Study On The Structure Change Of Bamboo Hemicellulose During Solid Alkali Pretreatment And The Conversion Of Its Downstream Product Furfural

The lignocellulosic biomass produces diverse basic chemicals through biorefinery,and therefore is considered as one of the renewable resources instead of traditional fossil resources.The lignocellulosic biomass is the most abundant biomass resource on the earth,and is mainly consists of cellulose,hemicellulose and lignin.Hemicellulose is second only to cellulose in content and is the second most abundant renewable natural biomass high polymer.Hemicellulose can be used to prepare high value-added xylooligosaccharides(XOS)by bio-enzymolysis and furfural(FF)by chemical catalysis,which is an important biomass-based platform molecule for the production of various liquid fuels,fuel additives,spices,pharmaceutical intermediates and other downstream chemicals.Based on the industrialization direction of the preparation of XOS,FF and its derivatives furfuryl alcohol(FFA)by the current development of hemicellulose utilization and the pretreatment technology of CAOSA developed by our group,the structural changes of bamboo powder hemicellulose during CAOSA process were analyzed,and the effects of the structure differences of hemicellulose between bamboo and corn stalk on the preparation of XOS were compared.Meanwhile,in order to solve the serious Cr pollution from Cu-Cr catalyst during the preparation of FFA by FF hydrogenation,a series of catalytic systems for the preparation of FFA were carried out.The main contents of this thesis are as follows:Firstly,the evolution of hemicellulose structural changes in bamboo powder during CAOSA process pretreatment was clarified.Most lignin were removed after CAOSA pretreatment,and the compact fiber structure of bamboo was destroyed,resulting in fluffy morphologies,and the cellulose crystal was partly damaged.The characterization analyses indicated that the bamboo hemicellulose is consisted of a(1?4)-?-D-xylose main chain and a side chain with glucuronic acid and/or arabinose linked by ?-(1?2)and/or ?-(1?3)glycosidic bonds.After the CAOSA pretreatment,the decrease of branching degree of bamboo hemicellulose,the destroying of more side chain and the hydrolysis of most acetyl groups were shown.The thermal stabilities of hemicellulose were much lower than that of the raw materials.Secondly,the feasibility of preparation of XOS from bamboo by CAOSA pretreatment was established by analyzing the influence of the structural differences of hemicellulose between corn stalk and bamboo on the preparation of XOS.The NMR analysis showed that the fiber structure of bamboo is more compact because the bamboo hemicellulose linked to other cellulose or lignin by the esterification of p-coumaric acid at y position.After the CAOSA treatment,the 19.60%hemicellulose in bamboo pulp was significantly higher than 8.24%in the corn stalk pulp.The analyses of monosaccharide composition and SEM showed that most side chains of hemicellulose were removed after CAOSA treatment,resulting in the thinner fiber and rougher surface.The results of the direct enzymolysis of two kinds of pretreatment materials showed that the bamboo pulp by CAOSA pretreatment had more advantages in the XOS yield and purity.Then,a catalytic system was developed by physical mixture CuO and Pd/C as catalysts for the catalytic transfer hydrogenation of FF and formic acid to FFA.The catalyst formation process was as follows:CuO was first dissolved by formic acid to form Cu2+ during the reaction,and then Cu2+ was reduced in situ to nano-Cu particles,and finally a stable Cu-Pd alloys were formed.The underlying reaction mechanism revealed that the Pd mainly promoted the FF conversion by enhancing hydrogen adsorption of the catalyst at a low temperature,and the nano Cu increased the FFA selectivity by the hydrogenation of carbonyl.In this process,the Cu-Pd alloys promoted the catalytic effect by a synergistic way.The 98.1%of FFA selectivity and complete FF conversion were achieved under 0.3 g FF and 0.58 g formic acid catalyzed by a direct mixture of 0.01 g CuO and 0.03 g Pd/C at 170? for 3 h.The in-situ formed Cu-Pd alloys present a high catalytic activity and an excellent stability based on a high yield of FFA after its reusing for 5 times.Furthermore,a method for preparing bimetallic catalysts by in-situ reduction of organic copper salt with the incorporation of alkali metal salts in formic acid system was developed.A reaction system for preparation FFA by formic acid transfer hydrogenation or exogenous hydrogen hydrogenation of FF was established.A highly active bimetallic Cu-BaCO3 catalyst was prepared by direct reduction of Cu(HCOO)2 and BaCO3 through one-pot method under the formic acid system.In the process of catalyst formation,some BaCO3 was first reacted with formic acid to form Ba(HCOO)2.With the formate was gradually decomposed by Cu and the production of CO2,Ba2+reacted with the CO2 to regenerate BaCO3.The homogeneous reduction of Cu2+to Cu and the presence of Ba(HCOO)2 intermediate in this process contributed to a close interaction and uniform dispersion of Cu and Ba on the catalyst surface.The incorporation of alkaline BaCO3 resulted in the decrease of Cu nanoparticles size and the surface acidity of Cu-based catalyst,and in the increase of the low-temperature hydrogen adsorption of the catalyst.Therefore,the FF hydrogenation activity and FFA selectivity of Cu-BaCO3 were improved at low temperatures.The in-situ Cu-BaCO3 exhibited an excellent catalytic activity in the catalytic FF transfer hydrogenation to FFA with a conversion of 98.5%and a selectivity over 99%at 170? for 2.5 h.The catalyst was reused without any reactivation processes and provided excellent recyclability.The catalytic hydrogenation performance of the in-situ prepared Cu-BaCO3 was significantly better than that of the Cu-BaCO3 prepared by coprecipitation under the relatively low external H2 pressure.Finally,the series of CuCs(x)-MCM catalysts with different metal ratios were prepared by using the molecular sieve MCM-41 as carrier and the catalytic system for the hydrogenation of FF to FFA under formic acid system was established.The analyses showed that the stable and ordered mesh structure of the carrier MCM-41 could stabilize the structure of the catalyst and improve its performance.The doping of Cs increased the dispersibility and specific surface area of Cu in the catalyst,thereby improving the availability of effective metals.The presence of Cs caused the electrons transfer on the surface of the catalyst,which reduced the acidity of the Cu-based catalyst to a certain extent and improved the catalytic activity.The complete conversion of FF and 99.6%of FFA yield were obtained by using the addition of CuCs(2)-MCM as the catalyst under 170? for 1 h.Compared with the Cu-Pd/C and Cu-BaCO3 catalysts,the shorter catalytic reaction time,the decreased effective addition of Cu,and a significant improvement of recyclability were presented under the CuCs(2)-MCM.The correlation analysis showed that the acidity and basicity of catalyst played a key role in FF hydrogenation to FFA,and others including dispersibility,specific surface area and active metal particle sizes played a secondary role.