Studies on tetraisobutylaluminoxane as a cocatalyst in a supported titanium system for propylene polymerization

Eur. Pa/w. J. Vol. 31. No. 2, pp. 145-148. 1995 Copyright ‘(’ 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0014-3057195 $9.50 + 0.00

Pergamon

STUDIES ON TETRAISOBUTYLALUMINOXANE AS A COCATALYST IN A SUPPORTED TITANIUM SYSTEM FOR PROPYLENE POLYMERIZATION* V. K. GUPTA, S. SATISHt and I. S. BHARDWAJ Research Centre, Indian Petrochemicals Corporation Ltd, PO: Petrochemicals, Vadodara 391 346, India (Received 26 October 1993; accepted in Jinal ,form 2 March 1994)

Abstract-A supported titanium catalyst was prepared using magnesium ethoxide as a precursor for the support. The catalyst was characterized by compositional analysis and specific surface area measurements. The propylene polymerization studies were carried out with a solid titanium catalyst [Mg, Ti] using tetraisobutylaluminoxane as cocatalyst. The results have been compared with a triisobutylaluminum based system. Both systems show a decay type kinetic profile. The [Mg, Ti]-tetraisobutylaluminoxane catalyst system shows high activity and high stereospecificity as compared to the [Mg, Ti]-triisobutyl aluminum.

INTRODUCTION

Polymerization of propylene using a supported titanium catalyst system continues to attract significant research efforts due to its commercial importance [l-7]. The solid titanium catalyst in conjunction with trialkylaluminum and alkoxysilane as a cocatalyst system polymerizes propylene with high activity and stereospecificity. The nature and concentration of cocatalyst governs the performance of the catalyst system. Various plausible mechanisms are propounded to explain the variations in the performance of the catalyst [8-151. The trialkylaluminum mainly generates and alkylates the active Tif3 species for propylene polymerization. The use of an external Lewis base, alkoxysilane, in conjunction with trialkyIaluminum results either in the poisoning of nonstereospecific Ti+3 species and/or activation of stereospecific Ti +3 sites, thereby leading to the improvement in the stereospecific character of the system. Aluminoxanes containing Al-O-Al bonds have received significant attention in recent years as cocatalysts for high activity homogeneous metallocene catalysts [16-221. Aluminoxanes are found to be more active than analogous aluminum alkyls for propylene polymerization involving cyclopentadienyl derivatives of titanium, zirconium and hafnium. There are, however, no published reports concerning the performance of supported titanium catalysts involving aluminoxanes. The patent reports describe the polymerization of propylene with a conventional TiCI, catalytic system containing aluminoxanes as cocatalyst [22-281. Our interest in the synthesis, characterization, and performance evaluation of supported titanium catalysts [29-311 have prompted us to examine the comparative performance of aluminoxanes

vis-a-vis analogous aluminumalkyls in the polymerization of propylene. The added advantage of aluminoxane as a cocatalyst is its capability to act as a electron donor via oxygen to Lewis acidic sites in the catalyst. We have selected tetraisobutylaluminoxane, (‘Bu,Al),O, for our present investigation due to its defined chemical composition as compared to the methyl and ethylaluminoxanes. In this paper, we report the synthesis and characterization of an [Mg, Ti] catalyst prepared from magnesium ethoxide as a starting material for the active support. The performance of the prepared catalyst was evaluated in conjunction with tetraisobutylaluminoxane for propylene polymerization. The results have been compared with triisobutylaluminum as cocatalyst.

