A Mechanism of Catalytic Cracking R . (1. H-4NSFORD Soconj - ?
~ C C I C I ILnhorrrtories, ~
Pntrlsboro. \ . J .
I'RO(:EI)I;RE
A iiiechanisni for the catalj tic crachinp of
hydrocarbons in the presence of tjpical catalysts of the silica-alumina type ic proposed, based 011 the acceptance of protons from the hydrocarbon bj adiorbecl water and on the donation of proton5 to the hydrocarbon bj the catalyst. Rater-deficient catalysts h a \ e l o w cracking acti\ity, and deuterium oxide adsorbed on craclcing catal? sfs readily exchanges deuterium for hydrogen i n adwrbetl h?drorarhonb at temperatures far helow thoce required for cracbitig. 'The high concentration of protons on the surface of silicaalumina a- compared to that on pure 4lic.a pel further -111)staniiates thr proposed mec.haniwn.
T
I 11.: r i i k p1:iycd t)y poriiu.. adsortx'iit catalysts of t h e silicaalumina type in the c m ~ n ion of hydrocarboris has long been a 1iuzzlc to eatal?-tic i n igators. Ordinarily, rraction mechai~i*msinvolving cattilysts are dcduced i'rolii reaCTioli kinetic i i i ( , : ~ ~ i ~ r ( ~ n i [from ' i i t ~ ,the irjolation and identification of waction iiiti~ririe~tliatcs. or ir~oma *tuti>- of the rraction using taggcd atonis (isotopic tra Iiy~li~iicarbo~is t l o c ~i i u t lent1 its iclt ic i i i i ~ t s u r ~ ~ i iii,r i 1ici~:ius:c~ it t lrc I mtiiiii gi~~icr:tlly is I i o t sufIii~ii~iitl>. cliffc~i,erit from tlic, t I i ( ~ i ~ i i : ii':iti' l t i ) :tllo\v conip1c~tetliffei~i,iiti:ttio~i. This i h pa;tioulai,ly t i ' u t ' uC tlic, cracking of piiixffim, ~ v l i i ~ rt l~i cc, ~c.at:ilv t ic reactiori rate dors i i o t iliffvr by inucli more thaii OIIC ordcs~,of magriit titic,, evcii for liy~I~oc~:ii~tio~i.~ boiling in tlic g:i,s-oil range ( 2I . R v a i , t i i r i i iiiti~i~iiirt1i:itc~~ iiivolvitig the catalyst n-ould appva~' to tw i)c~yilirclpi~iilxtliilit>.,if \vi' c ~ ~ i i s i d t(t ~i i xr c h c m i ~ t r yof d c a , aluniiti:), 01' ot 111~1~ ii\itlir i i i i s t i t l i t i l l y t l i t , 1)iiIli of crac*ltiiip catalysts I n .qiiti, IJi't h c a t l i f f i ~ i ~ i ~ n i .t+~ i ~ c r i r i i ~ t c ~i iii~ iiii~:i.-iii~ing c~ii catalytic, us(~luIiiii'oi~iiiatioii GLII bo derived cracliiiig kinc'tiw, ciiriritli~i,at)l(~ froni ..urli i i i ( ' : i . ~ u r ~ ' i i i i ' IOi ItI~ pur(' Iiytl~~oc~:ii~t~oil-. Thr est rniive work i i f (;rc~i~n>fc.lder anti :i~mri:ttc~.i f . 2 1 o i i I Iic c.:italytir :iiitl theriii:ii c~r:~cking of :t large 1ium1)c~rof pun' Iiytlroc~art~onha.. given us a m ~ a l t l iof inform on thc, rclativc. rates of cr:icliiiig of the various principal c1 [ I f h > ~ t l r o l ' ~ i r ~ ~ r l l.Illy lS. tllc'ol~>of catalytic, !7arkiiig iiiuht a ~ i : i l i l (i~i f iiitc~rprr~tiiig t h w , results, a,< ell a5 thc infornxition k i i o \ ~ - i iatxiut catalytic cracking of prtrokuni .twI- catalytic, c011 i ( i i i O i the 111 ini:ir>. p i ~ o d u i ~ t s of thei,niaI aiid catalytic c i i i i v ~ i ~ . ~ i iI~t ~may i . tic that, in n1ariy cases, the' primary thermal conversiou is thts rate-controlling step in catalytic cracking, particularly in th(a c t w of paraffins and cycloparaffins. The present paper prrwnts a study of the catalytic cracking of a simple paraffin, n-butane, including some' new d a t a on the exchange of deuterium and hydrogen between cracking catalysts and various classes of hydrocarbons. A general mechanism of catalytic cracking is presented, which is believed to explain the esprrinicntal facts 849
I \ CRACKING STUDIES
