Catalytic pyrolysis over transition metal-modified zeolites: A comparative study between catalyst activity and deactivation

Using metal-doped zeolites in catalytic pyrolysis of biomass is a well-known approach to promote the formation of certain compounds. One of the major technical issues with such catalysts is their rapid deactivation due to coke formation. However, little is known about how metal-doping influences the characteristics of coking, such as coking rate and its composition. In this study, four different materials were experimentally evaluated based on their catalytic activity and coking characteristics: HZSM-5, Fe/ZSM-5, Ni/ZSM-5 and FeNi/ZSM-5. The materials were prepared and characterized followed by screening in a bench-scale setup for in-situ catalytic pyrolysis. The mass balance and composition of pyrolysis products including catalyst coke were analyzed. It was found that metal-doping increases the concentration of aromatic hydrocarbons in the liquid product from 59.0 to 82.8% of GC/MS peak area, especially monoaromatic hydrocarbons (MAHs) and naphthalenes. Fe mainly increases the selectivity of MAHs whereas Ni additionally promotes naphthalenes. FeNi/ZSM-5 reflects the combined effect of Fe- and Ni-doped catalysts. Regarding the catalyst coke composition, metal-doped catalysts present an increased concentration of aromatic hydrocarbons. For each catalyst, the chemical composition of the coke reflects the catalytic activity seen in vapor upgrading. Based on the findings in this work, a reaction pathway for metal-doped ZSM-5 and HZSM-5 is proposed. The results also show that metal-doping increases the formation of catalyst coke, mainly due to a higher concentration of strong acid sites. Also, the severity of coking was found to be dependent on the strength of acid sites. The coke yield increased from 3.5 wt% in the case of HZSM-5 to maximum 7.2 wt% over Fe/ZSM-5. At the same time metal-doping reduces the temperature of catalyst regeneration and catalyzes the oxidation of coke. Overall, this work presents a comparative study between catalyst activity and deactivation during thermochemical conversion of biomass.