Preparation of isobutylene

Abstract: A method for the preparation of isobutylene substantially free from tertiary butyl formate and peroxides from a tertiary butyl alcohol feedstock contaminated with tertiary butyl formate and peroxides wherein the feedstock is continuously brought into contact with a bed of a catalyst consisting essentially of sulfuric acid impregnated alumina.

Description: This invention relates to a method for the preparation of isobutylene substantially free from tertiary butyl formate and peroxides from a tertiary butyl alcohol (TBA) feedstock contaminated with tertiary butyl formate, peroxides and ketones. In the process, the contaminated tertiary butyl alcohol feedstock is charged to a reactor and contacted therein with a bed of catalyst comprising sulfuric acid impregnated alumina. Preferred reaction conditions include a temperature of 150° to 400°C., a pressure of 0 to 2.1 x 10⁷ Pa gauge pressure (gp) (3000 psig). The reaction product is substantially completely free from tertiary butyl formate. The reaction is preferably carried out in a continuous manner at a feed rate of 0.5 to 10g of feedstock per hour per cc of catalyst. It is known to react isobutane with oxygen, either thermally or catalytically, to form a peroxidation reaction product wherein the principal peroxide that is formed is tertiary butyl hydroperoxide. It is also known to thermally or catalytically decompose the tertiary butyl hydroperoxide to form tertiary butyl alcohol. Thus, it is known that isobutane can be oxidized with molecular oxygen to form a corresponding tertiary butyl hydroperoxide and by-products and that the reaction can be promoted, for example, with an oxidation catalyst (see Johnson U. S. Patent No. 3,825,605 and Worrell U. S. Patent No. 4,296,263). A process for the manufacture of substituted epoxides from alpha olefins such as propylene is disclosed in Kollar U. S. Patent No. 3,351,635 which teaches that an organic peroxide compound such as tertiary butyl hydroperoxide can be reacted with an olefinically unsaturated compound such as propylene in the presence of a soluble molybdenum catalyst. The products of the reaction are propylene oxide and tertiary butyl alcohol. Thus, tertiary butyl alcohol can be prepared either by the direct thermal or catalytic reduction of tertiary butyl hydroperoxide to tertiary butyl alcohol or by the catalytic reaction of propylene with tertiary butyl hydroperoxide to provide propylene oxide and tertiary butyl alcohol. When isobutane is reacted with oxygen, a wide variety of oxidation by-products are also formed in small amounts including, for example, methyl formate, acetone, 2-butanone, isobutylene oxide, isobutyraldehyde, methanol, methyl tertiary butyl peroxide, isopropyl alcohol, tertiary butyl alcohol, ditertiary butyl peroxide, tertiary isopropyl peroxide and tertiary butyl formate. These oxidation by-products will remain with the tertiary butyl alcohol product that is formed by the direct or indirect decomposition of the tertiary butyl hydroperoxide. Sanderson et al. U. S. Patent No. 4,873,380 discloses, for example, that the peroxide-type contaminants can be removed from tertiary butyl alcohol by bringing the contaminated tertiary butyl alcohol into contact with a nickel, copper, chromium and barium catalyst. The resultant tertiary butyl alcohol product is substantially free from the other peroxide contaminants. Processes for the preparation of tertiary butyl alcohol from tertiary butyl hydroperoxide are disclosed, for example, in a series of Sanderson et al. patents (U. S. Patents Nos. 4,910,349; 4,912,266; 4,912,267; 4,922,033; 4,922,034; 4,922,035; 4,992,036; 4,992,602; and 5,025,113). It is also known that tertiary butyl alcohol can be dehydrated to form isobutylene. For example, Grane et al. U. S. Patent No. 3,665,048 discloses a process for obtaining essentially pure isobutylene by the controlled dehydration of tertiary butyl alcohol in the presence of an aluminum oxide catalyst. European Patent Application 0255948, filed May 8, 1987, by Mitsubishi Rayon Company, Ltd., in the names of Kazutaka Inoue et al. discloses a process for the production of isobutylene by the dehydration of tertiary butyl alcohol in gas phase over a fixed bed-type of silica alumina catalyst in the presence of added nonreactive gas to inhibit isobutylene polymer formation. The dehydration of tertiary butyl alcohol to isobutylene is also disclosed in an article by Heath et al. ("Acid Resin Catalysis: The Dehydration of t-butyl Alcohol", AICHE Journal (Vol. 18, No. 2, March 1972, pp. 321-326)). The prior art references directed to the dehydration of tertiary butyl alcohol to form isobutylene are not directed to the problem that is involved by the presence of tertiary butyl formate as a contaminant which is the case when the tertiary butyl alcohol is formed directly or indirectly from tertiary butyl hydroperoxide. Thus, the tertiary butyl formate will normally be present in the tertiary butyl alcohol feedstock and will also be present in the isobutylene reaction product as a contaminant. The prior art processes use essentially pure tertiary butyl alcohol as a product. If the isobutylene is reacted with methanol to provide methyl tertiary butyl ether, a motor fuel additive, the tertiary butyl formate and peroxides will be present in the methyl tertiary butyl ether motor fuel additive and would be very deleterious insofar as the octane enhancing qualities of the methyl tertiary butyl ether are concerned. Therefore, removal of the tertiary butyl formate and peroxides is very important when tertiary butyl alcohol is to be dehydrated to form isobutylene for use in the preparation of methyl tertiary butyl ether. An object of the present invention is to prepare isobutylene substantially free from tertiary butyl formate and peroxides from a contaminated TBA feedstock. The feedstock of the present invention comprises a tertiary butyl alcohol contaminated with tertiary butyl formate, such as a tertiary butyl alcohol product obtained by the thermal or catalytic decomposition of tertiary butyl hydroperoxide. It has been discovered in accordance with the present invention that when tertiary butyl alcohol is dehydrated to isobutylene over the sulfuric acid impregnated alumina, a surprising and unexpected result is obtained in that the tertiary butyl formate and peroxides are substantially completely converted to decomposition products so that the isobutylene that is recovered from the reactor is substantially completely free from tertiary butyl formate and peroxides, and no significant amount of oligomeric isobutylene product is formed. The catalyst to be used in accordance with the present invention comprises a sulfuric acid impregnated alumina catalyst such as a Porocel® alumina impregnated with from about 0.5 to about 5 wt.% of sulfuric acid. Activated bauxite impregnated with 0.5 to 5 wt.% of sulfuric acid is an example of another sulfuric acid impregnated alumina catalyst. The process of the present invention is preferably conducted on a continuous basis using a fixed bed of the sulfuric acid impregnated alumina catalyst. Thus, in accordance with the present invention, a tertiary butyl alcohol (TBA) feedstock contaminated with tertiary butyl formate and peroxides is charged to a reactor comprising a fixed bed of sulfuric acid impregnated alumina (e.g., Porocel® alumina). Suitably, the tertiary butyl formates contaminated tertiary butyl alcohol is charged to the reactor at the rate of 0.5 to 5g of tertiary butyl alcohol feedstock per hour per cc of catalyst and, more preferably, at the rate of 0.5 to 3g of contaminated tertiary butyl alcohol feedstock per cc of catalyst per hour. Reaction conditions within the reactor include suitably a temperature of 150° to 400°C and, more preferably, from 170° to 306°C and a pressure of 0 to 2.1 x 10⁷ Pa gauge pressure (gp) (3000 psig) preferably as a pressure of 0 to 1.05 x 10⁷ gp (1500 psig). The invention will be further illustrated by the following non-limiting specific examples. EXAMPLES Examples 1-6 A 300 cc electrically heated, stainless steel tubular down-flow reactor which had an inside diameter of 2.54cm (1 inch) was used. Crude TBA feed was pumped into the top of the reactor at WHSV of 1.0 g/hr-cc catalyst and atmospheric pressure. The products were analyzed by gas chromatograph. Engelhard's sulfuric acid on R. I. Porocel® alumina, sulfuric acid on H. A. Porocel® alumina and activated Baurite® were examined and the results are listed in Table I. Dehydration of TBA Example Catalyst Temp (°C) TBA (%) TBF (%) DTBP (%) Isobutylene Selec. (%) 98.9 0.432 0.057 - 1 None 200 95.4 0.410 0.056 - 2 None 220 94.8 0.400 0.039 - 3 Porocel 200 <1 0.003 - 97 4 Porocel 220 <1 - - 96 5 Activated Baurite® 200 <1 - - 99 6 Porocel 200 <1 - - 99 The porocel alumina that is treated with sulfuric acid will be composed primarily of alumina and other metal oxides in minor amounts such as iron oxide, titania and silica. However, the alumina will constitute about 70% to 90% of the support. Example 7 A 300 cc electrically heated, stainless steel tubular up-flow reactor which had an inside diameter of 2.54cm (1 inch) was used. Crude TBA feed was pumped into the bottom of the reactor at WHSV of 1.0 g/hr-cc catalyst, 5.6 x 10⁶ Pa gp (800 psig) and 230°C. Engelhard's surfuric acid on H. A. Porocel 20/60 was used. The reaction was run for 200 hours. About 95% of TBA conversion with 98% of isobutylene selectivity was obtained. Both TBF and DTBP was decomposed effectively. No catalyst deactivation was noted during the run.

