Formation Mechanism Of Gas-phase Products In Fe₂O₃ Catalytic Pyrolysis Of Tire Rubber

With the development of economy,the output of waste tires has increased year by year,which has become an important environmental problem.For a long time,a reasonable way to deal with waste tires has been looking for.The traditional solid waste treatment method will cause land occupation and environmental pollution.However,by pyrolysis,it can not only produce small pollution,but also produce products with high utility value.It is a good method to treat waste tires.Adding appropriate catalyst to the pyrolysis process can selectively change the product distribution.Therefore,catalytic pyrolysis is a hot topic in recent years.In this paper,Fe₂O₃was used as catalyst to study the micro formation mechanism of gas-phase products during catalytic pyrolysis of tire rubber by combining thermogravimetry,pyrolysis experiment and molecular dynamics simulation,which provides theoretical basis for the recycling of waste tires.Firstly,the pyrolysis process of tire rubber catalyzed by Fe₂O₃was studied.The results of thermogravimetry showed that Fe₂O₃can reduce the range of pyrolysis temperature,make the tire rubber pyrolysis more fully,and increase the pyrolysis rate and gas yield;the results of gas chromatography showed that CH₄,H₂and C₂H₄were the main gas products of tire rubber pyrolysis.Fe₂O₃had no effect on the type of gas products,however,it could change the gas products distribution of CH₄,H₂,C₂H₄,in which the proportion of CH₄and H₂were promoted while the proportion of C₂H₄was reduced.Thermogravimetry and gas chromatography results showed that the catalytic effect was more and more obvious with the increase of Fe₂O₃addition ratio in a certain range;from the perspective of economy,the best addition ratio of Fe₂O₃(rubber?Fe₂O₃)is between 2?1 and 1?1.Then the pyrolysis process of tire rubber and the catalytic pyrolysis process with Fe₂O₃were simulated by Materials Studio software.The simulation results showed that the main reaction in the low temperature pyrolysis stage was the long chain fracture of rubber to form monomers,and the main products were isoprene,styrene and 1,3-butadiene;the main reaction in the high temperature pyrolysis stage was that the monomers further generate small molecular gas,in which CH₄,H₂,C₂H₄account for the majority,and there was a small amount of C₂H₆,C₃H₆.Fe₂O₃could not only shorten the reaction time,but also reduce the termination temperature of pyrolysis.And Fe₂O₃could also increase the amount of uncondensated C₄H₆·at the end of the low temperature pyrolysis stage,change the distribution of gas products at the end of the high temperature pyrolysis stage,and increase the proportion of C1(a free radical with one C atom)and C3(a free radical with three C atoms)in the final products,but decrease the proportion of C2(a free radical with two C atoms).Moreover,the catalytic effect of Fe₂O₃on the pyrolysis of tire rubber mainly occur in the stage of high temperature pyrolysis.Then,according to the molecular chain breakage observed in the simulation stage of pyrolysis process,the transition states of three main free radicals H·,CH₃·,CH₂=CH·were searched before and after catalysis.After calculating and comparing the reaction energy barriers of all possible paths,it's demonstrated that Fe₂O₃presented good performance to reduce the energy barriers of six specific paths to make these reactions easier to occur.In the process of free radical formation,Fe₂O₃can promote isoprene monomer and styrene monomer to release more CH₃·and CH₂=CH·,respectively.In the process of free radical combination,Fe₂O₃can promote CH₃·attacking isoprene monomer and 1,3-butadiene to generate CH₄.Morever,Fe₂O₃can promote H·and CH₂=CH·attacking styrene monomer to generate H₂and CH₂=CH₂,respectively.The simulation results of pyrolysis process and reaction paths showed that styrene monomer was the most difficult part in tire rubber pyrolysis,and the addition of Fe₂O₃was conducive to the catalytic pyrolysis of styrene.And the catalytic effect of Fe₂O₃on the cracking of styrene monomer is mainly realized by promoting H·and CH₂=CH·to capture H·on styrene.Finally,comparing the catalytic pyrolysis effect of Fe₂O₃and Mn O₂,it is found that both of them can improve the gas yield and the proportion of H₂and CH₄in the pyrolysis gas products.The effect of Fe₂O₃is more obvious in reducing the pyrolysis termination temperature,and Mn O₂and Fe₂O₃are close to each other in reducing the energy barrier.