Synthesis of Gluconic Acid and its Salts by using Bimetallic Catalyst.
Carbohydrates are defined as compounds with molecular formula Cm(H2O)n, that is as hydrates of carbon. However the word "carbohydrate" is commonly used in a broader scene to designate substances composed of polyhydroxy aldehydes and ketones and their derivatives. Monosaccharides are carbohydrates which usually possess 3 to 9 carbon atoms  Among many sugar derivatives which find application in industry, medicine, food and feed are the mono basic sugars acids and their salts which are obtained by the mild oxidation of aldose sugar . Although these compounds have been known for long time and have been of great importance in acquiring our present knowledge of the molecular structure of various sugars . The sugar acids and their salts resemble the sugar in that they form soluble complex compounds with numerous substances.
Gluconic acid (also known as hexanoic acid, penta hydroxycaproic acid, IUPAC designation is 2,3,4,5,6-penta hydroxyl hexanoic acid) is a mild organic acid derivative from glucose by simple oxidation reaction. Oxidation of the aldehyde group on C-1 of b-d glucose to a carboxylic group results in the production of glucono lactone (C6H10O6) which is hydrolyzed to gluconic acid . Gluconic acid is a non corrosive, non volatile, non toxic mild organic acid. It impacts a refreshing sour taste in many food items such as wine, fruit juices etc. Sodium gluconate has high sequesting power. It is good chelator at alkaline pH. Its action is comparatively better than EDTA.. It is easily biodegradable. Concentrated gluconic solution contains certain lactone structures (neutral cyclic ester) showing antiseptic properties . Some scientists found that aldonic acid could be converted in parts into its epimers by heating a solution of the acid with pyridine or quinoline.
These have greater synthetic significance [7-12]. Gluconic acid has a carboxylic group and five hydroxyl groups, and thus is a good chelator particularly in alkaline conditions. It is used in chemotherapeutic treatments for metal poisoning. By forming stable water soluble complexes with multivalent metal ions, chelating agent prevent undesired interaction by blocking normal reactivity of metal ions. Heavy metals are chelated in alkaline solution and their interferences are eliminated by the gluconic acid. It chelates the anions of calcium, iron, aluminum, copper and many other heavy metals .
Results and Discussion
The laboratory prepared Palladium and Bismuth catalysts were used for the conversion of glucose into gluconic acid. A standard calibration graph was drawn by taking different concentration of gluconic acid (E.Merck). The progress of the reaction was then monitored by measuring UV absorption after every twenty minutes and a graph indicating the % of glucose converted into gluconic acid verses time was plotted as shown in Figure: 1, which indicate 40% of the glucose was converted into gluconic acid within 20 minutes. 58% of glucose was converted in to gluconic acid in 80 minutes and the reaction became very slow after 80 minutes. The 58% bimetallic catalytic conversion of glucose into gluconic acid illustrates acceptable catalytic activity of our laboratory prepared catalysts.
UV Spectrum (taken by UV Spectrometer Model Labomed UVD 3500) of the iron gluconate shows the lmax at 239nm (Figure-3), while that of zinc gluconate was 229nm. The % of metal in the metallic salt was calculated through AAS. The percentage of Fe(I) obtained in the salt was up to 11.5%, which is very close to the calculated value of 12.47%. Similarly the % of Zn obtained from AAS data was 13.5% which is also in good agreement to the calculated value of 14.29%. Both results indicate that metal to gluconic Acid ratio is 1:2. The proposed structure and molecular weight depends upon the mole ratio, one mole of metal and two moles of gluconic acid to form coordinate covalent bond. The proposed formula is (C6H12O7)2Fe and (C6H12O7)2Zn for Fe and Zn salts, respectively.
Preparation of Palladium and Bismuth Catalyst
Palladium and Bismuth metals were reduced on graphite by using graphite rods as cathode and an anode. 0.987 V was applied for 5minutes, to the solution containing 25mL of 0.1M palladium Chloride (E.Merck), 2mL of concentrated solution of HCl as supporting electrolyte was added in to it and the contents were further diluted to make the final volume to 100mL. Similarly 1.6V was applied for a period of five minutes, through the solution having 0.1M Bismuth chloride (E.Merck), 2mL of concentrated HCl was added in to it and the final volume was made to 100 mL. After reduction the electrodes were displaced, dried and then heated to 200 oC. The layer of nano particles of metal deposited on graphite rod was then scratched out and converted into fine powder in ball mill and used as a catalyst.
The amount of Palladium and bismuth was then determined by using Perkin Elmer Atomic Absorption Spertrophotometer model Series-100. Pd was determined by running standards of 4-16 ug/mL under acetylene air flame at 244.8nm. Similarly Bi was determined by using standards in the range of 10-40 ug/mL at 223.1nm under acetylene air flame. Sample solutions of Pd and Bi catalyst were prepared by dissolving appropriate amounts of the catalyst in HNO3. The results indicates that the amount of Palladium and Bismuth deposited on the glassy carbon was around 10 and23ug/g of glassy carbon respectively.
Synthesis of Gluconic Acid
The oxidation of 1M glucose solution was carried out in a 500 cc beaker equipped with a magnetic stirrer, an oxygen supply, a burette containing standard sodium hydroxide solution (0. 1M) and a pH electrode, 5g Palladium/Carbon and 5g Bismuth/Carbon catalyst was also added in to the reaction mixture. The gluconic acid formed during the oxidation of glucose was neutralized with NaOH solution in order to maintain the pH at 9 and move the reaction forward. The reaction was conducted at 60oC and oxygen was bubbled through the solution at about 100ccm/min. Number to samples were taken from the reaction mixture after every 20 minutes, filtered and analyzed by UV spectrometry at 200nm wavelength by using Labomed UV/Vis Spectrophotometer model UVD3500. The reaction was continued for about 120 minutes. The product was concentrated on water bath filtered and crystallized. The crystals were washed in acetone and used for the preparation of Iron and Zn Gluconates.
Preparation of Iron and Zn Gluconate
50 mL of 0. 1M solution of gluconic acid and FeSO4. 7H2O/ Zn Cl2 were refluxed for half an hour at a pH of 8-9 in two separate set of apparatuses in order to obtain Iron and Zinc gluconate. The solutions were then evaporated in china dish to final volume of 20 mL. The Iron gluconate and Zinc gluconate were then extracted in ethanol and finally evaporated to obtain crystals of metal-gluconate and re-crystalized in acetone.
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Institute of Chemistry, University of the Punjab, Lahore 54590, Pakistan.
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|Author:||Qadir, M. Abdul; Atta, Sadia; Jamshaid, Faisal|
|Publication:||Journal of the Chemical Society of Pakistan|
|Date:||Jun 30, 2012|
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