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Applied Catalysis A: General 197 (2000) 303–309

Intrinsic kinetics of MTBE synthesis from methanol andisobutene using a synthesised MFI type zeolite

M.A. Ali, B.J. Brisdon, W.J. Thomas∗Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK

Received 25 June 1999; received in revised form 29 October 1999; accepted 29 October 1999

Abstract

The intrinsic kinetics of the reaction of methanol with isobutene, catalysed by a synthesised MFI type zeolite ZCIC-10,was studied in a batch reactor operating in the range 15–20 bar and 70–100◦C. Intraparticle and interparticle mass transfereffects were eliminated by judicious choice of catalyst particle size and conditions of fluid agitation.

Orders of reaction with respect to the reactants and principal product methyl tertiary butyl ether (MTBE) were determined forthe range of temperature and pressure studied and curve fitting techniques were applied to the experimental concentration–timedata obtained. Langmuir–Hinshelwood kinetics in which the surface reaction is rate determining is the most satisfactory fitto the data. The activation energy for the reaction was found to be 140.8 kJ mol−1. © 2000 Elsevier Science B.V. All rightsreserved.

Keywords:MTBE; Synthesis; Kinetics; MFI zeolite

1. Introduction

Concomitant with the legislation and consequentdemand for unleaded motor fuels, there has been aresurgence in the production of suitable additives forblending with gasoline to form high-octane petroleumspirits for use in automobiles. Additives used includemethyl tertiary butyl ether (MTBE), ethyl tertiary butylether (ETBE), tertiary amyl methyl ether (TAME), ter-tiary amyl ethyl ether (TAEE) and di-isopropyl ether(DIPE). The most widely used of these additives isMTBE on account of performance and cost [1]. Com-mercial production of MTBE by the liquid phase re-

∗ Corresponding author. Tel.:+44-1225-826-575;fax: +44-1225-826-894.E-mail address:[email protected] (W.J. Thomas)

action of methanol with butene in the presence of asulphonated ion-exchange resin catalyst occurs below100◦C. Byproducts are di-isobutene and tertiary butylalcohol, the amounts of which increase with increasein the reaction temperature. Disadvantages in usingsuch an ion-exchange resin is its thermal instabilityand tendency to leach acid from the resin surface dur-ing reaction.

The batch reaction of methanol and butylene ona particular ion-exchange resin (Amberlyst 15) wasstudied by Al-Jarallah et al. [2] who showed thereaction to be first-order in methanol, half-order inisobutene and retarded by product. It was concludedthat the reaction kinetics could be represented by anEley–Rideal mechanism in which methanol, adsorbedon the resin, reacted with butylene absorbed by theliquid methanol. A similar mechanism was reported

0926-860X/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S0926-860X(99)00493-7

304 M.A. Ali et al. / Applied Catalysis A: General 197 (2000) 303–309

by Caetano et al. [3]. On the other hand, studies witha continuous differential reactor using a Bayer resincatalyst K2631 in the liquid phase showed the reactionto be first-order in isobutene [4]. The vapour phasesynthesis of MTBE using the Amberlyst 15 resinindicated that the catalyst is sensitive to methanolconcentration [5]. Analysis of reported researches ofthe vapour phase synthesis of MTBE over resin cat-alysts showed that Langmuir–Hinshelwood kineticsfitted the experimental results [6–11].

Notwithstanding the apparent discrepancies inmechanisms reported for the MTBE synthesis onresin catalysts in liquid and vapour phases, severalattempts have been made to seek more stable andactive catalysts using zeolitic and other microporousmaterials. Chu and Kuhl [12] investigated the syn-thesis of MTBE using a variety of zeolite catalysts,including ZMS-5, and Hoelderich [13] published acritical review of the use of zeolites for the formationand cleavage of C–O bonds in fine chemicals in gen-eral. More recently, Chang et al. [14] re-emphasizedthe potential of these materials for catalytic MTBEformation.

The study reported in this paper relates to the useof an MFI type synthesised zeolite (ZCIC 10) witha silicon to aluminium molar ratio of 10. This ratioof Si/Al was shown [15] to give the highest yield ofMTBE for a number of zeolite preparations with Si/Alratio ranging from 10 to 100.

