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E. F. PETERS, ALEX ZLETZ,
and B. L. EVERING
Research Department, Standard Oil Co. (Indiana), Whiting, Ind.
Solid Catalysts in Ethylene Polymerization Toughness and high crystallinity are but two of the desirable properties of these low-pressure polyethylenes made over solid catalysts
FOR
some 15 years polyethylene has been made at high pressure with peroxide catalysts. The cost of high-pressure operation has encouraged research toward a low-pressure process. Recently three such processes have been developed that use different catalysts to produce new, highly crystalline polymers of great current interest. The use of aluminum alkyls with titanium tetrachloride (6, 7) and of chromia-silicaalumina catalysts ( 7 ) has been described in the literature. O n the other hand, the earliest low-pressure process for polymerizing olefins over solid catalysts to high-molecular-weight polymers has appeared only in patents (2, 3, 4, 8 ) . Heterogeneous polymerization of olefins over solid catalysts to high-molecularweight polymer stemmed from a discovery made while investigating alkylation of ethylene to light hydrocarbons suitable for gasoline. When ethylene was converted over a cobalt-charcoal catalyst in the liquid phase with an aromatic solvent, the expected alkylation did not occur, but sufficient polyethylene was produced to justify further investigation. The catalytic activity of metals and metal oxides on various supports was therefore extensively explored. Relationships among the metal,
metal oxide, support, and oxidationreduction state of the catalyst were found to affect the activity of the catalyst and the properties of the polymer. A number of combinations were found to be active; nickel-charcoal and molybdenaalumina are good examples of the metal and metal-oxide catalyst systems, respectively. These two catalysts were investigated for ethylene polymerization in batch and flow reactors. The effects of temperature, pressure, catalyst particle size, and solvents on the yields and properties of the polymer were determined in batch reactors. Reactivation and regeneration of the catalyst were studied in a flow reactor.
Materials
Ethylene containing 25 p.p.m. or less of oxygen was used. Solvents were usually distilled and dried before use. The nickel-charcoal catalyst was prepared by impregnating a charcoal support with sufficient nickel nitrate to give 5% nickel content. Heating it to 260' C. converted the nickel nitrate to the oxide. Higher concentrations showed no beneficial effect. Coconut
charcoal leached with nitric acid was among the best supports. Although a number of charcoals of varying effectiveness were investigated, no simple correlation between surface area and pore structure was found. The molybdena-alumina catalyst contained 8% molybdena on alumina and was made either by impregnation or coprecipitation. It was finished by calcining a t 500' to 600' C. in the conventional manner and was generally used as either 6- to 14-mesh granules or powder. Although various other compositions were effective, best results were obtained with a maximum dispersion of molybdena on the support, which occurred in the range of 5 to 25%. Both catalysts have little activity in the oxidized state and must be partially reduced before use. Nickel-charcoal was activated by heating a t 200' to 260' C. with commercial hydrogen. Molybdena-alumina was activated by heating a t 430' to 480' C. with hydrogen a t atmospheric or elevated pressures. Activation occurs readily a t atmospheric pressure, but raising the pressure to 75 pounds per square inch increased the activation rate about 4Oy0 and further increases as high as 1000 pounds per square inch had no additional effect. VOL. 49, NO. 11
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For both 5% nickel-charcoal and 8% molybdena-alumina in granular form, a contact time of about 30 minutes was generally used. A study o f the activation variables on molybdena-alumina had established that optimum contact time varied with pressure, temperature, molybdena content, support, and particle size. High molybdena contents required longer contact; small particle sizes required shorter contact. Such other reducing agents as carbon monoxide, sulfur dioxide, and hydrocarbons also had an activating effect. The dependence of reduction on so many variables emphasized the criticality of placing the catalyst in the proper valence state for optimum results.
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TEMPERATURE, "C.
Figure 1 .
