Role of in situ (in contact with biomass) and ex situ (in contact with pyrolysis vapors) transition metal catalysts on pyrolysis of cherry pits

Abstract

While energy dense liquid biofuels can be produced from sustainable waste feedstocks via thermochemical conversion techniques, these biofuels are plagued by high acidity and low stability, largely due to the presence of oxygenated functional groups. By utilizing transition metal catalysts during pyrolysis either in situ within the biomass, or ex situ (directly downstream supported on an alumina substrate) acting on the pyrolysis vapors, the quality of the bio-oil can be improved via deoxygenation. Four transition metals (Cu, Mn, Ni, Zn) were either wet impregnated on cherry pit biomass, or were impregnated on alumina wool fibers placed downstream of the biomass to act on the devolatilized gases. The in situ catalysts Cu, Mn and Zn increase the pyrolysis bio-oil yield. The Mn, Ni and Zn used as ex situ catalysts increase the total non-condensable gas produced, suggesting that these downstream catalysts crack the pyrolyzed molecules into smaller organics and water. The ex situ catalysts result in larger quantities of especially oxygen-containing gases like CO2, which in turn lowered the bio-oil’s O/C ratio. However, such reactions also increase the water content of the resulting bio-oil. Both Mn and Zn used in either location enhance the concentration of alcohols relative to aromatic carbons, and when used ex situ, Mn shows a decrease in the ratio of aromatic to aliphatic carbon content.