The present invention relates to a process for the preparation of pivaloyl chloride, in which o-pivalolactone is reacted in the presence of a heterogeneous solid acid catalyst and with an aliphatic alcohol to produce pivaloyl chloride as the main product.
DE-A 26 38 463, for example, describes the reaction of pivalic acid esters with phosphorus pentoxide in the gas phase. A by-product is pivalic acid. In addition, this method requires the use of solvents, which are expensive.
According to Tetrahedron (1984), 40(22), 4003-14, pivalic acid esters are prepared from aliphatic aldehydes with diethyl malonate and an oxidizing agent. Here also, a comparatively expensive reagent, in the form of diethyl malonate, is used as the starting substance. A further disadvantage of this method is that the reaction is carried out under strongly acidic conditions, which causes considerable problems when converting pivalic acid esters into pivaloyl chloride.
JP-A 5 105 873 discloses the preparation of pivaloyl chloride in the gas phase from n-butyraldehyde.
JP-A 8-269 848 describes the preparation of pivaloyl chloride from aliphatic aldehydes in the gas phase, to which a solid acid catalyst is added, preferably a superacidic ion exchanger. A disadvantage of this method is that it is still carried out in the gas phase. This requires costly reactors and a comparatively high investment.
DE-A 25 05 076 describes the preparation of pivaloyl chloride from o-pivalolactone in the presence of acetic acid and sodium acetate in the gas phase at temperatures of 100-250xc2x0 C. In addition to the expensive acetic acid, which must be used in a large excess, this method also requires a reaction temperature above the boiling point of the starting compound o-pivalolactone.
A further method for the preparation of pivaloyl chloride is based on the reaction of pivalic acid with phosphorus pentoxide. The main product is a mixture of pivalic acid and pivaloyl chloride. Apart from the fact that pivaloyl chloride is obtained as a mixture with pivalic acid, which is not reactive under the process conditions, the yield of the reaction of pivalic acid with phosphorus pentoxide is very low and can amount to only 60 to 65%.
It is an object of the present invention to provide a novel process for the preparation of pivaloyl chloride in which o-pivalolactone is converted by simple means and in the absence of toxic solvents into pivaloyl chloride as the main product.
We have found that this object is achieved by a process for the preparation of pivaloyl chloride, in which o-pivalolactone is reacted in the presence of a heterogeneous solid acid catalyst with an aliphatic alcohol to produce pivaloyl chloride as the main product.
Surprisingly, this method yields a virtually complete conversion of the o-pivalolactone in the reaction with an aliphatic alcohol and affords a yield of more than 80% of pivaloyl chloride as the main product. Moreover, the method allows an almost complete utilization of the starting material employed in the form of o-pivalolactone since only a relatively small part is converted into by-products such as o-isopivalolactone or pivalic acid. The o-pivalolactone may be obtained in a simple manner in a technically acceptable manner from isophorone by hydroformylation or the lactonization with carbon monoxide.
The present invention therefore relates to a process for the preparation of pivaloyl chloride, in which o-pivalolactone is reacted in the presence of a heterogeneous solid acid catalyst and with an aliphatic alcohol to produce pivaloyl chloride as the main product, and o-pivalolactone is reacted with aliphatic alcohol, in the presence of a heterogeneous solid acid catalyst, to produce the corresponding pivaloyl alcohol, which can then be converted by simple esterification into pivaloyl chloride.
To improve the reproducibility of the reaction, it is advantageous to use a heterogeneous solid acid catalyst having a defined and narrow particle size distribution, preferably a spherical particle shape, in the process of the invention.
Suitable heterogeneous solid acid catalysts are those which are composed of amorphous (amorphous silica gel, for example) or crystalline (crystalline xcex3-alumina, for example) phases.
Preferred heterogeneous solid acid catalysts are heterogeneous catalysts which contain amorphous phases, preferably with a specific surface of 100-600 m2/g, preferably 200-400 m2/g. A particularly preferred heterogeneous solid acid catalyst is that having a silica gel content of 50-75% by weight.
