Aug. 6, 1946. K. H. W. TUERCKETAL 2,405,471 MANUFACTURE OF ACETIC ANHYDRIDE Filed July 17, 1945

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Patented Aug. 6, 1946 2,405,471

UNITED STATES PATENT OFFICE 2,405,471 MANUEFACTURE OF ACETIC ANHYDROE Karl Heinrich Walter Tuerck, Heathfield, Buckles Way, Banstead, and Eric Harvey Brittain, Epsom Downs, England, assignors to The Distillers Company Limited, Edinburgh, Scot land, a British company Original application October 29, 1942, Serial No. 463,819. Divided and this application July 17, 1945, Serial No. 605,607. In Great Britain July 23, 1941 8 Claims. (C. 260-546) 2 This invention relates to the manufacture of 89 to 91°/760 mm. This azeotropic mixture Sep acetic anhydride by direct oxidation of acetalde arates out into two layers on condensation. hyde With gaseous oxygen. It has already been It is therefore a further feature of our inven proposed in prior British Patent No. 461,808 to tion to subject the reaction mixture to distilla carry out this process in the presence of esters, tion so that the water formed during the reac especially ethyl-acetate, as diluents. Ethers tion is removed in the form of a low boiling have been considered as unsuitable for this pur fraction with paraldehyde. It is possible to ef pose as they tend to produce explosive peroxides fect this azeotropic distillation under reduced during the oxidation. pressure at temperatures below 50° C. Prefer Though the addition of esters improves the 10 ably the water is distilled from the reaction mix yield of acetic anhydride, it involves the disad ture at temperatures at which the rate of sapon vantage of the use of a substance which does ification of the anhydride is still relatively slow. not take part in the process and which, there The oxidation may be carried out in various fore has to be readily separable as such from ways. The acetaldehyde may be introduced into the other products, especially from acetic acid. 5 the reaction zone as vapour entrained in the The number of useful diluents is therefore al oxidising gas and/or dissolved in the paralde most entirely restricted to ethylacetate, which hyde. can be separated from acetic acid, but whose low One method is to use mixtures of acetaldehyde boiling point gives rise to difficulties during the and paraldehyde which have been prepared by oxidation and causes considerable losses during 20 depolymerisation of paraldehyde or polymerisa the distillation of the reaction products especially tion of acetaldehyde before the oxidation is under reduced pressure. started, e.g. paraldehyde is heated to 40 C. in We have now found that the manufacture of the presence of a trace of sulphuric acid and the acetic anhydride can be improved if the oxida resulting mixture of 20% acetaldehyde and 80% tion of acetaldehyde is carried out in the pres 25 paraldehyde is, after neutralisation of the cata ence of paraldehyde. lyst, subjected to the oxidation reaction. Though paraldehyde may be considered to pos Another method is to heat paralydehyde With SeSS the chemical structure of an ether, we have a mild polymerising agent to higher tempera found that under the conditions of the process of tures in order to depolymerise it to the desired the invention, the concentration or character of extent and thereafter to oxidise the mixture at the percompounds do not essentially differ from lower temperatures at which no further depoly those obtained when esters are used as diluents. merisation or polymerisation takes place. A Paraldehyde which is considerably less volatile suitable depolymerisation agent for this purpose than ethylacetate is readily obtained from or is vanadic acid which, we find, rapidly depoly converted to acetaldehyde by polymerisation and merises paraldehyde especially in presence of depolymerisation respectively, and it is an object 35 (some) acetic acid, at temperatures above 50 to of our invention to provide a process in which 60° C. e. g. 80°, and which can be used as an the diluent can be used as a raw material for the oxidation catalyst for the subsequent oxidation production of acetic anhydride. with oxygen at 30 to 45° C., at which tempera According to the present invention a mixture 40 tures polymerisation of the free acetaldehyde un of acetaldehyde and paraldehyde is oxidised with der the reaction conditions is negligible. , molecular oxygen or gases containing molecular As we have further found, it is possible to uti oxygen, e. g. air, at temperatures preferably be lise paraldehyde itself as the raw material for low 60° C. e. g. 30 to 50° C., and after the oxida oxidation to acetic anhydride and to obtain in tion the reaction mixture is worked up under this way acetic anhydride in satisfactory yields. conditions which minimize hydrolysis of the According to this method paraldehyde is subject formed acetic anhydride. The oxidation is pref ed to the action of molecular oxygen in the pres erably carried out under superatmospheric pres ence of a depolymerising agent which is able to Sure and in the presence of catalysts. depolymerise the paraldehyde under the condi We have found that paraldehyde is able to en tions at which the oxidation is carried out. train Water during the distillation of a mixture Such depolymerising agents are sulphuric acid, of acetic anhydride, acetic acid, paraldehyde and phosphoric acid, organic sulphonic acids, acidic Water, in the form of a minimum boiling mixture. salts such as zinc chloride, aluminum chloride, The pure azeotropic mixture contains 2.5 parts ferric chloride, and others. It may be noted that of paraldehyde and 1 part of water, and boils at 55 vanadic acid alone only functions as a depoly

