生物油模型化合物催化裂解机理

Catalytic cracking mechanism of bio-oil model compounds

为探索生物油催化裂解反应特性和催化作用机理,该文采用HZSM-5分子筛催化剂在550℃对生物油典型模型化合物(羟基丙酮、乙酸乙酯、愈创木酚)进行了催化裂解反应,研究模型化合物催化裂解特性和反应机理以及催化剂失活性质。试验结果显示:不含苯环的模型化合物催化裂解液体产物以芳烃为主,含氧化合物含量较低;羟基丙酮和乙酸乙酯的裂化气体产物分别以CO和烯烃为主。酚类模型化合物催化裂解液体产物仍以酚类为主,芳烃次之,说明苯酚类物质结构相对稳定,气体产物中烯烃含量约30%。根据气液产物分布,推测生物油催化裂解过程主要发生脱氧和环化反应,并对芳烃和烯烃有较好的选择性,为探索生物油催化裂解机理研究提供了理论依据。

Bio oil, produced by a fast pyrolysis of biomass, seems to be promising as the alternative to fossil fuel. However, bio oil has some undesired properties for fuel applications： high acidity, corrosiveness, low octane value, thermal instability, etc. These properties restrict its direct use for transportation, and require upgrading technology by reducing the oxygen content before use. Regarding to the complicated composition of bio oil, it is difficult to identify the reaction mechanism. Therefore, a model compound is widely used in bio oil upgrading research. Fluid catalytic cracking （FCC） experiments were performed in a quartz fluidized bed for a bio-oil model compound （acetol, ethyl acetate, guaiacol） over a HZSM-5 molecular sieve catalyst at 550℃, aiming to study the cracking characteristics of model compounds and the reaction mechanism as well as catalyst deactivation properties. The FCC facility mainly consisted of a gas-supplying unit, a pre-heater, a fluidized bed reactor with an inner diameter of 4cm and a height of 50 cm, a two-step condenser, an accumulative flowmeter and a gas-collecting unit. Gas products were quantified by GC 9800, liquid products quantified and qualified by Agilent 7890/5973-GC/MS, coke deposition detected by TG-DSC （Netsch STA409PC）. The liquid products of acetol cracking are composed of aromatics, phenols （5%）, and hydrocarbons, while oxygen is released in the form of CO、CO2、H2O. The liquid product of ethyl acetate cracking is mainly aromatics （65%）, oxygenated chemicals were less than 7%, and olefin content in the gas products is higher than 60 %, which shows that the HZSM-5 has good selectivity of aromatics and olefins and has good deoxygenation properties. The liquid products of guaiacol are phenols （~50%）, aromatics （~25%） and oxygenated chemicals （10%）, which proves the stable structure of phenol. The catalytic liquid product of a model compound without aromatic rings contains mainly aromatics and a low content of oxygenated chemicals. The catalytic gaseous products for acetol and ethyl acetate are mainly CO and olefin, respectively. The main catalytic product of phenols are phenolic compounds, followed by aromatics, which means phenols have relatively stable structures, while the olefin gaseous products are around 30%. The coke deposition rate is acetol guaiacol ethyl acetate. According to the catalytic cracking products distribution, the catalytic reaction pathways is speculated to illustrate that deoxygenation and cyclodehydration take place over a bio-oil catalytic cracking reaction, and show a good selectivity of aromatics and olefins, which provides a theoretical basis for the bio-oil catalytic cracking mechanism study.