- Title
- Hydrodeoxygenation of biocrude oil to value-added products
- Creator
- Yan, Penghui
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2020
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The utilisation of biomass has drawn widespread attention due to the fast depletion of fossil energy resources and environmental challenges. Biocrude oil, derived from pyrolysis or liquefaction of lignocellulosic biomass, contains a high level of oxygen leading to some detrimental properties (high viscosity, high corrosivity, low heating value and low thermal stability). Therefore, hydrodeoxygenation of the biocrude is necessary to remove the oxygen atoms and produce value-added fuels and chemicals. Hydrodeoxygenation (HDO) is a two-step process which involves the hydrogenation and deoxygenation. The hydrogenation commonly occurs in the presence of metal sites, and the acid sites play a crucial role in the deoxygenation. Therefore, the HDO catalysts are routinely composed of metals (such as Pt, Pd, Ru, and Ni) and supports (ZrO₂, Al₂O₃ and zeolites). Due to the complexity of biocrude oil, a model compound (guaiacol), which possesses two different oxygen-containing groups, was employed in this study. In the first step, the Ru/BEA and Ru/ZSM-5 with varied Si/Al ratios were studied to examine the influence of supports with varying acidity and pore size on HDO of biocrude oil and guaiacol in a batch-type reactor operated at 4.0 MPa hydrogen. It was observed that a decrease in the Si/Al ratio of the support generated an increase in the yield of cyclohexane and a decrease in the yield of 2-methoxycyclohexanol in HDO of guaiacol. Both Ru/BEA and Ru/ZSM-5, possessing low Si/Al ratios, displayed a high activity for HDO for guaiacol while only Ru/BEA catalyst exhibits a high activity for HDO of biocrude oil. Catalyst characterisation shows that the Ru/BEA catalyst, with a low Si/Al ratio, not only possesses strong B acid sites but also contains extensive mesoporosity. Notably, these mesopores appear to facilitate the hydrogenation, deoxygenation, and ring-opening of large oxygenated and condensed-ring hydrocarbons in biocrude oil which then leads to a high yield of cycloalkanes. As expected, the Ru/Al₂O₃ and Ru/SiO₂ catalysts exhibit a high hydrogenation activity but a low deoxygenation activity in the HDO of guaiacol and biocrude oil due to the absence of B acid sites. These results suggest that the larger pore support, with strong B acid sites, engendered the observed HDO activity. The reaction pathway for the main components of biocrude oil was proposed based on the observed reaction product distribution. Although Ru-based catalysts display a high HDO activity, the high cost could hinder their wide application in industry. Therefore, metallic Ni, which is low cost, non-sulfided, and has the advantage of high hydrogenation activity was employed as the main metal phase in this project. The influence of catalyst pore size and shape selectivity on the catalytic hydrodeoxygenation (HDO) of biocrude oil has been investigated by comparing the activity of nickel catalysts on the supports of different pore sizes towards model compounds of increasing dimension. Five model compounds (guaiacol, anisole, phenanthrene, pyrene, and 1,3,5-trimethoxybenzene), and five zeolite supports (small-pore ZSM-5, medium-pore MOR, large-pore Beta and Y, and mesoporous Al-MCM-41) have been investigated. Hydrodeoxygenation and hydrogenation activities were determined on the basis of the yield of the associated cycloalkanes. All catalysts show a high HDO activity for small molecules (anisole and guaiacol) while the catalysts with medium and large pore supports display high HDO activity for 1,3,5-trimethoxybenzene. Furthermore, the large pore catalysts (Ni/Y and Ni/Beta) exhibit high hydrogenation activity for phenanthrene, while only the extra-large pore size catalyst (Ni/Y) presents good hydrogenation and HDO activities for all model compounds. The mesoporous Ni/Al-MCM-41 catalyst shows low HDO and hydrogenation activities for large model compounds, which can be ascribed to its low metal dispersion and low concentration of acid sites. In addition, the Ni/Beta and Ni/ZSM-5 were also tested in HDO of biocrude oil. Ni/Beta catalyst displays a higher yield of cycloalkanes than that of Ni/ZSM-5, which confirms that the selection of catalyst support can have impact on the product distribution in HDO of biocrude. The BEA supported Ni catalyst, which possesses high concentration of acid sites and mesopores, displays a high HDO activity for guaiacol and biocrude oil, however, results from batch reactor cannot provide the information about turnover frequency of