The conversion of ethers to their corresponding alkenes and alkanols is an important reaction in a number of commercial processes. Thus, for example, this reaction is used to remove ethers, such as isopropyl ether, produced as the by-products of other processes, such as the hydration of propylene to produce isopropanol. In addition, an important route for the production of tertiary olefins involves reaction of mixed olefins with an alcohol over an acid catalyst to selectively produce a tertiary alkyl ether, separation of the ether from the remaining olefin stream, and then decomposition of the ether to the desired tertiary olefin. This latter process relies on the fact that tertiary olefins react with alcohols more rapidly than either secondary or primary olefins and hence provides an effective method for extracting tertiary olefins, such as isobutene and isoamylene, from a mixed olefin stream. For the purposes of this invention, a tertiary olefin or isoolefin will be understood to be an olefin containing at least one carbon atom that is covalently bonded to three other carbon atoms.
One commercial process for the selective decomposition of ethers, such as methyl tert-butyl ether (MTBE), is disclosed in U.S. Pat. No. 4,691,073 and employs a fluoride-treated clay, such as hydrofluoric acid (HF) treated attapulgite (HFA), as the catalyst. The process is typically operated at a starting temperature of about 340° F. (170° C.) but, since the catalyst loses its activity rapidly, run times are normally very short and the temperature has to be increased during the run to a final value of about 380° F. (193° C.) in order to maintain a constant MTBE conversion, typically around 90%. In fact, the cycle length of the HFA catalyst normally ranges from only a few weeks to 30+ days, which is a major disadvantage in that the loss of catalyst activity results in considerable losses in production time and leads to high catalyst replacement and disposal costs. Moreover, the relatively high temperatures required by the HFA catalyst tends to increase the concentration of impurities such as dimethyl ether (DME) and isobutane in the product, as well as promoting side reactions, for example, diisobutylene dehydrocyclization and isobutene oligomerization and polymerization, that lead to fouling of the catalyst.
Other solid acids have been proposed for the selective decomposition of tert-alkyl ethers to tertiary olefins. For example, U.S. Pat. No. 4,254,290 describes the use of solid acids such as SiO2/Al2O3, WO3/Al2O3, H2SO4-treated clay and acidic ion-exchange resins as catalysts for the decomposition of tert-alkyl ether alkanols. In U.S. Pat. Nos. 4,320,232 and 4,521,638, phosphoric acid on various supports is described as a catalyst suitable for the decomposition of tert-butyl alkyl ethers to isobutene and alcohols. The use of silica supported aluminum compounds as catalysts for the decomposition of alkyl tert-alkyl ethers is described in U.S. Pat. No. 4,398,051, whereas intermediate pore zeolites, such as ZSM-5 are employed for this purpose in U.S. Pat. No. 4,357,147.
An extensive discussion of catalysts for, and the mechanism of, the conversion of MTBE to isobutene is provided in an article entitled “Production D'Isobutene de Haute Puretépar Décomposition du MTBE” by P. B. Meunier et al. in Revue de L'Institut Francais du Petrole, vol. 46, No. 3, May 19991, pages 361 to 387. This document mentions the use of sulfonic resins, supported phosphoric acid, zeolites, silico-aluminas and modified silico-aluminas as catalysts for MTBE decomposition. According to this document, side-reactions can be limited by controlling the surface of the catalyst, its activity and the presence of impurities that can increase or decrease the catalyst acidity.
It is also known from, for example, U.S. Pat. No. 5,254,785, to employ calcium-exchanged zeolite Y as a catalyst in the conversion of dialkyl ethers to olefins. However, although pilot plant studies indicated that this catalyst would have a significantly lower aging rate than the HFA catalyst, the improved performance of the Ca—Y catalyst has to date never been achievable on a commercial scale.
U.S. Pat. No. 5,177,301 describes a two-step method for separating isobutylene from a C4 hydrocarbon fraction comprising (a) contacting the C4 fraction containing isobutylene with a glycol in the presence of a catalyst comprising a heteropoly acid on an inert support at a temperature of about 60° C. to 160° C. thereby reacting the isobutylene with the glycol to yield a glycol mono-t-butyl ether, and subsequently (b) reacting the glycol mono-t-butyl ether over the heteropoly acid on an inert support at a temperature of 150° C. to 220° C. to produce the separated isobutylene. Suitable heteropoly acids include 12-tungstophosphoric acid, 12-molybdophosphoric acid, molybdosilicic acid and 12-tungstosilicic acid on an inert support, such as silica, alumina, titania and zirconia.
U.S. Pat. No. 5,171,920 describes a process for obtaining a tertiary olefin, e.g. isobutylene, by decomposing the corresponding ether, e.g. methyl tert-butyl ether, in the presence of a catalyst comprising a silica support modified by the addition of at least one element or selected from the group constituted by rubidium, cesium, magnesium, calcium, strontium, barium, gallium, lanthanum, cerium, praseodymium, neodymium and uranium and optionally by the addition of at least one element selected from the group constituted by aluminum, titanium and zirconium. Modification of the silica support is effected by impregnating the support with at least one aqueous solution (or a solution in at least one appropriate solvent) containing the modifying element or elements it is desired to introduce.
