A new environmental friendly method for the preparation of sugar acids via catalytic oxidation on gold catalysts

Abstract

A novel and efficient way of producing of aldonic acids is reported. The selective catalytic oxidation of monosaccharides (arabinose, ribose, xylose, lyxose, mannose, rhamnose, glucose, galactose, N-acetyl-glucosamine) and disaccharides (lactose, maltose, cellobiose, melibiose) on Au, Pd and Pt catalysts was investigated. By using the gold catalyst, a total selectivity with respect to aldonic acids and a high catalytic activity for all investigated sugars was found. The reaction conditions for the production of maltobionic and lactobionic acids via oxidation on gold catalysts were optimised. A high long-term stability of two different gold catalysts for the oxidation of maltose and lactose, respectively, was found.

Introduction

Carbohydrates represent one of the most important sources of renewable materials. In spite of their large abundance, the carbohydrates are of limited use in fine chemistry. Their over-functionalization, and also their poor solubility in most of the commonly used organic solvents, narrow their use as raw materials. In the past years, special attention was paid to the selective catalytic oxidation of sugars. The aldonic acids obtained as main products found many applications in the food and detergent industries as well in cosmetics and medicine. For instance, gluconic acid is exceptionally well-suited for use in removing calcareous and rust deposits from metals or other surfaces [1]. In addition to its capability for chelating metals, gluconolactone, i.e., a derivative compound of gluconic acid, may also be used to scavenge free radicals, thereby protecting skin from some of the damaging effects of UV radiation [2]. Lactobionic acid, obtained by lactose oxidation, is the major component of the University of Wisconsin organ transplantation preservation fluid [3], displays prebiotic properties [4] and can provide benefits to skin as a result of its polyhydroxy acid structure. The presence of one aldonic acid, or a salt thereof, in the preparation of food under heat induces a decrease of the acrylamide contained in the food after cooking [5]. Moreover, carbohydrate-based products generally have advantageous biodegradability and biocompatibility properties.

Carbohydrate oxidation can be accomplished by chemical, biochemical and catalytic routes. The chemical and biochemical processes often display disadvantages as difficulties arise in the separation of the products, control of by-products or disposal of wastewater. In this regard, heterogeneous catalysis in aqueous solutions seems to be a valuable, environmentally friendly alternative to the chemical and enzymatic procedures for sugar oxidation. To make a catalytic process more competitive, it is essential to use highly selective, stable and active catalysts.

Starting with the early works of Wieland [6] and Heyns and Paulsen [7], [8] an oxidative dehydrogenation mechanism for the catalytic oxidation of sugars was proposed. Various works dealt with the study of glucose oxidation on Pt [9], [10], Pd [11], [12] and Au [13], [14] catalysts. Lactose was successfully oxidized to lactobionic acid on Pd/Bi [15] and Pt/Bi [16] catalysts in alkaline medium. In general, platinum catalysts displayed only a mediocre selectivity, an additional doping with bismuth or lead being necessary to obtain better catalytic properties [16]. In the oxidation process of glucose, leaching of bismuth was evidenced [17]. Prati and co-workers reported for the first time glucose oxidation on a gold catalyst. Glucose was oxidized on immobilized gold colloids on carbon and surprisingly, a total selectivity to gluconate was found. Nevertheless, a recycling test of the gold catalyst revealed that after four runs, a decrease in activity of about 50% occurred [13].

Historically, gold was regarded to be catalytically inert until Haruta discovered its surprising catalytic activity for low-temperature oxidation of CO [18]. Afterwards, it was shown that gold becomes active for many other reactions when nanoparticles are used [13], [14], [19]. The size of particle appears to be the most important parameter with respect to activity and selectivity in reactions catalyzed by gold. Further investigations showed that the particle size can be controlled by choosing appropriate preparation method of the catalyst as well as the suitable support [20].

In the present paper, the selective oxidation of aldoses (mono- and disaccharides) on Au, Pd and Pt catalysts is reported. The aim of this work was to compare the activity and the selectivity of three different catalyst metals, which can be used for the oxidation of carbohydrates. The effect of experimental conditions, i.e., temperature, pH, substrate concentration as well as the long-term stability of gold catalysts for the maltose and lactose oxidation were investigated.