摘要:
1974 465Thermal Decomposition and Autoxidation of Cobalt AcetylacetonatesBy G. Vasviiri, 1. P. Hajdu, and D. GBI," Central Research Institute for Chemistry, Budapest, HungaryThe thermal decomposition and oxidation of Co(acac), and Co(acac), have been investigated. It was establishedthat in the absence of oxygen between 100 and 130 "C Co(acac), was reduced to Co(acac),, while Co(acac), didnot undergo decomposition under similar conditions. The corresponding rate constants and activation energieswere determined. Both Co(acac), and Co(acac), are oxidized stepwise to Co(OAc),,xH,O in the presence ofoxygen. The intermediate of the oxidation process is presumably Co(acac) (OAc),. It was experimentallyproved that RO,*-type radicals generated in the system can interact with both acetylacetonates.The rate constantof the interaction between peroxy-radicals and Co(acac), has been calculated. Mechanisms for both the de-composition and oxidation reactions have been suggested.ACETYLACETONATES of transition metals, e.g., cobaltacetylacetonates, are excellent homogeneous catalystsin the liquid-phase oxidation of hydrocarbons. Inorder to obtain the detailed mechanism of such pro-cesses, the kinetics of the overall catalytic reactionshave been inve~tigated.l-~Since in these processes the catalysts themselvesundergo chemical changes, it seemed expedient to studytheir decomposition and autoxidation in the absence ofthe parent hydrocarbons, in inert solvents.The thermal decomposition and oxidation of cobaltacet ylacetonates and their interactions with peroxy-radicals in chlorobenzene were studied in order to find ananswer to the question: how much these reactions areto be taken into account in the interpretation of thecatalytic effect.Such processes were recently studied both in polarand non-polar solvents by several authors, usingdifferent metal acetylacetonates.During thermal de-composition in an inert atmosphere transition-metalcomplexes of acetylacetonates were found to yieldacetone and carbon dioxide,617 while their autoxidationin non-polar solvents resulted in the reduction of thecentral metal atoms of higher valency state, accom-panied by simultaneous oxidation of the ligand.8-llOxidation was essentially similar in polar solvents.12J3The present paper deals with the thermal decom-position and autoxidation of CoIII and CoII acetyl-acetonate complexes.EXPERIMENTALMaterials.-Reagent grade chlorobenzene was shakenwith aqueous NaHSO, (lo%), separated, dried (CaCl,), anddistilled.The solvent obtained (free from benzaldehyde)was used as inert solvent.1 P. George, E. Rideal, and A. Robertson, Proc. Roy. SOC.,1946, A , 185, 283.N. Ota and T. Tesuka, J . Chem. SOC. Japan, Ind. Chenz.Sect., 1954, 57, 641.3 J. Wibaut and A. Strong, PYOG. k. ned. Aknd. Wetenschap.,1951, 54, B, 102.4 I. Kamiya and K. U. Ingold, Canad. J . Chem., 1964, 42,1027.5 N. M. Emanuel, I. T. Denisov, and 2. I<. Maims, ' ChainReactions of Hydrocarbon Oxidation in Liquid Phase,' Nauka,Moscow, 1969 (in Russian).6 R.G. Charles, W. M. Hickam, and M. A. Pawlikowsky,J . Phys. Chem., 1958, 62, 440.7 R. G. Charles, W. M. Hickam, and M. A. Pawlikowsky,J . Phys. Chem., 1958, 62, 1098. * M. A. Mendelsohn, E. M. Arnett, and H. Freiser, J . Phys.Chem., 1960, 64, 660.Cobalt (111) acetylacetonate, Co(acac),, was prepared andpurified by the method of Bryant and Fernelius 14 (Found :C, 50.8; H, 6.0. Cak. for C1,H2,COO6: C, 50.57; H,5.97%).Cobalt(I1) acetylacetonate, Co(acac),, was synthesised asdescribed by Ellern and Ragsdale.16 Purity was checkedby i.r. spectra (no trace of ColI1 complex) (Found : C, 45.6 ;H, 5-65; Co, 18.6. Calc. for C,,Hl,Co04: C, 46.7; H,5.45; Co, 22.6%). [Co was determined by backweighingand i t is likely that the loss of Co is due to the sublimationof Co(acac),.]Procedure.