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In this thesis, chemical and novel Ni catalytic systems were explored for the valorization of lignin and bio-oil into high energy densed transportation fuel or arenes.
At the beginning, the effect of solvent on the activity of Raney Ni catalyst was studied using the hydrogenation of diphenyl ether as a model compound. It was discovered that the activity of Raney Ni depends strongly on the type of solvent. In addition, the conversion of diphenyl ether has a negative exponential relationship with the Lewis basicity of the solvent, as expressed by the donor number (DN). Lewis basic solvents (alcohols, ethers and esters) hinder the catalytic activity because of their strong interaction with Raney Ni. Among these solvents, the reaction carried out in 2-propanol and 2-butanol displays the higher conversions than predicted, because these two solvents worked as an H-donor even under low-severity conditions. Methanol and dioxane, the most commonly applied solvents for lignin hydrogenolysis, do not allow for high activity of Raney Ni. Unsaturated compounds such as phenols and other functionalized aromatics are the main products in these solvents. On the contrary, solvents showing no Lewis basicity (hydrocarbon solvents and hexafluoro-2-propanol) lead to quantitative conversion of diphenyl ether, yielding mainly saturated compounds (alcohols, alkanes and saturated ethers). When the system was applied to the hydrogenolysis of organosolv lignin (poplar), the reaction carried out in MCH resulted in cyclic alcohols and cyclic alkanes, whereas the reaction in 2-propanol led to cyclic alcohols, cyclic ketones and unsaturated products. The hydrogenolysis of lignin in methanol, however, produced mostly phenols. These findings clearly show that the solvent plays an essential role in steering the selectivity.
For the production of a higher energy-densed transportation fuel, further deoxygenation process is required for obtaining saturated alkanes. Ni catalysts supported on different functional carriers were prepared and examined on the HDO of diphenyl ether. It was found that when a solid acid was involved, the deoxygenation can be realized via acid-catalyzed dehydration. Full conversion of diphenyl ether into cyclic alkane was achieved when using a high surface area, acidic mesoporous material Al-SBA-15. The conversion of organosolv lignin was carried out with this reaction system. After 8 h of reaction at 300 °C under 70 bar H2 pressure (r.t.), 84 % conversion was achieved, with 99 % of the product detected being saturated hydrocarbons. This result demonstrates the high potential of the system for the direct conversion of lignin into liquid transportation fuels.
Besides lignin, phenolic bio-oil obtained from pyrolysis process of wood is also an attractive source for renewable transportation fuels and chemicals. H-transfer reactions provide an efficient approach for the upgrade of phenolic model substrates as well as bio-oil fractions, where the process can be realized under mild conditions. When catalyzed by Raney Ni, 2-propanol can work as an H-donor, being converted during the reaction into acetone upon donation of H-atoms to Raney Ni. The system was discovered to be active for the hydrogenation and defunctionalization of bio-oil model compounds with variety of functionalities. In particular, methoxyphenols can be easily converted into cyclic alcohols at 80-120 °C. Full saturation of bio-oil was achieved at 160 °C, providing cyclic alcohols and cyclohexane-1,2-diols.
In the last part of this thesis, the conversion of phenolic compounds into arenes was studied. In the reaction, lignin derived methoxyphenolic compounds were first hydrogenated into cyclic alcohols by H-transfer reaction over Raney Ni. The obtained alcohols were dehydrated in the presence of solid acid catalyst into cyclic alkenes, which further underwent dehydrogenation arenes. Yields of arenes as high as 90 % were obtained by this methodology at only 160 °C and autogenous solvent pressure. The reaction is much more efficient than the conventional sulfide catalyzed process, which involves very harsh conditions (300-450 °C and high pressure) producing much lower yield of arenes (< 60 %). Finally, bio-oil and lignin were successfully converted with this system, yielding a liquid mixture comprising nearly 80 % of the products being arenes. Such a process is a very attractive option in the production of biomass-based arenes, which are highly demanded in the bulk chemical industry as well as in the fuel industry. Moreover, the conversion of phenols or derived-structures into arenes represents an overall economy of hydrogen compared with the production of saturated hydrocarbons.
Overall, the valorization of lignin catalyzed by Ni catalysts was explored in this thesis. By carefully controlling the process, lignin was successfully converted into alkane mixtures, which is an ideal transportation fuel candidate, or arenes which are important bulk chemicals or fuel additives. In contrast to other previous works, which focused on the screening of the catalyst, this thesis suggests that significant attention should be paid to the other factors that can influence the interaction between the reactant and the catalyst, and thus determine the efficiency of the catalytic process. As a result, a new thinking in the design of the catalytic process is required for the future lignin-based biorefinery.
