Vinyl acetate synthesis

Vinyl acetate monomer (VAM) is an important chemical intermediate used in the production of several types of paints, adhesives as wallpaper paste and wood glue, and surface coatings such as the protective laminate films in automotive safety glass [2,104,237,238]. 

The commercial process for vinyl acetate production has evolved over the years. Early in the 1930s, Wacker developed a process based upon the gas-phase conversion of acetylene and acetic acid over a zinc acetate catalyst supported on activated carbon. Later, in 1960s, a more economically favourable gas-phase process was introduced involving the acetoxylation of ethene over a Pd-based silica supported catalyst. Ethene, acetic acid and oxygen reacted to form vinyl acetate and water [122,237-242]  

Most of current production of vinyl actetate is derived from this ethene-based process, which has been shown to be facilitated by many promoters. Gold and potassium acetate (KOAc) the most notable of these [237,241-243]. 

In consequence, Pd-Au silica-supported catalysts promoted with KOAc were also studied. The results obtained showed that addition of Au to Pd catalysts significantly improves productivity [237,238,243], whilst also improving the intrinsic selectivity to vinyl acetate (Table 6.2) [237,244]  

However, earlier findings [247] showed that very highly dispersed palladium catalysts had very low activity for VA formation. This was attributed to the very small particles being inaccessible to the ethene feed due to their being completely embedded within the acetic acid/acetate liquid layer (ethene has very low solubility in acetic acid). Vinyl acetate formation may therefore be restricted to the larger Pd-Au alloy particles accessible to gaseous ethene. This 

A gold-palladium catalyst which includes potassium acetate is very well established for the production of vinyl acetate monomer (VAM) from ethene, acetic acid and oxygen in selectivities as high as 96% (see Section 8.4). VAM is an important intermediate used in the production of polyvinyl acetate, polyvinyl butyral and a variety of other polymers, and the gold-catalysed process followed many years of industrially focused research and patent activity in a number of large industrial companies 39-43 

The role of gold has now emerged as having greater significance than was realised at the outset of these operations. Most of the commercial processes are fixed-bed, but at the end of 2001, BP commissioned the brand new plant in Hull, UK. This is the world s first fluidised-bed process for VAM, while 80% of today s VAM plants worldwide are more than 20 years old and use a fixed-bed process.44 

Moving from a fixed to a fluidised-bed operation also required a new catalyst, and the one selected was a supported gold palladium system in the form of very fine spheres, prepared in collaboration with Johnson Matthey. Hence, gold-based catalysts are being used for this new fluidised-bed process, and are well established in fixed-bed processes for the large-scale manufacture of VAM. 

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Discovering the Role of Au and KOAc in the Catalysis of Vinyl Acetate Synthesis... 

The chemistry of vinyl acetate synthesis from the gas-phase oxidative coupling of acetic acid with ethylene has been shown to be facilitated by many co-catalysts. Since the inception of the ethylene-based homogeneous liquid-phase process by Moiseev et al. (1960), the active c ytic species in both the liquid and gas-phase process has always been seen to be some form of palladium acetate [Nakamura et al, 1971 Augustine and Blitz, 1993]. Many co-catalysts which help to enhance the productivity or selectivity of the catalyst have appeared in the literature over the years. The most notable promoters being gold (Au) [Sennewald et al., 1971 Bissot, 1977], cadmium acetate (Cd(OAc)j) [Hoechst, 1967], and potassium acetate (KOAc) [Sennewald et al., 1971 Bissot, 1977]. 

In order to probe the influence of Au and KOAc on the vinyl acetate synthesis chemistry, four different catalysts were synthesized. All of these catalysts were prepared in a manner exemplified in prior patent technology [Bissot, 1977], and each contained the same palladium loading in an egg-shell layer on the surface of a spherical silica support. The palladium content in the catalyst was easily controlled by adjusting the solution strength of palladium chloride (PdClj) added to the porous silica beads prior to its precipitation onto the support by reaction with sodium metasilicate (Na SiOj). The other two catalyst components (Au and KOAc) were either present or absent in order to complete the independent evaluation of their effect on the process chemistry, e.g., (1) Pd-i-Au-hKOAc, (2) Pd-i-KOAc, (3) Pd-hAu, and (4) Pd only. 

The effect of the catalyst composition upon the catalyst activity, selectivity, and reaction pathways was examined using a conventional high pressure fixed reactor and a TAP reactor. Particular emphasis was placed upon the effect of Au and KOAc on the acceleration or impedance of the pathways associated with vinyl acetate synthesis. A summary of the key findings is given below ... 

Solidification Vinyl acetate synthesis Titanium dioxide Microorganism... 

Cupric Chloride can be used as a reoxidant in the vinyl acetate synthesis but other products are also produced. In fact, with increasing Cu(II) concentration, the side products can easily be made the major products 16>. The side products are chloro acetates and diacetates and they probably arise from a reaction of the palladium acetate-olefin adduct with cupric chloride or acetate. 

Cupric chloride, if present in concentrations above ca. 0.5 M, may cause side reactions to occur in the olefin aiylation reaction similar to those that occur with cupric chloride in the vinyl acetate synthesis mentioned above. The side reaction produces 2-arylethyl chlorides and these products may be made the major ones if cupric chloride is present to the extent of about 2M in 10% aqueous acetic acid solution 29>. The mixed solvent is required to obtain the necessary solubility of the cupric chloride. This is a general reaction useful for producing a variety of 2-arylalkyl halides. For example, 3-phenyl-2-chloropropionaldehyde is obtained in 63% yield by the reaction of phenylpalladium chloride , cupric chloride and acrolein. 

