improved heterogenized catalysts for selective propylene oligomerization to 2,3-dimethylbutenes...

Applied Catalysis A: General 207 (2001) 387–395

Improved heterogenized catalysts for selective propyleneoligomerization to 2,3-dimethylbutenes prepared by

oxidative addition of polymer-anchoredb-dithioacetylacetonateligands to nickel(0) complexes

Carlo Carlinia,∗, Mario Marchionnab, Renata Patrinib,Anna Maria Raspolli Gallettia, Glauco Sbranaa

a Dipartimento di Chimica e Chimica Industriale, University of Pisa, Via Risorgimento 35, I-56126 Pisa, Italyb Snamprogetti S.p.A., Via Maritano 26, I-20097 S. Donato Milanese, Milan, Italy

Received 7 December 1999; received in revised form 13 June 2000; accepted 15 June 2000

Abstract

A novel approach for obtaining heterogenized catalysts, via oxidative addition of polymer-boundb-dithioacetylacetonate(sacsac) ligands to nickel(0) complexes was reported and their performances were investigated in the propylene oligomeriza-tion, in terms of activity and selectivity to 2,3-dimethylbutenes (DMB). In particular, the effect of the nature of the ancillaryphosphine ligand as well as of the organoaluminium co-catalyst was studied. The influence of the addition to the reactionmedium of chlorinated promoters was also checked. The results indicated that the catalysts prepared by this route weremore active and selective towards DMB as compared with those previously prepared by an exchange reaction method. Whenbasic alkyl phosphines, such as tricyclohexylphosphine (PCy3) and triisopropylphosphine (PiPr3), and organoaluminiumco-catalysts with suitable Lewis acidity, such as Et2AlCl, were used, the resulting catalysts displayed a very high selectivity toDMB (∼80%) and an extremely high productivity (turnover frequency equal to 74,000 h−1), the latter being much higher ascompared with that previously obtained with the homogeneous counterparts. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Heterogenized nickel catalysts; Polymer-supportedb-dithioacetylacetonate nickel complexes; 2,3-Dimethylbutenes; Propyleneoligomerization; Trialkylphosphines; Organoaluminium co-catalysts

1. Introduction

Nickel(II) complexes (I ), characterized byb-dithioacetylacetonate (sacsac) as well as basic bulkyphosphine ligands (Chart 1) are known to affordin homogeneous phase extremely active catalystsfor the selective oligomerization of propylene to

∗ Corresponding author. Tel.:+39-50-918-222;fax: +39-50-918-260.E-mail address:[email protected] (C. Carlini).

2,3-dimethylbutenes (DMB), when activated by suit-able organoalumium co-catalysts [1,2].

The increased interest for DMB in the last yearsis justified by the fact that they are easily hydro-genated to 2,3-dimethylbutane, which in turn may besuccessfully applied as high octane gasoline blendingcomponent [3].

Very recently [4], we succeeded in anchoring sac-sac ligands on a macroporous styrene/divinylbenzeneresin (II ) and reacting its sodium salt (III ) withnickel(II) diphosphino dichloride complexes to

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0926-860X(00)00681-5

388 C. Carlini et al. / Applied Catalysis A: General 207 (2001) 387–395

Chart 1.

give the corresponding polymer-supported (sac-sac)(phosphino)nickel(II) chloride derivatives (IV ),which under propylene atmosphere and in the presenceof an organoaluminium compound were transformedin situ into the alkyl or hydride catalytic species (V)(Scheme 1, path a). This approach was adopted withthe aim to obtain heterogenized catalysts which maycombine the high activity and selectivity of the ho-mogeneous counterpart with the well known stabilityand recyclability of the heterogeneous systems.

