Ind. Eng. Chem. Res. 1998, 37, 3575-3581
3575
Catalytic Activity and Deactivation of Acidic Ion-Exchange Resins in Methyl tert-Butyl Ether Liquid-Phase Synthesis D. Parra,† J. F. Izquierdo,*,† F. Cunill,† J. Tejero,† C. Fite´ ,† M. Iborra,† and M. Vila‡ Chemical Engineering Department, Faculty of Chemistry, University of Barcelona, C/Martı´ i Franque´ s 1, 08028 Barcelona, Spain, and Escombreras Research Center, Repsol Petro´ leo S.A., 30350 Valle de Escombreras, Cartagena (Murcia), Spain
This paper presents a comparative study of catalytic activity of 12 different styrenedivinylbenzene ion-exchange resins as catalysts for MTBE (methyl tert-butyl ether) liquid-phase synthesis. Experiments were carried out at 1.6 MPa and 323 K using a stationary fixed-bed fed with methanol and C4 olefinic cut as the source of isobutylene. The isobutylene/methanol molar ratio studied was 0.91 and the LHSV employed in the continuous operation was 85 h-1. Experimental data show that ion exchangers with higher acidic capacity were the most active ones. It seems that this parameter plays the greatest influence on the catalytic activity for MTBE synthesis whereas other properties such as surface area and average pore diameter hardly influence it. Introduction It is hard to explain the kinetic mechanism of the etherification reaction of isobutylene with methanol in liquid phase. At low concentrations of methanol, the catalysis is by the polymer-bound -SO3H groups, which are very active, but if the concentration of methanol is increased, the alcohol dissociates the acid groups and solvated protons become now the catalytic agents, which are catalytically less active (Gates and Rodriguez, 1973; Chakrabarti and Sharma, 1993). A proof of the difficulty to explain methyl tert-butyl ether (MTBE) mechanism synthesis is found in the literature itself because researchers have used both pseudohomogeneous and heterogeneous kinetic models to describe the kinetics of this reaction (Voloch et al., 1986; Subramanian and Bathia, 1987; Colombo and Dalloro, 1983; Rehfinger and Hoffman, 1990; Gicquel and Tork, 1983; Parra et al., 1994; Tejero et al., 1996; Panneman and Beenackers, 1995a). Information in the available literature about the influence of catalyst morphology, acid concentration, and solvent effect on catalytic activity of an ion exchanger in MTBE synthesis is rather scarce (Colombo and Dalloro, 1983; Jera´bek et al., 1993; Rehfinger and Hoffmann, 1990; Panneman and Beenackers, 1995b). In general, the reaction rate of an ion-exchanger catalyzed reaction in a nonaqueous medium depends both on changes of the polymer structure and on changes in the concentration of accessible acid groups (Rodriguez and Setinek, 1975; Jera´bek and Setinek, 1987). In nonaqueous medium, the polymer matrix structure influences directly the reaction course since the possibility of forming undissociated sulfonic groups depends necessarily on the conformation of polymer chains. Depending on the polarity of the medium and on the size of the reactant and product molecules, steric restrictions may also influence the catalytic activity (Jera´bek and Setinek, 1987) because some of the inner active centers could not participate in the reaction. * To whom correspondence should be addressed. Fax: +343-4021291. E-mail:
[email protected]. † University of Barcelona. ‡ Escombreras Research Center, Repsol Petro ´ leo S.A.
