Microwave-assisted synthesis of NiO-MoO₃/H-MS catalysts: Engineering strong acid sites for efficient esterification of free fatty acids

This study reports the development of highly efficient NiO-MoO 3 bimetallic catalysts supported on modified molecular sieves (H-MS) for biodiesel production. Acid treatment of raw MS successfully increased the Si/Al ratio from 3.7 to 4.5 through dealumination, enhancing porosity (dP = 4.3 nm) while maintaining structural integrity. Microwave-assisted synthesis ensured high metal dispersion, as confirmed by XRD and TEM. Loading 10 wt.% NiO (10 NiO-H-MS) shifted the framework toward a mesopore-dominated structure (dP = 9.1 nm) with a total acidity of 2774 µmol NH3/g. For the bimetallic 5NiO-5MoO 3 /H-MS system, synergistic interactions between the metal oxides further expanded the average pore size to 10.6 nm and induced a unique optimization of the acid strength profile. Rather than relying on absolute quantity, the bimetallic synergy significantly enhanced the medium acid site density (827.7 µmol NH 3 /g) and restructured the strong acidity window; this was characterized by maintaining a prominent strong acid peak at 561.2 °C (976.8 µmol NH 3 /g) alongside the generation of a newly resolved, higher-temperature strong acid component at 644.3 °C (217.0 µmol NH 3 /g). Under optimized reaction conditions, the optimal 5NiO-5MoO 3 /H-MS catalyst demonstrated exceptional catalytic performance, achieving a maximum oleic acid conversion of 92.2% (significantly outperforming the monometallic counterpart) along with robust reusability over consecutive reaction cycles. These structural and chemical modifications successfully established a highly favorable environment characterized by a well-balanced distribution of enhanced medium-to-strong acidity and wide mesoporous channels, which are strategically configured to minimize internal mass transfer resistance for advanced catalytic applications.