Recoverable Ni-Based Catalyst for the Selective Hydrodeoxygenation of Vanillin Using Isopropanol as a Renewable LOHC

Nowadays, the definition of suitable methodologies to access high-value-added products from biomass valorization is of the utmost importance. In this context, the upgrading of lignin-derived vanillin into biofuels exploiting non-noble metal-based catalysts plays a central role. In particular, the use of nickel as the active site is limited to rare examples, especially in the presence of Liquid Organic Hydrogen Carriers (LOHCs) as the H-source. Herein, a series of eight heterogeneous catalytic systems, mainly based on nickel as the active site, was prepared using an ultrasound-assisted synthetic procedure. The materials were fully characterized and tested in the vanillin hydrodeoxygenation reaction to afford 2-methoxy-4-methylphenol (MMP) without the employment of a high-pressure and external hydrogen supply. Indeed, the selective production of MMP using isopropanol as both the reaction medium and H-source was achieved using Ni/Al2O3-B as a catalyst, which featured a metallic nickel surface rather than nickel oxide. The selected material was recovered and reused for consecutive cycles, showing good durability and stability. Besides the low metal leaching (<0.7%), the catalyst morphology after recovery remained unchanged, as evidenced by SEM-EDX, TEM, and XPS analyses performed on the spent catalyst, further confirming the stability of Ni/Al2O3-B. The green metric comparison further highlights the potential of our Ni/Al2O3-B catalyst in performing sustainable vanillin HDO.