Transesterification Kinetics of Soybean Oil FREEDMAN

8/11/2019 Transesterification Kinetics of Soybean Oil FREEDMAN

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ANALYSIS OF SOYBEAN OIL BY CGC

9. Nohl, A., Fast Capillary Chromatography, Spectra-Physics,3333 North First Street, San Jose ,CA (no date}.

10. D'Al onzo, R.P., W.J. Kozarek and H.W. Wharton, J. Amer.Oil Chem. Soc.58:215 (1981).

11. Devinat, G., J.L. Coustille, J.L. Perrin and A. Prevot ,Rev. Fr.Corps Gras 30:463 (1983).

12. See, J.B., Fette, Seifen, Anstrichm. 85:72 (1983).13. Goh, E.M., and R.E. Timms,J. Amer. Oil Chem. Soc.62:730

(1985).14. Haefelfinger,P., J. Chromatogr. 218:73(1981}.

[Received De ce mb er 13, 1985]

Transesterification Kinetics of S o y b e a n Oil1

Bernard Freedman*, Ro yd en O. Butterfield a nd Evereff H. Pryde~Northern Regional Research C enter , Agricult ural Research Service, U.S. De par tm ent of Agricu lture , Peoria, IL 61604

Tr ans es ter ific ation o f s o y b e a n o i l ( SB O) a n d o t h e r

triglycerides wit h alcohols, in th e presence ofa catalyst,yiel ds f a t t y este rs and glycerol. Di- and monoglycerides

are i n t e r m ed i a t e s. R e a ct i o ns are c o ns e cu t iv e a n d

reversi bl e. Rat e con st an t s h ave b een d et ermi ned for

each reaction wi th a computerized kine tic prog ram. Thee f f e c ts o f t h e t yp e o f alcohol, 1-butanol or met h an ol(MeOH); m ol ar rat i o of al coh ol t o S BO ; t ype an d amou n tof cat al yst ; an d react i on t emp erat u re on rat e con st an t s

and kinetic orde r were examined. Forward reactions

a pp ea r t o be p s eu do -f ir s t o r de r o r s e c on d o r d er

d e p en d i ng u p on c o n di t i on s u s e d. R e v er s e r e a c ti o n sa p p e a r t o be s e c on d o r de r. A t a m ol ar r a t io o f

M e O H/ S B O o f 6:1, a s h u n t reaction w a s o bs e rv e d.En ergy of activation was determined for all forward a ndr e v e rs e r e a c ti o n s u n d e r a v a ri e ty o f e x p e ri m e n ta lc o n d i t i o n s f r o m p l o t s o f log k vs 1/T. Values r a n g e d

f rom 8-20 kcal/mol.

T r a n s e st e r i f i c a ti o n o f v e g e t a b l e oils w i t h s i mp lealcohols has long been a pref erre d met hod for pre par ingf a t t y e st er s. T he se e s t e r s h av e g oo d p o t e n t i a l as a nalt ern ati ve or emergency fuel to replace #2 diesel oil infar m tra ct or s. Because of this potential, we have focusedour at ten tio n on variables tha t af fect the yield of theseeste rs {1,2} as well as analysis of intermediat es and

pro duc ts as d et er m i n e d b y a n I a t r o s c a n a n a l y z e r (3}. Inthe present pa pe r we report on a stu dy of the kinetics of

th e transesterification of soybean oil (SBO). A capi llar ygas chromatographic (CGC} me th od used to support thiss t ud y has been described (4). One of o ur objectives wasto determine how variations in type of alcohol, molar r a t io o f a lc oh ol t o SBO, c a t a l y s t t yp e a nd r ea ct ion

temperature affected kinetic order, reaction rates andenergies of activation. Another objective was to gain am or e f u n da m e n ta l u n d e r s t a n d i n g o f t h e c he m is t ry

involved in the transesterification of vegetable oils.Various mechanisms have been proposed for both

acid- and alkaline-catalyzed transesterification {5-7}. An u m b e r o f k in e t ic s tu di es h av e b ee n r e p o r t e d fo r t he

transesterification of simple {non-fatty) est ers witha l c oh o l s {6,8-10} a n d d i m e t h yl t e r e p h t ha l a t e w i t hethylene gl yc ol {11,12). Only a few kinet ic studies havedea lt wit h the transesterification of vegetable oil or fa tt ye st er s. T h es e i nc lu de t h e c on ve rs io n o f c a s t o r oil t o

*To whom correspondence should be addressed.'Presented at th e AOCS meeting in Philadelphia, PA, May, 1985.2Deceased.

met hyl ricinoleate {7), the glycerolysis of met hyl olea te(13) a n d t h e t ra n se s te r if i ca t io n o f t h e e s te r s o f 9(10)-carboxystearic acid (14} and me th yl 9(10}-car-

b o x y m e t h y l s t e a r a t e {15}. T h e p r e s e n t s t u d y wa sunde rta ken with a commercially impo rtant vegetableoil, SBO, utilizing a computerized kinetic pro gra m (16)to shed additional l igh t on the kin eti cs of transesterifica-tion reactions.

