Production d'isobutène de haute pureté par décomposition du MTBE
High-Purity Isobutene Production from Mtbe
Institut Français du Pétrole
La décomposition du MTBE en isobutène et méthanol s'accompagne de réactions secondaires (oligomérisation de l'isobutène, hydratation de l'isobutène, déshydratation du méthanol). Les différents types de catalyseurs utilisés, les mécanismes et les cinétiques suggérés, ainsi que les sites actifs et les espèces adsorbées proposés dans la littérature sont examinés dans le cas de la réaction principale, et des réactions secondaires. La formulation du catalyseur et la nature des sites actifs (Brönsted, Lewis) ont une incidence particulière sur la réaction. Les données de la littérature portent essentiellement sur des catalyseurs de type résines présentant une acidité de Brönsted. Sur catalyseurs de type oxydes il apparaît que les sites acides de Lewis, catalysent la réaction principale, tandis que les réactions secondaires sont essentiellement dues à la présence d'acidité de Brönsted. Un contrôle de l'acidité des formulations catalytiques est nécessaire afin de minimiser les réactions secondaires, et de produire de l'isobutène très pur.
Abstract
Under suitable conditions, methyl-tert-butyl ether (MTBE) is decomposed into isobutene (C4H8) and methanol (CH3OH). This decomposition is a reversible endothermic chemical reaction ((***) = 15. 6 kcal/mol in the gas phase). When this reaction is situated downstream from MTBE synthesis from a C4 cut, this results in the separation of the different isomers in this cut by a less costly method than the one now used, which consists of concentrated sulfuric-acid extraction. The isobutene obtained by MTBE decomposition is very pure and meets the specifications required for subsequent polymerization into butyl rubber or methyl methacrylate. The MTBE decomposition reaction is accompanied by secondary reactions such as the oligomerization of isobutene (mainly the formation of dimers), the dehydration of methanol into dimethylether, and the hydration of isobutene into tert-butyl alcohol. MTBE decomposition is catalyzed by solids with an acid nature. It has mainly been examined on catalysts of the sulfonic-resin type, but solid acid catalysts have recently appeared (zeolites, silica-alumina, supported phosphoric acid, etc. ). The different types of catalysts used are examined for the principal reaction and secondary reactions. The formulation of the catalyst and the nature of the active acid sites (Brönsted or Lewis) have great influence on the reaction. Data from the literature mainly concern catalysts of the resin type with Brönsted acidity. Concerning catalysts of the oxide type, mention is made of Lewis acid sites catalyzing the principal reaction. The species adsorbed, the mechanisms and kinetic investigations of MTBE decomposition have mainly been examined for sulfonic resins. The most probable mechanisms (mechanism B, page 371) is the following one :(a) Ether adsorption on a double center without dissociation. (b) Surface reaction between adsorbed ether and a free active center, to give rise to isobutene adsorbed on a double center without dissociation and methanol adsorbed on a single center. This stage is the one that limits the process from the kinetic standpoint. (c) Desorption of isobutene and methanol. The corresponding rate equation is given in Table III, and the adsorbed species are given on page . For solid acid catalysts, few data concerning the kinetics are available in the literature. A single equation (Eq. 4, page 371), which was determined experimentally on a gamma-AI2O3 catalyst modified on the surface by silica, is proposed. On a gamma-AI2O3 catalyst, the inhibiting influence of water has been shown for high contents. Secondary reactions are mainly due to the presence of Brönsted acidity. Indeed, the dimerization and trimerization of isobutene involve mechanisms that necessitate going via a carbonium-ion intermediary on Brönsted acid sites (mechanisms on pages 372 and 377). Likewise, the dehydration of methanol is enhanced by the presence of Bronsted acid sites with the participation of basic sites. But some authors note an influence or participation of Lewis acid sites during the dimerization of isobutene on TiO2 or the dehydration of methanol. Dehydration occurring on resins or an oxide catalyst is inhibited by the presence of water. On oxides the alkoxide species with surface CH3O- is revealed to be the adsorbed species. A check must be made of both the preparation and acidity of catalytic formulations to minimize secondary reactions and to produce very pure isobutene.
© IFP, 1991