Catalytic cracking of model compounds of bio-oil: Characteristics and mechanism research on guaiacol and acetic acid

The unstable oxygenates of raw bio-oil limit its high-value utilization, which renders the refinement process necessary. Catalytic cracking is one of the promising refining methods and HZSM-5 was found to be an effective catalyst. To investigate the reaction mechanism of catalytic cracking of the oxygenated components in bio-oil, guaiacol and acetic acid, are employed for the catalytic conversion over HZSM-5 and metal modified zeolite Zn/HZSM-5 in a self-designed micro-reactor. Herein, the mechanism of pyrolysis and catalytic cracking was calculated by density functional theory (DFT). For guaiacol, the selectivity of salicylaldehyde decreased from 57.41% to 41.84% and 40.31% over HZSM-5 and Zn/HZSM-5, respectively. In the catalytic cracking over Zn/HZSM-5, the yield of aromatic hydrocarbon increased. More specifically, benzene appeared in the liquid product. For acetic acid, the removal of the oxy-functional group was further promoted by Zn/HZSM-5, the as-formed ketene intermediate continues to react with zeolite catalysts, generating acetyl cation, and finally formed CO and ethylene. Zn/HZSM-5 has a better effect on promoting the reaction of ketene and methoxide since the energy barrier of the rate-determining step in this reaction decreased from 163.27 kJ/mol to 151.19 kJ/mol. Demethoxylation and dehydration were considered to be favored by Lewis acid sites.