EXPERIMENTAL

PROCEDURES

Materials

Magnesium ethoxide (Hills, Germany) and triisobutylaluminum (Ethyl Corp., U.S.A) were used as received. Titanium tetrachloride and diethyl phthalate were commercial products and used after distillation. Hexane, chlorobenzene and decane were used after drying over sodium wire. Propylene (polymerization grade from commercial PP plant) was used after passing through molecular sieve columns. All experimental manipulations were carried out under ultra high pure nitrogen atmosphere using a Vacuum Atmosphere Model HE-43-2 Dri lab equipped with a model HE 491 Dri train and Schlenk techniques. Catalyst preparation [Mg, Ti] (a) Reaction qf magnesium erhoxide with titanium tetrachloride and dierhyl phthalate. 5.5 ml of titanium tetrachlo-

ride in 55 ml chldrobenzene was added to 8.25g of Mg(OC,H,),. The temperature was raised to 120°C followed by the addition of 3.9 ml of diethyl phthalate. The mixture was stirred for 2 hr at 120 C. It was filtered and the solid product was washed with warm hexane (4 x IOOml). The solid product [Mg, Ti-I] was isolated.

*IPCL Communication No. 245. tTo whom all correspondence should be addressed. I45

V. K. Gupta

146

(b) Reaction qf [Mg, Ti-I] wilh tilanium rerrachloride. 6.Og of [Mg, Ti-I] product was mixed with 108 ml of titanium tetrachloride. The mixture was heated to 120 C with stirring for 2 hr. The unreacted titanium tetrachloride was siphoned off after settling of solid. Further the reaction was treated with 100 ml decane at l2O‘C with stirring. After

stirring was stopped, the solid material was allowed to settle down. The liquid content was siphoned OK. After decane washing, the temperature of the reaction mixture was brought down to 6o’C and 200 ml hexane was added with stirring and the whole mass was allowed to settle and hexane was removed by siphoning. The washing treatment with hexane was carried out twice. After that the slurry was stored in hexane. Synthesis

of rerrai.sohut~lalumB~~~.~an~~

The solid MgCl, 6HZ0 was added in a controlled manner with stirring into a hexane solution of triisobutylaluminum. The mole ratio of ‘Bu,AI to MgCl? .6H,O was kept at I :0.09. The reaction mixture was then allowed to stir for 6 hr. It was filtered and solvent was removed from the filtrate under reduced pressure. A viscous liquid product was obtained. The aluminum content was analyzed and found to be 17.8 wt%. Polymerization pr0cedure.r Polymerization of propylene was carried out in a 500 ml glass reactor equipped with a magnetic stirrer. A calculated amount of cocatalyst followed by the solid catalyst was added to the reactor containing 200 ml hexane. Propylene was continuously supplied under a total pressure of I bar for a fixed period and polymerization was terminated by the addition of methanolic hydrochloride. The polymer was dried in oacuo. Characterization In the catalyst, the titanium content was estimated by a spectrophotometric method [32]. Magnesium and chlorine contents were determined by titration [32]. The aluminum content in aluminoxane was determined by volumetric analysis [32]. The catalyst was hydrolyzed in an acidic solution to extract the organic residue for quantitative estimation. The BET surface area measurements were carried out on a Sorptomatic 1900 model of a Carlo Erba instrument. The polymer samples were extracted with boiling heptane in a Soxhlet apparatus. The isotactic index (1.1.) reported for each sample is the weight percentage of heptane insoluble polymer.

et ul. Table I. Characterization of magnesium ethoxide. intermediate product and catalyst Compositional analysis (wt%) Compound

Mg

Cl

Ti

Mg(OEt)? [Mg.Ti-I] IMg. Ti]

?I 15.0 16.8

57.4 61.1

5.9 3.6

~- Surface area Orgamc contents (m’ig) 78 ?I I6

20 70 I52

Analysis f 0.2 WI%, [Mg,Ti-I] product obtained by treatment of magnesium ethoxide with titanium tetrachloride and diethyl phthalate; [Mg. Ti] solid obtained by reaction of [Mg, X-I] with titanium tetrachloride.