The heterogeneous solid acid catalyst preferably has a specific surface area of about 100 to 600 m2/g, in particular about 200 to 400 m2/g. A very particularly preferred heterogeneous solid acid catalyst is an amorphous xcex3-alumina having a specific surface area of about 200 m2/g and an aluminum oxide content of about 20 to 80% by weight, preferably about 30 to 60% by weight.
The term xe2x80x9chomogeneous solid acid catalystxe2x80x9d as used in the process of the invention is understood as meaning a solid acid catalyst which is uniform, i.e. uniformly distributed throughout the reaction mixture in finely divided form (for example in the form of particles with a diameter of less than 5 xcexcm).
The heterogeneous solid acid catalysts used in the process of the invention are commercially available or are prepared by processes known in the literature.
In one particularly advantageous embodiment of the process of the invention, the o-pivalolactone is used in a form substantially free from water (for example liquid o-pivalolactone obtained in a technically and economically optimal manner from isophorone; by-product water is removed or prevented from entering the process). If water is present in the reaction mixture, this not only leads to unwanted by-products but also reduces the yield.
To convert the o-pivalolactone into pivaloyl chloride, a low-molecular-weight aliphatic alcohol, preferably a C1-C8-alkanol, in particular a C1-C4-alkanol, such as methanol, ethanol, n-propanol or n-butanol, is reacted with the o-pivalolactone in the presence of the heterogeneous solid acid catalyst in the process of the invention.
To produce pivaloyl chloride as the main product, reaction temperatures of xe2x88x9210 to +80xc2x0 C. are expediently employed, preferably xe2x88x9210 to +50xc2x0 C.
In one advantageous embodiment, the o-pivalolactone is reacted in the presence of a low-molecular-weight aliphatic alcohol in a temperature range of xe2x88x9210 to +60xc2x0 C.
The heterogeneous solid acid catalyst is preferably employed in an amount of 0.5 to 10% by weight, in particular about 0.75 to 1.0% by weight, based on o-pivalolactone.
In one preferred embodiment, the amount of low-molecular-weight aliphatic alcohol in the heterogeneous solid acid catalyst employed is 2 to 50% by weight, preferably 3 to 10% by weight.
In this context, it has proven advantageous for the heterogeneous solid acid catalyst to contain 1 to 25% by weight, preferably 2 to 20% by weight, of aliphatic alcohol.
For an optimal yield, it is advantageous if, based on the amount of o-pivalolactone, less than 20% by weight of a low-molecular-weight aliphatic alcohol is added to the heterogeneous solid acid catalyst before the reaction.
The heterogeneous solid acid catalyst preferably contains 2 to 25% by weight, in particular about 5 to 15% by weight, based on the aliphatic alcohol used.
In the process of the invention, it is advantageous if the o-pivalolactone is reacted in the presence of water. A particularly high conversion into pivaloyl chloride is achieved if the heterogeneous solid acid catalyst contains up to 100% by weight of water in the form of an aqueous solution (solution in water, emulsion in water).
In the process of the invention, the reaction is advantageously carried out in the presence of a basic compound or an acidic compound.
In one embodiment, when using a heterogeneous solid acid catalyst containing basic groups, compounds which contain basic nitrogen or sulfur atoms, for example basic amines or thiols, are preferably used as the basic compounds. Particular preference is given to basic compounds which contain amino groups and/or thiols, such as aminoalkylamides, aryl- or alkylsufonic amides and dithiocarbamates.
In a preferred embodiment, the basic compounds are mixed with the heterogeneous solid acid catalyst before the reaction. The heterogeneous solid acid catalyst is then used as a carrier for the basic compounds. Alternatively, a heterogeneous solid acid catalyst may be used which has been pretreated with the basic compounds.
According to a further advantageous embodiment of the process of the invention, the heterogeneous solid acid catalyst is present as a suspension or emulsion in an aliphatic alcohol and is used in this form in the reaction with o-pivalolactone.
In this case, the pivaloyl chloride product of the reaction is preferably extracted from the reaction mixture by distillation and purified.
In the process of the invention, the desired reaction product may be obtained
