Synthesis And Application Of Biomass-based Levulinic Acid Ketal Esters

The use of biomass-based platform chemicals to prepare new chemicals is an important way to achieve a green and sustainable development of the chemical industry.Alkyl levulinate esters and glycerol are two important types of biomass-based platform chemicals.Under the action of an acidic catalyst,the carbonyl group and hydroxyl group of them can be condensed to form a ketal ester with a cyclic structure.Levulinic acid glycerol ketal ester is a class of functional intermediate compounds whose derivatives are widely used in many fields,such as fuel additives,perfumes,plasticizers and polyols.In recent years,more and more researchers have devoted themselves to the exploration of biomass conversion to ketal substances to replace a variety of chemical products derived from fossil raw materials to alleviate the energy crisis and environmental pollution problems.In this paper,the reaction conditions of the condensation of alkyl levulinate and glycerol to produce levulinic acid glycerol ketal ester was mainly explored,and the kinetics of the reaction were also investigated.Furthermore,the ketal polyol was synthesized by transesterification with ketal and ethylene glycol,and the foaming experiment was carried out to verify the possibility of the ketal polyol for making polyurethane foams.The main conclusions are as follows.Firstly,the process conditions,optimization and reaction kinetics of ketalization of methyl levulinate and glycerol to form levulinate glycerol ketal methyl ester were studied,and the separation and purification method of ketal methyl ester was determined.A quantitative analysis method for ketal methyl ester was established by internal standard method.The experimental results show that the ultra-low acid system is more conducive to the formation of ketal methyl ester.The order of influence of single factor is temperature>molar ratio>vacuum degree.By optimization,the optimum process conditions for the reaction are molar ratio 4.4,reaction temperature123°C,vacuum degree 90 kPa,reaction time 30 min,sulfuric acid addition 2.7×10^-4mol（relative to the molar equivalent of glycerol）.The predicted yield of methyl levulinate glycerol ketal methyl ester was 97.5%.The yield of ketal methyl ester was consistent with the predicted value under the optimal process conditions by repeated verification.Secondly,the optimization of the process conditions and the kinetics of the reaction of ketalization of ethyl levulinate and glycerol to produce ethyl levulinate glycerol ketal were studied.The separation and purification method of ketal ethyl ester was determined.A gas chromatography quantitative analysis method for ketal ethyl ester was established within the internal standard.By optimization,the optimum process conditions for the reaction are molar ratio 5.1,reaction temperature 127°C,vacuum degree 91 kPa,reaction time 30 min,sulfuric acid addition 2.7×10^-4mol（relative to the molar equivalent of glycerol）.The predicted yield of ethyl levulinate glycerol ketal was 99.1%.By repeated verification,the yield of ketal ethyl ester was consistent with the predicted value under the optimum process conditions.Finally,the process conditions of synthesizing ketal polyols by transesterification of ketals with ethylene glycol were investigated.The effect of reaction time on ketal conversion and the effect of molar ratio on viscosity and hydroxyl value of ketal polyols were investigated.Under the condition that the amount of the catalyst（sodium hydroxide）is 3 wt%,the degree of vacuum is 20 kPa,and the reaction temperature is170°C,after 2.5 h of reaction time,the conversion rate of ketal polyol can reach 97%,the reaction is basically complete.When the molar ratio is 1:1,the hydroxyl value and viscosity of the ketal polyol are 536.3 mgKOH/g and 1432.2 mPa·s respectively.Characterization of the ketal polyol by FT-IR and high resolution mass spectrometry showed that the ketal polyol had two hydroxyl groups with a molar molecular weight of about 234 g/mol.The mechanical properties of the polyurethane foam synthesized by the ketal polyols were measured,and the results showed that the mechanical properties meet the requirements of national standard（GB/T 15718-2008,on-site foam packaging materials）..Among them,the composite polyurethane foam with 3%diatomaceous earth has the highest apparent density and compressive strength,being0.0452 g/cm³ and 277 kPa respectively.The thermogravimetric analysis showed that the thermal stability of the ketal polyol derived foam was slightly worse than that of the commercial polyol.The cell structure analysis of the polyurethane foam showed that the foam pore size which is prepared by the ketal polyol was smaller than the commercial derived.The cell distribution of the foam prepared by the single polyol is uniform and denser than the foam prepared by the compounding polyol,and the cell distribution of the foam prepared by adding the diatomaceous earth polyol is more finely distributed than other foams.