DIPOTASSIUM PENTAPROPIONATE AND METHOD OF PREPARING THE SAME

Abstract: Dipotassium pentapropionate and preparation of this by crystallisation from solution of potassium dipropionate.

Description: DIPOTASSIUM PENTAPROPIONATE AND METHOD OF PREPARING THE SAME The present invention relates to solid acid potassium propionates and a process for preparing the same. Acid salts formed from lower monocarboxylic acids and alkali metals are well known. Aqueous solutions of such acid salts are claimed and described in our British Patent Specification No: 1505388. Some of these acid salts have also been prepared in their solid form but procedures used to isolate the solids are rather complicated due to the relative instability of the salts. For example, anhydrous conditions are used in the process described by Becker et al in J.A.C.S., volume 85 (1963), pp 157-159. A method of producing some of these acid salts from aqueous solutions is disclosed in Deutsche Offenlegungsschrift 2432473. However, the technique used is complicated in that a fluidised bed and an inert atmosphere are necessary to obtain the desired product. The acid salts are very versatile and have immense commercial potential due to their reduced odour and corrosivity in comparison with free acids and their greater activity in relation to the neutral salts. For example, compositions containing these acid salts may be used as fungicides, mould inhibitors, crop preservatives, bread preservatives, setting accelerators for concrete, disinfectants and as dyeing auxiliaries. However, the difficulties encountered in producing relatively stable acid salts in a solid form necessitate the use of these salts as solutions. For some uses, e.g. preservation of compound animal feedstuffs, solid preservatives are preferred, because of the types of material, machinery and processing involved. It is an object of the present invention to produce a stable, solid acid salt. Accordingly, the present invention is solid dipotassium pentapropionate. According to a further embodiment the present invention is a process of producing dipotassium pentapropionate which comprises cooling an aqueous solution of potassium dipropionate of a concentration of at least about 50% w/w to a temperature at which dipotassium pentapropionate crystallises and recovering the solid, crystalline dipotassium pentapropionate. The starting material for the production of dipotassium pentapropionate according to the present invention is potassium dipropionate. Potassium dipropionate may be produced by reacting neutral potassium propionate with free propionic acid. The neutral potassium propionate may be in the form of solid salt or in aqueous solution. Alternatively, potassium hydroxide or potassium carbonate may be reacted with free propionic acid. The hydroxide or carbonate may be in the form of solids or in aqueous solutions. Whichever base is used it is reacted with an excess of propionic acid so as to form a solution of potassium dipropionate from which dipotassium pentapropionate is crystallised according to the process of the present invention. It is however preferable to form the potassium dipropionate by reacting the neutral salt with free propionic acid. This may be achieved by first reacting an aqueous solution of potassium hydroxide with propionic acid to obtain an aqueous solution of the neutral propionate. Water may then be evaporated from this solution, if necessary under- reduced pressure, e.g. by heating at 800C and 130 m bar pressure to form a concentrated solution of neutral potassium propionate. The additional propionic acid needed to form the potassium dipropionate is then added in the appropriate stoichiometric amount to the concentrated solution of the neutral salt. The concentration of the potassium diprol,ionate solution from which the dipotassium pentapropionate is to be crystallised is suitably between 60 and 95 % w/w, preferably between 70 and 90 % w/w. The solution may contain more or less than the stoichiometric proportion of propionic acid required to to convert the neutral propionate to the dipropionate. In carrying out the process of the invention the solution of potassium dipropionate is cooled to a temperature at which dipotassium pentapropionate crystallises out. The crystallisation temperature will vary depending on the concentration of the potassium dipropionate solution, and on the relative proportions of potassium dipropionate potassium neutral propionate and propionic acid. Thus where the potassium dipropionate solution contains more than the theoretical amount of propionic acid required to form the dipropionate, crystallisation of the dipotassium pentapropionate will occur at temperatures above OOC, e.g. at about 50C. On the other hand if the potassium dipropionate starting solution contains less than the theoretical amount of propionic acid required to form the dipropionate it is necessary to cool the solution to below OOC for crystallisation of the pentapropionate to occur. Where the concentration of the potassium dipropionate solution does not exceed 75% w/w, crystallisation of the dipotassium pentapropionate will occur directly, on cooling to the appropiate temperature. However on cooling potassium dipropionate solutions of concentrations exceeding 75% w/w crystals of potassium dipropionate initially separate out as platelets at ambient temperature and can be removed by filtration. On further cooling crystals of varying composition between the dipropionate and the pentapropionate separate out and again can be removed by filtration. On still further cooling pure dipotassium pentapropionate crystals separate out in the form of white needles. Precipitation of the desired dipotassium pentapropionate crystals may be facilitated by 'seeding' the solution with crystals of the pentapropionate. yet another method of producing dipotassium pentapropionate comprises reacting solid potassium dipropionate, e.g. separated as a by-product during the various crystallisation stages, with an appropriate stoichiometric amount of propionic acid in aqueous solution and finally cooling the reaction solution to ambient temperature allowing solid dipotassium pentapropionate to separate. Dipotassium pentapropionate can also be produced in amorphous non-crystalline form by reacting neutral potassium propionate with the stoichiometric amount of free propionic acid. The solid dipotassium pentapropionate can be represented by the molecular formula: (KO CO - CH2CH3)2.3CH3.CH2 COOH and is in the form of white crystalline needles or white amorphous powder, with a titratable propionic acid content of 49.8 - 0.6% w/w, a melting point at 940C and a solubility of 80% w/w in water at room temperature. The invention is further illustrated with reference to the following examples: Example 1 198.9g of propionic acid was added to 301g of a 50% aqueous potassium hydroxide solution, to produce the neutral salt. The temperature was raised to 80-9O0C and pressure reduced to 130 Mbar. Water was stripped from the solution until the neutral salt began to crystallise at the stated temperature (80-90 C). 198.9g of propionic acid was then added to produce an 89% w/w solution of the dipropionate. On cooling to 200C, 172g of platelets of potassium dipropionate containing 41.0% w/w titratable acid were obtained, and this was separated from the mother liquor by filtration. Further cooling of the resultant filtrate to OOC produced 71g of needles containing 45.0% w/w titratable acid representing mixed potassium dipropionate/dipotassium pentapropionate. This was again separated by filtration. On cooling the filtrate to -50C approximately 15g of needles of 49.8% w/w titratable acid separated reprgsenting pure dipotassium pentapropionate. Example 2 11.2g of solid potassium hydroxide was added to 30g of propionic acid such that a 91% solution of potassium dipropionate was produced. Cooling of this solution to 200C precipitated 2.3g of potassium dipropionate crystals of 39.7% w/w titratable acidity. Further cooling to successively low temperatures as in Example 1 above gave the following products: i) platelets at 110C of 42.7% w/w titratable acid ii) needles at 5PC of 47.0% w/w titratable acid iii) needles at -50C of 49.2% w/w titratable acid representing essentially pure dipotasium pentapropionate. Example 3 56g of a 50% w/w aqueous potassium hydroxide was reacted with 74g of propionic acid to produce a 71.5% w/w solution of potassium dipropionate. Cooling of this to 50C produced no crystals. Cooling to -50C produced 18g of needles of pure dipotassium pentapropionate equivalent to 49.8 w/w titratable acid. Example 4 15.0g of propionic acid was added to 13.8g of anhydrous potassium carbonate and the resultant solution dried at 1400C to produce 22.4g of the anhydrous neutral salt. 22.2g of propionic acid were then added to this neutral salt to produce 44.6g of solid amorphous dipotassium pentapropionate dhich could be ground to a fine free flowing powder of 49.8% w/w titratable acidity. Example 5 5.6g of solid potassium hydroxide were reacted with 18.5g of propionic acid to produce a solution of potassium dipropionate containing excess of propionic acid. Cooling of the solution to 250C precipitated 2.8g of a mixture of the dipropionate and pentapropionate of 47.0% titratable acidity. Filtration followed by further cooling to 50C precipitated 5g of pure dipotassium pentapropionate equivalent to 49.8% w/w titratable acid. Fungicidal Activitiy of Dipotassium Pentapropionate. Samples of potassium dipropionate and dipotassium pentapropionate were included in samples of a malt extract medium at concentrations of 0.1 arid 0.25% by weigEIt. Tile resulting plates were stah inoculated with Aspergillus flavus and Penicillium notatum and the colony diameters measured after incubation for 2, 3 and 6 days at 270C. The results are presented below: Inhibitor Average Colony Diameter (mm) 0.1% Potassium dipropionate 21.2 0.25% Potassium dipropionate 11.5 0.1% Dipotassium pentapropionate 19.4 0.25% Dipotassium pentapropionate 7.6 The results clearly indicate that the dipotassium pentapropionate has a higher fungicidal activity than potassium dipropionate.

