Wacker-Hoechst process

Balance reaction of the Wacker-Hoechst procedure

The Wacker-Hoechst process is a large-scale process in the chemical industry in which the oxidation of ethylene in the presence of palladium (II) chloride as a catalyst produces acetaldehyde . The oxygen of the resulting aldehyde function comes from the water used as solvent; the oxygen used in the process is used to reoxidize the catalyst.

The installed production capacity in 2009 was around two million tonnes per year. Acetaldehyde is an important raw material in the chemical industry and is mainly used in the production of acetic acid and acetic anhydride ; other secondary products are 1,3-butadiene , acrolein and pentaerythritol .

Walter Hafner , Jürgen Smidt , Reinhard Jira and other Wacker Chemie employees developed the process at the Wacker Research Center, the consortium for the electrochemical industry , in the late 1950s . It was the first to use a palladium compound as a catalyst on an industrial scale .

The publication of the process resulted in brisk research activity in the field of palladium-catalyzed olefin reactions in the technical and chemical-preparative field and led to numerous mechanistic studies of the reaction. In addition to the various synthetic routes that have been worked out for higher olefins, Hoechst developed a technical process based on the Wacker-Hoechst process in which the solvent acetone is obtained from propylene , which in turn is the starting material for many syntheses.

history

Acetaldehyde, the primary product of the Wacker-Hoechst process

The large-scale chemical processes put into operation in Germany immediately after the Second World War were still based on the coal chemistry of the pre-war and wartime periods. One of the basic building blocks of this chemistry was acetylene from the hydrolysis of calcium carbide , which in turn was obtained from calcium oxide and coal. Wacker-Chemie used acetylene to manufacture the important synthetic building block acetaldehyde using the first Wacker process .

The production of acetylene, however, required a high level of energy input, while olefins from petroleum became more accessible after the war and thus represented cheaper raw materials for chemical synthesis. Ethylene from petrochemical cracker plants was available in large quantities from the mid-1950s. Processes such as the oxidation of ethylene to acetylene used by the Azienda Nazionale Idrogenazione Combustibili (ANIC) in Ravenna , Italy , which in turn was converted to acetaldehyde using the First Wacker process, proved to be energetically unfavorable and did not find widespread use.

For the preparation of acetaldehyde, Wacker was therefore looking for a process based on the direct oxidation of ethylene. The starting point of this search was the stoichiometric oxidation of ethene to acetaldehyde using palladium (II) chloride in aqueous solution , which Francis C. Phillips had discovered since 1894 . In this reaction, the palladium salt was reduced to metallic palladium, precipitated and was not available for further oxidation reactions.

At the end of the 1950s, researchers at Wacker-Chemie were looking for a catalytic variant of this reaction. Initially, the search focused on a heterogeneous catalytic process. Hafner found olfactory traces of acetaldehyde in experiments in which he conducted ethylene and oxygen with traces of hydrogen over a heterogeneous palladium contact. In addition to the heterogeneous catalytic gas phase reactions, the consortium also investigated the reaction of ethylene in the aqueous phase. The observation that the metallic palladium can be oxidized again to the palladium (II) chloride with copper (II) chloride was a decisive factor in the development of the catalytic process in aqueous solution. The oxidation of copper (I) to copper (II) chloride by oxygen was known to be a rapid reaction. A similar method of catalyst regeneration, the reoxidation of metallic mercury using iron (III) chloride , was already known from the First Wacker process.

Wacker-Chemie decided to split the patent into three patent applications. The new plant to be built was to be built in Cologne near an Esso petrochemical plant that was supposed to supply the ethylene. Details of the proceedings were passed on to Hoechst in connection with the talks on the acquisition of the property, to which Hoechst, as a partner in Wacker-Chemie, had to agree. Hoechst then took up research activities on ethylene oxidation and placed its own patent between the second and third patent applications by Wacker-Chemie. This gave Hoechst a partial claim on the process, after which Wacker and Hoechst founded the joint venture Aldehyd GmbH , which took over the marketing of the patent. Since no pure oxygen was available at acceptable prices for the planned Cologne location , Wacker-Chemie subsequently developed a two-step process that worked with atmospheric oxygen as the oxidizing agent. The first system based on the two-stage process was put into operation in Cologne in January 1960 ( Lage ). Wolfgang Hafner and Jürgen Smidt were awarded the DECHEMA Prize in 1962 for their research and development of the process .

