I
MORRIS FELLER and EDMUND FIELD Research Department, Standard Oil Co. (Indiana), Whiting, Ind.
Promoted Metal Oxide Catalysts for Ethylene Polymerization The similarity of polymer structures resulting from various combinations of metal oxides, supports, and promoters suggests that a single mechanism is involved in all ethylene polymerizations in which these materials are used E T H Y L E N E can readily be converted to polyethylene over a variety of solid catalysts at pressures below 1000 p.s.i.e.g., nickel-charcoal (6), molybdenaalumina (6),and chromium oxide on silica-alumina (7). When used with molybdena catalysts, promoters such as sodium, calcium hydride. and lithium aluminum hydride greatly increased polymer production by reducing catalyst and scavenging poisons, as well as by participating in the reaction ( 3 ) . .4 wide variety of additional promoters effective with molybdena-alumina, as well as with other metal-oxide catalysts, have been studied (2, 4, 7). The large number of effective metal oxides, supports, and promoters can now be classified in a systematic manner.
Experimental
Catalysts were prepared by conventional impregnation techniques. Polymerizations were carried out batchwise, as previously described ( 3 ) , with a purified inert solvent such as benzene in 100 ml. or 250-ml. Magne-Dash reactors. Purified ethylene was used a t 900 to 1000 p.s.i. Polymerization temperature was 220' to 300" C., except with nickel and cobalt catalysts, for which 100" to 180" C. was used. Usual reaction time was 16 to 20 hours. The effectiveness of each catalyst system was expressed in terms of productivity, measured as grams of solid polymer produced per gram of catalyst.
Table I.
Classiflcation of Catalyst Systems
Metal Oxides. Metal oxides of Group \'A of the Periodic Table are good catalysts (Table I). Vanadia and niobia are about equally effective. Satisfactory vanadia catalysts have been prepared on alumina, silica, and clay supports. Although tantalum oxide is the least active member of this group, a significant amount of polyethylene was formed over it when silica or alumina was used as a support. Some of the more active catalysts are found among metal oxides from Group VIA (Table I). Molybdena is the most active; next is tungstia, which is quite active when suitably promoted. Catalysts prepared from chromium oxide on silica-alumina are known to give high prgductivity ( 7 ) ; however, chromium oxide on alumina gave relatively poor results under the polymerization conditions used here. The last portion of the Periodic Table from which active catalysts are derived is Group VI11 (Table I). Of this group, cobalt and nickel oxides, when supported on charcoal and prereduced with hydrogen a t 200' C., polymerize ethylene to solid form. Supports. Effective supports such as alumina, silica, zirconia, and charcoal are found in Groups I11 and IV. The supported catalysts were all prereduced with hydrogen at 350' to 480' C. and were tested with calcium hydride as a promoter. Even without a support.
Metal Oxide Catalysts in Ethylene Polymerization Molybdena i s the most active catalyst
Metal Oxide
molybdena (Table 11) gives a small amount of solid polymer when promoted with calcium hydride; when supported, it produces much more polymer. The generally improved results with supported molybdena apparently come in part from surface extension. However, the distinct superiority of alumina over charcoal indicates that the support also plays an intimate chemical role in the polymerization reaction. Promoters. Promoters increase the productivity of a supported metaloxide catalyst. However, they must also promote side reactions which can decrease productivity. The promoters studied are mainly metals from Groups 1.4 and IIA and their hydrides, as well as some complex hydrides from Group IIIA metals. In most instances, the promoter quantity used was 20y0 of the metal oxide catalyst charged. Alkali metals are effective promoters. Excellent polymer production was readily obtained with lithium and sodium (Table 111), and alkali metal hydrides are also good promoters. t\lkaline-earth metals are moderately effective (Table 111). With magnesium, a reaction temperature of 300' C. was necessary to initiate polymerization. Surprisingly, calcium carbide also is a promoter. The yield of 179 grams of polyethylene per gram of catalyst obtained with calcium hydride is considerably higher than that obtained in most other runs made with this promoter (3), indicating the order of catalyst life possi-
Wt. %
Support
Promoter
Productivity, Grams Polymer/Gram Catalyst
Table II. Effectiveness of 8% Molybdena" on Various Supports Even without a support a small amount of polymer was produced
Group VA V206
Nb205 Ta20s Group VIA CrO3 MOO8
wo3
Group VI11 NiO coo
10 10 10
Si02 Si02 Si02
LiBHI LiBH4 LiBHI
6.0 4.5 0.9 bupport
31 8 20
5 5
A1203 ZrO2
Na Na NaBHI
Charcoal Charcoal
NaH LiBH,
ALOa
0.3 49.6 2.5 3.8 0.4
None
1 .O
A1203
61.9 8.5 7.8
Si02 Clay Charcoal a
Productivity. Grams Polymer/Gram Catalyst
0.3
Prereduced and promoted with CaHr.
