The development of nickel-based heterogeneous catalysts with ultrasonic irradiation for the hydrogenation of plant-based feedstock

Lim, Mitchell Shyan Wei (2022) The development of nickel-based heterogeneous catalysts with ultrasonic irradiation for the hydrogenation of plant-based feedstock. PhD thesis, University of Nottingham.

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Nickel-based heterogeneous catalysts are characterised by their widespread utilisation in the hydrogenation of fats and oleochemicals. Nevertheless, various aspects of the catalyst are still in continuous development and improvement. Therefore, bridging these knowledge gaps concerning the functionality of the catalyst motivated the investigations constituting this thesis.

This thesis pertains to a coherent study in relation to the life cycle of nickel-based heterogeneous catalysts as the central theme. Several aspects and problem statements concerning the development of the catalyst are raised and addressed: (i) What are the predominant deactivation modes of nickel-based catalysts in the hydrogenation of palm-based liquid organic media? (ii) What are the improvements that can be made to nickel-based catalysts, and if so, can the incorporation of innovative technologies such as power ultrasound into the synthesis of such catalysts be of significance? (iii) What are the effects imparted by different ultrasonic synthesis configurations on nickel-based catalyst composites? (iv) How can the recovery of nickel from spent hydrogenation catalysts be improved?

To address problem (i), the hydrogenation of various organic media using nickel-based catalysts was studied to ascertain the dominant deactivation modes of the catalyst. Nickel lixiviation with Ostwald ripening and poisoning were found to be the principal deactivation mechanisms affecting the catalytic performance in the hydrogenation of palm-based liquid organic media. Nickel lixiviation increased 7-fold and 8-fold during the hydrogenation of split-refined, bleached and deodorised palm stearin (S-RBD-PS) when compared to that of refined, bleached and deodorised palm stearin (RBD-PS) at reaction temperatures of 195 and 210 °C, respectively. On the other hand, catalyst poisons such as S, P, Cl and N in the S-RBD-PS and RBD-PS feeds affected the catalytic activity negatively, as compared to the relatively clean distilled fatty acid blend feed.

To address problem (ii), silica-supported nickel catalysts were synthesised by introducing an alkaline metal dopant and ultrasonic irradiation. A Ni:Mg ratio of 3:1 showed the best catalyst properties, in which the catalyst was chosen for further studies by incorporating ultrasonic irradiation into its synthesis. Sonicated catalysts exhibited geometrical and electronic alterations, leading to improved reducibility, surface area and active site dispersion. The catalysts were evaluated for the partial hydrogenation of sunflower oil to ascertain their catalytic performance. At an ultrasonic intensity of 20.78 W cm-2, the nickel catalyst achieved a 22.6 % increase in hydrogenation activity, along with a 28.5 % decrease in trans-C18:1 yield at IV = 70.

To address problem (iii), the synthesis of nickel-silica composites was performed with ultrasonic irradiation incorporated into two different stages of the synthesis: the co-precipitation stage or the support loading/ageing stage, which were characterised for discrepancies in their textural, structural, morphological, reduction and surface composition properties. It was determined that sonication during the co-precipitation stage resulted in composites with wide pores of Type II and IV(a) isotherms, rough morphology, enriched superficial Mg and Si contents and increased reducibility. On the other hand, composites synthesised with sonication during silica loading/ageing had narrower pores of Type IV(a) isotherms, smooth morphology, decreased superficial Mg and Si contents and lowered reducibility. Ultimately, it was ascertained that the usage of ultrasound at different stages of the synthesis had profound impacts on the final properties of materials, establishing that the configuration of sono-syntheses also plays a significant role in influencing material characteristics, which is an important factor for future material scientists to consider when applying ultrasound for synthesis. It is also suggested that ultrasonic technology can be adopted to optimise and fine-tune the properties of nickel-silica composites for catalytic purposes.

To address problem (iv), acid leaching experiments were performed to assess the effectiveness of applying ultrasound irradiation in the recovery of nickel from spent catalysts. It was established that sonication expedited nickel recovery, allowing a 2 h time reduction in recovering ca. 70 % nickel. Besides, Ni recovery was more efficient when nitric acid was used as the leaching reagent compared to sulphuric acid, in which the former recovered ca. 20 % more nickel than the latter. Lastly, the process optimisation of nickel leaching via response surface methodology enabled a 99.37 % nickel recovery in 2 h.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Tiong, Timm Joyce
Lim, Siew Shee
Yap, Yeow Hong
Keywords: hydrogenation, nickel catalyst, ultrasound, sonochemistry, recovery, catalyst deactivation
Subjects: T Technology > TP Chemical technology
Faculties/Schools: University of Nottingham, Malaysia > Faculty of Science and Engineering — Engineering > Department of Chemical and Environmental Engineering
Item ID: 67396
Depositing User: LIM, Mitchell
Date Deposited: 27 Feb 2022 04:40
Last Modified: 27 Feb 2022 04:40

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