STERLING E. VOLTZ
756
Vol. 61
THE CATALYTIC PIiOPERTIES OF SUPPORTED SODIUM AND LITHIUM CATALYSTS BY STERLING E. VOLTZ Houdry Process Corporation, Marcus Hook, Pa. Received December 7,1066
The catalytic properties of supported alkali metal catalysts for hydrogen-deuterium exchange and ethylcne hydrogenation have been investigated. Sodium dispersed on dried alumina does not increase tho activity of the alumina for hydrogcndeuterium exchange. However, hydriding the sodium-alumina greatly increases the exchange activity, the hydrided catalyst being active even at 195'. Sodium-silica catalysts are much less active than the corresponding sodium-alumina catalysts. Supported sodium and lithium catalysts are also active for ethylene hydrogenation even below room temperature' in this case, however, hydrogen treatments have relatively minor effects on the activities. T h e supported alkali metal dataiysts are much more active than the bulk hydrides of sodium and lithium for both of these reactions. T h e mitjor role of the su port is probably t o increase the effective area of the alkali metal. The results of this stud suggest t h a t the mechanisms oractivation of hydrogen and ethylene on alkali metal hydrides are similar to those revinas& postulated for alkaline earth metal hydrides. T h e activations probably occur at metal sites at metal-metal hyfride interfaces. The results obtained with the bulk hydrides suggest that hydrogen activation takes place more readily a t lithium sites than a t sodium sites, and the reverse situation is likely for ethylene activation.
-
Introduction The recent development of the preparation of alkali metals dispersed on catalyst supports has made i t easier t o study the catdytic properties of these elements.' Only a few isolated references to the use of alkali metals dispersed over solids have been made previous1y.l This investigation was concerned with the catalytic properties of supported sodium and lithium. In particular, the activities of these catalysts for hydrogen-deuterium exchange and ethylene hydrogenation have been studied. T h e alkali metals and their hydrides have been used as reduction or hydrogenation catalysts.2-e They have also been used as catalysts for the alkylation of nitrogen ~ . o m p o u n d s ,condensation ~-~ and p o l y m e r i ~ a t i o ~ i The . ~ ~ ~com~~ pounds used by most of these workers were limited to those which are readily metnllated by the alkali metals. An exception is the use of cesium as a catalyst for the hydrogenation of e t h ~ 1 e n e . I ~ Investigations also have been made of the catalytic properties of the hydrides of other elements. I n particular, the recent study of the catalytic a,ctivities of barium and calcium hydrides has been useful in correlating activities with other properties of these substances.16 (1) "High Surface Sodium.'' bulletin by U. 8. Indristrial Chemical8 Co., New York. N . Y.,1953. (2) F. W. Bergstrom and J. F. Carson. J. A m . Chem. Soc., 6 3 , 2934
(1941). (3) 0 . Egloff, U. S. 1,950,721(1934); 1,984,477(1934); 1,954,478 (1934); and 1,9G2,182 (1934). (4) G. Hugel and co-workers, Chimie e l Industria, Special No. 128 (1929); Cnn. Chem. M e t . , 13, 5 (1929); Bull. aoc. c h i m . , 49, 1042 (1431); 61, 630 (1932); Ann.combusliblelirluide8,6, 1109(1931); 7 , 6 0 (1932): U. 8. 1,968,208 (1934). (5) A, M. Atrickenfuss, U. 8. 1,958,012(1934). (6) 0.Schmidt, 2. phiw'k. Chem., A166,209 (1933). (7) G.M. Whitman, U.8. 2,501,650(1950). (8) W.8. Fonee, J. Ore. Chem., 14, 1049 (1949). (9) H. A. Bruson, U. 8. 2,287,510(1942). (IO) G. H. Daub and W. S. Johnson, J. A m . Chem. Soc., 7 0 , 418 (1949); 72. 501 (1950). ( 1 1 ) N. Green and F. B . LaForge, J . A m . Chem. Soc.. 7 0 , 2287 (1948). (12) A. Morton, unrmhlished resulta. (13) K. Schirrnacher and L. Vnn Zutphen, U. 9. 1,838,234(1931): Brit,. 315,356 (1929). (14) D.G. Hill and G. B. Rbtiakowaky, J . A m . Chem. Soc., 6 2 , 892 (1930). (15) L. Wright and S. Weller, i b i d . , 76, 6302,5305,5948 (1854).
