Ethylene oxidative acetoxylation

Other large-volume esters are vinyl acetate [108-05-4] (VAM, 1.15 x 10 t/yr), methyl methacrylate [80-62-6] (MMA, 0.54 x 10 t/yr), and dioctyl phthalate [117-81-7] (DOP, 0.14 x 10 t/yr). VAM (see Vinyl polymers) is produced for the most part by the vapor-phase oxidative acetoxylation of ethylene. MMA (see Methacrylic polymers) and DOP (see Phthalic acids) are produced by direct esterification techniques involving methacryHc acid and phthaHc anhydride, respectively. 

Soon after the invention of the Wacker process the formation of vinyl acetate by the oxidative acetoxylation of ethylene using Pd(OAc)2 was discovered by Moiseev [16], and the industrial production of vinyl acetate based on this reation was developed. At present, vinyl acetate is produced commercially by a gas-phase reaction of ethylene, acetic acid and O using Pd catalyst supported on alumina or silica (eq. 1.11). 

The in situ regeneration of Pd(II) from Pd(0) should not be counted as being an easy process, and the appropriate solvents, reaction conditions, and oxidants should be selected to carry out smooth catalytic reactions. In many cases, an efficient catalytic cycle is not easy to achieve, and stoichiometric reactions are tolerable only for the synthesis of rather expensive organic compounds in limited quantities. This is a serious limitation of synthetic applications of oxidation reactions involving Pd(II). However it should be pointed out that some Pd(II)-promoted reactions have been developed as commercial processes, in which supported Pd catalysts are used. For example, vinyl acetate, allyl acetate and 1,4-diacetoxy-2-butene are commercially produced by oxidative acetoxylation of ethylene, propylene and butadiene in gas or liquid phases using Pd supported on silica. It is likely that Pd(OAc)2 is generated on the surface of the catalyst by the oxidation of Pd with AcOH and 02, and reacts with alkenes. 

Oxidative acetoxylation provides a direct access from alkenes to alkenyl esters the alkene molecule undergoes replacement of an H atom by an acetate (or generally OCOR) group in its vinylic (v), allylic (a), or homoallylic (h) position according to Scheme 1, where Ox is an oxidant such as O2, Cu p-benzoquinone, and Red a reduced form of Ox such as H2O, Cu hydroquinone. A typical example is the Pd-catalyzed co-oxidation of ethylene and acetic acid to vinyl acetate (eq. (D). 

The oxidative acetoxylation of ethylene was discovered while studying the reactivity of palladium(II) 7r-complexes [1]. It was found that 7r-ethyl-ene-palladium chloride, the so-called Kharash complex jt-C- 4 PdCl2)2 [3], is readily decomposed by hydroxyl-containing reagents such as water or alcohols, yielding Pd metal and acetaldehyde or acetyl, respectively (eq. (2)) [1, 4]. 

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]. 

In the well-known Wacker process ethylene is converted to acetaldehyde by aerobic oxidation in an aqueous medium in the presence of PdCl2 as catalyst and CuCl2 as cocatalyst [7], Terminal olefins afford the corresponding methyl ketones. Oxidative acetoxylation of olefins with Pd(II) salts as catalysts in acetic acid was first reported by Moiseev and coworkers [8], The addition of an alkali metal acetate, e. g. NaOAc, was necessary for the reaction to proceed. Palladium black was also found to be an active catalyst under mild conditions (40-70 °C, 1 bar) in the liquid phase, if NaOAc was added to the solution before reducing Pd(II) to Pd black, but not afterwards [9,10]. These results suggested that catalytic activity... 

In his pioneering contributions Moiseev has shown that giant cationic palladium clusters , e.g. Pd56iL6o(OAc)i8o (L = phenanthroline, bipyridine), characterized by use of high-resolution TEM, SAXS, EXAFS, IR and magnetic susceptibility data, catalyze, under mild conditions (293 363 K, 1 bar), the oxidative acetoxylation of ethylene into vinyl acetate, propylene into allyl acetate, and toluene into benzyl acetate. The oxidation of primary aliphatic alcohols to esters, and the conversion of aldehydes into acetals were also studied. ... 

In 1960, quickly after the introduction of the Celanese process, Wacker-Chemie commercialized a liquid phase vinyl acetate process which represented and extension of its earlier acetaldehyde process wherein acetic acid was simply substituted for water. (See equation [19]. This chemical transformation is also referred to as oxidative acetoxylation.) As shown in Figure 2, wherein R=Ac, the liquid phase oxidative acetoxylation of ethylene utilized the same catalytic cycle as the Wacker-Chemie acetaldehyde process. 

The production of vinylacetate by oxidative acetoxylation is one industrial process with ethylene and in the presence of a palladium catalyst as shown in eq. (20.56) [206]. 

