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Ethylene polymerization with a supported vanadium catalyst obtained by the molecular deposition method
European Pol).raer Journal, Vol. 15, pp. 1075 to 1078
0014-3057 79 1201-1075502.00~0
©Pergamon Press Ltd 1979. Printed in Great Britain
ETHYLENE POLYMERIZATION WITH A S U P P O R T E D V A N A D I U M CATALYST OBTAINED BY THE MOLECULAR DEPOSITION METHOD D. DAMYANOVand M. VELIKOVA Higher Institute of Chemical Technology, 8010 Bourgas, Bulgaria
(Received 21 May 1979) A~tract--An active-phase monolayer has been deposited on SiO2 using replacement of the surface OH groups by VOCI3 vapour. The amount of vanadium fixed on the SiO2 surface depends on the initial concentration of the silanol groups and ranges from 3.36 to 1.43~o. In combination with diethyl aluminium chloride, the products are active catalysts for ethylene polymerization. The effects of the reaction conditions (the time of catalyst-complex formation, the catalyst life time and the temperature of polymerization) as well as the effect of the vanadium content, the AI:V ratio and the presence of diphenyl magnesium on the activity of the catalyst system have been investigated. The catalyst activity was found to depend strongly on the amount of vanadium fixed on the support surface. The maximum productivity obtained is about 22,000 gPE/g vanadium. Some basic characteristics of the synthesized polymer such as tensile strength, elongation at break, density and crystallization degree are given.
INTRODUCTION
The interest in supported catalysts for polymerization of olefines has increased [1-3] recently. F r o m the concept about active centres, one should expect a higher catalytic activity to correspond to a larger surface area and a more uniform distribution of the active phase on it. However, with the classical methods of catalyst deposition (e.g. impregnation), it is difficult to regulate the active phase distribution on the carrier surface. In most cases, aggregates with different forms and sizes are obtained, making it difficult to investigate the structure and chemistry of supported catalysts in which the effect of interatomic surface interactions is decisive with respect to the catalytic properties. The molecular deposition method [4] leads to supported catalysts with new and interesting properties. As is known, this method is based on high reactivity of the surface O H groups of a suitable carrier. If the silanol groups are uniformly distributed on the SiO2 surface, then new groups formed by replacing the silanol groups by a volatile halide or oxyhalide will be uniformly distributed in a monolayer. On this basis, it is possible to obtain very active catalysts and to elucidate the mechanism of catalytic processes. Our previous studies [5-7] showed that when CRO2C12 or TiCI4 are fixed on the SiO2 surface by the molecular deposition method, the products are, in combination with diethyl aluminium chloride and diphenyl magnesium active catalysts for polyerization. On the other hand, Hanke et al. [8] showed that when the surface O H groups of aerosil are replaced by VOCI3, catalysts for the oxidation of butene and ethanol are obtained. The purpose of the present paper was to study the catalytic activity, with respect to ethylene polymerization, of vanadium catalyst monolayers obtained by the molecular deposition method on the SiO2 surface.
