Study On The Mechanism Of Oil Extraction From Microalgae Biomass By Deep Eutectic Solvent-assisted Hydrothermal Depolymerizatio

The conversion of microalgae with fast growth rate,no occupation of arable land and purification of the environment to produce biofuels has great development potential.Hydrothermal treatment（HTT）can directly utilize wet algae,thus significantly reducing the energy consumption of dehydration,which is one of the current research hotspots of microalgae energy conversion and utilization.In view of the degradation of the quality of microalgae lipid and the low efficiency of lipid separation in traditional hydrothermal treatment,this paper proposes an innovative method to promote the efficient separation of microalgae lipid by using deep eutectic solvent（DES）,reveals the microscopic mechanism of deep eutectic solvent as hydrogen bond donor and acceptor,co-solvent and catalyst to promote the separation of microalgae lipid,and constructs a life-cycle assessment system of deep eutectic solvent hydrothermal conversion of microalgae to produce biofuel.DES hydrothermal method was used to promote the depolymerization and dissolution of microalgae protein and carbohydrate into water,realizing the efficient and green extraction of microalgae lipid.With the assistance of DES,the lipid extraction efficiency after low-temperature hydrothermal treatment at 160°C was increased from 19.03%to 95.75%.The amount of protein dissolved in the hydrothermal phase increased from 53.97 mg/g DW to 351.10 mg/g DW,and the amount of reducing sugar dissolved increased from 13.29 mg/g DW to 50.70 mg/g DW,indicating that DES promoted the depolymerization and solubilization of microalgae biomass.The z-average molecular weight of solute in the hydrothermal phase increased from 1043 Da to 3203 Da,further demonstrating that DES promotes the dissolution of biological macromolecules.The solid residues were characterized by thermogravimetric analysis,Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,and elemental analysis,which demonstrated that DES disrupted the intermolecular interactions of microalgae biomass through the reconstruction of hydrogen bond networks,thereby promoting the extraction of lipid.With the help of quantum chemical density functional theory（DFT）,the microscopic mechanism of DES as a hydrogen bond donor,acceptor,co-solvent and catalyst promoting the separation of microalgae lipid under hydrothermal conditions was further revealed from the electronic level.Under the action of DES,the intermolecular interaction between microalgae biomacromolecules is significantly reduced（for example,the intermolecular interaction of tetrapeptides is reduced from-201.04 k J/mol to-166.19 k J/mol）.Meanwhile,new intermolecular interaction between DES and the tetrapeptide molecule was formed with a strength of-184.82 k J/mol.This finding indicates that DES effectively reduces the interaction between microalgae molecules and promotes the depolymerization of microalgae biomass by rebuilding the hydrogen bond network.DES significantly reduced the polarity difference between microalgae biomacromolecules and corresponding hydrolysates and water（for example,the polarity difference between glucose and water decreased from 7.32 kcal/mol to 3kcal/mol）,which proved that DES can be used as a cosolvent to promote the solubilization of microalgae biomacromolecules in the water phase.Under the action of DES,the hydrolysis energy barrier of microalgae biomacromolecules was significantly reduced from 193.74 k J/mol to 87.88 k J/mol,which proved that DES could be used as a catalyst to promote the depolymerization of microalgae biomass.On the basis of experimental data,a life-cycle assessment system for the conversion of microalgae to biofuel by DES hydrothermal method was established.The results showed that the addition of DES in the hydrothermal pretreatment process increased the energy output of biodiesel by 32.43%and biogas by 67.00%.Recrystallization and membrane filtration used alone or sequentially for DES recovery were also evaluated to reduce the energy consumption of DES-assisted HTT of wet algae.When DES was recovered by a combined method of membrane filtration and recrystallization,a low NER of 0.58 along with negative net GHG emissions(-0.77 g CO_2-eqMJ^-1)were achieved for microalgal biofuel production via DES-assisted HTT.Through the sensitivity analysis of the energy conversion characteristics of the system and the total greenhouse gas emissions of the system to the key parameters,it can be seen that the energy conversion characteristics of the system are sensitive to the change of the lipid recovery,and the total greenhouse gas emissions of the system are sensitive to the lipid recovery and methane yield.