Chemistry

MOLECULAR CATALYSIS
ELSEVIER Journal of MolecularCatalysis93 ( 1994) 289-304

JOURNAL OF

Infrared observation of the chemical consequences of cobalt catalyst produced in mixed solutions of Al(Et), and Co(acac),
J. Barrault *, M Blanchard, A Derouault, M. Ksibi, M. I. Zaki
Recewed 21 September 1993, revised 16 February 1994, accepted 3 June 1994

1

L.aboratowe de Catalyse, URA-CNRS 350, ESIP, 40 avenue Recteur Pmeau, 86022 Pomers Cedex, France

Abstract

Cobalt catalysts obtamed from the reduction of Co(acac)* with Al(Et)3 have heen studied by mfrared spectroscopy (m the selective hydrogenation of multlfunctlonal compounds) These solids prepared m situ were m suspension m a hqmd nuxture containing solvent and reagent In order to obtain mformatlon on the preparation and the composltlon of the catalyst we carned out an m-IR charactenzahon usmg a special device warrantmg sample manipulation m ar-free atmosphere At room temperature there were instantaneous and reductive hgand-exchange between Al(Et), and Co( acac), and formatlon of Coo particles, Coo soluble complexes and Al( Et),( acac) The mulhstep process may be mltlated through the formation of a donor-acceptor complex ( ( acac) Co( acac) + Al( Et) 3) The presence of CO in the gas phase ( H,) when heating the reaction nuxture up to 180°C enhances the reduction of cobalt and probes Coo m Qfferent coordmahon symmetnes Some of the Coo species could be surrounded with cobalt alkoxlde species and alummmm acetylacetonate
Keywords Acetylacetonate complexes, Cobalt, FT-IR spectroscopy, Reduction, Tnethylalummmm

1. Introduction Ziegler-type cobalt catalysts are highly active and selective m hquld phase polymenzatlon and hydrogenation processes of both mdustr& and technological importance Ohgomenzatlon of butadlene [ 11, copolymenzatlon of vmyl acetate with acrylomtnle [ 21, and synthesis of alkenes and alcohols via hydrogenation of CO (Fischer-Tropsch) [ 3,4] are Important examples of such processes, mvolvmg
* Corresponding author ’ On leave from Umverslty El Muna, Egypt
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~ls.-v~erFclence

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cobalt catalysts Moreover durmg the last decade, the catalysts have been found active m the selective hydrogenation of mtnle [5] and m the formation of fine chemicals via hydrogenation of (Y, P-unsaturated carbonyl denvatlves [ 63 The catalysts are often obtamed from the reaction of cobalt acetylacetonate [Co(acac), [7] orCo(acac), [8]]wlthalummmmtnalkyl [Al(R),] manorgamc solvent Reductive hgand-exchange reactions occur immediately at room temperature, whereby a range of soluble metal complexes (of Co, Al, and possibly CoAl), Coo metal particles and free hydrocarbons may be produced [ 8,9] The nature of these complexes are largely controlled by differences m preparation [ 8,9], VIZ , the Al/Co ratio, the sequence of reagent mixing, the nature of the alkyl group of Al(R) 3, the mltlal oxldatlon state of Co, and the nature of the solvent An additional parameter 1s high temperature treatment often necessitated by hydrogenation processes [ 3,4] Many studies, dealing with the catalyst, have been focused on the activity and selectivity as a function of preparation method (e g , [ 21) In contrast, few studies have tackled the formation and nature of the catalyst active species [ 81. The present mvestlgatlon alms basically to charactenze the chemical reactions occurrmg from the mlxmg of solutions of Co(acac), and AI(E and the formation of catalyst, used for the selective hydrogenation of @-unsaturated carbonyl denvatlves [ 5,6] It implements the potential of FT.-IR spectroscopy, m resolving the chemical complexity inherent to the preparation medium [ 8,9] This approach has been motivated by previous IR studies of analogous Ziegler-type catalysts [ 10,111 Moreover, the mvestlgatron adopted recommended techniques and condltlons so as to warrant sample mampulatlon and measurements m air-free atmosphere [ 121

