Characterizing Coke Deposition on ZSM-5 during Catalytic Fast Pyrolysis

Catalytic fast pyrolysis of biomass has gained interest as a pathway for producing bio-oil as a sustainable, cleaner alternative to traditional fossil fuels. One of the challenges facing the implementation of catalytic fast pyrolysis is the formation of carbonaceous coke during subsequent rounds of catalysis which leads to rapid deactivation. This coke may be combusted off, but this requires large amounts of energy. The precise mechanism of coke formation is not well understood, but by examining the coke formed under various conditions, we hope to understand more about how catalytic fast pyrolysis may be optimized to produce more hydrocarbons with less coking. For this research, a quartz annular flow horizontal packed bed reactor was used to pyrolyze ten quarts boats containing 50.25±0.25 mg biomass. The boats were pyrolyzed successively; the produced vapors were upgraded over 0.5 g HZSM-5 catalyst and then analyzed via molecular beam mass spectrometer. The overall biomass to catalyst ratio was 1:1 for each experiment. Eight experiments were conducted: four different biomasses were pyrolyzed and upgraded in the presence of hydrogen, then repeated in the absence of hydrogen. Biomasses sampled include microcrystalline cellulose, larch wood xylan, pine shavings, and red oak lignin. Following pyrolysis experiments, mass spectrometer results were analyzed for product formation and coked catalyst samples were analyzed for weight and acidity using thermogravimetric analysis and temperature programmed ammonia desorption. The catalyzed pyrolysis runs carried out in hydrogen were expected to have less coke formation, as that the effective hydrogen index would be increased. However, the zeolites in ZSM-5 were not able to activate the hydrogen, so addition of hydrogen had little effect on coke formation. Catalytic fast pyrolysis of xylan had the lowest overall coke formation, but produced low to moderate yield of hydrocarbons, similar to that of lignin. Lignin and Xylan consist of shorter polymers or amorphous resins and must build up a hydrocarbon pool within the pores of the catalyst before upgraded vapors are produced.  Catalytic fast pyrolysis of cellulose is capable of producing the greatest yield of hydrocarbon product, but also has the greatest deactivation due to coke formation.