Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2002-06-26
2003-07-01
Richter, Johann (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S863000, C568S868000, C568S869000
Reexamination Certificate
active
06586642
ABSTRACT:
The present invention relates to a process which affords polyhydric alcohols of low color index by hydrogenation.
Polyhydric alcohols are obtained on a large scale by condensing formaldehyde with higher CH-acidic aldehydes or with water and acrolein or 2-alkylacroleins. This reaction can be carried out according to two main procedural variants.
Firstly, there is the so-called Cannizzaro process, which is further subdivided into the inorganic and organic Cannizzaro processes. In the inorganic variant, excess formaldehyde is reacted with the appropriate alkanal in the presence of stoichiometric amounts of an inorganic base such as NaOH or Ca(OH)
2
. In the second step, the dimethylolbutanal formed in the first step reacts with the excess formaldehyde in a disproportionation reaction to give trimethylolpropane and the formate of the base used, i.e. sodium or calcium formate. The production of these salts is a disadvantage because they are not easy to separate from the reaction product; in addition, one equivalent of formaldehyde is lost.
In the organic Cannizzaro process, a tertiary alkylamine is used in place of an inorganic base, affording higher yields than with an inorganic base. Trialkylammonium formate is obtained as an unwanted by-product, so here again one equivalent of formaldehyde is lost.
The disadvantages of the Cannizzaro process are avoided in the so-called hydrogenation process, where formaldehyde is reacted with the appropriate aldehyde in the presence of catalytic amounts of an amine, the result being that the reaction stops at the alkylolated aldehyde stage. After separation of the formaldehyde, the reaction mixture—which, in addition to said alkylolated aldehyde, also contains small amounts of the corresponding polyhydric alcohol and acetals of the alcohols formed—is hydrogenated to give the desired polyhydric alcohol.
One particularly efficient process for the preparation of alcohols obtainable by condensing aldehydes with formaldehyde is described in WO 98/28253. This process affords high yields with the concomitant production of small amounts of coupling products. The procedure involves reacting the higher aldehyde with 2 to 8 times the amount of formaldehyde in the presence of a tertiary amine and separating the resulting reaction mixture into two solutions, one containing said fully methylolated alkanal and the other containing unreacted starting material. The latter solution is recycled into the reaction. The separation is effected by distillation or by simply separating the aqueous phase from the organic phase. The solution containing the product is subjected to a catalytic and/or thermal treatment to convert incompletely alkylolated alkanals to the desired fully methylolated compounds. By-product formed in this process is separated off by distillation and the resulting bottom product is subjected to catalytic hydrogenation to give the polyhydric alcohols.
examples of important alcohols prepared by the processes described are neopentyl glycol, pentaerythritol, trimethylolethane, trimethylolbutane and, in particular, trimethylolpropane (TMP).
TMP has become widely used as a crosslinking agent for polyesters and polyurethanes. However, commercially available grades of TMP have a more or less pronounced coloration, probably caused by the presence of impurities. This coloration is not a problem for many uses, but there are also applications for which it is desirable to use TMP with as little color as possible. A variety of processes aimed at improving the color index of TMP are described in the literature.
U.S. Pat. No. 3,097,245 describes a process for the preparation of trimethylolpropane with an APHA color index of between 50 and 200. This color index is achieved by observing specific reaction conditions in respect of temperature, reaction time, pH and concentration of the starting compounds. The reaction is also followed by treatment of the resulting solution with an ion exchange resin.
U.S. Pat. No. 5,603,835 discloses a process for the preparation of TMP with APHA color indices of <100. These are achieved by means of an extractive aftertreatment of the resulting crude TMP solutions with an ether or an ester. The TMP solutions used generally originate from the Cannizzaro process.
Both the processes described above have the disadvantage of being relatively expensive because specific conditions have to be observed precisely and it is necessary to add an ion exchange resin or introduce at least one solvent.
The literature only contains a small amount of information on the hydrogenation of products formed by condensing formaldehyde with higher aldehydes.
DE-A-17 68 259 discloses a process for the processing of the by-products formed when reacting formaldehyde with higher aldehydes to give polyhydric alcohols. The process consists in separating these by-products from the main product and then hydrogenating them to give comparatively large amounts of mainly aliphatic alcohols.
The hydrogenation process described in SU-A 125 552 is used to purify TMP obtained by the Cannizzaro process, said TMP being either a crude material in the form of an aqueous solution containing approx. 30% of TMP, or a purified material containing approx. 80% of TMP, from which water and formates have been removed. Hydrogenation on nickel, zinc, molybdenum and copper catalysts affords pure TMP with a content of approx. 98% after distillation. The pressures used are 1 to 250 bar, preferably 10 to 200 bar, and the temperatures are 20 to 200° C., preferably 100 to 150° C. The TMP obtained is said to be colorless, although no color index is mentioned.
It has been found, however, that the improvements in color index obtainable by this process are often inadequate for many purposes.
It is therefore an object of the present invention to provide a process which makes it possible to obtain polyhydric alcohols, especially TMP, with a low color index. APHA color indices of <20 should be achievable by this process.
We have found that this object is achieved by a process for improving the color index of polyhydric alcohols by catalytic hydrogenation, wherein the polyhydric alcohol used in the hydrogenation has been purified by distillation following its preparation.
The process according to the invention can be used to improve the color index of polyhydric alcohols, especially TMP, of any origin. Batches originating from the organic or inorganic Cannizzaro process can be used in the hydrogenation according to the present invention for improving the color index in just the same way as can alcohols originating from the hydrogenation process. It is important here, however, that the alcohol has been purified beforehand and is of a purity which is within an appropriate range and allows the color index to be improved by means of the process according to the invention. Particularly good results have been achieved in cases where the polyhydric alcohol used has originated from the hydrogenation process. The use of polyhydric alcohols, especially TMP, of this origin is usually preferred according to the invention.
It has been established that the use of already distilled polyhydric alcohol makes it possible to achieve an improvement in color index which is far greater than that achieved using an alcohol which has not been purified beforehand by distillation. Good results have been obtained with solutions containing >95% of alcohol.
If TMP is used in the hydrogenation, particularly good results can be achieved by using TMP solutions with a content of >98%.
The hydrogenation according to the invention is particularly applicable to any polyhydric alcohols which can be prepared by condensing formaldehyde with higher aldehydes, in the presence of catalytic amounts of trialkylamine, and then hydrogenating the products. Practically any alkanals with an acidic hydrogen atom in the &agr;-position to the carbonyl group are suitable higher aldehydes. Starting materials which can be used are aliphatic aldehydes having from 2 to 24 C atoms which can be linear or branched or can also contain alicyclic groups.
Dernbach Matthias
Haake Mathias
Koch Michael
Kratz Detlef
Schulz Gerhard
BASF - Aktiengesellschaft
Keil & Weinkauf
Price Elvis O.
Richter Johann
LandOfFree
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