DE2031921B2 - Process for the production of isoprene by dehydrating 3-methyl-3-buten-1-ol - Google Patents

Description

to 220 and J. Org. Chem. USSR 3 [1967] [8] p. 1328). According to these known processes, the reaction proceeds very slowly and with undesirable effects Side reactions, such as hydration and isomerization of 3-methyl-3-buten-l-ol or decomposition of the Alcohol in isobutene and formaldehyde.

The acids for the new process are monobasic or polybasic, inorganic or organic acids suitable which have a dissociation constant of at least 1 · 10 "in water, such as hydrochloric acid, Hydrobromic acid, hydriodic acid, hydrofluoric acid, sulfuric acid, phosphoric acid, Formic acid, acetic acid, p-toluenesulfonic acid, trichloroacetic acid, Oxalic acid, maleic acid, or ion exchange resins that contain sulfonic acid groups.

Substances that form acids in an aqueous medium are, for. B. aluminum trichloride, boron trifluoride, titanium tetrachloride or acyl halides such as acetyl chloride.

Mixtures of the acids and / or the acid-releasing substances can also be used. For economic For reasons, one will prefer the direct use of cheap acids.

Since the process according to the invention is carried out in the presence of water, the acids are expediently used from the outset as aqueous solutions. The concentration of the aqueous acids can vary within wide limits and is preferably between 0.01 and 40 percent by weight. Very strong acids, ie acids with dissociation constants of more than 10 " ¹ , are advantageously used in concentrations of 0.01 to 20, in particular 0.05 to 10 percent by weight, and acids with dissociation constants of less than 10 ^-1 , advantageously in concentrations of 1 to 40 , preferably 5 to 30 percent by weight, but the process can also be carried out outside the acid concentration limits specified here.

In general, when higher temperatures are used, low acid concentrations are preferred, and vice versa. For example, it may be useful when the temperature increases by, for. B. 30 ° C to reduce the acid concentration by a factor of 0.2 to 0.1.

Suitable salts are water-soluble salts which are derived from acids, such as those above To be defined. This means that the salts of acids with dissociation constants of at least Derive 1 · 10 ". The selection of the salt is to be made in such a way that that in cases where no hydrohalic acid is used, a salt is used derived from a hydrohalic acid. Suitable salts are, for example, sodium chloride, Potassium bromide, calcium iodide, sodium sulfate, sodium phosphate, calcium chloride, aluminum sulfate, Magnesium sulfate, zinc chloride and sodium oxalate. Since you can use all salts with about equally good success can - provided they are combined with the appropriate acid - are economic considerations suggest the use of cheap salts such as table salt, calcium chloride or Glauber's salt. It Mixtures of the salts can also be used. For example, seawater is a suitable medium to carry out the reaction.

The concentration of the salts in the aqueous dehydration medium becomes one upwards limit by ihfe solubility at the reaction temperature. But you can also use this upper limit exceed the dilution of the solution by the water produced during dehydration to counteract. Since the reaction rate decreases with increasing dilution, the The salt content in the aqueous acidic medium does not fall below 3 percent by weight, although this is also the case Area the dehydration versus the absence of salts by a factor of more than 100 is accelerated.

The dehydration is carried out according to the new process at temperatures between 40 and 100.degree by. It is expedient to choose temperatures which correspond at least to the boiling point of isoprene. It can be carried out at reduced or increased pressure. Preferably one takes the reaction at normal pressure between 60 and 100 ° C before.

To increase the yield, it is advantageous in many cases to add diluents to the reaction mixture. Thinners are in the frame of this invention under the conditions of dehydration inert liquid organic compounds, the expediently boil in the temperature range in which the dehydration is carried out. For example The following may be mentioned as diluents: hydrocarbons such as gasoline fractions in the boiling range of 50 up to 120 ° C, benzene, toluene, xylene, cyclohexane and methylcyclohexane. An addition of this has proven itself Diluents in amounts of 5 to 100 percent by weight, based on the acidic salt solution.