Indeed, a detailed investigation of the above cata-lysts [5] allowed to conclude that they, under suitablereaction conditions and in the presence of phosphineligands having appropriate basicity and bulkynessas well as of an organoaluminium co-catalyst withproper Lewis acidity, show a higher activity and com-parable selectivity to DMB with respect to the cor-responding homogeneous complexesI , accompaniedby a relevant stability, typical of systems working inheterogeneous phase.

However, the above procedure for affording cata-lysts V presents the drawback that two reaction steps

Scheme 1.

are necessary for obtainingIII and IV precursorsfrom sacsac-resinII , each of them requiring isola-tion and washing of the reacted resin before the finaladdition of the organoaluminium activator. Recently,an alternative route for obtaining catalystsV wassuccessfully applied by us [4]. This consists in thedirect reaction of resinII with a Ni(0) precursor, suchas bis(1,5-cyclooctadiene)nickel [Ni(cod)2], in thepresence of a free phosphine ancillary ligand underpropylene atmosphere, followed by the addition ofthe organoaluminium co-catalyst to afford in situ theheterogenized alkyl (hydride) speciesV (Scheme 1,path b). In principle, analogously to what known forthe corresponding homogeneous [6] and heteroge-nized [7] catalysts based onb-diketonate ligands,V would be active in the propylene oligomerizationwithout any addition of co-catalyst. However, pre-liminary catalytic experiments [4] indicated that theorganoaluminium compound largely increases the ac-tivity, probably preventing the decomposition to themetal.

Taking into account that this preparative methodappears more practical and also flexible, due to thepossibility to easily change the free phosphine ligandas well as the P/Ni molar ratio, this paper deals withthe investigation in more details of the heterogenizednickel catalystsV, prepared via oxidative addition,with the aim to optimize their formulation as well asreaction conditions and hence improve their perfor-mances in terms of activity and selectivity to DMB

C. Carlini et al. / Applied Catalysis A: General 207 (2001) 387–395 389

as compared with the same catalysts prepared by ex-change reaction.

2. Experimental

2.1. Materials

Anhydrous chlorobenzene (Aldrich) andN,N-dimethylformamide (Fluka) were obtained by distilla-tion under dry argon of the commercial products andstored on molecular sieves (4 Å).

Anhydrous toluene (Baker) was obtained by distil-lation on K/Na alloy under dry argon and stored onmolecular sieves (4 Å).

Propylene (UCAR), having a purity higher than99.95%, was used as received.

Diethylaluminium chloride (Et2AlCl) (Aldrich) andmonoethylaluminium dichloride (EtAlCl2) (Aldrich),in toluene (1.8 M) andn-hexane (1.0 M) solution, re-spectively, were stored under dry argon and used asreceived.

Diethylaluminium iodure (Et2AlI) (Witco, neat)has been distilled under reduced pressure and storedunder nitrogen.

Triethylaluminium (Et3Al) (Aldrich) in toluene so-lution (1.9 M) was stored under dry argon and usedas received.

Methylaluminoxane (MAO) (Witco, 4.5 M intoluene solution) was stored under dry argon and usedas received.

A DOW-C macroporous resin (Dow Chemi-cal), consisting of cross-linked poly[styrene (St)-co-divinylbenzene (DVB)] beads containing 8.0 mol% ofDVB co-units and having a 65 m2 g−1 surface area,was used for functionalization reactions. The resin wasdried under vacuum for 12 h at 60◦C before the use.

Tricyclohexylphosphine (PCy3) (Aldrich), triiso-propylphosphine (PiPr3) (Fluka) were stored underargon at−30◦C and used as received.

