One-pot Conversion of Biomass to Chemicals on Ni-Cu/ZnO Based Catalysts
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Optimization of the one-pot hydrothermal reaction of cellulose to polyols have been investigated. Ni-Cu-ZnO catalysts supported by carbon nanotubes (CNT) with varying nickel (Ni) and copper (Cu) mole ratio was synthesized by Pechini method, incipient wetness impregnation and several thermal methods. Characterization of the catalysts was performed, as well as testing of different reaction conditions. The optimized conditions were hereafter applied to investigate the impact of Ni and Cu ratio in the catalysts on the product distribution. A great effort has also been invested in developing a method for identification of carbon content in the product solution.X-ray diffraction (XRD) analysis confirmed the formation of Ni-Cu-Zn alloys in the core par- ticles of the catalyst, however X-ray photoelectron spectroscopy (XPS) analysis found metal oxides on the surface, which most likely originate from oxygen passivation of the reduced catalysts. Several characterization methods confirmed decreasing surface area and increas- ing crystalline size with amount of copper added. No outstanding trend between basic sites on the catalyst surface and product yield was observed.The optimal running conditions were found by the use of 6 hours reaction time, a temperature of 255 °C, 25 mL water and a feedstock:catalyst ratio of 3:1 in a 170 mL reactor pressurized with 50 bar (RT) H2 gas. Optimized conditions increased the conversion of cellulose to alcohols and polyols. Polyols as 1,2-propylen glycol, ethylene glycol,1,2-butylen glycol and 1,2 hexandiol was produced, as well as 2-butanol and terahydrofurfuryl alcohol. Use of a 4Cu:4ZnO/CNT catalyst obtained the total highest yield of alcohols and polyols of 56.11 %. Cu content clearly promoted the production of 1,2-hexandiol, but did also show a slightly effect on the ethylene glycol yield. However, the Cu catalyst did not achieve the highest yield of every product produced. An even ratio of Ni and Cu gave the highest 1,2-propylen glycol and 1,2-butylen glycol yield, increased Ni content promoted production of tetrahydro- furfuryl alcohol, and a 1Ni:3Cu:4ZnO/CNT catalyst was most promising for 2-butanol production. This alteration in the product distribution by changing the Ni and Cu content can therefore be used to tune the reaction towards a desired composition of the product solution. The 4Cu:4ZnO/CNT catalyst was applied to a reaction with woody biomass, pine wood, as feedstock. This resulted in a pine wood conversion of 82% and a production of diols in a total yield of 35.65 %. The diol yield may imply that formation of diols are favored over production of alcohols, which had a yield of only 2.65 %. This can indicate that hemicellulose is more easily converted into polyols than alcohols.