Catalytic co-pyrolysis of switchgrass and polyethylene over HZSM-5: catalyst deactivation and coke formation

Authors: Mullen, Charles; Dorado, Christina; Boateng, Akwasi

Submitted to: Journal of Analytical and Applied Pyrolysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/14/2017
Publication Date: 2/1/2018
Publication URL: http://handle.nal.usda.gov/10113/5869820
Citation: Mullen, C.A., Dorado, C., Boateng, A.A. 2018. Catalytic co-pyrolysis of switchgrass and polyethylene over HZSM-5: catalyst deactivation and coke formation. Journal of Analytical and Applied Pyrolysis. 129:195-203.

Interpretive Summary: Biomass, such as crop residues (such as corn stover), herbaceous grasses (such as switchgrass) and woody materials (such as willow and poplar), are the largest cellulosic based source available and can be readily converted to liquids, known as bio-oil, by rapid heating in the absence of air, otherwise known as fast pyrolysis. Bio-oil from these cellulosic sources is not miscible with current petroleum based fuels due to high water and oxygen content. These undesirable properties can be combatted by the use of catalysts, in combination with fast pyrolysis, that are selective for converting cellulosic biomass into compounds called aromatic hydrocarbons that are found in gasoline. However, catalysts used in this manner quickly become deactivated by the accumulation of carbon deposits on the catalyst; this could be alleviated by increasing the amount of hydrogen rich carbon molecules being reacted. Agricultural plastic waste could serve as a source for this hydrogen rich carbon and its participation in catalytic fast pyrolysis with cellulosic biomass would serve as a method for agricultural plastic waste removal from the environment. Previously, we reported that the addition of plastic was beneficial to the yield of hydrocarbons from fresh catalysts. In this study, for the first time, the role of plastic addition on the lifetime of the catalyst is studied. Results show that the catalyst remains more active initially due to lessening of solid carbon deposits from switchgrass on the catalysts, but over the long term the catalysts were still deactivated suggesting that use of catalyst can extend catalyst lifetime to a point. The results of this work will be useful to those designing pyrolysis based biorefineries and those addressing the issue of agricultural plastic waste disposal.

Technical Abstract: When conducted in the presence of a zeolite catalyst such as HZSM-5, fast pyrolysis of biomass can promote the rejection of oxygen and the formation of aromatic hydrocarbons in the organic liquid products. Unfortunately, this pathway removes hydrogen from the already hydrogen deficient biomass starting material, limiting the yield of hydrocarbons and leading to coke formation which results in catalyst deactivation. Co-pyrolysis of biomass and a low cost hydrogen rich material such as waste plastic has been considered as one strategy for mitigating hydrogen-deficiency with an added benefit of disposing of waste plastics effectively. Previous work from our lab showed an enhancement of aromatic yields when polyolefins were copyrolyzed with biomass over fresh HZSM-5, but studies on the effect of catalyst deactivation with repeated use of the catalyst are lacking. In this study, pyrolysis coupled with gas chromatography and mass spectrometry (py-GC/MS) with an external catalytic reactor was used to perform ex situ catalytic co-pyrolysis of switchgrass and polyethylene (1:1 w:w) in the presence of HZSM-5. The catalyst (approximately 15mg) was exposed to approximately 1mg biomass and/or plastic in a series of 30 or 60 pulsed experiments for a cumulative feedstock to catalyst ratio of 2:1 and 4:1, respectively. Results showed that the initial rate of catalyst deactivation (up to feed:catalyst 2:1), as measured by the decrease in production of aromatic hydrocarbons as the number of pulses increased was significantly decreased, estimated at only approximately 28% of the rate of deactivation that processing of switchgrass alone caused. However, over the continued use of the catalyst, up to a feed to catalyst ratio of 4:1, the rates of deactivation from the blended feedstock increased to near expected rates based on the rate of deactivation for the switchgrass and HDPE individually.