Diisopropyl-tert-butyl-phosphine (iPr2PtBu), diiso-propyl-sec-butyl-phosphine (iPr2PsBu) and diisopro-pyl-n-butyl-phosphine (iPr2PnBu) were prepared in54, 76.4 and 88.3 mol% yield, respectively, by re-acting the commercialiPr2PCl with the correspond-ing butyl lithium, according to the literature [8]({1H} 31P-NMR: δ=+33.0, +15.85 and 2.85 ppm,in that order, in good agreement with the calculatedchemical shifts with respect 85% H3PO4, as exter-

nal standard [9]). Dicyclohexyl-tert-butyl-phosphine(Cy2PtBu) and isopropyl-di-tert-butyl-phosphine(iPrPtBu2) were prepared by the above procedurestarting from tBuLi and iPrLi with Cy2PCl andtBu2PCl, respectively, these properly prepared [10]from PCl3 and two equivalents of the correspondingGrignard reagent. ({1H} 31P-NMR: δ=+26.94 and59.3 ppm, respectively).

Ni(cod)2 was prepared in 60 mol% yield as previ-ously described [11].

2.1.1. Synthesis of the polymeric ligandII

DOW-C resin was chloromethylated, according tothe literature [12], to afford a 20 mol% content offunctionalized co-units, the resulting chloromethy-lated resin containing 6 wt.% Cl (1.7 mmol g−1). Thenit was reacted with Na(sacsac), as previously de-scribed [4], to give sacsac-resinII which revealed theabsence of chlorine and a sulphur content of 4.3 wt.%,corresponding to 0.67 mmol of sacsac ligands/g offunctionalized resin. The absence of chlorine in thereacted resin may be addressed to the competitivehydrogenolysis of chloromethyl groups by the ex-cess NaBH4 used to generate in situ Na(sacsac) from3,5-dimethyl-1,2-dithiolium iodide [4].

2.1.2. Catalysts preparationThe heterogenized nickel catalystsV were pre-

pared in situ according to the synthetic procedure [4]represented in Scheme 1, path b. This consists in theoxidative addition of sacsac-resinII , suspended inanhydrous toluene, to Ni(cod)2 under propylene at-mosphere and in the presence of the appropriate freephosphine. The above reaction was not performed inchlorobenzene, the medium used for the catalytic ex-periments, because it is known [13] that chlorinatedcompounds may give oxidative addition to Ni(0)complexes, thus competing with sacsac ligands. Fi-nally, after replacing toluene with chlorobenzene theappropriate organoaluminium co-catalyst was added.

2.2. Catalytic experiments

Catalytic batch experiments were performed ina 250 ml mechanically stirred Büchi glass reactor,equipped with a jacket circulating cooling fluid inorder to maintain the reaction temperature at−15◦C.In a typical experiment, toluene (20 ml), sacsac-resin


II (0.28 g; 0.18 mmol of sacsac moieties), Ni(cod)2(0.25 mmol) and PR3 (0.37 mmol) were introducedin the reactor, in that order, under dry argon at 0◦C.The reaction mixture was left under stirring for 2 hat that temperature, then the solution containing theexcess of phosphine and soluble nickel complexeswas siphoned off. The heterogenized catalystV waswashed several times with portions of anhydrousfresh toluene, under propylene atmosphere. Then,after removal of the washings, 20 ml of chloroben-zene were added. Subsequently, the temperature wasprogressively lowered to−15◦C and the appropriateamount of the suitable organoaluminium co-catalystwas added under propylene atmosphere. The reac-tor was finally pressurized with propylene and thepressure manually held at 3 atm for 1–2 h.

The reaction was stopped by degasing unreactedpropylene from the reactor through a trap cooled at−10◦C and the liquid products collected and weighted.Subsequently, the olefinic reaction products were sub-mitted to catalytic hydrogenation and the resultingparaffins analyzed by gas-chromatography (GC).

Olefins hydrogenation was carried out in a 125 mlstainless-steel rocking autoclave by using 0.5 g of coalsupported 5% palladium (Baker) for 10 ml of olefinicproducts. The autoclave was then charged with hydro-gen up to 100 atm and heated with an oil bath at 130◦Cfor 12 h.

