Method for reducing the content of acetals or ketals in alcohol-containing reaction mixtures

Reduction in the content of acetals or ketone acetals in a reaction mixture containing at least 10 moles alcohol per mole acetal or ketone acetal can be achieved hydrogenolytically when the reaction mixture is hydrogenated at 80.degree. to 250.degree. C. at a hydrogen partial pressure of 0.5 to 30 MPa in the presence of activated carbon charged with noble metal as catalyst.

CROSS-REFERENCE TO RELATED APPLICATION
 This application is based on German Application DE 198 40 277.5, filed Sep.
 4, 1998, which disclosure is incorporated herein by reference.
 FIELD OF THE INVENTION
 The invention relates to a method for reducing the content of acetals or
 ketone acetals (ketals) in an aqueous reaction mixture containing at least
 10 moles of a mono- or polyvalent alcohol per mole of the acetal or ketal
 by heterogeneous, catalytic hydrogenation.
 BACKGROUND OF THE INVENTION
 During the hydrogenation of aldehydes and ketones to alcohols the formation
 of acetals and ketals readily occurs if the hydrogenation is carried out
 below pH 7 or in the presence of solid catalysts comprising acidic
 centers. The acetals and ketals cannot be satisfactorily converted into
 the alcohols forming the basis and formed from the carbonyl compound by
 mild hydrogenation. Stable cyclic acetals and ketals, namely,
 2-substituted 1,3-dioxolane- and 1,3-dioxan derivatives are produced in
 particular during the hydrogenation of .alpha.- and .beta.-hydroxycarbonyl
 compounds to 1,2- and 1.3-diols. In order to hydrogenate such acetals and
 ketals, rather high temperatures and/or special catalysts are required.
 One problem in reducing the content of acetals and ketals in aqueous,
 alcoholic reaction mixtures of differing provenance also resides
 additionally in the fact that there is a danger that the alcohol itself
 will be decomposed under the rather severe hydrogenation conditions
 required.
 3-Hydroxypropionaldehyde is reduced to 1,3-propane diol in the method of
 EP-A 0 572 812 in which a first hydrogenation stage for the purpose of
 reducing the content of residual carbonyl is followed by a second
 hydrogenation stage at a pH of 2.5 to 6.5 at an elevated temperature. The
 same hydrogenation catalyst was used in both temperature stages,
 preferably a Pt/TiO.sub.2 -carrier catalyst and Ni/Al.sub.2 O.sub.3
 /SiO.sub.2 -carrier catalyst. It was determined that only an
 unsatisfactory conversion can be achieved in the second stage with a noble
 metal catalyst on an oxide carrier, with a neutral or weakly acid pH
 range; in addition, the catalytic activity drops after a few hours of
 operation to a level which is lower than at the beginning.
 According to WO 97/01523 cyclic diethers can be hydrogenolytically split
 with a 1,3-dioxo group in an aqueous phase in the presence of a metallic
 catalyst such as Ru on activated carbon at 100.degree. to 200.degree. C.
 at a pH of 1 to 6. According to the examples the degree of conversion is
 not quantitative. In particular, this document furnishes no suggestion for
 degrading small amounts of acetal or ketal in addition to large amounts of
 an alcohol without also decomposing the alcohol at the same time by
 hydrogenolysis. In addition, a very high ratio of catalyst to substrate is
 required in order to bring about the conversion. In a similar manner the
 hydrogenolytic splitting of 1,3-dioxan derivatives is incomplete if Raney
 nickel is used as catalyst, according to U.S. Pat. No. 4,044,059.
 In order to reduce the carbonyl content of lauryl alcohol containing 0.1%
 by weight lauryl aldehyde by catalytic hydrogenation, nickel catalyst on
 kieselguhr diatomaceous earth is recommended as the catalyst according to
 the brochure "Carbonyl Removal from Oxoalcohols" (Engelhard Corp.) even
 though this company also supplies carrier-bound noble-metal catalysts.
 Nickel catalysts tend to leach metal at a pH below 7, which has a
 disadvantageous effect on the workup.
 Ind. Eng. Chem. Res. 33 (1994), 566-574 teaches the degradation of organic
 components such as sugar and glycols in aqueous media hydrogenolytically
 in the presence of Ni catalysts or ruthenium on various oxide carriers.
 When using such catalysts to reduce the acetal content or ketal content in
 an aqueous medium containing an excess of an alcohol with respect to the
 amount of the acetal or ketal a partial degradation of the alcohol must be
 expected in the process described.
 SUMMARY OF THE INVENTION
 The invention has the object of making available a method in which a low
 acetal content or ketal content in an aqueous reaction mixture, which has
 a high content of alcohols, which is acid or neutral at room temperature,
 can be further reduced essentially without degradation of the alcohols.
