Vinyl acetate, synthesis from ethylene

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. 

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]. 

Fig. 19. The ovoall catalytic route for the synthesis for vinyl acetate formation from ethylene, acetic acid in the absence of oxygen on a model Pd(l 11) sur ce.

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 ... 

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. 

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. 

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). 

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. 

Other catalytic reactions carried out in fluidized-bed reactors are the oxidation of naphthalene to phthalic anhydride [2, 6, 80] the ammoxidation of isobutane to mcthacrylonitrilc [2] the synthesis of maleic anhydride from the naphtha cracker C4 fraction (Mitsubishi process [81, 82]) or from n-butane (ALMA process [83], [84]) the reaction of acetylene with acetic acid to vinyl acetate [2] the oxychlorination of ethylene to 1,2-di-chloroethane [2, 6, 85, 86] the chlorination of methane [2], the reaction of phenol with methanol to cresol and 2,6-xylenol [2, 87] the reaction of methanol to gasoline... 

Even if new processes of synthesis were developed from alcohols, like catalytic vinylation with ethylene or vinyl exchange with vinyl acetate, the major commercial route for VE monomers seams to be still the Reppe method based on reaction, in basic conditions, of acetylene with the corresponding alcohols [96,97,100] ... 

Other oxygen nucleophiles are also effective, from alcohols to carboxylic acids. The conversion of ethylene to vinyl acetate and vinyl ethers is well documented, but applications in synthesis with more complex alkenes are few. 

Wender and co-workers have continued their elegant use of the meta photocycloaddition of ethylenes to arenes as the key step in the synthesis of several naturally occurring compounds. The major product from irradiation of benzene and vinyl acetate is the 1-endo acetate of (36), and using this as the starting material, the total synthesis of the ant defensive secretion, isoiridomyrmecin (50), has been accomplished in seven further steps." There... 

Preliminary mechanistic studies (36) on synthesizing vinyl acetate from ethylene have indicated that it probably proceeds in a manner similar to the synthesis of acetaldehyde from ethylene (19). 

Reaction Mechanism. Any mechanism proposed for the vinylation of acetic acid by the hexenes must be able to account for the production of the high boiling products, 1,2-hexandiol mono- and diacetates (VIII, IX and X), and possibly hexylidene diacetate, as well as the hexenyl acetates. The currently accepted mechanism for synthesizing vinyl acetate from ethylene and acetic acid is derived from that postulated by Henry (i, 19), based on studies of the Wacker acetaldehyde synthesis. The key step in this mechanism is an insertion reaction (18). 

However, less conjugated monomers such as vinyl acetate, vinyl chloride, and ethylene are still difficult to polymerize in a controlled way by metal-catalyzed polymerizations. This is most probably due to the difficulty in activation of their less reactive carbon-halogen bonds. The following sections will discuss these aspects from the viewpoint of the monomers listed in Figure 11. Functional monomers will be discussed later in another section, Precision Polymer Synthesis. 

Desulfurization of petroleum feedstock (FBR), catalytic cracking (MBR or FI BR), hydrodewaxing (FBR), steam reforming of methane or naphtha (FBR), water-gas shift (CO conversion) reaction (FBR-A), ammonia synthesis (FBR-A), methanol from synthesis gas (FBR), oxidation of sulfur dioxide (FBR-A), isomerization of xylenes (FBR-A), catalytic reforming of naphtha (FBR-A), reduction of nitrobenzene to aniline (FBR), butadiene from n-butanes (FBR-A), ethylbenzene by alkylation of benzene (FBR), dehydrogenation of ethylbenzene to styrene (FBR), methyl ethyl ketone from sec-butyl alcohol (by dehydrogenation) (FBR), formaldehyde from methanol (FBR), disproportionation of toluene (FBR-A), dehydration of ethanol (FBR-A), dimethylaniline from aniline and methanol (FBR), vinyl chloride from acetone (FBR), vinyl acetate from acetylene and acetic acid (FBR), phosgene from carbon monoxide (FBR), dichloroethane by oxichlorination of ethylene (FBR), oxidation of ethylene to ethylene oxide (FBR), oxidation of benzene to maleic anhydride (FBR), oxidation of toluene to benzaldehyde (FBR), phthalic anhydride from o-xylene (FBR), furane from butadiene (FBR), acrylonitrile by ammoxidation of propylene (FI BR)... 

This type of process is related in principle to those developed by HoechstfWacker to manufacture acetaldehyde (see Section 8X4) by the onee-through oxidation of ethylene by oxygen, in the presence of a redox system, or to produce acetone (Section 1023) from propylene The main reactions involved in the synthesis of vinyl acetate are the following ... 

