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Rate expression for THF synthesis on acidic ion exchange resin
Chemical Engineering Science 56 (2001) 2171}2178
Rate expression for THF synthesis on acidic ion exchange resin Uwe Limbeck, Carsten Altwicker, Ulrich Kunz, Ulrich Ho!mann* Technical University, Clausthal, Institut fu( r Chemische Verfahrenstechnik, Leibnizstr. 17, D-38678 Clausthal-Zellerfeld, Germany Received 15 June 2000; accepted 20 October 2000
Abstract The intrinsic kinetics for the liquid-phase cyclisation etheri"cation of 1,4-butanediol (BD) to tetrahydrofuran (THF) and water were determined in a stirred batch reactor in the temperature range from 373 to 393 K. A sulphonic acid ion exchange resin powder with a particle size of 3}7 m was manufactured and used as catalyst. The reaction can be regarded as irreversible and proceeds without di!usion limitations inside the catalytic microspheres. The formation of an intermediate containing 1,4-butanediol (BD) on an active site was determined to be the rate-determining step. The reaction product water acts as a strong inhibitor for the intermediate formation. The experimental results can be described by a three-parameter model based on Michaelis}Menten kinetics extended by an inhibition term for water. Within the developed rate equation, concentrations are expressed in liquid-phase activities from the UNIQUAC method in order to consider the nonidealities. The developed rate equation is valid in the whole range of possible concentrations and independent of the solvents used. During the experiments no formation of by-products was observed. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Tetrahydrofuran synthesis; Kinetics; Catalyst deactivation; Adsorption; Ion exchange
1. Introduction The great economic importance of the THF production is re#ected by the high world production capacities of 184,000 tonnes in 1994 (Weissermehl & Arpe, 1997). Because of the particular solvent properties, about 25% of the THF production is used as solvent for high polymers, such as PVC, rubber, Buna and others. However, most of the produced THF is used for polymerisation in order to satisfy the increasing demand for polytetramethylene glycol, a precursor for polyurethanes and Spandex "bres. Industrially, THF is manufactured in three di!erent ways based on the conversion of 1,4-butanediol, 1,4dichloro-2-butene or maleic anhydride. Splitting of water from butanediol in the presence of mineralic acid or ion exchange resins is the most important production
* Corresponding author. Tel.: #49-5323-72-2184; fax: #49-532372-2182. E-mail address: [email protected] (U. Ho!mann).
process for THF.
(1) Surprisingly, there are nearly no kinetic data published for this reaction. The only one that was found, describes the reaction mechanism of the 1,4-butanediol cyclisation on heteropoly acids (Baba & Ono, 1986). Two di!erent mechanisms are given for this catalyst, depending on whether the reaction proceeds in aqueous phase or in organic phase using dioxane as solvent. The aims of our investigations were the determination of a rate equation that describes the intrinsic kinetics of the butanediol cyclisation on ion exchange resins. An important requirement for rate equations is their validity in the whole concentration range and their independence from the used solvent. Therefore the rate equation should be given in terms of activities (De Donder, 1927; Kondepudi & Prigogine, 1998; Reh"nger, 1988). A further important aspect is the determination of the inhibition e!ect of water on ion exchange resin. For this
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reason the BD cyclisation is a useful model reaction for studying resin catalyst deactivation by water because the reaction can be regarded as irreversible and highly selective.
inner diameter"0.53 mm) using helium as carrier gas. Due to the water in the reaction mixture, a thermal conductivity detector was used. 2.3. Catalyst
2. Experimental 2.1. Procedure The reaction kinetics of the 1,4-butanediol cyclisation were studied in a stirred batch reactor (287 ml, stainless steel) as it is described in Fig. 1. Before closing the reactor, the reactant BD and eventually a solvent like THF, water or dioxane were fed into the reactor and the catalyst basket was "lled with the desired amount of catalyst (0.25}1 g). The temperature (from 373 to 393 K) was controlled within $0.1 K by immersing the whole reactor in a temperature-controlled oil bath. After the heating-up period, the magnetic-driven stirrer (60}2000 rpm) was switched on whereby the drop of the catalyst basket was initiated. This moment was considered to be the starting time of the reaction, because reactant and catalyst were mixed immediately. A sampling tube immersed into the liquid reaction mixture allowed to take samples due to excess pressure (0.12}0.35 MPa), which evolved by the vaporisation of a small part of the reaction mixture. In order to avoid the vaporisation of the reaction mixture during the withdrawal of samples, the sampling train was cooled. 2.2. Analytical methods The concentrations of the involved species butanediol, tetrahydrofuran, water and dioxane were analysed by gas chromatography. The separation was carried out on a Hewlett-Packard Innowax column (length"30 m,
A strongly acidic ion exchange resin, prepared in our laboratories, was used as a catalyst. This resin is manufactured by precipitation polymerisation of styrene and divinylbenzene (DVB) using para$n as solvent. After grinding a powder of small resin spheres with a diameter of 3}7 m (measured by scanning electron microscopy) is obtained by this method. More information about the preparation method and the incorporation of the resin in macroporous carrier materials is given in the literature (Kunz, 1998). For the kinetic experiments the pure resin is easier to handle and it has the same properties as the incorporated resin (Table 1). 2.4. Reactants These were 1,4-butanediol ('99.8%, H O (0.05%), tetrahydrofuran ('99.5%, H O (0.05%), 1,4-dioxane ('99.5%, H O (0.05%) and deionised water. 3. Results 3.1. External heat and mass transfer Special attention was paid to separate intrinsic kinetics from overlapping mass transport phenomena. The transport of the reactants from the bulk of the liquid reaction mixture to the surface of the catalyst particle and the transfer of reaction heat was taken into consideration. Therefore, the #ow condition around the catalyst particles was changed by variation of the rate of agitation. The experiments were started with pure butanediol, in order to achieve high reaction rates. For comparison, the conversion of butanediol after a reaction time of 180 min was determined. The results are presented in Fig. 2. Obviously, the measured butanediol conversions after 180 min were constant, when the rate of agitation was higher than 1000 rpm. The result presented complies with the assumption that the global reaction rate increases with the improvement of external mass transfer to
Table 1 Properties of the catalyst (Kunz, 1998)
Fig. 1. Scheme of the batch reactor.
