Biomass-Mediated Synthesis Of ZSM-5 And Catalytic Cracking Of Biomass And Plastics

With the goal of carbon neutrality and peak carbon emission,the petrochemical industry is facing the adverse situation of decelerated growth.In contrast,the demand for BTEX（benzene,toluene,ethylbenzene and xylene）and light olefins（ethylene,propylene and butene）,as important petrochemical products,keeps rising.In order to reduce China’s excessive dependence on petroleum resources in the future,increasing attention has been paid to the development of non-petroleum routes to produce aromatics and low-carbon olefins.The carbon-neutral energy that can be directly converted into fuels and chemicals,biomass has abundant,renewable,and cost-effective advantages.Catalytic cracking is one of the main technologies for converting biomass into aromatics and low-carbon olefins.However,biomass is oxygen-rich and hydrogen deficiency resource.Coke is easy to deposit on the catalyst during catalytic cracking of biomass,leading to rapid deactivation of the catalyst.Waste plastic（WP）is a kind of cheap and hydrogen-rich product.When biomass is co-pyrolyzed with WP,it is expectable that the defects of oxygen-rich and hydrogen deficiency can be balanced,and the utilization efficiency of biomass can be improved.ZSM-5 shows excellent catalytic performance among acid catalysts,but it has the following three main defects:（i）Expensive organic ammonia structure directing agent is required in ZSM-5 synthesis,which greatly increases the cost and hinders the large-scale commercial application of ZSM-5.（ii）Its strong acidity can easily lead to the excessive secondary reaction of products to form coke deposited on the catalyst surface.（iii）Its unique microporous structure is not conducive to the diffusion of macromolecular reactants in the pores.Aiming to solve the problem mentioned above,ZSM-5 was synthesized by modified cellulose template（MCT）method and active gel-hydrothermal crystallization（AGHC）method in the absence of adding any organic ammonia template.The ZSM-5 synthesized by AGHC method was also modified by adding eucalyptus sawdust（ES）,steam treatment,alkali treatment and La₂O₃metal loading to adjust its acidity,pore structure and specific surface area.The catalytic co-cracking of ES and WP,the catalytic cracking of waste cooking oil and its model compounds were also carried out to explore a new path to produce monocyclic aromatic hydrocarbons and low-carbon olefins.The main conclusions are as follows:（1）Microcrystalline cellulose（MCC）was modified by polyethylene imine（PEI）,and the modified MCC（PMCC）was used as a template to synthesize ZSM-5.After introducing PMCC,the crystallinity,BET surface area and acidity of ZSM-5 were all improved.The Z-8PMCC zeolite with a PMCC content of8%is more suitable for catalytic co-cracking of ES and WP to produce BTEX（20.9%）and light olefins（19.7%）at 500°C and ES/WP（mass ratio）of 1:1.Compared with commercial ZSM-5（CZ）,the BTEX carbon yield(Y_C-BTEX)of Z-8PMCC increased by 7.1%,and Z-8PMCC had stronger carbon deposition resistance.When the ratio of ES and WP is 1:1,the Y_C-BTEX obtained by catalytic co-cracking is higher than that from other ratios,which proves that there is a strong synergy between ES and WP.（2）ZSM-5 was prepared by a two-step method of AGHC.ES was also used to modify the acidity and pore size of ZSM-5.After a simple activation process of silicon aluminum gel,the active nucleation precursor can be highly formed,which creates favorable conditions for subsequent shortening of crystallization time.The crystallinity of AGZ reached 86.2%after 18 hours of crystallization.The BET surface area and acidity of ZSM-5 were also increased.In the prepared ZSM-5 with different ES dosages,AGZ-10（10%ES addition）obtained the highest Y_C-BTEX（18.9%）from catalytic co-cracking of ES and WP（ES/WP=1:1）at 500°C.Compared with ZSM-5 synthesized by adding organic amine template and MCT method,the cost of AGZ is lower（about 40 yuan/kg）.（3）For better approach industrial production,the pore size and acidity of low-cost AGZ zeolite,the steam treatment and alkali treatment on AGZ were carried out.It is found that the mesoporous ratio(V_meso/V_total)of AGZ can be effectively increased by increasing the steam space and temperature.Increasing the steam space velocity can gently regulate the crystallinity and acidity of AGZ than increasing steam temperature.The results of alkali treatment show that the weak alkali（Na₂CO₃）treatment can gently regulate the crystallinity and acidity of AGZ,and effectively increase the mesoporous structure.The results of co-cracking of ES and WP show that A-9-400 zeolite(steam treatment temperature:400°C,space velocity:9 h^-1)has the highest Y_C-BTEX（18.6%）over reaction temperature of 500°C and ES/WP ratio of 1:1.Under the same cracking reaction conditions,the A-NC-0.3 zeolite with Na₂CO₃ concentration of 0.3 mol/L showed the best Y_C-BTEX of 19.1%.After both steam and alkali treatment,AGZ showed a good synergistic effect in catalytic co-cracking of ES and WP.（4）AGZ was modified by La₂O₃,and the effect of metal loading on the acidity and pore size of AGZ and its catalytic cracking proformance were investigated.La₂O₃ modification has a significant regulatory effect on the pore size and acidity.When La₂O₃ loading was 6%and 10%,V_meso/V_total reached the maximum of 68.1%and 68.2%respectively,which increased by 4.5%compared with the unloaded AGZ.With the increase of La₂O₃ loading to 14%,the maximum amount of strong acid decreased by 15.1%.6La A zeolite with loading of 6%showed the highest Y_C-BTEX of 26.4%and 23.8%at 500°C from cracking of oleic acid and waste cooking oil,respectively.The highest Y_C-BTEX of 26.5%was obtained for cracking of methyl laurate at 600°C.With the reaction temperature reaching 600°C,the carbon yield of light olefins obtained from the three oil samples reached the maximum of 27.3%,24.7%and 26.6%,respectively.Compared with CZ,6La A shows more stable catalytic effect in cracking and recycling regeneration,and La₂O₃ loading can effectively improve the carbon deposition resistance of AGZ.（5）Steam catalytic cracking of ES-WP mixture and waste cooking oil was carried out over the AGZ zeolite,and the results were compared with those of N₂as carrier gas.The Y_C-BTEX from cracking of ES and WP（ES/WP=1:1）reached the maximum of 11.8%at water injection rate of 0.5 m L/min and 550°C.The Y_C-BTEX from cracking of waste cooking oil reached the maximum of 20.8%at the water injection rate of 0.5 m L/min and 500°C.When the reaction temperature is 600°C and the water injection rate is 1.0 m L/min,the carbon yield of light olefins from steam cracking of ES-WP（ES/WP=1:1）and waste cooking oil reach the maximum of 21.7%and 21.5%,respectively.Compared with N₂ as carrier gas,the carbon yield of light alkanes from steam cracking of ES-WP mixture and waste cooking oil are significantly reduced,which is more conducive to the separation and purification of light olefins.Moreover,the carbon yields of naphthalenes and indenes in the liquid products are significantly reduced,and the catalyst showed an excellent anti-coking ability.