4318
Ind. Eng. Chem. Res. 1995,34, 4318-4325
Synthesis of Methyl tert-Butyl Ether Catalyzed by Acidic Ion-Exchange Resins. Influence of the Proton Activity Henk-Jan Panneman and Antonie A. C. M. Beenackers* Department of Chemical Engineering, University of Groningen, Ncenborgh 4, 9747 AG Groningen, The Netherlands
The catalytic activity of various strong acid ion-exchange resins on the synthesis of methyl tertbutyl ether (MtBE) from methanol and isobutene has been investigated. Relative to Amberlyst 15, Kastel CS 381 and Amberlyst CSP have similar rate constants, whereas Duolite ES 276 and Amberlyst XE 307 have sigmficantly higher and Duolite C26 and Duolite C16P substantially lower rate constants. All resins show a great decrease in catalytic activity if part of the protons is exchanged by sodium ions. At 10% proton capacity the rate constants per equivalent acid are reduced by a factor of 9 (for Amberlyst XE 307 and Kastel CS 381) to more than a factor 20 for Amberlyst 15 and Duolite ES 276, resulting in 100-200 times lower MtBE production rates. Depending on the catalyst applied, mass transfer limitations start to occur between 50 and 80 and 4.1 "C. Values of the effective diffusion coefficient of isobutene varied between 0.4 x x m2 s-l at 80 "C.
Introduction The synthesis of methyl tert-butyl ether (MtBE) from isobutene and methanol catalyzed by acidic ion-exchange resins has gained spectacular commercial interest over the past decade. In light of these interests, the lack of information at least in the open literature, on the effects of catalytic activity as influenced by catalyst type, acid concentration, and solvent effects is surprising. In most experimental studies of the synthesis of MtBE, Amberlyst 15 was applied as the catalyst (Ancillotti et al., 1977, 1978; Giquel and Torck, 1983; AlJarallah et al., 1988; Subramaniam and Bhatia, 1987; Rehfinger and Hoffmann, 1990a). The catalytic activity of three commercial macroporous sulfonic acid resins, Amberlyst 15, Kastel C 350 P, and Lewatit SPC 118, has been compared by Colombo and Dalloro (1983). They found no significant difference in activity with respect to the synthesis of MtBE. However, they used an industrial C4 mixture, containing both bases and metals, resulting in a substantial loss of acidity. Below, we prove that the rate constants are very sensitive to a change in acidity. So, differences in reaction rates between various resins can be overwhelmed by a loss of acidity which might have a much stronger effect. Beasley and Jakovac (1984) reported about ionexchange resin catalysts having improved catalytic activity and enhanced thermal stability. Amberlite XE 383 reportedly gave higher conversions than Amberlyst 15, especially at low temperatures. Neither rate constants nor activation parameters were given. Rehfinger and Hoffmann (1990a) did experiments with both Amberlyst 15 and a self-prepared strong acidic macroporous resin. The latter was mainly used to study mass transfer effects on the rate of MtBE formation. Petrus (1982)experimentally determined the catalytic activity of strong acidic ion-exchange resins with respect to the hydration of isobutene. Petrus found significant differences in the activation parameters of Amberlyst 15, Amberlyst XE 307, and Imac C201P, which all are macroporous resins. Especially macroporous sulfonic acid ion-exchange resins are suitable for acid-catalyzed reactions such as
the MtBE synthesis. The permanent porosity of these resins does not require a strong polar solvent to promote swelling of a gellular resin. Organic binary mixtures often give rise to higher acidities than water, which can improve the catalytic activity of the resin. All industrially applied heterogeneous catalysts lose activity with time, due to both traces of metals and basic compounds in the C4-feed stock and too high temperatures during the reaction. Little is known about the decrease in rate constant for the MtBE synthesis due to a decrease in active groups of the catalyst and whether this decrease is solvent mixture and/or resin dependent. Ancillotti et al. (1978) were the first to determine the initial rate dependence on the acid concentration. They observed apparent third-order kinetics in the acid concentration in Amberlyst 15. Further, they concluded the reaction mechanism to be independent of acid concentration and the proton activity to follow a Hammett-type of acidity function, which means that the proton activity is not proportional to the acid molar concentration. Prokop and Setinek (1980) studied the synthesis of MtBE with partially exchanged resins, using various transition metals. They found that the partially exchanged resins gave higher yields of MtBE. These results, however, are dificult to interpret, because of the use of an electrical-heated packed-bed reactor, without heat removal. The conversions strongly decreased at higher temperatures, while the same flow rate was used. Probably, hot spots existed in this reactor, and the conversions were close t o equilibrium. With a partially exchanged resin the temperature rise in the reactor is less, giving a higher equilibrium conversion. Rehfinger and Hoffmann (1990a) found no influence of the degree of cross-linking, the internal surface area, and the exchange capacity of the resin on the rate of reaction. Here, we report initial reaction rate constants computed from small steady-state conversions in a packedbed reactor, operated between 20 and 80 "C, for several macroporous ion-exchange resins, and it appears that significant differences exist between the resins examined. We will show that the catalytic activity depends
