(617y) The Beneficial Effect of Fructose on the Catalytic Oxidation of Glucose to Gluconic Acid

In this study we analyzed the effect of different conditions of pH and temperature on the oxidation of glucose by air at P = 0.9 bar, using a commercial Pd-Pt-Bi/C catalyst in order to evaluate the best conditions for the formation of gluconic acid sodium salt. The reaction was carried out in a 0.5 L glass reactor provided a with Pt100 resistance thermometer to control temperature within 0.1 ^oC of thermostatic bath circulating water in the reactor jacket, a turbine impeller, baffles and a dip tube to introduce the NaOH solution near the impeller and a dip tube provided with porous stainless steel to introduce air. The initial volume of the reaction was 0.2 L of a solution 120 g/L of glucose and 1 g/L of the catalyst. Air was introduced in 0.5 L/min. The rotation impeller was high (2000-2200 rpm) so that the reaction were under kinetic control. The consumption of sodium hydroxide was monitored by a calibrated burette and the pH was maintained under manual control to vary as little as possible (+/- 0.1) during the tests.

The conversion of glucose was determined by means of stoichiometry (tritration) and uV-vis spectrophotometry. The gluconic acid content was determined by liquid chromatography (HPLC) equipped with HPX-87H columns and both refraction index and uV detectors. It was observed that the optimum reaction conditions after 70 minutes were pH ~ 9.5 and temperature ~ 55 °C and conversion of glucose was 77 % and gluconic acid the selectivity to was 84 %. The data also showed the formation of fructose, quantified by HPLC present in the products of reactions conducted mainly in pH of 10.5 in the temperatures range from 50-60 °C.

We studied the effect of adding fructose to glucose mixture in equimolar ratio on the development of the reaction in the optimized conditions (pH = 9.5 and temperature = 55 °C) using the described methodology. The results show an increase in the selectivity to 98 % based on the conversion of glucose but showed a lower conversion maintaining the same reaction time. The literature reports that one of the main by-products of catalytic oxidation reaction of pure glucose is fructose. In this study there was no pronounceable formation of fructose in experiments conducted with the mixtures of glucose and fructose. One possible explanation is the fact that the initial concentration of glucose and fructose are near to that of isomerization equilibrium between glucose and fructose and that the presence of fructose might occupy catalytic sites that could lead to other by products and influence positively the selectivity to gluconic acid formation in experimental conditions studied. A power law kinetics analysis to showed a fractionary order for glucose indicating a complex model which could be one of the three found in the literature (MVK, Eley Rideal of LHHW).