| Up to now,biomass is considered to be the only renewable resources in nature to complement or replace the traditional fossil fuels.Enlightened by the thought of petroleum refining,biorefinery concept has been proposed.That is people can get fuels,power,heat,materials and chemicals from biomass feedstocks without disobeying the principles of green,low-carbon,clean and sustainability.Lignocellulosic materials contributing the large proportion to the biomass resource are mainly composed of carbohydrate polymers(cellulose,hemicellulose),and aromatic macromolecules(lignin).Pre-fractionating lignocellulose is considered as the foundational step to establish an economical and sustainable lignocellulosic biorefinery.In 2012,our group reported a novel cooking process to fractionate lignocellulosic biomass which was abbreviated to CAOSA.According to a facility with CAOSA technique and prior-fractionation strategy,a series of studies on fractionating bamboo for the high-value valorization of hemicellulose,cellulose and lignin in the concept of lignocellulosic biorefinery were carried out in this thesis.Some outcomes have been made,as follows.Firstly,chemical structure changes of MgO during the CAOSA process have been explored as well as the solid alkali recycling method.After cooking,the products were divide into four types:gas,liquid,pulp and particles.Each of there products were analyzed at different cooking times.It was found that MgO was transformed into a new intermediate,magnesium oxide carbonate(Mg3O(CO3)2)at first,which was then dissolved into the cooking liquor via neutralizing with acidic products.It was also found that only under such in-situ generated but insufficient CO2 conditions could Mg3O(CO3)2 be formed.Mg3O(CO3)2 was stable and unable to react with acidic CO2 so that it could provide an indispensably weak but adequate alkaline environment to support the oxygen delignification proceeding.But there features were not available in commercial hydromagnesite(Mg5(CO3)4(OH)2).34.9 wt.%of MgO in the form of Mg-rich solid particles could be recovered easily from the fiber fraction by water-washing and from the liquor fraction by heating to reflux.The residual Mg could be precipitated by metathesis to result in a total Mg-rich solid particles yield of 94.6 wt.%(on Mg basis)by using 100.8 wt.%(54.5 mol%)hydrated lime.Mg-rich solid particles were dried and calcined at 900 0C for 1 h to give MgO with purity over 90%,which could be readily applied to next CAOSA process.Secondly,one-pot conversion of CAOSA pulp,namely bamboo cellulose into 5-[(formyloxy)methyl]furfural(FMF)was fulfilled in formic acid reaction system.Effects of catalyst type,water content,dosage,acids,reaction time and temperature on FMF yield were studied.By the addition of preferred NaBr catalyst,the yield of FMF was increased from 8.0 mol%to 28.1 mol%.It was confirmed that a neutral halide,especially bromide,was the preferred catalyst to catalyze cellulose degradation for the production of FMF in the formic acid reaction system.Bromide anions also played a role in promoting the esterification of 5-Hydroxymethylfurfural(HMF)into FMF.Up to 2 wt.%of water in this reaction system did not impact on the FMF yield,but too much water turned against cellulose conversion,inevitably reducing FMF productivity.High dosage of substrates were not favored by this reaction system,but a slightly higher yield of FMF was obtained when CAOSA pulp was feed at the same high concentration relative to glucose.The production of levulinic acid was significantly enhanced by both adding strong Bronsted and Lewis acids to the reaction system.Increasing the temperature for every 20 ? from 130 ? to 170 ? approximately halved the reaction time but did not change the maximum yield of FMF.A maximum FMF yield of 33.4 mol%was obtained,and a followed four steps processing gave a purified FMF yield of 29.4 mol%.FMF was more hydrophobic and thermostable than HMF,which made it better to be a platform molecule.Subsequently,efforts have attempted to depolymerize bamboo lignin for the production of monomeric phenols,and a synergetic effect of acidic zeolites and Raney Ni catalyst was found.Using either Raney Ni or acidic zeolites alone could catalytically liquefy lignin to bio-oil well,but ESI-MS and MALDI-TOF resluts indicated the existence of lignin oligomers rather than monophenols in bio-oil.The yields of monophenols were 12.9 wt.%and no more than 5.0 wt.%when catalyzed by Raney Ni and acidic zeolites,respectively.However,yields of monophenols as high as 21.0?27.9 wt.%were obtained when combining Raney Ni with acidic zeolite catalysts.A synergetic effect of acidic zeolites and Raney Ni catalyst was found,and the best Zeolite/Ni ratio was 2:1 by weight.Inadequate Raney Ni might depress lignin depolymerization,while inadequate acidic zeolites might result in not enough acid sites to block active lignin fragments.The yields of monophenols increased firstly and decreased later as the increase of reaction time.A higher reaction temperature benefited redox reactions,whereas excessively high temperature led to a significantly decrease of monomeric phenols.The in-situ generated hydrogen species in the case of lignin depolymerization showed better activity than extra hydrogen without regard to differences of solvent effects.Severe deactivation of the catalysts was observed possibly due to the fouling of the active sites by lignin-derived oligomers and carbon residuesAnd then,prior-fractionating bamboo hemicellulose to prepare value-added xylo-oligosaccharides(XOS)was evaluated.Effects of acid and base on hydrothermal autocatalysis were compared on the factor of Xylan extraction efficiency.Acid reinforced autocatalysis was preferred because acetyl group in bamboo hemicellulose was frequently difficult to hydrolyze.The highest Xylan yield of 12.2 wt.%could be obtained in 0.1 wt.%acetic acid aqueous solution at 180 ? for 100 min.Enzymolysis conditions and methods for decolorization,deproteinization,desalination and removal of monosaccharides were investigated to afford high purity XOS.Xylan was treated with 0.0125%xylanase at 50 ? for 4 h to give the optimum XOS distribution.Taking sugar loss into consideration,activated carbon of 0.5 v/v%was used for decolorization,and ultrafiltration membrane with MWCO 10 kD was selected for deproteinization.The most suitable dosage of positive ion exchange resin and negative ion exchange resin were 0.096 v/v%and 0.090 v/v%,respectively.In this step,the combined desalination rate was 80.0%with a sugar loss of 7.1%and a discoloration rate of 85.7%.10 L sugar solution was concentrated to 1.1 L via nanofiltration with a XOS loss of 9.4%,reducing the portion of monosaccharides from 14.6%to 6.7%.Eventually,integration of CAOSA technique and prior-fractionating hemicellulose for component separation of bamboo was achieved.This mehod contributed to improving bamboo hemicellulose value while decreasing solid alkali dosage from 15 wt.%to 12 wt.%.On the basis of the above-mentioned studies,a technology roadmap of fractionating bamboo for the high-value valorization of hemicellulose,cellulose and lignin was drafted to provide theoretical reference and technical support for lignocellulosic biorefinery in the future.Specifically,hemicellulose,mostly in xylo-oligosaccharide form,was extracted using an organic acid buffer solution recovered from the follow-up cooking liquor.The residual substrate was subjected to CAOSA for delignification to obtain cellulose-rich pulp and lignin-rich liquor.Solid alkali catalyst and the acid buffer solution used in the first pre-extraction step could be recovered from the liquor.In addition to contents mentioned in this thesis,the isolated hemicellulose,cellulose,and lignin in their processable form could be potentially converted to multiple products. |
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