Abstract
In this study, the contamination of H+ZSM-5 catalyst by calcium, potassium and sodium was investigated by deactivating the catalyst with various concentrations of these inorganics, and the subsequent changes in the properties of the catalyst are reported. Specific surface area analysis of the catalysts revealed a progressive reduction with increasing concentrations of the inorganics, which could be attributed to pore blocking and diffusion resistance. Chemisorption studies (NH3-TPD) showed that the Bronsted acid sites on the catalyst had reacted with potassium and sodium, resulting in a clear loss of active sites, whereas the presence of calcium did not appear to cause extensive chemical deactivation. Pyrolysis experiments revealed the progressive loss in catalytic activity, evident due the shift in selectivity from producing only aromatic hydrocarbons (benzene, toluene, xylene, naphthalenes and others) with the fresh catalyst to oxygenated compounds such as phenols, guaiacols, furans and ketones with increasing contamination by the inorganics. The carbon yield of aromatic hydrocarbons decreased from 22.3% with the fresh catalyst to 1.4% and 2.1% when deactivated by potassium and sodium at 2 wt %, respectively. However, calcium appears to only cause physical deactivation.
Highlights
Catalytic fast pyrolysis (CFP) has been investigated in recent years as a thermochemical conversion method for producing partially deoxygenated liquid fuel intermediates from biomass
We investigated the effect of individual biomass inorganics on the functionality of H+ZSM-5 catalyst during in-situ CFP
The accumulation of biomass inorganics and the resulting deactivation of the catalyst used in CFP is a critical issue that affects the commercial viability of the process
Summary
Catalytic fast pyrolysis (CFP) has been investigated in recent years as a thermochemical conversion method for producing partially deoxygenated liquid fuel intermediates from biomass. The improved thermal stability and lower oxygen content of bio-oil produced from CFP decreases the burden on the economically inefficient hydrotreating step, which utilizes expensive metal catalysts, high temperature and high pressure of hydrogen [5,6,7,8] Solid acid catalysts such as H+ZSM-5, Y-zeolite, β-zeolite are among the most commonly used materials, which transform the pyrolysis vapor by rejecting oxygen through dehydration (-H2O), decarboxylation (-CO) and decarbonylation (-CO2) reactions, leading to a product composed of aromatic hydrocarbons and olefins. Pyrolysis reactor systems typically counter this problem by employing a regeneration step wherein the coked catalyst is thermally oxidized to remove the carbon deposits in an effort to restore the original activity of the catalyst and provide process heat for the pyrolysis zone using the exothermic nature of the oxidation reaction Among these catalysts, H+ZSM-5 zeolite has been the most widely studied and considered unique due to its shape selectivity that suppresses the coke formation, while maximizing the conversion to aromatic hydrocarbons [11]
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