Process for the Preparation of Hydroxybenzaldehydes

The present invention relates to a process for the preparation of hydroxybenzaldehydes by oxidation of the corresponding hydroxybenzyl alcohols. More particularly, this invention relates to the preparation of salicylaldehyde from ortho-hydroxybenzyl alcohol (commonly called saligenol).

Various means for carrying out this oxidation have been proposed. Among those, there may especially be mentioned the oxidation of hydroxybenzyl alcohol by means of air or pure oxygen in the presence of a catalyst based on a noble metal such as platinum or palladium. The oxidation is generally carried out in the liquid phase at a low temperature in an aqueous, preferably alkaline, medium. Thus, in the case of ortho-hydroxybenzyl alcohol, the oxidation in an aqueous alkaline medium containing about 1% by weight of noble metal relative to the alcohol to be oxidized gives salicylaldehyde with yields which are between 70 and 80%, relative to the alcohol employed. However, it is found that this method needs extended reaction times, of the order of one to several hours, to give a high degree of conversion of the alcohol. Hence, the use of the method entails a reduction in the productivity of the apparatus. It was thus desirable to have available an oxidation process which makes it possible to overcome this disadvantage while retaining good selectivity of the oxidation reaction in respect of producing aldehyde.

We have now discovered a process which satisfactorily meets this objective.

It is, accordingly, an object of the present invention to provide a novel process for producing hydroxybenzaldehydes which overcomes the disadvantages of the prior art processes.

It is also an object of the present invention to provide a process for the production of hydroxybenzaldehydes which retains good selectivity in its production of the benzaldehydes and yet permits highly efficient use of the production apparatus.

Other objects of the invention will be apparent to those skilled in the art from the present disclosure.

The present invention provides a process for the preparation of hydroxybenzaldehydes which comprises liquid phase oxidation of the corresponding hydroxybenzyl alcohol with molecular oxygen or of a gas containing molecular oxygen in an aqueous medium containing an alkaline agent, in the presence of a platinum or palladium catalyst and in the presence of a co-catalyst containing bismuth. The process of the invention is particularly suitable for the production of salicylaldehyde from ortho-hydroxybenzyl alcohol.

It has been found that, by comparison with the oxidation processes carried out in the presence of platinum or of palladium as catalysts, the use of the catalyst systems of platinum/bismuth derivative or of palladium/bismuth derivative results in a significant improvement both in respect of the rate of reaction and in respect of the yields of aldehyde. Thus, with the first of these catalyst systems, a reduction is observed in the reaction time, which can become 20 times shorter, and an increase of 10 to 15% in the yields is observed. With palladium, the addition of the bismuth derivative or co-catalyst can make it possible to reduce the reaction time by a factor of about 8, with a increase of up to 10% in the yields. Furthermore, the use of the co-catalyst makes it possible significantly to reduce the amount of noble metal usually employed. For example, this quantity can be five to ten times smaller in the case of palladium and even twenty-five times smaller in the case of platinum. Thus, a savings in expensive catalyst is provided.

The co-catalyst generally employed is an inorganic or organic derivative of bismuth in which the bismuth atom is at an oxidation level greater than zero, for example, 2, 3, 4 or 5. The radical combined with the bismuth is not critical, provided it meets the above condition. The co-catalyst can be soluble or insoluble in the reaction medium.

Compounds which illustrate the co-catalysts which can be used in the process according to the present invention are bismuth oxides, bismuth hydroxides, salts of inorganic hydracids such as bismuth chloride, bromide, iodide, sulphide, selenide or telluride, salts of inorganic oxy-acids such as bismuth sulphite, sulphate, nitrite, nitrate, phosphite, phosphate, pyrophosphate, carbonate, perchlorate, antimonate, arsenate, selenite and selenate, salts of oxy-acids derived from transition metals such as bismuth vanadate, niobate, tantalate, chromate, molybdate, tungstate and permanganate.

Other suitable compounds are salts of aliphatic or aromatic organic acids such as bismuth acetate, propionate, benzoate, salicylate, oxalate, tartrate, lactate and citrate, and phenates such as bismuth gallate and bismuth pyrogallate. These salts and phenates can also be bismuthyl salts.

As further inorganic or organic compounds, it is possible to use binary combinations of bismuth with elements such as phosphorus and arsenic, and heteropolyacids containing bismuth, as well as their salts; aromatic and aliphatic bismuthines are also suitable.

By way of specific examples, the following may be mentioned:

As oxides: BiO, Bi.sub.2 O.sub.3, Bi.sub.2 O.sub.4 and Bi.sub.2 O.sub.5.