There are several published approaches to explain the behavior of an ion exchanger as a catalyst. From a morphological point of view, it is to be noted that in the model proposed by Guyot (1988), who claims that macroporous resins are composed of three families of pores, the structure contains large agglomerates of microspheres (100-200 nm), which look like cauliflowers, and each microsphere shows smaller nuclei (1030 nm) more or less fused together. In betweeen the nuclei, there is a family of very small pores (5-15 nm), which are mainly responsible for the high surface area (> 500 m2/g) of these materials. Between the microspheres, a second family of intermediate pores is observed (20-50 nm), which may account for moderate surface areas (up to 100 m2/g). A third family of large pores (50-100 nm) is located between the agglomerates; this kind of pores yields very low surface area but large pore volume up to 3 mL/g. Resins are able to swell to varying degrees in different solvent media. So, the catalytic behavior of a resin may be analyzed in terms of accessibility of its functional groups in different media taking into account their situation inside the resin. Other theories emphasize the dependence of the catalytic activity on the acidity and on the accessibility (Widdecke 1988; Buttersack, 1988). The acidity depends on the type and number of acid groups within the resin and is affected by the degree of cross-linking. The accessibility is related to the permeability of the resin and it can be influenced by the interaction of the solvent and adsorbed molecules with the functionalized polymer. So, it can be drawn from the general theories that the catalytic activity of the resins depends mainly on its initial morphology and on its interaction with the reacting medium including the solvent and the compounds that can be found in the reacting system. The properties of the MTBE reacting system change with the reaction progress. In this context, contradictory papers can be found, probably due to the different initial mixtures and kinds of resins used to study this reaction. Rehfinger and Hoffman (1990) and Jera´bek et al. (1993) found no influence of the degree of crosslinking, the internal surface area, and the exchange
S0888-5885(98)00007-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/14/1998
3576 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 Table 1. Physical Properties of Different Ion Exchangers, Methanol Conversions, and Reaction Rates Obtained at 323 K trade name
C (mequiv/g)
A (m2/g)
A′ (m2/g)
V (mL/g)
D (Å)
dp (mm)
XM (%)
r (mol/h‚mequiv)
Bayer K2631 Bayer OC-1501 Amberlyst 15 Amberlyst 35 Dowex M32 Purolite CT151 Purolite CT165 Purolite CT169 Purolite CT171 Purolite CT175 Purolite CT175/2824 Purolite CT179
4.83 5.47 4.75 5.32 4.78 5.40 5.00 4.90 4.94 4.98 5.30 5.25
41.5 25.0 42.0 34.0 29.0 25.0 6.2 48.1 31.0 29.0 24.1 35.0
---163.8 156.9 165.7
0.67 0.52 0.36 0.28 0.33 0.30 0.16 0.38 0.47 0.48 0.44 0.33
650 832 343 329 455 252 1148 342 597 662 745 386
0.63 0.66 0.74 0.51 0.63 0.43 0.43 0.43 0.40 0.40 0.43 0.43
6.9 11.0 7.2 10.4 6.8 8.1 4.1 7.7 7.8 8.2 10.4 9.6
0.0143 0.020 0.0151 0.0195 0.0143 0.0150 0.0081 0.0158 0.0158 0.0164 0.0196 0.0182
151.2
157.4 220.1
capacity of the resin on the rate of MTBE synthesis, but Panneman and Beenackers (1995b) showed that the catalytic activity of an ion-exchange resin on the same reaction depends on its three-dimensional structure, which is influenced by the level of cross-linking, the porosity, the internal surface area, the sulfonic acid content, and its distribution within the particles. On the other hand, another important factor in commercial operation that influences the catalytic activity is the deactivation of the catalyst. The acid sites of the cation-exchange resin catalyst may easily be poisoned by different ions that can be present in the feeds due to corrosion in pipelines or can be carried to the reactor through misoperations in preceding units. Brockwell et al. (1991) make a distinction between two different modes of catalyst deactivation. The first mode is plug flow neutralization and the second is diffused flow neutralization. With plug flow neutralization, the poison accumulates on the catalyst in a plugflow manner as it impinges on the catalyst bed. The uptake of deactivator is rapid and stoichiometric. As deactivation progresses, plug flow neutralization results in a foreshortening of the active catalyst bed