2,405,471. 3 4. merising agent at a temperature above 50 C. de umn acetaldehyde distils over. The acetalde polymerisation of the paraldehyde is however hyde obtained by these means is returned to the assisted by the formation of acetic acid as the oxidation, either as such or after having been reaction proceeds and the reaction temperature polymerised to paraldehyde. can therefore be reduced below 50° C. once the We have found that it is advantageous to carry reaction has started, out the distillation, When paraldehyde and acetic An important advantage of using paraldehyde acid are present and the paraldehyde is not to for the oxidation is that the acetaldehyde can be be depolymerised deliberately, under Vacuum so oxidised at the rate at which it is formed, so that that the distillation temperatures are below 70° C. the concentration of free acetaldehyde in the so O Generally speaking it is advisable to neutralise lution and consequently in the gas phase is at a any acidic depolymerising catalyst before distill minimum. - This reduces the danger of explosion ling the reaction products. It is possible, how which is an important factor in the manufacture ever, by reducing the temperature of the distill of acetic anhydride in view of the high preSSures iation to a sufficiently low value, e. g. below 30 of oxygen used. A further advantage is that the s C., SO that the depolymerisation catalyst is no percompounds are almost entirely present in the longer active, to distill without neutralisation. form of peracids, whereas in neutral solutions Owing to the reversibility of the depolymerisa peroxides are in higher concentrations. The tion of paraldehyde to acetaldehyde the present peracids are less stable and therefore easier to invention provides a, continuous process which is remove than the peroxides. 20 simple and entirely cyclic and which is now de In this atter process of direct oxidation. Of par Scribed with reference to the accompanying dia aldehyde the amount of catalyst required is pref grammatic drawing. From the reaction vessel. A erably not greater than that amount Which is in which a mixture of acetaldehyde and paralde necessary to induce depolymerisation. It is, in hyde is treated With oxygen, a certain amount of fact, a surprising observation that high yields of 25 the reaction liquid is continuously or periodically anhydride are obtained in the presence of acidic Withdrawn and introduced into a distillation col depolymerising agents which are known to be sa unni B which is so conducted that at its base the ponifying agents for acetic anhydride and that at concentrated water-free acetic anhydride collects, the low catalyst concentrations a sufficient rate whereas all the other reaction products, namely of depolymerisation can be effected. 30 paraldehyde, acetic acid, water and small The Oxidation catalysts used in the oxidation announts of acetaldehyde distill over at the top process should be of such a chemical nature as and are collected as condensate in a tank C. The not to interfere with the activity of the depoly Column is preferably kept under vacuum so that merising catalyst. For example, when adding an loSSes in acetic anhydride and depolymerisation acetate, Such as manganese or cobalt acetate, care 35 of paraldehyde are restricted. From the tank C. should be taken that the amount used is not such the mixture, which is free from anhydride, is as to neutralise the depolymerisation catalyst. paSSed into a second distillation column D, in Wanadic acid is found to be a specially suitable which the depolymerisation of the paraldehyde is oxidising catalyst particularly when activated by effected at normal pressure, small amounts of sul means of acetic acid. Wery efficient catalysts can 40 phuric acid being added if necessary. At the also be obtained by precipitating Vanadic acid bottom of the column the acetic acid and the from its Salts by means of acetic acid or any other Water are collected, whereas the acetaldehyde is acid directly in the reaction mixture or in acetic Condensed into a cooled receiver E. containing acid or other Organic solvent immediately prior paraldehyde and a polymerising agent. Accord to the commencement of the reaction. Active so ing to the temperature in the receiver E a mix lutions of Vanadic acid can be obtained by treat ture of paraldehyde and acetaldehyde of a defi ing vanadium pentoxide (for example in acetic nite composition is obtained which can be direct acid) With hydrogen peroxide or any other per ly returned to the reaction vessel A. In case the oxidic compound so that the Vanadium pentoxide Oxidation is to be carried out in the absence of an is caused to dissolve in acetic acid. Such a cata 50 acidic depolymerisation agent, the mixture is first