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Catalyst Systems Derived from Transition Metal Acetylacetonates for the Hydrogenation of Aromatics

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Catalyst Systems Derived from Transition Metal Acetylacetonates for the Hydrogenation of Aromatics ladl ••• Jouraal of Cbemlstry Vol. 14A, February 1976, pp. 107-111 Catalyst Systems Derived from Transition Metal Acetylacetonates for the Hydrogenation of Aromatics N. MURUGESAN & s. SARKAR Department of Chemical Engineering, Indian Institute of Technology, Bombay Received 29 [unuary 1975; accepted 24 March 1975 Acetylacetonate complexes of iron, cobalt and nickel on reduction with lithium aluminium hydride or sodium borohydrlde yield hiihly active catalyst systems for the hydrogenation of aromatic compounds. Cobalt and nickel based systems are more active than the iron based one. The systems derived from lithium aluminium hydride are much more active than those from sodium borohydride. The activities of the catalyst systems show a dependence on the ratio of the transition metal to the reducing agent, In the case of lithium aluminium hydride, solvents like tetrahydrofuran and diethyl ether lower the amount of the reducing agent required for the optimum catalytic activity. Usini tetrahydrofuran as the solvent, the effect of different variables, viz. [catalyst], temperature, pressure and additives, on the catalytic activity has been investigated. It has been possible to regenerate the lithium aluminium hydride derived cata- lysts after hydrogenation reactions. Based on the study of the infrared spectra of the precipi- tates from the reaction systems and also various other studies, a mechanism involving a transition metal hydride moiety has been suggested for the catalytic hydrogenation. ATALYSIS by coordination compounds is cobalt(III) and bis (acetylacetonato) nickel(II) dihy- now a recognized field of both theoretical and drate. C applied importance-->. A survey of transition All the solvents, viz. tetrahydrofuran, diethylether, metal complex catalysts shows that there are only a and 1,4-dioxane were dried over sodium wire and few catalysts available for the hydrogenation of freshly distilled over LAH. aromatic compounds+w, Of these, the costlier Commercial grade hydrogen supplied by the Indian rhodium complexes are active at room temperature Oxygen Limited was used without purification for and one atmosphere pressure. Dicobalt octacarbo- all the reactions in the autoclave. Its approximate nyl is not active for isolated benzene rings. The composition was given as: H2, 99'5%, Oz, 0·2%, CO2, Ziegler-Natta catalysts are active at high temperature 25 ppm and H20, 500 ppm. However, for reactions and pressure; moreover they require the use of organo- at room temperature and one atmosphere pressure, aluminium compounds which need special care in this hydrogen was purified using an active copper handling. In an earlier notel5 we reported the use catalysts? and then dried by passing through of lithium aluminium hydride (LAH) as a useful phosphorus pentoxide. alternative reducing agent for preparing catalysts Procedure for hydrogenation at room temperature similar to Ziegler-Natta type for the hydrogenation of and one atmosphere pressure - The apparatus con- benzene. In the present paper, are presented further sisted of a flat-bottomed flask fitted with a three- details on this newly developed catalyst system and way adaptor. One of the outlets was connected also another system prepared by using sodium boro- to the gas line. The second was closed with a hydride in place of LAH. The systems, bis stopper and the third was fitted with a pressure (acetylacetonato) cobalt(II) dihydrate-LAH and bis equilibrated addition funnel. The gas line connec- (acetylacetonato) nickel(II) dihydrate-LAH were ted a dibutyl phthalate-filled gas burette through investigated in greater detail. a two-way stopcock. There was another two-way stopcock connecting the system to pure, dry Materials and Methods hydrogen gas supply or vacuum. The other arm Cyclohexene (LR), benzene (AR), toluene (LR), of the two-way stopcock of the gas burette connected xylene (LR) and naphthalene (LR) were used as it to a separate hydrogen supply line. substrates. Lithium aluminium hydride (E. Merck) To start the experiment, the gas burette was filled and sodium borohydride (E. Merck) were used as the with hydrogen. The gas burette stopcock was reducing agents. The additives were of reagent closed. In the flask, the solution of the transition grade. metal complex in the solvent was taken. In the The following acetylacetonate complexes prepared addition funnel. the substrate was taken. The in the laboratory according to literature methodl6-19 system was filled with hydrogen by alternately eva- were used: tris(acetylacetonato) iron(III). bis (acetyl- cuating and filling with hydrogen. Then the stopper acetonato) cobalt(II) dihydrato, tris (acetylacetonato) in the system was opened with a continuous passage 107 INDIAN J. CHEM .