Study On Hydrolysis Of Dimethyl Terephthalate Over Solid Acid Catalysts

Poly（ethylene terephthalate）（PET）is the most widely produced polyester compound in plastics,and it has been widely used in daily life because of its excellent properties.More and more waste PET is produced as plastic waste.However,most of the PET waste can’t be recycled effectively,which leads to severe environmental pollution and causes much waste of resources.To realize the resource recovery and reuse of waste polyester,we hydrolyzed the monomer dimethyl terephthalate（DMT）depolymerized by PET methanol into terephthalic acid（TPA）.However,there are few studies on the hydrolysis of DMT to TPA.Therefore,the development of efficient catalysts is of great industrial significance to improve the yield of terephthalic acid and reduce the formation of intermediates.In this paper,the catalytic properties ofγ-Al₂O₃,HY,HZSM-5,and Hβsolid acid catalysts for the hydrolysis of DMT were investigated.Hβ（25）zeolite showed excellent catalytic performance because of its moderate pore size and abundant acid sites.Under the optimum reaction conditions（180°C,120 min）,the conversion of DMT was 98.6%,and the yield of monomer TPA was 91.9%.The catalytic performance of different Si/Al ratios on the hydrolysis of DMT was investigated.The results showed that HZSM-5 and Hβzeolite with a lower Si/Al ratio had the best catalytic effect.This is because the Br?nsted（B）acidic sites of zeolite are easier to promote the hydrolysis of DMT to TPA.In addition,the catalytic performance of Hβzeolite for DMT hydrolysis at different calcination temperatures was investigated.It was found that the Hβzeolite at 550°C had the most effective acid content and the best catalytic effect for DMT hydrolysis.Although the reaction performance of Hβ（25）zeolite decreased slightly after being recycled six times,the conversion of DMT remained stable.The synthesis of TPA by hydrolysis of DMT on Nb modified HZSM-5 zeolite was studied.Nb/HZSM-5 catalysts with different loading were prepared and characterized by XRD,XPS,SEM,NH₃-TPD,and Py-IR.The effects of different reaction conditions on the conversion of DMT and TPA yield were investigated.It was found that the yield of TPA strongly depended on the reaction conditions and the loading amount of Nb.The Nb component in the catalyst exists on zeolite in the form of Nb₂O₅and interacts with the surface of the HZSM-5 zeolite to form B acid active sites.The yield of TPA in the reaction is closely related to the concentration of B acid sites.On 0.6Nb/HZSM-5 catalyst,the yield of TPA is the highest,about 94%.The results of N₂isothermal adsorption-desorption and Py-IR show that when the loading of Nb is 0.6 wt%,the B acid content of Nb/HZSM-5catalyst is the highest,and the specific surface area and pore volume reach the maximum,which provides more effective catalytic active sites for DMT hydrolysis.By adjusting the calcination temperature,it was found that the addition of Nb significantly reduced the dehydroxylation of molecular sieves and enhanced the stability of the catalyst.In addition,the TPA obtained from the purification of the monomer product prepared in this study can be directly applied to the production of PET,which provides a feasible catalytic decomposition technology for the recovery and recycling of waste PET.