Isomerization of I=and 2-Pentenes A. G. OBLADI, JOSEPH U. -MESSENGER, .AND H. TRUEHEART BROWN2 Magnolia Petroleum Company, Dallas, Tex.
investigated the isomerization of 1-hexene over various acidic catalysts, such as acid-treated clay, acid-treated alumina, and the Universal Oil Products polymerization catalyst. These authors report extensive skeletal isomerization together with various amounts of cracking and polymerization, depending on the experimental conditions used. They employed a relatively limited range of space velocities and studied the reaction between 285 and 500' C. For some time an extensive study of alumina has been pursued in this laboratory and effort has been made to correlate ( a ) t h e various changes in physical properties-for example, surface area, density, and crystal structure-which alumina undergoes on heat treatment and ( b ) the chemical nature of the surface with the catalytic activity. The isomerization of 1-pentene under mild conditions was chosen as one of the routine tests of catalytic activity since the composition of the product could be follow-ed E C E N T workers in the field of catalytic vapor-phase olefin by its refractive index increase, provided no side reactions or isomerization are in substantial agreement about the general type of catalyst required for double bond shifting and for skeletal skeletal isomerization took place. This reaction proved to be clean cut and simple for most of t h e isomerization. There is, however, a i d e variance in the activities catalysts tested, since the temperatures employed were around of the catalysts employed. McCarthy and Turkevich ( 7 ) , using Alorco grade A activated alumina, report that substantially 230" C. and only double bond shifting took place. hluminas which had been acid-treated to reduce their sodium content equilibrium concentrations of 1-butene and 2-butene 15 ere rapidly showed activity for skeletal isomerization and could not be evaluattained a t 450" C. and more slowly a t 400" C. when the starting ated by using only the refractive index increase of the product. material was 2-butene. When the starting material was 1One catalyst mas found to give measurable conversion at 105' C., butene about 30 seconds ivere required t o reach equilibrium a t while others were quite effective at liquid space velocities above 450" C., and at 400' C. equilibrium was not reached because of 24 hour-'. excessive decomposition a t the necessarily long contact times. I n view of the high activity of pure alumina gel for simple These workers avoided skeletal isomerization by use of a nondouble bond isomerization and of the acid-treated alumina for acidic catalyst and claim t h a t polymerization took place only in skeletal isomerization, it was decided to make a more complete the runs using long contact times. Ewe11 and Hardy study of these reactions using t h e (S) studied the isomaio r e m e n t io n e d e r i z a t i o n of t h e P 9 types of catalysts, pentenes over a va,STEEL PISTON riety of catalysts inwith the objective of ,MERCURY S E A L obtaining first-hand cluding Alorco X and catalysts prepared by knowledge of t h e effect of the chemicoprecipitating cal nature of the suralumina and other face on the reaction. oxides by use of amThis paper covers monia. Equilibrium the results of this between 1-pentene DEWAR JACKET study. and.2-pentenewas attained a t temperaC.AT.ALYST TESTIKG tures ranging from PROCEDURES 265" to 365" C. and at very long contact The catalyst tcsttimes. Hay, Montingbpparatus was of gomery, and Coull(6) a conventional t y p e and is shown schc1 Present address, matically in Figure I. Houdry Process CorpoIt consistedof an elecration of Pennsylvania, Marcus Hook, Pa. trically heated lead 2 Present address, bath with a central Texas State Research well to acconimoFoundation, R e n n e r , date a glass reactor. Tex. Figure 1. Catalyst Testing Apparatus T h e vapor-phase isomerization of 1-pentene and 2pentene was studied over the temperature range 177" to 427' C. and at liquid space velocities from 0.5 to 24 hour-'. The catalysts employed were specially prepared aluminas of low sodium content and of acidic nature. Reactions varying from simple double bond shifting to skeletal isomerization were found to occur, depending on the chemical nature of the catalyst surface and the temperature of the reaction. Based on the apparent similarity between the nature of the solid catalysts studied and the acidic catalysts commonly used in liquid phase hydrocarbon reactions, a mechanism involving carbonium ions is proposed to explain the vapor phase isomerization of olefins.
