Preparation Of Carbon Manotubes By Catalytic Pyrolysis Of Waste Plastics And Their Adsorption Properties

Plastic with the advantages of lightness,durability and low manufacturing costs has a widely application in the production and daily life.However,the sharp increase of waste plastics have also be caused.In particular,this phenomenon has been exacerbated due to the outbreak of the new crown epidemic in 2019.Therefore,exploring the clean and high-value utilization of waste plastics should be explored.This is an urgent problem to be solved.In this paper,high value-added carbon nanotubes(CNTs)were prepared by chemical vapor deposition(CVD)with two-stage fixed-bed reactor using a Fe@Al₂O₃core-shell catalyst.The waste polypropylene was used as the raw material.Then,on this basis,the adsorption properties of carbon nanotubes on heavy metal ions were studied.The thesis includes the following three aspects:(1)During the pyrolysis of plastics,the release of carbonaceous gases such as C₁-C₄was investigated on a fixed-bed reactor using waste polypropylene as raw material.As the temperature increases,the acrylic volume reaches a maximum at500°C,and then the acrylic volume decrease with the increasing of temperature.At constant temperature for 60 min,the carbon yield increases by 0.23%compared to 30min.Under the optimal operating conditions of pyrolysis temperature of 500°C and a constant temperature for 30 min,the carbon conversion rate of the waste plastic is10.85%and the hydrocarbon gas yield is 1.65 L/g.(2)Fe@Al₂O₃core-shell catalyst was synthesized by sol-gel method.The carbon nanotube was prepared by polypropylene catalytic pyrolysis on a two-stage fixed-bed reactor.By characterizing the fresh and catalyst,SEM shows that as the Al content decreases,the catalyst changes from a dense distribution on the surface to a granular state,and the large agglomerated particles appear on the surface of the catalyst.With the increase of the catalytic temperature,the carbon yield increases to a certain extent.However,the high temperatures can cause the morphology change of the carbon nanotubes,with reducing the purity of the carbon nanotubes.Under these circumstances,the optimum catalytic temperature is 800°C.The purity of the carbon nanotubes can reach 0.99 at this temperature.Among the catalysts with different composition ratios,the AF1(Al/Fe=1)catalyst after the reaction has the highest quality carbon material and optimum reaction performance.Under the different quality ratio of PP:Cat(polypropylene:catalyst)experiments,when PP:Cat=7,the carbon nanotubes and amorphous carbon content are 46.88 mg/g_plasticand 137.81 mg/g_plastic,respectively.During the reaction,the carbon atoms was formed by the secondary cracking of hydrocarbons.Then,the carbon atoms were incorporated into iron,which was converted into Fe₃C.Because of the mutual diffusion and fusion adsorption between Fe₃C,the graphene was precipitated on the surface of catalyst.Finally,the carbon nanotubes were formed by irregular curling of graphene.(3)In order to improve the adsorption performance of carbon nanotubes on heavy metals in aqueous solution,the carbon nanotubes were modified.The results show that after modification,the average pore size and pore volume of carbon nanotubes increase.Meanwhile,the aromatic functional groups on the surface of modified carbon nanotubes are conducive to the improvement of adsorption performance.The adsorption isotherm shows that the adsorption of metal ions by carbon nanotubes can be described by langmuir model,indicating that the adsorption of carbon nanotubes is a single-molecular layer adsorption process.Meanwhile the adsorption molecules have the same activation energy.Kinetic studies have shown that the quasi-primary kinetic model has a relatively high degree of adsorption fit on carbon nanotubes.The adsorption process is proportional to the one-dimensional square concentration.Adsorption thermodynamics show that the adsorption on carbon nanotubes increase with the increase of temperature.Whether modified or not,the adsorption of Cu²⁺on carbon nanotubes is spontaneous.The competitive adsorption of Ni²⁺,Cu²⁺and Cd²⁺by modified carbon nanotubes show that the affinity of Ni²⁺,Cu²⁺and Cd²⁺is Ni²⁺>Cu²⁺>Cd²⁺.