Over the course of a few years, the Wacker-Hoechst process replaced alternative processes such as catalytic dehydrogenation or the oxidation of ethanol . It was one of the first chemical-technical processes to point out the importance of alkenes. As early as 1969, just a few years after the first plant was commissioned, the installed capacity was around 350,000 tons per year. At the turn of the millennium, the installed capacity was around two million tons per year. It was widely believed that “the invention of the Wacker process was a triumph of common sense”.

With the development of newer processes such as the Monsanto process and the Cativa process for the production of acetic acid from methanol and carbon monoxide , the Wacker-Hoechst process is becoming less important.

${\displaystyle \mathrm {CH_{3}OH+CO\longrightarrow CH_{3}COOH} }$

Acetic anhydride can be obtained from methyl acetate and carbon monoxide using the Tennessee-Eastman acetic anhydride process .

catalyst

Palladium (II) chloride is obtained by dissolving metallic palladium in aqua regia . It dissolves easily in hydrochloric acid to form tetrachloropalladate.

${\displaystyle \mathrm {PdCl_{2}+2\ Cl^{-}\longrightarrow [PdCl_{4}]^{2-}} }$

In addition to copper chloride, copper (II) acetate , copper (II) nitrate , iron (III) chloride and other oxidizing agents such as nitric acid or hydrogen peroxide are used as reoxidants for the metallic palladium produced in the process . When using chloride-free oxidizing agents, the formation of chlorinated by-products, including chloroacetaldehyde , is reduced.

Procedure

The Wacker-Hoechst process can be carried out in one or two stages. The yield based on ethylene for both processes is about 95%. By-products are chlorinated and more highly oxidized products of ethylene and acetaldehyde. In the two-stage process, the requirement for the purity of the ethene is lower than in the one-stage process. However, the investment costs in the systems and the energy consumption are higher in the two-stage process, which is why the one-stage process is preferred if the supply of pure oxygen is guaranteed.

One-step process

Block diagram of the one-step process

In the one-stage process, the oxidation of the ethene and the regeneration of the catalyst are carried out in a reactor at temperatures of about 120 to 130 ° C. and pressures of 3 to 4 bar. The regeneration takes place with pure oxygen, and the ethene used must be very pure. In order to stay below the explosion limit, an excess of ethene is used, which reduces the yield to around 35%. However, unconverted ethene can be fed back into the reactor.

The resulting crude acetaldehyde contains low-boiling components such as methyl chloride , ethyl chloride or carbon dioxide , which are separated off by extractive distillation. The resulting acetaldehyde fraction still contains higher boiling components such as chlorinated acetaldehyde, chloroform , methylene chloride , 1,2-dichloroethane , 2-chloroethanol and acetic acid, which are separated off by fractional distillation .

The reactors used are provided with a ceramic lining because of the corrosive properties of the catalyst solution. The pipes and pumps are made of titanium for the same reason . These poorly heat-conducting materials can be used because the heat of reaction can be dissipated through the evaporation of acetaldehyde and water.

Two-step process

Block diagram of the two-step process

In the two-stage process, the oxidation of the ethene and the regeneration of the catalyst are spatially separated. First, in the absence of oxygen, the ethene reacts stoichiometrically with the palladium chloride and the process water in the titanium oxidation reactor , usually a bubble column reactor , at pressures of around 10 bar and temperatures between 100 and 110 ° C to form acetaldehyde and palladium metal. After releasing the pressure to normal pressure, the crude acetaldehyde is separated from the process water and then purified by distillation.

The solution containing palladium and copper is pumped into another bubble column reactor and regenerated there. It can be regenerated with atmospheric oxygen.

Wacker-Chemie commissioned the first system based on the two-stage process in January 1960 at the Cologne-Merkenich plant. It was the world's first chemical plant with apparatus made of titanium.

Products

The main product of the Wacker-Hoechst process is acetaldehyde, which has a rich subsequent chemistry. Around 80% of the acetaldehyde produced is processed into acetic acid and acetic anhydride, and the remaining 20% ​​is used to manufacture a large number of other products. Acetic acid was produced by oxidizing acetaldehyde using manganese (II) acetate as a catalyst, a process that was also developed by Wacker-Chemie:

${\displaystyle \mathrm {2\ CH_{3}CHO+O_{2}\longrightarrow 2\ CH_{3}COOH} }$

The manganese salt catalyzes the decomposition of the peroxyacetic acid that is primarily formed .

For example, 1,3-butadiene was produced in the Lebedew process via the aldol addition of two molecules of acetaldehyde

to crotonaldehyde , which reacts with ethanol at temperatures around 380 to 500 ° C to form 1,3-butadiene, acetaldehyde and water. In a process similar to the Lebedev process, developed by the Russian chemist Iwan Ostromislenski , ethanol reacts directly with acetaldehyde at temperatures of around 325 to 350 ° C over a tantalum- modified silica catalyst to form 1,3-butadiene. Acrolein is formed from the aldol addition of formaldehyde to acetaldehyde. Pentaerythritol is also produced from the aldol addition of formaldehyde and acetaldehyde.