• VOL. 14A. FEBIWARY 1976 of hydrogen through the system and LAH was added. at a particular temperature. The time taken to When the reaction subsided. the stopper was closed reach the reaction temperature and then bring it and the stopcock of the gas burette opened to the back to the room temperature was neglected even system. When a steady reading was noted in the though some reaction might have taken place during gas burette. the substrate was added to the solution this time. The completion of reaction was indicated in the flask. Any hydrogen absorption was noted. by the constancy of pressure. When required. the gas burette was refilled with Isolation of theproducts - After the hydrogenation. hydrogen. without disturbing the system, through the reaction mixture, usually, had a black precipitate the separate hydrogen supply line. At the end of and a clear colourless solution. The clear solution the reaction. the whole system was flushed with at the end of the experiment was separated by filtra- nitrogen and the solution in the flask was taken for tion and washed with water to remove water-soluble further treatment. solvents such as tetrahydrofuran and 1,4-dioxane. Procedure for hydrogenation at medium temperature In the case of diethylether solvent, it was removed and high pressure - These experiments were carried by careful evaporation. The product was finally out in a i litre rotary autoclave provided with heat- dried over anhydrous calcium chloride. When no ing at a controlled temperature. solvent had been used, the product was freed from The hydrogenation reactions using a solvent were the precipitate by filtration and thereafter dried. performed in the following manner. The required In a few cases when the reaction mixture had amount of the transition metal complex was dissolv- a black solution as well as a black precipitate, the ed in the solvent (50 ml). The weighed amount solute was completely precipitated by adding some of LAH was added to this. When the reaction water to the solution. subsided, usually yielding a black or brownish black Analysis of the products - When the substrate solution for optimum complex to sodium hlyride was benzene, toluene or xylene, the product was ratio, the substrate was added and the Bask closed. analysed for the amount of aromatics and olefins After 10 min. this solution was fed to the autoclave by absorption with phosphorous pentoxide-sulphuric and the latter was closed with the lid. It was then acid absorption method'". In the case of cyclo- flushed with hydrogen several times and finally filled hexene and also the above mentioned substrates, with hydrogen up to the required pressure. All these the amount of olefin in the product was found by operations required about 20 min. The rotating a similar method using 84% sulphuric acid. The and heating of the autoclave were started. It took percentage of saturates in the products was deter- about 30 min to reach 100° from room temperature, mined by the method of difference. about 45 min to reach 150° and about 55 min to The hydrogenation products of naphthalene were reach 200°. The autoclave was maintained at the analysed by gas liquid chromatography. The latter required temperature till the pressure drop stopped. was not available in the benzene hydrogenation The time required for this was noted. For slower study which required a large number of experi- reactions indicated by prolonged period of pressure ments and formed the bulk of the present investi drop, the autoclave was run for 2 hr. After the gation. heating was stopped, the autoclave was kept over- The precipitates were isolated by centrifugation night for cooling. and washing with the solvent. The solvent was In hydrogenation reactions of naphthalene, the finally removed under reduced pressure and the preparation of the reaction solution was slightly precipitate dried in a vacuum desiccator over modified. The transition metal complex was reduced calcium chloride. For further reactions with this by LAH in tetrahydrofuran. This was added to precipitate, no special care was taken to protect it the naphthalene already taken in the autoclave. from the atmosphere. Then the autoclave was closed and pressurized. 'In hydrogenation experiments in the absence Results of solvent, the autoclave was filled in the following The recovery of the hydrogenated product ranged manner, The transition metal complex. the sub- from 80 to 85%. The saturated portion of the pro- strate and the saline hydride were placed in that duct was found to be cyclohexane when cyclohexene order in the autoclave. In experiments where the or benzene was used as the substrate. Ole fins were requirement of hydrogen was more than the totally absent. amount filled initially, the autoclave was repres- All the catalyst systems derived by reaction with surized with hydrogen. LAH were similar in many properties but slightly For the reactions carried out to find out the effect different in colour. The precipitates contained the of impurities on the catalyst system, the additive particular transition metal as well as lithium and was incorporated first to benzene and then the mix- aluminium. Elemental analyses of the precipitate, ture was introduced to the catalyst solution, both before and after exposure to atmosphere for However, in the case of water as the impurity, it a long time. showed the presence of carbon and was added to the reaction solution after the addition hydrogen.
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