As hydroxides: Bi(OH).sub.3.

As salts of inorganic hydracids: bismuth chloride BiCl.sub.3, bismuth bromide BiBr.sub.3, Bismuth iodide BiI.sub.3, bismuth sulphide Bi.sub.2 S.sub.3, bismuth selenide Bi.sub.2 Se.sub.3, and bismuth telluride Bi.sub.2 Te.sub.3.

As salts of inorganic oxy-acids: basic bismuth sulphite Bi.sub.2 (SO.sub.3).sub.3.sup. . Bi.sub.2 O.sub.3.sup. . 5H.sub.2 O, neutral bismuth sulphate Bi.sub.2 (SO.sub.4).sub.3, bismuthyl sulphate (BiO)HSO.sub.4, bismuthyl nitrite (BiO)NO.sub.2.sup.. O.sup.. 5H.sub.2 O, neutral bismuth nitrate Bi(NO.sub.3).sub.3.sup.. 5H.sub.2 O, the double nitrate of bismuth and magnesium 2 Bi(NO.sub.3).sub.3.sup.. 3 Mg(NO.sub.3).sub.2.sup.. 24H.sub.2 O, bismuthyl nitrate (BiO)NO.sub.3, bismuth phosphite Bi.sub.2 (PO.sub.3 H).sub.3.sup.. 3H.sub.2 O, neutral bismuth phosphate BiPO.sub.4, bismuth pyrophosphate Bi.sub.4 (P.sub.2 O.sub.7).sub.3, bismuthyl carbonate (BiO).sub.2 CO.sub.3.sup.. O.sup.. 5H.sub.2 O, neutral bismuth perchlorate Bi(ClO.sub.4).sub.3.sup.. 5H.sub.2 O, bismuthyl perchlorate (BiO)ClO.sub.4, bismuth antimonate BiSbO.sub.4, neutral bismuth arsenate Bi(AsO.sub.4).sub.3, bismuthyl arsenate BiO)AsO.sub.4.sup.. 5H.sub.2 O, and bismuth selenite Bi.sub.2 (SeO.sub.3).sub.3.

As salts of oxy-acids derived from transition metals: bismuth vanadate BiVO.sub.4, bismuth niobate BiNbO.sub.4, bismuth tantalate BiTaO.sub.4, neutral bismuth chromate Bi.sub.2 (CrO.sub.4).sub.3.sup.. 3.sup.. 5H.sub.2 O, neutral bismuthyl chromate (BiO).sub.2 CrO.sub.4, bismuthyl dichromate (BiO).sub.2 Cr.sub.2 O.sub.7, bismuthyl hydrogen chromate H(BiO)CrO.sub.4, bismuthyl potassium chromate (double salt) K(BiO)Cr.sub.3 O.sub.10, bismuth molybdate Bi.sub.2 (MoO.sub.4).sub.3, bismuth tungstate Bi.sub.2 (WO.sub.4).sub.3, bismuth sodium molybdate (double salt) NaBi(MoO.sub.4).sub.2, and basic bismuth permanganate Bi.sub.2 O.sub.2 (OH)MnO.sub.4.

As salts of aliphatic or aromatic organic acids: bismuth acetate Bi(C.sub.2 H.sub.3 O.sub.2).sub.3, bismuthyl propionate (BiO)C.sub.3 H.sub.5 O.sub.2, basic bismuth benzoate C.sub.6 H.sub.5 CO.sub.2 Bi(OH).sub.2, bismuthyl salicylate C.sub.6 H.sub.4 CO.sub.2 (BiO) (OH), bismuth oxalate (C.sub.2 O.sub.4).sub.3 Bi.sub.2, bismuth tartrate Bi.sub.2 (C.sub.4 H.sub.4 O.sub.6).sub.3.sup.. 6H.sub.2 O, bismuth lactate (C.sub.6 H.sub.9 O.sub.5)OBi.sup.. 7H.sub.2 O and bismuth citrate C.sub.6 H.sub.5 O.sub.7 Bi.

As phenates: basic bismuth gallate C.sub.7 H.sub.7 O.sub.7 Bi and basic bismuth pyrogallate C.sub.6 H.sub.3 (OH).sub.2 (OBi) (OH).

