Oct., 1961
COMMUNICATIONS TO THE EDITOR
1919
COMMUNICATIONS TO THE EDITOR ELECTRON SPIN RESONANCI1; OF AROMATIC HYDROCARBONS ON SILICA-ALUMINA CATALYSTS
Sir : It has been reported that the adsorption of certain polynuclear aromatic hydrocarbons on silicaalumina cracking catalysts results in the formation of a positive radical-ion from the hydrocarbon, which exhibits typical electron spin resonance spectra.l.2 It was tacitly assumed that this was the result of an interaction involving only the hydrocarbon and an acid site on the surface of the catalyst. Experiments in this laboratory have indicat'ed. that this may not be true in many cases, and that. the sometimes unsuspected presence of oxygen exerts a strong influence on the results. Oxygen is not eliminated easily from experimental materials, especially cat,alysts. sample of granulated silica-alumina cracking catalyst was contained in one lower arm of a Fi-mm. h-shaped glass tube and a small amount of perylerie was placcd in i:he other lower arm. The tube was attached t,o a high-vacuum system and the catalyst (but not, the perylene) was heated a t 500" until t,he pressure did not rise above one micron when the pump valve was closed for a few seconds. The perylene was diegassed by sublimation to the tube wall just above the lower end. The catalyst. was cooled to about 100' and t'he perylenc to -195' and the tube was exposed to vapor from degassed benzene in a.nother tube attached to the vacuum system. The benzene had been degassed by three cycles of alternate pumping while frozen and mehing under its own vapor pressure. When enough benzene had condensed in the perylene arm. the h-tube was removed from the vacuum system by sealing off above the juncture of the lower arms. N o resonance absorption could he detected in either the catalyst or the perylene-benzene arm a t t'his stage. The perylent then was transferred t'o the catalyst by means of t,hr:benzene as a solvent. The cat,alyst was instantly c:olored purple on contact' with t>he solution. The benzene mas returned to the other arm by (booling the arm in liquid nitrogen and tho catalyst, arni was sealed off just, below t~hrjunctiirc. The colored vatalyst then cxhibitd n !)-line electroil spin resonancr spect,rurn c~scnt~ially idciit.ical wit t i tJhosealrPady rcported. TI) a .similar experiment the ~ a t ~ a l y sarm t TVW provided with :I vent to atmosphere nf, t,he lower end, and hydrogen was passed over the catalyst at, 500' for t,wo hours before the vent was sealed arid evacuation was begun. I n this case the catalyst became only slightly colored on contact with the perylene solution and the spin resonance absorpt,ion was reduced in amplitude to one-third of t,he amplitude obtained \vit,h the unhydrogenated catalyst. J. .J. Rooney and T i . C. Pink. Proe. Chem. So,. .. 70 (1961). ( 2 ) 1). hl. Rrouncr, Chem. &- I n d . (London), 171, No. 6 (19tilj.
(1)
I n another experiment, anthracene was use(! with no solvent and was deposited on tho catalyst, under vacuum by heating a t 170". Very littlv spin resonance was found. Wheii air was admitted t,o the tube, a largc electron spin re,sonance absorption immediately appeared. These ohsrrvatioris and others along t,he samcb vein can perhaps be taken as evidence for the formntion of positive radical-ions by electron-transfer t o oxygc~nfrom a hydrocarbon-proton complex3 whivti might be expected from interaction of the hydrocarbon wit,h a protonic acid site on the catalyst. On the ot>herhand, while it is known that int.eracntions of several aromatic hydrocarbons with certain strong Lewis acids yield positive radiral-ions* which may he expected to be paramagnetic, it may \)o that oxygen can accept an electron from a covalciir complex of a hydrocarbon with a \ve&er I,ewis arid site, again forming a Paramagnetic radical-ion. In eit,hcr case it is obviously necessary in espcriments designed to examine the interaction of acidt'ype catalysts and hydrocarbons to control thr presence of oxygen (and presumably of othcr oxidizing agents) and to accwunt for its effects if present. UXIOIV RESEARCH CEXTER UNIOK O I L COMP.4NY O F C.4LIFOHNI.k RREA, C l l L I F O R N I A
JAMESK. Fooo
RECFIVEDJUNE: 15, 1961 (3) m-.11. Aaibersherg. J . Gani a n d IC. 1.. Mackor, J . Chem. Sor., 00.i (1961). (4) W. Ij. .ialbersb,arg, (1. J . Iloijtink. E. L. hfackor and W. 1'. Weijland, ?bid.,305.5 (1Y.59).
