Ethylene acetoxylation

Ethylene acetoxylation was also developed as a gas phase process following the liquid phase process and has been in commercial use since 1968. There is a notable difference between the two processes in the liquid phase the presence of palladium salts and redox systems results in the formation of both vinyl acetate and acetaldehyde, whereas in the gas phase process, using palladium metal,... 

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]. 

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. 

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. 

Ethylene glycol could also be obtained directly from ethylene by two methods, the Oxirane acetoxylation and the Teijin oxychlorination processes. The production of ethylene glycol from formaldehyde and carbon monoxide is noted in Chapter 5. 

The natural diterpenoid rostratone 16 is synthesized from ethylene ketal as shown in Scheme 17.162 In this synthesis, the Pd-mediated remote acetoxylation is achieved by G-H bond activation by Na2PdCl4 giving palladacycle dimers followed by treatment with pyridine and lead tetraacetate. 

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. 

Vinylic Acetoxylation. When alkenes are treated with Pd(II) compounds in the presence of acetic acid in a nonaqueous medium, acetoxylation takes place.495 498,499,501 503 567"569 Ethylene is converted to vinyl acetate in high yields and with high selectivity with PdCl2568,569 in the presence of added bases (NaOAc,568 Na2HP04569) or with Pd(OAc)2 570... 

Diethoxy-1,2-bis(trimethyl-silyloxy)ethylene, 108 Zirconium(IV) acetylacetonate, 351 Acyloxylation (see also Acetoxylation) t-Butyl perbenzoate, 58 Acyloxyselenylation (see Addition reactions to carbon-carbon multiple bonds)... 

First discovered by Moiseev et a/.,416 the palladium-catalyzed acetoxylation of ethylene to vinyl acetate has been the subject of very active investigations, particularly in industry, as shown by the considerable number of patents existing in this area. Vinyl acetate is an extremely important petrochemical product which is used for the synthesis of polymers such as poly(vinyl acetate) and poly(vinyl alcohol). Most of its annual production ( 2.6 Mt) results from the acetoxylation of ethylene (equation 160). 

A widely accepted mechanism for acetoxylation of ethylene is shown in equation (161) and consists of the nucleophilic attack of the acetate anion on the coordinated ethylene, followed by acetoxypalladation and /3-hydride elimination, giving vinyl acetate and palladium hydride.367... 

Such a stabilization of the palladium catalyst can also be achieved in homogeneous liquid phase by the use of appropriate ligands. Thus, it has recently been shown that palladium(II) hydroxamates are effective catalysts for the acetoxylation of ethylene with high selectivity and a high turnover (>200) (equation (162), whereas Pd(OAc)2 rapidly becomes deactivated and precipitates in the form of metallic palladium.419 It is probable that the bidentate hydroxamate ligand stabilizes the hydride Pd—H species and prevents palladium from precipitating. 

When palladium-catalyzed acetoxylation is carried out in the presence of nitrate or nitrite ions, ethylene glycol monoacetate (EGMA) results as the major product of the reaction (equation 163).420... 

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... 

A somewhat similar catalytic acetoxylation of ethylene to vinyl acetate by 02 has been carried out in acetic acid in the presence of a Pd(OAc)2-pyCo(TPP)N02 system.472 A stoichiometric epoxidation of alkenes such as 1-octene or propene by cobalt-nitro complexes has been shown to occur in the presence of thallium(III) benzoate. Oxygen labeling studies showed that the epoxide oxygen atom comes only from the nitro ligand (equation 197).473... 

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. 

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). 

The project deals with a VAM plant capacity of lOOkton per year for an effective operation time of8400 h. The process will be based on the acetoxylation of ethylene conducted in gas phase in the presence of a palladium-based solid catalyst. The case study will tackle the problem of process synthesis and energy integration, as well as the dynamics and control for ensuring flexibility in production rate of 10%, while preserving safety and environment protection. 

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