The recovered heterogenized catalyst was analyzedfor the determination of nickel content. The obtainedresults indicated that in all cases the average amountof nickel anchored to the resin was about 10% of thesacsac polymer-bound ligands. The content of nickelwas also determined on the liquid solution of theoligomeric products. In all experiments the amount ofreleased nickel was below the detectable limit of themethod (1–2 ppm).

2.3. Analytical procedures

Hydrogenated oligomers analysis was performed byGC on a Hewlett Packard 5890 flame ionization chro-matograph equipped with a HP PONA 50 m capillarycolumn and an HP 3396 integrator.n-Heptane wasused as internal standard.

Sulphur and chlorine elemental analyses wereperformed at Eniricerche Laboratories (San DonatoMilanese). The determination of nickel content in

the heterogenized catalysts as well as in the liquidoligomerization products was achieved by atomicabsorption spectroscopy at Eniricerche Laboratories(San Donato Milanese).

2.4. Physicochemical measurements

FT-IR (Fourier transform infrared) spectra were car-ried out on KBr pressed pellets of the samples by usinga Perkin-Elmer 1750 spectrophotometer. The spectraldata were processed by a IRDM Perkin-Elmer soft-ware.

1H-, and {1H}-31P-NMR (nuclear magnetic reso-nance) spectra were performed by a Varian XL Gem-ini 200 spectrometer operating at 200 and 80.95 MHz,respectively, on samples in CDCl3 solution. Tetram-ethylsilane (TMS) and 85% H3PO4 were used as in-ternal and external standards, respectively.

3. Results and discussion

The performances in the propylene oligomerizationof the heterogenized (sacsac)(phosphino)nickel cata-lysts V, obtained via oxidative addition, have beenchecked by varying the nature of the basic phosphineligand in terms of bulkyness and initial P/Ni molarratio, type of organoaluminium co-catalyst, includingAl/Ni molar ratio, and by adding chlorinated promot-ers. This approach was adopted with the aim of opti-mizing the process in terms of activity and selectivityto DMB and comparing these results with those pre-viously reported [5] by means of heterogenized cata-lystsV prepared by the exchange reaction. The effectof some reactions parameters, such as temperature,propylene pressure and nature of the reaction mediumwere not examined here because previous studies byusing catalystsV, prepared via exchange reaction, sug-gested that the best catalytic performances were ob-tained by adopting quite low temperature (−15◦C), amedium pressure of olefin (3 atm) and chlorobenzeneas reaction medium [5].

3.1. Effect of the nature of the phosphine ligandand of the P/Ni molar ratio on the performances ofcatalystsV

It is well known that for homogeneousp-allyl-[14,15] and b-dithioketonate-nickel catalysts [1,2]


Table 1Propylene oligomerization by heterogenized catalystsV, obtained in situ from resinII , Ni(cod)2 and different PR3, in the presence ofEt2AlCl as co-catalysta,b

Entry PR3 Dimers (%) C6c (%) C9 (%) Yd (%) TFe (h−1)

Type P/Ni DMB MPf HEXg

1 PCy3 1.5 79.0 19.9 1.1 71.3 26.0 56.3 740002 PCy3 1.0 75.5 23.5 1.0 70.1 23.0 52.9 521003 PCy3 2.0 72.3 26.2 1.5 75.8 20.4 54.8 496004 PCy3 2.7 66.7 31.5 1.8 76.5 22.8 51.0 366005 tBuPCy2 1.5 32.2 54.9 12.9 90.3 8.2 29.1 66006 PiPr3 1.5 74.5 24.0 1.5 76.4 19.5 56.9 566007 PiPr3 1.0 70.5 28.0 1.5 75.5 23.0 53.2 356008 PiPr3 2.0 66.9 31.1 2.0 78.3 18.5 52.4 335009 tBuPiPr2 1.5 68.9 28.9 2.2 84.0 13.2 57.9 17600