 A method has been found for reducing the content of acetals (except
 formals) or ketals in an aqueous reaction mixture containing at least 10
 moles of a monovalent or polyvalent alcohol per mole of the acetal or
 ketal by heterogeneous, catalytic hydrogenation of the reaction mixture
 with hydrogen, at a temperature of 80.degree. to 250.degree. C. and a
 hydrogen partial pressure of 0.5 to 30 MPa, particularly 1 to 15 MPa,
 which is characterized in that activated carbon charged with noble metal
 is used as catalyst.
 Preferred embodiments include the hydrogenation of reaction mixtures
 containing a cyclic acetal or ketal with a 1,3-dioxo structure, use of Pd
 and/or Ru on activated carbon as the catalyst, carrying out the method
 using a trickle-bed reactor and carrying out the method using a solution
 with a pH of between above 6.5 and 7.
 It is surprisingly possible to increase the amount of hydrogenation of the
 acetal or ketal contained in the reaction mixture at the same temperature
 and at the same pH without the alcohol present being decomposed to any
 appreciable extent at the same time, if an activated carbon charged with
 noble metal such as an activated carbon charged with one or more noble
 metals from the group Ru, Rh, Pd and Pt is used instead of an oxide
 catalyst charged with the noble metal. As follows from the examples, when
 Ru on TiO.sub.2 is used, a quantitative reaction of the acetal does take
 place but, at the same time, decomposition of the alcohol occurs to an
 intolerable extent. Other catalysts known for the hydrogenation of acetals
 and the carbonyl compound on which they are based, for example, Ni on
 SiO.sub.2 /Al.sub.2 O.sub.3 carrier, do not attack the alcohol; however,
 their hydrogenation activity is insufficient if only a small amount of
 acetal is contained in the reaction mixture in addition to a large amount
 of alcohol. Especially preferred catalysts to be used in accordance with
 the invention are Ru and/or Pd on activated carbon, wherein "activated
 carbon" includes all types of carbon suitable for catalytic purposes.
 The hydrogenation customarily takes place at 80.degree. to 250.degree. C.,
 preferably at 100.degree. to 180.degree.C. If the alcohol content drops
 during the hydrogenation, this reduction of alcohol content can generally
 be reduced or entirely avoided without any appreciable reduction of the
 hydrogenation conversion by reducing the reaction temperature.
 The hydrogenation usually takes place at a pH range from 1 to below 7,
 measured at the reaction temperature. The method of the invention has the
 advantage that the reaction mixture to be used can have a pH of between
 above 6.5 and 7 at room temperature since this simplifies the workup.
 The reaction mixture supplied to the hydrogenation reaction contains a high
 molar excess of alcohol in comparison to the acetal or ketal, namely, at
 least 10 moles and preferably at least 50 moles alcohol per mole acetal or
 ketal. The alcohol can be any alcohol since the source of the reaction
 mixture can be as desired; however, it is preferable that the alcohol is
 bound in the acetal or ketal and/or produced during the hydrogenation. The
 method is suited in particular for removing acetals, except for formals,
 or ketals of .alpha.- or .beta.-hydroxycarbonyl compounds out of reaction
 mixtures. The acetals and ketals are, in particular, cyclic compounds with
 a 1,3-dioxo structure, especially 1,3-dioxan compounds and 1,3-dioxolane
 compounds. During the hydrogenolytic splitting of the ring a 1,3-diol or
 1,2-diol and the alcohol formed from the aldehyde or ketone are produced.
 Cyclic acetals with a 1,3-dioxo structure which are not substituted in the
 2-position (this concerns formals) can be split only partially
 hydrogenolytically under customary conditions.
 The method can be carried out continuously or batchwise, e.g., in a
 suspension method or a fixed-bed method. It is especially preferable to
 carry out the hydrogenation using a trickle-bed reactor. The fixed-bed
 catalysts to be used therewith are preferably pellets with a diameter of
 0.5 to 5 mm, especially 1 to 3 mm, and with a length of 1 to 10 mm. The
 noble-metal content of such catalysts is in the customary range, usually
 0.5 to 5% by weight.
 The method of the invention is distinguished in that even a small content
 of acetals or ketals can be further reduced hydrogenolytically in the
 reaction mixture without any appreciable attack on the formed alcohol or
 alcohols present. This surprising effect is illustrated using the
 following examples and reference examples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 EXAMPLES
 A trickle-bed system with 15 ml reactor volume was used for the continuous
 hydrogenation in a trickle bed. The system included a liquid receiver, a
 pre-heater, a fixed-bed reactor and a liquid separator. The reactor was
 heated to the desired reaction temperature T.sub.R via a heating system
 using heat transfer oil. The hydrogen flow rate was 13 Ni/h. Pressure and
 hydrogen flow were regulated electronically. The aqueous educt solution
 was charged in front of the pre-heater to the hydrogen flow with a pump
 and the mixture added at the reactor head (trickle-bed method). After
 having passed through the reactor, the reaction product was removed at
 regular intervals (t.sub.R) from the liquid separator. The product was
 analyzed using gas chromatography to determine the amount of
 2-(2'-hydroxyethyl)-1,3-dioxan (HED) and 1,3-propane diol (PDO).
 Catalysts used: