Influence of carburization time on the activity of Mo2C/CNF catalysts for the HDO of guaiacol

Abstract

Carbon nanofiber (CNF) supported β-Mo2C-based catalysts were synthetized by carbothermal hydrogen reduction at 750 °C for different times (1, 2, 4, 6 and 18 h) in order to study both the carbide phase formation and the CNF stability during this key stage. Catalysts were characterized by N2 physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectrometry and scanning transmission electron microscopy. Subsequently, catalysts were evaluated in the hydrodeoxygenation (HDO) of guaiacol using a batch autoclave reactor at mild conditions of temperature and pressure (300 °C and 20 bar of H2). Liquid products were analysed after 2 h of reaction by gas chromatography. In all cases, β-Mo2C phase was formed during the carburization, and crystal sizes increased (from 6 to 18 nm as measured by XRD) as the carburization time did. However, a deep gasification of the support was observed in catalysts carburized above 6 h, resulting in a dramatic loss of specific surface area. In the guaiacol HDO reaction, higher conversions (72–80 %) and HDO ratios (53–63 %) were obtained using catalysts carburized for 2 h and onwards. Considering the carbide content in the catalyst, 1 and 2 h carburized catalysts exhibited the highest values of formation of HDO products: phenol, cyclohexane + benzene and anisole. Besides this, a clear relationship between the Mo2C surface area (SMo2C) and the product formation rate could be inferred for the catalysts carburized at 1, 2 and 4 h, in which the textural properties were not affected by the carburization time. On the other hand, longer carburization times had a worse performance despite the largest values of SMo2C, being this fact related to the dramatic loss of the catalyst textural properties.