Selective hydrodeoxygenation of monophenolics from lignin bio-oil for preparing cyclohexanol and its derivatives over Ni-Co/Al2O3-MgO catalyst

It is of significance to efficiently convert renewable and sustainable lignin bio-oil into cyclohexanol and its derivatives by selective hydrodeoxygenation to alleviate excessive dependence on fossil resources. However, the complicated components of lignin bio-oil hindered its application as bulk chemicals. A bimetallic Ni-Co catalyst supported on MgO-modified γ-Al2O3 was constructed for the catalytic hydrodeoxygenation of monophenolics from lignin bio-oil. The bimetallic catalyst with adding Co increased active centers and acid sites, improving the catalytic activity to the hydrodeoxygenation, compared to the Ni-based catalyst. The proper addition of MgO into the catalyst carrier increased the interplay among the Ni, Co particles and the catalyst carrier owing to the electron transfer from MgO into Ni and Co species. The enhanced interplay between active metal particles and the catalyst carrier benefited the dispersion of Ni, Co particles, and the average Ni and Co particle size decreased from 14 nm to 10 nm. Furthermore, MgO increased the basic sites, which effectively inhibited the elimination of the hydroxyl groups in phenols and preserved hydroxyl groups, resulting in a significant improvement in the selectivity of cyclohexanol and its derivatives. Ni-Co/Al2O3-MgO with good cycling stability contributed to a 65.6 wt% yield of cyclohexanol and its derivatives when the hydrodeoxygenation of monophenolics (such as guaiacol, 2,6-dimethoxyphenol and their alkyl substitutes) was pursued in isopropanol at 300 ℃ and 2 MPa H2 for 5 h. It was found that excessive dosage of the catalyst, higher temperature and hydrogen pressure, as well as longer reaction time, caused the excessive hydrodeoxygenation of monophenolics and hence decreased the selectivity to cyclohexanol and its derivatives. This work might provide a new idea for the high-value utilization of lignin bio-oil.