Acetoxylation of Olefins Over Supported Palladium Metal and Salts Catalysts*

Acetoxylation of Olefins over Supported Palladium Metal and Salts Catalysts* by Taiseki Kunugi**, HiromichiArai** and Kaoru Fujimoto** Summary: The acetoxylationof olefinsin thegaseous phase overpalladium metal and salts-active charcoalcatalysts was studiedusing a fixed bed reactor. Palladium salts adsorbedon activecharcoal are excellentcatalysts for this reactioneven in the absenceof any usual redoxreagents such as CuCl2. It was concludedthat the reoxidationof reducedpalladium is catalyzedby active charcoalin thepre- senceof oxygen. In the caseof acetoxylationof ethylenethe mainproduct was vinyl acetatefor both catalysts, but in the caseof propylene, allyl acetate,isopropenyl acetate and acetonewere mainlyproduced over palladium salts-activecharcoal catalysts, whereasallyl acetatewas the only principal product overpalladium metal-activecharcoal catalyst. The differencein theproduct distributionbetween these catalysts supposedly indicates that propylene forms a π-complex with palladium salts to give allyl acetate and isopropenylacetate whichwas hydrolyzedto form acetoneand, on the other hand, that propylene forms π-allyl complex with palladium metal to give allyl acetate. The kineticsof acetoxylationof ethylenewas studiedwith a differentialreactor for both catalysts and the reactionscheme was proposed for eachreaction system. 1 Introduction proceed by a similar mechanism2). The present authors have shown that palladium The oxidation of olefins to vinyl and allylic salts adsorbed on active charcoal are very effici- acetates has been reported recently. In the case ent catalysts for the Wacker reaction and for the of ethylene, as first reported by Moiseev et al.1) acetoxylation of olefins as well despite of the ab- the reaction is described by the following equation: sence of redox reagents such as CuCl27)～9). Ethyl- C2H4+Pd2++2OAc- ene reacts with acetic acid on palladium salts or →C2H3OAc+HOAc+Pd2 (1) palladium metal to produce vinyl acetate. Propyl- It has been generally accepted that the reaction ene however, reacts with acetic acid on palladium for the formation of vinyl acetate, proceeds by the salts, to produce allyl and isopropenyl acetates attack of the acetate anion on the palladium-ethyl- and on palladium metal to produce allyl acetate. ene-π-complex. The acetate anion reacts nucleo- In this paper, the mechanisms of acetoxylation philically with the carbon atom of the ethylenic catalyzed by palladium metal on γ-alumina or double bond from which a hydride ion can be palladium salts on active charcoal catalyst are removed most easily. discussed from the viewpoint of the kinetics and On the other hand, Henry offered a mechanism, the product distribution of the reactions. based on kinetic study, for the oxidation of ethylene 2 Experimental to acetaldehyde and suggested that the nucleo- philic addition of the hydroxide ion took place 2.1 Apparatus and Procedure predominantly on the carbon atom of the: π- The apparatus used was a conventional tubular complex to give hydroxy-π-complex. Subsequent flow reactor equipped with devices for measuring rearrangement of the π-complex to σ-complex the flow rates of reactants and products. The was the rate determining step, and the σ-complex reactor was a glass tube (50mm long and 18 decomposed rapidly to acetaldehyde through a mm i. d.), with a thermo-couple sheath placed hydride shift. In addition, he postulated that the along the central axis. It was heated with an formation of vinyl esters in acetic acid could easily electric furnace. The catalyst bed was fixed * Received November 22, 1969. and the amount of catalyst used was 5-20g. ** Department of Synthetic Chemistry, Faculty of The remaining parts of the reactor, above and Engineering, University of Tokyo (Hongo, Bunkyo- below the catalyst bed, were filled with glass ku, Tokyo, Japan) Volume 12-May 1970 98 Kunugi, Arai and Fujimoto: Acetoxylation of Olefins beads of 1-2mm diameter. ladium diacetate was completely adsorbed on the Liquid acetic acid was fed by micro-feeder and active charcoal. Acetic acid was filtered off and vaporized in the upper part of the reactor. The the catalyst was dried at 130℃ for 5 hours in mixture of gaseous reactants was then passed over vacuo. the catalyst bed. The liquid products were Other palladium salts were adsorbed on active collected in traps cooled by water or dry ice- charcoal in their acidic solutions, and treated methanol. similarly to palladium chloride. 