Dissolution Characteristics Of Hemicellulose In Ionic Liquids And Preparation Of Terminal Alkyne Hemicellulose

Hemicellulose is a renewable energy resource and its storage ranks second in biomass energy.Hemicellulose is a heterogeneous polymer which composed of multiple different types of monosaccharides and with kinds of reactive groups and chemical reaction behaviors,modification of hemicellulose has become a hot research issue.At present,most of the research on hemicellulose was used NaOH solution as solvent.Although hemicellulose exhibited good solubility in NaOH solution,the reaction in the dissolution process was violent and the reactive groups of hemicellulose were broken obviously,which limited its high-valued application.The terminal alkyne hemicellulose refers to a terminal alkyne hemicellulose having a terminal acetylene bond obtained by substituting an acetylenic bond for a hemicellulose terminal hydroxyl group by a substitution reaction.The compound can be used as a acetylenic bond monomer in a "click reaction",and can be synthesized by a polymerization reaction with other chemicals to synthesize a novel material having various special properties such as light,electricity,and magnetism.Aiming at the limitations of hemicellulose modified solvent system,this paper regarded the major component of hemicellulose�xylan as research object,used ionic liquids as solvent,detected the dissolving property of hemicellulose and the preparation of terminal alkyne hemicellulose,evaluated the performance of the modified product,and illustrated the influence of solvent system on physicochemical properties of hemicellulose modified products.In order to find an appropriate solvent system for hemicellulose,a series of ionic liquids with different cationic side chain hydrocarbon chain lengths have been synthesized.The solubility of hemicellulose in different cationic side chain hydrocarbon length ionic liquids and the influence of the molecular structure of regenerative hemicellulose on the ionic liquid solvent system were examined.The results indicated that the length of the side chain substituent had a significant effect on the solubility of hemicellulose.The solubility of hemicellulose increased with a rise of temperature and a decrease of hydrocarbon chain length.The maximum solubility of hemicellulose reached 5.27 g/100 g at 80 �C and 1-Ethyl-3-methylimidazolium acetate([Emim] AC)solvent,providing basic data and references for searching appropriate solvent system for hemicellulose.The heat stability,molecular weight and dispersibility of the hemicellulose regenerated by ionic liquid were declined slightly,but the main structure of hemicellulose had no significant change.Compared with the hemicellulose regenerated by the NaOH solution,the main active groups such as hydroxyl group and carbonyl group were retained,which provided a powerful technical guarantee for the subsequent modification of the hemicellulose.According to the characteristics and the rule of ionic liquids dissolving hemicellulose,the ionic liquid [Amim]Cl was selected as the reaction solvent to synthesized terminal alkyne hemicellulose,and the solubility of hemicellulose was 11.3 g/100 g.The effects of microwave reaction temperature,reactant ratio,reaction time on the yield and degree of substitution of the terminal alkyne hemicellulose were examined respectively,and the optimal conditions for synthesizing terminal alkyne hemicellulose were: the ratio of xylan sugar unit and bromopropyne molar,was1:2,the reaction temperature was 60 �C,and the microwave reaction time was 1 h.At this condition,the yield of terminal alkyne hemicellulose was 87.2%,and the degree of terminal alkyne substitution DSc was 0.8838,which was obviously superior to the conventional alkali solvent system.The experiments were carried on the terminal alkyne hemicellulose synthesized in different solvent systems to characterize their physicochemical properties.It is considered that uses of ionic liquid as solvent was superior to uses of the alkali as solvent synthesized terminal alkyne hemicellulose in the retention of the reactive groups and thermal stability.And the ionic liquid could be recycled for 5-7times,showing good cycle recycling behavior.Finally,a modified product of terminal alkyne hemicellulose in ionic liquid solvent was used to synthesize the palladium catalyst carrier-hemicellulose-g-chitosan for Suzuki cross-coupling reaction.It was found that the hemicellulose-g-chitosan manufactured by the terminal alkyne hemicellulose which used ionic liquid as solvent had a higher loading ratio(1.02%)and thermal stability of hemicellulose-g-chitosan than that of other methods(0.54%),and higher terminal alkyne substitution was beneficial to the loading of palladium.The performance of the catalyst was evaluated by the yield of the Suzuki cross-coupling reaction product of the halogenated aromatic hydrocarbon and phenylboronic acid.The product yield of the palladium catalyst synthesized from the terminal alkyne hemicellulose which modified in the ionic liquid in the Suzuki cross-coupling reaction was about 90%,with a good catalytic activity for Suzuki reaction,and the catalytic activity was superior to that of the existing.In addition,the hemicellulose-g-chitosan supported palladium catalyst after 5-6 times of recycling remained a good catalytic efficiency and had a promising application prospects.