Patent GB-734377-A (GB)
Title: Improvements in or relating to the manufacture of reductic acid
Filed: 1953-03-18
Abstract: Reductic acid is isolated from crude impure solutions thereof containing strong digesting acid by treating such solutions with a colour-adsorbing agent, removing the strong digesting acid, contacting the so treated solution with a strongly acidic cation exchange resin substantially saturated with hydrogen ions, separating the reductic acid-containing solution from the cation exchange resin, treating the separated reductic acid containing solution with a strongly basic anion exchange resin substantially saturated with hydroxyl ions, discarding the reductic acid-free solution, displacing the adsorbed reductic acid from the anion exchange resin by means of a displacing acid, removing the displacing acid from the solution and crystallizing out the reductic acid.  The crude solutions of reductic acid are prepared by digesting furfural, pentose, pentosan, polyuronic acid (or salts thereof), partial methyl ester of polyuronic acid or material (especially plant material) containing pentosan, polyuronic acid (or salts thereof) or partial methyl ester of polyuronic acid, with an aqueous solution of an inorganic or organic acid (or compounds yielding such acids) for several hours under pressure and at temperatures above 100 DEG  C. and preferably 120-150 DEG  C.  Digesting acids are, for example phosphoric, sulphuric, sulphurous, hydrochloric, oxalic, tartaric, citric, lactic and lower fatty acids, preferred strengths (except for treating furfural) being 5-15 per cent by weight. Colour-adsorbing agents are, for example, adsorbing carbons or charcoals and decolorizing resins such as phenol-formaldehyde or metaphenyl-enediamine - formaldehyde condensation products.  The digesting acid is preferably removed from the decolorized solution in such a way that the reductic acid remains in solution. This is achieved either by eliminating volatile digesting acid in vacuo or by converting non-volatile acids to their insoluble salts (preferably calcium salts).  Strongly acidic cation-exchange resins are, for example, sulphonated styrenedivinylbenzene copolymers or sulphonated phenol-formaldehyde condensation products; and strongly basic anion exchange resins are, for example, quaternary ammonium bases of a styrene-divinylbenzene copolymer.  Reductic acid is displaced from the anion exchange resin by percolation with an easily volatile or easily precipitable displacing acid (e.g. those used as digesting acids and especially formic acid) of strength preferably between 0.04 and 0.3 normal. These displacing acids may be removed from the reductic acid solution by distillation in vacuo or by precipitation as insoluble calcium salts.  After filtration the final purified solution is evaporated, preferably in vacuo until crystals of 98.3-99.7 per cent reductic acid separate out in high yields.  These crystals may be further purified by recrystallization from a mixture of ethyl acetate and ethyl alcohol, or from a mixture of dioxane and methyl alcohol. Optional treatment with colour adsorbing agents may be applied between the successive treatment with cation and anion exchange resins and also prior to crystallization of reductic acid from solution.  The examples describe the isolationof reductic acid from solutions prepared by digesting dry sugar beet pulp with (1) phosphoric acid, and (2) sulphuric acid, and by digesting (1) dried apple pomace, (2) pectin, (3) brown seaweeds, (4) alginic acid and (5) furfural with phosphoric acid.