Ind. Eng. Chem. Res. 2000, 39, 1235-1241
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Conversion, Selectivity, and Kinetics of the Addition of Isopropanol to Isobutene Catalyzed by a Macroporous Ion-Exchange Resin Fidel Cunill,* Montserrat Iborra, Carles Fite´ , Javier Tejero, and Jose´ -Felipe Izquierdo Departament d’Enginyeria Quı´mica i Metalurgia, Universitat de Barcelona, C/Martı´ i Franque` s, 1, E-08028 Barcelona, Spain
Equilibrium conversion, selectivity, and kinetics of the liquid-phase synthesis of isopropyl tertbutyl ether (IPTBE) from 2-propanol and isobutene were studied experimentally over a commercial ion-exchange resin in the temperature range 303-353 K at 1.6 MPa. The isobutene equilibrium conversion hardly changes upon varying the initial molar ratio of 2-propanol to isobutene. The IPTBE yield is very high and is independent of temperature for an initial molar ratio for 2-propanol to isobutene greater than 2. The best kinetic model stems from a mechanism in which 2-propanol, adsorbed on one center, reacts either with isobutene adsorbed on one adjacent center or with isobutene from solution to give the ether adsorbed on one center. The surface reaction is the rate-limiting step, in which three centers take part. An apparent activation energy of 75.5 kJ‚mol-1 was obtained. Isobutene Dimerization
Introduction Tertiary ethers such as methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME) are being used in gasolines to reduce evaporative and tailpipe emissions and as octane boosters. However, its growing demand, particularly for MTBE, the lack of cheap isobutene for MTBE and ETBE production, and the dependence of MTBE and TAME on methanol have promoted the evaluation of other ethers. Isopropyl tert-butyl ether (IPTBE) is a complete refinery-based ether because the source of oxygen is water. Furthermore, from a technical and emissions reduction point of view, IPTBE properties are slightly better than those of ethers mentioned above.1 On the other hand, IPTBE has a lower volatility and water solubility than diethyl ether and diisopropyl ether, which accounts for its potential use as a solvent in the chemical and pharmaceutical industries. Despite this, to date interest toward it has been only academic. Only a very few papers can be found in the literature concerning the etherification of isobutene with 2-propanol in the liquid phase.1-3 IPTBE can be prepared by adding 2-propanol to isobutene over an acidic exchange resin at operating temperatures between 40 and 80 °C and a pressure of about 1.5 MPa, enough to maintain the liquid phase.
(CH3)2CHOH + (CH3)2CdCH2 f (CH3)3 COCH(CH3)2 The reaction is less favored thermodynamically than the related reactions of MTBE and ETBE synthesis. As a consequence of the rather low isobutene equilibrium conversion, side reactions can easily take place, thus reducing the selectivity to IPTBE. The main side reactions involve * To whom correspondence should be addressed. Tel.: +3493-4021304.Fax: +34-93-4021291.E-mail:
[email protected].
2 isobutene f 2,4,4-trimethyl 1-pentene (DIS) 2 isobutene f 2,4,4-trimethyl 2-pentene (DIS) Isobutene Hydration isobutene + water f tert-butanol (TBA) Isopropanol Dehydration 2 isopropanol f diisopropyl ether (DIPE)+ water Etherification of 1-Butene 1-butene + isopropanol f isopropyl sec-butyl ether (IPSBE) Isopropanol Dehydration isopropanol f propene + water Decomposition of IPTBE IPTBE f TBA + propene The aim of the paper is to study the effect of the initial molar ratio of isopropanol to isobutene (R0A/O) and temperature (T) on isobutene conversion (XIB), on IPTBE yield (YIPTBE/IB), and on IPTBE selectivity with respect to isobutene dimers (SIPTBE/DIS). The influence of R0A/O on the initial reaction rate of IPTBE formation 0 (rIPTBE ) is studied. A kinetic model in terms of component activities for the commercial catalyst used is proposed to be used in designing the reactor. The apparent activation energy is also determinated. Experimental Section (i) Materials. 2-Propanol was supplied by Romil Chemicals Ltd. (Shepshed, England),with a purity of 99.8%, and stored over 3-Å molecular sieves before use. Isobutene was obtained from SEO (Barcelona, Spain) and used without further purification. The main impurities of isobutene were isobutane and linear butenes,
10.1021/ie990315r CCC: $19.00 © 2000 American Chemical Society Published on Web 03/31/2000
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Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000
