Catalyst development for the aqueous phase reforming of biorefinery wastewaters

Aqueous phase reforming (APR) is a catalytic hydrothermal process that operates at relatively low temperatures (approximately 250 °C) and pressures (around 50 bar) to convert oxygenated hydrocarbons in aqueous solutions into hydrogen-rich gas. This process shows significant potential for recovering valuable gas products from biorefinery wastewater streams, thereby enhancing the overall efficiency of these technologies. Although APR has been extensively studied, its practical application remains constrained by the limited availability of catalysts with sufficient hydrothermal stability. The harsh operational conditions inherent to APR lead to the deactivation of catalysts, presenting a major challenge for its broader industrial implementation.This doctoral thesis aims to investigate the barriers associated with catalyst stability and activity during APR explore potential solutions to address these challenges and assess the overall feasibility of implementing APR in hydrogen or energy production. The research was performed on two levels: (i) lab-scale experiments focused on catalyst testing and improving the stability and activity of catalysts and, (ii) investigate viability of APR, through process modelling using experimental data to validate APR.The study investigated the coupling of APR with Hydrothermal Liquefaction (HTL). However, using model component of HTL wastewater (glycolic acid) as a feedstock for APR highlighted significant challenges. Common HTL wastewater components are prone to polymerization and condensation reactions, which led to inconsistent results during APR reactions. Switching the feed from glycolic acid to glycerol increased the stability of liquid products and resulted in more repeatable tests.The primary catalyst used for APR is γ-alumina supported metal catalysts, although its deactivation through transformation into boehmite is a well-documented phenomenon. Despite this, the extent to which this structural transformation is influenced by reaction conditions, and the subsequent impact on catalyst performance, remains insufficiently explored. Present research confirmed that organic molecules present in biorefinery wastewaters increases the stability of γ -alumina. It was found that glycerol could inhibit the crystallization of alumina into boehmite, though it did not prevent hydrolysis. Additionally, tests with wastewater model compounds (guaiacol, lactic acid, and glycolic acid) demonstrated that the stabilizing effect is not unique to polyols; other components can exhibit even stronger stabilizing effects than glycerol.APR tests of glycerol were conducted using a Pt-alumina catalyst. Although glycerol conversion remained stable after catalyst reuse tests, deactivation was observed, through reduced gas formation rates and changes in product selectivity. As the conversion of γ-alumina into boehmite was not significant between runs the decline in activity was attributed more to the sintering and reversible blocking of metal particles rather than the conversion of the alumina support. To counteract this deactivation, a polydopamine carbon coating was applied to the catalyst. Interestingly, no significant difference in performance was noted in the batch experiments between the coated and uncoated catalysts. However, in the flow reactor system, the dopamine-coated catalyst exhibited 30 times higher hydrogen formation rates while the original benchmark Pt-alumina showed increased hydrocarbon selectivity. The coating could not prevent the formation of boehmite, it did maintain ten times higher metal dispersion and three times bigger surface area.La based perovskite materials (LaAlO3 and LaNiO3) showed conflicting stability results for the APR in previous studies of glycerol while showing good catalytic activity. In certain studies LaAlO3 catalyst was completely stable while other studies reported its conversion into LaCO3OH. This study present a systematic investigation of key reaction parameters (WHSV, hydrogen/gas–liquid ratio, time, pressure) on the activity and stability of Pt/LaAlO3 perovskite catalysts during the APR of glycerol. Reactor configuration (batch vs continuous) strongly influences product selectivity. While batch conditions promote lactic acid production which contributes to the complete decomposition of LaAlO3 phase and activity loss within 6 h, flow conditions facilitate the hydrogenation of hydroxyacetone and 1,2-propanediol intermediates and direct the progression of the reaction pathway that bypasses the lactic acid formation. After an initial induction period, associated with partial perovskite transformation into LaCO3OH and doubling of Pt nanoparticle size, catalysts remained stable over an extended 30 h experiment. Pt-LaAlO3 catalyst performance was further investigated in flow system and was found that activity increased with increasing gas–liquid phase ratio (co-feeding of argon). Additionally particular improvement was observed in catalyst performance after decreasing the reaction pressure into the 2-phase region (20 bar), resulting in 5-fold increase in conversion and complete retention of the LaAlO3 phase.A process simulation was conducted to evaluate the energetic feasibility of APR using experimental data. While APR has been reported to demonstrate good energetic sustainability, experimental studies often utilize an inert argon or nitrogen gas as an internal standard for gas analysis during the reaction. The inert gas does not take part in the reaction however it induces water evaporation, and at the highest gas-to-liquid ratio, it increases energy consumption by a factor of five. To provide a more realistic assessment, experimental results were used to estimate the reaction pathway, and these conditions were incorporated into the reactor settings to evaluate energetic sustainability and the potential for energy and hydrogen production. The modelling results indicated the potential for achieving energy self-sufficiency at conversion rate over 80% with a 10 wt.% glycerol feed if product gas is used for combustion. The potential for hydrogen production was compared to other technologies such as methane reforming (11-51 kW h kgH2- 1) and water electrolysis (36-65 kW h kgH2- 1) and showed comparable specific energy consumption then the fossil technologies at conversions over 80% (45-54 kW h kgH2- 1). Although the specific heat intensity of hydrogen production, based on experimental conversion rates, demonstrated a higher specific energy requirement (95-560 kW h kgH2- 1) than other technologies, APR could become competitive if the process reaches high conversion and hydrogen selectivity.