Improving aromatic hydrocarbon content from catalytic pyrolysis upgrading of biomass on a CaO/HZSM-5 dual-catalyst

Abstract

Pyrolysis of biomass with and without a catalyst was investigated using thermogravimetric analysis coupled with Pyrolysis-Gas Chromatography/Mass Spectrometry. The effect of two completely different catalysts (a basic catalyst: CaO, and an acidic catalyst: HZSM-5) on pyrolysis characteristics and the composition of pyrolysis vapors, as well as aromatic selectivity, were studied. A characteristic analysis of catalysts (using XRD, BET, FTIR, NH3-TPD and SEM), the effect of catalytic temperature, the CaO to HZSM-5 ratio and the biomass to catalyst ratio on the distribution of products were also investigated and analyzed. Coke formation and deposition mechanisms and the relationship between the formation and characteristics of the coke and the deactivation of the catalyst were also demonstrated. The results demonstrate that the pyrolysis characteristics of the biomass were changed by the addition of CaO (bond breaking) and HZSM-5 (deoxygenation) catalysts, causing the solid residue rate to increase significantly, the maximum weight loss rate to decrease and the biomass conversion rate to increase. The order of the activity of the catalysts is HZSM-5 > CaO/HZSM-5 > CaO; at the same time, characteristic parameters such as activation energy, activation enthalpy, activation entropy and the steric parameter increased significantly at the expense of the reaction ratio. Moreover, CaO mixed with HZSM-5 catalyst can improve the gas content and aromatic hydrocarbon content significantly. However, the content of bio-oil decreased slightly, and the maximum aromatic content was 72 wt% of peak area of that for pure HZSM-5 catalyst (69 wt% of peak area) at a pyrolysis temperature of 450 °C and a catalytic temperature of 550 °C. The ratio of CaO to HZSM-5 was 1:2, and the biomass to catalyst ratio was 1:2. In addition, the selectivities with toluene and xylene were 27% and 35%, respectively. This paper describes a process for the high-value utilization of biomass as a sustainable resource for the production of hydrocarbons, especially for the preparation of the high-quality chemical p-xylene.