Microalgal Jet Biofuel Production Through Catalytic Deoxygenation Over Nickel In Combination With Hydrocracking And Isomerization Over Strong Acid

Producing high grade jet biofuel from microalgal oil through deoxygenation and hydrocracking is of significance to guarantee the security of national liquid fuel energy and reduce the emission of CO₂.However,the intermediate processes of catalytic deoxygenation,hydrocracking,and isomerization reactions are still unclear,and the interaction mechanisms between the catalytic active sites and the reactants are not revealed.Moreover,the components of the jet biofuel product are complex,so that the catalyst needs to be adjusted to achieve directional selectivities for specific hydrocarbons.In this paper,the competitive reaction mechanisms of catalytic deoxygenation over nickel metal active sites in combination with hydrocracking and isomerization over strong acid sites to produce microalgal jet biofuel was analyzed through quantum chemistry calculations.By modifying and improving the metal active sites,acid strength,and porous structure of the catalyst,the directional selectivity of jet fuel range hydrocarbons was raised and the composition distribution of the product was improved.The competitive reaction mechanisms of the hydroprocessing deoxygenation and hydrocraking of methyl palmitate,the main component of microalgal biodiesel,were revealed through quantum chemistry calculations,that is,the catalytic deoxygenation over nickel metal active sites to produce pentadecane in combination with hydrocracking and isomerization over acid sites to produce jet fuel range hydrocarbons.The calculation analysis showed that the hydrodeoxygenation conversion pathway of methyl palmitate to produce pentadecane was of the lowest energy barrier,and the overall reaction enthalpy change was-130.2 k J/mol,which was lower than that to produce palmitic acid?-90.1 k J/mol?through hydrogenolysis pathway and that to produce hexadecane?-68.6 k J/mol?through direct decarboxylation pathway.In the process of nickel-catalyzed hydrodeoxygenation of methyl palmitate,the energy of each step in the hydrodecarboxylation pathway?releasing CO₂?was lower than that in the hydrodecarbonylation pathway?releasing CO?,and the differences in enthalpy were 49.6¹87.7 k J/mol,so that oxygen was inclined to be removed along the hydrodecarboxylation pathway in the form of CO₂.The selectivities of hydrocracking products with odd carbons?2n-1,n=5,6,7,8?were higher than those with even carbons?2n?,because the covalent bonds hydrocracked to remove even carbons were 0.001%⁰.149% longer in bond length and 0.012%¹.697% lower in bond order than those hydrocracked to remove odd carbons.In order to improve reaction safety and reduce life cycle carbon emission,the alteration of reaction pathways was investigated in the conversion of saturated fatty acid methyl ester to jet biofuel under the condition of CO₂ atmosphere instead of H₂.Thermodynamic analysis showed that there was an endothermic reaction pathway for methyl palmitate to hydrocracking in the form of ethane from the end of aliphatic hydrocarbon group to produce methyl tetradecenoate.When the reaction temperature increased to 470 °C,the Gibbs free energy change reduced from 26.8 k J/mol at normal temperature and pressure to-7.0 k J/mol,which made the reaction shift to spontaneous.The phosphotungstic acid?HPW?was supported on nickel-based molecular sieve to generate strong acidic bifunctional catalyst,which effectively strengthened the hydrocracking and isomerization pathways in the production of jet biofuel.It was found that the chemisorption activation energy of 1-pentadecene,the intermediate of deoxygenation,onto acid sites significantly reduced from 203.9 k J/mol with molecular sieve to 81.6 k J/mol with HPW.Since bond order of ?-scission site in 1-pentadecene over HPW was 0.93% lower than that over molecular sieve,and Hirshfeld atomic charge of carbenium ion in 1-pentadecene over HPW?0.0722?was much higher,HPW showed better catalytic activities than molecular sieve in both hydrocracking and isomerization processes.The desilication method was applied to effectively construct mesoporous channels of 2¹0 nm within the faujasite structure of the microporous Y molecular sieve,which increased the specific surface area of the catalyst by 13.8% to 520.3 m²/g and the specific pore volume by 16.4% to 0.3182 cm3/g.The selectivity of jet fuel range hydrocarbons increased to 64.8%.Comparing three competitive reaction pathways,i.e.hydrodecarboxylation,hydrogenolysis,and direct decarboxylation,pentadecane as the main product of hydrodecarboxylation was always of the highest relative content within the liquid products.After loaded with 4% HPW,the strong acid amount of the nickel-based hierarchical mesoporous Y molecular sieve catalyst increased by 30.6%.In the jet biofuel obtained by continuous hydroprocessing conversion of microalgal biodiesel,the iso-alkane content significantly increased to 32.5%.The freezing point was accordingly reduced to-35.54 °C comparing to 2.08 °C for microalgal biodiesel,and its low-temperature flow characteristics were efficiently enhanced.The selectivity of the products with odd carbons within the liquid products was higher than that of the products with even adjacent carbons.Within the gas products,the selectivity of the hydrodecarboxylation by-product CO₂?19.7%?was much higher than that of the hydrodecarbonylation by-product CO?5.6%?,which was consistent with the results of quantum chemistry calculations.The application of mesoporous MCM-41 molecular sieve with higher specific surface area and wider average pore width as the supporter further enhanced the HPW loading to 20%.The overall selectivity of jet fuel range hydrocarbons within the liquid products of catalytic deoxygenation and hydrocraking reached as high as 86.1%.To strengthen the dispersion and stability of metal active sites,a nickel metal organic framework?MOF?,Ni-1,3,5-benzenetricarboxylate?Ni-BTC?,was utilized for loading in replacement of nickel nanoparticles.A novel type of Ni-BTC@MCM-41 bifunctional catalyst was developed for jet biofuel production.The distance between Ni atoms?0.98 nm?within nickel metal organic framework,which was much larger than that?0.20 nm?within nickel nanoparticles,enabled methyl palmitate with maximum molecule width of 0.68 nm to go through nickel metal organic framework crystalline plane and get access to metal active sites more easily.Among them,the nickel metal organic framework loading on MCM-41 molecular sieve with 2.5 wt.% nickel?2.5Ni-BTC@MCM-41?catalyst reduced the nickel metal consumption by 75% comparing to nickel nanoparticle loading,but reached a higher specific surface area?1014.2 m2/g?and specific pore volume?0.939 cm3/g?.In the catalytic hydroprocessing conversion of methyl palmitate,the significantly improved alkane selectivity?45.2%?and reduced arene selectivity?8.7%?resulted in an overall jet biofuel selectivity of 71.0%.It was further applied to the catalytic conversion in a CO₂ atmosphere,and it was found that the catalyst prepared with N,N-Dimethylformamide?DMF?as the solvent maintained the most stable textural characteristics after carbonization.When the reaction temperature increased to 470 °C,the conversion rate of methyl palmitate reached 90.0%,and the selectivity of jet fuel range hydrocarbons reached 72.5%.