Particle size Exchange capacity Upper operating temperature limit BET surface area Degree of crosslinking Density
3}7 m 4.55 meq/g 1203C 29.5 m/g 7.5% DVB by weight 1.050 g/cm
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Because C (0.53 there is a slight in#uence of pore 5. di!usion for special reaction conditions. On the other hand, the estimation of the Weisz}Prater criterion includes again some assumptions like the e!ective di!usivity. So the exact con"rmation of the neglect of macroporeand gel-phase di!usion is a task of future work. 3.3. Nonideality of the liquid phase
Fig. 2. Butanediol conversions after 180 min in dependence on rate of agitation. Conditions: 1203C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol. "
an asymptotic limit. On the other hand, grinding of the catalyst by the stirrer limits the maximum rate of agitation. During our experiments no grinding e!ects were observed, so that all further experiments were carried out with 1500 rpm. 3.2. Exclusion of macropore- and gel-phase diwusion The exclusion of these internal transport resistances is based on the experimental results of Reh"nger and Ho!mann (1990a) on the MTBE synthesis. They examined the in#uence of the catalyst structure on the reaction rate, expressed as turnover number. They found out, that the reaction is in#uenced neither by the microsphere di!usion nor by the internal surface area, if the particles are "ne enough. For their comparison, they used Amberlyst 15 (20% DVB, 0.042 mm(d (0.180 mm) and the N resin described in this work, but with a larger particle size (7.5% DVB, 0.053 mm(d (0.156 mm). The ion exN change catalyst used in our experiments was much smaller and had a particle size of 3}7 m. With regard to the results of Reh"nger and Ho!mann (1990b), it can be assumed, that the whole microsphere contributes to the reaction with an e!ectiveness factor of 1. Moreover, the assumption can be validated by the appreciation of the Weisz}Prater criterion (Fogler, 1992) r R C " . "0.53, 5. D c C Q with r"163;10\ mol/g s, lowest rate of disappearance of species BD per mass of catalyst, "1.050 g/cm, density of catalyst, . R"3.5 m, largest radius of spherical catalyst particle, D "10\ m/s e!ective di!usivity of BD in catalyst, C and c "3.96 mol/l, concentration of BD at the surface of Q the catalyst particle.
The description of the nonidealities of the liquid phase was based on the UNIQUAC method (Gmehling & Onken, 1977). The parameters used are listed in Table 2. Unfortunately, interaction parameters for the systems dioxane/THF and butanediol/dioxane were not available. So these systems were calculated using the group contribution method mod. UNIFAC (Do.) (Gmehling & Kolbe, 1992). In order to simplify calculations UNIQUAC parameters were "tted to the estimated UNIFAC data. 3.4. Equilibrium The value of the equilibrium constant K was evaluated in a temperature range from 293 to 393 K based on data from the literature (Reid, Prausnitz, & Poling, 1987; Ullmann, 1987; NIST, 1999). Due to the low reaction enthalpy, the equilibrium constant can be regarded as constant in a vast temperature range as presented in Fig. 3. If pure BD is the only reactant, we expect the equilibrium mole fraction of BD to be x "0.0025 at 1203C, " based on "0.5294, "1.5124 and "1.8478 " 2&$ &as activity coe$cients. This result is in contrast to comments in the literature where it is mentioned that the reaction products have to be removed from the reaction mixture in order to achieve high conversion (Reppe, 1955) by shifting the equilibrium composition. Experiments for the investigation of the backward reaction were carried out by the present authors. THF and H O were fed into the reactor in stoichiometric ratio and
Table 2 UNIQUAC interaction parameters (Gmehling, Onken, & Arlt, 1981; Gmehling, Onken, & Weidlich, 1982; Gmehling, Onken, & Rarey-Nies, 1988). 1"BD, 2"THF, 3"H O, 4"dioxane a GH
(cal/mol)
a
a a a a a a
!133.3164 470.3109 156.1217 !183.4339 835.2626 10.5163
a a a a a a
GH
Based on mod. UNIFAC (Do.) calculations.
(cal/mol) 50.8926 239.2393 !323.0695 !264.9392 !444.0130 1302.038
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Fig. 3. Equilibrium constant in dependence on temperature for the liquid-phase butanediol cyclisation.
Fig. 4. Initial reaction rates in dependence on initial compositions. Conditions: 1203C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
toluene}sulphonic acid was used as a homogeneous catalyst. After a reaction time of 6 h at a temperature of 1203C, it was not possible to analyse any BD. Based on our equilibrium calculations and our experiments, the butanediol cyclisation is regarded as an irreversible reaction. Beyond this it is important to answer the question why it is important to remove the reaction products from the reaction mixture (Reppe, 1955). 3.5. Model equation In order to determine concentration dependencies on the reaction rate, initial rates r were measured at di!er ent compositions. The experiments were carried out at a temperature of 1203C and with a constant amount of catalyst of 4.55 mmol eq on 2 moles of reactants. In Fig. 4 initial rates are compared when the reactant butanediol was initially diluted with one of the reaction products THF or water. Surprisingly, the dilution e!ects of THF and water are completely di!erent. The presence of a small amount of water leads to a tremendous decrease of the reaction rate. Further addition of water only reduces the rate slightly. In contrast, the dilution of BD with THF has nearly no e!ect on the initial rate until the activity of THF approaches 0.75. Further dilution causes the decrease of the rate that tends of course to zero for the case of pure THF. Analogous rates were observed when dioxane was used as a solvent and consequently the reaction rate decrease does not depend on the solvent used, but on the dilution of the reaction mixture by an inert component like THF or dioxane (Fig. 10). So the experimental data based on THF dilution can be used to determine the dependence of the rate on the activity of BD. Fig. 5 shows a steep slope of the initial rate at low BD activities and afterwards an approximately constant
Fig. 5. Initial reaction rates in dependence on BD dilution with THF. Conditions: 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
value for BD activities larger than 0.6. The results obtained are comparable to Michaelis}Menten kinetics (Michaelis & Menten, 1913). Perhaps this behaviour should better be named saturation kinetics since this is not an enzyme-catalysed reaction. This means that BD forms in a "rst reversible reaction step an intermediate with one active site (S). During a subsequent irreversible reaction step, the ring closure takes place. Here THF and water are formed and the active site is free again for further reactive conversions. This reaction mechanism can be described by the following equations: BD#S & BD ' SPTHF#H O#S, (2) (3) n "n #n ' . 1 1 "1 Here n , n and n ' are the total number of active sites, 1 1 "1 the number of free active sites and the number of active sites occupied by the intermediate, respectively. The resulting rate equation is equivalent to a monomolecular Langmuir adsorption of BD multiplied by the rate
