Co-production Of Biodiesel And Diacylglycerol Using Switchable-plarity Solvent DBU As Catalyst

The global economic development has stimulated the continuous growth of energy demand.Under the background of increasing environmental protection,biodiesel has attracted attention as a renewable bio-based fuel with its excellent environmental protection and ease of use.The traditional chemical method for producing biodiesel has the problems of high cost,environmentally unfriendly in production process,and difficulty in recycling by-product glycerol.In this paper,1,8-Diazabicyclo[5.4.0]undec-7-ene(DBU)was used as a catalyst and a reaction solvent to prepare FAEE(biodiesel)and diacylglycerol(DAG),and the biodiesel by-product glycerol was directly used to convert it into a high value-added DAG,and established a series of methods to separate,recover and reuse DBU to achieve the coupling of biodiesel preparation process and high value-added conversion process.Then,based on the optimization of biodiesel preparation conditions,the kinetics of DBU-catalyzed transesterification of soybean oil with ethanol to prepare biodiesel was studied.In the process of DBU catalyzing the conversion of crude glycerol and soybean oil into DAG,the effect of reaction temperature on the glycerolysis reaction equilibrium and the relationship to the acyl transfer within the DAG molecule were studied.Firstly,in the process of DBU catalysing the transesterification of soybean oil and ethanol,the four reaction conditions were optimized using a single factor method:the molar ratio of alcohol to oil,the molar ratio of DBU to oil,the reaction time and the reaction temperature.When nEtOH:nOil=12:1,nDBU:nOil=2.0:1,the reaction was carried out at 70℃ for 2 h,and the yield of FAEEs was 97.9%.It is indicated that as a catalyst,DBU has good performance in catalyzing the transesterification of soybean oil with ethanol.On this basis,Box-Behnken response surface optimization was performed on the three factors:alcohol-oil molar ratio,DBU-oil molar ratio,and reaction time.When nEtOH:nOil=13.8:1,nDBU:nOil=1.4:1,the reaction was carried out at 70℃ for 1.9 h,and the yield of FAEEs was 98.6%.At the same time,the Arrhenius equation was obtained by mathematical processing using the curve of FAEEs yield obtained under the four temperature conditions of 55,60,65 and 70℃,v=561.31e-16.768/RTCTAG0.7 the apparent activation energy was 16.763 kJ mol-1,the pre-factor was 561.31 mol 0.3 mL-0.3 min-1,and the reaction order was 0.7.In the process of DBU-catalyzed transesterification of soybean oil with glycerol,four reaction conditions of soybean oil-glycerol molar ratio,DBU-glycerol molar ratio,reaction time and reaction temperature were optimized by single factor method.When nOil:nGlycerol=2:1,nDBU:nGlycerol=2.5:1,the reaction was carried out at 130℃ for 25 min to obtain DAG content in the system was 67.6%,at this time,in the DAG product,1,2-DAG accounted for 89.3%and 1,3-DAG accounted for 10.7%.It is indicated that DBU has good catalytic performance for catalyzing the glycerol reaction of soybean oil with glycerol,and its catalytic product has obvious tendency to 1,2-DAG.Based on the single-factor optimization results,the Soybean oil-glycerol molar ratio,DBU-glycerol molar ratio,and reaction time were optimized by Box-Behnken response surface.When nOil:nGlycerol=1.8,nDBU:nGlycerol=2.5:1,the reaction was performed at 130℃ for 32.4 min,and the actual content of DAG in the system was 67.8%.Next,a method for separating,recovering,and reusing the catalyst DBU from the FAEE preparation system was established,and the preparation process of FAEE and DAG was coupled through the recovery and reuse of DBU.When the reaction for the preparation of FAEE was completed,after DBU was separated from the product and returned to a non-polar state,soybean oil was directly added to the system to react with crude glycerol to produce DAG,and the reaction conditions of the process were single factor optimization.The results showed that the molar ratio of DBU to crude glycerol in the coupling system was about 2.04:1 at the end of the FAEE production;when the highest DAG content was 62.3%,the molar ratio of oil to glycerol was 2:1.The reaction time was 30 minutes and the reaction temperature was 130℃.The proportions of 1,2-and 1,3-DAG in the DAG products were 84.7%and 15.3%,respectively.The proportion of DAG produced by DBU to 1,2-DAG has obvious tendency.In order to understand the relationship between the time required for the equilibrium of the glycerolysis reaction catalyzed by DBU and the reaction temperature,the study found that the time required for the DAG content to reach the highest value at 130,150,and 180℃ was 30,25,and 20 minutes,respectively;The initial stable time range is 40-45 min,which indicated that increasing the temperature can shorten the time required for the glycerolysis reaction to reach an equilibrium state,which was beneficial to the DAG content of the target product to reach a stable state quickly.The relationship between the reaction temperature and the choice of DAG products,and the relationship between the reaction temperature and the time required for the acyl transfer reaction inside the DAG molecule to reach equilibrium were investigated.At the temperature of 130,150 and 180℃,intercepted for a period of 60～240 min,and the average value of the proportion of 1,2-DAG is used to characterize the degree of influence of temperature on the content of 1,2-DAG.The mean value decreased with the increase of temperature,indicating that the increase in temperature was favorable for the conversion of DAG products to 1,3-DAG.The interval of 60～240 min was used to characterize the degree of influence of temperature on the internal acyl transfer balance of the DAG molecule by the standard deviation of the proportion of 1,2-DAG.The standard deviation of 1,2-DAG content decreased as the temperature increased,indicating that increasing the temperature helps shorten the time required for the acyl transfer within the DAG molecule to reach equilibrium.Finally,the catalytic stability of catalyst DBU reuse was investigated in three systems(FAEE preparation system,DAG preparation system and coupling system).The results showed that:in the FAEE preparation system,the catalyst DBU was repeatedly used for 10 cycles,and the average of FAEE yield was 95.5 ±3.3%,and the FAEE yield was not significantly decreased within 8 times of reuse.After 5 cycles of repeated use,the DBU recovery was 97.3 ± 1.0%,and the average DAG content was 67.4 ±0.5%.In summary,the results in this article show that DBU can well catalyze transesterification and glycerolysis reactions to produce FAEE and DAG.Its reusable characteristics enable coupling production of FAEE and DAG,and the conversion of crude glycerol produced by FAEE to high value-added DAG becomes possible,which can alleviate the current situation of high cost of biodiesel and insufficient market competitiveness to a certain extent,and has a positive significance for reducing the cost of biodiesel production.