VOL. 51, NO. 2
FEBRUARY 1959
155
C0,Ll
00,No
K
CS
PAULlNG E L E C T R O N E G A T I V I T Y
Figure 1. The most effective promoters lie in the electronegativity range of 0.85 to 1.05
-- -. - - - -. .
Toto1 conversion By-products (side reactions) N e t difference between total conversion and by-product curves
ble when an active and selective promoter is used with molybdena-alumina in a poison-free polymerization system. Alkaline earth hydrides are more active than the metals at no sacrifice in selectivity. Complex metal hydrides (Table 111)) also effective promoters, include metal aluminum hydrides and metal borohydrides. Thus, elements participating in ethylene polymerization can be broadly grouped by role according to the Periodic Table. Promoters came from Groups I, 11, and 111, supports from 111 and IV, and reducible metal oxides from VA, VIA, and VIII. A search for more detailed correlations led to the consideration of electronegativity as a means of comparing promoters. Correlation of Promotion with Electronegativity
The authors propose the use of Pauling’s electronegativity scale (5) for comparing performance of metal promoters.
Table 111.
The curve for total conversion of ethylene (Figure 1) is thought to be related primarily to the ability of the promoter to participate in the principal catalytic process. In addition, promoters may simultaneously accelerate side reactions which produce polymers of low molecular weight and other by-products. Side reactions increase as the electronegativity decreases (Figure 1) ; the net difference between these curves is represented by a third curve (Figure l), which indicates that the most effective promoters lie approximately in the electronegativity range of 0.85 to 1.05. Metals in this range are sodium, barium, calcium, and lithium. Magnesium falls a t the low activity end of the scale, in line with the need to employ very high temperatures to initiate reaction when it is used as a promoter. At the other extreme is potassium, which is very active but causes extensive side reactions. O n the basis of performance in many polymerization reactions, arbitrary positions on the electronegativity scale can be assigned to the simple and complex hydrides that were used as promoters (Figure 1, top). Most effective are lithium borohydride, calcium hydride, lithium hydride, barium hydride, sodium hydride, and lithium aluminum hydride. Sodium borohydride gives few side reactions but low ethylene conversion, while sodium aluminum hydride gives higher conversion, less selectivity. Polymer Properties
Essentially the same kind of highly crystalline, high-density polyethylene is produced by all the promoted catalyst systems investigated (Table IV). Promoted nickel on charcoal produces polymer of the same 0.95 density as is obtained with the unpromoted catalyst ( 6 ) . However, higher molecular weights are somewhat easier to obtain when the catalyst is promoted. Groups V and V I metal oxides give
Promoters in Ethylene Polymerization
Catalyst
Despite the variety of combinations, the polyethylene produced was essentially the same
Metal Oxide Ni 0 Ni 0
Support Charcoal Charcoal AlzOa Si02
TazOs CrOa MOO8 MOO8 MOO3 Moos Moos WOa
Si02 NzOa &Oa
vios vzos
a
ZrOz Unannealed.
