Hydrodeoxygenation of fatty acid esters for biofuel production over sulfur-free catalysts

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Yuwei Bie Name of the doctoral thesis Hydrodeoxygenation of fatty acid esters for biofuel production over sulfur-free catalysts Publisher School of Chemical Engineering Unit Chemical and Metallurgical Engineering Series Aalto University publication series DOCTORAL THESES 173/2022 Field of research Chemical Engineering Manuscript submitted 11 July 2022 Date of the defence 2 December 2022 Permission for public defence granted (date) 31 October 2022 Language English Monograph Article thesis Essay thesis Abstract Liquid biofuels are playing an important role to reduce the carbon footprint in the road transportation and aviation sectors. Catalytic hydrodeoxygenation (HDO) is an attractive technology to upgrade various bio-based feedstock such as triglyceride-based oils and fats or lignocellulose-based bio-oils into drop-in hydrocarbon biofuels. The main objective of this thesis was to gain new mechanistic insights into the HDO reactions over sulfur-free catalysts using aliphatic methyl esters and fatty acids as model components. The main focus was on Rh/ZrO2 catalyst. Supported cobalt catalysts were also tested to explore the effect of support acid-base property for the deoxygenation pathway. In addition, we were aiming for developing a suitable kinetic model for supporting the design of the HDO process. HDO experiments of methyl heptanoate, methyl palmitate and their corresponding fatty acids were conducted in a batch reactor under different temperatures and pressures. Reaction pathways for the HDO of methyl ester over Rh/ZrO catalyst were proposed. The fatty acid was initially formed 2 by the hydrogenolysis reaction, which was further converted into C alkane by the sequential n-1 hydrogenation-decarbonylation reaction steps via forming aldehyde as a surface intermediate. The catalytic activity of ZrO support was studied, based on which the active sites of Rh/ZrO catalyst 2 2 were discussed. Both Rh metal and ZrO were found to contribute to catalyzing the HDO reactions, 2 indicating the multifunctional nature of Rh/ZrO catalyst for HDO application. The interface 2 between rhodium nanoparticle and ZrO was advocated to be the most active zone. Experimental 2 results with cobalt catalysts indicate that acidic support was favorable for the deoxygenation pathway to alkane products, whereas basic support favored the formation of alcohol and suppressed the deoxygenation pathway. Kinetic experiments for the HDO of methyl heptanoate and palmitate were studied over Rh/ZrO2 catalyst in a batch reactor at a wide range of temperatures (250-330 °C) and pressures (40-100 bar). Power-law type of kinetic model and a more detailed mechanistic model were both successfully applied to fit the kinetic data. A surface reaction mechanism was proposed, based on which the mechanistic model was developed. The model includes two types of catalytic active sites: one for oxygenated components and the other for hydrogen. During the estimation of kinetic parameters, vapor-liquid equilibrium and the thermodynamic non-ideality were taken into consideration in a batch reactor model for mass balance calculation. The modeling methodology and reaction mechanism insights are applicable for further studies of more complex HDO process of biomass-derived feeds.