n
1462
November 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY
1463
small concentrations of some components-that is, 3-methyl-lbutene and 1-pentene-the presence of nhich was not a l u a y s apparent from the distillation curve. Figure 2 shows a sample distillation of the product from a high temperature run over a hydrofluoric acid-treated alumina catalyst. I n a few instances the break betn een 2-methyl-1-butene and 1-pentene was obscured by too rapid take-off in the operation of the column, and the resulting liquid cuts were too wide for more than a rough approximation of their composition from boiling point and refractive index. I n these few instances, since the analysis for the more abundant component, 2-methyl-2-butcne, n as reliable, equilibrium ratios taken from Ewe11 and Hardy ( 3 ) were used t o calculate the amount of 2-methyl-1-butene present in the 1-peritene cut. When this correction was applied it was found t h a t the ratio of 1-pentene to 2-pentene in the product Tvas brought into fair agreement with t h a t deterinincd b y En ell and Hardy. It is recognized that the analytical procedure used to identify the products leaves much to be desired Tvith respect to precision; nevertheless it is felt that the analyses justify the conclusions drawn from the work. Cracking of the pentenes took place in the run, as shown by the presence of lon boiling paraffins and isopentane in Figure 2 and by the presence of a small amount of carbonaceous residue on the catalyst. One of the branched pentenes probably acted as a hydrogen acceptor; this would account for the isopentane which was found. CATALYSTS
Figure 2.
Typical Distillation Plot
T h e temperature \vas controlled manually, and the catalyst zone was maintained t o within * 2 3 C. of the desired temperature. T h e catalyst bed was about 20 em. long and was preceeded b y a 10-'cm. bed of crushed quartz which served t o preheat t h e pentene feed. T h e catalyst was mised with small glass beads during runs a t high space velocity in order to keep the bed depth constant and to avoid using escessive amounts of feed. A fresh sample of catalyst Ivas used in every run, and no attempt was made t o study the active life of these catalysts. Phillips' technical 2-pentene (n2,6 1.3778) and Phillips' technical 1-pentene (a2: 1.3675-1.3682) were used in this study. T h e pentene at 0 " C. was displaced from a thermostated reservoir by a steel plunger lowered a t a constant rate by means of a motor-driven windlass. This feed device ( I O ) was found t o be satisfactory a n d could be niade t o deliver from 7 to 200 cc. per hour by changes in the driving gear. T h e liquid products !yere collected at 0 O C. in a vessel located just below t h e condenser. Gaseous products were collected in two calibrated gas burets by the displacement of saturated brine. ANALYTICAL METHODS.The vaporous and liquid products of the isomerization studies were combined and analyzed by fractionation on a special precision column (8, 4 feet long and 0.416 inch in diameter, containing 0.~16-inch-diametertruncated conical packing of 50 X 70 mesh stainless stcrl wire gauze. This column is equipped tvith a vapor take-off, and, in order to avoid condensation of the vapors, the take-off lines are heated electrically by mrans of Sichronie ribbon. The column efficiency was determined to be approsimately eighty theoretical plates using a n-heptane-methylcyclohexane mixture. L o n boiling hgdrocarhoris were measured volumetrically as vapor and later condensed for refractive index determination. Higher boiling materials vxre taken overhead also as vapors, and condensed for measurement a s a liquid and for refractive index determination. Components n-ere identified by boiling points and by refractive indices. D a t a on the performance of this column on alkylates have been given b - Gorin, Kuhn, and Miles (4). Table I, n-hich contains tlie boiling points and refractive indices of the various pentene isomers, shows t h a t the analysis of these products was difficult. Complication was caused by the
~
The catalysts used in this study were specially prepared aluminas of lovi sodium content a n d of acidic or neutral character. S e u t r a l alumina indicates that the alumina contained no added acid and was substantially pure alumina. Catalyst A was prepared by leaching rilorco grade A alumina with 0.1 9acetic acid until the d i u m content was reduced from about 0 . 4 9 9 t o less than O.OSC, bv weight. The alumina was dried at 100" C., activated a t 500" C. for 2 hours, impregnated with 5 q hvdrofluoric acid, and t h m reactivatcd at 500" C.
PESTESEISOMERS (1) T ~ R LI.E D.IT.I o s T'.~RIOVS B.P.,
Compound
3-?iIethyl-l-butene 1-Pentene 2-Methyl-1-butene trans-2-Pentene ris-2-Pentene 2-AIethyl-2-butene
O
C.
