Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2000-10-11
2002-02-26
Richter, Johann (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S852000, C568S861000
Reexamination Certificate
active
06350924
ABSTRACT:
The invention relates to a process for preparing 1,4-butanediol plus if desired tetrahydrofuran (THF) and y&ggr;butyrolactone (GBL) from waste gases from the oxidation of butane and/or benzene by absorbing maleic anhydride (MA) with high-boiling solvents, esterifying the maleic anhydride with an alcohol to form the diester, and catalytically hydrogenating the maleic diester.
1,4-Butanediol is an important starting material in preparing polyesters, such as polybutylene terephthalate, and also &ggr;-butyrolactone and THF. Therefore, there are a large number of known processes for preparing 1,4-butanediol and mixtures comprising it from MA obtained by catalytic oxidation of hydrocarbons.
MA, for instance, can be prepared by gas phase oxidation of benzene or n-butane. Along with the MA the gaseous oxidation product generally includes water, oxygen, nitrogen, carbon dioxide, unreacted benzene or butane, and small amounts of formic, acetic, acrylic and propionic acids. These are accompanied by oxidation products of the impurities in the oxidation substrates, such as isobutane. Examples of these products are citraconic anhydride, furan, acrolein, crotonaldehyde, crotonic acid and methacrolein.
Since industry generally requires 1,4-butanediol with purities of more than 99.9%, it has to date been common to achieve this by purifying the MA before subjecting it, for example, to hydrogenation in ester form to give 1,4-butanediol.
WO 97/43242 describes a process for preparing 1,4-butanediol, &ggr;-butyrolactone and THF without pre-purifying the maleic anhydride, which is obtained by catalytic oxidation from benzene, mixed C
4
-olefinic compounds or n-butane.
In this process there is an absorption zone in which MA from an MA-comprising vapor stream is brought into contact with an organic solvent whose boiling point is at least 30° C. higher than that of the maleic diester. A waste gas stream is retained, and the MA in the high-boiling solvent is reacted with an appropriate C
1
-C
4
alcohol under esterifying conditions in an esterification zone to form the diester. The resultant diester is stripped from the organic solvent with a stream of hydrogen and is hydrogenated in the gas phase over a heterogeneous hydrogenation catalyst. The products of value, 1,4-butanediol, &ggr;-butyrolactone and THF, can be recovered from the hydrogenation product.
This process entails certain disadvantages. When the maleic diester is stripped from the high-boiling solvent it is accompanied by relatively low-boiling compounds originating from impurities in the maleic anhydride, and by solvent traces. These components pass into the hydrogenation phase, with the consequence that the separation method required following hydrogenation is higher.
Furthermore, stripping must be carried out at the same pressure as the gas phase hydrogenation. This requires the circulation of a considerable stream of hydrogen (in the example of WO 97/43242, 320 mol of hydrogen to each mole of diester). The high stripping temperatures which occur may cause cracking processes to take place.
Further disadvantages arise in the gas phase hydrogenation. For instance, the temperature of the gas stream must be kept above the dew point of the maleic diester. This requires a considerable hydrogen stream, and the recycling of the excess hydrogen entails high energy costs. The short residence time during the gas phase hydrogenation leads to the formation of by-products and thus to a reduction in yield and an increase in the separation method at the distillation stage. The catalyst loadings (space velocities) possible in the gas phase hydrogenation of maleic diesters are low: otherwise, the adiabatic temperature increase over the reactor becomes too great and leads to a reduction in selectivity. Countering this by raising the amount of hydrogen, although possible, is uneconomic owing to the high costs of circulated gas.
It is an object of the present invention to provide a process for preparing 1,4-butanediol and, if desired, &ggr;-butyrolactone and THF which produces 1,4-butanediol in a good yield and high purity without the need to purify the maleic anhydride beforehand and which is simple and cost-effective.
We have found that this object is achieved by a process for preparing 1,4-butanediol and, if desired, &ggr;-butyrolactone and THF by oxidizing n-butane or benzene to form a product stream including maleic anhydride, absorbing maleic anhydride from the product stream with a high-boiling inert solvent in an absorption stage to give a liquid absorption product, esterifying the liquid absorption product with a C
1
-C
5
esterifying alcohol in an esterification stage to form an esterification product comprising the corresponding diester and high-boiling inert solvent, then hydrogenating the esterification product to give a hydrogenation product which comprises the products of value, 1,4-butanediol and, if desired, &ggr;-butyrolactone and THF and which is separated by distillation into the products of value and the esterifying alcohol, and recycling the esterifying alcohol to the esterification zone. The process of the invention then comprises separating the esterification product into the diester and the inert solvent by distillation under reduced pressure prior to the hydrogenation, recycling the inert solvent to the absorption stage, and hydrogenating the diester in the liquid phase over a fixed-bed catalyst.
This process enables the obtention from maleic anhydride of the products of value, of 1,4-butanediol and, if desired, &ggr;-butyrolactone and THF, in cost-effective manner and in good yield in a few simple steps. Because of the distillative purification of the esterification product prior to hydrogenation, the products of value can be obtained in high purity following the hydrogenation. Liquid rather than gas phase hydrogenation allows high catalyst loadings, further increasing the economy of the process of the invention.
Following the distillation of the hydrogenation product it is possible by virtue of the process of the invention to obtain 1,4-butanediol in a purity of more than 99.9 mol %, preferably at least 99.95 mol %.
The highly pure 1,4-butanediol recovered is out-standingly useful as a starting material for preparing polyesters such as polybutylene terephthalate and for preparing &ggr;-butyrolactone and THF.
The preparation of the maleic anhydride takes place by oxidizing benzene or n-butane, generally in the gas phase over a vanadium oxide based catalyst. The process can be carried out exactly as described in WO 97/43242, or similarly. The partial oxidation of benzene is customarily performed using a supported vanadium pentoxide catalyst which is activated, for example, with MoO
3
. The reaction temperature is from 400 to 455° C. and the reaction pressure from about 1 bar to about 3 bar. The amount of air used is about four times that dictated by theory, so as not to exceed the explosion limits. The contact time is approximately 0.1 second.
If n-butane is used as starting material, it is common to use vanadium pentoxide as catalyst at a temperature from about 350 to about 450° C. and a pressure from about 1 bar to about 3 bar. If a suitable reactor is used the ratio of air to n-butanol can be about 20:1, despite the fact that per se this ratio leads to a mixture which is easily ignited.
The result is a gaseous oxidation product which generally comprises not only maleic anhydride but also water, oxygen, nitrogen, carbon dioxide, unreacted benzene or n-butane and fairly small amounts of organic impurities such as formic, acetic, acrylic and propionic acids. There may also be oxidation products of impurities from the starting materials in the oxidation product.
The absorption of the maleic anhydride contained in the gaseous oxidation product from the oxidation of benzene or n-butane can take place in an absorption stage in exactly the manner described in WO 97/43242, or similarly, using a high-boiling inert organic solvent. For this purpose the MA-comprising gaseous oxidation product is brought into contact with a high-boiling solv
Fischer Rolf
Kaibel Gerd
Pinkos Rolf
Rahn Ralf-Thomas
BASF - Aktiengesellschaft
Keil & Weinkauf
Price Elvis O.
Richter Johann
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