Al Jarallah1988

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Kinetics of Methyl Tertiary Butyl Ether SynthesisCatalyzed by Ion Exchange ResinADNAN M. AL-JARALLAH, MOHAM MED A . B . SIDDIQUI, and A. K . K . LEE

Department of Chem ical Engine ering and the Research Instihrte, King Fahd U niversityof Petroleum & Minerals, Dhahran31261, Saudi ArabiaThis paper presents the results of an experimental investigation of the kinetics of liquid phase reaction between methanoland isobu tene, catalyzed by an acidic ion-exchange resin, to form methyl tertiary butyl ether MTBE). A one litre Parrbatch reactor was used. Experiments were carried out at 70, 80, 90 nd 100C and at pressures sufficient to maintainliquid phase at those temperatures. Initial methanol/isobutene mole ratios of 1 O and 2.0 were used. The catalyst amountwas also varied.These kinetic data were used to model the reaction kinetics, by non-linear least squares regression technique. Thereaction was found to follow Rideal-Eley kinetics. The values of the rate constants are reported.On presente dans cet article les rtsultats dune recherche ex ptrimen tale sur la cinttique de reaction en phase liquideentre le methanol et Iisobuttne, catalysts par une rtsine tchangeuse ions acide, pour former du methyl tertiaire butylether MIB E). On a utilist u n rtacteur discontinu Parr de 1 litre. Les exptriences ont ttt mentes a des temperaturesde 70, 80, 90 t 100C et ies pressions suffisantes pour maintenir la phase liquide a ces temperatures. Des rapportsmolaires initiaux mtthanollisobuthe de 1 O et 2 O ont ttt utilists. La quantitt de catalyseur varie tgalement.Ces donntes de cinttique ont ett utilistes afin de modtliser la cinktique de reaction, par la technique de regressiondes moindres carres non linbaire. On a trouvt que la rtaction suit la cinttique de Rideal-Eley. Les valeurs des con-

stantes de vitesse sont egalement donntes.Keywords: methyl tertiary butyl ether synthesis, MTBE kinetics, ion exchange resin catalysis.

ethyl tertiary butyl ether has received attention inM ecent years as an important alternative to lead alkylsas a gasoline additive to increase the octane number. Unlikelead alkyl additives which cau se air pollution and ar e toxic,MTBE is non-toxic and non-polluting according to studiesby Csikos et al. 1976), Torck et al. 1982), and Furey andKing 1980).MT BE is produced by reacting methanol Me OH) withisobutene i-Bu) in the presence of an acidic catalyst, suchas sulfuric acid, acidic ion-exchange resins, o r other acidiccatalysts:

MeO H i-Bu MT BE . . . . . . . . . . . . . . . . . . 1)Th e reaction is reversible and exothe rmic, with a heat of reac-tion of -37.2 kJ/mol in the liquid phase at 25C.Since the discovery of the etherification reaction betweenalcohols and olefins by Reyc hler 1907), very little scien-tific work has been published on the re action. Only limitedkinetic inform ation on the reaction was published by Eva nsand Edlund 1936) and , recently, by Ancillotti et al. 1977and 1978), Gicquel and Torck 1983), Csikos et al. 1979),and by Chu and Kuhl l987). In the two latter investigations,sulfuric acid and zeolite were used a s catalysts respective ly,while in the first three recent studies the ion exchang e resinAmb erlyst 15) was used as catalyst. Actually this is the mostwidely used catalyst in industrial produc tions of MT BE .Ancillotti et al. 1977) studied this reaction with Amber-lyst 15 catalyst and reported a zero ord er dependence of rateon methanol concentration, for concentrations greater than4 mol/litre, with negative orders at lower concentrations anda first order dependence of rate on isobutene concentrationbased on analyses of the initial rates of the reaction. T he sam eauthors in 1978 examined th e influence of methanol concen-tration of the activity of Amberlyst 15 resin. Gicquel andTor ck 198 3) investigated this reaction and reported that the

reaction follows Langmuir-Hinshelwood kinetics. Theyreported relative values of rate and adsorption equilibriumconstan ts. A lot of information on reaction cond itions, con-versions, and selectivity in MTBE synthesis can be foundin patents. A comprehensive review of MTBE patents,production technolo gies and econom ics is given by Le e andAl-Jarallah 1986).In this stu dy, rate equa tions describing the kinetics of theMTBE synthesis reaction, catalyzed by ion exchange resin,have been developed and presented with the values of all therate constants involved.Experimental