length. Examples of plug-flow deactivators are all salts and other Lewis and Bro¨nsted bases. Diffused flow neutralization is a mode of deactivation in which catalytic sites are neutralized uniformly throughout the bed rather than in progression from the point of impingement. Diffusional deactivators are generally very weak bases that either weakly bind the catalytic sites or are species that generate more strongly binding species in situ. Examples of diffusional deactivators are propionitrile, dimethylformamide, dimethyl sulfide, hydrogen sulfide, and amine formers such as acetonitrile, which is responsible for most of the catalyst deactivation experienced in MTBE reactors due to formation of ammonium ion (Marston, 1994). It has not been possible to find experimental studies about the catalytic behavior of an ion exchanger partly deactivated by ammonium or Fe(III) in MTBE synthesis. Krause and Hammarstro¨m (1987) propose that the loss of the activity of the resin partly deactivated by sodium or ammonium ions is the same for tert-amyl methyl ether (TAME) synthesis in liquid phase, and Kmosta´k and Setinek (1981) found that sodium ions deactivate the catalyst more than Fe(III) ions for isomerization of cyclohexene and dehydration of 1-propanol in the gas phase. So, the first aim of this work is to contribute further to the elucidation of the effect of different morphologic properties of an ion exchanger (surface area, average pore diameter, and acidic capacity) on its catalytic activity in MTBE synthesis. For this purpose, a number
of resins that differed markedly in their properties were compared at the same experimental conditions. The second goal of this paper is to know the deactivating effect of ammonium and Fe(III) on the etherification reaction. To test catalytic activity of partially sulfonated ion exchangers, reaction rates were compared at the same experimental conditions. Experimental Section (i) Continuous Operation. Twelve commercial macroporous styrene-divinylbenzene ion exchangers were used as catalysts in MTBE synthesis. Their morphological properties are summarized in Table 1. Prior to use, all ion exchangers were washed with distilled water and dried at 110 °C for more than 14 h at atmospheric pressure. Afterward they were stored in a desiccator over sulfuric acid (98%). The water content in all resins, measured by Karl Fischer titration, was less than 3%. Before the experiment, the catalyst bed was preheated in the reactor at operating temperature in the presence of the methanol stream to ensure that an amount of methanol greater than 10 times the reactor volume was passed through it. As a result, the residual water content in the resin was reduced to less than 1.6%. tert-Butyl alcohol produced from this amount of water was not detected in the experiments. Other byproducts (dimethyl ether and diisobutylene) were not found in the runs. In all experiments, carried out in the steady state, commercial particle sizes of different resins have been used to study their behavior in operating conditions of industrial reactors. Fresh dry catalyst sample (0.874 g) was always placed in a continuous packed-bed microreactor (15 cm length; 4.4 mm i.d.) and the small remaining reactor volume was filled with quartz of 0.63 < dp < 0.8 mm to avoid bed expansion. The experiments in the continuous regime were carried out in a setup made of stainless steel at 1.6 MPa and 323 K by feeding an appropriate methanol-C4 olefinic cut mixture. This pressure was enough to maintain the reacting system in liquid phase and this temperature was chosen to avoid temperature axial gradients. The methanol molar flow rate was always 0.874 mol/h, the molar ratio of isobutylene/methanol employed was 0.91, and the liquid hourly space velocity (LHSV) used was 85 h-1. C4 cut from a steam cracking (SC) plant containing 45-55% isobutylene was supplied by Repsol Petro´leo S.A. Methanol for high-performance liquid chromatography (HPLC) ( 60 h-1 reaction rate does not depend on flow rate. As a result, a LHSV ) 85 h-1 was used. Fourteen different replicated experiments were made to calculate the experimental error of methanol conversion ((0.2% with a probability level of 95%). Table 1 shows the morphological properties of ion exchangers used along with the results of methanol conversion and reaction rates obtained after the experiments carried out in steady-state mode. The reaction rates were calculated by taking into account that for the methanol conversion values obtained, the reactor was differential (Parra et al., 1994). So, the reaction rates were determined by