10 sBuPiPr2 1.5 63.3 34.5 2.2 85.1 12.8 53.9 1640011 nBu2PiPr2 1.5 49.4 45.2 5.4 88.9 10.4 43.9 2810012 tBuPiPr 1.5 52.0 39.6 8.4 93.5 6.5 48.6 260013h – – – – – – – – 0.014i PCy3 1.5 – – – – – – 0.015j PCy3 1.5 44.3 49.6 6.1 94.7 5.3 41.9 560k

a Effect of phosphine type and P/Ni molar ratio.b Reaction conditions: resinII : 0.18 mmol of sacsac moieties; chlorobenzene: 20 ml; Et2AlCl co-catalyst (1.8 M toluene solution): 0.5 ml

(0.9 mmol); Ni(cod)2: 0.22 mmol; reacted sacsac moieties:∼0.018 mmol; Al/anchored Ni=50 mol mol−1; T: −15◦C; PC3H6: 3 atm; time: 1 h.c When C6+C9<100 also C12 are present.d Overall yield to DMB, determined as: fraction of DMB in C6 cut multiplied by % of C6 cut.e Turnover frequency, expressed as: moles of converted propylene/(moles of anchored Ni×h).f 2- and 4-Methylpentenes.g n-Hexenes.h No free PR3 was used.i No Et2AlCl was used.j No resin II was used.k Determined as moles of converted propylene/(moles of Ni(cod)2×h).

the nature of the ancillary phosphine ligand stronglyaffects the selectivity of propylene oligomerization,in the sense that more basic and bulky phosphinesusually orientate the dimerization towards the predo-minant formation of DMB. An analogous trend wasalso observed for heterogenized catalystsV, preparedby exchange reaction (Scheme 1, path a) [5]. How-ever, in order to have a better correlation betweenactivity and selectivity of the catalysts and type ofphosphine used, a larger screening of phosphine lig-ands was carried out. Indeed, the new heterogenizationpathway, via oxidative addition to Ni(0) precursors(Scheme 1, path b), allows to change very easily thephosphine (used as free ancillary ligand) as comparedwith the former route where a nickel(II) diphos-phino dichloride precursor, not always available, wasemployed.

As reported in Table 1, when PCy3 and PiPr3 wereused at the same adopted value of the initial P/Nimolar ratio (1.5), (entries 1 and 6) an extremely highactivity and a very good selectivity to DMB were ob-served. However, a deeper examination of the resultsindicated that the former catalyst system shows a bet-ter regioselectivity within the C6 cut (79%) than thelatter (74.5%) and a higher productivity, TF valuesbeing 74,000 against 56,600 h−1.

Indeed, PCy3 has a higher basicity than PiPr3[16,17] and provides a higher steric hindrance, asmeasured by the cone angle parameter [16]. How-ever, when the bulkyness of the phosphine is furtherincreased as in the case oftBuPCy2 (entry 5), the ba-sicity remaining substantially the same with respectto that of PCy3, a drop of both catalyst selectivityto DMB (32.2%) and activity (TF=6600 h−1) was


obtained, thus suggesting that the best performancesto DMB are achieved when the phosphine ligand isstrongly basic and its bulkyness is not excessivelyhigh. The above trend is better pointed out by movingfrom PiPr3 to tBu2PiPr throughtBuPiPr2 (entries 6, 9and 12, respectively), i.e. progressively increasing thebulkyness of the phosphine ligand, their basicity al-most remaining the same. In fact, the activity stronglydecreases in the same order, TF values passing from56,600 to 2600 through 17,600 h−1. Accordingly, acontemporary reduction of regioselectivity was ob-served. Finally, in the seriestBuPiPr2, sBuPiPr2 andnBuPiPr2 (entries 9–11) the drop of selectivity for thislast catalyst ligand may be connected with the sharpdecrease of steric hindrance and the higher flexibilityof n-butyl group. It is to underline that when verybulky phosphines were used (entries 5 and 9) a remark-able increase of the dimers in the oligomeric mixturewas achieved (>90%), although this high chemose-lectivity was accompanied by a strong reduction ofregioselectivity within C6 cut and a poorer activity.