active sites. Therefore BEA zeolite, with different Si/Al ratios (12.5, 25, 175) and varying metal loadings (2.3 ~23.4 wt%), was studied in a flow reactor to examine the influence of catalyst acidity and the structure of Ni on the performance, with particular emphasis on the change in product selectivity. The ratio of cyclohexane formation rate to the concentration of acid sites in reduced catalysts was found to be roughly constant when HDO of guaiacol in a flow reactor over 15.7 wt% Ni/BEA catalysts with different acid site concentrations, demonstrating the deoxygenation activity increases with increasing number of acid sites. On the other hand, the materials with the majority of isolated Ni sites (prepared by ion exchange) showed no cyclohexane yield at 230 degree. However, at higher temperatures, the formation of cyclohexane was observed, as a result of a consecutive reaction where catechol was generated by acid sites and subsequently reduced to cyclohexane on isolated Ni sites. With respect to catalysts with higher Ni-loading, the selectivity towards cyclohexane increases with an increased Ni loading up to 15.7 wt%. This is attributed to the formation of larger Ni nanoparticles upon H₂ reduction. A higher concentration of nickel hydrides compared to isolated Ni sites was observed by H₂-TPD and H₂-FTIR. The nickel hydrides are believed to be crucial intermediates in the hydrogenation reaction. Based on the product distribution over catalysts containing mainly isolated Ni sites and the Ni nanoparticles, two different reaction pathways were proposed. Traditionally, catalysts with high metal loadings are indispensable for improved catalytic performance. Supported metal catalysts are typically prepared by incipient wetness impregnation, which inevitably gives rise to inhomogeneous metal distribution and leads to large metal particles formed via agglomeration. Therefore, highly dispersed Ni/BEA catalysts prepared via ion-exchange-deposition-precipitation (IDP) utilising careful pH control were conducted. In comparison, catalysts prepared by incipient wetness impregnation (IWI) and deposition-precipitation (DP) methods were also investigated and IDP catalysts were shown to have higher dispersion. A significantly increased selectivity toward hydrocarbons was observed over these carefully prepared catalysts. The presence of nickel hydrides was confirmed by H₂-TPD and H₂-FTIR. IDP catalyst exhibits a higher metal dispersion and higher concentration of nickel hydrides than impregnated and DP catalysts, while larger Ni nanoparticles formed in impregnated catalysts show a higher concentration of nickel hydrides per surface Ni. The guaiacol conversion was not significantly affected by the catalyst preparation method, while the product selectivity was altered. Higher cyclohexane formation rate was detected over IDP catalysts compared to DP and impregnated catalysts. Besides, cyclohexane formation rate presents a positive linear correlation with the concentration of nickel hydrides, suggesting nickel hydrides play a crucial role in the hydrodeoxygenation reaction. The factors affecting catalyst deactivation during HDO in a continuous-flow reactor were investigated in order to gain insight into the deactivation mechanism of BEA-supported Ni catalyst. Phenolic-OH group moieties accelerate catalyst deactivation through the production of condensed-ring compounds, which then leads to the blockage of pores. In contrast, the yield of cycloalkane did not change with time-on-stream when using toluene, cyclohexanol and anisole as feeds, suggesting aromatic-ring, alkyl-OH and aromatic-OCH₃ have negligible effect on catalyst deactivation. Operation at low weight hour space velocity (WHSV) values increase the yield of cycloalkane and reduce the rate of catalyst deactivation. High metal loadings can increase selectivity to cyclohexane, however, the production of condensed-ring products is also enhanced, which is likely to be the result of an increased concentration of surface cyclohexane carbocations. Furthermore, high metal loadings play a limited role in preventing the catalyst from deactivation. In addition, the activity and stability of catalysts were positively affected by an increase in reaction temperature (up to 230°C). Based on the product distribution observed, a coupling reaction pathway (leading to the formation of condensed-ring products and cycloalkanes) is proposed.
- Subject
- hydrodeoxygenation; nickel; ruthenium; zeolites; heterogeneous catalysis
- Identifier
- http://hdl.handle.net/1959.13/1411892
- Identifier
- uon:36398
- Rights
- Copyright 2020 Penghui Yan
- Language
- eng
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