Experimental And Mechanism Study Of Microwave-induced Cracking Of Waste Plastics For Hydrogn Production With Iron-based Catalysts

With the rapid development of the plastic industry,the production of waste plastics has been increasing year by year.A large amount of plastic waste not only wastes resources,but also causes serious environmental pollution.The current situation between resources and the environment is becoming increasingly acute,efficient and clean treatment and resource utilization of waste plastics have become a global challenge.The use of microwave heating technology with selective catalytic characteristics to decompose waste plastics into hydrogen and carbon materials for resource utilization can greatly improve the cracking efficiency and product quality of waste plastics,which is an innovative solution to address challenges.However,this technology still faces issues such as low catalytic activity and energy conversion efficiency,as well as unclear catalytic mechanisms.Improving the reaction activity of catalysts in the microwave field,revealing the reaction pathway and mechanism of the cracking process play an important role in promoting the research and application of microwave induced cracking of waste plastics for hydrogen production.In response to the above issues,this study was based on the optimization of transition metal catalysts and the exploration of catalytic cracking reaction rules.Fe/FeAl2O4 composite catalysts with good absorption and catalytic performance were succes sfully prepared,and the cracking characteristics of waste plastics were systematically studied under various reaction conditions;The catalytic cracking behavior of various plastic polymers in microwave fields was investigated through ReaxFF molecular dynamics(ReaxFF MD),revealing the catalytic cracking reaction path of waste plastics,optimizing the reaction process,and finally analyzing its economy based on the design of waste plastic cracking process,laying a theoretical foundation for its promotion and application.The main work of the entire article is as follows:Based on density functional theory(DFT)simulation calculations,transition metal catalyst optimization experiments were carried out,and analyzed the polarization mechanism of plastic polymer monomers(ethylene,propylene,styrene)on the catalyst surface(Fe(110),Ni(111),Co(111).The study found that the transition metal adsorption capacity order is Fe>Co>Ni.After the molecules are adsorbed on the metal surface,electrons are transferred from the metal to the adsorbed molecules.The adsorbed molecules are negatively charged,and the metal is positively charged.After adsorption,a bond occurs between C atoms and surface metal atoms.The stronger the adsorption energy,the shorter the M-C bond length,which is conducive to the further bonding of H atoms in the molecule with the metal on the surface to form an M…H-C bond,and the H atom obtains more electrons from the C atom,polarizing and stretching the CH bond,thus revealing the polarized C-H bond mechanism of transition metals.Among these three types of olefin molecules,the difficulty in dissociating C-H bonds increases in the order of ethylene<propylene<styrene,and the C-H bonds on the carbon atoms on the C=C bond are easily polarized.Based on the above analysis,Fe-based catalysts are preferred for further research.The transition metal(Fe,Co,Ni)catalysts with low dielectric constant Al2O3 and high dielectric constant SiC and activated carbon(AC)were selected as carriers,and the experimental reaction rules of microwave-induced catalytic cracking of waste plastics for hydrogen production were investigated.The fitness of heating characteristics and catalytic characteristics of different supports and active site catalysts in the microwave field was evaluated.Research has found that M/Al2O3(M=Fe,Ni.Co)catalysts have low microwave conversion efficiency in multi-mode reactors,and the wave absorption performance of the catalyst needs to be optimized;M/AC catalysts have good wave absorption properties.Rapid pyrolysis of HDPE in a multi-membrane reactor can be achieved,and the selectivity of transition metals to H2 is in the order of Fe>Co>Ni.However,the catalytic active sites cannot effectively adsorb pyrolysis gas,which reduces the conversion rate of it.Therefore,AC is not suitable as a catalyst carrier for plastic cracking to produce hydrogen.The main factor in the difference in heating rate of M/SiC catalysts is the type of transition metal.Transition metals in quasi-catalysts are both absorbing sites and catalytic sites,and have good absorbing and catalytic properties.Among them,Fe/SiC achieved the highest H2 concentration with a yield of 34.1 8 mmol/gplastics.However,none of the above catalysts can achieve a higher hydrogen yield,and the competition between the catalytic performance and absorption performance of catalysts has not been resolved.Further research is needed to improve the compatibility of the catalyst.In order to further improve the fitness between catalytic performance and absorption performance,and enhance hydrogen production from waste plastics via microwave cracking,a performance modulation test of Fe/FeAl2O4 composite catalysts was conducted,investigating synergisms between wave-absorbing and catalytic performances.Under conventional heating,the hydrogen yield of the 30%Fe composite was only 23.57 mmol/gplastics.However,it exhibited excellent absorbing and catalytic properties under microwave heating,achieving high hydrogen enrichment with a concentration of 84.96 vol.