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constant r K a " " , r " "k (4) n 1#K a 1 " " where r is the initial rate of reaction related to the number of sulphonic groups (turnover number), K the " equilibrium sorption constant of BD, and k the rate constant of the reaction. The values for K "1.8878 " and k"59.77 mmol s\ eq\ at 1203C were "tted to the initial rates presented in Fig. 5. Special attention has to be paid to the inhibiting properties of water that have already been described above. In order to separate dilution and inhibition e!ects, the measured initial reaction rates in water presented in Fig. 4 have to be corrected. Therefore the rate equation is extended by the inhibition factor that contains all inhibition e!ects of the water while the last term describes the dilution e!ects of BD K a " " . (5) r "(a )k & - 1#K a " " Surely, is just dependent on the activity of water a and has to tend to 1 for the case of the absence of &water. Due to experimental data for initial rates when BD was diluted with water, the inhibition factor is described in dependence on a in Fig. 6. After the exclu&sion of dilution e!ects the main characteristic of the water inhibition remains. So small amounts of water a!ect the initial rate markedly while the concentration dependence for higher water contents is rather low. Without dilution e!ects the quantitative description of the inhibition factor can be given by the following equation: 1 , (6) (a )" &1#K (a &&where the proportionality factor K is comparable to &a sorption constant for water. The value for K " &2.877 and the exponential value for the water activity a were determined during the "tting procedure of &experimental data that were obtained at 1203C. A comparison of the model equation and the experimental data in Fig. 6 shows good agreement. The validity of the introduced inhibition term in the most signi"cant range for a (0.2 is con"rmed within the following simula&tions of long-time batch experiments. For comparison the inhibition factor 1/(1#Kc ) for &the butanediol cyclisation on heteropoly acids in dioxane was found in the literature (Baba & Ono, 1986). The same authors forgo an inhibition factor when the reaction proceeds in water as solvent and use a di!erent rate equation whether the reaction proceeds in aqueous or non-aqueous phase. Their results correspond to the observation that small amounts of water inhibit the reaction strongly while for high water concentration there is a constant inhibition e!ect.
Fig. 6. Inhibition factor in dependence on initial water activities. Conditions: 1203C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
A qualitative explanation for the inhibiting properties of water is also found in the literature (Gates, 1992). It is mentioned that water has such a great a$nity for the !SO H groups that it excludes the reactants and suppresses the catalytic reaction almost completely. The attribution of physicochemical phenomena to the observed inhibition e!ect of water is a subject of future work. One of these phenomena will be the change of the catalytic activity of sulphonic acid groups by di!erent solvation of protons depending on the composition of the liquid phase in the gel particles. Other important phenomena are gel phase swelling and gel phase di!usion which are not accounted for in the proposed rate equation. Nevertheless, the determined inhibition factor presents a simple and quantitative empirical possibility to take the complicate inhibition e!ects into account. After the description of the initial rates at di!erent initial compositions the validity of the achieved rate equation K a k " " r" 1#K a 1#K (a " " &&-
(7)
has to be tested for long-time batch experiments and changing compositions. Comparisons between experimental results and simulations are given in Figs. 7}9. It is obvious that the relations that were determined for the initial rates are extendable to all reaction conditions that were achieved during the experiments. The introduction of an intensive rate (turnover number) is justi"ed due to the simulation results in Fig. 7 where the amount of catalyst is varied by a factor of four. The good agreement of experimental data and simulation demonstrates that the developed rate equation works in the entire concentration range of the ternary reaction system BD, THF, water.
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Fig. 7. BD mole fraction pro"les of batch experiments. Variation of catalyst concentration and dilution of initial reaction mixture with THF. Conditions: 1203C, particle size: 3}7 m, n "2 mol.
Fig. 8. BD mole fraction pro"les of batch experiments for di!erent initial dilutions with THF. Conditions: 1203C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
Fig. 9. BD mole fraction pro"les of batch experiments for di!erent initial dilutions with H O. Conditions: 1203C, 4.55 meq catalyst, par ticle size: 3}7 m, n "2 mol.
Fig. 10. BD mole fraction pro"les of batch experiments for initial dilutions with THF and dioxane. Conditions: 1203C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
3.6. Ewect of temperature
During a further experiment dioxane was added to the ternary system as solvent. The experimental results and simulations are given in Fig. 10. The experimental data con"rm that regardless of whether BD is initially diluted by THF or dioxane the concentration pro"les are similar. In contrast to the experimental data, the calculated pro"les for the reaction in THF or in dioxane as solvent di!er a little. This surprising result may be explained by the problems to get reasonable UNIQUAC interaction parameters for the quaternary system. With regard to the experimental data, the rate in dioxane can be better simulated by assuming that all dioxane molecules behave like THF molecules.
The temperature dependencies of the three parameters k, K and K used in the rate equation developed " &were determined due to further experiments carried out at 110 and 1003C. Lower temperatures were not used, because the rate got too slow for reasonable experimental times. On the other hand, the maximum temperature limit of the catalyst is 1203C. Due to the small temperature interval and the small number of experimental data it does not seem to be reasonable to use the kinetic data outside the given temperature range. For example a particularly high value for the activation energy E was obtained. The values for k and K were deter" mined by "tting initial rates, when BD is diluted with THF. The results are presented in Fig. 5. The
U. Limbeck et al. / Chemical Engineering Science 56 (2001) 2171}2178
temperature-dependent K was "tted to experimental &data of BD mole fraction pro"les from batch experiments described in Figs. 11 and 12. For the quantitative description of temperature dependencies, Arrhenius equations can be used. Subsequently, the de"ned equations and the corresponding parameters are listed.