2. Experimental 2 I Materials The cobalt acetylacetonate (Co(acac),, acac = C5H802; 98 wt%-Co) alummmm tnethyl ( Al( Et) 3, Et = C2H5, 93 wt%-Al), acetylacetonate (Al( acac) 3, 98 wt%Al) and lsopropoxlde ( Al( 1Pr)3, 1Pr= C3H70) were Merck products Co( acac) 2 and Al( acac)3 was dned at lo-’ Torr and 55°C for 8 h, pnor to application The solvents used were benzene (Bz), (99% Prolabo) for low-temperature handling, and dodecane (Dod ) , (99% Interchlm) for high-temperature treatments Benzene was distilled (under Nz) m the presence of sodium metal and benzophenone, according to a standard procedure [ 131 2 2 Co(acac), reductzon 8 71 g of dned Co(acac), were dissolved m 220 ml of dned benzene at room temperature (RT) , under argon atmosphere After coolmg down to 6”C, a 5 ml

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portion of Al( Et) 3 (Al/Co = 1) was added by means of an ar-tight syrmge. The colour darkened immediately and a suspension of fine particles was formed A 60 ml portion of dodecane was added and benzene was eliminated by dlstlllatlon at 60°C under hydrogen A slurry was obtamed which was then transferred mto an all-Pyrex glass reactor and the temperature was rased slowly up to 18OOC under Hz The slurry was mamtamed at this temperature for 2 h pnor to any further change In certain instances, reduction condltlons were vaned the Al/Co ratio (0 5-l 5), the reduction temperature (RT-180°C) and duration (5 mm, 18 h) In order to monitor changes m the nature of the species formed durmg this preparation, we also studied the adsorption-reduction of carbon monoxide diluted m hydrogen (25-30 ~01%) at atmosphenc pressure 2 3 Samplmg procedures and tools for IR analysis Recommendations and techniques put forward by De Llefde Meyer et al [ 121 for IR samplmg under absolute au-free condltlons were stnctly considered A SPECACIR-cell of 0 025 mm thickness, equipped with KBr wmdows, was placed mslde an evacuable desiccator, together with a small beaker. The desiccator was equipped with two ground onfices terminated with greaseless taps, as well as fittmgs necessary for connection to a gas/vacuum lme The desiccator and the fittings were subjected to three successive cycles of evacuation-Ar flushing Finally, an Arpressure was mamtamed to avoid readmlsslon of air The test sample was then delivered mto the cell by means of a dry, hypodermic synnge. For the sake of cautiousness, the very first drops of the sample were dispensed mto the beaker With the help of special tongs, the cell ports were blocked with Ar-flushed Teflon corks Eventually, the cell was transferred, m au-, mto the sample compartment of the IR spectrophotometer (vlde mfra) , which was contmuously bemg purged with dry Nz 2 4. hfrared spectroscopy IR absorption spectra were taken, over the frequency range from 400 to 4800 cm-‘, using a N,-purged model 1OMX Nlcolet FT-spectrophotometer equipped with a data acqulsltlon and handling system All spectra were taken at beam temperature, 1 e close to RT, and observed frequencies were accurate to wlthm f 2 cm-* The chemical complexity of test samples was resolved by means of difference spectra These were obtamed by absorption subtraction of spectra recorded under identical spectroscopic condltlons The subtraction was faclhtated by the interfaced data handlmg system It 1s worth reporting, that gas phase components m the test samples were undetectable, for the very short optical pathlength of the cell employed (0 025 mm thick)

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2.5. Gas chromatography Gases formed durmg the reaction of Co( acac) 2 which Al( Et) 3 were sampled and analyzed with a 3300 Varmn gas chromatograph equipped with a 25 m Chrompack ISG cap&.ry column

