ACETALDEHYDE

Acetaldehyde is one of the few industrial chemicals whose production has shrunk in the past 15 years. Its decline has been paralleled only by acetylene and more recently by US petrochemical ethanol. In the United States in 1969, 1.65 billion lb of acetaldehyde was manufactured. By the late 1980s, this had decreased to an estimated 650 million lb and growth was not foreseen.

Acetaldehyde was manufactured by an ingenious process, the Wacker reaction. Its demise was caused by the discovery of equally ingenious processes for the preparation

of the two chemicals for which it served as precursor, n-butanol and, more important, acetic acid. These three processes are examples of shutdown economics (see Appendix 1). n-Butanol is no longer made by the old process, but a little acetaldehyde is still oxidized to acetic acid in Europe. The plants still operate because the cash cost of operating them is lower than the total cost of the newer processes.

Acetaldehyde was originally made by the hydration of acetylene over an oxidation–reduction catalyst, mercurous–mercuric sulfate buffered by ferric sulfate. Vinyl alcohol is assumed to form momentarily and to rearrange to acetaldehyde at atmospheric pressure and 95°C. Ethylene became much cheaper than acetylene in the early 1960s and the above route was displaced by the oxidation of ethylene-based ethanol at 450°C and 3 bar with air over a silver gauze catalyst. Alternatively, the ethanol may be dehydrogenated over a chromium oxide activated copper catalyst at 270–300°C. This is a more attractive process if a use exists for the byproduct hydrogen. By 1974, only 15% of acetaldehyde was made from acetylene.

These routes in turn gave way to the Wacker process described by Parshall as “a triumph of common sense.” It is based on the observation that ethylene is oxidized by palladium chloride to acetaldehyde. As indicated, stoichiometric quantities of palladium chloride are required. In the 1950s, chemists at Wacker Chemie in Germany converted the palladium salt from a stoichiometric to a catalytic component by including cupric chloride, oxygen, and hydrogen chloride in the reaction mixture. Each atom of palladium, when  formed, is then oxidized back to palladium chloride. There is general agreement about the formation of the π-complex. Step is the OH addition. There is controversy as to how this step takes place. Thereafter the mechanism is straightforward, with hydrogen abstraction by the palladium followed by rearrangement (Step 4) and acetaldehyde formation. The palladium chloride–copper chloride mixture is analogous to the oxychlorination system. n-Butanol was originally made from acetaldehyde by an aldol condensation. Today it is made from propylene by

hydroformylation. Acetic acid was made by oxidation of acetaldehyde with either air or oxygen over a manganese or cobalt acetate catalyst at 60°C. The oxidation takes place by a radical mechanism in which peracetic acid is the intermediate. The peracetic acid in turn reacts preferentially with acetaldehyde to give α-hydroxyethylperacetate, which decomposes through a cyclic transition state to 2 mol of acetic acid. The reaction goes without a catalyst at room temperature and 25– 40 bar in a solvent such as ethyl acetate. Cheap naphtha in Europe in the 1950s motivated the development of the primary flash

distillate route to acetic acid. This process is still in use in Europe because it gives valuable byproducts including formic acid and propionic acid.

n-Butane is oxidized in a related process in the United States giving methyl ethyl ketone, propionic acid, and formic acid byproducts. Cheap methanol in the 1970ssimilarly led to the development of methanol carbonylation, a process whose economics are so good that it shut down every US manufacturer using the acetaldehyde route.

In spite of the apparently unbeatable economics of methanol carbonylation, Showa Denko in Japan developed a one-step vapor-phase process for acetic acid production by direct oxidation of ethylene and, in 1997, they constructed a 100,000 tonne/year plant (Section 10.5.2.2). The reaction is based on a supported palladium catalyst and takes place in a fixed-bed reactor at about 150–160°C and 9 bar. The gases fed to the reactor are ethylene, oxygen, steam, and nitrogen. Selectivity to acetic acid based on ethylene is reported to be about 86–88% with ethylene conversion per pass of 7–8%.

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