| To reduce the greenhouse gas footprint of the transportation industry and prepare for the economic impacts of global instability and oil scarcity, technologies and strategies must be developed to transition from petroleum-derived fuels to biomass-derived liquid fuels. For this transition to occur successfully in the short term, the biofuel must be a liquid hydrocarbon. Lignocellulosic biomass is an abundant, inexpensive biomass resource that does not compete with food production, making it an ideal candidate for biofuel production. Although many technologies are advancing towards this goal, catalytic fast pyrolysis (CFP) using ZSM-5 catalyst is extremely promising. During CFP, biomass is rapidly heated (~1000 °C/s) in the absence of oxygen to a reaction temperature of 400-600°C with a short residence time of 1--2 s. The resulting vapors are upgraded (deoxygenated) upon contact with a catalyst prior to condensation.;CFP, performed with ZSM-5 catalyst, allows for the direct generation of aromatic hydrocarbons (primarily: benzene, toluene, xylene, and naphthalenes) from lignocellulosic biomass, without requiring pretreatment. This conversion occurs via a two-step series of reactions, consisting of initial cracking reactions on the ZSM-5 surface followed by deoxygenation and aromatization reactions occurring within the ZSM-5 pores. A primary hurdle for the large-scale deployment of CFP as a biofuel production method is coke formation on the catalyst. Coke is a solid, carbonaceous deposit which forms on the catalyst and causes a reversible deactivation, leading to the formation of oxygenated products. The carbon lost to coke also limits the overall conversion to desired products. The coke formation problem is exacerbated by the low hydrogen-to-carbon ratio in biomass. Despite the varied levels of deactivation experienced by individual catalyst particles within large-scale CFP reactors, little research has been conducted on ZSM-5 deactivation during CFP of biomass.;The overarching goal of this work is to advance the understanding of catalytic fast pyrolysis of biomass with ZSM-5 catalyst, in particular the process of catalyst deactivation, and guide future work towards enhancing its viability as a biofuel production process. This is accomplished by a) exploring how the products of catalytic fast pyrolysis change as the ZSM-5 catalyst progressively deactivates, b) studying the impact of catalyst properties (silica-to-alumina ratio and binder) on CFP, c) investigating the role of the two main component groups of lignocellulosic biomass (cellulose and lignin) in the deactivation of ZSM-5, and d) exploring metal modification of ZSM-5 and hydrogen addition in an effort to increase the hydrocarbon yield and improve catalyst performance.;Deactivation of ZSM-5 during CFP was studied by pyrolyzing and upgrading successive samples of biomass over a bed of catalyst and monitoring the products as the cumulative ratio of biomass pyrolyzed to catalyst bed weight (biomass:catalyst) increased, as well as characterizing and comparing fresh and post-reaction catalyst samples.;It was found that as the catalyst deactivated, the formation of fully upgraded aromatics decreased and that of primary pyrolysis vapors increased. In addition, products formed at intermediate levels of catalyst deactivation that did not form at low or high biomass:catalyst, including phenol and alkylated phenols. These were shown to be the results of the catalytic upgrading process and not a result of partial deoxygenation of existing phenolic components in the pyrolysis vapors, as they were produced during the upgrading of cellulose vapors as well. A silica-to-alumina ratio of 30 within the ZSM-5 crystal gave the highest yield of t aromatics due to an optimal level of deoxygenation capability while not leading to excessive coke formation. The binder choice for ZSM-5 catalyst particles was also shown to be important, as alumina resulted in a significant decrease in catalyst efficacy, due to the imparted acidity from the binder leading to excessive cracking.;The study of deactivation by individual biopolymer revealed two types of coke-induced deactivation occurring during CFP of lignocellulosic biomass, one caused by the upgrading of lignin-derived pyrolysis vapors and the other caused by the upgrading of cellulose-derived pyrolysis vapors. Cellulose-induced deactivation occurs by the formation of coke resulting from an extension of the aromatization and ring-growth reactions. This coke prevents the secondary ring-growth step of upgrading, reducing the formation of aromatics, by blocking micropores and obstructing access to acid sites. Lignin-induced deactivation, caused by monomer deposition and coupling, inhibits the initial surface cracking reactions, limiting the material which can be further upgraded in the catalyst, and leading to the breakthrough of primary pyrolysis products. However, the catalyst's acid sites remained accessible and active, allowing a stable yield of hydrocarbons to be produced. The loss of aromatic hydrocarbon formation during CFP of whole biomass is mainly the result of deactivation by the cellulosic components. (Abstract shortened by ProQuest.). |
