| As a class of new green solvent, ionic liquids have been successfully used in many fields because of their unique properties such as very low vapor pressure, high thermostability, non-flammability, strong dissolving ability, easy recovery, and structure and property tunability. In recent years, many important progresses have been made in the dissolution and separation of bimass with ionic liquids as solvent. However, most of the studies are focused on the dissolution of cellulose, semicellulose dissolution is rarely reported, and the dissolution mechanism of cellulose in ionic liquids is not well understood. In this thesis, a series of ionic liquids with different anionic and cationic structures were designed and synthesized, and the influence of cationic structures on cellulose dissollution was studied. The influences of structures of ionic liquids and addition of water on xylan(model compound of semicellulose) dissolution were also investigated. The possible mechanism of cellulose and xylan dissolution in ILs was studied. The difference in the dissolution mechanism of cellulose and xylan has been used to explain the promotion action of water for the dissolution of xylan in the ionic liquids. The main contents are as follows.1. 30 kinds of ILs with different structures were designed and synthesized, such as imidazolium carboxylate, morpholium carboxylate, piperidium carboxylate, pyrrolidinium carboxylate, benzimidazolium carboxylate and benzotriazolium carboxylate. These ILs were characterized by means of 1H NMR spectra. Thermal physical properties(such as glass transition temperature Tg, melting point Tm and thermal decomposition temperature Td) and K-T parameters(α, β and π*) of the ILs were also determined. The results indicate that Tg, Tm and Td values of the ILs were decreased with the increase of the delocalization of positive charges on the ionic liquid cations. The presence of polar groups on the alkyl chains of cations would increase the values of Tg, Tm and Td of the ILs. The α parameters of these ILs depended on the structures of the cations and were affected by temperature. The β parameters of the ILs were determined by structures of the anions and not sensitive to temperature. Ionic liquids with saturated heterocyclic cations were found to have low α parameter and high π* value duo to their localized positive charges.2. The influence of cationic structures of the ILs on cellulose solubility was investigated systematically. The possible mechanism for cellulose dissollution was probed by means of K-T parameters and 13 C NMR chemical shift. It was shown that both cations and anions of the ILs have important function for cellulose dissolution. Formation of hydrogen bonds between oxygen atoms of ionic liquids anions and OH hydrogen atoms of cellulose, as well as that between acidic hydrogen atoms of ionic liquids cations and ether oxygen atoms of cellulose, destroyed the primary hydrogen bonding network in cellulose, thus resulting in the efficient dissolution of cellulose. The interactions of the OH protons of cellulose with anion of the IL would be much stronger than those of the cellulose oxygen with protons of cation of the IL. Thus, although the role of anion of ILs is predominant in the cellulose dissolution process, but the contribution from cations of the ILs is indispensible.3. The influence of ILs structures on xylan solubility was investigated. The possible mechanism for xylan dissolluyion was studied by using FT-IR and 13 C NMR spectroscopy, as well as the K-T parameters. It was found that both cations and anions of the ILs have important contribution to xylan dissolution. Hydrogen bond interactions of the acidic hydrogen atoms of ionic liquids cations with ether oxygen atoms and bridging oxygen atoms of xylan, and those o f oxygen atoms of ionic liquids anions with hydroxyl protons of xylan, destroyed the primary hydrogen bonding network of xylan. Unlike cellulose dissolution, the interactions of the xylan oxygen with protons of cation of the IL would be much stronger than those of the OH protons of xylan with anion of the IL in the xylan dissolution process.4. The influence of water addition on xylan solubility in ionic liquids was investigated. The possible mechanisms of xylan dissolving in the ILs and the action of water in xylan dissolution were studied by FT-IR spectroscopy and 1H NMR chemical shift. The results indicated that the presence of water in the ILs increased xylan solubility, which reached the maximum at water mole fraction of 0.4. The addition of water enhanced xylan dissolution in two ways: to form hydrogen bonds with xylan, and to promote dissociation of cation- anion pairs of the ILs by ionic hydration. These findings are important for the development of new pretreatment technology of semicellulose. |
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