20,20 30.1 31.1 36.0 37.0 38.45
n
as
....
1.3680 1,3745 1.3764 1.3789 1.3841
Catalyst B v a s a gel-type alumina prepared from pure aluniiiium metal b y peptization n i t h dilute acetic acid (6). T h e 1,esulting sol ~ a a sthick sirupy liquid Lvhich was dried to give a granular alumina of high catalytic activity. Before use the catal?-st w ~ activated s at 500" C. for 2 hours. This alumina was not treated ivith a mineral acid, but it did contain a small amount of acetic acid until the high temperature activation step d e s t r o y d the organic material. Catalyst C was prepared liy impregnating catalyst 13 with 5 % aqueous hydrofluoric acid, drying a t 100" C., and activating a t 500" C. for 2 hours. In catalysts A and C the fluorine content was reduced about 470 by the activation trcatmcnt give11 them. Prolonged heat treatnicnt vas found to remove most of the hydrogen fluoride from thcw catalysts. E X P E R I I I E S T A L RESULTS
Isomerization runs xcre niade a t various temperatures ranging from 177" t o 427" C. and a t liquid space velocities from 0.5 to 2 1 hour-'. The results of these experiments are presented in Tables I1 and 111.
TABLE 11. Reaction Conditions Temp., L.H. ' C. S.V."
ISOMERIZATION O F 1 - P E X T E T E
Compn. of Product, W t . Yo Cd & Pen- Poly- Material lighter tenes mer balance
o\ E H C A T l L Y s T 177 177 177 260 260 260 316 316 316 371 371 371 427 427
260 260 260 260 316 316 316 37 1 371 371 371
1
0 0.. 7 0.3 0. I 0.4 0.2 0.4 1.5 0.8 7 3 5,9 2.4 25.8 8 5
6 24 0.5 3 12 1 6 24 1 6
24 6 24 0.5 1. o 1.0 6.0 0.5 3.0 6.0 0.5 1.0 3.0 6.0
0
0 0 0 0 0.4 0 4 3 9 1.9
Reaction Conditions Temp., L.H. O C. S.V.0
10.5 0.7 0.0 19 6 2 7 2.2 23.3 12.6 7.3
1 1
12 24 6 12 24
tine
-Nk;K ~ o R \ i a T I o s . I ~ i y u r t4~shotvs t i i t , f'elativt! anioulith C 0 111pn. of I'eii t cue oi polynier produced during pentent' isomc~rization ovcr t he c u t , K t . '1 -~ t h w c catalysts. Cat,alyst B yielded about 5c< C:eT at tempera1-pen- 2-pen- Branched tene tene pentenes t urc'z I X J I I ~ 260" to 371 O C. Products from runs with catal 11.0 67.0 22 0 .1 arid C shoivtd a maximum polymer concentration of about 13 0 80.4 6.fi 7.3 84.0 8 7 28(1 at 371' arid 316" respectively. I t is interosting t o note 0.0 28.6 71 4 that 371" C. is the temperature above which extensive cracking 19 0 52 3 28 i 14.2 69.3 16,:3 ta1x.h place x i t h catalyst .I. Similarly, it may be said tliat 0 13.7 g3.3 0 22.7 i r . 3 cracakiiig trvrrbalanc~i~s polyniixi, formation on catalyst C at s o t w 8 0 46.2 45 8
Liquid hourly space velocity.
-
The t \ v o acid-tlratcd cntaly-th .1 anti (' \ v v r ~niuch more active in this respect, catalyst .I yielding 30':; cracked product a t 425" and catalyst C yielding 30% a t 360" C. -%t all temperatures studied catalyst C showed more cracking activity than did A , but i t is possible that any real difference in activity may be masked by the fact t h a t the runs with catalyst C were made starting with 2-pentene. It should be noted, however, that near equilibrium ratios of I-pcntene to 2-pentene were achieved with -st C at 260" C. arid a space velocity of 2-1 hour-'; this that the, rc>action1-pi~ntt~ne e 2-pc.ntcne is a rapid one.
