I n d . E n g . Chem. Res. 1989, 28, 1269-1277
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KINETICS AND CATALYSIS Vapor-Phase Addition of Methanol to Isobutene on a Macroporous Resin. A Kinetic Study Javier Tejero,* Fidel Cunill, and Jose F. Izquierdo Chemical Engineering Department, University of Barcelona, Mart; i Franquds 1 , 08028 Barcelona, Spain T h e kinetics of the vapor-phase addition of methanol to isobutene to give methyl tert-butyl ether (MTBE) on the ion-exchange resin Amberlyst 15 has been studied. Rate data were obtained in a continuous differential reactor operated at atmospheric pressure and 41-61.5 O C . T h e best-fitting Langmuir-Hinshelwood-Hougen-Watson (LHHW) rate equation is derived from a mechanism whose rate-determining step is the reaction between the methanol adsorbed molecularly on one center and the isobutene adsorbed on two centers. This mechanism is thermodynamically consistent. MTBE, a good octane booster (Reynolds et al., 1975; Taniguchi and Johnson, 1979; Talbot, 1979; Csikos et al., 1980; Tejero et al., 1987), is formed by addition of methanol to isobutene on acid catalysts. The reaction is reversible and fairly exothermic. In industry, the reaction for obtaining MTBE could be of interest as a means for the quantitative separation of isobutene from 1-butene of C4 cuts (Clementi et al., 1979; Convers et al., 1981; Fattore et al., 1981). MTBE is obtained in the liquid phase below 100 "C and 14 atm with yields of up to 99% on sulfonicion-exchange resins (mainly Amberlyst 15 and Lewatit SPC 118). A t the selected reaction conditions, all C4's other than isobutene are inert. Recovery of isobutene could be accomplished by the vapor-phase decomposition of MTBE. In spite of the industrial interest of the reaction, literature about MTBE synthesis is scarce. Some attention has been paid to the kinetics of the reaction in the liquid phase on Amberlyst 15. Gicquel and Torck (1983) reported the strong dependence of the rate of MTBE synthesis and MTBE decomposition on methanol concentration at 50-95 "C. Ancillotti et al. (1977), moreover, at 60-80 "C showed a zero-order dependence of the rate on methanol concentration for concentrations greater than 4 mol.L-', with negative orders at lower concentrations, a first-order dependence of the rate on isobutene, and a strong dependence, about third order, of the rate on acid group concentration. On the basis of previous work, an ionic mechanism has been proposed by Ancillotti et al. (1978). The reaction for obtaining MTBE in the vapor phase, however, has been the subject of little work. This work has been devoted to testing the selectivity of the reaction. High selectivity was obtained by Moore and O'Donnell (1951) at 13 atm and 200 "C on a KOH-MgO catalyst, by Igarashi et al. (1979) at 1 atm and 90-130 "C on heteropoly acids, namely, 12-molybdosilicic and 12-molybdophosphoric acids, supported on silica gel, and by Chu and Kuhl (1987) at 82-93 "C on ZSM-5 and ZSM-11 zeolites. However, the conversions to MTBE reported are low because the equilibrium is not favorable a t temperatures higher than 100 "C (Tejero et al., 1988). At temperatures up to 100 "C, ion-exchange resins are effective catalysts. Thus, Setinek et al. (1977) obtained good yields at 1 atm
and 85 "C, as well as Baba et al. (1985) at 50 "C, maintaining a good selectivity. Although nowadays vapor-phase synthesis of MTBE has little industrial interest, MTBE decomposition in the vapor phase is really interesting for the recovery of pure isobutene. MTBE synthesis can occur on ion-exchange resins at milder conditions than on metallic oxides. Therefore, in this work, we have studied the vapor-phase addition of methanol to isobutene on Amberlyst 15 a t atmospheric pressure and 61.5 "C in order to round off our study of the MTBE decomposition a t 41-61.5 OC (Cunill et al., 1987) and to determine a Langmuir-Hinshelwood-HougenWatson (LHHW) kinetic equation that represents the whole reaction (forward and reverse).