sclicmatic diagram of t h r all-glass apparatus used in the crarking studies I\-ith n-butant, is shown in Figure 1. T h e catalyst hult) had a volumcx of approximately 300 cc. T h e manifold tutjirip was 2-nini. capiilarl- t o pcriiiit ready evacuation of the t i , m \rhilt, the, (lead space is still kept as small as possible. aixatiori of the catalyst bulb and connecting manifold was r i d out with a 1Iegavac pump at about 0.001 mni. of mercury for 30-60 miriutos prior to admission of the gasrous hydrocarbon froin t lit. stiir:igr tiulb. The' vatalyst hulb \\-a5 filled with n-butane initially at a presS U I ~ V ~ i about ' 500 m m . of nirrcury. This corresponds t o a ivoipht rutin of catalyst: butane of 700: 1. Thc reaction was c*arricdiiui at rorist ant volume, thc pressur? increase hcing measu r t d :is :i iurirtion of time, on the m t ~ c u r ynianomctcr. Variation in the c k g r c ~of' lletl hy the time during which thia rrwtiiiii 11i. Tiniw of 10, 20, 30, n-liich the reaction misaiid 60 niiriutc's \vow gcn ture \vas c~rp:ind(drapidly into tht, rvacuatecl rccrivcr bulb, aided oiitli~ri~ation i n the liquid air-coolcd rccc.ivclr. When the urc i n the, catalyst bulb had t w r t r i reduccd to about 1 nun. of reactor \\-as isolattd from thc, Iiiarrifold and the
tulic, placid :it thi, (,sit of the catalyst bulb.
and layg[c'cl ivitli (iiiic~hc~s c)f high trmprraturv ineulation. T h c ~ ~n-liich t h c c:it:tlyst bulb \vas niiddle 8 iiichcr oI' t h c i u i ~ i i a c iin placed, had a tcnipc:rat ure gradient of *0.5' C. atmve and belowwriter. Control of the him; hmting load on thv furriace was by I7:iriac5, xiid fine control lvas effvctcd by an air thcrmomctcr tiulb placwl at the cent(3r of t h r furnace. Slight pwss-urc fluctuations due. t o temprrature changes in thc bulb actuated a mercury inanonictt'r, which regulated the control 1iratt:r cirruit through a niagrictic relay. Temperaturc control W:IS * 0.5' ('. at 462" (',, ant1 tcmpcLrature measureinent \viis hy means of a calibrated t hi~i~iiioc~iuple and a I.crds 8: Xorthrup 'Type I< potentiometer. T h c ~ri-t)utane usrd \v than O:l,fjc; p u w by 11 pu~,ific~ition \vas niaili'. y i l t hiit ic silira-aluniiria crackirig cataly matcxly 12'; .\l?Oi and 88c1 Si02 icli:y ositltb eniplii?-cBcl i i i t t i c rsrhangc* c ~ s ~ i c ~ r i i i i !%r; D10. RESL'LTS O F TESTS
-4t the outset difficult>- wvas encountered because of erratic activity of the eaTalyst from run to run uiitkr supposedly identical conditions. This was believed to he due to variable amounts of water associated with the catalyst after regeneration (variable air humidity, variable catalyst deposits, etc.). T h a t this was the case was confirnicd tjy thc experiments shown in Table I. Partial desiccation of the catalyst by passing a stream of predried air through the catalyst a t 482' C. resulted in a conversion of n-butane in 30 minutes a t 481.7" * 0.5" C. amounting to 2829wc',. T h e adsorption of water on this same catalyst from an
850
air stream saturated at 30" c'. i n quantitics of 0.13-0.2!)'; I I ~ weight of the catalJ-st raised the decomposition activity to 3G4OC;. This observation forms an essential part of tlic h - i s ior the inechanisni of catalytic cracliilig proposed later. In the suhsequcrit study of the crarking of v-hutaiir, thi, c:i~:ilyst v a s preconditioiicd after each rryc~nc~ration by vluililii,at iilg with a s,ibstantially coiistaiit 1iarti:il p r c ~ s u r eof \ m t c i ' vapor t 3 0 11ini.)in air passecl ovcr t h e catalyst a t 482" ( ' . Figuic 1 SII[J!V~ the prelimiriary cracking (lata 011 ti-hutaiie a: 482" f l . o i n :in investigation (as j-et iiicoiiiplc~te) of the 1;irii.tics of catnlyt ic. cracking of siiiiple h>-drorarhoiis. Table I1 li the experiiiieiits from 15-hich these data were i)l)taiiic~(l. T t i i x ~ data were plotted according t o the iiiethod proposctl hy Srh~ic~iilct. and Frolich ( 5 ) to give thc primary products of thc rc>:irtioli. Admittedly the method may tie opcn to some quebtioii \\-lirn applied to catalytic cracking, \There the possibilit3- of cxttwiiely fast secondary reactions makes extrapolation to zero decompo-ition of doubtful validity in some cases. H o ~ c v c r certain , 1)ri)ilucts, such as hydrogen, methane, and other paraffinq, may he considered t o he relatively unsusceptihle to such fast wcondary reactions. The data shown have a n important bcariiig on the mechanism to be proposed. The marked influence of relatively small amounts of adiorlicd (I.