2. Experimental

The vessel in which the reaction between liquidmethanol and butylene was effected consisted of aclosed batch autoclave equipped with an impeller andinternal cooling coil. The reactor head assembly wasprovided with a gas inlet valve, a tube for taking liq-uid samples from the reaction mixture, a gas sam-pling valve and a thermowell. The complete autoclavewas electrically heated and the internal pressure regis-tered with a pressure gauge. Temperature of the react-ing mixture was controlled by circulating oil throughthe internal cooling coil to remove heat produced bythe exothermic reaction. The MFI type zeolite cat-alyst was prepared by rapid crystallisation, and cal-cined at 873 K in air, prior to ion-exchange to removesodium ions followed by further calcination. The Si/Al

ratio of this catalyst was 10 : 1 and it had a surfacearea of 321 m2 g−1 and a median pore diameter of5× 10−5 m.

Following the introduction of a measured volumeof methanol and mass of catalyst into the autoclave,initial temperature was set and the vessel pressurisedto a pressure corresponding to the selected reactiontemperature and at which the isobutylene would be inthe liquid phase (15–20 bar). A measured volume ofliquid isobutene was then fed by nitrogen displace-ment to the vessel contents via a preheated inlet tubeand valve. Progress of the reaction was monitored bycollecting, at predetermined time intervals, samples ofthe reaction mixture in a stainless steel sampling tube.These samples were weighed and chemically analysedusing a high-pressure gas chromatography column.The HP chromatographic column operated at 30◦Cwas a 50 m× 0.32 mm glass capillary with a 0.5mmfilm thickness of polydimethylsiloxane stationaryphase.

3. Results and discussion

Intrinsic kinetics are often obscured or completelymasked by intra and interparticle mass transport ef-fects. It was therefore considered necessary to searchfor experimental conditions which would at leastminimise or, at best, eliminate transport resistances.Suitable conditions which normally minimise in-terparticle transport effects in a batch reactor arevigorous fluid agitation. Intraparticle diffusion ef-fects may be avoided by judicious choice of catalystparticle size. Small sized particles are preferred, buttoo small a size can lead to loss of particulates dur-ing sampling. Once appropriate experimental condi-tions have been chosen, intrinsic kinetics would beobserved.

3.1. Elimination of transport resistances

Interparticle transport effects were examined bymeasuring the initial reaction rate at three differentspeeds of rotation of the fluid impeller. The data rep-resented in Fig. 1 was obtained by first observing theincrease in MTBE concentration as a function of timefor each of the stirring speeds adopted. Each one ofthe three curves were then fitted to a polynomial and

M.A. Ali et al. / Applied Catalysis A: General 197 (2000) 303–309 305

Fig. 1. Initial reaction rate as a function of impeller speed.

differentiated with respect to time to obtain an initialreaction rate for each speed of agitation. It is clearfrom Fig. 1 that there is a decrease in initial reactionrate below stirring speeds of approximately 950 rpm,indicating that transport of reactants from fluid tosolid particulates has an effect on the intrinsic kinet-ics of reaction. Above 1000 rpm, however, the effectsbecome negligible. The impeller rotational speed wasthus chosen as 1000 rpm in order to measure kineticsunimpeded by fluid to solid transport resistances.

Resistance to intraparticle diffusion, on the otherhand, may be reduced to negligible proportions byusing sufficiently small catalyst particles. The criteriafor observing chemically controlled reaction rates isthat the Thiele modulus8, in terms of measurableparameters, should be [16]

8 = dp

2

(R

cDe

)1/2

< 0.2

where dp is the catalyst particle size (m),R thereaction rate (measured on the basis of unit massof catalyst) andc the concentration expressed asmoles per unit catalyst mass (mol g−1). Utilisingconservative values from the preliminary experi-ments [15] withdp = 10−4 m, De (for Knudsen dif-fusion in a microporous zeolite [17])= 10−7 m2 s−1,R= 15× 10−3 mol g−1s−1 andc= 10−2 mol g−1; thevalue of the Thiele modulus is 0.003, well within

the criterion for minimal intraparticle diffusion resis-tance. It was therefore concluded that the preparedzeolite of particle size 5× 10−5 m was appropriate fordetermining the intrinsic kinetics of MTBE synthesis.

3.2. Order of reaction

The reaction orders with respect to each of the com-ponents methanol and isobutene were determined byobserving the initial rate of MTBE formation with, inturn, one of reactants in excess. Representing the rateof reaction by a power law, the net rate of formationof MTBE would be

dcC

dt= k ca

AcbB − k′cc

C

where C represents MTBE, A methanol and B isobuty-lene. The indicesa, b, andc are the reaction orderswith respect to each component. Initial rates wereevaluated so that the influence of the product C wasnegligible. A plot of log (initial rate) versus the con-centration of the component not in excess thereforeyields the indicesa andb. Initial rates were found bycomputing gradients, determined at zero time, to poly-nomial representations of MTBE concentration as afunction of time. Figs. 2 and 3 yield average slopes of0.94 for methanol and 0.95 for isobutylene for the four

Fig. 2. Log–log plot of initial rate of MTBE formation as a functionof methanol concentration.