Experimental Section Materials. The sulfonic-ion-exchange resin Amberlyst 15 (Rohm and Haas Co.) was used as the catalyst. This is a macroporous copolymer of styrene-divinylbenzene containing 20% divinylbenzene with a surface area (BET method) of 43 f 1 m2-g-', a mean pore diameter of 240 A, and an exchange capacity determined by titration against standard base of 4.8 mequiv of HSO3.g-' of dry resin. Methanol (Scharlau, Barcelona), with a minimum purity of 99% containing less than 0.1% water, and isobutene (SEO, Barcelona), with a minimum purity of 99%, were used without further purification and fed into the reactor diluted in nitrogen to obtain the highest possible variation of their partial pressures, as well as to facilitate evaporation and transport of the methanol. Nitrogen (SEO, Barcelona), with a minimum purity of 99.998%, was used. Apparatus. The experiments were carried out in a tubular fixed bed microreactor (downflow) preceded by a feed supply for each of the gases and the liquid and followed by a flowmeter and a sampling and analysis system. Nitrogen and isobutene were fed independently into the vaporizer from pressure bottles through a flow control valve and an orifice meter. Methanol was pumped through a rotameter and a flow control valve to the vaporizer. In the electrically heated vaporizer, packed with metallic Raschig rings, the mixture of nitrogen, isobutene, and vaporized methanol was homogenized and from there passed to the reactor. The reaction chamber was a jack-
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1270 Ind. Eng. Chem. Res., Vol. 28, No. 9, 1989
eted Pyrex tube (length, 45 cm; internal diameter, 1.2 cm) with a porous plate to hold the catalyst bed. Water containing propylene glycol (30-50 vol % ) was pumped from a thermostatic bath to the reactor jacket to keep the catalyst bed isothermal. The inside-outside temperature gradient of the reactor chamber was measured by means of two platinum resistance thermometers, the first one inserted axially in the center of the catalyst bed and the second one located in the reactor jacket close to the catalyst bed. In the temperature range explored, the temperature gradients were not higher than 0.4 "C. Moreover, the absence of axial temperature profiles was checked by ranging the height of the thermometer in the bed. A sampling valve at the reactor outlet allowed the analysis of the reactor effluent by means of an HP-5830A gas chromatograph equipped with a heat conductivity sensor. Analysis. The sample valve injected 0.5 cm3 of product stream into the gas chromatograph. Helium (SEO, Barcelona), with a minimum purity of 99.998%, was used as carrier gas at a flow rate of 30 cm3.min-'. A 3-m X 3.2mm-0.d. stainless steel GLC column packed with Chromosorb 101 (80/lOO mesh) was used. The column temperature was held a t 110 "C for 4 min, increased at a rate of 15 "C-min-l up to 220 "C, and held for 2 min. The column allowed the separation of isobutene, methanol, MTBE, water, and possible reaction byproducts, namely, diisobutene, dimethyl ether, and tert-butyl alcohol. However, in the kinetic runs, no byproducts were detected in the effluent of the reactor. Procedure. The catalyst was crushed, sieved, dried at 110 "C for 12-14 h, and stored in a desiccator over concentrated sulfuric acid (98 wt % ) for a t least 2 weeks. A water content in the dried resin of less than 2.9 wt % (1 mol of H 2 0 / 3 mol of sulfonic groups) was determinated by Karl Fischer titration. Surface area and exchange capacity measurements of the crushed resin gave the same values as those of the commercial particles. Catalyst samples (0.1 g) with a particle diameter between 0.063 and 0.1 mm were selected to avoid the internal mass-transport resistance in the macroporous phase on the reaction rate. The catalyst was diluted in quartz (1vol of catalyst/5 vol of quartz) in order to obtain a catalytic bed of sufficient length to guarantee a good contact pattern between reactants and catalyst, avoiding backmixing and channeling, and to make it isothermal. The catalytic bed prepared in this manner (length 2 cm) was preheated in the reactor for 2 h a t 110 "C in a stream of nitrogen at 10 cm3-s-l. As a result, the water content in the resin was approximately reduced to 1.4 wt % (1mol of H20/6 mol of sulfonic groups) according to Buttersack et al. (1987). Differential conversions (