%,and a yield of 47.03 mmol/gplastics.COMSOL electromagnetic field simulations supported that Fe in the composites was the primary microwave absorption site.The hydrogen yields of HDPE,LDPE,PP,and PS were 47.0,42.4,38.0,and 22.5 mmol/gplastics.The order of C-H bond dissociation difficulty was PE<PP<PS,which is consistent with the previous simulation results,proving the accuracy of the simulation.The growth mechanism of carbon nanotube(CNT)during plastic cracking was studied,and we find that top growth on Fe particles and bottom growth on FeAl2O4,with FeAl2O4 more conducive to CNT growth.The maximum CNT yield was 154 mg/gplastic.The research showed that microwave heating exhibited non-thermal effects with the magnetic field inhibiting catalyst metal particle agglomeration and benefiting Fe-based catalyst plastic hydrogen production reactions.Based on the research above,the Fe/FeAl2O4 composite material control strategy successfully improved fitness between catalytic and wave-absorbing performances.The reaction mechanism of microwave-enhanced Fe-catalyzed pyrolysis of waste plastics to produce a high concentration of H2 was elucidated by the ReaxFF MD simulation method.It was found that Fe atoms were highly selective for C-H bonds on C=C bonds,but the selectivity of Fe weakened with increasing temperature.Compared with conventional heating,the microwave enhanced the binding energy between Fe and C and promoted the breakage of the C-H bond,reducing the C-H bond dissociation energy from 88.01 kJ/mol under conventional simulated heating to 65.17 kJ/mol.Meanwhile,the lower temperature of the reaction reduced C-C bond breaking,leading to a decrease in the yield of other carbonaceous gas,thus demonstrating the "strong catalysis-weak pyrolysis" of microwaves and realizing a positive effect on selective hydrogen production through plastic cracking.However,the "non-thermal effect" of microwaves was gradually weakened with increasing temperature.It is found that microwaves can increase the bond angle and reduce the degree of hybridization of the C-H bond,also increase the collision frequency between the catalyst and the reactants,accelerate the chemical reaction,which is the main form of microwave "non-thermal effect".Finally,using molecular inverse tracking statistics,it is found that there are four H2 generation paths,and 68%of the H2 generated by the collision of H radicals in Fe catalyst through microwave excitation is the main H2 generation path.Thus,the reaction mechanism of microwave induced Fe catalyst cracking waste plastics to produce high concentration H2 was revealed.By optimizing and regulating the microwave-induced Fe/FeAl2O4 catalytic plastic pyrolysis process,the optimal reaction temperature range from 300℃ to 330℃.Within this window,"strong catalysis-weak pyrolysis" can be achieved,and the pyrolysis product tar is further cracked to produce hydrogen,boosting yields of hydrogen and residual carbon.Based on this,a process model for producing H2 and CNTs by microwave pyrolysis of waste plastics was constructed.Referring to NREL’s technical and economic analysis method,combined with the optimized operating conditions and product yield data obtained in this experiment,the economic feasibility of the process was analyzed.Utilizing valley power or curtailed new energy wind and electricity to generate microwaves,while matching and optimizing the system’s waste heat and energy,can further reduce generation costs and carbon emissions.Microwave catalytic cracking of waste plastics to produce hydrogen and high-value carbon is a novel approach for efficient resource utilization of waste plastics.Through appropriate catalyst selection and synergistic optimization with external fields,improving the conversion rate and product selectivity and quality of waste plastic cracking,enhancing the overall energy utilization rate of the system.This is of great significance for promoting the efficient disposal and resource utilization of waste plastics,solving the contradiction between waste plastic resources and the environment,and achieving green and sustainable development.