It is worth noting that, despite their heterogeneity,the catalystsV (R=Cy, iPr) show a much higher ac-tivity and comparable selectivity to DMB than thosereported for the corresponding homogeneous systems[1,2], probably due to a higher stability. Moreover, theabove catalysts display also a higher activity with re-spect to the corresponding catalystsV, prepared by theexchange reaction procedure, thus suggesting that thetwo synthetic approaches are not equivalent, a longerinduction period probably being necessary in the lattersystems for their activation by the organoaluminiumco-catalyst.

When the P/Ni molar ratio of catalystV (R=Cy)was varied from 1.0 to 2.7 (entries 1–4) both activ-ity and selectivity to DMB showed a maximum for avalue equal to 1.5. An analogous trend was observedfor V (R=iPr) (entries 6–8). These data seem to sug-gest that, although a P/Ni ratio equal to 1 is in prin-ciple enough to give the structureV, a slight excessof phosphine is required to stabilize the active sites,probably due to an equilibrium between phosphino-coordinated and -uncoordinated nickel species. How-ever, when the excess of phosphine ligand is furtherincreased, the activity of the catalyst decreases, prob-ably due to a coordinative loading of the active sitesby more than one phosphine ligand per metal atom,thus preventing propylene coordination.

Finally, blank experiments clearly showed that inabsence either of free phosphine (entry 13) or oforganoaluminium co-catalyst (entry 14) no activity atall was observed, the heterogenized catalysts beingunstable under the above conditions (formation ofmetal nickel was observed). When no sacsac-resinIIwas used (entry 15), the homogeneous catalyst deriv-ing from Ni(cod)2 as well as the phosphine ligand andthe aluminium co-catalyst showed a certain activity.However, a completely different oligomeric mixturewas formed as compared with that of catalystV (en-try 1), thus indirectly confirming that nickel specieseventually released in solution play a minor role inthe catalytic process.

3.2. Effect of chlorinated promoters and of thenature of the organoaluminium co-catalyst on theperformances of catalystV (R=Cy)

Recently, the use of chlorophenols has been claimed[18,19] to improve the performances of nickel basedcatalytic systems in the selective dimerization toDMB. Previous studies [2] concerning the use ofpentachlorophenol (PCP) as chlorinated promoterin the propylene oligomerization by homogeneousI /Et2AlCl catalysts indicated that the presence of anexcess of PCP with respect to the organoaluminiumco-catalyst did not appreciably affect the activity ofthe system, whereas a large influence on selectivitywas observed, a drastic reduction of dimers in theoligomeric mixture and also of DMB within C6 cutbeing obtained. This was addressed to the formation,under reaction conditions, of EtAl(OC6Cl5)Cl orAl(OC6Cl5)2Cl, depending on the relative amounts ofPCP against Et2AlCl, characterized by higher Lewisacidities with respect to Et2AlCl.

When the heterogenized catalytic systemV(R=Cy)/Et2AlCl was used, an analogous trend wasobserved. Indeed, on increasing the relative amount ofPCP with respect to Et2AlCl (entries 16–18, Table 2),the activity did not appreciably changed whereas aprogressive lowering of the DMB content within theC6 cut, accompanied by an increase of the relativeamount of higher oligomers was found. This was par-ticularly evident when an excess of PCP with respectto Et2AlCl was adopted (entry 18). However, the dropof selectivity was less relevant than that observed forthe corresponding homogeneousI /Et2AlCl catalysts,


Table 2Propylene oligomerization by heterogenized catalystV, obtained in situ from sacsac-resinII , Ni(cod)2, PCy3 and Et2AlCl, in the presenceof variable amounts of pentachlorophenol (PCP) as promotera