0888-588519512634-4318$09.00/00 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 12, 1995 4319 on the three-dimensional structure of the resin, which is influenced by the level of cross-linking, the porosity, and the internal surface area and by the sulfonic acid content and its distribution within the particles. Further, we report initial rate constants of the MtBE formation as a function of feed composition for a number of commercially available macroporous sulfonic acid ionexchange resins. It appears that the energies of activation change with feed composition and are resin dependent. We can attribute these changes to resindependent changes in Gibbs energy of the activated complex. Finally, the dependence of the initial rate constants on the remaining hydrogen exchange capacity of various partially neutralized resins is reported. Also here, large differences between various macroporous resins appear to exist. Experiments with catalysts a t full hydrogen capacity showed mass transfer limitations above 70 "C. By taking both external and internal mass transfer limitations into account, the effectiveness factor could be computed and effective diffusion coefficients in various resins could be obtained.
Theory We have shown (Panneman and Beenackers, 1995) that the MtBE synthesis can be described by a pseudohomogeneous model with first-order reversible reaction kinetics in isobutene and MtBE. The mechanism consists of the protonation of the olefin by methanolsolvated protons, followed by the reaction of the carbonium ion with methanol, available in large excess in the resin pores. The formation rate of MtBE for the reaction mixtures applied is (Panneman and Beenackers, 1995): -?'IB
= rMt& = k*CH+CIB
- k%H+cMtBE
(1)
and
Values of K, were computed from the equilibrium constant based on mole fractions (Panneman and Beenackers, 1995). The initial rate constants were computed from the performance equation of a packed-bed reactor with firstorder reversible reaction (Panneman and Beenackers, 1995):
At each temperature the isobutene conversion was determined for at least four volumetric flow rates. The initial concentrations were calculated from the volumetric flow rates, densities, and the temperature. Small conversions kept the variation in mixture composition as small as possible and allowed us to compute initial rate constants. These experimentally obtained apparent rate constants, k", can be used to compute intrinsic rate constants, k*, which are free from both heat and mass transfer effects (Panneman and Beenackers, 1995). Apparent rate constants a t temperatures between 20 and 50 "C were fitted according to both the Arrhenius equation:
k* = k, exp(-EAIR13
(4)
and the Eyring equation (Connors, 1990; Panneman and Beenackers, 1995):
k* =
h
exp(AS*lR)exp(-&lRT)
(5)
Ion-ExchangeResin. We investigated only macroporous styrene-divinylbenzene-based ion-exchange resins with respect to their reactivity toward the MtBE synthesis. Their catalytic properties depend on the level of dross-linking (determining the porosity and internal surface area), the sulfonic acid content and distribution, and the possible substitution of another functional group. Within the particles there might be an inhomogeneity in the cross-linking and the sulfonic acid distribution. Therefore, no uniform acidity exists in a resin particle, and differences are still possible between chemically identical macroporous resins. Below, we try t o explain the experimentally observed differences in activation parameters of various resins, in terms of differences in resin structure and variations in intraparticle distribution of the sulfonic acid groups. Experimental Section Equipment. The experiments were performed in a packed-bed reactor, immersed in a thermostated water bath. Initial forward reaction rate constants were computed from the small stationary conversion a t the reactor outlet; at least three different flow rates were used at each temperature. For further details on equipment used and the analytical techniques applied, see Panneman and Beenackers (1995) and Marsman et al. (19891, respectively. Reagents. Isobutene, reagent grade, was supplied by Matheson, Gent, Belgium. Methanol and MtBE are both reagent-grade and were obtained from Janssen Chimica. The resins were converted into the acidic form by standard procedures. The catalysts were sieve-analyzed, and the different sieve fractions were separated. To prepare ion exchangers with protons partially substituted by sodium ions, we took the resin in the acid form with an exchange capacity known from titration, allowed it t o swell in water, and added to it a calculated amount of sodium chloride. The system was allowed to stand for 24 h to reach equilibrium, and the ion exchanger was then washed with distilled water and dried at 75 "C and reduced pressure. The remaining hydrogen capacity was determined by accurate titration. It is assumed that the remaining protons will realize the energetically most favorable distribution (the Gibbs energy will be minimal). Dried resin particles bursted if a strong polar liquid such as water or methanol was added. To avoid this problem, the catalyst particles were first wetted in 2-propanol. Then the propanol was displaced by methanol. All but one of the catalysts applied are commercially available sulfonated styrene-divinylbenzene (DVB) copolymers. Amberlyst XE 307, not commercially available (Rohm and Haas), was used because of its high thermal stability (150 "C), caused by chlorine in the polymer matrix. All catalysts were macroreticular. The available manufacturers information on chemical and physical properties of the resins is rather limited.
