(38b) Model Aldehyde Chemistry Using Cezrox Catalyst for Hydrothermal Liquefaction Upgrading

Green processes that upgrade biomass feedstocks and recycled bio-waste can both help meet our growing energy demands as well as reduce our overall carbon footprint. One promising technology is hydrothermal liquefaction (HTL), which breaks down wet organic solids at moderate temperatures and high pressures to form a carbon rich oil phase. While the bio-oil formed has a high heating value, the process often produces other byproducts such as solid biochar, carbon dioxide and wide distribution of organics that are water soluble. To design an economical HTL process, the water soluble organic byproducts must be minimized to increase oil product yields and reduce wastewater generation. Metal oxide catalysts are a promising option for HTL upgrading chemistry due to their catalytic activity as well as stability in hot liquid water conditions. CeZrO_x, in particular, is active for acid coupling reactions which simultaneously increase the carbon number of the product and decrease its oxygen to carbon ratio, both of which act to increase the oil yield and decrease the carbon loss to the aqueous phase. In our study, we sought to understand the key reactions that take place in the HTL process when using CeZrO_x as a catalyst.

In preliminary runs, use of CeZrO_x increased oil yields relative to other commonly used catalysts in the HTL conversion of food waste at 300 °C and 20.7 MPa. Subsequently, tests were performed to expose CeZrO_x to hot liquid water conditions at 300 °C and 20.7 MPa for up to 165 hours. Post-run characterization was performed using both XRD and XPS to verify the hydrothermal stability of CeZrO_x, Model HTL reactions using CeZrO_x were also performed with water soluble alcohols, aldehydes, ketones and carboxylic acids as reactants. CeZrO_x had minimal activity for catalyzing reactions involving alcohol-alcohol, carboxylic acid-carboxylic acid or ketone-ketone coupling. In contrast, CeZrO_x was highly active – and selective – for aldehyde coupling reactions, with the primary product being alkenes. For example, pentanal self-condensation reactions under HTL conditions produced a decene product that phase separated into an oil phase at room conditions. On-going work is focused on optimizing the catalyst properties and catalytic reactor conditions.