Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
1999-08-11
2001-03-13
Dentz, Bernard (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C549S509000, C528S417000
Reexamination Certificate
active
06201137
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for purifying crude tetrahydrofurans such as tetrahydrofuran or alkyl tetrahydrofurans. The purified tetrahydrofurans (hereinafter shortened to THF) obtained by this invention are suitable to use as starting materials in the preparation of a polyether polyols. Such polyether polyols are very important because these materials are used in the preparation of polyurethane or polyester for use in elastomers or spantex. Polyurethane resins obtained by the co-polymerization of alkyl THF and THF exhibit superior elastic properties, resistance to lower temperature and hydrolysis, and consequently, are industrially useful substances in the chemical industry.
2. Description of the Prior Art
Many methods have been proposed for the purification of THF. For example, through the use of bleaching earth, Japanese Patent Publication 61-54029 obtained THF having a low carbonyl, bromine and peroxide content. There is, however, neither a stated objective of purifying nor disclosure directed to any kind of impurity. The removal of another THF impurity, THF peroxide, has been described in Japanese Patent Laid Open Publication 54-88256, by treatment with an activated carbon layer.
Further, aromatic aldehydes are an impurity in crude THF. A method to obtain THF from which aldehydes, such as n-buthylaldehyde, are removed is disclosed in Japanese Patent Publication 52-29. It involves a distillation following treatment with a primary amine having a high boiling point and non-volatile acid. These latter disclosed methods may be characterized as targeting a specified impurity, however, the procedure by which THF before purifying is prepared and the usage of the purified THF are not disclosed. Still further, when a primary amine of a high boiling point and non volatile acid are used alone, obtaining a purified THF would not be expected. Indeed, as disclosed in the aforesaid Japanese patent publication, they must be used together in order.
Aldehydes are also an impurity in THF that must be removed. For example, a method to obtain THF having less than 50 ppm aldehyde impurity is disclosed in Japanese Patent Laid open publication 54-88256. It involves treatment with boron hydride and afterwards distillation. In this publication, it is disclosed that when poly-oxitetramethyleneglycol (hereinafter shortened to PTMG) is prepared using THF which contains aldehydes, PTMG becomes colored.
A reported purification method directed towards THF obtained by the Reppe method comprises a distillation, a contact hydration in the presence of Raney nickel and a subsequent distillation is described in Japanese Patent Laid open publication 57-28076. In this publication, substances are listed which cause discoloration of the glycol polymer produced from THF, however, the substance mainly responsible for discoloration is not disclosed. In addition, expensive materials must be used when purifying THF by this method.
THF purified by the method disclosed in Japanese Patent Publication 10-29280 is obtained in greater than 99.9% purity. However, the disclosed process is lengthy and uses expenses materials. Crude THF is contacted with water and/or acetic acid in the presence of a high acid cationic ion exchanging resin. The acetic is then removed by distillation. The THF is then hydrated in the presence of a precious metal catalyst, and then purified by further distillation. In this method, the impurities to be removed are limited to dihydrofuran and N-butylaldehyde, and it is reported that PTMG obtained from this high purity THF is not colored. However, this reference provides no clarification as to which compound is responsible for the discoloration.
SUMMARY OF THE INVENTION
Methods for purifying THF which prevents the coloring of PTMG obtained by a ring-opening polymerization have been reported in many papers, as mentioned above. However, in the case of producing high purity THF, which must reliably prevent coloring, long and expensive procedures must be used. In contrast, tetrahydrofurans used for the preparation of polyether polyols need only be sufficiently pure to prevent the polyether polyol from coloring, and need not necessarily be of high purity.
To discover an economic and facile method for purifying crude THF, the inventors investigated the mechanism of coloring of polyether polyol to identify which impurity among the many impurities must be removed in order to prevent the polyether polyol from being discolored. In addition, it was also necessary to clarify the cause of coloring of alkyl THF and to establish a method for purifying alkyl THF. The impurities contained in alkyl THF more seriously affect the coloring problem of obtained polyether polyol than the impurities contained in THF.
Further, when PTMG obtained by a ring-opening polymerization and polyether polyol obtained by a copolymerization of alkyl THF and THF are compared, the latter is more easily handled because it has a lower melting point and is liquid at the room temperature. These compounds also form desirable polyurethanes because of their physical properties in the low temperature region. Consequently, advantageous elastic recovery ratios, as well as greater strength and elongation, are present in polyurethanes obtained alkyl THF copolymers.
After extensive studies it has been found that the impurities responsible for polyether polyol coloration of alkyl THF and the coloring of THF are vary similar. After investing the coloring mechanism of polyether polyol it was found that dihydrofuran is the impurity which mainly causes the coloring problem. It was further found that mineral acid can satisfactorily remove these undesirable dihydrofuran compounds, and thus accomplished the present invention.
An object of the present invention is to provide a sufficient method for purifying THF to obtain compounds suitable for use as starting materials in the synthesis of polyether polyols. Another object of the present invention is to provide a method for purifying THF compounds to remove the impurity causing the discoloration of polyether polyol, regardless the preparation method of crude THF.
These and other objects are achieved by the present method for purifying tetrahydrofurans suitable for use in the synthesis of polyether polyols. This method comprises contacting crude tetrahydrofurans represented by general formula (1)
wherein, R
1
and R
2
represent, independently, a hydrogen atom or an alkyl group of 1 to 5 carbon atoms, with mineral acid or a high acid cationic exchanging resin, separating the acidic material from the THF, followed by recovery of the THF by simple distillation and/or rectification to obtain the purified THF.
In another embodiment, crude tetrahydrofurans may be purified by contacting said tetrahydrofurans with a mineral acid which is then neutralized by adding an aqueous alkaline solution containing more than one equivalent by volume, relative to mineral acid, of an alkali metal salt or an alkaline-earth metal salt, followed by simple distillation and/or a rectification.
In the present invention, the entire process for purifying THF can be carried out in series, in whole or part.
Further, the present invention also concerns the tetrahydrofurans suitable for use in the synthesis of polyether polyols, which tetrahydrofurans are purified by a simple distillation and/or a rectification after the crude tetrahydrofurans represented by general formula (1) are contacted with a mineral acid or a high acid cationic ion exchanging resin and separated therefrom. In general formula (1),
R
1
and R
2
independently represent a hydrogen atom or an alkyl group having from 1 to 5 carbon atoms.
Furthermore, the purified tetrahydrofurans produced by this invention, which may be used as starting material in the synthesis of polyether polyols, can also be prepared by a method comprising contacting crude tetrahydrofurans with a solution of 0.1 to 15 percent by weight mineral acid followed by a simple distillation and/or a rectification. Still more preferably the acid s
Kubota Toshiro
Nakaoka Hiroshi
Dentz Bernard
Hodogaya Chemical Co. Ltd.
Pillsbury Madison & Sutro LLP
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