Th e liquid phase reaction between methanol and isobutenewas carried out in a standard on e liter Parr pre ssure reacto r.Batchwise experiments were performed. The Parr pressurereactor was equipped with magnetic stirrer and internalcooling coil in addition to the necessary a ccesso ries such asinlet valve, sampling valve, pressure gau ge, thermowell andheater jacket.A measured volume of methanol was introduced into thereactor and a weighed quantity of the ion-exchange resincatalyst w as added to it . The contents were heated up to thedesired temperature. Pure liquid isobutene was then fed inand the reactor was pressurised with nitrogen to maintainliquid phase. T he whole mixture wa s stirred at lo00 r/min.to eliminate the effect of agitation on mass transfer whichis significant at speed below 6 r/min. T he temperature wasmaintained at the desired set point by circulation of thecooling water through the internal cooling coils. The timeof addition of isobutene was taken as the starting time ofthe reaction. The reaction was allowed to run and liquidsamples were collected at regular intervals. Details of theexperimental procedure have been described by Siddiqui

1987).802 THE CANA DIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 66. OCTOBER, 1988


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A

o o O0 06 0

wa.W4 0 Alt I QF A2m X 0 2 5 g c o t

A S 2 9 C o t02 0 - .+ A7 6 9 c o t+

X x 12 4 g c o t0 20 0 9 COl

00 4 0 80 1 2 0 160 2 0 0

TIME, m l n AMOUNT OF CATALYST, I

Figure 1 - Isobuteneconversion versus time at methanol/isobu-tene = 2 and 80C for different amounts of catalyst. Figure 2 - nitial rate of isobutene conversion versus amount ofcatalyst at 80C and methanol/isobutene = 2.

The ion exchange resin catalyst Amberlyst 15) was sup-plied by Rohm & Haas. The resin is a mac roreticular nuclearsulfonated copolymer of styrene and divinylbenzene. Thecatalytically active group is the nuclear sulfonic acid. Its ion-exchange capacity is 4.9 milli-equivalent/g of dry resin.AnalyticalOne microlitreof each sam ple was ana lysed by a Varian 3700gas chromatograph GC ). The GC was equipped with a flameionisation d etector. A 3 .2 m long, 4 mm ID, stainless steelcolum n, packed with 10%dinonyl pthalate on chrom osorbW-HP, was used. Th e detector was connected to a CDS 1 1 1integrator directly giving the area of the different peaks. TheGC was calibrated with pure compounds, and the amountof each compound present in the product sample was thendetermined from the respective area counts using this calibra-tion. Most of the isobutene escaped to the air a s the sam plewas depre ssurised; therefore the amount of isobutene wascalculated from the stoichiometry of the reaction while thatof methanol and MTBE was determined by GC analysis.Diisobutene was not found in product samples, becauseof the very low rates of its formation and excess or equi-molar m ethanol at the conditions of these experim ents.Ancillotti et al. (1978) showed that at 60C the initial ratesof isobutene dimeriza tion in MTBE synthesis were insigni-ficant at metha nolho buten e molar ratios greater than 0.30.Results and discussionKINETICDATA