Other inorganic or organic compounds which are also suitable are bismuth phosphide BiP, bismuth arsenide Bi.sub.3 As.sub.4, sodium bismuthate NaBiO.sub.3, the bismuth-thiocyanic acids H.sub.2 [Bi(CNS).sub.5 ] and H.sub.3 [Bi(CNS).sub.6 ], and their sodium and potassium salts, trimethylbismuthine Bi(CH.sub.3).sub.3 and triphenylbismuthine Bi(C.sub.6 H.sub.5).sub.3.

The bismuth derivatives used preferentially for carrying out the process according to the invention are the bismuth oxides, the bismuth hydroxides, the bismuth or bismuthyl salts of inorganic hydracids, the bismuth or bismuthyl salts of inorganic oxy-acids, the bismuth or bismuthyl salts of aliphatic or aromatic organic acids; and the bismuth or bismuthyl phenates.

A group of co-catalysts which are particularly suitable for carrying out the invention consists of the bismuth oxides Bi.sub.2 O.sub.3 and Bi.sub.2 O.sub.4, bismuth hydroxide Bi(OH).sub.3, neutral bismuth sulphate Bi.sub.2 (SO.sub.4).sub.3, bismuth chloride BiCl.sub.3, bismuth bromide BiBr.sub.3, bismuth iodide BiI.sub.3, neutral bismuth nitrate Bi(NO.sub.3).sub.3.sup.. 5H.sub.2 O, bismuthyl carbonate (BiO).sub.2 CO.sub.3.sup.. O.sup.. 5H.sub.2 O, bismuth acetate Bi(C.sub.2 H.sub.3 O.sub.2).sub.3 and bismuthyl salicylate C.sub.6 H.sub.4 CO.sub.2 (BiO) (OH).

The amount of co-catalyst used, expressed as the amount of metallic bismuth contained in the co-catalyst, relative to the weight of the noble metal employed can vary within wide limits. For example, this amount can be as low as 0.1% and can be as high as the weight of noble metal employed, and can even, without disadvantage, exceed the latter.

More particularly, this amount is chosen so as to introduce into the oxidation medium from 10 to 900 ppm by weight of metallic bismuth relative to the ortho-hydroxybenzyl alcohol. In this connection, larger amounts of co-catalyst, of the order of 900 to 1,500 ppm, can, of course, be used, but without major additional advantage.

As regards the noble metals used conjointly for catalyzing the reaction, that is to say platinum and palladium, they may be in various forms such as, for example, platinum black, palladium black, platinum oxide, palladium oxide or the noble metal itself, deposited on various supports such charcoal, calcium carbonate, activated aluminas and silicas, or equivalent materials. Catalyst compositions based on charcoal are particularly suitable.

The amount of this catalyst to be employed, expressed in weight of metallic platinum or metallic palladium relative to that of the alcohol to be oxidized, can vary from 0.01 to 4% and preferably from 0.04 to 2%.

According to the process of the invention, the oxidation is carried out in an aqueous medium containing an alkaline reagent in solution. In this context, sodium hydroxide or potassium hydroxide is generally employed as the alkaline reagent. The proportion of inorganic base to be used can be between 0.5 and 3 mols of inorganic base per mol of alcohol to be oxidized. The concentration by weight of alcohol in the aqueous medium is usually between 1% and 60% and preferably between 2% and 30%.

In practice, one way of carrying out the process consists of bringing the aqueous solution containing the alcohol to be oxidized, the alkaline reagent, the catalyst based on a noble metal and the co-catalyst based on a bismuth derivative, in the proportions indicated above, into contact with molecular oxygen or a gas containing the latter. The process is carried out at atmospheric pressure but can also, if required, be carried out under pressure. The mixture is then stirred at the desired temperature until an amount of oxygen corresponding to that required to convert the alcohol to the aldehyde has been consumed.

In general terms, the reaction is carried out in a temperature range extending from 10.degree. C. to 100.degree. C. and preferably extending from 20.degree. C. to 60.degree. C.

Thereafter, having cooled the batch if appropriate, the catalyst mass is separated from the reaction mixture, for example, by filtration, and the resulting liquid is acidified by adding a proton acid of inorganic nature. Thereafter, it suffices to isolate the desired ortho-hydroxybenzaldehyde, for example, by extraction with a suitable solvent or by steam distillation, and to purify it in accordance with the known processes.

In order to disclose more clearly the nature of the present invention, the following examples illustrating the invention are given. It should be understood, however, that this is done solely by way of example and is intended neither to delineate the scope of the invention nor limit the ambit of the appended claims. In the examples which follow, and throughout the specification, the quantities of material are expressed in terms of parts by weight, unless otherwise specified.