EXPERIMENTAL
Preliminary purified amorphous SiO 2 (Vulcasil-S, Bayer, GFR; grain size 20-25 nm) was used as a carrier. A series of samples with various contents of silanol groups were obtained by calcination (48 hr) in a muffle furnace at temperatures between 200 and 800 °. The amount of silanol groups was calculated by extrapolation with respect to the specific surface areas and to the amount of surface OH groups in a Vulcasil sample without intraglobular water. The VOCI3 vapour (Merck, GFR) came into contact, at 150° for 2hr, with the samples preliminary treated in vacuum. The reactor has been described [9]. The Cl-ions in the modified samples were determined by the method of Mohr; V (after extraction in a solution) was determined by atomic absorption spectometry. A 0.5 M solution of diethyl aluminium chloride (Shering A.G., GFR) in isooctane was used as cocatalyst. Diphenyl magnesium (a 2Vo solution in chlorobenzene) synthesized in our laboratory [10] was the second cocatalyst. The reaction medium consisted of purified iso-octane and benzene purified with nitrogen for complete removal of air and dried with molecular sieves Type 4A to a residual water content below 0.001Vo. The monomer had 99.9~ purity. Polymerization was carried out in a 11. stainless-steel reactor permitting continuous stirring and easy control of pressure and temperature. The physico-mechanical characteristics of the polymer were investigated on 1 ram-thick plates obtained by pressing the polymer powder for 5 rain at 190° and 180 kgf/cm 2. The degree of crystallization eas calculated [11] from the density. RESULTS AND DISCUSSION The concentration of silanol groups and their distribution on the SiO2 surface can have a considerable effect on the mechanism of the "secondary" surface reactions involving VOCI3 [12]. For this reason, the amount of surface O H groups of the carrier and its specific surface area should be quantitatively determined. The corresponding data, depending on the 1075
1076
D. DAblYANOVand M. VELIKOVA Table 1. Structural and chemical characteristics of the initial samples and those modified with VOCI3 vapour Precalcination temperature t(°C)
Initial samples ~H Sinitia I (rag eq/g) (m2/g)
200 300 400 500 600 700 800
1.93 1,70 1.52 1.19 1.05 0.82 0.60
161 166 167 160 157 156 137
Modified samples Sin,dr' CI V (m2/g) (rag at/g) 154 149 149 154 152 148 135
0.92 0.88 0.83 0.70 0.55 0.49 0.40
0.66 0.62 0.59 0.45 0.39 0.33 0.28
Cl/V 1.39 1.42 1.41 1.56 1.42 1.47 1.43
tions [12], one can assume that two competitive reactions proceed on the SiO2 surface: - Si---OH + VOCI3---~=Si---OVOCI2+ HCI - Si---OH = Si----O _= Si---OH + VOCI3 ~ Si---O > VOCI + 2HCI.
7
t [*c] o r t'mln] •
8
5
I 3
o i 20
i 40
i 60
J 80
Fig. 1. Dependence of the productivity of the catalyst system on: (a) the life time of the catalyst (V = 0.66 mg at/g, P = 10kgf/cm 2, AI/V = 24, t = 60°); (b) the polymerization temperature (V=0.66mg at/g; P = 10kgf/cm 2, AI/V = 24, Tp = 40 rain).
temperature of precalcination of Vulcasil, are given in the first two columns of Table 1. The same table includes data on the specific surface area, the C1 and V contents and the ratio between them for samples modified with VOCI3. It is evident that the specific surface area of the unmodified samples is practically constant at temperatures from 200 to 700 °. A more pronounced change of the specific surface area is observed after thermal treatment of at 800 °. This may be due to some sintering of the SiO2 skeleton. Comparison of the specific surface areas of the modified and initial samples shows that the deposition of a monolayer of the active phase on the carrier surface causes only negligible changes in the structural characteristics of the samples. Most probably this results from "smoothening" of the surface due to filling of the narrow pores. Table 1 shows also that decreasing amount of silanol groups determines the decrease of the CI and V contents fixed on the surface. This is not surprising as the O H groups are responsible for the surface interactions. This was confirmed by the i.r. spectroscopic studies: no band for OH groups was observed at 3750cm-~ in the spectrum of samples modified with VOCI3 vapour. In accordance with our previous investiga-
(1) (2)
This is confirmed by the fact that the V fixed on the surface is almost always pentavalent, and the values of the CI:V ratio exceed 1. The catalytic behaviour of the synthesized samples is of great interest. By analogy with a similar Ti catalyst system [6], polymerization was carried out at a pressure of 10 kgf/cm 2. Under our experimental conditions (apparatus and hydrodynamic conditions), the catalyst lifetime (,p) was 4 0 m i n (Fig. la). Hence, the polymer yield i.e. the catalyst productivity which is determined by the polyethylene amount corresponding to ! g V can be used as a parameter for comparing catalyst system activities under various polymerization conditions. It was found that the time of catalyst-complex formation (~s) has no effect on its activity (Table 2). However, hydrolysis of the synthesized surface V compounds with water vapour leads, irrespective of the subsequent calcination of the samples at 170 ° for 6 hr, to complete deactivation of the V catalyst. The effect of the temperature of polymerization on the productivity during the lifetime of the catalyst was studied over the usual temperature range (30--80 °, Fig. lb). The pronounced maximum at 60 ° may be due to decreasing solubility of ethylene in the reaction mixture at higher temperatures. For this reason, polymerization was carried out under the following conditions: P = 10 kgf/cm2; t = 60°; zp = 40 min. Table 2. Dependence of the polyethylene yield and the catalyst system productivity on the time of catalyst-complex formation (Lr) at V = 0.66mg at/g, P = 10kgf/cm 2, t = 60°, AI:V = 12, zp = 40min Catalystcomplex formationtime ~f(min)
Yield (g PE/g cat)
Productivity (kg PE/g V)
0 5
101.4 108.2
3.01 3.21
10
117.9
3.50
20 30 Average
100.1 103.3 105.2
2.97 3.21 3.18
Ethylene polymerization with a supported vanadium catalyst
10
1077
20
8
15
6
I0
z. 5 2'0
40 '
6'0
8
80 ' ' 100 ' ~,L/ V
16
2/.
