Catalytic Pyrolysis Of Biomass Over Seaweed-Derived Biochar Catalyst:Towards Enhanced Monophenol Production

The efficient and environmentally friendly energy utilization from biomass resources is of great significance for our country to improving the environment,ensuring energy demand and realizing sustainable development.Catalytic pyrolysis of biomass for producing high quality biofuels and platform chemicals is one of the current research hotspots.Amongst,the biochar-based catalyst showed high reaction activity and selectivity during the conversion of biomass into bio-monophenols.Due to the complexity of biochar catalyst,the research of identifying reactive sites and exploring the catalytic mechanism has not yet achieved remarkable results.In present work,a series of nitrogen/oxygen doped carbocatalysts was developed from seaweed biomass,which were further employed for enhancing bio-monophenols,including phenol,methyl phenol and ethyl phenol from pyrolysis of lignocellulosic biomass.A systematic study was carried out which focused on catalytic reaction mechanism over those carbocatalyst by combining the experimental research and quantitative theoretical calculations.The main research contents of the dissertation are below:In terms of the carbonization-activation process,effects of different biomass species（seaweed,microalgae and woody biomass）,carbonization methods（hydrothermal and pyrolysis）,activation agents（KOH and Na OH）and reaction atmospheres（N₂ and CO₂）on physicochemical characteristics of carbocatalysts were systematically studied.Amongst,the performance of carbocatalyst obtained from seaweed biomass for enhancing monophenol production was the best,whose relative content was up to 54.64%.Importantly,the components in monophenols were mainly o-cresol,p-cresol,dimethyl phenol and ethyl phenol.When the activation conditions are the same,the carbocatalyst prepared by slow pyrolysis carbonization has more suitable mesoporous to microporous proportion and higher content of surface functional groups than that by hydrothermal carbonization.As a weak oxidizing atmosphere,CO₂ can improve the porous structure of catalyst to some extent,but its effect is lower than that of using chemical agents.The porous structure of carbocatalyst when Na OH was employed as the chemical activation agent is not as developed as that using KOH.However,Na OH can greatly increase the contents of surface oxygen and nitrogen groups,which will provide more active sites for catalytic reactions.The carbocatalyst ENPC800-Na OH-CO₂ was prepared by CO₂ and Na OH co-activation,which can effectively increase the content of monophenol in bio-oil（59.46%）.The effects of activation temperature,agent dosage and CO₂ residence time on the performance of carbocatalysts were investigated.Surface response methodology（RSM）and artificial neural network（ANN）were used for modeling and data processing.By comparing the indicators of R²、MSE、RMSE and MAE during RSM and ANN,it is found that both fitting and prediction accuracy of ANN is higher than those of RSM.The optimal activation conditions were predicted by ANN as follows;activation temperature was 798.3℃,Na HCO₃:ENPC500 was 1:2 and CO₂residence time was 30.1 min,which is close to the preparation condition of ENPC800-1:2-30.The physicochemical characteristics and catalytic performance of ENPC800-1:2-30 were further investigated.Results showed that the relative content of monophenols in bio-oil was up to 60%.This was mainly ascribed to the developed porous structure（1085.77 m²/g）,suitable pore size distribution（mesoporous to microporous ratio=35%:65%）and abundant surface-active groups.In order to further explore the catalytic mechanism of lignocellulosic biomass over carbocatalysts,the biomass components,namely cellulose,hemicellulose,and lignin were decoupled and the product distribution of three major components of biomass in the presence of carbocatalyst was investigated.Results showed that the pyrolysis volatiles from cellulose and hemicellulose were reformed by the carbocatalyst.The contents of acids and ketones decreased,and furans increased,while the phenolic content was basically unchanged.The content of Meo PHs in pyrolysis volatiles from lignin decreased from 57.27%to 3.37%after adding the carbocatalyst ENPC800-1:2-30,while the Me PHs was significantly increased up to 80.01%.In addition,the content of aromatics was also increased.It is proved that the carbocatalysts not only had excellent demethoxylation performance,but also had certain aromatization ability.There are many types of active groups in carbocatalysts.In order to identify which group play a key role in demethoxylation and aromatization,the relative contents of oxygen-containing groups such as-OH,C=O,C-O-C and nitrogen-containing groups such as pyridine-N,pyrrole-N,graphite-N and oxidizaed-N on the surface of carbon catalysts were further regulated.The results showed that the-OH and pyrrole-N groups played a prominent role.DFT calculation were carried out to reveal the interaction mechanism of various oxygen-nitrogen sites with typical model compounds.In order to realize the full component utilization of biomass,the route that aromatics and phenol were produced from woody biomass through stepwise catalytic pyrolysis technology over zeolite and carbocatalysts was proposed.Firstly,Py-GC/MS was used to investigate the product distribution of cellulose catalyzed by ZSM-5 at different reaction temperatures.Secondly,based on the experimental results,the basic unit structure of celluloseβ-D-glucopyranose was selected,and the typical T5 model was used to represent the active site of ZSM-5 to carry out density functional theory calculation.The reaction paths and energy barriers of direct pyrolysis and catalytic pyrolysis of cellulose were investigated detailly.Results showed that in the presence of the ZSM-5 catalyst,the energy barrier of the reaction was effectively reduced.Glucose is first converted to 5-HMF,then5-HMF to furfural,and finally furfural to benzene.Due to the low selectivity of zeolite catalyst for the DA reaction,aromatic compounds can only be formed during catalytic pyrolysis.ZSM-5 is directly involved in most of the pyrolysis reactions of cellulose,and the electrophilicity of acid sites plays an essential role in the reaction system.Finally,the effects of different pyrolysis temperatures on the yield of aromatics and phenol were investigated.The results showed that the phenol yield was the highest when it was firstly pyrolyzed at 300℃and then catalytic pyrolyzed at 600℃,and the monophenol content was as high as 76.2%,among which the relative contents of phenol was49.6%.The energy consumption and environmental effects of each stage of the process were calculated and analyzed by using life cycle assessment（LCA）.By compared with the traditional process of preparing aromatics and phenol,the prospect of large-scale application of the process was evaluated.The environmental indicators of the whole route during different biochar application scenarios were compared.The results showed that the whole process can reduce carbon emissions when the biochar was used as soil amendment.however,all the other indicators except GWP when biochar as solid fuel were lower than those when biochar as soil amendment.Compared with the traditional process,when 8.325 t aromatics and 1 t phenol were produced,the energy consumption and carbon emissions of biomass route were lower,but other indicators were higher than those of the traditional process.In order to further improve the whole stepwise catalytic pyrolysis process of biomass,the contributions of each main stage to the indicators were analyzed.Results showed that for the two biochar application scenarios,the first-stage catalytic pyrolysis process account for the highest proportion in all indicators,among which zeolite employment account for the highest proportion,while the second-stage catalytic pyrolysis process has little impact on the environment.The sensitivity analysis of each indicators affected by key process variables was conducted.In order to reduce the impact of biomass route on the environment,it is necessary to further optimize the whole process from the following aspects;1)developing high efficiency zeolite catalyst,2)reducing the amount of catalyst dosage and consumption,3)reducing the energy consumption of the first stage,and 4)increasing the yield of phenol.