r)
XMF 100CW
(1)
At first sight, the following differences about the properties of the resins must be pointed out: Purolite CT165 has a particular morphology, its BrunauerEmmett-Teller (BET) surface area (A) and pore volume are the smallest in the group but its average pore diameter is the highest one. It seems to be a microporous resin. Despite the fact that Purolite CT151 has a high acidic capacity, it has a surprisingly small average pore diameter in comparison with the other resins studied. Bayer OC-1501, Amberlyst 35, and Purolite CT175/2824 have the highest acidic capacity of ion exchangers selected. Finally, Purolite CT 169 has the largest value of surface area (A), but its other properties are similar to the other resins. As can be seen in Table 1, the catalytic behavior of the resins differs greatly. Methanol conversions vary from 4.1% (Purolite CT165) to 11.0% (Bayer OC-1501). Equal conversion values for Amberlyst 35 and Purolite CT175/2824 are found, close to that for Bayer OC-1501. Traditional catalysts in MTBE synthesis such as Bayer K2631 or Amberlyst 15 show lesser catalytic activities than the new generation of commercial resins from the same manufacturers, Bayer OC-1501 and Amberlyst 35. This fact could be explained because of the increase of density of sulfonic groups produced after a special sulfonation procedure which added a second sulfonic group to the aromatic rings already sulfonated rather than sulfonate the unsulfonated aromatic rings. So, this sulfonation procedure makes it possible to achieve a higher number of acidic groups per unit of mass of these new resins. To assess the quantitative influence of morphological properties in dried state of the different resins on methanol conversion, different polynomials were fitted to the data. From a statistical point of view, the best significant polynomial fitted to the data was
XM ) (2.924 × 10-3)C3A - (1.751 × 10-4)C2A2 + (1.598 × 10-5)C3D (2) This fourth-order polynomial only contains three variables, namely, acid capacity (C), BET surface area (A), and average pore diameter (D). We have not considered in this mathematic analysis pore volume (V), because it depends on the surface area and average pore diameter, or approximate average particle diameter (dp),
3578 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998
Figure 1. Reaction rates of MTBE synthesis at 323 K versus average pore diameter.
Figure 2. Reaction rates of MTBE synthesis at 323 K versus BET surface area.
because the values obtained from manufacturers are not accurate enough. It can be seen from this polynomial that the positive influence of acidic capacity on methanol conversion is stronger than that of the other two variables. It seems that the accessibility of methanol and isobutylene to the greater part of the acidic active centers is quite good for the majority of the resins chosen. Several authors assure that kinetics of MTBE synthesis catalyzed by strong acidic ion exchangers proceeds with the participation of ensembles of sulfonic groups (Parra et al., 1994; Tejero et al., 1996; Jera´bek et al., 1993). Most of these ensembles are placed within the polymeric matrix and their accessibility to reactants may be sometimes limited. On the whole, the strong influence of acidic capacity on catalytic activity of the resins studied for MTBE synthesis can be explained by supposing that the polymeric mass of ion exchanger, which suffers a moderate swelling due to the presence of significant quantities of methanol, allows reactants to permeate with relative freedom and to react faster as the density of active centers increases. Because of that, the weak influence of average pore diameter and BET surface area on catalytic activity, whose values have been obtained from dry state, could be explained by supposing that the initial structure of dry ion exchanger would have changed due to the reaction microenvironment, and so pore sizes and surface area would have changed after the moderate swelling. Figures 1 and 2 demonstrate this affirmation because there are not significant and defined relationships either between reaction rate and surface area or between reaction rate and average pore volume for the resins studied. The results of
Figure 3. Reaction rates of MTBE synthesis at 323 K as a function of acidic capacity.
