The catalytic dehydration of xylose and glucose into high value chemicals-furfural and 5-(hydroxymethyl) furfural

Xylose and glucose, are carbohydrates derived from lignocellulosic biomass which can be strategically valorised to a variety of platform chemicals and fuels. Catalytic dehydration of these carbohydrates leads to the formation of furfural and 5-hydroxymethylfurfural (HMF), which are potential substitutes for the displacement of petrochemicals. The challenges for the efficient production of these high-value chemicals are the development of a highly selective processes using cost-effective catalysts and reasonable reaction conditions on an industrial scale. Thus, this thesis is aimed at advancing the potential application of homogeneous and heterogeneous catalysis in aqueous phase systems for the synthesis of furfural and HMF under mild reaction conditions. In the initial stage, the catalytic effect of different metal tri-chlorides (FeCl3, AlCl3 and CrCl3) with formic acid (FA) was investigated for the selective conversion of D-xylose to furfural. Reactions were carried out at various temperatures (100-170 °C), metal chloride and FA concentrations. The catalytic mixture containing 0.4 M AlCl3 and 55 wt.% FA gave the highest furfural yield and it was successfully selective at low temperature (130 °C, selectivity ≈70%). Additionally, a kinetic model to simulate the experimental data was developed resulting in a realistic prediction tool. Likewise, the promotion of different metal tri-chlorides with formic acid as catalyst was investigated for the conversion of glucose to HMF. Similarly, the same AlCl3-FA mixture gave the highest HMF selectivity (130 °C, selectivity ≈30%). However, the kinetic model developed showed a limited capability to predict data at high FA concentration (> 30 wt.%). Thirdly, bulk and modified tin oxide catalysts (SnO2, SO42-/SnO2 and SO42-/Al2O3-SnO2) were synthesised and their catalyst performance were investigated for the conversion of glucose to HMF. The SO42-/Al2O3-SnO2 calcined at 350 °C showed the highest chemo-selectivity to HMF (120 °C, selectivity ≈53%). Moreover, recovery and the lifetime of the catalyst were evaluated. Despite applying simple regeneration methods, the decline in catalytic activity was inevitable with successive runs. In the last stage, sulphated bulk and binary-oxides catalyst, containing TiO2, SnO2, ZrO2 and Fe3O4, was explored for the production of furfural and HMF. The catalyst SO42-/SnO2-TiO2 composed of 7:3 molar ratio and calcined at 350 °C, presented advantageous catalyst features such as surface area, acid sites and high sulphur content; achieving better results to furfural and HMF (145 °C, selectivity ≈35% and 23% respectively) among the oxides tested. However, the internal transport phenomena was shown to limit this catalytic reaction system.