| With the increasingly serious white pollution,the depletion of fossil energy and the increasing awareness of environmental protection,scientists focus not only on the development of biobased polymer materials from renewable resources,but on the recycling of waste materials from the end use of polymer.This recycling development model on“renewable resources-polymer-monomer”can effectively reduce the exploitation and use of fossil energy,and promote the sustainable development of“chemistry-society-economy”.Therefore,the development of biobased polymer materials which can replace petroleum based polymer materials and effective recycling strategies for their final use have attracted great attention of academia and industry,becoming an important research direction of chemical industry and polymer industry in the new century.2,5-furandiformic acid(FDCA),as a derivative of 5-hydroxymethylfurfural(HMF),can be obtained from fructose which is a renewable resource,was selected as one of the12 most potential biobased aromatic platform compounds by the U.S.department of energy.Due to its similar chemical structure and higher reactivity with terephthalic acid(PTA),FDCA is expected to become an ideal substitute for petroleum-based aromatic platform compounds PTA.At present,poly(ethylene glycol 2,5-furanadecarboxylate)(PEF),which is prepared by direct esterification melt polycondensation from FDCA and ethylene glycol(EG),has attracted much attention due to its similar thermal and mechanical properties and better barrier properties with petroleum based polyethylene terephthalate(PET).Because of the angle between the two carbonyl groups in the furan ring(129.4°),PEF has a special spiral nonlinear structure.This spiral structure endows PEF excellent properties,such as high strength,chemical stability and outstanding barrier properties,which make PEF have many advantages in food packaging film,beverage bottle and other packaging fields.Avantium has reported its application in the packaging of soft drinks and alcoholic drinks,as well as in the packaging film as a textile fiber.Carlsberg group promotes the use of PEF and wood fiber as prototypes of“green fiber bottles”suitable for beer.As a result,PEF has the ability to compete with its petroleum based counterparts.However,the effective catalysts in the synthesis of PEF are metal catalysts,such as Ti-,Sn-,Sb-and Ge-based catalysts.Although metal catalysts show excellent catalytic activity in PEF synthesis,the residual metal catalysts can not be completely removed,which will have a negative impact on the performance of PEF,for example,the poor color,thermal stability and so on.And the accumulation of residual toxic metals in the final product will also bring harm to the ecological environment and human health.In addition,the future accumulation of PEF after commercialization,due to its limited or non biodegradability,will bring huge pollution from plastic wastes.Therefore,in order to solve the above problems of PEF,from the perspective of"green chemistry"and"sustainable society",combined with the structural characteristics of FDCA furan ring,we explores the use of metal free,green,efficient and stable ionic liquids(ILs)catalytic system instead of metal catalyst,using FDCA and EG as raw materials to synthesize high molecular weight PEF by direct esterification method.Furthermore,the eco-friendly metal free ionic liquids are used as catalyst to explore the chemical recovery of PEF,so as to provide theoretical guidance and technical solutions for the end use of furan based polyester materials,and provide a closed-loop strategy from"renewable resources-polymer-monomer"for the recycling of PEF materials.In this work,the green synthesis and chemical recovery of PEF are studied in the following aspects:1.In the first part,a series of tetrabutylphosphonium carboxylate ILs were successfully prepared by one-step neutralization method.The structure and stability of ILs were characterized by NMR and TGA.The catalytic activity of the ILs for the synthesis of PEF in the direct esterification method of FDCA and EG was investigated systematically.Taking([P4444][LAc])as an example,the stability of the IL in the reaction system was analyzed in detail and it was speculated that the real catalytic activity in the reaction system was tetrabutylphosphonium furan dicarboxylate([P4444]2[FDCAc]).Using[P4444]2[FDCAc]as catalyst,the number average molecular weight(Mn)of PEF was 4.31×104 g.mol-1 by optimizing the reaction conditions of FDCA and EG.According to the experimental results,ESI-MS and 1H NMR,a possible catalytic mechanism of[P4444]2[FDCAc]promoting the growth of PEF chain was proposed.2.In the second part,the synthesis of PEF with FDCA and EG was investigated by using environment-friendly imidazole inorganic acid salt ILs as catalyst.The catalytic activities of a series of imidazole cations combined with different anions in bulk polymerization of FDCA and EG were systematically studied.1-ethyl-3-methylimidazolium tetrafluoroborate([C2MIM]BF4)was identified as the optimum catalyst by 1H NMR,HPLC and GPC.[C2MIM]BF4 showed excellent catalytic activity,selectivity and stability even at low catalyst loading(0.1 mol%FDCA).Optimization of polymerization parameters(esterification temperature,catalyst dosage,molar ratio of alcohol to acid,polycondensation time and temperature),enabled the PEF wih a high Mn(5.25×104 g.mol-1).In addition,the Br(?)nsted acidity of[CnMIM]BF4(n=2,4,6,8)was determined by Hammett method,and the relationship between Br(?)nsted acidity and catalytic activity of[CnMIM]BF4 was studied.Based on the experimental results and density functional theory(DFT),a reasonable hydrogen bond induced electrophilic activation mechanism was proposed.3.In the third part,combined with the catalytic characteristics of tetrabutylphosphonate carboxylate ionic liquids and imidazole inorganic acid salt ILs,the two kinds of ILs explored in above work were used as catalysts for methanolysis of PEF to explore the depolymerization process and characteristics of PEF.A series of tetrabutylphosphonate carboxylate ILs with different steric hindrance,electronic effect and electronegativity were investigated.The results showed that the lower steric hindrance,no intramolecular hydrogen bonding anion with strong electronegativity was more effective for methanolysis of PEF.1-butyl-3-methylimidazolium acetate([BMIm][OAc])showed excellent catalytic activity in the methanolysis of PEF by adjusting the anionic and anionic structures of ILs.By optimizing the reaction parameters,PEF can be completely depolymerized with 0.15 mol%[BMIm][OAc]at130? for 30 min.The degradation rate of PEF was close to 100%,and the yield of dimethyl furan-2,5-dicarboxylate(DMFD)was 77.6%.The structure and purity of DMFD were confirmed by NMR,IR,XRD,ESI-MS and HPLC.The induction of hydrogen bond between[BMIm][OAc]and reactants was studied by NMR,IR and DFT.In addition,according to the experimental results and scanning electron microscopy(SEM),the characteristics of alcoholysis reaction between PEF and PET were compared,and it was confirmed that PEF with spiral structure could depolymerize under mild conditions.In summary,based on the interdiscipline fields of polymer chemistry,polymer physics,organic chemistry and so on,this work provides a new scientific research idea and theoretical basis for the construction of efficient and environmentally friendly metal free ionic liquid catalyst for the synthesis of environmental friendly bottle grade PEF and the recycling of PEF. |
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