4320 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 Table 1. Physical Properties of Three Types of Amberlyst Resins 1010 15 surface area, m2 g-1 55 570 average macropore 25 8 diameter, nm 1.4 skeletal density, g cm-3 1.53 80 percentage cross-linking, % 20 macropore porosity 0.32-0.36 0.47 mean microparticle 71-87 7-8 diameter, nm 1.8 exchange capacity, mequiv mL-1
80
60
40
2OoC
3.0
3.2
3.4
-11
XE307 28
-13
0.24
-15
1.2
Table 2. Experimentally Determined Total Exchange Capacity of the Resins Applied (Particle Sieve Fraction 0.71-0.81 mm, Unless Otherwise Noted) exchange capacity u resin manufacturing company (equiv kg-l) 4.6 Amberlyst 15 Rohm & Haas, U.S.A. 4.3 Amberlyst CSP Rohm & Haas, U.S.A. Rohm & Haas, U.S.A. 4.4 Amberlyst XE 307 4.2 Kastel CS 381 Ausino, Italy Duolite C26 Diamond Shamrock, U.S.A. 4.8 Diamond Shamrock, U.S.A. Duolite C16P 5.0 Duolite ES 276 Diamond Shamrock, U.S.A. 5.1 Duolite ES 276, Diamond Shamrock. U.S.A. 4.2 d p = 0.25-0.4 mm
More detailed information is available for Amberlyst 15 (O'Connor et al., 1985) and to some extent also for Amberlyst XE 307 (Kunin, 1973); see Table 1. For a good comparison also the data of the very high crosslinked Amberlyst 1010 are shown. The higher surface area and macropore porosity are caused by a higher percentage of cross-linking. Comparison of data of Amberlyst 15 and Amberlyst 1010 shows that an increase in surface area gives a decrease of the mean microparticle diameter with the same factor. From Table 1 we can see that the surface area and the macropore porosity of XE 307 are considerably lower than those of Amberlyst 15. From the data presented in Table 1 and from an elemental analysis of XE 307, it appears that XE 307 is a macroporous resin with a low cross-linking percentage ( 4 0 % DVB),which possesses quite large gellular microparticles and a large average macropore diameter. The total hydrogen exchange capacity of the dried resins was determined by titration. The results are shown in Table 2.
Results and Discussion The more prospective resins were selected from experiments with 2 mol % isobutene and 98 mol % methanol (about 0.5 kmol m-3 isobutene). The reaction rate constants were measured between 20 and 80 "C for resin particles of 0.71 .c d, .c 0.81 mm. Figure 1shows the results for the Duolites C26, C16P, and ES 276 and for the Amberlysts XE 307 and CSP. The points are the experimentally determined apparent rate constants. The results below 60 "C were fitted according to the Arrhenius equation and are represented by the lines. Figure 1 shows that above 60 "C the apparent rate constants become significantly lower than the computed intrinsic rate constants. Because of the large excess of methanol in the feed and of the large dipole moment of methanol, the swelling of the resin particles will be maximal, all sulfonic acid groups will be dissociated, and the protons
-17
2.8
I/T
( 1 0 .K'I) ~
Figure 1. Apparent and intrinsic rate constants for the MtBE formation rate in a mixture of 2 mol % isobutene and 98 mol % methanol. Symbols: apparent rate constants from experiments. Lines: intrinsic rate constants, fitted between 20 and 50 "C, extrapolated to higher temperatures.
-
-10
E
-12
3
A
0.25