3. Results and discussion 3 1 Chemical consequences of mzxzng Al(Et), and Co(acac), at room temperature 3 I 1 As afunctzon of the AVCo ratzo Fig 1 compares IR spectra of benzene solutions of Al( Et)3 and Co( acac)Z at vmous Al/Co ratios, namely at Al/Co of 0 5, 1 0 and 1 5. The figure also insets the spectrum taken from Co( acac)2 solution prror to addltlon of Al( Et)3, 1 e , at Al/Co = 0 0 The solvent contnbutlon was almost completely eliminated by subtraction Frequencies of charactenstlc absorptlons exhibited by separate solutions of Co ( acac) 2 and Al (Et) 3 below 2000 cm- ‘, are summarrzed m Table 1. The table also summarrzes frequencies of absorptlons observed at < 2000 cm- ’ for the reference compounds of Al( acac) 3, Al( Et) *( acac) and Al( 1Pr) 3 Fig 1 shows that the addition of Al(Et)3 to Co( acac)Z (Al/Co = 0.5) induced some detectable changes m the IR spectrum of Co(acac), The most important changes, are the emergence of new absorptlons at 1533,1289,1027,939, and 490 cm-’ Moreover, the absorption at 1401 cm-’ splits into two strongly overlappmg but weaker absorptlons at 1410 and 1391 cm-’ On the other hand, some of the mltlal absorptlons, due to Co(acac),, weaken slgmficantly (e.g , at 1519, 1260, 924 cm- ’ ) , whereas others undergo shght frequency shifts (e g , from 1591 to 1596 cm-l, from 1196 to 1194 cm-‘, and from 420 to 425 cm- ‘) and grow stronger In terms of the reference data compiled m Table 1 and other reports [ 151, these changes show that Al( Et)3 1s transformed mto Al( acac)s species The newly emerging absorptlons at 1533, 1289, and 490 cm- ’ are dlagnostlc to the formation of Al(acac)3 (Table 1) The 1533 and 1289 cm-’ absorptlons seem (Fig 1) to be high-frequency modlficatlons of Co( acac) 2 absorptlons at 15 19 ( Y(CO) + Y(CX) ) and 1260 cm- ’ ( Y(C-C) + v(C-CH~) ), respectively This implies that acetylacetonate hgands are now coordmated to a stronger Lewis acid centre (Al) than Co [ 161 The emergence of the 490 cm- ’ absorption mdlcates stretching vlbratlons of Al-O bonds [ 81, which can be correlated with the shght hlgh-freto 425 cm- ’ (out of plane quency shift of the 420 cm- ’ absorption ( v(Co-0)) bending of Al(acac),) [ 8,151

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fl
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Table 1 Frequency values ( f 2 cmcompounds m benzene Co(acac), 1609

’ ) of charactenstlc IR absorphons of the catalyst precursorsand some.other reference

1591 1519

1401

1364

1260

1196

1020

924

420

Al(acac), 1620 1600

1592 1586 1543 1520

1450 1416 1405

1385 1361

1289 1276

1190 1175

1022 1016

991 950 939 935

777 771

596 577

490 425

AI(Et)3 1480 1440 1401 Al(Et),(acac) a 1592 1556 1530 1480 1375 1354 1295 1289 1227 1193 1020 986 972 946 939 783 775 770 700 670 650 620 580 560 1385 1376 1228 1196 986 954 921 650 620 540 530 477 470

Al(O’=(CW,), 1525 1389 1374 1360 1347 1332 1261 1208 1182 1170 1137 1125 1120 950 860 835 700

’ Obtamed on addmg excess of Al(Et)3 to Al(acac)3 solution at room temperature,accordmg to Km11 and Naegelle [ 141

Upon mcreasmg the amount of Al(Et),, (Al/Co = 1 5), absorptlons due to Co( acac), are undetectable, absorptlons of Al(acac), weaken, and a new set of absorptlons emerges [at 1450,1295,1227,1160,988,951 and 905 cm-‘] These results imply that the hgands of Co( acac) 2 have been largely exchanged for, and that new species have been formed most hkely at the expense of Al( acac) 3. On basis of the reference data exhibited m Table 1, the newly emerging absorptlons may account for the formatlon of Al(Et),( acac) It has been found previously [14],thatmthepresenceofA1(Et),,Al(acac)3~stransformedmtoAl(Et)(acac), and at finally mto Al( Et)*( acac) This result was confirmed m a specific reaction of Al( Et)3 with Al( acac)S followed by IR On the other hand formatlon of stable Co ( Et) ( acac ) or Co ( Et) 2 species can be ruled out m view of the data reported and discussed elsewhere [ 171 GC analysis of gas phase products resultmg from mteractlons between Al( Et)3 and Co( acac)* at room temperature and Al/Co = 10 and 1 5, are summmzed m Table 2 It 1s clear that m both cases ethane 1s the major product (mole fraction > 96%) This result implies that a major dlsproportlonatlon (hydrogen-abstrac-

