Reaction and characterization studies of the conversion of biomass-derived carbohydrates on supported metal catalysts

The decomposition of carbohydrates and polyols (C:O = 1) on metal surfaces occurs through C-H, C-C and C-O bond cleavage reactions. A decomposition mechanism that is dominated by C-C cleavages will yield primarily CO and H2 (and CO2 if water-gas shift is occurring) whereas high rates of C-O cleavage produce alkanes. If C-O cleavage reactions do not proceed to completion, intermediate products with lower functionality (C:O <1) are formed. To this end, the conversion of sorbitol and glucose on carbon supported Pt and PtRe catalysts at 483-523 K and 17-27 bar integrates hydrogen generating C-C cleavage reactions (reforming) with hydrogen-consuming C-O cleavage reactions (deoxygenation) to yield hydrophobic monofunctional species.;These monofunctional molecules include C4-C6 alcohols, ketones, carboxylic acids and heterocyclic compounds, which separate spontaneously from an aqueous phase containing more hydrophilic reaction products. At 503 K and 18 bar, over a PtRe catalyst, 52% of the carbon contained in a 60 wt% sorbitol solution can be converted into a hydrophobic liquid phase rich in the aforementioned monofunctional species. At these conditions, 65% of the energy contained in the feed is stored in the hydrophobic products. This resulting mixture represents a petroleum-like platform that can be fractionated to obtain useful commodity chemicals or chemical intermediates, or it can be subjected to a variety of C-C coupling and isomerization processes to produced gasoline or diesel fuel components.;The addition of rhenium to platinum promotes both reforming and deoxygenation reactions, leading to greater yields of mono-functional species from polyol conversion. Characterization results suggest intimate interaction between platinum and rhenium. Platinum catalyzes the reduction of volatile rhenium oxide precursor species to the metallic state during catalyst pre-treatment. The resulting particles possess a bimetallic composition and are 1-2 nm in diameter. Furthermore, rhenium prevents the sintering of catalyst particles during exposure to reaction conditions. Based on reactivity and characterization studies, a model of the PtRe active surface site was proposed, in which Pt sites neighbor partially oxidized Re species. The Re acts to reduce the CO binding enthalpy of the nearby Pt sites, promote water-gas shift and participates in hydrogenolysis (C-O) cleavage reactions.