Since more than 60% of the EO production is converted directly to EG, the obvious question some macho chemist might ask is why don t we do an end run and just convert ethylene directly to EG Skip the oxidation step. Research starting 50 years ago led to several promising commercial processes, oxychlorination and acetoxylation. Exotic catalysts were used, and both avoided the EO step. But neither process was quite effective enough to replace the ethylene-to-EO-to-MEG route, which predominates today. 

In 1960, Moiseev and coworkers reported that benzoquinone (BQ) serves as an effective stoichiometric oxidant in the Pd-catalyzed acetoxylation of ethylene (Eq. 2) [19,20]. This result coincided with the independent development of the Wacker process (Eq. 1, Scheme 1) [Ij. Subsequently, BQ was found to be effective in a wide range of Pd-catalyzed oxidation reactions. Eor example, BQ was used to achieve Wacker-type oxidation of terminal alkenes to methyl ketones in aqueous DMF (Eq. 3 [21]), dehydrogenation of cyclohexanone (Eq. 4 [22]), and alcohol oxidation (Eq. 5 [23]). In the final example, 1,4-naphthoquinone (NQ) was used as the stoichiometric oxidant. 

Palladium-catalyzed addition of oxygen nucleophiles to alkenes dates back to the Wacker process and acetoxylation of ethylene (Sects. 1 and 2). In contrast, catalytic methods for intermolecular oxidative amination of alkenes (i.e., aza-Wacker reactions) have been identified only recently. Both O2 and BQ have been used as oxidants in these reactions. 

The formation of vinyl acetate via the oxidative coupling of ethylene and acetic acid was among the earliest Pd-catalyzed reactions developed (Sect. 2) [19,20]. Subsequent study of this reaction with higher olefins revealed that, in addition to C-2 acetoxylation, allylic acetoxylation occurs to generate products with the acetoxy group at the C-1 and C-3 positions (Scheme 14). The synthetic utihty of these products imderhes the substantial historical interest in these reactions, and both BQ and dioxygen have been used as oxidants. 

In contrast to ethylene, which gives only vinylic or oxidative addition products, the acetoxylation of higher alkenes results in the formation of a mixture of allylic and vinylic acetates.367 The... 

The industrially important acetoxylation consists of the aerobic oxidation of ethylene into vinyl acetate in the presence of acetic acid and acetate. The catalytic cycle can be closed in the same way as with the homogeneous Wacker acetaldehyde catalyst, at least in the older liquid-phase processes (320). Current gas-phase processes invariably use promoted supported palladium particles. Related fundamental work describes the use of palladium with additional activators on a wide variety of supports, such as silica, alumina, aluminosilicates, or activated carbon (321-324). In the presence of promotors, the catalysts are stable for several years (320), but they deactivate when the palladium particles sinter and gradually lose their metal surface area. To compensate for the loss of acetate, it is continuously added to the feed. The commercially used catalysts are Pd/Cd on acid-treated bentonite (montmorillonite) and Pd/Au on silica (320). 

Figure 28 shows that the chemistry involved in the Wacker process could in principle be extended to other nucleophiles. The modern catalytic manufacturing process making vinyl acetate from ethylene and acetic acid is based on the observation that palladium catalyzed oxidation of ethylene to acetaldehyde can be converted into an acetoxylation reaction if carried out in a solution of acetic acid and in the presence of sodium acetate (Equation 42). 

Acetoxylations (oxyacylations) have to be seen in context with olefin oxidation to carbonyl compounds (Wacker process, Section 2.4.1). With the lowest olefin, ethylene, acetaldehyde is formed. In water-free acetic acid no reaction takes place. Only in the presence of alkali acetates - the acetate ion shows higher nu-cleophilicity than acetic acid - ethylene reacts with palladium salts (eq. (1)) to give vinyl acetate, the expected product, as first reported by Moiseev et al. [1]. Stem and Spector [2] independently used [HP04] as base in a mixture of isooctane and acetic acid. This reaction could be exploited for a commercial process to produce vinyl acetate and closed the last gap replacing acetylene by the cheaper ethylene, a petrochemical feed material, for the production of large-tonnage chemical intermediates. 

As a major step in the evaluation of the above mentioned high-throughput tools and techniques, a scale-down of different types of catalysts for several applications was performed. For that purpose, two well established commercial catalysts, one of the mixed metal oxide type for selective olefin oxidation and one impregnated catalyst for ethylene acetoxylation to vinyl acetate monomer (VAM), respectively, were prepared in the small-scale and their catalytic performance was compared. As shown in Fig. 1 with the selective oxidation catalyst, the scale-down of this catalyst was successful, since both, the commercial and the high-throughput prepared catalyst are showing identical performances. Regarding the calcination procedure one can point out, that only if this step is carried out in the 5-fold rotary kiln, equal catalysts were obtained. 

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