Entry PCP/Al (mol mol−1) Dimers (%) C6b (%) C9 (%) C12 (%) Yc (%) TFd (h−1)

DMB MPe HEXf

1 0 79.0 19.9 1.1 71.3 26.0 2.7 56.3 7400016 0.4 72.8 25.8 1.4 64.5 28.3 5.0 47.0 7420017 1.0 69.7 28.3 2.0 60.2 28.5 7.0 42.0 7760018 2.0 66.4 29.8 3.8 41.0 28.4 17.8 18.9 77000

a Reaction conditions: resinII : 0.18 mmol of sacsac moieties; chlorobenzene: 20 ml; Ni(cod)2: 0.22 mmol; PCy3: 0.33 mmol; Et2AlClco-catalyst (1.8 M toluene solution): 0.5 ml (0.9 mmol); reacted sacsac moieties:∼0.018 mmol; Al/anchored Ni: 50 mol mol−1; T: −15◦C;PC3H6: 3 atm; time: 1 h.

b When C6+C9+C12<100 C15+ are present.c Overall yield to DMB, determined as: fraction of DMB in C6 cut multiplied by % of C6 cut.d Turnover frequency, expressed as: moles of converted propylene/(moles of anchored Ni×h).e 2- and 4-Methylpentenes.f n-Hexenes.

thus suggesting that a matrix effect on selectivity maybe also present.

In this context, the effect of the nature of the alu-minium co-catalyst and of its relative amount withrespect to the anchored-nickel on catalystV (R=Cy)

Table 3Propylene oligomerization by heterogenized catalystsV, obtained in situ from sacsac-resinII , Ni(cod)2, PCy3 and different organoaluminiumco-catalystsa

Entry Al co-catalyst Dimers C6b (%) C9 (%) Yc (%) TFd (h−1)

Type Al/Nie (mol mol−1) DMB (%) MPf (%) HEXg (%)

1 Et2AlCl 50 79.0 19.9 1.1 71.3 26.0 56.3 7400019 Et2AlCl 25 74.7 24.1 1.2 79.0 17.6 59.0 9180020 Et2AlCl 15 78.3 20.7 1.0 77.5 18.2 60.7 5100021 EtAlCl2 50 36.3 26.6 37.1 20.9 26.8h 7.6 22300022 Et3Al2Cl3i 50 73.1 23.2 3.7 67.0 27.1j 49.0 14400023 Et3Al 50 – – – – – – 0.024 Et3Al/Et2AlCl (1:1) 50 54.3 45.6 0.1 100.0 0.0 54.3 ∼025 MAO 50 39.5 48.8 11.7 78.0 19.2 30.8 ∼026 Et2AlCl/MAO (1:1) 50 78.5 20.1 1.4 72.5 25.5 56.9 700027 Et2AlI 50 63.8 33.5 2.7 95.2 4.8 60.7 ∼0

a Reaction conditions: resinII : 0.18 mmol of sacsac moieties; chlorobenzene: 20 ml; Ni(cod)2: 0.22 mmol; PCy3: 0.33 mmol; reactedsacsac moieties:∼0.018 mmol;T: −15◦C; PC3H6: 3 atm; time: 1 h.

b When C6+C9<100 only C12 are usually present.c Overall yield to DMB, determined as: fraction of DMB in C6 cut multiplied by % of C6 cut.d Turnover frequency, expressed as: moles of converted propylene/(moles of anchored Ni×h).e With respect to the anchored nickel.f 2- and 4-Methylpentenes.g n-Hexenes.h In addition to C12 (31.2%) C15+ (21.1%) were present.i Prepared as 1 to 1 molar mixture of Et2AlCl and EtAlCl2.j In addition to C12 (3.9%) C15+ (2.0%) were present.

performances was also studied. When the molar ra-tio Et2AlCl/anchored Ni was varied from 50 to 15(entries 1,19,20, Table 3) the activity of the systemremained extremely high. A maximum was observedfor an Al/Ni ratio equal to 25 mol mol−1, although


accompanied by a slightly lower regioselectivity toDMB within C6 cut. These data however indicate thatthe variation of the Al/Ni molar ratio, at least in theadopted range, did not remarkably change the catalyticperformances, thus suggesting that the heterogenizedcatalyst, differently from the homogeneous counter-part [2], is less sensitive to protic impurities in thereaction medium.