-Experiments were carried out in a silicone-coated glass vessel provided with a reflux condenser.Thegases (oxygen or argon) were introduced through a capillarytube a t the bottom of the reaction vessel and samples weretaken through a side arm provided with a stopcock.Stirring was by the bubbling gas. For the kinetic runschlorobenzene (30-50 cm3) was placed in the reactionvessel which was immersed in a thermostat. The gasbubbling was already started in the warm-up time and thecobalt complex was added to the solvent after thermalequilibrium was reached. The flow rate of oxygen in theautoxidation runs was varied in the range 6-18 1 h-l toensure saturation and in the thermal decompositions argonwas bubbled through the liquid.Analysis.-The changes in the valency state of thecatalysts were followed spectrophotometrically by use of aSpectromom 401 instrument. Two versions of the analysiswere applied.( A ) 3.5 cm3 of the sample was centrifuged to separatethe precipitate (if any) from the liquid.The precipitate,which proved to be CoII acetate, Co(OAc),, as reportedpreviously l6 was washed with benzene, dissolved inaqueous ~M-NH,SCN (1 cm3) and after the addition ofacetone (2 cm3) the absorption of the resulting blue solutionwas measured at 620 nm (E 1740). The Co(acac), contentof the green supernatant liquid was measured directlya t 620 nm (E 113). For the determination of the Co(acac),,9 E. M. Arnett, H.Freiser, and M. A. Mendelsohn, J . Amer.l o E. M. Arnett and M. A. Mendelsohn, J . Amer. Chem. SOC.,11 E. M. Arnett and M. A. Mendelsohn, J . Amev. Cham. SOC.,l2 D. Banerjea and S. Dutta Chandhuri, J . Inorg. Nuclearl 3 Tadashi Koga and Tadashi Hara, Bull. Chem. SOC. Japan.l4 B. F. Bryant and W. C. Fernelius, Inorg. Synth., 1967, 5,l5 J. B. Ellern and R. 0. Ragsdale, Inovg. Synth., 1968, 11,l6 G. VasvAri, P. Hajdu, I. Kende, and D. GAl, Magyar KkmChem. SOC., 1962, 84, 2482.1962, 84, 3821.1962, 84, 3824.Chem., 1971, 33, 515.1966, 39, 664.188.82.Folybirat, 1971, 77, 625466 J.C.S. Dalton3 c1n3 of this green solution was extracted three times withaqueous IM-NH,SCN (1.2, 1, and 1 cm3). Each extract wasdiluted with double the amount of acetone and the ab-sorptions of the blue solutions were measured at 620 nm[since the complex measured is the same as in the case ofCo(OAc), the value of E is also the same].(B) Co(OAc), was determined according to ( A ) .Thcsupernatant liquid was extracted three times with watercontaining 1 yo Na,SO, to reduce frothing (1.2, 1, and 1 om3),and the absorptions of the combined green extracts weremeasured directly at 620 nni [Co(acac)(OAc),, E 1331. Thenext step was the reduction of Co(acac)(OAc), by one dropof liydrazine hydrate. After adding a mixture of Oil(' partof lix-NH,SCN and two parts of acetone to the reducedextract we measured the blue solution at 620 nm [Co(acac), +reduced Co(acac)(0~4c),]. The organic layer of the cs-tracted supernatant liquid wliich contained the Co(ncnc)was also measured at 620 nm after drying (Ka2S04).RESULTSwere carried out in chlorobenzene under argon.centration of the chelate was varied between 3.3 vand 1 x' 10-2~ in the temperature range 100--130 O C .TherwzZ Decomibosition of Co(acac),.-The experimentsThe con-Time / hFIGURE 1 Dccomposition o f Lo(acac), in chlorobeiizciic atdiffereni temperatures, -1, 100 "C; B, 110 "C; C, 120 "C;and D, 130 "C; [Co(acac),lp = 1 :,: 10-331.