The effects of three variables on the kinetics of MTBE syn-thesis have been investigated. These variables are tem pera-ture (70 to 100 C), amount of catalyst (2.5 to 20.0 g)corresponding to 1 10% by weight, and initialmethanol/isobutene molar ratio 1 O and 2.0).For studyingthese effects, only the approp riate parameter was varied while

the other two were kept constant.The first param eter that was tested was the catalyst amountin order to determine the optimum catalyst amount to usefor studying other parameters. Figure 1 shows the conver-sion of isobutene vs. time for different catalyst amounts ata tem perature of 80C and an initial reactants ratio of 2.0.In this figure, the slope of the curve at any time is an indica-tion of the rate of conversion of isobutene. Figure 2 showsthat the initial rate of isobutene conversion increased as theamount of solid catalyst increased from 2 . 5 gm to 12.4gmand was practically the same when the amount was increasedfrom 12.4gm to 20.0 gm. At low catalyst amount, the highconcentration of methanol inside the resin reacts with the acidgroups form ing solvated protons which become the catalyticagent. Th e solvated proton is a less active acid species thanthe acid group S03H, therefore the rate is slower accordingto Gates and Rodriguez (1973). As the methanol concentra-tion de creases relative to the am ount of catalyst because ofincreasing the am ount of catalyst (2.5 gm to 12.4 gm), themechanism gradually shifts to catalysis by S0 3H . At a verylarge amoun t of catalyst 20 gm). there are so many S 0 3 Hgroups that the rate now only depends on the rate of pr oto-nation of the isobutene. Howe ver, the conv ersion of isobu-tene at equilibrium should be independent of the catalystamount. The optimum catalyst amount of 12.4 g was usedfor testing other parameters.The reaction was investigated at temperature s of 70, 80,90, nd 100 C. A sam ple graph show ing the changing con-centration of methanol, MTBE and isobutene with time atone temperature 80C) is given in Figure 3. The initial molarratio of methanolhsobutene was 2.0and the catalyst amountwas 12.4 g in this experiment. This figure shows that theMTBE product con centration increases m onotonically withtime and approa ches asymptotically to a final value; this isa typical behavior of batchw ise operations. S ince the initialmolar ratio of m ethanol to isobutene is two, there is alwa ysa cons iderable amount of methanol in the reaction mixture,while isobutene decrease to a very low concentration.

Figure 4 shows the conve rsion of isobutene with time forTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 66, OCTOBER, 1988 803


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METHANOLA MTB E0 IlODUTENE

0.30-i 0 .24

gLI2 0.18U

0.12

0 . 0 8

0 ' ' ' ' ' ' ' 1 ' 1 1 '0 4 0 80 120 180 200 240 280

TIME, min

Figure 3 - Concentration of methanol, isobutene and MTBEversus time at 80 C , methanol/isobutene = 2, and 12.4 g catalyst.

different temperatures. From this figure it can be seen thatthe initial increa se in isobutene conversion is faster as thetempera ture increases. The final conversions were lowe r athigher temperatures, since the equilibrium constant of thesynthesis reaction decreases with increase in temperaturebecause the synthesis reaction is exothermic. N evertheless,high isobutene conversion about 95 9 ) has been obtained.Figure 5 shows the effect of the initial molar ratio ofmethanol to isobutene. From this figure it is seen that theinitial rate as well as the final conversion of isobutene ishigher when the ratio is one. Since Amberlyst 15 catalystacts through the intermediary sulfonic groups S0 3H )bonded to insoluble macromolecule, these groups providefor the protonation of isobutene and the reaction proceedsto form MTBE. The catalytic mechanism occurring in thepresence of this resin depends on the polarity of the reactionmedium, according to Gates and Rodriguez 1973) andThornton and Gates 1974). At low alcohol concentrations,the resin retains a network of hy drogen bonds between thesulfonic groups alone, or between these groups and thealcohol, while at high alcohol concentrations the protons aresolvated and the H-bonded network disappears. In the presentstudy it seems that for the lower alcohol concentration molarratio of m ethanol to isobutene of 1 O the protons w ere notsolvated and the isobutene can take the p roton directly fromthe sulfonic group. According to G ates and Rodriguez 1973)the sulfonic group S0 3H ) is a more acidic species than thesolvated proton, and this can account for the increased ratefor the lower mo lar ratio. N evertheless, excess methanol isoften used to suppress side reactions forming isobutenedimers.KINETICMODEL