Figure 12. Photoconversion of a-(o-tolyl)acetophenone as a function of absorbed dose in benzene solution and in solid EVA beads. Reprinted with permission from J. E. Guillet, W. K. Macinnis, and A. E. Redpath, Prospects for solar synthesis. II. Study of the photocyclization of a-(o-tolyl)acetophenone in solution and in crosslinked ethylene-vinyl acetate beads. Canadian Journal of Chemistry, 63, 1333 (1985).

The period from 1970 to 1985 saw radical changes in the production of acetic acid and acetic anhydride. By 1985, both products would be generated not from ethylene, but from synthesis gas which in turn could be generated fi om abundant resources such as coal, natural gas, and in the future, biomass. At the end of this period, acetaldehyde became a very small contributor to the total acetyl product stream since it was no longer required to make acetic acid or acetic anhydride and ethylene would only be required to produce vinyl acetate and to meet a much diminished acetaldehyde market. These advances were the result of two significant process breakthroughs - the Monsanto Acetic Acid Process and the Eastman Chemical Company Acetic Anhydride Process which will be discussed below. 

As of 1985, production of the acetyl chemicals had changed entirely. From that time forward, the preferred processes would contain primarily synthesis gas. Ethylene was only needed for vinyl acetate production and to address the rapidly diminishing market for acetaldehyde. (Remember that the primary outlet for acetaldehyde was to make acetic acid and acetic anhydride, so it was no longer needed for these purposes with the advent of the new technologies.) A summary of the preferred production methods as of 1985 appears in Figure 6. 

Vinyl acetate, CH2=CH—OOC—CH3, is produced from ethylene and acetic acid or from acetaldehyde and acetic anhydride. The previously extensively used synthesis from acetylene and acetic acid is now too expensive. 

Ethylene from cracking of the alkane gas mixtures or the naphtha fraction can be directly polymerized or converted into useful monomers. (Alternatively, the ethane fraction in natural gas can also be converted to ethylene for that purpose). These include ethylene oxide (which in turn can be used to make ethylene glycol), vinyl acetate, and vinyl chloride. The same is true of the propylene fi action, which can be converted into vinyl chloride and to ethyl benzene (used to make styrene). The catalytic reformate has a high aromatic fi action, usually referred to as BTX because it is rich in benzene, toluene, and xylene, that provides key raw materials for the synthesis of aromatic polymers. These include p-xylene for polyesters, o-xylene for phthalic anhydride, and benzene for the manufacture of styrene and polystyrene. When coal is used as the feedstock, it can be converted into water gas (carbon monoxide and hydrogen), which can in turn be used as a raw material in monomer synthesis. Alternatively, acetylene derived from the coal via the carbide route can also be used to synthesize the monomers. Commonly used feedstock and a simplified diagram of the possible conversion routes to the common plastics are shown in Figure 2.1. 

Depending on the reaction conditions, ethylidine diacetate can be the major product of the metal-catalyzed reaction of acetylene with acetic acid and is also a byproduct of the oxidative acylation of ethylene. In addition, ethylidine diacetate is readily prepared by the reaction of acetaldehyde with acetic anhydride (37). A commercial-scale synthesis of vinyl acetate developed and piloted by the Celenese Corporation involved the pyrolysis of ethylidine diacetate obtained from acetaldehyde (38) [219,220]. 

Bubble columns, in which the liquid is the continuous phase, are used for slow reactions. Drawbacks with respect to packed columns are the higher pressure drop and the important degree of axial and radial mixing of both the gas and the liquid, which may be detrimental for the selectivity in complex reactions. On the other hand, they may be used when the fluids carry solid impurities that would plug packed columns. In fact, many bubble column processes involve a finely divided solid catalyst that is kept in suspension, such as the Rheinpreussen Fischer-Tropsch synthesis, described by Kolbel [1971], or the former LG. Farben coal hydrogenation process or vegetable oil hardening processes. Several oxidations are carried out in bubble columns the production of acetaldehyde from ethylene, of acetic acid from C4 fractions, of vinyl chloride from ethylene by oxychlorination, and of cyclohexanone from cyclohexanol. 

Clearly, the slate of chemicals produced from coal-derived synthesis gas will expand as new technologies are developed, and supplies of petroleum and natural gas dwindle. The most likely such chemicals are those for which existing processes have been demonstrated but which presently lack economic merit. Relatively small improvements in technology, shifts in feedstock availability, capital costs, or political factors could enhance the viability of coal-based processes for the production of methanol, ethanol, and higher alcohols, vinyl acetate, ethylene glycol, carboxylic acids, and light olefins. 

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