"
"K
"
(8)
H " , exp ! R¹
where k "8.63;10 mmol/s eq, E "152 kJ/mol, K "6.29;10\, H "!11.1 kJ/mol, " " K "2.43, H "!6.36 kJ/mol. & - &3.7. Further possible reaction mechanisms
E k"k exp ! , R¹ K
2177
(9)
H K "K exp ! & - , && - R¹
(10)
Several rate equations based on Langmuir}Hinshelwood theory were tested and compared with experimental data. Special attention was paid to a competitive sorption of water and BD on the active sites in order to describe the inhibition e!ects. Unfortunately, all rate equations based on such a sorption mechanism failed. So it was not possible to introduce the inhibition e!ects of water in a sorption model for the description of the rate equation. Because of the high value of the equilibrium constant, the consideration of the backward reaction was not reasonable for the description of the reaction kinetics.
4. Conclusions
Fig. 11. BD mole fraction pro"les of batch experiments for di!erent initial dilutions with THF. Conditions: 1103C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
By this work we succeeded for the "rst time in expressing the intrinsic kinetics of the BD cyclisation on ion exchange resins in one rate equation that is valid in aqueous and in non-aqueous phases, covering the whole concentration range. To achieve this it was necessary to express the non-idealities of the liquid phase in terms of activities. Moreover, the presented rate expression is independent of the solvents used. The in#uence of inhibiting water was quantitatively described. An important aspect is that the inhibition e!ects cannot be described by a Langmuir}Hinshelwood mechanism, but by the presented inhibition factor. This procedure could be a guideline to other comparable liquid-phase reactions.
Notation
a G a GH d N C 5. c G c Q
Fig. 12. BD mole fraction pro"les of batch experiments for di!erent initial dilutions with THF. Conditions: 1003C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
D C E H G H 0
liquid-phase activity of component A G UNIQUAC interaction parameter of binary system i/j, cal/mol arithmetic mean sphere volume equivalent diameter, mm constant in Weisz}Prater criterion concentration of component i, mol/l concentration of BD at the surface of the catalyst particle, mol/l e!ective di!usivity of BD in catalyst, m s\ activation energy, kJ mol\ heat of absorption, kJ mol\ standard enthalpy change of reaction, kJ/mol
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K K G K G k k n R r r r r x G
U. Limbeck et al. / Chemical Engineering Science 56 (2001) 2171}2178
chemical equilibrium constant equilibrium sorption constant of component i equilibrium sorption constant of component i for ¹PR rate constant, mmol s\ eq\ rate constant for ¹PR, mmol s\ eq\ amount of substance, mol largest radius of spherical catalyst particle, m reaction rate related to the number of acid groups, mmol s\ eq\ lowest rate of disappearance of species BD per mass of catalyst, mol g\ s\ initial reaction rate related to the number of acid groups, mmol s\ eq\ reaction rate, mmol s\ eq\ mole fraction of component i
Greek letters G .
activity coe$cient of component i inhibition factor density of catalyst, g cm\
Subscripts 0
initial condition
Abbreviations BD 1,4-butanediol BD ' S intermediate of butanediol molecule and active site DVB divinylbenzene THF tetrahydrofuran S active site"acid group
References Baba, T., & Ono, Y. (1986). Kinetic studies in liquid phase dehydration cyclization of 1,4-butanediol to tetrahydrofuran with heteropoly acids. Journal of Molecular Catalysis, 37, 317}326.
De Donder, T. (1927). L'Aznite& . Paris: Gauthiers-Villars. Fogler, H. S. (1992). Elements of chemical reaction engineering (2nd ed.). (p. 625). New Jersey: Prentice-Hall. Gates, B. C. (1992). Catalytic chemistry (pp. 220}221). New York: Wiley. Gmehling, J., & Kolbe, B. (1992). Thermodynamik, Zweite u( berarbeitete Auyage (pp. 246}255). Weinheim: VCH. Gmehling, J., & Onken, U. (1977). Vapour-Liquid Equilibrium Data Collection, Chemistry Data Series. Frankfurt am Main: DECHEMA. Gmehling, J., Onken, U., & Arlt, W. (1981). Vapor-Liquid Equilibrium Data Collection, Chemistry Data Series, vol. I, Part 1a. Frankfurt/M: DECHEMA. Gmehling, J., Onken, U., & Rarey-Nies, J. R. (1988). Vapor-Liquid Equilibrium Data Collection, Chemistry Data Series, vol. I, Part 1b. Frankfurt/M: DECHEMA. Gmehling, J., Onken, U., & Weidlich, U. (1982). Vapor-Liquid Equilibrium Data Collection, Chemistry Data Series, vol. I, Part 2d. Frankfurt/M: DECHEMA. Kondepudi, D., & Prigogine, I. (1998). Modern thermodynamics (p. 199). Chichester: Wiley. Kunz, U. (1998). Entwicklung neuartiger Polymer/Tra( ger-Ionenaustauscher als Katalysatoren fu( r chemische Reaktion in Fu( llko( rperkolonnen. Habilitation, CUTEC-Schriftenreihe Nr. 34, ISBN 3-89720-225-5 (pp. 46}71). Michaelis, L., & Menten, M. L. (1913). Biochemische Zeitschrift, 49, 333. NIST. (1999). National Institute of Standards and Technology. Websites. http://webbook.nist.gov. Reh"nger, A. (1988). Reaktionstechnische Untersuchungen zur Flu( ssigphasesynthese von Methyl-tert.-buthylether (MTBE) an einem starksauren makroporo( sen Ionenaustauscherharz als Katalysator. Dissertation, TU Clausthal (pp. 96}100). Reh"nger, A., & Ho!mann, U. (1990a). Kinetics of methyl tertiary butyl ether liquid phase synthesis catalyzed by ion exchange resin } I. Intrinsic rate expression in liquid phase activities. Chemical Engineering Science, 45, 1605}1617. Reh"nger, A., & Ho!mann, U. (1990b). Kinetics of methyl tertiary butyl ether liquid phase synthesis catalyzed by ion exchange resin } II. Macropore di!usion of methanol as rate controlling step. Chemical Engineering Science, 45, 1619}1626. Reid, R. C., Prausnitz, J. M., & Poling, B. E. (1987). The properties of gases and liquids (4th ed.). Boston: Mc Graw-Hill. Reppe, W. (1955). AG thinylierung. Justus Liebigs Annalen der Chemie, 596, 82. Ullmann. (1987). Ullmann's encyclopedia of industrial chemistry A4. Weinheim: VCH. Weissermehl, K., & Arpe, H. J. (1997). Industrial organic chemistry (3rd ed.). (pp. 101}102). Weinheim: VCH.