surface area for the swollen state of some of the resins studied (Jera´bek, 1997) by inverse steric exclusion chromatography (ISEC) corroborate this affirmation (see Table 1). As can be seen, there is not a relationship between the BET surface area, determined in dry state, and the surface area in the swollen state. After this, we studied in detail the influence of acidic capacity on reaction rate for all resins and we found out some behavior deviations. Figure 3 shows the reaction rate against the acidic capacities of the resins studied. As a general rule, it can be seen that the higher the acidic capacity, the higher the reaction rate, but there are two resins that do not fulfill this trend, namely, Purolite CT 151 and especially Purolite CT 165. Despite the fact that Purolite CT 151 has a high acidic capacity, it has the smallest average pore diameter in the dry state. It seems that this resin could not suffer a significant swelling in contact with the MTBE reaction mixture, and so the accessibility of reactants to the ensembles of sulfonic groups placed inside the polymer mass is not satisfactory. Purolite CT 165 has a high acidic capacity and the smallest BET surface area, though its average pore diameter in the dry state is the largest one. It seems that this resin, which is microporous (Jera´bek, 1997), could have a small density of accessible sulfonic groups and so concerted mechanisms are difficult to form. (ii) Deactivated Catalysts. Four representative resins (Bayer K 2631, Bayer OC-1501, Amberlyst 35, and Purolite CT 175) were chosen to be partially neutralized by Fe(III) and ammonium ions in batch operation. These cations were selected because they can frequently be found in industrial reactors for MTBE synthesis, so it is expedient to measure the loss of activity of these resins due to partial neutralization by both cations. Two neutralization standards for Fe(III) and only one for ammonium were used. To measure the deactivation produced, initial reaction rates of MTBE formation were determined from the slope of the curve at zero time of experimental values of methanol conversion versus time. Figure 4 shows the plots of methanol conversion against time for fresh and partly deactivated resin K2631. It can be seen that the initial slope of the curve for fresh resin is higher than those for resins neutralized by, respectively, low standard of Fe(III), high standard of Fe(III), and ammonium. Some experiments were repeated to evaluate the experimental error on initial reaction rate, which was (0.0011 mol h-1 mequiv-1. The concentrations of pollutants in the contaminated resins, the remaining acidic capacity after deactivation,
Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 3579
Figure 4. Methanol conversion values versus time at 333 K for fresh and deactivated K2631. Table 2. Remainig Acidic Capacities and Initial Reaction Rates at 333 K for the Four Resins Used in MTBE Synthesis as a Function of Fe(III) and Ammonium Contents
resin Bayer K-2631
pollutant
cFe(III)
cam
4.83 4.72 4.16
0.037 0.036 0.033
ammonium 8433 Bayer OC-1501 none low Fe(III) 3152 high Fe(III) 14368
4.38 5.47 5.33 4.92
0.026 0.041 0.041 0.040
ammonium 9653 Purolite CT175 none low Fe(III) 2915 high Fe(III) 13630
4.59 4.98 4.85 4.41
0.027 0.037 0.036 0.034
ammonium 9696 none low Fe(III) 3386 high Fe(III) 13890 ammonium 9822
4.57 5.32 5.18 4.96 4.90
0.025 0.042 0.041 0.035 0.031
Amberlyst 35
none low Fe(III) 3613 high Fe(III) 15904
acidic capacity r0 (mol/ (mequiv/g) h‚mequiv)
and the initial reaction rate values can be seen in Table 2. From these data it can be pointed up that fresh Bayer OC-1501 and Amberlyst 35 are more active (higher initial reaction rate) than Purolite CT175 and Bayer K2631. This fact was already found above in continuous operation. While the deactivation by low concentrations of Fe(III) did not affect significantly the reaction rate, high concentration values of Fe(III) led to a different decrease in both exchange capacity and initial reaction rate. For this high iron contamination, the resin Bayer OC-1501 was the strongest one; its loss of activity was only 2%, whereas Amberlyst 35 suffered a loss of 17%. The loss of acidic capacity varied from 7% to 