The new process for the production of isoprene can be carried out batchwise. Advantageous however, you run it continuously. According to a particularly expedient continuous procedure the 3-methyl-3-buten-l-ol is charged into the with the acidic aqueous salt solution and on the Reaction temperature brought reaction vessel with stirring to the extent in which the formed Isoprene can be removed from a fractionation column attached to the top of the reaction vessel can. The water released during dehydration is also continuously removed from the acidic salt solution, for example by distillation, removed. A consumption of the in the reaction vessel remaining catalyst consisting of the acidic aqueous salt solution does not occur or only occurs to a small extent.

Isoprene is an important monomer for the production of polymers and a valuable intermediate product z. B. for the production of terpenes.

The parts mentioned in the example are parts by weight. They relate to parts of volume like kilograms Liter.

example

We obtained the results given in the table in the following test arrangement:

A cylindrical reaction vessel provided with a stirrer and having a capacity of 4 parts by volume is charged with the acidic salt solution, which is prepared by mixing salt, acid and water. The mixture is heated with stirring at 90 to 95 ⁰ C. The reaction vessel is provided with a at 40 to 45 ⁰ C and a reflux condenser connected thereto operated descending condenser, in which condensing the resultant isoprene. In experiment No. 8, in addition to the acidic salt solution, 0.5 volume

parts methylcyclohexane used. Experiment No. 9 is a comparative experiment in which in the absence is worked by salt.

3-Methyl-3-buten-1-ol is added continuously over the specified period. After completion the methylbutenol feed becomes complete

the reaction was held at reaction temperature for a further 2 hours. The amount of isoprene that is produced on average already has a purity of 94 to 96% and only something that has not condensed in the dephlegmator Contains methylbutenol, was determined by gas chromatography.

Volume parts
water Parts acid Parts
salt methyl
Parts ) utenol
admitted
in hours Parts
Isoprene Parts attempt
No. 0.6 0.007 hydrochloric acid, 35% 0.3 CaCl ₂ 1 3 0.72 heavier
boiling
Next to
Products 1 0.6 0.04 p-toluenesulfonic acid 0.3 CaCl ₂ 1 2 0.70 0.1 2 1.2 0.2 oxalic acid 0.6 NaCl 5 10 3.7 0.12 3 1.2 0.68 phosphoric acid, 0.4 NaCl 4th 12th 2.9 0.4 4th 85% 0.3 1.2 0.06 hydrochloric acid, 35% 0.5NH ₄ Cl 2 3 1.5 5 1.2 0.36 hydrochloric acid, 35% 0.4 Na ₃ PO ₄ 4th 9 3.8 0.15 6th 1.2 0.08 phosphoric acid, 0.4 NaBr 2 8th 1.5 0.36 7th 85% 0.15 1 0.06 phosphoric acid, 0.8 NaCl 5 11 3.8 8th 85% 0.43 0.6 0.007 hydrochloric acid, 35% - 1 3 0.001 9 0.5 0.01 hydrochloric acid, 35% 0.25 1 8th 0.74 - 10 NH ₄ Cl 0.05 1.0 0.02 boron trifluoride 0.5 NaCl 2 15th 1.45 11 etherate BF ₃ O (C ₂ H ₅ ) ₂ 0.12 0.6 0.4 glacial acetic acid 0.3 NaCl 0.425 20th 0.31 12th 0.025

Claims (2)

