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Title: Aqueous phase hydrogenation of succinic acid using mono-and bi-metallic ruthenium-based catalysts
Author: Bashal, Ali Habib
ISNI:       0000 0004 7428 670X
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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Most of the succinic acid hydrogenation in the literature have been done in high reaction conditions and with a help of organic solvent instead of a green solvent such as a water. Therefore, one of the main objectives of this study is to develop catalytic systems to improve reaction rates for the hydrogenation of succinic acid in water at mild reaction condition, also to find a promoter to affect the catalyst activity so that selectivity to 1,4-butanediol can be achieved. The increasing social concern about environmental problems and energy demand have urged scientists to put their attention on the catalytic conversion of renewable biomass to produce various chemicals. Carboxylic acids, i.e., succinic acid can be obtained via fermentation of biomass. To produce significant chemicals from bio-renewable and lowcost carboxylic acids is highly desirable. The hydrogenation of carboxylic acids suffers from lack of selectivity so an active catalyst is needed to overcome those unwelcomed issues. Diols have a massive demand in the plastic industries due to their use as monomers to produce polyesters. Diols are considered a significant product amongst other products (lactones and cyclic ethers). It is hard to target diols since undesired-straight chain alkanes, acids, and, alcohols are produced by over-hydrogenation and C-C cleavage reactions which affect the selectivity. The choice of a catalyst under suitable experimental conditions would shift the selectivity to the diols. Therefore, to fulfil the necessity towards these diols, more efficient catalysts have been developed in this thesis. We demonstrated the significance of the support in the hydrogenation of succinic acid using different metals. The potential of tin as a promoter towards the formation of diols from the hydrogenation of succinic acid and other biomass-derived acids was successfully achieved. Liquid-phase hydrogenation of succinic acid was carried out over different reactors, highpressure conventional batch and microwave reactors. In the conventional reactor, the reaction conditions were optimized by varying the hydrogen pressure and reaction temperature to maximize the conversion of succinic acid and to avoid undesirable hydrogenolysis. Increasing the reaction pressure enhanced the hydrogenation activity and improved the yield of alcohols. Increasing temperature raised the rate of the reaction but negatively impacted the yield of the alcohols. The location of Ru nanoparticles on carbon nanotubes (CNTs), as well as the support's properties, had significant effects on the catalytic performance. We have demonstrated that the catalytic performance of Ru catalysts is strongly supportdependent. CNTs were found to increase the reactivity of all tested supported metals (Ru, Pt, and Pd) in the hydrogenation of succinic acid in both reactors systems. However, the use of CNTs as supports for Ru produced significantly superior rates of reaction as compared to other supports such as activated carbon, silica or Al 2O3. .5% Ru/CNT proved to be the best catalyst for this reaction and, at mild conditions (150 °C, 50 bar hydrogen), delivered 95.4 % conversion of succinic acid after 11 h in water. Furthermore, we reported for the first time that the use of pressurized microwave reactors resulted in a 3-fold reduction of the reaction time compared to the conventional heating reactor (pressure autoclave). All catalysts were characterized by X-ray diffraction, temperature programmed reduction, electron microscopy, CO chemisorption and X-ray photoelectron spectroscopy to be able to understand the relationship between the structure-activity and selectivity. Based on the XPS results, it is suggested that the electronic promotional effect along with the high accessibility of pores in the carbon nanotubes seem to be responsible for the finer activity of the catalyst. No sign of crystalline phases of metallic Ru in all reduced Ru based catalysts was observed, suggesting that the Ru nanoparticles are too small to be characterized by XRD. Linked to XRD, analysis of the TEM images showed that Ru catalysts (AC and CNT-based) and Pt/CNT were well dispersed while Pd/CNT exhibited a bimodal distribution showing agglomeration to big particles between 6 and 13 nm apart from the small 2±0.7 nm nanoparticles. On the other hand, the selectivity towards a single product remained an issue in both reactors but even more in the microwave reactor where less optimization work was carried out. At the completion of the reaction, the conventional reactor gave a combined yield of alcohols (1, 4-butanediol, n-propanol, n-butanol) of ~40 % whereas in the microwave reactor the yield drops to 18.2 %. Future work will determine the reasons behind this, but at this stage, we could hypothseze that the local hot spots could be responsible. Therefore, as the second objective of this thesis, we worked towards the selectivity to 1,4-butanediol. A wide range of Ru/Sn bimetallic catalysts were prepared on carbon and CNTs and tested. We found that Sn could substantially increase selectivity at iso-conversion levels as compared to the monometallic analogue. The incorporation the Sn into the Ru-based catalyst (2% Sn-5% Ru/CNT) results showed the almost complete elimination of the unwanted C-C cleavage reactions, thus the percentage of C-C cleavage products obtained decreased from 100 % to 7.7 % in which the yield of 1,4-butanediol increased from 4 to ~80 %. Based on XPS and TPR results, envision the formation of a new type of active site, potentially Ru-Sn alloy on CNT which has a high impact to produce diols from the hydrogenation of different carboxylic acids. The third objective of this thesis was to explore the activity of the Ru-Sn/CNT system to other hydrogenation reactions attempting to further capitalize in a combined higher selectivity offered by Ru-Sn while enhancing reactivity using CNTs as support. The system was very efficient in the production of adipic acid, caprolactone, and 1,6-hexanediol from the hydrogenation of cis,cis-muconic acid at 200 °C. High selectivity for these products was accomplished by altering the hydrogen pressure for the hydrogenation reaction. Also, the levulinic acid hydrogenation was successfully performed towards to 1,4- pentanediol using the same system.
Supervisor: Lopez-Sanchez, Jose A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral