Study On Pyrolysis Of Polyolefins Catalyzed By Modified Mesoporous Molecular Sieves

There is 2000MT/a waste plastics material generated in China at present,and China has become the largest waste plastics market and the largest country of recycling waste plastics. The lower biodegradability of polyolefins creates a serious environmental problem that is directing the governments to propose a solution to solve it based on source reduction,to reuse and recycle the waste plastics.If the materials can be chemically recycled,they will become a cheap and abundant source for useful chemicals and energy.Among various recycling methods,the chemical method,which converts waste plastics to useful hydrocarbons,has been recognized as a promising approach.In this study,several modified mesoporous molecular sieves such as Al-MCM-41, Zr-MCM-41 and Mo-MCM-41,were synthesized by hydrothermal system and some acid groups like BO₃^3-,SO₄^2- and MoO₃ and base group like K₂O were incorporated into the frame of the mesoporous molecular sieves to obtain the acid or base mesoporous molecular sieve catalysts.The obtained materials were characterized by XRD,FT-IR,BET,UV-VIS and NH₃-TPD.The results show that the obtained materials are of the characteristics of mesoporous molecular sieves,better long-range order and crystallinity,and that acid and base groups have been combined with the frame of mesoporous molecular sieve so that the acidity and base of the sieve is improved.The catalytic cracking performances of different modified mesoporous molecular sieves were evaluated in model reactions of pyrolysis of HDPE,LDPE,PP and PS according to the amount of the obtained yield of liquid fuel or monomer.According to the results of catalytic characterization,the relationships among structure,acidity,base and catalytic performance of different acid mesoporous molecular sieves were studied.The obtained results are as followed.Al-MCM-41 is of the best catalytic activity for the pyrolysis of HDPE and LDPE among the prepared catalysts.Under the optimum conditions,the conversion and the yield of liquid products for LDPE and HDPE are 97.1%and 83.6%,87.5%and 74.9%,respectively.The mole ratio of n(Si):n(Al) affects the results of pyrolysis of HDPE and LDPE,which means the acidity of catalyst greatly impacts the cracking reaction.For the pyrolysis of LDPE and HDPE,the catalytic activity of HZSM-5 is better than that of Al-MCM-41,because the acid of Al-MCM-41 is weaker than that of HZSM-5.However,the yield of liquid,specially the component of the high carbon material(kerosene),is better than that of HZSM-5 because the larger pore of mesoporouse molecular sieves.The catalytic activity of acid modified Al-MCM-41 is better than that of unmodified one.However,the yield of liquid product decreases due to the effect of introduced acid group on the structure of sieve.For other acid modified M-MCM-41(M=Zr,Ti,Sn,Mo),both the catalytic activity and yield of liquid are improved,but the activity is lower than that of Al-MCM-41 for the pyrolysis of LDPE and HDPE,especially for the HDPE that is of excellent regular structure.However,for the catalytic pyrolysis of sterically hindered PP,the effect of mesoporous molecular sieve sructure on pyrolysis results of PP is more important than that of mesoporous molecular sieve acidity.Al-MCM-41,ZrO₂/Mo-MCM-41,ZrO₂/Ti-MCM-41 and SO₄^2-/ Zr-MCM-41 are of the best catalytic activity for the pyrolysis of PP among the prepared catalysts.Under the optimum conditions,the conversion and the yield of liquid products are 94.9%and 84.3%,96.6%and 88.9%,91.2%and 83.6%,92.0%and 84.2%,respectively.For the pyrolysis of PP,not only the catalytic activity but also the yield of liquid of HZSM-5 are weaker than that of the above mentioned mesoporous molecular sieves catalysts,specially for the component of the high carbon material(kerosene),because the mesoporouse molecular sieves catalysts are of the large pore and surface,which can improve the selectivity for the high carbon material in the pyrolysis of PP.To obtain styrene monomer by the catalytic pyrolysis of PS,the alkali strength of catalyst plays an important role.K₂O/Si-MCM-41 is of better catalytic performance for pyrolysis of PS.Under the optimum conditions,the conversion of PS is 90.5%;the yield of liquid products is 85.7%in which the concentration of styrene reaches 80.5%.Compared with the results of thermal degradation and acid pyrolysis,K₂O/Si-MCM-41 is of better catalytic activity and selectivity to styrene.In addition,the selectivity to styrene decreases with the increasing of acid strength of catalyst.This is due to the different mechanism of pyrolysis with different catalysts.