32 M g / v
Fig. 2. Dependence of the catalyst system productivity on the amount of diethyl aluminium chloride (V = 0.66 mg at/g, P = 10 k~/cm2; t = 60°; Tp = 40 min).
Fig. 3. Effect of the Mg:V ratio on the catalyst system productivity (V = 0.39 mg at/g); P = 10 kgf/cm2; t = 60°; rp = 40 min; AI/V = 50).
Under the effect of the diethyl aluminium chloride solution, the V compounds fixed on the carrier surface become highly active in the formation of polymer chains. According to current opinion [1,3], one can suppose that diethyl aluminium chloride reduces a certain amount of V s+, the extent being determined by the solvent nature, the AI :V ratio and the temperature of polymerization. In this connection, Fig. 2 shows the dependence of the system productivity on the AI:V ratio. It is obvious that, with increasing amount of diethyl aluminium chloride, the catalytic activity considerably increases up to an AI:V ratio of about 30; with further increase in the cocatalyst amount, the catalytic activity shows a smoother increase. In a recent study [7], the catalytic properties of molecularly deposited Ti-layers on SiO2 were discussed with the aid of the Cossee's model 1"13] and the results of Pack 1,14] concerning the coordination of the metal atoms on the surface after deposition. According to Cossee, the active centre represents a metal ion in octahedral coordination, which has one alkyl group and one vacant place in its coordination sphere. Pack has shown that the V atoms fixed by the molecular deposition method on silica gel, are coordinatively unsaturated: they can additionally accept one or two molecules. Therefore, one can assume that along with the reduction of V s÷, the surface V ions accept molecules of diethyl aluminium chloride during polymerization. On the basis of the theory about the structure of the active centre, the results agree well
with the decrease in catalytic activity after hydrolysis. This can be due to restoration of part of the silanol groups as a result of breaking of some of the Si--4)--V bonds as well as to coordinative saturation of the V ions with water molecules, which excludes the possibility of interaction with the organo-metallic cocatalysts and the olefine. The application of diphenyl magnesium as a second cocatalyst in the system increases its effectiveness up to a Mg:V ratio of 12 (Fig. 3). At higher values of the Mg:V ratio, the productivity considerably decreases. It could be assumed that, at lower Mg:V ratio values, the diphenyl magnesium is incorporated in the first coordination sphere of the active centre and immediately participates in the distribution of the electron density, leading to additional weakening of the transition metal-carbon bond. For the present it is difficult to explain the deactivating role of the diphenyl magnesium at high Mg:V ratio values. • As already pointed out, the preliminary thermal treatment of the support can lead to the desired density of the silanol groups, and hence, on modification with VOCI3 vapour, the necessary concentration of the transition element can be achieved (Table 1). This concentration can have a significant effect on the catalytic activity 1"8]. For this reason, the dependence of the catalyst system productivity on the temperature of preliminary dehydroxylation of the Vulcasil, i.e. on the content of deposited V 5+, was studied (Fig. 4). Comparison of the data from chemical analysis of the samples with data on their catalytic activity shows
Table 3. Some characteristics of the synthesized polyethylene depending on the thermal pretreatment of the support (P = 10 kgf/cm2, t = 60°, rp = 40 min, AI/V = 60)
No.