Fig. 3. Log–log plot of initial rate of MTBE formation as a functionof isobutylene concentration.

temperatures of operation. The reaction order withrespect to each of the reactants is therefore virtuallyunity.

3.3. The reaction profile

For each of the four reaction temperatures stud-ied, the variation of concentration as a function oftime was observed for both the two reactants and theproduct. Fig. 4 is a typical profile obtained at 100◦Cand initial concentrations of 1.0 mol g−1 methanol and10 mol g−1 isobutene. Similar profiles were obtainedfor 70, 80 and 90◦C. In each case, the reaction wasallowed to proceed until the MTBE concentration wasasymptotic to its equilibrium value. All of the pro-files were curve fitted by a cubic polynomial of theform

cC = (α + βt + γ t2 + δt3) mol g−1

Values of the constantsα, β, γ and δ which werefound are given in Table 1. The polynomials weresubsequently differentiated to give reaction rates as afunction of time to produce observed reaction rates formatching against those predicted by a suitable model.The object of the rate measurements conducted wasnot to establish a molecular mechanism but to searchfor a suitable expression which will represent the

kinetics in the temperature range studied. It is there-fore assumed, purely for pragmatic purposes, that aLangmuir–Hinshelwood or Eley–Rideal kinetic modelis a reasonable starting point for matching experi-mental results with a mathematical model. Bearing inmind that experiments to determine reaction ordersfrom measured initial reaction rates showed that theorder with respect to each reactant is 1 (i.e. initiallya second-order reaction rate), five rate models of theLangmuir–Hinshelwood and Eley–Rideal type whichwould conform to second-order initial kinetics wereexamined. Parameters in each of five models exam-ined [15] were (i) the surface reaction rate constantKS, (ii) the adsorption equilibrium constantsKA, KBandKC, and (iii) the thermodynamic equilibrium con-stantK and (iv) the reaction ordersa, b andc. Valuesof a, b and c were chosen over the range 0.5–3.0while the thermodynamic equilibrium constant wascalculated from the equation

lnK = 4.8 + (7300/T ) + 1.17× 10−2T

−4.8 × 10−6T 2 + 2.5 × 10−9T 3

previously utilised by others [10,11,12].A computer software package ‘Sigma Plot’ was

used for model parameter estimation. The software

Fig. 4. Profile of reaction showing component concentrations as afunction of time.


Table 1Constants in polynomial fitting MTBE concentration vs. timea

T (K) α × 104 mol g−1 β × 103 mol g−1 h−1 γ × 104 mol g−1 h−2 δ × 10 mol g−1 h−3

343 4.94 4.10 −1.63 −2.19353 4.15 6.80 −7.93 −3.68363 15.0 12.4 −35.1 30.4373 22.0 19.3 −79.0 80.4

a Initial concentrations: 1 mol g−1 methanol, 10 mol g−1 isobutene; temperature: 100◦C.

finds the coefficients of the independent variableswhich give the best fit between the chosen modeland the experimental data expressed in terms ofrate-concentrations data. The software programmeis interactive and it was run until convergence wasobtained. A non-linear least squares regression algo-rithm contained within the software enabled residuals1, which are the difference between estimated andactual values of parameters, to be calculated andthe square root of the absolute sum of squares ofresiduals minimised. Of the five models tested, aLangmuir–Hinshelwood model with rate determin-ing surface reaction between adsorbed methanol andadsorbed isobutene gives the most satisfactory agree-ment with the observed results,

√612 ranging from

1.63× 10−3 to 8.9× 10−3, thus representing a verygood fit. Adsorption of each reactant on adjacent sur-face sites might possibly occur by virtue of bondingbetween the oxygen of methanol and the hydrogen ofisobutene to hydrogen and oxygen sites, respectively,of the acidic silica. The rate expression for this modelis

−rA = KSKaAKb

B(ca

AcbB − cc

C)/K

(1 + KAcA + KBcB + KCcC)a+b

Criteria for model discrimination included (i) the rateconstantKS and the adsorption equilibrium constantsKA, KB and KC should be positive; (ii) the constantKS should increase with increase in temperature; (iii)the adsorption equilibrium constantsKA, KB andKCshould decrease with increase in temperature in ac-cordance with the thermodynamic requirement of thevan’t Hoff equation. The best fit to the experimentaldata was obtained fora= b= 1, as indeed, the exper-iments designed to ascertain reaction orders revealed.The best value forc was 2.5 in contrast to the value of

