Hydrodeoxygenation of 2-ethylphenol as a model compound of bio-crude over sulfided Mo-based catalysts: Promoting effect and reaction mechanism

The hydrodeoxygenation of 2-ethylphenol was carried out under 7 MPa of total pressure and at 340 °C in a fixed-bed reactor over unpromoted Mo/Al2O3 catalyst and over two promoted catalysts (CoMo/Al2O3 and NiMo/Al2O3). For all experiments, dimethyldisulfide was added to the feed to maintain the sulfidation state of the catalysts. On these sulfided catalysts, the transformation of 2-ethylphenol is considered to proceed by three pathways: (1) prehydrogenation of the aromatic ring followed by a dehydration reaction leading to a mixture of alkenes (1-ethylcyclohexene and 3-ethylcyclohexene) and after hydrogenation leading to ethylcyclohexane (HYD pathway); (2) direct cleavage of the Csp2–O bond leading to ethylbenzene (DDO pathway); (3) disproportionation and isomerization reactions leading to oxygenated products (phenol, isomers of 2-ethylphenol and diethylphenols) and their deoxygenated products (ACI pathway). The production of those oxygenated compounds mainly involved the support acidity. The presence of nickel and cobalt allowed an increase of the deoxygenation rate. Nickel only promoted the HYD pathway whereas cobalt promoted both the HYD and DDO pathways. Consequently, the DDO/HYD selectivity was very dependent on the catalyst used. The highest DDO/HYD selectivity was obtained for CoMo/Al2O3. Sulfur vacancies are proposed as active sites for both deoxygenation pathways of 2-ethylphenol, although other active sites (e.g. brim sites) could be involved to explain that the HYD pathway was always predominant. For both deoxygenation pathways, two probable mechanisms are described. The adsorption mode of the molecule most likely determines the deoxygenation route.