with titanium tetrachloride and diethyl phthalate increases the surface area of the product [Mg, Ti-I] to 70m*/g. Further treatment of [Mg,Ti-I] with titanium tetrachloride gave a solid catalyst [Mg, Ti] with increased surface area (Table 1). These data indicated that the [Mg, Ti] catalyst had an approx. &fold higher surface area as compared to the initial starting material-magnesium ethoxide. Propylene

polymerization

studies

The activity of the catalyst [Mg, Ti] was examined for propylene polymerization as a function of time using tetraisobutylaluminoxane as cocatalyst. The kinetic curve (Fig. 1) shows a decay type profile indicating a decrease of activity with increasing polymerization time. The higher cocatalyst concentration leads to the improvement of the activity of the catalyst due to a higher initial rate of polymerization. The equimmolar substitution of tetraisobutylaluminoxane by triisobutylaluminum reduces the polymerization rate (Fig. 2). It can be attributed to low activity and decreased rate of polymerization with time. The comparison of the kinetic profile of both systems reveals that the [Mg, Ti]/tetraisobutyl-

240

RESULTS AND DISCUSSION Preparation

and characterization

of’ the cat&w

Magnesium ethoxide was reacted with titanium tetrachloride and diethyl phthalate sequentially. This process facilitated the generation of a chlorinated support by exchange of an ethoxy group with chlorine [33-351. Subsequently it also lead to the incorporation of diethyl phthalate and a titanium (IV) compound on the chlorinated support. The isolated product was treated a second time with titanium tetrachloride. The obtained solid [Mg, Ti] had a lower titanium content than [Mg, Ti-I] (Table 1). It can be explained on the basis of the removal of titanium species present on the support during the second treatment. There are reports that the second treatment of catalyst with titanium tetachloride removes inactive titanium compound which results in the improvement of the activity of the catalyst [36,37]. The BET surface area of magnesium ethoxide is found to be 20m*/g. The treatment of the support

40

30

60 Time

90

120

(min)

catalyst system: [Mg. Ti]/(‘Bu,Al),O, catalyst = 40 mg. hexane = 200 ml, Pc; = I atm, temperature = 30 C. cocatalyst amount 0 = 3.0 mmol, @ = 6.0 mmol, 0 = 9.0 mmol. Fig. I. Polymerization

rate vs time curve.

supported titanium system for propylene polymerization

A

147

Table 3. Effects of trisobutylaluminum concentration on the performance of the caklyst* Polymer yield (gPP/gCat) ‘Bu,Al (mm011

Total

lsotaclic

Atactic

Yield ratio

1.1. (%I

3.0 6.0 9.0

121 148 206

86 99 130

35 49 76

2.5 2.0 1.;

71 67 63

*Polymerization conditions as mentioned in Table ratio = isotactic/atactic polymer yield.

30

60

90

Time

120

(min)

Fig. 2. Polymerization rate vs time curve, catalyst system: [Mg, Ti]/‘Bu,Al, catalyst = 40 mg, hexane = 200 ml, Pc; = 1atm, temperature = 3o”C, cocatalyst amount 0 = 3.0 mmol, @ = 6.0 mmol, l = 9.0 mmol.

aluminoxane system shows an approximately two fold higher rate of polymerization as compared to [Mg, Ti]/triisobutyl aluminum. The performance of the [Mg, Ti]/tetraisobutylaluminoxane catalyst system was evaluated at different cocatalyst concentrations (Table 2). The increase of cocatalyst amount from 3.0 to 6.0 mmol results in an increased polymer yield by 15 wt%. However, the isotactic index reduced by about 2 wt%. A further increase of cocatalyst concentration improves polymer yield with a minor decrease in the isotactic index of the polymer. Overall data revealed that an increase in the amount of tetraisobutylaluminoxane from 3.0 to 9.0 mmol considerably improved the polymer yield i.e. by 32 wt% coupled with about a 3 wt% reduction in the isotactic index. The higher polymer yield is due to more formation of both isotactic and atactic polymeric contents. However, the lowering in the ratio of the isotactic/atactic polymer yield from 5.3 to 4.3 results in a reduction of the isotactic index of polypropylene. The productivity of the [Mg, Ti] catalyst was also investigated using triisobutylaluminum as cocatalyst. The results (Table 3) indicate that an increasing amount of cocatalyst brings enhancement in the yield of total, isotactic and atactic polymers while a decreasing trend is observed for the isotactic index. The total polymer yield is increased by 70 wt% and the isotactic index reduces by 11 wt% by increasing the amount of triisobutylaluminum from 3.0 to 9.0 mmol. The decrease in the isotactic index is due to a reduction in the ratio of isotacticjatactic polymer yield from 2.5 to 1.7. Table 2. Effect of tetraisobutylaluminoxane concentration on the ,X~fO!Xl~“Ci Of the cdlalyst*