EXPERIMENTAL

Ma te ri al s. R e f in e d S BO was o b t a i n e d f r o m C e n t r a lSoy a, Chattanooga, Tennessee. The molecular weightw as a s s u me d t o be 872.4 . M e t h a no l ( Me OH ) a n d1-butanol (BuOH) were MCB Omn isolv (spectrograde)an d were stored over mole cula r sieves 4A. A solution o f

s o d i u m b u t o x i d e ( Na O B u ) w as p r e p a r e d b y t he r e a c t i o nof 240 mg of sod ium with 10 ml of BuOH at 59-99 C withstirring. Sulfuric acid was purchased from B&A, Allie d

Chemical, Morristown, New Jerse y. Sodi um methoxide(NaOCH3) was obtained from Al dri ch Chemical Com-p a n y , Milwaukee, Wisc onsin.

Rea ction co nd it io ns empl oy ed in ki ne ti c st ud ie s. Thes co pe o f o ur k i n et i c s tu di es is o ut li ne d in T a b l e 1.Experiments were designed to determine kinetic ord erand rate constants using two simple alcohols, two molarratios of alcohol to SBO, acidic vs alkaline catalysis, twow e i g h t p er ce nt ag es o f N aO B u a nd two t e m p e r a t u r era ng es each containing five temperatures. All reactionsstudied were conducted at atmospheric pressure.

Transesterification reaction and sampling. T h e f i r s ttwo of five systems shown in Table 1 utilized a 250-mlround-bottomed four-necked flask, equipped with am e c h a ni c a l s t i r r er , t h e rm o me t e r, c on de ns er a n d / o r d r y i n g t ub e a nd s to pp er (f or s a m p l e removal}. T h er em ai n in g s y s te m s in T ab le 1 e m pl o y ed a 400 -mlc u st o m -m a de r e a ct i o n f l a sk w i th f i ve n e c k s . T h ea d d i t i o n a l n ec k was e qu ip pe d w i t h a t hermocoup leconnected t o a d i gi t al p y r o me t er . T hi s p y r om e t e r

provid ed ra pid t emp er at u re rea din gs t h a t were ess ent ialto follow th e very fa st reactions. The larger size flaske n ab l ed u s t o s ca le u p t h e r e ac t io n and p o ss i bl y

pe rm i tt e d mo re vig oro us agitation.The reaction flas k was immersed in a Polyscience

Series 9000 refrigera ted constant temperature bat h andcirculator. The oper atin g ran ge of this bat h was -1 5 t o+1 50 C, w i t h a c on tr ol a cc ur ac y o f +_0.02 C. T h et em pe ra tu re c o nt r o l o f t h e r ea c ti o n m i x t u r e w asgenerally ca. 0.1 C.

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B. FREEDMAN, R.O. BUTTERFIELD AND E.H. PRYDE

TABLE 1

R e a c t i o n C o n d i t i o n s E mp l o y e d i n K i n e t i c S t ud i e s

Alcohol used

Molar ratio Weig ht of reactantsof

alcohol/SBO Alcohol(g) SBO (g) Catalyst Temperature (C)a

1-Butanol 30:1 51 201-Butanol 30:1 51 201-Butanol 30:1 102 401-Butanol 6:1 51 100Methanol 6:1 22 100

1% H~SO41% NaOBu

0.5% NaOBu1% NaOBu

0.5% NaOCH3

77,87,97, 107, 11720, 30, 40, 50, 6020, 30, 40, 50, 6020, 30, 40, 50, 6020, 30, 40, 50, 60

aTemperature control: 0.1 C.

The rea ction flas k and its associated e quip ment ando t h e r g l a s s a p p ar a t us used f o r h an d li n g s t ar t in gmaterials were fir st he at ed to expel moisture. Calculatedamounts of SBO and alcohol were added to the reaction

flas k tha t was br oug ht to a predetermined temperatureu s i n g t h e c o n s t a n t t e m pe r a tu r e b a t h . A calculatedamou nt of cat al yst was quickly added to the reactants,and timing o f the reaction was begun.

Samp les (0.5 ml each} were rem ov ed fro m the reactionmixture at various intervals, depending upon reacti onconditions. Fo r example, alkali-catalyzed reactions at60 C were sam ple d initially a t 15-sec intervals and thenover lo ng er intervals du ri ng the course of one hr. F or theacid-catalyzed reaction at 117 C, early samples weretake n a t 6-min intervals, and lat er samples were ta kena t 30- min i nt er va ls d u r in g t h e c ou rs e o f t h r ee h r.Samples were immediately quenched in 0.5 ml water,thus stopping the reaction. Because the oil la ye r did not

always cleanly se pa ra te from the water, s alt ing out andc e n t r i f u g i n g w e re u s e d t o e nh an ce s ep ar at io n. T heresu ltin g oil was used for both thin laye r chromatog-ra ph y (TLC) and CGC analysis.