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Table 2 Analysis results (% mole fraction) of the gas phase products released on nuxmg benzene solutions of Co( acac), and Al(Et)3 at room temperature and under H, atmosphere, as a function of the Al/Co rat10 and the time of rmxmg Al/Co Time/mm
Gas

10 5

60

15 5

60

C,
c*= C2 c,= C3 -X4 =c,

01 06 96 2 ;1 27

05 919 02 15 E

03 01 98 7 E E 09

03 98 4 01 11 E

tlon) reaction amongst the ethyl groups, occurs accordmg to the followmg equation,

2C2H5-+CzHg+C2H4

(1)

The fact that the C2HJC2H6 ratio m the gas phase released 1s close to zero (Table 2) may have two confllctmg lmphcatlons (1) ethane 1s not formed via reaction ( 1) , or (11) it is, but the CzH4 1s dissolved m benzene and/or bound to Co Quantitative calculations based on the mltlal concentration of Al(Et), and the GC analysis results, revealed that only 30% of the ethyl groups were released as CzH6 molecules, I e , one group out of three The remammg ethyl are mostly retamed by Al(Et),(acac) species, whereas the released ethyl are obtamed from the reduction of cobalt This result indicates that 50-60% of the cobalt 1s reduced to the metallic state and that there are two types of cobalt contammg species. reduced ( Coo-contammg) and unreduced (Co”-contammg) species On the other hand, the molar ratio of C2H,/C2H, m the gas phase products would Indicate, if reaction ( 1) was the actual route to C& that C,H, 1s largely retained m the reaction medium The solublhty m benzene cannot compensate fully for the expected amount of C2H4 Therefore, the results would imply the mvolvement of C2H4 m formation of the soluble cobalt complexes According to earher reports [ 191, C,H, coordmates to metal atoms via a 67~ bondmg system. The &ondmg becomes more important as the oxldatlon state of the metal becomes lower, smce the metal must back-donate more electrons to olefin It has also been found [ 19,201 that most of the IR absorptlons of free C2H, undergo significant frequency shifts upon bonding to metal atoms These absorptlonsoccurat 1623 (y(C=C)), 1342 (6, (CH,)), 1236and810 (p, (CH,)), 1007 (p, (CH,)) and943 cm-’ ( pW( CH2) ) [ 19,201 Upon bonding to Pt, for example, these absorptlons shift to 1526, 1418, 1251,844,730 and 1023 cm-‘, respectively [20] If they were occurrmg, all of these absorptlons, except for the ones at 844 and 730 cm-‘, would have been completely shrouded by absorptlons of Al( acac) 3

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and Al (Et) 2(acac) (Fig 1) A thorough mspectlon of the frequency range at 90& 500 cm- ’ (not shown m Fig 1) revealed the display of a sharp absorption at 860 cm - ’ and a broad one at 850-760 cm- ’ m the spectrum taken followmg the reaction at Al/Co = 1 5, notwlthstandmg that such an absorption might be related to vlbratlons of olefimc species bound to Coo atoms, and mdlcated the formatlon of Co( C,H,), species So, IR observations accumulated m the present study can help to make concrete conclusions as to the nature of the soluble Coo complexes 3 1 2 As a function of time In order to check on the importance of the reaction duration, IR spectra were taken followmg the elapse of mcreasmg penods (from 25 mm to 18 h) of mlxmg benzene solutions of Al( Et) 3 and Co( acac)* (Al/Co = 1 0) at room temperature Spectra obtained at 1320-l 160 cm- ’ are represented m Fig 2. The spectra show clearly that the chemical consequences of mixing the reagents, 1.e , the appearance

,

I

I

1340

1300

1260

1220 (cm-‘)

1180

WAVlCNUMBERS

Ftg 2 IR spectra taken from a 1 1 nuxed solution of Al(Et)3 and Co(acac)t, at room temperature Spectra of (b) t= 25 mm, (c) t= 18 separate solutions of the precursors are mset for compamtwe purposes (a) Co(acac),, h, (d) Al(acac),

