High Performance Of Nickel Based Materials For In-situ Hydrogenation Of Algae Based Fatty Acids To Long Chain Alkanes

Aviation kerosene（aviation kerosene）is an essential high-performance fuel for commercial airliners,and its global demand is increasing year by year.At present,the vast majority of aviation coal production depends on petroleum based fossil fuels,which not only deepens the panic of global fossil energy shortage,but also adds a large number of greenhouse gas（CO₂）emissions.Therefore,in recent decades,the synthesis of biological aviation coal from renewable and carbon neutral biomass resources has attracted extensive attention and research.It is an important research field in line with China’s national strategy of"carbon peaking and carbon neutralization".Microalgae biomass has the characteristics of fast growth and high oil content.A large amount of bio-oil can be obtained by appropriate cell wall breaking separation technology.Because the long-chain fatty acid structure of bio-oil（triglyceride）is similar to that of aviation coal（long-chain alkane）,it is of great economic,energy and environmental significance to synthesize long-chain alkanes with cheap algae based bio-oil.Among them,the synthesis of long-chain alkanes by catalytic hydrodeoxygenation of higher fatty acids is the key step and research difficulty of aviation kerosene.Compared with noble metals and gaseous hydrogen,using cheap Ni based materials as catalysts and using liquid hydrogen donors to realize the hydrogenation and decarboxylation of fatty acids,long-chain alkanes have obvious advantages in cost and safety,which is a research hotspot in this field at present.However,the low catalytic activity of Ni based materials,poor product selectivity and weak dehydrogenation ability of hydrogen donors limit the application and popularization of cheap metal catalytic materials in the preparation of alkanes from higher fatty acids.In view of the above problems,purposeful modification and structural regulation of Ni based materials are the key to improve the catalytic activity and selectivity.Therefore,this paper has carried out the following research:Firstly（Chapter 2）,because the size and surface oxygen defect of the catalyst are important factors affecting the catalytic hydrogenation activity and product selectivity,Ni Al bimetallic precursor was prepared by hydrothermal method,and Al O_x modified Ni composite catalyst with high Ni loading was obtained by hydrogen thermal reduction method.Through the characterization of material morphology,it is found that the introduction of a small amount of Al precursor in the preparation process can effectively reduce the size of Ni nanoparticles,which is conducive to increase the active site of the catalyst;In addition,compared with pure Ni powder and Ni/Al₂O₃-IMP prepared by traditional impregnation method,it is found that there is obvious electron transfer and higher oxygen vacancy concentration between Ni and Al in Ni-Al O_x composites,and the density functional theory（DFT）calculation shows that the surface of Ni（111）modified by Al O_x containing oxygen vacancy has higher adsorption energy for fatty acid molecules,It is conducive to the adsorption and activation of fatty acid molecules.On this basis,this chapter tests the catalytic decarboxylation performance of fatty acids with stearic acid as the model compound of higher fatty acids and isopropanol as the liquid hydrogen donor.The effects of Ni Al catalysts prepared by different Ni/Al ratios and different synthesis methods on the conversion of stearic acid and the selectivity of hydrocarbon products are studied.It is found that Ni-Al O_x with Ni/Al of 1/0.33 prepared by hydrothermal method has the highest catalytic activity,and combined with the previous material physicochemical characterization and theoretical calculation results,it is clear that the oxygen defect on the surface of Ni-Al_0.33O_x plays an important role in the hydrogenation and decarboxylation of fatty acids;Then,through the optimization of reaction temperature,time and other parameters,under the condition of 250°C and 8hours of reaction,the highest yield of heptadecane is 93.2%,and can be stably recycled for at least 4 times.Finally（Chapter 3）,due to the research findings in Chapter 2 that Ni Al materials with high Ni loading are easy to be oxidized and inactivated in air,it is of great scientific significance to develop catalytic materials with low Ni loading for fatty acid decarboxylation.In this chapter,a series of carbon supported Ni based catalysts with different Ni loading and nitrogen doping were prepared.It is found that the catalytic material with low Ni loading（10%）still shows good dehydrogenation ability for isopropanol and can provide sufficient hydrogen source for fatty acid hydrodeoxygenation,but the nitrogen content will slightly weaken the dehydrogenation performance of Ni catalyst for isopropanol;In addition,in the study of the hydrodeoxygenation performance of fatty acids,it is found that the introduction of nitrogen can provide basic sites for the catalyst,so as to inhibit the reaction between fatty acids and isopropanol to produce ester by-products and improve the selectivity of alkane products.Therefore,under the same Ni loading（10%）and the same reaction conditions,the heptadecane yield of nitrogen doped Ni based carbon materials is about 7 times higher than that of non nitrogen doped materials.In conclusion,Ni based catalytic materials with good fatty acid hydrodecarboxylation were prepared based on oxygen vacancy regulation and n-doped alkaline site introduction strategy,which provided a new idea and method for the synthesis of high-performance bio-oil to aviation kerosene catalyst.