Sample
1 2 3 4 5 6 7 Average
VS-200 VS-300 VS-400 VS-500 VS-600 VS-700 VS-800
Tensile strength of(kgf/cm 2)
Elongation at break E(~o)
Density d (g/cm3)
246 264 266 239 311 273 266 267
241 243 211 228 257 217 239 233
0.931 0.933 0.935 0.934 0.931 0.933 0.933 0..933
Crystallization degree ~(9/o) 52.5 54.0 55.5 54.7 52.5 - 54.0 53.8 53.9
1078
D. DAMYANOVand M. VELIKOVA
t.U
22 1B
10
36o
sGo 6oo
Fig. 4. Dependence of the catalyst system productivity on the amount of vanadium fixed on Vulcasil precalcinated at different temperatures (AI:V = 60; P = 10kgf/cm2; t = 60°; zp = 40 min). that, with increasing temperature of precalcination up to 500°; both the CI:V ratio and the catalyst system activity increase. The polyethylene yield with the sample treated at 500 ° is more than twice that with the sample heated at 200 °. It is difficult to explain this fact without additional investigations on the structure of the active phase formed on the carrier surface. The synthesized polyethylene has a high molecular mass ( ~ 2 x 10-6). The data on some basic characteristics of the polymer presented in Table 3 indicate some of its properties. Evidently, the polymer samples have not the physico-mechanical indices characteristic of polyethylene with such a high molecular mass. The crystallization degree corresponds to that of ultra high-molecular polyethylene. However, it should be taken into account that the results concern samples obtained by hot (190 °) pressing. Owing to the very large molecular mass of the samples which are not completely soluble in decalin at 135 °, they cannot be homogeneously melted, which results in low physicomechanical indices of the polymer. In contrast, the method of cold pressing [15] leads to considerably higher strength parameters, confirmed by the good characteristics of the samples prepared by us according to this method.
It should be pointed out that molecular deposition method is very promising with respect to the preparation of effective catalysts for ethylene polymerization. On the basis of the present investigations, however, it is difficult to give a more detailed description of the structure of the compounds being formed in the catalyst system and the mechanism of the processes taking place there. Additional studies of the degree of V 5÷ reduction, aiming at identifying the surface compounds being formed during the activation, should be carried out. Our results demonstrate, above all, the applicability of the molecular deposition method to the synthesis of new kinds of catalysts for the polymerization of olefines.
REFERENCES
I. R. Kroker, M. Schneider and K. Hamann. Usp. Khim. 43, 349 (1974). 2. G. V. Grozdova, Khim. Prom. Rub. 1, 26 (1974). 3. N. M. Chirkov, P. E. Matkovskii and F. S. Dyachkovskii, Polimerizatsiya na kompleksnykh metalloroanicheskikh katalizatorakh. Khimiya, Moskva (1976). 4. S. I. Kortsov, G. N. Kuznetsova and V. B. Aleskovskii, Zh. priM. Khim. 40, 2774 (1967). 5. D. Mehandjiev, S. An#cloy and D. Damyanov, Proc. 2nd Int. Syrup. Sci. Bases Prep. Het. Catalysts. 4-7. September 1978, Louvain-La-Neuve, p. 605. 6. D. Damyanov, M. Velikova and L. Petkov, Eur. Polym. J. 15, 233 (1979). 7. D. Damyanov, M. Velikova, S. Angelov and D. Mehandjiev, Proc 4th Int. Symp. Heterogeneous Catalysis, Varna, 2-5 October, 2, 139 (1979). 8. W. Hanke, K. Heise, H.-G. Jerschkewitz, G. Lischke, G. Ohlmann and B. Parlitz, Z. anor#, all#. Chem. 438, 176 (1978). 9. D. Damyanov and Ts. Obretenov, God. VKhTI Bur#as 12, 159 (1977). 10. Yu. N. Baryshnikov and A. A. Kvasov, Trudy khim. khim. Tekh. 2, 154 (1966). 11. E. W. Fischer and G. F. Schmidt, Anoew. Chem. 74, 551 (1962). 12. D. Damyanov and M. Velikova, Submitted for publication. 13. P. Cossee, J. Catalys. 3, 80 (1964). 14, V. N. Pack, Zh.fiz. khim..~0, 1404 (1976). 15. Bulg. Patent No. 23601 (1977).