These four examples employ, as the catalyst based on a noble metal, palladium in the form of a catalyst comprising 10% by weight of the metal deposited on charcoal (specific surface area 1,100 sq. meters per gram); the amount employed, expressed in weight of palladium relative to that of the alcohol to be oxidized, is 1%. These examples are carried out by varying the amount of bismuth derivative used, which in the case in question is neutral bismuth sulphate Bi.sub.2 (SO.sub.4).sub.3.

The method of working for each example is as follows:

A 100 cm..sup.3 glass flask is used which is equipped with a central stirring system, a heating device and a thermometer and which is connected to a source of pure oxygen so equipped as to make it possible to read off the volume of gas absorbed over a period of time.

The following are introduced into this reactor: the chosen amount of Bi.sub.2 (SO.sub.4).sub.3, 8 cm..sup.3 of an aqueous 4 N solution of NaOH, 0.4 g. of the catalyst based on palladium (representing 0.04 g. of palladium metal), 4 g. (0.0323 mol) of ortho-hydroxybenzyl alcohol and 34 cm..sup.3 of water.

The reactor is flushed with oxygen and is connected to the source of oxygen, while setting up a slight pressure corresponding to the weight of a 30 cm. column of water. The reaction mixture is heated to a temperature of 45.degree. C. and the stirring is then started (1,000 rpm). The mixture is kept stirred at the above temperature for a specific period of time, until the volume of oxygen absorbed corresponds to the amount theoretically required (0.0162 mol) to convert the alcohol to salicylaldehyde. At that instant, the operation is stopped and the reactor is flushed with nitrogen.

By way of comparison, these experiments were repeated, starting from the palladium-based catalyst, but in the absence of the co-catalyst Bi.sub.2 (SO.sub.4).sub.3 (experiment A).

In the absence of palladium deposited on charcoal, but in the presence of Bi.sub.2 (SO.sub.4).sub.3, no absorption of oxygen is observed, even for amounts of co-catalyst as high as 50 mg. (experiment B).

The results of the reactions, that is to say the yields of salicylaldehyde relative to the alcohol employed, are determined by analysis by vapor phase chromatography, carried out on the reaction mixture. To do this, a sample of 6.5 g. is taken, which is acidified with 1 cm..sup. 3 of an aqueous solution of HCl containing 36% by weight of pure acid, and the aldehyde is then extracted with 4 times 10 cm..sup.3 of toluene. The toluene extracts are introduced into a 50 cm..sup.3 calibrated flask; the volume of the organic phase is brought to precisely 50 cm..sup.3 with toluene and the salicylaldehyde is determined by vapor phase chromatography, with the aid of standard solutions containing given quantities of aldehyde.

The results are summarized in Table 1 which follows:

The procedure followed is as indicated in the preceding examples, but using bismuth derivatives of different nature. The amount of co-catalyst employed is so chosen that it introduces into the medium about 600 ppm by weight of metallic bismuth relative to the alcohol to be oxidized (the conditions of Example 3).

The results are summarized in Table 2 which follows:

These examples employ, as the catalyst based on noble metal, platinum in the form of a catalyst containing 4.76% by weight of the metal deposited on charcoal (specific surface area 1,100 m..sup.2 /g.). The amount employed, expressed in weight of platinum relative to that of the alcohol to be oxidized, is 1%.

These examples are carried out by varying the amount of bismuth derivative used, which in the case in question is Bi.sub.2 (SO.sub.4).sub.3.

The following are introduced into a flask equipped as in Example 1: the chosen amount of Bi.sub.2 (SO.sub.4).sub.3, 8 cm..sup.3 of an aqueous 4 N solution of NaOH, 0.84 g. of the platinum based catalyst (representing 0.04 g. of platinum metal), 4 g. (0.0323 mol) of ortho-hydroxybenzyl alcohol and 34 cm..sup.3 of water.

Thereafter, the procedure followed is in every respect as indicated in Example 1.

By way of comparison, an experiment was carried out, starting from the platinum-based catalyst, without introducing Bi.sub.2 (SO.sub.4).sub.3 (experiment C).

The results are summarized in Table 3 which follows:

The preceding Example 3 is repeated, but using an amount of palladium metal, relative to the ortho-hydroxybenzyl alcohol, which is smaller by a factor of 5 (Example 14) or by a factor of 10 (Example 15).

Example 12 is also repeated, but using an amount of platinum metal, relative to the alcohol, which is smaller by a factor of 25 (Example 16).

The results are as follows:

In the foregoing examples, the bismuth co-catalyst may be replaced with an equivalent amount of any of the bismuth compounds disclosed hereinabove.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.