Study On Preparation Of Light Aromatic Hydrocarbons From Cellulose Pyrolysis Vapor Catalyzed By Porous Molecular Sieves

The escalating utilization of traditional fossil fuels has resulted in the production of greenhouse gas emissions and the degradation of air quality.However,biomass,a promising alternative to fossil fuels,can be transformed into biofuels and chemicals through thermochemical conversion techniques such as pyrolysis.Specifically,catalytic fast pyrolysis(CFP)of cellulose,the most abundant biomass fraction,has recently gained considerable attention.Nevertheless,the commercialization of fast pyrolysis products is hampered by low calorific value,low stability,high oxygen content,and acidity.Consequently,efforts have been made to search for novel catalysts that can promote cellulose conversion and the production of biofuels and renewable chemicals from cellulose pyrolysis using acidic catalysts.This approach produces biofuels containing approximately50% aromatic hydrocarbons(AHs),and the strong acid sites on the catalysts promote high selectivity of AHs in the pyrolysis oil.To develop effective catalysts for the production of light aromatic hydrocarbons(LAHs)and suppress carbon accumulation,the tuning of pore structure and acid center of ZSM-5 is a necessary strategy.In this study,a core-shell structured ZSM-5@MCM-41 catalyst was synthesized and used for the preparation of LAHs from CFP of cellulose.The successful synthesis of graded core-shell ZSM-5@MCM-41 was confirmed by N2-physisorption,Xray diffraction(XRD),and transmission electron microscopy(TEM)analysis.A 1.0SHZ5@M41 catalyst was obtained for the production of AHs,with the highest relative content of 98.76% achieved for AHs production.The relative content of BTEX was also high,reaching 89.83%,which was 2.39 times higher than that of conventional microporous ZSM-5.A quadratic polynomial was developed to correlate the Br?nsted-to-Lewis acid site strength ratio(BLR)and the mesopore-to-micropore volume ratio(MMR)with the relative content of BTEX.The study revealed the mechanism of in situ catalytic reforming of cellulose pyrolysis steam over the ZSM-5@MCM-41 catalyst.Due to the high selectivity for LAHs and the low yield of carbon accumulation,ZSM-5@MCM-41 provides a better option for the preparation of LAHs from cellulosic biomass.Cellulose and other biomasses are characterized as "oxygen-rich and low-hydrogen" materials,which presents a natural disadvantage for the preparation of AHs.Conversely,waste plastics,which are commonly regarded as "hydrogen-rich" materials,can compensate for this deficiency of cellulose.Consequently,in situ catalytic co-pyrolysis(CCP)of both cellulose and waste plastics presents a promising technology for the production of AHs.In this study,the formation characteristics and mechanism of AHs during CCP were investigated,focusing on mass transfer and free radicals.The results demonstrate that a 20.44wt% yield of BTEX is achieved at 500°C when the cellulose/polyethylene/cellulose(C-P-C)mass ratio is 0.5:1:0.5.The free radicals generated from the broken bonds of polyethylene promote the transfer of hydrogen atoms to the cellulose cleavage intermediates during the CCP process,resulting in a mass transfer effect.Furthermore,the C-P-C loading mode facilitates the hydrogen atoms to pass through the cellulose without escaping.Increasing the proportion of polyethylene increases the number of hydrogen radicals,which regulate the hydrogen radical-mass transfer efficiency.However,the high concentration of hydrogen radicals inhibits the pyrolysis of cellulose,which results in lower volatile fraction content and higher biochar content.The ZSM-5@MCM-41 catalyst exhibits a higher catalytic capacity for radical-carrying co-pyrolysis products than ZSM-5 due to the conjugated double size effect.The BTEX yield of 1.0SH-Z5@M41 is 3.15 times higher than that of ZSM-5,and the coke yield is only 14.70% of ZSM-5.Furthermore,the functional group changes of furfural,the main component of cellulose,over the catalyst were observed using infrared diffuse reflectance characterization,which led to the proposed chemical process of cellulose to AHs conversion and the proposed reaction pathway of polyethylene promoted cellulose pyrolysis and hydrogen radical-transfer co-pyrolysis mechanism.