Japanese Published Patent Application No. JP-A-06072904, published Mar. 15, 1994, describes a process for obtaining a tertiary olefin by decomposing the corresponding alkyl tert-alkyl ether over a catalyst composition having the formula SiaAlbZrcXdOe where X is an element selected from sodium, potassium, cesium, cerium, zinc, magnesium and calcium; a, b, c, d and e are the atomic ratios of their respective elements and when a is 1, b is 0.01–1, c is 0.001–1, d is 0.001–1 and e designates the number of oxygen atoms necessary to satisfy the valence of the other components.
In addition, Japanese Published Patent Application No. JP-A-59010528, published Jan. 20, 1984, describes a process for thermally decomposing a tertiary ether to a tertiary olefin in the presence of a titanium or zirconium oxide catalyst containing 0.1 to 20 wt % of SO4 groups. The catalyst activity is said to be high even at low temperatures thereby allowing co-production of the corresponding alcohol with negligible etherification.
It has now been found that certain mixed metal oxides comprising at least one metal from Group 4 of the Periodic Table of Elements, at least one metal from Group 3 (including the Lanthanides and Actinides) and Group 6 of the Periodic Table of Elements, and optionally at least one metal from Groups 7, 8, and 11 of the Periodic Table of Elements exhibit both high selectivity and long catalyst lifetime when used as ether decomposition catalysts.
U.S. Pat. No. 5,607,892 discloses a zirconium/cerium mixed oxide having a specific surface area of greater than 10 m2/g. The mixed oxide is produced by intimately admixing a zirconium sol with a cerium sol, wherein the ratio of the mean diameter r1 of the particles of the zirconium sol to the mean diameter r2 of the particles of the cerium sol is at least 5, adding a precipitating amount of a base, such as aqueous ammonia, sodium hydroxide, or potassium hydroxide to the mixture, recovering the precipitate thus formed and calcining the precipitate at a temperature of 700 to 1,000° C. The mixed oxide is said to be useful as a catalyst or catalyst support for carrying out a variety of reactions, such as dehydration, hydrosulfurization, hydrodenitrification, desulfurization, hydrodesulfurization, dehydrohalogenation, reforming, steam reforming, cracking, hydrocracking, hydrogenation, dehydrogenation, isomerization, dismutation, oxychlorination, dehydrocyclization of hydrocarbons or other organic compounds, oxidation and/or reduction reactions, the Claus reaction, treatment of exhaust gases emanating from internal combustion engines, demetallation, methanation or shift conversion.
U.S. Pat. No. 6,150,299 discloses a cerium- and zirconium-based mixed oxide containing sulfur, which is said to be active as an exhaust gas purification catalyst and which comprises 50 to 79% by weight cerium oxide, 20 to 49% by weight zirconium oxide and 1 to 5% by weight sulfate (SO4). In Example 1, the mixed oxide was produced by dispersing cerous sodium sulfate double salt (containing 75 g as cerium oxide) in 1,000 g of water and adding an aqueous solution of zirconium nitrate (containing 25 g as zirconium oxide). Then, an aqueous solution of sodium hydroxide was added until the pH of the mixture became 13.5, whereby a precipitate was obtained. This precipitate was separated from the mixture and heated in the air at 600° C. for 5 hours. Analysis showed the resultant mixed oxide to contain 73.9% by weight cerium oxide, 24.1% by weight zirconium oxide and 2.0% by weight sulfate.
International Patent Publication No. WO 03/37506, published May 8, 2003, discloses a promoter or catalyst support for an automobile exhaust gas system comprising a zirconium-cerium-based mixed oxide produced by reacting an alkali with an aqueous solution of a zirconium salt containing 0.42–0.7 mole of sulfate anion per mole of zirconium cation at a temperature not greater than 50° C. in the presence of a cerium salt to form a mixed cerium-zirconium hydroxide and then calcining the hydroxide at a temperature of 500 to 1000° C., such as 650 to 850° C.
U.S. Pat. No. 6,124,232 discloses a tungsten-modified zirconia catalyst produced by coprecipitating zirconia with an anion or oxyanion of tungsten in the presence of ammonium sulfate to obtain a sulfate-containing product, steaming the sulfate-containing product; recovering the sulfate-containing product by filtration, washing the product with water in order to remove the sulfate ions and calcining the product to produce a catalyst that is essentially free of sulfate ions. The catalyst is said to be active in the isomerization of paraffins.
U.S. Pat. No. 6,162,757 discloses a synthesis of a solid acid containing zirconium, in addition to a rare earth element, such as cerium, useful for isomerization of paraffins, ring opening of cyclics, hydrocracking, alkylation, hydrogenation of polynuclear aromatics, selective catalytic reduction of nitrogen peroxides, and oligomerization of light olefins.
U.S. Pat. No. 6,297,406 discloses a process for producing phenol and acetone from cumene hydroperoxide, in which cumene hydroperoxide is contact with a solid acid catalyst comprising a mixed oxide of cerium and a Group IVB metal.