- , calculatcclcurves; 0, experimental pointsIt has been shown that in this temperature regionCo(acac), absorption decreased with time a t a rate in-creasing with temperature. The initial rate of Co(acac),consumption was proportional t o its initial concentration.Based on analysis ( A ) i t was established that no precipitatewas formed during decoiiiposition and the total amountof Co(acac), consumed was recovered in the form ofCo(acac),.Transformation never exceeded 50% [in arti-ficial mixtures of Co(acac), and Co(acac), (1 : 1) no tle-composition was observed].The temperature dependence of transformation is shownin Figure 1.'I'he experiments pointed to the opening of one chelatering of the octahedral Co(acac), followed by the cleavage ofthe C-0 bond of the ligaiid, yielding Co(acac),.R. C . Fay, A. Y . Girgis, and 11. Klabuiide, J . A m e ~ . C'hciij.SOC., 1970, 92, 7056.'This assumption is strongly supported by the results ofFay et ~ 1 . ~ ~ 9 who also supposed short-lived intermediatesin the optical inversion and geometrical racemization ofCo(acac), and related compounds.The Co(acac), formedwas capable of dimerization with the ' opened-up ' Co(acac),This mechanism explains the fact that the reaction is o fthe first order with respect t o Co(acac), and that the Co71rconsumed was recovered as CoJI.The formation of radicals in the decomposition of CoilLchelates was proved by Kastning et a1.l9 in the CoJ1l-initiated polymerization of styrene and ethylene. There-after the mechanism (1)-(4) can he assumed for tliek ,k-1I? 3Co(acac), + Co (acac) +icac ( i )(- 1 ) C o (acac) zacac --)- C o (acac)C~(acac)~acac + Co(acac), + acac. (2)k 1 Co(acac),acac -; Co(acac), ---f DL' acac- + stablc products(:I)(4k ,decomposition procebs, where Co(acac),acac represents the' opened-up ' chelate and D is the stabilized dimer.If we consider that the rate-determining step is reaction( 2 ) and apply the steady-state treatment we obtainequations (5) and (6).Using the rate values of our deconi-position experiments and the value o f k , in the literature l7Co(acac),] (5) II'l , ._ ._ -tl [Co (acac) ,]d t 1 2 h -we found that 30 < /<--l/2k2 < 81, clepending on the tern-perattare and so 1 can be neglected in the denominator ofequations ( 5 ) and (6). After integration of the correspond-ing differcntial equations we obtain ( 7 ) . The absorptionmeasured during tlie rcxaction, if we neglect the absorptioncaused by the Co(acac), a t 620 nm, is due to the ' free 'Co(acac), as we11 t o the Co(acac), in dimeric formrequation (S)] .The activation cnergy of the overall decompositionreaction is 26 kcal mol-l and the ratio of the rate constantsi i l to k , is given in equation (9) if we accept equation (10)12 *k2- = 9-12 >: lo6 es1974 467decomposition curves a t different teuipc.1-aturcs are given inFigure 1 .This mechanism is supported by the esperirnents wheret ht.ad-azobisisobutyronitrile (AIBK) initiator was addedto the reaction mixture during thermal decomposition.These runs were carried out at 100 "C where the thermaldecomposition of Co(acac), was negligible compared withthat of the initiator. The decomposition rate of Co(acac),remain unchanged, that is, radicals generated from thea zobisisobutyronitrile did not induce decomposition.Thermal Decomposition of Co(acac),.-Experiments werecarried out under the same conditions used for the study ofthe thermal decomposition of Co(acac),.In inert solvent,ind atmosphere 1 1 0 measurable changes could be observed"1' to I20 "CIn order to obtain the activation energy of oxidation, thelogarithm of the initial oxidation rates are plotted against1/T (Figure 4). The Arrhenius dependence is satisfactory,jok 91\ I \IIO' A 1:o ,kl 2:o 4 0 3LoTirne/minFIGURE 2 C)sitlation o f Co(acac), in chlorobenzene a t 120 "C-, calculated curveA utoxidation of Co(acac),.