14%. Surprisingly, the behavior of the resins was different when they were deactivated by ammonium ion. In general, the loss of activity was more pronounced, especially for Bayer OC-1501. The decrease on initial reaction rates (catalytic activities) for the four resins was now from 26% to 34%, whereas the loss of exchange capacity ranged from 8% to 16%. It must be considered that the standard of ammonium employed in deactivating the resins was less than the high iron standard (see Table 2) used and also that the exchange number for Fe(III) is 3 times higher than that of ammonium. When deactivation by ammonium is compared to that produced by Fe(III) high standard, it could be said that ammonium ion leads to a deactivation of the catalyst
Figure 5. Initial reaction rates of MTBE synthesis at 333 K versus acidic capacities of the resins before and after being polluted by iron and ammonium. Table 3. Loss of Activity for Different Resins Partially Neutralized by Iron and the Corresponding Values of Effective Exchange Capacity for Fe(III) resin Bayer K-2631 Bayer OC-1501 Purolite CT 175 Amberlyst 35
ppm Fe(III)
L (mequiv/g)
nFe(III)
3613 15904 3152 14368 2915 13630 3386 13890
0.11 0.67 0.14 0.55 0.13 0.57 0.14 0.36
1.70 2.35 2.48 2.13 2.48 2.34 2.31 1.45
significantly greater than that of Fe(III). An explanation for this fact is that the presence of ammonium in the catalyst besides neutralization could lead to the formation of hydrogen bonds with some sulfonic groups. Clearly Fe(III) is incapable of doing so. Furthermore, it is known that the ionic radius of Fe(III) is 0.064 nm and the ionic radius of ammonium is 0.143 nm. So it is likely that a part of the so-called active sites, situated inside the polymer mass, were difficult to reach for the reactants due to steric hindrance produced by ammonium ion placed at external layers of the polymeric mass. Figure 5 shows that there exists a linear relationship between the initial rates and the acidic capacities of the fresh resins, but it can be also seen that the catalytic activity of the resins polluted by iron follows the same trend as the fresh resins. Table 3 shows the effective exchange capacity values for Fe(III) obtained after analyzing the relationship between the loss (L) of acidic capacity (difference between the acidic capacity of fresh resin and the remaining acidic capacity determined after the neutralization by iron) of the resins and their Fe(III) content. To calculate the effective exchange capacity of Fe(III), the following expression has been used:
nFe(III) )
(1000)(55.85)L cFe(III)
(3)
From Table 3 we can see that values of nFe(III) varies from an unexpected value of 1.45 for Amberlyst 35 at high standard of Fe(III) to 2.48 for Bayer OC1501 and Purolite CT 175 at low standard of Fe(III), but for the most part values are close to 2. So, an average value of
3580 Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998
the reactants with Fe(III) and especially with nitrogen organic compounds should be avoided in industrial operation.
Table 4. Catalytic Exchange Capacity and Fictitious Loss of Activity Values for Different Resins Partially Neutralized by Ammonium and the Corresponding Values of Effective Exchange Capacity for This Ion resin
ppm ammonium
CEC (mequiv/g)
L′ (mequiv/g)
nam
Bayer K-2631 Bayer OC-1501 Purolite CT 175 Amberlyst 35
8433 9653 9696 9822
3.40 3.54 3.27 4.06
1.42 1.93 1.71 1.26
3.04 3.60 3.17 2.31
2 can be considered as the effective exchange capacity of Fe(III) for these resins. The results of initial rates for ammonium are significantly lower than for iron and it was found that there is a certain linear relationship between initial rates and the remaining acidic capacities of the resins polluted by ammonium (Figure 5). We can see in Table 2 that resins polluted by ammonium really have lesser effective exchange capacities, from a kinetic standpoint, than the ones obtained by titration. It can be inferred again that ammonium not only neutralizes sulfonic groups but also obstructs the access of the reactants into the polymer mass where there is a high density of active sites. This conclusion can be confirmed after calculating the acidic capacities that resins would have, taking into account the initial rates obtained (catalytic exchange capacity) and analyzing the relationship between the fictitious loss of activity and the quantities of ammonium ion found in them. The fictitious loss of activity (L′) of a resin can be determined as a difference between the acidic capacity of the fresh resin and the catalytic exchange capacity calculated by using the significant linear correlation between initial rates and acidic capacities of fresh resins and polluted by Fe(III). The appropriate expression to calculate the catalytic exchange capacity (CEC) values is