Reaction mechanism

Mechanism of the Wacker process for the production of aldehydes from ethene

The reaction mechanism has been studied intensively for decades, not only because of the industrial importance of the reaction, but also because it marked the beginning of catalytic palladium chemistry.

In the first step of the reaction mechanism, palladium (II) chloride, ethene and a chloride ion form an anionic palladium (II) -ethene complex equivalent to Zeise's salt , the trichloridoethylene palladinate (II) anion (top right in the figure). From studies of substitution reactions of the Zeise salt it was known that the ethene ligand has a strong trans effect . This weakens the binding of the trans chloride ligand and facilitates the addition of water (an aquo ligand) with displacement of a chloride ligand. This exchange creates a neutral palladium (II) -ethene complex (figure, center right). An anionic ethene-hydroxo complex is created again by deprotonation. Adding water and inserting the ethene ligand into the Pd-OH bond creates a 2-hydroxyethyl complex.

Another important step in the Wacker process is the migration of hydrogen from oxygen to chloride and the formation of the CO double bond (figure bottom to top left). The exact mechanism of this step is not known. One hypothesis is a β-hydride elimination in a cyclic four-membered transition state:

However, calculations showed that a four-membered transition state with β-hydride elimination is energetically unfavorable for this reaction step (151 kJ / mol). A five-membered transition state supported by water molecules with subsequent reductive elimination is energetically more favorable (78.6 kJ / mol) and is therefore viewed as more likely:

The ongoing catalytic cycle (see figure) can be summarized in the following balance equations:

${\displaystyle \mathrm {(a)\ [PdCl_{4}]^{2-}+C_{2}H_{4}+H_{2}O\rightarrow CH_{3}CHO+Pd+2\ HCl+2\ Cl^{-}} }$

${\displaystyle \mathrm {(b)\ Pd+2\ CuCl_{2}+2\ Cl^{-}\rightarrow [PdCl_{4}]^{2-}+2\ CuCl} }$

${\displaystyle \mathrm {(c)\ 2\ CuCl+1/2\ O_{2}+2\ HCl\rightarrow 2\ CuCl_{2}+H_{2}O} }$

with the overall balance:

${\displaystyle \mathrm {C_{2}H_{4}+1/2\ O_{2}\rightarrow CH_{3}CHO} }$

The partial reactions a) to c) can be represented as coupled partial reactions:

The exact mechanism of the reaction has long remained the subject of kinetic and mechanistic studies. Early kinetic studies provided a rate equation that was first order with respect to tetrachloropalladate and ethylene, and showed first order inhibition with respect to the proton and second order inhibition with respect to the chloride ion.

${\displaystyle v={\frac {k[\mathrm {PdCl_{4}^{2-}} ]\cdot [\mathrm {C_{2}H_{4}} ]}{[\mathrm {H^{+}} ]\cdot [\mathrm {Cl^{-}} ]^{2}}}}$

While the inhibiting effect of the chloride is due to the rapid equilibrium reactions

${\displaystyle \mathrm {\ [PdCl_{4}]^{2-}+C_{2}H_{4}\rightleftharpoons [PdCl_{3}(C_{2}H_{4})]^{-}+Cl^{-}} }$

${\displaystyle \mathrm {\ [PdCl_{3}(C_{2}H_{4})]^{-}+H_{2}O\rightleftharpoons [PdCl_{2}(C_{2}H_{4})(H_{2}O)]+Cl^{-}} }$

explained, there were several explanations for the inhibiting effect of the proton.

The oxidation of non-deuterated ethylene C ₂ H ₄ in heavy water D ₂ O only led to the formation of non-deuterated acetaldehyde CH ₃ CHO. Conversely, the oxidation of deuterated ethylene C ₂ D ₄ in normal water H ₂ O only led to the formation of deuterated acetaldehyde CD ₃ CDO. Through these experiments, the thesis that the process takes place via a vinyl alcohol intermediate stage, in which a hydrogen atom in the acetaldehyde, for example via a keto-enol tautomerism , should come from the water, was refuted.

Process variants

If the Wacker-Hoechst process uses a mixture of acetic acid and alkali acetates instead of water as the solvent, vinyl acetate is the main product, which is the monomer for the production of polyvinyl acetate and thus polyvinyl alcohol .