Productivity, Grams Polymer/Gram Catalyst
Alkali m e t a l s and hydrides Li Na K NaH
M o Oa-A1zOa Moos-AlzOs Moos-AlzOa MOOa-AlzOa
29.8 49.6 2.0 11.2
Alkaline earths Mg Ca CaHz CaC2
MoOs-AlzOa MOOa-AlzOa MOOa-A1z0s MOOa-A1zoa
6.5 7.3 179.0 4.2
Complex hydrides LiAlH4 NaAlH4 NaBHd LiBH4
MOOs-&08 MOOa-A1zoa MOOs-A1zOs 20% WO8-ZrOz
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Promoter LiH
0.953 0.946 0.981 0.966 0.952 0.955 0.963 0.960 0.968 0.963 0.956 0.936
NaBHa LlBH4 LiAlH4 LiAlHa BaHz Na NaH CaHz NaAlHi MgAlHi CaHl
polymers of higher density than those from nickel oxide, regardless of the promoter employed. An exception is the highly acidic tungsten-zirconia catalyst, which forms largely liquid polymers in the absence of a promoter; it makes a solid polymer of 0.94 density when calcium hydride is used as the promoter. Polymers with 0.96 density typically contain about two double bonds per 1000 carbon atoms. These are predominantly trans-internal; the distribution is approximately 7270 trans-internal, 2.57, vinyl, and 3% vinylidene.
9.4 1.5 3.2 4.5
literature Cited (1) Clark, Alfred, Hogan, J. P., Banks, R. L.. Lanninp. W. C . . IND ENC. C H E M . ’1152 ~~, v956). (2) Feller, Morris, Field, Edmund to Standard Oil Co. (Indiana) U. S. 2
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-
Patents 2,717,888, 2,717,889 (gept. 13, 1955); 2,791,575 (May 7. 1957); 2,795,574 ?June 11, ‘1957) ; 2,802,814 (Aug. 13, 1957); 2,834,769 (May 13, 1958).
(3) Field, Edmund, Feller, Morris, IND. ENC.CHEM.49,1883 (1957). (4) Field, Edmund, Feller, Morris to Standard Oil Co. (Indiana), U. S. Patents 2,691,647 (Oct. 12, 1954) ; 2,726,231, 2,726,234 (Dec. 6, 1955) ; 2,727,024 (Dec. 13, 1955); 2,728,757, 2,728,758 (Dec. 27, 1955); 2,731,452, 2,731,453 (Jan. 1 , 1 9 5 6 ) ; 2,767,160 (Oct. 16, 1956); 2,771,463 (Nov. 20, 1956); 2,773,053 (Dec. 4, 1956); 2,791,576 (May 7 , 1 9 5 7 ) . (5) Pauling, Linus, “The Nature of the Chemical Bond,” 2nd ed., Cornel1 University Press, Ithaca, N. Y.,1940. (6) Peters, E. F., Zletz, Alex, Evering, B. L., IND.ENG.C ~ ~ ~ . 4 9 , 1 8(1957). 79 (7) Seelig, H. S. [to Standard Oil Co. (Indiana)], U. S. Patent 2,710,854 (June 14,1955). RECEIVED for review April 12, 1958 ACCEPTEDOctober 6, 1958 8th Canadian High Polymer Forum, MacDonald College, Ste. Anne de Bellevue, Province of Quebec, Canada, Komarewsky Commemorative Meeting, Illinois Institute of Technology, Chicago, Ill., May 1958.
1 56
A1203 Si02 AlzOa Al2os
‘Polyethylenea Density (24’/4O C.), Grams/Cc.
0
Yield with calcium hydride indicates the catalyst activity possible when an active, selective promoter i s used with 8 % molybdeno-alumina
Promoter
Table IV. Effect of Catalyst System on Polyethylene Density
