Engineering Metal–Acid Interactions in Ni–Re/HZSM-5 for Selective Hydrodeoxygenation of Oleic Acid: The Effect of Impregnation Sequence

Abstract The performance of bifunctional Ni–Re/zeolite catalysts in fatty-acid deoxygenation depends on the spatial organization of the metallic and acidic functions. In this work, the preparation sequence of bifunctional Ni–Re/HZSM-5 catalysts was varied to determine how metal organization and accessible acidity affect oleic acid hydrotreatment. Catalysts nominally containing 6 wt% Ni and 2 wt% Re were prepared by co-impregnation and by sequential Ni-first or Re-first impregnation. Additional calcination steps decreased Re retention from 2.0 wt% for NiRe/HZSM-5 to 1.5 and 1.0 wt% for Ni–Re/HZSM-5 and Re–Ni/HZSM-5, respectively. Co-impregnation gave the smallest XRD-derived Ni crystallites (9.2 nm), the highest apparent H 2 -chemisorption-derived dispersion of the series (2%), and a comparatively uniform metallic distribution. At 340 °C, 30 bar H 2 , and WHSV = 5 h⁻ 1 , all bimetallic catalysts maintained high oleic acid conversion, but the condensable-product distribution depended strongly on preparation route. NiRe/HZSM-5 initially favored alkyl aromatics, whereas Re–Ni/HZSM-5 favored C 1 ₇–C 18 paraffins and isoparaffins. After approximately 24 h, stearic acid accumulated despite sustained oleic acid conversion, indicating preferential loss of the deoxygenation function. Apparent coke contents followed Ni–Re/HZSM-5 (7.58%) > NiRe/HZSM-5 (6.32%) > Re–Ni/HZSM-5 (4.49%). The results show that impregnation sequence controls Re retention, accessible metallic surface, relative Brønsted/Lewis character, secondary conversion, and coke formation; none of the routes simultaneously maximized oxygen removal, hydrocarbon formation, and stability in the studied conditions.