0014-3057 79 1201-1075502.00~0
©Pergamon Press Ltd 1979. Printed in Great Britain
ETHYLENE POLYMERIZATION WITH A S U P P O R T E D V A N A D I U M CATALYST OBTAINED BY THE MOLECULAR DEPOSITION METHOD D. DAMYANOVand M. VELIKOVA Higher Institute of Chemical Technology, 8010 Bourgas, Bulgaria
(Received 21 May 1979) A~tract--An active-phase monolayer has been deposited on SiO2 using replacement of the surface OH groups by VOCI3 vapour. The amount of vanadium fixed on the SiO2 surface depends on the initial concentration of the silanol groups and ranges from 3.36 to 1.43~o. In combination with diethyl aluminium chloride, the products are active catalysts for ethylene polymerization. The effects of the reaction conditions (the time of catalyst-complex formation, the catalyst life time and the temperature of polymerization) as well as the effect of the vanadium content, the AI:V ratio and the presence of diphenyl magnesium on the activity of the catalyst system have been investigated. The catalyst activity was found to depend strongly on the amount of vanadium fixed on the support surface. The maximum productivity obtained is about 22,000 gPE/g vanadium. Some basic characteristics of the synthesized polymer such as tensile strength, elongation at break, density and crystallization degree are given.
INTRODUCTION
The interest in supported catalysts for polymerization of olefines has increased [1-3] recently. F r o m the concept about active centres, one should expect a higher catalytic activity to correspond to a larger surface area and a more uniform distribution of the active phase on it. However, with the classical methods of catalyst deposition (e.g. impregnation), it is difficult to regulate the active phase distribution on the carrier surface. In most cases, aggregates with different forms and sizes are obtained, making it difficult to investigate the structure and chemistry of supported catalysts in which the effect of interatomic surface interactions is decisive with respect to the catalytic properties. The molecular deposition method [4] leads to supported catalysts with new and interesting properties. As is known, this method is based on high reactivity of the surface O H groups of a suitable carrier. If the silanol groups are uniformly distributed on the SiO2 surface, then new groups formed by replacing the silanol groups by a volatile halide or oxyhalide will be uniformly distributed in a monolayer. On this basis, it is possible to obtain very active catalysts and to elucidate the mechanism of catalytic processes. Our previous studies [5-7] showed that when CRO2C12 or TiCI4 are fixed on the SiO2 surface by the molecular deposition method, the products are, in combination with diethyl aluminium chloride and diphenyl magnesium active catalysts for polyerization. On the other hand, Hanke et al. [8] showed that when the surface O H groups of aerosil are replaced by VOCI3, catalysts for the oxidation of butene and ethanol are obtained. The purpose of the present paper was to study the catalytic activity, with respect to ethylene polymerization, of vanadium catalyst monolayers obtained by the molecular deposition method on the SiO2 surface.
EXPERIMENTAL
Preliminary purified amorphous SiO 2 (Vulcasil-S, Bayer, GFR; grain size 20-25 nm) was used as a carrier. A series of samples with various contents of silanol groups were obtained by calcination (48 hr) in a muffle furnace at temperatures between 200 and 800 °. The amount of silanol groups was calculated by extrapolation with respect to the specific surface areas and to the amount of surface OH groups in a Vulcasil sample without intraglobular water. The VOCI3 vapour (Merck, GFR) came into contact, at 150° for 2hr, with the samples preliminary treated in vacuum. The reactor has been described [9]. The Cl-ions in the modified samples were determined by the method of Mohr; V (after extraction in a solution) was determined by atomic absorption spectometry. A 0.5 M solution of diethyl aluminium chloride (Shering A.G., GFR) in isooctane was used as cocatalyst. Diphenyl magnesium (a 2Vo solution in chlorobenzene) synthesized in our laboratory [10] was the second cocatalyst. The reaction medium consisted of purified iso-octane and benzene purified with nitrogen for complete removal of air and dried with molecular sieves Type 4A to a residual water content below 0.001Vo. The monomer had 99.9~ purity. Polymerization was carried out in a 11. stainless-steel reactor permitting continuous stirring and easy control of pressure and temperature. The physico-mechanical characteristics of the polymer were investigated on 1 ram-thick plates obtained by pressing the polymer powder for 5 rain at 190° and 180 kgf/cm 2. The degree of crystallization eas calculated [11] from the density. RESULTS AND DISCUSSION The concentration of silanol groups and their distribution on the SiO2 surface can have a considerable effect on the mechanism of the "secondary" surface reactions involving VOCI3 [12]. For this reason, the amount of surface O H groups of the carrier and its specific surface area should be quantitatively determined. The corresponding data, depending on the 1075
