Experimental Study On Preparation Of Carbon Nanotubes By Catalytic Cracking Of Waste Hard Plastics

Plastics,as the main material in the 21 st century,have been widely used in production and life,but at the same time,a large number of plastic wastes have been discarded and disposed.Among them,the waste hard plastics have the characteristics of large volume,light weight,non-decay and non-decomposition.They may last for hundreds or even thousands of years,and the soil and environmental pollution problems caused by them are particularly serious.Therefore,the efficient recycling and recycling of waste hard plastics is particularly important for the protection of environmental ecology and the development of manufacturing industry.In this paper,main components of waste hard plastics-phenol formaldehyde resin(PF)and typical waste hard plastic-plastic handle were used as research objects and a large number of characterization and analysis were carried out.Using PF and waste plastic handle as carbon source,high value-added carbon nanotubes(CNTs)were prepared by two-stage catalytic cracking chemical vapor deposition(CVD)using transition metal catalysts.At the same time,the quality parameters of CNTs prepared by catalytic cracking of PF and waste plastic handles by two main transition metal catalysts were compared and analyzed.Firstly,the experimental study found that the carbon content of main components of waste hard plastics-PF was higher than 60%,which indicated that waste hard plastics had the potential to prepare CNTs.The process of mass loss can be roughly divided into three stages.The second stage is the main stage of PF thermal decomposition,which releases a large number of pyrolysis gas products,and the mass loss rate is about 20%.At this stage,when the temperature is about 500 °C,the concentration of the carboncontaining gas is the highest.The pyrolysis gas phase components released during the entire cracking stage are H2 O,alcohols,LMP,CO2,CO and LMH including CH4,etc.Characterization analysis results showed that the ether bond and the unreacted terminal methylol group(-CH2-OH)in the waste hard plastic are degraded,accompanied by the release of CH3 OH and H2 O,and additional crosslinks are formed between methylene(=CH2)and phenolic hydroxyl(-OH)in the pyrolysis process of waste plastics.The crosslinked structure is broken to form a thermally crosslinked intermediate structure.At the same time,the methylene group(=CH2)is cleaved to a methyl group(-CH3)group.The hydroxyl radical(-OH)provides an oxidizing atmosphere for the methylene group(=CH2),and oxidation of the methylene group(=CH2)results in the formation of carbon CO and CO2.The phenolic hydroxyl group(-OH)undergoes dehydration cyclization and eventually forms coke.Secondly,CNTs were prepared by catalytic cracking of PF with chemical vapor deposition(CVD)in two-stage catalytic cracking tubular furnace.The characterization of the catalyst and CNTs showed that the multi-walled carbon nanotubes(MWCNTs)produced by the Fe catalysts were more abundant and smoother than the Ni catalysts.MWCNTs produced by Fe catalysts are more complete in structure and smaller in outer diameter than that of Ni catalysts.The outer diameter of MWCNTs produced by Fe catalysts is about 12 nm,while that of MWCNTs produced by Ni catalysts is about 15 nm.Meanwhile,the G:D ratio of MWCNTs prepared by Ni catalysts is 1.37,the average yield is 24.07%,and the average purity is 93.13%.The G:D ratio of MWCNTs prepared by Fe catalysts is 1.81,the average yield is 34.39%,and the average purity is 97.45%.Fe catalysts produce more and purer MWCNTs than Ni catalysts.Finally,a two-stage catalytic cracking experiment was also carried out on a typical waste hard plastic-plastic handle using the most optimal metal Fe catalysts to prepare high value-added CNTs.It is found that the plastic handle is mainly composed of C,H,O and S.The carbon content of the plastic handle is higher than that of the PF,reaching more than 70%.Moreover,compared with PF,the plastic handle has more elements,including more S elements and trace Br and Cl elements.MWCNTs were also obtained,but compared with the MWCNTs prepared by PF catalytic pyrolysis,the length of MWCNTs is longer.More importantly,straight-chain MWCNTs were produced.The wall layer is more uniform and continuous,and the winding,curling and stacking of MWCNTs are correspondingly less.The G:D ratio of straight-chain MWCNTs is 1.14,the average yield is 36.60%,and the average purity is 94.14%.At the same time,the effects of S element,water vapor and particle size of plastics on the preparation of straight-chain MWCNTs were studied : Within the experimental range,the graphitization degree,yield and purity of MWCNTs increased firstly and reduced as the increase of SO2 concentration.When SO2 concentration reaches 60 ppm,the quality of MWCNTs is the best.The G:D ratio of MWCNTs reaches 1.48,the yield reaches 39.24%,and the purity reaches 96.86%.Within the experimental range,when the water content is 2%,the graphitization degree,yield and purity of MWCNTs are relatively high.The G:D ratio of MWCNTs reaches 1.38,the yield reaches 34.78%,and the purity reaches 98.72%.However,with the increase of water content,the graphitization degree,yield and purity of MWCNTs decrease greatly.When water content is 10%,the G:D ratio of MWCNTs is even less than 1,the yield is 8.28%,and the purity is only 71.62%.Within the experimental range,the graphitization degree and purity of MWCNTs increased slowly with the increase of the plastic particle mesh in plastic handle.When the mesh of plastic particles reaches 50 meshes,MWCNTs have the greatest graphitization degree,the G:D ratio of MWCNTs is 1.42,and the purity of MWCNTs is 96.86%.However,the yield of MWCNTs hardly changed,and only a small range of fluctuations occurred.The results show that the graphitization degree,yield and purity of MWCNTs can only be improved by appropriate amount of S element.Too much or too little will affect the quality of MWCNTs.Only a small amount of water vapor will increase the quality of MWCNTs,and an excess will seriously hinder the growth of MWCNTs.The smaller the particle size of waste hard plastics,the better the preparation of MWCNTs.