I
EDMUND FIELD and MORRIS FELCER Research Department, Standard Oil Co. (Indiana), Whiting, Ind.
Promoted Molybdena-Alumina Catalysts in Ethylene Polymerization With these promoters, the amount of polymer produced without catalyst regeneration may be increased as much as 200-fold
T H E polymerization of ethylene over a molybdena-alumina catalyst by a regenerative process has already been described ( 3 ) . This process requires hydrogen treatment of the catalyst after each cycle, as well as an occasional oxygen treatment to remove carbon. Both the reduction and the oxidation stages require temperatures well above those needed for polymerization. The need for regeneration has been almost eliminated by the discovery that electropositive metals and their hydrides are excellent promoters for the molybdenaalumina catalyst (7). To elucidate the role of these promoters, they were used in a wide variety of polymerization experiments carried out in the laboratory. Many promoters for molybdena-alumina were tested, of which sodium, calcium hydride, ahd lithium aluminum hydride are typical. Promoters were evaluated mainly in terms of the amount of polyethylene produced. No significant effect on polymer structure was observed.
Experimental
Commercial molybdena-alumina hydroforming catalyst that had been ground to 20 to 30 mesh was used for these studies. In contrast to the case with an unpromoted system, reduction of the molybdena-alumina is optional. However, prereduction was preferred and was carried out in a glass tube by passing dry hydrogen at atmospheric pressure through 0.1 to 10 grams of catalyst a t a rate of 5 to 10 liters per
hour at 480' C. for 16 hours. After it had cooled to room temperature, the catalyst was transferred under an inert atmosphere to a Magne-Dash reactor ( 2 ) which contained the solvent. For 0.1 gram of molybdena-alumina, typical charges of solvent and promoter were 50 ml. of purified benzene and 0.08 gram of C.P. sodium. After being pressure tested for leaks, the reactor was heated to 230' C. with sufficient agitation to keep the catalyst uniformly suspended in the solvent. At this point, purified ethylene was slowly admitted to the reactor to allow saturation at 1000 pounds per square inch. When polymerization reduced the pressure to about 900 pounds per square inch, more ethylene was introduced to restore it to 1000 pounds per square inch. With this procedure, 0.1 gram of catalyst produced 2 to 10 grams of polyethylene in 17 hours. When the polymerization ended, the cooled contents of the reactor were transferred to a container of wet alcohol, which destroyed any excess sodium. A sequence of water and acetone washes produced a polymer-catalyst mass from which polyethylene was extracted with boiling toluene. About 200 ml. of toluene was required to completely dissolve each gram of polyethylene. Then the clear polymer solution was slowly poured into an equal volume of acetone and allowed to cool. The recovered polyethylene was washed with acetone, granulated, and dried in vacuum a t 110' C. to constant weight. Upon distilling the combined acetone washes, a grease-
like residue consisting of low-melting polymer and side-reaction products was obtained. Results of these studies indicate that the promoters serve three functions. They reduce and thereby activate fresh and spent catalysts. They scavenge catalyst poisons present in the system. Most importantly, they enter directly into the catalytic process. Reduction
Promoters have the ability to activate fresh catalyst much like hydrogen but a t lower temperatures. Hydrogen reduces hexavalent molybdenum to an intermediate valence state, and promoters probably act similarly. Table I presents the results of experiments in which unreduced molybdena-alumina was charged to the reactor together with various promoters. No polymer was produced in the absence of a promoter. Although sodium, calcium hydride, and lithium aluminum hydride are not equally powerful reducing agents, each is capable of activating the catalyst.
Table 1.
