Kinetics of Methyl Tertiary Butyl Ether Synthesis Catalyzed by Ion Exchange Resin

ADNAN M. AL-JARALLAH, MOHAMMED A. B. SIDDIQUI, and A . K. K . LEE

Department of Chemical Engineering and the Research Instihrte, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

This paper presents the results of an experimental investigation of the kinetics of liquid phase reaction between methanol and isobutene, catalyzed by an acidic ion-exchange resin, to form methyl tertiary butyl ether (MTBE). A one litre Parr batch reactor was used. Experiments were carried out at 70, 80, 90 and 100C and at pressures sufficient to maintain liquid phase at those temperatures. Initial methanol/isobutene mole ratios of 1 .O and 2.0 were used. The catalyst amount was also varied.

These kinetic data were used to model the reaction kinetics, by non-linear least squares regression technique. The reaction was found to follow Rideal-Eley kinetics. The values of the rate constants are reported.

On presente dans cet article les rtsultats dune recherche exptrimentale sur la cinttique de reaction en phase liquide entre le methanol et Iisobuttne, catalysts par une rtsine tchangeuse ions acide, pour former du methyl tertiaire butyl ether (MIBE). On a utilist un rtacteur discontinu Parr de 1 litre. Les exptriences ont ttt mentes a des temperatures de 70, 80, 90 et 100C et ?i des pressions suffisantes pour maintenir la phase liquide a ces temperatures. Des rapports molaires 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 regression des 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 in M recent years as an important alternative to lead alkyls as a gasoline additive to increase the octane number. Unlike lead alkyl additives which cause air pollution and are toxic, MTBE is non-toxic and non-polluting according to studies by Csikos et al. (1976), Torck et al. (1982), and Furey and King (1980).

MTBE is produced by reacting methanol (MeOH) with isobutene (i-Bu) in the presence of an acidic catalyst, such as sulfuric acid, acidic ion-exchange resins, o r other acidic catalysts:

MeOH + i-Bu MTBE . . . . . . . . . . . . . . . . . . . (1) The reaction is reversible and exothermic, with a heat of reac- tion of -37.2 kJ/mol in the liquid phase at 25C.

Since the discovery of the etherification reaction between alcohols and olefins by Reychler (1907), very little scien- tific work has been published on the reaction. Only limited kinetic information on the reaction was published by Evans and Edlund (1936) and, recently, by Ancillotti et al. (1977 and 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 as catalysts respectively, while in the first three recent studies the ion exchange resin (Amberlyst 15) was used as catalyst. Actually this is the most widely used catalyst in industrial productions of MTBE.

Ancillotti et al. (1977) studied this reaction with Amber- lyst 15 catalyst and reported a zero order dependence of rate on methanol concentration, for concentrations greater than 4 mol/litre, with negative orders at lower concentrations and a first order dependence of rate on isobutene concentration based on analyses of the initial rates of the reaction. The same authors in 1978 examined the influence of methanol concen- tration of the activity of Amberlyst 15 resin. Gicquel and Torck (1983) investigated this reaction and reported that the

reaction follows Langmuir-Hinshelwood kinetics. They reported relative values of rate and adsorption equilibrium constants. A lot of information on reaction conditions, con- versions, and selectivity in MTBE synthesis can be found in patents. A comprehensive review of MTBE patents, production technologies and economics is given by Lee and Al-Jarallah (1986).

In this study, rate equations describing the kinetics of the MTBE synthesis reaction, catalyzed by ion exchange resin, have been developed and presented with the values of all the rate constants involved.

Experimental

The liquid phase reaction between methanol and isobutene was carried out in a standard one liter Parr pressure reactor. Batchwise experiments were performed. The Parr pressure reactor was equipped with magnetic stirrer and internal cooling coil in addition to the necessary accessories such as inlet valve, sampling valve, pressure gauge, thermowell and heater jacket.

A measured volume of methanol was introduced into the reactor and a weighed quantity of the ion-exchange resin catalyst was added to it. The contents were heated up to the desired temperature. Pure liquid isobutene was then fed in and the reactor was pressurised with nitrogen to maintain liquid phase. The whole mixture was stirred at lo00 r/min. to eliminate the effect of agitation on mass transfer which is significant at speed below 600 r/min. The temperature was maintained at the desired set point by circulation of the cooling water through the internal cooling coils. The time of addition of isobutene was taken as the starting time of the reaction. The reaction was allowed to run and liquid samples were collected at regular intervals. Details of the experimental procedure have been described by Siddiqui (1987).

802 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 66. OCTOBER, 1988

A

o o O 0 0,

60

w a. W

4 0 A 0 I l% t I

Q+ 0 F' A 2 - 0 m 0 !!