Experimental The sup orted sodium and lithium catalysts were prepared by &persing the molten metal ovcr powdered alumina or silica which had been dt,ied by evacuation at 500" for about 16 hours. I n a typical preparation (sodiiimalumina) the dried alumina and sodium were placed in a high vacuum r e d o r equipped with a magnetic stirrer. Transfers of materials to the reactor were made in a dry box in dry nitrogen. The reactor was heated lowly under evacuation while the solids werc stirred. When the sodium melted, it dispersed over the aliimiila powder. The reactor was heated to about 150" and kept at this temperature (under evacuat>ion and with stirring) for at least one-half hour. Small amounts of gaseous producats were given off in some preparations when the molten alkali metal disperscd over the powder. I n the prepamtimi of lithium-alumina catnIysts, the reactor was he:ttecl to iil)orit 280' because of the higher melting point of lithium (186"). I3irlk sodiuni and lithium hytlritles were obtained from Metal Hydrides, Inc., Beverly, Mass. Hydrogen-deuterium exchange react,ions were carried out in a high vacuum apparatus equipped with an electromagnetic circulating pump. A fitatic high vacuum system , was used for ethylene hydrogenntions. Catalysts were evacuated a t 200' for two hours prior to being tested for activity. Equimolecular mixtures of hydrogen-deuterium or ethylenehydrogen were charged t.o the catalysts at 600 mni. Gas samples were analyzed by mas3 spectrometry. The rate consttrnts for hydrogen-deuterium exchange reactions were mlculatecl from a first-order rate equation. I n all ethylene hydrogenations, the rate const,:tnt,s were calculated from a second-order rate equation for equimoleculnr concentrations of the two reactants.
Results and Discussion I. Properties of Supported Alkali Metal Catalysts.-Impregnat8ion of a support wit'h a molten alkali metal decreases the surface area. For example, the surface area of an alumina support decreased from 81 to 48 n7.2/g. when 4 weight a/n sodium was dispersed on it. In the preparation of a sodium-silica catalyst, the surface area of the silica gel was decreased from 454 to 272 m.2/g. The sodium probably blocks some of the small pores of the supports. Similar results are observed with lithium. The average pore diameter of a silica gel is generally sma,ller than that, of an a h ininn support. The most effective support for alltali metals would probablv be a high area material which contains primarily large pores. Supported alkali metal catalysts show large differences in their reactions with hydrogen. SOdium-alumina, sodium-silica and lithium-alumina all react with hydrogen a t 300" but the rates of re-
Jiine, 19.57
t ~ T A T I Y T I CP R O P E R T I E S OF
757
S I J F P O R T E D S O D l U M A N D L I T H I U M CATALYSTS
n,ction diffcr mn,rkedly. Sodiiim-nliimina react,s very rn,pidly; dl tjhe hydrogcn is sorbed wit,hin 20 minutcs. Jn cont,rnst, both sodiiim-silica and lit~tiium-n,liiininn rc:n,c*t, slowly witJh hydrogen for ovcr R n w k . 11. Hydrogen-Deuterium Exchange.-The activi t8y of a t8ypiwl sodium-alrimina (4 weight 70 sodium) cahlyst, for hydrogen-deuterium exchmign is shown in Table I. The activily of alumina is a l w i t thc same as that of sodium-alumina. An earlittr s t ~ i d yof tliix rearction ovcr very piire 0 ,h i in inn ( t r n , l ~mi i nil m isopropoxide) gave simi Inr n.ctivitjy n,nd t,rinprrnt,urc! dcpendence.16 T h e activit,y of t)lic socliuin--n,liiininn catalyst is enornioiisly incrensr:tl hy h.vdrogen trent'ment,. The h ydr i dctf t x f :i.Ivst, i H a.ctd ve for h ydrogcn--delitJeriiiin c:sc:IinrigcCVCII n,t - 105"; the other catalyst,s are innct)ive nf8this low t.empera.ture. The hydrogoii h k c n up by the hydrided cat,alyst,mas equivd r n t , t,o convrrsion of 13 weight yo of sodium t o sotliiirn Iiydricle. 'l'ticrc is no evidence of deuterium e x h a n g e with cat'nlyst hydrogen in the experi inen t8s. J M h sodium-silica &nd hydrided sodium-silica n.rc less actjive for hydrogen-deuterium exchange t,tinn the corresponding sodium-alumina cntJttlystjs. Tlic former n.rc innc.t,ive a t -78" and slightly active at, 0" and higher.
TABLE I1 IIYDROGEN--DEUTERIUM
Temp. ( ' C . )
5 niin.
OVER L I T H I U M H Y DRIDE" H @ ( i n o l ~%) 15 min. 30 min.
EXCHANQE
30 5.1 8.3 100 17.0 25.3 a 1.9 milliinolrs Iiydrogen-dcutRriiiiii cliargad to 3 g. lithium hydritlt:.