2. Yield

The mixture of tetraisobutylaluminoxane and triisobutylaluminum is used as a cocatalyst for studying its influence on the performance of the [Mg,Ti] catalyst. The results show (Table 4) that addition of 1.Ommol of triisobutylaluminum in 2.0 mmol of tetraisobutylalumoxane brings a 21 wt% reduction in the polymer yield and about 5 wt% decrease in the isotactic index as compared to the performance of a catalyst with 3.0 mmol of tetraisobutylaluminoxane. A further reduction in the amount of tetraisobutylaluminoxane results in a low polymer yield and isotactic index. These data indicate that the equimolar substitution of tetraisobutylaluminoxane by triisobutylaluminum lowers the activity as well as stereospecificity of the supported titanium catalyst. The comparative performance of tetraisobutylaluminoxane and triisobutylaluminum as cocatalyst with the [Mg, Ti] catalyst for propylene polymerization is shown in Table 5. At 3.0 mmol cocatalyst amount, the [Mg, Ti]/tetraisobutylaluminoxane shows 93% improvement in the polymer yield as well as an 18% increase in the isotactic index of polypropylene as compared to [Mg, Ti]/triisobutylaluminum. However, the degree of enhancement varies with concentration of cocatalyst. It indicates that the cocatalyst concentration also controls the improvement in the activity and stereospecificity of the [Mg, Ti]/tetraisobutylaluminoxane system over [Mg, Ti]/ triisobutylaluminum. In conclusion, the results indicate that use of tetraisobutylaluminoxane in place of triisobutylaluminum influences the performance of the supported Table 4. Efiects of a mixture of tetraisobutylaluminoxane and triisobutylaluminum on the performance of the catalyst (‘Bu,Al),O (mmol)

‘Bu,AI (mmol)

Polymer yield kPP/&dt)

1.1. (%)

1.0 2.0 3.0

285 225 210 200

84 80 74 71

3.0 2.0 1.0

Polymerization conditions as mentioned in Table time = 2 hr.

2 except

Table 5. Comparative performance of [Mg, Ti]/tetrdisobulyl aluminoxane and [Mg, Ti]/triisobutylaluminum for propylene polymerization* Cdldl)‘St

System

Polymer yield (gPP/gCat) (‘BU,AI),O (mm011

Total

Isotactic

Atactic

Yield ratio

1.1. (%)

3.0 6.0 9.0

234 270 310

197 221 251

37 49 59

5.3 4.5 4.3

84 82 81

*Polymerization conditions: PC;= I atm, catalyst = 40 f 2 mg, temp. = 30 C. hexane = 200 ml, time = I hr; 1.1. = lsotactic Index, Yield ratio = isotactic/atactic polymer yield.

[Mg, Ti]/‘Bu,Al

[Mg. Ti]/(‘BuzAl),O

Cocatalyst amount (mmol)

Polymer yield (gPP/gcdt)

1.1. (%)

Polymer yield (gPP/gmt)

1.1. (%)

3.0 6.0 9.0

I21 148 206

71 67 63

234 270 310

84 82 XI

‘Polymerizalion conditions as mentioned in Table 2.

148

V. K. Gupta et al.

titanium catalyst. A higher activity and stereospecificity is observed for the [Mg, Ti] catalyst with tetraisobutylaluminoxane as compared with triisobutylaluminum as cocatalyst. Acknowledgemenr-Authors wish to acknowledge the experimental assistance rendered by Mr A. N. Baria. REFERENCES

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