Hydrodeoxygenation (HDO) catalysts Characterization, reaction and deactivation studies
Characterization, reaction and deactivation studies
Department of Chemistry and Chemical Engineering
Chalmers University of Technology
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.
Keywords: Renewable diesel, Hydrodeoxygenation (HDO), Hydroconversion, Ring opening, Carbon deposition, Sulfided phase, Deactivation, Poisoning, Brønsted acid sites
Brønsted acid sites
Chalmers, Kemi och kemiteknik, Kemiteknik, Kemisk reaktionsteknik
Producing biofuel from renewable sources such as biomass can reduce the reliance on fossil fuels as well as the emission of carbon dioxide (CO2) which is a greenhouse gas. The produced CO2 from biofuels is recycled in the earth’s biosphere therefore biofuels emissions are considered carbon neutral. Sustainable biomass resources include wood and wood wastes, agricultural crops and their waste by-products, animal wastes, wastes from food processing, aquatic plants and algae. Tall oil is a by-product from the alkaline pulping (Kraft pulping) process in the pulp and paper industry which is used as a feedstock for the production of renewable diesel fuels. Tall oil is an American term adopted from the Swedish word "tallolja" (pine oil). Tall oil is a sustainable resource that can also be produced on a cost competitive basis compared to other bio-sources in countries with a high capacity for Kraft pulping such as Sweden, Finland, United States, Canada, Russia and China.
Tall oil contains free fatty acids (FFA), rosin acids (RA) and unsaponifiables compounds (neutrals). The FA as well as RA portions of tall oil can be hydroprocessed via catalytic hydrodeoxygenation (HDO) to produce highly paraffinic diesel like hydrocarbons. Catalytic HDO is an efficient upgrading treatment that is used to reduce the oxygen contents of both fossil based and bio-based feedstocks. During an HDO process, the oxygen is removed by hydrogen at elevated temperatures in the form of CO, CO2 and water.
HDO provides an important and effective route for producing high quality renewable fuels from bio-resources. However, change in catalyst activity and selectivity have raised concerns from the economic and technological perspectives. The aim of this study was to gain a better understanding of the HDO catalyst, and then to assess the effect of different pretreatment and operational conditions on the catalyst activity and selectivity. The results shows that addition of dimethyl disulfide (DMDS) to an oleic acid feed clearly promoted maintenance of the active sulfided phases and more importantly, decreased the amount of carbon deposition on the alumna supported NiMoS catalyst. Iron impurity in a FA acid feed evidently decreased the oxygenate conversion activity and changed the selectivities towards the final products. Addition of RA acid to a FA feed inhibited the deoxygenation rate of the FA and increase the amount of carbon deposition on the catalyst. Finally, it is shown that the dispersion and Brønsted acidity of the supported NiMoS catalysts could be optimized to achieve a satisfactory level of deoxygenation, ring opening and cracking of the rosin acid while avoiding excessive coke formation.
Innovation och entreprenörskap
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4385
Chalmers tekniska högskola
KB-salen, Kemigården 4, Göteborg
Opponent: Professor Anker Degn Jensen, Department of Chemical and Biochemical Engineering, DTU, Denmark