'I'FIE ISFI,CEN(.:E OF X-RAYS ON C.ITAI,YTI(' ACTIVITY .4S IiELiTED TO INCORPORATED R;4DIO:iCTIT.'ITY Sir: It has been reported in the that t,hr addition of millicurie amounts of radioactive S35 to h4gS04-Na2S04catalysts markedly increases their :ic%ivitj*for the drhydration of cyclohexanol a t 365 t o 315'. Thc percentage increase in reaction rate was rcported proportional to the logarithm of tohe spec4ir activity, reaching 171yoat. an :wt,ivit,y of 105.3 Inc. ' g . On t,he ot,hcr hand, irr:tdiattion of a MgSO, catalyst witJhcst~crnallyproduced 800-key. c.lrc*troris while the reaction was proceeding was wportrcl to have had 110 effect. Since pure MgS04 \vas uscd in the Iattcr tcsts while MgSO, containing 0..52 to 21.87% Na2S04was used in the former, and since the range of 800-kev. electrons in MgS04 is of the order of oiily 0.08 cm.. thc question of ( 1 ) .\. A. Balandin, 1'. I. Spitsyn, N. P. 1)obroselskaya and 1. Pi. Rlikhailenko, Doklady A k a d . N a u k S.S.S.R., 121, 495 (1958). (2) V. 1. Spitsyn, h e s t . -4kad. N a u k S.S.LS.R..Otdel. Rhim. .Vauk, 1296 (1958). (3) A . .4. Balandin, V. I. Spitsyn. N. F'. Dobroaelskaya. 1). Vrvclitschinski and P. Gloaounov, Paper 68, 2nd International C'nngross un Catalysia, Pans, 1980.
1920
Vol. 65
COMMUNICATIONS M THE EDITOR I
I
I
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30 20
10
8 6 E 4 I-: o
9
Z 0
U
8 2
--
1
I
1.46
I-
1.50
1.54
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1.58
1.62
1.66
10S/T. Vig. 1.-Arrhcniuu plot: catalyst, 0.0500 g. MgSO,; 0 no radiation; 0 irradiated, 180 kv. 15 nia., 123 rad./min.; A irradiated, 300 kv., 5 ma., 113r:td./min.
whether ex teriully supplied radiation could c:iusc an effect similar to that resulting from the incorporation of radioactive sulfur remained in doubt. In this work catalysts composed of MgSOc and MgSO1--Na&Oc have been irradiated while the dehydration reaction was taking place using a Uorelco hIG.300 constant potential X-ray machine. Electrons were thus generated throughout the catdyst bed to simulate more closely the incorporation of SY5. ‘fhe X-ray beam attenuation was less thaii 10%. Dose rates of 113 rad./rniIi. (300 Lv., i ma.) a i d 123 rad./min. (180 kv., 15 ma.), as nic:tsured by Fricke dosimeters and corrected for electron density, were used. On the basis of total energy absorbed, using 50 kev. RS the avepage rncrgy of the Sj5 beta particle, this corrcsponds to -67 mc./ g 01 S?5. Compton and photoelec1,ron energy-spec trum calculations, however, show i,hat rclatively fen. electrons of energies greater than 100 kev. are generated by the X-rays. If it is postulated that only these higher energy electrons are effective in producing changes in catalytic activity, the X-rays are equivalent t o -14 mc./g.
r
1I
3
1.52
1.56
1.60
I
1.64
1
103/1‘. Fig. 2.-Arrhenius plot: catalyst, 0.1500 g. R/lgS04-4% Na,SO,; 0 no radiation; 0 irradiated, 180 kv., 1.5 ma., 123 mcl./min.; A irradiated, 300 kv., 5 ma., 113 rad./min.
of 836.The reported data indicated that 67 mc./g. of S35would produce a 154% increase in rate and 14 rnc./g. a 97% increase in rate, so that in either case the effect should be easily observable. No such rate increases were found, however, on either fresh catalyst or catalyst brought to equilibrium activity by several hours’ use (Fig. 1 and a), It thus appears that the effect of the additioii of radioactive sulfur to these catalysts cannot hc caused by any mechanism involviiig only the beta, particles, but must be a result of physicxl or chemical changes related to the conversion of siilfur ntonis to atoms of chlorine. 1)GPARTMENT O F CHEMISTRY
ZJNIVERSITY OF TENNESSEE
KNOXVILLE,TENNESSEE, ANI) NORMAN A. K R O ~ I N OAK RIDGENATIONAL LABORATORY POSTOFFICEBox X IIILTON A. SMITH OAK RIDQE,TENNBSSEIC RECEIVED JULY 14, 1961