--Experiments were performeda t 100-120 OC and the initial concentration of Co(acac),was varied between 1 x and 1 x 10-3~1.The flow-rate of oxygen \\-as 6 1 1i-I.Varying the chelate concentration and following analysisby version ( A ) we can see the decrease in Co(acac), concen-tration in Figure 2.A CoII compound [probably Co(acac),] can be observedi n the solution after a certain extent of conversion. Thesolution becomes turbid owing to the formation of ;tprecipitate, which turned out to be Co(OAc),. Completeanalysis of thc system with time is shown in Figure 3. Itfollows from Figure 3 that Co(acac), is an intermediate ofthe oxidation in thc course of which the ligands are oxidizedstepwise and the initial Co(acac), is reduced to Co(OAc),Being insolublc, the hydrate of the latter precipitated.The hydrate water arises from the oxidation of theligand~.~-' The initial rate of oxidation is proportional tothe initial concentration of Co(acac),.U 2 4 6 8 10Oxidation in chlorobenzene at 120 "C of A, Co(OAc),;Time / hFIGURE 3R, Co(acac),; C, Co(acac),; .--- , calculated curvealthough the temperature region studied was rathernarrow (20 K) since below 100 "C the reaction was veryslow and above 120 "C the loss was too high owing to thelow b.p. of the solvent. The activation energy was E =46-8 kcal mol-l.I_ I10'K ITThe log of thc initial ratcs of the Co(acac), oxidationagainst reciprocal temperature in chlorobenzenc : [~'~(acac),],I;IGURB 4- 1 Y 10-3~'The mechanism ( 11)-( 15) has been assumed for theoxidation. This mechanism represents the sequence oftransformation Co(acac), --+ Co(acac), + Co(OAc),.The esistence of adductx resembling the comple468Co(acac), - - - 0, has been proved so far only for nitrogen-containing ligands though in oxidations i t was assumedearlier.20y 21Co(acac), -1- 0, -% ~o(acac), -+- a c a c ~ , .(-o(acac), Co(acac) 2 -I- (acac) 2 0 2(11)kl,( 12)acacO,*0, + stable products (13)acacO,.h’, tCo(acac), 4- 0, -, Co(acac), * - - 0, (14)Co(acac), - - 0, Co(OAc), -t stable products (15)Since in the oxidations of hydrocarbons peroxy-radicalsplay an important role their effect exerted on the oxidationof Co(acac), has been also investigated.In order togenerate peroxy-radicals the azobisisobutyronitrile initiatoro/ I I { I OLhM 0 120 ! 150 180Time / minF r G c K E 5 Oxidation of Co(acac), in chlorobeiizene at 100 “C;introduction of azobisiscbutyronitrile marked by an arroiv ;A, CG(OAC),; B, Co(acac),; C, Co(acac),was used a t temperatures where the oxidation rate of thechelate can he neglected compared with the decompositionrate of the initiator.Results are shown in Figure 5 ,whencc i t can be established that peroxy-radicals markedlyaccelerate Co(acac), consumption and the formation of bothCo(acac), and Co(OAc),.1 hese experiments support the existence of step (12)according to which peroxy-type radicals are capable ofreducing Co(acac), to Co(acac),.The higher overall oxidation rates of Co(acac) , comparedwith its decomposition rates indicate that step (1 1) is fastcompared with (2).Accordingly, the differential equationsE. T. Denisov and N. M. Emanuel, Zkw. 3:. Khim., 1956,r .