Effect of temperature on polymer yield
Equipment and Procedures
The batch reactors were presnure vessels holding 183 ml. and were agitated in a small rocker. The flow reactor was a stainless steel tube 1 inch in diameter and 32 inches long; it held about 200 ml. of catalyst around a coaxial thermowell. I n batch runs, 25 grams of granular or powdered catalyst was charged to the reactor and activated with hydrogen. The temperature was lowered to the desired range, 100 ml. of solvent was added, and ethylene was pressured into 1000 pounds per square inch. Additional ethylene was added when the pressure dropped to 800 pounds per square inch. Reaction time was usually 2 hours. Polymer produced was extracted from the catalyst with boiling xylene. In the flow run, 200 ml. of 8% molybdena-alumina was charged to the reactor and activated a t 455' C. with hydrogen at 200 pounds per square inch for 30 minutes. Ethylene and solvent were mixed in the desired ratio in a vessel equipped with a Jerguson gage. The feed was pumped into the bottom of the reactor, which was operated upflow so as to be liquid-full a t all times. Reactor pressure was 800 to 1000 pounds per square inch. The product and solvent were collected at atmospheric pressure; any condensible gases were collected in a receiver cooled with dry ice, and fixed gases were measured in a wet-test meter. The polymer was recovered from the reactor effluent essentially free of catalyst. Polymer produced in both the batch and flow experiments was separated into three fractions: solid that precipitated from xylene or other solvents on cooling, semisolid that remained after stripping off the solvent, and liquid that was fractionated from the solvent. On occasion, further solid was separated from the semisolid fraction by redissolving in xylene and precipitating with acetone. The quality of the polymer was deter-
1880
Molybdena-alumina catalyst
mined from the specific viscosity and density. The specific viscosities were measured by the Staudinger method (5); a solution of 0.125 grams of polyethylene in 100 ml. of xylene a t 110' C. was used. Melt viscosity, melting point, methylene-to-methyl ratio, position of double bond, and crystallinity were also determined occasionally. The methylene-to-methyl ratio and position of double bond were measured by infrared absorption. Effect of Process Variables
The process variables were evaluated in terms of the yield and specific viscosity of the solid polymer. Because conditions best suited for studying the process variables were chosen, yields were not necessarily optimum. Temperature, pressure, catalyst particle size, and solvent were studied in the batch reactors. The poisoning effect of trace amounts o f feed impurities was also examined.
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Temperature had a pronounced effect on the distribution of products. Figure 1 shows the linear decrease in the yield of polyethylene obtained with molybdenaalumina catalyst as temperature was increased. O n the other hand, the yield of semisolid and oil rose with increase in temperature; below 200' C. it was small, but a t 300' C. it rose rapidly, and above 320' C. the product was predominantly liquid. Temperature also had a pronounced effect on the specific viscosity of the polymer. Figure 2 shows the marked decrease in specific viscosity with increase in temperature. Molybdenaalumina tended to produce polymers of higher specific viscosity than nickelcharcoal, even a t much higher temperatures. Pressure affected the yield of polyethylene in batch reactors, where both liquid and gas phases were present. The yield was low at atmospheric pressure, increased rapidly as pressure was raised
9 MOLYBDENA ALUMINA
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Effect of temperature and catalyst on polymer
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ETHYLENE P O L Y M E R I Z A T I O N perature had the greatest influence on polymer properties. I t largely controlled the specific viscosity of the polymer. As high a temperature as possible should be used to obtain the maximum solvent action for keeping the catalyst free of polymer while maintaining the specific viscosity in the desired range. This balance was more rapidly obtained with molybdena-alumina than with nickel-charcoal because it produced polymer three- to fivefold higher in molecular weight. More attention was therefore directed to the molybdenaalumina system.
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Reactivation and Regeneration
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Solubility of ethylene in xylene
to 300 pounds per square inch, and then leveled out. Experience in a flow reactor, operated in the liquid phase, has shown that pressure is not critical; only enough is needed to maintain the desired ethylene concentration in solution. The minimum pressures needed are given in Figure 3 in terms of the solubility of ethylene in xylene. Solubility increased with pressure, rapidly a t low temperatures and more slowly at high temperature. Size of the catalyst particles affected the specific viscosity of the polymer, as shown for nickel-charcoal in Figure 4. In passing from granules of 6- to 14mesh to powder of 400-mesh, a linear increase occurred in the specific viscosity of the polymer. The low-viscosity polymer was brittle, whereas the high-viscosity polymer was tough and flexible. The yield of polymer was unaffected by particle size. Under optimum conditions, powdered catalyst produced polymer with a specific viscosiiy of 0.2 in much higher yields. Solvents varied in ability to dissolve polyethylene and apparently affected the specific viscosity of the polymer. An important requisite in the production of polyethylene over solid catalyst is the use of an efficient solvent that will remove the polyethylene as formed so as not to reduce the activity of the catalyst. Solvent type affected specific viscosity of the polymer, especially that from nickel-charcoal catalyst which formed polymer of lower molecular weight. The solvent not only dissolved the polyethylene, it seemed to act also as a reducing medium that established the valence level of the catalyst system. The reducing action of hydrocarbons is easily observed with vanadia and chromia catalysts; in the presence of paraffin solvents under reaction conditions, they undergo color changes that indicate reduction to a lower valence state.