The MTBE synthesis reaction can be represented by:

(2)B C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

100

A ~ : A A+ + + + +

8 0z0z: 0>z85Wz

4 0ms

o hx x X

0 80 120 180TIME min

240 300

Figure 4 - sobutene conversion versus time at rnethanol/isobu-tene = 2 and 12.4 g catalyst for different temperatures.

where, A, B and C denote methanol, isobutene and MTBErespectively. In general, the forward reaction is order a inA and order b in B, and the rever se reaction is order c in C.The ra te of su rface reaction, r,, is assumed to be the ratecontrolling step, as there were no mass transfer lim itations.There ar e two possible mechanisms by which this surfa cereaction takes place:1) Reaction between adsorbed molecules of both A andB on adjacent active centers, and2) Reaction between one adsorbed reactant and the otherreactant in solution.The first mechanism is the Langmuir-Hinshelwoodmechanism and the second one is the Rideal-Eley mechanismas discussed in Smith 1981) and Satterfield 1980). In thesereferences the reaction is assumed to be a simple reaction,that is, the reac tion is first order in all species . The followingrate equations were derived for general orders of reactiona, b and c . For a Langm uir-Hinshelwood model, the rateof reaction can be represented by the following equation:r, = k , K j K ;

. . . (3)j C i C : /K[ 1 KACA KBCB KCCC)~For the case of the Rideal-Eley mechanism, there are twopossibilities in which either one of the two reactants isadsorbed on the catalyst and then reacts with the othe r reac-tant in solution. For the case when the methanol A) isadsorbed and reacted with the isobutene B) in solution, thefinal rate equation is:

. . . . . . .j C; CZIK1 KACA KcCc)a

r, = k, K j 4)8 4 THE CANADIAN JOURNAL OF CHEMICA L ENGINEERING, VOLUME 66, OCTOBER, 1988


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TABLEReaction Equilibrium Constant, Rate Constant, and Equilibrium Adsorption Constants forEq. (4) with a = 1 b = 0.5 and c = 1.5T

( C) K k.s KA K C70 38.0 0 .512 359.8 202.18 15.8 1.065 159.8 73 .390 13.0 2.537 47.6 18.5

100 6 .9 6 .080 25.5 7 .64

9 0

0 -

70

OQ

5 0 -

4 0

3 0

20

10-

0

3- O

hA A

A

~

A-A

- hA

YLTliAm LIISOIUTLNL 2 9

~YETl4ANOLI ISODUTLI IC' I I

30 60 90 110 180

iQlnKW>

YWImB

TIME mlnFigure 5 - Comparison of isobutene conversion versus time at80C and 5 wt% catalyst for methanol/isobutene ratios of 1 and 2 .

For the case when isobutene is adsorbed and reacted withmethanol in solution, the final rate equation is:

. . . . . . .i C j - C:/K(1 KBCB K c C ~ ) ~r, = k K jFor a given set of a, b and c the unknown parameters inEquations (3), 4) and 5 ) are the surface reaction rate cons-tant, k,, the equilibrium adsorption constants K A , KB nd

Kc and the thermodynamic equilibrium constant, K. Thisequilibrium constant can be calculated from experimentalconcentration data in which concentration equilibrium hasbeen reached.SinceK = K ,K , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ( 6 )

K was calculated from the mole fractions of componentsfrom experimental data and K, was calculated from theUNIFAC method as described by Colombo et al. (1983).Thus K values obtained at 70, 80, 90, and 100C are 38.0,15.8, 13.0, and 6.9, respectively.In data analysis the experimental concentrations versustime for methanol, isobutene, and MTBE were fitted to poly-nomials so that a polynomial i s obtained for each compo-nent for each temperature. These polynomials were used to

P ? 2.0

I IT x 103, ~ - 1Figure 6 - Arrhenius plot for k,.

