Research On Metal Free Ionic Liquids And Deep Eutectic Solvents For PET Glycolysis

Poly（ethylene terephthalate）（PET）is thermoplastic polyester with excellent properties,such as high structural strength,chemical stability and safety.Consequently,PET can be widely used in synthetic fibers,bottles,food packaging and so on.As the economy develops,the demand for PET is growing day by day.Meanwhile,more and more waste PET are accumulated,which putting huge pressure on the environment.However,producing PET requires the consumption of non-renewable oil resources.In summary,taking both environmental protection and cost saving into account,PET recycling is of great significance.Chemical recycling of waste polyester is consistent with the concept of sustainable development.Glycolysis is a method that converts waste PET into monomer bis（hydroxyethyl）terephthalate（BHET）through EG.The operation condition is mild,and it is suitable for continuous production.Glycolysis routes have been developed into commercialized processes by many famous companies.Catalysts can significantly improve the rate of glycolysis.Hence,designing high activity catalysts will further promote recycling of PET.In this thesis,a series of novel Ionic liquids（ILs）and deep eutectic solvents（DES）catalyst were put forward.UV irradiation was introduced into reaction system to enhance the glycolysis efficiency.The main innovative and results are as follows:（1）A series of ChCl based DES were synthesized,including Acetic Acid/ChCl/Levulinic acid（Ac/ChCl/La）,ChCl/Levulinic acid（ChCl/La）,ChCl/Acetic Acid（ChCl/Ac）and ChCl/Formic Acid（ChCl/Fa）.Among them,ChCl/Fa exhibited the highest catalytic activity.For ChCl/Fa,ChCl:Fa（molar ratio）is 1:2.The optimized experimental condition is as follows;195℃,5 g PET,20 g EG,0.25 g ChCl/Fa,3 h,the conversion of PET is 100%,the yield of BHET is 80.8%.ChCl/Fa could be reused at least six times without any noticeable decline in catalytic activity.The process conforms to first order model.The activation energy（E_a）is 188.12 k J/mol.The reaction process was tracked by in suit IR.A possible catalytic mechanism was proposed as follows;the Fa in ChCl/Fa can form hydrogen bonds（H-bonds）with EG,the H-bonds elongate the length of the O–H bond of hydroxyl in EG.The ChCl in ChCl/Fa can protonate the carbon of carbonyl group in PET,which makes it more electrophilic.The degradation product BHET was used to produce recycled PET（r-PET）.The r-PET was analyzed by TGA,GPC and chroma.The experimental results indicated that the thermal stability and molecular weight of r-PET were similar with raw PET.The chroma of r-PET reached food-grade packaging standards.（2）The reaction time of ChCl/Fa is relatively long,which is the common disadvantage for metal free catalysts.Hence,UV irradiation was introduced the glycolysis process to improve the degradation efficiency.Some reports indicated that basic catalysts are more active than others.A series DBU based ionic liquids were prepared with Imidazole and its derivate.DBU is organic strong basic.When Imidazole loses H⁺,it will have week basic.So,the ILs we prepared are dibasic.1,8-diazabicyclo[5,4,0]undec-7-ene imidazole（[HDBU]Im）exhibited the best catalytic activity.The optimized operation condition is as follows;under 10000μW·cm^-2 UV irradiation,5 g PET,20 g EG,0.25 g[HDBU]Im,reacted for 90 min,the conversion of PET is 100%,the yield of BHET is 88.9%.Under optimized experimental condition,[HDBU]Im could be reused at least six times without any obvious decline in catalytic activity.With UV irradiation or without,the kinetic of the reaction is first order.The Ea with UV irradiation is 113.82 k J/mol,and that without UV irradiation is 166.50 k J/mol,which indicates that UV can reduce Ea obviously.The degradation mechanism was explored,[HDBU]⁺and Im^-play synergistic role.（3）The research on controllable PET glycolysis was carried out.Controllable PET degradation means obtaining the oligomers with a certain molecular weight.To achieve this goal,the catalyst with moderately active should be designed and the experimental condition should be adjusted.The acid-base property of catalyst has great influence on the degree of degradation.ChCl/1,3-DMU is chosen as the optimal catalyst.The oligomers with number-average molecular weight（M_n）of 1000 g·mol^-1and 2000 g·mol^-1can be obtained by lowering the temperature,extending reaction time and adjusting the amount of EG.Controllable glycolysis can gain more details about reaction process.（4）ChCl/Fa and[HDBU]Im are both liquid at room temperature,and they can dissolve in EG.The catalyst/EG mixture can be regarded as the reaction medium for PET glycolysis.In this thesis,the densities and viscosities were determined.The interaction between EG and catalyst can be revealed by calculating molar volume and apparent molar volume.Viscosity can affect the mass transfer among PET,EG and catalyst directly.The viscosity deviation and the activation energy of viscous flow were calculated.It can be inferred that EG can form hydrogen bond with the cations and anions of[HDBU]Im and ChCl/Fa can interact with EG by Vander Waals force.