1 2 according to the dehydration of 2-methyl-3-butene claims: 2-oI and mixtures of 2- and 3-methyl-3-buten-2-ol when using magnesium sulfate as de- 1. Process for the preparation of isoprene by hydration catalyst Isoprene yields of dehydration of 3-methyl-3-buten-l-ol with 5 95% of theory are achieved, although very good acids, characterized in that isoprene yields are known. The starting materials to be used according to this Verman 3-methyl-3-buten-l-ol at temperatures can be between 40 and 100 ⁰ C with the acids or with, however, only in a cumbersome way in little host substances, the acids in the aqueous medium form, are produced socially. in the presence of water and salts and ίο Next is in British Patent 846 847 if necessary treated with diluents, the dehydration of 3-methyl-l-buten-3-ol under where the acids or the acids forming the use of a salt of a weak aromatic Substances and the salts are chosen so that either the base and a strong acid are described. This However, the anion of the acid or the anion of the salt or process differs significantly from the both anions are halides. 15 The method of the present invention in which a 2. The method according to claim 1, characterized in that another starting material is used and characterized in that the dehydration is carried out in the case of Nor- dehydration in the presence of an aqueous painting pressure and temperatures between 60 and a solution of acids or of substances in aqueous 100 ⁰ C makes. Medium form acids and carried out by salts so becomes, whereby one forms the acids or the acids Chooses substances and salts so that either that Anion of the acid or the anion of the salt or both anions are halides. The 3-methyl to be used according to the British patent There are already several processes for producing as buten-l-ol-3 is a tertiary alcohol with an allyl known from isoprene. This is how the diolefin is placed in a permanent hydroxyl group. It is against it large scale by dehydrogenation of isopentene in the process according to the invention or isopentane. It was also the starting material 3-methyl-3-butene to be used propose isoprene by dimerizing propylene l-ol around a primary alcohol without allylic and subsequent elimination of methane from the methyl hydroxyl group. It is common knowledge that to manufacture penten. Furthermore, isoprene can be obtained from iso- teitiary alcohol with allylic hydroxyl butene and formaldehyde via the stage of the 4,4-di- group easily using an acid catalyst win away methyl-1,3-dioxane. Isoprene allows sators to dehydrate (see E. S. Gould, also through dehydration of 2-methyl mechanism and structure in organic chemistry, 3-buten-2-ol, which is obtained from acetone, acetylene and 35 Verlag Chemie GmbH, Weinheim-Bergstr., 1962, Can produce hydrogen. The mentioned p. 563, lines 16 to 19). In contrast, the Processes are unsatisfactory in many ways. For the use of 3-methyl-2-buten-l-ol no acid In part, the starting materials used are only to be expected in catalyzed dehydration, as it is limited amount or only in a laborious way with this starting material to a primary alcohol available, the reaction to isoprene 40 is also, the hydroxyl group of which is not through a unselective or with an insufficiently high yield. a-vinyl group is activated. Based on the stand In addition, the insufficient purity of the technology (cf. E. S. Gould, Mechanism of the produced isoprene a deficiency, which is ins- and structure in organic chemistry, publishing house especially in polymerizations with organometallic chemistry GmbH, Weinheim-Bergstr., 1962, p. 568 bis Catalysts has a detrimental effect. 45 571) would have at most an alkali-catalyzed elimination It has now been found that isoprene can be considered by an approximate reaction. Dehydration of 3-methyl-3-buten-l-ol with Also British patent 846 847, in which the Can produce acids particularly advantageous when dehydrating olefinic alcohols too if the 3-methyl-3-buten-l-ol is described at temperatures diolefins in the liquid phase, between 40 and 100 ° C with the acids or with 50 does not affect the dehydration of 3-methyl- Substances that form acids in an aqueous medium, in 3-buten-l-ol, but from alcohols like that Presence of water and salts and optionally 3-methyl-1-buten-1-ol activated by a «vinyl group treated by diluents, one being the fourth. For dehydration, according to Acids or the acid-forming substances and British Patent 846 847 as catalysts Salts so choose that either the anion of the acid 55 strong mineral acids or corresponding salts or or the anion of the salt or both anions halo- other known dehydration catalysts too are genius. use. However, it is not from this patent specification The inventive method is known from, the dehydration in the presence of a a starting material which, according to the German patent, is permeable to an aqueous solution of an acid and a salt Supply 1,275,049 by reacting isobutene and form-60. aldehyde in a simple and economical way. It was surprising that isoprene is available after the is, isoprene of high purity technically according to the invention in such excellent processes easily obtained with almost quantitative yield. Yields is obtained because it is from the literature From the publication in J. Am. Soc. 63 (1941), known that 3-methyl-3-buten-l-ol with both P. 2756, right column, penultimate paragraph, according to which 65 acids in the liquid phase as well as in the gas the dehydration of 3-methyl-l-buten-ol-3 Ausphase over fixed bed catalysts is very difficult and yields of 88% of theory of isoprene obtained can be converted into isoprene with poor yields and U.S. Patent 2,967,897, where- (cf. Petroleum Chemistry USSR 4 [1965], p. 211