CEC )
r0 - 0.00012 0.0076
(4)
To calculate the effective exchange capacity of ammonium we used the expression:
nam )
(1000)(18)L′ cam
(5)
Results obtained for CEC, L′ and nam are presented in Table 4. It can be seen that the values of effective exchange capacity for ammonium range from 2.31 for Amberlyst 35 to 3.60 for Bayer OC-1501 but an average value of 3 could be considered as the representative one. If we compare both average values of effective exchange capacity for Fe(III) and ammonium, it can be inferred that deactivation by ammonium is surprisingly much higher than by Fe(III). The standard exchange capacity of Fe(III) is 3 and the effective value obtained was only 2. So it can be thought that an important amount of Fe(III) deactivates the resin by ion exchanging but there is a small part of it placed into the resin that does not affect the active centers of the catalyst. On the other hand, the standard exchange capacity of ammonium is 1 and the effective value found was 3. So this ion deactivates the resin not only by neutralization but also by blocking sulfonic groups. So contact of
Conclusions After testing of 12 macroporous resins as catalysts on MTBE synthesis, it has been found that the acidic capacity is the most significant variable for this reaction This conclusion agrees with other similar studies (Krause and Hammarstro¨m, 1987; Panneman and Beenackers, 1995b). It seems that the resins which have greater density of sulfonic groups were the most active because the reaction of MTBE synthesis involves a concerted mechanism where some adjacent sulfonic groups participate. Four resins were partially deactivated using Fe(III) and ammonium ions to evaluate the influence of these ions on the activity of the resins. The results showed that for both ions there is a linear relation between the initial reaction rates and the remaining acidic capacities of the resins, but while iron fulfils the same correlation as the fresh resins, ammonium produces standards of activity much smaller than those produced by Fe(III). It seems that ammonium could provoke steric hindrance due to the formation of hydrogen bonds with sulfonic groups located in outer layers of the polymeric mass of the resins studied. Acknowledgment We thank Repsol Petro´leo S.A. for its financial support and Mr. A. Gonzalez, M. Somoza, and J. A. Gonzalez for their technical support. Nomenclature A ) BET surface area, m2/g A′ ) ISEC surface area, m2/g C ) acidic capacity, mequiv/g cam ) concentration of ammonium in the resin, ppm cFe(III) ) concentration of Fe(III) in the resin, ppm CEC ) catalytic exchange capacity, mequiv.g-1 D ) average pore diameter, Å dp ) approximate value of average particle diameter, mm F ) methanol flow rate, mol.h-1 L ) loss of acidic capacity by Fe(III), meq.g-1 L′ ) loss of acidic capacity by ammonium, L′ ) C - CEC, mequiv‚g-1 nam ) effective exchange capacity for ammonium, eq/at-g ammonium nFe(III) ) effective exchange capacity for Fe(III), eq/at-g iron r ) reaction rate, mol h-1 mequiv-1 r0 ) initial reaction rate, mol h-1 mequiv-1 V ) pore volume, mL/g W ) mass of dry resin, g XM ) methanol conversion, %
Literature Cited Bayer. Product and Operation Information, 1990. Brockwell, H. L.; Sarathy, P. R.; Trotta, R. Synthesize Ethers. Hydrocarbon Process. 1991, 70, 133. Buttersack, C.; Accessibility and Catalytic Activity of Sulfonic Acid Ion-Exchange Resins in Different Solvents. React. Polym. 1988, 10, 143. Chakrabarti, A.; Sharma M. M. Cationic Ion Exchange Resins as Catalyst. React. Polym. 1993, 20, 1. Colombo, F.; Dalloro, L. MTBE Synthesis: Activity of Different Catalysts and Kinetics. Geterog. Katal. 1983, 2, 407.
