Graphene Catalyzed Biodiesel Production From Lipids In Wet Microalgae And Biocrude Production From Microalgal Residue

The conversion of microalgae biomass with high photosynthetic efficiency,short cycling time and high lipid content into biofuels is a hotspot in the field of sustainable energy around the world.In order to solve the problems during the process,such as high energy consumption for drying wet microalge,strong toxicity of chloroform extraction solvent,difficult recovery of liquid acid catalyst,and utilization of algae residue after oil extraction,etc.,this study took wet microalgae as the research object,and sulfonated graphene oxide(SGO)was first proposed as solid acid catalyst for microwave-assisted transesterification to transform lipids in wet microalgae into biodiesel,which solve the problem of separating and recycling corrosive wastewater produced by concentrated sulfuric acid catalyst.Batch reactors were used to investigate the effects of subcritical hydrothermal pretreatment on breaking the microalgal cell wall and hydrolysis of triglyceride and the esterification efficiency of the hydrothermal hydrolyzed lipids was improved significantly without the use of highly toxic chloroform extractant.The typical maillard reaction of protein and carbohydrate in algal residue during hydrothermal liquefaction to produce biocrude was revealed,which solved the problem of energy utilization of algae residue after microalgal lipids extraction.SGO,graphene oxide(GO),sulfonated graphene(SG)and sulfonated active carbon(SAC)were first proposed as solid acid catalysts to convert lipids in wet microalgae into biodiesel,which solve the problem of separating and recycling corrosive wastewater produced by concentrated sulfuric acid catalyst.Wet microalgae cells were adsorbed on hydrophilic graphene derivatives' surfaces covered with many OH groups.Lipids extracted by chloroform from microalgal cells were then transformed into fatty acids methyl esters(FAMEs)through transesterification catalyzed by the acid centers(-SO3 H groups)in solid acid catalysts.The lipids conversion efficiency into FAMEs reached the highest in microwave-assisted transesterification reactions of 5 wt.% catalyst at 90 ? for 40 min.Among the four solid acid catalysts,SGO provided the highest conversion efficiency(84.6% of sulfuric acid),whereas SAC converted few lipids into FAME.Much higher hydrophilic hydroxyl content in SGO catalyst resulted in a considerable higher conversion efficiency of lipids to FAME than that(48.6%)catalyzed by SG,although SO3 H groups(0.44 mmol/g)in SGO were less than those(1.69 mmol/g)in SG.Given its higher SO3 H group content than GO(0.38 mmol/g),SGO had higher conversion efficiency than GO(73.1%),when they had similar hydrophilic hydroxyl contents.Mild hydrothermal pretreatment(245?)was used to break microalgal cell walls for lipid hydrolysis and the esterification efficiency of the hydrothermal hydrolyzed lipids was improved significantly without the use of highly toxic chloroform extractant.Microalgal biocrude obtained through hydrothermal pretreatment for 10 min,had the highest heating value(33.83 MJ/kg),the highest carbon and hydrogen contents(69.63% and 9.39%),as well as the lowest nitrogen content(3.56%).Fourier transform infrared spectroscopy showed that treated microalgal biocrude contained increased long-chain alkane peaks and no observable carbohydrate peaks,as compared to the untreated microalgal biomass.Thermogravimetric analysis showed that triglycerides in microalgal biomass were hydrolyzed into fatty acids by hydrothermal pretreatment,while fatty acids and protein hydrolysates reacted to produce amides with higher boiling points.The efficiency of solvent-free lipid extraction reached 95.0 wt.% after subcritical hydrothermal pretreatment for 10 min(with a microalgal solid concentration of 5% at 245?,)and esterification reactions with methanol and H2SO4 catalysts.The resulting main components of microalgal biodiesel were C18:0,C18:1 and C20:5,which reached maximum abundances of 22.7%,24.1% and 16.2% of the total microalgal lipid content,respectively,much higher than the content of C20:5 in hydrolysis biocrude obtained at 260?.In order to solved the problem of energy utilization of algae residue after microalgal lipids extraction,the typical maillard reaction of protein and carbohydrate in algal residue during hydrothermal liquefaction(320?)to produce biocrude was revealed: the ammonia produced by the leucine deamination reaction and the amine group produced by the decarboxylation reaction polymerized with the cyclic oxygenate degraded by glucose to produce pyrazine derivatives.GC-MS results revealed that N&O-heterocyclic compounds,organic acids,and carbonyls/imine/amine were the main components of both biocrude and aqueous fraction.The optimal reaction temperature,reaction time,leucine to glucose weight ratio and solid concentration for biocrude production are 320 ?,60 min,2:5,and 20 wt.%,respectively.The obtained biocrude,presented large amount of pyrazine derivatives(84.2% peak area),yielded 47.6 wt.% of leucine and glucose feedstock.Food waste was mixed with microalgal biomass to regulate the C: N ratio,which promoted biodiesel production from lipids through transesterification and bio-crude production from carbohydrates and proteins through hydrothermal liquefaction.The cytoplasms of microalgal biomass,which reduces polarity differences between lipids and the methanol reactant,improved transesterification reactions of lipids in food waste,thereby promoting biodiesel production by 13.3%.The optimized Maillard reactions between the carbohydrates and proteins(C:N ratio= 6.2)promoted bio-crude production from the biomass residue by 13.0% after biodiesel extraction.The highest weight ratio of biodiesel(with a higher heating value of 41.0 MJ/kg)through the transesterification reaction to dry feedstock(composed of microalgae and food waste with an optimized weight ratio of 1:1)was obtained at 23.2%(wt.).A high-weight ratio of bio-crude(Higher heating value: 35.1 MJ/kg)to biomass residue after biodiesel extraction was achieved at 16.6%(wt.).