which do not react under the investigated reaction conditions. IPTBE (99.8% pure, GC) was prepared in our laboratory. tert-Butyl alcohol (>99%) was supplied by Merck-Schuchardt (Hohenbrunn, Germany). Isobutene dimers and DIPE were supplied by Fluka. All these byproducts were used for analysis. The catalyst was the commercial ion-exchange resin Bayer K2631 from Bayer AG (Leverkusen, Germany). The degree of cross-linking is 15-25 wt % DVB and its ion-exchange capacity determined by titration is 4.8 equiv of H+ kg-1 of dry resin. The average bead size is 0.73 mm. Before use the catalyst was dried at 378 K for about 14 h and the residual water content, titrated by the Karl-Fisher method, was less than 3 wt %. (ii) Apparatus. The experiments were carried out in a stainless steel jacketed 300-mL autoclave operating in batch mode. The reaction medium was agitated at 500 rpm by a magnetic-drive turbine and mixing baffles on the reactor walls were used to improve the mixing. To maintain the reacting mixture in the liquid phase over the whole temperature range, the pressure was set at 1.6 MPa by means of N2. (iii) Analysis. The composition of the reacting mixture was analyzed by injecting 0.2 µL of pressurized liquid using a split mode operation into a gas chromatograph (HP5890A) equipped with a FID detector. A 50 m × 0.2 mm × 0.5 µm methyl silicone capillary column was used to separate and determine 2-propanol, isobutene, IPTBE, TBA, DIPE, and isobutene dimers. The column was temperature-programmed with a 7-min initial hold at 313 K followed by a 20 K‚min-1 ramp up to 433 K and was held there for 10 min. Helium was the carrier gas, whose flow rate was 30 mL‚ min-1. (iv) Procedure. A calculated amount of 2-propanol and 0.5-10 g of dry resin were first charged into the reactor and, after looking for leakages, heated to the desired temperature. Then, the corresponding amount of isobutene liquid was volumetrically measured at 0.8 MPa in a buret and charged into the reactor using nitrogen as the carrier, and the pressure was set to 1.6 MPa. The time of introduction of the isobutene into the reactor was taken as the starting point for the reaction. The catalyst loading ranged from 1 to 10 wt % of the liquid mixture so that the reaction rate was suitable for following the concentration variation of the chemicals with time. The reaction was allowed to reach the chemical equilibrium for IPTBE synthesis, which was checked by periodically removing samples until a quasistationary composition was obtained. For more detailed information about the Experimental Section, see Calderon et al.1 Results and Discussion (i) Isobutene Equilibrium Conversion. A set of experiments was carried out in the temperature range 303-353 K with R0A/O ranging from 1.0 to 4.25. Figure 1 shows the isobutene equilibrium conversions obtained. As can be seen, the isobutene equilibrium conversions decreased as the temperature increased, as expected for an exothermic reaction1,4 (∆H° ) -22.9 and -25.5 kJ‚mol-1). The effect of the R0A/O is rather unexpected because the isobutene equilibrium conversion does not increase as R0A/O increases according to LeChatelier’s principle, even it seems that isobutene equilibrium conversion is maximum for R0A/O ) 2. An explanation of this experimental fact can be found in the nonideal
Figure 1. Isobutene equilibrium conversion versus temperature at different R0A/O values. Table 1. Isobutene Experimental Equilibrium Conversion and Kγ Values at 343 K R0A/O
Kγ
XIB
1.03 1.97 3.0 4.25
0.594 0.877 1.04 1.11
0.55 0.57 0.57 0.56
behavior of the system. Thus, if we consider that the thermodynamic equilibrium constant for a liquid-phase reaction of a nonideal system is given by
Ka ) KxKγ )
xIPTBE γIPTBE xIPAxIB γIPAγIB
(1)
the isobutene equilibrium conversion, if there is no ether initially and there are no byproducts, will be given by the positive solution of the equation
XIB2 - (1 + R0A/O)XIB +
Ka R0 ) 0 Ka + Kγ A/O
(2)
So, this way XIB not only depends on R0A/O and on Ka, of course, but on Kγ also. When R0A/O is increased and the UNIFAC predictions are considered, Kγ also increases and, as a result, when eq 2 is solved, the XIB value hardly changes. Table 1 shows the Kγ and experimental XIB values for different values of R0A/O at 343 K. Considering that isobutene equilibrium conversion is much lower than those of MTBE and ETBE, IPTBE synthesis becomes an ideal candidate to take place industrially by catalytic distillation. The formed IPTBE would be removed from the reaction zone, thus suppressing the reverse reaction and allowing higher IPTBE conversions.5 (ii) IPTBE Yield and Selectivities. The effect of R0A/O on IPTBE yield, defined as moles of IPTBE generated per mole of isobutene consumed, at different temperatures is shown in Figure 2. As can be seen, the IPTBE yield is very high and that for R0A/O > 2 is independent of temperature. Thus, in the range of R0A/O’s and temperatures explored the majority of reacted isobutene is converted to IPTBE and the presence of all byproducts is
