Acetaldehyde, from synthesis gas

Since 1960, the Hquid-phase oxidation of ethylene has been the process of choice for the manufacture of acetaldehyde. There is, however, stiU some commercial production by the partial oxidation of ethyl alcohol and hydration of acetylene. The economics of the various processes are strongly dependent on the prices of the feedstocks. Acetaldehyde is also formed as a coproduct in the high temperature oxidation of butane. A more recently developed rhodium catalyzed process produces acetaldehyde from synthesis gas as a coproduct with ethyl alcohol and acetic acid (83—94). 

From Synthesis Gas. A rhodium-catalyzed process capable of converting synthesis gas directly into acetaldehyde in a single step has been reported (83,84). 

Cyclopentadiene itself has been used as a feedstock for carbon fiber manufacture (76). Cyclopentadiene is also a component of supported metallocene—alumoxane polymerization catalysts in the preparation of syndiotactic polyolefins (77), as a nickel or iron complex in the production of methanol and ethanol from synthesis gas (78), and as Group VIII metal complexes for the production of acetaldehyde from methanol and synthesis gas (79). 

Other synthetic methods have been investigated but have not become commercial. These include, for example, the hydration of ethylene in the presence of dilute acids (weak sulfuric acid process) the conversion of acetylene to acetaldehyde, followed by hydrogenation of the aldehyde to ethyl alcohol and the Fischer-Tropsch hydrocarbon synthesis. Synthetic fuels research has resulted in a whole new look at processes to make lower molecular weight alcohols from synthesis gas. 

Other Methods of Preparation. In addition to the direct hydration process, the sulfuric acid process, and fermentation routes to manufacture ethanol, several other processes have been suggested. These include the hydration of ethylene by dilute acids, the hydrolysis of ethyl esters other than sulfates, the hydrogenation of acetaldehyde, and the use of synthesis gas. None of these methods has been successfilUy implemented on a commercial scale, but the route from synthesis gas has received a great deal of attention since the 1974 oil embargo. 

Acetic acid is also produced hy the oxidation of acetaldehyde and the oxidation of n-hutane. However, acetic acid from the carhonylation route has an advantage over the other commercial processes because both methanol and carbon monoxide come from synthesis gas, and the process conditions are quite mild. 

The low-pressure acetic acid process was developed by Monsanto in the late 1960s and proved successful with commercialization of a plant producing 140 X 10 metric tons per year in 1970 at the Texas City (TX, USA) site [21]. The development of this technology occurred after the commercial implementation by BASF of the cobalt-catalyzed high-pressure methanol carbonylation process [22]. Both carbonylation processes were developed to utilize carbon monoxide and methanol as alternative raw materials, derived from synthesis gas, to compete with the ethylene-based acetaldehyde oxidation and saturated hydrocarbon oxidation processes (cf. Sections 2.4.1 and 2.8.1.1). Once the Monsanto process was commercialized, the cobalt-catalyzed process became noncom-... 

The direct production of acetic acid from synthesis gas [80] instead of methanol as feedstock has demonstrated selectivities up to 80% using rhodium fixed-bed catalysts with Group IIIA-VIIIA promoters and alkaline metals. Other C2 compounds were also formed (acetaldehyde, ethanol, and ethyl acetate) [129],... 

Most work has used CH3OH as a feedstock, since it is readily obtained from synthesis gas. Addition of an iodide promoter, solvent, and a group V promoter markedly increases the rate and, under controlled conditions, the selectivity to acetaldehyde. In general, the best acetaldehyde rates and selectivities, typically 3-5 Mhr and 80-90%, are obtained with cobalt l2-PPh3 in ether or polyether solvents at 170° and 34.5 MPa (H2 C0 = 1 1). ... 

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. 

The major route for the industrial production of vinyl acetate, the monomer of polyvinyl acetate (emulsion paints, adhesives) and its hydrolysis product, polyvinyl alcohol (textiles, food packaging) is closely related to the Wacker acetaldehyde process, but the industrial catalysts are heterogeneous. A mixture of ethene, oxygen and acetic acid is passed over a palladium catalyst supported on alumina at 100-200°C. The overall reaction is H C=CH2-hCHjCO H-hyO ->H2C=CHC02CH3 -hH O. Ethene is no longer cheap, so that work is being pursued to make vinyl acetate from synthesis gas (p. 384). 

The price of acetaldehyde duriag the period 1950 to 1973 ranged from 0.20 to 0.22/kg. Increased prices for hydrocarbon cracking feedstocks beginning in late 1973 resulted in higher costs for ethylene and concurrent higher costs for acetaldehyde. The posted prices for acetaldehyde were 0.26/kg in 1974, 0.78/kg in 1985, and 0.92/kg in 1988. The future of acetaldehyde growth appears to depend on the development of a lower cost production process based on synthesis gas and an increase in demand for processes based on acetaldehyde activation techniques and peracetic acid. 

Currently, almost all acetic acid produced commercially comes from acetaldehyde oxidation, methanol or methyl acetate carbonylation, or light hydrocarbon Hquid-phase oxidation. Comparatively small amounts are generated by butane Hquid-phase oxidation, direct ethanol oxidation, and synthesis gas. Large amounts of acetic acid are recycled industrially in the production of cellulose acetate, poly(vinyl alcohol), and aspirin and in a broad array of other... 

Acetaldehyde is obtained from the reaction of synthesis gas with methanol, methyl ketals or methyl esters. The reactions are carried out with an iodide-promoted Co catalyst at 180-200 °C and 2000-5000 psig. In comparing the various feedstocks, the best overall process to make acetaldehyde involves the reductive carbonylation of methyl esters. In this case, acetaldehyde selec-tivities are > 95% ut acceptable rates and conversion. 

Reductive Carbonylation of Methanol. The reductive carbonylation of methanol (solvent free) was studied at variable I/Co, PPh,/I, temperature, pressure, synthesis gas ratio and methanol conversion (gas uptake) in the batch reactor, A summary of the results is given in Table I. In general, the acetaldehyde rate and selectivity increase with increasing I/Co. The PPh /I ratio has little effect except in run //7 where the rate is drastically reduced at I/Co =3.5 and PPh /I r 2. A good set of conditions is I/Co =3 5 and PPh /I = 1,T where the acetaldehyde rate and selectivity is 7.6 M/nr and 765 at 170 °C and 5000 psig. The effect of methanol conversion at these conditions is obtained by compearing runs 13, 1, 14, and 15. The gas uptake was varied from 14000 to 4000 psi, which corresponds to observed methanol conversions of 68% to 38 te. 

Butane from natural gas is cheap and abundant in the United States, where it is used as an important feedstock for the synthesis of acetic acid. Since acetic acid is the most stable oxidation product from butane, the transformation is carried out at high butane conversions. In the industrial processes (Celanese, Hills), butane is oxidized by air in an acetic acid solution containing a cobalt catalyst (stearate, naphthenate) at 180-190 °C and 50-70 atm.361,557 The AcOH yield is about 40-45% for ca. 30% butane conversion. By-products include C02 and formic, propionic and succinic acids, which are vaporized. The other by-products are recycled for acetic acid synthesis. Light naphthas can be used instead of butane as acetic adic feedstock, and are oxidized under similar conditions in Europe where natural gas is less abundant (Distillers and BP processes). Acetic acid can also be obtained with much higher selectivity (95-97%) from the oxidation of acetaldehyde by air at 60 °C and atmospheric pressure in an acetic acid solution and in the presence of cobalt acetate.361,558... 

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