Dual-Promoted Trimetallic CoMo-Ni/Al2O3-K2O Catalysts: Impact of K2O Doping on Guaiacol Hydrodeoxygenation Selectivity

The influence of potassium oxide (K2O) doping on the hydrodeoxygenation (HDO) performance of trimetallic CoMo–Ni/Al2O3 catalysts was systematically investigated using guaiacol as a lignin-derived model compound. Catalysts containing 0, 1, 3, and 5 wt% K2O were synthesized and characterized by SEM-EDS, N2 physisorption, XRD, FTIR, and HRTEM. SEM micrographs showed homogeneous morphologies with no significant agglomeration, while EDS analysis confirmed elemental compositions close to nominal values, with K2O contents increasing proportionally and maintaining uniform surface distribution. Adsorption–desorption isotherms confirmed mesoporous structures with specific surface areas ranging from 258 to 184 m2 g−1, decreasing with increasing K2O loading. XRD revealed γ-Al2O3, NiO, (NH4)3[CoMo6O24H6]·7H2O, and K2O phases, with slight peak shifts indicating surface modification rather than lattice incorporation of K+. FTIR spectra evidenced characteristic polyoxomolybdate vibrations and metal–oxygen interactions with alumina. HRTEM revealed MoS2 slab lengths between 1.85 and 2.51 nm, stacking numbers from 2.08 to 3.17, and Mo edge-to-corner ratios (fe/fc) between 1.39 and 2.43, corresponding to dispersions of 0.45–0.57. Guaiacol conversion remained high (≥95%) for all catalysts, while HDO selectivity strongly depended on K2O content. At 5 wt% K2O, cyclohexane selectivity reached 81.3% with an HDO degree of 65%, compared to 52.0% and 31% for the undoped catalyst. Pseudo-first-order kinetic analysis revealed that potassium promotes demethylation and demethoxylation steps while suppressing rearrangement pathways, steering the reaction network toward direct deoxygenation. These results demonstrate that K2O acts as an efficient structural and electronic promoter, enabling precise control of HDO selectivity without compromising catalytic activity.