T LC w a s u se d i n it i al l y t o d e t e r m i n e t h e q u a l i t a t i v ecomposition of each sample of the react ion mixture andto determine which samples were to be quantitativelya na ly zed . This T LC p r o c e d u r e h as b ee n d es cr ib ed

previously {2). The r e a c t i o n was st op pe d whe n TL Cindicated high conversion to the ester. F ou rt ee n sampleswere ta ke n, of wh ic h nine were quantitatively analyzed

b y the CGC . T h e CGC m e t h o d pro vided a q u a n t i t a t i v eanalysis of t he triglyceride (TG), diglyceride (DG),monoglyceride (MG) and es te r composition needed for

the kinetic study.Pr ep ar at io n o f CG C sa mp les an d th eir an al ys is .S am pl es f r om t h e oil l a ye r s o b t a i n e d a b o v e wereaccurately weighed and silylated, inter nal st and ar d wasadded, and the concentration was adjusted and thena n al y z ed b y CGC as p r ev i o us l y d e s c r i b e d (4). Acomputerized prog ram was used to analyze CGC dat a.The out pu t from this p rog ram gave wt %, ~mol or mmol,sta ndar d deviation and relative sta nda rd deviation for each component.

Co mp ute rize d k in e tic p rog ram. A c o m pu t e r iz e dkinetic program, described previously 06), was used todetermine whe the r a proposed kinetic ord er appeared

pos sible . The p r o g r a m requires t h a t a specific kin eti cscheme be proposed for each reaction be in g studied. Thereactions studied are shown in Figure 1. Additional

Ov era ll Reaction:

Triglyceride (TG) + 3ROHc a t a l y s t

3R'CO~R + glycer ol (GL)

S t e p w i s e Reactlons:

TG-DG(k,)1. TG + RO H _

DG-TG(k,)

2 , DG + ROHDG-MG(k2)

MG DG(ks)

MG-GL(k3)

diglceride (DG) + R'CO2R

monoglyceride (MG) + R'C%R

3, MG + ROH- ~ gl y ce r ol + R'CO2R GL-MG(k~)

FIG. 1. Chemistry of t h e t r a n s e s t e r i f i c a t i o n o f v e g e t a b l e o i l s .

100

80 x". * / , . , - / " /

~ 4o/ . . - - / . ," . ,/

0 1 , . ~ , , i l l . . . . . . . . I

0.1 0.2 0.5 1 2 5 10 20

Time, hr

FIG. 2. Effect of t i m e a n d t e m pe r at u re o n e s te r f o r m at i o n .B u O H / S B O(30:D, 1% H~SO4, 77-117 C. X, 117 C; o, 107 C; a , 97 C;m, 8 7 C; + , 7 7 C .

in put consists of the concentration of each component

w i t h t i m e . C on ce nt ra ti on u n i t s were e i t h e r m m o l o r ~mol/ml depending on requ irem ents of the program. Th e

p r o g r a m ut ili ze s t h e s e d a t a t o p r o d u c e p l o t s o fconcentration vs time, whic h are shown later. A se pa ra te

plot is produ ced f o r each t e m p e r a t u r e . B a s e d up on t h e

k i n e t i c s c h e m e b e i n g t e s t e d , t h e p r o g r a m a t t e m p t s t odraw a line through the points. A close fit of the lines toth e poin ts suggests tha t the kinetic scheme employe d isadequate, and fr om this a kinetic or de r can be proposed.O n t h e o t h e r h a n d , a p oo r f it o f th e l ines t o t he p o i n t s

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TRANSESTERIFICATION KINETICS OF SBO

suggests tha t an incorr ect scheme was proposed and/ort h e d a t a a r e f a u l t y .

T he p r o g r a m also r ep or ts t wo t y pe s o f e r ro rsassociated with t he fit of th e l ines to the points at eachtemperature. The first is the "summed squared error,"w h i c h is t he s u m o f all v e r t i c al dis tanc es s qu ar ed

b e t w e e n p o i n t s a n d line s. The g r e a t e r t h e coincidenc eb e t w e e n p o i n t s an d lines, the s mal ler t h e error. Thesecond errorreportedis the" averageerror," whichis thesquare root of the summed squared error divided by thenum ber of dat a points (concentrations). By com paringt h e t o t a l o f e i t h e r t h e s u m m e d s q ua re d o r a v e r a g e e r r o r s

of the various schemes be in g te st ed at all temperatures,o ne c an d e t e r m i n e w h i c h s c he m e g iv es t he b e st fit.F i n a l l y , t h e c o m p u t e r p r o g r a m r e p o r t s r a t e c on st an tsfor each react ion at each temperatu re.

RESULTS AND DISCUS SION

Chemistry of the transesterif ication of vegetable oils.