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of charactenstlc absorptlons of Al(acac),, occur shortly after mlxmg at room temperature. The same was Indicated by the GC analysis of gas phase products, which showed the completion of maximum production of &I-& (Table 2) after the elapse of 3 mm of mlxmg These results indicate, m line with earlier reports [ 11, that reductive hgand-exchange mteractlons mvolvmg Al( Et) 3 and Co( acac)z are instantaneous 3 2 Chemical consequences of heatmg mzxed solutions of Al(Et), and Co(acac), at high temperatures The synthesis of the catalytic active species m the Al( Et)J and Co( acac)z system imposes the followmg alterations (see Expenmental) replacement of benzene by dodecane, swltchmg the gas atmosphere from Ar (or N,) mto Hz and heating up to 180°C Therefore, expenments were designed to let IR observe chemical changes accompanymg heating the catalyst precursor system to various mtermedlate temperatures up to 180°C This was accomphshed by taking spectra after heatmg at the desired temperature (60, 100, 120, 160 and 180°C) for 1 to 4 h and subsequent coolmg to room temperature The resultmg spectra were compared with the spectra obtamed pnor to heatmg This compmson revealed that differences appeared only at T > 100°C Therefore, Fig 3 only compares the spectra obtamed before and after heating a mixture of Al(Et), and Co(acac)z (Al/Co = 10) at 180°C. The companson reveals that charactenstlc absorptlons displayed for the initial reaction products at room temperature are very much weakened on heating up to 180°C. Alternatively, several new absorptlons emerged Some of these absorptlons could be attibuted to metal-CJ& (e g , at 2966,2929,1525,844 and 738 cm- ’ [ 201) and some others to alkoxlde-type species (e g , at 2955, 2916, 2910, 1525, 1465, 1369,1340,1261,1240,1177-1138,955,844and770cm-’ [21,22]).Absorptlon frequencies complied m Table 1 for Al ( IPI-)3 lend support to the assignment given to the latter set of absorptlons It 1s worth notmg, however, that absorptlons dlsplayed m spectrum (b) of Fig 3 (1 e , followmg heating at lSO”C), part~ularly those at 1650-1500 cm-‘, still account for metal-acac species Nevertheless, they cannot support that these species are associated with Co( acac) 2 or Al (acac) 3 The acac species are presumably conmbutmg to a sort of mixed Co-Al complexes, or to mixed hgand complexes of Coo In order to identify reasons behind these remarkable, heat induced changes m the chemical composltlon of the reaction medium, the followmg expenment was camed out A 1 5 1 mixed solution of Al(Et)3 and Al( acac)3 was heated, under H2 at 180°C for 1 h, and then cooled to room temperature. A spectrum taken thereof 1s compared, m Fig 4, with that obtamed from the same solution, but pnor to heating The comparison shows that Al(Et),( acac) decomposed (ehmmatlon of the charactenstlc absorptlons at 1295 and 1227 cm- ‘) , whereas Al (acac) 3persisted (absorptlons at 1289 and 1220 cm - ’ )

298

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19ZI t

-

KlNVa2lrOSfD’

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299

1680

1650

1620

I590

1560

1530

Is00 1340 (cm-l)

13Ou

1260

I220

1190

WAVENUMBEBS

Fig 4 IR spectra exlublted by a 1 5 1 muted solution of Al( Et)3 and Al( acac)3 (a) before and (b) after heating at 180°C for 60 mm, under H, atmosphere

GC analysis of gaseous products obtained after heatmg a 1 1 mixture of Al( Et) 3 and Co( acac) 2 at various temperatures up to 18O”C,are given m Table 3 The results indicate that the proportion of C2H6,the major product at room temperature, decreases (from 97 9 to 69 3%) with temperature mcrease In the meantime, C1 and C&I, products increase. These results reveal the occurrence of catalytic hydrogenolysls ( C2 + C, ) and ollgomenzatlon ( C2 + C3-C5) m the presence of reduced
Table 3 Analysis results (% mole fraction) of the gas phase products released on nuxmg 1 1 dodecane solutions of Co(acac), and Al(Et)3 under H, atmosphere, as a fun&on of the temperature ’ TemperaturePC
Gas