Ind. Eng. Chem. Res., Vol. 37, No. 9, 1998 3581 Gates, B. C.; Rodriguez, W. General and Specific Acid Catalysis in Sulfonic Acid Resin. J. Catal. 1973, 31, 27. Gicquel, A.; Torck, B. Synthesis of MTBE Catalyzed by IonExchange Resin. Influence of Methanol Concentration and Temperature. J. Catal. 1983, 83, 9. Guyot, A. Syntheses and Separations using Functional Polymers; John Wiley & Sons Ltd.: Chichester, England, 1988; Chapt. 1, p 1. Izquierdo, J.F.; Cunill, F.; Vila, M.; Tejero, J.; Iborra, M. Equilibrium Constants for Methyl tert-Butyl Ether Liquid-Phase Synthesis. J. Chem. Eng. Data 1992, 37, 339. Jera´bek, K. Personal communication, 1997. Jera´bek, K.; Setinek, K. Polymer Matrix Influence on Ion Exchange Resin-Catalyzed Reactions. J. Mol. Catal. 1987, 39, 161. Jera´bek, K.; Hochmann, T.; Prokop, Z. Influence of Polymer Support Morphology on Ion-Exchanger Catalysts Activity in tert-Alkyl Methyl Ethers Synthesis. New Frontiers in Catalysis 1993; Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary; Elsevier Scientific Publishers: Amsterdam, 1993. Kmosta´k, S.; Setı´nek, K. The Effect of Partial Exchange of Sulphonated Macroporous Styrene-Divinylbenzene Copolymers for Different Ions on their Catalytic Activity.Collect. Czechosl. Chem. Commun. 1981, 46, 2354. Krause, A. O. I.; Hammarstro¨m, L. G. Etherification of Isoamylenes with Methanol. Appl. Catal. 1987, 30, 313. Marston, C. R. Improve Etherification Plant Efficiency and Safety. Fuel Reform. 1994, 4, 42. Panneman, H. J.; Beenackers, A. A. C. M. Influence of the Isobutene/Methanol Ratio and of the Methyl tert-Butyl Ether Content on the Reaction Rate of the Synthesis of Methyl tertButyl Ether. Ind. Eng. Chem. Res. 1995a, 34, 3817.
Panneman, H. J.; Beenackers, A. A. C. M. Synthesis of Methyl tert-Butyl Ether Catalyzed by Acidic Ion-Exchange Resins. Influence of the Proton Activity. Ind. Eng. Chem. Res. 1995b, 34, 4318. Parra, D.; Tejero, J.; Cunill, F.; Iborra, M.; Izquierdo, J.F. Kinetic Study of MTBE Liquid-Phase Synthesis using C4 Olefinic Cut. Chem. Eng. Sci. 1994, 49, 4563. Rehfinger, A.; Hoffmann, U. Kinetics of Methyl tert-Butyl Ether Liquid-Phase Synthesis Catalyzed by Ion Exchange ResinsI. Intrinsic Rate Expresion in Liquid-Phase Activities.Chem. Eng. Sci. 1990, 45, 1605. Rodriguez, O.; Setinek, K. Dependence of Etherification Rates on Cross-Linking of Ion Exchange Resins Used as Solid Catalysts. J. Catal. 1975, 39, 449. Subramanian, C.; Bathia, S. Liquid-Phase Synthesis of MTBE Catalyzed by Ion Exchange Resin. Can. J. Chem. Eng. 1987, 65, 613. Tejero, J.; Cunill, F.; Izquierdo, J. F.; Iborra, M.; Fite´, C.; Parra, D. Scope and Limitations of Mechanistic Inferences from Kinetic Studies on Acidic Macroporous Resins. The MTBE Liquid-Phase Synthesis Case. Appl. Catal. 1996, 134, 21. Voloch, M.; Ladisch, M. R.; Tsao, G. T. Methyl tert-butyl Ether (MTBE) Process Catalyst Parameters. React. Polym. 1986, 4, 91. Widdecke, H. Syntheses and Separations using Functional Polymers; John Wiley & Sons Ltd.; Chichester, England, 1988; Chapt. 4, p 149.
Received for review January 7, 1998 Revised manuscript received June 8, 1998 Accepted June 10, 1998 IE980007D
