Interconnected hierarchical HUSY zeolite-loaded Ni nano-particles probed for hydrodeoxygenation of fatty acids, fatty esters, and palm oil

Abstract

Hierarchical H-style ultra-stable Y (HUSY) zeolites with abundant interconnected mesopores have been prepared using a sequential post-synthesis strategy that includes steaming dealumination and mixed-alkali desilication. The steaming treatment generates a broad size range of intra-mesopores (around 25 and 45 nm) and a moderate Si/Al ratio of 13.4 in the HUSY, which provides optimal material precursors for the ensuing subsequent alkaline desilication. N₂ adsorption–desorption isotherms and X-ray diffractometry results indicate that the sample treated with pyridine/sodium hydroxide (HUSY-4) has a larger external surface area and a higher relative crystallinity. Infrared spectra of adsorbed pyridine show that HUSY-4 contains substantial Brønsted acid sites. The ²⁷Al and ²⁹Si nuclear magnetic resonance spectra show that HUSY-4 possesses few extra-framework alumina species. Infrared spectra in a vacuum show that the peak intensities of HUSY-4 in the bridged hydroxyl group (at 3560 and 3631 cm⁻¹) are much stronger than those of the sample treated with tetrapropylammonium hydroxide (HUSY-3), indicating that the framework integrity of HUSY-4 is better. Differences in treatments with tetrapropylammonium hydroxide/sodium hydroxide and pyridine/sodium hydroxide treatments are attributed to the fact that the pyridine molecule (0.54 nm) can pass through the supercages (0.74 nm) to protect the zeolite framework from deep desilication, whereas the tetrapropylammonium hydroxide molecule (0.85 nm) is adsorbed only on the external surface. Eventually, a HUSY zeolite with a high external surface area, inter-connectedness and hierarchical mesopores (10, 25, and 45 nm) is prepared by initial high-temperature steaming, which is followed by desilication using a mixed alkali solution containing pyridine and sodium hydroxide. High-dispersion (5.5%), high-content (35 wt%), small Ni nanoparticles (4.9 ± 1.2 nm) are loaded onto and into the external surface areas and interpores of the hierarchical HUSY by the deposition–precipitation method. The resultant Ni/HUSY-4 shows an ultra-high efficiency in the hydrodeoxygenation of fatty acids, esters, and palm oil, and achieves high initial rates (60 g g⁻¹ h⁻¹) and a high C₁₈ alkane selectivity (96%), which may be attributed to the enhanced Brønsted acid and adjacent Lewis acid (confirmed by the ¹H DQ MAS NMR spectrum) together with the substantial dispersive Ni nanoparticles loaded onto/into the interconnected pores of the hierarchical HUSY support.