Synthesis and structure of zirconium tetrachloride ester complexes, ethylene oligomerization catalyst precursors

Journal of Molecular

Catalysis,

53 (1989)

433 - 442

433

SYNTHESIS AND STRUCTURE OF ZIRCONIUM TETRACHLORIDE ESTER COMPLEXES, ETHYLENE OLIGOMERIZATION CATALYST PRECURSORS DAVID A. YOUNG Intermediates Technology Division, Baton Rouge, LA 70821 (U.S.A.) (Received January 17,1989;

Exxon Chemical Company,

P.O. Box 241,

accepted April 24,1989)

The reaction of organic esters with zirconium tetrachloride produced dimeric, chloride-bridged adducts. Structural characterization by singlecrystal X-ray diffraction determined the following for the n-hexyl acetate derivative, [p-Cl] 2[ZrC1sCHsC02(CH,),CHJ~ : triclinic crystal system, space group Pl - C’i (No. 2) with 2 = 2 and unit cell dimensions a = 9.636(3) A, p = 106.53(2)“, 7= b = 14.837(4) A, c = 12.920(4) A, (Y= 116.34(2)“, 88.79(2)“, and V= 1575(l) A3. The structure was refined to Ri = 0.050 and R2 = 0.059 for 2102 independent reflections. The complex contains two chloride-bridged zirconium atoms in octahedral configuration. Each zirconium is bound to three terminal chloride atoms located in one axial and two equatorial positions. Each ester is coordinated to one zirconium via carbonyl oxygen in an axial position tmns across the dimer. Ethylene was oligomerized to Shulz-Flory distributions of a-olefins using the isodecyl acetate complex and diethylaluminum chloride at 130 to 150 “C. The olefins contained four to about forty carbons, with over 90% linear isomers. Catalyst turnover numbers ranged from 4.1 X lo4 to 8.0 X lo4 mol ethylene per mol zirconium. Polyethylene comprised about 0.5 wt.% of the product when the water concentration was below 50 ppb, and became the major product at concentrations above 100 to 200 ppb.

Introduction Most reports of zirconium Ziegler-Natta catalysis employ tetraalkyls, tetraalkoxides, or zirconocenes soluble in nonpolar organic solvents [l]. However, insoluble zirconium tetrachloride has been reported to react slowly into solution with organoaluminum chloride cocatalyst and ethylene, either with [2,3] or without [2] added Lewis base, to form a very active ethylene oligomerization catalyst. The latter observation is consistent with the report of Attridge and coworkers of ethylene oligomerization catalysts formed from zirconium tetraalkyls. Their most active catalysts were those having the 0304-5102/89/$3.50

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434

highest chloride content in the organoaluminum cocatalyst [4]. Attridge proposed that catalytic activity was enhanced by successive replacement of alkyl groups on zirconium with chloride from aluminum, forming a polar, binuclear complex of aluminum and zirconium. The higher catalytic activity was attributed to the increasing positive charge on zirconium as chloride substitution increased. We now report the synthesis of zirconium tetrachloride ester complexes that are very soluble in nonpolar solvents, and the activation of these complexes for Zeigler-Natta catalysis. We also offer some proposals concerning the nature of the active catalyst based upon our observations.