I'nrtially d e h y d r a t e d in dry air, 16, hr. at 482' C. P a r t i a l l y d e h y d r a r e d in d r y a i r , 40 h r a t 482' C. R e h y d r a t e d i n wet air, 0.13'; H?O adsorbed i i f 482' C . 2nd r e h y d r a t i o n in wet a i r , O.lSc; H20 adsorbed at 4 8 ? O C 3 r d r e h y d r a t i o n i n w e t air, 0 2 9 ' t H:O adsorbed a t 482' c'
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1947
deuterium exchange in anj- of the products, notnithstandiiig a considerable degree of decomposition. .is Taylor pointed out (61, an exchange reaction offers a delicate means for detecting the nce of activated adsorption irhere conventional methods fail. ; i threshold temperature of c,\;c.liaiige is deterniiiied in the present investigation for a number of typical liydrocarbom This w:xs tlcfiiied arbitrarily as the Ion-est teiiijx'rarure :it which deuterium escliange could 1x2 d e r c c t r c l in the product :ifter the hytlroe:xrl~on vapor liad been iri contact Jvitli a deuterated cat:il.vst fur 1 hour. The app t eiiip(>r:iture,s for >ix hydrocar rz-tiutaiie. 205'; i>obutane,35 I I - ~ P X X I I C ' . 260" : cyclohesatie, 315'; a11tlI>elize11-dinus oxides function equally \veil i n catalytic r r x k i i i g , nor do those that exhibit activity all produce t h e same type's of products. Physical structure does l i n t appi~arto be the aiisn.er, since a .-ilic:t gel impregnated with 1 (or even lee>' of alunlilla is linon-n to be a n efficient catalyst The pliysical structure of such tin impregnated gcl cariiiot differ ni:irkedly from t h a t of tlie relatively inactiT-c, liasc. The clwiiiical Iiatui'cs of silica-alumiiia surfxces appears t o hold the key t o thc p i i z z l ( ~ ,although this sec'nis to have been largely neglected 11y previous investigators. Tur1;evich and Smith (;,I rc:ceritly p r ~ s e n t t da so-c.allt~Iunitary theory ~f catalytic liytlro-
C,H
3
c,I1 :I rg ed
1 1 2 8 1 4 1 6 1 0 3 3 2 6 88 i 0 8 0.4 0 0 0 1 lloles p r o d u c t 100 iiicrles C4H.n decomposed IIydropeii 10 2 Iletiione 24 9 Et h r l e tie 1" 0 Ethane 13.8 91' Propylene I'rupane 31 3 Iwburnne 23 1 Rurenes ( t o t a l ) 7 4 Iqopenrane 3 7 7 1- P e n t a ne 0 0 P e n r enei t I o t:i 1 I 0.9
.,
L' 5 6
2.5 3.1
1 0
8 0 5 s i8 0 0 9
0.5 0 2 (1 1 9.8
25 5 113 14 2 4 4
36 26 3 2 1 0
3 4 9 5 0 5
?!E i
,
1s
4 2 0 , 12 3 8 1 71.6 0 9
0
$1
0 3
12 17 0 9 0 22 9 52 0
4
1 9 0 6 6 8 1
s
0 9
0 1
0 4 0 1
12 i 26.9 6 3 15 8 2 3 43 1 28.5 31' 3 2 1.2 0.4
25.9 35.8 1 9 18 8 1 3 4i 2 20 7 1 6 1 9 0 8 0.3
;\lumina may be incorporated into the catalyst by siiiiplc impregnation; this results finally in the splitting out of water bet r e e n the aluminum hydrate and an hydroxyl group of tlie silica gel surface. The resulting catalyst surface may be pictured as follon-s : 011
852
INDUSTRIAL AND ENGINEERING CHEMISTRY
This coordination of aluminum with the adjaceiit oxygeii atom is responsible for the high activity of silica-alumina catalysts toward hydrocartion reactions catalyzed by protons-that is, acidcatalyzed hydrocarbon reactions such ah polymerization alkylation, and isomerization. This investigation showed that watcsr niarkedly affects thc activity of silica-alumina catalysts and that there is an eschangv of hydrogen between t,he catalyst and hydrocarbon, even at tvmperatures where no measurable cracking occurs. This eschangt is probably of a n ionic nature, since it occurs Kith adsorbed water. I t is postulated that adsorbed water can accept a proton from a hydrocarbon, so that a negative hydrocarbon ion is formid. This ion may rearrange under the influence of the catalyst anti high temperatures to form a branched structurc, or i t may de,compose to a smaller ion of similar nature and a hydrogcn-deficient fragment. Keutralization of the negative ion with a prtiton from the catalyst occurs to regenerate a stablcz molecul(,. This mechanism is applicable t o reactions of paraffins principally. I t is a difficult and s l o proceys ~ requiring high encxrgics of activation; this accounts for the slow rate of catalytic cracking of paraffins. The relatively high tenipvratures required n.ith paraffins and cycloparaffins for deuterium excharigca tend to C O I I firm this. The principal reactions of ti-butane o n silica-aluniiiia catalyst, may he illustrated by the following equations: ~SO31ERIZ.4TIOS