Space
CRACKING.T h e ainounti of clacked product produced tiuiirig the isomerization of 1-pentene ovcr cataljsts -1 and z( and 01 2-pentene over catalyst C, all at a liquid space velocity of 1houi-1, are shown in Figure 3. The pule alumina (catal>st B ) shoved little activity for cracking a t tempeiatures u p to 317" C . and and only slight cracking activity at 371 C.
cotitlit ions. I'KuuI ('TIOS O F ~ - P E S T E S EOf . thc thrt!tB catal vt,stigatetl, thv pure alumina gel (catalyst Hi gave t clc:iti-cui waction. Figure 5 slio~vsthat between 260" and 316' C'. :tiid :it I hour-' space velocity, catalyst B gavel a product c(111Cut, the ba~aric*t~ raining at)(JUt 85'; 2-prntc~iiein tho pc~ritt~ne being I-pc~iitr~ne.Catall-st A produc*id a pentene cut ( X I I I raining approxiniately 555: 2-pt~nteiicat 177" and 260" C. (1 IIOUY - I cpaco velocity), tht, rt~niaintler twing 1-pentenc aiitl branc'tictl-ehaiii pentenes. .Ibove 260" C. the concentration of' 2peritt~iic~ dropped off sharply because of the formation of branchc,tl pt~ritc~rit~s. Runs with catalyst C x e r e made starting with 2pt~iitt.nt~, and the results were similar to those obtained with cataly,st B in t h a t the concentration of 2-pentene in the p e n t c w cut dropped off sharply with increasing temperature. The Iiiglit.st concentration of 2-pentene in the pentene cut was ohtairied with catalyst B , which yielded about 90po at 316" C. ant1 6 hour-' space velocity. BRAX~HEI) PESTAZIEJ. T h e concentrations of the branched aniylcnes, 3-methyI-l-butene, 2-methyl-l-butene, and 2-methyl2;butene, were combined and plotted as a function of tcmperature in Figure 6. These products resulted from runs with 1H and with 2-pentene o w r C'. pC'Ilt(5111?OVCr CatalyistS -2 a The most active catalyst for letal isomerization vias the acitltreated alumina gel (catalyst C ) , which yieldedover 90yobranchrd pentixties in the pentene cut at 360" C. arid 1 hour-' spaw vclority. Catalyst A appeared t o show a linear function of artivity with tcmpclratures and yielded 70% branched pentencs a t 427' C. The neutral alumina gel (catalyst B ) producid practically 110 branching at 260" and 316" C. but did beconic active in this respect at 371 C. EFFECTO F SPACEVELOCITY.The chief effect of incrrasing
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1947
space velocity \vas to suppress cracking arid pol~-merformation Branched pentenes were produced a t all temperatures studied over catalysts d and C and at temperatures above 316" C. for catalyst B . I t n-as found t h a t the yields of branched pentrnea and 2-pentene could be controlled when using catalyst A a t 316" C.: 65% branched and 3 0 7 2-pentenc at 1 hour-' space wlocity, and 3 0 7 branched and 55% 2-pentene at 24 hour-' >pacevelocity. ;it higher temperatures the amount ,q of branched prntenes and 2-pentene in the pentene cut were relativrly unaffected by varying the space velocity in the range from 1 t o 24 hour-', although the over-all vieid of amyltnes was gicatly improved by high space velocitifs liecause of the suppression of (,racking and polymerization. Catalyst C showed similar behavior at 371" C. in that th(, yield of branched pentenes was decreased from about 90'; at t i !lour-' space velocity to about ,507 at 24 hour-'spacp wlority.
1465
high tempcraturcs arc necessary t,o rcmovr these substances from the alumina. The mechanism oi the isomc~rizationof nornial olefins is pojtulated, bearing in mind the statemerits made in previous paragraphs, as follows: Under conditioris prc,vailing on a neutral or acid-treated alumina surface, carhoniuni ions are formed by the addition of a proton to the double bord of the olrfin. The carbonium ion at its momrnt of formation is bound t o the surface of the alumina by mcans of rhr interaction brtween the positive carbonium ion arid thc electronegative proton donor. The lat'ter is either an oxygcn atom or an acid anion. .It temperatures
W W
z
>IECHANIS>I O F 1SO~lERIZ.ATIOS
z a W
liecent papers (2, 4, 9, 11) have proposed mechanisnis for a number of liquid-phase hydrocarbon reactions such as alkylation of isoparaffins. polymerization and isomerization of olefins, and isomerization of n-paraffins as catalyzed by anhydrous hydrogen fluoride, sulfuric acid, and promoted aluminum halides. Only the promoted aluminum halides are active for the i s o m t ~ i zation of the normal paraffins. Oblad and Gorin (9) suggest that a solid surface or interface having the proper dielectric properties is necessary for the existence of the carbonium ions involvcd in the mechanism of n-butane isomerization.
z G?