Rate expression for THF synthesis on acidic ion exchange resin Uwe Limbeck, Carsten Altwicker, Ulrich Kunz, Ulrich Ho!mann* Technical University, Clausthal, Institut fu( r Chemische Verfahrenstechnik, Leibnizstr. 17, D-38678 Clausthal-Zellerfeld, Germany Received 15 June 2000; accepted 20 October 2000
Abstract The intrinsic kinetics for the liquid-phase cyclisation etheri"cation of 1,4-butanediol (BD) to tetrahydrofuran (THF) and water were determined in a stirred batch reactor in the temperature range from 373 to 393 K. A sulphonic acid ion exchange resin powder with a particle size of 3}7 m was manufactured and used as catalyst. The reaction can be regarded as irreversible and proceeds without di!usion limitations inside the catalytic microspheres. The formation of an intermediate containing 1,4-butanediol (BD) on an active site was determined to be the rate-determining step. The reaction product water acts as a strong inhibitor for the intermediate formation. The experimental results can be described by a three-parameter model based on Michaelis}Menten kinetics extended by an inhibition term for water. Within the developed rate equation, concentrations are expressed in liquid-phase activities from the UNIQUAC method in order to consider the nonidealities. The developed rate equation is valid in the whole range of possible concentrations and independent of the solvents used. During the experiments no formation of by-products was observed. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Tetrahydrofuran synthesis; Kinetics; Catalyst deactivation; Adsorption; Ion exchange
1. Introduction The great economic importance of the THF production is re#ected by the high world production capacities of 184,000 tonnes in 1994 (Weissermehl & Arpe, 1997). Because of the particular solvent properties, about 25% of the THF production is used as solvent for high polymers, such as PVC, rubber, Buna and others. However, most of the produced THF is used for polymerisation in order to satisfy the increasing demand for polytetramethylene glycol, a precursor for polyurethanes and Spandex "bres. Industrially, THF is manufactured in three di!erent ways based on the conversion of 1,4-butanediol, 1,4dichloro-2-butene or maleic anhydride. Splitting of water from butanediol in the presence of mineralic acid or ion exchange resins is the most important production
* Corresponding author. Tel.: #49-5323-72-2184; fax: #49-532372-2182. E-mail address: [email protected] (U. Ho!mann).
process for THF.
(1) Surprisingly, there are nearly no kinetic data published for this reaction. The only one that was found, describes the reaction mechanism of the 1,4-butanediol cyclisation on heteropoly acids (Baba & Ono, 1986). Two di!erent mechanisms are given for this catalyst, depending on whether the reaction proceeds in aqueous phase or in organic phase using dioxane as solvent. The aims of our investigations were the determination of a rate equation that describes the intrinsic kinetics of the butanediol cyclisation on ion exchange resins. An important requirement for rate equations is their validity in the whole concentration range and their independence from the used solvent. Therefore the rate equation should be given in terms of activities (De Donder, 1927; Kondepudi & Prigogine, 1998; Reh"nger, 1988). A further important aspect is the determination of the inhibition e!ect of water on ion exchange resin. For this
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reason the BD cyclisation is a useful model reaction for studying resin catalyst deactivation by water because the reaction can be regarded as irreversible and highly selective.
inner diameter"0.53 mm) using helium as carrier gas. Due to the water in the reaction mixture, a thermal conductivity detector was used. 2.3. Catalyst
2. Experimental 2.1. Procedure The reaction kinetics of the 1,4-butanediol cyclisation were studied in a stirred batch reactor (287 ml, stainless steel) as it is described in Fig. 1. Before closing the reactor, the reactant BD and eventually a solvent like THF, water or dioxane were fed into the reactor and the catalyst basket was "lled with the desired amount of catalyst (0.25}1 g). The temperature (from 373 to 393 K) was controlled within $0.1 K by immersing the whole reactor in a temperature-controlled oil bath. After the heating-up period, the magnetic-driven stirrer (60}2000 rpm) was switched on whereby the drop of the catalyst basket was initiated. This moment was considered to be the starting time of the reaction, because reactant and catalyst were mixed immediately. A sampling tube immersed into the liquid reaction mixture allowed to take samples due to excess pressure (0.12}0.35 MPa), which evolved by the vaporisation of a small part of the reaction mixture. In order to avoid the vaporisation of the reaction mixture during the withdrawal of samples, the sampling train was cooled. 2.2. Analytical methods The concentrations of the involved species butanediol, tetrahydrofuran, water and dioxane were analysed by gas chromatography. The separation was carried out on a Hewlett-Packard Innowax column (length"30 m,
A strongly acidic ion exchange resin, prepared in our laboratories, was used as a catalyst. This resin is manufactured by precipitation polymerisation of styrene and divinylbenzene (DVB) using para$n as solvent. After grinding a powder of small resin spheres with a diameter of 3}7 m (measured by scanning electron microscopy) is obtained by this method. More information about the preparation method and the incorporation of the resin in macroporous carrier materials is given in the literature (Kunz, 1998). For the kinetic experiments the pure resin is easier to handle and it has the same properties as the incorporated resin (Table 1). 2.4. Reactants These were 1,4-butanediol ('99.8%, H O (0.05%), tetrahydrofuran ('99.5%, H O (0.05%), 1,4-dioxane ('99.5%, H O (0.05%) and deionised water. 3. Results 3.1. External heat and mass transfer Special attention was paid to separate intrinsic kinetics from overlapping mass transport phenomena. The transport of the reactants from the bulk of the liquid reaction mixture to the surface of the catalyst particle and the transfer of reaction heat was taken into consideration. Therefore, the #ow condition around the catalyst particles was changed by variation of the rate of agitation. The experiments were started with pure butanediol, in order to achieve high reaction rates. For comparison, the conversion of butanediol after a reaction time of 180 min was determined. The results are presented in Fig. 2. Obviously, the measured butanediol conversions after 180 min were constant, when the rate of agitation was higher than 1000 rpm. The result presented complies with the assumption that the global reaction rate increases with the improvement of external mass transfer to
Table 1 Properties of the catalyst (Kunz, 1998)
Fig. 1. Scheme of the batch reactor.