The stoichiometry of t he overall transesterificationre ac ti on requires 1 mol of TG for 3 mol of alcohol to give3 mol of es te r and 1 mol of glycerol (GL) ( Fig. 1). Thisreversible reaction, whi ch is e it her acid- or alkaline-catalyzed, involves stepwise conversions of TG to DG toMG to GL producing 3 mol of ester in the process. Oneo b j ec t i ve o f this s t u d y was t o d e t e rm i ne t h e r a teconstants for all forward and rever se reactions. Rath ert h a n u s e k , t o d es ign ate t he r a t e c on st an t for t heconversion of the TG to DG, we have chosen to use thedesignation TG-DG. Similarly, designations for theot he r rat e constants are shown in Figure 1. It should beno te d tha t the transesterification react ion consists of consecutive as well as reversible reactions th at mu st be

accounted fo r in any kinetic study.E f f e c t o f ti me an d tem pe rat ure on ester for ma ti on . Todetermine the effect of temperature on este r formationfor th e reaction of BuOH wit h SBO a t a molar ratio of 30:1 catalyzed b y 1% H2SO4, five te mp er at ur es ran gi ngfrom 77-117 C were examined. The results of this studyshowed tha t est er formation is essentially complet e in3 hr a t 117 C, com par ed to 20 hr at 77 C (Fig. 2). It will be

no te d th at the curves have spacing which indicates adist inct rate change for every 10 C temperature rise . Thecurves also indicate the reaction begins at a slow rate,

proceed s a t a f a s t e r r a t e and th en slows a gai n as the

rea cti on nea rs completion. The shapes of the curves att w o a dj ac en t t e m p e r a t u r e s e na bl ed u s t o p r e d i c t t h ea p p r o xi m a t e s ha pe s o f t he c ur ve s f o r t h e r e m a i n i ngthr ee temperatures. This informat ion was valuable fordetermini ng both when samples should be take n andwhen to stop the reaction. Similarly shaped curves forth e esters were observed for alkaline-catalyzed reac-

tions.Kin et ic s of acid-catalyzed tr an se st er if ic at io n reac-

tions. W e u n d e r t o o k a k i n e ti c s t u d y o f t h e r e a c t i o n o f B u OH w i t h S B O u s i n g a 30:1 m o l ar r a ti o w i t h 1%H2SO4 at five temperatures. A 30:1 molar ratio was

c h o s e n b e c a u s e a t m o l a r ra t io s of 6 :1 a n d 20:1,conversion to es te r was unsatisfactory (1). The resul tsobtained at 117 C are shown in Figure 3. Because weused a larg e excess of alcohol over th at req uir ed by the

stoichiometry, we expect ed the forward reactions tofollow pseudo-first-order kinetics. The rev ers e reac-

60

50

4O

J

J

~ - ~ - - - L i - . . . . . . . . IO ~ ~ - J I I ~ L - ~ l ~ - - i - ~

0 25 50 75 100 125 150 175 200

Time,min

F I G . 3 . C h a n g e o f c o mp o n e n t c o n c e n t r a t i o n w i t h t i me f o r a mo l a r

r a t i o o f B u O H / S B O ( 30:1), 1 % H2SO4, 1 17 C . . , S o y b e a n b u t y le s t e r s ; A , s o y b e a n o il ; X , d i g l y c er i d e ; I , mo n o g l y c e r i d e .

100

BO

~ 4[1

20

f i , ~ A r . . ~ ~ - - ~ - - . . . . . . . . . . . . . . . . . . . .

t~,?n

- j !

L i I i I I

2 4 6 8 10 12

Time,rain

F I G . 4 . C h a n g e o f c o mp o n e n t c o n c e n t r at i o n w i t h t i me f o r a mo l a rratio of BuOH/SBO (6:2) , 1% NaOBu, 30 C. e , Soybean butyl esters;I , mo n o g l y c e r i d e X 1 0; A , s o y b e a n o il ; X , d i g l y c e r i d e .

tions would be expec ted to be second orde r. The fit of thelines to th e poi nts was excel lent for all four components.Similarly, good fits were obtained for t he ot he r fourt e mp e ra t ur e s s t u d i e d . T h e s e f i nd i n gs s u p p o r t o u r k in et ic a s si g n me n t f o r t h e f or wa rd a n d r e v er s ereactions. The curves for all four components sh own inFigure 3 are characteristic of consecutive reactionsa n d c o nf i rm t h a t t h e t r a ns e s te r i fi c a ti o n r e a ct i o nconsists of a series of consecutive reactions. It should ben o t e d t h a t b o t h t h e f o r w a r d a n d r ev er se r ea ct io ns a r e

c on sec ut iv e. Th e r a t e c on st an ts o b t a i n e d f o r t h e s ereactions are discussed later.

K i n e t i c s of al kal in e- ca ta ly zed tr an se st er if ic at io nreactions. Alkaline-catalyzed transesterifications pro-ceed at considerably fas ter rates than acid-catalyzedtransesterifications (2,17). Partly for this re as on and

part ly be ca us e alk ali ne c a t a l y s t s ar e less corrosi ve t oindu stri aleq uipme nt th an acid catalysts, mo st commer-cial transesterifications are conducted with alkalinec a t a ly s t s. S o di u m a lk ox id es a re a m on g t h e m o s tefficient ca ta ly st s used for this purpose (5,6,18) an d were

therefore used in ou r study.The molar ratio of alcohol to SBO has an impor tant

b e a r i n g on the ki ne tic o r d e r of the reaction. W e have

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s h o w n t h a t acid c a ta l ys i s o f t h e f or wa rd r e a ct i o nemploying a 30:1 molar ratio of BuOH to SBO followed