Claims: A method for the preparation of isobutylene substantially free from tertiary butyl formate from a tertiary butyl alcohol feedstock contaminated with tertiary butyl formate which comprises charging a said feedstock to a reactor and contacting it therein with a bed of a catalyst comprising sulfuric acid impregnated alumina to form a reaction product substantially completely free from tertiary butyl formate. A method as claimed in Claim 1 wherein the catalyst consists of an alumina support impregnated with from 0.5 to 5 wt.% of sulfuric acid, based on the weight of the support. A method as claimed in Claim 2 wherein the catalyst support is a porous support comprising 70 to about 90 wt.% of alumina, the balance of the support comprising metal oxides. A method as claimed in any one of Claims 1 to 3 wherein the reaction conditions comprise a temperature of 150° to 400°C and a pressure of 0 to 2.1 x 10⁷ Pa gauge pressure (3000 psig). A method as claimed in any one of Claims 1 to 4 wherein the method is continuous. A method as claimed in Claim 5 wherein the feed rate is 0.5 to 10g of feedstock per hour per cc of catalyst. A method as claimed in Claim 5 wherein the tertiary butyl alcohol feedstock is brought into contact with the sulfuric acid impregnated catalyst under reaction conditions including a temperature of 170° to 300°C and a pressure of 0 to 1.05 x 10⁷ Pa gauge pressure (1500 psig) at the rate of about 0.5 to about 3.0 g of feedstock per hour per cc of catalyst.

Inventors: SMITH WILLIAM ALAN, SU WEI-YANG, TOOLOIAN ROYA, SMITH, WILLIAM ALAN, SU, WEI-YANG, TOOLOIAN, ROYA
Applicants: TEXACO CHEMICAL, TEXACO CHEMICAL COMPANY
Application year: 1992
Main classification: C07C11
IPC codes: C07C11, C07B61, B01J27, C07C1