Particle size Exchange capacity Upper operating temperature limit BET surface area Degree of crosslinking Density
3}7 m 4.55 meq/g 1203C 29.5 m/g 7.5% DVB by weight 1.050 g/cm
U. Limbeck et al. / Chemical Engineering Science 56 (2001) 2171}2178
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Because C (0.53 there is a slight in#uence of pore 5. di!usion for special reaction conditions. On the other hand, the estimation of the Weisz}Prater criterion includes again some assumptions like the e!ective di!usivity. So the exact con"rmation of the neglect of macroporeand gel-phase di!usion is a task of future work. 3.3. Nonideality of the liquid phase
Fig. 2. Butanediol conversions after 180 min in dependence on rate of agitation. Conditions: 1203C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol. "
an asymptotic limit. On the other hand, grinding of the catalyst by the stirrer limits the maximum rate of agitation. During our experiments no grinding e!ects were observed, so that all further experiments were carried out with 1500 rpm. 3.2. Exclusion of macropore- and gel-phase diwusion The exclusion of these internal transport resistances is based on the experimental results of Reh"nger and Ho!mann (1990a) on the MTBE synthesis. They examined the in#uence of the catalyst structure on the reaction rate, expressed as turnover number. They found out, that the reaction is in#uenced neither by the microsphere di!usion nor by the internal surface area, if the particles are "ne enough. For their comparison, they used Amberlyst 15 (20% DVB, 0.042 mm(d (0.180 mm) and the N resin described in this work, but with a larger particle size (7.5% DVB, 0.053 mm(d (0.156 mm). The ion exN change catalyst used in our experiments was much smaller and had a particle size of 3}7 m. With regard to the results of Reh"nger and Ho!mann (1990b), it can be assumed, that the whole microsphere contributes to the reaction with an e!ectiveness factor of 1. Moreover, the assumption can be validated by the appreciation of the Weisz}Prater criterion (Fogler, 1992) r R C " . "0.53, 5. D c C Q with r"163;10\ mol/g s, lowest rate of disappearance of species BD per mass of catalyst, "1.050 g/cm, density of catalyst, . R"3.5 m, largest radius of spherical catalyst particle, D "10\ m/s e!ective di!usivity of BD in catalyst, C and c "3.96 mol/l, concentration of BD at the surface of Q the catalyst particle.
The description of the nonidealities of the liquid phase was based on the UNIQUAC method (Gmehling & Onken, 1977). The parameters used are listed in Table 2. Unfortunately, interaction parameters for the systems dioxane/THF and butanediol/dioxane were not available. So these systems were calculated using the group contribution method mod. UNIFAC (Do.) (Gmehling & Kolbe, 1992). In order to simplify calculations UNIQUAC parameters were "tted to the estimated UNIFAC data. 3.4. Equilibrium The value of the equilibrium constant K was evaluated in a temperature range from 293 to 393 K based on data from the literature (Reid, Prausnitz, & Poling, 1987; Ullmann, 1987; NIST, 1999). Due to the low reaction enthalpy, the equilibrium constant can be regarded as constant in a vast temperature range as presented in Fig. 3. If pure BD is the only reactant, we expect the equilibrium mole fraction of BD to be x "0.0025 at 1203C, " based on "0.5294, "1.5124 and "1.8478 " 2&$ &as activity coe$cients. This result is in contrast to comments in the literature where it is mentioned that the reaction products have to be removed from the reaction mixture in order to achieve high conversion (Reppe, 1955) by shifting the equilibrium composition. Experiments for the investigation of the backward reaction were carried out by the present authors. THF and H O were fed into the reactor in stoichiometric ratio and
Table 2 UNIQUAC interaction parameters (Gmehling, Onken, & Arlt, 1981; Gmehling, Onken, & Weidlich, 1982; Gmehling, Onken, & Rarey-Nies, 1988). 1"BD, 2"THF, 3"H O, 4"dioxane a GH
(cal/mol)
a
a a a a a a
!133.3164 470.3109 156.1217 !183.4339 835.2626 10.5163
a a a a a a
GH
Based on mod. UNIFAC (Do.) calculations.
(cal/mol) 50.8926 239.2393 !323.0695 !264.9392 !444.0130 1302.038
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Fig. 3. Equilibrium constant in dependence on temperature for the liquid-phase butanediol cyclisation.
Fig. 4. Initial reaction rates in dependence on initial compositions. Conditions: 1203C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
toluene}sulphonic acid was used as a homogeneous catalyst. After a reaction time of 6 h at a temperature of 1203C, it was not possible to analyse any BD. Based on our equilibrium calculations and our experiments, the butanediol cyclisation is regarded as an irreversible reaction. Beyond this it is important to answer the question why it is important to remove the reaction products from the reaction mixture (Reppe, 1955). 3.5. Model equation In order to determine concentration dependencies on the reaction rate, initial rates r were measured at di!er ent compositions. The experiments were carried out at a temperature of 1203C and with a constant amount of catalyst of 4.55 mmol eq on 2 moles of reactants. In Fig. 4 initial rates are compared when the reactant butanediol was initially diluted with one of the reaction products THF or water. Surprisingly, the dilution e!ects of THF and water are completely di!erent. The presence of a small amount of water leads to a tremendous decrease of the reaction rate. Further addition of water only reduces the rate slightly. In contrast, the dilution of BD with THF has nearly no e!ect on the initial rate until the activity of THF approaches 0.75. Further dilution causes the decrease of the rate that tends of course to zero for the case of pure THF. Analogous rates were observed when dioxane was used as a solvent and consequently the reaction rate decrease does not depend on the solvent used, but on the dilution of the reaction mixture by an inert component like THF or dioxane (Fig. 10). So the experimental data based on THF dilution can be used to determine the dependence of the rate on the activity of BD. Fig. 5 shows a steep slope of the initial rate at low BD activities and afterwards an approximately constant
Fig. 5. Initial reaction rates in dependence on BD dilution with THF. Conditions: 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
value for BD activities larger than 0.6. The results obtained are comparable to Michaelis}Menten kinetics (Michaelis & Menten, 1913). Perhaps this behaviour should better be named saturation kinetics since this is not an enzyme-catalysed reaction. This means that BD forms in a "rst reversible reaction step an intermediate with one active site (S). During a subsequent irreversible reaction step, the ring closure takes place. Here THF and water are formed and the active site is free again for further reactive conversions. This reaction mechanism can be described by the following equations: BD#S & BD ' SPTHF#H O#S, (2) (3) n "n #n ' . 1 1 "1 Here n , n and n ' are the total number of active sites, 1 1 "1 the number of free active sites and the number of active sites occupied by the intermediate, respectively. The resulting rate equation is equivalent to a monomolecular Langmuir adsorption of BD multiplied by the rate
U. Limbeck et al. / Chemical Engineering Science 56 (2001) 2171}2178
2175
