| Quinoa straw contains rich bast fibre and lignocellulose.Quinoa straw can be converted to bioethanol by saccharification and fermentation,which is an effective way for utilization of quinoa straw.Due to the complex structure and low direct enzymatic efficiency of quinoa straw,it is necessary to adopt suitable pretreatment methods to change the complex structure of quinoa straw and improve the efficiency of enzymatic saccharification.In this study,three kinds of quinoa straws,including Haili quinoa straw,Gannan quinoa straw and Geermu quinoa straw,were used as experimental materials.The composition and structure analysis of them were conducted to compare the difference of three kinds of quinoa straws.Quinoa straws were pretreated by hydrotherm,dilute alkali,and 1-butyl-3-methylimidazolium chloride([Bmim]Cl).The pretreatment conditions of the three pretreatment methods were optimized.Then the microstructure of quinoa straw before and after pretreatment was characterized through the Scanning electronic microscopy(SEM),Fourier transform infrared spectra(FT-IR)and X-ray diffraction(XRD).Finally,a feasible pretreatment method was selected.The main research results were as follows:(1)There are differences in the composition and structure of three kinds of quinoa straws.The cellulose content of Haili quinoa straw was highest(29.89%),and the lignin content was lowest(18.60%).SEM,FT-IR,and XRD analysis demonstrated that the structure of Haili quinoa straw was looser,and the crystallinity was lower than other quinoa straws,which made it obtain a high pretreatment efficiency.(2)Hydrothermal pretreatment pressure showed significant influence on the cellulose content and weight loss of pretreated quinoa straw.The order of the main effect on pretreatment efficiency was pretreatment pressure > pretreatment duration > liquid/solid ratio.The optimal pretreatment conditions were pretreatment pressure of 130 k Pa,duration of60 min,and liquid to solid ratio of 20:1.The cellulose content from pretreated quinoa straw was 46.57%,which was increased by 55.80% compared with untreated straw.SEM analysis showed that hydrothermal pretreatment destroyed and loosed the dense structure of quinoa straw,which showed that hydrothermal pretreatment efficiency was significant.FT-IR and XRD analysis showed that the functional groups and crystal structure of quinoa straw were not changed after hydrothermal pretreatment.After pretreatment,a part of hemicellulose was removed,the content of lignin was increased slightly,and crystallinity was increased.(3)The effect of pretreatment temperature on the cellulose content and weight loss of dilute alkali pretreated quinoa straw was significant.The effects of pretreatment duration and sodium hydroxide concentration(Na OH)were not significant.After pretreatment at pretreatment temperature of 140℃,sodium hydroxide(Na OH)concentration of 3.5% for 2h,a maximum cellulose content(63.54%)of pretreated straw was obtained,being 112.58%higher than that of untreated straw.The surface of quinoa straw became rougher,and the fiber bundle structure was damaged seriously after pretreatment.The effect of dilute alkali pretreatment was remarkable.FT-IR and XRD analysis showed that no new functional groups were formed in quinoa straw after pretreatment.A major part of lignin in quinoa straw was degraded.The crystallinity of pretreated straw was higher than that of untreated one.(4)[Bmim]Cl pretreatment temperature showed significant influence on the reducing sugar yield and weight loss of pretreated quinoa straw.The impact of pretreatment duration was minimal.A maximum reducing sugar yield of 33.22% from pretreated quinoa straw was obtained at pretreatment temperature of 100℃,pretreatment duration of 5h,and liquid/solid ratio of 15:1,being 20.93% higher than that of untreated straw.After [Bmim]Cl pretreatment,the intact structure of quinoa straw was destroyed and became loose,cracks and holes were appeared.FT-IR and XRD analysis showed that [Bmim]Cl pretreatment degraded a part of lignin and hemicellulose,and increased the crystallinity of quinoa straw.(5)The composition analysis of three kinds of pretreated quinoa straws showed that the cellulose content of dilute alkali pretreated quinoa straw was highest(64.54%),and the lignin content was lowest(16.94%).The reducing sugar yield of dilute alkali pretreated quinoa straw was the highest(40.92%),which was increased by 48.96% compared with untreated quinoa straw.The reducing sugar yield of hydrothermal and [Bmim]Cl pretreated quinoa straws were 41.10% and 20.93% higher than that of untreated quinoa straw,respectively.SEM analysis showed that the enzymatic hydrolysis extent of dilute alkali pretreated quinoa straw was greater than that of quinoa straws after hydrothermal pretreatment and [Bmim]Cl pretreatment.FT-IR and XRD analysis showed that the functional groups of hydrolysis residues from three pretreated quinoa straws were consistent.Dilute alkali pretreatment removed most of the lignin and hemicellulose and seriously damaged the hydrogen bond between cellulose molecules.The crystallinity of dilute alkali pretreated quinoa straw(49.35%)after enzymatic hydrolysis was higher than that of hydrothermal pretreated straw(43.75%)and [Bmim]Cl pretreated straw(41.5%).The efficiency of dilute alkali pretreatment was the most significant among three kinds of pretreatment methods. |
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