Heterogeneous catalytic conversion of biomass to added-value biofuels and chemicals
Lignin is a complex, interlinked, heterogeneous biopolymer found in biomass which consists of phenylpropane subunits. The phenolic monomers of lignin are linked via various ether and carbon-carbon bonds. It is the second most abundant biopolymer after cellulose, accounting for 15-30 % of the lignocellulosic constituent with the potential to be a renewable source of small aromatic feedstock molecules. The catalytic conversion of lignin from renewable biomass sources to added-value biofuels and chemicals is important in meeting the biorefinery concept. One of the main aims of this research was to investigate halide salts to enhance the stability and longevity of catalysts as well as for the in situ redispersion of deactivated/sintered catalyst in a hydrogenolysis reaction. Hence, this thesis reports the conversion of lignocellulosic biomass to added-value biofuels and chemicals using primarily platinum group and related metal catalysts on a range of supports. The major biofuels/biochemicals extracted from the hydrogenolysis of solid digestate (SD) were ethylguaiacol, propylguaiacol and dihydroconiferyl alcohol. Without the addition of halides there is little or no catalytic activity after 1 h based on the quantity of bio-oil produced. In the absence of halide salts the catalysts were found to have completely deactivated/sintered after 4 h reaction time; though the catalysts could be redispersed post reaction while activity could also be prolonged using in situ halide salts with the ability to redisperse being in the order NaI> KI> NaBr> KBr > NaCl> KCl. Rh/C, Pd/C, Ru/C, Pt/C, Ni/ZrO2 and Cu/Al2O3 catalysts were used for the hydrogenolysis reaction with Ni/ZrO2 performing the best in terms of bio-oil yield. The addition of halide led to an increase in bio-oil yield for all the studied catalysts with a 91 % increase in yield of bio-oil for Rh/C after 4 h. Importantly, it was found that only a very small amount (0.006 %) of the Rh leached into the bulk medium, which can be considered negligible. The performance of the halide salts is attributed to their solubility which is intrinsically linked to the metal-halide bond dissociation energy, with NaI having the highest solubility (3.0 kg/L) and lowest bond dissociation energy (304.2 ± 2.0 kJ/mol). Also, the pyrolysis kinetic triplet of residual SD and thermal predictions was reported for the first time using Advanced Kinetics and Technology Solutions (AKTS) thermokinetics software to model and calculate the activation energy (Ea) and other kinetic parameters. ASTM-E698, Ozawa-Flynn-Wall and Differential iso-conversional (model-free) methods were used and the Ea values calculated from each model were 169.8, 75 – 175.0, and 85 – 190.0 kJ mol-1, respectively. The kinetic triplet can be used in the scale-up or designing of reactor systems considering SD as a feedstock. The kinetic prediction of isothermal pyrolysis of SD indicated that a temperature higher than 210 °C is required for onset of decomposition in the sample. Furthermore, SD has the potential to produce an additional ~27.9 MJ per day at a 500 kW standalone on-farm AD plant. These results have the potential to have a significant impact on the design and operation of heterogeneous catalytic processes for biomass valorisation. Thesis embargoed until 31 December 2026.
Biofuels/chemicals, biomass, catalysis, solid digestate, AKTS simulation, redispersion
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