Experimental Materials Sublimed zirconium tetrachloride was purchased from Teledyne Wah Chang Albany Corp. and used as received. Zirconium tetra-n-propoxide was purchased from Kay-Fries Corp. and rigorously purified of n-propanol by vacuum stripping and of oxygen-bridged zirconium impurities by multiple recrystallizations from dry n-heptane, final m.p. 138 - 141.5 “C in a sealed capillary tube under argon. n-Hexyl acetate was purchased from Aldrich Chemical Co. and isodecyl acetate, trade name EXXATE 1000, from Exxon Chemical Co.; both were used as received after drying, see below. Diethylaluminum chloride, a 20 wt.% solution in n-heptane, was used as received from Ethyl Corp. Chemically pure ethylene and reagent grade n-heptane and toluene were used for the synthesis and oligomerization reactions after drying. Procedures The esters, solvents and ethylene were dried before use to less than 50 ppb water content using beds of 3A molecular sieves. The sieves were activated by drying at 300 “C under vacuum. All materials used for the synthesis and oligomerization catalyst feed solutions were prepared and handled in a dry box filled with argon containing less than 1 ppm oxygen and water. All equipment was dried prior to use at 200 “C under vacuum. Synthesis and catalysis A slurry of zirconium tetrachloride powder in n-heptane solvent was reacted under dry argon with an equimolar amount of ester at room temperature, producing a slight exotherm and a solution of the dimer complex. A very small amount of haze in the product solution, impurity remaining from the zirconium tetrachloride, was removed by filtration through a dry glass frit. The n-hexyl acetate complex was then recovered in essentially quantitative yield as colorless crystals by removal of solvent under vacuum. The product was purified by recrystallization from n-heptane solution, m.p. 98 - 98.5 “C in a sealed capillary tube under argon. The isodecyl acetate

435 TABLE 1 Crystallographic data for [E.(-C~]~ [ ZrC1sCHsC02(CH2)sCHs]~ Formula M.W. Crystal system Space group a, b, c (A) ff, 0, Y (deg) v (A3) z

C1&32~4CWr2

754.5 triclinic Pl - C’i (NO. 2) 9.636(3), 14.837(4), 12.920(4) 116.34(2), 106.53(2), 88.79(2) 1575(l) 2 0.71073 1.593 colorless 0.27 x 0.54 x 0.72 rectangular parallelepiped MO Ko (graphite monochromated) 1.356 3.0 - 39.7, 39.7 - 50.7 direct methods, SHELXTL for 30 nonhydrogens 5774 2102 0.050 0.059

h (4

Calcd. density (g cmV3) Crvstal color Crystal size (mm) Crystal shape Radiation Lin. abs. coef. (mm-‘) 26 ranges (deg) Refinement No. measd. reflecns. No. ind. reflecns. used Era R2

b

ark = Zll~,l

- IF,il/I’ZlF,I

bR,= [Zw(lF,I - IFcl)2/ZwIFo~2]1’2 TABLE 2 Atomic coordinates for nonhydrogen atoms in [p-Cl12[ ZrC13CHsC02(CH&CH3]2 Atom typeb

Zrl Zr.2 Clbl Clb2 Cl11 Cl12 Cl13 Cl21 Cl22 cl23 011 012 Cl1 Cl2 Cl3 Cl4

Fractional coordinates 104 x

104 y

1042

3294( 1) -514(l) 1929(3) 863(3) 4239(5) 5407(4) 2242(4) -1478(4) -2623(4) 533(4) 3807(7) 4474(8) 3919(11) 3439(14) 4865( 14) 5764(15)

-2170(l) -1022(l) -843( 2) -2358(2) -3290(3) -1602(3) -3418(3) 58(3) -1545(3) 231(2) -962(5) 413(6) -508(8) -1009(8) 934(9) 1997(10)

2224( 1) 1886(l) 3519(2) 585( 2) 715(4) 3922(4) 2529(4) 3393(4) 213(4) 1591(3) 1894(6) 1821(7) 1297(10) -32(9) 3121(11) 3522(12)

a

Equivalent isotropic thermal parameter, B (A2 x 1O)C 71(l) 65(l) 67(l) 80(l) 137(3) 123(2) 115(2) 120(2) 142(2) 87(2) 71(3) 77(4) 68(5) 96(6) 96(7) 136(8) (continued)

436 TABLE 2 (continued) Atom typeb

Cl5 cl6 Cl7 Cl8 021 022 c21 c22 c23 c24 c2s c26 c27 C28

104y

104 z

Equivalent isotropic thermal parameter, E (A* x 10)C

2592(10) 3616(10) 4095(14) 4834( 13) -2240(5) -3215(7) - 3090(9) -3966( 8) -2317(12) -2632(14) -2998(15) -3347(15) -3934(15) -4342( 10)