25

120

180

G
c*= C2 c3= C3 ZG SC,

05 919 02 15 E

19 3 12 4 12 61 07

130 10 69 3 19 28 68 52

’ After elapse of 60 mm at 25”C, but 120 mm at each of the higher temperatures

300

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cobalt species and H2 Thus, changes occurrmg m the gas phase composltlon are m line with those taking place m the liquid phase, and confirm the catalytic mterventlon of reduced cobalt species 3 3 Carbonylatlon and nature of metal centres

3 3 I IR spectroscopy After mixing Al( Et) 3 and Co( acac)2 at Al/Co = 1 and room temperature, a mixture of CO and Hz (30% CO) was bubbled through the reactlon medium at atmosphenc pressure IR spectra were then taken as a function of time-on-stream ( 15-270 mm) at room temperature Subsequently, the temperature was raked up to 8O”C, while the gas mixture was flowed. IR spectra were also taken as a function of time-on-stream In a second set of expenments A1(Et)3 and Co(acac), (Al/ Co = 1) were mixed at room temperature under H2 atmosphere and the temperature was increased up to 180°C At each step, a (CO, H,) mixture (30% CO) was contacted with the solution and CO spectra were taken at room temperature and 80°C In the frequency range 1750-2000 cm-’ correspondmg to V(CO) vlbratlons of chemlsorbed carbonyl groups [ 23,251 we observed slgmficant changes (Fig 5) The room temperature carbonylatlon of the reaction mixture pretreated at room temperature results m a spectrum (a, Fig 5) dlsplaymg weak V(CO) absorptlons at 1991,195O and 1790 cm- ’ The former two absorptlons are assignable to termmal carbonyls (M-CO) coordmated to two different types of metal atoms, whereas the latter absorption 1s mdlcatlve of multlcentered carbonyls ( (M),-CO) [ 23,251. When the carbonylatlon was camed out at 80°C (spectra c), the termmal-CO absorptlons mtenslfied considerably, with the maximum bemg shlfted to 1996 cm-’ In the meantime, the multlcentered carbonyls transformed mto bndgmgCO, ( (M) &O) , as mdlcated by the high frequency shift to 1850 cm- ’ [ 23,251 Since the V(CO) of free CO molecules occurs at 2143 cm- ’ [ 231, the frequencies observed at room temperature and 80°C account for carbonyl groups coordinated to electron-nch sites with of back donating electrons to the 27~* anti-bondmg orbital of the CO molecule [24] These are most hkely associated with reduced cobalt (Co’) rather than with Alu’ or Co” Thus, the CO chemlsorptlon probes the exlstence of coordmatlvely unsaturated (cus) Coo atoms resultmg from the reductive hgand-exchange reactions referred to above The results also mdlcate that these reactions are further activated at the higher temperature of 80°C. The question that may be rased at this stage 1s whether the CO coordmatton centres are associated with Coo exposed on metallic particles or contamed m soluble complexed species The highest v(C0) frequency observed for the terminal-CO (at 1991 cm- * ) 1s slgmficantly less than that reported [ 231 for CO termmallybound to cus sites exposed on Coo particles (at 2095-2030 cm- ‘> This fact may Imply that the CO coordmatlon centres m the present case are more electron-nch (and hence more electron-donatmg) than those normally exposed on Coo metal

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301

co:8oT

I 0.t
I .

a)

1wc

“i /

I

2050

2ooo

1950

1900

1850

1800

1750

WAVENUMBZRS

(cm-‘)

Fig 5 IR spectra taken (at beam temperature) from a 1 1 nuxed solution of Al(Et), and Co(acac), pretreated ( 1) at room temperature under hydrogen followmg by a (CO, Hz) adsorption at room temperature (spectrum a) or at 80°C (spectrum c), (2) at 180°C under hydrogen followmg a (CO, Hz) adsorphon at rOOmtemperature (spectrum b) or at 80°C (spectrum d)

particles This IS what might be expected if the Coo IS mamtamed atomically dispersed by electron-nch hgands such as C&L+and alkoxldes, rather than the electron-wlthdrawmg alummmm alkyls [ 91 Carbonylatlon of the hydrogen-reduced reactlon nuxture at 180°C led to spectra b (room temperature) and d (80°C) shown m Fig 5 Both spectra are smnlar m