lyst may be also admixed with other catalysts neutralised or a sufficient amount of neutralising Such as Cobalt acetate or manganese acetate, but agent is constantly kept in the oxidation vessel. the amount of these latter substances should be Such a cyclic process makes it possible to work Smaller than the amount of vanadic acid used. economically even with small conversions (e. g. The utilisation of paraldehyde in the present 55 leSS than 30%) in the oxidation. This means process enables the manufacture of acetic anhy high yields in acetic anhydride and low concen dride to be carried out in the form of a simple trations of percompounds. It is advisable to se cyclic process. lect the size of the various columns so that the For example the reaction mixture is introduced contents of the reaction vessel can be cycled into a column at the top of which the mixture of through the apparatus in a to 1 hour. paraldehyde and water, together with small 0. The following examples in which quantities are amounts of acetaldehyde if present, is drawn off. calculated by weight illustrate the manner in The paraldehyde in the distillate is returned to which the invention may be carried into effect. the oxidation. The remaining mixture, which Eacample I-A mixtures of 80% paraldehyde runs down the column, contains acetic acid, par 65 and 20% acetaldehyde is treated with oxygen in aldehyde and acetic anhydride. a reaction vessel at 45°. C. in the presence of 0.1% By further fractionation a mixture of acetic Vanadic acid. After 42 hour the whole amount acid and paraldehyde, can be isolated and either of acetaldehyde is practically oxidised, and a returned to the oxidation or separated by distilla fresh neutral mixture of paraldehyde and acet tion with water or by distilation with Small aldehyde is introduced while the corresponding amounts of a depolymerising catalyst so that volume of liquid is continuously withdrawn and acetaldehyde distils over. The separation of the Worked up as described above. 60% of the con mixture of paraldehyde and acetic acid can also verted aldehyde can be obtained as acetic an be effected by distilling it slowly at at least at hydride, while the unchanged paraldehyde is dis moSpheric pressure so that at the top of the col 5 tilled off from the acetic acid as acetaldehyde,

2,405,471. 5 6 which is condensed at 40° C. in paraldehyde in a residue of acetic anhydride, and thereafter Sep the presence of 0.1% sulphuric acid so as to give arating the paraldehyde from its admixture with a mixture of 80% paraldehyde and 20% acetalde acetic acid by depolymerising said paraldehyde hyde, which after being neutralised with solid to acetaldehyde and distilling off the formed acet Sodium acetate, is returned to the oxidation ves aldehyde. Sel. 3. A process for the continuous production of Eacample 2.-Paraldehyde containing 0.01% sul acetic anhydride which comprises treating a mix phuric acid and 0.1% of vanadic acid (made by ture of acetaldehyde and paraldehyde with Oxy heating ammonium vana.date with acetic acid and gen at a temperature between 30 and 50° C., redispersing the red-brown precipitate in acetic 0 withdrawing a portion of the reaction products acid) was treated with oxygen at 32 C, and 1300 to a distillation column wherein the formed acetic mm. Hg for 30 minutes. During the oxidation anhydride is separated from the lower-boiling the acetaldehyde content was found to be below substances in the reaction product, which latter 1.5%. 67.5% by weight of the paraldehyde was substances are condensed in a vessel from which converted into a mixture of acetic acid and acetic 5 they are passed to a second distillation column anhydride, the latter constituting 59% of the oxi in which paraldehyde is depolymerised, the pro dation products. The unchanged paraldehyde to duced acetaldehyde being condensed in a cooled gether with the water formed is distilled off in receiver containing paraldehyde and a polymer vacuo at 29 C., leaving a residue of acetic acid ising agent, from which receiver the paraldehyde and acetic anhydride. 20 and acetaldehyde thus obtained is returned to Eacample 3.-A mixture, prepared by adding to the oxidation reaction, the acetic acid and water a solution containing 50 parts paraldehyde, 50 being withdrawn from said second distillation parts acetic acid and 0.1 part sulphuric acid, a column. Solution of 0.2 part cobalt acetate and 0.2 part 4. A process for the production of acetic an Copper acetate in acetic acid, was oxidised for 20 hydride which comprises treating a mixture of minutes at 38° C. The oxidation was stopped acetaldehyde and paraldehyde with oxygen in the when 40% by Weight of the paraldehyde had been presence of a neutralising agent, distilling the converted. Of the products of conversion, 45% reaction mixture to remove water and acetic by Weight was acetic anhydride and the remain acid therefrom in the form of a vapour in ad der acetic acid. The mixture was diluted with 30 mixture with paraldehyde and to leave a residue 100 parts of paraldehyde and distilled, so that of acetic anhydride, thereafter separating the first a fraction containing paraldehyde and Wa paraldehyde from its mixture with acetic acid by ter, then a fraction of paraldehyde and acetic acid depolymerising the paraldehyde to acetaldehyde, distilled over. The latter fraction was used as condensing said acetaldehyde in a cooled receiver Such in the next oxidation. 