Glycolic acid electro-membrane synthesis

The regularities of glycolic acid from sodium glycolate by means of electro-membrane synthesis are represented in the article. Glycolic acid is a raw material to produce surgical suture materials. Optimal conditions of sodium glycolate conversion to glycolic acid such as current density and salt concentration are determined. The maximum current yield (75%) in the bipolar membrane FBM FuMA-Tech GmbH is observed for cation-exchange membrane MK-40. Glycolic (hydroxyacetic) acid (HO-CH2-COOH) is the first member of a-hydroxycarboxylic acids homologous series. Due to its properties glycolic acid is widely used in industry: food, oil and gas, textile, tanning, and other industries. It is used in medicine to produce biodegradable polymers and co-polymers (with lactic acid) for surgical fibers and bioresorbable prostheses. Polymers based on glycolic acid can be used as barrier materials which impede the oxygen permeation into the pack. The glycolic acid molecule has a low radius thus it has high penetration power. This property caused the glycolic acid use for cosmetology to normalize the processes of skin microcirculation and regeneration. Besides glycolic acid has low toxicity so it can be used as the oxalic acid substitute in cleanser and disinfectant industry. Glycolic acid can be obtained due to the Cannizzaro reaction from glyoxal, i.e. by means of intermolecular disproportionation in the presence of alkali. The process has several steps: first of all, a glyoxal solution undergoes alkali treatment (with low alkali excess to achieve the molar ratio of 1:1.05) under 10°С. At this step sodium glycolate is produced. The sodium glycolate solution undergoes electro-membrane conversion to obtain glycolic acid. This process is realized by means of electrodialysis with bipolar homo- and heterogeneous membranes the filter-press-type electrodialyzer. The laboratory electrodialyzer was used to investigate this process. Glycolic acid, glyoxal and other substances were analyzed chromatographically. It is showed that sodium glycolate yield increases in the low excess of alkali (molar ratio 1:1.05) and constant stirring of the reaction mixture chilled to 0-10°C. The excess of alkali leads to 100% glyoxal conversion and provides lesser ohmic resistance in the electrodialyzer. The decreasing of ohmic resistance in the electrodialyzer leads to heat release decreasing. Thus, the technological process is simplified and the cost of glycolic acid decreases. Besides the optimal concentration of sodium glycolate was determined. When bipolar membranes (MK-40/ MB-3; MK-40 based on cationic and anionic resins RALEX and MK-40/FBM FuMA-Tech GmbH) are used the C2H3O3Na concentration is 0.5-0.65 mol/l. The optimal current density is 100-200 mA/cm2. All the parameters of sodium glycolate conversion to glycolic acid over the electrodialyzer are linearly scalable. The maximum yield of glycolic acid is achieved when MK-40 and homogeneous bipolar FBM FuMA-Tech GmbH membranes are used. These membranes have low ohmic resistance and largest transfer numbers.

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