Solvent-free catalytic deoxygenation of oleic acid via nano-Ni/HZSM-5: Effect of reaction medium and coke characterization

Non-H2 and solvent-free catalytic conversion of waste lipids to liquid hydrocarbons is of great interest because of the associated low-cost, full-safety, and easy-operability. In this work, a solvent-free catalytic process for deoxygenating oleic acid was performed in H2 and non-H2 (N2, CH4, and CO2) media over nano-Ni/HZSM-5. 8-Heptadecene, a primary product obtained at 320 °C in four media, was derived from the straightforward decarboxylation of oleic acid. In non-H2 medium at 360 °C, the formation of C8-C15 alkanes was enhanced, with yields of ca. 65.05 mol%, 70.71 mol%, and 73.09 mol% for N2, CH4, and CO2, respectively. A low yield of 49.67 mol% C8-C15 alkanes in H2 medium suggested that the H2 medium reduced catalytic cracking. This was due to the preferential formation of stable heptadecane from 8-heptadecene in the presence of abundant H2. The absence of H2 favored the cracking of 8-heptadecene. These cracked products were further converted to C8-C15 alkanes, for which a mass supply of hydrogen was mandatory. Gas-phase reactions including methanation, Fisher-Tropsch (F-T) synthesis, and water-gas shift (WGS) reactions between deoxygenated gas products and reaction media provided significant pathways for the hydrogen required for the formation of alkanes. The CH4 medium also acted as a hydrogen source due to its decomposition, catalyzed by nano-Ni/HZSM-5. Coke was substantially formed in all the four media. It was highlighted that the medium of H2 favored the formation of aliphatic cokes, whereas the non-H2 media, particularly the CO2 medium, facilitated aromatic coke species on account of its weak oxidizability. Additionally, there was a severe loss of loaded Ni nanoparticles under H2 medium but a slight Ni loss in CO2 medium, which was proposed to the reason of aromatic cokes formation at the catalyst surface, acting as a trap for the loaded Ni nanoparticles.