Experimental Study On The Synthesis Of Liquid Fuel Ethyl Levulinate By Dilute Acid-catalyzed Alcoholysis Of Cellulose Biomass

Ethyl levulinate(EL)is a green biofuel and fuel additive,and can also be used as a plasticizer and pharmaceutical raw material.Converting cellulosic biomass base into EL is beneficial to both waste biomass resource utilization and carbon neutrality as well as ecological protection.In this paper,EL was synthesized from cellulose and corn straw under the catalytic action of dilute and mixed acids.The paper firstly used ultra-low concentration sulfuric acid to catalyze the alcoholysis of cellulose to synthesize EL,and on this basis,the direct conversion of corn straw after steam explosion pretreatment catalyzed by mixed acids to generate EL was investigated,and the following conclusions were obtained:(Ⅰ)The effects of sulfuric acid dosage,reaction time,reaction temperature and substrate concentration on the preparation of EL from ultra-low concentration sulfuric acid-catalyzed cellulose were investigated.The response surface Box-Behnken model was applied to optimize the EL synthesis process,and better process conditions were obtained for the preparation of EL from ultra-low concentration sulfuric acid-catalyzed cellulose: sulfuric acid dosage of 0.5%,reaction temperature of 204°C,reaction time of240 min,and substrate concentration of 29 g/L.The average yield of EL was 66.7%,with a relative error of 2.24% from the theoretical prediction;The distribution of alcoholysis products was analyzed by gas chromatography-mass spectrometry(GCMS),and a possible reaction pathway for ultra-low acid-catalyzed cellulose alcoholysis was proposed.(Ⅱ)The pretreatment of corn straw was carried out using steam explosion technology,and the effects of explosion pressure and holding time on the pretreatment of corn straw were investigated and characterized using scanning electron microscopy(SEM),Fourier infrared spectroscopy(FTIR),X-ray diffraction(XRD),specific surface area analysis(BET)and thermogravimetric analysis(TG).The results showed that steam explosion pretreatment could effectively break the entangled structure of cellulose,hemicellulose and lignin,promote the degradation of hemicellulose and the reorganization of lignin in corn straw,and reduce the conversion difficulty of corn straw.With the explosion pretreatment pressure of 1.5 MPa and a holding time of 10 min,the relative cellulose content of corn straw was elevated by 30.4% and the hemicellulose degradation rate was 47.3% after explosion.(Ⅲ)The synthesis of EL from alcoholysis of corn straw after steam explosion pretreatment was investigated using a mixed acid system consisting of ultra-low concentration sulfuric acid with metal salts and heteropolyacids as catalysts.The effects of reaction temperature,substrate concentration,reaction time and catalyst dosage on the EL yield were investigated,and the better reaction conditions were obtained as follows: substrate concentration 30 g/L,reaction temperature 205°C,reaction time 90 min,Br?nsted acid/Lewis acid = 20:1,and EL yield 24.55%.The reaction kinetic results showed that the activation energy for the conversion of straw to EL was E1(110.45 k J·mol-1)higher than that of the by-products(glucose,fructose and humus)E2(56.80 k J·mol-1),and K1/K2 increased with increasing temperature.The results indicated that the degradation of cellulose to glucose and fructose required lower energy,while the production of EL required higher temperature.The reaction pathway for the synthesis of EL from the alcoholysis of corn straw catalyzed by silicotungstic acid and sulfuric acid was proposed by GC-MS analysis of the liquid phase products.