13.5 38.6 ( a t 600 niin.)
hydrided sodium-&lumina for et,hylcne hydrogenation is small (:ompared t o the difference observed for hydrogen--deiit,eriiim exchange. Sodirim-alumina is more Rct)ive than a.luminn for ethylene hydrogenation whereas they have similar activities for hydrogen-dauterium exchange. Sodium react's readily with certain olefins and ethylene is prohably more easily act,ivated on sodium-alumina than alumina. TABLE I11 ETHYLENE HYDROOENATION Catalyst0
(mm.-l
Activityb - mine-1 X IO')
A1203 .. 0.0 Na-AI20s 0.43 2.2 Ilydrided Na-A1t03" 1.2 2.9 TIi-Al2O3 0.12 0.38 Hydrided Li-A190< 0.73 1. B SiOl .. 0.03 TABLE I Na-Si02 2.9 4.4 H Y D R O n ~ N - I ) E l J T E R I U h I EXCHANGE O V E R SOI)IIJM-AI,UMINA Hydridcd Na-SiOzn * . 0.30 T e m p . of Catalysts were prepared by disprrsin alkali metal CRtRlyat exchange ("C.) Activity ( m i u . - l P on 9 g. alumina or silica. b About 5 mmok,',kan cquimolwN;L-AI~O~~ - 100 0.019 rilar mixture of ethylene and hydrogcn ( a t 600 mm.) were N:L-AlzOrh - 123 0.0070 charged to tho reactor in ench experiment. Duplirato riins N &--AI ZO:? - 195 111ac t.ive were made in most instances; tthe above V R I I I F L ~ repremnt Treated with hydrogeii ut tlic averages for these runs. tlydriclo Nn--A120,c - 195 0.0030 300". AlzOrd - 100 0.020 AlzOsd - 123 0,0074 Lithium-alumina is less act,ive for ethylene iiy0 The qiinntitien of hydi,ogcn-deut,erinm nhnrgcd were 2.4, drogenation than hydrided lithium-aliimina; this 2.7 'and 4.4 mrnolas at, -100, -123 and -I%i", rcfqwctively. * 5 g. of sotlirim ( 4 Wright yo)dispersed on alumina is similar t,o the result, observed with sodium-(dried by evacuation overnight a t 500"). NR-AIZOS W'RR alumina. Sodium-silica is more active than sotrcatrd with hydrogeii nt BOO". Evarrinted nt 500" for 16 dium-alumina or lithium-a81umina, but treatment hours. with hydrogen decreases the activity of sodium(I
Bulk sodium hydride is only slightlv fictive for hydrogen-deuterium exchange (1.0% ITD in 0.5 hour) at 100". At this temperature there is no exchange between gaseous deuterium and catalyst hydrogen (hydride ions). At 125 and IliO", sodium hydride is very active for hydrogen-deuterium exchange and appreciable exchange occii rs between gaseous deuterium and catalyst hydrogen. Bulk lithium hydride is more Rct8ivefor hydrogen -deuterium exchange than sodium hydride. Typical data for a sample of lithium hydride are given in Table 11. The atom per cent. darit,crium in the gas phase remained constant during both of these runs, which means tmhatno exc,hange occurs between ga,seous deut)eriurn and catJalyst hydrogen under these conditions. 111. Ethylene Hydrogenation.-Sod ium-alumina, lithium-alumina and sodium-silica are active for the hydrogcnation of ethylen'e. Data for these catalysts are summarized in Table IH. The difference in activities hotwecn sodium-nlumina and (16) 5.
(1956),
G .Hindin a n d 5. W. Weller, T H I BJ O U R N A L ,
60, 1501. 1506
.
silica for ethylene hydrogenation, possibly because of pore blockage by sodium hydride. The activity of the silica gel base is negligible under these conditions. The extents of hydriding for tJhe three catalysts in Table 111 were 15, 11 and 10 weight % ' of the alkali metal for sodium-alumina, lithium-alumina and sodium-silica, respectively. The activities of bulk sodium and lithium hydrides for ethylene hydrogenation are given in Table IV. T h e activity of the bulk sodium hydride is considerably less than sodium-alumina or hydrided sodium-alumina. From the kinetic data for sodium hydride between 100-200" the energy of activation is 8 kcal./mole. The energy of activation over lithium hydride is 17 kcal./mole which is double the value for sodium hydride. T h e energies of act,iva,tionof the supported alkali meta.1 catalysts listed in Table 111 are between 2-0 kcal./mole. T h e slight activities (for ethylene hydrogenation) of some of these catalysts (such as sodium hydride) at high temperatures with no measurable
TARLE I\'
11 Y l ~ l U l ~ ~ l C N , 4 l ' l O0N1 ' '
l