W
60
53
z
W
t
z
L W
0 W
30
I
0
z 4
23
m LL
*.
3 0 1%
200
250
300
TEMPERATURE.
350
+do
45c
C.
Figure 6. irnount of Branched Pentenes in Pentene Cut at Various Temperatures for Space Yelocity of 1 Aoiir-'
L 30
*. 20
I
G4TALtST C
I 01 150
I 200
I 210
I 300
TEMPERATURE,
I 350
1 400
I 450
C.
Figure 5 . .irnount of 2-Pentene in Pentene Cut at Various Temperatures for Space Velority of 1 Hour-'
The present authors beli that the' riiechanisni of thts isomcrization of normal olefins by contact catalysts occurs in a manner quite similar t o that of alkylation, isomerization, and similar reactions described previously, and that the necessary carbonium ions are readily formed under the conditions existing on t h r surface of the various aluminas employed in the present st,udy. The alumina surface is envisioned as being covered t o some degree with protons attached to oxygen atoms in thc alumina latticr or attached t o fluoride ion.- chemisorbed on the alumina surface. T h e protons and the fluoride anions come from t,he acid used in preparing or trrlating the alumina. I n the case of the neutral alumina the protons are present because of the slightl?. acidic nature of hydrated alumina, which persists to some cstent even after calcination at 500" C. The bulk of the water remaining on the alumina after activation probably exists on the surface. Thus the surface alumina is hydrated to a large estent. The fluoride ions may be replaced by the negative ions in hydrochloric, sulfuric, phosphoric, and other acids. T h e acids are strongly adsorbed on t h r alumina, since long heating tinws and
belori 250-300 C. the principal reaction occurring on the surface of the alumina is believed to he rearrangement of the adsorbed carbonium ion by a proton shift, which moves the positive charge t o a more centrally located carbon atom. This arrangement is more stable tlierniodynamicall~~ than the original adsorbed carbonium ion. Loss of a proton from a neighboring carbon atom to the donor group at the instant of desorption leaves the double bond c1osc.r to thc ctxnter of the molecule. At higher temperatures the mow difficult skeletal rearrangements occur to a n increasing estrnt. This reaction is much slower than a double bond shift and involves the migration of a methyl group. The latter is helieved to occur in a manner similar to t h a t which has bcen postulated for n-paraffin isomerization (9). The double bond shift is so rapid at the higher temperatures t h a t i t makes little differiwe whether 1-pentene or 2-pentene is t,he starting material x h e n skeletal isomerization is predomin'ant,. Formation arid adsorption of the carbonium ion from the olefin feed is folloned by rearrangement to a secondary carbonium ion in the rase of 1-pentene. T h e secondary carbonium ion in any case undergoes a nipthy1 group-proton exchange to a more stable twtiary carbonium ion, and this is followed b y loss of a proton to a donor group at the moment of desorption to yield a branched pentene. The composition of the product depends on the relativt, ratm of formation and desorption of the various branched pentents, and if Fufficient reaction time is allowed a n equilibrium amount of each possible product will hc obtained at each temperature studied. These suppositions are in substantial agreement, with the ohs;ervations that the isomerization of 1-pentene to an equilibrium misture of 1-pentcne and 2-pentene is much more rapid than formation of branched-chain pentenes, and t h a t the formation of the latter is greatly accelerated by a n increase in temperahre. The concentration of protons on the surface is also import,ant>, since it has bcen found that the acid-treated aluminas are more activr as catalysts than aril the neutral aluminas (catalyst. R ) .
