Submitted to: ACS Sustainable Chemistry & Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2017
Publication Date: 1/22/2017
Citation: Mullen, C.A., Tarves, P.C., Boateng, A.A. 2017. Role of potassium exchange in catalytic pyrolysis of biomass over ZSM-5: Formation of alkyl phenols and furans. ACS Sustainable Chemistry & Engineering. 5:2154-2162.
Interpretive Summary: The development of bio-based alternatives to petrochemicals is necessary to economically develop biorefineries that produce both fuels and chemicals. A processing method called catalytic fast pyrolysis, which involves the rapid heating of biomass such as wood, crop residues or grasses in the absence of oxygen over a solid catalytic material, can produce aromatic hydrocarbons, chemicals that are found in gasoline. In conventional petroleum refining, the majority of the products produced are fuels, but a majority of the profits rather come from petrochemicals. Therefore if biorefineries are to compete in the commodity marketplace then they must also produce valuable chemical co-products. In this study we have modified a catalyst commonly used for catalytic fast pyrolysis, i.e., a zeolite called HZSM-5 to include some potassium. Using these modified catalysts we were able to achieve about 3-4 fold increases in the yields of chemical compounds called alkyl phenols and furans, which command higher prices than the aromatic hydrocarbons used in fuels. These results also suggest that the products produced in the processing of biomass can be adjusted by simple variations in the catalyst. This information will be useful to those considering development and economics of a pyrolysis based bio-refinery.
Technical Abstract: Catalytic fast pyrolysis of biomass with ZSM-5 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, 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-5). The catalyst:biomass ratio and pyrolysis temperature were optimized for the production of monomeric phenols. The KZSM-5 provided approximately 3-4-fold increases in the yields of both alkyl phenols and 2-methylfuran when compared to the non-catalytic and high acidity HZSM-5 catalyzed experiments while decreasing the yield of mono-aromatic hydrocarbons. The observations made in the course of this study 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.