1076
D. DAblYANOVand M. VELIKOVA Table 1. Structural and chemical characteristics of the initial samples and those modified with VOCI3 vapour Precalcination temperature t(°C)
Initial samples ~H Sinitia I (rag eq/g) (m2/g)
200 300 400 500 600 700 800
1.93 1,70 1.52 1.19 1.05 0.82 0.60
161 166 167 160 157 156 137
Modified samples Sin,dr' CI V (m2/g) (rag at/g) 154 149 149 154 152 148 135
0.92 0.88 0.83 0.70 0.55 0.49 0.40
0.66 0.62 0.59 0.45 0.39 0.33 0.28
Cl/V 1.39 1.42 1.41 1.56 1.42 1.47 1.43
tions [12], one can assume that two competitive reactions proceed on the SiO2 surface: - Si---OH + VOCI3---~=Si---OVOCI2+ HCI - Si---OH = Si----O _= Si---OH + VOCI3 ~ Si---O > VOCI + 2HCI.
7
t [*c] o r t'mln] •
8
5
I 3
o i 20
i 40
i 60
J 80
Fig. 1. Dependence of the productivity of the catalyst system on: (a) the life time of the catalyst (V = 0.66 mg at/g, P = 10kgf/cm 2, AI/V = 24, t = 60°); (b) the polymerization temperature (V=0.66mg at/g; P = 10kgf/cm 2, AI/V = 24, Tp = 40 rain).
temperature of precalcination of Vulcasil, are given in the first two columns of Table 1. The same table includes data on the specific surface area, the C1 and V contents and the ratio between them for samples modified with VOCI3. It is evident that the specific surface area of the unmodified samples is practically constant at temperatures from 200 to 700 °. A more pronounced change of the specific surface area is observed after thermal treatment of at 800 °. This may be due to some sintering of the SiO2 skeleton. Comparison of the specific surface areas of the modified and initial samples shows that the deposition of a monolayer of the active phase on the carrier surface causes only negligible changes in the structural characteristics of the samples. Most probably this results from "smoothening" of the surface due to filling of the narrow pores. Table 1 shows also that decreasing amount of silanol groups determines the decrease of the CI and V contents fixed on the surface. This is not surprising as the O H groups are responsible for the surface interactions. This was confirmed by the i.r. spectroscopic studies: no band for OH groups was observed at 3750cm-~ in the spectrum of samples modified with VOCI3 vapour. In accordance with our previous investiga-
(1) (2)
This is confirmed by the fact that the V fixed on the surface is almost always pentavalent, and the values of the CI:V ratio exceed 1. The catalytic behaviour of the synthesized samples is of great interest. By analogy with a similar Ti catalyst system [6], polymerization was carried out at a pressure of 10 kgf/cm 2. Under our experimental conditions (apparatus and hydrodynamic conditions), the catalyst lifetime (,p) was 4 0 m i n (Fig. la). Hence, the polymer yield i.e. the catalyst productivity which is determined by the polyethylene amount corresponding to ! g V can be used as a parameter for comparing catalyst system activities under various polymerization conditions. It was found that the time of catalyst-complex formation (~s) has no effect on its activity (Table 2). However, hydrolysis of the synthesized surface V compounds with water vapour leads, irrespective of the subsequent calcination of the samples at 170 ° for 6 hr, to complete deactivation of the V catalyst. The effect of the temperature of polymerization on the productivity during the lifetime of the catalyst was studied over the usual temperature range (30--80 °, Fig. lb). The pronounced maximum at 60 ° may be due to decreasing solubility of ethylene in the reaction mixture at higher temperatures. For this reason, polymerization was carried out under the following conditions: P = 10 kgf/cm2; t = 60°; zp = 40 min. Table 2. Dependence of the polyethylene yield and the catalyst system productivity on the time of catalyst-complex formation (Lr) at V = 0.66mg at/g, P = 10kgf/cm 2, t = 60°, AI:V = 12, zp = 40min Catalystcomplex formationtime ~f(min)
Yield (g PE/g cat)
Productivity (kg PE/g V)
0 5
101.4 108.2
3.01 3.21
10
117.9
3.50
20 30 Average
100.1 103.3 105.2
2.97 3.21 3.18
Ethylene polymerization with a supported vanadium catalyst
10
1077
20
8
15
6
I0
z. 5 2'0
40 '
6'0
8
80 ' ' 100 ' ~,L/ V
16
2/.
