Design and synthesis of novel catalysts for liquid phase biomass upgradation

The continuous diminishment of fossil fuels has led to significant interest in alternative energy sources, which in turn has boosted modern-day research on potential sources of renewable energy such as biomass, solar, hydrothermal etc. Among the aforementioned, biomass-based green energy, which is mainly related to the conversion of biomass into fuels, has received a very high amount of interest due to the high accessibility of biomass and potentially lesser expense involved in the conversion of biomass into fuels as compared to some other forms of renewable energy. In the past decades, first-generation biofuels (ethanol and bio-diesel) were manufactured from edible biomass. Modern research however has been concentrated on the transformation of lignocellulose biomass which originates from non-edible resources into fuels and chemicals. Among the many types of processes that have been researched for the conversion of biomass to fuels, catalytic conversion has been deemed to have the most potential due to environmental and economic advantages. The development of new catalysts for the selective upgradation of biomass derived chemicals under ambient reaction conditions is still challenging however due to the active nature of the compounds in the biomass (high oxygen content). In this research project the main aim was to investigate and develop novel catalysts for the conversion of compounds readily derived from biomass. The catalysts that were investigated were composed with both noble (Pd, Ru and Ag) and non-noble metals (Cu, Fe and Co) with various supports (metal oxide, heteropoly acids and carbon). Some new approaches for preparing these types of catalysts have been offered in this thesis, such as the use of a heteropoly acids in hydrogenation catalysts, metal/metal oxide composites derived from metal organic frameworks and a core-shell nanoalloy as hydrodeoxygenation catalyst which are extraordinarily stable and recyclable as well as provide the selective catalytic efficiency under moderate reaction conditions towards the desired product. <br><br> In this thesis, chapter 3 and 4 concentrated on the hydrogenation and hydrodeoxygenation of furfural into tetrahydrofurfuryl alcohol (THFAL) and methyl furan (MF) over catalysts that were comprised of palladium exchanged heteropoly acid supported on different metal oxides and non-noble metal-based (metal/metal oxide derived from metal organic framework) catalysts respectively. In chapter 5 and 6 the research reported focused on the conversion of levulinic acid (LA) which is a significant biomass derived platform chemical into gammavalerolactone (GVL) over Ru exchanged tungstophosphoric acid and a combination of noble and non-noble metal based (different ratio of Ag and Co supported on alumina) catalysts respectively. The catalysts studied were prepared by a simple wet impregnation method, thermal pyrolysis and reduction procedures which can be easily approachable and beneficial for the large-scale production of catalysts. The influence of the variation of reaction parameters such as temperature, pressure, reaction time, catalyst weight etc. on the selectivity of the targeted product was investigated thoroughly and the reaction conditions were optimized for the selective transformation of the reactant into a particular product. To explain the catalytic proficiency and the interconnection between the structure and catalytic activity a range of physicochemical studies for instance XRD, temperature programmed reduction (H2-TPR), CO pulse chemisorption, NH3-TPD, Pyridine absorbed FT-IR, Raman, insitu ATR-IR. BET Surface area, SEM, TEM EXAFS and XPS have been conducted. Because, all of the reactions involved a hydrogenation step, the active metal surface area which is measured from chemisorption, was one of the contributing factors for high catalytic conversion as the metallic surface area basically activates the breakage of H-H bonds. For example, the PdMPAV2/Al2O3 catalyst, Cu/CuFe2O4@C-A, Ru3TPA and 5-15 catalyst which exhibited comparatively higher active metal surface area than comparable catalysts that were studied was attributed to the better catalytic efficiency observed for these catalysts. In addition, the quantitative percentage of metallic state (oxidation state) on the catalyst's surface which is calculated by XPS was also found to influence the activity of the catalysts studied. For instance, the 5-15 catalyst which contained the highest amount of metallic cobalt, which is the active metal in the hydrogenation reaction, showed maximum conversion of levulinc acid (LA) into gamma-valerolactone (GVL) and a similar observation was noticed for the Cu/CuFe2O4@C-A catalyst. The acidity of the catalysts also played a significant role in the hydrodeoxygenation reaction. NH3-TPD studies showed that weak acidic/Lewis acidic sites facilitated the deoxygenation reaction that generated the products in the hydrodeoxygenation process. The Cu/CuFe2O4@C-A catalyst which exhibited a higher amount of Lewis acidic sites as compared to the as-synthesized conventional graphite supported catalyst (Cu/CuFe2O4@C-B) showed better conversion of furfural into MF that is produced via the HDO pathway. Similar findings were also observed for the Ru3TPA and 5-15 catalyst which contained a considerably higher amount of Lewis/weak acidic sites displayed higher catalytic efficiency for the hydrodeoxygenation reaction of LA into GVL. The stabilization of the metallic nanoparticles in the support matrix was also identified as one of the key aspects for the effectiveness and the stability of the catalyst in the reaction media. Different approaches were implemented in this thesis work such as encapsulation of metal/metal oxide in a carbon framework or the cobalt silver alloy formation to prevent the leaching of the active metal in the liquid phase. The encapsulation effect or the strong synergistic effect which is generated due to the aforementioned alloy formation can also modulate the electronic structure or environment which can be evidenced by the binding energy shifting in the XPS spectra of the metals such as copper/iron and cobalt /silver respectively. EXAFS studies also suggested that the formation led to a unique geometric effect in the catalysts. The morphology of the catalysts which was investigated by electron microscopic techniques such as TEM and FE-SEM showed that the catalysts which were composed with well dispersed uniform nanoparticles displayed high catalytic performance. The effect of different supports, the proportion of metals (composition), the reduction and calcination temperature also were found to have a huge impact on the performance of the catalyst and these effects were investigated in this project. <br><br> Kinetic studies were conducted for selected systems (furfural to methyl furan conversion and levulinic acid to gamma-valerolactone conversion) to evaluate the activation energy of the particular reaction. The results obtained suggested that the catalysts with high productivity exhibited low activation energy. The probable reaction mechanism was also proposed depending on the active component of the catalyst.  Some in-situ studies such as in-situ ATRIR were conducted to understand the interaction of the active component with the reactant. Recyclability tests were conducted to confirm the heterogenicity of the catalyst.  <br><br> This thesis offers a strategic and sustainable approach to synthesize highly recyclable catalysts for the selective upgradation of biomass-derived platform molecules into versatile chemicals, fuels and fuel additives. The detailed studies also provide a deep understanding between the structure activity relationship and the reaction mechanistic pathway that assist to select the active component of the catalyst for a specific reaction. This research opens the new opportunity for the development of catalysts in the biomass refinery industries which is the main platform to produce alternative fuels.