pse udo -fi rst o r d e r kinetics. This was e x p e c t e d be ca us eo f t h e l a r g e m o l a r e x c e s s o f alcohol. S im il ar ly , w eex pe ct ed alkaline-catalyzed transesterifications us in g a30:1 molar ratio of alcohol to SBO to follow pseudo-first

ord er kinetics. We conducted two studies, both us ingm o l a r r at io s o f BuOH t o SBO o f 30 : 1, a t f ivet em pe ra tu re s b e t we e n 20 a n d 60 C i n w h i ch t h ecatalysts were 1% and 0.5% NaOBu by weight. The da taobtained from th es e experiments were used in compu-terized plots of concentration vs t ime as previouslydiscussed. These plo ts strongly supported ou r assump-tion th at t he NaOBu-catalyzed forward reactions at30:1 followed pseudo-first ord er kinetics.

We next studied the effect oflowering th e molar ratiof r o m 30:1 t o 6:1 w i t h 1% N a OB u a s c at al ys t. I t h as

previously been sho wn t h a t wi th a lka li ne cat aly sis , a 6:1molar ratio o f alcohol to vegetable oil results in highconversions (93-9 8%) o f vegetable oil to est er , whereas

bel ow t h e 6:1 ra ti o con ver sio n dec reases (1). A 6:1 mol arratio would therefore proba bly be t he preferr ed ratio incommercial operations. At this ratio a second-orderreaction appeared more likely than a pseudo-first-orderreaction because the exces s alcohol was only twice th atof theory compared to 10 times that of theory for a 30:1r a t i o . B o t h s ch em es , h ow ev er, were t e s t e d u s i n g t h ecomputerized kinetic program. Again it was ass ume dt h a t t h e c on se cu ti ve , r e v e r s e r ea ct io ns were s ec on dorder. This prog ram indicated a bet te r fit of lines to

p o i n t s f o r t h e second- order r e a c t i o n t h a n f o r t h ep s e u d o - f i r s t - o r d e r r e a c t i o n . T h e s u m me d s q u a r e d

errors for the second-order reactions at 20 -6 0 C were46.5, 3.4, 17.4, 27.7 and 22.7, for a to ta l of 117.7. Thes u m m e d s q u a r e d e r r o r s f or t h e p se ud o- fi rs t- or de r reactions at 2 0 - 60 C were 235.2, 7.1, 33.2, 44.2 and 25.6,f o r a t o t a l o f 3 45 .3 . T h e a v e r ag e e r ro rs f o r t h esecond-order reactions were 1.3, 0.4, 0.8, 1.0 and 0.9, fora t ot al of 4.4. The average error s for the pseudo-first-or de r reactions were 2.9, 0.5, 1.1, 1.3 and 1.0, for a to ta lof 6.8. Thus both the tot al of th e summed squared error sand th e ave rag e errors were lower for the second-orderreact ion compare d to the pseudo-first-order reaction.These differences ap pe ar to be significant and were the

ba si s f o r choosing the secon d-ord er reaction. A plot o fdata fo r component concentration vs time for a 6:1 mol arr at io of B uO H to S BO w it h 1% N aO Bu a t 30 C,as su min g a second-order reaction, is shown in Figure 4.Th e f it o f th e l i n es t o t he p o i n t s i n this c as e wasexcellent. Good fits were also observed at the ot he r fourtemperatures. These resul ts suggest th at at a 6:1 molar ratio with 1% NaOBu, the reaction of BuOH with SBOfollows consecutive, second-order kinetics.

We also expected the reactions of MeO H with SBO ata 6:1 m o l a r r a ti o w i t h 0 . 5% N a O C H3 a t 2 0 - 6 0 C t o b esecond ord er. MeO H, however, behaved differently fro mBuOH in this re gar d. Although a second-order schemewas evaluated by our com put er progr am for the reactionof MeOH/SBO a t 6:1, the average error obtained wass o m e w h a t l a r g e r t h a n t h a t o bs er ve d w i t h Bu O H / SB O a t6:1. This i nc re as e i n e r r o r is b ec au se o f t h e r a p i da pp ea ra nc e o f e s t e r w i t h o u t a c or re sp on di ng r a p i di n cr e as e a n d d e c re a se o f D G a n d MG. B e ca u se

c ons ec ut iv e r e a c t i o n s c o u l d n o t e xp la in t h e r a p i d

a pp ea ra nc e o f e s t e r w i t h o u t t h e a pp ea ra nc e o f i n t e r -m e d i a t e D G a nd M G, a s hu nt -r ea ct io n s c h e m e wastested. This shunt-reaction scheme compr ise d both thesecond-order reactions and a s hu nt reactio n in which 3m o l o f M e O H d i r ec t l y a t t a c k e d 1 m o l o f T G. B y u s i n g t h eshunt-reaction scheme, we obtained m uch bet te r fits of

t h e d a t a . T hu s, t h e k i n e ti c s wh i c h b es t descr ibe t h er e a c t i o n o f M e O H / SB O a t 6 :1 m o l a r r a t io c on si st o f acombination of second-order consecutive and fou rth -ord er sh un t reactions.