${\displaystyle \mathrm {C_{2}H_{4}+CH_{3}COOH+1/2\ O_{2}\longrightarrow CH_{3}COOCHCH_{2}+H_{2}O} }$

This homogeneous catalytic process was replaced early on by a heterogeneous catalytic oxidation process based on a palladium contact. Various solvents other than acetic acid can be used. With alcohols as solvents, acetals are usually formed , with diols the formation of 1,3-dioxolanes.

Propylene can be oxidized to acetone using the Wacker-Hoechst process. The reaction takes place at a temperature of about 110 to 120 ° C. and a pressure of 10 to 14 bar. As with the oxidation of ethylene, the metallic palladium obtained is reoxidized with copper chloride. The selectivity for acetone is 92%. The byproduct propionaldehyde . The process can be carried out in one or two stages. The only industrial plants using this process were built and operated in Japan. A process developed by Wacker for the oxidation of 1-butene to methyl ethyl ketone could not be established.

Wacker oxidation

In addition to the large-scale industrial processes, the reaction on which the Wacker-Hoechst process is based is known as Wacker oxidation and is used in laboratory syntheses, for example in the form of Wacker-Tsuji oxidation . This involves the oxidation of higher α-olefins to methyl ketones, such as the oxidation of 1-decene to 2-decanone .

In general, more highly substituted alkenes react faster than unsubstituted ones and dienes with conjugated double bonds react more slowly than non-conjugated systems. The product formation follows the Markovnikov rule , according to which the hydrogen atom is always bound to the already hydrogen-rich carbon atom.

On a laboratory scale, the reaction is usually carried out at room temperature with palladium (II) chloride and copper (II) chloride under an oxygen atmosphere. To avoid chlorination reactions, other soluble palladium systems such as palladium (II) acetate or bis (acetonitrile) chloronitropalladium (II) are used in addition to the classic palladium / copper chloride system. Strong inorganic acids such as perchloric acid sometimes increase the reaction rate. The reaction rate of the oxidation of poorly water-soluble higher alkenes can be increased by using mixtures of water and polar organic solvents such as dimethylformamide or N- methyl-2-pyrrolidone or by using surface-active substances such as sodium lauryl sulfate . Double bond isomerization can be observed as a common side reaction; the rate of reaction of this side reaction seems to be strongly dependent on the choice of solvent. In addition to the classic copper chloride, various oxidizing agents are used under laboratory conditions to re-oxidize the resulting palladium metal.

literature

• Patrick Henry: Palladium Catalyzed Oxidation of Hydrocarbons (Catalysis by Metal Complexes) , Verlag Springer, 1980, ISBN 978-94-009-9448-5

