Does a Strong Oxophilic Promoter Enhance Direct Deoxygenation? A Study of NiFe, NiMo, and NiW Catalysts in p-Cresol Conversion

A bifunctional catalyst, composed of a hydrogenation active metal and an oxophilic promoter for CAr–O bond weakening, is effective in the deoxygenation of lignin derivatives due to a contact synergy. A systematic comparison of the physicochemical properties of Ni-based catalysts promoted by metal oxides (i.e., FeOx, MoOx, and WOx) with different extents of oxophilicity was conducted, and their catalytic behaviors were evaluated in the conversion of p-cresol. Ni promoted by WOx displays the highest oxophilicity among all promoters and is intrinsically more active than NiMo and NiFe in direct deoxygenation (DDO); this characteristic is possibly related to its great hydride mobility. However, in a H2-pressured system, Ni promoted by MoOx, which has moderate oxophilicity compared with the employed promoters, generated more DDO product (toluene) than NiFe and NiW. This study discovered that this discrepancy, between the result of the chemical test under normal pressure and the result of the reaction test in an authentic pressurized system concerning the relation between toluene yield and a promoter's oxophilicity, is related to different hydrogenation rates of toluene. Toluene derived from DDO of p-cresol can be subsequently hydrogenated and thus be turned into methylcyclohexane more rapidly by the NiW catalyst than by NiMo and NiFe catalysts. Hence, both the oxophilicity of a promoter and the inertness of hydrogenation of toluene should be considered in designing a bimetallic catalyst in the conversion of lignin derivatives.