Oxidation of Carbohydrates on Metal Catalysts

The metal-catalyzed oxidation of carbohydrates with molecular oxygen is a remarkable example of green chemistry because reactants are obtained from renewable resources, processes are conducted under mild conditions with air as oxidizing agent and water as solvent, and reaction products are environmentally benign because of their biodegradability. In addition oxidized carbohydrate derivatives can often be obtained with high selectivity, and the catalysts are recyclable. These catalytic processes are, therefore, potentially very attractive for the preparation of specialties or intermediates employed in the food, cosmetic, pharmaceutical, and chemical industries. 

The first example of carbohydrate oxidation on platinum catalysts was reported by von Gorup-Besanz in 1861 [1], but the field received little attention until the work of Heyns and coworkers [2-4]. These authors found the following reactivity scale for the different functional groups  

During the last twenty years, studies conducted by academic groups at Eindhoven [5-24], Delft [25-36], Ziirich [37-42], Lyon [43-45], and in industrial research and development laboratories [46-49] have resulted in a better knowledge of these reactions and in industrial developments on the pilot or industrial stage. 

Carbohydrate oxidation on metal catalysts has been reviewed [29,50-53]. In the first part of this section, the general features of carbohydrate oxidation on metal catalysts will be described. Illustrative examples of carbohydrate oxidation with high selectivity or of process innovation will then be examined in more detail. 

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It was found as early as the middle of the 19th century that oxidation of alcohols to aldehydes and acids with molecular oxygen is catalyzed by platinum metals, but these reactions have been comparatively little studied. After the pioneering work of Heyns et al. [1,2] most of the reports in the open literature were published by a small number of research groups at Eindhoven [3-24], Delft [25-39], Zurich [40-56], and Villeurbanne [57-68], and the oxidation of glycerol and derivatives was studied by Kimura [69-74]. A few review papers were devoted to the liquid-phase oxidation of alcohols and carbohydrates on metal catalysts [31,48, 75-77]. 

Liquid-phase oxidation of carbohydrates on supported metal catalysts results in high selectivity which occasionally, e. g. in glucose oxidation, can match or surpass that of enzymatic processes. Metal-catalyzed oxidation also affords high productivity, e. g. up to 8 mol h (gpd) for oxidation of glucose on Pd-Bi catalysts [39]. These processes have the important advantages of high simplicity of operation ( one-pot reaction) and environmental acceptability, because almost no harmful effluents are generated. 

As compared to conventional petrochemicals, the significant hindrance of carbohydrates induces many diffusional limitations and activity of solid catalysts is obviously strictly governed by the accessibility of the catalytic sites. In this context, the porosity of commonly used siliceous-based catalysts or metal oxides is not crucial since, because of the steric hindrance of carbohydrates, the catalytic reaction mainly takes place on the catalyst surface. In the case of organic polymers, utilization of flexible polymeric chains considerably improves the accessibility of the catalytic sites. 

Mild reaction conditions using catalytic triflates of rare earth metals were also developed. This was based on the better Lewis acid properties of the catalysts, their ready availability and easy handling. An alternative is the use iron(iii) triflate. In carbohydrate chemistry, iron(iii) triflate has only been used for oxidative C-C bond cleavage, thioglycosylation of peracetylated glycosides and type I Ferrier rearrangement of glucal. ... 

These processes involve a multistep transformation from the carbohydrate fraction to the value-added products which makes most of them far from commercialization. Hence, intensive efforts are stiU required to enable scale up of synthetic protocols developed on a lab-scale into industrial processes. Some of the current drawbacks might be overcome by the one-pot transformation of lignocellulose carbohydrates in value-added chemicals without isolation of the intermediate platform molecules (Delidovich et al., 2014). Moreover, nanoporous materials, such as acidic, basic or metallic catalysts (zeolites, mesoporous silicas, microporous/mesoporous carbons, resins, metal oxides, etc.), wUl play a crucial role in this biomass transformation (Wang and Xiao, 2015). 

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