Patent GB-797726-A (GB)
Title: Improvements in or relating to the decomposition of coal oxidation products
Inventor: GROSSKINSKY OTTO
IPC: C07C51/42
Filed: 1955-02-10
Abstract: A process of decomposing crude oxidation products which are soluble in water and in polar organic solvents and are obtained by oxidizing hard coals with nitric acid, into dark-coloured high molecular weight carboxylic acids which are insoluble in water or in polar organic solvents and light-coloured low molecular weight carboxylic acids which remain dissolved in water or polar organic solvents, comprises subjecting the crude oxidation products to a thermal treatment at a temperature above 100 DEG  C., whereby the aforesaid high molecular weight carboxylic acids are rendered insoluble in water or in polar organic solvents, digesting the thermally treated oxidation products in water or in a polar organic solvent, separating the insoluble substances, evaporating the remaining solution and recovering low molecular weight carboxylic acids.  The crude mixture of acids may be thermally treated in the dry state or in the presence of a liquid medium. In a modification of the process the crude oxidation products are dissolved in alcohols and subjected to the thermal treatment whereby the low-molecular weight carboxylic acids are esterified and the insoluble constituents separated, the organic solvent is distilled off and the residue subjected to high-vacuum distillation and a mixture of esters of low molecular weight carboxylic acids is recovered.  Superatmospheric pressures may be employed.  A non-polar organic solvent may be employed additionally in the modified process, such as an aliphatic hydrocarbon, e.g. gasoline or ligroin. Specified solvents include ketones, esters and ethers.  The thermal treatment may be applied to an extract obtained by digesting the crude mixture of carboxylic acids with an organic solvent which may be inert or reactive.  The mixtures of soluble acids obtained may be bleached, e.g. with decolourizing agents such as animal charcoal or bleaching earth or by means of oxidizing agents such as nitric acid, chlorine, chlorine dioxide, hydrogen peroxide, potassium permanganate or chromates.  Chlorination also takes place when chlorine is employed as bleaching agent.  The esters are similarly decolourized and chlorinated.  Aqueous solutions of the low molecular weight acids may be further purified by adding alkaline substances or salts such as sodium chloride or sulphate.  Such treatments may be applied before separating the acids rendered insoluble by the thermal treatment.  The low molecular weight acids may be separated from the aqueous solutions by extraction with organic solvents such as ketones, alcohols, esters and ethers.  The separated high molecular weight acids may be used as base exchangers or returned to the oxidation process.  The thermal treatment may be carried out in the presence of inorganic salts of metals, e.g. selected from the first, second and eighth group of the Periodic System such as copper, silver, zinc, cadmium, mercury, iron, cobalt and nickel which appear to promote decarboxylation of the dark-coloured high molecular weight acids. The lower carboxylic acids produced in the process may be esterified.  In examples: a dry mixture of carboxylic acids obtained by oxidizing hard coal with nitric acid at super-atmospheric pressure in the presence of oxygen and subsequently distilling off unconsumed nitric acid is heated on a layer of pumice in steam which carries away volatile constituents such as carbon dioxide and picric acid, the residue is digested with water, the insoluble part consisting of multinuclear aromatic acids is separated off and the aqueous liquor is evaporated to obtain a mixture of benzene di-, tri-, tetra- and pentacarboxylic acids (I); in a similar process the heating is carried out under pressure with the crude material dissolved in water (II); an aqueous solution containing partially chlorinated mellitic, pyromellitic, trimellitic, benzene pentacarboxylic, phthalic and succinic acids is obtained by heating a similar starting material under pressure in methanol, filtering off the precipitate, evaporating the methanol solution, decolourizing the esters obtained with chlorine and hydrolyzing with water at elevated temperature and pressure (III); in a similar process omitting the chlorination the residue obtained on evaporating the methanol solution is distilled in vacuo, the residue is heated with nitric acid, the mixture obtained is diluted with water and heated under pressure to obtain an aqueous solution of carboxylic acids (IV); in a process similar to (III) propanol is employed instead of methanol and before the chlorination the mixture of esters is dried with sodium sulphate and heated (V); a crude ester mixture, which is purified by distillation and a pitch-like solid are obtained by heating the product obtained by oxidizing coal with nitric acid, with water, butanol and a petroleum fraction of boiling range 100-120 DEG  C. in an autoclave whilst nitrogen and hydrogen are added if necessary (VI) and (VII). Specification 693,262 is referred to.