*O pu’. TJri, Xature, 1956, 177, 1177.30, 2499.J.C.S. Dalton(16)-(18) can be deduced. If i n the initial stages of__ - K,,[Co (acac) ,] LO,] --d[Co (acac) ,]tlt-- ._h,,~Co(acac),~~acacO,~] (16)k,,[C~(acac)~]~acacO~~] - K,,h,,[Co(acac),]~0,] (17)oxidation, the consumption of radicals due to recombinationis neglected the equations can be reduced to (19)-(21).d[Co( acac) 3] __ _ _dtd[Co(acac) ,]dt- ___. -tl [Co( OAc) ,]d t2h,,[Co (acac) ,j -I<, ,A 15 iC o ( ac a c ) ?] [ 0 ,] ( 2 0)K,,K,,[Co(acac) ,] [O,] (21)Xfter integration we obtain equations (22)-(24). Froin[Co(acac),ll = ;Co(acac),], exp (- %K,,[O,]t) (22)[Co(acac),j, 2kll [Co(acac),lt = 2kll - Kl,tkl,(exp (- K14h15[02]t) - esp ( - 2kll[O,]t)) (23)Kl4A 15esp ( - E<1,h,5[0,]t)exp+ 2h,, - K14h15[Co (OAc) ,I1 = iCo( acac),],(- 2k11[02]t) - 2kll2k11 - K14k15thc values obtained for k,,120 = 2-08 Y and(h714k15)120 = 2-54 x 10-2 1 mol-1 s-1 the calculated curvesof concentrations against reaction tinie shown in Figures 2and 3 agree well with the experimental points (solubility ofoxygen in clilorobenzene at the temperature of the experi-ments was taken as 0.77 x 10-2sr).If the oxidation of thc chelate procceds in oxidizablesol\rents capable of yielding liO,.-tj~pe radicals, dependingon the concentration of the latter, reaction (12) and/or(12’) should be taken into account. Naturally when thisk,lfCo(acnc), + 110,..-w Co(acac), + 1COOacac (12’)occurs reaction ( 1 3) and/or reaction ( 13’) i d 1 be the rate-kist“KO,* RI)_ 0, -{- stable products (13’)controlling termination. From Figure 5 i t seems verylikely that the sudden drop in the concentration of Co(acac),after the introduction of azobisisobnt?ironitrile is the directconsequence of reaction ( 12’). ’l‘herefore an approximatecalculation of k,,, can be carried out through equations(2t5)-(27). Thc following values ivcre used: e -- 0.8;tl [Co(acac) ,]d t I_-- - ii,,,!Co(acac),iITCO,.] ( 2 6 1974 469/ij"" = 1-24 10 3 5-1; and A,,! = 1.6 x 107 1 niol-l s - ~ .The corresponding value of the rate constant is given byequation (28) a t 120 "C.h , , ~ 1 x lo3 1 mol-l s-l (28)A ztloxidafion of Co(acac) ,.-The initial concentration oftlic chelate was varied between 1 x lop4 and 1 x 1 0 - 2 ~ ~while the reaction temperature changed between 80 and120 "C.The flow rate of oxygen was 6-18 1 h-1 dependingon the catalyst concentration.The originally pink solution became first colourless thengrcen. According to analysis ( A ) Co(acac), was formed.'I'lie kinetic curves are shown in Figure 6. However, thedeficicncy of cobalt in the material balance indicates thatan unknown compound was formed during the oxidation.Analysis ( B ) gave the following results: the unknowncompound is a water-soluble, greenish material containingCo711. It can oxidize instantly KI in acidic solution toiodine. Its aqueous solution can be reduced with hydrazinehydrate even a t low temperatures accompanied by thesimultaneous disappearance of its greenish colour.Wesucceeded in precipitating this compound from the oxidationmixture, b u t the purity of the solid sample was not satis-factory and it decomposed a t room temperature (smellingof acetylacetone) . 1.r. spectroscopy 22 indicated the pre-sencc of CO'~', acetylacetone, and acetate anion groups.Based on the similarity to the ccjmpounds reported intlie literature 23-29 and because of the lack of the OH groupit was assumed that its composition was Co(acac) (OAc),.111 order to prepare such a compound we oxidized Co(OAc),SF \Time / minFIGURE 6 Oxidation of Co(acac), in chlorobenzene a t 100 "C;[Co(acac),], = G x 1 0 - 4 ~ ; A, Co(OAc),; B, Co(acac),; C ,Co(acac), ; the broken line represents the deficiency in cobaltin chlorobenzene with H,O, in the presence of acetyl-acetone. The i.r.spectrum of the freshly prepared com-pound was identical with that of the compound precipitatedfrom the oxidation.22 S. Holly, personal communication.23 J . Minczmvski and M. Poronicka, Chem. analit., 1964, 9,** J . Budesinskv, J. Dolezal, B. Sramkova, and J. Zyka,25 C . E. Pricker and L. J. Loefler, Analyt. Clzenz., 1955, 27,26 J . A. Sharp and Ll. G. White, J. Chem. Soc., 1952, 110.?' E. Koubek and J . 