Claims: Claims: 1. Solid dipotassium pentapropionate. 2. Solid, crystalline dipotassium pentapropionate. 3. The process of producing solid dipotassium pentapropionate, which comprises cooling an aqueous solution of potassium dipropionate of a concentration of at least about 50% w/w to a temperature at which dipotassium pentapropionate crystallises and recovering the solid, crystalline dipotassium pentapropionate. 4. A process according to claim 3 wherein the solution of potassium dipropionate is produced by reacting neutral potassium propionate with propionic acid. 5. A process according to either of the preceding claims 3 or 4 in which the concentration of the solution of potassium dipropionate is between 60 and 95% w/w. 6. A process according to any of the preceding claims 3 to 5 in which the concentration of the solution of potassium dipropionate is between 70 to 90% w/w. 7. A method of producing solid crystalline dipotassium pentapropionate which comprises cooling a solution of potassium dipropionate of a concentration exceeding 75% w/w to a temperature at which crystals of potassium dipropionate separate, removing the crystals of potassium dipropionate from the mother liquor, further cooling the mother liquor to a temperature at which crystals of dipotassium pentapropionate separate, and recovering the crystalline dipotassium pentapropionate. 8. A method of producing solid crystalline dipotassium pentapropionate, said method comprising reacting a solution of neutral potassium propionate with propionic acid, the relative amounts of the reactants being such that the theoretical concentration of the potassium dipropionate in the reaction solution does no exceed 75% w/w, cooling the solution to a temperature at which crystalline dipotassium pentapropionate separates and recovering said dipotassium pentapropionate. 9. A method of producing solid dipotassium pentapropionate, said method comprising reacting neutral potassium propionate with the stoichiometric amount of propionic acid required to form solid dipotassium pentapropionate.

Inventors: BRANKLING, DAVID
Applicants: BP CHEMICALS LIMITED
Application year: 1981
Main classification: C07C53
IPC codes: C07C67, C07C51, A01N37, C07C53