3140(13) 3520(18) 2855( 25) 2758(19) 2181(6) 2995(11) 2049(12) 867(12) 4188(15) 5060( 15) 5576(19) 6434( 19) 6773(23) 7474( 12)

160(9) 171(13) 263(20) 221(16) 79(4) 103(6) 83(7) 130(8) 127(10) 203(14) 177(14) 183(15) 237(19) 159(10)

Fractional coordinates 104x 4966( 17)

5783(21) 5123(29) 5484(22) -1061(7) -1398(9) -1438(11) -1954(14) -882(17) -1176(20) -435(23) -768(24) -267(31) -494(20)

aThe numbers in parentheses are the estimated standard deviations in the last significant digit. bAtoms are labeled in agreement with Fig. 1. V?his is one-third of the trace of the orthogonalized Bu tensor.

TABLE 3 Selected bond lengths and bond angles for nonhydrogen [ ZrClsCHsC02( CH&CH& a

atoms in crystalline [/kCl]2-

Length (A)

Tywb

Length (A)

zrl-clbl Zrr-Clb2

2.574(3) 2.594(3)

a2-clb2

2.601(3) 2.591(3)

Zrr--Cl11 Zrl--C112 Zrr-Cl13

2.337(5) 2.358(3) 2.361(5)

Zrz-Cl21 Zrs-Clz2 Zr2--Cl23

2.329(4) 2.342(4) 2.355(4)

Zrl--011

2.113(9)

Zr2--021

2.111(9)

021-c21

(A*--Cl1

1.26(2) 1.27(l)

O2rC21

1.24(2) 1.31(2)

012~Cl3

1.44(2)

O22-C23

1.47(2)

011-Cl1

Typeb

ClblZdb2 ClblZrlClll ClblZrlCll2 ~blZrlC113

Angle (“) 79.0(l) 167.5(2) 89.8(l) 89.8(l)

Tywb Zr+&

Speb

r

Angle (“) 78.5(l) 90.5(l) 167.8(l) 88.9(l) (continued)

437 TABLE 3 (continued)

Speb

Angle (“) 90.2(l) 166.7(l) 88.9(l) 100.2(l) 96.1(2) 98.2(2) 81.7(2) 81.6(2) 90.7(2) 89.8(2) 168.4( 2)

166.6(l) 90.9(l) 90.2(l) 99.0(l) 97.3(l) 97.4(2) 82.7(2) 81.6(2) 89.6(2) 89.7( 2) 169.2(2)

101.4(l)

101.1(l)

158.3(6)

164.2(9)

121(l) 122(l) 117(l)

120(l) 123(2) 118(l)

117(l)

117(l)

aThe numbers in parentheses are the estimated standard deviations in the last significant digit. bAtoms are labeled in agreement with Fig. 1.

complex was used for ethylene oligomerization reactions after filtration of the reaction product solution without further purification. Typical ethylene oligomerization conditions were: 500 - 600 ml of reaction solution in a 1 1 stirred autoclave, 130 - 150 “C, 6.9 X 10” kPa ethylene pressure, n-heptane solvent and 15 to 30 min residence time. The continuous reaction feed rates, all per hour, to the autoclave were: 800 to 1000 g solvent, 830 g ethylene, 0.17 to 0.28 meq zirconium tetrachloride isodecyl acetate complex and 2.1 to 2.6 meq aluminum cocatalyst. The catalyst components were fed separately to the autoclave as solutions in nheptane solvent. The autoclave and associated feed lines were initially dried by circulating dry solvent through them at 150 “C. All water concentrations were measured with a Panametrics Corp. hygrometer using aluminum oxide sensors. The oligomerization products were analyzed using standard capillary gas chromatographic techniques with known standards and measured response factors. X-ray crystallographic study The structure of [P-Cl] 2[ ZrCl,CH,CO,( CH2),CHs] 2 was determined by Crystallytics Co., Lincoln, NE. Suitable single crystals were grown from nheptane solvent by slowly cooling a warm saturated solution. The crystallo-

438

Fig. 1. The molecular structure of &Cl ]2[ ZrC13CH~CO,(CH~)FH3]2 using arbitrarify small spheres for hydrogen atoms and 50% probability ellipsoids for ail other atoms.

graphic data are given in Table 1, the atomic coordinates for nonhydrogen atoms in Table 2 and selected bond lengths and bond angles in Table 3. Supplementary material is available from the author.

