Refinement Of Crude Glycerol And Heterogeneous Catalytic Synthesis Of Its Downstream Product Glycerol Monocaprate

Crude glycerol is a brown,high-viscosity,oily and glycerin-rich viscous liquid,which is the main by-product of biodiesel,accounting for about 10%of biodiesel production.However,due to its extremely complex composition,it is often treated as waste oil.In recent years,with the introduction of the carbon neutral development strategy,the world has paid more and more attention to the development and utilization of resources and environmental protection issues.How to purify a large amount of crude glycerol and develop downstream products with high added value has become two major research hotspots to promote the development of biodiesel industry.In this article,the crude glycerol is purified through a series of purification processes to make it reach the standard of first-class refined glycerol,and glycerol monocaprate is synthesized from the refined glycerol to further enhance the application value of refined glycerol.On the basis of analyzing and summarizing the synthesis of glycerides and the preparation of solid acid catalysts at home and abroad,a series of sulfonic acid-type silicon-based solid acids were prepared by using cheap silicon-based materials as carriers and strongly acidic sulfonic acid groups as acid donor groups.Then to investigate their catalytic activity,reusability,physicochemical stability and practicality in the above esterification reaction.First,the crude glycerol raw material was pretreated with calcium oxide as an alkali treatment agent and potassium aluminum sulfate dodecahydrate as a degumming agent.Under the conditions that pretreatment temperature of 80℃,calcium oxide content of 0.25 wt%,alkali treatment time of 10 min,and aluminum potassium sulfate dodecahydrate content of 7 wt%,the saponification equivalent of crude glycerol was reduced by 54%.The filtrate obtained from the pretreatment was subjected to vacuum distillation,and the glycerol was distilled out at about178℃under the pressure of 1.03 KPa.The glycerol content after vacuum distillation can reach the standard of second-class refined glycerol.Molecular sieve,bentonite,attapulgite and activated carbon were used as decolorizing agents to decolorize glycerol obtained by vacuum distillation.It has found that when 200 mesh coal-based activated carbon was used as the decolorizing agent,the decolorization temperature is 80℃,the amount of decolorizing agent is 2 wt%,and the decolorizing time is 1 h,the decolorization effect of glycerol is the best,and the chroma could reach the standard of first-class refined glycerol.Finally,the heavy metal lead in glycerol was removed by ion exchange.Under the conditions that lead removal temperature of 35℃,cation exchange resin addition of 1.0 g/L,and decolorization time of 120 min,the lead removal rate reach 95%,and first-class refined glycerol has been obtained.Secondly,on the basis of the preparation of refined glycerol,catalytic synthesis of refined glycerol downstream product glycerol monocaprate.Mesoporous silica-based molecular sieves Ti-SBA-15 was prepared by co-condensation method of using tetrabutyl titanate as titanium source and ethyl orthosilicate as silicon source.Using KH-580 as silane coupling agent and hydrogen peroxide as oxidant,the sulfonic acid functionalized silicon-based solid acid Ti-SBA-15-SO₃H was prepared by graft oxidation method.The catalysts were analyzed by a series of characterization methods including X-ray diffraction analysis（XRD）,specific surface area adsorption（BET）,scanning electron microscope（SEM）,Fourier transform infrared（FT-IR）,X-ray energy dispersive analysis（EDS）,thermogravimetric analyzer（TGA）and infrared（Py-IR）and acid-base titration,etc.The results show that the catalyst has a hexagonal mesoporous structure,the sulfonic acid group is successfully connected to the hydroxyl group on the silicon surface,contains both Lewis acid sites and Bronsted acid sites,and exhibits good thermal stability below 350℃.The esterification reaction of refined glycerol and capric acid to glycerol monocaprate was used as probe reaction to investigate the catalytic performance and reuse of the catalyst.The reaction conditions were optimized by single factor and response surface methodology,and it is found that the reaction temperature has the greatest influence on the actual yield of glycerol monocaprate.When the reaction temperature is 152.2℃,the alkyd molar ratio is 4.1:1,the catalyst dosage is 4.3 wt%and the reaction time is 5.1 h,the highest yield of glycerol monocaprate is obtained.Carry out 3 parallel experiments with this reaction condition,the average yield of gained glycerol monocaprate reaches more than 77%.After Ti-SBA-15-SO₃H has used repeatedly for 5 times under this condition,the yield of glycerol monocaprate can still reach 72%.It shows that Ti-SBA-15-SO₃H has good catalytic performance and reusability in catalyzing the esterification of refined glycerol.Finally,on the basis of the excellent catalytic performance of the above-mentioned sulfonic acid-type silicon-based solid acid,in order to improve the practicability of silicon-based carriers and simplify the loading process of sulfonic acid groups.Fe₃O₄@Si O₂magnetic particles with core-shell structure were prepared by co-precipitation method and sol-gel method,and Fe₃O₄@Si O₂@Ts OH was prepared by loading p-toluenesulfonic acid on Fe₃O₄@Si O₂ by impregnation method.Then the sulfonic acid type acidic ionic liquid[（H₃CO）₃Si（CH₂）₃NH₂（CH₂）₃SO₃H⁺][HSO₄^-]was prepared by two-step method,The ionic liquid was supported by grafting on the silicon surface hydroxyl groups on Fe₃O₄@Si O₂ core-shell to prepare Fe₃O₄@Si O₂-NH₂-IL.Fe₃O₄@Si O₂@Ts OH and Fe₃O₄@Si O₂-NH₂-IL were analyzed by a series of characterization methods such as XRD,TGA,FT-IT,SEM,TEM,BET,vibrating sample magnetism（VSM）and acid-base titration.TEM characterization shows that the magnetic core（Fe₃O₄）is completely covered by amorphous silica,which can effectively prevent Fe₃O₄ from being eroded by acidic liquids,while providing a high specific surface area and a certain mesoporous structure for the carrier.These two catalysts were used to catalyze the esterification of refined glycerol with capric acid to synthesize glycerol monocaprate,and their reaction conditions were optimized by single factor.The results show that when the reaction time is 3 h,the reaction temperature is 160℃,the catalyst dosage is 3 wt%,and the alkyd molar ratio is 3:1,in the catalytic reaction using Fe₃O₄@Si O₂@Ts OH and Fe₃O₄@Si O₂-NH₂-IL as catalysts,The conversion rates of capric acid reached 94.83%and 98.23%,respectively,and the yields of glycerol monocaprate were 76.61%and 72.96%,respectively.The two catalysts were separated with a magnet,and the stronger magnetic Fe₃O₄@Si O₂@Ts OH is found to be easier to separate.After they were reused 4 times according to the above reaction conditions,the yield of glycerol monocaprate of Fe₃O₄@Si O₂@Ts OH is lower than 40%,while the yield of glycerol monocaprate of Fe₃O₄@Si O₂-NH₂-IL is still 64.74%.It shows that both catalysts have good catalytic activity in the esterification of refined glycerol,and Fe₃O₄@Si O₂-NH₂-IL has better reusability.