Catalytic cracking mechanism of bio-oil model compounds

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.