Effect of Ni, Zn and Fe on hydrothermal liquefaction of cellulose: Impact on bio-crude yield and composition

The knowledge of cellulose behavior in hydrothermal conditions assisted by different transition metals, represents a necessary step towards a full comprehension of the more complex lignocellulosic biomass liquefaction mechanism and the consequent effect on the yield and quality of the bio-crude produced. Hydrothermal liquefaction (HTL) of cellulose was carried out with the addition of the metals Ni, Fe and Zn. Ni is a well-known hydrogenation catalyst while Fe and Zn are considered potential indirect hydrogenating agents, producing active hydrogen in situ through redox reaction with water. Among the tested metals Fe showed the best performances, by increasing the bio-crude yield from 17.4 % of the blank test to 26.5 % and by raising its higher heating value (HHV) from 27.0 MJ/kg to 29.7 MJ/kg. Using Zn an increase of the water-soluble products was obtained but only a slight increase in the bio-crude amount was observed. With all the tested transition metals the H/C ratio and HHV of the obtained bio-crude was significantly raised. Fe and Zn reduced furan derivatives and increased the 2-cyclopenten-1-ones and aromatic compounds in the bio-crude. The possible reaction pathway of cellulose HTL was also reported in this article.