Research On Biodiesel Production And Hydrothermal Conversion Of By-product Glycerol Catalyzed By Sodium Silicate

Biodiesel,as an environmental-friendly renewable energy,receives continuous attention and is extensively studied.How to improve the efficiency of biodiesel production is a hot research topic.As the biodiesel industry grows rapidly,the discharge of main by-product glycerol increases substantially.Therefore,utilizing by-product glycerol to produce high value-added products has become the new focus of attention.In this work,the two aspects-catalytic and intensified synthesis of biodiesel and efficient conversion of glycerol were studied.(1)Microwave-and ultrasound-assisted synthesis of biodiesel using sodium silicateSolid sodium silicate catalyst was prepared by a simple calcination method,and used to catalyze the transesterification of rapeseed and Jatropha oils to biodiesel under microwave irradiation.Single-factor experimental results showed that biodiesel yields of 97.0%and 90.1%were achieved from rapeseed and Jatropha oils,respectively(microwave power of 400 W,methanol/oil molar ratio of 11:1,catalyst amount of 4 wt%and reaction time of 5 min).The catalyst could be reused at least four times with biodiesel yield of more than 80.0%.Fresh and reused sodium silicate catalysts were characterized by Brunauer-Emmett-Teller surface area(BET),X-ray diffraction(XRD),scanning electron microscopy(SEM)and CO2-temperature programmed desorption(CO2-TPD).It was found that the agglomeration and leaching of basic species resulted in the loss of catalytic activity.Solid sodium silicate catalyst(micro size)was further prepared by calcination followed ball-milling treatment,and used to catalyze the transesterification of rapeseed oil to biodiesel under ultrasound irradiation.Orthogonal experimental results showed that reaction temperature is the most important influence factor on biodiesel yield,and a biodiesel yield of 97.5%was achieved under the optimized conditions(reaction temperature of 60 ℃,ultrasonic power of 350 W,reaction time of 40 min,catalyst amount of 5 wt%and methanol/oil molar ratio of 9:1).The catalyst could be reused at least two times under ultrasound irradiation.Howerver,biodiesel yield was reduced to less than 80.0%at the third recycle.Fresh and reused sodium silicate catalysts and carbonized product samples were characterized by SEM and energy dispersive X-ray spectroscopy(EDX),which indicated that the leaching of active sites resulted in the loss of catalytic activity.It was also proved that the catalytic synthesis of biodiesel over solid sodium silicate catalyst at room temperature was feasible with the help of ultrasound.(2)Hydrothermal conversion/gasification of by-product glycerol to lactic acid and hydrogenSodium silicate was used to catalyze the hydrothermal conversion of glycerol to lactic acid.The results showed that glycerol conversion rate of nearly 100%and lactic acid yield of 90.7%were obtained under the conditions(substrate concentration of 0.3 mol/L,catalyst concentration of 0.625 mol/L,reaction temperature of 300 ℃,and reaction time of 90 min).As reaction temperature or reaction time increased,lactic acid decomposition reaction occurred,which led to the decreased yield.The deactivated sodium silicate from biodiesel production was used as catalyst for the hydrothermal conversion of glycerol,and the glycerol conversion rate and lactic acid yield were reduced to 92.8%and 80.5%,respectively.A lactic acid yield of 88.7%was achieved by using crude glycerol as raw material(substrate concentration of 3 mol/L,catalyst concentration of 1.5 mol/L,reaction temperature of 300 ℃,and reaction time of 90 min).Other components of crude glycerol had no obvious effect on lactic acid yield.Ni-based catalyst and sodium silicate were used to catalyze the hydrothermal gasification of glycerol to hydrogen.The results showed that a hydrogen yield of 85.2%with a gaseous concentration of 86.2%(mol/mol)was achieved by using Ni-based catalyst(40 wt%)and sodium silicate catalyst(160 wt%)under the gasification conditions(temperature of 350 ℃,heating and reaction time of 30 min).In the case of catalysis with sodium silicate,the concentration of hydrogen increased substantially due to carbon dioxide absorption by alkaline solution,which was formed by the hydration of sodium silicate.The decrease in the yield(82.8%)and gaseous concentration(73.6%,mol/mol)of hydrogen was observed by using Ni-based catalyst and deactivated sodium silicate from biodiesel production.There was no significant change in gas compositions(hydrogen concentration of 85.3%,mol/mol)by using crude glycerol as raw material due to that glycerol component was mainly responsible for gas products in the hydrothermal gasification.This revealed the feasibility of producing high-pure hydrogen through the direct hydrothermal gasification of waste glycerol from biodiesel industry.It can be concluded that microwave or ultrasound assistance can effectively promote biodiesel production over solid sodium silicate catalyst compared with conventional heating such as oil-bath.In addition,high-value products lactic acid and hydrogen can be obtained through the hydrothermal conversion/gasification of by-product glycerol using deactivated sodium silicate and with Ni-based catalysts(for gasification).In this study,sodium silicate catalyst and low-cost by-product glycerol were fully and appropriately utilized,and efficient biodiesel production and high value-added conversion of by-product glycerol were accomplished.