Wacker process
The Wacker process or the Hoechst-Wacker process is an industrial chemical reaction: the aerobic oxidation of ethylene to acetaldehyde in the presence of catalytic, aqueous palladium chloride and copper chloride.
The Tsuji-Wacker oxidation refers to a family of reactions inspired by the Wacker process. In Tsuji-Wacker reactions, palladium catalyzes transformation of α-olefins into carbonyl compounds in various solvents.
The development of the Wacker process popularized modern organopalladium chemistry, and Tsuji-Wacker oxidations remain in use today.
History
The Wacker process was one of the first homogeneous catalysis with organopalladium chemistry applied on an industrial scale.In an 1893 doctoral dissertation on Pennsylvanian natural gas, Francis Clifford Phillips had reported that palladium chloride oxidized ethylene to acetaldehyde, but the reaction required stoichiometric quantities of palladium. It remained a niche curiosity until Wacker Chemie began developing its eponymous process in 1956.
At the time, many industrial compounds were produced via acetaldehyde from acetylene, itself from calcium carbide. The overall route exhibited poor thermodynamic efficiency and required great expense. Esso sought to market waste olefins from a new, under-construction oil refinery in Cologne close to a Wacker site. Wacker realized that ethylene would be a cheaper feedstock than acetylene, and began to investigate catalytic oxidation to ethylene oxide.
To Wacker's surprise, they smelled not ethylene oxide but acetaldehyde in the product stream. From Phillips' dissertation, known properties of Zeise's salt, and transformation of the catalyst over the course of a batch reaction, Wacker realized that they needed to reoxidize the palladium to close the catalytic cycle. They began publishing the process outline in 1957. However, poor patenting strategy allowed parent corporation Hoechst AG to outrace Wacker to the optimal catalysis conditions.
Wacker-Hoechst began jointly constructing pilot plants in 1958, but the relatively aggressive reaction conditions required the first large-scale use of titanium metal in the European chemical industry to protect against corrosion. Production plants started operation in 1960.
The process also sparked a boom in organopalladium chemistry. Studies from the 1960s elucidated several key points about the reaction mechanism through kinetic isotope effects and stereochemistry. Many focused on the hydroxypalladation step, which forms the C-O bond. Early reactions used conditions much milder than the industrial plants and obtained contradictory results; the modern consensus is that the step's stereochemistry is quite sensitive to chloride concentrations.
Other studies investigated reaction's application to more complex terminal olefins. High-order olefins are insoluble in water, but Clement and Selwitz found that aqueous DMF as solvent allowed for the oxidation of 1-dodecene to 2-dodecanone. Fahey noted the use of 3-methylsulfolane in place of DMF as solvent increased the yield of oxidation of 3,3-Dimethylbut-1-ene. Two years after, Tsuji applied the Clement-Selwitz conditions for selective oxidations of terminal olefins with multiple functional groups, and demonstrated its utility in synthesis of complex substrates.
Carbonylation has mainly superseded the Wacker process for modern bulk chemical synthesis, but small-scale Tsuji-Wacker reactions remain important for fine chemical and laboratory-scale syntheses.
Reaction mechanism
The reaction mechanism for the industrial Wacker process has received significant attention for several decades. Aspects of the mechanism are still debated. A modern formulation is described below:This reaction can also be described as follows:
followed by reactions that regenerate the Pd catalyst:
Only the alkene and oxygen are consumed. Without copper chloride as an oxidizing agent, Pd metal would precipitate, stopping Philips' reaction after one cycle. Air, pure oxygen, or a number of other reagents can then oxidise the resultant CuCl-chloride mixture back to CuCl2, allowing the cycle to continue.
High concentrations of chloride and copper chloride favor formation of a new product, ethylene chlorohydrin.
Evidence
Evidence for the overall mechanism includes:- No H/D exchange effects. Experiments with C2D4 in water generate CD3CDO, and runs with C2H4 in D2O generate CH3CHO. Thus, keto-enol tautomerization is not a possible mechanistic step.
- Negligible kinetic isotope effect with fully deuterated reactants. Hence hydride transfer is not rate-determining.
- Significant competitive isotope effect with C2H2D2,, suggests that the rate determining step precedes acetaldehyde formation.
The ethylene ligand's hydroxylation is typically a slow process. Depending on experimental conditions, it can occur either intramolecularly, from a palladium-bound hydroxido ligand, or intermolecularly. In the former case the hydroxylation is anti; in the latter, syn. Assuming small amounts of copper, experiments have shown that syn addition occurs at low chloride concentrations and anti addition occurs at high concentrations. The pathway change is probably due to chloride ions saturating the catalyst. However, under strictly copper-free conditions, anti addition always occurs, and the rate no longer depends on the ethylene hydrogen isotopes.
Another key step in the Wacker process is the migration of the hydrogen from oxygen to chloride, followed by reductive elimination to form the C-O double bond. This step is generally thought to proceed through a so-called β-hydride elimination:
The cyclic four-membered transition state shown above is unlikely. In silico studies argue that the transition state for this reaction step likely involves a 7-membered ring with a water molecule acting as a catalyst.
Industrial process
Two routes are commercialized for the production of acetaldehyde: one-stage process and two-stage. The acetaldehyde yield is about 95% in either, and byproducts are chlorinated hydrocarbons, chlorinated acetaldehydes, and acetic acid. In general, 100 parts of ethene gives:- 95 parts acetaldehyde
- 1.9 parts chlorinated aldehydes
- 1.1 parts unconverted ethene
- 0.8 parts carbon dioxide
- 0.7 parts acetic acid
- 0.1 parts chloromethane
- 0.1 parts ethyl chloride
- 0.3 parts ethane, methane, crotonaldehyde
The production costs are virtually the same across the two processes; the advantage of using dilute gases in the two-stage method is balanced by higher investment costs. Due to the corrosive nature of the catalyst, either process requires a reactor lined with acid-proof ceramic and titanium tubing, but the two-stage process requires more reactors and piping. Generally, the choice of method is governed by the raw material and energy situations as well as by the availability of oxygen at a reasonable price.