Hydrodeoxygenation (HDO) catalysts Characterization, reaction and deactivation studies

Doctoral thesis, 2018

Production of biomass derived fuels such as renewable diesel, are primarily intended to reduce the reliance of the conventional engines on petroleum fuels as well as the emission of CO2 from fossil hydrocarbons. Hydrodeoxygenation (HDO) is a powerful technique that has been regularly used in the fuel upgrading processes. Sulfided molybdenum catalysts, supported on alumina and promoted by cobalt or nickel, are frequently used in the HDO processes. Despite the high efficiency of the HDO, catalyst activity and selectivity have raised major concerns from the economic and technological perspectives. The aim of this study was to gain a better understanding of the HDO catalyst structure, and then to assess the effect of different pretreatment and operational conditions on the catalyst activity and selectivity.

The effect of preparation and pretreatment conditions on hydrogen uptake capacity and dispersion of the prepared Ni, Co and Mo containing catalysts was evaluated by using several characterization techniques such as BET, ICP-SFMS, SEM, TEM, TPO, ethylamine-TPD, XPS and H2-chemisorption. The H2-chemisorption, XPS and SEM results confirmed the detrimental effect of calcination on hydrogen uptake capacity of catalysts. The effect of pH of the impregnating solutions on the dispersion of the metal phases was also assessed from the TEM experiments. Moreover, HDO reactions of oleic acid and abietic acid over a prepared sulfided NiMo catalyst were studied. The results show that addition of DMDS to an oleic acid feed clearly promoted maintenance of the active sulfided phases on the NiMo catalysts. Higher concentration of DMDS also promoted the decarbonylation/decarboxylation (DCOx) route, and more importantly, decreased the amount of carbon deposition on the NiMo catalyst. On the other hand, addition of abietic acid to an oleic acid feed, is shown to decrease the deoxygenation rate of the oleic acid and increase the amount of carbon deposition on the catalyst. The inhibition effect of abietic acid on the HDO of oleic acid was related to stronger adsorption of the bulkier abietic acid molecules on the active sites compared to oleic acid that may have sterically hindered adsorption of oleic acid on neighboring sites.

Furthermore, the poisoning effect of iron on the HDO of oleic acid over sulfided NiMo and Mo catalysts was investigated. It is shown that addition of iron to an oleic acid feed decreased the oxygenate conversion activity of both catalysts and changed their selectivities towards the final products. TEM results of the poisoned spent NiMo catalyst revealed that iron was mainly deposited on and in the vicinity of the Ni particles. This may also indicate that iron has reacted with Ni phase and as a result modified the catalyst activity. Finally, hydroconversion of rosin acids over supported NiMoS catalysts on alumina, USY-zeolite and mixed alumina/USY-zeolite was investigated. Various catalyst properties such as dispersion of the NiMo phases and Brønsted acidity affected the selectivity for the products. The results also indicate that the Brønsted acidity of the support could be optimized by the USY-zeolite content of the catalyst to achieve a satisfactory level of deoxygenation, ring opening and cracking of the rosin acid while avoiding excessive coke formation.