1.5 found by Al-Jararallah et al. [2] for an acid resincatalyst. This result, although inconsistent with theprinciple of microscopic reversibility, merely repre-sents an artefact of the computational modelling ratherthan any mechanistic revelation. Fig. 5 demonstratesthat the surface reaction rate constantKS increaseswith temperature as required, the constant obeying theequation

KS = 3.92× 1019 exp

(−14935

T

)mol g−1 s−1

The activation energy for reaction extracted from theabove equation is 140.8 kJ mol−1. This value may becompared with an activation energy of 130.2 kJ mol−1

found for the batch synthesis of MTBE from methanol

Fig. 5. Variation of rate constantKS with absolute temperatureT.


Fig. 6. Variation of adsorption equilibrium constantKA with ab-solute temperatureT.

Fig. 7. Variation of adsorption equilibrium constantKB with ab-solute temperatureT.

and isobutylene [3]. Figs. 6–8 show thatKA, KB andKC all decrease with temperature and are representedby

KA = 1.36× 10−19 exp

(16492

T

)g mol−1

Fig. 8. Variation of adsorption equilibrium constantKC with ab-solute temperatureT.

KB = 5.14× 10−23 exp

(19170

T

)g mol−1

KC = 1.74× 10−19 exp

(14522

T

)g mol−1

4. Conclusions

The experimental work reported demonstrates thatan MFI synthesised zeolite (ZCIC 10) with a siliconto aluminium molar ratio of 10 is a good catalystfor the synthesis of MTBE from isobutylene andmethanol. In a batch reactor operating in the tem-perature range 70–100◦C, and pressures between 15and 20 bar, equilibrium concentrations of the reactioncomponent are established within 50 h. Analysis ofthe time dependent concentration of each reactioncomponent in the approach to equilibrium indicatesthat the synthesis reaction can be described by aLangmuir–Hinshelwood model, the parameters ofwhich are comparable with previously reported work[2,3]. A recently published paper [18] on the otherhand reports apparent activation energies of only32 kJ mol−1 for the synthesis reaction and 51 kJ mol−1

for the decomposition of MTBE. These values were


obtained using a temperature relaxation techniqueusing solid NMR spectroscopy in which conditionswere neither isothermal nor isobaric, and hence, donot provide a fair comparison. For batch liquid phaseexperiments, values of 130.2 kJ mol−1 obtained for anAmberlyst resin catalyst [3] and 92.4 kJ mol−1 for adifferent Amberlyst resin catalyst [19] compare morefavorably with the results quoted in this paper.

References

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[2] A.M. Al-Jarallah, M.A.B. Siddiqui, A.K. Lee, Can. J. Chem.Eng. 66 (1988) 802.

[3] N.S. Caetano, J.M. Loureiro, A.E. Rodriguez, Chem. Eng.Sci. 49 (1994) 4589.

[4] D. Parra, J. Tejero, F. Cunill, M. Iborra, J.E. Izquierdo, Chem.Eng. Sci. 49 (1994) 4563.

[5] A. Giquel, B. Torck, J. Catal. 83 (1983) 9.[6] S. Subramanian, S. Bhatia, Can. J. Chem. Eng. 45 (1987)

613.[7] A. Ali, S. Bhatia, Chem. Eng. J. 44 (1990) 97.[8] T. Zhang, R. Dutta, Ind. Eng. Chem. Res. 34 (1995) 730.[9] J. Tejero, F. Cunill, M. Iborn, J. Mol. Catal. 42 (1987) 257.

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[12] P. Chu, G.H. Kuhl, Ind. Eng. Chem. Res. 26 (1987) 365.[13] W.F. Hoelderich, Catal. Sci. Technol. 1 (1991) 31.[14] K.-H. Chang, G.T. Kim, W.S. Ahn, Ind. Eng. Chem. Res. 31

(1992) 125.[15] M.A. Ali, Ph.D. Thesis, University of Bath, Bath, 1998.[16] J.J. Carberry, Chemical and Catalytic Reaction Engineering,

McGraw-Hill, New York, 1976.[17] C.N. Satterfield, Mass Transfer in Heterogeneous Catalysis,

MIT Press, Cambridge, 1970.[18] T. Mildner, H. Ernst, D. Freude, W.F. Hölderich, J.A.C.S.

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