Web links

Commons : Wacker-Hoechst process  - collection of images, videos and audio files

Individual evidence

1. ↑ ^{a b c d e f g h} Reinhard Jira: Acetaldehyde from Ethylene - a look back at the discovery of the Wacker process. In: Angewandte Chemie. 121, 2009, pp. 9196-9199, doi : 10.1002 / anie.200903992 .
2. ^ Albert Hester, Karl Himmler: Chemicals from Acetaldehyde . In: Industrial and Engineering Chemistry , 51.12, 1959, pp. 1424-1430.
3. ^ Francis C. Phillips, Am. Chem. J., 1894, 16, pp. 255-277.
4. ↑ ^{a b c d e} People • Markets • Molecules: The Wacker Chemie Formula for Success 1914–2014, p. 154 and p. 175–185 ( pdf ) .
5. ↑ ^{a b c d} John A. Keith, Patrick M. Henry: On the mechanism of the Wacker reaction: two hydroxypalladations !. In: Angewandte Chemie. 121, 2009, pp. 9200-9212, doi : 10.1002 / anie.200902194 .
6. ↑ The DECHEMA award winners since 1951 .
7. ^ ^{A b} Marc Eckert, Gerald Fleischmann, Reinhard Jira, Hermann M. Bolt, Klaus Golka: Acetaldehyde . In: Ullmann's Encyclopedia of Industrial Chemistry , 2005, Weinheim, Wiley-VCH, doi : 10.1002 / 14356007.a01_031.pub2 .
8. ↑ James A. Kent (Ed.): Riegel's Handbook of Industrial Chemistry , 7th edition, Verlag Van Nostrand Reinhold, ISBN 0-442-24347-2 , p. 790.
9. ↑ George W. Parshall, Steven D. Ittel: Homogeneous Catalysis. Wiley, New York 1992, ISBN 978-0-471-53829-5 , p. 138.
10. ↑ Glenn J. Sunley, Derrick J. Watson: High productivity methanol carbonylation catalysis using iridium. In: Catalysis Today. 58, 2000, pp. 293-307, doi : 10.1016 / S0920-5861 (00) 00263-7 .
11. ^ Joseph R. Zoeller, Victor H. Agreda, Steven L. Cook, Norma L. Lafferty, Stanley W. Polichnowski, David M. Pond: Eastman chemical company acetic anhydride process. In: Catalysis Today. 13, 1992, pp. 73-91, doi : 10.1016 / 0920-5861 (92) 80188-S .
12. ^ Adolph Segnitz: s-Organocobalt, - rhodium, -iridium, -nickel, -palladium compounds. In: Houben-Weyl Methods of Organic Chemistry Vol. XIII / 9b, 4th Edition , Georg Thieme Verlag, 1984, ISBN 978-3-13-215004-1 , p. 882.
13. ↑ ^{a b c} Jürgen Smidt, W. Hafner, R. Jira, R. Sieber, J. Sedlmeier, A. Sabel: The Oxidation of Olefins with Palladium Chloride Catalysts. In: Angewandte Chemie International Edition in English. 1, 1962, pp. 80-88, doi : 10.1002 / anie.196200801 .
14. ^ ^{A b} Wilhelm Keim, Arno Behr and Günter Schmitt: Fundamentals of industrial chemistry. Technical products and processes , ISBN 3-7935-5490-2 (Salle), ISBN 3-7941-2553-3 (Sauerländer), pp. 217-223.
15. ↑ Boy Cornils, Wolfgang A. Herrmann, Chi-Huey Wong, Horst-Werner Zanthoff: Catalysis from A to Z: A Concise Encyclopedia . Verlag Wiley-VCH, 2012, ISBN 3-527-33307-X , p. 337.
16. ↑ H. Schulz, H. Wagner: Synthesis and conversion products of acrolein. In: Angewandte Chemie. 62, 1950, pp. 105-118, doi : 10.1002 / anie.19500620502 .
17. ^ P. Rave, B. Tollens: XXXIV. About the penta-erythritol (tetramethylolmethane). In: Justus Liebig's Annals of Chemistry. 276, 1893, pp. 58-69, doi : 10.1002 / jlac.18932760106 .
18. ↑ Ido Leden, Joseph Chatt: The stability constants of some platinous halide complexes. In: Journal of the Chemical Society. 1955, pp. 2936-2943, doi : 10.1039 / JR9550002936 .
19. ↑ ^{a b c} Dirk Steinborn: Fundamentals of organometallic complex catalysis. Teubner, Wiesbaden 2007, ISBN 978-3-8351-0088-6 , pp. 283-292.
20. ↑ John A. Keith, Jonas Oxgaard, William A. Goddard: Inaccessibility of β-Hydride Elimination from −OH Functional Groups in Wacker-Type Oxidation. In: Journal of the American Chemical Society. 128, 2006, pp. 3132-3133, doi : 10.1021 / ja0533139 .
21. ^ Wilhelm Keim: Homogeneous Catalysis in Germany: Past and Present . In: Studies in Surface Science and Catalysis , 44, 1989, pp. 321-331.
22. ↑ W. Schwerdtel: vinyl acetal based on ethylene in the gas phase. In: Chemical Engineer Technology. 40, 1968, pp. 781-784, doi : 10.1002 / cite.330401602 .
23. ^ William G. Lloyd, BJ Luberoff: Oxidations of olefins with alcoholic palladium (II) salts. In: The Journal of Organic Chemistry. 34, 1969, pp. 3949-3952, doi : 10.1021 / jo01264a043 .
24. ↑ Gian Paolo Chiusoli, Peter M. Maitlis: metal catalysis in Industrial Organic Processes . RSC Publishing, 2008, ISBN 978-0-85404-150-3 , p. 69.
25. ↑ Jiro Tsuji, Hideo Nagashima, Hisao Nemoto: A General Synthetic Method for the Preparation of Methyl Ketones from Terminal Olefins: 2-Decanone. In: Organic Syntheses. 62, 1984, p. 9, doi : 10.15227 / orgsyn.062.0009 .
26. ↑ Michael B. Smith: Organic Synthesis. 1st edition, McGraw-Hill, 1994, ISBN 0-07-113909-5 , pp. 1350-1353.
27. ↑ ^{a b} James Takacs, Xun-tian Jiang: The Wacker Reaction and Related Alkenes Oxidation Reactions. In: Current Organic Chemistry. 7, 2003, pp. 369-396, doi : 10.2174 / 1385272033372851 .