A Study On Catalytic Decarboxylation Of Fatty Acids To Long-chain N-alkanes

With the depletion of fossil resources, issues emerging of greenhouse gas emission, and growing demand of aviation fuels, the preparation technology of aviation bio-fuels has become a hot research topic in the biomass field. Currently, hydrodeoxygenation of refined vegetable oil is the main approach to produce aviation bio-fuel from greases. However, it has the issues such as high cost of raw material and high hydrogen consumption. Aiming at these two issues, the approach that low-price and low-grade greases were hydrolyzed and hydrogen-free decarboxlated to prepare long-chains alkanes, have gained more attention and development recently.Focusing on the research topic of preparing long-chains alkanes from the hydrogen-free decarboxylation of fatty acid, systematic study on hydrogen-free decarboxylation of saturated fatty acid and in-situ hydrogenation-decarboxylation of unsaturated fatty acid to prepare long-chain n-alkanes was performed in this dissertation. It included catalyst preparation, catalyst evaluation, study on reaction regularity of hydrogen-free decarboxylation of saturated fatty acid, and study on reaction mechanism of in-situ hydrogenation and decarboxylation of unsaturated fatty acid. Based on these researches, the non-precious supported catalysts efficient for in-situ hydrogenation and decarboxylation were developed. A new low-cost and hydrogen-free method was established for efficient production of long-chain n-alkanes from greases. The main contents are generalized as follows:Firstly, the catalytic activities for decarboxylation of lauric acid, myristic acid, palmic acid, stearic acid, arachic acid and behenic acid as model compounds over six non-noble metal catalysts without any solvent were evaluated. Then, Ni/C catalyst, which performed a better catalytic decarboxylation compared to other catalysts, was characterized by a series of analytical method. Thereafter, the effects of nickel loading, catalyst loading, temperature, and different carbon number of saturated fatty acid were investigated. The results indicate that the activity of nickel was superior to copper and cobalt for decarboxylation. At 370℃, complete conversion of stearic acid was achieved within 5 h over Ni/C, and the selectivity to heptadecane was around 80%. Besides, Ni/C exhibited excellent reusability. The products from cracking increased with the increase of nickel loading or catalyst loading, which leads to a lower selectivity of heptadecane. The conversion of stearic acid significantly increased with the rise of reaction temperature, while the selectivity to heptadecane remained almost the same. The different fatty acids with varying carbon numbers can also be well catalyzed by Ni/C for decarboxylation. At the same reaction condition, the conversions of the fatty acids with smaller carbon numbers were higher than those with larger carbon numbers, and the fatty acids with large carbon numbers tend to be cracked.Secondly, in this article, in-situ hydrogenation and decarboxylation of unsaturated fatty acid were studied for further reducing hydrogen consumption and producing long-chain n-alkanes from grease without any hydrogen cost. Hydrothermal in-situ hydrogenation and decarboxylation of oleic acid as a model compound was studied over Pt/C with methanol as a hydrogen donor and water as reaction medium. The effects of methanol loading on the reactions of oleic acid, stearic acid and octadecanol were investigated followed by discussing the role of methanol as hydrogen donor based on the experimental results. Furthermore, the regulation mechanism of in-situ hydrogenation and decarboxlytion of oleic acid by the addition amount of methanol was proposed. The results indicate that a moderate methanol can greatly increase the rate of decarboxylation of oleic acid. Without the addition of methanol, the molar yield of heptadecane was only 6.8% at 330℃ for 1 h, compared to the heptadecane molar yield of 72.2% with 5 mg methanol loading at the same reaction condition. The hydrogenation of carbonyl group to form octadecanol occurs firstly, followed by reforming of octadecanol to heptadecane during the process of decarboxylation of stearic acid in hydrothermal conditions. Methanol would compete with octadecanol on the surface of catalyst active sites, so excessive methanol loading would inhabit the production of heptadecane from octadecanol, resulting in the decrease of the molar yield of heptadecane.The suitable addition amount of methanol could rightly fulfill the hydrogenation of C-C double bonds by aqueous phase reforming of methanol, while it didn’t affect the decarboxylation of stearic acid.Finally, in order to reduce the cost of aviation bio-fuel and develop the non-noble metal catalysts with high activity of in-situ hydrogenation and decarboxylation, a series of catalysts of Cu/Al2O3, Ni/Al2O3 and bimetallic Cu-Ni/Al2O3 with different Cu/Ni ratios were prepared. The catalytic activities of these catalysts for the in-situ hydrogenation and decarboxylaion were evaluated, and the in-situ hydrogenation and decarboxylation of hydrolyzates (stearic acid) of gutter oil and microalgal lipids were also performed. The results indicated that the bimetallic Cu-Ni/Al2O3 exhibited higher activity than Cu/Al2O3 and Ni/Al2O3 for the in-situ hydrogenation and decarboxylation of oleic acid, and the activity of Cu-Ni/Al2O3 was similar with those of noble metal catalysts. The catalytic activity of Cu-Ni/Al2O3 with different Cu/Ni ratios showed a great difference, and 20% Cu-40% Ni/Al2O3 revealed the best results for the in-situ hydrogenation and decarboxylation of oleic acid. When the catalyst/substrate ratio was 30%, complete conversions of oleic acid and stearic acid were acheieved within 1 h at 330℃, and the molar yield of heptadecane was more than 90%. Cu-Ni/Al2O3 also showed high catalytic activity on the hydrolyzates of gutter oil and microalgae lipids. The saturated and unsaturated fatty acids, existing in hydrolyzates of gutter oil and microalgae lipids, were substantially converted to the corresponding alkanes within 1 h at the same reaction conditions.