Results and discussion The reaction of organic esters with zirconium ~~ac~loride produced chloride-bridged dimeric adducts in quantitative yields. The molecular structure (see Fig. 1) of the n-hexyl acetate derivative, determined from singlecrystal X-ray diffraction data, contains two chloride-bridged zirconium atoms in octahedral configuration. Each zirconium is bound to three terminal chloride atoms located in one axial and two equatorial positions. Each ester is coordinated to one zirconium via carbonyl oxygen in an axial position trans across the dimer. Slight deviations exist in the zirconium octahedral coordination. The angles between the equatorial terminal chlorides are expanded from the expected 90” to 99.0” and 100.2”. Associated compressions are present in the (bridge Cl)-Zr-(equatorial Cl) bond angles which range from 166.6 - 16’7.8” instead of the expected 180”. The carbonyl oxygens and axial chlorides all lean towards the second zirconium atom, producing 0--Zr-(axial Cl) bond angles of 168.4” and 169.2” instead of the

439

expected 180”. In spite of these distortions, the equatorial and axial planes are positioned at 90.6” to each other and contain only minor nonplanar deviations. The largest of these deviations is -0.134 A for Zr, and +0.140 A for Zr, in the equatorial plane, and +0.017 A for Oii in the axial plane. This structure is similar to structures reported for the dimeric adducts of titanium tetrachloride with ethyl acetate [ 51, p-ethyl anisate [ 61 and ethyl benzoate [7]. The ester bond lengths in the titanium complexes led Bassi and coworkers to propose the following electron delocalization: M+-O--C(R)= 0+--R 161. The zirconium complex is similar in this respect. The C=O bonds are 0.01 and 0.03 A longer, the =C-Obonds 0.05 and 0.09 A shorter, and the -0-R bonds 0.01 and 0.04 A longer than the average values found in free esters [6,8]. These zirconium complexes are very soluble in aliphatic and aromatic solvents and provide effective precursors for Ziegler-Natta catalysts. Ethylene was converted to linear cr-olefins by activating the isodecyl acetate complex with diethylaluminum chloride. The oligomer products were ShulzFlory distributions of a-olefins, from four to about forty carbons in length, with greater than 90% linear isomers. Only a few percent of branched and internal olefin isomers were produced. As the aluminum:zirconium molar ratio was varied from 7.5 to 15.3 in these experiments, the average oligomer molecular weights varied from 140 to 96 respectively. The catalytic activity, measured in catalyst turnover numbers, ranged from 4.1 X lo4 - 8.0 X lo4 mol ethylene per mol zirconium (1.2 X lo3 - 2.3 X 10’ g product per mmol zirconium) at 150 “C. After washing the oligomerization product with dilute aqueous caustic solution and water, we determined by gas chromatography that the ester was present in the amount initially charged to the reaction as catalyst complex. For comparison, the continuous ethylene oligomerization reaction was repeated under the same conditions with zirconium tetra-n-propoxide substituted for zirconium tetrachloride isodecyl acetate complex. The same product distribution was obtained, with almost identical molar turnover numbers. The only difference observed was the requirement of an additional 3 - 4 mol diethylaluminum chloride per mol zirconium to achieve the same average oligomer molecular weight that was obtained for a given aluminum: zirconium molar ratio with the chloride ester complex. Our observed molar turnover numbers are similar to those reported for batch experiments using both zirconium tetra-n-propoxide [ 21, and zirconium tetrachloride reacted into solution with the cocatalyst and ethylene, without ligands or added Lewis base, during an induction period [2,9]. A water concentration below about 50 ppb in the oligomerization reactions was necessary to avoid the production of more than 0.5 wt.% of polyethylene byproduct. This polyethylene had an average molecular weight of about 2 X 105, and was produced in increasing amounts as the water concentration increased. Polyethylene became the major product if water was present at concentrations greater than about 100 to 200 ppb. This effect of water was observed regardless of the source. Wet ethylene feed, wet solvent

440

or the formation of reaction products of water with either catalyst component prior to injection into the autoclave all led to polyethylene production.

