Ni-Based Catalyst on Citric Acid-Modified ZSM-22 Promoting the Highly Selective Hydroisomerization of <i>n</i> -Hexadecane to Monobranched <i>i</i> -Hexadecane

Addressing the cold-flow limitations of Hydrotreated Vegetable Oil is a significant challenge in biofuel production, which can be mitigated through hydroisomerization. In this work, we design a 15% Ni/ZSM-22-C catalyst using citric acid modification for the hydroisomerization reaction of n-hexadecane, which is a model reaction for biodiesel upgrading. Under solvent-free conditions (300 °C, 3 MPa H2, substrate/catalyst mass ratio of 116:1), the optimal catalyst achieves an i-hexadecane yield of 85.1% with a hydrocracking yield of <10.0%. The stable needle-shaped morphology of 15% Ni/ZSM-22-C is confirmed by scanning electron microscopy (SEM). Mechanistic studies (Bhore’s delplot and β-scission type analyses) show that the 15% Ni/ZSM-22-C catalyst works via a pore-mouth mechanism. Here, a balanced metal–acid synergy (with an optimal metal–acid site ratio of CNi/CB = 1.24) and shape-selective confinement in the TON (International Zeolite Association code) channels of 15% Ni/ZSM-22-C can suppress β-scission and enable sequential hydroisomerization (from monobranched isomers to multibranched isomers). This non-noble metal and mild modification-supported catalyst demonstrates a green pathway to enhance the biodiesel cold-flow performance while minimizing the impact on energy density.