Study On Co-Pyrolysis-Vapor Phase Hydrogenation Of Polystyrene And Lignin To Produce Cycloalkanes

The unsustainability of fossil energy,huge carbon emissions,and environmental pollution urgently require breakthroughs in renewable alternative energy and low-carbon environmental transition technologies.Biomass is a plant resource consisting of organic carbon from a wide range of sources and is the only renewable resource that can be used to prepare liquid fuels.Thermochemical conversion is one of the most effective ways to upcycle biomass into fuels and/or value-added chemicals.However,the complex cross-linked structure,low effective hydrogen to carbon ratio,and low energy density of biomass lead to high oxygen content,low calorific value,and poor stability of pyrolysis oil,etc.It is necessary to improve the quality of biomass pyrolysis oil.The Indirect hydrodeoxygenation of bio-oil causes problems such as secondary energy consumption and repolymerization of oxygenated products.To address the problems of high oxygen content of biomass pyrolysis products,as well as the low energy utilization rate and mass extraction and secondary polymerization of products,we propose a conversion method of pressurized co-hydrogenolysis coupled with vapor-phase hydrogenation of biomass and hydrogen-enriched solid waste by integrating fast pyrolysis and catalytic upgrading technologies.The introduction of hydrogen-rich solid waste can supply active atmosphere and hydrogen source for vapor-phase hydrogenation of biomass pyrolysis primary products,which is beneficial to reduce the consumption of hydrogen and the cost of hydrogenation improvement.The introduction of catalysts with deep deoxygenation hydrogenation activity into vapor-phase hydrogenation to regulate the hydrogenation reaction pathway of pyrolysis product intermediates from the source and selectively adjust the ratio of pyrolysis product components to maximize the production of cycloalkanes as the target product.Based on this,a basic study on the preparation of cycloalkanes from polystyrene and lignin by co-pyrolysis coupled with vapor-phase hydrogenation on a two-stage pressurized fixed-bed reactor was carried out,focusing on the design of the catalytic system and the optimization of the reaction process.The main research contents and results are summarized as follows:The effect of supported Ni-based catalysts on the vapor-phase hydrodeoxygenation of guaiacol was investigated using the lignin model compound guaiacol,and the stability of the catalysts was examined.The results showed that 10%Ni/DNZ-H exhibited excellent high catalytic performance,mainly attributed to the appropriate acid strength and specific surface area,the rich mesoporous structure.The conversion rate of guaiacol reached 100%,and the complete HDO product cyclohexane was formed with a relative content of 98%under the pyrolysis temperature of 240℃,vapor-phase hydrogenation temperature of 220℃,low reaction pressure of 1.5 bar and catalyst loading of 7 mg.The catalyst of 10%Ni/DNZ-H showed good stability and the conversion of guaiacol remained at 88%after 47 cycles.As a comparison,10%Ni/HZSM-5（27）showed a rapid decrease in guaiacol conversion from 83%to 26%after 6 cycles.The behavior of PS pyrolysis and the hydrogenation of pyrolysis intermediates over Fe₂N@C and 10%Ni/DNZ-H were investigated using polystyrene（PS）-based waste plastics,and the effect of vapor-phase hydrogenation temperature on the distribution of PS degradation products was examined.The Fe₂N@C showed good catalytic performance in selective hydrogenation,and the yield of ethylbenzene obtained from Fe₂N@C catalysis was 81 wt%,which is much higher than that of Fe₂N（63.3 wt%）and commercial iron nitride（Fe N,4.9 wt%）.10%Ni/DNZ-H showed excellent catalytic activity in deep hydrogenation and the PS pyrolysis products were completely hydrogenated to cycloalkanes（mainly ethylcyclohexane）with a relative content of 99%at a pyrolysis temperature of 480°C,hydrogenation temperature of220°C,reaction pressure of 1.5 bar,and catalyst loading of 20 mg.With the increase of hydrogenation temperature,the relative content of cycloalkanes gradually decreased from 99%at 220℃to 29.5%at 320℃,a decrease of 69.5%,and the relative content of aromatics increased by 40.4%.We investigated the behavior of PS pyrolysis and the hydrogenation of pyrolysis intermediates on Fe2N@C and 10%Ni/DNZ-H using polystyrene（PS）-based waste plastics,and examined the effect of vapor-phase hydrogenation temperature on the distribution of PS degradation products.The results showed that co-pyrolysis of lignin and PS could effectively inhibit the formation of oxygenated compounds and promote the formation of cycloalkanes and aromatic hydrocarbons.The co-pyrolysis temperature in the first reactor,hydrogenation temperature in the second reactor,reaction pressure and catalyst loading were optimized at 480℃、220℃and 20 mg,respectively,and the relative content of cycloalkanes was the largest at 94.5%.The co-pyrolysis temperature had an important effect on the product distribution,and the co-pyrolysis temperature of 480℃is conducive to the formation of ethylcyclohexane with a relative content of 84%.The maximum dimethylcyclohexane yield of 34.7%was obtained at 530℃.Co-pyrolysis of different types of PS-based waste plastics with lignin has a small effect on the relative content of cycloalkanes.The relative content of cycloalkanes obtained from the co-pyrolysis of PS foam,ABS,AS and HIPS with lignin was 94.5%,87.3%,83.8%and 94.7%,respectively.