Conclusion We have shown that the simple equimolar addition of organic esters to zirconium tetrachloride produces dimeric adducts in high yield. The molecular structure of the n-hexyl acetate complex has been determined and is similar to structures previously reported for equimolar ester complexes of titanium tetrachloride. These complexes were shown to be convenient precursors to ethylene oligomerization catalysts, producing Shulz-Flory distributions of cx-olefins with over 90% linear structure. We have also demonstrated the crucial effect of water in transforming the ethylene oligomerization catalyst into a polymerization catalyst at very low concentrations. Regrettably, we have no molecular structure information concerning the interaction of the two catalyst components under the oligomerization reaction conditions*. Therefore, we may only speculate about the nature of the active form of the catalyst. We have observed similar catalyst turnover numbers for ethylene oligomerization using both the ester complex of zirconium tetrachloride and zirconium tetra-n-propoxide. These numbers are also similar to those previously reported for zirconium tetra-n-propoxide and ligand-free zirconium tetrachloride reacted into solution with cocatalyst in the presence of ethylene [2,9]. We suspect, therefore, that the active catalyst contains no ester or propoxide ligand, but is probably a complex, perhaps polynuclear, of zirconium chloride and cocatalyst. If this were the case, both ester and n-propoxide ligands must exchange off zirconium (the npropoxide being replaced by chloride from aluminum) and remain complexed to the excess cocatalyst. A useful model for this chemistry is Attridge’s proposal of zirconium tetraalkyls forming polar, chloride-bridged, binuclear complexes of alkylated zirconium chloride with aluminum cocatalyst [ 41. This hypothesis is supported by our observation that zirconium tetra-n-propoxide required an additional 3 - 4 mol more of aluminum cocatalyst per mol zirconium, over the amount required by the zirconium tetrachloride ester complex, to produce the same oligomer average molecular weight. The acceleration effect of water on ethylene polymerization rates, due to the formation of noncoordinating aluminoxane anions, has been well documented for Ziegler-Nat& catalysts [lo, 111. We propose, without *Colorless microcrystalline products were isolated from the room temperature reaction of the zirconium tetrachloride n-hexyl acetate complex with equimolar and excess dimethylaluminum chloride; typical elemental analysis: Zrr.m, Alr_e,, Cls.84, Ce.42, He.68. While these results indicate the possibility of forming ester-free complexes from these materials, they offer no conclusive information concerning the structure of the active catalyst. Repeated attempts to recrystallize these materials were unsuccessful.

441

proof, that water present in our oligomerization reacts with the aluminum cocatalyst to form such anions. The result could be the formation of chargeseparated ion pairs. The resulting more positively charged zirconium catalyst site should have a much higher rate of ethylene insertion with respect to @hydride elimination and produce polyethylene at the expense of oligomer. This type of charge separation has recently been shown to be necessary for the onset of ethylene polymerization with titanocene catalysts activated by methylaluminum dichloride [ 121 and with cationic zirconocene catalysts

P31. Acknowledgements The author is indebted to Drs. Terry J. Burkhardt, Arthur W. Langer, Howard W. Turner and Professor John E. Bercaw for valuable discussions, to Drs. Larry 0. Jones and Troy J. Campione for engineering assistance with the catalytic reactions, and to Dr. Cynthia S. Day, Crystalytics Co., for the X-ray structure determination. This publication is dedicated to the late Dr. Arthur W. Langer. Supplementary material available Complete X-ray crystal data, including experimental procedures, tables of anisotropic thermal parameters and structure factor amplitudes, are available from the author.

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