32 M g / v
Fig. 2. Dependence of the catalyst system productivity on the amount of diethyl aluminium chloride (V = 0.66 mg at/g, P = 10 k~/cm2; t = 60°; Tp = 40 min).
Fig. 3. Effect of the Mg:V ratio on the catalyst system productivity (V = 0.39 mg at/g); P = 10 kgf/cm2; t = 60°; rp = 40 min; AI/V = 50).
Under the effect of the diethyl aluminium chloride solution, the V compounds fixed on the carrier surface become highly active in the formation of polymer chains. According to current opinion [1,3], one can suppose that diethyl aluminium chloride reduces a certain amount of V s+, the extent being determined by the solvent nature, the AI :V ratio and the temperature of polymerization. In this connection, Fig. 2 shows the dependence of the system productivity on the AI:V ratio. It is obvious that, with increasing amount of diethyl aluminium chloride, the catalytic activity considerably increases up to an AI:V ratio of about 30; with further increase in the cocatalyst amount, the catalytic activity shows a smoother increase. In a recent study [7], the catalytic properties of molecularly deposited Ti-layers on SiO2 were discussed with the aid of the Cossee's model 1"13] and the results of Pack 1,14] concerning the coordination of the metal atoms on the surface after deposition. According to Cossee, the active centre represents a metal ion in octahedral coordination, which has one alkyl group and one vacant place in its coordination sphere. Pack has shown that the V atoms fixed by the molecular deposition method on silica gel, are coordinatively unsaturated: they can additionally accept one or two molecules. Therefore, one can assume that along with the reduction of V s÷, the surface V ions accept molecules of diethyl aluminium chloride during polymerization. On the basis of the theory about the structure of the active centre, the results agree well
with the decrease in catalytic activity after hydrolysis. This can be due to restoration of part of the silanol groups as a result of breaking of some of the Si--4)--V bonds as well as to coordinative saturation of the V ions with water molecules, which excludes the possibility of interaction with the organo-metallic cocatalysts and the olefine. The application of diphenyl magnesium as a second cocatalyst in the system increases its effectiveness up to a Mg:V ratio of 12 (Fig. 3). At higher values of the Mg:V ratio, the productivity considerably decreases. It could be assumed that, at lower Mg:V ratio values, the diphenyl magnesium is incorporated in the first coordination sphere of the active centre and immediately participates in the distribution of the electron density, leading to additional weakening of the transition metal-carbon bond. For the present it is difficult to explain the deactivating role of the diphenyl magnesium at high Mg:V ratio values. • As already pointed out, the preliminary thermal treatment of the support can lead to the desired density of the silanol groups, and hence, on modification with VOCI3 vapour, the necessary concentration of the transition element can be achieved (Table 1). This concentration can have a significant effect on the catalytic activity 1"8]. For this reason, the dependence of the catalyst system productivity on the temperature of preliminary dehydroxylation of the Vulcasil, i.e. on the content of deposited V 5+, was studied (Fig. 4). Comparison of the data from chemical analysis of the samples with data on their catalytic activity shows
Table 3. Some characteristics of the synthesized polyethylene depending on the thermal pretreatment of the support (P = 10 kgf/cm2, t = 60°, rp = 40 min, AI/V = 60)
No.