constant r K a " " , r " "k (4) n 1#K a 1 " " where r is the initial rate of reaction related to the number of sulphonic groups (turnover number), K the " equilibrium sorption constant of BD, and k the rate constant of the reaction. The values for K "1.8878 " and k"59.77 mmol s\ eq\ at 1203C were "tted to the initial rates presented in Fig. 5. Special attention has to be paid to the inhibiting properties of water that have already been described above. In order to separate dilution and inhibition e!ects, the measured initial reaction rates in water presented in Fig. 4 have to be corrected. Therefore the rate equation is extended by the inhibition factor that contains all inhibition e!ects of the water while the last term describes the dilution e!ects of BD K a " " . (5) r "(a )k & - 1#K a " " Surely, is just dependent on the activity of water a and has to tend to 1 for the case of the absence of &water. Due to experimental data for initial rates when BD was diluted with water, the inhibition factor is described in dependence on a in Fig. 6. After the exclu&sion of dilution e!ects the main characteristic of the water inhibition remains. So small amounts of water a!ect the initial rate markedly while the concentration dependence for higher water contents is rather low. Without dilution e!ects the quantitative description of the inhibition factor can be given by the following equation: 1 , (6) (a )" &1#K (a &&where the proportionality factor K is comparable to &a sorption constant for water. The value for K " &2.877 and the exponential value for the water activity a were determined during the "tting procedure of &experimental data that were obtained at 1203C. A comparison of the model equation and the experimental data in Fig. 6 shows good agreement. The validity of the introduced inhibition term in the most signi"cant range for a (0.2 is con"rmed within the following simula&tions of long-time batch experiments. For comparison the inhibition factor 1/(1#Kc ) for &the butanediol cyclisation on heteropoly acids in dioxane was found in the literature (Baba & Ono, 1986). The same authors forgo an inhibition factor when the reaction proceeds in water as solvent and use a di!erent rate equation whether the reaction proceeds in aqueous or non-aqueous phase. Their results correspond to the observation that small amounts of water inhibit the reaction strongly while for high water concentration there is a constant inhibition e!ect.
Fig. 6. Inhibition factor in dependence on initial water activities. Conditions: 1203C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
A qualitative explanation for the inhibiting properties of water is also found in the literature (Gates, 1992). It is mentioned that water has such a great a$nity for the !SO H groups that it excludes the reactants and suppresses the catalytic reaction almost completely. The attribution of physicochemical phenomena to the observed inhibition e!ect of water is a subject of future work. One of these phenomena will be the change of the catalytic activity of sulphonic acid groups by di!erent solvation of protons depending on the composition of the liquid phase in the gel particles. Other important phenomena are gel phase swelling and gel phase di!usion which are not accounted for in the proposed rate equation. Nevertheless, the determined inhibition factor presents a simple and quantitative empirical possibility to take the complicate inhibition e!ects into account. After the description of the initial rates at di!erent initial compositions the validity of the achieved rate equation K a k " " r" 1#K a 1#K (a " " &&-
(7)
has to be tested for long-time batch experiments and changing compositions. Comparisons between experimental results and simulations are given in Figs. 7}9. It is obvious that the relations that were determined for the initial rates are extendable to all reaction conditions that were achieved during the experiments. The introduction of an intensive rate (turnover number) is justi"ed due to the simulation results in Fig. 7 where the amount of catalyst is varied by a factor of four. The good agreement of experimental data and simulation demonstrates that the developed rate equation works in the entire concentration range of the ternary reaction system BD, THF, water.
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Fig. 7. BD mole fraction pro"les of batch experiments. Variation of catalyst concentration and dilution of initial reaction mixture with THF. Conditions: 1203C, particle size: 3}7 m, n "2 mol.
Fig. 8. BD mole fraction pro"les of batch experiments for di!erent initial dilutions with THF. Conditions: 1203C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
Fig. 9. BD mole fraction pro"les of batch experiments for di!erent initial dilutions with H O. Conditions: 1203C, 4.55 meq catalyst, par ticle size: 3}7 m, n "2 mol.
Fig. 10. BD mole fraction pro"les of batch experiments for initial dilutions with THF and dioxane. Conditions: 1203C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
3.6. Ewect of temperature
During a further experiment dioxane was added to the ternary system as solvent. The experimental results and simulations are given in Fig. 10. The experimental data con"rm that regardless of whether BD is initially diluted by THF or dioxane the concentration pro"les are similar. In contrast to the experimental data, the calculated pro"les for the reaction in THF or in dioxane as solvent di!er a little. This surprising result may be explained by the problems to get reasonable UNIQUAC interaction parameters for the quaternary system. With regard to the experimental data, the rate in dioxane can be better simulated by assuming that all dioxane molecules behave like THF molecules.
The temperature dependencies of the three parameters k, K and K used in the rate equation developed " &were determined due to further experiments carried out at 110 and 1003C. Lower temperatures were not used, because the rate got too slow for reasonable experimental times. On the other hand, the maximum temperature limit of the catalyst is 1203C. Due to the small temperature interval and the small number of experimental data it does not seem to be reasonable to use the kinetic data outside the given temperature range. For example a particularly high value for the activation energy E was obtained. The values for k and K were deter" mined by "tting initial rates, when BD is diluted with THF. The results are presented in Fig. 5. The
U. Limbeck et al. / Chemical Engineering Science 56 (2001) 2171}2178
temperature-dependent K was "tted to experimental &data of BD mole fraction pro"les from batch experiments described in Figs. 11 and 12. For the quantitative description of temperature dependencies, Arrhenius equations can be used. Subsequently, the de"ned equations and the corresponding parameters are listed.
"
"K
"
(8)
H " , exp ! R¹
where k "8.63;10 mmol/s eq, E "152 kJ/mol, K "6.29;10\, H "!11.1 kJ/mol, " " K "2.43, H "!6.36 kJ/mol. & - &3.7. Further possible reaction mechanisms
E k"k exp ! , R¹ K
2177
(9)
H K "K exp ! & - , && - R¹
(10)
Several rate equations based on Langmuir}Hinshelwood theory were tested and compared with experimental data. Special attention was paid to a competitive sorption of water and BD on the active sites in order to describe the inhibition e!ects. Unfortunately, all rate equations based on such a sorption mechanism failed. So it was not possible to introduce the inhibition e!ects of water in a sorption model for the description of the rate equation. Because of the high value of the equilibrium constant, the consideration of the backward reaction was not reasonable for the description of the reaction kinetics.
