The sustainability of human civilization primarily depends on whether the energy requirements of its increasing population can be satisfied in the future. While the establishment of the exact date of the depletion of fossil fuels seems difficult, skyrocketing oil and gas prices could come much earlier than 2050, as predicted by several studies.1 Efficient conversion of solar energy to electricity could open the way to the production of increasing amounts of hydrogen and finally to the development of a hydrogen economy.1 Of course, the increasing population will demand increasingly larger volumes of carbon-based consumer products. Carbon dioxide is the simplest renewable carbon source, and there are several studies on the hydrogenation of CO2.2 Since there is no effective direct CO2-based process known to produce large amounts of organic chemicals, nature can help to convert CO2 to biomass, which could serve as the renewable carbon resource. The hydrogenation of carbohydrates could be one of the approaches to produce key carbon-based intermediates.3 If the currently forecasted transition to a hydrogen economy materializes in the coming decades,1 hydrogen would be available in large quantities at reasonable prices for the conversion of carbohydrates to a variety of industrial chemical products, including oxygenates or even hydrocarbons such as hexanes and pentanes. The latter could truly be considered as a "renewable oil," ready to be used by existing technologies.
The acid-catalyzed dehydration of carbohydrates giving 5-hydroxymethylfurfural4 (Figure 4.1) and levulinic acid5 (Figure 4.2) is one of the most studied and used conversions of carbohydrates. Since it is known that these acid-catalyzed dehydrations proceed through C,C- and C,O-double bonds containing intermediates, we have been investigating the possibility of combining the dehydration of carbohydrates with in situ hydrogenation of the reaction intermediates.6
Since carbohydrates are water soluble and the expected products have lower solubility in aqueous phase, the use of water-soluble catalysts for hydrogenation offers the possibility of facile product separation and catalyst recycling.7 When sucrose (0.6 mol/l) was dissolved in an aqueous solution of sulfuric acid (1.8 mol/l) and treated with hydrogen (85 bar) at 140°C for 4 h in the presence of an in situ-generated Ru
-oh ho ho hoh2c ho ho oh
o ho cho cho oh
FIGURE 4.1 Formation of 5-hydroxymethylfurfural from glucose.
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