1. Field of the Invention
The present invention relates to a novel settling accelerator for the epoxidation of a cyclic, at least monounsaturated alkene.
2. Description of the Background
Numerous processes for the epoxidation of alkenes are known and it is possible to use a wide range of different reaction or catalyst systems. The epoxidation of alkenes in a homogeneous, liquid phase using organic hydroperoxides in the presence of catalysts based on molybdenum, tungsten or vanadium is carried out industrially. However, epoxide production is accompanied by formation of equivalent or even greater amounts of the alcohol corresponding to the hydroperoxide, and the need to utilize or recirculate this alcohol greatly restricts industrial use of this process. For this reason, alternative processes for the more direct oxidation (epoxidation) of alkenes are being developed to an increasing extent.
One such process is epoxidation by means of molecular oxygen using silver catalysts. However, this process has only been able to be employed successfully for ethene and has not been able to be applied analogously to the epoxidation of other alkenes of the interest (for example propene).
Another process for direct oxidation of alkenes to epoxides is reaction with hydrogen peroxide. The process has been proposed for various epoxidation reactions because of, in particular, the positive aspect of the use of the oxidant in significantly reducing environmental pollution. Since the activity of hydrogen peroxide toward alkenes is only low, sometimes even completely absent, it is necessary to use activating agents, usually organic acids such as formic acid, acetic acid, etc., in organic solvents to facilitate the oxidation reaction. The acids form peracids in the reaction medium, which represent the actual reactive epoxidation agent, in situ. These processes, too, do not appear to be particularly successful, because it is difficult to obtain the peracids and because of the instability of the epoxides in acidic media, as a result of which varying convenient process conditions are necessary.
Still another method is the oxidation of alkenes by reaction with highly concentrated hydrogen peroxide in a homogeneous, i.e. exclusively organic, liquid phase in the presence of soluble catalyst systems based on elements of Groups 4, 5 and 6 of the Periodic Table (Ti, V, Mo, W) in combination with elements selected from the group consisting of Pb, Sn, As, Sb, Bi, Hg, and the like. Here too, the results of the process do not permit implementation as an industrial process. This is firstly because the reaction proceeds slowly, and secondly the preparation of the catalyst systems, which generally consist of very complicated organic metal compounds and additionally have to be soluble in the organic reaction medium, is complicated and expensive. Furthermore, the use of highly concentrated hydrogen peroxide ( greater than 70%) involves considerable safety risks which cannot easily be overcome in an economical manner.
These processes of the prior art clearly show that the oxidation of alkenes by means of hydrogen peroxide is self-contradictory because the best working conditions in respect of the catalyst system and hydrogen peroxide involve an aqueous, acidic medium while the factors of the oxidation reaction itself and the stability of the epoxide are favored in a neutral organic medium. For this reason, further processes for the epoxidation of alkenes using hydrogen peroxide have been developed, in which either an improved catalyst system based on TiO2/SiO2 in an aqueous phase with addition of primary or secondary alcohols (see EP 0 987 259 A1) or a two phase system containing a catalyst comprising tungstic acid, a quaternary ammonium salt and a phosphorous compound (for example DE 30 27 349) is used.
In the case of alkenes whose epoxides are not hydrolysis-labile and in which the olefinic double bond is not sterically hindered (e.g. cyclic at least monounsaturated alkenes), the known epoxidation using hydrogen peroxide and a tungsten catalyst is the most economical alternative.
For the epoxidation reaction by means of hydrogen peroxide to proceed sufficiently quickly, a phase transfer catalyst (for example, Aliquat(copyright) 336) is usually used in the case of very lipophilic alkenes (for example cyclododecene)(Angew. Chem. (1991), 103(12), 1706-9). However, the desired strong acceleration of the epoxidation by means of the phase transfer catalyst leads to the phases being significantly more difficult to separate after the reaction, because of emulsion formation; the corresponding settling times increase greatly. In addition, the organic phase usually remains very turbid after the separation. To achieve virtually complete phase separation, it is necessary to use either phase separators having a very large volume or suitable centrifuges.
The increased settling times in this process greatly reduce its attractiveness for continuous, industrial-scale use. In particular, the process can usually not be implemented at all in existing plants because of space problems caused by the need for larger phase separators. The use of centrifuges is of little interest in view of the power costs and the maintenance requirement required by moving parts.
It can be said quite generally that settling times of less than 2 minutes are industrially desirable. If, on the other hand, the settling times are more than 4 minutes, an industrial-scale continuous process is difficult to operate economically.
DE 30 27 349 describes a process for the epoxidation of alkenes using hydrogen peroxide, a tungsten compound, a phosphorus compound and a phase transfer catalyst. In this process, solvents such as alkanes or cycloalkanes are absolutely necessary. These solvents are always added to the reaction mixture in large amounts and generally serve either to dissolve a solid material and thus allow it to react, or to improve the reaction conditions, for example to achieve better heat removal.
However, the dilution of starting materials with non-reactive substances such as solvents is undesirable since, firstly, the dilution leads to a reduction in the space-time yield and, secondly, a further separation operation is necessary after the reaction.
Accordingly, one object of the present invention is to provide a settling accelerator, i.e. a compound, which leads to industrially acceptable settling times of the heterogeneous catalyst-containing reaction mixture in the epoxidation of alkenes, which ensures a sufficiently high space-time yield of epoxide, so that this process using the settling accelerator makes it possible for such epoxides to be prepared on an industrial scale.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a method of improving settling times of catalyst in the epoxidation of a cyclic, at least monounsaturated alkene, comprising:
epoxidizing a cyclic, at least monounsaturated alkene having from 8 to 20 carbon atoms in the ring in a reaction medium containing an oxidant and a catalyst system comprising at least one metal of Groups 4, 5 and 6 of the Periodic Table of the Elements, phosphoric acid and a phase transfer catalyst and a cyclic alkane having from 8 to 20 carbon atoms in the ring, which corresponds to the alkene reactant, as settling accelerator in the epoxidation reaction.