obtain values of concentrations at different times. The poly-nomial for MTBE was differentiated in order to calculatethe rate of MTBE formation at different times.Non-linear least square regression analysis was then usedto determine the rate constant and the equilibrium adsorp-tion constants for integral and half-integral values of the expo-nents a, b and c ) ranging from zero to three. The kineticdata were fitted to different combinations of a, b and c forall the three possible models above (Equations 3, 4 and 5 ) .The criteria for the acceptance of the model were:1) The estimated rate constant, k , , and the adsorptionequilibrium constants should be positive.2) A plot of the logarithm of the rate constant, In k , ,versus 1/T (Arrhenius plot) should be linear with anegative slope.3) A plot of the logarithm of each adsorption constantversus l/T (van t Hoff plot) should be linear with apositive slope, except when chemisorption isendothermic, and4) The goodness of the fit as indicated by the statisticalpercentage absolute average deviation.Regression analysis was carried out for various sets of a,b and c. Based on the above criteria, rate Equations 3)and5 ) were rejected. Equation 4) met the above mentionedcriteria and gave the best fit for a = 1.0, b = 0.5 and c= 1.5. The parameters k , , KA and Kc for Equation 4) atdifferent temperatures are given in Table 1.

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. 00 -

5.20 -aYc-

4 .40 -

2 . 0 0 L I I I I2.eo 2.68 2.70 2.84 e.92 3

Figure 7 - van? Hoff plot for K,.

I

The dependence of the rate constant, k,, on temperaturewas determined from the Arrhenius equation,. . . . . . . . . . . . . . . . . . . . . . ., = k,r,lexp -EIIRT ) 7)

The values of k,, and E l were found from the least squaresfit of Equation 7) as shown in Figu re 6. Thus:8), = 1.2 x 1013 exp -87,900/RT) . . . . . . . . . . . .

The activation energy 87.9 kJ/mol) is similar to the valuesin other homoge neous and heterogeneous investigations assummarized by Gicquel and Torck 1983).The depend ence of the adsorption constants, KA and Kc,was determined from the vant Hoff equation,

(9). . . . . . . . . . . . . . . . . . .A = KAoexp -AHA/RT)and

. . . . . . . . . . . . . . . . . .c = Kco exp -A HclRT) 10)Th e value s of KA,, Kco, AHA and A Hc were obtained fromthe least squares fit of the above two equations as shown inFigures 7 and 8. Thus:

KA = 5.1 x exp 97,500/RT) . . . . . . . . . . . 1 1)and

KC = 1.6 X 10-16exp 119,000/RT) . . . . . . . . . . 12)Conclusion

This investigation showed that in the range of conditionsstudied the reaction kinetics for MTBE synthesis can berepresented by a Rideal-Eley model. Methanol is preferen-tially a dsorbed in the ion-exchange resin catalyst. The catalystis more active at low methanol/isobutene ratios.The rate constant increases with increase in temperature.The reaction has an activation energy of 87.9 kl/mol. Thethermodynamic equilibrium constant and the adsorptionequilibrium constants for methanol and MTBE d ecrease with

t

2.80 2.68 2.78 2.04 2.92 3.00

i T ,to3, K-Figure 8 - ant Hoff plot for K,.

increases in temperature.The heterogeneous catalyzed reaction is a complex reac-tion. The reaction is first order in methanol, half order inisobutene and 1.5 order in MTBE.Acknowledgement

We acknowledge with thanks the financial support of this projectNo. AR-6-133 by King Abdul Aziz City for Science and Tech-nology. We also gratefully acknowledge the support and encourage-ment by K ing Fahd University of Petroleum & Minerals, Dhahran,Saudi Arabia.Nomenclaturea, b, c = order of reaction of species A, B and C, respectively.C ,C ,C,ElAHAHKK,K BK ,K,KKAo = preexponential factorkS = preexponential factorK,, = preexponential factork S = surface reaction rate constant forw ard), (mol/grsRT = temperature, KX i = mole fractionyi = activity coefficientReferences