4. Conclusions
Fig. 11. BD mole fraction pro"les of batch experiments for di!erent initial dilutions with THF. Conditions: 1103C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
By this work we succeeded for the "rst time in expressing the intrinsic kinetics of the BD cyclisation on ion exchange resins in one rate equation that is valid in aqueous and in non-aqueous phases, covering the whole concentration range. To achieve this it was necessary to express the non-idealities of the liquid phase in terms of activities. Moreover, the presented rate expression is independent of the solvents used. The in#uence of inhibiting water was quantitatively described. An important aspect is that the inhibition e!ects cannot be described by a Langmuir}Hinshelwood mechanism, but by the presented inhibition factor. This procedure could be a guideline to other comparable liquid-phase reactions.
Notation
a G a GH d N C 5. c G c Q
Fig. 12. BD mole fraction pro"les of batch experiments for di!erent initial dilutions with THF. Conditions: 1003C, 4.55 meq catalyst, particle size: 3}7 m, n "2 mol.
D C E H G H 0
liquid-phase activity of component A G UNIQUAC interaction parameter of binary system i/j, cal/mol arithmetic mean sphere volume equivalent diameter, mm constant in Weisz}Prater criterion concentration of component i, mol/l concentration of BD at the surface of the catalyst particle, mol/l e!ective di!usivity of BD in catalyst, m s\ activation energy, kJ mol\ heat of absorption, kJ mol\ standard enthalpy change of reaction, kJ/mol
2178
K K G K G k k n R r r r r x G
U. Limbeck et al. / Chemical Engineering Science 56 (2001) 2171}2178
chemical equilibrium constant equilibrium sorption constant of component i equilibrium sorption constant of component i for ¹PR rate constant, mmol s\ eq\ rate constant for ¹PR, mmol s\ eq\ amount of substance, mol largest radius of spherical catalyst particle, m reaction rate related to the number of acid groups, mmol s\ eq\ lowest rate of disappearance of species BD per mass of catalyst, mol g\ s\ initial reaction rate related to the number of acid groups, mmol s\ eq\ reaction rate, mmol s\ eq\ mole fraction of component i
Greek letters G .
activity coe$cient of component i inhibition factor density of catalyst, g cm\
Subscripts 0
initial condition
Abbreviations BD 1,4-butanediol BD ' S intermediate of butanediol molecule and active site DVB divinylbenzene THF tetrahydrofuran S active site"acid group
References Baba, T., & Ono, Y. (1986). Kinetic studies in liquid phase dehydration cyclization of 1,4-butanediol to tetrahydrofuran with heteropoly acids. Journal of Molecular Catalysis, 37, 317}326.
De Donder, T. (1927). L'Aznite& . Paris: Gauthiers-Villars. Fogler, H. S. (1992). Elements of chemical reaction engineering (2nd ed.). (p. 625). New Jersey: Prentice-Hall. Gates, B. C. (1992). Catalytic chemistry (pp. 220}221). New York: Wiley. Gmehling, J., & Kolbe, B. (1992). Thermodynamik, Zweite u( berarbeitete Auyage (pp. 246}255). Weinheim: VCH. Gmehling, J., & Onken, U. (1977). Vapour-Liquid Equilibrium Data Collection, Chemistry Data Series. Frankfurt am Main: DECHEMA. Gmehling, J., Onken, U., & Arlt, W. (1981). Vapor-Liquid Equilibrium Data Collection, Chemistry Data Series, vol. I, Part 1a. Frankfurt/M: DECHEMA. Gmehling, J., Onken, U., & Rarey-Nies, J. R. (1988). Vapor-Liquid Equilibrium Data Collection, Chemistry Data Series, vol. I, Part 1b. Frankfurt/M: DECHEMA. Gmehling, J., Onken, U., & Weidlich, U. (1982). Vapor-Liquid Equilibrium Data Collection, Chemistry Data Series, vol. I, Part 2d. Frankfurt/M: DECHEMA. Kondepudi, D., & Prigogine, I. (1998). Modern thermodynamics (p. 199). Chichester: Wiley. Kunz, U. (1998). Entwicklung neuartiger Polymer/Tra( ger-Ionenaustauscher als Katalysatoren fu( r chemische Reaktion in Fu( llko( rperkolonnen. Habilitation, CUTEC-Schriftenreihe Nr. 34, ISBN 3-89720-225-5 (pp. 46}71). Michaelis, L., & Menten, M. L. (1913). Biochemische Zeitschrift, 49, 333. NIST. (1999). National Institute of Standards and Technology. Websites. http://webbook.nist.gov. Reh"nger, A. (1988). Reaktionstechnische Untersuchungen zur Flu( ssigphasesynthese von Methyl-tert.-buthylether (MTBE) an einem starksauren makroporo( sen Ionenaustauscherharz als Katalysator. Dissertation, TU Clausthal (pp. 96}100). Reh"nger, A., & Ho!mann, U. (1990a). Kinetics of methyl tertiary butyl ether liquid phase synthesis catalyzed by ion exchange resin } I. Intrinsic rate expression in liquid phase activities. Chemical Engineering Science, 45, 1605}1617. Reh"nger, A., & Ho!mann, U. (1990b). Kinetics of methyl tertiary butyl ether liquid phase synthesis catalyzed by ion exchange resin } II. Macropore di!usion of methanol as rate controlling step. Chemical Engineering Science, 45, 1619}1626. Reid, R. C., Prausnitz, J. M., & Poling, B. E. (1987). The properties of gases and liquids (4th ed.). Boston: Mc Graw-Hill. Reppe, W. (1955). AG thinylierung. Justus Liebigs Annalen der Chemie, 596, 82. Ullmann. (1987). Ullmann's encyclopedia of industrial chemistry A4. Weinheim: VCH. Weissermehl, K., & Arpe, H. J. (1997). Industrial organic chemistry (3rd ed.). (pp. 101}102). Weinheim: VCH.