X 0 2 5 g c o t

A S 2 9 C o t 0 2 0 - .+ A 0 + 7 6 9 c o t + 0 X x 12 4 g cot

0 20 0 9 COl

0 0 4 0 80 1 2 0 160 2 0 0

TIME, mln AMOUNT OF CATALYST, (I

Figure 1 - Isobutene conversion versus time at methanol/isobu- tene = 2 and 80C for different amounts of catalyst.

Figure 2 - Initial rate of isobutene conversion versus amount of catalyst at 80C and methanol/isobutene = 2.

The ion exchange resin catalyst (Amberlyst 15) was sup- plied by Rohm & Haas. The resin is a macroreticular nuclear sulfonated copolymer of styrene and divinylbenzene. The catalytically active group is the nuclear sulfonic acid. Its ion- exchange capacity is 4.9 milli-equivalent/g of dry resin.

Analytical

One microlitre of each sample was analysed by a Varian 3700 gas chromatograph (GC). The GC was equipped with a flame ionisation detector. A 3.2 m long, 4 mm ID, stainless steel column, packed with 10% dinonyl pthalate on chromosorb W-HP, was used. The detector was connected to a CDS 1 1 1 integrator directly giving the area of the different peaks. The GC was calibrated with pure compounds, and the amount of each compound present in the product sample was then determined from the respective area counts using this calibra- tion. Most of the isobutene escaped to the air as the sample was depressurised; therefore the amount of isobutene was calculated from the stoichiometry of the reaction while that of methanol and MTBE was determined by GC analysis.

Diisobutene was not found in product samples, because of the very low rates of its formation and excess or equi- molar methanol at the conditions of these experiments. Ancillotti et al. (1978) showed that at 60C the initial rates of isobutene dimerization in MTBE synthesis were insigni- ficant at methanolhobutene molar ratios greater than 0.30.

Results and discussion

KINETIC DATA

The effects of three variables on the kinetics of MTBE syn- thesis have been investigated. These variables are tempera- ture (70 to 100"C), amount of catalyst (2.5 to 20.0 g) corresponding to 1 % - 10% by weight, and initial methanol/isobutene molar ratio (1 .O and 2.0). For studying these effects, only the appropriate parameter was varied while

the other two were kept constant. The first parameter that was tested was the catalyst amount

in order to determine the optimum catalyst amount to use for studying other parameters. Figure 1 shows the conver- sion of isobutene vs. time for different catalyst amounts at a temperature 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 shows that the initial rate of isobutene conversion increased as the amount of solid catalyst increased from 2.5 gm to 12.4 gm and was practically the same when the amount was increased from 12.4 gm to 20.0 gm. At low catalyst amount, the high concentration of methanol inside the resin reacts with the acid groups forming solvated protons which become the catalytic agent. The solvated proton is a less active acid species than the acid group S03H, therefore the rate is slower according to Gates and Rodriguez (1973). As the methanol concentra- tion decreases relative to the amount of catalyst because of increasing the amount of catalyst (2.5 gm to 12.4 gm), the mechanism gradually shifts to catalysis by S03H. At a very large amount of catalyst (20 gm). there are so many S03H groups that the rate now only depends on the rate of proto- nation of the isobutene. However, the conversion of isobu- tene at equilibrium should be independent of the catalyst amount. The optimum catalyst amount of 12.4 g was used for testing other parameters.

The reaction was investigated at temperatures of 70, 80, 90, and 100C. A sample graph showing the changing con- centration of methanol, MTBE and isobutene with time at one temperature (80C) is given in Figure 3. The initial molar ratio of methanolhsobutene was 2.0 and the catalyst amount was 12.4 g in this experiment. This figure shows that the MTBE product concentration increases monotonically with time and approaches asymptotically to a final value; this is a typical behavior of batchwise operations. Since the initial molar ratio of methanol to isobutene is two, there is always a considerable amount of methanol in the reaction mixture, while isobutene decrease to a very low concentration.

Figure 4 shows the conversion of isobutene with time for

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 66, OCTOBER, 1988 803

0 METHANOL

A MTBE

0 IlODUTENE

0 0.30 , - E i 0.24 g L I- 2 0.18 U

0 U

0.12

0 . 0 8

0 ' ' ' ' ' ' ' 1 ' 1 1 " ' 0 40 80 120 180 200 240 280

TIME, min

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

different temperatures. From this figure it can be seen that the initial increase in isobutene conversion is faster as the temperature increases. The final conversions were lower at higher temperatures, since the equilibrium constant of the synthesis reaction decreases with increase in temperature because the synthesis reaction is exothermic. Nevertheless, high isobutene conversion (about 95 9%) has been obtained.