Poisons must be excluded from the catalyst system. The relative poisoning effects of oxygen, water, and acetylene were determined by adding small amounts in standard reference runs. The results with molybdena-alumina a t 232' C. are shown in Figure 5. As little as 20 mole yooxygen, based on the molybdena content of the catalyst, cut the activity by 90% and lowered the specific viscosity of the polymer. Water was far less detrimental. Nevertheless, the solvent and ethylene should be carefully dried, and precautions should be taken to strip off any water absorbed on the catalyst as a result of reduction. Acetylene could be tolerated. Of all the variables studied, tem-
Figure 4. Effect of particle size
The decline in catalyst activity and reactivation of the catalyst were demonstrated by operation in the flow reactor. Feed consisting of 4.4% ethylene in xylene was pumped upflow at a space velocity of 2.0 and a temperature of 237' C. When conversion dropped to about 25%, the catalyst was reactivated by heating to 455' C. for 5 minutes with hydrogen a t 200 pounds per square inch. Figure 6 shows a run carried through four reactivations. Catalyst activity was completely restored in each case, and cycle length even increased slightly as the catalyst aged. Because the solvent and ethylene were not recycled, the catalyst was subjected to maximum contact with any poison in the feed. Although cycle length was relatively short, the cycle could be repeated many times. However, reactivation of the catalyst eventually became
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Figure 5. Effect of poisons on molybdena catalyst
Table 1.
Polymerization Type
Density, Grams/&.
Crystallinity, % '
0.96 0.95 0.92
87 80 65
Molybdena-alumina Nickel-charcoal Conventional high pressure a
Properties of Ethylene Polymers
Initial Yield Modulus", Total Strengtha, Lb./Sq. In. Elongationa, Lb./Sq. In. X lo 4000 3000
15.5 10.6
1000
1.8
680 460 675
ASTM D 882 test method.
less effective because of buildup of cokelike deposits. Spent catalyst was readily regenerated to its original activity by burning off the deposits with air at 430" C. and reducing with hydrogen.
nickel-charcoal polymers. The methylene-to-methyl ratio, which increased somewhat with increase in specific viscosity, was about 50 for the nickelcharcoal polymer and as high as 500 for rnolybdena-alumina polymers. Double bonds were about 707, internal and 30% terminal in the molybdena-alumina polymer and evenly distributed in the-nickel-charcoal poltmer. Chain transfer with the solvent did not occur in the process because no aromatic groups were detected in the polymer when aromatic solvents were used. Polymers obtained from these cata-
Nature of Polymer
The specific viscosity of the polymers produced over nickel-charcoal was generally 0.1 to 0.2; that of polymer produced over molybdena-alumina ranged from 0.2 to 0.6 and even higher. The degree of branching was greater for
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Response of catalyst to reactivation
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lysts were highly crystalline. The density of 0.95 for the nickel-charcoal polymers corresponds to about 80y0 crystallinity. Polymers from molybdenaalumina catalyst had a density range of 0.94 to 0.98, which corresponds to a crystallinity range of 76 to %yo. The nickel-charcoal polymers were softer, more flexible, and more soluble than those from molybdena-alumina. Polymers obtained by such low-pressure polymerization over solid catalysts have higher methylene-to-methyl ratios than the present-day high-pressure polyethylene. Low branching accounts for the higher density and crystallinity of the low-pressure polymer. Table I compares the properties of the lowpressure and high-pressure polymers. Because of high crystallinity, the lowpressure polymers exhibit much higher yield strengths and initial moduli. The properties of the nickel-charcoal polymer are intermediate but closer to the molybdena-alumina polymer than the high-pressure polymer. Molybdena-alumina polymers melt a t about 130" C. The high-melting point is a valuable property that permits sterilization and high temperature uses of molded objects. This new class of polymers will have many uses where important considerations are low permeability to liquids and gases, high softening temperatures. and increased rigidity, toughness, strength, and chemical resistance. Acknowledgment
The present work represents the combined efforts of many members of the research staff of the Standard Oil Co. (Indiana). The authors especially acknowledge the contributions of A. K. Roebuck, D. R. Carrnody, UT. S . Higley, A. A . Harban, R. R. Hopkins, Mary Elizabeth Turney, Herman Hoeksema, and Harold Shalit. Literature Cited
Clark, .4lfred, Hogan, J. P., Banks, R. L., Lanning, W. C., IND.END. CHEW48,1152 (1956). Peters, E. F., Everin , B. L. [to Standard Oil Co. (Ind$], U. S. Patent 2,658,059 (Kov. 3, 1953). (3) Ibid., 2,692,261 (Oct. 19, 1954). (4) Roebuck, A. K., Zletz, Alex [to Standard Oil Co. (Ind.)], U. S. Patent 2,692,258 (Oct. 19, 1954). Staudinger, H., Heuer, W., Z . Phvs. Chem. 171A, 129 (1934). Ziegler, K., Belgian Patent 533,362 (Nov. 16,1954). Zieyler, K., Holzkamp, E., Breil, H., Martin, H., Angew. Chem. 67, 541 (1955). Zletz, Alex [to Standard Oil Co. (Ind.)], U. S. Patent 2,692,257 (Oct. 19,1954). RECEIVED for review October 4, 1956 h C E P T E D April 26, 1957 Division of Polymer Chemistry, 130th Meeting, ACS, Atlantic City, N. J., September 1956.
