The depleting supply of petroleum has motivated research on developing alternative technologies from renewable resources (biomass) for fuels and chemicals. Hydrogenolysis of biomass-derived sugars and sugar alcohols is an important class of reactions to produce valuable megaton products such as 1,2 or 1,3-propanediol, ethylene glycol, lactic acid, hexanediols and hydrocarbons. Although, hydrogenolysis of glycerol has been extensively studied in recent years , relatively limited work exists on the hydrogenolysis of C5 and C6 sugars and corresponding polyols. A major challenge in these processes is to achieve high selectivity to desired products and hence to design appropriate catalyst formulations. For example, hydrogenolysis of C5 and C6 sugars/polyols involve a number of consecutive and parallel reaction steps, such as C-C and C-O bond breakage, dehydration, aldol condensation and dehydrogenation. These steps are affected to different degrees by the catalytic components, preparation methods and structural characteristics. Hydrogenolysis of xylitol and sorbitol follows retro-aldol mechanism[2,3] over monometallic catalysts (Ru or Ni) in basic medium, which leads to C-C and C-O bond breakage. The complex reaction mechanism underlying the wide product distribution observed in these hydrogenolysis reactions is not yet well understood. In this presentation, the role of mono and bimetallic Ru and Cu based catalysts, acid and base promoters and reaction conditions on hydrogenolysis of sorbitol, xylitol and glucose will be systematically addressed with emphasis on discerning the reaction pathways. Particularly, the activity/selectivity behavior will be discussed.
Our initial results showed that the presence of a solid base enhances the activity of Cu and Ru monometallic catalysts in sorbitol hydrogenolysis. Cu activity is increased by more than two fold by using a solid base promoter. Similarly, the activity (expressed as TOF) of Ru/C catalysts increased from 110 h-1 to 214h-1 and 145 h-1 in the presence of MgO and CeO2 respectively, at pH=7~9 and 483 K. With bimetallic RuRe and RuCo catalysts, the catalytic activity further increased when CeO2 was used as a promoter. Secondly, it was observed that the presence of Re also increases the selectivity of C3 products (glycerol and 1,2-propanediols) by approximately 10% compared to monometallic Ru catalysts while RuCo shows similar product distribution as with Ru/C. This observation provides important information for reaction pathways of sorbitol hydrogenolysis. Further work on reaction pathways, kinetics and mechanism, and catalyst characterization is in progress, and the results of which will be presented.
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