Figure 5 shows the effect of the initial molar ratio of methanol to isobutene. From this figure it is seen that the initial rate as well as the final conversion of isobutene is higher when the ratio is one. Since Amberlyst 15 catalyst acts through the intermediary sulfonic groups (S03H) bonded to insoluble macromolecule, these groups provide for the protonation of isobutene and the reaction proceeds to form MTBE. The catalytic mechanism occurring in the presence of this resin depends on the polarity of the reaction medium, according to Gates and Rodriguez (1973) and Thornton and Gates (1974). At low alcohol concentrations, the resin retains a network of hydrogen bonds between the sulfonic groups alone, or between these groups and the alcohol, while at high alcohol concentrations the protons are solvated and the H-bonded network disappears. In the present study it seems that for the lower alcohol concentration (molar ratio of methanol to isobutene of 1 .O) the protons were not solvated and the isobutene can take the proton directly from the sulfonic group. According to Gates and Rodriguez (1973) the sulfonic group (S03H) is a more acidic species than the solvated proton, and this can account for the increased rate for the lower molar ratio. Nevertheless, excess methanol is often used to suppress side reactions forming isobutene dimers.

KINETIC MODEL

The MTBE synthesis reaction can be represented by:

(2) A + B C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0 0 0 0 100

+ A ~ : A A + + + + +

8 0

z 0 z

: 80 > z 8

5 W z

4 0

m s 0

o h x x X

0 80 120 180

TIME, min

240 300

Figure 4 - Isobutene 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 MTBE respectively. In general, the forward reaction is order a in A and order b in B, and the reverse reaction is order c in C.

The rate of surface reaction, r,, is assumed to be the rate controlling step, as there were no mass transfer limitations. There are two possible mechanisms by which this surface reaction takes place:

1) Reaction between adsorbed molecules of both A and B on adjacent active centers, and

2) Reaction between one adsorbed reactant and the other reactant in solution.

The first mechanism is the Langmuir-Hinshelwood mechanism and the second one is the Rideal-Eley mechanism as discussed in Smith (1981) and Satterfield (1980). In these references the reaction is assumed to be a simple reaction, that is, the reaction is first order in all species. The following rate equations were derived for general orders of reaction a, b and c. For a Langmuir-Hinshelwood model, the rate of reaction can be represented by the following equation:

r, = k,KjK;

. . . (3) 1 Cj Ci - C:/K [ (1 + KACA + KBCB + KCCC)~'' For the case of the Rideal-Eley mechanism, there are two possibilities in which either one of the two reactants is adsorbed on the catalyst and then reacts with the other reac- tant in solution. For the case when the methanol (A) is adsorbed and reacted with the isobutene (B) in solution, the final rate equation is:

. . . . . . . 1 Cj C; - CZIK (1 + KACA + KcCc)a r, = k, Kj (4) 804 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 66, OCTOBER, 1988

TABLE 1 Reaction Equilibrium Constant, Rate Constant, and Equilibrium Adsorption Constants for

Eq. (4) with a = 1 , b = 0.5 and c = 1.5

T ("C) K k.s KA K C

70 38.0 0.512 359.8 202.1 80 15.8 1.065 159.8 73.3 90 13.0 2.537 47.6 18.5

100 6.9 6.080 25.5 7.64

90

0 -

70

OQ

5 0 -

4 0

3 0 -

20

10-

0'

3

- 0 0 O 0 0 0

h

0 A A A

~

A -

0

A

- h

A

- 0 YLTliAm)LIISOIUTLNL 2 9

~YETl4ANOLIISODUTLIIC : 2

' I I 30 60 90 110 180

i Q ln K W > z 0 0

Y W I- 3

0 m

B

TIME, mln

Figure 5 - Comparison of isobutene conversion versus time at 80C and 5 wt% catalyst for methanol/isobutene ratios of 1 and 2.

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

. . . . . . . 1 C i C j - C:/K (1 + KBCB + K c C ~ ) ~ r, = k, K j For a given set of a, b and c the unknown parameters in

Equations (3), (4) and ( 5 ) are the surface reaction rate cons- tant, k,, the equilibrium adsorption constants K A , KB and Kc, and the thermodynamic equilibrium constant, K. This equilibrium constant can be calculated from experimental concentration data in which concentration equilibrium has been reached. Since

K = K,K, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (6)

K, was calculated from the mole fractions of components from experimental data and K, was calculated from the UNIFAC 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 versus time for methanol, isobutene, and MTBE were fitted to poly- nomials so that a polynomial is obtained for each compo- nent for each temperature. These polynomials were used to

P.? 2.0

I IT x 103, ~ - 1

Figure 6 - Arrhenius plot for k,.

obtain values of concentrations at different times. The poly- nomial for MTBE was differentiated in order to calculate the rate of MTBE formation at different times.