2.2 Analytical Liquid products were analyzed by gas chromato- 3 Results graphy with a 3m column packed with 20% 3.1 Catalysis of Palladium Metal and Pal- di-nonyl phthalate on C-22. Columns packed ladium Salts with Molecular Sieve and silica gel were used to Propylene reacts with acetic acid and oxygen analyze the noncondensable gases. to give propenyl acetates and water according 2.3 Materials to the following equations: C Ethylene and propylene (99.9% purity) were 3H6+AcOH+1/2O2 fed to the reactor after being passed through calcium chloride and active charcoal. CH3-C=CH2→H2OCH3COCH3+AcOH (2) Commercially obtained oxygen and nitrogen OAc were also passed through calcium chloride and Isopropenyl Acetate active charcoal before feeding. The acetic acid CH2=CH-CH2 (3) purity was 99.9%, and was used without any -H2O→ further purification. OAc Allyl Acetate 2.4 Catalysts The metallic palladium catalysts which were CH3-CH=CH (4) made by Nihon Engelhald Co. were 0.5wt% OAc on r-alumina and 0.5w% on active charcoal. n-Propenyl Acetate (cisor trans) With these catalysts, a clear pattern of palladium The product distribution is discussed in terms of metal was observed by X-ray diffraction. the selectivity as defined below: Palladium salts-active charcoal catalysts were A(mol/hr) Product Distribution of A(%)= (5) prepared by the methods described below. B(mol/hr) PdCl2-A.C.: Palladium dichloride (0.84 grams) B(mol/hr)=isopropenyl acetate (mol/hr) was dissolved into 300ml of 1 N hydrochloric +acctone (mol/hr)+allyl acetate(mol/hr) acid (Solution A). Acitive charcoal (50 grams) +n-propenyl acetate(mol/hr) was boiled for ten minutes in 200ml of 1N hydro- +diacetate compounds (mol/hr) chloric acid (Mixture A). After Mixture A was +isopropyl acetate(mol/hr) cooled to room temperature, Solution A was The chemical composition of the carrier used added with stirring (Mixture B). Mixture B was left to stand for 24-48 hours at room tem- has a profound effect on carbon dioxide formation under the conditions investigated. Hence, the perature. During this period, palladium chloride in the solution was adsorbed completely on the amount of carbon dioxide formation is excluded active charcoal. The mother liquid was filtered Table 1 Conversion and Product Distribution in off. The remaining mixture was washed with Acetoxylation of Ethylene 200ml of pure water, then dried at 150℃ for 6 hours in vacuo. Pd(OAc)2-A.C.: Palladium diacetate (1.01 grams) was dissolved into 300ml of pure acetic acid (Solution B). Active charcoal was boiled in 100ml of acetic acid for a short time and cooled (Mixture C). Solution B was mixed with Mixture C and allowed to stand for 48 hours. In some Reaction condition: W/F=100g-cat.hr/mol, tempera- cases, the mixture was heated at 100-110℃ for ture=135℃, feed composition in mole: ethylene: several hours to complete the adsorption. Pal- oxygen: acctic acid=5:2:5 Bulletin of The Japan Petroleum Institute over Supported Palladium Metal and Salts Catalysts 99 Table 2 Conversion and Product Distribution in Acetoxylation of Propylene Reaction condition: W/F=100g-cat・hr/mol, temperature=135℃, feed composition in mole: propylene: oxygen: acetic acid=5:2:5 from "B" in the above equation. because of the difficulty in complete reducing Ethylene reacts with acetic acid on either the palladium salt. palladium salts or palladium metal catalyst to The ratio of acetone to allyl acetate decreased give vinyl acetate as a major product, as is gradually as the reduced state of the catalyst shown in Table 1. The difference in reaction increased as is shown in Fig. 1. This suggests mechanisms over these catalysts can not be fully that the catalyst is gradually reduced during this interpreted from the product distribution. Table reaction. Over palladium metal supported on 2, however, shows that propylene reacts with active charcoal soaked in the aqueous solution of acetic acid in the presence of oxygen to give a lithium chloride or copper chloride and dried characteristic product distribution. On pal- at 150℃, the amount of acetone increased while ladium salts, the major product is acetone which that of allyl acetate decreased as is shown in Table is the hydrolysis product of isopropenyl acetate. 3. Minor products are allyl acetate and isopropenyl It was previously known that the ethylene- acetate. However, over palladium metal cata- palladium salt complex may react with water lyst, allyl acetate amounts to about 90% of the products with the remaining being diacetate Table 3 Effect of Added Salts on Palladium Catalyst compounds, isopropenyl acetate and acetone. In the case of palladium metal on active charcoal , a small amount of palladium salt would have remained on the surface of the active charcoal, Reaction condition: W/F=50g-cat・hr/mol , temperature=135℃, feed composition in mole: propylene: oxygen: acetic acid: nitrogen=2:4:5:6, molar ratio of added salts: Li/Pd=3, Cu/Pd=3 Table 4 Hydrolysis of Isopropenyl Acetate Reaction condition: W/F=100g-cat・hrlmol , temperature=135℃, feed composition in mole: isopropyl acetate: water: acetic acid: oxygen: nitrogen=1:1 .3:40: 16:40 Fig. 1 Selectivity Change vs. Reaction Time Volume 12-May 1970 100 Kunugi, Arai and Fujimoto: Acetoxylation of Olefins to give acetaldehyde, while vinyl acetate may reaction again. Results are shown in Table 6 also react with water to give acetaldehyde. In with PdCl2-A.C. and Pd(OAc)2-A.C. catalysts. the case of propylene, the ratio of acetone to No activity decrease was observed with PdCl2- isopropenyl acetate increased with the increase of A.C.

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