The shunt-reaction scheme was fu rt her supported bycom par ing certa in res ult s obtained fr om kinetic studieso f b ot h t h e s h u n t a n d s ec ond -o rd er s ch em es . Twocriteria were used in this comparison. The firs t was a

comparison of sum med squared and aver ag e errors. Thesummed squared errors for the second-order scheme at2 0 - 6 0 C were 1.5, 3.4, 2.9, 6.7 and 4.1, all values X 10-5,for a to ta l of 18.6 10-5. The su mmed squared err ors f or

th e shunt-reaction scheme at 20 - 6 0 C were 1.5, 0.4, 1.9,1.8 and 1.5, all values 10-5, fo r a t ot al of 7.1 X 10-5. The

average errors for the second-order scheme were 0.8,1.1, 1.0, 1.6 and 1.3, all values 10-3, fo r a to ta l of 5.8 10-3. The average er ror s for the shunt-reaction schemewere 0.8, 0.4, 0.8, 0.8 and 0.8, all va lu es 10-3, for a to t a lof 3.6 10-3. Thus bo th the to ta l of the s umme d squarede rr or s a n d t h e a v e r ag e e rr or s we re l o we r f o r t h eshunt-reaction scheme compa red t o the second-orderscheme. A plot o f da ta for component concentration vst im e f or M e O H / S BO a t a 6 :1 m o l a r r a t i o w i t h 0.5%

Na OC H3 a t 40 C, a s s u m i n g t h e shu nt- reacti on sch eme ,

is shown in Figure 5. The close fit of lines t o points, att hi s t e mp e ra t ur e a nd t h e o t h e r four t e mp e ra t ur e s

studied, strongly supports th e vali dity of this scheme.T h e s e c o n d c r it e ri o n w as c o rr e la t io n c o ef f ic i en t s

obtained by linea r regression from plo ts o f log k vs 1/T.These plo ts were used to dete rmin e en er gy of activation,E, and are discussed later . The correlation coefficientsobtained wit h th e shunt-reaction sch eme were generally

hi gh er th an those from the second-order scheme. Thus,the sh un t react ion mus t be used in conjunction with thesecond-order rea cti on to fully explain the kinetics fort h e r eac tio n o f M e O H w i t h S BO a t a 6:1 m o l a r r a t i o .

Sh un t reactions have been observed in kinetic studies ofth e hydrogenation of conjugated die ne (19) and SBO andlinseed oils (20).

We next reexamined the reactionof BuOH/SBO at 6:1

0.12

4 ~ f t r

0.09 - o f

0.06

L~_

0.03 - - m ,

/

0 . 0 1 I I I I

5 10 15 20 25 30Time, min

F I G . 5 . C h a n g e o f c o mp o n e n t c o n c e n t r a t i o n w i t h t ime f o r a mo l a r

ratio of MeOH/SBO (6:2) , 0.5% Na OCH3, 40 C. s , Soyb ean m ethyle s t e r s ; X , d i g l y c e r i d e X 1 0; l , mo n o g l y c e r i d e X 1 0; b , s o y b e a n o i l.

JAOCS, Vol. 63, no. 10 (October 1986)


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1379

TRANSESTERIFICATION KINETICS OF SBO

to determine if the shunt-reaction scheme was applic-

able. Our computer pr og ra m calculated a zero reaction

ra te , clearly indicating that the sh un t reaction did not

ex is t for the BuOH reaction. Why was the transester-

ification of SBO wit h MeOH different from that with

BuOH at the 6:1 mol ar ratio? Two possible explanations

can be propo sed. The smaller size of the MeOH moleculerelative to that of BuOH could facilitate the simultane-

ous a tt ac k of MeOH on all th re e chains of the TG. Also,

the greater pola rity of the methoxy anion compared to the

b u t o x y a n i o n m i g h t pr omo te its a t t a c k on th e es te r

carbonyl according t o established mechanisms.

R a t e co ns ta nt s. Rate constants were calculated by ourc o mp u te r p r o g r a m f o r a ll r ea ct i on s i nc lu de d i n o u r

k in et ic s t u d y . T h e r a t e c o ns t a nt s d et er mi ne d f o r th ereaction of BuOH wit h SBO at a 30:1 mola rr at io us in g

v ar io us c a t al y s ts a re s h ow n i n T a b l e 2. B ec au se t h e

forward and reverse reactions are not of the same kinetic

or de r at a 30:1 mo lar ratio, a direct comparison of t he se

rate constants ca nno t be mad e. The alkaline-catalyzed

reactions had much lar ger rate constants than the acid-c at aly zed r e a c t i o n s for b ot h f o r w a r d and r e v e r s e

reactions, as expected. The higher rate constants for the

T A B L E 2

E f f ec t o f V ar i o u s C a t a l y s t Systems on R a t e C o n s t a n t s

BuOH/SBO, 30:1, rate constants X 10~

Rate 1% H~SO4, 1%NaOBu, 0.5%NaOBu,

designation 77 C 60 C 60 C

TG-DG 3 3,822 26,626DG-MG 8 1,215 3,584

MG-GL 7 792 2,373DG-TG 0.02 121 439MG-DG 0.05 7 8

GL-MG 0.03 11 7

For the forward reactions units are min-L For the reverse reactionsunits are ml mmol-~ rain-' for the acid-catalyzed reaction, and ml

~no1-1 rain-' for the alkali-catalyzed reaction. TG, triglyceride; DG,diglyceride; MG, monoglyceride; GL, glycerol.

