| With fossil energy being gradually exhausted,the growing demand for renewable energy has become more and more urgent.Biodiesel,fatty acid methyl esters,has been successfully commercialized for many years,but there are inevitable problems,such as corrosiveness of equipment and huge amount of discharge of waste water resulting from sulfuric acid catalyst for esterification.In order to overcome the drawbacks of the traditional technology,a green synthesis process of fatty acid methyl esters was studied,in which a deep eutectic solvent(DES)composed of choline chloride and p-toluene sulfonic acid was used as esterification catalyst replacing for sulfuric acid.This paper covers following four aspects:1)The preparation and physical properties of DES were studied with choline chloride as hydrogen bond acceptor and ethylene glycol as hydrogen bond donor.A colorless transparent solution is formed by mixturing the hydrogen bond donor and the hydrogen bond acceptor at a certain molar ratio.Its physical properties are closely related to molar ratio of the hydrogen bond donor to the hydrogen bond acceptor.The smaller is the mole fraction of ethylene glycol in the DES-EG,the greater the density and the viscosity of the DES-EG.The freezing point of the DES decreased firstly with increasing content of ethylene glycol in the DES-EG.When the molar ratio of choline chloride to ethylene glycol was 1:2,the freezing point reached a minimum of-66?.Afterwards the freezing point of the deep eutectic solvent increased with the rising content of ethylene glycol.The thermal decomposition temperature of DES-EG is between both of them and its thermal stability is relatively poor compared with ionic liquids.2)DES formed by mixturing choline chloride and p-toluene sulfonic acid was used as a replacement of sulfuric acid catalyst,esterifications of oleic acid with methanol,diethylene glycol,glycerol was investigated,respectively.Experimental results indicated that the conversion of oleic acid with methanol was the highest.Optimum process parameters were obtained after the optimization of the esterification of oleic acid with methanol.Under the conditions of the molar ratio of choline chloride/p-toluene sulfonic acid 1:3,the reaction temperature 85?,stirring speed 400 r/min,reaction time is 4h,the acid value of resulting product is 2.68 mgKOH/g,esterification conversion reached 98.3%.With DES and p-toluene sulfonic acid being investigated,respectively,the obtained oleic acid conversion were similar,but the product obtained by the DES catalyst exhibited a lighter color,which simplifies the subsequent bleaching process.This is the place where the DES type catalyst is better than the traditional method.3)After the esterification reaction,the purification of products is necessary to remove the residual catalyst in the products.Experimental comparisons indicate that the new method is superior to the traditional washing method,the optimum technological conditions are as follows: temperature is 60?,stirring speed is 200 r/min,mole ratio of urea and choline chloride is 3:1,the volume ratio of DES-Urea and oleic acid methyl ester is 1:1.4)After each extraction of the crude oleic acid methyl ester,the catalyst was recovered and recycled.The experimental data shows that in the sixth extraction effect is significantly lower than the first five times.It can be concluded that the deep eutectic solvent can be recycled 5 times.A green process with no environmental problem of waste acid and waste water will be realized. |
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