Activation of Fresh Catalyst by Promoters
Promoter None Na CaHl LiAIH4
VOL. 49, NO. 1 1
Polyethylene, Tempera- Grams/Gram ture, ' C. Catalyst 230 0.0 230 3.4 262
222
1.1 4.3
NOVEMBER 1957
1883
The promoters can also activate spent, unpromoted catalysts. Such a catalyst was removed from the reactor at the end of seven cycles. After extraction to remove all polymer and contaminants, but without carbon being burned off, it was transferred to another reaction vessel together with lithium aluminum hydride. In the single cycle that followed, the catalyst produced more polyethylene than it had made in all the previous, unpromoted cycles. Scavenging
Since sodium, calcium hydride, and lithium aluminum hydride readily react with such impurities as water, hydrogen sulfide, mercaptans, and carbon dioxide, the role of the promoters as scavengers is not unexpected. They function by converting these poisons to relatively harmless compounds. Water, the most prevalent poison, is encountered in every part of the system. Sulfur compounds usually occur as impurities in the solvent, whereas oxygen, carbon monoxide, and carbon dioxide are most often found in the ethylene. Promoters vary in ability to cope with these poisons in situ. For example, calcium hydride is more suitable for scavenging water than is sodium. However, in the case of mercaptans, sodium is the more suitable promoter. For the sake of increased efficiency, each part of the reaction system was freed of poisons in advance by a suitable method. The final stage of purification was completed by the promoter in the reactor. Inner surfaces of the reactor were eliminated as sources of poisons by heating in a vacuum. This precaution was taken when only small amounts of catalyst and promoter were used. When large amounts were present, minor amounts of impurities could be safely ignored. The only harmful impurity known to be originally present in the catalyst was physically adsorbed water, including that derived from reduction with hydrogen. Water was reduced to a low level by pretreating the catalyst in a stream of atmospheric hydrogen a t 480' C. for 16 hours. I n addition to increased yields, reproducibility of both the amount and the molecular weight of the polymer was greatly increased as a result of this pretreatment. Commercial ethylene containing small amounts of water, oxygen, and carbon dioxide was purified by consecutively passing it through a heated bed of reduced copper oxide, a concentratedcaustic scrubber, and an alumina drying tower. The purified ethylene was transferred to a large cylinder at 1200 pounds per square inch, from which it was drawn as needed. As a further precaution, the ethylene was passed through
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a high-pressure Ascarite-Drierite trap in the feed line to the reactor. Benzene, toluene, and xylene were the solvents most frequently used, although alkanes and alkenes were generally also satisfactory. Although these materials were always of reagent grade, best results were obtained only after further purification. Thiophene-free benzene usually requires only thorough drying, although more vigorous purification methods were also employed. Table I1 shows the effect of pretreatment on the amount of polyethylene produced. The various drying procedures reduced the water content to less than 15 p.p,m. The smaller yield of polyethylene obtained after treating with sodium a t 230' C. may have been due to poisonous by-products.
the catalyst and scavenging impurities. In view of the nature of the evidence, a more direct approach was sought. A promotion effect was demonstrated by polymerizing ethylene over molybdena-alumina and calcium hydride in the absence of solvent. Omitting the solvent decreased the possibility of interaction between promoter and catalyst to a negligible amount. Table I11 shows the results obtained both in the absence and presence of calcium hydride. These results definitely indicate promotion by interaction with the ethylene. Supporting evidence suggesting that promoters participate directly in the reaction has also been obtained from flow runs wherein the rate of polymerization increased as the amount of promoter was increased.
Effect of Purifying Benzene Polyethylene, Grams/Gram Treatment Catalyst
Table 111. Promoter Action of Calcium Hydride at 250" C. in Absence of Solvent Reduced Grams MolybdenaPolymer/ Alumina, Gram CaHz, Gram Catalyst Gram None 0.06 0.5
Table II.
None Sodium at 230' C . n CaHz at 2 5 O C. a
1.6 50 87
Benzene maintained in liquid state by
pressure.
Xylene and toluene did not respond to a simple drying step as well as benzene. Measures of increased intensity were needed. Because the amounts of impurities present before treatment were frequently a t the lower limits of detection, analysis before and after treatment showed only slight differences in composition. The only significant variation appeared to be a change in sulfur level from 12 to 4 p.p.m. upon refluxing with lithium aluminum hydride and molybdena-alumina. When xylene was subjected only to drying with sodium, 5 grams of polyethylene per gram of catalJ-st were obtained in the standard procedure. However, when the xylene was first refluxed with lithium aluminum hydride and then distilled over sodium, the standard procedure produced 72 grams of polyethylene per gram of catalyst.
0.5
1.0
2.1
Conclusion
The use of promoters in conjunction with molybdena-alumina in the polymerization of ethylene has resulted in a production of polyethylene as high as 180 grams per gram o f catalyst in a single cycle. This remarkable improvement in the performance of the molybdena-alumina catalyst led to a consideration of additional catalysts which might be suitable for ethylene polymerization if properly promoted. A number of such catalysts, along with new promoters, have actually been found. Acknowledgment
The authors wish particularly to acknowledge the contributions of H. N. Friedlander, R. A. Mosher, and Edward Pramuk of this laboratory.
Promotion
literature Cited
The remarkable increase in polymer production obtained by means of promoters led to speculation about the increase that would be obtained in highly purified systems when no promoter was present. Therefore, a number of particularly vigorous and thorough purification techniques were utilized in order to obtain these highly purified systems, In every instance, the increase in polyethylene production was negligible under normal operating conditions. These results imply that the promoter must be doing more than just reducing
(1) Field, Edmund, Feller, Morris [to Standard Oil Co. (Ind,)], U. S. Patents 2,691,647 (Oct. 12, 1954), 2,726,231,2,726,234 (Dec. 6, 1955), 2,731,452,2,731,453(Jan. 17, 1956), 2,791,576(May 7,1957). (2) Kuentzel, W. E., Field, Edmund [to Standard Oil Co. (Ind.)], Ibid., 2,631,091 (March 10, 1953). ( 3 ) Peters, E. F., Zletz, A., Evering, B. L., IND.ENC.CHEW49, 1879 (1 957).
INDUSTRIAL AND ENGINEERING CHEMISTRY
RECEIVED for review October 4, 195G ACCEPTED April 26, 1957 Division of Polymer Chemistry, 130th Meeting, ACS, Atlantic City, N. J., September 1956.