Non-linear least square regression analysis was then used to 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 kinetic data were fitted to different combinations of a, b and c for all 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 adsorption

equilibrium 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 a negative slope.

3) A plot of the logarithm of each adsorption constant versus l/T (van't Hoff plot) should be linear with a positive slope, except when chemisorption is endothermic, and

4) The goodness of the fit as indicated by the statistical percentage absolute average deviation.

Regression analysis was carried out for various sets of a, b and c. Based on the above criteria, rate Equations (3) and ( 5 ) were rejected. Equation (4) met the above mentioned criteria 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) at different temperatures are given in Table 1.

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 66, OCTOBER, 1988 805

r 8.00 -

5.20 -

a Y c -

4.40 -

2.00 L I I I I 2.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 temperature was determined from the Arrhenius equation,

. . . . . . . . . . . . . . . . . . . . . . . k, = k,r,l exp(-EIIRT) (7)

The values of k,, and E l were found from the least squares fit of Equation (7) as shown in Figure 6. Thus:

(8) k, = 1.2 x 1013 exp(-87,900/RT) . . . . . . . . . . . .

The activation energy (87.9 kJ/mol) is similar to the values in other homogeneous and heterogeneous investigations as summarized by Gicquel and Torck (1983).

The dependence of the adsorption constants, KA and Kc, was determined from the vant Hoff equation,

(9) .................... KA = KAo exp( -AHA/RT)

and

. . . . . . . . . . . . . . . . . . Kc = Kco exp( -A HclRT) (10)

The values of KA,, Kco, A HA and A Hc were obtained from the least squares fit of the above two equations as shown in Figures 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 conditions studied the reaction kinetics for MTBE synthesis can be represented by a Rideal-Eley model. Methanol is preferen- tially adsorbed in the ion-exchange resin catalyst. The catalyst is 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. The thermodynamic equilibrium constant and the adsorption equilibrium constants for methanol and MTBE decrease with

t

2.80 2.68 2.78 2.04 2.92 3.00

i T ,to3, K-

Figure 8 - vant 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 in isobutene and 1.5 order in MTBE.

Acknowledgement

We acknowledge with thanks the financial support of this project No. AR-6-133 by King Abdul Aziz City for Science and Tech- nology. We also gratefully acknowledge the support and encourage- ment by King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia.

Nomenclature

a, b, c = order of reaction of species A, B and C, respectively. C, C, C, El AH, AH, K K, K B K , K, K, KAo = preexponential factor kS0 = preexponential factor K,, = preexponential factor k S = surface reaction rate constant (forward), (mol/g

rs R T = temperature, K Xi = mole fraction yi = activity coefficient

References

= 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 Exchange Resin catalysed Addition of Alcohols to Olefins, J. Catal. 46, 49-57 (1977).

806 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 66. OCTOBER. 1988

Ancillotti, F., M. M. Mauri and E. Pescarollo, Mechanisms in the Reaction Between Olefins and Alcohols Catalyzed by Ion Exchange Resins, J. Mol. Catal. 4, 37-48 (1978).

Chu, P., and G. H. Kuhl, Preparation of Methyl tert-Butyl Ether (MTBE) over Zeolite Catalysts, Ind. Eng. Chem. Res. 26,

Colombo, F., L. Corl, L. Dalloro and P. Delogu, Equilibrium Constants for the Methyl Tertiary Butyl Ether Liquid 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 of Tenth 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 Emissions from Cars Fueled with Gasoline Containing Ethanol or Methyl Tert-Butyl Ether, Paper 800261 presented at the Congress and Exposition of the Society of Automotive Engineers, Detroit, Michigan, February (1980).

Gates B. C. and W. Rodriguez, General 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 Butyl Ether Catalysed by Ion-Exchange Resin. Influence of Methanol Concentration and Temperature, J. Catal. 83, 9-18 (1983).

Lee, A. and A. Al-Jarallah, MTBE Production technologies and Economics, 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. A. B.; Kinetics of MTBE Synthesis by Homogeneous

and Heterogeneous Catalysis M. Sc. Thesis, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia (1987).

Smith, J. M., Chemical Engineering Kinetics, McGraw-Hill, New York (1981).

Thornton, R. and B. C. Gates, Catalysis by Matrix-Bound Sul- fonic Acid groups: Olefin and Paraffin Formation from Butyl Alcohols, J. Catal. 34, 275-287 (1974).

Torck, B., A. Convers and A. Chauvel, Methanol for Motor Fuel Via the Ethers Route, Chem. Eng. Prog. 78, (8), 36-45 (August 1982).

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

THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 66, OCTOBER, 1988 807