Microwave-assited Lignin Degradation And Lipids Extraction From Microalgae

The industrial development of human society depends on fossil resources to a large extent. With the increasing speed of petrochemical resources consumption, existing unrenewable resources are not enough to meet human needs. Searching for alternative sources of fossil resources becomes the common demands for the world’s scientific research personnel. Biomass resources are renewable and remarkably abundant in nature, in the cultivation process of which CO2 can be fixed by photosynthesis. Regarding these advantages, biomass resources are considered to be one of the solutions to the energy crisis. Waste water from paper industry and agricultural wastes contain a large amount of lignin. Lignin is consisted of benzene propane units and is a promising source of aromatic compounds. Microalgae contain a good amount of bio-oil which can be extracted and converted into biodiesel. The efficient utilization of the two kinds of biomass which can solve the problems of energy crisis and climate change has very important significance.Compared with conventional heating, microwave heating directly acts on the molecular/ion by electromagnetic radiation. Polar molecules in microwave field will rotate in high-speed. Ions in microwave field will migrate as the electromagnetic field swings. Thus the electromagnetic energy will transfer into heat energy through friction between molecules/ions. This energy transfer mode is more efficient and faster than that of the traditional heating. In addition, there is no temperature gradient within the reaction system with microwave heating. Therefore, the side reaction with microwave heating is less and the selectivity of the reaction with microwave heating is higher. Microwave technology can be used in the degradation of lignin to accelerate the reaction rate, decrease the activation energy of reaction and improve the selectivity of the reaction. For microalgae lipids extraction, microwave technology can improve the cell disruption, accelerate the extraction and increase the lipids extraction yield. Focusing on lignin degradation and lipids extraction from microalgae, this research is summarized as follows:(1) Degradation of phenethyl phenyl ether with microwave irradiation in hydrogen donor solvent was studied. The effects of phenethyl phenyl ether initial concentration and p-toluenesulfonic acid concentration on phenethyl phenyl ether conversion and phenol yield were investigated. The results showed that as phenethyl phenyl ether concentration increased, the phenethyl phenyl ether conversion and the phenol yield decreased; as p-toluenesulfonic acid concentration increased, the phenethyl phenyl ether conversion and the phenol yield increased. Degradation kinetics of microwave heating and conventional heating were compared and fitted by Arrhenius equation. The activation energy with microwave irradiation was 81.97 kJ·mol-1, with conventional heating was 111.37 kj·mol-1. The former was lower than the latter by 26.4%. Microwave irradiation can speed up the reaction, reduce the activation energy and improve the reaction selectivity. According to the mechanism of degradation, tetralin gives a hydrogen radical and reacted with the free radicals from the degradation of lignin in the system, producting complex compounds.(2) Benzyl phenyl ether and guaiacol were selected as lignin model compounds, and were degradated with microwave irradiation in 29 kinds of ionic liquids. The results showed that for benzyl phenyl ether degradation, [CmMIM]BF4, [BMIM]HSO4 and [BsMIM]HSO4 had the highest catalytic activity; for guaiacol degradation, [CmMIM]Cl, [BMIM]HSO4 and [BsMIM]HSO4 had the highest catalytic activity. The ionic liquids with the highest acidity were [CmMIM]BF4, [CmMIM]Cl, [BMIM]HSO4 and [BsMIM]HSO4, which indicated that the higher the acidity of the ionic liquid, the better the catalytic effect. The correlation between the conversion of model compounds and the acidity of ionic liquids was not strict, so the acidity is not the only factor that affecting the degradation. Microwave accelerated the degradation of benzyl phenyl ether by 6.5 times, acclerated the degradation of guaiacol by 11 times, and also increased target product selectivity of the degradation reaction. According to results of the GC-MS analysis of the degradation products, the degradation mechanisms of two model compounds were proposed, In the degradation of benzyl phenyl ether, both SN2 and Sn1 mechanisms existed. In the degradation of graiacol, only the Sn2 mechanism existed. Ionic liquid can be recycled. With four times recycling, ionic liquid structure did not change, and the catalytic activity did not reduced.(3) A systematic study on microwave-assisted oxidative degradation of lignin model compounds, such as 2-phenoxy-1-phenylethanol, vanillyl alcohol, and 4-hydroxybenzyl alcohol, was performed by evaluating the catalytic activity of 14 types of metal salts. The combination of microwave and metal salts performed very well. Microwave irradiation can improve the degradation of three model compounds. In the 14 metal salt catalysts, CrCls, MnCl2 and CuCl2 were the best catalysts for 2-phenoxy-1-phenylethanol oxidation degradation, CrCl3, MnCl2, and CoCl2 for vanillyl alcohol, CaCl2, CrCl3, MnCl2, and ZnCl2 for 4-hydroxybenzyl alcohol. The relationship between model compounds conversion and the acidity of metal salts satisfied the following law:the higher the metal salt acidity, the higher the model compounds conversion.(4) Chlorella sorokiniana, Nannochloropsis salina and Galdieria sulphuraria were used as lipids sources. [BMIM][HSO4] was the extraction solvent and catalyst. The effects of microwave irradiation, ultrasonic and conventional heating on extraction were compared, and the composition of the lipids was analyzed by GC-MS. The results showed that, the combination of microwave and ionic liquid performed very well for lipids extraction from Chlorella sorokiniana, Nannochloropsis salina and Galdieria sulphuraria. Microwave radiation improved 20 times lipids extraction for Chlorella sorokiniana,4 times for Nannochloropsis salina and 12 times for Galdieria sulphuraria. Mineral acids also improved the lipids extraction, presumably because of mineral acid promotes cell disruption and breakage of hydrogen bonds in cell wall, thereby contributing to the formation of ionic liquid non-polar channel.