Chemical Basis

The principle of the process currently in general use - the partial oxidation of ethylene to - is based on the observation made by F. C. Phillips back in 1894 that platinum metal salts stoichiometrically oxidize ethylene selectively to acetaldehyde while themselves being reduced to the metal. However, industrial application was first possible only after the discovery by Wacker of a catalytic process using a redox system and the developmen of a commercial process by Wacker and Hoechst.

The total process, developed by Wacker and Hoechst between 1957 and 1959, can be depicted as an exothermic catalytic direct oxidation:

The catalyst is a two-component system consisting of PdCl₂ and CuCl₂. PdCl₂ functions as the actual catalyst in a process involving ethylene complex formation and ligand exchange. The important elementary steps in the mechanism are seen as the formation of a π charge-transfer complex, rearrangement to a σ complex, and its decomposition into the final products:

CuCl₂ reoxidizes the nonvalent palladium to the divalent state. Although numerous other oxidizing agents can also convert Pd⁰ into Pd²⁺, the copper redox system has the advantage that Cu⁺ can be easily reoxidized to Cu²⁺ with O₂. Recently, a new catalyst development using a phosphorous-molybdenum-vanadium-polyoxoanion system for the reoxidation of Pd⁰ has been disclosed by Catalytica in the USA. There are several advantages, including a higher selectivity in the absence of chlorinated coproducts. This new catalyst has been demonstrated commercially.

The previous net equation summarizes the various reactions taking place. They can be formally divided into the rapid olefin oxidation:



and the rate determining regeneration:

The relative rates of the partial reactions can be determined by adjusting the HCl content, the regeneration being accelerated by a higher HCl concentration.

Thus the quantity of palladium salt required for the selective oxidation of ethylene can be limited to catalytic amounts by using a large excess of CuCl₂.

 Process Operation

The large-scale manufacture of acetaldehyde takes place in a two-phase, i. e., gadliquid, system. The gaseous reaction components - ethylene, and air or O₂ - react with the acidic (HCl) aqueous catalyst solution in a titanium or lined bubble column reactor.

Two versions of the process were developed at the same time:
1. Single-step process - in which the reaction and regeneration are conducted simultaneously in the same reactor. O₂ is used as the oxidizing agent.
2. Two-step process - in which the reaction and regeneration take place separately in two reactors. In this case, air can be used for the oxidation.

In the single-step process, ethylene and O₂ are fed into the catalyst solution at 3 bar and 120- 130°C, where 35-45% of the ethylene is converted. The resulting heat of reaction is utilized to distill off acetaldehyde and water from the catalyst solution, which must be recycled to the reactor. In this way, around 2.5-3.0 m³ H₂O per tonne acetaldehyde are recycled. It is necessary to use a pure O₂ and ethylene (99.9 vol%) feed to avoid ethylene losses which would otherwise occur on discharging the accumulated inert gas.

In the two-step process, ethylene is almost completely converted with the catalyst solution at 105- 110 °C and 10 bar. After reducing the pressure and distilling off an acetaldehyde/H₂O mixture, the catalyst solution is regenerated with air at 100°C and 10 bar in the oxidation reactor and then returned to the reactor. Since the O₂ in the air is largely removed, a residual gas with a high N₂ content is obtained which can be used as an inert gas. The advantages of total ethylene conversion and the use of air contrast with the disadvantages of a greater investment arising from the double reactor system at higher pressure and the catalyst circulation.

In both processes the aqueous crude aldehyde is concentrated and byproducts such as acetic acid crotonaldehyde and chlorine-containing compounds are removed in a two-step distillation. The selectivities are almost equal (94%).

Currently, the Wacker-Hoechst process accounts for 85 % of the worldwide production capacity for acetaldehyde.