Vol. 39, No. 7
11 aiiy ot1ii.r reactioiis, such as polymerization, hydrogen trarisfer, cyclization, alkylation, dealkylation of aromatics, etc., may he pictured t o proceed through the ions formed as a result of protori exchange betLvecn the catalyst and hydrocarbon. The foriiiat ion of isoparaffins from higher parafins and olefins probably
o(2rurb through intraionic rearrangement followed by splitting to a n ir;oparafin anti hydrogen-deficient fragment (intraniolccular hydrogen exchange), In the catalytic cracking of criiiiniercial petroleum charge stocks it is believed t h a t thernial cracking plays an impoi,tant rolc, in the over-all conversion. This does not mean deep therniul cracking which leads to vep- light products, but the initial split tiiig of large paraffinic molecules and deallcl-lation of long romatics and like ?roducts--reactioii~ akin I "viscosity breaking" operations. 1 lie i'aatiwt catalytic cracking reactions are ivith liydrocarboris \\ 1iic.h art' capable of T ~ ) a i t~o dparaffins appe puzzling on this basis. biiiw temperatures re( m exchange are even higher than for n-paraffins. Perhaps the tendency for the cyclohc~sarirx ring to isomerize to the cyclopentane structure at high teiiipcTratures in the presciicc of acid catalysts produces a react ivi, inlc~rmi~tliate which can form a carboniumi l i i ) t ('niptBra1i m involved ~ in catal-tic cracking th split rather than t o bcc.oriic~stabilized through rearrangement and Ims of a proton. Partial thermal dehydrogenation prior to ratalytic coiivcrsir~fi\vi11 lead to the sanic result. Furthci, \\ark \vi11 h:ivc, t ( )~) I % iIiiii[, t~ijfoi,r,this point is c.lrai.. 7 7
,.
C Hi ( S K I >G
LChhVU LEUGIIEYT
1 lit. autlior \vislies i o express his appreciation to 11. J . Sullivaii. l didi niuch ~ of the expr.rimcnta1 work reported, and to David [ 'liristisoii his pcrsonal attention to the mas> spcctroineter analyses, particularly in the deuterium exchange work. l l a n y liclpful coIiinients and suggestions from various members of the l a h m t i j i , y itaff art' HIM acknowledged. I .
~
1,o)ver olefiis produced by thernial crackiiig art' ret'ori1icd. :Isho~vvnbelow for olefiiis generally. lIethaIie, rtlianr., m t l hyilrogel1 produced bl- themial cararking largoly >uivive furt li(,i, r(xction and accumulate as the dccompositioii ticgree iiicrcla. The deuterium t~xchangc~ esperimeiits with paraffins and c , ~ . c , l i ~ parafiin, ShOTV that fairly high tcmpcraturrs are required t o activate proton removal. 011 thib other hand, excharige occurs with olcfiris and aromatics at niuch lower temperatures. Proton donation by the catalyat is the important, mechanism involved with these hydrocarbons.
(1931). (6) Taylor, H. S., "Frontiers in Chemistry," Vol. 111, p. 24, S e u -
DOI-BLE ROSD ISO>II;RIZATIOS +€I c,k,-c~,-~w=c~, -
1168 (1915). (3) Pauling, L., "h-ature of the C'iieriiir~alBond," p . 37ti, New Yorh Cornel1 Univ. Press, 1939. ( 4 ) Pitzer, E. C., "Advancing Front, in Chemistry," Vol. 1, p . 3 3 Sca- York, Rcinhold Pub. Corp., 1945. (5j Schneider, V., and Frolich, P. K., IND. ESG. CHEX,,23. 1405
CH 3-("2-C
--+
CB
:H-('H
j
Tork, Interscience Publishers. Inc., 1945. (7) Turkevich, J.,and Smith, R. K., Div. of Physical and Inorga~ii(. Chem., 109th Meeting of A.C.S., Atlantic City, N.J. I'RLSEYTLD befure t h e Diyision of Petroleum Chemistry a t t h e 110th X e e t i u g of t h e . ~ U E R I C A S CHEMICAL SOCIETT, C h i c a g o , 111.