= bulk concentration of A , mol A/g cat= bulk concentration of B, mol B/g cat= bulk concentration of C, mol Clg cat= activation energy of the forward reaction, J/mol= heat of adsorption of methanol, J/mol= heat of adsorption of MTBE, J/mol= equilibrium constant for the overall reaction= equilibrium adsorption constant for A, g cat/rnol= equilibrium adsorption constant for B, g cat/mol= equilibrium adsorption constant for C , g cat/mol= mole fraction equilibrium ratio= ratio of activity coefficients at equilibrium

cat) .5/h= rate of surface reaction, (mol/g cat)/h= gas constant, 8.314 Jho1.K

Ancillotti, F., M . M. Mauri and E. Pescarollor, Ion ExchangeResin catalysed Addition of Alcohols to Olefins, J. Catal. 4649-57 1977).

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Ancillotti, F., M. M. Mauri and E. Pescarollo, Mechanisms inthe Reaction Between Olefins and Alcohols Catalyzed by IonExchange Resins, J. Mol. Catal. 4, 37-48 1978).Chu, P ., and G. H. Kuhl, Preparation of Methyl tert-Butyl EtherMTBE) over Zeolite Catalysts, Ind. Eng. Chem . Res. 26,Colombo, F., L. Corl, L. Dalloro and P. Delogu, EquilibriumConstants for the Methyl Tertiary Butyl Ether L iquid Phase Syn-thesis by Use of UNIFAC, Ind. Eng. Chem. Fund. 22,Csikos, R., I. Pallay, J . Laky, E. D. Radcsenke, B. A . Englin and

Roberts, J. A . Low-lead Fuel with MTBE and C, Alcohols,Hydrocarbon Processing, 121-125 July 1976).Csikos, R., I. Pallay and J. Laky, Practical Use of Methyl Ter-tiary Butyl Ether Produced from C, Fraction, Proceedings ofTenth World Petroleum Congress, Bucharest, 167-175 1979).Evans, T. W. and K. R. Edlund, Tertiary Alkyl Ethers, Prepara-tion and Properties, Ind. Eng. Chem. 28, 1186-1 188 1936).Furey, R. L. and J. B. King, Evaporative and Exhaust Emissionsfrom Cars Fueled with Gasoline Containing Ethanol or MethylTert-Butyl Ethe r, Paper 800261 presented at the Congress andExposition of the Society of Automotive Engineers, Detroit,Michigan, February 1980).Gates B. C. and W. R odriguez, Ge ner al and Specific Acid Catal-ysis in Sulfonic Acid Resin, J. Catal. 31, 27-31 1973).

365-369 1987).

219-223 1983).

Gicquel, A. and B. Torck, Synthesis of Methyl Tertiary ButylEther Catalysed by Ion-Exchange Resin. Influence of MethanolConcentration and Temperature, J. Catal. 83, 9-18 1983).Lee, A. and A. Al-Jarallah, MT BE Production technologies andEconomics, Chemical Economy and Engineering Review, 18,9 , 25-34 1986).Reychler, A. Bull. SOC Chem. Belg., 21, 71 1907).Satterfield, C. N., Heterogeneous Catalysis in Practice,McGraw-Hill, New York 1980).Siddiqui,M. . B.; Kinetics of MTBE Synthesis by Homogene ousand Heterogeneous Catalysis M. Sc. The sis, King FahdUniversity of Petroleum and Minerals, Dhahran 31261, SaudiArabia 1987).Smith, J. M., Chemical Engineering Kinetics, McGraw-Hill,New York 1981).Thornton, R nd B. C. Gates, Catalysis by Matrix-Bound Sul-fonic Acid groups: Olefin and Paraffin Formation from ButylAlcohols, J. Catal. 34 75-287 1974).Torck, B., A. Convers and A. Chauvel, M ethanol for Motor FuelVia the Ethers Route, Chem. Eng. Prog. 78, (8), 36-45August 1982).

Manuscript received November 12, 1987; revised manuscriptreceived March 4, 1988; accepted for publication April 19, 1988.

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