35 containing paraldehyde and an acidic polymeris If the concentration of percompounds in the ing agent thereby polymerising it to a mixture reaction product exceeds the limit which is usu of paraldehyde and acetaldehyde and recycling ally regarded to be safe for working up, it is pos said mixture to the oxidation stage. sible to reduce the amount of percompounds, say 5. A process for the production of acetic an to below 0.5%, by diluting the reaction mixture 40 hydride which comprises treating a mixture of with paraldehyde and to work up the mixture acetaldehyde and paraldehyde with oxygen in the in one of the ways described above. During the presence of a neutralising agent, distilling the re Working up, the percompounds decompose grad action mixture in vacuo to remove Water and ually without undue rise in temperature, and the acetic acid therefrom in the form of a vapour in excess of paraldehyde is recovered together With 5 admixture with paraldehyde and to leave a residue the original amount. of acetic anhydride, thereafter separating the If large amounts of the percompounds are pres paraldehyde from its mixture with acetic acid ent in the reaction product and have to be re by depolymerising the paraldehyde to acetalde moved, we have found that this can be achieved hyde, condensing said acetaldehyde in a cooled by adding a Small amount of manganese acetate receiver containing paraldehyde and an acidic to the reaction mixture after Oxidation. If an polymerising agent thereby polymerising it to a acidic depolymerising agent is present in the oxi mixture of paraldehyde and acetaldehyde and re dation mixture, We prefer to add the exact amount cycling said mixture to the oxidation stage. of manganese acetate Which is necessary to neu 6. A process for the continuous production of tralise the depolymerising agent. 5 5 acetic anhydride which comprises treating a mix This application is a division of application No. ture of acetaldehyde and paraldehyde with Oxy 463,819, filed October 29, 1942. gen, Withdrawing a portion of the reaction pro What We claim is: ducts to a distillation column wherein the formed 1. In a process for the manufacture of acetic acetic anhydride is separated from the remainder anhydride by the liquid phase oxidation of acet 60 of the substances in the reaction product, which aldehyde with molecular oxygen, the steps of ef latter substances are condensed in a vessel from fecting the oxidation in the presence of paralde which they are passed to a second distillation col hyde, distilling the reaction mixture to remove umn in which paraldehyde is depolymerised by water and acetic acid therefrom in the form of a heating at normal pressure in the presence of a vapor in admixture with paraldehyde and to leave Small amount of Sulphuric acid, the produced a residue of acetic anhydride and thereafter sep acetaldehyde being condensed in a cooled receiver arating the paraldehyde from its admixture With containing paraldehyde and a polymerising agent acetic acid. t from which receiver the paraldehyde and acetal 2. In a process for the manufacture of acetic dehyde thus obtained is, after neutralisation, re anhydride by the liquid phase oxidation of acet turned to the oxidation reaction, the acetic acid aldehyde with molecular oxygen, the steps of ef and water being withdrawn from said second dis fecting the oxidation in the presence of paralde tillation column. hyde, distilling the reaction mixture to remove 7. A process for the continuous production of water and acetic acid therefrom in the form of a acetic anhydride which comprises treating a mix Vapor in admixture with paraldehyde and to leave 75 ture of acetaldehyde and paraldehyde With oxy

2,405,471 7 8 gen, withdrawing a portion of the reaction prod agent in the acetaldehyde-paraldehyde mixture ucts to a distillation column wherein the formed being recycled is maintained, the acetic acid and acetic anhydride is separated from the remainder Water being withdrawn from said second dis of the substances in the reaction product, which tillation column. latter substances are condensed in a vessel from 8. In a process for the manufacture of acetic which they are passed to a second distillation col anhydride by the liquid phase oxidation of acet umn in which paraldehyde is depolymerised by aldehyde with molecular oxygen, the steps of heating at normal pressure in the presence of a effecting the oxidation in the presence of paral small amount of sulphuric acid, the produced dehyde, distilling the reaction mixture to remove acetaldehyde being condensed in a cooled receiver O Water and acetic acid therefrom in the form of containing paraldehyde and a polymerising agent a vapor in admixture with paraldehyde and to from which receiver the paraldehyde and acet leave a residue of acetic anhydride. aldehyde thus obtained is returned to the Oxida tion reaction wherein an amount of neutralising KARL, HEINRICH WALTER TUERCK. agent sufficient to neutralise the polymerising 5 ERIC HARVEY BRITIAIN.