Production And Structural Characterization Of Xylo-oligosaccharides From Bamboo (Dendrocalamus Giganteus Munro)

Xylo-oligosaccharides from lingocellulosic biomass autohydrolysis is one of the important approaches for biomass utilization,which attracts extensive attentions and research interests.As a potential prebiotic,the role of xylo-oligosaccharides in promoting human health has been recognized.However,it is difficult to control the autohydrolysis conditions and products,resulting in complex structures of xylo-oligosaccharides,which led to difficulties in separation and purification.In order to overcome these issues and analyze the structure of xylo-oligosaccharides prepared by autohydrolysis,in this work autohydrolysis reaction of bamboo(Dendrocalamus giganteus Munro)culm was carried out for the production of xylo-oligosaccharides.The obtained xylo-oligosaccharides were then separated by membrane separation and gel permeation chromatography for the structural characterization,which contributed to the future study of the relationship between the structure and prebiotics activity of xylo-oligosaccharides.Moreover,the residues obtained in xylo-oligosaccharides production can be converted into bio-oil by liquefaction/pyrolysis.Further upgrading of bio-oil by catalytic deoxygenation reaction will generate fuels.However,the reaction deoxygenation mechanism is unclear.Thus,the reaction mechanism of biomass catalytic deoxygenation was conducted by a combined theoretical and experimental study,which determined the biomass catalytic deoxygenation mechanism over reducible metal oxides.Bamboo powder(10.0 g)was subjected to autohydrolysis with a solid to liquid ratio of 1:10 under non-isothermal conditions to produce xylo-oligosaccharides with degrees of polymerization of 2 to 6.The experiment was performed with two independent variables(reaction temperature 152-208? and reaction time 1.72-58.28 min)to optimize the reaction condition by response surface methodology based on central composite design.The analysis of variance of the regression model of xylo-oligosaccharides yield was in good agreement with the experimental results,and the predicted optimal condition for the production of xylo-oligosaccharides was observed at 182? for 31 min with the yield of 36.4%.Under the optimal reaction condition for the production of XOS,relatively low concentrations of monosaccharides and byproducts were observed.The investigation of antioxidant activity revealed that XOS by autohydrolysis exhibited a comparable scavenging activity with commercial antioxidant in superoxide and hydroxyl radicals.Gel permeation chromatography was used to efficiently separate and purify the xylo-oligosaccharides produced by bamboo autohydrolysis for the chemical structural evaluation.Results showed that the separation obtained high purity(85.2?94.5%)xylo-oligosaccharides with individual component with a good recovery(71?86%).The separated samples were further characterized by ESI-MS and NMR,which revealed that the samples were non-substituted xylobiose and acetylated xylo-oligosaccharides with degree of polymerization from 3-6.The combined 1H,13C and two-dimensional proton-detected heteronuclear single quantum(2D HSQC)NMR spectra results suggested that acetyl groups only attached to the internal units of xylo-oligosaccharides after autohydrolysis,and no acetylated non-reducing ends were detected.Bamboo autohydrolysis liquor was separated by ultrafiltration and nanofiltration with multiple molecular weight cut off of 10 kDa,3 kDa,1 kDa,and 250 Da,and various fractions with different molecular weights were obtained.The membrane with a cut off of 1000 Da retained the xylo-oligosaccharides fraction with a degree of polymerization(DP)greater than 6.While further separation was conduct by a nanofiltration membrane with 250 Da cut off,obtaining xylo-oligosaccharides fraction with a degree of polymerization between 2 and 6 in retentate and the ratio increased from 63.2%to 90.3%after separation.The results indicated that membrane separation is highly efficient for separating autohydrolysis xylo-oligosaccharides.The structural characterization by ESI-MS showed that the main component of the permeate separated by the nanofiltration with 250 Da cut off was xylose,while the ingredient of the retentate was the acetylated xylo-oligosaccharides with DP 2-6.Reducible metal oxides have shown the ability to catalytically remove oxygen even at low exogenous H2 pressures.As opposed to a traditional surface reaction,it is possible that oxygen vacancies provide the active sites in such catalysts,in a Mars-van Krevelen(MvK)-like mechanism.In this work,a combination of theoretical calculations and experimental measurements was used to provide conclusive evidence that this reaction proceeds through such a vacancy-driven mechanism,for the example system of acetic acid deoxygenation to acetaldehyde over reducible zinc oxide surfaces.Density functional theory(DFT)calculations suggest that the catalyst without an oxygen vacancy on the surface was relatively unreactive,due to a strong energetic penalty of breaking the C-0 bond,while the presence of the vacancy provides the reducing power to facilitate this elementary step.Experimentally,to examine the role of vacancies directly,the comparison of rates and selectivities of acetic acid deoxygenation for standard hydrodeoxygenation conditions with rates in H2-free conditions starting with reduced Zn metal was carried out,in which presumably the only active sites are vacancies created through diffusion.A striking similarity in the product distribution had been found,suggesting a common mechanism in both cases.Specifically,metallic zinc with numerous vacant sites on the surface showed a high activity in promoting the deoxygenation reaction,while oxidized zinc(without oxygen vacancies)was fully deactivated.This study suggests that a unique vacancy-driven mechanism is responsible for the reactivity of reducible metal oxide catalysts.