Cellulosic biomass provides renewable alternatives to fossil-fuel resources for the sustainable production of liquid fuels and valuable chemicals. The challenge for the effective utilization of biomass resources is to develop cost-effective processing methods for the transformation of carbohydrates into value-added chemicals. Carbohydrates, predominantly cellulose, represent the largest fraction of biomass, and various strategies for their efficient use as a commercial chemical feedstock are currently being established with the aim to supplement and ultimately replace fossil fuels. To achieve this goal, it is pivotal to develop more efficient and environmental-friendly methods to convert cellulose into useful chemicals. One possible conversion route of cellulose is through the synthesis of 5-hydroxymethyfurfural (HMF), which the US Department of Energy has classified as one of the most promising renewable molecules.
This thesis is aimed at advancing the potential application of heterogeneous catalysis and water-organic biphase system for the synthesis of HMF directly from solid biomass. Firstly, we undertook a literature survey of the current strategies employed for HMF production with more attention on solid catalysis.
Secondly, we explored the catalytic potential of non-toxic TiO2 nanoparticle prepared by sol-gel technique on glucose conversion into HMF in an aqueous reaction system. Catalytic performance of TiO2 was modified by introducing a second metal oxide (ZrO2) to form binary oxides. Compared to pure TiO2, the binary oxide displayed better activity in terms of HMF yield, which was attributed to optimum balance of basic and acid sites for the tandem isomerization-dehydration reactions. Solvent effect was also studied in order to promote the reaction preferentially towards target product. Consequently, a biphase system of water-organic solvent mix was utilized and in combination with TiO2-ZrO2/Amberlyst 70 catalyst system, a remarkable yield of HMF can be produced from glucose.
Thirdly, catalytic properties of TiO2 nanoparticle were fine-tuned to develop a single solid acid bifunctional (Lewis and Brønsted acidity) catalyst. To achieve this goal, TiO2 was modified with phosphate anion and evaluated as catalysts for the conversion of glucose to HMF in a water-butanol biphasic system. Catalyst synthesis protocol was optimized by varying loading amount of phosphate anion and calcination temperature. X-ray spectroscopic analysis confirmed phosphorus incorporation into the TiO2 framework giving rise to small-sized nanocrystals with high surface acidity. Pyridine-infrared (Py-IR) spectroscopy on the TiO2 catalyst confirmed the presence of bifunctional Lewis and Brønsted acid sites as compared to pure TiO2 with only Lewis acidity. The phosphate modified TiO2 demonstrated excellent catalytic performance in terms of both activity and selectivity, as well as good recyclability.
Fourthly, the robustness of phosphated TiO2 catalyst to transform a variety of sugar moieties ranging from simple to complex ones was evaluated in a water-THF biphasic system. Addition of N-methyl-2-pyrrolidone (NMP) to the biphasic medium enhanced the overall effectiveness of the reaction process to selectively produce HMF through: a) suppressing humin formation and b) preventing rehydration of HMF. The sugar moieties were successfully transformed giving ≥ 80% HMF yield. In the case of cellulose, its crystalline structure strongly inhibited its reactivity. Because of its poor reactivity, structural deconstruction of cellulose was carried out by acid catalyzed solid state depolymerization using a ball milling system (mechanocatalysis). Subsequently, conversion of the highly reactive water soluble oligomers that resulted from the mechanocatalytic deconstructed cellulose, gave improved yield of HMF. To simulate an industrial production of HMF under a continuous process, a bench scale biphasic flow reactor was designed and utilized for the conversion of solubilized cello-oligomers into HMF under which a reasonable yield of HMF (53%) can be produced.
Lastly, the potential realization of an efficient eco-friendly catalytic process for converting cheap source of renewable biomass-derived carbohydrates into 5-hydroxymethylfurfural was explored. A more facile synthesis of the highly active phosphated–TiO2 nanomaterial was designed by combining the previously two-stage technique to a one-pot route. Acid-catalyzed dehydration of cellulosic biomass (sugarcane bagasse and rice husk) into HMF was carried out in the presence of water–methyltetrahydrofuran (water–MeTHF) biphasic system modified with N-methyl-pyrrolidone (NMP). Chemical transformation of biomass to HMF was facilitated by solid state depolymerization. Eventually, sugarcane bagasse and rice husk were effectively converted to produce 72% and 65% yields of HMF, respectively. Even though production of HMF in a biphasic system is well documented, kinetic study of this reaction over a solid acid in a biphasic reaction system has never been reported. Therefore, kinetics study of cellulose–to–HMF reaction was conducted and a simplified kinetic model comprising of two reaction steps was developed: (a) hydrolysis of cello-oligomers to glucose; and (b) glucose dehydration to HMF. Finally, the obtained reaction rate parameters from the kinetic analysis could potentially serve as a valuable analogy in developing models for a scale-up biphasic process of HMF production.