Catalytic Conversion Of Cellulose Extracted From Straw To 5-Hydroxymethyl Furfural

With the depletion of fossil resources,the conversion of straw into platform compounds to alternative fossil fuels has attracted widespread attention.Straw is an important renewable resource for the preparation of fuels and chemicals due to it savailability and low cost.In this paper,straws such as rice straw and wheat straw were employed to extract cellulose,which was further converted to microcrystalline cellulose.The convertion of microcrystalline cellulose into 5-hydroxymethylfurfural(HMF)was also investigated.The main research contents are as follows:Toluene-ethanol mixture was first employed to remove waxy and soluble impurity from straws.The straw crude cellulose was then obtained by organic solvent extraction,in which most of lignin and hemicellulose was removed.The residual lignin,hemicellulosesand silicon was further removed in alkaline condition,thus giving straw cellulose.The as-obtained straw cellulose was further hydrolyzed to prepare microcrystalline cellulose in the presence of acid catalyst.Variables such as type of solvent,concentration of alkali/hydrogen peroxide,time and temperature were investigated.The structure,morphology and thermal property of straws and their derived samplesincluding crude cellulose,cellulose and microcrystalline cellulosewere characterized by infrared spectroscopy(IR),X-ray diffraction(XRD),scanning electron microscopy(SEM)and thermogravimetric analysis(TG).The results showed that the rice straw was extracted by N-methylpyrrolidone/water with volume ratio of 1:1 and bleached by Na2CO3/H2O2 solution,giving product with cellulose content of 90.8 wt%.The removal rates of hemicellulose,lignin and silicon were 92.5,94.5 and 98.8%,respectively.Microcrystalline cellulose derived from rice straw was obtained via the hydrolysis of crude cellulose at 80℃ for 3 h in the presence of solid catalyst Amberlyst-15.The yield of microcrystalline cellulose and degree of polymerization were 79.4%and 261,respectively.The cellulose content of microcrystalline cellulose was 92.3 wt%.The crude cellulose of wheat straw was extracted by a mixed solvent of methanol/water withvolume ratio of 1:2,and the lignin,hemicellulose and silicon were further removed by bleaching in potassium hydroxide/H2O2 solution.The content of cellulose in wheat straw cellulose was 93.4 wt%,and the removal rates of hemicellulose,lignin and silicon were 93.1 96.3 and 96.6%,respectively.Microcrystalline cellulose derived from wheat straw was also obtained in the presence of solid catalyst Amberlyst-15.The yield of microcrystalline cellulose and degree of polymerization were 91.9%and 209,respectively.The cellulose content of microcrystalline cellulose was 94.0 wt%.The synthesis of 5-hydroxymethyl furfural(HMF)from microcrystalline cellulose was investigated.To optimize reactionconditions,celluloseand glucose were first converted to HMF separatelyin tetrahydrofuran(THF)and N,N-dimethylformamide(DMF)via a one-step processusing hosphotungstic acid as catalyst.The quantitative analysis of HMF was determined by high performance liquid chromatography(HPLC).Based on the synthesis of HMF from cellulose and glucose,the direct conversion of cellulose to HMF was then performed via a two-step process.The first step and the second step were carried out in THF and the mixture solvent composed of THF/DMF,respectively.The conversion of cellulse to HMF and glucose were significantly improved by the two-step process,and the total yield of HMF and glucose was elevated from 52.1 to 97.0%.Using wheat straw microcrystalline cellulose as raw material,the yields of HMF and glucose reached 37.2 and 56.9%,respectively,and the total yield reached 94.1%.The products were qualitative analyzed by liquid chromatography-high resolution mass spectrometry(LC-HRMS)and gas chromatography-mass spectrometry(GC-MS).A possible mechanism for the formation of HMF from cellulose via the two-step process was also proposed.It is supporsed that 1,6-anhydroglucopyranose is an intermediate for the synthesis of HMF from cellulose via the two-step process.