Abstract
The central point of this work is the metal catalyzed liquid phase oxidation of glucose to gluconic acid. During recent years, this reaction has indeed received much attention, since gluconic acid is a fine chemical which finds many industrial applications, mainly as water soluble cleansing agent and as additive for food and beverages.
In this study, the glucose oxidation is performed starting from an alkaline sugar solution. However, no basic solution (NaOH for example) is added to the reaction mixture to maintain the pH at a fixed value; the reaction is therefore carried out at uncontrolled pH.
The reaction is first performed in a batch reactor. Au, Pd and Pt nanoparticles immobilized on metal oxides, resins and porous carbons are used as catalysts; among them, carbon supported metal materials, mainly prepared according to the sol immobilization procedure [1], are the most used ones. After performing the glucose oxidation with varying temperature, pressure and oxidizing agent (pure O2 or air), it can be observed that, at 70°C and 3 bar pure O2, SX carbon supported Au(1wt%) catalyst shows the best performance. Indeed, already after 30 minutes, glucose is almost fully converted into gluconic acid (98% yield).
As the maximum gluconic acid formation is achieved in a very short time, the carbon supported Au catalyst might be successfully used also in a continuous system, i.e. in a trickle bed reactor (TBR). However, as powdered catalysts like Au(1wt%)/SX are difficult to handle in TBRs, a “in home” carbon (IHC) in grain form is chosen as support for the Au(1wt%) catalyst for use in the TBR. The glucose oxidation is performed with varying liquid and gas flow rate, temperature and initial glucose concentration; the optimal reaction conditions, which allow to achieve 81.5% yield of gluconic acid, are 20 ml/h (1.2 minutes as average residence time) and 575 ml/min as liquid and gas flow rate, respectively, 70°C and 5wt% starting concentration of glucose.
Both under batch conditions and in the trickle bed reactor, with carbon supported Au catalysts, high gluconic acid yields were obtained in a very short time and without pH control during the reaction.