When Et2AlCl was replaced by EtAlCl2 (entry 21,Table 3), as previously observed for both homoge-neous catalysts based onI [2] and heterogenizedV cat-alysts prepared by the exchange method [5], a dramaticincrease of activity was observed (TF=223,000 h−1),although a drop of selectivity to both dimers and DMBwas contemporarily found. It is noteworthy that whenEt3Al2Cl3 was used (entry 22, Table 3) a selectivityquite close to that observed in entry 1 with Et2AlClalone was obtained, but with an almost double activity(TF=144,000 h−1). The use of Et3Al or MAO (entries23 and 25, Table 3) in combination withV, accord-ingly to what previously found for the homogeneouscounterpart [1,2], gave a completely or almost com-pletely inactive catalyst in that order. Moreover, thetraces of oligomers obtained in the presence of MAOallowed also to conclude that the regioselectivity andthe overall yield (Y) to DMB were quite poor. An anal-ogous result was obtained when Et2AlI was used asco-catalyst (entry 27, Table 3). The presence of Et3Alin equimolar mixture with Et2AlCl (entry 24, Table 3)substantially inhibited the oligomerization activity ofthe catalystV, whereas an equimolar amount of MAOwith Et2AlCl (entry 26, Table 3) afforded a catalyticsystem with a selectivity of the same order of magni-tude as that of entry 1, although the activity was ratherlow (TF=7000 h−1).

All these data indicate that also for these heterog-enized systems it is possible, by varying the type oforganoaluminum co-catalyst and its relative amountwith respect to the active nickel species, to modu-late the catalytic performances and hence to orientatethe oligomerization reaction towards the formation ofspecific target products.

4. Conclusions

On the basis of the results obtained the followingconcluding remarks can be drawn:

1. Heterogenized catalysts, obtained by oxidative ad-dition of polymer-boundb-dithioacetylacetonateligands to nickel(0) complexes, when combinedwith suitable ancillary phosphine ligands andorganoaluminium compounds, display very highactivity in the propylene oligomerization.

2. The above catalysts show improved performancesas compared with the corresponding heterogenizedsystems prepared by an exchange reaction betweenthe sodium salt of the polymer-bound sacsac lig-ands and nickel(II) diphosphino dichloride com-plexes. Moreover, the novel preparative approachvia oxidative addition appears more attractive asit allows to modify very easily the catalyst char-acteristics by changing the free ancillary phos-phine ligand and the initial P/Ni molar ratio, thusreadily modulating its performances and orientat-ing the selectivity towards the target oligomericproducts.

3. When basic phosphine ligands with a relativelyhigh bulkyness, such as PCy3 and PiPr3, andorganoaluminium co-catalysts of suitable Lewisacidity, such as Et2AlCl, are used, the aboveheterogenized catalysts display a very high re-gioselectivity to 2,3-dimethylbutenes (∼80%) andan extremely high activity (TF=74,000 h−1), thusshowing also superior performances as comparedwith the corresponding homogeneous catalysts.

4. Finally, taking into account that the amount ofnickel in the liquid reaction products in all batchexperiments was almost negligible (<1–2 ppm), asignificant metal leaching from the polymer ma-trix can be excluded, at least under the adoptedconditions, thus confirming the analogous re-sults found for the same heterogenized sys-tems prepared by the exchange reaction method[5].

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improved heterogenized catalysts for selective propylene oligomerization to 2,3-dimethylbutenes...

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