Role of Potassium Exchange in Catalytic Pyrolysis of Biomass over ZSM-5: Formation of Alkyl Phenols and Furans

Catalytic fast pyrolysis of biomass with ZSM-S type zeolites is a commonly considered in situ upgrading technique for the production of partially deoxygenated bio-oils. The acidity and structure of ZSM-5 catalysts favor the production of aromatic hydrocarbons from oxygenates present in the pyrolysis vapors, such as acids, anhydrosugars, ketones and aldehydes. However, Biomass these acid catalyzed deoxygenation pathways remove functional groups from the aromatic ring structure, which makes the liquid bio-oil more amenable to processing in current petroleum refineries while subsequently decreasing the value of the compounds as industrial chemical feedstocks. The observation of improved yields of alkyl phenols and furans during catalyst deactivation suggests a method for tuning the product distribution by altering the parent zeolite catalyst via changes in acidity and/or incorporation of alkali metals known to accumulate on the catalyst during pyrolysis. Here, we report the catalytic fast pyrolysis of three biomass components (cellulose, xylan, and lignin) and switchgrass with two different HZSM-5 catalysts and their corresponding potassium exchanged counterparts (KZSM-S). The catalyst:biomass ratio and pyrolysis temperature were optimized for the production of monomeric phenols. The KZSM-S provided similar to 3-4-fold increases in the yields of both alkyl phenols and 2-methylfuran when compared to the noncatalytic and high acidity HZSM-5 catalyzed experiments while decreasing the yield of monoaromatic hydrocarbons. The observations made in the course of this study suggest that the main role of K exchange was to attenuate the Bronsted acidity of the zeolite and show that zeolites can be tuned to produce specific classes of compounds for use as renewable chemical feedstocks and can be used to develop catalytic conditions to further improve the yields of these valuable chemicals.