Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
1999-08-26
2001-07-24
O'Sullivan, Peter (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S854000
Reexamination Certificate
active
06265623
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application relates to German Application DE 198 40 276.7, filed Sep. 4, 1998, which disclosure is incorporated herein by reference.
FIELD OF THE INVENTION
The invention relates to a process for the reductive cleavage of linear and cyclic acetals, especially formals. The invention is directed especially towards the hydrogenolytic cleavage of cyclic formals in an aqueous medium in the presence of a formate, especially an ammonium, alkali metal or alkaline earth metal formate, wherein 1,3-diols and methanol are formed.
BACKGROUND OF THE INVENTION
Even though the hydrolysis of formals and other acetals is one of the fundamental reactions of organic chemistry, problems can still occur where the equilibrium position is one-sided, as is the case, for example, with cyclic acetals having a 1,3-dioxo structure, owing to inadequate selectivity and side reactions, including thermal decomposition.
Linear and cyclic acetals, especially formals, are formed as undesired secondary products in the large-scale preparation of polyhydric alcohols, such as pentaerythritol, trimethylol-propane, trimethylol-ethane, neopentyl glycol and oligomers of those alcohols, which includes an aldol addition and a Cannizzaro reaction. In the Cannizzaro reaction, which is carried out in the presence of an alkali metal, alkaline earth metal or ammonium hydroxide, the corresponding formate is formed. The formate is present in the various stages of working up of the polyols in varying amounts.
WO 97/01523 describes a process for the cleavage, by hydrogenation, of cyclic formals, such as are formed as secondary products in the case of an aldol addition with a subsequent Cannizzaro reaction to prepare polyhydric alcohols, such as pentaerythritol and trimethylolpropane, to the underlying diols and methanol. The cleavage takes place in the aqueous phase at a pH of from 1 to 6 in the presence of a heterogeneous metal catalyst at from 100° to 200° C. under a hydrogen atmosphere. As is shown by Example 1 of that document, a previously acidified aqueous reaction mixture from the preparation of di-trimethylolpropane (di-TMP), which also contains sodium formate in addition to the cyclic formals di-TMP and tri-TMP to be cleaved and unidentified compounds, is hydrogenolyzed in the presence of Ru on activated carbon at 67 bar H
2
and 130° C.
As was discovered by the inventors of the present Application when re-working WO 97/01523, the hydrogenolysis under the mentioned pressure and temperature conditions can only be carried out successfully if it is based on a very high weight ratio of catalytically active metal to cyclic formal. This ratio is 0.91 in the Example mentioned. The formate contained in the reaction mixture evidently acts as a catalyst poison. However, such a high relative amount of catalyst used renders the process uneconomical. Therefore, the object of the invention is to improve the above-mentioned process.
SUMMARY OF THE INVENTION
In the process in question, the acetal cleavage is to take place as quantitatively as possible but, at the same time, the formation of unsaturated and color-giving compounds is to be avoided.
The object is achieved by a process for the reductive cleavage of linear and cyclic acetals, especially formals, in an aqueous medium containing a formate, in the presence of a heterogeneous hydrogenation catalyst and hydrogen, at a pH value of less than 7, a pressure of from 0.1 to 30 MPa and a temperature of over 100° C., which process is characterized in that the reaction is carried out at a temperature of over 200° up to 300° C., wherein, in the case of a suspension procedure, the weight ratio of metal having hydrogenating activity to acetal is less than 0.1.
It has been found that the catalyst-poisoning effect of the formates can, surprisingly, be eliminated merely by raising the temperature. In this manner, it is possible to carry out the hydrogenolytic cleavage of linear and cyclic acetals with a high degree of selectivity for the 1,3-diol underlying the acetal, using an amount of catalyst that is substantially lower as compared with the prior art. In that process it is possible, in the presence of a formate, to cleave not only the acetals per se, but also linear and cyclic formals, such as may be contained as secondary products in reaction mixtures from the combined aldol addition and Cannizzaro reaction.
Studies show that the negative influence of the formate on the hydrogenation catalyst diminishes almost abruptly only at or above a temperature of 160° C., and the cleavage even of cyclic formals to the polyhydric alcohol and methanol proceeds sufficiently rapidly and completely. In general, the temperature is from above 200° to 300° C. and the reaction takes place especially preferably at from above 200° to 280° C.
Suprisingly, it is also possible to feed to the cleavage according to the invention an aqueous solution, containing an acetal, which is very weakly acid (pH>6) or even neutral at room temperature. It is, therefore, not absolutely necessary to adjust the pH to an acid value, as was necessary in the prior-known process. A pH value of the solution of 7 (at 20° C.) leads to a value of less than 7 at the reaction temperature used. The presence of an acid catalyst in the process according to the invention does not have an adverse effect but, rather, speeds up the reaction slightly. The acids may be protic inorganic or organic acids, or acid fixed catalysts whose H
o
value of the Hammett acidity function is less than +2, preferably less than −3 (Tanabe et al. in “Surface Science and Catalysis” Vol. 51 (1989),5). Mineral acids are less preferred, because they must be neutralized after the reaction and the salts must be separated from the reaction mixture and disposed of. Where an acid is used, preference is given to lower carboxylic acids, especially formic acid. A pH value of, for example, from 1 to 6 is suitable. Where acid fixed catalysts are used, for example those from the group of natural and synthetic silicate-like substances, such as mordenite, montmorillonite and acid zeolites, and oxides, mixed oxides, cation exchangers, the amount used is determined by the activity of the catalyst at the chosen reaction temperature. Acid fixed catalysts can be used in suspension form or in the form of a fixed bed.
The process according to the invention is carried out in the presence of a conventional hydrogenation catalyst at a pressure in the range of from 0.1 to 30 MPa. The hydrogen partial pressure is advantageously in the range of from 0.5 to 15 MPa, preferably from 1 to 5 MPa and especially from 1 to 3 MPa. Heterogeneous hydrogenation catalysts are preferred because they allow the catalyst to be separated from the reaction mixture in a simple manner—separation of a suspension catalyst by filtration, or arrangement of the catalyst as a fixed bed, for example in the case of a trickling bed or bubble column procedure.
Conventional hydrogenation catalysts contain as the active component a noble metal from the group Ru, Rh, Pd and Pt, or a transition metal from the group Cu, Cr, Co, Ni, Fe, including especially Raney catalysts and chromite catalysts; bimetallic catalysts of a transition metal and a noble metal may also be used, although they are not preferred because they have a limited useful life. The use of a hydrogenation catalyst containing one or more transition metals is advantageous only if the catalyst is sufficiently stable to acid under the reaction conditions.
Preferred hydrogenation catalysts for the process according to the invention are noble metal catalysts in metallic form, such as so-called blacks of Ru, Rh and, especially, Pd and Pt, or in a form bonded to a support. Suitable support materials for Ru, Rh, Pd and Pt are activated carbon, aluminium oxide, SiO
2
, TiO
2
and other metal oxides, as well as silicates. The amount of noble metal in noble metal catalysts bonded to a support is usually in the range of from 0.0001 to 10 wt. %, preferably in the range of from 0.01 to 5 wt. %. The optimum amount of noble me
Burkhardt Olaf
Haas Thomas
Morawietz Marcus
Vanheertum Rudolf
O'Sullivan Peter
Perstorp Specialty Chemicals AB
Pillsbury & Winthrop LLP
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