbonylations, the presence of water is a strong inhibitor of the reaction and favors the side-formation of C02 (equation 178). Dehydrating agents such as triethyl orthoformate or boric anhydride are necessary to prevent water formation and subsequent deactivation of the... 

In order to support the proposed mechanism for the Wacker Process, there has recently been growing interest in the preparation of stable n-vinyl alcohol complexes, preferably containing palladium or one of the other metals of the platinum group which show activity as Wacker catalysts. Until recently, the only well-characterized n complexes of vinyl alcohol have contained iron. One of the first reports of the formation of a stable 7r-vinyl alcohol complex by Ariyaratne and Green described the preparation of 7r-cyclopentadienyldicarbonyl(/3 oxoethyl)iron (39)... 

The main disadvantage of Wacker chemistry is the chloride medium in which the reaction occurs. It is corrosive and leads to formation of a significant number of chlorinated by-products the yield of acetaldehyde is approximately 95% (304). For obvious reasons, attemps were reported to anchor a chloride-free variant of the traditional homogeneous Wacker catalyst. 

Pd/Cu Ion-Exchanged Zeolites as Green Variants of the Wacker Catalyst... 

The zeolite-based Wacker catalyst, however, shows the following characteristics ... 

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... 

The mechanism of deactivation of supported Wacker catalysts in the oxidation of 1-butene... 

The decrease in activity of heterogeneous Wacker catalysts in the oxidation of 1-butene is caused by two processes. The catalyst, based on PdS04 deposited on a vanadium oxide redox layer on a high surface area support material, is reduced under reaction conditions, which leads to an initial drop in activity. When the steady-state activity is reached a further deactivation is observed which is caused by sintering of the vanadium oxide layer. This sintering is very pronounced for 7-alumina-supported catalysts. In titania (anatase)-supported catalysts deactivation is less due to the fact that the vanadium oxide layer is stabilized by the titania support. After the initial decrease, the activity remains stable for more than 700 h. 

A rather interesting application of zeolite-based alkene oxidation catalysis has been demonstrated by Japanese workers (46, 47). In particular, a Pd2 +, Cu2 +Y zeolite was shown to be an active and stable heterogeneous oxidation catalyst which is analogous to the well-known homogeneous Wacker catalyst system containing PdCl2 and CuCl2 (48). Under Wacker conditions (i.e., alkene/02/H20) the zeolite Y catalyst was shown to convert ethylene to acetaldehyde and propylene to acetone with selectivities in excess of 90% with C02 as the major by-product. 

Nowinska, K., Dudko, D. Mn(5.n/2HnPMOV2 as re-oxidant of palladium(ll) in solid Wacker catalysts. React. Kinet. Catai. Lett. 1997, 61,187-192. 

Francis, J. W., Henry, P. M. Oxidation of olefins by palladium(ll). Part XIV. Product distribution and kinetics of the oxidation of ethene by PdCl3(pyridine)- in aqueous solution in the presence and absence of CUCI2 a modified Wacker catalyst with altered reactivity. J. Mol. Catal. A Chemical 1995, 99, 77-86. 

Rao, V., Datta, R. (1988). Development of a supported molten-salt Wacker catalyst for the oxidation of ethylene to acetaldehyde. Journal of Catalysis, 114(2), 377—387. 

The carbonylative oxidation of alkenes catalyzed by palladium catalysts has been extensively studied owing to its industrial importance. The conversion of ethylene to acrylic acid has been developed into a commercial process by Union Oil (Scheme 10). The reaction is performed in a mixed solvent of acetic acid and acetic anhydride in the presence of a Wacker catalyst under high pressure of ethylene. 

---