| The conversion of microalgal biomass with fast growth rate and high lipid content into biodiesel will play an important role in achieving national carbon neutrality and energy security.However,the extraction of lipids from microalgal cells requires energy-intensive processes of biomass drying and cell wall disruption.Volatile toxic solvents such as chloroform need to be used during extraction.Biodiesel production usually requires a complex two-step process of free fatty acids(FFAs)esterification with acid catalysts to reduce the acid value followed by triglycerides(TGs)transesterification with base catalysts.To address these problems,the switchable polarity solvent was synthesized to replace the traditional organic solvents,which achieved cell wall disruption of wet microalgae and in-situ extraction of microalgal lipids,avoiding the huge energy consumption of biomass drying and cell wall disruption,and the pollution caused by volatile organic solvents.By optimizing the structure of cobalt-based zeolitic imidazolate frameworks to release active sites,highly efficient acid-base bifunctional catalysts were synthesized to achieve the simultaneous conversion of FFAs and TGs,which greatly simplified the technical process.The mechanism of microalgal cells destruction and lipids extraction using switchable polarity solvent N,N,N’,N’-tetraethyl-1,3-propanediamine(TEPDA)was investigated.A small amount of cations dissociated from TEPDA interacted with hydroxyl groups in cellulose to form electron donor-acceptor(EDA)complex,destroying hydrogen bonds between the cellulose chains in the cell wall and weakening the thickness of the cell wall.TEPDA cations neutralized the negatively charged phospholipid bilayers,and the local electrostatic equilibrium led to rearrangement and destruction of the cell membrane.After 24 h of extraction,the cell wall thickness decreased from 141 nm to 68.6 nm,and the fractal dimension increased from 1.45 to 1.73,indicating the cell destruction was intensified.The heat generated by cavitation of ultrasound destroyed the pectin layer in cell wall,while mechanical shear stress promoted the stripping of the cellulose chains by TEPDA cations.The ultrasound-assisted TEPDA reduced lipid extraction time from 24 h to 2 h and achieved the high lipid extraction efficiency of 98.2%.An acid-base bifunctional catalyst was synthesized by modifying zeolitic imidazolate framework(ZIF-67)with phosphotungstic acid(HPW)to achieve high conversion efficiency of high-acid-value microalgal lipids to biodiesel with 94.2%.The terminal W=O groups in HPW interacted with N extremities in ZIF-67 via covalent W-O-N bonds,which achieved the low solubility in polar solvents,thermal stability,and recyclability of HPW.Meanwhile,HPW destroyed the coordinatively saturated Co-N4 structure in ZIF-67 to release coordinatively unsaturated Co cations and N extremities,which enhanced the Lewis acidity and basicity of the catalyst.The ratio of Lewis acid sites to Br(?)nsted acid sites(L/B)increased from 0.1 to 0.66 and Lewis base sites increased from 0.45 to 4.53 mmol/g.To address the problem of poor utilization of active sites inside the crystal due to the microporous structure of ZIF-67 limiting the diffusion of reactant molecules,the catalyst with coordinatively unsaturated cobalt single-atom active sites and mesoporous transport networks was synthesized by fabricating high-density crystal structure defects using pyrolysis to improve active site utilization.After pyrolysis at 450°C,the coordinatively saturated Co-N4 structure in ZIF-67 was destroyed,whose content decreased from 78.4% to 29.6%.The released coordinatively unsaturated Co-Nx and uncoordinated N sites enhanced Lewis acid-base properties,increasing the L/B acidity ratio from 0.1 to 11.45 and the Lewis base sites from 0.96 to 6.05 mmol/g.Density functional theory(DFT)calculations showed that the adsorption energy of FFA on Co-N2 site(-1.003 e V)exceeded that on Co-N4 site(-0.271 e V).The stronger interaction between the Co-N2 site and FFA molecule led to the migration of electrons,reducing the electron cloud density near the carbonyl carbon atom of FFA and increased its electropositivity from 1.379 e V to 1.529 e V.The higher positive charge of the carbocation means that it is more easily attacked by the nucleophilic methanol,thus promoting the esterification reaction.The free energy change for the adsorption of methanol on pyridinic N sites and further conversion to activated methoxy anions(+1.686 e V)was lower than that on pyrrolic N(+3.705 e V)and graphitic N(+3.279 e V),indicating that the pyridinic N sites exhibited the best catalytic activity on promoting the attack of methoxy anions to carbonyl groups in TGs to form fatty acid methyl esters(FAMEs).The crystal defects generated by pyrolysis formed more mesopores structures,which promoted the diffusion and reaction of lipid molecules inside the crystals and increased the conversion efficiency of lipids to 96.7%.To address the problems of ZIF-67 crystal loss during pyrolysis and the decrease in metal atoms utilization caused by the agglomeration of Co single atoms into Co nanoparticles,two-dimensional ultrathin nanosheets with thickness of only 5 nm were synthesized by controlling crystal growth via coordination ion competition and solvent interface effect.The metal atoms inside the crystal were released to crystal surface to form Co single-atom active sites that were highly accessible to reactant molecules,improving the utilization rate of Co atoms.The coordination competition between deprotonated 2-methylimidazole and hydroxyl anion with Co2+ generated more crystal defects,forming more coordinatively unsaturated Co-Nx and pyridinic N sites to enhace the acid-base properties of the catalyst.To address the problem that water molecules in the interlayer of two-dimensional ultrathin nanosheet catalyst inhibiting the esterification reaction of FFAs,ultrafine ZIF-67 was synthesized by using low-polarity solvent ethanol,avoiding the presence of interlayer water while improving atom utilization.The ultrafine crystal increased the structural defects density on the crystal surface.With reducing ZIF-67 crystal size from 2.3 μm to 30 nm,coordinatively saturated Co-N4 content decreased from 81.3% to 56.2%,while unsaturated Co-Nx content increased from 18.7% to 43.8%,resulting in an increased L/B acidity ratio from 0.10 to 15.89 and total basicity from 0.69 to 1.89 mmol/g.The improved metal atoms utilization achieved high conversion efficiency of FFAs and TGs to 99.3% and 97.7%,respectively,at a lower catalyst concentration(1wt.%).Using the ultra-low-emission coal-fired flue gas CO2 from National Energy Group Penglai Power Generation Co.,Ltd,the project demonstration of microalgae fixing flue gas CO2 and the conversion of microalgae biomass to biodiesel was built in Yantai Hearol Biotechnology Co.,Ltd.The project demonstration achieved the large-scale production of microalgae with high lipid content above 30 wt.% and the biodiesel production capacity of 100 tons/year.The test indicators of biodiesel products reached the China BD100 quality standard. |
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