0.5% NaOBu- vs 1% NaOBu-catalyzed reactions was

unexpected. This difference probably resulted because

t he 0 .5 %- a nd l %- ca ta ly ze d r ea ct io ns w er e r un o n

different scales (see Tab le 1) and with different reactors(see Experimental). The large r size of the 400-ml fl as k

m ay h av e p e rm i tt e d m or e v ig or ou s a gi ta ti on , w h i c h

could have influenced reaction rates. Had the reactionsbeen ru n u n d e r id enti cal co nd it io ns , r a t e s fo r th e

l%-catalyzed reactions should have been highe r than

t h o s e f or t h e 0 . 5 %- c at a ly z ed r ea ct i on s. A n u m b e r o f

kinetic studies have shown that reaction rate s increasewit h increasing catalyst concentration (7,8,14).

E n e r g y o f ac ti va ti on . T h e r el at io ns hi p b e t w e e nreaction rate constant, k, and temperature is giv en bythe integrated form of the Arrhenius equation: log,ok =

(-Eo/2.303R)/T + Cw he re E, is the ene rgy of activation,

R the gas constant in calories mo1-1 degree-1, T the

absolute temperature and C a constant. From a plot of

log k vs l /T, the slope can be determined. This slope is

eq ua l to (-E,/2.303R). Thus, E, = - 4 . 5 8 (slope).

We determined energ y of activation in this m an ne r formost of the reactions. An exa mpl e is sh ow n in Fig ure 6

f or t he a ci d- ca ta ly ze d b ut an ol ys is o f T G t o D G at

77-117 C. Line ar regression analysis of t he s e data gave

a s l op e o f - 3 2 5 8 . 1 w i t h a c or re la ti on c oe ff ic ien t o f

- 1 .5 117 ~ ' ~ 1

-1.7 07 C

\-1.9 ~ 9 7 C

\

-2.~ \

-2.3

- 2 . 5 L__ L ___~- J _ _ _ 0.25 0.26 0.27 0.28 0.29

lIT x 10~

FIG. 6. Plot o f l o g k v s l i T f o r t h e b u t a n o l y s i s o f t r i g l y c e r i d e t od i g l y c e r i d e w i t h 1% H2804 at 77-117 C.

T A B L E3

Energy of Activation (E.) Under Studied Condit ions

E. (caYmol): reaction conditions

BuOH/SBO, 30:1

1% H2SO4 1%NaOBuRate designation 77-117 C 20-60 C

BuOH/SBO,6:1 MeOH/SBO,6:1

0.5% NaOBu 1% NaOBu 0.5% NaOCH3

20-60 C 20-60 C 20-60 C

TG-DG 14,922 15,360 15,662 17,092 16,062DG-MG 16,435 11,199 13,053 12,137 17,247MG-GL 15,067 11,621 13,395 10,693 --

DG-TG 19,895 17,195 15,587 15,925 15,843MG-DG 16,885 -- 13,336 15,816 13,571GL-MG 12,196 -- 13,110 8,181 --

TG-GL . . . . 20,022

TG, triglyceride; DG, diglyceride; MG, monoglyceride; GL, glycerol.

JAOCS, Vol . 63, no. 10 ( O c t o b e r 1986)


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]380


--0.9964. By us in gthis slope in the equation E, = - 4 . 5 8

(slope), an Ea of 14,922 cal/mol was calculated for this

r ea ct io n. C or re la ti on c oe ff ic ie nt s f o r t h e o t h e r f iv e

reactions of BuOH/SBO, 30:1 catalyzed by 1% H2SO4were - 0 .9 9 18 , - 0 .9 9 65 , - 0 .8 9 18 , - 0 .9 7 95 and

- 0 . 9 9 0 5 .Ener gies of activation determined for ot he r reactions

in our study ranged from 8,000-20,000 cal/mol (Table 3).

Ot he r investigators have reported Eovalu es within this

ran ge for other transesterification reactions (6,8,9-12).E o f or t he s h u n t r ea ct io n T G -G L h ad a v a l u e o f 2 0

kcal/mol. The spread of values seen in Ta bl e 3 for Ea is

d ue p ar t ly t o e xp er im en ta l e rr or . I n a dd it io n, t he

s imp li fi ed m o d e l u se d i n t he e q u a t i o n log,oK =

(-Eo/2.30R)/T + C may not be adequate t o account for

all the variables involved. This mi ght also explain why

Eo for some reverse reactions are greater than those of

the corresponding forward reactions. Fur th er research is

neededto develop a model that might provide a narrower

range of Eovalues.

ACKNOWLEDGMENTS

Haifa Khoury assisted in the laboratory and in manuscriptpreparation; Wilma Bailey gave statistical assistance.

