Mechanism of Catalyzed Alkylation J
J
of Isobutane with Olefins 31. H. GORIN, C. S. KUHN, JR., AND C. B. MILES Ifugnoliu P e t r o l e u m C o m p a n y , Dullus, Zeras
At tlic eiitl of an cxperiiiit,ut t,he hydrocarbons were n-asiied free of cata1y.t with water (after separation from the liquid catalyst phasr in the case of hytlrogen fluoride), and a sample of the tot,al hydrocarbons i n s arialyzetl b)y fractionation on a Podhielniak Hydrobot column (Heligrid packing) to clctcrniiiic the content of iiidivitlual light paraffin hydromrboris through t lic pentanes. ; i second samplo was staliilized Iiy rrmoral of r l i e bulk of the isobutane and lighter hytlrocark~onson a l o x tciriperature column, and was then fractionated on a .Tpccial preciniun column (4 feet long and 0.5 inch in diameter) containing 0.5-iiic-li diameter conical packing coristriictetl of 40 >< 60 mesh stainlws stecl gauze. T h e column elficiency was approximately eighty theoretical plates. Components xwre identified by boiling ctive inrlex (ng) val onally employed. It account for a t least 9 boiling in the range of the component. T h e possihility of thc presence of small quantities of unrecorded components is, thewfore, not escludrd.
Supporting e\ idelice is presented for a simple iiiec1ianislii u hich explains the product- of the primary ally lation
reaction of isobutane \+ ith olefins, catalj zed \+ ith acid* and aluminum halides. This iiiechaniani ewludes certain products which are not e\perimeiitally detected in important quantities. Primarj all\:latioii products are defined as those alkanes which are fornied preferentiall: and in high yield4 at low temperatures and short residence times. Formation of the-e products and the secoiidar? isomeric products is ascribed to the siniple addition of methyl and isopropyl fragments (from isobutaiie) to the \arioiis permissible fornib of an olefin-catall st compley.
T
H E nieclianisin of the alkylation of isoparaffins with olefiiir has received much attention in recent yea (1, 4, 10) have frequently adapted K h i t m o r “carbonium ion” t o this reaction Tyith considerable success. I n criticism of these theories, i t may be contended t h a t they are too Iiroad in a l l o n h g the formation of almost, any alkane isomer by nieaiis of methyl group or hydi~ideshifts within the intermediate i,arbonium ion. In order t o explain the observations made in our hhoratory under various conditions during an extensive study of alkylation reactions catalyzed wit.h both hydrogen fluoride and :tJuminuin bromide, i t has been convenient to differentiate beI\yeen the possible mechanisms by which “primary” antl “secondxry” isomeric products are formed and t o introduce tlie concept CJf “direct” alkylation as t h a t process which leads t o the prefert.ntial formation of a single primary alkane isomer. This theowtical situation has been approached closely by proper adjustment of the experimental conditions employed in conducting t h e reactions. ri mechanism for isobutane alkylation is presented where t h e relative amounts of the principal alkane isomers are qualitatively accounted for on t h e basis of a n approach to thernioc Iynamic equilibrium of the permissible product isomers via (a) .imple one-carbon hydride shifts in the catalpt-olefin complex xiid ( b ) addition t o this complex of methyl and isopropyl fragiiients from tlic polarized isobutane molecule nr from the primary product. T h e theory is substantiated by pertinent experimental [lata.
DIRECT ALKYLATION
-kt an early stage in the development of proccsws ior t l i p iiiariufacture of aviation alkylates by t h e acid-catalyzed rcact ion of isobutane with butenes, a milarity iri cornpiiaition of thr. products obtained x i t h the butene iscmniers v a s obsi:rvc,d. This similarity ivas a s c r i l d . other things, t o isomerization of the olefins or rapid equilibration of intermediate carboniuni ions prior to alkylation. Such c w m p t s implicitly dircoiintetl t lip idea of producing a single alkylate isomer in high yield antl cotiscquently led to either a n overhimplified or a n overly complex picture of the mechanism. 1Iore recently, experimental Lvork carried out in this laboratory ha3 shown t h a t the nature of the protlucts olitained from the alk~-lationreaction with both hy(lrofluoric acid and aluminum brriniitle catalysts i:i strongly influencer1 hy the operating variables, particularly cont:ac-t time, tcniperat i r e , and catalyst concentration’. Thhs, by operating a t short contact times and low temperatures x i t h relativc~lylorv rntios of hydrogen fluoriclr t o hydrocarbon, niarketl differences were ohtaincd in the compoqition of tlic prut1uct.c from tlict i i i ( l i r i ( l i i : i l I)utcmw (1-butene, Zhutene, and isohutcne). :b Ta1)lr I ~ l i o ~ats . -10” (’. and a contacmt iinie uf 5 millute;: n .single octane isoriier. 2~3-tlinic~tliylliox:~n~. c.ons;titiitt.~Ialiorlt GOc; of tlic, octanc, Elaction oi’tlie pi,011 1111 l - t l u t ~ ~ n c ’\vlll~rlQs , t n i c iiomer coriititutctl o n l ~ a- h ) )i tlie Iiriit1ui.t troni :it :ill in the, prmlu(>tfrom iso2-hutcnr and \vas not di~tc~c.tcd pontlingly, i i i i 1 1 ~ ~t 1Iic ~ Imii(litioiis 2,2,4-triI lies oct:ine fraction niethylpcntanp constituted ahoni 7.5 from isobutcne, only 1AcC ( i f il i t , o c t : i n ( ~I m r t i n t i ii,c~ni1-bittriic~, niid .?$Ic; of t l i r octanc~r’i~a~~tion froni ~ - ~ I ! I ~ V I I I > . Fili:llly, 11nilc’r
PROCEDURE
T h e expcriinents u-ere ior the most part c o n d u r t e d in a snitill 2.8-liter capacity) stecl batch reactor prorided n-ith a nicrcur--ealed motor-driven agitator and n-irh 3 jacket thIough n-hich :I fluid heat, transfer medium v-as p ed t o control reaction temperature. Accessory equipment included pressure burets for measuring the desired quantities of hydrocgrbons into the reactor and special facilities for c!i:irging hydrogen fluoride or aluminuni broniidc. \Tith the except i l t i nf the experiments in which olefins n-ere treated with hytlrogen fiuoride prior t o alkylation, the standard procedure consisted of adding the olefin a t a controlled rate t o a xell agitated mixture of isobutane and catalyst. Although paraffin-olefin ratios and catalyst concentrations were studied indeprndently, these variables n-ere held substantially constant in most of t h e studies reported here. I n the case of investigations nit,h hydrogen fluoride a s t’he catalyst, the usual paraffinolefin mole ratio vias about 4.0 and the acid content of the react inn mixture was ahout 20 liquid volume 70.
1 I n the aluniiriult: liroiiiiile t e u i . ci,’L,il>..t ~ ‘ ~ j t , ~ , ~ , i ~expressed i ~ ~ t i ~ , as il weight pep cent A1r3r3