0. Edwards, J . Inovg. Nitcleav ChewJ.,28 R. G. D u r r a t , J. Clzeun. SOC., 1905, 87, 1781.785, 947.Micvochem. J . , 1971, 16, 121.1419.1063, 25, 1401.The order of the reaction concerning the consumption ofCo(acac), has been calculated from the rate maxima and1 O L4L \ I L- x0 30 60 9O"lOO 120 140 160 180Oxidation of Co(acac), in chlorobenzenc a t 80 "C;introduction of azobisisobutyronitrile marked by an arrow ;A, Co(OAc),; B, Co(acac),; C, Co(acac),; D, Co(acac)(OAc),found to be 2 with respect to the initial CoT1 conccntration.The activation energy was 16.5 kcal mol-I.It should be noted that Co(acac), formation could beobserved during oxidation and its concentration was pro-portional to the initial concentration of Co(acac),.Peroxy-radicals generated in the system [containingCo(acac), and oxygen] yielded curves shown in Figure 7.The radicals were generated a t the 90th minute of the re-action introducing azobisisobutyronitrile initiator.Fromthis time on, the decrease in Co(acac), concentration wasrather steep while the decrease in the concentration ofCo(acac) (OAc), was less marked. There was no immediatechange as might have been expected in the amount ofCo(acac),; later, however, a slight decrease could beobserved. It may be assumed that the expected decreasehas been compensated by its formation from Co(acac),under the influence of the radicals.Time / m i nFIGURE 7DISCUSSIONOur results yield the following picture for the oxidationof Co(acac),. The ligands of the Co(acac), are oxidizedstepwise and transformed into Co(OAc),. The inter-mediate of the oxidation is presumably Co(acac) (OAc),.Taking into account the second order of the oxidationand the corresponding literature we can29 E.G. V. Percival and W. Wardlaw, J. Chem. SOC., 1929,2628.30 R. G. Wilkins, Adv. Chenz. Sev., 1971, 100, 111.31 G. L. Goodman, H. G. Hecht, and J. A. Wil, Adv. Chem.32 M. Nakai, S. Ssto, and Y . Fujimura, Chubzc Kogyo Daigalzzt33 Y . Fujimura and T. Mitsui, Chubu Kogyo Daigakit liyo,34 D. Bekoraglu, B. Erdem, and S. Fallab, Istaxbd Unid.Sev., 1962, 36, 90.Kyo, 1969, 5, 1.51.19G9, 6, 81.FenFak. Mecmussi, Sev. C , 1964, 29, 16470 J.C.S. Daltonassume the network of Scheme 1. This scheme seeinsto be consistent with our results ; verification would,Co(acac), @. Co,jacac), - - . 0, + Co(acac)(OAc), \ Co (acac) "O(i )Ak)zSCHEME 1however, require further experiments. Further it canbe applied for the oxidation of Co(acac), if completedas in Scheme 2.Co(acac) 3 c o (( ) Ac) ,SCHEMX 2Co(acac), 12 Co,(acac), . . . 0,Co(acac)(@,4~)~This means that the compound Co(acac)(OAc),assumed should also be formed during the oxidation ofCo(acac),. In order to support this suggestion analysis(B) was used for the oxidation of Co(acac), giving resultsshown in Figure 8. The intermediate can be observed,though in small quantity.This investigation is part of our studies in thefield of catalytic oxidation of hydrocarbons. We cannow state that the transformation of a catalyst cannotbe considered exclusively as interaction with the parenthydrocarbon and its derivatives. Its decompositionand autoxidation could play a role in the course ofhydrocarbon oxidation and this role is even more pro-nounced in their interaction with peroxy-radicals.'"r 1L--I: 12-\ - 0 *- 10-si8-6 -- 20- 18- 16-14 - 0b2-12 5110 30 50Time / minFIGURE 8 Oxidation of Co(acac), i n chlorobenzene at 120 "C:[C~(acac)~], = 20 x IO-*M; A, Co(OAc),; B, Co(acac),;C, Co(acac),; D, Co(acac)(OAc),; E, Co(OAc), + Co(acac), + Co(acac) ( 0 . 4 ~ ) ~We thank Dr. JA. SimAndy for discussions and Dr. S.[3/342 Kpcciwd, 14th February, 19731Holly for help with i.r. spectra
ISSN:1477-9226
DOI:10.1039/DT9740000465
出版商:RSC
年代:1974
数据来源: RSC