Fast pyrolysis oil stabilization kinetics over a Ni-Cu catalyst using propionic acid as a model compound

Propionic acid hydrotreatment was investigated as a model reaction for fast pyrolysis oil stabilization over a Ni-Cu/SiO2-ZrO2 catalyst. Intrinsic kinetics were acquired within a wide range of operating conditions resulting in an extended dataset of 37 experiments. Two major Ni-catalyzed conversion pathways, being hydrogenation to propanol and decarbonylation into ethane and carbon monoxide, were identified. Propanol, formed on the Ni sites after a series of hydrogenation steps, was found to also undergo decarbonylation at the investigated operating conditions. Additionally, the metal oxide support, catalyzed the esterification between propionic acid and propanol. The observation of only traces of decarboxylation and ketonization products indicated that these conversion pathways were not particularly pronounced in the investigated range of operating conditions. Due to the experimentally observed high methanation activity of Ni-Cu/SiO2-ZrO2, the hydrogenation route, which was favored at lower temperatures and higher total and partial hydrogen pressure was determined to be the preffered conversion pathway as compared to the decarbonylation route. The acquired experimental dataset was utilized for the construction of a comprehensive hydrodeoxigenation kinetic model based on elementary steps. The first step, which is a joint one in the decarbonylation and hydrogenation route, was found to be rate determining with activation energy of 118 kJ mol−1. The higher selectivity to propyl propionate compared to propane was ascribed to the higher affinity of propanol for the metal oxide support than for metal active sites. © 2018 The Authors