Modern catalysts for vinyl-acetate synthesis contain Au in the chemical formulation, which manifests in much higher activity and selectivity. This is reflected by fundamental changes in the kinetics, such as for example switching the reaction order of ethylene from negative to positive [8]. As a consequence, in more recent studies the formation of vinyl acetate can be described conveniently by a power-law kinetics involving only ethylene and oxygen ... 

Figure 10,4 First separation step and gas separation section at vinyl acetate synthesis.

Vinyl acetate synthesis 1968 Polymers C2H4 -b CH3COOH -b I/2O2 — CH3C00CH=CH2 -b H2O Pd on Si02 or -Al203... 

Some prominent industrial examples of packed-bed reactors are in ammonia, methanol or vinyl acetate synthesis, and in ethylene, methanol, naphthalene, xylene or SO2 oxidation. In recent years (since the 1975 model year), an important application of packed-bed reactors has been as catalytic converters for pollution control from automotive exhausts. 

In some of the studies on the vinyl acetate synthesis from ethylene, high boiling products were reported (9, 10, 24, 25, 26). These included ethylidene diacetate (III), ethylene glycol monoacetate (IV) and ethylene glycol diacetate (V). Little attention has been given to the reactions by which these products are formed. No dioxygenated products have been reported previously when higher olefins have been used. 

Moiseev and Vargaftik (36) have reported that deuterium is not incorporated into either vinyl acetate or ethylidene diacetate (III) when the vinyl acetate synthesis reaction is carried out in deuteroacetic acid. They considered this observation as evidence for a process such as Reaction 4 in which the final carbonium ion (VII) is forming products either by losing a proton or by reacting with acetate to give ethylidene diacetate (III). 

In vinyl acetate synthesis in the presence of cupric chloride according to eq. (4), other by-products are mono- and diacetates of glycol and /5-chloroethyl acetate which under certain conditions become the predominant products [22]. In order to avoid side reactions, chloride-free catalyst systems such as Pd"/H9PMo6V6O40 have been described [23],... 

Table 1. Kinetic parameters for vinyl acetate synthesis over a Pd/Au/Si02 catalyst [9,10]...

The case study of vinyl acetate synthesis emphasises the benefits of an integrated process design and plantwide control strategy based on the analysis of the Reactor / Separation / Recycles structure. The core is the chemical reactor, whose behaviour in recycle depends on the kinetics and selectivity of the catalyst, as well as on safety and technological constraints. Moreover, the recycle policy depends on the reaction mechanism of the catalytic reaction. 

Acetoxylation vinyl acetate synthesis from ethene and acetic acid. 

Palladous acetate has been widely studied in connection with vinyl acetate synthesis (page 798). It acts to some extent like Hg11 and Pblv acetates in attacking benzene and other aromatic hydrocarbons in acid media.11 Thus, in acetic acid, it specifically attacks the side chain of toluene. 

First-principle quantum chemical methods have advanced to the stage where they can now offer qualitative, as well as, quantitative predictions of structure and energetics for adsorbates on surfaces. Cluster and periodic density functional quantum chemical methods are used to analyze chemisorption and catalytic surface reactivity for a series of relevant commercial chemistries. DFT-predicted adsorption and overall reaction energies were found to be within 5 kcal/mol of the experimentally known values for all systems studied. Activation barriers were over-predicted but still within 10 kcal/mol. More specifically we examined the mechanisms and reaction pathways for hydrocarbon C-H bond activation, vinyl acetate synthesis, and ammonia oxidation. Extrinsic phenomena such as substituent effects, bimetallic promotion, and transient surface precursors, are found to alter adsorbate-surface bonding and surface reactivity. 

The foregoing analysis can be extended from the chemistry of ammonia to a more complex catalytic system such as vinyl acetate synthesis. Vinyl acetate is produced by the acetoxylation of ethylene in the presence of oxygen over supported Pd/Au particles. While this is a well-established commercial route, the mechanism is still poorly understood. It was postulated that the chemistry could occur in the liqiud layer via homogeneous solublized Pd-acetate complexes. Recent evidence, however, indicate that the chemistry occurs on the Pd metal surface rather than on Pd(2+) particles. While we have explored both homogeneous as well as heterogeneous [36,47, 118] mechaiusms, we discuss only the heterogeneous results here. 

Fig. 20. The overall catalytic route for oxygen-assisted vinyl acetate synthesis on a model Pd( 111) surface.

Vinyl Acetate synthesis Acetic acid/ethylene/oxygen process... 

OAc +HOAc+OAc + Pd Scheme 8.3 Proposed liquid phase type mechanism in vinyl acetate synthesis [7]. 

It should be noted that despite the attention given to the liquid HO Ac layer, more traditional surface-mediated mechanisms remain the conventional model in the literature. For example, recent kinetic studies on vinyl acetate synthesis over Pd-only [9] and Pd/Au [10] catalysts suggest that the enhanced activity of the Pd/Au catalyst originates from its ability to provide more binding sites for O2 and thus increase the O2 mobility on the catalyst surface. 

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