Sample
1 2 3 4 5 6 7 Average
VS-200 VS-300 VS-400 VS-500 VS-600 VS-700 VS-800
Tensile strength of(kgf/cm 2)
Elongation at break E(~o)
Density d (g/cm3)
246 264 266 239 311 273 266 267
241 243 211 228 257 217 239 233
0.931 0.933 0.935 0.934 0.931 0.933 0.933 0..933
Crystallization degree ~(9/o) 52.5 54.0 55.5 54.7 52.5 - 54.0 53.8 53.9
1078
D. DAMYANOVand M. VELIKOVA
t.U
22 1B
10
36o
sGo 6oo
Fig. 4. Dependence of the catalyst system productivity on the amount of vanadium fixed on Vulcasil precalcinated at different temperatures (AI:V = 60; P = 10kgf/cm2; t = 60°; zp = 40 min). that, with increasing temperature of precalcination up to 500°; both the CI:V ratio and the catalyst system activity increase. The polyethylene yield with the sample treated at 500 ° is more than twice that with the sample heated at 200 °. It is difficult to explain this fact without additional investigations on the structure of the active phase formed on the carrier surface. The synthesized polyethylene has a high molecular mass ( ~ 2 x 10-6). The data on some basic characteristics of the polymer presented in Table 3 indicate some of its properties. Evidently, the polymer samples have not the physico-mechanical indices characteristic of polyethylene with such a high molecular mass. The crystallization degree corresponds to that of ultra high-molecular polyethylene. However, it should be taken into account that the results concern samples obtained by hot (190 °) pressing. Owing to the very large molecular mass of the samples which are not completely soluble in decalin at 135 °, they cannot be homogeneously melted, which results in low physicomechanical indices of the polymer. In contrast, the method of cold pressing [15] leads to considerably higher strength parameters, confirmed by the good characteristics of the samples prepared by us according to this method.
It should be pointed out that molecular deposition method is very promising with respect to the preparation of effective catalysts for ethylene polymerization. On the basis of the present investigations, however, it is difficult to give a more detailed description of the structure of the compounds being formed in the catalyst system and the mechanism of the processes taking place there. Additional studies of the degree of V 5÷ reduction, aiming at identifying the surface compounds being formed during the activation, should be carried out. Our results demonstrate, above all, the applicability of the molecular deposition method to the synthesis of new kinds of catalysts for the polymerization of olefines.
REFERENCES
I. R. Kroker, M. Schneider and K. Hamann. Usp. Khim. 43, 349 (1974). 2. G. V. Grozdova, Khim. Prom. Rub. 1, 26 (1974). 3. N. M. Chirkov, P. E. Matkovskii and F. S. Dyachkovskii, Polimerizatsiya na kompleksnykh metalloroanicheskikh katalizatorakh. Khimiya, Moskva (1976). 4. S. I. Kortsov, G. N. Kuznetsova and V. B. Aleskovskii, Zh. priM. Khim. 40, 2774 (1967). 5. D. Mehandjiev, S. An#cloy and D. Damyanov, Proc. 2nd Int. Syrup. Sci. Bases Prep. Het. Catalysts. 4-7. September 1978, Louvain-La-Neuve, p. 605. 6. D. Damyanov, M. Velikova and L. Petkov, Eur. Polym. J. 15, 233 (1979). 7. D. Damyanov, M. Velikova, S. Angelov and D. Mehandjiev, Proc 4th Int. Symp. Heterogeneous Catalysis, Varna, 2-5 October, 2, 139 (1979). 8. W. Hanke, K. Heise, H.-G. Jerschkewitz, G. Lischke, G. Ohlmann and B. Parlitz, Z. anor#, all#. Chem. 438, 176 (1978). 9. D. Damyanov and Ts. Obretenov, God. VKhTI Bur#as 12, 159 (1977). 10. Yu. N. Baryshnikov and A. A. Kvasov, Trudy khim. khim. Tekh. 2, 154 (1966). 11. E. W. Fischer and G. F. Schmidt, Anoew. Chem. 74, 551 (1962). 12. D. Damyanov and M. Velikova, Submitted for publication. 13. P. Cossee, J. Catalys. 3, 80 (1964). 14, V. N. Pack, Zh.fiz. khim..~0, 1404 (1976). 15. Bulg. Patent No. 23601 (1977).