Hydrodeoxygenation of guaiacol over Mo, W and Ta modified supported nickel catalysts

The thermochemical conversion of lignocellulosic biomass including lignin leads to the generation of a complex mixture of oxygenated aromatic monomers. Further processing of this mixture is essential for the production of value-added chemicals. The present investigation is therefore dedicated to the hydrodeoxygenation (HDO) of guaiacol as model compound over molybdenum (Mo), tungsten (W) and tantalum (Ta) modified nickel catalysts supported on Al2O3, ZrO2, TiO2 and SiO2 in a batch reactor. All the catalysts were prepared by co-impregnation method and characterized by several techniques such as BET, powder-XRD, H-2-TPR, UV-vis, FTIR and pyridine-FTIR to understand the interaction of nickel and modifier and their role in activity and product selectivity. Among the various catalysts studied, molybdenum modified nickel catalyst (Ni-Mo) supported on zirconia exhibits superior activity and selectivity. Detail characterization results revealed that various reducible surface species were formed depending on nickel to molybdenum mole ratio, total metal loading, reduction temperature and supports. The Ni-Mo/Zr catalysts are associated with metallic nickel and partially reduced Mo4+ species which acts as Lewis acid sites. The proportion of Ni/Mo4+ varies with the nickel to molybdenum mole ratio and reduction temperature. The Ni-Mo/Zr catalyst of equimolar Ni-Mo composition showed higher HDO activity of guaiacol and selectivity of phenol due to the presence of optimum proportion of Ni and Mo4+ species. During HDO of guaiacol, phenol, catechol, anisole, cresols, cyclohexanol, cyclohexanone, cyclohexene and cyclohexane were identified as products and phenol was the primary product with higher selectivity. A possible reaction pathway is proposed based on the product distribution obtained under wide range of process parameters and using different catalysts. The structural informations of catalysts obtained from various characterization studies are co-related with the HDO activity and selectivity of products.