Study On Microwave-assisted Ionic Liquid Treatment On Bamboo

In order to alleviate the dual pressure from energy and environment, countriesaround the world have payed much attention to renewable resources, especially thedevelopment and utilization of biomass resources. As the ideal substitute forpetroleum, coal, and other fossil resources, biomass resources have attractedextensive attention. Lignocellulosic biomass is a natural resource and has the largestannual production of up to10～50×109tons, which accounts for about50%of totalglobal production of biomass. Due to the enormous yield, renewability and otheradvantages of lignocellulosic biomass, its application as raw materials for producingfunctional materials, fuel, and platform chemicals has become an important researchdirection in the21st century. Bamboo shows the characteristics of high productivity,rapid growth, and widely planting around the world. The planting area of bamboo inJiangxi province has reached626,000hm2. Thus, it is important to develop thebiomass refining technology of bamboo biomass for the biomass conversion ofbamboo resourse. Ionic liquids (ILs) are salts with melting point lower than100C and have become a promising substitution for tranditional solvent due to theirnontoxicity, low volatility, and high thermal stability.In the present study, ball milled and dewaxed bamboo powder was dissolved in1-ethyl-3-methylimidazolium acetate (EmimOAc) assisted by microwave irradiation.The physical and chemical characteristics of the lignin and cellulose riched materials(CRM) obtained from bamboo were analyzed. The structual changes of ligninextracted by alkali and the structural changes of lignin and CRM extracted bytranditional heating method were also compared. The process of bamboo powderdissolution in ionic liquid assisted by microwave was evaluated on the basis of relatedexperiments.24%of the bamboo powders was dissolved in IL after oil heating at110℃for16hrs, while the dissolution after heated by microwave for60min increased to100%.Due to the high inside-out heating efficiency of microwave, dissolution of biomasswas significantly enhanced with much shorter heating time compared with thetraditional heating method. In the regeneration process, water as anti-solvent wasadded into the biomass-IL solution, the ions of the ionic liquid were extracted into theaqueous phase through hydrogen bonding, dipolar and Coulombic forces. Hydrogenbond among cellulose was reconstructed, leading to the precipitation of CRM.Therefore it was clear that the yield of CRM depended on the dissolution of biomass.It was quite obvious that the lignin content in ILR and CRM decreased aftermicrowave treatment. In specific, after receiving microwave for80min, the lignin content of CRM decreased to11.1%, which was about half of the lignin content in theoriginal biomass.The structural changes after ionic liquid pretreatment were analyzed using XRD.The original biomass showed two main peaks at22.0°and18.0°. And these twopeaks became weaker after ILR, which indicates a transformation from cellulose I tocellulose II. In addition, the peak at18.0°disappeared in CRM, which means thatafter regeneration, cellulose II of low crysatllinity was recovered. According to SEManalysis, the compact framework of bamboo became highly disordered and loosebecause of the swelling of cellulose and delignification by EmimOAc. CRMs showedthe same morphology with a conglomerate texture in which wood fibers were fusedinto a relatively more homogeneous macrostructure.Structural features of lignin extracted by ionic liquid and NaOH weredetermined using FT-IR and NMR techniques. The results indicated that the bamboolignin was GSH type and the most abundant linkage between lignin uints is β-O-4′substructure. It was also found that ionic liquid treatment can cause the cleavage ofβ-β′, β-1′, and β-5′substructures.The morphology of the lignins was observed by scanning electron microscopy(SEM). The results showed that alkali lignin had a globular appearance, while ionicliquid lignin had an irregular appearance and bigger shape.With the increase ofmicrowave radiation time, the particles of lignin became larger. The thermalgravimetric analysis (TGA) was used to analyze the pyrolysis characteristics ofbamboo lignin extracted by alkali and ionic liquid. The results indicated that thelignin pyrolysis could be divided into three stages and the main pyrolysis occurred inthe range from120to400℃. TG-DTG curves showed that the maximum weight lossoccurred between the temperature of296and352℃. The alkali lignin had a higherthermal stability.