1466
INDUSTRIAL AND ENGINEERING CHEMISTRY
Neutralization of the acid groups with alkali destroys the activity of the alumina completely for isomerization. Likewise, the commercial aluminas which contain sodium have not been found active a t the experimental conditions used in this study. P1.olonged heat treatment of the hydrogen fluoride-treated catalyst was found to drive off the hydrogen fluoride and thus reduce the isomerization activity of the catalysts eventually to that of pure alumina. Likewise. loss of water from phosphoric acid polymerization catalyst reduces the activitl- of this catalyst for isomerization, alkylation, and polymerization. Concentration of the surface protons (as xell as temperatures) is important from the standpoint of polymerization of the olefin feed. If the acid groups are numerous the concentration of adsorbed carbonium ions will bc high, and the chances for polymerization are good. This is in partial agreement vl-ith the observation t h a t high boiling material is formed over the acidtreated catalysts, \Thereas essentially only the double bond shift takes place over the neutral alumina. Thus, it is possible to explain the mechanism of isomerization and polymerization of olefin hydrocarbons by aluminas by extending the ideas of the carbonium ion theory of reactions. These same ideas could be extended to the cracking of hydrocarbons by various catalysts such as the well known silicaalumina catalyst and the hydrofluoric acid-treated alumina. T h e latter catalyst has been found to be a potcnt cracking catalyst at. temperatures above 425" C. for paraffin hydrocarbons. T h e data presented in t.his paper show the acid-treated aluminas t o be active for the cracking of olefins at a temperature as l o i r ar: 350' C. SUMMARY
The react,ions of 1-pentene and 2-pentene were studied over a limited range of temperatures and space velocities with the use of three different catalysts. These catalysts were siniilar in t h a t thev contained verv little sodium and were either acidic or
Vol. 39, No. 11
neutral in composition. The pure alumina gel catalyst B showed high activity for double bond shifting at low temperatures and produced little cracking or polymerization. Acid-treated aluminaq were active in skeletal isomerization and gave good yields of branchcd pcntenes even at very high space velocities. Shifting of thc, tloublc bond in the n-pentene molecule appeaxs to be a rapid reaction which is catalyzed by neutral alumina even at loir- temperatures. Skeletal isomerization requires higher temperatures and catalysts of more acid character. Branchedchain olefins may be prepared from either 1-pentene or 2-pcntene, since equilibrium is rapidly established between these straightchain olefins. A reaction mechanism is proposed in which carbonium ions are formed by reaction betveen the olefin and protons on the acidic alumina surface. Tlie distribution of products is explained by the various thermodynamic stabilities of the carbonium ions thus formed. LITERATURE CITED
(1) American Petroleum Institute Project 44. (2) Bloch, B. S., Pines, Herman, and Scherling, Louib J . A m . Chem. Soc., 68,153 (1946). 13) Ewell. R. H.. and Hardv. P. E.. Ibid.. 63.3460 11941). (4) Gorin, M . H., Kuhn, C S., and Miles, C. B., IND. EXG.CWEM., 38, i 9 5 (1946). (5) Hay, R. G., hfontgorneiy, C. W. and Coull, James, I b i d . , 37, 335 (1945). (6) Heard, Llewellyn, U. S. Patent 2,274,634 (March 3, 1942); Reissue 22,196 (Oct. 6, 1942). (7) LLIcCarthy, W. W., and Turkevich, John, J . Chem. Phys , 12, 405 (1944). (S) Magnolia Petroleum Co., unpublished work. (9) Oblad, 8.G., and Gorin, M. H., IND.ENG. CHEM., 38, 822 (19461. (10) Oblad, A. G., hIarschner, R. F., and Heard, Llemellyn, J Am. Chem Soc.. 62, 2066 (1940). (11) Whitmole, F. C., Ibid., 54, 3274 (1932). RECEIVEDhlav 31. 1946. Presented at the Texas Reeionai RIeetine of the ANERICANC H E ~ I X CSOCIETY, AL .4ustin, Texas, December 8 , 1945.
Selenium Dioxide as a Lubricant
Additive RAY E. HEIKS i N D FRASK C. CROXTOS Battelle .llemoricd I n s t i t u t e , Columbus, Ohio Seleniuiii dioxide, although \+ell hnown as an oxidizing agent, has been shown to retard the oxidation of drjiiig oils. With se\eral typical alcohols to increase its solubilit?, selenium diotide was found to act as an antiotidant for lubricating oils. This paper describes the results of a number of bench and engine tests which indicate that the effecti?eness of selenium dioxide is comparable with that of certain commercial antioxidant
