Hydrodeoxygenation activity of W modified Ni/H-ZSM-5 catalyst for single step conversion of levulinic acid to pentanoic acid: An insight on the reaction mechanism and structure activity relationship

Direct conversion of levulinic acid (LA) to pentanoic acid (PA) was achieved over the W modified Ni supported on SiO2, Al2O3 and H-ZSM-5 catalysts at 270 °C and ambient H2 pressure in a continuous flow fixed-bed reactor. The interaction between Ni and W was the key which is rationalized by XRD, XPS, TGA-TPD of NH3, DRIFTS, TEM, CO-pulse chemisorption and H2-TPR analyses. PA rate of ∼1.24 μmol s−1 gcat−1 (with highest LA conversion ∼40% and selectivity ∼65% after 6 h of continuous operation) achieved over W modified Ni/H-ZSM-5 catalyst compared to others. The better PA rate on Ni-W/H-ZSM-5 catalyst was manifested by the presence of not only strong acid sites and also due to a higher number surface ionic Ni species which were responsible for the ring opening of γ-valerolactone (GVL) and the β-proton abstraction that occurred on basic sites. On the contrary, Ni-W/SiO2, Ni-W/Al2O3 and Ni/H-ZSM-5 were found to be less selective towards PA and more selective towards GVL, exemplifying the role of support and W for the PA selectivity. The surface active sites for the PA formation are illustrated by using probes such as pyridine (strong base) and/or acetone (mild base) and formic acid adsorbed DRIFT spectroscopy. Based on the product distribution, a plausible surface mechanism was also elucidated and discussed.