302

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showing CO absorptlons due to terminal and bndgmg carbonyls bound to complexed Coo atoms They also show that the HZ-treatment at 180°C improves considerably the amount of the Co coordmatlon centres In contrast to the room temperature carbonylatlon, the carbonylatlon at 80°C activates two different types of coordmatlon centres for termmal-CO hgands This 1s corroborated by the two absorptlons at 1992 and 1960 cm-’ (spectrum d, Fig. 5) These centres are most likely associated with complexed Coo atoms m different coordmatlon symmetnes. 3 4 Chromatographlc analysis of gaseous products Table 4 compares GC analysis of the gaseous products formed when heatmg a 1 1 mixture of Al( Et) 3 and Co( acac) 2,under a (CO + H,) atmosphere (25% CO) In compmson with the results obtamed when heatmg the same reagents under pure Hz atmosphere (Table 3)) one can find out the followmg CO-mduced effects. (1) C,H6 remams the dominant gas phase product (75-80%) at lOO-160°C (Table 4) but we observed m this expenment a very important formation of C4 hydrocarbons, (11)at 18O”C,the formation of &I-& 1svery drastically decreased (from 80 to 18%) to a proportion that indicates a much stronger suppression than m the absence of CO (from 98 to 69%) These results reveal that at lOO-16O”C,the presence of CO enhances the catalytic ohgomerlzatlon to higher hydrocarbons However at 18O”C, the presence of CO enhances the catalytic hydrogenolysls of C,I-& to CI-L, Of course, these effects are closely related to modlficatlons of catalytic species, Coo metal particles and soluble Coo complexed species It 1s obvious from Fig 5, that the presence of CO m the reducing atmosphere improves the reduction of cobalt at 180°C It also probes the avalablllty of at least two different types of v of Coo centres, followmg the reduction at 18O”C,as implied by the two UC0 IR absorptlons of terminal-CO at 1992 and 1960 cm- ’ (spectrum d, Fig 5) In contrast, one type of Coo centres, namely that responsible for the carbonyl absorption at 1996 cm- ’ (spectrum c, Fig. 5)) IS produced upon reduction at temperatures lower than 180°C
Table 4 Analysis results (% mole fraction) of the gas phase products released on nuxmg 1 1 dodecane solutions of Co( acac)* and Al( Et) 3 under Hz atmosphere (25% CO), asa function of the temperature * TemperatureP’C 120 180

GZiS G
c,= CZ c3= C3 z’c, -G

01
76 3 02 216 11

79 0 17 6 _ 06 27 12

’ After elapse of 60 mm at 25’C, but 120 mm at each of the higher temperatures

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303

It 1s worth mentlomng that the CO-probed coordmatron centres are those exclusively associated with soluble Coo complexed species This should not exclude the Coo particles from playing, both actively and selectively, a role m the catalytic events observed

4. Conclusion The followmg conclusions can be drawn from the above results ( 1) At room temperature, there are reductive hgand-exchange reactions between Al( Et)3 and Co( acac)Z and formation of Coo particles, Coo soluble complexes, Al( acac) 3, and Al( Et) 2( acac) m the hquld phase, &I-& bemg the mam gas phase product (2) The multistep transformation of reagent may be initiated through the formation of a donor-acceptor complex ( (acac) Co( acac) + Al( Et) 3) [ 8,9] mvolvmg the migration of an ethyl group from alummmm to cobalt [ 181 (3) The mam product (C&J m the gas phase produced durmg the reaction comes from an hydrogen-abstraction mechanism (2 CzHs + C& + C,H,) mvolvmg ethyl groups bound to alummmm and cobalt, m a donor-acceptor complex as (Et)Co(acac) +Al(Et)(acac), (4) The presence of CO m the gas phase (H,) when heating the reaction mixture up to 18O”C,enhances the reduction of cobalt and shows that Coo atoms are m different coordmatlon symmetnes wlthm soluble complexed species ( 5) These Coo species exhibit different activities and selectrvltles towards hydrocarbon (C,H,) hydrogenolysls (to C, ) and polymenzatlon (to C&T,), depending on the reaction temperature

Acknowledgements

M I 2 thanks the C N R S of France for a research assoclateshlp

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