REFERENCES

1. Freedman, B., E.H. Pryde and T.L. Mounts, J . A m e r . O il C he m . Soc . 61:1638 (1984}.

2. Freedman, B., and E.H. Pryde, in Vegetable Oil Fuels,

Pr oc ee di ng s o f th e In te rn at io na l Co nf er en ce on P l a n t andVegetable Oils as Fuels, American Society of AgriculturalEngineers, St. Joseph, MI, 1982, p. 117.

3. Freedman, B., E.H. Pryde and W.F. Kwolek, J . A me r . O il C he m . Soc . 61:1215 {1984}.

4. Freedman, B., W.F. Kwolek and E.H. Pryde, Ibid. 62:663{1985}.

5. Markley, K.S., in F a t t y A c i d s , T he i r C he mi st ry , P rope rt ie s,Pr od uc ti on and Use s, edited by K.S. Markley, 2nd ed.,Interscience Publishers, New York, 1961, Part 2, p. 862.

6. Sridharan, R., and I.M. Mathai, J. Sci . Ind. Res. 33:183 {1974}.7. Cotes, A.C., and L.A. Wanzel, L at . A m . J . C he m . E ng. A ppl .

C he m . 4:126 11974).8. Davies, B., and G.V. Jeffreys, T r ans . I n s t . C he m . E ng. 51:271

{1973).9. Farkas, L.,O. Schachter and B.H. Vromen,J . A m . Chem. Soc.

71:1991 11949}.10. Baker, G.E., and R.R. White,Ch em. E ng. P rog. S y m p . Ser. No.

4 48:75 (1952).11. Ravindranath, K., and R.A. Mashelkar, J . P ol y m . Sc i. P ol y m .

C he m . E d.20:3447 {1982}.12. Tomita, K., and H. Ida, Polymer14:55 (1973}.13. Pore, J., J.P. Houis and I. Rasori, Rev. Tech. Ind. Cuir 73:164

(1981).

14. Dufek, E. J. , R.O. Butterfield and E.N. Frankel, J . A m e r . O i l C he m . Soc . 49:302 {1972}.

15. Dufek, E.J ., Ibid. 55:337 (1978}.16. Butterfield, R.O.,Ibid. 46:429 (1969}.17. Reid, E.E., A m . C h e m . J. 45:506 {1911}.18. Formo, M.W.,J. Am er . Oil C he m . So c. 31:548 {1954}.19. Koritala, S., R.O. Butterfield and H.J. Dutton, Ibid. 50:317

{1973}.20. Vigeron, P.Y., S. Koritala, R.O. Butterfield and H.J. Dutton,

Ib id . 49:371 ~1972).

[Received December 17, 1985]

.S tud y on the Oxi dative Rate and ProoxJdant Activity

of Free Fatty Acids

Kazuo Miyashita an d Tom Takagi*Department of Chemistry, Faculty of Fisheries, Hokkaido University, Hak oda te, Jap an 0 41

Oleic, linoleic a nd linolenic acids were autoxidized more

rap idl y tha n their corresponding m et hyl esters. Addi tion

of st ea ri c aci d accel erat ed t he rate of au t oxi d at i on of

meth yl linoleate and the decompos ition of meth yl linole-

ate hydroperoxides. Therefore, the higher oxidative rate

o f F F A ' s than t he ir m e t h y l e s t e r s c o u l d be due t o t h e

cat al yt i c ef f ect of t h e carb oxyl grou ps on t h e f ormat i on

of free radic als by the dec omp osi tio n of hydroperoxides.

Add iti on of stearic acid also accelerated the oxidat ive rate

of soy bea n oil. This result su gg es ts th at particular atten-

t i on sh ou l d b e p ai d t o t h e FFA con t en t t h at af f ect s t h e

oxi d at i ve st ab i l i t y of oi l s.

A few pape rs (1,2) have been published on t he comparison

of the oxidative rate of free fatt y acids (FFA) and their

esters. Holman et al. (2,3) reported that FFA's were ox-

idized more rapidly than their esters, and he suggested

that this effect probably was due to participation of the

*To whom correspondence should be addressed.

carboxyl groups in the decomposition of peroxides. He

also said in this review (3) that this idea has been sup-

po rte d by th e wo rk of Pr ive tt et al., in w h i c h ad di ti on of

linoleic acid to methyl linoleate peroxide accelerated its

decomposition. However, i t was not cle ar whether i ts

decomposition was promoted by the catalytic action of

the carboxyl group in linoleic acid or oxidized products

of this acid, and the effect of F F A ' s on the autoxidation

of esters has not been investigated.In this paper, we report the detailed data for the dif-

ference in oxidative rates between FF A' s and their methyl

e s te r s w i t h p er i od ic m ea su re me nt o f t h e u no xi di ze d

substrate content by GLC and POV, and elucidate the

catalytic action of the carboxyl group in FF A' s on the

autoxidation of oils by the use of stearic acid as a cata lystwhich is not autoxidized un de r the conditions of the pres-

ent experiment.

MATERIALS AND METHODS

P r e p a r a t i o n o f m a t e r i a l s . Oleic,linoleic and linolenic acids

JAOCS, Vol, 63, no, ]O (October ]986)

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